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diff --git a/doc/autoconf.texi b/doc/autoconf.texi new file mode 100644 index 0000000..34ca213 --- /dev/null +++ b/doc/autoconf.texi @@ -0,0 +1,26667 @@ +\input texinfo @c -*-texinfo-*- +@comment ======================================================== +@comment %**start of header +@setfilename autoconf.info +@include version.texi +@settitle Autoconf +@setchapternewpage odd +@ifnothtml +@setcontentsaftertitlepage +@end ifnothtml +@finalout + +@c @ovar(ARG) +@c ---------- +@c The ARG is an optional argument. To be used for macro arguments in +@c their documentation (@defmac). +@macro ovar{varname} +@r{[}@var{\varname\}@r{]}@c +@end macro + +@c @dvar(ARG, DEFAULT) +@c ------------------- +@c The ARG is an optional argument, defaulting to DEFAULT. To be used +@c for macro arguments in their documentation (@defmac). +@macro dvar{varname, default} +@r{[}@var{\varname\} = @samp{\default\}@r{]}@c +@end macro + +@c Handling the indexes with Texinfo yields several different problems. +@c +@c Because we want to drop out the AC_ part of the macro names in the +@c printed manual, but not in the other outputs, we need a layer above +@c the usual @acindex{} etc. That's why we first define indexes such as +@c acx meant to become the macro @acindex. First of all, using ``ac_'' +@c does not work with makeinfo, and using ``ac1'' doesn't work with TeX. +@c So use something more regular ``acx''. Then you finish with a printed +@c index saying ``index is not existent''. Of course: you ought to use +@c two letters :( So you use capitals. +@c +@c Second, when defining a macro in the TeX world, following spaces are +@c eaten. But then, since we embed @acxindex commands that use the end +@c of line as an end marker, the whole things wrecks itself. So make +@c sure you do *force* an additional end of line, add a ``@c''. +@c +@c Finally, you might want to get rid of TeX expansion, using --expand +@c with texi2dvi. But then you wake up an old problem: we use macros +@c in @defmac etc. where TeX does perform the expansion, but not makeinfo. + +@c Define an environment variable index, for variables users may set +@c in their environment or on the configure command line. +@defcodeindex ev +@c Define an output variable index, for commonly AC_SUBST'ed variables. +@defcodeindex ov +@c Define a cache variable index, for variables matching *_cv_*. +@defcodeindex CA +@c Other shell variables not fitting the above categories should be +@c listed in the predefined vrindex, which we merge in the concept index. +@syncodeindex vr cp +@c Define a CPP preprocessor macro index, for #define'd strings. +@defcodeindex cv +@c Define an Autoconf macro index that @defmac doesn't write to. +@defcodeindex AC +@c Define an Autotest macro index that @defmac doesn't write to. +@defcodeindex AT +@c Define an M4sugar macro index that @defmac doesn't write to. +@defcodeindex MS +@c Define an index for *foreign* programs: `mv' etc. Used for the +@c portability sections and so on. +@defindex pr + +@c shortindexflag +@c -------------- +@c Shall we factor AC_ out of the Autoconf macro index etc.? +@iftex +@set shortindexflag +@end iftex + +@c @acindex{MACRO} +@c --------------- +@c Registering an AC_\MACRO\. +@ifset shortindexflag +@macro acindex{macro} +@ACindex \macro\ +@c +@end macro +@end ifset +@ifclear shortindexflag +@macro acindex{macro} +@ACindex AC_\macro\ +@end macro +@end ifclear + +@c @ahindex{MACRO} +@c --------------- +@c Registering an AH_\MACRO\. +@macro ahindex{macro} +@ACindex AH_\macro\ +@c +@end macro + +@c @asindex{MACRO} +@c --------------- +@c Registering an AS_\MACRO\. +@ifset shortindexflag +@macro asindex{macro} +@MSindex \macro\ +@c +@end macro +@end ifset +@ifclear shortindexflag +@macro asindex{macro} +@MSindex AS_\macro\ +@end macro +@end ifclear + +@c @atindex{MACRO} +@c --------------- +@c Registering an AT_\MACRO\. +@ifset shortindexflag +@macro atindex{macro} +@ATindex \macro\ +@c +@end macro +@end ifset +@ifclear shortindexflag +@macro atindex{macro} +@ATindex AT_\macro\ +@end macro +@end ifclear + +@c @auindex{MACRO} +@c --------------- +@c Registering an AU_\MACRO\. +@macro auindex{macro} +@ACindex AU_\macro\ +@c +@end macro + +@c @hdrindex{MACRO} +@c ---------------- +@c Indexing a header. +@macro hdrindex{macro} +@prindex @file{\macro\} +@c +@end macro + +@c @msindex{MACRO} +@c --------------- +@c Registering an m4_\MACRO\. +@ifset shortindexflag +@macro msindex{macro} +@MSindex \macro\ +@c +@end macro +@end ifset +@ifclear shortindexflag +@macro msindex{macro} +@MSindex m4_\macro\ +@end macro +@end ifclear + + +@c @caindex{VARIABLE} +@c ------------------ +@c Registering an ac_cv_\VARIABLE\ cache variable. +@ifset shortindexflag +@macro caindex{macro} +@CAindex \macro\ +@end macro +@end ifset +@ifclear shortindexflag +@macro caindex{macro} +@CAindex ac_cv_\macro\ +@end macro +@end ifclear + +@c Define an index for functions: `alloca' etc. Used for the +@c portability sections and so on. We can't use `fn' (aka `fnindex), +@c since `@defmac' goes into it => we'd get all the macros too. + +@c FIXME: Aaarg! It seems there are too many indices for TeX :( +@c +@c ! No room for a new @write . +@c l.112 @defcodeindex fu +@c +@c so don't define yet another one :( Just put some tags before each +@c @prindex which is actually a @funindex. +@c +@c @defcodeindex fu +@c +@c +@c @c Put the programs and functions into their own index. +@c @syncodeindex fu pr + +@comment %**end of header +@comment ======================================================== + +@copying + +This manual (@value{UPDATED}) is for GNU Autoconf +(version @value{VERSION}), +a package for creating scripts to configure source code packages using +templates and an M4 macro package. + +Copyright @copyright{} 1992-1996, 1998-2012 Free Software Foundation, +Inc. + +@quotation +Permission is granted to copy, distribute and/or modify this document +under the terms of the GNU Free Documentation License, +Version 1.3 or any later version published by the Free Software +Foundation; with no Invariant Sections, no Front-Cover texts, and +no Back-Cover Texts. A copy of the license is included in the section +entitled ``GNU Free Documentation License.'' +@end quotation +@end copying + + + +@dircategory Software development +@direntry +* Autoconf: (autoconf). Create source code configuration scripts. +@end direntry + +@dircategory Individual utilities +@direntry +* autoscan: (autoconf)autoscan Invocation. + Semi-automatic @file{configure.ac} writing +* ifnames: (autoconf)ifnames Invocation. Listing conditionals in source. +* autoconf-invocation: (autoconf)autoconf Invocation. + How to create configuration scripts +* autoreconf: (autoconf)autoreconf Invocation. + Remaking multiple @command{configure} scripts +* autoheader: (autoconf)autoheader Invocation. + How to create configuration templates +* autom4te: (autoconf)autom4te Invocation. + The Autoconf executables backbone +* configure: (autoconf)configure Invocation. Configuring a package. +* autoupdate: (autoconf)autoupdate Invocation. + Automatic update of @file{configure.ac} +* config.status: (autoconf)config.status Invocation. Recreating configurations. +* testsuite: (autoconf)testsuite Invocation. Running an Autotest test suite. +@end direntry + +@titlepage +@title Autoconf +@subtitle Creating Automatic Configuration Scripts +@subtitle for version @value{VERSION}, @value{UPDATED} +@author David MacKenzie +@author Ben Elliston +@author Akim Demaille +@page +@vskip 0pt plus 1filll +@insertcopying +@end titlepage + +@contents + + +@ifnottex +@node Top +@top Autoconf +@insertcopying +@end ifnottex + +@c The master menu, created with texinfo-master-menu, goes here. + +@menu +* Introduction:: Autoconf's purpose, strengths, and weaknesses +* The GNU Build System:: A set of tools for portable software packages +* Making configure Scripts:: How to organize and produce Autoconf scripts +* Setup:: Initialization and output +* Existing Tests:: Macros that check for particular features +* Writing Tests:: How to write new feature checks +* Results:: What to do with results from feature checks +* Programming in M4:: Layers on top of which Autoconf is written +* Programming in M4sh:: Shell portability layer +* Writing Autoconf Macros:: Adding new macros to Autoconf +* Portable Shell:: Shell script portability pitfalls +* Portable Make:: Makefile portability pitfalls +* Portable C and C++:: C and C++ portability pitfalls +* Manual Configuration:: Selecting features that can't be guessed +* Site Configuration:: Local defaults for @command{configure} +* Running configure Scripts:: How to use the Autoconf output +* config.status Invocation:: Recreating a configuration +* Obsolete Constructs:: Kept for backward compatibility +* Using Autotest:: Creating portable test suites +* FAQ:: Frequent Autoconf Questions, with answers +* History:: History of Autoconf +* GNU Free Documentation License:: License for copying this manual +* Indices:: Indices of symbols, concepts, etc. + +@detailmenu + --- The Detailed Node Listing --- + +The GNU Build System + +* Automake:: Escaping makefile hell +* Gnulib:: The GNU portability library +* Libtool:: Building libraries portably +* Pointers:: More info on the GNU build system + +Making @command{configure} Scripts + +* Writing Autoconf Input:: What to put in an Autoconf input file +* autoscan Invocation:: Semi-automatic @file{configure.ac} writing +* ifnames Invocation:: Listing the conditionals in source code +* autoconf Invocation:: How to create configuration scripts +* autoreconf Invocation:: Remaking multiple @command{configure} scripts + +Writing @file{configure.ac} + +* Shell Script Compiler:: Autoconf as solution of a problem +* Autoconf Language:: Programming in Autoconf +* Autoconf Input Layout:: Standard organization of @file{configure.ac} + +Initialization and Output Files + +* Initializing configure:: Option processing etc. +* Versioning:: Dealing with Autoconf versions +* Notices:: Copyright, version numbers in @command{configure} +* Input:: Where Autoconf should find files +* Output:: Outputting results from the configuration +* Configuration Actions:: Preparing the output based on results +* Configuration Files:: Creating output files +* Makefile Substitutions:: Using output variables in makefiles +* Configuration Headers:: Creating a configuration header file +* Configuration Commands:: Running arbitrary instantiation commands +* Configuration Links:: Links depending on the configuration +* Subdirectories:: Configuring independent packages together +* Default Prefix:: Changing the default installation prefix + +Substitutions in Makefiles + +* Preset Output Variables:: Output variables that are always set +* Installation Directory Variables:: Other preset output variables +* Changed Directory Variables:: Warnings about @file{datarootdir} +* Build Directories:: Supporting multiple concurrent compiles +* Automatic Remaking:: Makefile rules for configuring + +Configuration Header Files + +* Header Templates:: Input for the configuration headers +* autoheader Invocation:: How to create configuration templates +* Autoheader Macros:: How to specify CPP templates + +Existing Tests + +* Common Behavior:: Macros' standard schemes +* Alternative Programs:: Selecting between alternative programs +* Files:: Checking for the existence of files +* Libraries:: Library archives that might be missing +* Library Functions:: C library functions that might be missing +* Header Files:: Header files that might be missing +* Declarations:: Declarations that may be missing +* Structures:: Structures or members that might be missing +* Types:: Types that might be missing +* Compilers and Preprocessors:: Checking for compiling programs +* System Services:: Operating system services +* Posix Variants:: Special kludges for specific Posix variants +* Erlang Libraries:: Checking for the existence of Erlang libraries + +Common Behavior + +* Standard Symbols:: Symbols defined by the macros +* Default Includes:: Includes used by the generic macros + +Alternative Programs + +* Particular Programs:: Special handling to find certain programs +* Generic Programs:: How to find other programs + +Library Functions + +* Function Portability:: Pitfalls with usual functions +* Particular Functions:: Special handling to find certain functions +* Generic Functions:: How to find other functions + +Header Files + +* Header Portability:: Collected knowledge on common headers +* Particular Headers:: Special handling to find certain headers +* Generic Headers:: How to find other headers + +Declarations + +* Particular Declarations:: Macros to check for certain declarations +* Generic Declarations:: How to find other declarations + +Structures + +* Particular Structures:: Macros to check for certain structure members +* Generic Structures:: How to find other structure members + +Types + +* Particular Types:: Special handling to find certain types +* Generic Types:: How to find other types + +Compilers and Preprocessors + +* Specific Compiler Characteristics:: Some portability issues +* Generic Compiler Characteristics:: Language independent tests and features +* C Compiler:: Checking its characteristics +* C++ Compiler:: Likewise +* Objective C Compiler:: Likewise +* Objective C++ Compiler:: Likewise +* Erlang Compiler and Interpreter:: Likewise +* Fortran Compiler:: Likewise +* Go Compiler:: Likewise + +Writing Tests + +* Language Choice:: Selecting which language to use for testing +* Writing Test Programs:: Forging source files for compilers +* Running the Preprocessor:: Detecting preprocessor symbols +* Running the Compiler:: Detecting language or header features +* Running the Linker:: Detecting library features +* Runtime:: Testing for runtime features +* Systemology:: A zoology of operating systems +* Multiple Cases:: Tests for several possible values + +Writing Test Programs + +* Guidelines:: General rules for writing test programs +* Test Functions:: Avoiding pitfalls in test programs +* Generating Sources:: Source program boilerplate + +Results of Tests + +* Defining Symbols:: Defining C preprocessor symbols +* Setting Output Variables:: Replacing variables in output files +* Special Chars in Variables:: Characters to beware of in variables +* Caching Results:: Speeding up subsequent @command{configure} runs +* Printing Messages:: Notifying @command{configure} users + +Caching Results + +* Cache Variable Names:: Shell variables used in caches +* Cache Files:: Files @command{configure} uses for caching +* Cache Checkpointing:: Loading and saving the cache file + +Programming in M4 + +* M4 Quotation:: Protecting macros from unwanted expansion +* Using autom4te:: The Autoconf executables backbone +* Programming in M4sugar:: Convenient pure M4 macros +* Debugging via autom4te:: Figuring out what M4 was doing + +M4 Quotation + +* Active Characters:: Characters that change the behavior of M4 +* One Macro Call:: Quotation and one macro call +* Quoting and Parameters:: M4 vs. shell parameters +* Quotation and Nested Macros:: Macros calling macros +* Changequote is Evil:: Worse than INTERCAL: M4 + changequote +* Quadrigraphs:: Another way to escape special characters +* Balancing Parentheses:: Dealing with unbalanced parentheses +* Quotation Rule Of Thumb:: One parenthesis, one quote + +Using @command{autom4te} + +* autom4te Invocation:: A GNU M4 wrapper +* Customizing autom4te:: Customizing the Autoconf package + +Programming in M4sugar + +* Redefined M4 Macros:: M4 builtins changed in M4sugar +* Diagnostic Macros:: Diagnostic messages from M4sugar +* Diversion support:: Diversions in M4sugar +* Conditional constructs:: Conditions in M4 +* Looping constructs:: Iteration in M4 +* Evaluation Macros:: More quotation and evaluation control +* Text processing Macros:: String manipulation in M4 +* Number processing Macros:: Arithmetic computation in M4 +* Set manipulation Macros:: Set manipulation in M4 +* Forbidden Patterns:: Catching unexpanded macros + +Programming in M4sh + +* Common Shell Constructs:: Portability layer for common shell constructs +* Polymorphic Variables:: Support for indirect variable names +* Initialization Macros:: Macros to establish a sane shell environment +* File Descriptor Macros:: File descriptor macros for input and output + +Writing Autoconf Macros + +* Macro Definitions:: Basic format of an Autoconf macro +* Macro Names:: What to call your new macros +* Reporting Messages:: Notifying @command{autoconf} users +* Dependencies Between Macros:: What to do when macros depend on other macros +* Obsoleting Macros:: Warning about old ways of doing things +* Coding Style:: Writing Autoconf macros @`a la Autoconf + +Dependencies Between Macros + +* Prerequisite Macros:: Ensuring required information +* Suggested Ordering:: Warning about possible ordering problems +* One-Shot Macros:: Ensuring a macro is called only once + +Portable Shell Programming + +* Shellology:: A zoology of shells +* Invoking the Shell:: Invoking the shell as a command +* Here-Documents:: Quirks and tricks +* File Descriptors:: FDs and redirections +* Signal Handling:: Shells, signals, and headaches +* File System Conventions:: File names +* Shell Pattern Matching:: Pattern matching +* Shell Substitutions:: Variable and command expansions +* Assignments:: Varying side effects of assignments +* Parentheses:: Parentheses in shell scripts +* Slashes:: Slashes in shell scripts +* Special Shell Variables:: Variables you should not change +* Shell Functions:: What to look out for if you use them +* Limitations of Builtins:: Portable use of not so portable /bin/sh +* Limitations of Usual Tools:: Portable use of portable tools + +Portable Make Programming + +* $< in Ordinary Make Rules:: $< in ordinary rules +* Failure in Make Rules:: Failing portably in rules +* Special Chars in Names:: Special Characters in Macro Names +* Backslash-Newline-Empty:: Empty lines after backslash-newline +* Backslash-Newline Comments:: Spanning comments across line boundaries +* Long Lines in Makefiles:: Line length limitations +* Macros and Submakes:: @code{make macro=value} and submakes +* The Make Macro MAKEFLAGS:: @code{$(MAKEFLAGS)} portability issues +* The Make Macro SHELL:: @code{$(SHELL)} portability issues +* Parallel Make:: Parallel @command{make} quirks +* Comments in Make Rules:: Other problems with Make comments +* Newlines in Make Rules:: Using literal newlines in rules +* Comments in Make Macros:: Other problems with Make comments in macros +* Trailing whitespace in Make Macros:: Macro substitution problems +* Command-line Macros and whitespace:: Whitespace trimming of values +* obj/ and Make:: Don't name a subdirectory @file{obj} +* make -k Status:: Exit status of @samp{make -k} +* VPATH and Make:: @code{VPATH} woes +* Single Suffix Rules:: Single suffix rules and separated dependencies +* Timestamps and Make:: Subsecond timestamp resolution + +@code{VPATH} and Make + +* Variables listed in VPATH:: @code{VPATH} must be literal on ancient hosts +* VPATH and Double-colon:: Problems with @samp{::} on ancient hosts +* $< in Explicit Rules:: @code{$<} does not work in ordinary rules +* Automatic Rule Rewriting:: @code{VPATH} goes wild on Solaris +* Tru64 Directory Magic:: @command{mkdir} goes wild on Tru64 +* Make Target Lookup:: More details about @code{VPATH} lookup + +Portable C and C++ Programming + +* Varieties of Unportability:: How to make your programs unportable +* Integer Overflow:: When integers get too large +* Preprocessor Arithmetic:: @code{#if} expression problems +* Null Pointers:: Properties of null pointers +* Buffer Overruns:: Subscript errors and the like +* Volatile Objects:: @code{volatile} and signals +* Floating Point Portability:: Portable floating-point arithmetic +* Exiting Portably:: Exiting and the exit status + +Integer Overflow + +* Integer Overflow Basics:: Why integer overflow is a problem +* Signed Overflow Examples:: Examples of code assuming wraparound +* Optimization and Wraparound:: Optimizations that break uses of wraparound +* Signed Overflow Advice:: Practical advice for signed overflow issues +* Signed Integer Division:: @code{INT_MIN / -1} and @code{INT_MIN % -1} + +Manual Configuration + +* Specifying Target Triplets:: Specifying target triplets +* Canonicalizing:: Getting the canonical system type +* Using System Type:: What to do with the system type + +Site Configuration + +* Help Formatting:: Customizing @samp{configure --help} +* External Software:: Working with other optional software +* Package Options:: Selecting optional features +* Pretty Help Strings:: Formatting help string +* Option Checking:: Controlling checking of @command{configure} options +* Site Details:: Configuring site details +* Transforming Names:: Changing program names when installing +* Site Defaults:: Giving @command{configure} local defaults + +Transforming Program Names When Installing + +* Transformation Options:: @command{configure} options to transform names +* Transformation Examples:: Sample uses of transforming names +* Transformation Rules:: Makefile uses of transforming names + +Running @command{configure} Scripts + +* Basic Installation:: Instructions for typical cases +* Compilers and Options:: Selecting compilers and optimization +* Multiple Architectures:: Compiling for multiple architectures at once +* Installation Names:: Installing in different directories +* Optional Features:: Selecting optional features +* Particular Systems:: Particular systems +* System Type:: Specifying the system type +* Sharing Defaults:: Setting site-wide defaults for @command{configure} +* Defining Variables:: Specifying the compiler etc. +* configure Invocation:: Changing how @command{configure} runs + +Obsolete Constructs + +* Obsolete config.status Use:: Obsolete convention for @command{config.status} +* acconfig Header:: Additional entries in @file{config.h.in} +* autoupdate Invocation:: Automatic update of @file{configure.ac} +* Obsolete Macros:: Backward compatibility macros +* Autoconf 1:: Tips for upgrading your files +* Autoconf 2.13:: Some fresher tips + +Upgrading From Version 1 + +* Changed File Names:: Files you might rename +* Changed Makefiles:: New things to put in @file{Makefile.in} +* Changed Macros:: Macro calls you might replace +* Changed Results:: Changes in how to check test results +* Changed Macro Writing:: Better ways to write your own macros + +Upgrading From Version 2.13 + +* Changed Quotation:: Broken code which used to work +* New Macros:: Interaction with foreign macros +* Hosts and Cross-Compilation:: Bugward compatibility kludges +* AC_LIBOBJ vs LIBOBJS:: LIBOBJS is a forbidden token +* AC_ACT_IFELSE vs AC_TRY_ACT:: A more generic scheme for testing sources + +Generating Test Suites with Autotest + +* Using an Autotest Test Suite:: Autotest and the user +* Writing Testsuites:: Autotest macros +* testsuite Invocation:: Running @command{testsuite} scripts +* Making testsuite Scripts:: Using autom4te to create @command{testsuite} + +Using an Autotest Test Suite + +* testsuite Scripts:: The concepts of Autotest +* Autotest Logs:: Their contents + +Frequent Autoconf Questions, with answers + +* Distributing:: Distributing @command{configure} scripts +* Why GNU M4:: Why not use the standard M4? +* Bootstrapping:: Autoconf and GNU M4 require each other? +* Why Not Imake:: Why GNU uses @command{configure} instead of Imake +* Defining Directories:: Passing @code{datadir} to program +* Autom4te Cache:: What is it? Can I remove it? +* Present But Cannot Be Compiled:: Compiler and Preprocessor Disagree +* Expanded Before Required:: Expanded Before Required +* Debugging:: Debugging @command{configure} scripts + +History of Autoconf + +* Genesis:: Prehistory and naming of @command{configure} +* Exodus:: The plagues of M4 and Perl +* Leviticus:: The priestly code of portability arrives +* Numbers:: Growth and contributors +* Deuteronomy:: Approaching the promises of easy configuration + +Indices + +* Environment Variable Index:: Index of environment variables used +* Output Variable Index:: Index of variables set in output files +* Preprocessor Symbol Index:: Index of C preprocessor symbols defined +* Cache Variable Index:: Index of documented cache variables +* Autoconf Macro Index:: Index of Autoconf macros +* M4 Macro Index:: Index of M4, M4sugar, and M4sh macros +* Autotest Macro Index:: Index of Autotest macros +* Program & Function Index:: Index of those with portability problems +* Concept Index:: General index + +@end detailmenu +@end menu + +@c ============================================================= Introduction. + +@node Introduction +@chapter Introduction +@cindex Introduction + +@flushright +A physicist, an engineer, and a computer scientist were discussing the +nature of God. ``Surely a Physicist,'' said the physicist, ``because +early in the Creation, God made Light; and you know, Maxwell's +equations, the dual nature of electromagnetic waves, the relativistic +consequences@enddots{}'' ``An Engineer!,'' said the engineer, ``because +before making Light, God split the Chaos into Land and Water; it takes a +hell of an engineer to handle that big amount of mud, and orderly +separation of solids from liquids@enddots{}'' The computer scientist +shouted: ``And the Chaos, where do you think it was coming from, hmm?'' + +---Anonymous +@end flushright +@c (via Franc,ois Pinard) + +Autoconf is a tool for producing shell scripts that automatically +configure software source code packages to adapt to many kinds of +Posix-like systems. The configuration scripts produced by Autoconf +are independent of Autoconf when they are run, so their users do not +need to have Autoconf. + +The configuration scripts produced by Autoconf require no manual user +intervention when run; they do not normally even need an argument +specifying the system type. Instead, they individually test for the +presence of each feature that the software package they are for might need. +(Before each check, they print a one-line message stating what they are +checking for, so the user doesn't get too bored while waiting for the +script to finish.) As a result, they deal well with systems that are +hybrids or customized from the more common Posix variants. There is +no need to maintain files that list the features supported by each +release of each variant of Posix. + +For each software package that Autoconf is used with, it creates a +configuration script from a template file that lists the system features +that the package needs or can use. After the shell code to recognize +and respond to a system feature has been written, Autoconf allows it to +be shared by many software packages that can use (or need) that feature. +If it later turns out that the shell code needs adjustment for some +reason, it needs to be changed in only one place; all of the +configuration scripts can be regenerated automatically to take advantage +of the updated code. + +@c "Those who do not understand Unix are condemned to reinvent it, poorly." +@c --Henry Spencer, 1987 (see http://en.wikipedia.org/wiki/Unix_philosophy) +Those who do not understand Autoconf are condemned to reinvent it, poorly. +The primary goal of Autoconf is making the @emph{user's} life easier; +making the @emph{maintainer's} life easier is only a secondary goal. +Put another way, the primary goal is not to make the generation of +@file{configure} automatic for package maintainers (although patches +along that front are welcome, since package maintainers form the user +base of Autoconf); rather, the goal is to make @file{configure} +painless, portable, and predictable for the end user of each +@dfn{autoconfiscated} package. And to this degree, Autoconf is highly +successful at its goal --- most complaints to the Autoconf list are +about difficulties in writing Autoconf input, and not in the behavior of +the resulting @file{configure}. Even packages that don't use Autoconf +will generally provide a @file{configure} script, and the most common +complaint about these alternative home-grown scripts is that they fail +to meet one or more of the GNU Coding Standards (@pxref{Configuration, , , +standards, The GNU Coding Standards}) that users +have come to expect from Autoconf-generated @file{configure} scripts. + +The Metaconfig package is similar in purpose to Autoconf, but the +scripts it produces require manual user intervention, which is quite +inconvenient when configuring large source trees. Unlike Metaconfig +scripts, Autoconf scripts can support cross-compiling, if some care is +taken in writing them. + +Autoconf does not solve all problems related to making portable +software packages---for a more complete solution, it should be used in +concert with other GNU build tools like Automake and +Libtool. These other tools take on jobs like the creation of a +portable, recursive makefile with all of the standard targets, +linking of shared libraries, and so on. @xref{The GNU Build System}, +for more information. + +Autoconf imposes some restrictions on the names of macros used with +@code{#if} in C programs (@pxref{Preprocessor Symbol Index}). + +Autoconf requires GNU M4 version 1.4.6 or later in order to +generate the scripts. It uses features that some versions of M4, +including GNU M4 1.3, do not have. Autoconf works better +with GNU M4 version 1.4.14 or later, though this is not +required. + +@xref{Autoconf 1}, for information about upgrading from version 1. +@xref{History}, for the story of Autoconf's development. @xref{FAQ}, +for answers to some common questions about Autoconf. + +See the @uref{http://@/www.gnu.org/@/software/@/autoconf/, +Autoconf web page} for up-to-date information, details on the mailing +lists, pointers to a list of known bugs, etc. + +Mail suggestions to @email{autoconf@@gnu.org, the Autoconf mailing +list}. Past suggestions are +@uref{http://@/lists.gnu.org/@/archive/@/html/@/autoconf/, archived}. + +Mail bug reports to @email{bug-autoconf@@gnu.org, the +Autoconf Bugs mailing list}. Past bug reports are +@uref{http://@/lists.gnu.org/@/archive/@/html/@/bug-autoconf/, archived}. + +If possible, first check that your bug is +not already solved in current development versions, and that it has not +been reported yet. Be sure to include all the needed information and a +short @file{configure.ac} that demonstrates the problem. + +Autoconf's development tree is accessible via @command{git}; see the +@uref{http://@/savannah.gnu.org/@/projects/@/autoconf/, Autoconf +Summary} for details, or view +@uref{http://@/git.sv.gnu.org/@/gitweb/@/?p=autoconf.git, the actual +repository}. Anonymous CVS access is also available, see +@file{README} for more details. Patches relative to the +current @command{git} version can be sent for review to the +@email{autoconf-patches@@gnu.org, Autoconf Patches mailing list}, with +discussion on prior patches +@uref{http://@/lists.gnu.org/@/archive/@/html/@/autoconf-@/patches/, +archived}; and all commits are posted in the read-only +@email{autoconf-commit@@gnu.org, Autoconf Commit mailing list}, which is +also @uref{http://@/lists.gnu.org/@/archive/@/html/@/autoconf-commit/, +archived}. + +Because of its mission, the Autoconf package itself +includes only a set of often-used +macros that have already demonstrated their usefulness. Nevertheless, +if you wish to share your macros, or find existing ones, see the +@uref{http://@/www.gnu.org/@/software/@/autoconf-archive/, Autoconf Macro +Archive}, which is kindly run by @email{simons@@cryp.to, +Peter Simons}. + + +@c ================================================= The GNU Build System + +@node The GNU Build System +@chapter The GNU Build System +@cindex GNU build system + +Autoconf solves an important problem---reliable discovery of +system-specific build and runtime information---but this is only one +piece of the puzzle for the development of portable software. To this +end, the GNU project has developed a suite of integrated +utilities to finish the job Autoconf started: the GNU build +system, whose most important components are Autoconf, Automake, and +Libtool. In this chapter, we introduce you to those tools, point you +to sources of more information, and try to convince you to use the +entire GNU build system for your software. + +@menu +* Automake:: Escaping makefile hell +* Gnulib:: The GNU portability library +* Libtool:: Building libraries portably +* Pointers:: More info on the GNU build system +@end menu + +@node Automake +@section Automake + +The ubiquity of @command{make} means that a makefile is almost the +only viable way to distribute automatic build rules for software, but +one quickly runs into its numerous limitations. Its lack of +support for automatic dependency tracking, recursive builds in +subdirectories, reliable timestamps (e.g., for network file systems), and +so on, mean that developers must painfully (and often incorrectly) +reinvent the wheel for each project. Portability is non-trivial, thanks +to the quirks of @command{make} on many systems. On top of all this is the +manual labor required to implement the many standard targets that users +have come to expect (@code{make install}, @code{make distclean}, +@code{make uninstall}, etc.). Since you are, of course, using Autoconf, +you also have to insert repetitive code in your @file{Makefile.in} to +recognize @code{@@CC@@}, @code{@@CFLAGS@@}, and other substitutions +provided by @command{configure}. Into this mess steps @dfn{Automake}. +@cindex Automake + +Automake allows you to specify your build needs in a @file{Makefile.am} +file with a vastly simpler and more powerful syntax than that of a plain +makefile, and then generates a portable @file{Makefile.in} for +use with Autoconf. For example, the @file{Makefile.am} to build and +install a simple ``Hello world'' program might look like: + +@example +bin_PROGRAMS = hello +hello_SOURCES = hello.c +@end example + +@noindent +The resulting @file{Makefile.in} (~400 lines) automatically supports all +the standard targets, the substitutions provided by Autoconf, automatic +dependency tracking, @code{VPATH} building, and so on. @command{make} +builds the @code{hello} program, and @code{make install} installs it +in @file{/usr/local/bin} (or whatever prefix was given to +@command{configure}, if not @file{/usr/local}). + +The benefits of Automake increase for larger packages (especially ones +with subdirectories), but even for small programs the added convenience +and portability can be substantial. And that's not all@enddots{} + +@node Gnulib +@section Gnulib + +GNU software has a well-deserved reputation for running on +many different types of systems. While our primary goal is to write +software for the GNU system, many users and developers have +been introduced to us through the systems that they were already using. + +@cindex Gnulib +Gnulib is a central location for common GNU code, intended to +be shared among free software packages. Its components are typically +shared at the source level, rather than being a library that gets built, +installed, and linked against. The idea is to copy files from Gnulib +into your own source tree. There is no distribution tarball; developers +should just grab source modules from the repository. The source files +are available online, under various licenses, mostly GNU +GPL or GNU LGPL. + +Gnulib modules typically contain C source code along with Autoconf +macros used to configure the source code. For example, the Gnulib +@code{stdbool} module implements a @file{stdbool.h} header that nearly +conforms to C99, even on old-fashioned hosts that lack @file{stdbool.h}. +This module contains a source file for the replacement header, along +with an Autoconf macro that arranges to use the replacement header on +old-fashioned systems. + +@node Libtool +@section Libtool + +Often, one wants to build not only programs, but libraries, so that +other programs can benefit from the fruits of your labor. Ideally, one +would like to produce @emph{shared} (dynamically linked) libraries, +which can be used by multiple programs without duplication on disk or in +memory and can be updated independently of the linked programs. +Producing shared libraries portably, however, is the stuff of +nightmares---each system has its own incompatible tools, compiler flags, +and magic incantations. Fortunately, GNU provides a solution: +@dfn{Libtool}. +@cindex Libtool + +Libtool handles all the requirements of building shared libraries for +you, and at this time seems to be the @emph{only} way to do so with any +portability. It also handles many other headaches, such as: the +interaction of Make rules with the variable suffixes of +shared libraries, linking reliably with shared libraries before they are +installed by the superuser, and supplying a consistent versioning system +(so that different versions of a library can be installed or upgraded +without breaking binary compatibility). Although Libtool, like +Autoconf, can be used without Automake, it is most simply utilized in +conjunction with Automake---there, Libtool is used automatically +whenever shared libraries are needed, and you need not know its syntax. + +@node Pointers +@section Pointers + +Developers who are used to the simplicity of @command{make} for small +projects on a single system might be daunted at the prospect of +learning to use Automake and Autoconf. As your software is +distributed to more and more users, however, you otherwise +quickly find yourself putting lots of effort into reinventing the +services that the GNU build tools provide, and making the +same mistakes that they once made and overcame. (Besides, since +you're already learning Autoconf, Automake is a piece of cake.) + +There are a number of places that you can go to for more information on +the GNU build tools. + +@itemize @minus + +@item Web + +The project home pages for +@uref{http://@/www@/.gnu@/.org/@/software/@/autoconf/, Autoconf}, +@uref{http://@/www@/.gnu@/.org/@/software/@/automake/, Automake}, +@uref{http://@/www@/.gnu@/.org/@/software/@/gnulib/, Gnulib}, and +@uref{http://@/www@/.gnu@/.org/@/software/@/libtool/, Libtool}. + +@item Automake Manual + +@xref{Top, , Automake, automake, GNU Automake}, for more +information on Automake. + +@item Books + +The book @cite{GNU Autoconf, Automake and +Libtool}@footnote{@cite{GNU Autoconf, Automake and Libtool}, +by G. V. Vaughan, B. Elliston, T. Tromey, and I. L. Taylor. SAMS (originally +New Riders), 2000, ISBN 1578701902.} describes the complete GNU +build environment. You can also find +@uref{http://@/sources.redhat.com/@/autobook/, the entire book on-line}. + +@end itemize + +@c ================================================= Making configure Scripts. + +@node Making configure Scripts +@chapter Making @command{configure} Scripts +@cindex @file{aclocal.m4} +@cindex @command{configure} + +The configuration scripts that Autoconf produces are by convention +called @command{configure}. When run, @command{configure} creates several +files, replacing configuration parameters in them with appropriate +values. The files that @command{configure} creates are: + +@itemize @minus +@item +one or more @file{Makefile} files, usually one in each subdirectory of the +package (@pxref{Makefile Substitutions}); + +@item +optionally, a C header file, the name of which is configurable, +containing @code{#define} directives (@pxref{Configuration Headers}); + +@item +a shell script called @file{config.status} that, when run, recreates +the files listed above (@pxref{config.status Invocation}); + +@item +an optional shell script normally called @file{config.cache} +(created when using @samp{configure --config-cache}) that +saves the results of running many of the tests (@pxref{Cache Files}); + +@item +a file called @file{config.log} containing any messages produced by +compilers, to help debugging if @command{configure} makes a mistake. +@end itemize + +@cindex @file{configure.in} +@cindex @file{configure.ac} +To create a @command{configure} script with Autoconf, you need to write an +Autoconf input file @file{configure.ac} (or @file{configure.in}) and run +@command{autoconf} on it. If you write your own feature tests to +supplement those that come with Autoconf, you might also write files +called @file{aclocal.m4} and @file{acsite.m4}. If you use a C header +file to contain @code{#define} directives, you might also run +@command{autoheader}, and you can distribute the generated file +@file{config.h.in} with the package. + +Here is a diagram showing how the files that can be used in +configuration are produced. Programs that are executed are suffixed by +@samp{*}. Optional files are enclosed in square brackets (@samp{[]}). +@command{autoconf} and @command{autoheader} also read the installed Autoconf +macro files (by reading @file{autoconf.m4}). + +@noindent +Files used in preparing a software package for distribution, when using +just Autoconf: +@example +your source files --> [autoscan*] --> [configure.scan] --> configure.ac + +@group +configure.ac --. + | .------> autoconf* -----> configure +[aclocal.m4] --+---+ + | `-----> [autoheader*] --> [config.h.in] +[acsite.m4] ---' +@end group + +Makefile.in +@end example + +@noindent +Additionally, if you use Automake, the following additional productions +come into play: + +@example +@group +[acinclude.m4] --. + | +[local macros] --+--> aclocal* --> aclocal.m4 + | +configure.ac ----' +@end group + +@group +configure.ac --. + +--> automake* --> Makefile.in +Makefile.am ---' +@end group +@end example + +@noindent +Files used in configuring a software package: +@example +@group + .-------------> [config.cache] +configure* ------------+-------------> config.log + | +[config.h.in] -. v .-> [config.h] -. + +--> config.status* -+ +--> make* +Makefile.in ---' `-> Makefile ---' +@end group +@end example + +@menu +* Writing Autoconf Input:: What to put in an Autoconf input file +* autoscan Invocation:: Semi-automatic @file{configure.ac} writing +* ifnames Invocation:: Listing the conditionals in source code +* autoconf Invocation:: How to create configuration scripts +* autoreconf Invocation:: Remaking multiple @command{configure} scripts +@end menu + +@node Writing Autoconf Input +@section Writing @file{configure.ac} + +To produce a @command{configure} script for a software package, create a +file called @file{configure.ac} that contains invocations of the +Autoconf macros that test the system features your package needs or can +use. Autoconf macros already exist to check for many features; see +@ref{Existing Tests}, for their descriptions. For most other features, +you can use Autoconf template macros to produce custom checks; see +@ref{Writing Tests}, for information about them. For especially tricky +or specialized features, @file{configure.ac} might need to contain some +hand-crafted shell commands; see @ref{Portable Shell, , Portable Shell +Programming}. The @command{autoscan} program can give you a good start +in writing @file{configure.ac} (@pxref{autoscan Invocation}, for more +information). + +Previous versions of Autoconf promoted the name @file{configure.in}, +which is somewhat ambiguous (the tool needed to process this file is not +described by its extension), and introduces a slight confusion with +@file{config.h.in} and so on (for which @samp{.in} means ``to be +processed by @command{configure}''). Using @file{configure.ac} is now +preferred. + +@menu +* Shell Script Compiler:: Autoconf as solution of a problem +* Autoconf Language:: Programming in Autoconf +* Autoconf Input Layout:: Standard organization of @file{configure.ac} +@end menu + +@node Shell Script Compiler +@subsection A Shell Script Compiler + +Just as for any other computer language, in order to properly program +@file{configure.ac} in Autoconf you must understand @emph{what} problem +the language tries to address and @emph{how} it does so. + +The problem Autoconf addresses is that the world is a mess. After all, +you are using Autoconf in order to have your package compile easily on +all sorts of different systems, some of them being extremely hostile. +Autoconf itself bears the price for these differences: @command{configure} +must run on all those systems, and thus @command{configure} must limit itself +to their lowest common denominator of features. + +Naturally, you might then think of shell scripts; who needs +@command{autoconf}? A set of properly written shell functions is enough to +make it easy to write @command{configure} scripts by hand. Sigh! +Unfortunately, even in 2008, where shells without any function support are +far and few between, there are pitfalls to avoid when making use of them. +Also, finding a Bourne shell that accepts shell functions is not trivial, +even though there is almost always one on interesting porting targets. + +So, what is really needed is some kind of compiler, @command{autoconf}, +that takes an Autoconf program, @file{configure.ac}, and transforms it +into a portable shell script, @command{configure}. + +How does @command{autoconf} perform this task? + +There are two obvious possibilities: creating a brand new language or +extending an existing one. The former option is attractive: all +sorts of optimizations could easily be implemented in the compiler and +many rigorous checks could be performed on the Autoconf program +(e.g., rejecting any non-portable construct). Alternatively, you can +extend an existing language, such as the @code{sh} (Bourne shell) +language. + +Autoconf does the latter: it is a layer on top of @code{sh}. It was +therefore most convenient to implement @command{autoconf} as a macro +expander: a program that repeatedly performs @dfn{macro expansions} on +text input, replacing macro calls with macro bodies and producing a pure +@code{sh} script in the end. Instead of implementing a dedicated +Autoconf macro expander, it is natural to use an existing +general-purpose macro language, such as M4, and implement the extensions +as a set of M4 macros. + + +@node Autoconf Language +@subsection The Autoconf Language +@cindex quotation + +The Autoconf language differs from many other computer +languages because it treats actual code the same as plain text. Whereas +in C, for instance, data and instructions have different syntactic +status, in Autoconf their status is rigorously the same. Therefore, we +need a means to distinguish literal strings from text to be expanded: +quotation. + +When calling macros that take arguments, there must not be any white +space between the macro name and the open parenthesis. + +@example +AC_INIT ([oops], [1.0]) # incorrect +AC_INIT([hello], [1.0]) # good +@end example + +Arguments should +be enclosed within the quote characters @samp{[} and @samp{]}, and be +separated by commas. Any leading blanks or newlines in arguments are ignored, +unless they are quoted. You should always quote an argument that +might contain a macro name, comma, parenthesis, or a leading blank or +newline. This rule applies recursively for every macro +call, including macros called from other macros. For more details on +quoting rules, see @ref{Programming in M4}. + +For instance: + +@example +AC_CHECK_HEADER([stdio.h], + [AC_DEFINE([HAVE_STDIO_H], [1], + [Define to 1 if you have <stdio.h>.])], + [AC_MSG_ERROR([sorry, can't do anything for you])]) +@end example + +@noindent +is quoted properly. You may safely simplify its quotation to: + +@example +AC_CHECK_HEADER([stdio.h], + [AC_DEFINE([HAVE_STDIO_H], 1, + [Define to 1 if you have <stdio.h>.])], + [AC_MSG_ERROR([sorry, can't do anything for you])]) +@end example + +@noindent +because @samp{1} cannot contain a macro call. Here, the argument of +@code{AC_MSG_ERROR} must be quoted; otherwise, its comma would be +interpreted as an argument separator. Also, the second and third arguments +of @samp{AC_CHECK_HEADER} must be quoted, since they contain +macro calls. The three arguments @samp{HAVE_STDIO_H}, @samp{stdio.h}, +and @samp{Define to 1 if you have <stdio.h>.} do not need quoting, but +if you unwisely defined a macro with a name like @samp{Define} or +@samp{stdio} then they would need quoting. Cautious Autoconf users +would keep the quotes, but many Autoconf users find such precautions +annoying, and would rewrite the example as follows: + +@example +AC_CHECK_HEADER(stdio.h, + [AC_DEFINE(HAVE_STDIO_H, 1, + [Define to 1 if you have <stdio.h>.])], + [AC_MSG_ERROR([sorry, can't do anything for you])]) +@end example + +@noindent +This is safe, so long as you adopt good naming conventions and do not +define macros with names like @samp{HAVE_STDIO_H}, @samp{stdio}, or +@samp{h}. Though it is also safe here to omit the quotes around +@samp{Define to 1 if you have <stdio.h>.} this is not recommended, as +message strings are more likely to inadvertently contain commas. + +The following example is wrong and dangerous, as it is underquoted: + +@example +AC_CHECK_HEADER(stdio.h, + AC_DEFINE(HAVE_STDIO_H, 1, + Define to 1 if you have <stdio.h>.), + AC_MSG_ERROR([sorry, can't do anything for you])) +@end example + +In other cases, you may have to use text that also resembles a macro +call. You must quote that text even when it is not passed as a macro +argument. For example, these two approaches in @file{configure.ac} +(quoting just the potential problems, or quoting the entire line) will +protect your script in case autoconf ever adds a macro @code{AC_DC}: + +@example +echo "Hard rock was here! --[AC_DC]" +[echo "Hard rock was here! --AC_DC"] +@end example + +@noindent +which results in this text in @file{configure}: + +@example +echo "Hard rock was here! --AC_DC" +echo "Hard rock was here! --AC_DC" +@end example + +@noindent +When you use the same text in a macro argument, you must therefore have +an extra quotation level (since one is stripped away by the macro +substitution). In general, then, it is a good idea to @emph{use double +quoting for all literal string arguments}, either around just the +problematic portions, or over the entire argument: + +@example +AC_MSG_WARN([[AC_DC] stinks --Iron Maiden]) +AC_MSG_WARN([[AC_DC stinks --Iron Maiden]]) +@end example + +However, the above example triggers a warning about a possibly +unexpanded macro when running @command{autoconf}, because it collides +with the namespace of macros reserved for the Autoconf language. To be +really safe, you can use additional escaping (either a quadrigraph, or +creative shell constructs) to silence that particular warning: + +@example +echo "Hard rock was here! --AC""_DC" +AC_MSG_WARN([[AC@@&t@@_DC stinks --Iron Maiden]]) +@end example + +You are now able to understand one of the constructs of Autoconf that +has been continually misunderstood@enddots{} The rule of thumb is that +@emph{whenever you expect macro expansion, expect quote expansion}; +i.e., expect one level of quotes to be lost. For instance: + +@example +AC_COMPILE_IFELSE(AC_LANG_SOURCE([char b[10];]), [], + [AC_MSG_ERROR([you lose])]) +@end example + +@noindent +is incorrect: here, the first argument of @code{AC_LANG_SOURCE} is +@samp{char b[10];} and is expanded once, which results in +@samp{char b10;}; and the @code{AC_LANG_SOURCE} is also expanded prior +to being passed to @code{AC_COMPILE_IFELSE}. (There was an idiom common +in Autoconf's past to +address this issue via the M4 @code{changequote} primitive, but do not +use it!) Let's take a closer look: the author meant the first argument +to be understood as a literal, and therefore it must be quoted twice; +likewise, the intermediate @code{AC_LANG_SOURCE} macro should be quoted +once so that it is only expanded after the rest of the body of +@code{AC_COMPILE_IFELSE} is in place: + +@example +AC_COMPILE_IFELSE([AC_LANG_SOURCE([[char b[10];]])], [], + [AC_MSG_ERROR([you lose])]) +@end example + +@noindent +Voil@`a, you actually produce @samp{char b[10];} this time! + +On the other hand, descriptions (e.g., the last parameter of +@code{AC_DEFINE} or @code{AS_HELP_STRING}) are not literals---they +are subject to line breaking, for example---and should not be double quoted. +Even if these descriptions are short and are not actually broken, double +quoting them yields weird results. + +Some macros take optional arguments, which this documentation represents +as @ovar{arg} (not to be confused with the quote characters). You may +just leave them empty, or use @samp{[]} to make the emptiness of the +argument explicit, or you may simply omit the trailing commas. The +three lines below are equivalent: + +@example +AC_CHECK_HEADERS([stdio.h], [], [], []) +AC_CHECK_HEADERS([stdio.h],,,) +AC_CHECK_HEADERS([stdio.h]) +@end example + +It is best to put each macro call on its own line in +@file{configure.ac}. Most of the macros don't add extra newlines; they +rely on the newline after the macro call to terminate the commands. +This approach makes the generated @command{configure} script a little +easier to read by not inserting lots of blank lines. It is generally +safe to set shell variables on the same line as a macro call, because +the shell allows assignments without intervening newlines. + +You can include comments in @file{configure.ac} files by starting them +with the @samp{#}. For example, it is helpful to begin +@file{configure.ac} files with a line like this: + +@example +# Process this file with autoconf to produce a configure script. +@end example + +@node Autoconf Input Layout +@subsection Standard @file{configure.ac} Layout + +The order in which @file{configure.ac} calls the Autoconf macros is not +important, with a few exceptions. Every @file{configure.ac} must +contain a call to @code{AC_INIT} before the checks, and a call to +@code{AC_OUTPUT} at the end (@pxref{Output}). Additionally, some macros +rely on other macros having been called first, because they check +previously set values of some variables to decide what to do. These +macros are noted in the individual descriptions (@pxref{Existing +Tests}), and they also warn you when @command{configure} is created if they +are called out of order. + +To encourage consistency, here is a suggested order for calling the +Autoconf macros. Generally speaking, the things near the end of this +list are those that could depend on things earlier in it. For example, +library functions could be affected by types and libraries. + +@display +@group +Autoconf requirements +@code{AC_INIT(@var{package}, @var{version}, @var{bug-report-address})} +information on the package +checks for programs +checks for libraries +checks for header files +checks for types +checks for structures +checks for compiler characteristics +checks for library functions +checks for system services +@code{AC_CONFIG_FILES(@r{[}@var{file@dots{}}@r{]})} +@code{AC_OUTPUT} +@end group +@end display + + +@node autoscan Invocation +@section Using @command{autoscan} to Create @file{configure.ac} +@cindex @command{autoscan} + +The @command{autoscan} program can help you create and/or maintain a +@file{configure.ac} file for a software package. @command{autoscan} +examines source files in the directory tree rooted at a directory given +as a command line argument, or the current directory if none is given. +It searches the source files for common portability problems and creates +a file @file{configure.scan} which is a preliminary @file{configure.ac} +for that package, and checks a possibly existing @file{configure.ac} for +completeness. + +When using @command{autoscan} to create a @file{configure.ac}, you +should manually examine @file{configure.scan} before renaming it to +@file{configure.ac}; it probably needs some adjustments. +Occasionally, @command{autoscan} outputs a macro in the wrong order +relative to another macro, so that @command{autoconf} produces a warning; +you need to move such macros manually. Also, if you want the package to +use a configuration header file, you must add a call to +@code{AC_CONFIG_HEADERS} (@pxref{Configuration Headers}). You might +also have to change or add some @code{#if} directives to your program in +order to make it work with Autoconf (@pxref{ifnames Invocation}, for +information about a program that can help with that job). + +When using @command{autoscan} to maintain a @file{configure.ac}, simply +consider adding its suggestions. The file @file{autoscan.log} +contains detailed information on why a macro is requested. + +@command{autoscan} uses several data files (installed along with Autoconf) +to determine which macros to output when it finds particular symbols in +a package's source files. These data files all have the same format: +each line consists of a symbol, one or more blanks, and the Autoconf macro to +output if that symbol is encountered. Lines starting with @samp{#} are +comments. + +@command{autoscan} accepts the following options: + +@table @option +@item --help +@itemx -h +Print a summary of the command line options and exit. + +@item --version +@itemx -V +Print the version number of Autoconf and exit. + +@item --verbose +@itemx -v +Print the names of the files it examines and the potentially interesting +symbols it finds in them. This output can be voluminous. + +@item --debug +@itemx -d +Don't remove temporary files. + +@item --include=@var{dir} +@itemx -I @var{dir} +Append @var{dir} to the include path. Multiple invocations accumulate. + +@item --prepend-include=@var{dir} +@itemx -B @var{dir} +Prepend @var{dir} to the include path. Multiple invocations accumulate. +@end table + +@node ifnames Invocation +@section Using @command{ifnames} to List Conditionals +@cindex @command{ifnames} + +@command{ifnames} can help you write @file{configure.ac} for a software +package. It prints the identifiers that the package already uses in C +preprocessor conditionals. If a package has already been set up to have +some portability, @command{ifnames} can thus help you figure out what its +@command{configure} needs to check for. It may help fill in some gaps in a +@file{configure.ac} generated by @command{autoscan} (@pxref{autoscan +Invocation}). + +@command{ifnames} scans all of the C source files named on the command line +(or the standard input, if none are given) and writes to the standard +output a sorted list of all the identifiers that appear in those files +in @code{#if}, @code{#elif}, @code{#ifdef}, or @code{#ifndef} +directives. It prints each identifier on a line, followed by a +space-separated list of the files in which that identifier occurs. + +@noindent +@command{ifnames} accepts the following options: + +@table @option +@item --help +@itemx -h +Print a summary of the command line options and exit. + +@item --version +@itemx -V +Print the version number of Autoconf and exit. +@end table + +@node autoconf Invocation +@section Using @command{autoconf} to Create @command{configure} +@cindex @command{autoconf} + +To create @command{configure} from @file{configure.ac}, run the +@command{autoconf} program with no arguments. @command{autoconf} processes +@file{configure.ac} with the M4 macro processor, using the +Autoconf macros. If you give @command{autoconf} an argument, it reads that +file instead of @file{configure.ac} and writes the configuration script +to the standard output instead of to @command{configure}. If you give +@command{autoconf} the argument @option{-}, it reads from the standard +input instead of @file{configure.ac} and writes the configuration script +to the standard output. + +The Autoconf macros are defined in several files. Some of the files are +distributed with Autoconf; @command{autoconf} reads them first. Then it +looks for the optional file @file{acsite.m4} in the directory that +contains the distributed Autoconf macro files, and for the optional file +@file{aclocal.m4} in the current directory. Those files can contain +your site's or the package's own Autoconf macro definitions +(@pxref{Writing Autoconf Macros}, for more information). If a macro is +defined in more than one of the files that @command{autoconf} reads, the +last definition it reads overrides the earlier ones. + +@command{autoconf} accepts the following options: + +@table @option +@item --help +@itemx -h +Print a summary of the command line options and exit. + +@item --version +@itemx -V +Print the version number of Autoconf and exit. + +@item --verbose +@itemx -v +Report processing steps. + +@item --debug +@itemx -d +Don't remove the temporary files. + +@item --force +@itemx -f +Remake @file{configure} even if newer than its input files. + +@item --include=@var{dir} +@itemx -I @var{dir} +Append @var{dir} to the include path. Multiple invocations accumulate. + +@item --prepend-include=@var{dir} +@itemx -B @var{dir} +Prepend @var{dir} to the include path. Multiple invocations accumulate. + +@item --output=@var{file} +@itemx -o @var{file} +Save output (script or trace) to @var{file}. The file @option{-} stands +for the standard output. + +@item --warnings=@var{category} +@itemx -W @var{category} +@evindex WARNINGS +Report the warnings related to @var{category} (which can actually be a +comma separated list). @xref{Reporting Messages}, macro +@code{AC_DIAGNOSE}, for a comprehensive list of categories. Special +values include: + +@table @samp +@item all +report all the warnings + +@item none +report none + +@item error +treats warnings as errors + +@item no-@var{category} +disable warnings falling into @var{category} +@end table + +Warnings about @samp{syntax} are enabled by default, and the environment +variable @env{WARNINGS}, a comma separated list of categories, is +honored as well. Passing @option{-W @var{category}} actually behaves as if +you had passed @option{--warnings syntax,$WARNINGS,@var{category}}. To +disable the defaults and @env{WARNINGS}, and then +enable warnings about obsolete constructs, use @option{-W +none,obsolete}. + +@cindex Back trace +@cindex Macro invocation stack +Because @command{autoconf} uses @command{autom4te} behind the scenes, it +displays a back trace for errors, but not for warnings; if you want +them, just pass @option{-W error}. @xref{autom4te Invocation}, for some +examples. + +@item --trace=@var{macro}[:@var{format}] +@itemx -t @var{macro}[:@var{format}] +Do not create the @command{configure} script, but list the calls to +@var{macro} according to the @var{format}. Multiple @option{--trace} +arguments can be used to list several macros. Multiple @option{--trace} +arguments for a single macro are not cumulative; instead, you should +just make @var{format} as long as needed. + +The @var{format} is a regular string, with newlines if desired, and +several special escape codes. It defaults to @samp{$f:$l:$n:$%}; see +@ref{autom4te Invocation}, for details on the @var{format}. + +@item --initialization +@itemx -i +By default, @option{--trace} does not trace the initialization of the +Autoconf macros (typically the @code{AC_DEFUN} definitions). This +results in a noticeable speedup, but can be disabled by this option. +@end table + + +It is often necessary to check the content of a @file{configure.ac} +file, but parsing it yourself is extremely fragile and error-prone. It +is suggested that you rely upon @option{--trace} to scan +@file{configure.ac}. For instance, to find the list of variables that +are substituted, use: + +@example +@group +$ @kbd{autoconf -t AC_SUBST} +configure.ac:2:AC_SUBST:ECHO_C +configure.ac:2:AC_SUBST:ECHO_N +configure.ac:2:AC_SUBST:ECHO_T +@i{More traces deleted} +@end group +@end example + +@noindent +The example below highlights the difference between @samp{$@@}, +@samp{$*}, and @samp{$%}. + +@example +@group +$ @kbd{cat configure.ac} +AC_DEFINE(This, is, [an +[example]]) +$ @kbd{autoconf -t 'AC_DEFINE:@@: $@@} +*: $* +%: $%' +@@: [This],[is],[an +[example]] +*: This,is,an +[example] +%: This:is:an [example] +@end group +@end example + +@noindent +The @var{format} gives you a lot of freedom: + +@example +@group +$ @kbd{autoconf -t 'AC_SUBST:$$ac_subst@{"$1"@} = "$f:$l";'} +$ac_subst@{"ECHO_C"@} = "configure.ac:2"; +$ac_subst@{"ECHO_N"@} = "configure.ac:2"; +$ac_subst@{"ECHO_T"@} = "configure.ac:2"; +@i{More traces deleted} +@end group +@end example + +@noindent +A long @var{separator} can be used to improve the readability of complex +structures, and to ease their parsing (for instance when no single +character is suitable as a separator): + +@example +@group +$ @kbd{autoconf -t 'AM_MISSING_PROG:$@{|:::::|@}*'} +ACLOCAL|:::::|aclocal|:::::|$missing_dir +AUTOCONF|:::::|autoconf|:::::|$missing_dir +AUTOMAKE|:::::|automake|:::::|$missing_dir +@i{More traces deleted} +@end group +@end example + +@node autoreconf Invocation +@section Using @command{autoreconf} to Update @command{configure} Scripts +@cindex @command{autoreconf} + +Installing the various components of the GNU Build System can be +tedious: running @command{autopoint} for Gettext, @command{automake} for +@file{Makefile.in} etc.@: in each directory. It may be needed either +because some tools such as @command{automake} have been updated on your +system, or because some of the sources such as @file{configure.ac} have +been updated, or finally, simply in order to install the GNU Build +System in a fresh tree. + +@command{autoreconf} runs @command{autoconf}, @command{autoheader}, +@command{aclocal}, @command{automake}, @command{libtoolize}, and +@command{autopoint} (when appropriate) repeatedly to update the +GNU Build System in the specified directories and their +subdirectories (@pxref{Subdirectories}). By default, it only remakes +those files that are older than their sources. The environment variables +@env{AUTOM4TE}, @env{AUTOCONF}, @env{AUTOHEADER}, @env{AUTOMAKE}, +@env{ACLOCAL}, @env{AUTOPOINT}, @env{LIBTOOLIZE}, @env{M4}, and @env{MAKE} +may be used to override the invocation of the respective tools. + +If you install a new version of some tool, you can make +@command{autoreconf} remake @emph{all} of the files by giving it the +@option{--force} option. + +@xref{Automatic Remaking}, for Make rules to automatically +rebuild @command{configure} scripts when their source files change. That +method handles the timestamps of configuration header templates +properly, but does not pass @option{--autoconf-dir=@var{dir}} or +@option{--localdir=@var{dir}}. + +@cindex Gettext +@cindex @command{autopoint} +Gettext supplies the @command{autopoint} command to add translation +infrastructure to a source package. If you use @command{autopoint}, +your @file{configure.ac} should invoke both @code{AM_GNU_GETTEXT} and +@code{AM_GNU_GETTEXT_VERSION(@var{gettext-version})}. @xref{autopoint +Invocation, , Invoking the @code{autopoint} Program, gettext, +GNU @code{gettext} utilities}, for further details. + +@noindent +@command{autoreconf} accepts the following options: + +@table @option +@item --help +@itemx -h +Print a summary of the command line options and exit. + +@item --version +@itemx -V +Print the version number of Autoconf and exit. + +@item --verbose +@itemx -v +Print the name of each directory @command{autoreconf} examines and the +commands it runs. If given two or more times, pass @option{--verbose} +to subordinate tools that support it. + +@item --debug +@itemx -d +Don't remove the temporary files. + +@item --force +@itemx -f +Remake even @file{configure} scripts and configuration headers that are +newer than their input files (@file{configure.ac} and, if present, +@file{aclocal.m4}). + +@item --install +@itemx -i +Install the missing auxiliary files in the package. By default, files +are copied; this can be changed with @option{--symlink}. + +If deemed appropriate, this option triggers calls to +@samp{automake --add-missing}, +@samp{libtoolize}, @samp{autopoint}, etc. + +@item --no-recursive +Do not rebuild files in subdirectories to configure (see @ref{Subdirectories}, +macro @code{AC_CONFIG_SUBDIRS}). + +@item --symlink +@itemx -s +When used with @option{--install}, install symbolic links to the missing +auxiliary files instead of copying them. + +@item --make +@itemx -m +When the directories were configured, update the configuration by +running @samp{./config.status --recheck && ./config.status}, and then +run @samp{make}. + +@item --include=@var{dir} +@itemx -I @var{dir} +Append @var{dir} to the include path. Multiple invocations accumulate. +Passed on to @command{aclocal}, @command{autoconf} and +@command{autoheader} internally. + +@item --prepend-include=@var{dir} +@itemx -B @var{dir} +Prepend @var{dir} to the include path. Multiple invocations accumulate. +Passed on to @command{autoconf} and @command{autoheader} internally. + +@item --warnings=@var{category} +@itemx -W @var{category} +@evindex WARNINGS +Report the warnings related to @var{category} (which can actually be a +comma separated list). + +@table @samp +@item cross +related to cross compilation issues. + +@item obsolete +report the uses of obsolete constructs. + +@item portability +portability issues + +@item syntax +dubious syntactic constructs. + +@item all +report all the warnings + +@item none +report none + +@item error +treats warnings as errors + +@item no-@var{category} +disable warnings falling into @var{category} +@end table + +Warnings about @samp{syntax} are enabled by default, and the environment +variable @env{WARNINGS}, a comma separated list of categories, is +honored as well. Passing @option{-W @var{category}} actually behaves as if +you had passed @option{--warnings syntax,$WARNINGS,@var{category}}. To +disable the defaults and @env{WARNINGS}, and then +enable warnings about obsolete constructs, use @option{-W +none,obsolete}. +@end table + +If you want @command{autoreconf} to pass flags that are not listed here +on to @command{aclocal}, set @code{ACLOCAL_AMFLAGS} in your @file{Makefile.am}. +Due to a limitation in the Autoconf implementation these flags currently +must be set on a single line in @file{Makefile.am}, without any +backslash-newlines. + +@c ========================================= Initialization and Output Files. + +@node Setup +@chapter Initialization and Output Files + +Autoconf-generated @command{configure} scripts need some information about +how to initialize, such as how to find the package's source files and +about the output files to produce. The following sections describe the +initialization and the creation of output files. + +@menu +* Initializing configure:: Option processing etc. +* Versioning:: Dealing with Autoconf versions +* Notices:: Copyright, version numbers in @command{configure} +* Input:: Where Autoconf should find files +* Output:: Outputting results from the configuration +* Configuration Actions:: Preparing the output based on results +* Configuration Files:: Creating output files +* Makefile Substitutions:: Using output variables in makefiles +* Configuration Headers:: Creating a configuration header file +* Configuration Commands:: Running arbitrary instantiation commands +* Configuration Links:: Links depending on the configuration +* Subdirectories:: Configuring independent packages together +* Default Prefix:: Changing the default installation prefix +@end menu + +@node Initializing configure +@section Initializing @command{configure} + +Every @command{configure} script must call @code{AC_INIT} before doing +anything else that produces output. Calls to silent macros, such as +@code{AC_DEFUN}, may also occur prior to @code{AC_INIT}, although these +are generally used via @file{aclocal.m4}, since that is implicitly +included before the start of @file{configure.ac}. The only other +required macro is @code{AC_OUTPUT} (@pxref{Output}). + +@anchor{AC_INIT} +@defmac AC_INIT (@var{package}, @var{version}, @ovar{bug-report}, @ + @ovar{tarname}, @ovar{url}) +@acindex{INIT} +Process any command-line arguments and perform initialization +and verification. + +Set the name of the @var{package} and its @var{version}. These are +typically used in @option{--version} support, including that of +@command{configure}. The optional argument @var{bug-report} should be +the email to which users should send bug reports. The package +@var{tarname} differs from @var{package}: the latter designates the full +package name (e.g., @samp{GNU Autoconf}), while the former is meant for +distribution tar ball names (e.g., @samp{autoconf}). It defaults to +@var{package} with @samp{GNU } stripped, lower-cased, and all characters +other than alphanumerics and underscores are changed to @samp{-}. If +provided, @var{url} should be the home page for the package. + +The arguments of @code{AC_INIT} must be static, i.e., there should not +be any shell computation, quotes, or newlines, but they can be computed +by M4. This is because the package information strings are expanded at +M4 time into several contexts, and must give the same text at shell time +whether used in single-quoted strings, double-quoted strings, quoted +here-documents, or unquoted here-documents. It is permissible to use +@code{m4_esyscmd} or @code{m4_esyscmd_s} for computing a version string +that changes with every commit to a version control system (in fact, +Autoconf does just that, for all builds of the development tree made +between releases). + +The following M4 macros (e.g., @code{AC_PACKAGE_NAME}), output variables +(e.g., @code{PACKAGE_NAME}), and preprocessor symbols (e.g., +@code{PACKAGE_NAME}), are defined by @code{AC_INIT}: + +@table @asis +@item @code{AC_PACKAGE_NAME}, @code{PACKAGE_NAME} +@acindex{PACKAGE_NAME} +@ovindex PACKAGE_NAME +@cvindex PACKAGE_NAME +Exactly @var{package}. + +@item @code{AC_PACKAGE_TARNAME}, @code{PACKAGE_TARNAME} +@acindex{PACKAGE_TARNAME} +@ovindex PACKAGE_TARNAME +@cvindex PACKAGE_TARNAME +Exactly @var{tarname}, possibly generated from @var{package}. + +@item @code{AC_PACKAGE_VERSION}, @code{PACKAGE_VERSION} +@acindex{PACKAGE_VERSION} +@ovindex PACKAGE_VERSION +@cvindex PACKAGE_VERSION +Exactly @var{version}. + +@item @code{AC_PACKAGE_STRING}, @code{PACKAGE_STRING} +@acindex{PACKAGE_STRING} +@ovindex PACKAGE_STRING +@cvindex PACKAGE_STRING +Exactly @samp{@var{package} @var{version}}. + +@item @code{AC_PACKAGE_BUGREPORT}, @code{PACKAGE_BUGREPORT} +@acindex{PACKAGE_BUGREPORT} +@ovindex PACKAGE_BUGREPORT +@cvindex PACKAGE_BUGREPORT +Exactly @var{bug-report}, if one was provided. Typically an email +address, or URL to a bug management web page. + +@item @code{AC_PACKAGE_URL}, @code{PACKAGE_URL} +@acindex{PACKAGE_URL} +@ovindex PACKAGE_URL +@cvindex PACKAGE_URL +Exactly @var{url}, if one was provided. If @var{url} was empty, but +@var{package} begins with @samp{GNU }, then this defaults to +@samp{http://@/www.gnu.org/@/software/@/@var{tarname}/}, otherwise, no URL is +assumed. +@end table +@end defmac + +If your @command{configure} script does its own option processing, it +should inspect @samp{$@@} or @samp{$*} immediately after calling +@code{AC_INIT}, because other Autoconf macros liberally use the +@command{set} command to process strings, and this has the side effect +of updating @samp{$@@} and @samp{$*}. However, we suggest that you use +standard macros like @code{AC_ARG_ENABLE} instead of attempting to +implement your own option processing. @xref{Site Configuration}. + +@node Versioning +@section Dealing with Autoconf versions +@cindex Autoconf version +@cindex version, Autoconf + +The following optional macros can be used to help choose the minimum +version of Autoconf that can successfully compile a given +@file{configure.ac}. + +@defmac AC_PREREQ (@var{version}) +@acindex{PREREQ} +@cindex Version +Ensure that a recent enough version of Autoconf is being used. If the +version of Autoconf being used to create @command{configure} is +earlier than @var{version}, print an error message to the standard +error output and exit with failure (exit status is 63). For example: + +@example +AC_PREREQ([@value{VERSION}]) +@end example + +This macro may be used before @code{AC_INIT}. +@end defmac + +@defmac AC_AUTOCONF_VERSION +@acindex{AUTOCONF_VERSION} +This macro was introduced in Autoconf 2.62. It identifies the version +of Autoconf that is currently parsing the input file, in a format +suitable for @code{m4_version_compare} (@pxref{m4_version_compare}); in +other words, for this release of Autoconf, its value is +@samp{@value{VERSION}}. One potential use of this macro is for writing +conditional fallbacks based on when a feature was added to Autoconf, +rather than using @code{AC_PREREQ} to require the newer version of +Autoconf. However, remember that the Autoconf philosophy favors feature +checks over version checks. + +You should not expand this macro directly; use +@samp{m4_defn([AC_AUTOCONF_VERSION])} instead. This is because some +users might +have a beta version of Autoconf installed, with arbitrary letters +included in its version string. This means it is possible for the +version string to contain the name of a defined macro, such that +expanding @code{AC_AUTOCONF_VERSION} would trigger the expansion of that +macro during rescanning, and change the version string to be different +than what you intended to check. +@end defmac + +@node Notices +@section Notices in @command{configure} +@cindex Notices in @command{configure} + +The following macros manage version numbers for @command{configure} +scripts. Using them is optional. + +@defmac AC_COPYRIGHT (@var{copyright-notice}) +@acindex{COPYRIGHT} +@cindex Copyright Notice +State that, in addition to the Free Software Foundation's copyright on +the Autoconf macros, parts of your @command{configure} are covered by the +@var{copyright-notice}. + +The @var{copyright-notice} shows up in both the head of +@command{configure} and in @samp{configure --version}. +@end defmac + + +@defmac AC_REVISION (@var{revision-info}) +@acindex{REVISION} +@cindex Revision +Copy revision stamp @var{revision-info} into the @command{configure} +script, with any dollar signs or double-quotes removed. This macro lets +you put a revision stamp from @file{configure.ac} into @command{configure} +without RCS or CVS changing it when you check in +@command{configure}. That way, you can determine easily which revision of +@file{configure.ac} a particular @command{configure} corresponds to. + +For example, this line in @file{configure.ac}: + +@c The @w prevents RCS from changing the example in the manual. +@example +AC_REVISION([@w{$}Revision: 1.30 $]) +@end example + +@noindent +produces this in @command{configure}: + +@example +#!/bin/sh +# From configure.ac Revision: 1.30 +@end example +@end defmac + + +@node Input +@section Finding @command{configure} Input + +@anchor{AC_CONFIG_SRCDIR} +@defmac AC_CONFIG_SRCDIR (@var{unique-file-in-source-dir}) +@acindex{CONFIG_SRCDIR} +@var{unique-file-in-source-dir} is some file that is in the package's +source directory; @command{configure} checks for this file's existence to +make sure that the directory that it is told contains the source code in +fact does. Occasionally people accidentally specify the wrong directory +with @option{--srcdir}; this is a safety check. @xref{configure +Invocation}, for more information. +@end defmac + + +@c FIXME: Remove definitively once --install explained. +@c +@c Small packages may store all their macros in @code{aclocal.m4}. As the +@c set of macros grows, or for maintenance reasons, a maintainer may prefer +@c to split the macros in several files. In this case, Autoconf must be +@c told which files to load, and in which order. +@c +@c @defmac AC_INCLUDE (@var{file}@dots{}) +@c @acindex{INCLUDE} +@c @c FIXME: There is no longer shell globbing. +@c Read the macro definitions that appear in the listed files. A list of +@c space-separated file names or shell globbing patterns is expected. The +@c files are read in the order they're listed. +@c +@c Because the order of definition of macros is important (only the last +@c definition of a macro is used), beware that it is @code{AC_INIT} that +@c loads @file{acsite.m4} and @file{aclocal.m4}. Note that +@c @code{AC_INCLUDE}ing a file before @code{AC_INIT} or within +@c @file{aclocal.m4} is different from doing so after @code{AC_INIT}: in +@c the latter case, non-macro lines from included files may end up in the +@c @file{configure} script, whereas in the former case, they'd be discarded +@c just like any text that appear before @code{AC_INIT}. +@c @end defmac + +Packages that do manual configuration or use the @command{install} program +might need to tell @command{configure} where to find some other shell +scripts by calling @code{AC_CONFIG_AUX_DIR}, though the default places +it looks are correct for most cases. + +@defmac AC_CONFIG_AUX_DIR (@var{dir}) +@acindex{CONFIG_AUX_DIR} +Use the auxiliary build tools (e.g., @file{install-sh}, +@file{config.sub}, @file{config.guess}, Cygnus @command{configure}, +Automake and Libtool scripts, etc.)@: that are in directory @var{dir}. +These are auxiliary files used in configuration. @var{dir} can be +either absolute or relative to @file{@var{srcdir}}. The default is +@file{@var{srcdir}} or @file{@var{srcdir}/..} or +@file{@var{srcdir}/../..}, whichever is the first that contains +@file{install-sh}. The other files are not checked for, so that using +@code{AC_PROG_INSTALL} does not automatically require distributing the +other auxiliary files. It checks for @file{install.sh} also, but that +name is obsolete because some @command{make} have a rule that creates +@file{install} from it if there is no makefile. + +The auxiliary directory is commonly named @file{build-aux}. +If you need portability to DOS variants, do not name the +auxiliary directory @file{aux}. @xref{File System Conventions}. +@end defmac + +@defmac AC_REQUIRE_AUX_FILE (@var{file}) +@acindex{REQUIRE_AUX_FILE} +Declares that @var{file} is expected in the directory defined above. In +Autoconf proper, this macro does nothing: its sole purpose is to be +traced by third-party tools to produce a list of expected auxiliary +files. For instance it is called by macros like @code{AC_PROG_INSTALL} +(@pxref{Particular Programs}) or @code{AC_CANONICAL_BUILD} +(@pxref{Canonicalizing}) to register the auxiliary files they need. +@end defmac + +Similarly, packages that use @command{aclocal} should declare where +local macros can be found using @code{AC_CONFIG_MACRO_DIR}. + +@defmac AC_CONFIG_MACRO_DIR (@var{dir}) +@acindex{CONFIG_MACRO_DIR} +Specify @var{dir} as the location of additional local Autoconf macros. +This macro is intended for use by future versions of commands like +@command{autoreconf} that trace macro calls. It should be called +directly from @file{configure.ac} so that tools that install macros for +@command{aclocal} can find the macros' declarations. + +Note that if you use @command{aclocal} from Automake to generate +@file{aclocal.m4}, you must also set @code{ACLOCAL_AMFLAGS = -I +@var{dir}} in your top-level @file{Makefile.am}. Due to a limitation in +the Autoconf implementation of @command{autoreconf}, these include +directives currently must be set on a single line in @file{Makefile.am}, +without any backslash-newlines. +@end defmac + + +@node Output +@section Outputting Files +@cindex Outputting files + +Every Autoconf script, e.g., @file{configure.ac}, should finish by +calling @code{AC_OUTPUT}. That is the macro that generates and runs +@file{config.status}, which in turn creates the makefiles and any +other files resulting from configuration. This is the only required +macro besides @code{AC_INIT} (@pxref{Input}). + +@anchor{AC_OUTPUT} +@defmac AC_OUTPUT +@acindex{OUTPUT} +@cindex Instantiation +Generate @file{config.status} and launch it. Call this macro once, at +the end of @file{configure.ac}. + +@file{config.status} performs all the configuration actions: all the +output files (see @ref{Configuration Files}, macro +@code{AC_CONFIG_FILES}), header files (see @ref{Configuration Headers}, +macro @code{AC_CONFIG_HEADERS}), commands (see @ref{Configuration +Commands}, macro @code{AC_CONFIG_COMMANDS}), links (see +@ref{Configuration Links}, macro @code{AC_CONFIG_LINKS}), subdirectories +to configure (see @ref{Subdirectories}, macro @code{AC_CONFIG_SUBDIRS}) +are honored. + +The location of your @code{AC_OUTPUT} invocation is the exact point +where configuration actions are taken: any code afterwards is +executed by @command{configure} once @command{config.status} was run. If +you want to bind actions to @command{config.status} itself +(independently of whether @command{configure} is being run), see +@ref{Configuration Commands, , Running Arbitrary Configuration +Commands}. +@end defmac + +Historically, the usage of @code{AC_OUTPUT} was somewhat different. +@xref{Obsolete Macros}, for a description of the arguments that +@code{AC_OUTPUT} used to support. + + +If you run @command{make} in subdirectories, you should run it using the +@command{make} variable @code{MAKE}. Most versions of @command{make} set +@code{MAKE} to the name of the @command{make} program plus any options it +was given. (But many do not include in it the values of any variables +set on the command line, so those are not passed on automatically.) +Some old versions of @command{make} do not set this variable. The +following macro allows you to use it even with those versions. + +@anchor{AC_PROG_MAKE_SET} +@defmac AC_PROG_MAKE_SET +@acindex{PROG_MAKE_SET} +@ovindex SET_MAKE +If the Make command, @code{$MAKE} if set or else @samp{make}, predefines +@code{$(MAKE)}, define output variable @code{SET_MAKE} to be empty. +Otherwise, define @code{SET_MAKE} to a macro definition that sets +@code{$(MAKE)}, such as @samp{MAKE=make}. Calls @code{AC_SUBST} for +@code{SET_MAKE}. +@end defmac + +If you use this macro, place a line like this in each @file{Makefile.in} +that runs @command{MAKE} on other directories: + +@example +@@SET_MAKE@@ +@end example + + + +@node Configuration Actions +@section Performing Configuration Actions +@cindex Configuration actions + +@file{configure} is designed so that it appears to do everything itself, +but there is actually a hidden slave: @file{config.status}. +@file{configure} is in charge of examining your system, but it is +@file{config.status} that actually takes the proper actions based on the +results of @file{configure}. The most typical task of +@file{config.status} is to @emph{instantiate} files. + +@acindex{CONFIG_@var{ITEMS}} +This section describes the common behavior of the four standard +instantiating macros: @code{AC_CONFIG_FILES}, @code{AC_CONFIG_HEADERS}, +@code{AC_CONFIG_COMMANDS} and @code{AC_CONFIG_LINKS}. They all +have this prototype: + +@c FIXME: Can't use @ovar here, Texinfo 4.0 goes lunatic and emits something +@c awful. +@example +AC_CONFIG_@var{ITEMS}(@var{tag}@dots{}, @r{[}@var{commands}@r{]}, @r{[}@var{init-cmds}@r{]}) +@end example + +@noindent +where the arguments are: + +@table @var +@item tag@dots{} +A blank-or-newline-separated list of tags, which are typically the names of +the files to instantiate. + +You are encouraged to use literals as @var{tags}. In particular, you +should avoid + +@example +@dots{} && my_foos="$my_foos fooo" +@dots{} && my_foos="$my_foos foooo" +AC_CONFIG_@var{ITEMS}([$my_foos]) +@end example + +@noindent +and use this instead: + +@example +@dots{} && AC_CONFIG_@var{ITEMS}([fooo]) +@dots{} && AC_CONFIG_@var{ITEMS}([foooo]) +@end example + +The macros @code{AC_CONFIG_FILES} and @code{AC_CONFIG_HEADERS} use +special @var{tag} values: they may have the form @samp{@var{output}} or +@samp{@var{output}:@var{inputs}}. The file @var{output} is instantiated +from its templates, @var{inputs} (defaulting to @samp{@var{output}.in}). + +@samp{AC_CONFIG_FILES([Makefile:boiler/top.mk:boiler/bot.mk])}, +for example, asks for +the creation of the file @file{Makefile} that contains the expansion of the +output variables in the concatenation of @file{boiler/top.mk} and +@file{boiler/bot.mk}. + +The special value @samp{-} might be used to denote the standard output +when used in @var{output}, or the standard input when used in the +@var{inputs}. You most probably don't need to use this in +@file{configure.ac}, but it is convenient when using the command line +interface of @file{./config.status}, see @ref{config.status Invocation}, +for more details. + +The @var{inputs} may be absolute or relative file names. In the latter +case they are first looked for in the build tree, and then in the source +tree. Input files should be text files, and a line length below 2000 +bytes should be safe. + +@item commands +Shell commands output literally into @file{config.status}, and +associated with a tag that the user can use to tell @file{config.status} +which commands to run. The commands are run each time a @var{tag} +request is given to @file{config.status}, typically each time the file +@file{@var{tag}} is created. + +The variables set during the execution of @command{configure} are +@emph{not} available here: you first need to set them via the +@var{init-cmds}. Nonetheless the following variables are precomputed: + +@table @code +@item srcdir +@vrindex srcdir +The name of the top source directory, assuming that the working +directory is the top build directory. This +is what the @command{configure} option @option{--srcdir} sets. + +@item ac_top_srcdir +@vrindex ac_top_srcdir +The name of the top source directory, assuming that the working +directory is the current build directory. + +@item ac_top_build_prefix +@vrindex ac_top_build_prefix +The name of the top build directory, assuming that the working +directory is the current build directory. +It can be empty, or else ends with a slash, so that you may concatenate +it. + +@item ac_srcdir +@vrindex ac_srcdir +The name of the corresponding source directory, assuming that the +working directory is the current build directory. + +@item tmp +@vrindex tmp +The name of a temporary directory within the build tree, which you +can use if you need to create additional temporary files. The +directory is cleaned up when @command{config.status} is done or +interrupted. Please use package-specific file name prefixes to +avoid clashing with files that @command{config.status} may use +internally. +@end table + +@noindent +The @dfn{current} directory refers to the directory (or +pseudo-directory) containing the input part of @var{tags}. For +instance, running + +@example +AC_CONFIG_COMMANDS([deep/dir/out:in/in.in], [@dots{}], [@dots{}]) +@end example + +@noindent + with @option{--srcdir=../package} produces the following values: + +@example +# Argument of --srcdir +srcdir='../package' +# Reversing deep/dir +ac_top_build_prefix='../../' +# Concatenation of $ac_top_build_prefix and srcdir +ac_top_srcdir='../../../package' +# Concatenation of $ac_top_srcdir and deep/dir +ac_srcdir='../../../package/deep/dir' +@end example + +@noindent +independently of @samp{in/in.in}. + +@item init-cmds +Shell commands output @emph{unquoted} near the beginning of +@file{config.status}, and executed each time @file{config.status} runs +(regardless of the tag). Because they are unquoted, for example, +@samp{$var} is output as the value of @code{var}. @var{init-cmds} +is typically used by @file{configure} to give @file{config.status} some +variables it needs to run the @var{commands}. + +You should be extremely cautious in your variable names: all the +@var{init-cmds} share the same name space and may overwrite each other +in unpredictable ways. Sorry@enddots{} +@end table + +All these macros can be called multiple times, with different +@var{tag} values, of course! + + +@node Configuration Files +@section Creating Configuration Files +@cindex Creating configuration files +@cindex Configuration file creation + +Be sure to read the previous section, @ref{Configuration Actions}. + +@anchor{AC_CONFIG_FILES} +@defmac AC_CONFIG_FILES (@var{file}@dots{}, @ovar{cmds}, @ovar{init-cmds}) +@acindex{CONFIG_FILES} +Make @code{AC_OUTPUT} create each @file{@var{file}} by copying an input +file (by default @file{@var{file}.in}), substituting the output variable +values. +@c Before we used to have this feature, which was later rejected +@c because it complicates the writing of makefiles: +@c If the file would be unchanged, it is left untouched, to preserve +@c timestamp. +This macro is one of the instantiating macros; see @ref{Configuration +Actions}. @xref{Makefile Substitutions}, for more information on using +output variables. @xref{Setting Output Variables}, for more information +on creating them. This macro creates the directory that the file is in +if it doesn't exist. Usually, makefiles are created this way, +but other files, such as @file{.gdbinit}, can be specified as well. + +Typical calls to @code{AC_CONFIG_FILES} look like this: + +@example +AC_CONFIG_FILES([Makefile src/Makefile man/Makefile X/Imakefile]) +AC_CONFIG_FILES([autoconf], [chmod +x autoconf]) +@end example + +You can override an input file name by appending to @var{file} a +colon-separated list of input files. Examples: + +@example +AC_CONFIG_FILES([Makefile:boiler/top.mk:boiler/bot.mk] + [lib/Makefile:boiler/lib.mk]) +@end example + +@noindent +Doing this allows you to keep your file names acceptable to +DOS variants, or +to prepend and/or append boilerplate to the file. +@end defmac + + + +@node Makefile Substitutions +@section Substitutions in Makefiles +@cindex Substitutions in makefiles +@cindex Makefile substitutions + +Each subdirectory in a distribution that contains something to be +compiled or installed should come with a file @file{Makefile.in}, from +which @command{configure} creates a file @file{Makefile} in that directory. +To create @file{Makefile}, @command{configure} performs a simple variable +substitution, replacing occurrences of @samp{@@@var{variable}@@} in +@file{Makefile.in} with the value that @command{configure} has determined +for that variable. Variables that are substituted into output files in +this way are called @dfn{output variables}. They are ordinary shell +variables that are set in @command{configure}. To make @command{configure} +substitute a particular variable into the output files, the macro +@code{AC_SUBST} must be called with that variable name as an argument. +Any occurrences of @samp{@@@var{variable}@@} for other variables are +left unchanged. @xref{Setting Output Variables}, for more information +on creating output variables with @code{AC_SUBST}. + +A software package that uses a @command{configure} script should be +distributed with a file @file{Makefile.in}, but no makefile; that +way, the user has to properly configure the package for the local system +before compiling it. + +@xref{Makefile Conventions, , Makefile Conventions, standards, The +GNU Coding Standards}, for more information on what to put in +makefiles. + +@menu +* Preset Output Variables:: Output variables that are always set +* Installation Directory Variables:: Other preset output variables +* Changed Directory Variables:: Warnings about @file{datarootdir} +* Build Directories:: Supporting multiple concurrent compiles +* Automatic Remaking:: Makefile rules for configuring +@end menu + +@node Preset Output Variables +@subsection Preset Output Variables +@cindex Output variables + +Some output variables are preset by the Autoconf macros. Some of the +Autoconf macros set additional output variables, which are mentioned in +the descriptions for those macros. @xref{Output Variable Index}, for a +complete list of output variables. @xref{Installation Directory +Variables}, for the list of the preset ones related to installation +directories. Below are listed the other preset ones, many of which are +precious variables (@pxref{Setting Output Variables}, +@code{AC_ARG_VAR}). + +The preset variables which are available during @file{config.status} +(@pxref{Configuration Actions}) may also be used during +@command{configure} tests. For example, it is permissible to reference +@samp{$srcdir} when constructing a list of directories to pass via +option @option{-I} during a compiler feature check. When used in this +manner, coupled with the fact that @command{configure} is always run +from the top build directory, it is sufficient to use just +@samp{$srcdir} instead of @samp{$top_srcdir}. + +@c Just say no to ASCII sorting! We're humans, not computers. +@c These variables are listed as they would be in a dictionary: +@c actor +@c Actress +@c actress + +@defvar CFLAGS +@evindex CFLAGS +@ovindex CFLAGS +Debugging and optimization options for the C compiler. If it is not set +in the environment when @command{configure} runs, the default value is set +when you call @code{AC_PROG_CC} (or empty if you don't). @command{configure} +uses this variable when compiling or linking programs to test for C features. + +If a compiler option affects only the behavior of the preprocessor +(e.g., @option{-D@var{name}}), it should be put into @code{CPPFLAGS} +instead. If it affects only the linker (e.g., @option{-L@var{directory}}), +it should be put into @code{LDFLAGS} instead. If it +affects only the compiler proper, @code{CFLAGS} is the natural home for +it. If an option affects multiple phases of the compiler, though, +matters get tricky. One approach to put such options directly into +@code{CC}, e.g., @code{CC='gcc -m64'}. Another is to put them into both +@code{CPPFLAGS} and @code{LDFLAGS}, but not into @code{CFLAGS}. + +However, remember that some @file{Makefile} variables are reserved by +the GNU Coding Standards for the use of the ``user''---the person +building the package. For instance, @code{CFLAGS} is one such variable. + +Sometimes package developers are tempted to set user variables such as +@code{CFLAGS} because it appears to make their job easier. However, the +package itself should never set a user variable, particularly not to +include switches that are required for proper compilation of the +package. Since these variables are documented as being for the package +builder, that person rightfully expects to be able to override any of +these variables at build time. If the package developer needs to add +switches without interfering with the user, the proper way to do that is +to introduce an additional variable. Automake makes this easy by +introducing @code{AM_CFLAGS} (@pxref{Flag Variables Ordering, , , +automake, GNU Automake}), but the concept is the same even if +Automake is not used. +@end defvar + +@defvar configure_input +@ovindex configure_input +A comment saying that the file was generated automatically by +@command{configure} and giving the name of the input file. +@code{AC_OUTPUT} adds a comment line containing this variable to the top +of every makefile it creates. For other files, you should +reference this variable in a comment at the top of each input file. For +example, an input shell script should begin like this: + +@example +#!/bin/sh +# @@configure_input@@ +@end example + +@noindent +The presence of that line also reminds people editing the file that it +needs to be processed by @command{configure} in order to be used. +@end defvar + +@defvar CPPFLAGS +@evindex CPPFLAGS +@ovindex CPPFLAGS +Preprocessor options for the C, C++, Objective C, and Objective C++ +preprocessors and compilers. If +it is not set in the environment when @command{configure} runs, the default +value is empty. @command{configure} uses this variable when preprocessing +or compiling programs to test for C, C++, Objective C, and Objective C++ +features. + +This variable's contents should contain options like @option{-I}, +@option{-D}, and @option{-U} that affect only the behavior of the +preprocessor. Please see the explanation of @code{CFLAGS} for what you +can do if an option affects other phases of the compiler as well. + +Currently, @command{configure} always links as part of a single +invocation of the compiler that also preprocesses and compiles, so it +uses this variable also when linking programs. However, it is unwise to +depend on this behavior because the GNU Coding Standards do +not require it and many packages do not use @code{CPPFLAGS} when linking +programs. + +@xref{Special Chars in Variables}, for limitations that @code{CPPFLAGS} +might run into. +@end defvar + +@defvar CXXFLAGS +@evindex CXXFLAGS +@ovindex CXXFLAGS +Debugging and optimization options for the C++ compiler. It acts like +@code{CFLAGS}, but for C++ instead of C. +@end defvar + +@defvar DEFS +@ovindex DEFS +@option{-D} options to pass to the C compiler. If @code{AC_CONFIG_HEADERS} +is called, @command{configure} replaces @samp{@@DEFS@@} with +@option{-DHAVE_CONFIG_H} instead (@pxref{Configuration Headers}). This +variable is not defined while @command{configure} is performing its tests, +only when creating the output files. @xref{Setting Output Variables}, for +how to check the results of previous tests. +@end defvar + +@defvar ECHO_C +@defvarx ECHO_N +@defvarx ECHO_T +@ovindex ECHO_C +@ovindex ECHO_N +@ovindex ECHO_T +How does one suppress the trailing newline from @command{echo} for +question-answer message pairs? These variables provide a way: + +@example +echo $ECHO_N "And the winner is... $ECHO_C" +sleep 100000000000 +echo "$@{ECHO_T@}dead." +@end example + +@noindent +Some old and uncommon @command{echo} implementations offer no means to +achieve this, in which case @code{ECHO_T} is set to tab. You might not +want to use it. +@end defvar + +@defvar ERLCFLAGS +@evindex ERLCFLAGS +@ovindex ERLCFLAGS +Debugging and optimization options for the Erlang compiler. If it is not set +in the environment when @command{configure} runs, the default value is empty. +@command{configure} uses this variable when compiling +programs to test for Erlang features. +@end defvar + +@defvar FCFLAGS +@evindex FCFLAGS +@ovindex FCFLAGS +Debugging and optimization options for the Fortran compiler. If it +is not set in the environment when @command{configure} runs, the default +value is set when you call @code{AC_PROG_FC} (or empty if you don't). +@command{configure} uses this variable when compiling or linking +programs to test for Fortran features. +@end defvar + +@defvar FFLAGS +@evindex FFLAGS +@ovindex FFLAGS +Debugging and optimization options for the Fortran 77 compiler. If it +is not set in the environment when @command{configure} runs, the default +value is set when you call @code{AC_PROG_F77} (or empty if you don't). +@command{configure} uses this variable when compiling or linking +programs to test for Fortran 77 features. +@end defvar + +@defvar LDFLAGS +@evindex LDFLAGS +@ovindex LDFLAGS +Options for the linker. If it is not set +in the environment when @command{configure} runs, the default value is empty. +@command{configure} uses this variable when linking programs to test for +C, C++, Objective C, Objective C++, Fortran, and Go features. + +This variable's contents should contain options like @option{-s} and +@option{-L} that affect only the behavior of the linker. Please see the +explanation of @code{CFLAGS} for what you can do if an option also +affects other phases of the compiler. + +Don't use this variable to pass library names +(@option{-l}) to the linker; use @code{LIBS} instead. +@end defvar + +@defvar LIBS +@evindex LIBS +@ovindex LIBS +@option{-l} options to pass to the linker. The default value is empty, +but some Autoconf macros may prepend extra libraries to this variable if +those libraries are found and provide necessary functions, see +@ref{Libraries}. @command{configure} uses this variable when linking +programs to test for C, C++, Objective C, Objective C++, Fortran, and Go +features. +@end defvar + +@defvar OBJCFLAGS +@evindex OBJCFLAGS +@ovindex OBJCFLAGS +Debugging and optimization options for the Objective C compiler. It +acts like @code{CFLAGS}, but for Objective C instead of C. +@end defvar + +@defvar OBJCXXFLAGS +@evindex OBJCXXFLAGS +@ovindex OBJCXXFLAGS +Debugging and optimization options for the Objective C++ compiler. It +acts like @code{CXXFLAGS}, but for Objective C++ instead of C++. +@end defvar + +@defvar GOFLAGS +@evindex GOFLAGS +@ovindex GOFLAGS +Debugging and optimization options for the Go compiler. It acts like +@code{CFLAGS}, but for Go instead of C. +@end defvar + +@defvar builddir +@ovindex builddir +Rigorously equal to @samp{.}. Added for symmetry only. +@end defvar + +@defvar abs_builddir +@ovindex abs_builddir +Absolute name of @code{builddir}. +@end defvar + +@defvar top_builddir +@ovindex top_builddir +The relative name of the top level of the current build tree. In the +top-level directory, this is the same as @code{builddir}. +@end defvar + +@defvar top_build_prefix +@ovindex top_build_prefix +The relative name of the top level of the current build tree with final +slash if nonempty. This is the same as @code{top_builddir}, except that +it contains zero or more runs of @code{../}, so it should not be +appended with a slash for concatenation. This helps for @command{make} +implementations that otherwise do not treat @file{./file} and @file{file} +as equal in the toplevel build directory. +@end defvar + +@defvar abs_top_builddir +@ovindex abs_top_builddir +Absolute name of @code{top_builddir}. +@end defvar + +@defvar srcdir +@ovindex srcdir +The name of the directory that contains the source code for +that makefile. +@end defvar + +@defvar abs_srcdir +@ovindex abs_srcdir +Absolute name of @code{srcdir}. +@end defvar + +@defvar top_srcdir +@ovindex top_srcdir +The name of the top-level source code directory for the +package. In the top-level directory, this is the same as @code{srcdir}. +@end defvar + +@defvar abs_top_srcdir +@ovindex abs_top_srcdir +Absolute name of @code{top_srcdir}. +@end defvar + +@node Installation Directory Variables +@subsection Installation Directory Variables +@cindex Installation directories +@cindex Directories, installation + +The following variables specify the directories for +package installation, see @ref{Directory Variables, , Variables for +Installation Directories, standards, The GNU Coding +Standards}, for more information. Each variable corresponds to an +argument of @command{configure}; trailing slashes are stripped so that +expressions such as @samp{$@{prefix@}/lib} expand with only one slash +between directory names. See the end of this section for +details on when and how to use these variables. + +@defvar bindir +@ovindex bindir +The directory for installing executables that users run. +@end defvar + +@defvar datadir +@ovindex datadir +The directory for installing idiosyncratic read-only +architecture-independent data. +@end defvar + +@defvar datarootdir +@ovindex datarootdir +The root of the directory tree for read-only architecture-independent +data files. +@end defvar + +@defvar docdir +@ovindex docdir +The directory for installing documentation files (other than Info and +man). +@end defvar + +@defvar dvidir +@ovindex dvidir +The directory for installing documentation files in DVI format. +@end defvar + +@defvar exec_prefix +@ovindex exec_prefix +The installation prefix for architecture-dependent files. By default +it's the same as @code{prefix}. You should avoid installing anything +directly to @code{exec_prefix}. However, the default value for +directories containing architecture-dependent files should be relative +to @code{exec_prefix}. +@end defvar + +@defvar htmldir +@ovindex htmldir +The directory for installing HTML documentation. +@end defvar + +@defvar includedir +@ovindex includedir +The directory for installing C header files. +@end defvar + +@defvar infodir +@ovindex infodir +The directory for installing documentation in Info format. +@end defvar + +@defvar libdir +@ovindex libdir +The directory for installing object code libraries. +@end defvar + +@defvar libexecdir +@ovindex libexecdir +The directory for installing executables that other programs run. +@end defvar + +@defvar localedir +@ovindex localedir +The directory for installing locale-dependent but +architecture-independent data, such as message catalogs. This directory +usually has a subdirectory per locale. +@end defvar + +@defvar localstatedir +@ovindex localstatedir +The directory for installing modifiable single-machine data. +@end defvar + +@defvar mandir +@ovindex mandir +The top-level directory for installing documentation in man format. +@end defvar + +@defvar oldincludedir +@ovindex oldincludedir +The directory for installing C header files for non-GCC compilers. +@end defvar + +@defvar pdfdir +@ovindex pdfdir +The directory for installing PDF documentation. +@end defvar + +@defvar prefix +@ovindex prefix +The common installation prefix for all files. If @code{exec_prefix} +is defined to a different value, @code{prefix} is used only for +architecture-independent files. +@end defvar + +@defvar psdir +@ovindex psdir +The directory for installing PostScript documentation. +@end defvar + +@defvar sbindir +@ovindex sbindir +The directory for installing executables that system +administrators run. +@end defvar + +@defvar sharedstatedir +@ovindex sharedstatedir +The directory for installing modifiable architecture-independent data. +@end defvar + +@defvar sysconfdir +@ovindex sysconfdir +The directory for installing read-only single-machine data. +@end defvar + + +Most of these variables have values that rely on @code{prefix} or +@code{exec_prefix}. It is deliberate that the directory output +variables keep them unexpanded: typically @samp{@@datarootdir@@} is +replaced by @samp{$@{prefix@}/share}, not @samp{/usr/local/share}, and +@samp{@@datadir@@} is replaced by @samp{$@{datarootdir@}}. + +This behavior is mandated by the GNU Coding Standards, so that when +the user runs: + +@table @samp +@item make +she can still specify a different prefix from the one specified to +@command{configure}, in which case, if needed, the package should hard +code dependencies corresponding to the make-specified prefix. + +@item make install +she can specify a different installation location, in which case the +package @emph{must} still depend on the location which was compiled in +(i.e., never recompile when @samp{make install} is run). This is an +extremely important feature, as many people may decide to install all +the files of a package grouped together, and then install links from +the final locations to there. +@end table + +In order to support these features, it is essential that +@code{datarootdir} remains defined as @samp{$@{prefix@}/share}, +so that its value can be expanded based +on the current value of @code{prefix}. + +A corollary is that you should not use these variables except in +makefiles. For instance, instead of trying to evaluate @code{datadir} +in @file{configure} and hard-coding it in makefiles using +e.g., @samp{AC_DEFINE_UNQUOTED([DATADIR], ["$datadir"], [Data directory.])}, +you should add +@option{-DDATADIR='$(datadir)'} to your makefile's definition of +@code{CPPFLAGS} (@code{AM_CPPFLAGS} if you are also using Automake). + +Similarly, you should not rely on @code{AC_CONFIG_FILES} to replace +@code{bindir} and friends in your shell scripts and other files; instead, +let @command{make} manage their replacement. For instance Autoconf +ships templates of its shell scripts ending with @samp{.in}, and uses a +makefile snippet similar to the following to build scripts like +@command{autoheader} and @command{autom4te}: + +@example +@group +edit = sed \ + -e 's|@@bindir[@@]|$(bindir)|g' \ + -e 's|@@pkgdatadir[@@]|$(pkgdatadir)|g' \ + -e 's|@@prefix[@@]|$(prefix)|g' +@end group + +@group +autoheader autom4te: Makefile + rm -f $@@ $@@.tmp + srcdir=''; \ + test -f ./$@@.in || srcdir=$(srcdir)/; \ + $(edit) $$@{srcdir@}$@@.in >$@@.tmp +@c $$ restore font-lock + chmod +x $@@.tmp + chmod a-w $@@.tmp + mv $@@.tmp $@@ +@end group + +@group +autoheader: $(srcdir)/autoheader.in +autom4te: $(srcdir)/autom4te.in +@end group +@end example + +Some details are noteworthy: + +@table @asis +@item @samp{@@bindir[@@]} +The brackets prevent @command{configure} from replacing +@samp{@@bindir@@} in the Sed expression itself. +Brackets are preferable to a backslash here, since +Posix says @samp{\@@} is not portable. + +@item @samp{$(bindir)} +Don't use @samp{@@bindir@@}! Use the matching makefile variable +instead. + +@item @samp{$(pkgdatadir)} +The example takes advantage of the variable @samp{$(pkgdatadir)} +provided by Automake; it is equivalent to @samp{$(datadir)/$(PACKAGE)}. + +@item @samp{/} +Don't use @samp{/} in the Sed expressions that replace file names since +most likely the +variables you use, such as @samp{$(bindir)}, contain @samp{/}. +Use a shell metacharacter instead, such as @samp{|}. + +@item special characters +File names, file name components, and the value of @code{VPATH} should +not contain shell metacharacters or white +space. @xref{Special Chars in Variables}. + +@item dependency on @file{Makefile} +Since @code{edit} uses values that depend on the configuration specific +values (@code{prefix}, etc.)@: and not only on @code{VERSION} and so forth, +the output depends on @file{Makefile}, not @file{configure.ac}. + +@item @samp{$@@} +The main rule is generic, and uses @samp{$@@} extensively to +avoid the need for multiple copies of the rule. + +@item Separated dependencies and single suffix rules +You can't use them! The above snippet cannot be (portably) rewritten +as: + +@example +autoconf autoheader: Makefile +@group +.in: + rm -f $@@ $@@.tmp + $(edit) $< >$@@.tmp + chmod +x $@@.tmp + mv $@@.tmp $@@ +@end group +@end example + +@xref{Single Suffix Rules}, for details. + +@item @samp{$(srcdir)} +Be sure to specify the name of the source directory, +otherwise the package won't support separated builds. +@end table + +For the more specific installation of Erlang libraries, the following variables +are defined: + +@defvar ERLANG_INSTALL_LIB_DIR +@ovindex ERLANG_INSTALL_LIB_DIR +@acindex{ERLANG_SUBST_INSTALL_LIB_DIR} +The common parent directory of Erlang library installation directories. +This variable is set by calling the @code{AC_ERLANG_SUBST_INSTALL_LIB_DIR} +macro in @file{configure.ac}. +@end defvar + +@defvar ERLANG_INSTALL_LIB_DIR_@var{library} +@ovindex ERLANG_INSTALL_LIB_DIR_@var{library} +@acindex{ERLANG_SUBST_INSTALL_LIB_SUBDIR} +The installation directory for Erlang library @var{library}. +This variable is set by using the +@samp{AC_ERLANG_SUBST_INSTALL_LIB_SUBDIR} +macro in @file{configure.ac}. +@end defvar + +@xref{Erlang Libraries}, for details. + + +@node Changed Directory Variables +@subsection Changed Directory Variables +@cindex @file{datarootdir} + +In Autoconf 2.60, the set of directory variables has changed, and the +defaults of some variables have been adjusted +(@pxref{Installation Directory Variables}) to changes in the +GNU Coding Standards. Notably, @file{datadir}, @file{infodir}, and +@file{mandir} are now expressed in terms of @file{datarootdir}. If you are +upgrading from an earlier Autoconf version, you may need to adjust your files +to ensure that the directory variables are substituted correctly +(@pxref{Defining Directories}), and that a definition of @file{datarootdir} is +in place. For example, in a @file{Makefile.in}, adding + +@example +datarootdir = @@datarootdir@@ +@end example + +@noindent +is usually sufficient. If you use Automake to create @file{Makefile.in}, +it will add this for you. + +To help with the transition, Autoconf warns about files that seem to use +@code{datarootdir} without defining it. In some cases, it then expands +the value of @code{$datarootdir} in substitutions of the directory +variables. The following example shows such a warning: + +@example +$ @kbd{cat configure.ac} +AC_INIT +AC_CONFIG_FILES([Makefile]) +AC_OUTPUT +$ @kbd{cat Makefile.in} +prefix = @@prefix@@ +datadir = @@datadir@@ +$ @kbd{autoconf} +$ @kbd{configure} +configure: creating ./config.status +config.status: creating Makefile +config.status: WARNING: + Makefile.in seems to ignore the --datarootdir setting +$ @kbd{cat Makefile} +prefix = /usr/local +datadir = $@{prefix@}/share +@end example + +Usually one can easily change the file to accommodate both older and newer +Autoconf releases: + +@example +$ @kbd{cat Makefile.in} +prefix = @@prefix@@ +datarootdir = @@datarootdir@@ +datadir = @@datadir@@ +$ @kbd{configure} +configure: creating ./config.status +config.status: creating Makefile +$ @kbd{cat Makefile} +prefix = /usr/local +datarootdir = $@{prefix@}/share +datadir = $@{datarootdir@} +@end example + +@acindex{DATAROOTDIR_CHECKED} +In some cases, however, the checks may not be able to detect that a suitable +initialization of @code{datarootdir} is in place, or they may fail to detect +that such an initialization is necessary in the output file. If, after +auditing your package, there are still spurious @file{configure} warnings about +@code{datarootdir}, you may add the line + +@example +AC_DEFUN([AC_DATAROOTDIR_CHECKED]) +@end example + +@noindent +to your @file{configure.ac} to disable the warnings. This is an exception +to the usual rule that you should not define a macro whose name begins with +@code{AC_} (@pxref{Macro Names}). + + + +@node Build Directories +@subsection Build Directories +@cindex Build directories +@cindex Directories, build + +You can support compiling a software package for several architectures +simultaneously from the same copy of the source code. The object files +for each architecture are kept in their own directory. + +To support doing this, @command{make} uses the @code{VPATH} variable to +find the files that are in the source directory. GNU Make +can do this. Most other recent @command{make} programs can do this as +well, though they may have difficulties and it is often simpler to +recommend GNU @command{make} (@pxref{VPATH and Make}). Older +@command{make} programs do not support @code{VPATH}; when using them, the +source code must be in the same directory as the object files. + +If you are using GNU Automake, the remaining details in this +section are already covered for you, based on the contents of your +@file{Makefile.am}. But if you are using Autoconf in isolation, then +supporting @code{VPATH} requires the following in your +@file{Makefile.in}: + +@example +srcdir = @@srcdir@@ +VPATH = @@srcdir@@ +@end example + +Do not set @code{VPATH} to the value of another variable (@pxref{Variables +listed in VPATH}. + +@command{configure} substitutes the correct value for @code{srcdir} when +it produces @file{Makefile}. + +Do not use the @command{make} variable @code{$<}, which expands to the +file name of the file in the source directory (found with @code{VPATH}), +except in implicit rules. (An implicit rule is one such as @samp{.c.o}, +which tells how to create a @file{.o} file from a @file{.c} file.) Some +versions of @command{make} do not set @code{$<} in explicit rules; they +expand it to an empty value. + +Instead, Make command lines should always refer to source +files by prefixing them with @samp{$(srcdir)/}. For example: + +@example +time.info: time.texinfo + $(MAKEINFO) '$(srcdir)/time.texinfo' +@end example + +@node Automatic Remaking +@subsection Automatic Remaking +@cindex Automatic remaking +@cindex Remaking automatically + +You can put rules like the following in the top-level @file{Makefile.in} +for a package to automatically update the configuration information when +you change the configuration files. This example includes all of the +optional files, such as @file{aclocal.m4} and those related to +configuration header files. Omit from the @file{Makefile.in} rules for +any of these files that your package does not use. + +The @samp{$(srcdir)/} prefix is included because of limitations in the +@code{VPATH} mechanism. + +The @file{stamp-} files are necessary because the timestamps of +@file{config.h.in} and @file{config.h} are not changed if remaking +them does not change their contents. This feature avoids unnecessary +recompilation. You should include the file @file{stamp-h.in} in your +package's distribution, so that @command{make} considers +@file{config.h.in} up to date. Don't use @command{touch} +(@pxref{touch, , Limitations of Usual Tools}); instead, use +@command{echo} (using +@command{date} would cause needless differences, hence CVS +conflicts, etc.). + +@example +@group +$(srcdir)/configure: configure.ac aclocal.m4 + cd '$(srcdir)' && autoconf + +# autoheader might not change config.h.in, so touch a stamp file. +$(srcdir)/config.h.in: stamp-h.in +$(srcdir)/stamp-h.in: configure.ac aclocal.m4 + cd '$(srcdir)' && autoheader + echo timestamp > '$(srcdir)/stamp-h.in' + +config.h: stamp-h +stamp-h: config.h.in config.status + ./config.status + +Makefile: Makefile.in config.status + ./config.status + +config.status: configure + ./config.status --recheck +@end group +@end example + +@noindent +(Be careful if you copy these lines directly into your makefile, as you +need to convert the indented lines to start with the tab character.) + +In addition, you should use + +@example +AC_CONFIG_FILES([stamp-h], [echo timestamp > stamp-h]) +@end example + +@noindent +so @file{config.status} ensures that @file{config.h} is considered up to +date. @xref{Output}, for more information about @code{AC_OUTPUT}. + +@xref{config.status Invocation}, for more examples of handling +configuration-related dependencies. + +@node Configuration Headers +@section Configuration Header Files +@cindex Configuration Header +@cindex @file{config.h} + +When a package contains more than a few tests that define C preprocessor +symbols, the command lines to pass @option{-D} options to the compiler +can get quite long. This causes two problems. One is that the +@command{make} output is hard to visually scan for errors. More +seriously, the command lines can exceed the length limits of some +operating systems. As an alternative to passing @option{-D} options to +the compiler, @command{configure} scripts can create a C header file +containing @samp{#define} directives. The @code{AC_CONFIG_HEADERS} +macro selects this kind of output. Though it can be called anywhere +between @code{AC_INIT} and @code{AC_OUTPUT}, it is customary to call +it right after @code{AC_INIT}. + +The package should @samp{#include} the configuration header file before +any other header files, to prevent inconsistencies in declarations (for +example, if it redefines @code{const}). + +To provide for VPATH builds, remember to pass the C compiler a @option{-I.} +option (or @option{-I..}; whichever directory contains @file{config.h}). +Even if you use @samp{#include "config.h"}, the preprocessor searches only +the directory of the currently read file, i.e., the source directory, not +the build directory. + +With the appropriate @option{-I} option, you can use +@samp{#include <config.h>}. Actually, it's a good habit to use it, +because in the rare case when the source directory contains another +@file{config.h}, the build directory should be searched first. + + +@defmac AC_CONFIG_HEADERS (@var{header} @dots{}, @ovar{cmds}, @ovar{init-cmds}) +@acindex{CONFIG_HEADERS} +@cvindex HAVE_CONFIG_H +This macro is one of the instantiating macros; see @ref{Configuration +Actions}. Make @code{AC_OUTPUT} create the file(s) in the +blank-or-newline-separated list @var{header} containing C preprocessor +@code{#define} statements, and replace @samp{@@DEFS@@} in generated +files with @option{-DHAVE_CONFIG_H} instead of the value of @code{DEFS}. +The usual name for @var{header} is @file{config.h}. + +If @var{header} already exists and its contents are identical to what +@code{AC_OUTPUT} would put in it, it is left alone. Doing this allows +making some changes in the configuration without needlessly causing +object files that depend on the header file to be recompiled. + +Usually the input file is named @file{@var{header}.in}; however, you can +override the input file name by appending to @var{header} a +colon-separated list of input files. For example, you might need to make +the input file name acceptable to DOS variants: + +@example +AC_CONFIG_HEADERS([config.h:config.hin]) +@end example + +@end defmac + +@defmac AH_HEADER +@ahindex{HEADER} +This macro is defined as the name of the first declared config header +and undefined if no config headers have been declared up to this point. +A third-party macro may, for example, require use of a config header +without invoking AC_CONFIG_HEADERS twice, like this: + +@example +AC_CONFIG_COMMANDS_PRE( + [m4_ifndef([AH_HEADER], [AC_CONFIG_HEADERS([config.h])])]) +@end example + +@end defmac + +@xref{Configuration Actions}, for more details on @var{header}. + +@menu +* Header Templates:: Input for the configuration headers +* autoheader Invocation:: How to create configuration templates +* Autoheader Macros:: How to specify CPP templates +@end menu + +@node Header Templates +@subsection Configuration Header Templates +@cindex Configuration Header Template +@cindex Header templates +@cindex @file{config.h.in} + +Your distribution should contain a template file that looks as you want +the final header file to look, including comments, with @code{#undef} +statements which are used as hooks. For example, suppose your +@file{configure.ac} makes these calls: + +@example +AC_CONFIG_HEADERS([conf.h]) +AC_CHECK_HEADERS([unistd.h]) +@end example + +@noindent +Then you could have code like the following in @file{conf.h.in}. +The @file{conf.h} created by @command{configure} defines @samp{HAVE_UNISTD_H} +to 1, if and only if the system has @file{unistd.h}. + +@example +@group +/* Define as 1 if you have unistd.h. */ +#undef HAVE_UNISTD_H +@end group +@end example + +The format of the template file is stricter than what the C preprocessor +is required to accept. A directive line should contain only whitespace, +@samp{#undef}, and @samp{HAVE_UNISTD_H}. The use of @samp{#define} +instead of @samp{#undef}, or of comments on the same line as +@samp{#undef}, is strongly discouraged. Each hook should only be listed +once. Other preprocessor lines, such as @samp{#ifdef} or +@samp{#include}, are copied verbatim from the template into the +generated header. + +Since it is a tedious task to keep a template header up to date, you may +use @command{autoheader} to generate it, see @ref{autoheader Invocation}. + +During the instantiation of the header, each @samp{#undef} line in the +template file for each symbol defined by @samp{AC_DEFINE} is changed to an +appropriate @samp{#define}. If the corresponding @samp{AC_DEFINE} has not +been executed during the @command{configure} run, the @samp{#undef} line is +commented out. (This is important, e.g., for @samp{_POSIX_SOURCE}: +on many systems, it can be implicitly defined by the compiler, and +undefining it in the header would then break compilation of subsequent +headers.) + +Currently, @emph{all} remaining @samp{#undef} lines in the header +template are commented out, whether or not there was a corresponding +@samp{AC_DEFINE} for the macro name; but this behavior is not guaranteed +for future releases of Autoconf. + +Generally speaking, since you should not use @samp{#define}, and you +cannot guarantee whether a @samp{#undef} directive in the header +template will be converted to a @samp{#define} or commented out in the +generated header file, the template file cannot be used for conditional +definition effects. Consequently, if you need to use the construct + +@example +@group +#ifdef THIS +# define THAT +#endif +@end group +@end example + +@noindent +you must place it outside of the template. +If you absolutely need to hook it to the config header itself, please put +the directives to a separate file, and @samp{#include} that file from the +config header template. If you are using @command{autoheader}, you would +probably use @samp{AH_BOTTOM} to append the @samp{#include} directive. + + +@node autoheader Invocation +@subsection Using @command{autoheader} to Create @file{config.h.in} +@cindex @command{autoheader} + +The @command{autoheader} program can create a template file of C +@samp{#define} statements for @command{configure} to use. +It searches for the first invocation of @code{AC_CONFIG_HEADERS} in +@file{configure} sources to determine the name of the template. +(If the first call of @code{AC_CONFIG_HEADERS} specifies more than one +input file name, @command{autoheader} uses the first one.) + +It is recommended that only one input file is used. If you want to append +a boilerplate code, it is preferable to use +@samp{AH_BOTTOM([#include <conf_post.h>])}. +File @file{conf_post.h} is not processed during the configuration then, +which make things clearer. Analogically, @code{AH_TOP} can be used to +prepend a boilerplate code. + +In order to do its job, @command{autoheader} needs you to document all +of the symbols that you might use. Typically this is done via an +@code{AC_DEFINE} or @code{AC_DEFINE_UNQUOTED} call whose first argument +is a literal symbol and whose third argument describes the symbol +(@pxref{Defining Symbols}). Alternatively, you can use +@code{AH_TEMPLATE} (@pxref{Autoheader Macros}), or you can supply a +suitable input file for a subsequent configuration header file. +Symbols defined by Autoconf's builtin tests are already documented properly; +you need to document only those that you +define yourself. + +You might wonder why @command{autoheader} is needed: after all, why +would @command{configure} need to ``patch'' a @file{config.h.in} to +produce a @file{config.h} instead of just creating @file{config.h} from +scratch? Well, when everything rocks, the answer is just that we are +wasting our time maintaining @command{autoheader}: generating +@file{config.h} directly is all that is needed. When things go wrong, +however, you'll be thankful for the existence of @command{autoheader}. + +The fact that the symbols are documented is important in order to +@emph{check} that @file{config.h} makes sense. The fact that there is a +well-defined list of symbols that should be defined (or not) is +also important for people who are porting packages to environments where +@command{configure} cannot be run: they just have to @emph{fill in the +blanks}. + +But let's come back to the point: the invocation of @command{autoheader}@dots{} + +If you give @command{autoheader} an argument, it uses that file instead +of @file{configure.ac} and writes the header file to the standard output +instead of to @file{config.h.in}. If you give @command{autoheader} an +argument of @option{-}, it reads the standard input instead of +@file{configure.ac} and writes the header file to the standard output. + +@command{autoheader} accepts the following options: + +@table @option +@item --help +@itemx -h +Print a summary of the command line options and exit. + +@item --version +@itemx -V +Print the version number of Autoconf and exit. + +@item --verbose +@itemx -v +Report processing steps. + +@item --debug +@itemx -d +Don't remove the temporary files. + +@item --force +@itemx -f +Remake the template file even if newer than its input files. + +@item --include=@var{dir} +@itemx -I @var{dir} +Append @var{dir} to the include path. Multiple invocations accumulate. + +@item --prepend-include=@var{dir} +@itemx -B @var{dir} +Prepend @var{dir} to the include path. Multiple invocations accumulate. + +@item --warnings=@var{category} +@itemx -W @var{category} +@evindex WARNINGS +Report the warnings related to @var{category} (which can actually be a +comma separated list). Current categories include: + +@table @samp +@item obsolete +report the uses of obsolete constructs + +@item all +report all the warnings + +@item none +report none + +@item error +treats warnings as errors + +@item no-@var{category} +disable warnings falling into @var{category} +@end table + +@end table + + + +@node Autoheader Macros +@subsection Autoheader Macros +@cindex Autoheader macros + +@command{autoheader} scans @file{configure.ac} and figures out which C +preprocessor symbols it might define. It knows how to generate +templates for symbols defined by @code{AC_CHECK_HEADERS}, +@code{AC_CHECK_FUNCS} etc., but if you @code{AC_DEFINE} any additional +symbol, you must define a template for it. If there are missing +templates, @command{autoheader} fails with an error message. + +The template for a @var{symbol} is created +by @command{autoheader} from +the @var{description} argument to an @code{AC_DEFINE}; +see @ref{Defining Symbols}. + +For special needs, you can use the following macros. + + +@defmac AH_TEMPLATE (@var{key}, @var{description}) +@ahindex{TEMPLATE} +Tell @command{autoheader} to generate a template for @var{key}. This macro +generates standard templates just like @code{AC_DEFINE} when a +@var{description} is given. + +For example: + +@example +AH_TEMPLATE([CRAY_STACKSEG_END], + [Define to one of _getb67, GETB67, getb67 + for Cray-2 and Cray-YMP systems. This + function is required for alloca.c support + on those systems.]) +@end example + +@noindent +generates the following template, with the description properly +justified. + +@example +/* Define to one of _getb67, GETB67, getb67 for Cray-2 and + Cray-YMP systems. This function is required for alloca.c + support on those systems. */ +#undef CRAY_STACKSEG_END +@end example +@end defmac + + +@defmac AH_VERBATIM (@var{key}, @var{template}) +@ahindex{VERBATIM} +Tell @command{autoheader} to include the @var{template} as-is in the header +template file. This @var{template} is associated with the @var{key}, +which is used to sort all the different templates and guarantee their +uniqueness. It should be a symbol that can be defined via @code{AC_DEFINE}. +@end defmac + + +@defmac AH_TOP (@var{text}) +@ahindex{TOP} +Include @var{text} at the top of the header template file. +@end defmac + + +@defmac AH_BOTTOM (@var{text}) +@ahindex{BOTTOM} +Include @var{text} at the bottom of the header template file. +@end defmac + + +Please note that @var{text} gets included ``verbatim'' to the template file, +not to the resulting config header, so it can easily get mangled when the +template is processed. There is rarely a need for something other than + +@example +AH_BOTTOM([#include <custom.h>]) +@end example + + + +@node Configuration Commands +@section Running Arbitrary Configuration Commands +@cindex Configuration commands +@cindex Commands for configuration + +You can execute arbitrary commands before, during, and after +@file{config.status} is run. The three following macros accumulate the +commands to run when they are called multiple times. +@code{AC_CONFIG_COMMANDS} replaces the obsolete macro +@code{AC_OUTPUT_COMMANDS}; see @ref{Obsolete Macros}, for details. + +@anchor{AC_CONFIG_COMMANDS} +@defmac AC_CONFIG_COMMANDS (@var{tag}@dots{}, @ovar{cmds}, @ovar{init-cmds}) +@acindex{CONFIG_COMMANDS} +Specify additional shell commands to run at the end of +@file{config.status}, and shell commands to initialize any variables +from @command{configure}. Associate the commands with @var{tag}. +Since typically the @var{cmds} create a file, @var{tag} should +naturally be the name of that file. If needed, the directory hosting +@var{tag} is created. This macro is one of the instantiating macros; +see @ref{Configuration Actions}. + +Here is an unrealistic example: +@example +fubar=42 +AC_CONFIG_COMMANDS([fubar], + [echo this is extra $fubar, and so on.], + [fubar=$fubar]) +@end example + +Here is a better one: +@example +AC_CONFIG_COMMANDS([timestamp], [date >timestamp]) +@end example +@end defmac + +The following two macros look similar, but in fact they are not of the same +breed: they are executed directly by @file{configure}, so you cannot use +@file{config.status} to rerun them. + +@c Yet it is good to leave them here. The user sees them together and +@c decides which best fits their needs. + +@defmac AC_CONFIG_COMMANDS_PRE (@var{cmds}) +@acindex{CONFIG_COMMANDS_PRE} +Execute the @var{cmds} right before creating @file{config.status}. + +This macro presents the last opportunity to call @code{AC_SUBST}, +@code{AC_DEFINE}, or @code{AC_CONFIG_@var{ITEMS}} macros. +@end defmac + +@defmac AC_CONFIG_COMMANDS_POST (@var{cmds}) +@acindex{CONFIG_COMMANDS_POST} +Execute the @var{cmds} right after creating @file{config.status}. +@end defmac + + + + +@node Configuration Links +@section Creating Configuration Links +@cindex Configuration links +@cindex Links for configuration + +You may find it convenient to create links whose destinations depend upon +results of tests. One can use @code{AC_CONFIG_COMMANDS} but the +creation of relative symbolic links can be delicate when the package is +built in a directory different from the source directory. + +@anchor{AC_CONFIG_LINKS} +@defmac AC_CONFIG_LINKS (@var{dest}:@var{source}@dots{}, @ovar{cmds}, @ + @ovar{init-cmds}) +@acindex{CONFIG_LINKS} +@cindex Links +Make @code{AC_OUTPUT} link each of the existing files @var{source} to +the corresponding link name @var{dest}. Makes a symbolic link if +possible, otherwise a hard link if possible, otherwise a copy. The +@var{dest} and @var{source} names should be relative to the top level +source or build directory. This macro is one of the instantiating +macros; see @ref{Configuration Actions}. + +For example, this call: + +@example +AC_CONFIG_LINKS([host.h:config/$machine.h + object.h:config/$obj_format.h]) +@end example + +@noindent +creates in the current directory @file{host.h} as a link to +@file{@var{srcdir}/config/$machine.h}, and @file{object.h} as a +link to @file{@var{srcdir}/config/$obj_format.h}. + +The tempting value @samp{.} for @var{dest} is invalid: it makes it +impossible for @samp{config.status} to guess the links to establish. + +One can then run: +@example +./config.status host.h object.h +@end example +@noindent +to create the links. +@end defmac + + + +@node Subdirectories +@section Configuring Other Packages in Subdirectories +@cindex Configure subdirectories +@cindex Subdirectory configure + +In most situations, calling @code{AC_OUTPUT} is sufficient to produce +makefiles in subdirectories. However, @command{configure} scripts +that control more than one independent package can use +@code{AC_CONFIG_SUBDIRS} to run @command{configure} scripts for other +packages in subdirectories. + +@defmac AC_CONFIG_SUBDIRS (@var{dir} @dots{}) +@acindex{CONFIG_SUBDIRS} +@ovindex subdirs +Make @code{AC_OUTPUT} run @command{configure} in each subdirectory +@var{dir} in the given blank-or-newline-separated list. Each @var{dir} should +be a literal, i.e., please do not use: + +@example +@c If you change this example, adjust tests/torture.at:Non-literal AC_CONFIG_SUBDIRS. +if test "x$package_foo_enabled" = xyes; then + my_subdirs="$my_subdirs foo" +fi +AC_CONFIG_SUBDIRS([$my_subdirs]) +@end example + +@noindent +because this prevents @samp{./configure --help=recursive} from +displaying the options of the package @code{foo}. Instead, you should +write: + +@example +if test "x$package_foo_enabled" = xyes; then + AC_CONFIG_SUBDIRS([foo]) +fi +@end example + +If a given @var{dir} is not found at @command{configure} run time, a +warning is reported; if the subdirectory is optional, write: + +@example +if test -d "$srcdir/foo"; then + AC_CONFIG_SUBDIRS([foo]) +fi +@end example + +@c NB: Yes, below we mean configure.in, not configure.ac. +If a given @var{dir} contains @command{configure.gnu}, it is run instead +of @command{configure}. This is for packages that might use a +non-Autoconf script @command{Configure}, which can't be called through a +wrapper @command{configure} since it would be the same file on +case-insensitive file systems. Likewise, if a @var{dir} contains +@file{configure.in} but no @command{configure}, the Cygnus +@command{configure} script found by @code{AC_CONFIG_AUX_DIR} is used. + +The subdirectory @command{configure} scripts are given the same command +line options that were given to this @command{configure} script, with minor +changes if needed, which include: + +@itemize @minus +@item +adjusting a relative name for the cache file; + +@item +adjusting a relative name for the source directory; + +@item +propagating the current value of @code{$prefix}, including if it was +defaulted, and if the default values of the top level and of the subdirectory +@file{configure} differ. +@end itemize + +This macro also sets the output variable @code{subdirs} to the list of +directories @samp{@var{dir} @dots{}}. Make rules can use +this variable to determine which subdirectories to recurse into. + +This macro may be called multiple times. +@end defmac + +@node Default Prefix +@section Default Prefix +@cindex Install prefix +@cindex Prefix for install + +By default, @command{configure} sets the prefix for files it installs to +@file{/usr/local}. The user of @command{configure} can select a different +prefix using the @option{--prefix} and @option{--exec-prefix} options. +There are two ways to change the default: when creating +@command{configure}, and when running it. + +Some software packages might want to install in a directory other than +@file{/usr/local} by default. To accomplish that, use the +@code{AC_PREFIX_DEFAULT} macro. + +@defmac AC_PREFIX_DEFAULT (@var{prefix}) +@acindex{PREFIX_DEFAULT} +Set the default installation prefix to @var{prefix} instead of +@file{/usr/local}. +@end defmac + +It may be convenient for users to have @command{configure} guess the +installation prefix from the location of a related program that they +have already installed. If you wish to do that, you can call +@code{AC_PREFIX_PROGRAM}. + +@anchor{AC_PREFIX_PROGRAM} +@defmac AC_PREFIX_PROGRAM (@var{program}) +@acindex{PREFIX_PROGRAM} +If the user did not specify an installation prefix (using the +@option{--prefix} option), guess a value for it by looking for +@var{program} in @env{PATH}, the way the shell does. If @var{program} +is found, set the prefix to the parent of the directory containing +@var{program}, else default the prefix as described above +(@file{/usr/local} or @code{AC_PREFIX_DEFAULT}). For example, if +@var{program} is @code{gcc} and the @env{PATH} contains +@file{/usr/local/gnu/bin/gcc}, set the prefix to @file{/usr/local/gnu}. +@end defmac + + + +@c ======================================================== Existing tests + +@node Existing Tests +@chapter Existing Tests + +These macros test for particular system features that packages might +need or want to use. If you need to test for a kind of feature that +none of these macros check for, you can probably do it by calling +primitive test macros with appropriate arguments (@pxref{Writing +Tests}). + +These tests print messages telling the user which feature they're +checking for, and what they find. They cache their results for future +@command{configure} runs (@pxref{Caching Results}). + +Some of these macros set output variables. @xref{Makefile +Substitutions}, for how to get their values. The phrase ``define +@var{name}'' is used below as a shorthand to mean ``define the C +preprocessor symbol @var{name} to the value 1''. @xref{Defining +Symbols}, for how to get those symbol definitions into your program. + +@menu +* Common Behavior:: Macros' standard schemes +* Alternative Programs:: Selecting between alternative programs +* Files:: Checking for the existence of files +* Libraries:: Library archives that might be missing +* Library Functions:: C library functions that might be missing +* Header Files:: Header files that might be missing +* Declarations:: Declarations that may be missing +* Structures:: Structures or members that might be missing +* Types:: Types that might be missing +* Compilers and Preprocessors:: Checking for compiling programs +* System Services:: Operating system services +* Posix Variants:: Special kludges for specific Posix variants +* Erlang Libraries:: Checking for the existence of Erlang libraries +@end menu + +@node Common Behavior +@section Common Behavior +@cindex Common autoconf behavior + +Much effort has been expended to make Autoconf easy to learn. The most +obvious way to reach this goal is simply to enforce standard interfaces +and behaviors, avoiding exceptions as much as possible. Because of +history and inertia, unfortunately, there are still too many exceptions +in Autoconf; nevertheless, this section describes some of the common +rules. + +@menu +* Standard Symbols:: Symbols defined by the macros +* Default Includes:: Includes used by the generic macros +@end menu + +@node Standard Symbols +@subsection Standard Symbols +@cindex Standard symbols + +All the generic macros that @code{AC_DEFINE} a symbol as a result of +their test transform their @var{argument} values to a standard alphabet. +First, @var{argument} is converted to upper case and any asterisks +(@samp{*}) are each converted to @samp{P}. Any remaining characters +that are not alphanumeric are converted to underscores. + +For instance, + +@example +AC_CHECK_TYPES([struct $Expensive*]) +@end example + +@noindent +defines the symbol @samp{HAVE_STRUCT__EXPENSIVEP} if the check +succeeds. + + +@node Default Includes +@subsection Default Includes +@cindex Default includes +@cindex Includes, default + +Several tests depend upon a set of header files. Since these headers +are not universally available, tests actually have to provide a set of +protected includes, such as: + +@example +@group +#ifdef TIME_WITH_SYS_TIME +# include <sys/time.h> +# include <time.h> +#else +# ifdef HAVE_SYS_TIME_H +# include <sys/time.h> +# else +# include <time.h> +# endif +#endif +@end group +@end example + +@noindent +Unless you know exactly what you are doing, you should avoid using +unconditional includes, and check the existence of the headers you +include beforehand (@pxref{Header Files}). + +Most generic macros use the following macro to provide the default set +of includes: + +@defmac AC_INCLUDES_DEFAULT (@ovar{include-directives}) +@acindex{INCLUDES_DEFAULT} +Expand to @var{include-directives} if defined, otherwise to: + +@example +@group +#include <stdio.h> +#ifdef HAVE_SYS_TYPES_H +# include <sys/types.h> +#endif +#ifdef HAVE_SYS_STAT_H +# include <sys/stat.h> +#endif +#ifdef STDC_HEADERS +# include <stdlib.h> +# include <stddef.h> +#else +# ifdef HAVE_STDLIB_H +# include <stdlib.h> +# endif +#endif +#ifdef HAVE_STRING_H +# if !defined STDC_HEADERS && defined HAVE_MEMORY_H +# include <memory.h> +# endif +# include <string.h> +#endif +#ifdef HAVE_STRINGS_H +# include <strings.h> +#endif +#ifdef HAVE_INTTYPES_H +# include <inttypes.h> +#endif +#ifdef HAVE_STDINT_H +# include <stdint.h> +#endif +#ifdef HAVE_UNISTD_H +# include <unistd.h> +#endif +@end group +@end example + +If the default includes are used, then check for the presence of these +headers and their compatibility, i.e., you don't need to run +@code{AC_HEADER_STDC}, nor check for @file{stdlib.h} etc. + +These headers are checked for in the same order as they are included. +For instance, on some systems @file{string.h} and @file{strings.h} both +exist, but conflict. Then @code{HAVE_STRING_H} is defined, not +@code{HAVE_STRINGS_H}. +@end defmac + +@node Alternative Programs +@section Alternative Programs +@cindex Programs, checking + +These macros check for the presence or behavior of particular programs. +They are used to choose between several alternative programs and to +decide what to do once one has been chosen. If there is no macro +specifically defined to check for a program you need, and you don't need +to check for any special properties of it, then you can use one of the +general program-check macros. + +@menu +* Particular Programs:: Special handling to find certain programs +* Generic Programs:: How to find other programs +@end menu + +@node Particular Programs +@subsection Particular Program Checks + +These macros check for particular programs---whether they exist, and +in some cases whether they support certain features. + +@defmac AC_PROG_AWK +@acindex{PROG_AWK} +@ovindex AWK +@caindex prog_AWK +Check for @code{gawk}, @code{mawk}, @code{nawk}, and @code{awk}, in that +order, and set output variable @code{AWK} to the first one that is found. +It tries @code{gawk} first because that is reported to be the +best implementation. The result can be overridden by setting the +variable @code{AWK} or the cache variable @code{ac_cv_prog_AWK}. + +Using this macro is sufficient to avoid the pitfalls of traditional +@command{awk} (@pxref{awk, , Limitations of Usual Tools}). +@end defmac + +@defmac AC_PROG_GREP +@acindex{PROG_GREP} +@ovindex GREP +@caindex prog_GREP +Look for the best available @code{grep} or @code{ggrep} that accepts the +longest input lines possible, and that supports multiple @option{-e} options. +Set the output variable @code{GREP} to whatever is chosen. +@xref{grep, , Limitations of Usual Tools}, for more information about +portability problems with the @command{grep} command family. The result +can be overridden by setting the @code{GREP} variable and is cached in the +@code{ac_cv_path_GREP} variable. +@end defmac + +@defmac AC_PROG_EGREP +@acindex{PROG_EGREP} +@ovindex EGREP +@caindex prog_EGREP +Check whether @code{$GREP -E} works, or else look for the best available +@code{egrep} or @code{gegrep} that accepts the longest input lines possible. +Set the output variable @code{EGREP} to whatever is chosen. The result +can be overridden by setting the @code{EGREP} variable and is cached in the +@code{ac_cv_path_EGREP} variable. +@end defmac + +@defmac AC_PROG_FGREP +@acindex{PROG_FGREP} +@ovindex FGREP +@caindex prog_FGREP +Check whether @code{$GREP -F} works, or else look for the best available +@code{fgrep} or @code{gfgrep} that accepts the longest input lines possible. +Set the output variable @code{FGREP} to whatever is chosen. The result +can be overridden by setting the @code{FGREP} variable and is cached in the +@code{ac_cv_path_FGREP} variable. +@end defmac + +@defmac AC_PROG_INSTALL +@acindex{PROG_INSTALL} +@ovindex INSTALL +@ovindex INSTALL_PROGRAM +@ovindex INSTALL_DATA +@ovindex INSTALL_SCRIPT +@caindex path_install +Set output variable @code{INSTALL} to the name of a BSD-compatible +@command{install} program, if one is found in the current @env{PATH}. +Otherwise, set @code{INSTALL} to @samp{@var{dir}/install-sh -c}, +checking the directories specified to @code{AC_CONFIG_AUX_DIR} (or its +default directories) to determine @var{dir} (@pxref{Output}). Also set +the variables @code{INSTALL_PROGRAM} and @code{INSTALL_SCRIPT} to +@samp{$@{INSTALL@}} and @code{INSTALL_DATA} to @samp{$@{INSTALL@} -m 644}. + +@samp{@@INSTALL@@} is special, as its value may vary for different +configuration files. + +This macro screens out various instances of @command{install} known not to +work. It prefers to find a C program rather than a shell script, for +speed. Instead of @file{install-sh}, it can also use @file{install.sh}, +but that name is obsolete because some @command{make} programs have a rule +that creates @file{install} from it if there is no makefile. Further, this +macro requires @command{install} to be able to install multiple files into a +target directory in a single invocation. + +Autoconf comes with a copy of @file{install-sh} that you can use. If +you use @code{AC_PROG_INSTALL}, you must include either +@file{install-sh} or @file{install.sh} in your distribution; otherwise +@command{configure} produces an error message saying it can't find +them---even if the system you're on has a good @command{install} program. +This check is a safety measure to prevent you from accidentally leaving +that file out, which would prevent your package from installing on +systems that don't have a BSD-compatible @command{install} program. + +If you need to use your own installation program because it has features +not found in standard @command{install} programs, there is no reason to use +@code{AC_PROG_INSTALL}; just put the file name of your program into your +@file{Makefile.in} files. + +The result of the test can be overridden by setting the variable +@code{INSTALL} or the cache variable @code{ac_cv_path_install}. +@end defmac + +@defmac AC_PROG_MKDIR_P +@acindex{PROG_MKDIR_P} +@ovindex MKDIR_P +@caindex path_mkdir +Set output variable @code{MKDIR_P} to a program that ensures that for +each argument, a directory named by this argument exists, creating it +and its parent directories if needed, and without race conditions when +two instances of the program attempt to make the same directory at +nearly the same time. + +This macro uses the @samp{mkdir -p} command if possible. Otherwise, it +falls back on invoking @command{install-sh} with the @option{-d} option, +so your package should +contain @file{install-sh} as described under @code{AC_PROG_INSTALL}. +An @file{install-sh} file that predates Autoconf 2.60 or Automake 1.10 +is vulnerable to race conditions, so if you want to support parallel +installs from +different packages into the same directory you need to make sure you +have an up-to-date @file{install-sh}. In particular, be careful about +using @samp{autoreconf -if} if your Automake predates Automake 1.10. + +This macro is related to the @code{AS_MKDIR_P} macro (@pxref{Programming +in M4sh}), but it sets an output variable intended for use in other +files, whereas @code{AS_MKDIR_P} is intended for use in scripts like +@command{configure}. Also, @code{AS_MKDIR_P} does not accept options, +but @code{MKDIR_P} supports the @option{-m} option, e.g., a makefile +might invoke @code{$(MKDIR_P) -m 0 dir} to create an inaccessible +directory, and conversely a makefile should use @code{$(MKDIR_P) -- +$(FOO)} if @var{FOO} might yield a value that begins with @samp{-}. +Finally, @code{AS_MKDIR_P} does not check for race condition +vulnerability, whereas @code{AC_PROG_MKDIR_P} does. + +@samp{@@MKDIR_P@@} is special, as its value may vary for different +configuration files. + +The result of the test can be overridden by setting the variable +@code{MKDIR_P} or the cache variable @code{ac_cv_path_mkdir}. +@end defmac + +@anchor{AC_PROG_LEX} +@defmac AC_PROG_LEX +@acindex{PROG_LEX} +@ovindex LEX +@ovindex LEXLIB +@cvindex YYTEXT_POINTER +@ovindex LEX_OUTPUT_ROOT +@caindex prog_LEX +If @code{flex} is found, set output variable @code{LEX} to @samp{flex} +and @code{LEXLIB} to @option{-lfl}, if that library is in a standard +place. Otherwise set @code{LEX} to @samp{lex} and @code{LEXLIB} to +@option{-ll}, if found. If neither variant is available, set @code{LEX} +to @samp{:}; for packages that ship the generated @file{file.yy.c} +alongside the source @file{file.l}, this default allows users without a +lexer generator to still build the package even if the timestamp for +@file{file.l} is inadvertently changed. + +Define @code{YYTEXT_POINTER} if @code{yytext} defaults to @samp{char *} instead +of to @samp{char []}. Also set output variable @code{LEX_OUTPUT_ROOT} to +the base of the file name that the lexer generates; usually +@file{lex.yy}, but sometimes something else. These results vary +according to whether @code{lex} or @code{flex} is being used. + +You are encouraged to use Flex in your sources, since it is both more +pleasant to use than plain Lex and the C source it produces is portable. +In order to ensure portability, however, you must either provide a +function @code{yywrap} or, if you don't use it (e.g., your scanner has +no @samp{#include}-like feature), simply include a @samp{%noyywrap} +statement in the scanner's source. Once this done, the scanner is +portable (unless @emph{you} felt free to use nonportable constructs) and +does not depend on any library. In this case, and in this case only, it +is suggested that you use this Autoconf snippet: + +@example +AC_PROG_LEX +if test "x$LEX" != xflex; then + LEX="$SHELL $missing_dir/missing flex" + AC_SUBST([LEX_OUTPUT_ROOT], [lex.yy]) + AC_SUBST([LEXLIB], ['']) +fi +@end example + +The shell script @command{missing} can be found in the Automake +distribution. + +Remember that the user may have supplied an alternate location in +@env{LEX}, so if Flex is required, it is better to check that the user +provided something sufficient by parsing the output of @samp{$LEX +--version} than by simply relying on @code{test "x$LEX" = xflex}. + +To ensure backward compatibility, Automake's @code{AM_PROG_LEX} invokes +(indirectly) this macro twice, which causes an annoying but benign +``@code{AC_PROG_LEX} invoked multiple times'' warning. Future versions +of Automake will fix this issue; meanwhile, just ignore this message. + +As part of running the test, this macro may delete any file in the +configuration directory named @file{lex.yy.c} or @file{lexyy.c}. + +The result of this test can be influenced by setting the variable +@code{LEX} or the cache variable @code{ac_cv_prog_LEX}. +@end defmac + +@anchor{AC_PROG_LN_S} +@defmac AC_PROG_LN_S +@acindex{PROG_LN_S} +@ovindex LN_S +If @samp{ln -s} works on the current file system (the operating system +and file system support symbolic links), set the output variable +@code{LN_S} to @samp{ln -s}; otherwise, if @samp{ln} works, set +@code{LN_S} to @samp{ln}, and otherwise set it to @samp{cp -pR}. + +If you make a link in a directory other than the current directory, its +meaning depends on whether @samp{ln} or @samp{ln -s} is used. To safely +create links using @samp{$(LN_S)}, either find out which form is used +and adjust the arguments, or always invoke @code{ln} in the directory +where the link is to be created. + +In other words, it does not work to do: +@example +$(LN_S) foo /x/bar +@end example + +Instead, do: + +@example +(cd /x && $(LN_S) foo bar) +@end example +@end defmac + +@defmac AC_PROG_RANLIB +@acindex{PROG_RANLIB} +@ovindex RANLIB +@c @caindex prog_RANLIB +@c @caindex prog_ac_ct_RANLIB +Set output variable @code{RANLIB} to @samp{ranlib} if @code{ranlib} +is found, and otherwise to @samp{:} (do nothing). +@end defmac + +@defmac AC_PROG_SED +@acindex{PROG_SED} +@ovindex SED +@caindex path_SED +Set output variable @code{SED} to a Sed implementation that conforms to +Posix and does not have arbitrary length limits. Report an error if no +acceptable Sed is found. @xref{sed, , Limitations of Usual Tools}, for more +information about portability problems with Sed. + +The result of this test can be overridden by setting the @code{SED} variable +and is cached in the @code{ac_cv_path_SED} variable. +@end defmac + +@defmac AC_PROG_YACC +@acindex{PROG_YACC} +@evindex YACC +@evindex YFLAGS +@ovindex YACC +@caindex prog_YACC +If @code{bison} is found, set output variable @code{YACC} to @samp{bison +-y}. Otherwise, if @code{byacc} is found, set @code{YACC} to +@samp{byacc}. Otherwise set @code{YACC} to @samp{yacc}. +The result of this test can be influenced by setting the variable +@code{YACC} or the cache variable @code{ac_cv_prog_YACC}. +@end defmac + +@node Generic Programs +@subsection Generic Program and File Checks + +These macros are used to find programs not covered by the ``particular'' +test macros. If you need to check the behavior of a program as well as +find out whether it is present, you have to write your own test for it +(@pxref{Writing Tests}). By default, these macros use the environment +variable @env{PATH}. If you need to check for a program that might not +be in the user's @env{PATH}, you can pass a modified path to use +instead, like this: + +@example +AC_PATH_PROG([INETD], [inetd], [/usr/libexec/inetd], + [$PATH$PATH_SEPARATOR/usr/libexec$PATH_SEPARATOR]dnl +[/usr/sbin$PATH_SEPARATOR/usr/etc$PATH_SEPARATOR/etc]) +@end example + +You are strongly encouraged to declare the @var{variable} passed to +@code{AC_CHECK_PROG} etc.@: as precious. @xref{Setting Output Variables}, +@code{AC_ARG_VAR}, for more details. + +@anchor{AC_CHECK_PROG} +@defmac AC_CHECK_PROG (@var{variable}, @var{prog-to-check-for}, @ + @var{value-if-found}, @ovar{value-if-not-found}, @dvar{path, $PATH}, @ + @ovar{reject}) +@acindex{CHECK_PROG} +@caindex prog_@var{variable} +Check whether program @var{prog-to-check-for} exists in @var{path}. If +it is found, set @var{variable} to @var{value-if-found}, otherwise to +@var{value-if-not-found}, if given. Always pass over @var{reject} (an +absolute file name) even if it is the first found in the search path; in +that case, set @var{variable} using the absolute file name of the +@var{prog-to-check-for} found that is not @var{reject}. If +@var{variable} was already set, do nothing. Calls @code{AC_SUBST} for +@var{variable}. The result of this test can be overridden by setting the +@var{variable} variable or the cache variable +@code{ac_cv_prog_@var{variable}}. +@end defmac + +@anchor{AC_CHECK_PROGS} +@defmac AC_CHECK_PROGS (@var{variable}, @var{progs-to-check-for}, @ + @ovar{value-if-not-found}, @dvar{path, $PATH}) +@acindex{CHECK_PROGS} +@caindex prog_@var{variable} +Check for each program in the blank-separated list +@var{progs-to-check-for} existing in the @var{path}. If one is found, set +@var{variable} to the name of that program. Otherwise, continue +checking the next program in the list. If none of the programs in the +list are found, set @var{variable} to @var{value-if-not-found}; if +@var{value-if-not-found} is not specified, the value of @var{variable} +is not changed. Calls @code{AC_SUBST} for @var{variable}. The result of +this test can be overridden by setting the @var{variable} variable or the +cache variable @code{ac_cv_prog_@var{variable}}. +@end defmac + +@defmac AC_CHECK_TARGET_TOOL (@var{variable}, @var{prog-to-check-for}, @ + @ovar{value-if-not-found}, @dvar{path, $PATH}) +@acindex{CHECK_TARGET_TOOL} +Like @code{AC_CHECK_PROG}, but first looks for @var{prog-to-check-for} +with a prefix of the target type as determined by +@code{AC_CANONICAL_TARGET}, followed by a dash (@pxref{Canonicalizing}). +If the tool cannot be found with a prefix, and if the build and target +types are equal, then it is also searched for without a prefix. + +As noted in @ref{Specifying Target Triplets}, the +target is rarely specified, because most of the time it is the same +as the host: it is the type of system for which any compiler tool in +the package produces code. What this macro looks for is, +for example, @emph{a tool @r{(assembler, linker, etc.)}@: that the +compiler driver @r{(@command{gcc} for the GNU C Compiler)} +uses to produce objects, archives or executables}. +@end defmac + +@defmac AC_CHECK_TOOL (@var{variable}, @var{prog-to-check-for}, @ + @ovar{value-if-not-found}, @dvar{path, $PATH}) +@acindex{CHECK_TOOL} +@c @caindex prog_@var{VARIABLE} +@c @caindex prog_ac_ct_@var{VARIABLE} +Like @code{AC_CHECK_PROG}, but first looks for @var{prog-to-check-for} +with a prefix of the host type as specified by @option{--host}, followed by a +dash. For example, if the user runs +@samp{configure --build=x86_64-gnu --host=i386-gnu}, then this call: +@example +AC_CHECK_TOOL([RANLIB], [ranlib], [:]) +@end example +@noindent +sets @code{RANLIB} to @file{i386-gnu-ranlib} if that program exists in +@var{path}, or otherwise to @samp{ranlib} if that program exists in +@var{path}, or to @samp{:} if neither program exists. + +When cross-compiling, this macro will issue a warning if no program +prefixed with the host type could be found. +For more information, see @ref{Specifying Target Triplets}. +@end defmac + +@defmac AC_CHECK_TARGET_TOOLS (@var{variable}, @var{progs-to-check-for}, @ + @ovar{value-if-not-found}, @dvar{path, $PATH}) +@acindex{CHECK_TARGET_TOOLS} +Like @code{AC_CHECK_TARGET_TOOL}, each of the tools in the list +@var{progs-to-check-for} are checked with a prefix of the target type as +determined by @code{AC_CANONICAL_TARGET}, followed by a dash +(@pxref{Canonicalizing}). If none of the tools can be found with a +prefix, and if the build and target types are equal, then the first one +without a prefix is used. If a tool is found, set @var{variable} to +the name of that program. If none of the tools in the list are found, +set @var{variable} to @var{value-if-not-found}; if @var{value-if-not-found} +is not specified, the value of @var{variable} is not changed. Calls +@code{AC_SUBST} for @var{variable}. +@end defmac + +@defmac AC_CHECK_TOOLS (@var{variable}, @var{progs-to-check-for}, @ + @ovar{value-if-not-found}, @dvar{path, $PATH}) +@acindex{CHECK_TOOLS} +Like @code{AC_CHECK_TOOL}, each of the tools in the list +@var{progs-to-check-for} are checked with a prefix of the host type as +determined by @code{AC_CANONICAL_HOST}, followed by a dash +(@pxref{Canonicalizing}). If none of the tools can be found with a +prefix, then the first one without a prefix is used. If a tool is found, +set @var{variable} to the name of that program. If none of the tools in +the list are found, set @var{variable} to @var{value-if-not-found}; if +@var{value-if-not-found} is not specified, the value of @var{variable} +is not changed. Calls @code{AC_SUBST} for @var{variable}. + +When cross-compiling, this macro will issue a warning if no program +prefixed with the host type could be found. +For more information, see @ref{Specifying Target Triplets}. +@end defmac + +@anchor{AC_PATH_PROG} +@defmac AC_PATH_PROG (@var{variable}, @var{prog-to-check-for}, @ + @ovar{value-if-not-found}, @dvar{path, $PATH}) +@acindex{PATH_PROG} +@caindex path_@var{variable} +Like @code{AC_CHECK_PROG}, but set @var{variable} to the absolute +name of @var{prog-to-check-for} if found. The result of this test +can be overridden by setting the @var{variable} variable. A positive +result of this test is cached in the @code{ac_cv_path_@var{variable}} +variable. +@end defmac + +@anchor{AC_PATH_PROGS} +@defmac AC_PATH_PROGS (@var{variable}, @var{progs-to-check-for}, @ + @ovar{value-if-not-found}, @dvar{path, $PATH}) +@acindex{PATH_PROGS} +@caindex path_@var{variable} +Like @code{AC_CHECK_PROGS}, but if any of @var{progs-to-check-for} +are found, set @var{variable} to the absolute name of the program +found. The result of this test can be overridden by setting the +@var{variable} variable. A positive result of this test is cached in +the @code{ac_cv_path_@var{variable}} variable. +@end defmac + +@defmac AC_PATH_PROGS_FEATURE_CHECK (@var{variable}, @ + @var{progs-to-check-for}, @var{feature-test}, @ + @ovar{action-if-not-found}, @dvar{path, $PATH}) +@acindex{PATH_PROGS_FEATURE_CHECK} +@caindex path_@var{variable} +@vrindex ac_path_@var{variable} +@vrindex ac_path_@var{variable}_found +This macro was introduced in Autoconf 2.62. If @var{variable} is not +empty, then set the cache variable @code{ac_cv_path_@var{variable}} to +its value. Otherwise, check for each program in the blank-separated +list @var{progs-to-check-for} existing in @var{path}. For each program +found, execute @var{feature-test} with @code{ac_path_@var{variable}} +set to the absolute name of the candidate program. If no invocation of +@var{feature-test} sets the shell variable +@code{ac_cv_path_@var{variable}}, then @var{action-if-not-found} is +executed. @var{feature-test} will be run even when +@code{ac_cv_path_@var{variable}} is set, to provide the ability to +choose a better candidate found later in @var{path}; to accept the +current setting and bypass all further checks, @var{feature-test} can +execute @code{ac_path_@var{variable}_found=:}. + +Note that this macro has some subtle differences from +@code{AC_CHECK_PROGS}. It is designed to be run inside +@code{AC_CACHE_VAL}, therefore, it should have no side effects. In +particular, @var{variable} is not set to the final value of +@code{ac_cv_path_@var{variable}}, nor is @code{AC_SUBST} automatically +run. Also, on failure, any action can be performed, whereas +@code{AC_CHECK_PROGS} only performs +@code{@var{variable}=@var{value-if-not-found}}. + +Here is an example, similar to what Autoconf uses in its own configure +script. It will search for an implementation of @command{m4} that +supports the @code{indir} builtin, even if it goes by the name +@command{gm4} or is not the first implementation on @env{PATH}. + +@example +AC_CACHE_CHECK([for m4 that supports indir], [ac_cv_path_M4], + [AC_PATH_PROGS_FEATURE_CHECK([M4], [m4 gm4], + [[m4out=`echo 'changequote([,])indir([divnum])' | $ac_path_M4` + test "x$m4out" = x0 \ + && ac_cv_path_M4=$ac_path_M4 ac_path_M4_found=:]], + [AC_MSG_ERROR([could not find m4 that supports indir])])]) +AC_SUBST([M4], [$ac_cv_path_M4]) +@end example +@end defmac + +@defmac AC_PATH_TARGET_TOOL (@var{variable}, @var{prog-to-check-for}, @ + @ovar{value-if-not-found}, @dvar{path, $PATH}) +@acindex{PATH_TARGET_TOOL} +Like @code{AC_CHECK_TARGET_TOOL}, but set @var{variable} to the absolute +name of the program if it is found. +@end defmac + +@defmac AC_PATH_TOOL (@var{variable}, @var{prog-to-check-for}, @ + @ovar{value-if-not-found}, @dvar{path, $PATH}) +@acindex{PATH_TOOL} +Like @code{AC_CHECK_TOOL}, but set @var{variable} to the absolute +name of the program if it is found. + +When cross-compiling, this macro will issue a warning if no program +prefixed with the host type could be found. +For more information, see @ref{Specifying Target Triplets}. +@end defmac + + +@node Files +@section Files +@cindex File, checking + +You might also need to check for the existence of files. Before using +these macros, ask yourself whether a runtime test might not be a better +solution. Be aware that, like most Autoconf macros, they test a feature +of the host machine, and therefore, they die when cross-compiling. + +@defmac AC_CHECK_FILE (@var{file}, @ovar{action-if-found}, @ + @ovar{action-if-not-found}) +@acindex{CHECK_FILE} +@caindex file_@var{file} +Check whether file @var{file} exists on the native system. If it is +found, execute @var{action-if-found}, otherwise do +@var{action-if-not-found}, if given. The result of this test is cached +in the @code{ac_cv_file_@var{file}} variable, with characters not +suitable for a variable name mapped to underscores. +@end defmac + +@defmac AC_CHECK_FILES (@var{files}, @ovar{action-if-found}, @ + @ovar{action-if-not-found}) +@acindex{CHECK_FILES} +@caindex file_@var{file} +Executes @code{AC_CHECK_FILE} once for each file listed in @var{files}. +Additionally, defines @samp{HAVE_@var{file}} (@pxref{Standard Symbols}) +for each file found. The results of each test are cached in the +@code{ac_cv_file_@var{file}} variable, with characters not suitable for +a variable name mapped to underscores. +@end defmac + + +@node Libraries +@section Library Files +@cindex Library, checking + +The following macros check for the presence of certain C, C++, Fortran, +or Go library archive files. + +@anchor{AC_CHECK_LIB} +@defmac AC_CHECK_LIB (@var{library}, @var{function}, @ + @ovar{action-if-found}, @ovar{action-if-not-found}, @ovar{other-libraries}) +@acindex{CHECK_LIB} +@caindex lib_@var{library}_@var{function} +Test whether the library @var{library} is available by trying to link +a test program that calls function @var{function} with the library. +@var{function} should be a function provided by the library. +Use the base +name of the library; e.g., to check for @option{-lmp}, use @samp{mp} as +the @var{library} argument. + +@var{action-if-found} is a list of shell commands to run if the link +with the library succeeds; @var{action-if-not-found} is a list of shell +commands to run if the link fails. If @var{action-if-found} is not +specified, the default action prepends @option{-l@var{library}} to +@code{LIBS} and defines @samp{HAVE_LIB@var{library}} (in all +capitals). This macro is intended to support building @code{LIBS} in +a right-to-left (least-dependent to most-dependent) fashion such that +library dependencies are satisfied as a natural side effect of +consecutive tests. Linkers are sensitive to library ordering +so the order in which @code{LIBS} is generated is important to reliable +detection of libraries. + +If linking with @var{library} results in unresolved symbols that would +be resolved by linking with additional libraries, give those libraries +as the @var{other-libraries} argument, separated by spaces: +e.g., @option{-lXt -lX11}. Otherwise, this macro may fail to detect +that @var{library} is present, because linking the test program can +fail with unresolved symbols. The @var{other-libraries} argument +should be limited to cases where it is desirable to test for one library +in the presence of another that is not already in @code{LIBS}. + +@code{AC_CHECK_LIB} requires some care in usage, and should be avoided +in some common cases. Many standard functions like @code{gethostbyname} +appear in the standard C library on some hosts, and in special libraries +like @code{nsl} on other hosts. On some hosts the special libraries +contain variant implementations that you may not want to use. These +days it is normally better to use @code{AC_SEARCH_LIBS([gethostbyname], +[nsl])} instead of @code{AC_CHECK_LIB([nsl], [gethostbyname])}. + +The result of this test is cached in the +@code{ac_cv_lib_@var{library}_@var{function}} variable. +@end defmac + +@anchor{AC_SEARCH_LIBS} +@defmac AC_SEARCH_LIBS (@var{function}, @var{search-libs}, @ + @ovar{action-if-found}, @ovar{action-if-not-found}, @ovar{other-libraries}) +@acindex{SEARCH_LIBS} +@caindex search_@var{function} +Search for a library defining @var{function} if it's not already +available. This equates to calling +@samp{AC_LINK_IFELSE([AC_LANG_CALL([], [@var{function}])])} first with +no libraries, then for each library listed in @var{search-libs}. + +Prepend @option{-l@var{library}} to @code{LIBS} for the first library found +to contain @var{function}, and run @var{action-if-found}. If the +function is not found, run @var{action-if-not-found}. + +If linking with @var{library} results in unresolved symbols that would +be resolved by linking with additional libraries, give those libraries +as the @var{other-libraries} argument, separated by spaces: +e.g., @option{-lXt -lX11}. Otherwise, this macro fails to detect +that @var{function} is present, because linking the test program +always fails with unresolved symbols. + +The result of this test is cached in the +@code{ac_cv_search_@var{function}} variable as @samp{none required} if +@var{function} is already available, as @samp{no} if no library +containing @var{function} was found, otherwise as the +@option{-l@var{library}} option that needs to be prepended to @code{LIBS}. +@end defmac + + + +@node Library Functions +@section Library Functions + +The following macros check for particular C library functions. +If there is no macro specifically defined to check for a function you need, +and you don't need to check for any special properties of +it, then you can use one of the general function-check macros. + +@menu +* Function Portability:: Pitfalls with usual functions +* Particular Functions:: Special handling to find certain functions +* Generic Functions:: How to find other functions +@end menu + +@node Function Portability +@subsection Portability of C Functions +@cindex Portability of C functions +@cindex C function portability + +Most usual functions can either be missing, or be buggy, or be limited +on some architectures. This section tries to make an inventory of these +portability issues. By definition, this list always requires +additions. A much more complete list is maintained by the Gnulib +project (@pxref{Gnulib}), covering @ref{Function Substitutes, , +Current Posix Functions, gnulib, GNU gnulib}, @ref{Legacy Function +Substitutes, , Legacy Functions, gnulib, GNU gnulib}, and @ref{Glibc +Function Substitutes, , Glibc Functions, gnulib, GNU gnulib}. Please +help us keep the gnulib list as complete as possible. + +@table @asis +@item @code{exit} +@c @fuindex exit +@prindex @code{exit} +On ancient hosts, @code{exit} returned @code{int}. +This is because @code{exit} predates @code{void}, and there was a long +tradition of it returning @code{int}. + +On current hosts, the problem more likely is that @code{exit} is not +declared, due to C++ problems of some sort or another. For this reason +we suggest that test programs not invoke @code{exit}, but return from +@code{main} instead. + +@item @code{free} +@c @fuindex free +@prindex @code{free} +The C standard says a call @code{free (NULL)} does nothing, but +some old systems don't support this (e.g., NextStep). + +@item @code{isinf} +@itemx @code{isnan} +@c @fuindex isinf +@c @fuindex isnan +@prindex @code{isinf} +@prindex @code{isnan} +The C99 standard says that @code{isinf} and @code{isnan} are +macros. On some systems just macros are available +(e.g., HP-UX and Solaris 10), on +some systems both macros and functions (e.g., glibc 2.3.2), and on some +systems only functions (e.g., IRIX 6 and Solaris 9). In some cases +these functions are declared in nonstandard headers like +@code{<sunmath.h>} and defined in non-default libraries like +@option{-lm} or @option{-lsunmath}. + +The C99 @code{isinf} and @code{isnan} macros work correctly with +@code{long double} arguments, but pre-C99 systems that use functions +typically assume @code{double} arguments. On such a system, +@code{isinf} incorrectly returns true for a finite @code{long double} +argument that is outside the range of @code{double}. + +The best workaround for these issues is to use gnulib modules +@code{isinf} and @code{isnan} (@pxref{Gnulib}). But a lighter weight +solution involves code like the following. + +@smallexample +#include <math.h> + +#ifndef isnan +# define isnan(x) \ + (sizeof (x) == sizeof (long double) ? isnan_ld (x) \ + : sizeof (x) == sizeof (double) ? isnan_d (x) \ + : isnan_f (x)) +static inline int isnan_f (float x) @{ return x != x; @} +static inline int isnan_d (double x) @{ return x != x; @} +static inline int isnan_ld (long double x) @{ return x != x; @} +#endif + +#ifndef isinf +# define isinf(x) \ + (sizeof (x) == sizeof (long double) ? isinf_ld (x) \ + : sizeof (x) == sizeof (double) ? isinf_d (x) \ + : isinf_f (x)) +static inline int isinf_f (float x) +@{ return !isnan (x) && isnan (x - x); @} +static inline int isinf_d (double x) +@{ return !isnan (x) && isnan (x - x); @} +static inline int isinf_ld (long double x) +@{ return !isnan (x) && isnan (x - x); @} +#endif +@end smallexample + +Use @code{AC_C_INLINE} (@pxref{C Compiler}) so that this code works on +compilers that lack the @code{inline} keyword. Some optimizing +compilers mishandle these definitions, but systems with that bug +typically have many other floating point corner-case compliance problems +anyway, so it's probably not worth worrying about. + +@item @code{malloc} +@c @fuindex malloc +@prindex @code{malloc} +The C standard says a call @code{malloc (0)} is implementation +dependent. It can return either @code{NULL} or a new non-null pointer. +The latter is more common (e.g., the GNU C Library) but is by +no means universal. @code{AC_FUNC_MALLOC} +can be used to insist on non-@code{NULL} (@pxref{Particular Functions}). + +@item @code{putenv} +@c @fuindex putenv +@prindex @code{putenv} +Posix prefers @code{setenv} to @code{putenv}; among other things, +@code{putenv} is not required of all Posix implementations, but +@code{setenv} is. + +Posix specifies that @code{putenv} puts the given string directly in +@code{environ}, but some systems make a copy of it instead (e.g., +glibc 2.0, or BSD). And when a copy is made, @code{unsetenv} might +not free it, causing a memory leak (e.g., FreeBSD 4). + +On some systems @code{putenv ("FOO")} removes @samp{FOO} from the +environment, but this is not standard usage and it dumps core +on some systems (e.g., AIX). + +On MinGW, a call @code{putenv ("FOO=")} removes @samp{FOO} from the +environment, rather than inserting it with an empty value. + +@item @code{realloc} +@c @fuindex realloc +@prindex @code{realloc} +The C standard says a call @code{realloc (NULL, size)} is equivalent +to @code{malloc (size)}, but some old systems don't support this (e.g., +NextStep). + +@item @code{signal} handler +@c @fuindex signal +@prindex @code{signal} +@prindex @code{sigaction} +Normally @code{signal} takes a handler function with a return type of +@code{void}, but some old systems required @code{int} instead. Any +actual @code{int} value returned is not used; this is only a +difference in the function prototype demanded. + +All systems we know of in current use return @code{void}. The +@code{int} was to support K&R C, where of course @code{void} is not +available. The obsolete macro @code{AC_TYPE_SIGNAL} +(@pxref{AC_TYPE_SIGNAL}) can be used to establish the correct type in +all cases. + +In most cases, it is more robust to use @code{sigaction} when it is +available, rather than @code{signal}. + +@item @code{snprintf} +@c @fuindex snprintf +@prindex @code{snprintf} +@c @fuindex vsnprintf +@prindex @code{vsnprintf} +The C99 standard says that if the output array isn't big enough +and if no other errors occur, @code{snprintf} and @code{vsnprintf} +truncate the output and return the number of bytes that ought to have +been produced. Some older systems return the truncated length (e.g., +GNU C Library 2.0.x or IRIX 6.5), some a negative value +(e.g., earlier GNU C Library versions), and some the buffer +length without truncation (e.g., 32-bit Solaris 7). Also, some buggy +older systems ignore the length and overrun the buffer (e.g., 64-bit +Solaris 7). + +@item @code{sprintf} +@c @fuindex sprintf +@prindex @code{sprintf} +@c @fuindex vsprintf +@prindex @code{vsprintf} +The C standard says @code{sprintf} and @code{vsprintf} return the +number of bytes written. On some ancient systems (SunOS 4 for +instance) they return the buffer pointer instead, but these no +longer need to be worried about. + +@item @code{sscanf} +@c @fuindex sscanf +@prindex @code{sscanf} +On various old systems, e.g., HP-UX 9, @code{sscanf} requires +that its +input string be writable (though it doesn't actually change it). This +can be a problem when using @command{gcc} since it normally puts +constant strings in read-only memory (@pxref{Incompatibilities, +Incompatibilities of GCC, , gcc, Using and +Porting the GNU Compiler Collection}). Apparently in some cases even +having format strings read-only can be a problem. + +@item @code{strerror_r} +@c @fuindex strerror_r +@prindex @code{strerror_r} +Posix specifies that @code{strerror_r} returns an @code{int}, but many +systems (e.g., GNU C Library version 2.2.4) provide a +different version returning a @code{char *}. @code{AC_FUNC_STRERROR_R} +can detect which is in use (@pxref{Particular Functions}). + +@item @code{strnlen} +@c @fuindex strnlen +@prindex @code{strnlen} +AIX 4.3 provides a broken version which produces the +following results: + +@example +strnlen ("foobar", 0) = 0 +strnlen ("foobar", 1) = 3 +strnlen ("foobar", 2) = 2 +strnlen ("foobar", 3) = 1 +strnlen ("foobar", 4) = 0 +strnlen ("foobar", 5) = 6 +strnlen ("foobar", 6) = 6 +strnlen ("foobar", 7) = 6 +strnlen ("foobar", 8) = 6 +strnlen ("foobar", 9) = 6 +@end example + +@item @code{sysconf} +@c @fuindex sysconf +@prindex @code{sysconf} +@code{_SC_PAGESIZE} is standard, but some older systems (e.g., HP-UX +9) have @code{_SC_PAGE_SIZE} instead. This can be tested with +@code{#ifdef}. + +@item @code{unlink} +@c @fuindex unlink +@prindex @code{unlink} +The Posix spec says that @code{unlink} causes the given file to be +removed only after there are no more open file handles for it. Some +non-Posix hosts have trouble with this requirement, though, +and some DOS variants even corrupt the file system. + +@item @code{unsetenv} +@c @fuindex unsetenv +@prindex @code{unsetenv} +On MinGW, @code{unsetenv} is not available, but a variable @samp{FOO} +can be removed with a call @code{putenv ("FOO=")}, as described under +@code{putenv} above. + +@item @code{va_copy} +@c @fuindex va_copy +@prindex @code{va_copy} +The C99 standard provides @code{va_copy} for copying +@code{va_list} variables. It may be available in older environments +too, though possibly as @code{__va_copy} (e.g., @command{gcc} in strict +pre-C99 mode). These can be tested with @code{#ifdef}. A fallback to +@code{memcpy (&dst, &src, sizeof (va_list))} gives maximum +portability. + +@item @code{va_list} +@c @fuindex va_list +@prindex @code{va_list} +@code{va_list} is not necessarily just a pointer. It can be a +@code{struct} (e.g., @command{gcc} on Alpha), which means @code{NULL} is +not portable. Or it can be an array (e.g., @command{gcc} in some +PowerPC configurations), which means as a function parameter it can be +effectively call-by-reference and library routines might modify the +value back in the caller (e.g., @code{vsnprintf} in the GNU C Library +2.1). + +@item Signed @code{>>} +Normally the C @code{>>} right shift of a signed type replicates the +high bit, giving a so-called ``arithmetic'' shift. But care should be +taken since Standard C doesn't require that behavior. On those +few processors without a native arithmetic shift (for instance Cray +vector systems) zero bits may be shifted in, the same as a shift of an +unsigned type. + +@item Integer @code{/} +C divides signed integers by truncating their quotient toward zero, +yielding the same result as Fortran. However, before C99 the standard +allowed C implementations to take the floor or ceiling of the quotient +in some cases. Hardly any implementations took advantage of this +freedom, though, and it's probably not worth worrying about this issue +nowadays. +@end table + + +@node Particular Functions +@subsection Particular Function Checks +@cindex Function, checking + +These macros check for particular C functions---whether they exist, and +in some cases how they respond when given certain arguments. + +@anchor{AC_FUNC_ALLOCA} +@defmac AC_FUNC_ALLOCA +@acindex{FUNC_ALLOCA} +@cvindex C_ALLOCA +@cvindex HAVE_ALLOCA_H +@ovindex ALLOCA +@c @fuindex alloca +@prindex @code{alloca} +@hdrindex{alloca.h} +@c @caindex working_alloca_h +Check how to get @code{alloca}. Tries to get a builtin version by +checking for @file{alloca.h} or the predefined C preprocessor macros +@code{__GNUC__} and @code{_AIX}. If this macro finds @file{alloca.h}, +it defines @code{HAVE_ALLOCA_H}. + +If those attempts fail, it looks for the function in the standard C +library. If any of those methods succeed, it defines +@code{HAVE_ALLOCA}. Otherwise, it sets the output variable +@code{ALLOCA} to @samp{$@{LIBOBJDIR@}alloca.o} and defines +@code{C_ALLOCA} (so programs can periodically call @samp{alloca (0)} to +garbage collect). This variable is separate from @code{LIBOBJS} so +multiple programs can share the value of @code{ALLOCA} without needing +to create an actual library, in case only some of them use the code in +@code{LIBOBJS}. The @samp{$@{LIBOBJDIR@}} prefix serves the same +purpose as in @code{LIBOBJS} (@pxref{AC_LIBOBJ vs LIBOBJS}). + +This macro does not try to get @code{alloca} from the System V R3 +@file{libPW} or the System V R4 @file{libucb} because those libraries +contain some incompatible functions that cause trouble. Some versions +do not even contain @code{alloca} or contain a buggy version. If you +still want to use their @code{alloca}, use @code{ar} to extract +@file{alloca.o} from them instead of compiling @file{alloca.c}. + +Source files that use @code{alloca} should start with a piece of code +like the following, to declare it properly. + +@example +@group +#ifdef STDC_HEADERS +# include <stdlib.h> +# include <stddef.h> +#else +# ifdef HAVE_STDLIB_H +# include <stdlib.h> +# endif +#endif +#ifdef HAVE_ALLOCA_H +# include <alloca.h> +#elif !defined alloca +# ifdef __GNUC__ +# define alloca __builtin_alloca +# elif defined _AIX +# define alloca __alloca +# elif defined _MSC_VER +# include <malloc.h> +# define alloca _alloca +# elif !defined HAVE_ALLOCA +# ifdef __cplusplus +extern "C" +# endif +void *alloca (size_t); +# endif +#endif +@end group +@end example +@end defmac + +@defmac AC_FUNC_CHOWN +@acindex{FUNC_CHOWN} +@cvindex HAVE_CHOWN +@c @fuindex chown +@prindex @code{chown} +@caindex func_chown_works +If the @code{chown} function is available and works (in particular, it +should accept @option{-1} for @code{uid} and @code{gid}), define +@code{HAVE_CHOWN}. The result of this macro is cached in the +@code{ac_cv_func_chown_works} variable. +@end defmac + +@anchor{AC_FUNC_CLOSEDIR_VOID} +@defmac AC_FUNC_CLOSEDIR_VOID +@acindex{FUNC_CLOSEDIR_VOID} +@cvindex CLOSEDIR_VOID +@c @fuindex closedir +@prindex @code{closedir} +@caindex func_closedir_void +If the @code{closedir} function does not return a meaningful value, +define @code{CLOSEDIR_VOID}. Otherwise, callers ought to check its +return value for an error indicator. + +Currently this test is implemented by running a test program. When +cross compiling the pessimistic assumption that @code{closedir} does not +return a meaningful value is made. + +The result of this macro is cached in the @code{ac_cv_func_closedir_void} +variable. + +This macro is obsolescent, as @code{closedir} returns a meaningful value +on current systems. New programs need not use this macro. +@end defmac + +@defmac AC_FUNC_ERROR_AT_LINE +@acindex{FUNC_ERROR_AT_LINE} +@c @fuindex error_at_line +@prindex @code{error_at_line} +@caindex lib_error_at_line +If the @code{error_at_line} function is not found, require an +@code{AC_LIBOBJ} replacement of @samp{error}. + +The result of this macro is cached in the @code{ac_cv_lib_error_at_line} +variable. + +The @code{AC_FUNC_ERROR_AT_LINE} macro is obsolescent. New programs +should use Gnulib's @code{error} module. @xref{Gnulib}. +@end defmac + +@defmac AC_FUNC_FNMATCH +@acindex{FUNC_FNMATCH} +@c @fuindex fnmatch +@prindex @code{fnmatch} +@caindex func_fnmatch_works +If the @code{fnmatch} function conforms to Posix, define +@code{HAVE_FNMATCH}. Detect common implementation bugs, for example, +the bugs in Solaris 2.4. + +Unlike the other specific +@code{AC_FUNC} macros, @code{AC_FUNC_FNMATCH} does not replace a +broken/missing @code{fnmatch}. This is for historical reasons. +See @code{AC_REPLACE_FNMATCH} below. + +The result of this macro is cached in the @code{ac_cv_func_fnmatch_works} +variable. + +This macro is obsolescent. New programs should use Gnulib's +@code{fnmatch-posix} module. @xref{Gnulib}. +@end defmac + +@defmac AC_FUNC_FNMATCH_GNU +@acindex{FUNC_FNMATCH_GNU} +@c @fuindex fnmatch +@prindex @code{fnmatch} +@caindex func_fnmatch_gnu +Behave like @code{AC_REPLACE_FNMATCH} (@emph{replace}) but also test +whether @code{fnmatch} supports GNU extensions. Detect common +implementation bugs, for example, the bugs in the GNU C +Library 2.1. + +The result of this macro is cached in the @code{ac_cv_func_fnmatch_gnu} +variable. + +This macro is obsolescent. New programs should use Gnulib's +@code{fnmatch-gnu} module. @xref{Gnulib}. +@end defmac + +@anchor{AC_FUNC_FORK} +@defmac AC_FUNC_FORK +@acindex{FUNC_FORK} +@cvindex HAVE_VFORK_H +@cvindex HAVE_WORKING_FORK +@cvindex HAVE_WORKING_VFORK +@cvindex vfork +@c @fuindex fork +@prindex @code{fork} +@c @fuindex vfork +@prindex @code{vfork} +@hdrindex{vfork.h} +@c @caindex func_fork +@c @caindex func_fork_works +This macro checks for the @code{fork} and @code{vfork} functions. If a +working @code{fork} is found, define @code{HAVE_WORKING_FORK}. This macro +checks whether @code{fork} is just a stub by trying to run it. + +If @file{vfork.h} is found, define @code{HAVE_VFORK_H}. If a working +@code{vfork} is found, define @code{HAVE_WORKING_VFORK}. Otherwise, +define @code{vfork} to be @code{fork} for backward compatibility with +previous versions of @command{autoconf}. This macro checks for several known +errors in implementations of @code{vfork} and considers the system to not +have a working @code{vfork} if it detects any of them. It is not considered +to be an implementation error if a child's invocation of @code{signal} +modifies the parent's signal handler, since child processes rarely change +their signal handlers. + +Since this macro defines @code{vfork} only for backward compatibility with +previous versions of @command{autoconf} you're encouraged to define it +yourself in new code: +@example +@group +#ifndef HAVE_WORKING_VFORK +# define vfork fork +#endif +@end group +@end example + +The results of this macro are cached in the @code{ac_cv_func_fork_works} +and @code{ac_cv_func_vfork_works} variables. In order to override the +test, you also need to set the @code{ac_cv_func_fork} and +@code{ac_cv_func_vfork} variables. +@end defmac + +@defmac AC_FUNC_FSEEKO +@acindex{FUNC_FSEEKO} +@cvindex _LARGEFILE_SOURCE +@cvindex HAVE_FSEEKO +@c @fuindex fseeko +@prindex @code{fseeko} +@c @fuindex ftello +@prindex @code{ftello} +@c @caindex sys_largefile_source +If the @code{fseeko} function is available, define @code{HAVE_FSEEKO}. +Define @code{_LARGEFILE_SOURCE} if necessary to make the prototype +visible on some systems (e.g., glibc 2.2). Otherwise linkage problems +may occur when compiling with @code{AC_SYS_LARGEFILE} on +largefile-sensitive systems where @code{off_t} does not default to a +64bit entity. All systems with @code{fseeko} also supply @code{ftello}. +@end defmac + +@defmac AC_FUNC_GETGROUPS +@acindex{FUNC_GETGROUPS} +@cvindex HAVE_GETGROUPS +@ovindex GETGROUPS_LIBS +@c @fuindex getgroups +@prindex @code{getgroups} +@caindex func_getgroups_works +If the @code{getgroups} function is available and works (unlike on +Ultrix 4.3, where @samp{getgroups (0, 0)} always fails), define +@code{HAVE_GETGROUPS}. Set @code{GETGROUPS_LIBS} to any libraries +needed to get that function. This macro runs @code{AC_TYPE_GETGROUPS}. +@end defmac + +@anchor{AC_FUNC_GETLOADAVG} +@defmac AC_FUNC_GETLOADAVG +@acindex{FUNC_GETLOADAVG} +@cvindex SVR4 +@cvindex DGUX +@cvindex UMAX +@cvindex UMAX4_3 +@cvindex HAVE_NLIST_H +@cvindex NLIST_NAME_UNION +@cvindex GETLOADAVG_PRIVILEGED +@cvindex NEED_SETGID +@cvindex C_GETLOADAVG +@ovindex LIBOBJS +@ovindex NEED_SETGID +@ovindex KMEM_GROUP +@ovindex GETLOADAVG_LIBS +@c @fuindex getloadavg +@prindex @code{getloadavg} +Check how to get the system load averages. To perform its tests +properly, this macro needs the file @file{getloadavg.c}; therefore, be +sure to set the @code{AC_LIBOBJ} replacement directory properly (see +@ref{Generic Functions}, @code{AC_CONFIG_LIBOBJ_DIR}). + +If the system has the @code{getloadavg} function, define +@code{HAVE_GETLOADAVG}, and set @code{GETLOADAVG_LIBS} to any libraries +necessary to get that function. Also add @code{GETLOADAVG_LIBS} to +@code{LIBS}. Otherwise, require an @code{AC_LIBOBJ} replacement for +@samp{getloadavg} with source code in @file{@var{dir}/getloadavg.c}, and +possibly define several other C preprocessor macros and output +variables: + +@enumerate +@item +Define @code{C_GETLOADAVG}. + +@item +Define @code{SVR4}, @code{DGUX}, @code{UMAX}, or @code{UMAX4_3} if on +those systems. + +@item +@hdrindex{nlist.h} +If @file{nlist.h} is found, define @code{HAVE_NLIST_H}. + +@item +If @samp{struct nlist} has an @samp{n_un.n_name} member, define +@code{HAVE_STRUCT_NLIST_N_UN_N_NAME}. The obsolete symbol +@code{NLIST_NAME_UNION} is still defined, but do not depend upon it. + +@item +Programs may need to be installed set-group-ID (or set-user-ID) for +@code{getloadavg} to work. In this case, define +@code{GETLOADAVG_PRIVILEGED}, set the output variable @code{NEED_SETGID} +to @samp{true} (and otherwise to @samp{false}), and set +@code{KMEM_GROUP} to the name of the group that should own the installed +program. +@end enumerate + +The @code{AC_FUNC_GETLOADAVG} macro is obsolescent. New programs should +use Gnulib's @code{getloadavg} module. @xref{Gnulib}. +@end defmac + +@anchor{AC_FUNC_GETMNTENT} +@defmac AC_FUNC_GETMNTENT +@acindex{FUNC_GETMNTENT} +@cvindex HAVE_GETMNTENT +@c @fuindex getmntent +@prindex @code{getmntent} +@caindex search_getmntent +Check for @code{getmntent} in the standard C library, and then in the +@file{sun}, @file{seq}, and @file{gen} libraries, for UNICOS, +IRIX 4, PTX, and UnixWare, respectively. Then, if +@code{getmntent} is available, define @code{HAVE_GETMNTENT} and set +@code{ac_cv_func_getmntent} to @code{yes}. Otherwise set +@code{ac_cv_func_getmntent} to @code{no}. + +The result of this macro can be overridden by setting the cache variable +@code{ac_cv_search_getmntent}. +@end defmac + +@defmac AC_FUNC_GETPGRP +@acindex{FUNC_GETPGRP} +@cvindex GETPGRP_VOID +@c @fuindex getpgid +@c @fuindex getpgrp +@prindex @code{getpgid} +@prindex @code{getpgrp} +@caindex func_getpgrp_void +Define @code{GETPGRP_VOID} if it is an error to pass 0 to +@code{getpgrp}; this is the Posix behavior. On older BSD +systems, you must pass 0 to @code{getpgrp}, as it takes an argument and +behaves like Posix's @code{getpgid}. + +@example +#ifdef GETPGRP_VOID + pid = getpgrp (); +#else + pid = getpgrp (0); +#endif +@end example + +This macro does not check whether +@code{getpgrp} exists at all; if you need to work in that situation, +first call @code{AC_CHECK_FUNC} for @code{getpgrp}. + +The result of this macro is cached in the @code{ac_cv_func_getpgrp_void} +variable. + +This macro is obsolescent, as current systems have a @code{getpgrp} +whose signature conforms to Posix. New programs need not use this macro. +@end defmac + +@defmac AC_FUNC_LSTAT_FOLLOWS_SLASHED_SYMLINK +@acindex{FUNC_LSTAT_FOLLOWS_SLASHED_SYMLINK} +@cvindex LSTAT_FOLLOWS_SLASHED_SYMLINK +@c @fuindex lstat +@prindex @code{lstat} +@caindex func_lstat_dereferences_slashed_symlink +If @file{link} is a symbolic link, then @code{lstat} should treat +@file{link/} the same as @file{link/.}. However, many older +@code{lstat} implementations incorrectly ignore trailing slashes. + +It is safe to assume that if @code{lstat} incorrectly ignores +trailing slashes, then other symbolic-link-aware functions like +@code{unlink} also incorrectly ignore trailing slashes. + +If @code{lstat} behaves properly, define +@code{LSTAT_FOLLOWS_SLASHED_SYMLINK}, otherwise require an +@code{AC_LIBOBJ} replacement of @code{lstat}. + +The result of this macro is cached in the +@code{ac_cv_func_lstat_dereferences_slashed_symlink} variable. + +The @code{AC_FUNC_LSTAT_FOLLOWS_SLASHED_SYMLINK} macro is obsolescent. +New programs should use Gnulib's @code{lstat} module. @xref{Gnulib}. +@end defmac + +@defmac AC_FUNC_MALLOC +@acindex{FUNC_MALLOC} +@cvindex HAVE_MALLOC +@cvindex malloc +@c @fuindex malloc +@prindex @code{malloc} +@caindex func_malloc_0_nonnull +If the @code{malloc} function is compatible with the GNU C +library @code{malloc} (i.e., @samp{malloc (0)} returns a valid +pointer), define @code{HAVE_MALLOC} to 1. Otherwise define +@code{HAVE_MALLOC} to 0, ask for an @code{AC_LIBOBJ} replacement for +@samp{malloc}, and define @code{malloc} to @code{rpl_malloc} so that the +native @code{malloc} is not used in the main project. + +Typically, the replacement file @file{malloc.c} should look like (note +the @samp{#undef malloc}): + +@verbatim +#include <config.h> +#undef malloc + +#include <sys/types.h> + +void *malloc (); + +/* Allocate an N-byte block of memory from the heap. + If N is zero, allocate a 1-byte block. */ + +void * +rpl_malloc (size_t n) +{ + if (n == 0) + n = 1; + return malloc (n); +} +@end verbatim + +The result of this macro is cached in the +@code{ac_cv_func_malloc_0_nonnull} variable. +@end defmac + +@defmac AC_FUNC_MBRTOWC +@acindex{FUNC_MBRTOWC} +@cvindex HAVE_MBRTOWC +@c @fuindex mbrtowc +@prindex @code{mbrtowc} +@caindex func_mbrtowc +Define @code{HAVE_MBRTOWC} to 1 if the function @code{mbrtowc} and the +type @code{mbstate_t} are properly declared. + +The result of this macro is cached in the @code{ac_cv_func_mbrtowc} +variable. +@end defmac + +@defmac AC_FUNC_MEMCMP +@acindex{FUNC_MEMCMP} +@ovindex LIBOBJS +@c @fuindex memcmp +@prindex @code{memcmp} +@caindex func_memcmp_working +If the @code{memcmp} function is not available, or does not work on +8-bit data (like the one on SunOS 4.1.3), or fails when comparing 16 +bytes or more and with at least one buffer not starting on a 4-byte +boundary (such as the one on NeXT x86 OpenStep), require an +@code{AC_LIBOBJ} replacement for @samp{memcmp}. + +The result of this macro is cached in the +@code{ac_cv_func_memcmp_working} variable. + +This macro is obsolescent, as current systems have a working +@code{memcmp}. New programs need not use this macro. +@end defmac + +@defmac AC_FUNC_MKTIME +@acindex{FUNC_MKTIME} +@ovindex LIBOBJS +@c @fuindex mktime +@prindex @code{mktime} +@caindex func_working_mktime +If the @code{mktime} function is not available, or does not work +correctly, require an @code{AC_LIBOBJ} replacement for @samp{mktime}. +For the purposes of this test, @code{mktime} should conform to the +Posix standard and should be the inverse of +@code{localtime}. + +The result of this macro is cached in the +@code{ac_cv_func_working_mktime} variable. + +The @code{AC_FUNC_MKTIME} macro is obsolescent. New programs should +use Gnulib's @code{mktime} module. @xref{Gnulib}. +@end defmac + +@anchor{AC_FUNC_MMAP} +@defmac AC_FUNC_MMAP +@acindex{FUNC_MMAP} +@cvindex HAVE_MMAP +@c @fuindex mmap +@prindex @code{mmap} +@caindex func_mmap_fixed_mapped +If the @code{mmap} function exists and works correctly, define +@code{HAVE_MMAP}. This checks only private fixed mapping of already-mapped +memory. + +The result of this macro is cached in the +@code{ac_cv_func_mmap_fixed_mapped} variable. +@end defmac + +@defmac AC_FUNC_OBSTACK +@acindex{FUNC_OBSTACK} +@cvindex HAVE_OBSTACK +@cindex obstack +@caindex func_obstack +If the obstacks are found, define @code{HAVE_OBSTACK}, else require an +@code{AC_LIBOBJ} replacement for @samp{obstack}. + +The result of this macro is cached in the @code{ac_cv_func_obstack} +variable. +@end defmac + +@defmac AC_FUNC_REALLOC +@acindex{FUNC_REALLOC} +@cvindex HAVE_REALLOC +@cvindex realloc +@c @fuindex realloc +@prindex @code{realloc} +@caindex func_realloc_0_nonnull +If the @code{realloc} function is compatible with the GNU C +library @code{realloc} (i.e., @samp{realloc (NULL, 0)} returns a +valid pointer), define @code{HAVE_REALLOC} to 1. Otherwise define +@code{HAVE_REALLOC} to 0, ask for an @code{AC_LIBOBJ} replacement for +@samp{realloc}, and define @code{realloc} to @code{rpl_realloc} so that +the native @code{realloc} is not used in the main project. See +@code{AC_FUNC_MALLOC} for details. + +The result of this macro is cached in the +@code{ac_cv_func_realloc_0_nonnull} variable. +@end defmac + +@defmac AC_FUNC_SELECT_ARGTYPES +@acindex{FUNC_SELECT_ARGTYPES} +@cvindex SELECT_TYPE_ARG1 +@cvindex SELECT_TYPE_ARG234 +@cvindex SELECT_TYPE_ARG5 +@c @fuindex select +@prindex @code{select} +@c @caindex func_select_args +Determines the correct type to be passed for each of the +@code{select} function's arguments, and defines those types +in @code{SELECT_TYPE_ARG1}, @code{SELECT_TYPE_ARG234}, and +@code{SELECT_TYPE_ARG5} respectively. @code{SELECT_TYPE_ARG1} defaults +to @samp{int}, @code{SELECT_TYPE_ARG234} defaults to @samp{int *}, +and @code{SELECT_TYPE_ARG5} defaults to @samp{struct timeval *}. + +This macro is obsolescent, as current systems have a @code{select} whose +signature conforms to Posix. New programs need not use this macro. +@end defmac + +@defmac AC_FUNC_SETPGRP +@acindex{FUNC_SETPGRP} +@cvindex SETPGRP_VOID +@c @fuindex setpgrp +@prindex @code{setpgrp} +@caindex func_setpgrp_void +If @code{setpgrp} takes no argument (the Posix version), define +@code{SETPGRP_VOID}. Otherwise, it is the BSD version, which takes +two process IDs as arguments. This macro does not check whether +@code{setpgrp} exists at all; if you need to work in that situation, +first call @code{AC_CHECK_FUNC} for @code{setpgrp}. + +The result of this macro is cached in the @code{ac_cv_func_setpgrp_void} +variable. + +This macro is obsolescent, as current systems have a @code{setpgrp} +whose signature conforms to Posix. New programs need not use this macro. +@end defmac + +@defmac AC_FUNC_STAT +@defmacx AC_FUNC_LSTAT +@acindex{FUNC_STAT} +@acindex{FUNC_LSTAT} +@cvindex HAVE_STAT_EMPTY_STRING_BUG +@cvindex HAVE_LSTAT_EMPTY_STRING_BUG +@c @fuindex stat +@prindex @code{stat} +@c @fuindex lstat +@prindex @code{lstat} +@caindex func_stat_empty_string_bug +@caindex func_lstat_empty_string_bug +Determine whether @code{stat} or @code{lstat} have the bug that it +succeeds when given the zero-length file name as argument. The @code{stat} +and @code{lstat} from SunOS 4.1.4 and the Hurd (as of 1998-11-01) do +this. + +If it does, then define @code{HAVE_STAT_EMPTY_STRING_BUG} (or +@code{HAVE_LSTAT_EMPTY_STRING_BUG}) and ask for an @code{AC_LIBOBJ} +replacement of it. + +The results of these macros are cached in the +@code{ac_cv_func_stat_empty_string_bug} and the +@code{ac_cv_func_lstat_empty_string_bug} variables, respectively. + +These macros are obsolescent, as no current systems have the bug. +New programs need not use these macros. +@end defmac + +@anchor{AC_FUNC_STRCOLL} +@defmac AC_FUNC_STRCOLL +@acindex{FUNC_STRCOLL} +@cvindex HAVE_STRCOLL +@c @fuindex strcoll +@prindex @code{strcoll} +@caindex func_strcoll_works +If the @code{strcoll} function exists and works correctly, define +@code{HAVE_STRCOLL}. This does a bit more than +@samp{AC_CHECK_FUNCS(strcoll)}, because some systems have incorrect +definitions of @code{strcoll} that should not be used. + +The result of this macro is cached in the @code{ac_cv_func_strcoll_works} +variable. +@end defmac + +@defmac AC_FUNC_STRERROR_R +@acindex{FUNC_STRERROR_R} +@cvindex HAVE_STRERROR_R +@cvindex HAVE_DECL_STRERROR_R +@cvindex STRERROR_R_CHAR_P +@c @fuindex strerror_r +@caindex func_strerror_r_char_p +@prindex @code{strerror_r} +If @code{strerror_r} is available, define @code{HAVE_STRERROR_R}, and if +it is declared, define @code{HAVE_DECL_STRERROR_R}. If it returns a +@code{char *} message, define @code{STRERROR_R_CHAR_P}; otherwise it +returns an @code{int} error number. The Thread-Safe Functions option of +Posix requires @code{strerror_r} to return @code{int}, but +many systems (including, for example, version 2.2.4 of the GNU C +Library) return a @code{char *} value that is not necessarily equal to +the buffer argument. + +The result of this macro is cached in the +@code{ac_cv_func_strerror_r_char_p} variable. +@end defmac + +@anchor{AC_FUNC_STRFTIME} +@defmac AC_FUNC_STRFTIME +@acindex{FUNC_STRFTIME} +@cvindex HAVE_STRFTIME +@c @fuindex strftime +@prindex @code{strftime} +Check for @code{strftime} in the @file{intl} library, for SCO Unix. +Then, if @code{strftime} is available, define @code{HAVE_STRFTIME}. + +This macro is obsolescent, as no current systems require the @file{intl} +library for @code{strftime}. New programs need not use this macro. +@end defmac + +@defmac AC_FUNC_STRTOD +@acindex{FUNC_STRTOD} +@ovindex POW_LIB +@c @fuindex strtod +@prindex @code{strtod} +@caindex func_strtod +@caindex func_pow +If the @code{strtod} function does not exist or doesn't work correctly, +ask for an @code{AC_LIBOBJ} replacement of @samp{strtod}. In this case, +because @file{strtod.c} is likely to need @samp{pow}, set the output +variable @code{POW_LIB} to the extra library needed. + +This macro caches its result in the @code{ac_cv_func_strtod} variable +and depends upon the result in the @code{ac_cv_func_pow} variable. + +The @code{AC_FUNC_STRTOD} macro is obsolescent. New programs should +use Gnulib's @code{strtod} module. @xref{Gnulib}. +@end defmac + +@defmac AC_FUNC_STRTOLD +@acindex{FUNC_STRTOLD} +@cvindex HAVE_STRTOLD +@prindex @code{strtold} +@caindex func_strtold +If the @code{strtold} function exists and conforms to C99, define +@code{HAVE_STRTOLD}. + +This macro caches its result in the @code{ac_cv_func_strtold} variable. +@end defmac + +@defmac AC_FUNC_STRNLEN +@acindex{FUNC_STRNLEN} +@cvindex HAVE_STRNLEN +@c @fuindex strnlen +@prindex @code{strnlen} +@caindex func_strnlen_working +If the @code{strnlen} function is not available, or is buggy (like the one +from AIX 4.3), require an @code{AC_LIBOBJ} replacement for it. + +This macro caches its result in the @code{ac_cv_func_strnlen_working} +variable. +@end defmac + +@anchor{AC_FUNC_UTIME_NULL} +@defmac AC_FUNC_UTIME_NULL +@acindex{FUNC_UTIME_NULL} +@cvindex HAVE_UTIME_NULL +@c @fuindex utime +@prindex @code{utime} +@caindex func_utime_null +If @samp{utime (@var{file}, NULL)} sets @var{file}'s timestamp to +the present, define @code{HAVE_UTIME_NULL}. + +This macro caches its result in the @code{ac_cv_func_utime_null} +variable. + +This macro is obsolescent, as all current systems have a @code{utime} +that behaves this way. New programs need not use this macro. +@end defmac + +@anchor{AC_FUNC_VPRINTF} +@defmac AC_FUNC_VPRINTF +@acindex{FUNC_VPRINTF} +@cvindex HAVE_VPRINTF +@cvindex HAVE_DOPRNT +@c @fuindex vprintf +@prindex @code{vprintf} +@c @fuindex vsprintf +@prindex @code{vsprintf} +If @code{vprintf} is found, define @code{HAVE_VPRINTF}. Otherwise, if +@code{_doprnt} is found, define @code{HAVE_DOPRNT}. (If @code{vprintf} +is available, you may assume that @code{vfprintf} and @code{vsprintf} +are also available.) + +This macro is obsolescent, as all current systems have @code{vprintf}. +New programs need not use this macro. +@end defmac + +@defmac AC_REPLACE_FNMATCH +@acindex{REPLACE_FNMATCH} +@c @fuindex fnmatch +@prindex @code{fnmatch} +@hdrindex{fnmatch.h} +@caindex func_fnmatch_works +If the @code{fnmatch} function does not conform to Posix (see +@code{AC_FUNC_FNMATCH}), ask for its @code{AC_LIBOBJ} replacement. + +The files @file{fnmatch.c}, @file{fnmatch_loop.c}, and @file{fnmatch_.h} +in the @code{AC_LIBOBJ} replacement directory are assumed to contain a +copy of the source code of GNU @code{fnmatch}. If necessary, +this source code is compiled as an @code{AC_LIBOBJ} replacement, and the +@file{fnmatch_.h} file is linked to @file{fnmatch.h} so that it can be +included in place of the system @code{<fnmatch.h>}. + +This macro caches its result in the @code{ac_cv_func_fnmatch_works} +variable. + +This macro is obsolescent, as it assumes the use of particular source +files. New programs should use Gnulib's @code{fnmatch-posix} module, +which provides this macro along with the source files. @xref{Gnulib}. +@end defmac + + + +@node Generic Functions +@subsection Generic Function Checks + +These macros are used to find functions not covered by the ``particular'' +test macros. If the functions might be in libraries other than the +default C library, first call @code{AC_CHECK_LIB} for those libraries. +If you need to check the behavior of a function as well as find out +whether it is present, you have to write your own test for +it (@pxref{Writing Tests}). + +@anchor{AC_CHECK_FUNC} +@defmac AC_CHECK_FUNC (@var{function}, @ovar{action-if-found}, @ + @ovar{action-if-not-found}) +@acindex{CHECK_FUNC} +@caindex func_@var{function} +If C function @var{function} is available, run shell commands +@var{action-if-found}, otherwise @var{action-if-not-found}. If you just +want to define a symbol if the function is available, consider using +@code{AC_CHECK_FUNCS} instead. This macro checks for functions with C +linkage even when @code{AC_LANG(C++)} has been called, since C is more +standardized than C++. (@pxref{Language Choice}, for more information +about selecting the language for checks.) + +This macro caches its result in the @code{ac_cv_func_@var{function}} +variable. +@end defmac + +@anchor{AC_CHECK_FUNCS} +@defmac AC_CHECK_FUNCS (@var{function}@dots{}, @ovar{action-if-found}, @ + @ovar{action-if-not-found}) +@acindex{CHECK_FUNCS} +@cvindex HAVE_@var{function} +For each @var{function} enumerated in the blank-or-newline-separated argument +list, define @code{HAVE_@var{function}} (in all capitals) if it is available. +If @var{action-if-found} is given, it is additional shell code to +execute when one of the functions is found. You can give it a value of +@samp{break} to break out of the loop on the first match. If +@var{action-if-not-found} is given, it is executed when one of the +functions is not found. + +Results are cached for each @var{function} as in @code{AC_CHECK_FUNC}. +@end defmac + +@defmac AC_CHECK_FUNCS_ONCE (@var{function}@dots{}) +@acindex{CHECK_FUNCS_ONCE} +@cvindex HAVE_@var{function} +For each @var{function} enumerated in the blank-or-newline-separated argument +list, define @code{HAVE_@var{function}} (in all capitals) if it is available. +This is a once-only variant of @code{AC_CHECK_FUNCS}. It generates the +checking code at most once, so that @command{configure} is smaller and +faster; but the checks cannot be conditionalized and are always done once, +early during the @command{configure} run. +@end defmac + +@sp 1 + +Autoconf follows a philosophy that was formed over the years by those +who have struggled for portability: isolate the portability issues in +specific files, and then program as if you were in a Posix +environment. Some functions may be missing or unfixable, and your +package must be ready to replace them. + +Suitable replacements for many such problem functions are available from +Gnulib (@pxref{Gnulib}). + +@defmac AC_LIBOBJ (@var{function}) +@acindex{LIBOBJ} +@ovindex LIBOBJS +Specify that @samp{@var{function}.c} must be included in the executables +to replace a missing or broken implementation of @var{function}. + +@vrindex ac_objext +Technically, it adds @samp{@var{function}.$ac_objext} to the output +variable @code{LIBOBJS} if it is not already in, and calls +@code{AC_LIBSOURCE} for @samp{@var{function}.c}. You should not +directly change @code{LIBOBJS}, since this is not traceable. +@end defmac + +@defmac AC_LIBSOURCE (@var{file}) +@acindex{LIBSOURCE} +Specify that @var{file} might be needed to compile the project. If you +need to know what files might be needed by a @file{configure.ac}, you +should trace @code{AC_LIBSOURCE}. @var{file} must be a literal. + +This macro is called automatically from @code{AC_LIBOBJ}, but you must +call it explicitly if you pass a shell variable to @code{AC_LIBOBJ}. In +that case, since shell variables cannot be traced statically, you must +pass to @code{AC_LIBSOURCE} any possible files that the shell variable +might cause @code{AC_LIBOBJ} to need. For example, if you want to pass +a variable @code{$foo_or_bar} to @code{AC_LIBOBJ} that holds either +@code{"foo"} or @code{"bar"}, you should do: + +@example +AC_LIBSOURCE([foo.c]) +AC_LIBSOURCE([bar.c]) +AC_LIBOBJ([$foo_or_bar]) +@end example + +@noindent +There is usually a way to avoid this, however, and you are encouraged to +simply call @code{AC_LIBOBJ} with literal arguments. + +Note that this macro replaces the obsolete @code{AC_LIBOBJ_DECL}, with +slightly different semantics: the old macro took the function name, +e.g., @code{foo}, as its argument rather than the file name. +@end defmac + +@defmac AC_LIBSOURCES (@var{files}) +@acindex{LIBSOURCES} +Like @code{AC_LIBSOURCE}, but accepts one or more @var{files} in a +comma-separated M4 list. Thus, the above example might be rewritten: + +@example +AC_LIBSOURCES([foo.c, bar.c]) +AC_LIBOBJ([$foo_or_bar]) +@end example +@end defmac + +@defmac AC_CONFIG_LIBOBJ_DIR (@var{directory}) +@acindex{CONFIG_LIBOBJ_DIR} +Specify that @code{AC_LIBOBJ} replacement files are to be found in +@var{directory}, a name relative to the top level of the +source tree. The replacement directory defaults to @file{.}, the top +level directory, and the most typical value is @file{lib}, corresponding +to @samp{AC_CONFIG_LIBOBJ_DIR([lib])}. + +@command{configure} might need to know the replacement directory for the +following reasons: (i) some checks use the replacement files, (ii) some +macros bypass broken system headers by installing links to the +replacement headers (iii) when used in conjunction with Automake, +within each makefile, @var{directory} is used as a relative path +from @code{$(top_srcdir)} to each object named in @code{LIBOBJS} and +@code{LTLIBOBJS}, etc. +@end defmac + +@sp 1 + +It is common to merely check for the existence of a function, and ask +for its @code{AC_LIBOBJ} replacement if missing. The following macro is +a convenient shorthand. + +@defmac AC_REPLACE_FUNCS (@var{function}@dots{}) +@acindex{REPLACE_FUNCS} +@cvindex HAVE_@var{function} +@ovindex LIBOBJS +Like @code{AC_CHECK_FUNCS}, but uses @samp{AC_LIBOBJ(@var{function})} as +@var{action-if-not-found}. You can declare your replacement function by +enclosing the prototype in @samp{#ifndef HAVE_@var{function}}. If the +system has the function, it probably declares it in a header file you +should be including, so you shouldn't redeclare it lest your declaration +conflict. +@end defmac + +@node Header Files +@section Header Files +@cindex Header, checking + +The following macros check for the presence of certain C header files. +If there is no macro specifically defined to check for a header file you need, +and you don't need to check for any special properties of +it, then you can use one of the general header-file check macros. + +@menu +* Header Portability:: Collected knowledge on common headers +* Particular Headers:: Special handling to find certain headers +* Generic Headers:: How to find other headers +@end menu + +@node Header Portability +@subsection Portability of Headers +@cindex Portability of headers +@cindex Header portability + +This section documents some collected knowledge about common headers, +and the problems they cause. By definition, this list always requires +additions. A much more complete list is maintained by the Gnulib +project (@pxref{Gnulib}), covering @ref{Header File Substitutes, , +Posix Headers, gnulib, GNU gnulib} and @ref{Glibc Header File +Substitutes, , Glibc Headers, gnulib, GNU gnulib}. Please help us keep +the gnulib list as complete as possible. + +@table @asis + +@item @file{limits.h} +C99 says that @file{limits.h} defines @code{LLONG_MIN}, +@code{LLONG_MAX}, and @code{ULLONG_MAX}, but many almost-C99 +environments (e.g., default GCC 4.0.2 + glibc 2.4) do not +define them. + +@item @file{inttypes.h} vs.@: @file{stdint.h} +@hdrindex{inttypes.h} +@hdrindex{stdint.h} +The C99 standard says that @file{inttypes.h} includes +@file{stdint.h}, so there's no need to include @file{stdint.h} +separately in a standard environment. Some implementations have +@file{inttypes.h} but not @file{stdint.h} (e.g., Solaris 7), but we don't +know of any implementation that has @file{stdint.h} but not +@file{inttypes.h}. + +@item @file{linux/irda.h} +@hdrindex{linux/irda.h} +It requires @file{linux/types.h} and @file{sys/socket.h}. + +@item @file{linux/random.h} +@hdrindex{linux/random.h} +It requires @file{linux/types.h}. + +@item @file{net/if.h} +@hdrindex{net/if.h} +On Darwin, this file requires that @file{sys/socket.h} be included +beforehand. One should run: + +@example +AC_CHECK_HEADERS([sys/socket.h]) +AC_CHECK_HEADERS([net/if.h], [], [], +[#include <stdio.h> +#ifdef STDC_HEADERS +# include <stdlib.h> +# include <stddef.h> +#else +# ifdef HAVE_STDLIB_H +# include <stdlib.h> +# endif +#endif +#ifdef HAVE_SYS_SOCKET_H +# include <sys/socket.h> +#endif +]) +@end example + +@item @file{netinet/if_ether.h} +@hdrindex{netinet/if_ether.h} +On Darwin, this file requires that @file{stdio.h} and +@file{sys/socket.h} be included beforehand. One should run: + +@example +AC_CHECK_HEADERS([sys/socket.h]) +AC_CHECK_HEADERS([netinet/if_ether.h], [], [], +[#include <stdio.h> +#ifdef STDC_HEADERS +# include <stdlib.h> +# include <stddef.h> +#else +# ifdef HAVE_STDLIB_H +# include <stdlib.h> +# endif +#endif +#ifdef HAVE_SYS_SOCKET_H +# include <sys/socket.h> +#endif +]) +@end example + +@item @file{stdint.h} +See above, item @file{inttypes.h} vs.@: @file{stdint.h}. + +@item @file{stdlib.h} +@hdrindex{stdlib.h} +On many systems (e.g., Darwin), @file{stdio.h} is a prerequisite. + +@item @file{sys/mount.h} +@hdrindex{sys/mount.h} +On FreeBSD 4.8 on ia32 and using gcc version 2.95.4, +@file{sys/params.h} is a prerequisite. + +@item @file{sys/ptem.h} +@hdrindex{sys/ptem.h} +On Solaris 8, @file{sys/stream.h} is a prerequisite. + +@item @file{sys/socket.h} +@hdrindex{sys/socket.h} +On Darwin, @file{stdlib.h} is a prerequisite. + +@item @file{sys/ucred.h} +@hdrindex{sys/ucred.h} +On Tru64 5.1, @file{sys/types.h} is a prerequisite. + +@item @file{X11/extensions/scrnsaver.h} +@hdrindex{X11/extensions/scrnsaver.h} +Using XFree86, this header requires @file{X11/Xlib.h}, which is probably +so required that you might not even consider looking for it. + +@example +AC_CHECK_HEADERS([X11/extensions/scrnsaver.h], [], [], +[[#include <X11/Xlib.h> +]]) +@end example +@end table + + +@node Particular Headers +@subsection Particular Header Checks + +These macros check for particular system header files---whether they +exist, and in some cases whether they declare certain symbols. + +@defmac AC_CHECK_HEADER_STDBOOL +@acindex{CHECK_HEADER_STDBOOL} +@cvindex HAVE__BOOL +@hdrindex{stdbool.h} +@caindex header_stdbool_h +Check whether @file{stdbool.h} exists and conforms to C99, and cache the +result in the @code{ac_cv_header_stdbool_h} variable. If the type +@code{_Bool} is defined, define @code{HAVE__BOOL} to 1. + +This macro is intended for use by Gnulib (@pxref{Gnulib}) and other +packages that supply a substitute @file{stdbool.h} on platforms lacking +a conforming one. The @code{AC_HEADER_STDBOOL} macro is better for code +that explicitly checks for @file{stdbool.h}. +@end defmac + +@defmac AC_HEADER_ASSERT +@acindex{HEADER_ASSERT} +@cvindex NDEBUG +@hdrindex{assert.h} +Check whether to enable assertions in the style of @file{assert.h}. +Assertions are enabled by default, but the user can override this by +invoking @command{configure} with the @option{--disable-assert} option. +@end defmac + +@anchor{AC_HEADER_DIRENT} +@defmac AC_HEADER_DIRENT +@acindex{HEADER_DIRENT} +@cvindex HAVE_DIRENT_H +@cvindex HAVE_NDIR_H +@cvindex HAVE_SYS_DIR_H +@cvindex HAVE_SYS_NDIR_H +@hdrindex{dirent.h} +@hdrindex{sys/ndir.h} +@hdrindex{sys/dir.h} +@hdrindex{ndir.h} +Check for the following header files. For the first one that is +found and defines @samp{DIR}, define the listed C preprocessor macro: + +@multitable {@file{sys/ndir.h}} {@code{HAVE_SYS_NDIR_H}} +@item @file{dirent.h} @tab @code{HAVE_DIRENT_H} +@item @file{sys/ndir.h} @tab @code{HAVE_SYS_NDIR_H} +@item @file{sys/dir.h} @tab @code{HAVE_SYS_DIR_H} +@item @file{ndir.h} @tab @code{HAVE_NDIR_H} +@end multitable + +The directory-library declarations in your source code should look +something like the following: + +@example +@group +#include <sys/types.h> +#ifdef HAVE_DIRENT_H +# include <dirent.h> +# define NAMLEN(dirent) strlen ((dirent)->d_name) +#else +# define dirent direct +# define NAMLEN(dirent) ((dirent)->d_namlen) +# ifdef HAVE_SYS_NDIR_H +# include <sys/ndir.h> +# endif +# ifdef HAVE_SYS_DIR_H +# include <sys/dir.h> +# endif +# ifdef HAVE_NDIR_H +# include <ndir.h> +# endif +#endif +@end group +@end example + +Using the above declarations, the program would declare variables to be +of type @code{struct dirent}, not @code{struct direct}, and would access +the length of a directory entry name by passing a pointer to a +@code{struct dirent} to the @code{NAMLEN} macro. + +This macro also checks for the SCO Xenix @file{dir} and @file{x} libraries. + +This macro is obsolescent, as all current systems with directory +libraries have @code{<dirent.h>}. New programs need not use this macro. + +Also see @code{AC_STRUCT_DIRENT_D_INO} and +@code{AC_STRUCT_DIRENT_D_TYPE} (@pxref{Particular Structures}). +@end defmac + +@anchor{AC_HEADER_MAJOR} +@defmac AC_HEADER_MAJOR +@acindex{HEADER_MAJOR} +@cvindex MAJOR_IN_MKDEV +@cvindex MAJOR_IN_SYSMACROS +@hdrindex{sys/mkdev.h} +@hdrindex{sys/sysmacros.h} +If @file{sys/types.h} does not define @code{major}, @code{minor}, and +@code{makedev}, but @file{sys/mkdev.h} does, define +@code{MAJOR_IN_MKDEV}; otherwise, if @file{sys/sysmacros.h} does, define +@code{MAJOR_IN_SYSMACROS}. +@end defmac + +@defmac AC_HEADER_RESOLV +@acindex{HEADER_RESOLV} +@cvindex HAVE_RESOLV_H +@hdrindex{resolv.h} +Checks for header @file{resolv.h}, checking for prerequisites first. +To properly use @file{resolv.h}, your code should contain something like +the following: + +@verbatim +#ifdef HAVE_SYS_TYPES_H +# include <sys/types.h> +#endif +#ifdef HAVE_NETINET_IN_H +# include <netinet/in.h> /* inet_ functions / structs */ +#endif +#ifdef HAVE_ARPA_NAMESER_H +# include <arpa/nameser.h> /* DNS HEADER struct */ +#endif +#ifdef HAVE_NETDB_H +# include <netdb.h> +#endif +#include <resolv.h> +@end verbatim +@end defmac + +@anchor{AC_HEADER_STAT} +@defmac AC_HEADER_STAT +@acindex{HEADER_STAT} +@cvindex STAT_MACROS_BROKEN +@hdrindex{sys/stat.h} +If the macros @code{S_ISDIR}, @code{S_ISREG}, etc.@: defined in +@file{sys/stat.h} do not work properly (returning false positives), +define @code{STAT_MACROS_BROKEN}. This is the case on Tektronix UTekV, +Amdahl UTS and Motorola System V/88. + +This macro is obsolescent, as no current systems have the bug. +New programs need not use this macro. +@end defmac + +@defmac AC_HEADER_STDBOOL +@acindex{HEADER_STDBOOL} +@cvindex HAVE_STDBOOL_H +@cvindex HAVE__BOOL +@hdrindex{stdbool.h} +@caindex header_stdbool_h +If @file{stdbool.h} exists and conforms to C99, define +@code{HAVE_STDBOOL_H} to 1; if the type @code{_Bool} is defined, define +@code{HAVE__BOOL} to 1. To fulfill the C99 requirements, your +program could contain the following code: + +@example +@group +#ifdef HAVE_STDBOOL_H +# include <stdbool.h> +#else +# ifndef HAVE__BOOL +# ifdef __cplusplus +typedef bool _Bool; +# else +# define _Bool signed char +# endif +# endif +# define bool _Bool +# define false 0 +# define true 1 +# define __bool_true_false_are_defined 1 +#endif +@end group +@end example + +Alternatively you can use the @samp{stdbool} package of Gnulib +(@pxref{Gnulib}). It simplifies your code so that it can say just +@code{#include <stdbool.h>}, and it adds support for less-common +platforms. + +This macro caches its result in the @code{ac_cv_header_stdbool_h} +variable. + +This macro differs from @code{AC_CHECK_HEADER_STDBOOL} only in that it +defines @code{HAVE_STDBOOL_H} whereas @code{AC_CHECK_HEADER_STDBOOL} +does not. +@end defmac + +@anchor{AC_HEADER_STDC} +@defmac AC_HEADER_STDC +@acindex{HEADER_STDC} +@cvindex STDC_HEADERS +@hdrindex{stdlib.h} +@hdrindex{stdarg.h} +@hdrindex{string.h} +@hdrindex{float.h} +@hdrindex{ctype.h} +@caindex header_stdc +Define @code{STDC_HEADERS} if the system has C header files +conforming to ANSI C89 (ISO C90). +Specifically, this macro checks for @file{stdlib.h}, @file{stdarg.h}, +@file{string.h}, and @file{float.h}; if the system has those, it +probably has the rest of the C89 header files. This macro also +checks whether @file{string.h} declares @code{memchr} (and thus +presumably the other @code{mem} functions), whether @file{stdlib.h} +declare @code{free} (and thus presumably @code{malloc} and other related +functions), and whether the @file{ctype.h} macros work on characters +with the high bit set, as the C standard requires. + +If you use this macro, your code can refer to @code{STDC_HEADERS} to +determine whether the system has conforming header files (and probably C +library functions). + +This macro caches its result in the @code{ac_cv_header_stdc} variable. + +This macro is obsolescent, as current systems have conforming header +files. New programs need not use this macro. + +@hdrindex{string.h} +@hdrindex{strings.h} +Nowadays @file{string.h} is part of the C standard and declares functions like +@code{strcpy}, and @file{strings.h} is standardized by Posix and declares +BSD functions like @code{bcopy}; but +historically, string functions were a major sticking point in this area. +If you still want to worry about portability to ancient systems without +standard headers, there is so much variation +that it is probably easier to declare the functions you use than to +figure out exactly what the system header files declare. Some ancient systems +contained a mix of functions from the C standard and from BSD; +some were mostly standard but lacked @samp{memmove}; some defined the +BSD functions as macros in @file{string.h} or +@file{strings.h}; some had only the BSD functions but +@file{string.h}; some declared the memory functions in @file{memory.h}, +some in @file{string.h}; etc. It is probably sufficient to check for +one string function and one memory function; if the library had the +standard versions of those then it probably had most of the others. +If you put the following in @file{configure.ac}: + +@example +# This example is obsolescent. +# Nowadays you can omit these macro calls. +AC_HEADER_STDC +AC_CHECK_FUNCS([strchr memcpy]) +@end example + +@noindent +then, in your code, you can use declarations like this: + +@example +@group +/* This example is obsolescent. + Nowadays you can just #include <string.h>. */ +#ifdef STDC_HEADERS +# include <string.h> +#else +# ifndef HAVE_STRCHR +# define strchr index +# define strrchr rindex +# endif +char *strchr (), *strrchr (); +# ifndef HAVE_MEMCPY +# define memcpy(d, s, n) bcopy ((s), (d), (n)) +# define memmove(d, s, n) bcopy ((s), (d), (n)) +# endif +#endif +@end group +@end example + +@noindent +If you use a function like @code{memchr}, @code{memset}, @code{strtok}, +or @code{strspn}, which have no BSD equivalent, then macros don't +suffice to port to ancient hosts; you must provide an implementation of +each function. An easy +way to incorporate your implementations only when needed (since the ones +in system C libraries may be hand optimized) is to, taking @code{memchr} +for example, put it in @file{memchr.c} and use +@samp{AC_REPLACE_FUNCS([memchr])}. +@end defmac + +@defmac AC_HEADER_SYS_WAIT +@acindex{HEADER_SYS_WAIT} +@cvindex HAVE_SYS_WAIT_H +@hdrindex{sys/wait.h} +@caindex header_sys_wait_h +If @file{sys/wait.h} exists and is compatible with Posix, define +@code{HAVE_SYS_WAIT_H}. Incompatibility can occur if @file{sys/wait.h} +does not exist, or if it uses the old BSD @code{union wait} instead +of @code{int} to store a status value. If @file{sys/wait.h} is not +Posix compatible, then instead of including it, define the +Posix macros with their usual interpretations. Here is an +example: + +@example +@group +#include <sys/types.h> +#ifdef HAVE_SYS_WAIT_H +# include <sys/wait.h> +#endif +#ifndef WEXITSTATUS +# define WEXITSTATUS(stat_val) ((unsigned int) (stat_val) >> 8) +#endif +#ifndef WIFEXITED +# define WIFEXITED(stat_val) (((stat_val) & 255) == 0) +#endif +@end group +@end example + +@noindent +This macro caches its result in the @code{ac_cv_header_sys_wait_h} +variable. + +This macro is obsolescent, as current systems are compatible with Posix. +New programs need not use this macro. +@end defmac + +@cvindex _POSIX_VERSION +@hdrindex{unistd.h} +@code{_POSIX_VERSION} is defined when @file{unistd.h} is included on +Posix systems. If there is no @file{unistd.h}, it is definitely +not a Posix system. However, some non-Posix systems do +have @file{unistd.h}. + +The way to check whether the system supports Posix is: + +@example +@group +#ifdef HAVE_UNISTD_H +# include <sys/types.h> +# include <unistd.h> +#endif + +#ifdef _POSIX_VERSION +/* Code for Posix systems. */ +#endif +@end group +@end example + +@anchor{AC_HEADER_TIME} +@defmac AC_HEADER_TIME +@acindex{HEADER_TIME} +@cvindex TIME_WITH_SYS_TIME +@hdrindex{time.h} +@hdrindex{sys/time.h} +@caindex header_time +If a program may include both @file{time.h} and @file{sys/time.h}, +define @code{TIME_WITH_SYS_TIME}. On some ancient systems, +@file{sys/time.h} included @file{time.h}, but @file{time.h} was not +protected against multiple inclusion, so programs could not explicitly +include both files. This macro is useful in programs that use, for +example, @code{struct timeval} as well as +@code{struct tm}. It is best used in conjunction with +@code{HAVE_SYS_TIME_H}, which can be checked for using +@code{AC_CHECK_HEADERS([sys/time.h])}. + +@example +@group +#ifdef TIME_WITH_SYS_TIME +# include <sys/time.h> +# include <time.h> +#else +# ifdef HAVE_SYS_TIME_H +# include <sys/time.h> +# else +# include <time.h> +# endif +#endif +@end group +@end example + +@noindent +This macro caches its result in the @code{ac_cv_header_time} variable. + +This macro is obsolescent, as current systems can include both files +when they exist. New programs need not use this macro. +@end defmac + + +@defmac AC_HEADER_TIOCGWINSZ +@acindex{HEADER_TIOCGWINSZ} +@cvindex GWINSZ_IN_SYS_IOCTL +@hdrindex{sys/ioctl.h} +@hdrindex{termios.h} +@c FIXME: I need clarifications from Jim. +If the use of @code{TIOCGWINSZ} requires @file{<sys/ioctl.h>}, then +define @code{GWINSZ_IN_SYS_IOCTL}. Otherwise @code{TIOCGWINSZ} can be +found in @file{<termios.h>}. + +Use: + +@example +@group +#ifdef HAVE_TERMIOS_H +# include <termios.h> +#endif + +#ifdef GWINSZ_IN_SYS_IOCTL +# include <sys/ioctl.h> +#endif +@end group +@end example +@end defmac + +@node Generic Headers +@subsection Generic Header Checks + +These macros are used to find system header files not covered by the +``particular'' test macros. If you need to check the contents of a header +as well as find out whether it is present, you have to write your own +test for it (@pxref{Writing Tests}). + +@anchor{AC_CHECK_HEADER} +@defmac AC_CHECK_HEADER (@var{header-file}, @ovar{action-if-found}, @ + @ovar{action-if-not-found}, @ovar{includes}) +@acindex{CHECK_HEADER} +@caindex header_@var{header-file} +If the system header file @var{header-file} is compilable, execute shell +commands @var{action-if-found}, otherwise execute +@var{action-if-not-found}. If you just want to define a symbol if the +header file is available, consider using @code{AC_CHECK_HEADERS} +instead. + +@var{includes} is decoded to determine the appropriate include +directives. If omitted or empty, @file{configure} will check for both header +existence (with the preprocessor) and usability (with the compiler), +using @code{AC_INCLUDES_DEFAULT} for the compile test. If +there is a discrepancy between the results, a warning is issued to the +user, and the compiler results are favored (@pxref{Present But +Cannot Be Compiled}). In general, favoring the compiler results means +that a header will be treated as not found even though the file exists, +because you did not provide enough prerequisites. + +Providing a non-empty @var{includes} argument allows the code to provide +any prerequisites prior to including the header under test; it is common +to use the argument @code{AC_INCLUDES_DEFAULT} (@pxref{Default +Includes}). With an explicit fourth argument, no preprocessor test is +needed. As a special case, an @var{includes} of exactly @samp{-} +triggers the older preprocessor check, which merely determines existence +of the file in the preprocessor search path; this should only be used as +a last resort (it is safer to determine the actual prerequisites and +perform a compiler check, or else use @code{AC_PREPROC_IFELSE} to make +it obvious that only a preprocessor check is desired). + +This macro caches its result in the @code{ac_cv_header_@var{header-file}} +variable, with characters not suitable for a variable name mapped to +underscores. +@end defmac + +@anchor{AC_CHECK_HEADERS} +@defmac AC_CHECK_HEADERS (@var{header-file}@dots{}, @ + @ovar{action-if-found}, @ovar{action-if-not-found}, @ + @ovar{includes}) +@acindex{CHECK_HEADERS} +@cvindex HAVE_@var{header} +@caindex header_@var{header-file} +For each given system header file @var{header-file} in the +blank-separated argument list that exists, define +@code{HAVE_@var{header-file}} (in all capitals). If @var{action-if-found} +is given, it is additional shell code to execute when one of the header +files is found. You can give it a value of @samp{break} to break out of +the loop on the first match. If @var{action-if-not-found} is given, it +is executed when one of the header files is not found. + +@var{includes} is interpreted as in @code{AC_CHECK_HEADER}, in order to +choose the set of preprocessor directives supplied before the header +under test. + +This macro caches its result in the @code{ac_cv_header_@var{header-file}} +variable, with characters not suitable for a variable name mapped to +underscores. +@end defmac + +Previous versions of Autoconf merely checked whether the header was +accepted by the preprocessor. This was changed because the old test was +inappropriate for typical uses. Headers are typically used to compile, +not merely to preprocess, and the old behavior sometimes accepted +headers that clashed at compile-time (@pxref{Present But Cannot Be +Compiled}). If you need to check whether a header is preprocessable, +you can use @code{AC_PREPROC_IFELSE} (@pxref{Running the Preprocessor}). + +Actually requiring a header to compile improves the robustness of the +test, but it also requires +that you make sure that headers that must be included before the +@var{header-file} be part of the @var{includes}, (@pxref{Default +Includes}). If looking for @file{bar.h}, which requires that +@file{foo.h} be included before if it exists, we suggest the following +scheme: + +@verbatim +AC_CHECK_HEADERS([foo.h]) +AC_CHECK_HEADERS([bar.h], [], [], +[#ifdef HAVE_FOO_H +# include <foo.h> +#endif +]) +@end verbatim + +The following variant generates smaller, faster @command{configure} +files if you do not need the full power of @code{AC_CHECK_HEADERS}. + +@defmac AC_CHECK_HEADERS_ONCE (@var{header-file}@dots{}) +@acindex{CHECK_HEADERS_ONCE} +@cvindex HAVE_@var{header} +For each given system header file @var{header-file} in the +blank-separated argument list that exists, define +@code{HAVE_@var{header-file}} (in all capitals). +This is a once-only variant of @code{AC_CHECK_HEADERS}. It generates the +checking code at most once, so that @command{configure} is smaller and +faster; but the checks cannot be conditionalized and are always done once, +early during the @command{configure} run. Thus, this macro is only safe +for checking headers that do not have prerequisites beyond what +@code{AC_INCLUDES_DEFAULT} provides. +@end defmac + +@node Declarations +@section Declarations +@cindex Declaration, checking + +The following macros check for the declaration of variables and +functions. If there is no macro specifically defined to check for a +symbol you need, then you can use the general macros (@pxref{Generic +Declarations}) or, for more complex tests, you may use +@code{AC_COMPILE_IFELSE} (@pxref{Running the Compiler}). + +@menu +* Particular Declarations:: Macros to check for certain declarations +* Generic Declarations:: How to find other declarations +@end menu + +@node Particular Declarations +@subsection Particular Declaration Checks + +There are no specific macros for declarations. + +@node Generic Declarations +@subsection Generic Declaration Checks + +These macros are used to find declarations not covered by the ``particular'' +test macros. + +@defmac AC_CHECK_DECL (@var{symbol}, @ovar{action-if-found}, @ + @ovar{action-if-not-found}, @dvar{includes, AC_INCLUDES_DEFAULT}) +@acindex{CHECK_DECL} +@caindex have_decl_@var{symbol} +If @var{symbol} (a function, variable, or constant) is not declared in +@var{includes} and a declaration is needed, run the shell commands +@var{action-if-not-found}, otherwise @var{action-if-found}. +@var{includes} is a series of include directives, defaulting to +@code{AC_INCLUDES_DEFAULT} (@pxref{Default Includes}), which are used +prior to the declaration under test. + +This macro actually tests whether @var{symbol} is defined as a macro or +can be used as an r-value, not whether it is really declared, because it +is much safer to avoid introducing extra declarations when they are not +needed. In order to facilitate use of C++ and overloaded function +declarations, it is possible to specify function argument types in +parentheses for types which can be zero-initialized: + +@example +AC_CHECK_DECL([basename(char *)]) +@end example + +This macro caches its result in the @code{ac_cv_have_decl_@var{symbol}} +variable, with characters not suitable for a variable name mapped to +underscores. +@end defmac + +@anchor{AC_CHECK_DECLS} +@defmac AC_CHECK_DECLS (@var{symbols}, @ovar{action-if-found}, @ + @ovar{action-if-not-found}, @dvar{includes, AC_INCLUDES_DEFAULT}) +@acindex{CHECK_DECLS} +@cvindex HAVE_DECL_@var{symbol} +@caindex have_decl_@var{symbol} +For each of the @var{symbols} (@emph{comma}-separated list with optional +function argument types for C++ overloads), define +@code{HAVE_DECL_@var{symbol}} (in all capitals) to @samp{1} if +@var{symbol} is declared, otherwise to @samp{0}. If +@var{action-if-not-found} is given, it is additional shell code to +execute when one of the function declarations is needed, otherwise +@var{action-if-found} is executed. + +@var{includes} is a series of include directives, defaulting to +@code{AC_INCLUDES_DEFAULT} (@pxref{Default Includes}), which are used +prior to the declarations under test. + +This macro uses an M4 list as first argument: +@example +AC_CHECK_DECLS([strdup]) +AC_CHECK_DECLS([strlen]) +AC_CHECK_DECLS([malloc, realloc, calloc, free]) +AC_CHECK_DECLS([j0], [], [], [[#include <math.h>]]) +AC_CHECK_DECLS([[basename(char *)], [dirname(char *)]]) +@end example + +Unlike the other @samp{AC_CHECK_*S} macros, when a @var{symbol} is not +declared, @code{HAVE_DECL_@var{symbol}} is defined to @samp{0} instead +of leaving @code{HAVE_DECL_@var{symbol}} undeclared. When you are +@emph{sure} that the check was performed, use +@code{HAVE_DECL_@var{symbol}} in @code{#if}: + +@example +#if !HAVE_DECL_SYMBOL +extern char *symbol; +#endif +@end example + +@noindent +If the test may have not been performed, however, because it is safer +@emph{not} to declare a symbol than to use a declaration that conflicts +with the system's one, you should use: + +@example +#if defined HAVE_DECL_MALLOC && !HAVE_DECL_MALLOC +void *malloc (size_t *s); +#endif +@end example + +@noindent +You fall into the second category only in extreme situations: either +your files may be used without being configured, or they are used during +the configuration. In most cases the traditional approach is enough. + +This macro caches its results in @code{ac_cv_have_decl_@var{symbol}} +variables, with characters not suitable for a variable name mapped to +underscores. +@end defmac + +@defmac AC_CHECK_DECLS_ONCE (@var{symbols}) +@acindex{CHECK_DECLS_ONCE} +@cvindex HAVE_DECL_@var{symbol} +For each of the @var{symbols} (@emph{comma}-separated list), define +@code{HAVE_DECL_@var{symbol}} (in all capitals) to @samp{1} if +@var{symbol} is declared in the default include files, otherwise to +@samp{0}. This is a once-only variant of @code{AC_CHECK_DECLS}. It +generates the checking code at most once, so that @command{configure} is +smaller and faster; but the checks cannot be conditionalized and are +always done once, early during the @command{configure} run. +@end defmac + + +@node Structures +@section Structures +@cindex Structure, checking + +The following macros check for the presence of certain members in C +structures. If there is no macro specifically defined to check for a +member you need, then you can use the general structure-member macros +(@pxref{Generic Structures}) or, for more complex tests, you may use +@code{AC_COMPILE_IFELSE} (@pxref{Running the Compiler}). + +@menu +* Particular Structures:: Macros to check for certain structure members +* Generic Structures:: How to find other structure members +@end menu + +@node Particular Structures +@subsection Particular Structure Checks + +The following macros check for certain structures or structure members. + +@defmac AC_STRUCT_DIRENT_D_INO +@acindex{STRUCT_DIRENT_D_INO} +@cvindex HAVE_STRUCT_DIRENT_D_INO +@c @caindex header_dirent_dirent_h +@c @caindex member_struct_dirent_d_ino +Perform all the actions of @code{AC_HEADER_DIRENT} (@pxref{Particular +Headers}). Then, if @code{struct dirent} contains a @code{d_ino} +member, define @code{HAVE_STRUCT_DIRENT_D_INO}. + +@code{HAVE_STRUCT_DIRENT_D_INO} indicates only the presence of +@code{d_ino}, not whether its contents are always reliable. +Traditionally, a zero @code{d_ino} indicated a deleted directory entry, +though current systems hide this detail from the user and never return +zero @code{d_ino} values. +Many current systems report an incorrect @code{d_ino} for a directory +entry that is a mount point. +@end defmac + +@defmac AC_STRUCT_DIRENT_D_TYPE +@acindex{STRUCT_DIRENT_D_TYPE} +@cvindex HAVE_STRUCT_DIRENT_D_TYPE +@c @caindex header_dirent_dirent_h +@c @caindex member_struct_dirent_d_type +Perform all the actions of @code{AC_HEADER_DIRENT} (@pxref{Particular +Headers}). Then, if @code{struct dirent} contains a @code{d_type} +member, define @code{HAVE_STRUCT_DIRENT_D_TYPE}. +@end defmac + +@anchor{AC_STRUCT_ST_BLOCKS} +@defmac AC_STRUCT_ST_BLOCKS +@acindex{STRUCT_ST_BLOCKS} +@cvindex HAVE_STRUCT_STAT_ST_BLOCKS +@cvindex HAVE_ST_BLOCKS +@ovindex LIBOBJS +@caindex member_struct_stat_st_blocks +If @code{struct stat} contains an @code{st_blocks} member, define +@code{HAVE_STRUCT_STAT_ST_BLOCKS}. Otherwise, require an +@code{AC_LIBOBJ} replacement of @samp{fileblocks}. The former name, +@code{HAVE_ST_BLOCKS} is to be avoided, as its support will cease in the +future. + +This macro caches its result in the @code{ac_cv_member_struct_stat_st_blocks} +variable. +@end defmac + +@defmac AC_STRUCT_TM +@acindex{STRUCT_TM} +@cvindex TM_IN_SYS_TIME +@hdrindex{time.h} +@hdrindex{sys/time.h} +If @file{time.h} does not define @code{struct tm}, define +@code{TM_IN_SYS_TIME}, which means that including @file{sys/time.h} +had better define @code{struct tm}. + +This macro is obsolescent, as @file{time.h} defines @code{struct tm} in +current systems. New programs need not use this macro. +@end defmac + +@anchor{AC_STRUCT_TIMEZONE} +@defmac AC_STRUCT_TIMEZONE +@acindex{STRUCT_TIMEZONE} +@cvindex HAVE_DECL_TZNAME +@cvindex HAVE_STRUCT_TM_TM_ZONE +@cvindex HAVE_TM_ZONE +@cvindex HAVE_TZNAME +@c @caindex member_struct_tm_tm_zone +@c @caindex struct_tm +Figure out how to get the current timezone. If @code{struct tm} has a +@code{tm_zone} member, define @code{HAVE_STRUCT_TM_TM_ZONE} (and the +obsoleted @code{HAVE_TM_ZONE}). Otherwise, if the external array +@code{tzname} is found, define @code{HAVE_TZNAME}; if it is declared, +define @code{HAVE_DECL_TZNAME}. +@end defmac + +@node Generic Structures +@subsection Generic Structure Checks + +These macros are used to find structure members not covered by the +``particular'' test macros. + +@defmac AC_CHECK_MEMBER (@var{aggregate}.@var{member}, @ + @ovar{action-if-found}, @ovar{action-if-not-found}, @ + @dvar{includes, AC_INCLUDES_DEFAULT}) +@acindex{CHECK_MEMBER} +@caindex member_@var{aggregate}_@var{member} +Check whether @var{member} is a member of the aggregate @var{aggregate}. +If no @var{includes} are specified, the default includes are used +(@pxref{Default Includes}). + +@example +AC_CHECK_MEMBER([struct passwd.pw_gecos], [], + [AC_MSG_ERROR([we need `passwd.pw_gecos'])], + [[#include <pwd.h>]]) +@end example + +You can use this macro for submembers: + +@example +AC_CHECK_MEMBER(struct top.middle.bot) +@end example + +This macro caches its result in the +@code{ac_cv_member_@var{aggregate}_@var{member}} variable, with +characters not suitable for a variable name mapped to underscores. +@end defmac + +@anchor{AC_CHECK_MEMBERS} +@defmac AC_CHECK_MEMBERS (@var{members}, @ovar{action-if-found}, @ + @ovar{action-if-not-found}, @dvar{includes, AC_INCLUDES_DEFAULT}) +@acindex{CHECK_MEMBERS} +@cvindex HAVE_@var{aggregate}_@var{member} +Check for the existence of each @samp{@var{aggregate}.@var{member}} of +@var{members} using the previous macro. When @var{member} belongs to +@var{aggregate}, define @code{HAVE_@var{aggregate}_@var{member}} (in all +capitals, with spaces and dots replaced by underscores). If +@var{action-if-found} is given, it is executed for each of the found +members. If @var{action-if-not-found} is given, it is executed for each +of the members that could not be found. + +@var{includes} is a series of include directives, defaulting to +@code{AC_INCLUDES_DEFAULT} (@pxref{Default Includes}), which are used +prior to the members under test. + +This macro uses M4 lists: +@example +AC_CHECK_MEMBERS([struct stat.st_rdev, struct stat.st_blksize]) +@end example +@end defmac + + +@node Types +@section Types +@cindex Types +@cindex C types + +The following macros check for C types, either builtin or typedefs. If +there is no macro specifically defined to check for a type you need, and +you don't need to check for any special properties of it, then you can +use a general type-check macro. + +@menu +* Particular Types:: Special handling to find certain types +* Generic Types:: How to find other types +@end menu + +@node Particular Types +@subsection Particular Type Checks + +@hdrindex{sys/types.h} +@hdrindex{stdlib.h} +@hdrindex{stdint.h} +@hdrindex{inttypes.h} +These macros check for particular C types in @file{sys/types.h}, +@file{stdlib.h}, @file{stdint.h}, @file{inttypes.h} and others, if they +exist. + +The Gnulib @code{stdint} module is an alternate way to define many of +these symbols; it is useful if you prefer your code to assume a +C99-or-better environment. @xref{Gnulib}. + +@anchor{AC_TYPE_GETGROUPS} +@defmac AC_TYPE_GETGROUPS +@acindex{TYPE_GETGROUPS} +@cvindex GETGROUPS_T +@caindex type_getgroups +Define @code{GETGROUPS_T} to be whichever of @code{gid_t} or @code{int} +is the base type of the array argument to @code{getgroups}. + +This macro caches the base type in the @code{ac_cv_type_getgroups} +variable. +@end defmac + +@defmac AC_TYPE_INT8_T +@acindex{TYPE_INT8_T} +@cvindex HAVE_INT8_T +@cvindex int8_t +@caindex c_int8_t +If @file{stdint.h} or @file{inttypes.h} does not define the type +@code{int8_t}, define @code{int8_t} to a signed +integer type that is exactly 8 bits wide and that uses two's complement +representation, if such a type exists. +If you are worried about porting to hosts that lack such a type, you can +use the results of this macro in C89-or-later code as follows: + +@example +#if HAVE_STDINT_H +# include <stdint.h> +#endif +#if defined INT8_MAX || defined int8_t + @emph{code using int8_t} +#else + @emph{complicated alternative using >8-bit 'signed char'} +#endif +@end example + +This macro caches the type in the @code{ac_cv_c_int8_t} variable. +@end defmac + +@defmac AC_TYPE_INT16_T +@acindex{TYPE_INT16_T} +@cvindex HAVE_INT16_T +@cvindex int16_t +@caindex c_int16_t +This is like @code{AC_TYPE_INT8_T}, except for 16-bit integers. +@end defmac + +@defmac AC_TYPE_INT32_T +@acindex{TYPE_INT32_T} +@cvindex HAVE_INT32_T +@cvindex int32_t +@caindex c_int32_t +This is like @code{AC_TYPE_INT8_T}, except for 32-bit integers. +@end defmac + +@defmac AC_TYPE_INT64_T +@acindex{TYPE_INT64_T} +@cvindex HAVE_INT64_T +@cvindex int64_t +@caindex c_int64_t +This is like @code{AC_TYPE_INT8_T}, except for 64-bit integers. +@end defmac + +@defmac AC_TYPE_INTMAX_T +@acindex{TYPE_INTMAX_T} +@cvindex HAVE_INTMAX_T +@cvindex intmax_t +@c @caindex type_intmax_t +If @file{stdint.h} or @file{inttypes.h} defines the type @code{intmax_t}, +define @code{HAVE_INTMAX_T}. Otherwise, define @code{intmax_t} to the +widest signed integer type. +@end defmac + +@defmac AC_TYPE_INTPTR_T +@acindex{TYPE_INTPTR_T} +@cvindex HAVE_INTPTR_T +@cvindex intptr_t +@c @caindex type_intptr_t +If @file{stdint.h} or @file{inttypes.h} defines the type @code{intptr_t}, +define @code{HAVE_INTPTR_T}. Otherwise, define @code{intptr_t} to a +signed integer type wide enough to hold a pointer, if such a type +exists. +@end defmac + +@defmac AC_TYPE_LONG_DOUBLE +@acindex{TYPE_LONG_DOUBLE} +@cvindex HAVE_LONG_DOUBLE +@caindex type_long_double +If the C compiler supports a working @code{long double} type, define +@code{HAVE_LONG_DOUBLE}. The @code{long double} type might have the +same range and precision as @code{double}. + +This macro caches its result in the @code{ac_cv_type_long_double} +variable. + +This macro is obsolescent, as current C compilers support @code{long +double}. New programs need not use this macro. +@end defmac + +@defmac AC_TYPE_LONG_DOUBLE_WIDER +@acindex{TYPE_LONG_DOUBLE_WIDER} +@cvindex HAVE_LONG_DOUBLE_WIDER +@caindex type_long_double_wider +If the C compiler supports a working @code{long double} type with more +range or precision than the @code{double} type, define +@code{HAVE_LONG_DOUBLE_WIDER}. + +This macro caches its result in the @code{ac_cv_type_long_double_wider} +variable. +@end defmac + +@defmac AC_TYPE_LONG_LONG_INT +@acindex{TYPE_LONG_LONG_INT} +@cvindex HAVE_LONG_LONG_INT +@caindex type_long_long_int +If the C compiler supports a working @code{long long int} type, define +@code{HAVE_LONG_LONG_INT}. However, this test does not test +@code{long long int} values in preprocessor @code{#if} expressions, +because too many compilers mishandle such expressions. +@xref{Preprocessor Arithmetic}. + +This macro caches its result in the @code{ac_cv_type_long_long_int} +variable. +@end defmac + +@defmac AC_TYPE_MBSTATE_T +@acindex{TYPE_MBSTATE_T} +@cvindex mbstate_t +@hdrindex{wchar.h} +@caindex type_mbstate_t +Define @code{HAVE_MBSTATE_T} if @code{<wchar.h>} declares the +@code{mbstate_t} type. Also, define @code{mbstate_t} to be a type if +@code{<wchar.h>} does not declare it. + +This macro caches its result in the @code{ac_cv_type_mbstate_t} +variable. +@end defmac + +@anchor{AC_TYPE_MODE_T} +@defmac AC_TYPE_MODE_T +@acindex{TYPE_MODE_T} +@cvindex mode_t +@caindex type_mode_t +Define @code{mode_t} to a suitable type, if standard headers do not +define it. + +This macro caches its result in the @code{ac_cv_type_mode_t} variable. +@end defmac + +@anchor{AC_TYPE_OFF_T} +@defmac AC_TYPE_OFF_T +@acindex{TYPE_OFF_T} +@cvindex off_t +@caindex type_off_t +Define @code{off_t} to a suitable type, if standard headers do not +define it. + +This macro caches its result in the @code{ac_cv_type_off_t} variable. +@end defmac + +@anchor{AC_TYPE_PID_T} +@defmac AC_TYPE_PID_T +@acindex{TYPE_PID_T} +@cvindex pid_t +@caindex type_pid_t +Define @code{pid_t} to a suitable type, if standard headers do not +define it. + +This macro caches its result in the @code{ac_cv_type_pid_t} variable. +@end defmac + +@anchor{AC_TYPE_SIZE_T} +@defmac AC_TYPE_SIZE_T +@acindex{TYPE_SIZE_T} +@cvindex size_t +@caindex type_size_t +Define @code{size_t} to a suitable type, if standard headers do not +define it. + +This macro caches its result in the @code{ac_cv_type_size_t} variable. +@end defmac + +@defmac AC_TYPE_SSIZE_T +@acindex{TYPE_SSIZE_T} +@cvindex ssize_t +@caindex type_ssize_t +Define @code{ssize_t} to a suitable type, if standard headers do not +define it. + +This macro caches its result in the @code{ac_cv_type_ssize_t} variable. +@end defmac + +@anchor{AC_TYPE_UID_T} +@defmac AC_TYPE_UID_T +@acindex{TYPE_UID_T} +@cvindex uid_t +@cvindex gid_t +@caindex type_uid_t +Define @code{uid_t} and @code{gid_t} to suitable types, if standard +headers do not define them. + +This macro caches its result in the @code{ac_cv_type_uid_t} variable. +@end defmac + +@defmac AC_TYPE_UINT8_T +@acindex{TYPE_UINT8_T} +@cvindex HAVE_UINT8_T +@cvindex uint8_t +@caindex c_uint8_t +If @file{stdint.h} or @file{inttypes.h} does not define the type +@code{uint8_t}, define @code{uint8_t} to an +unsigned integer type that is exactly 8 bits wide, if such a type +exists. +This is like @code{AC_TYPE_INT8_T}, except for unsigned integers. +@end defmac + +@defmac AC_TYPE_UINT16_T +@acindex{TYPE_UINT16_T} +@cvindex HAVE_UINT16_T +@cvindex uint16_t +@caindex c_uint16_t +This is like @code{AC_TYPE_UINT8_T}, except for 16-bit integers. +@end defmac + +@defmac AC_TYPE_UINT32_T +@acindex{TYPE_UINT32_T} +@cvindex HAVE_UINT32_T +@cvindex uint32_t +@caindex c_uint32_t +This is like @code{AC_TYPE_UINT8_T}, except for 32-bit integers. +@end defmac + +@defmac AC_TYPE_UINT64_T +@acindex{TYPE_UINT64_T} +@cvindex HAVE_UINT64_T +@cvindex uint64_t +@caindex c_uint64_t +This is like @code{AC_TYPE_UINT8_T}, except for 64-bit integers. +@end defmac + +@defmac AC_TYPE_UINTMAX_T +@acindex{TYPE_UINTMAX_T} +@cvindex HAVE_UINTMAX_T +@cvindex uintmax_t +@c @caindex type_uintmax_t +If @file{stdint.h} or @file{inttypes.h} defines the type @code{uintmax_t}, +define @code{HAVE_UINTMAX_T}. Otherwise, define @code{uintmax_t} to the +widest unsigned integer type. +@end defmac + +@defmac AC_TYPE_UINTPTR_T +@acindex{TYPE_UINTPTR_T} +@cvindex HAVE_UINTPTR_T +@cvindex uintptr_t +@c @caindex type_uintptr_t +If @file{stdint.h} or @file{inttypes.h} defines the type @code{uintptr_t}, +define @code{HAVE_UINTPTR_T}. Otherwise, define @code{uintptr_t} to an +unsigned integer type wide enough to hold a pointer, if such a type +exists. +@end defmac + +@defmac AC_TYPE_UNSIGNED_LONG_LONG_INT +@acindex{TYPE_UNSIGNED_LONG_LONG_INT} +@cvindex HAVE_UNSIGNED_LONG_LONG_INT +@caindex type_unsigned_long_long_int +If the C compiler supports a working @code{unsigned long long int} type, +define @code{HAVE_UNSIGNED_LONG_LONG_INT}. However, this test does not test +@code{unsigned long long int} values in preprocessor @code{#if} expressions, +because too many compilers mishandle such expressions. +@xref{Preprocessor Arithmetic}. + +This macro caches its result in the @code{ac_cv_type_unsigned_long_long_int} +variable. +@end defmac + +@node Generic Types +@subsection Generic Type Checks + +These macros are used to check for types not covered by the ``particular'' +test macros. + +@defmac AC_CHECK_TYPE (@var{type}, @ovar{action-if-found}, @ + @ovar{action-if-not-found}, @dvar{includes, AC_INCLUDES_DEFAULT}) +@acindex{CHECK_TYPE} +@caindex type_@var{type} +Check whether @var{type} is defined. It may be a compiler builtin type +or defined by the @var{includes}. @var{includes} is a series of include +directives, defaulting to @code{AC_INCLUDES_DEFAULT} (@pxref{Default +Includes}), which are used prior to the type under test. + +In C, @var{type} must be a type-name, so that the expression @samp{sizeof +(@var{type})} is valid (but @samp{sizeof ((@var{type}))} is not). The +same test is applied when compiling for C++, which means that in C++ +@var{type} should be a type-id and should not be an anonymous +@samp{struct} or @samp{union}. + +This macro caches its result in the @code{ac_cv_type_@var{type}} +variable, with @samp{*} mapped to @samp{p} and other characters not +suitable for a variable name mapped to underscores. +@end defmac + + +@defmac AC_CHECK_TYPES (@var{types}, @ovar{action-if-found}, @ + @ovar{action-if-not-found}, @dvar{includes, AC_INCLUDES_DEFAULT}) +@acindex{CHECK_TYPES} +@cvindex HAVE_@var{type} +For each @var{type} of the @var{types} that is defined, define +@code{HAVE_@var{type}} (in all capitals). Each @var{type} must follow +the rules of @code{AC_CHECK_TYPE}. If no @var{includes} are +specified, the default includes are used (@pxref{Default Includes}). If +@var{action-if-found} is given, it is additional shell code to execute +when one of the types is found. If @var{action-if-not-found} is given, +it is executed when one of the types is not found. + +This macro uses M4 lists: +@example +AC_CHECK_TYPES([ptrdiff_t]) +AC_CHECK_TYPES([unsigned long long int, uintmax_t]) +AC_CHECK_TYPES([float_t], [], [], [[#include <math.h>]]) +@end example + +@end defmac + +Autoconf, up to 2.13, used to provide to another version of +@code{AC_CHECK_TYPE}, broken by design. In order to keep backward +compatibility, a simple heuristic, quite safe but not totally, is +implemented. In case of doubt, read the documentation of the former +@code{AC_CHECK_TYPE}, see @ref{Obsolete Macros}. + + +@node Compilers and Preprocessors +@section Compilers and Preprocessors +@cindex Compilers +@cindex Preprocessors + +@ovindex EXEEXT +All the tests for compilers (@code{AC_PROG_CC}, @code{AC_PROG_CXX}, +@code{AC_PROG_F77}) define the output variable @code{EXEEXT} based on +the output of the compiler, typically to the empty string if +Posix and @samp{.exe} if a DOS variant. + +@ovindex OBJEXT +They also define the output variable @code{OBJEXT} based on the +output of the compiler, after @file{.c} files have been excluded, typically +to @samp{o} if Posix, @samp{obj} if a DOS variant. + +If the compiler being used does not produce executables, the tests fail. If +the executables can't be run, and cross-compilation is not enabled, they +fail too. @xref{Manual Configuration}, for more on support for cross +compiling. + +@menu +* Specific Compiler Characteristics:: Some portability issues +* Generic Compiler Characteristics:: Language independent tests and features +* C Compiler:: Checking its characteristics +* C++ Compiler:: Likewise +* Objective C Compiler:: Likewise +* Objective C++ Compiler:: Likewise +* Erlang Compiler and Interpreter:: Likewise +* Fortran Compiler:: Likewise +* Go Compiler:: Likewise +@end menu + +@node Specific Compiler Characteristics +@subsection Specific Compiler Characteristics + +Some compilers exhibit different behaviors. + +@table @asis +@item Static/Dynamic Expressions +Autoconf relies on a trick to extract one bit of information from the C +compiler: using negative array sizes. For instance the following +excerpt of a C source demonstrates how to test whether @samp{int} objects are 4 +bytes wide: + +@example +static int test_array[sizeof (int) == 4 ? 1 : -1]; +@end example + +@noindent +To our knowledge, there is a single compiler that does not support this +trick: the HP C compilers (the real ones, not only the +``bundled'') on HP-UX 11.00. +They incorrectly reject the above program with the diagnostic +``Variable-length arrays cannot have static storage.'' +This bug comes from HP compilers' mishandling of @code{sizeof (int)}, +not from the @code{? 1 : -1}, and +Autoconf works around this problem by casting @code{sizeof (int)} to +@code{long int} before comparing it. +@end table + +@node Generic Compiler Characteristics +@subsection Generic Compiler Characteristics + +@anchor{AC_CHECK_SIZEOF} +@defmac AC_CHECK_SIZEOF (@var{type-or-expr}, @ovar{unused}, @ + @dvar{includes, AC_INCLUDES_DEFAULT}) +@acindex{CHECK_SIZEOF} +@cvindex SIZEOF_@var{type-or-expr} +@caindex sizeof_@var{type-or-expr} +Define @code{SIZEOF_@var{type-or-expr}} (@pxref{Standard Symbols}) to be +the size in bytes of @var{type-or-expr}, which may be either a type or +an expression returning a value that has a size. If the expression +@samp{sizeof (@var{type-or-expr})} is invalid, the result is 0. +@var{includes} is a series of include directives, defaulting to +@code{AC_INCLUDES_DEFAULT} (@pxref{Default Includes}), which are used +prior to the expression under test. + +This macro now works even when cross-compiling. The @var{unused} +argument was used when cross-compiling. + +For example, the call + +@example +@c If you change this example, adjust tests/semantics.at:AC_CHECK_SIZEOF struct. +AC_CHECK_SIZEOF([int *]) +@end example + +@noindent +defines @code{SIZEOF_INT_P} to be 8 on DEC Alpha AXP systems. + +This macro caches its result in the @code{ac_cv_sizeof_@var{type-or-expr}} +variable, with @samp{*} mapped to @samp{p} and other characters not +suitable for a variable name mapped to underscores. +@end defmac + +@defmac AC_CHECK_ALIGNOF (@var{type}, @dvar{includes, AC_INCLUDES_DEFAULT}) +@acindex{CHECK_ALIGNOF} +@cvindex ALIGNOF_@var{type} +@caindex alignof_@var{type-or-expr} +Define @code{ALIGNOF_@var{type}} (@pxref{Standard Symbols}) to be the +alignment in bytes of @var{type}. @samp{@var{type} y;} must be valid as +a structure member declaration. If @samp{type} is unknown, the result +is 0. If no @var{includes} are specified, the default includes are used +(@pxref{Default Includes}). + +This macro caches its result in the @code{ac_cv_alignof_@var{type-or-expr}} +variable, with @samp{*} mapped to @samp{p} and other characters not +suitable for a variable name mapped to underscores. +@end defmac + +@defmac AC_COMPUTE_INT (@var{var}, @var{expression}, @ + @dvar{includes, AC_INCLUDES_DEFAULT}, @ovar{action-if-fails}) +@acindex{COMPUTE_INT} +Store into the shell variable @var{var} the value of the integer +@var{expression}. The +value should fit in an initializer in a C variable of type @code{signed +long}. To support cross compilation (in which case, the macro only works on +hosts that use twos-complement arithmetic), it should be possible to evaluate +the expression at compile-time. If no @var{includes} are specified, the +default includes are used (@pxref{Default Includes}). + +Execute @var{action-if-fails} if the value cannot be determined correctly. +@end defmac + +@defmac AC_LANG_WERROR +@acindex{LANG_WERROR} +Normally Autoconf ignores warnings generated by the compiler, linker, and +preprocessor. If this macro is used, warnings count as fatal +errors for the current language. This macro is useful when the +results of configuration are used where warnings are unacceptable; for +instance, if parts of a program are built with the GCC +@option{-Werror} +option. If the whole program is built using @option{-Werror} it is +often simpler to put @option{-Werror} in the compiler flags (@code{CFLAGS}, +etc.). +@end defmac + +@defmac AC_OPENMP +@acindex{OPENMP} +@cvindex _OPENMP +@ovindex OPENMP_CFLAGS +@ovindex OPENMP_CXXFLAGS +@ovindex OPENMP_FFLAGS +@ovindex OPENMP_FCFLAGS +@caindex prog_c_openmp +@caindex prog_cxx_openmp +@caindex prog_f77_openmp +@caindex prog_fc_openmp +@uref{http://@/www.openmp.org/, OpenMP} specifies extensions of C, C++, +and Fortran that simplify optimization of shared memory parallelism, +which is a common problem on multicore CPUs. + +If the current language is C, the macro @code{AC_OPENMP} sets the +variable @code{OPENMP_CFLAGS} to the C compiler flags needed for +supporting OpenMP@. @code{OPENMP_CFLAGS} is set to empty if the +compiler already supports OpenMP, if it has no way to activate OpenMP +support, or if the user rejects OpenMP support by invoking +@samp{configure} with the @samp{--disable-openmp} option. + +@code{OPENMP_CFLAGS} needs to be used when compiling programs, when +preprocessing program source, and when linking programs. Therefore you +need to add @code{$(OPENMP_CFLAGS)} to the @code{CFLAGS} of C programs +that use OpenMP@. If you preprocess OpenMP-specific C code, you also +need to add @code{$(OPENMP_CFLAGS)} to @code{CPPFLAGS}. The presence of +OpenMP support is revealed at compile time by the preprocessor macro +@code{_OPENMP}. + +Linking a program with @code{OPENMP_CFLAGS} typically adds one more +shared library to the program's dependencies, so its use is recommended +only on programs that actually require OpenMP. + +If the current language is C++, @code{AC_OPENMP} sets the variable +@code{OPENMP_CXXFLAGS}, suitably for the C++ compiler. The same remarks +hold as for C. + +If the current language is Fortran 77 or Fortran, @code{AC_OPENMP} sets +the variable @code{OPENMP_FFLAGS} or @code{OPENMP_FCFLAGS}, +respectively. Similar remarks as for C hold, except that +@code{CPPFLAGS} is not used for Fortran, and no preprocessor macro +signals OpenMP support. + +For portability, it is best to avoid spaces between @samp{#} and +@samp{pragma omp}. That is, write @samp{#pragma omp}, not +@samp{# pragma omp}. The Sun WorkShop 6.2 C compiler chokes on the +latter. + +This macro caches its result in the @code{ac_cv_prog_c_openmp}, +@code{ac_cv_prog_cxx_openmp}, @code{ac_cv_prog_f77_openmp}, or +@code{ac_cv_prog_fc_openmp} variable, depending on the current language. +@end defmac + +@node C Compiler +@subsection C Compiler Characteristics + +The following macros provide ways to find and exercise a C Compiler. +There are a few constructs that ought to be avoided, but do not deserve +being checked for, since they can easily be worked around. + +@table @asis +@item Don't use lines containing solitary backslashes +They tickle a bug in the HP-UX C compiler (checked on +HP-UX 10.20, +11.00, and 11i). When given the following source: + +@example +#ifdef __STDC__ +/\ +* A comment with backslash-newlines in it. %@{ %@} *\ +\ +/ +char str[] = "\\ +" A string with backslash-newlines in it %@{ %@} \\ +""; +char apostrophe = '\\ +\ +'\ +'; +#endif +@end example + +@noindent +the compiler incorrectly fails with the diagnostics ``Non-terminating +comment at end of file'' and ``Missing @samp{#endif} at end of file.'' +Removing the lines with solitary backslashes solves the problem. + +@item Don't compile several files at once if output matters to you +Some compilers, such as HP's, report names of files being +compiled when given more than one file operand. For instance: + +@example +$ @kbd{cc a.c b.c} +a.c: +b.c: +@end example + +@noindent +This can cause problems if you observe the output of the compiler to +detect failures. Invoking @samp{cc -c a.c && cc -c b.c && cc -o c a.o +b.o} solves the issue. + +@item Don't rely on @code{#error} failing +The IRIX C compiler does not fail when #error is preprocessed; it +simply emits a diagnostic and continues, exiting successfully. So, +instead of an error directive like @code{#error "Unsupported word size"} +it is more portable to use an invalid directive like @code{#Unsupported +word size} in Autoconf tests. In ordinary source code, @code{#error} is +OK, since installers with inadequate compilers like IRIX can simply +examine these compilers' diagnostic output. + +@item Don't rely on correct @code{#line} support +On Solaris, @command{c89} (at least Sun C 5.3 through 5.8) +diagnoses @code{#line} directives whose line +numbers are greater than 32767. Nothing in Posix +makes this invalid. That is why Autoconf stopped issuing +@code{#line} directives. +@end table + +@defmac AC_PROG_CC (@ovar{compiler-search-list}) +@acindex{PROG_CC} +@evindex CC +@evindex CFLAGS +@ovindex CC +@ovindex CFLAGS +@caindex prog_cc_c89 +Determine a C compiler to use. If @code{CC} is not already set in the +environment, check for @code{gcc} and @code{cc}, then for other C +compilers. Set output variable @code{CC} to the name of the compiler +found. + +This macro may, however, be invoked with an optional first argument +which, if specified, must be a blank-separated list of C compilers to +search for. This just gives the user an opportunity to specify an +alternative search list for the C compiler. For example, if you didn't +like the default order, then you could invoke @code{AC_PROG_CC} like +this: + +@example +AC_PROG_CC([gcc cl cc]) +@end example + +If the C compiler does not handle function prototypes correctly by +default, try to add an option to output variable @code{CC} to make it +so. This macro tries various options that select standard-conformance +modes on various systems. + +After calling this macro you can check whether the C compiler has been +set to accept ANSI C89 (ISO C90); if not, the shell +variable +@code{ac_cv_prog_cc_c89} is set to @samp{no}. See also +@code{AC_C_PROTOTYPES} below. + +If using the GNU C compiler, set shell variable @code{GCC} to +@samp{yes}. If output variable @code{CFLAGS} was not already set, set +it to @option{-g -O2} for the GNU C compiler (@option{-O2} on systems +where GCC does not accept @option{-g}), or @option{-g} for +other compilers. If your package does not like this default, then it is +acceptable to insert the line @samp{: $@{CFLAGS=""@}} after @code{AC_INIT} +and before @code{AC_PROG_CC} to select an empty default instead. + +Many Autoconf macros use a compiler, and thus call +@samp{AC_REQUIRE([AC_PROG_CC])} to ensure that the compiler has been +determined before the body of the outermost @code{AC_DEFUN} macro. +Although @code{AC_PROG_CC} is safe to directly expand multiple times, it +performs certain checks (such as the proper value of @env{EXEEXT}) only +on the first invocation. Therefore, care must be used when invoking +this macro from within another macro rather than at the top level +(@pxref{Expanded Before Required}). +@end defmac + +@anchor{AC_PROG_CC_C_O} +@defmac AC_PROG_CC_C_O +@acindex{PROG_CC_C_O} +@cvindex NO_MINUS_C_MINUS_O +@caindex prog_cc_@var{compiler}_c_o +If the C compiler does not accept the @option{-c} and @option{-o} options +simultaneously, define @code{NO_MINUS_C_MINUS_O}. This macro actually +tests both the compiler found by @code{AC_PROG_CC}, and, if different, +the first @code{cc} in the path. The test fails if one fails. This +macro was created for GNU Make to choose the default C compilation +rule. + +For the compiler @var{compiler}, this macro caches its result in the +@code{ac_cv_prog_cc_@var{compiler}_c_o} variable. +@end defmac + + +@defmac AC_PROG_CPP +@acindex{PROG_CPP} +@evindex CPP +@ovindex CPP +Set output variable @code{CPP} to a command that runs the +C preprocessor. If @samp{$CC -E} doesn't work, @file{/lib/cpp} is used. +It is only portable to run @code{CPP} on files with a @file{.c} +extension. + +Some preprocessors don't indicate missing include files by the error +status. For such preprocessors an internal variable is set that causes +other macros to check the standard error from the preprocessor and +consider the test failed if any warnings have been reported. +For most preprocessors, though, warnings do not cause include-file +tests to fail unless @code{AC_PROG_CPP_WERROR} is also specified. +@end defmac + +@defmac AC_PROG_CPP_WERROR +@acindex{PROG_CPP_WERROR} +@ovindex CPP +This acts like @code{AC_PROG_CPP}, except it treats warnings from the +preprocessor as errors even if the preprocessor exit status indicates +success. This is useful for avoiding headers that generate mandatory +warnings, such as deprecation notices. +@end defmac + + +The following macros check for C compiler or machine architecture +features. To check for characteristics not listed here, use +@code{AC_COMPILE_IFELSE} (@pxref{Running the Compiler}) or +@code{AC_RUN_IFELSE} (@pxref{Runtime}). + +@defmac AC_PROG_CC_STDC +@acindex{PROG_CC_STDC} +@caindex prog_cc_stdc +If the C compiler cannot compile ISO Standard C (currently +C99), try to add an option to output variable @code{CC} to make it work. +If the compiler does not support C99, fall back to supporting +ANSI C89 (ISO C90). + +After calling this macro you can check whether the C compiler has been +set to accept Standard C; if not, the shell variable +@code{ac_cv_prog_cc_stdc} is set to @samp{no}. +@end defmac + +@defmac AC_PROG_CC_C89 +@acindex{PROG_CC_C89} +@caindex prog_cc_c89 +If the C compiler is not in ANSI C89 (ISO C90) mode by +default, try to add an option to output variable @code{CC} to make it +so. This macro tries various options that select ANSI C89 on +some system or another, preferring extended functionality modes over +strict conformance modes. It considers the compiler to be in +ANSI C89 mode if it handles function prototypes correctly. + +After calling this macro you can check whether the C compiler has been +set to accept ANSI C89; if not, the shell variable +@code{ac_cv_prog_cc_c89} is set to @samp{no}. + +This macro is called automatically by @code{AC_PROG_CC}. +@end defmac + +@defmac AC_PROG_CC_C99 +@acindex{PROG_CC_C99} +@caindex prog_cc_c99 +If the C compiler is not in C99 mode by default, try to add an +option to output variable @code{CC} to make it so. This macro tries +various options that select C99 on some system or another, preferring +extended functionality modes over strict conformance modes. It +considers the compiler to be in C99 mode if it handles @code{_Bool}, +@code{//} comments, flexible array members, @code{inline}, signed and +unsigned @code{long long int}, mixed code and declarations, named +initialization of structs, +@code{restrict}, @code{va_copy}, varargs macros, variable declarations +in @code{for} loops, and variable length arrays. + +After calling this macro you can check whether the C compiler has been +set to accept C99; if not, the shell variable +@code{ac_cv_prog_cc_c99} is set to @samp{no}. +@end defmac + +@defmac AC_C_BACKSLASH_A +@acindex{C_BACKSLASH_A} +@cvindex HAVE_C_BACKSLASH_A +Define @samp{HAVE_C_BACKSLASH_A} to 1 if the C compiler understands +@samp{\a}. + +This macro is obsolescent, as current C compilers understand @samp{\a}. +New programs need not use this macro. +@end defmac + +@anchor{AC_C_BIGENDIAN} +@defmac AC_C_BIGENDIAN (@ovar{action-if-true}, @ovar{action-if-false}, @ + @ovar{action-if-unknown}, @ovar{action-if-universal}) +@acindex{C_BIGENDIAN} +@cvindex WORDS_BIGENDIAN +@cindex Endianness +If words are stored with the most significant byte first (like Motorola +and SPARC CPUs), execute @var{action-if-true}. If words are stored with +the least significant byte first (like Intel and VAX CPUs), execute +@var{action-if-false}. + +This macro runs a test-case if endianness cannot be determined from the +system header files. When cross-compiling, the test-case is not run but +grep'ed for some magic values. @var{action-if-unknown} is executed if +the latter case fails to determine the byte sex of the host system. + +In some cases a single run of a compiler can generate code for multiple +architectures. This can happen, for example, when generating Mac OS X +universal binary files, which work on both PowerPC and Intel +architectures. In this case, the different variants might be for +different architectures whose endiannesses differ. If +@command{configure} detects this, it executes @var{action-if-universal} +instead of @var{action-if-unknown}. + +The default for @var{action-if-true} is to define +@samp{WORDS_BIGENDIAN}. The default for @var{action-if-false} is to do +nothing. The default for @var{action-if-unknown} is to +abort configure and tell the installer how to bypass this test. +And finally, the default for @var{action-if-universal} is to ensure that +@samp{WORDS_BIGENDIAN} is defined if and only if a universal build is +detected and the current code is big-endian; this default works only if +@command{autoheader} is used (@pxref{autoheader Invocation}). + +If you use this macro without specifying @var{action-if-universal}, you +should also use @code{AC_CONFIG_HEADERS}; otherwise +@samp{WORDS_BIGENDIAN} may be set incorrectly for Mac OS X universal +binary files. +@end defmac + +@anchor{AC_C_CONST} +@defmac AC_C_CONST +@acindex{C_CONST} +@cvindex const +@caindex c_const +If the C compiler does not fully support the @code{const} keyword, +define @code{const} to be empty. Some C compilers that do +not define @code{__STDC__} do support @code{const}; some compilers that +define @code{__STDC__} do not completely support @code{const}. Programs +can simply use @code{const} as if every C compiler supported it; for +those that don't, the makefile or configuration header file +defines it as empty. + +Occasionally installers use a C++ compiler to compile C code, typically +because they lack a C compiler. This causes problems with @code{const}, +because C and C++ treat @code{const} differently. For example: + +@example +const int foo; +@end example + +@noindent +is valid in C but not in C++. These differences unfortunately cannot be +papered over by defining @code{const} to be empty. + +If @command{autoconf} detects this situation, it leaves @code{const} alone, +as this generally yields better results in practice. However, using a +C++ compiler to compile C code is not recommended or supported, and +installers who run into trouble in this area should get a C compiler +like GCC to compile their C code. + +This macro caches its result in the @code{ac_cv_c_const} variable. + +This macro is obsolescent, as current C compilers support @code{const}. +New programs need not use this macro. +@end defmac + +@defmac AC_C_RESTRICT +@acindex{C_RESTRICT} +@cvindex restrict +@caindex c_restrict +If the C compiler recognizes a variant spelling for the @code{restrict} +keyword (@code{__restrict}, @code{__restrict__}, or @code{_Restrict}), +then define @code{restrict} to that; this is more likely to do the right +thing with compilers that support language variants where plain +@code{restrict} is not a keyword. Otherwise, if the C compiler +recognizes the @code{restrict} keyword, don't do anything. +Otherwise, define @code{restrict} to be empty. +Thus, programs may simply use @code{restrict} as if every C compiler +supported it; for those that do not, the makefile +or configuration header defines it away. + +Although support in C++ for the @code{restrict} keyword is not +required, several C++ compilers do accept the keyword. +This macro works for them, too. + +This macro caches @samp{no} in the @code{ac_cv_c_restrict} variable +if @code{restrict} is not supported, and a supported spelling otherwise. +@end defmac + +@defmac AC_C_VOLATILE +@acindex{C_VOLATILE} +@cvindex volatile +If the C compiler does not understand the keyword @code{volatile}, +define @code{volatile} to be empty. Programs can simply use +@code{volatile} as if every C compiler supported it; for those that do +not, the makefile or configuration header defines it as +empty. + +If the correctness of your program depends on the semantics of +@code{volatile}, simply defining it to be empty does, in a sense, break +your code. However, given that the compiler does not support +@code{volatile}, you are at its mercy anyway. At least your +program compiles, when it wouldn't before. +@xref{Volatile Objects}, for more about @code{volatile}. + +In general, the @code{volatile} keyword is a standard C feature, so +you might expect that @code{volatile} is available only when +@code{__STDC__} is defined. However, Ultrix 4.3's native compiler does +support volatile, but does not define @code{__STDC__}. + +This macro is obsolescent, as current C compilers support @code{volatile}. +New programs need not use this macro. +@end defmac + +@anchor{AC_C_INLINE} +@defmac AC_C_INLINE +@acindex{C_INLINE} +@cvindex inline +If the C compiler supports the keyword @code{inline}, do nothing. +Otherwise define @code{inline} to @code{__inline__} or @code{__inline} +if it accepts one of those, otherwise define @code{inline} to be empty. +@end defmac + +@anchor{AC_C_CHAR_UNSIGNED} +@defmac AC_C_CHAR_UNSIGNED +@acindex{C_CHAR_UNSIGNED} +@cvindex __CHAR_UNSIGNED__ +If the C type @code{char} is unsigned, define @code{__CHAR_UNSIGNED__}, +unless the C compiler predefines it. + +These days, using this macro is not necessary. The same information can +be determined by this portable alternative, thus avoiding the use of +preprocessor macros in the namespace reserved for the implementation. + +@example +#include <limits.h> +#if CHAR_MIN == 0 +# define CHAR_UNSIGNED 1 +#endif +@end example +@end defmac + +@defmac AC_C_STRINGIZE +@acindex{C_STRINGIZE} +@cvindex HAVE_STRINGIZE +If the C preprocessor supports the stringizing operator, define +@code{HAVE_STRINGIZE}. The stringizing operator is @samp{#} and is +found in macros such as this: + +@example +#define x(y) #y +@end example + +This macro is obsolescent, as current C compilers support the +stringizing operator. New programs need not use this macro. +@end defmac + +@defmac AC_C_FLEXIBLE_ARRAY_MEMBER +@acindex{C_FLEXIBLE_ARRAY_MEMBER} +@cvindex FLEXIBLE_ARRAY_MEMBER +If the C compiler supports flexible array members, define +@code{FLEXIBLE_ARRAY_MEMBER} to nothing; otherwise define it to 1. +That way, a declaration like this: + +@example +struct s + @{ + size_t n_vals; + double val[FLEXIBLE_ARRAY_MEMBER]; + @}; +@end example + +@noindent +will let applications use the ``struct hack'' even with compilers that +do not support flexible array members. To allocate and use such an +object, you can use code like this: + +@example +size_t i; +size_t n = compute_value_count (); +struct s *p = + malloc (offsetof (struct s, val) + + n * sizeof (double)); +p->n_vals = n; +for (i = 0; i < n; i++) + p->val[i] = compute_value (i); +@end example +@end defmac + +@defmac AC_C_VARARRAYS +@acindex{C_VARARRAYS} +@cvindex HAVE_C_VARARRAYS +If the C compiler supports variable-length arrays, define +@code{HAVE_C_VARARRAYS}. A variable-length array is an array of automatic +storage duration whose length is determined at run time, when the array +is declared. +@end defmac + +@defmac AC_C_TYPEOF +@acindex{C_TYPEOF} +@cvindex HAVE_TYPEOF +@cvindex typeof +If the C compiler supports GCC's @code{typeof} syntax either +directly or +through a different spelling of the keyword (e.g., @code{__typeof__}), +define @code{HAVE_TYPEOF}. If the support is available only through a +different spelling, define @code{typeof} to that spelling. +@end defmac + +@defmac AC_C_PROTOTYPES +@acindex{C_PROTOTYPES} +@cvindex PROTOTYPES +@cvindex __PROTOTYPES +@cvindex PARAMS +If function prototypes are understood by the compiler (as determined by +@code{AC_PROG_CC}), define @code{PROTOTYPES} and @code{__PROTOTYPES}. +Defining @code{__PROTOTYPES} is for the benefit of +header files that cannot use macros that infringe on user name space. + +This macro is obsolescent, as current C compilers support prototypes. +New programs need not use this macro. +@end defmac + +@anchor{AC_PROG_GCC_TRADITIONAL} +@defmac AC_PROG_GCC_TRADITIONAL +@acindex{PROG_GCC_TRADITIONAL} +@ovindex CC +Add @option{-traditional} to output variable @code{CC} if using the +GNU C compiler and @code{ioctl} does not work properly without +@option{-traditional}. That usually happens when the fixed header files +have not been installed on an old system. + +This macro is obsolescent, since current versions of the GNU C +compiler fix the header files automatically when installed. +@end defmac + + +@node C++ Compiler +@subsection C++ Compiler Characteristics + + +@defmac AC_PROG_CXX (@ovar{compiler-search-list}) +@acindex{PROG_CXX} +@evindex CXX +@evindex CXXFLAGS +@ovindex CXX +@ovindex CXXFLAGS +Determine a C++ compiler to use. Check whether the environment variable +@code{CXX} or @code{CCC} (in that order) is set; if so, then set output +variable @code{CXX} to its value. + +Otherwise, if the macro is invoked without an argument, then search for +a C++ compiler under the likely names (first @code{g++} and @code{c++} +then other names). If none of those checks succeed, then as a last +resort set @code{CXX} to @code{g++}. + +This macro may, however, be invoked with an optional first argument +which, if specified, must be a blank-separated list of C++ compilers to +search for. This just gives the user an opportunity to specify an +alternative search list for the C++ compiler. For example, if you +didn't like the default order, then you could invoke @code{AC_PROG_CXX} +like this: + +@example +AC_PROG_CXX([gcc cl KCC CC cxx cc++ xlC aCC c++ g++]) +@end example + +If using the GNU C++ compiler, set shell variable @code{GXX} to +@samp{yes}. If output variable @code{CXXFLAGS} was not already set, set +it to @option{-g -O2} for the GNU C++ compiler (@option{-O2} on +systems where G++ does not accept @option{-g}), or @option{-g} for other +compilers. If your package does not like this default, then it is +acceptable to insert the line @samp{: $@{CXXFLAGS=""@}} after @code{AC_INIT} +and before @code{AC_PROG_CXX} to select an empty default instead. + +@end defmac + +@defmac AC_PROG_CXXCPP +@acindex{PROG_CXXCPP} +@evindex CXXCPP +@ovindex CXXCPP +Set output variable @code{CXXCPP} to a command that runs the C++ +preprocessor. If @samp{$CXX -E} doesn't work, @file{/lib/cpp} is used. +It is portable to run @code{CXXCPP} only on files with a @file{.c}, +@file{.C}, @file{.cc}, or @file{.cpp} extension. + +Some preprocessors don't indicate missing include files by the error +status. For such preprocessors an internal variable is set that causes +other macros to check the standard error from the preprocessor and +consider the test failed if any warnings have been reported. However, +it is not known whether such broken preprocessors exist for C++. +@end defmac + +@defmac AC_PROG_CXX_C_O +@acindex{PROG_CXX_C_O} +@cvindex CXX_NO_MINUS_C_MINUS_O +Test whether the C++ compiler accepts the options @option{-c} and +@option{-o} simultaneously, and define @code{CXX_NO_MINUS_C_MINUS_O}, +if it does not. +@end defmac + + +@node Objective C Compiler +@subsection Objective C Compiler Characteristics + + +@defmac AC_PROG_OBJC (@ovar{compiler-search-list}) +@acindex{PROG_OBJC} +@evindex OBJC +@evindex OBJCFLAGS +@ovindex OBJC +@ovindex OBJCFLAGS +Determine an Objective C compiler to use. If @code{OBJC} is not already +set in the environment, check for Objective C compilers. Set output +variable @code{OBJC} to the name of the compiler found. + +This macro may, however, be invoked with an optional first argument +which, if specified, must be a blank-separated list of Objective C compilers to +search for. This just gives the user an opportunity to specify an +alternative search list for the Objective C compiler. For example, if you +didn't like the default order, then you could invoke @code{AC_PROG_OBJC} +like this: + +@example +AC_PROG_OBJC([gcc objcc objc]) +@end example + +If using the GNU Objective C compiler, set shell variable +@code{GOBJC} to @samp{yes}. If output variable @code{OBJCFLAGS} was not +already set, set it to @option{-g -O2} for the GNU Objective C +compiler (@option{-O2} on systems where @command{gcc} does not accept +@option{-g}), or @option{-g} for other compilers. +@end defmac + +@defmac AC_PROG_OBJCPP +@acindex{PROG_OBJCPP} +@evindex OBJCPP +@ovindex OBJCPP +Set output variable @code{OBJCPP} to a command that runs the Objective C +preprocessor. If @samp{$OBJC -E} doesn't work, @file{/lib/cpp} is used. +@end defmac + + +@node Objective C++ Compiler +@subsection Objective C++ Compiler Characteristics + + +@defmac AC_PROG_OBJCXX (@ovar{compiler-search-list}) +@acindex{PROG_OBJCXX} +@evindex OBJCXX +@evindex OBJCXXFLAGS +@ovindex OBJCXX +@ovindex OBJCXXFLAGS +Determine an Objective C++ compiler to use. If @code{OBJCXX} is not already +set in the environment, check for Objective C++ compilers. Set output +variable @code{OBJCXX} to the name of the compiler found. + +This macro may, however, be invoked with an optional first argument +which, if specified, must be a blank-separated list of Objective C++ compilers +to search for. This just gives the user an opportunity to specify an +alternative search list for the Objective C++ compiler. For example, if you +didn't like the default order, then you could invoke @code{AC_PROG_OBJCXX} +like this: + +@example +AC_PROG_OBJCXX([gcc g++ objcc++ objcxx]) +@end example + +If using the GNU Objective C++ compiler, set shell variable +@code{GOBJCXX} to @samp{yes}. If output variable @code{OBJCXXFLAGS} was not +already set, set it to @option{-g -O2} for the GNU Objective C++ +compiler (@option{-O2} on systems where @command{gcc} does not accept +@option{-g}), or @option{-g} for other compilers. +@end defmac + +@defmac AC_PROG_OBJCXXCPP +@acindex{PROG_OBJCXXCPP} +@evindex OBJCXXCPP +@ovindex OBJCXXCPP +Set output variable @code{OBJCXXCPP} to a command that runs the Objective C++ +preprocessor. If @samp{$OBJCXX -E} doesn't work, @file{/lib/cpp} is used. +@end defmac + + +@node Erlang Compiler and Interpreter +@subsection Erlang Compiler and Interpreter Characteristics +@cindex Erlang + +Autoconf defines the following macros for determining paths to the essential +Erlang/OTP programs: + +@defmac AC_ERLANG_PATH_ERLC (@ovar{value-if-not-found}, @dvar{path, $PATH}) +@acindex{ERLANG_PATH_ERLC} +@evindex ERLC +@evindex ERLCFLAGS +@ovindex ERLC +@ovindex ERLCFLAGS +Determine an Erlang compiler to use. If @code{ERLC} is not already set in the +environment, check for @command{erlc}. Set output variable @code{ERLC} to the +complete path of the compiler command found. In addition, if @code{ERLCFLAGS} +is not set in the environment, set it to an empty value. + +The two optional arguments have the same meaning as the two last arguments of +macro @code{AC_PATH_PROG} for looking for the @command{erlc} program. For +example, to look for @command{erlc} only in the @file{/usr/lib/erlang/bin} +directory: + +@example +AC_ERLANG_PATH_ERLC([not found], [/usr/lib/erlang/bin]) +@end example +@end defmac + +@defmac AC_ERLANG_NEED_ERLC (@dvar{path, $PATH}) +@acindex{ERLANG_NEED_ERLC} +A simplified variant of the @code{AC_ERLANG_PATH_ERLC} macro, that prints an +error message and exits the @command{configure} script if the @command{erlc} +program is not found. +@end defmac + +@defmac AC_ERLANG_PATH_ERL (@ovar{value-if-not-found}, @dvar{path, $PATH}) +@acindex{ERLANG_PATH_ERL} +@evindex ERL +@ovindex ERL +Determine an Erlang interpreter to use. If @code{ERL} is not already +set in the +environment, check for @command{erl}. Set output variable @code{ERL} to the +complete path of the interpreter command found. + +The two optional arguments have the same meaning as the two last arguments of +macro @code{AC_PATH_PROG} for looking for the @command{erl} program. For +example, to look for @command{erl} only in the @file{/usr/lib/erlang/bin} +directory: + +@example +AC_ERLANG_PATH_ERL([not found], [/usr/lib/erlang/bin]) +@end example +@end defmac + +@defmac AC_ERLANG_NEED_ERL (@dvar{path, $PATH}) +@acindex{ERLANG_NEED_ERL} +A simplified variant of the @code{AC_ERLANG_PATH_ERL} macro, that prints an +error message and exits the @command{configure} script if the @command{erl} +program is not found. +@end defmac + + +@node Fortran Compiler +@subsection Fortran Compiler Characteristics +@cindex Fortran +@cindex F77 + +The Autoconf Fortran support is divided into two categories: legacy +Fortran 77 macros (@code{F77}), and modern Fortran macros (@code{FC}). +The former are intended for traditional Fortran 77 code, and have output +variables like @code{F77}, @code{FFLAGS}, and @code{FLIBS}. The latter +are for newer programs that can (or must) compile under the newer +Fortran standards, and have output variables like @code{FC}, +@code{FCFLAGS}, and @code{FCLIBS}. + +Except for the macros @code{AC_FC_SRCEXT}, @code{AC_FC_FREEFORM}, +@code{AC_FC_FIXEDFORM}, and @code{AC_FC_LINE_LENGTH} (see below), the +@code{FC} and @code{F77} macros behave almost identically, and so they +are documented together in this section. + + +@defmac AC_PROG_F77 (@ovar{compiler-search-list}) +@acindex{PROG_F77} +@evindex F77 +@evindex FFLAGS +@ovindex F77 +@ovindex FFLAGS +@caindex f77_compiler_gnu +@caindex prog_f77_g +Determine a Fortran 77 compiler to use. If @code{F77} is not already +set in the environment, then check for @code{g77} and @code{f77}, and +then some other names. Set the output variable @code{F77} to the name +of the compiler found. + +This macro may, however, be invoked with an optional first argument +which, if specified, must be a blank-separated list of Fortran 77 +compilers to search for. This just gives the user an opportunity to +specify an alternative search list for the Fortran 77 compiler. For +example, if you didn't like the default order, then you could invoke +@code{AC_PROG_F77} like this: + +@example +AC_PROG_F77([fl32 f77 fort77 xlf g77 f90 xlf90]) +@end example + +If using @code{g77} (the GNU Fortran 77 compiler), then +set the shell variable @code{G77} to @samp{yes}. +If the output variable @code{FFLAGS} was not already set in the +environment, then set it to @option{-g -02} for @code{g77} (or @option{-O2} +where @code{g77} does not accept @option{-g}). Otherwise, set +@code{FFLAGS} to @option{-g} for all other Fortran 77 compilers. + +The result of the GNU test is cached in the +@code{ac_cv_f77_compiler_gnu} variable, acceptance of @option{-g} in the +@code{ac_cv_prog_f77_g} variable. +@end defmac + +@defmac AC_PROG_FC (@ovar{compiler-search-list}, @ovar{dialect}) +@acindex{PROG_FC} +@evindex FC +@evindex FCFLAGS +@ovindex FC +@ovindex FCFLAGS +@caindex fc_compiler_gnu +@caindex prog_fc_g +Determine a Fortran compiler to use. If @code{FC} is not already set in +the environment, then @code{dialect} is a hint to indicate what Fortran +dialect to search for; the default is to search for the newest available +dialect. Set the output variable @code{FC} to the name of the compiler +found. + +By default, newer dialects are preferred over older dialects, but if +@code{dialect} is specified then older dialects are preferred starting +with the specified dialect. @code{dialect} can currently be one of +Fortran 77, Fortran 90, or Fortran 95. However, this is only a hint of +which compiler @emph{name} to prefer (e.g., @code{f90} or @code{f95}), +and no attempt is made to guarantee that a particular language standard +is actually supported. Thus, it is preferable that you avoid the +@code{dialect} option, and use AC_PROG_FC only for code compatible with +the latest Fortran standard. + +This macro may, alternatively, be invoked with an optional first argument +which, if specified, must be a blank-separated list of Fortran +compilers to search for, just as in @code{AC_PROG_F77}. + +If using @code{gfortran} or @code{g77} (the GNU Fortran compilers), then +set the shell variable @code{GFC} to @samp{yes}. +If the output variable @code{FCFLAGS} was not already set in the +environment, then set it to @option{-g -02} for GNU @code{g77} (or +@option{-O2} where @code{g77} does not accept @option{-g}). Otherwise, +set @code{FCFLAGS} to @option{-g} for all other Fortran compilers. + +The result of the GNU test is cached in the @code{ac_cv_fc_compiler_gnu} +variable, acceptance of @option{-g} in the @code{ac_cv_prog_fc_g} +variable. +@end defmac + +@defmac AC_PROG_F77_C_O +@defmacx AC_PROG_FC_C_O +@acindex{PROG_F77_C_O} +@acindex{PROG_FC_C_O} +@cvindex F77_NO_MINUS_C_MINUS_O +@cvindex FC_NO_MINUS_C_MINUS_O +@caindex prog_f77_c_o +@caindex prog_fc_c_o +Test whether the Fortran compiler accepts the options @option{-c} and +@option{-o} simultaneously, and define @code{F77_NO_MINUS_C_MINUS_O} or +@code{FC_NO_MINUS_C_MINUS_O}, respectively, if it does not. + +The result of the test is cached in the @code{ac_cv_prog_f77_c_o} or +@code{ac_cv_prog_fc_c_o} variable, respectively. +@end defmac + +The following macros check for Fortran compiler characteristics. +To check for characteristics not listed here, use +@code{AC_COMPILE_IFELSE} (@pxref{Running the Compiler}) or +@code{AC_RUN_IFELSE} (@pxref{Runtime}), making sure to first set the +current language to Fortran 77 or Fortran via @code{AC_LANG([Fortran 77])} +or @code{AC_LANG(Fortran)} (@pxref{Language Choice}). + + +@defmac AC_F77_LIBRARY_LDFLAGS +@defmacx AC_FC_LIBRARY_LDFLAGS +@acindex{F77_LIBRARY_LDFLAGS} +@ovindex FLIBS +@acindex{FC_LIBRARY_LDFLAGS} +@ovindex FCLIBS +@caindex prog_f77_v +@caindex prog_fc_v +@caindex f77_libs +@caindex fc_libs +Determine the linker flags (e.g., @option{-L} and @option{-l}) for the +@dfn{Fortran intrinsic and runtime libraries} that are required to +successfully link a Fortran program or shared library. The output +variable @code{FLIBS} or @code{FCLIBS} is set to these flags (which +should be included after @code{LIBS} when linking). + +This macro is intended to be used in those situations when it is +necessary to mix, e.g., C++ and Fortran source code in a single +program or shared library (@pxref{Mixing Fortran 77 With C and C++, , , +automake, GNU Automake}). + +For example, if object files from a C++ and Fortran compiler must be +linked together, then the C++ compiler/linker must be used for linking +(since special C++-ish things need to happen at link time like calling +global constructors, instantiating templates, enabling exception +support, etc.). + +However, the Fortran intrinsic and runtime libraries must be linked in +as well, but the C++ compiler/linker doesn't know by default how to add +these Fortran 77 libraries. Hence, this macro was created to determine +these Fortran libraries. + +The macros @code{AC_F77_DUMMY_MAIN} and @code{AC_FC_DUMMY_MAIN} or +@code{AC_F77_MAIN} and @code{AC_FC_MAIN} are probably also necessary to +link C/C++ with Fortran; see below. Further, it is highly recommended +that you use @code{AC_CONFIG_HEADERS} (@pxref{Configuration Headers}) +because the complex defines that the function wrapper macros create +may not work with C/C++ compiler drivers. + +These macros internally compute the flag needed to verbose linking +output and cache it in @code{ac_cv_prog_f77_v} or @code{ac_cv_prog_fc_v} +variables, respectively. The computed linker flags are cached in +@code{ac_cv_f77_libs} or @code{ac_cv_fc_libs}, respectively. +@end defmac + +@defmac AC_F77_DUMMY_MAIN (@ovar{action-if-found}, @dvar{action-if-not-found, @ + AC_MSG_FAILURE}) +@defmacx AC_FC_DUMMY_MAIN (@ovar{action-if-found}, @dvar{action-if-not-found, @ + AC_MSG_FAILURE}) +@acindex{F77_DUMMY_MAIN} +@cvindex F77_DUMMY_MAIN +@acindex{FC_DUMMY_MAIN} +@cvindex FC_DUMMY_MAIN +@caindex f77_dummy_main +@caindex fc_dummy_main +With many compilers, the Fortran libraries detected by +@code{AC_F77_LIBRARY_LDFLAGS} or @code{AC_FC_LIBRARY_LDFLAGS} provide +their own @code{main} entry function that initializes things like +Fortran I/O, and which then calls a user-provided entry function named +(say) @code{MAIN__} to run the user's program. The +@code{AC_F77_DUMMY_MAIN} and @code{AC_FC_DUMMY_MAIN} or +@code{AC_F77_MAIN} and @code{AC_FC_MAIN} macros figure out how to deal with +this interaction. + +When using Fortran for purely numerical functions (no I/O, etc.)@: often +one prefers to provide one's own @code{main} and skip the Fortran +library initializations. In this case, however, one may still need to +provide a dummy @code{MAIN__} routine in order to prevent linking errors +on some systems. @code{AC_F77_DUMMY_MAIN} or @code{AC_FC_DUMMY_MAIN} +detects whether any such routine is @emph{required} for linking, and +what its name is; the shell variable @code{F77_DUMMY_MAIN} or +@code{FC_DUMMY_MAIN} holds this name, @code{unknown} when no solution +was found, and @code{none} when no such dummy main is needed. + +By default, @var{action-if-found} defines @code{F77_DUMMY_MAIN} or +@code{FC_DUMMY_MAIN} to the name of this routine (e.g., @code{MAIN__}) +@emph{if} it is required. @var{action-if-not-found} defaults to +exiting with an error. + +In order to link with Fortran routines, the user's C/C++ program should +then include the following code to define the dummy main if it is +needed: + +@example +@c If you change this example, adjust tests/fortran.at:AC_F77_DUMMY_MAIN usage. +#ifdef F77_DUMMY_MAIN +# ifdef __cplusplus + extern "C" +# endif + int F77_DUMMY_MAIN () @{ return 1; @} +#endif +@end example + +(Replace @code{F77} with @code{FC} for Fortran instead of Fortran 77.) + +Note that this macro is called automatically from @code{AC_F77_WRAPPERS} +or @code{AC_FC_WRAPPERS}; there is generally no need to call it +explicitly unless one wants to change the default actions. + +The result of this macro is cached in the @code{ac_cv_f77_dummy_main} or +@code{ac_cv_fc_dummy_main} variable, respectively. +@end defmac + +@defmac AC_F77_MAIN +@defmacx AC_FC_MAIN +@acindex{F77_MAIN} +@cvindex F77_MAIN +@acindex{FC_MAIN} +@cvindex FC_MAIN +@caindex f77_main +@caindex fc_main +As discussed above, many Fortran libraries allow you to provide an entry +point called (say) @code{MAIN__} instead of the usual @code{main}, which +is then called by a @code{main} function in the Fortran libraries that +initializes things like Fortran I/O@. The +@code{AC_F77_MAIN} and @code{AC_FC_MAIN} macros detect whether it is +@emph{possible} to utilize such an alternate main function, and defines +@code{F77_MAIN} and @code{FC_MAIN} to the name of the function. (If no +alternate main function name is found, @code{F77_MAIN} and @code{FC_MAIN} are +simply defined to @code{main}.) + +Thus, when calling Fortran routines from C that perform things like I/O, +one should use this macro and declare the "main" function like so: + +@example +@c If you change this example, adjust tests/fortran.at:AC_F77_DUMMY_MAIN usage. +#ifdef __cplusplus + extern "C" +#endif +int F77_MAIN (int argc, char *argv[]); +@end example + +(Again, replace @code{F77} with @code{FC} for Fortran instead of Fortran 77.) + +The result of this macro is cached in the @code{ac_cv_f77_main} or +@code{ac_cv_fc_main} variable, respectively. +@end defmac + +@defmac AC_F77_WRAPPERS +@defmacx AC_FC_WRAPPERS +@acindex{F77_WRAPPERS} +@cvindex F77_FUNC +@cvindex F77_FUNC_ +@acindex{FC_WRAPPERS} +@cvindex FC_FUNC +@cvindex FC_FUNC_ +@caindex f77_mangling +@caindex fc_mangling +Defines C macros @code{F77_FUNC (name, NAME)}, @code{FC_FUNC (name, NAME)}, +@code{F77_FUNC_(name, NAME)}, and @code{FC_FUNC_(name, NAME)} to properly +mangle the names of C/C++ identifiers, and identifiers with underscores, +respectively, so that they match the name-mangling scheme used by the +Fortran compiler. + +Fortran is case-insensitive, and in order to achieve this the Fortran +compiler converts all identifiers into a canonical case and format. To +call a Fortran subroutine from C or to write a C function that is +callable from Fortran, the C program must explicitly use identifiers in +the format expected by the Fortran compiler. In order to do this, one +simply wraps all C identifiers in one of the macros provided by +@code{AC_F77_WRAPPERS} or @code{AC_FC_WRAPPERS}. For example, suppose +you have the following Fortran 77 subroutine: + +@example +@c If you change this example, adjust tests/fortran.at:AC_F77_DUMMY_MAIN usage. + subroutine foobar (x, y) + double precision x, y + y = 3.14159 * x + return + end +@end example + +You would then declare its prototype in C or C++ as: + +@example +@c If you change this example, adjust tests/fortran.at:AC_F77_DUMMY_MAIN usage. +#define FOOBAR_F77 F77_FUNC (foobar, FOOBAR) +#ifdef __cplusplus +extern "C" /* prevent C++ name mangling */ +#endif +void FOOBAR_F77 (double *x, double *y); +@end example + +Note that we pass both the lowercase and uppercase versions of the +function name to @code{F77_FUNC} so that it can select the right one. +Note also that all parameters to Fortran 77 routines are passed as +pointers (@pxref{Mixing Fortran 77 With C and C++, , , automake, GNU +Automake}). + +(Replace @code{F77} with @code{FC} for Fortran instead of Fortran 77.) + +Although Autoconf tries to be intelligent about detecting the +name-mangling scheme of the Fortran compiler, there may be Fortran +compilers that it doesn't support yet. In this case, the above code +generates a compile-time error, but some other behavior +(e.g., disabling Fortran-related features) can be induced by checking +whether @code{F77_FUNC} or @code{FC_FUNC} is defined. + +Now, to call that routine from a C program, we would do something like: + +@example +@c If you change this example, adjust tests/fortran.at:AC_F77_DUMMY_MAIN usage. +@{ + double x = 2.7183, y; + FOOBAR_F77 (&x, &y); +@} +@end example + +If the Fortran identifier contains an underscore (e.g., @code{foo_bar}), +you should use @code{F77_FUNC_} or @code{FC_FUNC_} instead of +@code{F77_FUNC} or @code{FC_FUNC} (with the same arguments). This is +because some Fortran compilers mangle names differently if they contain +an underscore. + +The name mangling scheme is encoded in the @code{ac_cv_f77_mangling} or +@code{ac_cv_fc_mangling} cache variable, respectively, and also used for +the @code{AC_F77_FUNC} and @code{AC_FC_FUNC} macros described below. +@end defmac + +@defmac AC_F77_FUNC (@var{name}, @ovar{shellvar}) +@defmacx AC_FC_FUNC (@var{name}, @ovar{shellvar}) +@acindex{F77_FUNC} +@acindex{FC_FUNC} +Given an identifier @var{name}, set the shell variable @var{shellvar} to +hold the mangled version @var{name} according to the rules of the +Fortran linker (see also @code{AC_F77_WRAPPERS} or +@code{AC_FC_WRAPPERS}). @var{shellvar} is optional; if it is not +supplied, the shell variable is simply @var{name}. The purpose of +this macro is to give the caller a way to access the name-mangling +information other than through the C preprocessor as above, for example, +to call Fortran routines from some language other than C/C++. +@end defmac + +@defmac AC_FC_SRCEXT (@var{ext}, @ovar{action-if-success}, @ + @dvar{action-if-failure, AC_MSG_FAILURE}) +@defmacx AC_FC_PP_SRCEXT (@var{ext}, @ovar{action-if-success}, @ + @dvar{action-if-failure, AC_MSG_FAILURE}) +@acindex{FC_SRCEXT} +@acindex{FC_PP_SRCEXT} +@caindex fc_srcext_@var{ext} +@caindex fc_pp_srcext_@var{ext} +By default, the @code{FC} macros perform their tests using a @file{.f} +extension for source-code files. Some compilers, however, only enable +newer language features for appropriately named files, e.g., Fortran 90 +features only for @file{.f90} files, or preprocessing only with +@file{.F} files or maybe other upper-case extensions. On the other +hand, some other compilers expect all source files to end in @file{.f} +and require special flags to support other file name extensions. The +@code{AC_FC_SRCEXT} and @code{AC_FC_PP_SRCEXT} macros deal with these +issues. + +The @code{AC_FC_SRCEXT} macro tries to get the @code{FC} compiler to +accept files ending with the extension @file{.@var{ext}} (i.e., +@var{ext} does @emph{not} contain the dot). If any special compiler +flags are needed for this, it stores them in the output variable +@code{FCFLAGS_@var{ext}}. This extension and these flags are then used +for all subsequent @code{FC} tests (until @code{AC_FC_SRCEXT} or +@code{AC_FC_PP_SRCEXT} is called another time). + +For example, you would use @code{AC_FC_SRCEXT(f90)} to employ the +@file{.f90} extension in future tests, and it would set the +@code{FCFLAGS_f90} output variable with any extra flags that are needed +to compile such files. + +Similarly, the @code{AC_FC_PP_SRCEXT} macro tries to get the @code{FC} +compiler to preprocess and compile files with the extension +@file{.@var{ext}}. When both @command{fpp} and @command{cpp} style +preprocessing are provided, the former is preferred, as the latter may +treat continuation lines, @code{//} tokens, and white space differently +from what some Fortran dialects expect. Conversely, if you do not want +files to be preprocessed, use only lower-case characters in the file +name extension. Like with @code{AC_FC_SRCEXT(f90)}, any needed flags +are stored in the @code{FCFLAGS_@var{ext}} variable. + +The @code{FCFLAGS_@var{ext}} flags can @emph{not} be simply absorbed +into @code{FCFLAGS}, for two reasons based on the limitations of some +compilers. First, only one @code{FCFLAGS_@var{ext}} can be used at a +time, so files with different extensions must be compiled separately. +Second, @code{FCFLAGS_@var{ext}} must appear @emph{immediately} before +the source-code file name when compiling. So, continuing the example +above, you might compile a @file{foo.f90} file in your makefile with the +command: + +@example +foo.o: foo.f90 + $(FC) -c $(FCFLAGS) $(FCFLAGS_f90) '$(srcdir)/foo.f90' +@end example + +If @code{AC_FC_SRCEXT} or @code{AC_FC_PP_SRCEXT} succeeds in compiling +files with the @var{ext} extension, it calls @var{action-if-success} +(defaults to nothing). If it fails, and cannot find a way to make the +@code{FC} compiler accept such files, it calls @var{action-if-failure} +(defaults to exiting with an error message). + +The @code{AC_FC_SRCEXT} and @code{AC_FC_PP_SRCEXT} macros cache their +results in @code{ac_cv_fc_srcext_@var{ext}} and +@code{ac_cv_fc_pp_srcext_@var{ext}} variables, respectively. +@end defmac + +@defmac AC_FC_PP_DEFINE (@ovar{action-if-success}, @dvar{action-if-failure, @ + AC_MSG_FAILURE}) +@acindex{FC_PP_DEFINE} +@caindex fc_pp_define + +Find a flag to specify defines for preprocessed Fortran. Not all +Fortran compilers use @option{-D}. Substitute @code{FC_DEFINE} with +the result and call @var{action-if-success} (defaults to nothing) if +successful, and @var{action-if-failure} (defaults to failing with an +error message) if not. + +This macro calls @code{AC_FC_PP_SRCEXT([F])} in order to learn how to +preprocess a @file{conftest.F} file, but restores a previously used +Fortran source file extension afterwards again. + +The result of this test is cached in the @code{ac_cv_fc_pp_define} +variable. +@end defmac + +@defmac AC_FC_FREEFORM (@ovar{action-if-success}, @dvar{action-if-failure, @ + AC_MSG_FAILURE}) +@acindex{FC_FREEFORM} +@caindex fc_freeform + +Try to ensure that the Fortran compiler (@code{$FC}) allows free-format +source code (as opposed to the older fixed-format style from Fortran +77). If necessary, it may add some additional flags to @code{FCFLAGS}. + +This macro is most important if you are using the default @file{.f} +extension, since many compilers interpret this extension as indicating +fixed-format source unless an additional flag is supplied. If you +specify a different extension with @code{AC_FC_SRCEXT}, such as +@file{.f90}, then @code{AC_FC_FREEFORM} ordinarily succeeds without +modifying @code{FCFLAGS}. For extensions which the compiler does not +know about, the flag set by the @code{AC_FC_SRCEXT} macro might let +the compiler assume Fortran 77 by default, however. + +If @code{AC_FC_FREEFORM} succeeds in compiling free-form source, it +calls @var{action-if-success} (defaults to nothing). If it fails, it +calls @var{action-if-failure} (defaults to exiting with an error +message). + +The result of this test, or @samp{none} or @samp{unknown}, is cached in +the @code{ac_cv_fc_freeform} variable. +@end defmac + +@defmac AC_FC_FIXEDFORM (@ovar{action-if-success}, @dvar{action-if-failure, @ + AC_MSG_FAILURE}) +@acindex{FC_FIXEDFORM} +@caindex fc_fixedform + +Try to ensure that the Fortran compiler (@code{$FC}) allows the old +fixed-format source code (as opposed to free-format style). If +necessary, it may add some additional flags to @code{FCFLAGS}. + +This macro is needed for some compilers alias names like @command{xlf95} +which assume free-form source code by default, and in case you want to +use fixed-form source with an extension like @file{.f90} which many +compilers interpret as free-form by default. If you specify a different +extension with @code{AC_FC_SRCEXT}, such as @file{.f}, then +@code{AC_FC_FIXEDFORM} ordinarily succeeds without modifying +@code{FCFLAGS}. + +If @code{AC_FC_FIXEDFORM} succeeds in compiling fixed-form source, it +calls @var{action-if-success} (defaults to nothing). If it fails, it +calls @var{action-if-failure} (defaults to exiting with an error +message). + +The result of this test, or @samp{none} or @samp{unknown}, is cached in +the @code{ac_cv_fc_fixedform} variable. +@end defmac + +@defmac AC_FC_LINE_LENGTH (@ovar{length}, @ovar{action-if-success}, @ + @dvar{action-if-failure, AC_MSG_FAILURE}) +@acindex{FC_LINE_LENGTH} +@caindex fc_line_length + +Try to ensure that the Fortran compiler (@code{$FC}) accepts long source +code lines. The @var{length} argument may be given as 80, 132, or +unlimited, and defaults to 132. Note that line lengths above 254 +columns are not portable, and some compilers do not accept more than 132 +columns at least for fixed format source. If necessary, it may add some +additional flags to @code{FCFLAGS}. + +If @code{AC_FC_LINE_LENGTH} succeeds in compiling fixed-form source, it +calls @var{action-if-success} (defaults to nothing). If it fails, it +calls @var{action-if-failure} (defaults to exiting with an error +message). + +The result of this test, or @samp{none} or @samp{unknown}, is cached in +the @code{ac_cv_fc_line_length} variable. +@end defmac + +@defmac AC_FC_CHECK_BOUNDS (@ovar{action-if-success}, @ + @dvar{action-if-failure, AC_MSG_FAILURE}) +@acindex{FC_CHECK_BOUNDS} +@caindex fc_check_bounds + +The @code{AC_FC_CHECK_BOUNDS} macro tries to enable array bounds checking +in the Fortran compiler. If successful, the @var{action-if-success} +is called and any needed flags are added to @code{FCFLAGS}. Otherwise, +@var{action-if-failure} is called, which defaults to failing with an error +message. The macro currently requires Fortran 90 or a newer dialect. + +The result of the macro is cached in the @code{ac_cv_fc_check_bounds} +variable. +@end defmac + +@defmac AC_F77_IMPLICIT_NONE (@ovar{action-if-success}, @ + @dvar{action-if-failure, AC_MSG_FAILURE}) +@defmacx AC_FC_IMPLICIT_NONE (@ovar{action-if-success}, @ + @dvar{action-if-failure, AC_MSG_FAILURE}) +@acindex{F77_IMPLICIT_NONE} +@acindex{FC_IMPLICIT_NONE} +@caindex f77_implicit_none +@caindex fc_implicit_none + +Try to disallow implicit declarations in the Fortran compiler. If +successful, @var{action-if-success} is called and any needed flags +are added to @code{FFLAGS} or @code{FCFLAGS}, respectively. Otherwise, +@var{action-if-failure} is called, which defaults to failing with an error +message. + +The result of these macros are cached in the +@code{ac_cv_f77_implicit_none} and @code{ac_cv_fc_implicit_none} +variables, respectively. +@end defmac + +@defmac AC_FC_MODULE_EXTENSION +@acindex{FC_MODULE_EXTENSION} +@caindex fc_module_ext +@ovindex FC_MODEXT + +Find the Fortran 90 module file name extension. Most Fortran 90 +compilers store module information in files separate from the object +files. The module files are usually named after the name of the module +rather than the source file name, with characters possibly turned to +upper case, plus an extension, often @file{.mod}. + +Not all compilers use module files at all, or by default. The Cray +Fortran compiler requires @option{-e m} in order to store and search +module information in @file{.mod} files rather than in object files. +Likewise, the Fujitsu Fortran compilers uses the @option{-Am} option to +indicate how module information is stored. + +The @code{AC_FC_MODULE_EXTENSION} macro computes the module extension +without the leading dot, and stores that in the @code{FC_MODEXT} +variable. If the compiler does not produce module files, or the +extension cannot be determined, @code{FC_MODEXT} is empty. Typically, +the result of this macro may be used in cleanup @command{make} rules as +follows: + +@example +clean-modules: + -test -z "$(FC_MODEXT)" || rm -f *.$(FC_MODEXT) +@end example + +The extension, or @samp{unknown}, is cached in the +@code{ac_cv_fc_module_ext} variable. +@end defmac + +@defmac AC_FC_MODULE_FLAG (@ovar{action-if-success}, @ + @dvar{action-if-failure, AC_MSG_FAILURE}) +@acindex{FC_MODULE_FLAG} +@caindex fc_module_flag +@ovindex FC_MODINC +@ovindex ac_empty + +Find the compiler flag to include Fortran 90 module information from +another directory, and store that in the @code{FC_MODINC} variable. +Call @var{action-if-success} (defaults to nothing) if successful, and +set @code{FC_MODINC} to empty and call @var{action-if-failure} (defaults +to exiting with an error message) if not. + +Most Fortran 90 compilers provide a way to specify module directories. +Some have separate flags for the directory to write module files to, +and directories to search them in, whereas others only allow writing to +the current directory or to the first directory specified in the include +path. Further, with some compilers, the module search path and the +preprocessor search path can only be modified with the same flag. Thus, +for portability, write module files to the current directory only and +list that as first directory in the search path. + +There may be no whitespace between @code{FC_MODINC} and the following +directory name, but @code{FC_MODINC} may contain trailing white space. +For example, if you use Automake and would like to search @file{../lib} +for module files, you can use the following: + +@example +AM_FCFLAGS = $(FC_MODINC). $(FC_MODINC)../lib +@end example + +Inside @command{configure} tests, you can use: + +@example +if test -n "$FC_MODINC"; then + FCFLAGS="$FCFLAGS $FC_MODINC. $FC_MODINC../lib" +fi +@end example + +The flag is cached in the @code{ac_cv_fc_module_flag} variable. +The substituted value of @code{FC_MODINC} may refer to the +@code{ac_empty} dummy placeholder empty variable, to avoid losing +the significant trailing whitespace in a @file{Makefile}. +@end defmac + +@defmac AC_FC_MODULE_OUTPUT_FLAG (@ovar{action-if-success}, @ + @dvar{action-if-failure, AC_MSG_FAILURE}) +@acindex{FC_MODULE_OUTPUT_FLAG} +@caindex fc_module_output_flag +@ovindex FC_MODOUT + +Find the compiler flag to write Fortran 90 module information to +another directory, and store that in the @code{FC_MODOUT} variable. +Call @var{action-if-success} (defaults to nothing) if successful, and +set @code{FC_MODOUT} to empty and call @var{action-if-failure} (defaults +to exiting with an error message) if not. + +Not all Fortran 90 compilers write module files, and of those that do, +not all allow writing to a directory other than the current one, nor +do all have separate flags for writing and reading; see the description +of @code{AC_FC_MODULE_FLAG} above. If you need to be able to write to +another directory, for maximum portability use @code{FC_MODOUT} before +any @code{FC_MODINC} and include both the current directory and the one +you write to in the search path: + +@example +AM_FCFLAGS = $(FC_MODOUT)../mod $(FC_MODINC)../mod $(FC_MODINC). @dots{} +@end example + +The flag is cached in the @code{ac_cv_fc_module_output_flag} variable. +The substituted value of @code{FC_MODOUT} may refer to the +@code{ac_empty} dummy placeholder empty variable, to avoid losing +the significant trailing whitespace in a @file{Makefile}. +@end defmac + + +@node Go Compiler +@subsection Go Compiler Characteristics +@cindex Go + +Autoconf provides basic support for the Go programming language when +using the @code{gccgo} compiler (there is currently no support for the +@code{6g} and @code{8g} compilers). + +@defmac AC_PROG_GO (@ovar{compiler-search-list}) +Find the Go compiler to use. Check whether the environment variable +@code{GOC} is set; if so, then set output variable @code{GOC} to its +value. + +Otherwise, if the macro is invoked without an argument, then search for +a Go compiler named @code{gccgo}. If it is not found, then as a last +resort set @code{GOC} to @code{gccgo}. + +This macro may be invoked with an optional first argument which, if +specified, must be a blank-separated list of Go compilers to search for. + +If output variable @code{GOFLAGS} was not already set, set it to +@option{-g -O2}. If your package does not like this default, +@code{GOFLAGS} may be set before @code{AC_PROG_GO}. +@end defmac + + +@node System Services +@section System Services + +The following macros check for operating system services or capabilities. + +@anchor{AC_PATH_X} +@defmac AC_PATH_X +@acindex{PATH_X} +@evindex XMKMF +@cindex X Window System +Try to locate the X Window System include files and libraries. If the +user gave the command line options @option{--x-includes=@var{dir}} and +@option{--x-libraries=@var{dir}}, use those directories. + +If either or both were not given, get the missing values by running +@code{xmkmf} (or an executable pointed to by the @code{XMKMF} +environment variable) on a trivial @file{Imakefile} and examining the +makefile that it produces. Setting @code{XMKMF} to @samp{false} +disables this method. + +If this method fails to find the X Window System, @command{configure} +looks for the files in several directories where they often reside. +If either method is successful, set the shell variables +@code{x_includes} and @code{x_libraries} to their locations, unless they +are in directories the compiler searches by default. + +If both methods fail, or the user gave the command line option +@option{--without-x}, set the shell variable @code{no_x} to @samp{yes}; +otherwise set it to the empty string. +@end defmac + +@anchor{AC_PATH_XTRA} +@defmac AC_PATH_XTRA +@acindex{PATH_XTRA} +@ovindex X_CFLAGS +@ovindex X_LIBS +@ovindex X_EXTRA_LIBS +@ovindex X_PRE_LIBS +@cvindex X_DISPLAY_MISSING +An enhanced version of @code{AC_PATH_X}. It adds the C compiler flags +that X needs to output variable @code{X_CFLAGS}, and the X linker flags +to @code{X_LIBS}. Define @code{X_DISPLAY_MISSING} if X is not +available. + +This macro also checks for special libraries that some systems need in +order to compile X programs. It adds any that the system needs to +output variable @code{X_EXTRA_LIBS}. And it checks for special X11R6 +libraries that need to be linked with before @option{-lX11}, and adds +any found to the output variable @code{X_PRE_LIBS}. + +@c This is an incomplete kludge. Make a real way to do it. +@c If you need to check for other X functions or libraries yourself, then +@c after calling this macro, add the contents of @code{X_EXTRA_LIBS} to +@c @code{LIBS} temporarily, like this: (FIXME - add example) +@end defmac + +@anchor{AC_SYS_INTERPRETER} +@defmac AC_SYS_INTERPRETER +@acindex{SYS_INTERPRETER} +Check whether the system supports starting scripts with a line of the +form @samp{#!/bin/sh} to select the interpreter to use for the script. +After running this macro, shell code in @file{configure.ac} can check +the shell variable @code{interpval}; it is set to @samp{yes} +if the system supports @samp{#!}, @samp{no} if not. +@end defmac + +@defmac AC_SYS_LARGEFILE +@acindex{SYS_LARGEFILE} +@cvindex _FILE_OFFSET_BITS +@cvindex _LARGE_FILES +@ovindex CC +@cindex Large file support +@cindex LFS +Arrange for 64-bit file offsets, known as +@uref{http://@/www.unix-systems@/.org/@/version2/@/whatsnew/@/lfs20mar.html, +large-file support}. On some hosts, one must use special compiler +options to build programs that can access large files. Append any such +options to the output variable @code{CC}. Define +@code{_FILE_OFFSET_BITS} and @code{_LARGE_FILES} if necessary. + +Large-file support can be disabled by configuring with the +@option{--disable-largefile} option. + +If you use this macro, check that your program works even when +@code{off_t} is wider than @code{long int}, since this is common when +large-file support is enabled. For example, it is not correct to print +an arbitrary @code{off_t} value @code{X} with @code{printf ("%ld", +(long int) X)}. + +The LFS introduced the @code{fseeko} and @code{ftello} functions to +replace their C counterparts @code{fseek} and @code{ftell} that do not +use @code{off_t}. Take care to use @code{AC_FUNC_FSEEKO} to make their +prototypes available when using them and large-file support is +enabled. +@end defmac + +@anchor{AC_SYS_LONG_FILE_NAMES} +@defmac AC_SYS_LONG_FILE_NAMES +@acindex{SYS_LONG_FILE_NAMES} +@cvindex HAVE_LONG_FILE_NAMES +If the system supports file names longer than 14 characters, define +@code{HAVE_LONG_FILE_NAMES}. +@end defmac + +@defmac AC_SYS_POSIX_TERMIOS +@acindex{SYS_POSIX_TERMIOS} +@cindex Posix termios headers +@cindex termios Posix headers +@caindex sys_posix_termios +Check to see if the Posix termios headers and functions are available on the +system. If so, set the shell variable @code{ac_cv_sys_posix_termios} to +@samp{yes}. If not, set the variable to @samp{no}. +@end defmac + +@node Posix Variants +@section Posix Variants + +The following macro makes it possible to use features of Posix that are +extensions to C, as well as platform extensions not defined by Posix. + +@anchor{AC_USE_SYSTEM_EXTENSIONS} +@defmac AC_USE_SYSTEM_EXTENSIONS +@acindex{USE_SYSTEM_EXTENSIONS} +@cvindex _ALL_SOURCE +@cvindex _GNU_SOURCE +@cvindex _MINIX +@cvindex _POSIX_1_SOURCE +@cvindex _POSIX_PTHREAD_SEMANTICS +@cvindex _POSIX_SOURCE +@cvindex _TANDEM_SOURCE +@cvindex __EXTENSIONS__ +This macro was introduced in Autoconf 2.60. If possible, enable +extensions to C or Posix on hosts that normally disable the extensions, +typically due to standards-conformance namespace issues. This should be +called before any macros that run the C compiler. The following +preprocessor macros are defined where appropriate: + +@table @code +@item _GNU_SOURCE +Enable extensions on GNU/Linux. +@item __EXTENSIONS__ +Enable general extensions on Solaris. +@item _POSIX_PTHREAD_SEMANTICS +Enable threading extensions on Solaris. +@item _TANDEM_SOURCE +Enable extensions for the HP NonStop platform. +@item _ALL_SOURCE +Enable extensions for AIX 3, and for Interix. +@item _POSIX_SOURCE +Enable Posix functions for Minix. +@item _POSIX_1_SOURCE +Enable additional Posix functions for Minix. +@item _MINIX +Identify Minix platform. This particular preprocessor macro is +obsolescent, and may be removed in a future release of Autoconf. +@end table +@end defmac + + +@node Erlang Libraries +@section Erlang Libraries +@cindex Erlang, Library, checking + +The following macros check for an installation of Erlang/OTP, and for the +presence of certain Erlang libraries. All those macros require the +configuration of an Erlang interpreter and an Erlang compiler +(@pxref{Erlang Compiler and Interpreter}). + +@defmac AC_ERLANG_SUBST_ERTS_VER +@acindex{ERLANG_SUBST_ERTS_VER} +@ovindex ERLANG_ERTS_VER +Set the output variable @code{ERLANG_ERTS_VER} to the version of the +Erlang runtime system (as returned by Erlang's +@code{erlang:system_info(version)} function). The result of this test +is cached if caching is enabled when running @command{configure}. The +@code{ERLANG_ERTS_VER} variable is not intended to be used for testing +for features of specific ERTS versions, but to be used for substituting +the ERTS version in Erlang/OTP release resource files (@code{.rel} +files), as shown below. +@end defmac + +@defmac AC_ERLANG_SUBST_ROOT_DIR +@acindex{ERLANG_SUBST_ROOT_DIR} +@ovindex ERLANG_ROOT_DIR +Set the output variable @code{ERLANG_ROOT_DIR} to the path to the base +directory in which Erlang/OTP is installed (as returned by Erlang's +@code{code:root_dir/0} function). The result of this test is cached if +caching is enabled when running @command{configure}. +@end defmac + +@defmac AC_ERLANG_SUBST_LIB_DIR +@acindex{ERLANG_SUBST_LIB_DIR} +@ovindex ERLANG_LIB_DIR +Set the output variable @code{ERLANG_LIB_DIR} to the path of the library +directory of Erlang/OTP (as returned by Erlang's +@code{code:lib_dir/0} function), which subdirectories each contain an installed +Erlang/OTP library. The result of this test is cached if caching is enabled +when running @command{configure}. +@end defmac + +@defmac AC_ERLANG_CHECK_LIB (@var{library}, @ovar{action-if-found}, @ + @ovar{action-if-not-found}) +@acindex{ERLANG_CHECK_LIB} +@ovindex ERLANG_LIB_DIR_@var{library} +@ovindex ERLANG_LIB_VER_@var{library} +Test whether the Erlang/OTP library @var{library} is installed by +calling Erlang's @code{code:lib_dir/1} function. The result of this +test is cached if caching is enabled when running @command{configure}. +@var{action-if-found} is a list of shell commands to run if the library +is installed; @var{action-if-not-found} is a list of shell commands to +run if it is not. Additionally, if the library is installed, the output +variable @samp{ERLANG_LIB_DIR_@var{library}} is set to the path to the +library installation directory, and the output variable +@samp{ERLANG_LIB_VER_@var{library}} is set to the version number that is +part of the subdirectory name, if it is in the standard form +(@code{@var{library}-@var{version}}). If the directory name does not +have a version part, @samp{ERLANG_LIB_VER_@var{library}} is set to the +empty string. If the library is not installed, +@samp{ERLANG_LIB_DIR_@var{library}} and +@samp{ERLANG_LIB_VER_@var{library}} are set to @code{"not found"}. For +example, to check if library @code{stdlib} is installed: + +@example +AC_ERLANG_CHECK_LIB([stdlib], + [echo "stdlib version \"$ERLANG_LIB_VER_stdlib\"" + echo "is installed in \"$ERLANG_LIB_DIR_stdlib\""], + [AC_MSG_ERROR([stdlib was not found!])]) +@end example + +The @samp{ERLANG_LIB_VER_@var{library}} variables (set by +@code{AC_ERLANG_CHECK_LIB}) and the @code{ERLANG_ERTS_VER} variable (set +by @code{AC_ERLANG_SUBST_ERTS_VER}) are not intended to be used for +testing for features of specific versions of libraries or of the Erlang +runtime system. Those variables are intended to be substituted in +Erlang release resource files (@code{.rel} files). For instance, to +generate a @file{example.rel} file for an application depending on the +@code{stdlib} library, @file{configure.ac} could contain: + +@example +AC_ERLANG_SUBST_ERTS_VER +AC_ERLANG_CHECK_LIB([stdlib], + [], + [AC_MSG_ERROR([stdlib was not found!])]) +AC_CONFIG_FILES([example.rel]) +@end example + +@noindent +The @file{example.rel.in} file used to generate @file{example.rel} +should contain: + +@example +@{release, + @{"@@PACKAGE@@", "@@VERSION@@"@}, + @{erts, "@@ERLANG_ERTS_VER@@"@}, + [@{stdlib, "@@ERLANG_LIB_VER_stdlib@@"@}, + @{@@PACKAGE@@, "@@VERSION@@"@}]@}. +@end example +@end defmac + +In addition to the above macros, which test installed Erlang libraries, the +following macros determine the paths to the directories into which newly built +Erlang libraries are to be installed: + +@defmac AC_ERLANG_SUBST_INSTALL_LIB_DIR +@acindex{ERLANG_SUBST_INSTALL_LIB_DIR} +@ovindex ERLANG_INSTALL_LIB_DIR + +Set the @code{ERLANG_INSTALL_LIB_DIR} output variable to the directory into +which every built Erlang library should be installed in a separate +subdirectory. +If this variable is not set in the environment when @command{configure} runs, +its default value is @code{$@{libdir@}/erlang/lib}. +@end defmac + +@defmac AC_ERLANG_SUBST_INSTALL_LIB_SUBDIR (@var{library}, @var{version}) +@acindex{ERLANG_SUBST_INSTALL_LIB_SUBDIR} +@ovindex ERLANG_INSTALL_LIB_DIR_@var{library} + +Set the @samp{ERLANG_INSTALL_LIB_DIR_@var{library}} output variable to the +directory into which the built Erlang library @var{library} version +@var{version} should be installed. If this variable is not set in the +environment when @command{configure} runs, its default value is +@samp{$ERLANG_INSTALL_LIB_DIR/@var{library}-@var{version}}, the value of the +@code{ERLANG_INSTALL_LIB_DIR} variable being set by the +@code{AC_ERLANG_SUBST_INSTALL_LIB_DIR} macro. +@end defmac + + + + + +@c ========================================================= Writing Tests + +@node Writing Tests +@chapter Writing Tests + +If the existing feature tests don't do something you need, you have to +write new ones. These macros are the building blocks. They provide +ways for other macros to check whether various kinds of features are +available and report the results. + +This chapter contains some suggestions and some of the reasons why the +existing tests are written the way they are. You can also learn a lot +about how to write Autoconf tests by looking at the existing ones. If +something goes wrong in one or more of the Autoconf tests, this +information can help you understand the assumptions behind them, which +might help you figure out how to best solve the problem. + +These macros check the output of the compiler system of the current +language (@pxref{Language Choice}). They do not cache the results of +their tests for future use (@pxref{Caching Results}), because they don't +know enough about the information they are checking for to generate a +cache variable name. They also do not print any messages, for the same +reason. The checks for particular kinds of features call these macros +and do cache their results and print messages about what they're +checking for. + +When you write a feature test that could be applicable to more than one +software package, the best thing to do is encapsulate it in a new macro. +@xref{Writing Autoconf Macros}, for how to do that. + +@menu +* Language Choice:: Selecting which language to use for testing +* Writing Test Programs:: Forging source files for compilers +* Running the Preprocessor:: Detecting preprocessor symbols +* Running the Compiler:: Detecting language or header features +* Running the Linker:: Detecting library features +* Runtime:: Testing for runtime features +* Systemology:: A zoology of operating systems +* Multiple Cases:: Tests for several possible values +@end menu + +@node Language Choice +@section Language Choice +@cindex Language + +Autoconf-generated @command{configure} scripts check for the C compiler and +its features by default. Packages that use other programming languages +(maybe more than one, e.g., C and C++) need to test features of the +compilers for the respective languages. The following macros determine +which programming language is used in the subsequent tests in +@file{configure.ac}. + +@anchor{AC_LANG} +@defmac AC_LANG (@var{language}) +@acindex{LANG} +Do compilation tests using the compiler, preprocessor, and file +extensions for the specified @var{language}. + +Supported languages are: + +@table @samp +@item C +Do compilation tests using @code{CC} and @code{CPP} and use extension +@file{.c} for test programs. Use compilation flags: @code{CPPFLAGS} with +@code{CPP}, and both @code{CPPFLAGS} and @code{CFLAGS} with @code{CC}. + +@item C++ +Do compilation tests using @code{CXX} and @code{CXXCPP} and use +extension @file{.C} for test programs. Use compilation flags: +@code{CPPFLAGS} with @code{CXXCPP}, and both @code{CPPFLAGS} and +@code{CXXFLAGS} with @code{CXX}. + +@item Fortran 77 +Do compilation tests using @code{F77} and use extension @file{.f} for +test programs. Use compilation flags: @code{FFLAGS}. + +@item Fortran +Do compilation tests using @code{FC} and use extension @file{.f} (or +whatever has been set by @code{AC_FC_SRCEXT}) for test programs. Use +compilation flags: @code{FCFLAGS}. + +@item Erlang +@ovindex ERLC +@ovindex ERL +@ovindex ERLCFLAGS +Compile and execute tests using @code{ERLC} and @code{ERL} and use extension +@file{.erl} for test Erlang modules. Use compilation flags: @code{ERLCFLAGS}. + +@item Objective C +Do compilation tests using @code{OBJC} and @code{OBJCPP} and use +extension @file{.m} for test programs. Use compilation flags: +@code{CPPFLAGS} with @code{OBJCPP}, and both @code{CPPFLAGS} and +@code{OBJCFLAGS} with @code{OBJC}. + +@item Objective C++ +Do compilation tests using @code{OBJCXX} and @code{OBJCXXCPP} and use +extension @file{.mm} for test programs. Use compilation flags: +@code{CPPFLAGS} with @code{OBJCXXCPP}, and both @code{CPPFLAGS} and +@code{OBJCXXFLAGS} with @code{OBJCXX}. + +@item Go +Do compilation tests using @code{GOC} and use extension @file{.go} for +test programs. Use compilation flags @code{GOFLAGS}. +@end table +@end defmac + +@anchor{AC_LANG_PUSH} +@defmac AC_LANG_PUSH (@var{language}) +@acindex{LANG_PUSH} +Remember the current language (as set by @code{AC_LANG}) on a stack, and +then select the @var{language}. Use this macro and @code{AC_LANG_POP} +in macros that need to temporarily switch to a particular language. +@end defmac + +@defmac AC_LANG_POP (@ovar{language}) +@acindex{LANG_POP} +Select the language that is saved on the top of the stack, as set by +@code{AC_LANG_PUSH}, and remove it from the stack. + +If given, @var{language} specifies the language we just @emph{quit}. It +is a good idea to specify it when it's known (which should be the +case@dots{}), since Autoconf detects inconsistencies. + +@example +AC_LANG_PUSH([Fortran 77]) +# Perform some tests on Fortran 77. +# @dots{} +AC_LANG_POP([Fortran 77]) +@end example +@end defmac + +@defmac AC_LANG_ASSERT (@var{language}) +@acindex{LANG_ASSERT} +Check statically that the current language is @var{language}. +You should use this in your language specific macros +to avoid that they be called with an inappropriate language. + +This macro runs only at @command{autoconf} time, and incurs no cost at +@command{configure} time. Sadly enough and because Autoconf is a two +layer language @footnote{Because M4 is not aware of Sh code, +especially conditionals, some optimizations that look nice statically +may produce incorrect results at runtime.}, the macros +@code{AC_LANG_PUSH} and @code{AC_LANG_POP} cannot be ``optimizing'', +therefore as much as possible you ought to avoid using them to wrap +your code, rather, require from the user to run the macro with a +correct current language, and check it with @code{AC_LANG_ASSERT}. +And anyway, that may help the user understand she is running a Fortran +macro while expecting a result about her Fortran 77 compiler@enddots{} +@end defmac + + +@defmac AC_REQUIRE_CPP +@acindex{REQUIRE_CPP} +Ensure that whichever preprocessor would currently be used for tests has +been found. Calls @code{AC_REQUIRE} (@pxref{Prerequisite Macros}) with an +argument of either @code{AC_PROG_CPP} or @code{AC_PROG_CXXCPP}, +depending on which language is current. +@end defmac + + +@node Writing Test Programs +@section Writing Test Programs + +Autoconf tests follow a common scheme: feed some program with some +input, and most of the time, feed a compiler with some source file. +This section is dedicated to these source samples. + +@menu +* Guidelines:: General rules for writing test programs +* Test Functions:: Avoiding pitfalls in test programs +* Generating Sources:: Source program boilerplate +@end menu + +@node Guidelines +@subsection Guidelines for Test Programs + +The most important rule to follow when writing testing samples is: + +@center @emph{Look for realism.} + +This motto means that testing samples must be written with the same +strictness as real programs are written. In particular, you should +avoid ``shortcuts'' and simplifications. + +Don't just play with the preprocessor if you want to prepare a +compilation. For instance, using @command{cpp} to check whether a header is +functional might let your @command{configure} accept a header which +causes some @emph{compiler} error. Do not hesitate to check a header with +other headers included before, especially required headers. + +Make sure the symbols you use are properly defined, i.e., refrain from +simply declaring a function yourself instead of including the proper +header. + +Test programs should not write to standard output. They +should exit with status 0 if the test succeeds, and with status 1 +otherwise, so that success +can be distinguished easily from a core dump or other failure; +segmentation violations and other failures produce a nonzero exit +status. Unless you arrange for @code{exit} to be declared, test +programs should @code{return}, not @code{exit}, from @code{main}, +because on many systems @code{exit} is not declared by default. + +Test programs can use @code{#if} or @code{#ifdef} to check the values of +preprocessor macros defined by tests that have already run. For +example, if you call @code{AC_HEADER_STDBOOL}, then later on in +@file{configure.ac} you can have a test program that includes +@file{stdbool.h} conditionally: + +@example +@group +#ifdef HAVE_STDBOOL_H +# include <stdbool.h> +#endif +@end group +@end example + +Both @code{#if HAVE_STDBOOL_H} and @code{#ifdef HAVE_STDBOOL_H} will +work with any standard C compiler. Some developers prefer @code{#if} +because it is easier to read, while others prefer @code{#ifdef} because +it avoids diagnostics with picky compilers like GCC with the +@option{-Wundef} option. + +If a test program needs to use or create a data file, give it a name +that starts with @file{conftest}, such as @file{conftest.data}. The +@command{configure} script cleans up by running @samp{rm -f -r conftest*} +after running test programs and if the script is interrupted. + +@node Test Functions +@subsection Test Functions + +These days it's safe to assume support for function prototypes +(introduced in C89). + +Functions that test programs declare should also be conditionalized for +C++, which requires @samp{extern "C"} prototypes. Make sure to not +include any header files containing clashing prototypes. + +@example +#ifdef __cplusplus +extern "C" +#endif +void *valloc (size_t); +@end example + +If a test program calls a function with invalid parameters (just to see +whether it exists), organize the program to ensure that it never invokes +that function. You can do this by calling it in another function that is +never invoked. You can't do it by putting it after a call to +@code{exit}, because GCC version 2 knows that @code{exit} +never returns +and optimizes out any code that follows it in the same block. + +If you include any header files, be sure to call the functions +relevant to them with the correct number of arguments, even if they are +just 0, to avoid compilation errors due to prototypes. GCC +version 2 +has internal prototypes for several functions that it automatically +inlines; for example, @code{memcpy}. To avoid errors when checking for +them, either pass them the correct number of arguments or redeclare them +with a different return type (such as @code{char}). + + +@node Generating Sources +@subsection Generating Sources + +Autoconf provides a set of macros that can be used to generate test +source files. They are written to be language generic, i.e., they +actually depend on the current language (@pxref{Language Choice}) to +``format'' the output properly. + + +@defmac AC_LANG_CONFTEST (@var{source}) +@acindex{LANG_CONFTEST} +Save the @var{source} text in the current test source file: +@file{conftest.@var{extension}} where the @var{extension} depends on the +current language. As of Autoconf 2.63b, the source file also contains +the results of all of the @code{AC_DEFINE} performed so far. + +Note that the @var{source} is evaluated exactly once, like regular +Autoconf macro arguments, and therefore (i) you may pass a macro +invocation, (ii) if not, be sure to double quote if needed. + +This macro issues a warning during @command{autoconf} processing if +@var{source} does not include an expansion of the macro +@code{AC_LANG_DEFINES_PROVIDED} (note that both @code{AC_LANG_SOURCE} and +@code{AC_LANG_PROGRAM} call this macro, and thus avoid the warning). + +This macro is seldom called directly, but is used under the hood by more +common macros such as @code{AC_COMPILE_IFELSE} and @code{AC_RUN_IFELSE}. +@end defmac + +@defmac AC_LANG_DEFINES_PROVIDED +@acindex{LANG_DEFINES_PROVIDED} +This macro is called as a witness that the file +@file{conftest.@var{extension}} appropriate for the current language is +complete, including all previously determined results from +@code{AC_DEFINE}. This macro is seldom called directly, but exists if +you have a compelling reason to write a conftest file without using +@code{AC_LANG_SOURCE}, yet still want to avoid a syntax warning from +@code{AC_LANG_CONFTEST}. +@end defmac + +@defmac AC_LANG_SOURCE (@var{source}) +@acindex{LANG_SOURCE} +Expands into the @var{source}, with the definition of +all the @code{AC_DEFINE} performed so far. This macro includes an +expansion of @code{AC_LANG_DEFINES_PROVIDED}. + +In many cases, you may find it more convenient to use the wrapper +@code{AC_LANG_PROGRAM}. +@end defmac + +For instance, executing (observe the double quotation!): + +@example +@c If you change this example, adjust tests/compile.at:AC_LANG_SOURCE example. +AC_INIT([Hello], [1.0], [bug-hello@@example.org], [], + [http://www.example.org/]) +AC_DEFINE([HELLO_WORLD], ["Hello, World\n"], + [Greetings string.]) +AC_LANG([C]) +AC_LANG_CONFTEST( + [AC_LANG_SOURCE([[const char hw[] = "Hello, World\n";]])]) +gcc -E -dD conftest.c +@end example + +@noindent +on a system with @command{gcc} installed, results in: + +@example +@c If you change this example, adjust tests/compile.at:AC_LANG_SOURCE example. +@dots{} +# 1 "conftest.c" + +#define PACKAGE_NAME "Hello" +#define PACKAGE_TARNAME "hello" +#define PACKAGE_VERSION "1.0" +#define PACKAGE_STRING "Hello 1.0" +#define PACKAGE_BUGREPORT "bug-hello@@example.org" +#define PACKAGE_URL "http://www.example.org/" +#define HELLO_WORLD "Hello, World\n" + +const char hw[] = "Hello, World\n"; +@end example + +When the test language is Fortran, Erlang, or Go, the @code{AC_DEFINE} +definitions are not automatically translated into constants in the +source code by this macro. + +@defmac AC_LANG_PROGRAM (@var{prologue}, @var{body}) +@acindex{LANG_PROGRAM} +Expands into a source file which consists of the @var{prologue}, and +then @var{body} as body of the main function (e.g., @code{main} in +C). Since it uses @code{AC_LANG_SOURCE}, the features of the latter are +available. +@end defmac + +For instance: + +@example +@c If you change this example, adjust tests/compile.at:AC_LANG_PROGRAM example. +AC_INIT([Hello], [1.0], [bug-hello@@example.org], [], + [http://www.example.org/]) +AC_DEFINE([HELLO_WORLD], ["Hello, World\n"], + [Greetings string.]) +AC_LANG_CONFTEST( +[AC_LANG_PROGRAM([[const char hw[] = "Hello, World\n";]], + [[fputs (hw, stdout);]])]) +gcc -E -dD conftest.c +@end example + +@noindent +on a system with @command{gcc} installed, results in: + +@example +@c If you change this example, adjust tests/compile.at:AC_LANG_PROGRAM example. +@dots{} +# 1 "conftest.c" + +#define PACKAGE_NAME "Hello" +#define PACKAGE_TARNAME "hello" +#define PACKAGE_VERSION "1.0" +#define PACKAGE_STRING "Hello 1.0" +#define PACKAGE_BUGREPORT "bug-hello@@example.org" +#define PACKAGE_URL "http://www.example.org/" +#define HELLO_WORLD "Hello, World\n" + +const char hw[] = "Hello, World\n"; +int +main () +@{ +fputs (hw, stdout); + ; + return 0; +@} +@end example + +In Erlang tests, the created source file is that of an Erlang module called +@code{conftest} (@file{conftest.erl}). This module defines and exports +at least +one @code{start/0} function, which is called to perform the test. The +@var{prologue} is optional code that is inserted between the module header and +the @code{start/0} function definition. @var{body} is the body of the +@code{start/0} function without the final period (@pxref{Runtime}, about +constraints on this function's behavior). + +For instance: + +@example +AC_INIT([Hello], [1.0], [bug-hello@@example.org]) +AC_LANG(Erlang) +AC_LANG_CONFTEST( +[AC_LANG_PROGRAM([[-define(HELLO_WORLD, "Hello, world!").]], + [[io:format("~s~n", [?HELLO_WORLD])]])]) +cat conftest.erl +@end example + +@noindent +results in: + +@example +-module(conftest). +-export([start/0]). +-define(HELLO_WORLD, "Hello, world!"). +start() -> +io:format("~s~n", [?HELLO_WORLD]) +. +@end example + +@defmac AC_LANG_CALL (@var{prologue}, @var{function}) +@acindex{LANG_CALL} +Expands into a source file which consists of the @var{prologue}, and +then a call to the @var{function} as body of the main function (e.g., +@code{main} in C). Since it uses @code{AC_LANG_PROGRAM}, the feature +of the latter are available. + +This function will probably be replaced in the future by a version +which would enable specifying the arguments. The use of this macro is +not encouraged, as it violates strongly the typing system. + +This macro cannot be used for Erlang tests. +@end defmac + +@defmac AC_LANG_FUNC_LINK_TRY (@var{function}) +@acindex{LANG_FUNC_LINK_TRY} +Expands into a source file which uses the @var{function} in the body of +the main function (e.g., @code{main} in C). Since it uses +@code{AC_LANG_PROGRAM}, the features of the latter are available. + +As @code{AC_LANG_CALL}, this macro is documented only for completeness. +It is considered to be severely broken, and in the future will be +removed in favor of actual function calls (with properly typed +arguments). + +This macro cannot be used for Erlang tests. +@end defmac + +@node Running the Preprocessor +@section Running the Preprocessor + +Sometimes one might need to run the preprocessor on some source file. +@emph{Usually it is a bad idea}, as you typically need to @emph{compile} +your project, not merely run the preprocessor on it; therefore you +certainly want to run the compiler, not the preprocessor. Resist the +temptation of following the easiest path. + +Nevertheless, if you need to run the preprocessor, then use +@code{AC_PREPROC_IFELSE}. + +The macros described in this section cannot be used for tests in Erlang, +Fortran, or Go, since those languages require no preprocessor. + +@anchor{AC_PREPROC_IFELSE} +@defmac AC_PREPROC_IFELSE (@var{input}, @ovar{action-if-true}, @ + @ovar{action-if-false}) +@acindex{PREPROC_IFELSE} +Run the preprocessor of the current language (@pxref{Language Choice}) +on the @var{input}, run the shell commands @var{action-if-true} on +success, @var{action-if-false} otherwise. The @var{input} can be made +by @code{AC_LANG_PROGRAM} and friends. + +This macro uses @code{CPPFLAGS}, but not @code{CFLAGS}, because +@option{-g}, @option{-O}, etc.@: are not valid options to many C +preprocessors. + +It is customary to report unexpected failures with +@code{AC_MSG_FAILURE}. If needed, @var{action-if-true} can further access +the preprocessed output in the file @file{conftest.i}. +@end defmac + +For instance: + +@example +AC_INIT([Hello], [1.0], [bug-hello@@example.org]) +AC_DEFINE([HELLO_WORLD], ["Hello, World\n"], + [Greetings string.]) +AC_PREPROC_IFELSE( + [AC_LANG_PROGRAM([[const char hw[] = "Hello, World\n";]], + [[fputs (hw, stdout);]])], + [AC_MSG_RESULT([OK])], + [AC_MSG_FAILURE([unexpected preprocessor failure])]) +@end example + +@noindent +results in: + +@example +checking for gcc... gcc +checking for C compiler default output file name... a.out +checking whether the C compiler works... yes +checking whether we are cross compiling... no +checking for suffix of executables... +checking for suffix of object files... o +checking whether we are using the GNU C compiler... yes +checking whether gcc accepts -g... yes +checking for gcc option to accept ISO C89... none needed +checking how to run the C preprocessor... gcc -E +OK +@end example + +@sp 1 + +The macro @code{AC_TRY_CPP} (@pxref{Obsolete Macros}) used to play the +role of @code{AC_PREPROC_IFELSE}, but double quotes its argument, making +it impossible to use it to elaborate sources. You are encouraged to +get rid of your old use of the macro @code{AC_TRY_CPP} in favor of +@code{AC_PREPROC_IFELSE}, but, in the first place, are you sure you need +to run the @emph{preprocessor} and not the compiler? + +@anchor{AC_EGREP_HEADER} +@defmac AC_EGREP_HEADER (@var{pattern}, @var{header-file}, @ + @var{action-if-found}, @ovar{action-if-not-found}) +@acindex{EGREP_HEADER} +If the output of running the preprocessor on the system header file +@var{header-file} matches the extended regular expression +@var{pattern}, execute shell commands @var{action-if-found}, otherwise +execute @var{action-if-not-found}. +@end defmac + +@anchor{AC_EGREP_CPP} +@defmac AC_EGREP_CPP (@var{pattern}, @var{program}, @ + @ovar{action-if-found}, @ovar{action-if-not-found}) +@acindex{EGREP_CPP} +@var{program} is the text of a C or C++ program, on which shell +variable, back quote, and backslash substitutions are performed. If the +output of running the preprocessor on @var{program} matches the +extended regular expression @var{pattern}, execute shell commands +@var{action-if-found}, otherwise execute @var{action-if-not-found}. +@end defmac + + + +@node Running the Compiler +@section Running the Compiler + +To check for a syntax feature of the current language's (@pxref{Language +Choice}) compiler, such as whether it recognizes a certain keyword, or +simply to try some library feature, use @code{AC_COMPILE_IFELSE} to try +to compile a small program that uses that feature. + +@defmac AC_COMPILE_IFELSE (@var{input}, @ovar{action-if-true}, @ + @ovar{action-if-false}) +@acindex{COMPILE_IFELSE} +Run the compiler and compilation flags of the current language +(@pxref{Language Choice}) on the @var{input}, run the shell commands +@var{action-if-true} on success, @var{action-if-false} otherwise. The +@var{input} can be made by @code{AC_LANG_PROGRAM} and friends. + +It is customary to report unexpected failures with +@code{AC_MSG_FAILURE}. This macro does not try to link; use +@code{AC_LINK_IFELSE} if you need to do that (@pxref{Running the +Linker}). If needed, @var{action-if-true} can further access the +just-compiled object file @file{conftest.$OBJEXT}. + +This macro uses @code{AC_REQUIRE} for the compiler associated with the +current language, which means that if the compiler has not yet been +determined, the compiler determination will be made prior to the body of +the outermost @code{AC_DEFUN} macro that triggered this macro to +expand (@pxref{Expanded Before Required}). +@end defmac + +@ovindex ERL +For tests in Erlang, the @var{input} must be the source code of a module named +@code{conftest}. @code{AC_COMPILE_IFELSE} generates a @file{conftest.beam} +file that can be interpreted by the Erlang virtual machine (@code{ERL}). It is +recommended to use @code{AC_LANG_PROGRAM} to specify the test program, +to ensure that the Erlang module has the right name. + +@node Running the Linker +@section Running the Linker + +To check for a library, a function, or a global variable, Autoconf +@command{configure} scripts try to compile and link a small program that +uses it. This is unlike Metaconfig, which by default uses @code{nm} or +@code{ar} on the C library to try to figure out which functions are +available. Trying to link with the function is usually a more reliable +approach because it avoids dealing with the variations in the options +and output formats of @code{nm} and @code{ar} and in the location of the +standard libraries. It also allows configuring for cross-compilation or +checking a function's runtime behavior if needed. On the other hand, +it can be slower than scanning the libraries once, but accuracy is more +important than speed. + +@code{AC_LINK_IFELSE} is used to compile test programs to test for +functions and global variables. It is also used by @code{AC_CHECK_LIB} +to check for libraries (@pxref{Libraries}), by adding the library being +checked for to @code{LIBS} temporarily and trying to link a small +program. + +@anchor{AC_LINK_IFELSE} +@defmac AC_LINK_IFELSE (@var{input}, @ovar{action-if-true}, @ + @ovar{action-if-false}) +@acindex{LINK_IFELSE} +Run the compiler (and compilation flags) and the linker of the current +language (@pxref{Language Choice}) on the @var{input}, run the shell +commands @var{action-if-true} on success, @var{action-if-false} +otherwise. The @var{input} can be made by @code{AC_LANG_PROGRAM} and +friends. If needed, @var{action-if-true} can further access the +just-linked program file @file{conftest$EXEEXT}. + +@code{LDFLAGS} and @code{LIBS} are used for linking, in addition to the +current compilation flags. + +It is customary to report unexpected failures with +@code{AC_MSG_FAILURE}. This macro does not try to execute the program; +use @code{AC_RUN_IFELSE} if you need to do that (@pxref{Runtime}). +@end defmac + +The @code{AC_LINK_IFELSE} macro cannot be used for Erlang tests, since Erlang +programs are interpreted and do not require linking. + + + +@node Runtime +@section Checking Runtime Behavior + +Sometimes you need to find out how a system performs at runtime, such +as whether a given function has a certain capability or bug. If you +can, make such checks when your program runs instead of when it is +configured. You can check for things like the machine's endianness when +your program initializes itself. + +If you really need to test for a runtime behavior while configuring, +you can write a test program to determine the result, and compile and +run it using @code{AC_RUN_IFELSE}. Avoid running test programs if +possible, because this prevents people from configuring your package for +cross-compiling. + +@anchor{AC_RUN_IFELSE} +@defmac AC_RUN_IFELSE (@var{input}, @ovar{action-if-true}, @ + @ovar{action-if-false}, @dvar{action-if-cross-compiling, AC_MSG_FAILURE}) +@acindex{RUN_IFELSE} +Run the compiler (and compilation flags) and the linker of the current +language (@pxref{Language Choice}) on the @var{input}, then execute the +resulting program. If the program returns an exit +status of 0 when executed, run shell commands @var{action-if-true}. +Otherwise, run shell commands @var{action-if-false}. + +The @var{input} can be made by @code{AC_LANG_PROGRAM} and friends. +@code{LDFLAGS} and @code{LIBS} are used for linking, in addition to the +compilation flags of the current language (@pxref{Language Choice}). +Additionally, @var{action-if-true} can run @command{./conftest$EXEEXT} +for further testing. + +In the @var{action-if-false} section, the failing exit status is +available in the shell variable @samp{$?}. This exit status might be +that of a failed compilation, or it might be that of a failed program +execution. + +If cross-compilation mode is enabled (this is the case if either the +compiler being used does not produce executables that run on the system +where @command{configure} is being run, or if the options @code{--build} +and @code{--host} were both specified and their values are different), +then the test program is +not run. If the optional shell commands @var{action-if-cross-compiling} +are given, those commands are run instead; typically these commands +provide pessimistic defaults that allow cross-compilation to work even +if the guess was wrong. If the fourth argument is empty or omitted, but +cross-compilation is detected, then @command{configure} prints an error +message and exits. If you want your package to be useful in a +cross-compilation scenario, you @emph{should} provide a non-empty +@var{action-if-cross-compiling} clause, as well as wrap the +@code{AC_RUN_IFELSE} compilation inside an @code{AC_CACHE_CHECK} +(@pxref{Caching Results}) which allows the user to override the +pessimistic default if needed. + +It is customary to report unexpected failures with +@code{AC_MSG_FAILURE}. +@end defmac + +@command{autoconf} prints a warning message when creating +@command{configure} each time it encounters a call to +@code{AC_RUN_IFELSE} with no @var{action-if-cross-compiling} argument +given. If you are not concerned about users configuring your package +for cross-compilation, you may ignore the warning. A few of the macros +distributed with Autoconf produce this warning message; but if this is a +problem for you, please report it as a bug, along with an appropriate +pessimistic guess to use instead. + +To configure for cross-compiling you can also choose a value for those +parameters based on the canonical system name (@pxref{Manual +Configuration}). Alternatively, set up a test results cache file with +the correct values for the host system (@pxref{Caching Results}). + +@ovindex cross_compiling +To provide a default for calls of @code{AC_RUN_IFELSE} that are embedded +in other macros, including a few of the ones that come with Autoconf, +you can test whether the shell variable @code{cross_compiling} is set to +@samp{yes}, and then use an alternate method to get the results instead +of calling the macros. + +It is also permissible to temporarily assign to @code{cross_compiling} +in order to force tests to behave as though they are in a +cross-compilation environment, particularly since this provides a way to +test your @var{action-if-cross-compiling} even when you are not using a +cross-compiler. + +@example +# We temporarily set cross-compile mode to force AC_COMPUTE_INT +# to use the slow link-only method +save_cross_compiling=$cross_compiling +cross_compiling=yes +AC_COMPUTE_INT([@dots{}]) +cross_compiling=$save_cross_compiling +@end example + +A C or C++ runtime test should be portable. +@xref{Portable C and C++}. + +Erlang tests must exit themselves the Erlang VM by calling the @code{halt/1} +function: the given status code is used to determine the success of the test +(status is @code{0}) or its failure (status is different than @code{0}), as +explained above. It must be noted that data output through the standard output +(e.g., using @code{io:format/2}) may be truncated when halting the VM. +Therefore, if a test must output configuration information, it is recommended +to create and to output data into the temporary file named @file{conftest.out}, +using the functions of module @code{file}. The @code{conftest.out} file is +automatically deleted by the @code{AC_RUN_IFELSE} macro. For instance, a +simplified implementation of Autoconf's @code{AC_ERLANG_SUBST_LIB_DIR} +macro is: + +@example +AC_INIT([LibdirTest], [1.0], [bug-libdirtest@@example.org]) +AC_ERLANG_NEED_ERL +AC_LANG(Erlang) +AC_RUN_IFELSE( + [AC_LANG_PROGRAM([], [dnl + file:write_file("conftest.out", code:lib_dir()), + halt(0)])], + [echo "code:lib_dir() returned: `cat conftest.out`"], + [AC_MSG_FAILURE([test Erlang program execution failed])]) +@end example + + +@node Systemology +@section Systemology +@cindex Systemology + +This section aims at presenting some systems and pointers to +documentation. It may help you addressing particular problems reported +by users. + +@uref{http://@/www.opengroup.org/@/susv3, Posix-conforming systems} are +derived from the @uref{http://@/www.bell-labs.com/@/history/@/unix/, Unix +operating system}. + +The @uref{http://@/bhami.com/@/rosetta.html, Rosetta Stone for Unix} +contains a table correlating the features of various Posix-conforming +systems. @uref{http://@/www.levenez.com/@/unix/, Unix History} is a +simplified diagram of how many Unix systems were derived from each +other. + +@uref{http://@/heirloom.sourceforge.net/, The Heirloom Project} +provides some variants of traditional implementations of Unix utilities. + +@table @asis +@item Darwin +@cindex Darwin +Darwin is also known as Mac OS X@. Beware that the file system @emph{can} be +case-preserving, but case insensitive. This can cause nasty problems, +since for instance the installation attempt for a package having an +@file{INSTALL} file can result in @samp{make install} report that +nothing was to be done! + +That's all dependent on whether the file system is a UFS (case +sensitive) or HFS+ (case preserving). By default Apple wants you to +install the OS on HFS+. Unfortunately, there are some pieces of +software which really need to be built on UFS@. We may want to rebuild +Darwin to have both UFS and HFS+ available (and put the /local/build +tree on the UFS). + +@item QNX 4.25 +@cindex QNX 4.25 +@c FIXME: Please, if you feel like writing something more precise, +@c it'd be great. In particular, I can't understand the difference with +@c QNX Neutrino. +QNX is a realtime operating system running on Intel architecture +meant to be scalable from the small embedded systems to the hundred +processor super-computer. It claims to be Posix certified. More +information is available on the +@uref{http://@/www.qnx.com/, QNX home page}. + +@item Tru64 +@cindex Tru64 +@uref{http://@/h30097.www3.hp.com/@/docs/, +Documentation of several versions of Tru64} is available in different +formats. + +@item Unix version 7 +@cindex Unix version 7 +@cindex V7 +Officially this was called the ``Seventh Edition'' of ``the UNIX +time-sharing system'' but we use the more-common name ``Unix version 7''. +Documentation is available in the +@uref{http://@/plan9.bell-labs.com/@/7thEdMan/, Unix Seventh Edition Manual}. +Previous versions of Unix are called ``Unix version 6'', etc., but +they were not as widely used. +@end table + + +@node Multiple Cases +@section Multiple Cases + +Some operations are accomplished in several possible ways, depending on +the OS variant. Checking for them essentially requires a ``case +statement''. Autoconf does not directly provide one; however, it is +easy to simulate by using a shell variable to keep track of whether a +way to perform the operation has been found yet. + +Here is an example that uses the shell variable @code{fstype} to keep +track of whether the remaining cases need to be checked. Note that +since the value of @code{fstype} is under our control, we don't have to +use the longer @samp{test "x$fstype" = xno}. + +@example +@group +AC_MSG_CHECKING([how to get file system type]) +fstype=no +# The order of these tests is important. +AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <sys/statvfs.h> +#include <sys/fstyp.h>]])], + [AC_DEFINE([FSTYPE_STATVFS], [1], + [Define if statvfs exists.]) + fstype=SVR4]) +if test $fstype = no; then + AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <sys/statfs.h> +#include <sys/fstyp.h>]])], + [AC_DEFINE([FSTYPE_USG_STATFS], [1], + [Define if USG statfs.]) + fstype=SVR3]) +fi +if test $fstype = no; then + AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include <sys/statfs.h> +#include <sys/vmount.h>]])]), + [AC_DEFINE([FSTYPE_AIX_STATFS], [1], + [Define if AIX statfs.]) + fstype=AIX]) +fi +# (more cases omitted here) +AC_MSG_RESULT([$fstype]) +@end group +@end example + +@c ====================================================== Results of Tests. + +@node Results +@chapter Results of Tests + +Once @command{configure} has determined whether a feature exists, what can +it do to record that information? There are four sorts of things it can +do: define a C preprocessor symbol, set a variable in the output files, +save the result in a cache file for future @command{configure} runs, and +print a message letting the user know the result of the test. + +@menu +* Defining Symbols:: Defining C preprocessor symbols +* Setting Output Variables:: Replacing variables in output files +* Special Chars in Variables:: Characters to beware of in variables +* Caching Results:: Speeding up subsequent @command{configure} runs +* Printing Messages:: Notifying @command{configure} users +@end menu + +@node Defining Symbols +@section Defining C Preprocessor Symbols + +A common action to take in response to a feature test is to define a C +preprocessor symbol indicating the results of the test. That is done by +calling @code{AC_DEFINE} or @code{AC_DEFINE_UNQUOTED}. + +By default, @code{AC_OUTPUT} places the symbols defined by these macros +into the output variable @code{DEFS}, which contains an option +@option{-D@var{symbol}=@var{value}} for each symbol defined. Unlike in +Autoconf version 1, there is no variable @code{DEFS} defined while +@command{configure} is running. To check whether Autoconf macros have +already defined a certain C preprocessor symbol, test the value of the +appropriate cache variable, as in this example: + +@example +AC_CHECK_FUNC([vprintf], [AC_DEFINE([HAVE_VPRINTF], [1], + [Define if vprintf exists.])]) +if test "x$ac_cv_func_vprintf" != xyes; then + AC_CHECK_FUNC([_doprnt], [AC_DEFINE([HAVE_DOPRNT], [1], + [Define if _doprnt exists.])]) +fi +@end example + +If @code{AC_CONFIG_HEADERS} has been called, then instead of creating +@code{DEFS}, @code{AC_OUTPUT} creates a header file by substituting the +correct values into @code{#define} statements in a template file. +@xref{Configuration Headers}, for more information about this kind of +output. + +@defmac AC_DEFINE (@var{variable}, @var{value}, @ovar{description}) +@defmacx AC_DEFINE (@var{variable}) +@cvindex @var{variable} +@acindex{DEFINE} +Define @var{variable} to @var{value} (verbatim), by defining a C +preprocessor macro for @var{variable}. @var{variable} should be a C +identifier, optionally suffixed by a parenthesized argument list to +define a C preprocessor macro with arguments. The macro argument list, +if present, should be a comma-separated list of C identifiers, possibly +terminated by an ellipsis @samp{...} if C99 syntax is employed. +@var{variable} should not contain comments, white space, trigraphs, +backslash-newlines, universal character names, or non-ASCII +characters. + +@var{value} may contain backslash-escaped newlines, which will be +preserved if you use @code{AC_CONFIG_HEADERS} but flattened if passed +via @code{@@DEFS@@} (with no effect on the compilation, since the +preprocessor sees only one line in the first place). @var{value} should +not contain raw newlines. If you are not using +@code{AC_CONFIG_HEADERS}, @var{value} should not contain any @samp{#} +characters, as @command{make} tends to eat them. To use a shell +variable, use @code{AC_DEFINE_UNQUOTED} instead. + +@var{description} is only useful if you are using +@code{AC_CONFIG_HEADERS}. In this case, @var{description} is put into +the generated @file{config.h.in} as the comment before the macro define. +The following example defines the C preprocessor variable +@code{EQUATION} to be the string constant @samp{"$a > $b"}: + +@example +AC_DEFINE([EQUATION], ["$a > $b"], + [Equation string.]) +@end example + +If neither @var{value} nor @var{description} are given, then +@var{value} defaults to 1 instead of to the empty string. This is for +backwards compatibility with older versions of Autoconf, but this usage +is obsolescent and may be withdrawn in future versions of Autoconf. + +If the @var{variable} is a literal string, it is passed to +@code{m4_pattern_allow} (@pxref{Forbidden Patterns}). + +If multiple @code{AC_DEFINE} statements are executed for the same +@var{variable} name (not counting any parenthesized argument list), +the last one wins. +@end defmac + +@defmac AC_DEFINE_UNQUOTED (@var{variable}, @var{value}, @ovar{description}) +@defmacx AC_DEFINE_UNQUOTED (@var{variable}) +@acindex{DEFINE_UNQUOTED} +@cvindex @var{variable} +Like @code{AC_DEFINE}, but three shell expansions are +performed---once---on @var{variable} and @var{value}: variable expansion +(@samp{$}), command substitution (@samp{`}), and backslash escaping +(@samp{\}), as if in an unquoted here-document. Single and double quote +characters in the value have no +special meaning. Use this macro instead of @code{AC_DEFINE} when +@var{variable} or @var{value} is a shell variable. Examples: + +@example +AC_DEFINE_UNQUOTED([config_machfile], ["$machfile"], + [Configuration machine file.]) +AC_DEFINE_UNQUOTED([GETGROUPS_T], [$ac_cv_type_getgroups], + [getgroups return type.]) +AC_DEFINE_UNQUOTED([$ac_tr_hdr], [1], + [Translated header name.]) +@end example +@end defmac + +Due to a syntactical bizarreness of the Bourne shell, do not use +semicolons to separate @code{AC_DEFINE} or @code{AC_DEFINE_UNQUOTED} +calls from other macro calls or shell code; that can cause syntax errors +in the resulting @command{configure} script. Use either blanks or +newlines. That is, do this: + +@example +AC_CHECK_HEADER([elf.h], + [AC_DEFINE([SVR4], [1], [System V Release 4]) LIBS="-lelf $LIBS"]) +@end example + +@noindent +or this: + +@example +AC_CHECK_HEADER([elf.h], + [AC_DEFINE([SVR4], [1], [System V Release 4]) + LIBS="-lelf $LIBS"]) +@end example + +@noindent +instead of this: + +@example +AC_CHECK_HEADER([elf.h], + [AC_DEFINE([SVR4], [1], [System V Release 4]); LIBS="-lelf $LIBS"]) +@end example + +@node Setting Output Variables +@section Setting Output Variables +@cindex Output variables + +Another way to record the results of tests is to set @dfn{output +variables}, which are shell variables whose values are substituted into +files that @command{configure} outputs. The two macros below create new +output variables. @xref{Preset Output Variables}, for a list of output +variables that are always available. + +@defmac AC_SUBST (@var{variable}, @ovar{value}) +@acindex{SUBST} +Create an output variable from a shell variable. Make @code{AC_OUTPUT} +substitute the variable @var{variable} into output files (typically one +or more makefiles). This means that @code{AC_OUTPUT} +replaces instances of @samp{@@@var{variable}@@} in input files with the +value that the shell variable @var{variable} has when @code{AC_OUTPUT} +is called. The value can contain any non-@code{NUL} character, including +newline. If you are using Automake 1.11 or newer, for newlines in values +you might want to consider using @code{AM_SUBST_NOTMAKE} to prevent +@command{automake} from adding a line @code{@var{variable} = +@@@var{variable}@@} to the @file{Makefile.in} files (@pxref{Optional, , +Automake, automake, Other things Automake recognizes}). + +Variable occurrences should not overlap: e.g., an input file should +not contain @samp{@@@var{var1}@@@var{var2}@@} if @var{var1} and @var{var2} +are variable names. +The substituted value is not rescanned for more output variables; +occurrences of @samp{@@@var{variable}@@} in the value are inserted +literally into the output file. (The algorithm uses the special marker +@code{|#_!!_#|} internally, so neither the substituted value nor the +output file may contain @code{|#_!!_#|}.) + +If @var{value} is given, in addition assign it to @var{variable}. + +The string @var{variable} is passed to @code{m4_pattern_allow} +(@pxref{Forbidden Patterns}). +@end defmac + +@defmac AC_SUBST_FILE (@var{variable}) +@acindex{SUBST_FILE} +Another way to create an output variable from a shell variable. Make +@code{AC_OUTPUT} insert (without substitutions) the contents of the file +named by shell variable @var{variable} into output files. This means +that @code{AC_OUTPUT} replaces instances of +@samp{@@@var{variable}@@} in output files (such as @file{Makefile.in}) +with the contents of the file that the shell variable @var{variable} +names when @code{AC_OUTPUT} is called. Set the variable to +@file{/dev/null} for cases that do not have a file to insert. +This substitution occurs only when the @samp{@@@var{variable}@@} is on a +line by itself, optionally surrounded by spaces and tabs. The +substitution replaces the whole line, including the spaces, tabs, and +the terminating newline. + +This macro is useful for inserting makefile fragments containing +special dependencies or other @command{make} directives for particular host +or target types into makefiles. For example, @file{configure.ac} +could contain: + +@example +AC_SUBST_FILE([host_frag]) +host_frag=$srcdir/conf/sun4.mh +@end example + +@noindent +and then a @file{Makefile.in} could contain: + +@example +@@host_frag@@ +@end example + +The string @var{variable} is passed to @code{m4_pattern_allow} +(@pxref{Forbidden Patterns}). +@end defmac + +@cindex Precious Variable +@cindex Variable, Precious +Running @command{configure} in varying environments can be extremely +dangerous. If for instance the user runs @samp{CC=bizarre-cc +./configure}, then the cache, @file{config.h}, and many other output +files depend upon @command{bizarre-cc} being the C compiler. If +for some reason the user runs @command{./configure} again, or if it is +run via @samp{./config.status --recheck}, (@xref{Automatic Remaking}, +and @pxref{config.status Invocation}), then the configuration can be +inconsistent, composed of results depending upon two different +compilers. + +Environment variables that affect this situation, such as @samp{CC} +above, are called @dfn{precious variables}, and can be declared as such +by @code{AC_ARG_VAR}. + +@defmac AC_ARG_VAR (@var{variable}, @var{description}) +@acindex{ARG_VAR} +Declare @var{variable} is a precious variable, and include its +@var{description} in the variable section of @samp{./configure --help}. + +Being precious means that +@itemize @minus +@item +@var{variable} is substituted via @code{AC_SUBST}. + +@item +The value of @var{variable} when @command{configure} was launched is +saved in the cache, including if it was not specified on the command +line but via the environment. Indeed, while @command{configure} can +notice the definition of @code{CC} in @samp{./configure CC=bizarre-cc}, +it is impossible to notice it in @samp{CC=bizarre-cc ./configure}, +which, unfortunately, is what most users do. + +We emphasize that it is the @emph{initial} value of @var{variable} which +is saved, not that found during the execution of @command{configure}. +Indeed, specifying @samp{./configure FOO=foo} and letting +@samp{./configure} guess that @code{FOO} is @code{foo} can be two +different things. + +@item +@var{variable} is checked for consistency between two +@command{configure} runs. For instance: + +@example +$ @kbd{./configure --silent --config-cache} +$ @kbd{CC=cc ./configure --silent --config-cache} +configure: error: `CC' was not set in the previous run +configure: error: changes in the environment can compromise \ +the build +configure: error: run `make distclean' and/or \ +`rm config.cache' and start over +@end example + +@noindent +and similarly if the variable is unset, or if its content is changed. +If the content has white space changes only, then the error is degraded +to a warning only, but the old value is reused. + +@item +@var{variable} is kept during automatic reconfiguration +(@pxref{config.status Invocation}) as if it had been passed as a command +line argument, including when no cache is used: + +@example +$ @kbd{CC=/usr/bin/cc ./configure var=raboof --silent} +$ @kbd{./config.status --recheck} +running CONFIG_SHELL=/bin/sh /bin/sh ./configure var=raboof \ + CC=/usr/bin/cc --no-create --no-recursion +@end example +@end itemize +@end defmac + +@node Special Chars in Variables +@section Special Characters in Output Variables +@cindex Output variables, special characters in + +Many output variables are intended to be evaluated both by +@command{make} and by the shell. Some characters are expanded +differently in these two contexts, so to avoid confusion these +variables' values should not contain any of the following characters: + +@example +" # $ & ' ( ) * ; < > ? [ \ ^ ` | +@end example + +Also, these variables' values should neither contain newlines, nor start +with @samp{~}, nor contain white space or @samp{:} immediately followed +by @samp{~}. The values can contain nonempty sequences of white space +characters like tabs and spaces, but each such sequence might +arbitrarily be replaced by a single space during substitution. + +These restrictions apply both to the values that @command{configure} +computes, and to the values set directly by the user. For example, the +following invocations of @command{configure} are problematic, since they +attempt to use special characters within @code{CPPFLAGS} and white space +within @code{$(srcdir)}: + +@example +CPPFLAGS='-DOUCH="&\"#$*?"' '../My Source/ouch-1.0/configure' + +'../My Source/ouch-1.0/configure' CPPFLAGS='-DOUCH="&\"#$*?"' +@end example + +@node Caching Results +@section Caching Results +@cindex Cache + +To avoid checking for the same features repeatedly in various +@command{configure} scripts (or in repeated runs of one script), +@command{configure} can optionally save the results of many checks in a +@dfn{cache file} (@pxref{Cache Files}). If a @command{configure} script +runs with caching enabled and finds a cache file, it reads the results +of previous runs from the cache and avoids rerunning those checks. As a +result, @command{configure} can then run much faster than if it had to +perform all of the checks every time. + +@defmac AC_CACHE_VAL (@var{cache-id}, @var{commands-to-set-it}) +@acindex{CACHE_VAL} +Ensure that the results of the check identified by @var{cache-id} are +available. If the results of the check were in the cache file that was +read, and @command{configure} was not given the @option{--quiet} or +@option{--silent} option, print a message saying that the result was +cached; otherwise, run the shell commands @var{commands-to-set-it}. If +the shell commands are run to determine the value, the value is +saved in the cache file just before @command{configure} creates its output +files. @xref{Cache Variable Names}, for how to choose the name of the +@var{cache-id} variable. + +The @var{commands-to-set-it} @emph{must have no side effects} except for +setting the variable @var{cache-id}, see below. +@end defmac + +@defmac AC_CACHE_CHECK (@var{message}, @var{cache-id}, @ + @var{commands-to-set-it}) +@acindex{CACHE_CHECK} +A wrapper for @code{AC_CACHE_VAL} that takes care of printing the +messages. This macro provides a convenient shorthand for the most +common way to use these macros. It calls @code{AC_MSG_CHECKING} for +@var{message}, then @code{AC_CACHE_VAL} with the @var{cache-id} and +@var{commands} arguments, and @code{AC_MSG_RESULT} with @var{cache-id}. + +The @var{commands-to-set-it} @emph{must have no side effects} except for +setting the variable @var{cache-id}, see below. +@end defmac + +It is common to find buggy macros using @code{AC_CACHE_VAL} or +@code{AC_CACHE_CHECK}, because people are tempted to call +@code{AC_DEFINE} in the @var{commands-to-set-it}. Instead, the code that +@emph{follows} the call to @code{AC_CACHE_VAL} should call +@code{AC_DEFINE}, by examining the value of the cache variable. For +instance, the following macro is broken: + +@example +@c If you change this example, adjust tests/base.at:AC_CACHE_CHECK. +@group +AC_DEFUN([AC_SHELL_TRUE], +[AC_CACHE_CHECK([whether true(1) works], [my_cv_shell_true_works], + [my_cv_shell_true_works=no + (true) 2>/dev/null && my_cv_shell_true_works=yes + if test "x$my_cv_shell_true_works" = xyes; then + AC_DEFINE([TRUE_WORKS], [1], + [Define if `true(1)' works properly.]) + fi]) +]) +@end group +@end example + +@noindent +This fails if the cache is enabled: the second time this macro is run, +@code{TRUE_WORKS} @emph{will not be defined}. The proper implementation +is: + +@example +@c If you change this example, adjust tests/base.at:AC_CACHE_CHECK. +@group +AC_DEFUN([AC_SHELL_TRUE], +[AC_CACHE_CHECK([whether true(1) works], [my_cv_shell_true_works], + [my_cv_shell_true_works=no + (true) 2>/dev/null && my_cv_shell_true_works=yes]) + if test "x$my_cv_shell_true_works" = xyes; then + AC_DEFINE([TRUE_WORKS], [1], + [Define if `true(1)' works properly.]) + fi +]) +@end group +@end example + +Also, @var{commands-to-set-it} should not print any messages, for +example with @code{AC_MSG_CHECKING}; do that before calling +@code{AC_CACHE_VAL}, so the messages are printed regardless of whether +the results of the check are retrieved from the cache or determined by +running the shell commands. + +@menu +* Cache Variable Names:: Shell variables used in caches +* Cache Files:: Files @command{configure} uses for caching +* Cache Checkpointing:: Loading and saving the cache file +@end menu + +@node Cache Variable Names +@subsection Cache Variable Names +@cindex Cache variable + +The names of cache variables should have the following format: + +@example +@var{package-prefix}_cv_@var{value-type}_@var{specific-value}_@ovar{additional-options} +@end example + +@noindent +for example, @samp{ac_cv_header_stat_broken} or +@samp{ac_cv_prog_gcc_traditional}. The parts of the variable name are: + +@table @asis +@item @var{package-prefix} +An abbreviation for your package or organization; the same prefix you +begin local Autoconf macros with, except lowercase by convention. +For cache values used by the distributed Autoconf macros, this value is +@samp{ac}. + +@item @code{_cv_} +Indicates that this shell variable is a cache value. This string +@emph{must} be present in the variable name, including the leading +underscore. + +@item @var{value-type} +A convention for classifying cache values, to produce a rational naming +system. The values used in Autoconf are listed in @ref{Macro Names}. + +@item @var{specific-value} +Which member of the class of cache values this test applies to. +For example, which function (@samp{alloca}), program (@samp{gcc}), or +output variable (@samp{INSTALL}). + +@item @var{additional-options} +Any particular behavior of the specific member that this test applies to. +For example, @samp{broken} or @samp{set}. This part of the name may +be omitted if it does not apply. +@end table + +The values assigned to cache variables may not contain newlines. +Usually, their values are Boolean (@samp{yes} or @samp{no}) or the +names of files or functions; so this is not an important restriction. +@ref{Cache Variable Index} for an index of cache variables with +documented semantics. + + +@node Cache Files +@subsection Cache Files + +A cache file is a shell script that caches the results of configure +tests run on one system so they can be shared between configure scripts +and configure runs. It is not useful on other systems. If its contents +are invalid for some reason, the user may delete or edit it, or override +documented cache variables on the @command{configure} command line. + +By default, @command{configure} uses no cache file, +to avoid problems caused by accidental +use of stale cache files. + +To enable caching, @command{configure} accepts @option{--config-cache} (or +@option{-C}) to cache results in the file @file{config.cache}. +Alternatively, @option{--cache-file=@var{file}} specifies that +@var{file} be the cache file. The cache file is created if it does not +exist already. When @command{configure} calls @command{configure} scripts in +subdirectories, it uses the @option{--cache-file} argument so that they +share the same cache. @xref{Subdirectories}, for information on +configuring subdirectories with the @code{AC_CONFIG_SUBDIRS} macro. + +@file{config.status} only pays attention to the cache file if it is +given the @option{--recheck} option, which makes it rerun +@command{configure}. + +It is wrong to try to distribute cache files for particular system types. +There is too much room for error in doing that, and too much +administrative overhead in maintaining them. For any features that +can't be guessed automatically, use the standard method of the canonical +system type and linking files (@pxref{Manual Configuration}). + +The site initialization script can specify a site-wide cache file to +use, instead of the usual per-program cache. In this case, the cache +file gradually accumulates information whenever someone runs a new +@command{configure} script. (Running @command{configure} merges the new cache +results with the existing cache file.) This may cause problems, +however, if the system configuration (e.g., the installed libraries or +compilers) changes and the stale cache file is not deleted. + +If @command{configure} is interrupted at the right time when it updates +a cache file outside of the build directory where the @command{configure} +script is run, it may leave behind a temporary file named after the +cache file with digits following it. You may safely delete such a file. + + +@node Cache Checkpointing +@subsection Cache Checkpointing + +If your configure script, or a macro called from @file{configure.ac}, happens +to abort the configure process, it may be useful to checkpoint the cache +a few times at key points using @code{AC_CACHE_SAVE}. Doing so +reduces the amount of time it takes to rerun the configure script with +(hopefully) the error that caused the previous abort corrected. + +@c FIXME: Do we really want to document this guy? +@defmac AC_CACHE_LOAD +@acindex{CACHE_LOAD} +Loads values from existing cache file, or creates a new cache file if a +cache file is not found. Called automatically from @code{AC_INIT}. +@end defmac + +@defmac AC_CACHE_SAVE +@acindex{CACHE_SAVE} +Flushes all cached values to the cache file. Called automatically from +@code{AC_OUTPUT}, but it can be quite useful to call +@code{AC_CACHE_SAVE} at key points in @file{configure.ac}. +@end defmac + +For instance: + +@example +@r{ @dots{} AC_INIT, etc. @dots{}} +@group +# Checks for programs. +AC_PROG_CC +AC_PROG_AWK +@r{ @dots{} more program checks @dots{}} +AC_CACHE_SAVE +@end group + +@group +# Checks for libraries. +AC_CHECK_LIB([nsl], [gethostbyname]) +AC_CHECK_LIB([socket], [connect]) +@r{ @dots{} more lib checks @dots{}} +AC_CACHE_SAVE +@end group + +@group +# Might abort@dots{} +AM_PATH_GTK([1.0.2], [], [AC_MSG_ERROR([GTK not in path])]) +AM_PATH_GTKMM([0.9.5], [], [AC_MSG_ERROR([GTK not in path])]) +@end group +@r{ @dots{} AC_OUTPUT, etc. @dots{}} +@end example + +@node Printing Messages +@section Printing Messages +@cindex Messages, from @command{configure} + +@command{configure} scripts need to give users running them several kinds +of information. The following macros print messages in ways appropriate +for each kind. The arguments to all of them get enclosed in shell +double quotes, so the shell performs variable and back-quote +substitution on them. + +These macros are all wrappers around the @command{echo} shell command. +They direct output to the appropriate file descriptor (@pxref{File +Descriptor Macros}). +@command{configure} scripts should rarely need to run @command{echo} directly +to print messages for the user. Using these macros makes it easy to +change how and when each kind of message is printed; such changes need +only be made to the macro definitions and all the callers change +automatically. + +To diagnose static issues, i.e., when @command{autoconf} is run, see +@ref{Diagnostic Macros}. + +@defmac AC_MSG_CHECKING (@var{feature-description}) +@acindex{MSG_CHECKING} +Notify the user that @command{configure} is checking for a particular +feature. This macro prints a message that starts with @samp{checking } +and ends with @samp{...} and no newline. It must be followed by a call +to @code{AC_MSG_RESULT} to print the result of the check and the +newline. The @var{feature-description} should be something like +@samp{whether the Fortran compiler accepts C++ comments} or @samp{for +c89}. + +This macro prints nothing if @command{configure} is run with the +@option{--quiet} or @option{--silent} option. +@end defmac + +@anchor{AC_MSG_RESULT} +@defmac AC_MSG_RESULT (@var{result-description}) +@acindex{MSG_RESULT} +Notify the user of the results of a check. @var{result-description} is +almost always the value of the cache variable for the check, typically +@samp{yes}, @samp{no}, or a file name. This macro should follow a call +to @code{AC_MSG_CHECKING}, and the @var{result-description} should be +the completion of the message printed by the call to +@code{AC_MSG_CHECKING}. + +This macro prints nothing if @command{configure} is run with the +@option{--quiet} or @option{--silent} option. +@end defmac + +@anchor{AC_MSG_NOTICE} +@defmac AC_MSG_NOTICE (@var{message}) +@acindex{MSG_NOTICE} +Deliver the @var{message} to the user. It is useful mainly to print a +general description of the overall purpose of a group of feature checks, +e.g., + +@example +AC_MSG_NOTICE([checking if stack overflow is detectable]) +@end example + +This macro prints nothing if @command{configure} is run with the +@option{--quiet} or @option{--silent} option. +@end defmac + +@anchor{AC_MSG_ERROR} +@defmac AC_MSG_ERROR (@var{error-description}, @dvar{exit-status, $?/1}) +@acindex{MSG_ERROR} +Notify the user of an error that prevents @command{configure} from +completing. This macro prints an error message to the standard error +output and exits @command{configure} with @var{exit-status} (@samp{$?} +by default, except that @samp{0} is converted to @samp{1}). +@var{error-description} should be something like @samp{invalid value +$HOME for \$HOME}. + +The @var{error-description} should start with a lower-case letter, and +``cannot'' is preferred to ``can't''. +@end defmac + +@defmac AC_MSG_FAILURE (@var{error-description}, @ovar{exit-status}) +@acindex{MSG_FAILURE} +This @code{AC_MSG_ERROR} wrapper notifies the user of an error that +prevents @command{configure} from completing @emph{and} that additional +details are provided in @file{config.log}. This is typically used when +abnormal results are found during a compilation. +@end defmac + +@anchor{AC_MSG_WARN} +@defmac AC_MSG_WARN (@var{problem-description}) +@acindex{MSG_WARN} +Notify the @command{configure} user of a possible problem. This macro +prints the message to the standard error output; @command{configure} +continues running afterward, so macros that call @code{AC_MSG_WARN} should +provide a default (back-up) behavior for the situations they warn about. +@var{problem-description} should be something like @samp{ln -s seems to +make hard links}. +@end defmac + + + +@c ====================================================== Programming in M4. + +@node Programming in M4 +@chapter Programming in M4 +@cindex M4 + +Autoconf is written on top of two layers: @dfn{M4sugar}, which provides +convenient macros for pure M4 programming, and @dfn{M4sh}, which +provides macros dedicated to shell script generation. + +As of this version of Autoconf, these two layers still contain +experimental macros, whose interface might change in the future. As a +matter of fact, @emph{anything that is not documented must not be used}. + +@menu +* M4 Quotation:: Protecting macros from unwanted expansion +* Using autom4te:: The Autoconf executables backbone +* Programming in M4sugar:: Convenient pure M4 macros +* Debugging via autom4te:: Figuring out what M4 was doing +@end menu + +@node M4 Quotation +@section M4 Quotation +@cindex M4 quotation +@cindex quotation + +The most common problem with existing macros is an improper quotation. +This section, which users of Autoconf can skip, but which macro writers +@emph{must} read, first justifies the quotation scheme that was chosen +for Autoconf and then ends with a rule of thumb. Understanding the +former helps one to follow the latter. + +@menu +* Active Characters:: Characters that change the behavior of M4 +* One Macro Call:: Quotation and one macro call +* Quoting and Parameters:: M4 vs. shell parameters +* Quotation and Nested Macros:: Macros calling macros +* Changequote is Evil:: Worse than INTERCAL: M4 + changequote +* Quadrigraphs:: Another way to escape special characters +* Balancing Parentheses:: Dealing with unbalanced parentheses +* Quotation Rule Of Thumb:: One parenthesis, one quote +@end menu + +@node Active Characters +@subsection Active Characters + +To fully understand where proper quotation is important, you first need +to know what the special characters are in Autoconf: @samp{#} introduces +a comment inside which no macro expansion is performed, @samp{,} +separates arguments, @samp{[} and @samp{]} are the quotes +themselves@footnote{By itself, M4 uses @samp{`} and @samp{'}; it is the +M4sugar layer that sets up the preferred quotes of @samp{[} and @samp{]}.}, +@samp{(} and @samp{)} (which M4 tries to match by pairs), and finally +@samp{$} inside a macro definition. + +In order to understand the delicate case of macro calls, we first have +to present some obvious failures. Below they are ``obvious-ified'', +but when you find them in real life, they are usually in disguise. + +Comments, introduced by a hash and running up to the newline, are opaque +tokens to the top level: active characters are turned off, and there is +no macro expansion: + +@example +# define([def], ine) +@result{}# define([def], ine) +@end example + +Each time there can be a macro expansion, there is a quotation +expansion, i.e., one level of quotes is stripped: + +@example +int tab[10]; +@result{}int tab10; +[int tab[10];] +@result{}int tab[10]; +@end example + +Without this in mind, the reader might try hopelessly to use her macro +@code{array}: + +@example +define([array], [int tab[10];]) +array +@result{}int tab10; +[array] +@result{}array +@end example + +@noindent +How can you correctly output the intended results@footnote{Using +@code{defn}.}? + + +@node One Macro Call +@subsection One Macro Call + +Let's proceed on the interaction between active characters and macros +with this small macro, which just returns its first argument: + +@example +define([car], [$1]) +@end example + +@noindent +The two pairs of quotes above are not part of the arguments of +@code{define}; rather, they are understood by the top level when it +tries to find the arguments of @code{define}. Therefore, assuming +@code{car} is not already defined, it is equivalent to write: + +@example +define(car, $1) +@end example + +@noindent +But, while it is acceptable for a @file{configure.ac} to avoid unnecessary +quotes, it is bad practice for Autoconf macros which must both be more +robust and also advocate perfect style. + +At the top level, there are only two possibilities: either you +quote or you don't: + +@example +car(foo, bar, baz) +@result{}foo +[car(foo, bar, baz)] +@result{}car(foo, bar, baz) +@end example + +Let's pay attention to the special characters: + +@example +car(#) +@error{}EOF in argument list +@end example + +The closing parenthesis is hidden in the comment; with a hypothetical +quoting, the top level understood it this way: + +@example +car([#)] +@end example + +@noindent +Proper quotation, of course, fixes the problem: + +@example +car([#]) +@result{}# +@end example + +Here are more examples: + +@example +car(foo, bar) +@result{}foo +car([foo, bar]) +@result{}foo, bar +car((foo, bar)) +@result{}(foo, bar) +car([(foo], [bar)]) +@result{}(foo +define([a], [b]) +@result{} +car(a) +@result{}b +car([a]) +@result{}b +car([[a]]) +@result{}a +car([[[a]]]) +@result{}[a] +@end example + +@node Quoting and Parameters +@subsection Quoting and Parameters + +When M4 encounters @samp{$} within a macro definition, followed +immediately by a character it recognizes (@samp{0}@dots{}@samp{9}, +@samp{#}, @samp{@@}, or @samp{*}), it will perform M4 parameter +expansion. This happens regardless of how many layers of quotes the +parameter expansion is nested within, or even if it occurs in text that +will be rescanned as a comment. + +@example +define([none], [$1]) +@result{} +define([one], [[$1]]) +@result{} +define([two], [[[$1]]]) +@result{} +define([comment], [# $1]) +@result{} +define([active], [ACTIVE]) +@result{} +none([active]) +@result{}ACTIVE +one([active]) +@result{}active +two([active]) +@result{}[active] +comment([active]) +@result{}# active +@end example + +On the other hand, since autoconf generates shell code, you often want +to output shell variable expansion, rather than performing M4 parameter +expansion. To do this, you must use M4 quoting to separate the @samp{$} +from the next character in the definition of your macro. If the macro +definition occurs in single-quoted text, then insert another level of +quoting; if the usage is already inside a double-quoted string, then +split it into concatenated strings. + +@example +define([single], [a single-quoted $[]1 definition]) +@result{} +define([double], [[a double-quoted $][1 definition]]) +@result{} +single +@result{}a single-quoted $1 definition +double +@result{}a double-quoted $1 definition +@end example + +Posix states that M4 implementations are free to provide implementation +extensions when @samp{$@{} is encountered in a macro definition. +Autoconf reserves the longer sequence @samp{$@{@{} for use with planned +extensions that will be available in the future GNU M4 2.0, +but guarantees that all other instances of @samp{$@{} will be output +literally. Therefore, this idiom can also be used to output shell code +parameter references: + +@example +define([first], [$@{1@}])first +@result{}$@{1@} +@end example + +Posix also states that @samp{$11} should expand to the first parameter +concatenated with a literal @samp{1}, although some versions of +GNU M4 expand the eleventh parameter instead. For +portability, you should only use single-digit M4 parameter expansion. + +With this in mind, we can explore the cases where macros invoke +macros@enddots{} + +@node Quotation and Nested Macros +@subsection Quotation and Nested Macros + +The examples below use the following macros: + +@example +define([car], [$1]) +define([active], [ACT, IVE]) +define([array], [int tab[10]]) +@end example + +Each additional embedded macro call introduces other possible +interesting quotations: + +@example +car(active) +@result{}ACT +car([active]) +@result{}ACT, IVE +car([[active]]) +@result{}active +@end example + +In the first case, the top level looks for the arguments of @code{car}, +and finds @samp{active}. Because M4 evaluates its arguments +before applying the macro, @samp{active} is expanded, which results in: + +@example +car(ACT, IVE) +@result{}ACT +@end example + +@noindent +In the second case, the top level gives @samp{active} as first and only +argument of @code{car}, which results in: + +@example +active +@result{}ACT, IVE +@end example + +@noindent +i.e., the argument is evaluated @emph{after} the macro that invokes it. +In the third case, @code{car} receives @samp{[active]}, which results in: + +@example +[active] +@result{}active +@end example + +@noindent +exactly as we already saw above. + +The example above, applied to a more realistic example, gives: + +@example +car(int tab[10];) +@result{}int tab10; +car([int tab[10];]) +@result{}int tab10; +car([[int tab[10];]]) +@result{}int tab[10]; +@end example + +@noindent +Huh? The first case is easily understood, but why is the second wrong, +and the third right? To understand that, you must know that after +M4 expands a macro, the resulting text is immediately subjected +to macro expansion and quote removal. This means that the quote removal +occurs twice---first before the argument is passed to the @code{car} +macro, and second after the @code{car} macro expands to the first +argument. + +As the author of the Autoconf macro @code{car}, you then consider it to +be incorrect that your users have to double-quote the arguments of +@code{car}, so you ``fix'' your macro. Let's call it @code{qar} for +quoted car: + +@example +define([qar], [[$1]]) +@end example + +@noindent +and check that @code{qar} is properly fixed: + +@example +qar([int tab[10];]) +@result{}int tab[10]; +@end example + +@noindent +Ahhh! That's much better. + +But note what you've done: now that the result of @code{qar} is always +a literal string, the only time a user can use nested macros is if she +relies on an @emph{unquoted} macro call: + +@example +qar(active) +@result{}ACT +qar([active]) +@result{}active +@end example + +@noindent +leaving no way for her to reproduce what she used to do with @code{car}: + +@example +car([active]) +@result{}ACT, IVE +@end example + +@noindent +Worse yet: she wants to use a macro that produces a set of @code{cpp} +macros: + +@example +define([my_includes], [#include <stdio.h>]) +car([my_includes]) +@result{}#include <stdio.h> +qar(my_includes) +@error{}EOF in argument list +@end example + +This macro, @code{qar}, because it double quotes its arguments, forces +its users to leave their macro calls unquoted, which is dangerous. +Commas and other active symbols are interpreted by M4 before +they are given to the macro, often not in the way the users expect. +Also, because @code{qar} behaves differently from the other macros, +it's an exception that should be avoided in Autoconf. + +@node Changequote is Evil +@subsection @code{changequote} is Evil +@cindex @code{changequote} + +The temptation is often high to bypass proper quotation, in particular +when it's late at night. Then, many experienced Autoconf hackers +finally surrender to the dark side of the force and use the ultimate +weapon: @code{changequote}. + +The M4 builtin @code{changequote} belongs to a set of primitives that +allow one to adjust the syntax of the language to adjust it to one's +needs. For instance, by default M4 uses @samp{`} and @samp{'} as +quotes, but in the context of shell programming (and actually of most +programming languages), that's about the worst choice one can make: +because of strings and back-quoted expressions in shell code (such as +@samp{'this'} and @samp{`that`}), and because of literal characters in usual +programming languages (as in @samp{'0'}), there are many unbalanced +@samp{`} and @samp{'}. Proper M4 quotation then becomes a nightmare, if +not impossible. In order to make M4 useful in such a context, its +designers have equipped it with @code{changequote}, which makes it +possible to choose another pair of quotes. M4sugar, M4sh, Autoconf, and +Autotest all have chosen to use @samp{[} and @samp{]}. Not especially +because they are unlikely characters, but @emph{because they are +characters unlikely to be unbalanced}. + +There are other magic primitives, such as @code{changecom} to specify +what syntactic forms are comments (it is common to see +@samp{changecom(<!--, -->)} when M4 is used to produce HTML pages), +@code{changeword} and @code{changesyntax} to change other syntactic +details (such as the character to denote the @var{n}th argument, @samp{$} by +default, the parentheses around arguments, etc.). + +These primitives are really meant to make M4 more useful for specific +domains: they should be considered like command line options: +@option{--quotes}, @option{--comments}, @option{--words}, and +@option{--syntax}. Nevertheless, they are implemented as M4 builtins, as +it makes M4 libraries self contained (no need for additional options). + +There lies the problem@enddots{} + +@sp 1 + +The problem is that it is then tempting to use them in the middle of an +M4 script, as opposed to its initialization. This, if not carefully +thought out, can lead to disastrous effects: @emph{you are changing the +language in the middle of the execution}. Changing and restoring the +syntax is often not enough: if you happened to invoke macros in between, +these macros are lost, as the current syntax is probably not +the one they were implemented with. + +@c FIXME: I've been looking for a short, real case example, but I +@c lost them all :( + + +@node Quadrigraphs +@subsection Quadrigraphs +@cindex quadrigraphs +@cindex @samp{@@S|@@} +@cindex @samp{@@&t@@} +@c Info cannot handle `:' in index entries. +@ifnotinfo +@cindex @samp{@@<:@@} +@cindex @samp{@@:>@@} +@cindex @samp{@@%:@@} +@cindex @samp{@@@{:@@} +@cindex @samp{@@:@}@@} +@end ifnotinfo + +When writing an Autoconf macro you may occasionally need to generate +special characters that are difficult to express with the standard +Autoconf quoting rules. For example, you may need to output the regular +expression @samp{[^[]}, which matches any character other than @samp{[}. +This expression contains unbalanced brackets so it cannot be put easily +into an M4 macro. + +Additionally, there are a few m4sugar macros (such as @code{m4_split} +and @code{m4_expand}) which internally use special markers in addition +to the regular quoting characters. If the arguments to these macros +contain the literal strings @samp{-=<@{(} or @samp{)@}>=-}, the macros +might behave incorrectly. + +You can work around these problems by using one of the following +@dfn{quadrigraphs}: + +@table @samp +@item @@<:@@ +@samp{[} +@item @@:>@@ +@samp{]} +@item @@S|@@ +@samp{$} +@item @@%:@@ +@samp{#} +@item @@@{:@@ +@samp{(} +@item @@:@}@@ +@samp{)} +@item @@&t@@ +Expands to nothing. +@end table + +Quadrigraphs are replaced at a late stage of the translation process, +after @command{m4} is run, so they do not get in the way of M4 quoting. +For example, the string @samp{^@@<:@@}, independently of its quotation, +appears as @samp{^[} in the output. + +The empty quadrigraph can be used: + +@itemize @minus +@item to mark trailing spaces explicitly + +Trailing spaces are smashed by @command{autom4te}. This is a feature. + +@item to produce quadrigraphs and other strings reserved by m4sugar + +For instance @samp{@@<@@&t@@:@@} produces @samp{@@<:@@}. For a more +contrived example: + +@example +m4_define([a], [A])m4_define([b], [B])m4_define([c], [C])dnl +m4_split([a )@}>=- b -=<@{( c]) +@result{}[a], [], [B], [], [c] +m4_split([a )@}@@&t@@>=- b -=<@@&t@@@{( c]) +@result{}[a], [)@}>=-], [b], [-=<@{(], [c] +@end example + +@item to escape @emph{occurrences} of forbidden patterns + +For instance you might want to mention @code{AC_FOO} in a comment, while +still being sure that @command{autom4te} still catches unexpanded +@samp{AC_*}. Then write @samp{AC@@&t@@_FOO}. +@end itemize + +The name @samp{@@&t@@} was suggested by Paul Eggert: + +@quotation +I should give some credit to the @samp{@@&t@@} pun. The @samp{&} is my +own invention, but the @samp{t} came from the source code of the +ALGOL68C compiler, written by Steve Bourne (of Bourne shell fame), +and which used @samp{mt} to denote the empty string. In C, it would +have looked like something like: + +@example +char const mt[] = ""; +@end example + +@noindent +but of course the source code was written in Algol 68. + +I don't know where he got @samp{mt} from: it could have been his own +invention, and I suppose it could have been a common pun around the +Cambridge University computer lab at the time. +@end quotation + + +@node Balancing Parentheses +@subsection Dealing with unbalanced parentheses +@cindex balancing parentheses +@cindex parentheses, balancing +@cindex unbalanced parentheses, managing + +One of the pitfalls of portable shell programming is that @command{case} +statements require unbalanced parentheses (@pxref{case, , Limitations of +Shell Builtins}). With syntax highlighting +editors, the presence of unbalanced @samp{)} can interfere with editors +that perform syntax highlighting of macro contents based on finding the +matching @samp{(}. Another concern is how much editing must be done +when transferring code snippets between shell scripts and macro +definitions. But most importantly, the presence of unbalanced +parentheses can introduce expansion bugs. + +For an example, here is an underquoted attempt to use the macro +@code{my_case}, which happens to expand to a portable @command{case} +statement: + +@example +AC_DEFUN([my_case], +[case $file_name in + *.c) echo "C source code";; +esac]) +AS_IF(:, my_case) +@end example + +@noindent +In the above example, the @code{AS_IF} call underquotes its arguments. +As a result, the unbalanced @samp{)} generated by the premature +expansion of @code{my_case} results in expanding @code{AS_IF} with a +truncated parameter, and the expansion is syntactically invalid: + +@example +if :; then + case $file_name in + *.c +fi echo "C source code";; +esac) +@end example + +If nothing else, this should emphasize the importance of the quoting +arguments to macro calls. On the other hand, there are several +variations for defining @code{my_case} to be more robust, even when used +without proper quoting, each with some benefits and some drawbacks. + +@itemize @w{} +@item Creative literal shell comment +@example +AC_DEFUN([my_case], +[case $file_name in #( + *.c) echo "C source code";; +esac]) +@end example +@noindent +This version provides balanced parentheses to several editors, and can +be copied and pasted into a terminal as is. Unfortunately, it is still +unbalanced as an Autoconf argument, since @samp{#(} is an M4 comment +that masks the normal properties of @samp{(}. + +@item Quadrigraph shell comment +@example +AC_DEFUN([my_case], +[case $file_name in @@%:@@( + *.c) echo "C source code";; +esac]) +@end example +@noindent +This version provides balanced parentheses to even more editors, and can +be used as a balanced Autoconf argument. Unfortunately, it requires +some editing before it can be copied and pasted into a terminal, and the +use of the quadrigraph @samp{@@%:@@} for @samp{#} reduces readability. + +@item Quoting just the parenthesis +@example +AC_DEFUN([my_case], +[case $file_name in + *.c[)] echo "C source code";; +esac]) +@end example +@noindent +This version quotes the @samp{)}, so that it can be used as a balanced +Autoconf argument. As written, this is not balanced to an editor, but +it can be coupled with @samp{[#(]} to meet that need, too. However, it +still requires some edits before it can be copied and pasted into a +terminal. + +@item Double-quoting the entire statement +@example +AC_DEFUN([my_case], +[[case $file_name in #( + *.c) echo "C source code";; +esac]]) +@end example +@noindent +Since the entire macro is double-quoted, there is no problem with using +this as an Autoconf argument; and since the double-quoting is over the +entire statement, this code can be easily copied and pasted into a +terminal. However, the double quoting prevents the expansion of any +macros inside the case statement, which may cause its own set of +problems. + +@item Using @code{AS_CASE} +@example +AC_DEFUN([my_case], +[AS_CASE([$file_name], + [*.c], [echo "C source code"])]) +@end example +@noindent +This version avoids the balancing issue altogether, by relying on +@code{AS_CASE} (@pxref{Common Shell Constructs}); it also allows for the +expansion of @code{AC_REQUIRE} to occur prior to the entire case +statement, rather than within a branch of the case statement that might +not be taken. However, the abstraction comes with a penalty that it is +no longer a quick copy, paste, and edit to get back to shell code. +@end itemize + + +@node Quotation Rule Of Thumb +@subsection Quotation Rule Of Thumb + +To conclude, the quotation rule of thumb is: + +@center @emph{One pair of quotes per pair of parentheses.} + +Never over-quote, never under-quote, in particular in the definition of +macros. In the few places where the macros need to use brackets +(usually in C program text or regular expressions), properly quote +@emph{the arguments}! + +It is common to read Autoconf programs with snippets like: + +@example +AC_TRY_LINK( +changequote(<<, >>)dnl +<<#include <time.h> +#ifndef tzname /* For SGI. */ +extern char *tzname[]; /* RS6000 and others reject char **tzname. */ +#endif>>, +changequote([, ])dnl +[atoi (*tzname);], ac_cv_var_tzname=yes, ac_cv_var_tzname=no) +@end example + +@noindent +which is incredibly useless since @code{AC_TRY_LINK} is @emph{already} +double quoting, so you just need: + +@example +AC_TRY_LINK( +[#include <time.h> +#ifndef tzname /* For SGI. */ +extern char *tzname[]; /* RS6000 and others reject char **tzname. */ +#endif], + [atoi (*tzname);], + [ac_cv_var_tzname=yes], + [ac_cv_var_tzname=no]) +@end example + +@noindent +The M4-fluent reader might note that these two examples are rigorously +equivalent, since M4 swallows both the @samp{changequote(<<, >>)} +and @samp{<<} @samp{>>} when it @dfn{collects} the arguments: these +quotes are not part of the arguments! + +Simplified, the example above is just doing this: + +@example +changequote(<<, >>)dnl +<<[]>> +changequote([, ])dnl +@end example + +@noindent +instead of simply: + +@example +[[]] +@end example + +With macros that do not double quote their arguments (which is the +rule), double-quote the (risky) literals: + +@example +AC_LINK_IFELSE([AC_LANG_PROGRAM( +[[#include <time.h> +#ifndef tzname /* For SGI. */ +extern char *tzname[]; /* RS6000 and others reject char **tzname. */ +#endif]], + [atoi (*tzname);])], + [ac_cv_var_tzname=yes], + [ac_cv_var_tzname=no]) +@end example + +Please note that the macro @code{AC_TRY_LINK} is obsolete, so you really +should be using @code{AC_LINK_IFELSE} instead. + +@xref{Quadrigraphs}, for what to do if you run into a hopeless case +where quoting does not suffice. + +When you create a @command{configure} script using newly written macros, +examine it carefully to check whether you need to add more quotes in +your macros. If one or more words have disappeared in the M4 +output, you need more quotes. When in doubt, quote. + +However, it's also possible to put on too many layers of quotes. If +this happens, the resulting @command{configure} script may contain +unexpanded macros. The @command{autoconf} program checks for this problem +by looking for the string @samp{AC_} in @file{configure}. However, this +heuristic does not work in general: for example, it does not catch +overquoting in @code{AC_DEFINE} descriptions. + + +@c ---------------------------------------- Using autom4te + +@node Using autom4te +@section Using @command{autom4te} + +The Autoconf suite, including M4sugar, M4sh, and Autotest, in addition +to Autoconf per se, heavily rely on M4. All these different uses +revealed common needs factored into a layer over M4: +@command{autom4te}@footnote{ +@c +Yet another great name from Lars J. Aas. +@c +}. + +@command{autom4te} is a preprocessor that is like @command{m4}. +It supports M4 extensions designed for use in tools like Autoconf. + +@menu +* autom4te Invocation:: A GNU M4 wrapper +* Customizing autom4te:: Customizing the Autoconf package +@end menu + +@node autom4te Invocation +@subsection Invoking @command{autom4te} + +The command line arguments are modeled after M4's: + +@example +autom4te @var{options} @var{files} +@end example + +@noindent +@evindex M4 +where the @var{files} are directly passed to @command{m4}. By default, +GNU M4 is found during configuration, but the environment +variable +@env{M4} can be set to tell @command{autom4te} where to look. In addition +to the regular expansion, it handles the replacement of the quadrigraphs +(@pxref{Quadrigraphs}), and of @samp{__oline__}, the current line in the +output. It supports an extended syntax for the @var{files}: + +@table @file +@item @var{file}.m4f +This file is an M4 frozen file. Note that @emph{all the previous files +are ignored}. See the option @option{--melt} for the rationale. + +@item @var{file}? +If found in the library path, the @var{file} is included for expansion, +otherwise it is ignored instead of triggering a failure. +@end table + +@sp 1 + +Of course, it supports the Autoconf common subset of options: + +@table @option +@item --help +@itemx -h +Print a summary of the command line options and exit. + +@item --version +@itemx -V +Print the version number of Autoconf and exit. + +@item --verbose +@itemx -v +Report processing steps. + +@item --debug +@itemx -d +Don't remove the temporary files and be even more verbose. + +@item --include=@var{dir} +@itemx -I @var{dir} +Also look for input files in @var{dir}. Multiple invocations +accumulate. + +@item --output=@var{file} +@itemx -o @var{file} +Save output (script or trace) to @var{file}. The file @option{-} stands +for the standard output. +@end table + +@sp 1 + +As an extension of @command{m4}, it includes the following options: + +@table @option +@item --warnings=@var{category} +@itemx -W @var{category} +@evindex WARNINGS +@c FIXME: Point to the M4sugar macros, not Autoconf's. +Report the warnings related to @var{category} (which can actually be a +comma separated list). @xref{Reporting Messages}, macro +@code{AC_DIAGNOSE}, for a comprehensive list of categories. Special +values include: + +@table @samp +@item all +report all the warnings + +@item none +report none + +@item error +treats warnings as errors + +@item no-@var{category} +disable warnings falling into @var{category} +@end table + +Warnings about @samp{syntax} are enabled by default, and the environment +variable @env{WARNINGS}, a comma separated list of categories, is +honored. @samp{autom4te -W @var{category}} actually +behaves as if you had run: + +@example +autom4te --warnings=syntax,$WARNINGS,@var{category} +@end example + +@noindent +For example, if you want to disable defaults and @env{WARNINGS} +of @command{autom4te}, but enable the warnings about obsolete +constructs, you would use @option{-W none,obsolete}. + +@cindex Back trace +@cindex Macro invocation stack +@command{autom4te} displays a back trace for errors, but not for +warnings; if you want them, just pass @option{-W error}. + +@item --melt +@itemx -M +Do not use frozen files. Any argument @code{@var{file}.m4f} is +replaced by @code{@var{file}.m4}. This helps tracing the macros which +are executed only when the files are frozen, typically +@code{m4_define}. For instance, running: + +@example +autom4te --melt 1.m4 2.m4f 3.m4 4.m4f input.m4 +@end example + +@noindent +is roughly equivalent to running: + +@example +m4 1.m4 2.m4 3.m4 4.m4 input.m4 +@end example + +@noindent +while + +@example +autom4te 1.m4 2.m4f 3.m4 4.m4f input.m4 +@end example + +@noindent +is equivalent to: + +@example +m4 --reload-state=4.m4f input.m4 +@end example + +@item --freeze +@itemx -F +Produce a frozen state file. @command{autom4te} freezing is stricter +than M4's: it must produce no warnings, and no output other than empty +lines (a line with white space is @emph{not} empty) and comments +(starting with @samp{#}). Unlike @command{m4}'s similarly-named option, +this option takes no argument: + +@example +autom4te 1.m4 2.m4 3.m4 --freeze --output=3.m4f +@end example + +@noindent +corresponds to + +@example +m4 1.m4 2.m4 3.m4 --freeze-state=3.m4f +@end example + +@item --mode=@var{octal-mode} +@itemx -m @var{octal-mode} +Set the mode of the non-traces output to @var{octal-mode}; by default +@samp{0666}. +@end table + +@sp 1 + +@cindex @file{autom4te.cache} +As another additional feature over @command{m4}, @command{autom4te} +caches its results. GNU M4 is able to produce a regular +output and traces at the same time. Traces are heavily used in the +GNU Build System: @command{autoheader} uses them to build +@file{config.h.in}, @command{autoreconf} to determine what +GNU Build System components are used, @command{automake} to +``parse'' @file{configure.ac} etc. To avoid recomputation, +traces are cached while performing regular expansion, +and conversely. This cache is (actually, the caches are) stored in +the directory @file{autom4te.cache}. @emph{It can safely be removed} +at any moment (especially if for some reason @command{autom4te} +considers it trashed). + +@table @option +@item --cache=@var{directory} +@itemx -C @var{directory} +Specify the name of the directory where the result should be cached. +Passing an empty value disables caching. Be sure to pass a relative +file name, as for the time being, global caches are not supported. + +@item --no-cache +Don't cache the results. + +@item --force +@itemx -f +If a cache is used, consider it obsolete (but update it anyway). +@end table + +@sp 1 + +Because traces are so important to the GNU Build System, +@command{autom4te} provides high level tracing features as compared to +M4, and helps exploiting the cache: + +@table @option +@item --trace=@var{macro}[:@var{format}] +@itemx -t @var{macro}[:@var{format}] +Trace the invocations of @var{macro} according to the @var{format}. +Multiple @option{--trace} arguments can be used to list several macros. +Multiple @option{--trace} arguments for a single macro are not +cumulative; instead, you should just make @var{format} as long as +needed. + +The @var{format} is a regular string, with newlines if desired, and +several special escape codes. It defaults to @samp{$f:$l:$n:$%}. It can +use the following special escapes: + +@table @samp +@item $$ +@c $$ restore font-lock +The character @samp{$}. + +@item $f +The file name from which @var{macro} is called. + +@item $l +The line number from which @var{macro} is called. + +@item $d +The depth of the @var{macro} call. This is an M4 technical detail that +you probably don't want to know about. + +@item $n +The name of the @var{macro}. + +@item $@var{num} +The @var{num}th argument of the call to @var{macro}. + +@item $@@ +@itemx $@var{sep}@@ +@itemx $@{@var{separator}@}@@ +All the arguments passed to @var{macro}, separated by the character +@var{sep} or the string @var{separator} (@samp{,} by default). Each +argument is quoted, i.e., enclosed in a pair of square brackets. + +@item $* +@itemx $@var{sep}* +@itemx $@{@var{separator}@}* +As above, but the arguments are not quoted. + +@item $% +@itemx $@var{sep}% +@itemx $@{@var{separator}@}% +As above, but the arguments are not quoted, all new line characters in +the arguments are smashed, and the default separator is @samp{:}. + +The escape @samp{$%} produces single-line trace outputs (unless you put +newlines in the @samp{separator}), while @samp{$@@} and @samp{$*} do +not. +@end table + +@xref{autoconf Invocation}, for examples of trace uses. + +@item --preselect=@var{macro} +@itemx -p @var{macro} +Cache the traces of @var{macro}, but do not enable traces. This is +especially important to save CPU cycles in the future. For instance, +when invoked, @command{autoconf} preselects all the macros that +@command{autoheader}, @command{automake}, @command{autoreconf}, etc., +trace, so that running @command{m4} is not needed to trace them: the +cache suffices. This results in a huge speed-up. +@end table + +@sp 1 + +@cindex Autom4te Library +Finally, @command{autom4te} introduces the concept of @dfn{Autom4te +libraries}. They consists in a powerful yet extremely simple feature: +sets of combined command line arguments: + +@table @option +@item --language=@var{language} +@itemx -l @var{language} +Use the @var{language} Autom4te library. Current languages include: + +@table @code +@item M4sugar +create M4sugar output. + +@item M4sh +create M4sh executable shell scripts. + +@item Autotest +create Autotest executable test suites. + +@item Autoconf-without-aclocal-m4 +create Autoconf executable configure scripts without +reading @file{aclocal.m4}. + +@item Autoconf +create Autoconf executable configure scripts. This language inherits +all the characteristics of @code{Autoconf-without-aclocal-m4} and +additionally reads @file{aclocal.m4}. +@end table + +@item --prepend-include=@var{dir} +@itemx -B @var{dir} +Prepend directory @var{dir} to the search path. This is used to include +the language-specific files before any third-party macros. + +@end table + +@cindex @file{autom4te.cfg} +As an example, if Autoconf is installed in its default location, +@file{/usr/local}, the command @samp{autom4te -l m4sugar foo.m4} is +strictly equivalent to the command: + +@example +autom4te --prepend-include /usr/local/share/autoconf \ + m4sugar/m4sugar.m4f --warnings syntax foo.m4 +@end example + +@noindent +Recursive expansion applies here: the command @samp{autom4te -l m4sh foo.m4} +is the same as @samp{autom4te --language M4sugar m4sugar/m4sh.m4f +foo.m4}, i.e.: + +@example +autom4te --prepend-include /usr/local/share/autoconf \ + m4sugar/m4sugar.m4f m4sugar/m4sh.m4f --mode 777 foo.m4 +@end example + +@noindent +The definition of the languages is stored in @file{autom4te.cfg}. + +@node Customizing autom4te +@subsection Customizing @command{autom4te} + +One can customize @command{autom4te} via @file{~/.autom4te.cfg} (i.e., +as found in the user home directory), and @file{./.autom4te.cfg} (i.e., +as found in the directory from which @command{autom4te} is run). The +order is first reading @file{autom4te.cfg}, then @file{~/.autom4te.cfg}, +then @file{./.autom4te.cfg}, and finally the command line arguments. + +In these text files, comments are introduced with @code{#}, and empty +lines are ignored. Customization is performed on a per-language basis, +wrapped in between a @samp{begin-language: "@var{language}"}, +@samp{end-language: "@var{language}"} pair. + +Customizing a language stands for appending options (@pxref{autom4te +Invocation}) to the current definition of the language. Options, and +more generally arguments, are introduced by @samp{args: +@var{arguments}}. You may use the traditional shell syntax to quote the +@var{arguments}. + +As an example, to disable Autoconf caches (@file{autom4te.cache}) +globally, include the following lines in @file{~/.autom4te.cfg}: + +@verbatim +## ------------------ ## +## User Preferences. ## +## ------------------ ## + +begin-language: "Autoconf-without-aclocal-m4" +args: --no-cache +end-language: "Autoconf-without-aclocal-m4" +@end verbatim + + +@node Programming in M4sugar +@section Programming in M4sugar + +@cindex M4sugar +M4 by itself provides only a small, but sufficient, set of all-purpose +macros. M4sugar introduces additional generic macros. Its name was +coined by Lars J. Aas: ``Readability And Greater Understanding Stands 4 +M4sugar''. + +M4sugar reserves the macro namespace @samp{^_m4_} for internal use, and +the macro namespace @samp{^m4_} for M4sugar macros. You should not +define your own macros into these namespaces. + +@menu +* Redefined M4 Macros:: M4 builtins changed in M4sugar +* Diagnostic Macros:: Diagnostic messages from M4sugar +* Diversion support:: Diversions in M4sugar +* Conditional constructs:: Conditions in M4 +* Looping constructs:: Iteration in M4 +* Evaluation Macros:: More quotation and evaluation control +* Text processing Macros:: String manipulation in M4 +* Number processing Macros:: Arithmetic computation in M4 +* Set manipulation Macros:: Set manipulation in M4 +* Forbidden Patterns:: Catching unexpanded macros +@end menu + +@node Redefined M4 Macros +@subsection Redefined M4 Macros + +@msindex{builtin} +@msindex{changecom} +@msindex{changequote} +@msindex{debugfile} +@msindex{debugmode} +@msindex{decr} +@msindex{define} +@msindex{divnum} +@msindex{errprint} +@msindex{esyscmd} +@msindex{eval} +@msindex{format} +@msindex{ifdef} +@msindex{incr} +@msindex{index} +@msindex{indir} +@msindex{len} +@msindex{pushdef} +@msindex{shift} +@msindex{substr} +@msindex{syscmd} +@msindex{sysval} +@msindex{traceoff} +@msindex{traceon} +@msindex{translit} +With a few exceptions, all the M4 native macros are moved in the +@samp{m4_} pseudo-namespace, e.g., M4sugar renames @code{define} as +@code{m4_define} etc. + +The list of macros unchanged from M4, except for their name, is: +@itemize @minus +@item m4_builtin +@item m4_changecom +@item m4_changequote +@item m4_debugfile +@item m4_debugmode +@item m4_decr +@item m4_define +@item m4_divnum +@item m4_errprint +@item m4_esyscmd +@item m4_eval +@item m4_format +@item m4_ifdef +@item m4_incr +@item m4_index +@item m4_indir +@item m4_len +@item m4_pushdef +@item m4_shift +@item m4_substr +@item m4_syscmd +@item m4_sysval +@item m4_traceoff +@item m4_traceon +@item m4_translit +@end itemize + +Some M4 macros are redefined, and are slightly incompatible with their +native equivalent. + +@defmac __file__ +@defmacx __line__ +@MSindex __file__ +@MSindex __line__ +All M4 macros starting with @samp{__} retain their original name: for +example, no @code{m4__file__} is defined. +@end defmac + +@defmac __oline__ +@MSindex __oline__ +This is not technically a macro, but a feature of Autom4te. The +sequence @code{__oline__} can be used similarly to the other m4sugar +location macros, but rather than expanding to the location of the input +file, it is translated to the line number where it appears in the output +file after all other M4 expansions. +@end defmac + +@defmac dnl +@MSindex dnl +This macro kept its original name: no @code{m4_dnl} is defined. +@end defmac + +@defmac m4_bpatsubst (@var{string}, @var{regexp}, @ovar{replacement}) +@msindex{bpatsubst} +This macro corresponds to @code{patsubst}. The name @code{m4_patsubst} +is kept for future versions of M4sugar, once GNU M4 2.0 is +released and supports extended regular expression syntax. +@end defmac + +@defmac m4_bregexp (@var{string}, @var{regexp}, @ovar{replacement}) +@msindex{bregexp} +This macro corresponds to @code{regexp}. The name @code{m4_regexp} +is kept for future versions of M4sugar, once GNU M4 2.0 is +released and supports extended regular expression syntax. +@end defmac + +@defmac m4_copy (@var{source}, @var{dest}) +@defmacx m4_copy_force (@var{source}, @var{dest}) +@defmacx m4_rename (@var{source}, @var{dest}) +@defmacx m4_rename_force (@var{source}, @var{dest}) +@msindex{copy} +@msindex{copy_force} +@msindex{rename} +@msindex{rename_force} +These macros aren't directly builtins, but are closely related to +@code{m4_pushdef} and @code{m4_defn}. @code{m4_copy} and +@code{m4_rename} ensure that @var{dest} is undefined, while +@code{m4_copy_force} and @code{m4_rename_force} overwrite any existing +definition. All four macros then proceed to copy the entire pushdef +stack of definitions of @var{source} over to @var{dest}. @code{m4_copy} +and @code{m4_copy_force} preserve the source (including in the special +case where @var{source} is undefined), while @code{m4_rename} and +@code{m4_rename_force} undefine the original macro name (making it an +error to rename an undefined @var{source}). + +Note that attempting to invoke a renamed macro might not work, since the +macro may have a dependence on helper macros accessed via composition of +@samp{$0} but that were not also renamed; likewise, other macros may +have a hard-coded dependence on @var{source} and could break if +@var{source} has been deleted. On the other hand, it is always safe to +rename a macro to temporarily move it out of the way, then rename it +back later to restore original semantics. +@end defmac + +@defmac m4_defn (@var{macro}@dots{}) +@msindex{defn} +This macro fails if @var{macro} is not defined, even when using older +versions of M4 that did not warn. See @code{m4_undefine}. +Unfortunately, in order to support these older versions of M4, there are +some situations involving unbalanced quotes where concatenating multiple +macros together will work in newer M4 but not in m4sugar; use +quadrigraphs to work around this. +@end defmac + +@defmac m4_divert (@var{diversion}) +@msindex{divert} +M4sugar relies heavily on diversions, so rather than behaving as a +primitive, @code{m4_divert} behaves like: +@example +m4_divert_pop()m4_divert_push([@var{diversion}]) +@end example +@noindent +@xref{Diversion support}, for more details about the use of the +diversion stack. In particular, this implies that @var{diversion} +should be a named diversion rather than a raw number. But be aware that +it is seldom necessary to explicitly change the diversion stack, and +that when done incorrectly, it can lead to syntactically invalid +scripts. +@end defmac + +@defmac m4_dumpdef (@var{name}@dots{}) +@defmacx m4_dumpdefs (@var{name}@dots{}) +@msindex{dumpdef} +@msindex{dumpdefs} +@code{m4_dumpdef} is like the M4 builtin, except that this version +requires at least one argument, output always goes to standard error +rather than the current debug file, no sorting is done on multiple +arguments, and an error is issued if any +@var{name} is undefined. @code{m4_dumpdefs} is a convenience macro that +calls @code{m4_dumpdef} for all of the +@code{m4_pushdef} stack of definitions, starting with the current, and +silently does nothing if @var{name} is undefined. + +Unfortunately, due to a limitation in M4 1.4.x, any macro defined as a +builtin is output as the empty string. This behavior is rectified by +using M4 1.6 or newer. However, this behavior difference means that +@code{m4_dumpdef} should only be used while developing m4sugar macros, +and never in the final published form of a macro. +@end defmac + +@defmac m4_esyscmd_s (@var{command}) +@msindex{esyscmd_s} +Like @code{m4_esyscmd}, this macro expands to the result of running +@var{command} in a shell. The difference is that any trailing newlines +are removed, so that the output behaves more like shell command +substitution. +@end defmac + +@defmac m4_exit (@var{exit-status}) +@msindex{exit} +This macro corresponds to @code{m4exit}. +@end defmac + +@defmac m4_if (@var{comment}) +@defmacx m4_if (@var{string-1}, @var{string-2}, @var{equal}, @ovar{not-equal}) +@defmacx m4_if (@var{string-1}, @var{string-2}, @var{equal-1}, @ + @var{string-3}, @var{string-4}, @var{equal-2}, @dots{}, @ovar{not-equal}) +@msindex{if} +This macro corresponds to @code{ifelse}. @var{string-1} and +@var{string-2} are compared literally, so usually one of the two +arguments is passed unquoted. @xref{Conditional constructs}, for more +conditional idioms. +@end defmac + +@defmac m4_include (@var{file}) +@defmacx m4_sinclude (@var{file}) +@msindex{include} +@msindex{sinclude} +Like the M4 builtins, but warn against multiple inclusions of @var{file}. +@end defmac + +@defmac m4_mkstemp (@var{template}) +@defmacx m4_maketemp (@var{template}) +@msindex{maketemp} +@msindex{mkstemp} +Posix requires @code{maketemp} to replace the trailing @samp{X} +characters in @var{template} with the process id, without regards to the +existence of a file by that name, but this a security hole. When this +was pointed out to the Posix folks, they agreed to invent a new macro +@code{mkstemp} that always creates a uniquely named file, but not all +versions of GNU M4 support the new macro. In M4sugar, +@code{m4_maketemp} and @code{m4_mkstemp} are synonyms for each other, +and both have the secure semantics regardless of which macro the +underlying M4 provides. +@end defmac + +@defmac m4_popdef (@var{macro}@dots{}) +@msindex{popdef} +This macro fails if @var{macro} is not defined, even when using older +versions of M4 that did not warn. See @code{m4_undefine}. +@end defmac + +@defmac m4_undefine (@var{macro}@dots{}) +@msindex{undefine} +This macro fails if @var{macro} is not defined, even when using older +versions of M4 that did not warn. Use + +@example +m4_ifdef([@var{macro}], [m4_undefine([@var{macro}])]) +@end example + +@noindent +if you are not sure whether @var{macro} is defined. +@end defmac + +@defmac m4_undivert (@var{diversion}@dots{}) +@msindex{undivert} +Unlike the M4 builtin, at least one @var{diversion} must be specified. +Also, since the M4sugar diversion stack prefers named +diversions, the use of @code{m4_undivert} to include files is risky. +@xref{Diversion support}, for more details about the use of the +diversion stack. But be aware that it is seldom necessary to explicitly +change the diversion stack, and that when done incorrectly, it can lead +to syntactically invalid scripts. +@end defmac + +@defmac m4_wrap (@var{text}) +@defmacx m4_wrap_lifo (@var{text}) +@msindex{wrap} +@msindex{wrap_lifo} +These macros correspond to @code{m4wrap}. Posix requires arguments of +multiple wrap calls to be reprocessed at EOF in the same order +as the original calls (first-in, first-out). GNU M4 versions +through 1.4.10, however, reprocess them in reverse order (last-in, +first-out). Both orders are useful, therefore, you can rely on +@code{m4_wrap} to provide FIFO semantics and @code{m4_wrap_lifo} for +LIFO semantics, regardless of the underlying GNU M4 version. + +Unlike the GNU M4 builtin, these macros only recognize one +argument, and avoid token pasting between consecutive invocations. On +the other hand, nested calls to @code{m4_wrap} from within wrapped text +work just as in the builtin. +@end defmac + + +@node Diagnostic Macros +@subsection Diagnostic messages from M4sugar +@cindex Messages, from @command{M4sugar} + +When macros statically diagnose abnormal situations, benign or fatal, +they should report them using these macros. For issuing dynamic issues, +i.e., when @command{configure} is run, see @ref{Printing Messages}. + +@defmac m4_assert (@var{expression}, @dvar{exit-status, 1}) +@msindex{assert} +Assert that the arithmetic @var{expression} evaluates to non-zero. +Otherwise, issue a fatal error, and exit @command{autom4te} with +@var{exit-status}. +@end defmac + +@defmac m4_errprintn (@var{message}) +@msindex{errprintn} +Similar to the builtin @code{m4_errprint}, except that a newline is +guaranteed after @var{message}. +@end defmac + +@anchor{m4_fatal} +@defmac m4_fatal (@var{message}) +@msindex{fatal} +Report a severe error @var{message} prefixed with the current location, +and have @command{autom4te} die. +@end defmac + +@defmac m4_location +@msindex{location} +Useful as a prefix in a message line. Short for: +@example +__file__:__line__ +@end example +@end defmac + +@anchor{m4_warn} +@defmac m4_warn (@var{category}, @var{message}) +@msindex{warn} +Report @var{message} as a warning (or as an error if requested by the +user) if warnings of the @var{category} are turned on. If the message +is emitted, it is prefixed with the current location, and followed by a +call trace of all macros defined via @code{AC_DEFUN} used to get to the +current expansion. You are encouraged to use standard categories, which +currently include: + +@table @samp +@item all +messages that don't fall into one of the following categories. Use of an +empty @var{category} is equivalent. + +@item cross +related to cross compilation issues. + +@item obsolete +use of an obsolete construct. + +@item syntax +dubious syntactic constructs, incorrectly ordered macro calls. +@end table +@end defmac + + +@node Diversion support +@subsection Diversion support + +M4sugar makes heavy use of diversions under the hood, because it is +often the case that +text that must appear early in the output is not discovered until late +in the input. Additionally, some of the topological sorting algorithms +used in resolving macro dependencies use diversions. However, most +macros should not need to change diversions directly, but rather rely on +higher-level M4sugar macros to manage diversions transparently. If you +change diversions improperly, you risk generating a syntactically +invalid script, because an incorrect diversion will violate assumptions +made by many macros about whether prerequisite text has been previously +output. In short, if you manually change the diversion, you should not +expect any macros provided by the Autoconf package to work until you +have restored the diversion stack back to its original state. + +In the rare case that it is necessary to write a macro that explicitly +outputs text to a different diversion, it is important to be aware of an +M4 limitation regarding diversions: text only goes to a diversion if it +is not part of argument collection. Therefore, any macro that changes +the current diversion cannot be used as an unquoted argument to another +macro, but must be expanded at the top level. The macro +@code{m4_expand} will diagnose any attempt to change diversions, since +it is generally useful only as an argument to another macro. The +following example shows what happens when diversion manipulation is +attempted within macro arguments: + +@example +m4_do([normal text] +m4_divert_push([KILL])unwanted[]m4_divert_pop([KILL]) +[m4_divert_push([KILL])discarded[]m4_divert_pop([KILL])])dnl +@result{}normal text +@result{}unwanted +@end example + +@noindent +Notice that the unquoted text @code{unwanted} is output, even though it +was processed while the current diversion was @code{KILL}, because it +was collected as part of the argument to @code{m4_do}. However, the +text @code{discarded} disappeared as desired, because the diversion +changes were single-quoted, and were not expanded until the top-level +rescan of the output of @code{m4_do}. + +To make diversion management easier, M4sugar uses the concept of named +diversions. Rather than using diversion numbers directly, it is nicer +to associate a name with each diversion. The diversion number associated +with a particular diversion name is an implementation detail, and a +syntax warning is issued if a diversion number is used instead of a +name. In general, you should not output text +to a named diversion until after calling the appropriate initialization +routine for your language (@code{m4_init}, @code{AS_INIT}, +@code{AT_INIT}, @dots{}), although there are some exceptions documented +below. + +M4sugar defines two named diversions. +@table @code +@item KILL +Text written to this diversion is discarded. This is the default +diversion once M4sugar is initialized. +@item GROW +This diversion is used behind the scenes by topological sorting macros, +such as @code{AC_REQUIRE}. +@end table + +M4sh adds several more named diversions. +@table @code +@item BINSH +This diversion is reserved for the @samp{#!} interpreter line. +@item HEADER-REVISION +This diversion holds text from @code{AC_REVISION}. +@item HEADER-COMMENT +This diversion holds comments about the purpose of a file. +@item HEADER-COPYRIGHT +This diversion is managed by @code{AC_COPYRIGHT}. +@item M4SH-SANITIZE +This diversion contains M4sh sanitization code, used to ensure M4sh is +executing in a reasonable shell environment. +@item M4SH-INIT +This diversion contains M4sh initialization code, initializing variables +that are required by other M4sh macros. +@item BODY +This diversion contains the body of the shell code, and is the default +diversion once M4sh is initialized. +@end table + +Autotest inherits diversions from M4sh, and changes the default +diversion from @code{BODY} back to @code{KILL}. It also adds several +more named diversions, with the following subset designed for developer +use. +@table @code +@item PREPARE_TESTS +This diversion contains initialization sequences which are executed +after @file{atconfig} and @file{atlocal}, and after all command line +arguments have been parsed, but prior to running any tests. It can be +used to set up state that is required across all tests. This diversion +will work even before @code{AT_INIT}. +@end table + +Autoconf inherits diversions from M4sh, and adds the following named +diversions which developers can utilize. +@table @code +@item DEFAULTS +This diversion contains shell variable assignments to set defaults that +must be in place before arguments are parsed. This diversion is placed +early enough in @file{configure} that it is unsafe to expand any +autoconf macros into this diversion. +@item HELP_ENABLE +If @code{AC_PRESERVE_HELP_ORDER} was used, then text placed in this +diversion will be included as part of a quoted here-doc providing all of +the @option{--help} output of @file{configure} related to options +created by @code{AC_ARG_WITH} and @code{AC_ARG_ENABLE}. +@item INIT_PREPARE +This diversion occurs after all command line options have been parsed, +but prior to the main body of the @file{configure} script. This +diversion is the last chance to insert shell code such as variable +assignments or shell function declarations that will used by the +expansion of other macros. +@end table + +For now, the remaining named diversions of Autoconf, Autoheader, and +Autotest are not documented. In other words, +intentionally outputting text into an undocumented diversion is subject +to breakage in a future release of Autoconf. + +@defmac m4_cleardivert (@var{diversion}@dots{}) +@msindex{cleardivert} +Permanently discard any text that has been diverted into +@var{diversion}. +@end defmac + +@defmac m4_divert_once (@var{diversion}, @ovar{content}) +@msindex{divert_once} +Similar to @code{m4_divert_text}, except that @var{content} is only +output to @var{diversion} if this is the first time that +@code{m4_divert_once} has been called with its particular arguments. +@end defmac + +@defmac m4_divert_pop (@ovar{diversion}) +@msindex{divert_pop} +If provided, check that the current diversion is indeed @var{diversion}. +Then change to the diversion located earlier on the stack, giving an +error if an attempt is made to pop beyond the initial m4sugar diversion +of @code{KILL}. +@end defmac + +@defmac m4_divert_push (@var{diversion}) +@msindex{divert_push} +Remember the former diversion on the diversion stack, and output +subsequent text into @var{diversion}. M4sugar maintains a diversion +stack, and issues an error if there is not a matching pop for every +push. +@end defmac + +@defmac m4_divert_text (@var{diversion}, @ovar{content}) +@msindex{divert_text} +Output @var{content} and a newline into @var{diversion}, without +affecting the current diversion. Shorthand for: +@example +m4_divert_push([@var{diversion}])@var{content} +m4_divert_pop([@var{diversion}])dnl +@end example + +One use of @code{m4_divert_text} is to develop two related macros, where +macro @samp{MY_A} does the work, but adjusts what work is performed +based on whether the optional macro @samp{MY_B} has also been expanded. +Of course, it is possible to use @code{AC_BEFORE} within @code{MY_A} to +require that @samp{MY_B} occurs first, if it occurs at all. But this +imposes an ordering restriction on the user; it would be nicer if macros +@samp{MY_A} and @samp{MY_B} can be invoked in either order. The trick +is to let @samp{MY_B} leave a breadcrumb in an early diversion, which +@samp{MY_A} can then use to determine whether @samp{MY_B} has been +expanded. + +@example +AC_DEFUN([MY_A], +[# various actions +if test -n "$b_was_used"; then + # extra action +fi]) +AC_DEFUN([MY_B], +[AC_REQUIRE([MY_A])dnl +m4_divert_text([INIT_PREPARE], [b_was_used=true])]) +@end example + +@end defmac + +@defmac m4_init +@msindex{init} +Initialize the M4sugar environment, setting up the default named +diversion to be @code{KILL}. +@end defmac + +@node Conditional constructs +@subsection Conditional constructs + +The following macros provide additional conditional constructs as +convenience wrappers around @code{m4_if}. + +@defmac m4_bmatch (@var{string}, @var{regex-1}, @var{value-1}, @ + @ovar{regex-2}, @ovar{value-2}, @dots{}, @ovar{default}) +@msindex{bmatch} +The string @var{string} is repeatedly compared against a series of +@var{regex} arguments; if a match is found, the expansion is the +corresponding @var{value}, otherwise, the macro moves on to the next +@var{regex}. If no @var{regex} match, then the result is the optional +@var{default}, or nothing. +@end defmac + +@defmac m4_bpatsubsts (@var{string}, @var{regex-1}, @var{subst-1}, @ + @ovar{regex-2}, @ovar{subst-2}, @dots{}) +@msindex{bpatsubsts} +The string @var{string} is altered by @var{regex-1} and @var{subst-1}, +as if by: +@example +m4_bpatsubst([[@var{string}]], [@var{regex}], [@var{subst}]) +@end example + +@noindent +The result of the substitution is then passed through the next set of +@var{regex} and @var{subst}, and so forth. An empty @var{subst} implies +deletion of any matched portions in the current string. Note that this +macro over-quotes @var{string}; this behavior is intentional, so that +the result of each step of the recursion remains as a quoted string. +However, it means that anchors (@samp{^} and @samp{$} in the @var{regex} +will line up with the extra quotations, and not the characters of the +original string. The overquoting is removed after the final +substitution. +@end defmac + +@defmac m4_case (@var{string}, @var{value-1}, @var{if-value-1}, @ + @ovar{value-2}, @ovar{if-value-2}, @dots{}, @ovar{default}) +@msindex{case} +Test @var{string} against multiple @var{value} possibilities, resulting +in the first @var{if-value} for a match, or in the optional +@var{default}. This is shorthand for: +@example +m4_if([@var{string}], [@var{value-1}], [@var{if-value-1}], + [@var{string}], [@var{value-2}], [@var{if-value-2}], @dots{}, + [@var{default}]) +@end example +@end defmac + +@defmac m4_cond (@var{test-1}, @var{value-1}, @var{if-value-1}, @ + @ovar{test-2}, @ovar{value-2}, @ovar{if-value-2}, @dots{}, @ovar{default}) +@msindex{cond} +This macro was introduced in Autoconf 2.62. Similar to @code{m4_if}, +except that each @var{test} is expanded only when it is encountered. +This is useful for short-circuiting expensive tests; while @code{m4_if} +requires all its strings to be expanded up front before doing +comparisons, @code{m4_cond} only expands a @var{test} when all earlier +tests have failed. + +For an example, these two sequences give the same result, but in the +case where @samp{$1} does not contain a backslash, the @code{m4_cond} +version only expands @code{m4_index} once, instead of five times, for +faster computation if this is a common case for @samp{$1}. Notice that +every third argument is unquoted for @code{m4_if}, and quoted for +@code{m4_cond}: + +@example +m4_if(m4_index([$1], [\]), [-1], [$2], + m4_eval(m4_index([$1], [\\]) >= 0), [1], [$2], + m4_eval(m4_index([$1], [\$]) >= 0), [1], [$2], + m4_eval(m4_index([$1], [\`]) >= 0), [1], [$3], + m4_eval(m4_index([$1], [\"]) >= 0), [1], [$3], + [$2]) +m4_cond([m4_index([$1], [\])], [-1], [$2], + [m4_eval(m4_index([$1], [\\]) >= 0)], [1], [$2], + [m4_eval(m4_index([$1], [\$]) >= 0)], [1], [$2], + [m4_eval(m4_index([$1], [\`]) >= 0)], [1], [$3], + [m4_eval(m4_index([$1], [\"]) >= 0)], [1], [$3], + [$2]) +@end example +@end defmac + +@defmac m4_default (@var{expr-1}, @var{expr-2}) +@defmacx m4_default_quoted (@var{expr-1}, @var{expr-2}) +@defmacx m4_default_nblank (@var{expr-1}, @ovar{expr-2}) +@defmacx m4_default_nblank_quoted (@var{expr-1}, @ovar{expr-2}) +@msindex{default} +@msindex{default_quoted} +@msindex{default_nblank} +@msindex{default_nblank_quoted} +If @var{expr-1} contains text, use it. Otherwise, select @var{expr-2}. +@code{m4_default} expands the result, while @code{m4_default_quoted} +does not. Useful for providing a fixed default if the expression that +results in @var{expr-1} would otherwise be empty. The difference +between @code{m4_default} and @code{m4_default_nblank} is whether an +argument consisting of just blanks (space, tab, newline) is +significant. When using the expanding versions, note that an argument +may contain text but still expand to an empty string. + +@example +m4_define([active], [ACTIVE])dnl +m4_define([empty], [])dnl +m4_define([demo1], [m4_default([$1], [$2])])dnl +m4_define([demo2], [m4_default_quoted([$1], [$2])])dnl +m4_define([demo3], [m4_default_nblank([$1], [$2])])dnl +m4_define([demo4], [m4_default_nblank_quoted([$1], [$2])])dnl +demo1([active], [default]) +@result{}ACTIVE +demo1([], [active]) +@result{}ACTIVE +demo1([empty], [text]) +@result{} +-demo1([ ], [active])- +@result{}- - +demo2([active], [default]) +@result{}active +demo2([], [active]) +@result{}active +demo2([empty], [text]) +@result{}empty +-demo2([ ], [active])- +@result{}- - +demo3([active], [default]) +@result{}ACTIVE +demo3([], [active]) +@result{}ACTIVE +demo3([empty], [text]) +@result{} +-demo3([ ], [active])- +@result{}-ACTIVE- +demo4([active], [default]) +@result{}active +demo4([], [active]) +@result{}active +demo4([empty], [text]) +@result{}empty +-demo4([ ], [active])- +@result{}-active- +@end example +@end defmac + +@defmac m4_define_default (@var{macro}, @ovar{default-definition}) +@msindex{define_default} +If @var{macro} does not already have a definition, then define it to +@var{default-definition}. +@end defmac + +@defmac m4_ifblank (@var{cond}, @ovar{if-blank}, @ovar{if-text}) +@defmacx m4_ifnblank (@var{cond}, @ovar{if-text}, @ovar{if-blank}) +@msindex{ifblank} +@msindex{ifnblank} +If @var{cond} is empty or consists only of blanks (space, tab, newline), +then expand @var{if-blank}; otherwise, expand @var{if-text}. Two +variants exist, in order to make it easier to select the correct logical +sense when using only two parameters. Note that this is more efficient +than the equivalent behavior of: +@example +m4_ifval(m4_normalize([@var{cond}]), @var{if-text}, @var{if-blank}) +@end example +@end defmac + +@defmac m4_ifndef (@var{macro}, @var{if-not-defined}, @ovar{if-defined}) +@msindex{ifndef} +This is shorthand for: +@example +m4_ifdef([@var{macro}], [@var{if-defined}], [@var{if-not-defined}]) +@end example +@end defmac + +@defmac m4_ifset (@var{macro}, @ovar{if-true}, @ovar{if-false}) +@msindex{ifset} +If @var{macro} is undefined, or is defined as the empty string, expand +to @var{if-false}. Otherwise, expands to @var{if-true}. Similar to: +@example +m4_ifval(m4_defn([@var{macro}]), [@var{if-true}], [@var{if-false}]) +@end example +@noindent +except that it is not an error if @var{macro} is undefined. +@end defmac + +@defmac m4_ifval (@var{cond}, @ovar{if-true}, @ovar{if-false}) +@msindex{ifval} +Expands to @var{if-true} if @var{cond} is not empty, otherwise to +@var{if-false}. This is shorthand for: +@example +m4_if([@var{cond}], [], [@var{if-false}], [@var{if-true}]) +@end example +@end defmac + +@defmac m4_ifvaln (@var{cond}, @ovar{if-true}, @ovar{if-false}) +@msindex{ifvaln} +Similar to @code{m4_ifval}, except guarantee that a newline is present +after any non-empty expansion. Often followed by @code{dnl}. +@end defmac + +@defmac m4_n (@var{text}) +@msindex{n} +Expand to @var{text}, and add a newline if @var{text} is not empty. +Often followed by @code{dnl}. +@end defmac + + +@node Looping constructs +@subsection Looping constructs + +The following macros are useful in implementing recursive algorithms in +M4, including loop operations. An M4 list is formed by quoting a list +of quoted elements; generally the lists are comma-separated, although +@code{m4_foreach_w} is whitespace-separated. For example, the list +@samp{[[a], [b,c]]} contains two elements: @samp{[a]} and @samp{[b,c]}. +It is common to see lists with unquoted elements when those elements are +not likely to be macro names, as in @samp{[fputc_unlocked, +fgetc_unlocked]}. + +Although not generally recommended, it is possible for quoted lists to +have side effects; all side effects are expanded only once, and prior to +visiting any list element. On the other hand, the fact that unquoted +macros are expanded exactly once means that macros without side effects +can be used to generate lists. For example, + +@example +m4_foreach([i], [[1], [2], [3]m4_errprintn([hi])], [i]) +@error{}hi +@result{}123 +m4_define([list], [[1], [2], [3]]) +@result{} +m4_foreach([i], [list], [i]) +@result{}123 +@end example + +@defmac m4_argn (@var{n}, @ovar{arg}@dots{}) +@msindex{argn} +Extracts argument @var{n} (larger than 0) from the remaining arguments. +If there are too few arguments, the empty string is used. For any +@var{n} besides 1, this is more efficient than the similar +@samp{m4_car(m4_shiftn([@var{n}], [], [@var{arg}@dots{}]))}. +@end defmac + +@defmac m4_car (@var{arg}@dots{}) +@msindex{car} +Expands to the quoted first @var{arg}. Can be used with @code{m4_cdr} +to recursively iterate +through a list. Generally, when using quoted lists of quoted elements, +@code{m4_car} should be called without any extra quotes. +@end defmac + +@defmac m4_cdr (@var{arg}@dots{}) +@msindex{cdr} +Expands to a quoted list of all but the first @var{arg}, or the empty +string if there was only one argument. Generally, when using quoted +lists of quoted elements, @code{m4_cdr} should be called without any +extra quotes. + +For example, this is a simple implementation of @code{m4_map}; note how +each iteration checks for the end of recursion, then merely applies the +first argument to the first element of the list, then repeats with the +rest of the list. (The actual implementation in M4sugar is a bit more +involved, to gain some speed and share code with @code{m4_map_sep}, and +also to avoid expanding side effects in @samp{$2} twice). +@example +m4_define([m4_map], [m4_ifval([$2], + [m4_apply([$1], m4_car($2))[]$0([$1], m4_cdr($2))])])dnl +m4_map([ m4_eval], [[[1]], [[1+1]], [[10],[16]]]) +@result{} 1 2 a +@end example +@end defmac + +@defmac m4_for (@var{var}, @var{first}, @var{last}, @ovar{step}, @ + @var{expression}) +@msindex{for} +Loop over the numeric values between @var{first} and @var{last} +including bounds by increments of @var{step}. For each iteration, +expand @var{expression} with the numeric value assigned to @var{var}. +If @var{step} is omitted, it defaults to @samp{1} or @samp{-1} depending +on the order of the limits. If given, @var{step} has to match this +order. The number of iterations is determined independently from +definition of @var{var}; iteration cannot be short-circuited or +lengthened by modifying @var{var} from within @var{expression}. +@end defmac + +@defmac m4_foreach (@var{var}, @var{list}, @var{expression}) +@msindex{foreach} +Loop over the comma-separated M4 list @var{list}, assigning each value +to @var{var}, and expand @var{expression}. The following example +outputs two lines: + +@example +m4_foreach([myvar], [[foo], [bar, baz]], + [echo myvar +])dnl +@result{}echo foo +@result{}echo bar, baz +@end example + +Note that for some forms of @var{expression}, it may be faster to use +@code{m4_map_args}. +@end defmac + +@anchor{m4_foreach_w} +@defmac m4_foreach_w (@var{var}, @var{list}, @var{expression}) +@msindex{foreach_w} +Loop over the white-space-separated list @var{list}, assigning each value +to @var{var}, and expand @var{expression}. If @var{var} is only +referenced once in @var{expression}, it is more efficient to use +@code{m4_map_args_w}. + +The deprecated macro @code{AC_FOREACH} is an alias of +@code{m4_foreach_w}. +@end defmac + +@defmac m4_map (@var{macro}, @var{list}) +@defmacx m4_mapall (@var{macro}, @var{list}) +@defmacx m4_map_sep (@var{macro}, @var{separator}, @var{list}) +@defmacx m4_mapall_sep (@var{macro}, @var{separator}, @var{list}) +@msindex{map} +@msindex{mapall} +@msindex{map_sep} +@msindex{mapall_sep} +Loop over the comma separated quoted list of argument descriptions in +@var{list}, and invoke @var{macro} with the arguments. An argument +description is in turn a comma-separated quoted list of quoted elements, +suitable for @code{m4_apply}. The macros @code{m4_map} and +@code{m4_map_sep} ignore empty argument descriptions, while +@code{m4_mapall} and @code{m4_mapall_sep} invoke @var{macro} with no +arguments. The macros @code{m4_map_sep} and @code{m4_mapall_sep} +additionally expand @var{separator} between invocations of @var{macro}. + +Note that @var{separator} is expanded, unlike in @code{m4_join}. When +separating output with commas, this means that the map result can be +used as a series of arguments, by using a single-quoted comma as +@var{separator}, or as a single string, by using a double-quoted comma. + +@example +m4_map([m4_count], []) +@result{} +m4_map([ m4_count], [[], + [[1]], + [[1], [2]]]) +@result{} 1 2 +m4_mapall([ m4_count], [[], + [[1]], + [[1], [2]]]) +@result{} 0 1 2 +m4_map_sep([m4_eval], [,], [[[1+2]], + [[10], [16]]]) +@result{}3,a +m4_map_sep([m4_echo], [,], [[[a]], [[b]]]) +@result{}a,b +m4_count(m4_map_sep([m4_echo], [,], [[[a]], [[b]]])) +@result{}2 +m4_map_sep([m4_echo], [[,]], [[[a]], [[b]]]) +@result{}a,b +m4_count(m4_map_sep([m4_echo], [[,]], [[[a]], [[b]]])) +@result{}1 +@end example +@end defmac + +@defmac m4_map_args (@var{macro}, @var{arg}@dots{}) +@msindex{map_args} +Repeatedly invoke @var{macro} with each successive @var{arg} as its only +argument. In the following example, three solutions are presented with +the same expansion; the solution using @code{m4_map_args} is the most +efficient. +@example +m4_define([active], [ACTIVE])dnl +m4_foreach([var], [[plain], [active]], [ m4_echo(m4_defn([var]))]) +@result{} plain active +m4_map([ m4_echo], [[[plain]], [[active]]]) +@result{} plain active +m4_map_args([ m4_echo], [plain], [active]) +@result{} plain active +@end example + +In cases where it is useful to operate on additional parameters besides +the list elements, the macro @code{m4_curry} can be used in @var{macro} +to supply the argument currying necessary to generate the desired +argument list. In the following example, @code{list_add_n} is more +efficient than @code{list_add_x}. On the other hand, using +@code{m4_map_args_sep} can be even more efficient. + +@example +m4_define([list], [[1], [2], [3]])dnl +m4_define([add], [m4_eval(([$1]) + ([$2]))])dnl +dnl list_add_n(N, ARG...) +dnl Output a list consisting of each ARG added to N +m4_define([list_add_n], +[m4_shift(m4_map_args([,m4_curry([add], [$1])], m4_shift($@@)))])dnl +list_add_n([1], list) +@result{}2,3,4 +list_add_n([2], list) +@result{}3,4,5 +m4_define([list_add_x], +[m4_shift(m4_foreach([var], m4_dquote(m4_shift($@@)), + [,add([$1],m4_defn([var]))]))])dnl +list_add_x([1], list) +@result{}2,3,4 +@end example +@end defmac + +@defmac m4_map_args_pair (@var{macro}, @dvar{macro-end, macro}, @ + @var{arg}@dots{}) +@msindex{map_args_pair} +For every pair of arguments @var{arg}, invoke @var{macro} with two +arguments. If there is an odd number of arguments, invoke +@var{macro-end}, which defaults to @var{macro}, with the remaining +argument. + +@example +m4_map_args_pair([, m4_reverse], [], [1], [2], [3]) +@result{}, 2, 1, 3 +m4_map_args_pair([, m4_reverse], [, m4_dquote], [1], [2], [3]) +@result{}, 2, 1, [3] +m4_map_args_pair([, m4_reverse], [, m4_dquote], [1], [2], [3], [4]) +@result{}, 2, 1, 4, 3 +@end example +@end defmac + +@defmac m4_map_args_sep (@ovar{pre}, @ovar{post}, @ovar{sep}, @var{arg}@dots{}) +@msindex{map_args_sep} +Expand the sequence @code{@var{pre}[@var{arg}]@var{post}} for each +argument, additionally expanding @var{sep} between arguments. One +common use of this macro is constructing a macro call, where the opening +and closing parentheses are split between @var{pre} and @var{post}; in +particular, @code{m4_map_args([@var{macro}], [@var{arg}])} is equivalent +to @code{m4_map_args_sep([@var{macro}(], [)], [], [@var{arg}])}. This +macro provides the most efficient means for iterating over an arbitrary +list of arguments, particularly when repeatedly constructing a macro +call with more arguments than @var{arg}. +@end defmac + +@defmac m4_map_args_w (@var{string}, @ovar{pre}, @ovar{post}, @ovar{sep}) +@msindex{map_args_w} +Expand the sequence @code{@var{pre}[word]@var{post}} for each word in +the whitespace-separated @var{string}, additionally expanding @var{sep} +between words. This macro provides the most efficient means for +iterating over a whitespace-separated string. In particular, +@code{m4_map_args_w([@var{string}], [@var{action}(], [)])} is more +efficient than @code{m4_foreach_w([var], [@var{string}], +[@var{action}(m4_defn([var]))])}. +@end defmac + +@defmac m4_shiftn (@var{count}, @dots{}) +@defmacx m4_shift2 (@dots{}) +@defmacx m4_shift3 (@dots{}) +@msindex{shift2} +@msindex{shift3} +@msindex{shiftn} +@code{m4_shiftn} performs @var{count} iterations of @code{m4_shift}, +along with validation that enough arguments were passed in to match the +shift count, and that the count is positive. @code{m4_shift2} and +@code{m4_shift3} are specializations +of @code{m4_shiftn}, introduced in Autoconf 2.62, and are more efficient +for two and three shifts, respectively. +@end defmac + +@defmac m4_stack_foreach (@var{macro}, @var{action}) +@defmacx m4_stack_foreach_lifo (@var{macro}, @var{action}) +@msindex{stack_foreach} +@msindex{stack_foreach_lifo} +For each of the @code{m4_pushdef} definitions of @var{macro}, expand +@var{action} with the single argument of a definition of @var{macro}. +@code{m4_stack_foreach} starts with the oldest definition, while +@code{m4_stack_foreach_lifo} starts with the current definition. +@var{action} should not push or pop definitions of @var{macro}, nor is +there any guarantee that the current definition of @var{macro} matches +the argument that was passed to @var{action}. The macro @code{m4_curry} +can be used if @var{action} needs more than one argument, although in +that case it is more efficient to use @var{m4_stack_foreach_sep}. + +Due to technical limitations, there are a few low-level m4sugar +functions, such as @code{m4_pushdef}, that cannot be used as the +@var{macro} argument. + +@example +m4_pushdef([a], [1])m4_pushdef([a], [2])dnl +m4_stack_foreach([a], [ m4_incr]) +@result{} 2 3 +m4_stack_foreach_lifo([a], [ m4_curry([m4_substr], [abcd])]) +@result{} cd bcd +@end example +@end defmac + +@defmac m4_stack_foreach_sep (@var{macro}, @ovar{pre}, @ovar{post}, @ovar{sep}) +@defmacx m4_stack_foreach_sep_lifo (@var{macro}, @ovar{pre}, @ovar{post}, @ + @ovar{sep}) +@msindex{stack_foreach_sep} +@msindex{stack_foreach_sep_lifo} +Expand the sequence @code{@var{pre}[definition]@var{post}} for each +@code{m4_pushdef} definition of @var{macro}, additionally expanding +@var{sep} between definitions. @code{m4_stack_foreach_sep} visits the +oldest definition first, while @code{m4_stack_foreach_sep_lifo} visits +the current definition first. This macro provides the most efficient +means for iterating over a pushdef stack. In particular, +@code{m4_stack_foreach([@var{macro}], [@var{action}])} is short for +@code{m4_stack_foreach_sep([@var{macro}], [@var{action}(], [)])}. +@end defmac + +@node Evaluation Macros +@subsection Evaluation Macros + +The following macros give some control over the order of the evaluation +by adding or removing levels of quotes. + +@defmac m4_apply (@var{macro}, @var{list}) +@msindex{apply} +Apply the elements of the quoted, comma-separated @var{list} as the +arguments to @var{macro}. If @var{list} is empty, invoke @var{macro} +without arguments. Note the difference between @code{m4_indir}, which +expects its first argument to be a macro name but can use names that are +otherwise invalid, and @code{m4_apply}, where @var{macro} can contain +other text, but must end in a valid macro name. +@example +m4_apply([m4_count], []) +@result{}0 +m4_apply([m4_count], [[]]) +@result{}1 +m4_apply([m4_count], [[1], [2]]) +@result{}2 +m4_apply([m4_join], [[|], [1], [2]]) +@result{}1|2 +@end example +@end defmac + +@defmac m4_count (@var{arg}, @dots{}) +@msindex{count} +This macro returns the decimal count of the number of arguments it was +passed. +@end defmac + +@defmac m4_curry (@var{macro}, @var{arg}@dots{}) +@msindex{curry} +This macro performs argument currying. The expansion of this macro is +another macro name that expects exactly one argument; that argument is +then appended to the @var{arg} list, and then @var{macro} is expanded +with the resulting argument list. + +@example +m4_curry([m4_curry], [m4_reverse], [1])([2])([3]) +@result{}3, 2, 1 +@end example + +Unfortunately, due to a limitation in M4 1.4.x, it is not possible to +pass the definition of a builtin macro as the argument to the output of +@code{m4_curry}; the empty string is used instead of the builtin token. +This behavior is rectified by using M4 1.6 or newer. +@end defmac + +@defmac m4_do (@var{arg}, @dots{}) +@msindex{do} +This macro loops over its arguments and expands each @var{arg} in +sequence. Its main use is for readability; it allows the use of +indentation and fewer @code{dnl} to result in the same expansion. This +macro guarantees that no expansion will be concatenated with subsequent +text; to achieve full concatenation, use @code{m4_unquote(m4_join([], +@var{arg@dots{}}))}. + +@example +m4_define([ab],[1])m4_define([bc],[2])m4_define([abc],[3])dnl +m4_do([a],[b])c +@result{}abc +m4_unquote(m4_join([],[a],[b]))c +@result{}3 +m4_define([a],[A])m4_define([b],[B])m4_define([c],[C])dnl +m4_define([AB],[4])m4_define([BC],[5])m4_define([ABC],[6])dnl +m4_do([a],[b])c +@result{}ABC +m4_unquote(m4_join([],[a],[b]))c +@result{}3 +@end example +@end defmac + +@defmac m4_dquote (@var{arg}, @dots{}) +@msindex{dquote} +Return the arguments as a quoted list of quoted arguments. +Conveniently, if there is just one @var{arg}, this effectively adds a +level of quoting. +@end defmac + +@defmac m4_dquote_elt (@var{arg}, @dots{}) +@msindex{dquote_elt} +Return the arguments as a series of double-quoted arguments. Whereas +@code{m4_dquote} returns a single argument, @code{m4_dquote_elt} returns +as many arguments as it was passed. +@end defmac + +@defmac m4_echo (@var{arg}, @dots{}) +@msindex{echo} +Return the arguments, with the same level of quoting. Other than +discarding whitespace after unquoted commas, this macro is a no-op. +@end defmac + +@defmac m4_expand (@var{arg}) +@msindex{expand} +Return the expansion of @var{arg} as a quoted string. Whereas +@code{m4_quote} is designed to collect expanded text into a single +argument, @code{m4_expand} is designed to perform one level of expansion +on quoted text. One distinction is in the treatment of whitespace +following a comma in the original @var{arg}. Any time multiple +arguments are collected into one with @code{m4_quote}, the M4 argument +collection rules discard the whitespace. However, with @code{m4_expand}, +whitespace is preserved, even after the expansion of macros contained in +@var{arg}. Additionally, @code{m4_expand} is able to expand text that +would involve an unterminated comment, whereas expanding that same text +as the argument to @code{m4_quote} runs into difficulty in finding the +end of the argument. Since manipulating diversions during argument +collection is inherently unsafe, @code{m4_expand} issues an error if +@var{arg} attempts to change the current diversion (@pxref{Diversion +support}). + +@example +m4_define([active], [ACT, IVE])dnl +m4_define([active2], [[ACT, IVE]])dnl +m4_quote(active, active) +@result{}ACT,IVE,ACT,IVE +m4_expand([active, active]) +@result{}ACT, IVE, ACT, IVE +m4_quote(active2, active2) +@result{}ACT, IVE,ACT, IVE +m4_expand([active2, active2]) +@result{}ACT, IVE, ACT, IVE +m4_expand([# m4_echo]) +@result{}# m4_echo +m4_quote(# m4_echo) +) +@result{}# m4_echo) +@result{} +@end example + +Note that @code{m4_expand} cannot handle an @var{arg} that expands to +literal unbalanced quotes, but that quadrigraphs can be used when +unbalanced output is necessary. Likewise, unbalanced parentheses should +be supplied with double quoting or a quadrigraph. + +@example +m4_define([pattern], [[!@@<:@@]])dnl +m4_define([bar], [BAR])dnl +m4_expand([case $foo in + m4_defn([pattern])@@:@}@@ bar ;; + *[)] blah ;; +esac]) +@result{}case $foo in +@result{} [![]) BAR ;; +@result{} *) blah ;; +@result{}esac +@end example +@end defmac + +@defmac m4_ignore (@dots{}) +@msindex{ignore} +This macro was introduced in Autoconf 2.62. Expands to nothing, +ignoring all of its arguments. By itself, this isn't very useful. +However, it can be used to conditionally ignore an arbitrary number of +arguments, by deciding which macro name to apply to a list of arguments. +@example +dnl foo outputs a message only if [debug] is defined. +m4_define([foo], +[m4_ifdef([debug],[AC_MSG_NOTICE],[m4_ignore])([debug message])]) +@end example + +Note that for earlier versions of Autoconf, the macro @code{__gnu__} can +serve the same purpose, although it is less readable. +@end defmac + +@defmac m4_make_list (@var{arg}, @dots{}) +@msindex{make_list} +This macro exists to aid debugging of M4sugar algorithms. Its net +effect is similar to @code{m4_dquote}---it produces a quoted list of +quoted arguments, for each @var{arg}. The difference is that this +version uses a comma-newline separator instead of just comma, to improve +readability of the list; with the result that it is less efficient than +@code{m4_dquote}. +@example +m4_define([zero],[0])m4_define([one],[1])m4_define([two],[2])dnl +m4_dquote(zero, [one], [[two]]) +@result{}[0],[one],[[two]] +m4_make_list(zero, [one], [[two]]) +@result{}[0], +@result{}[one], +@result{}[[two]] +m4_foreach([number], m4_dquote(zero, [one], [[two]]), [ number]) +@result{} 0 1 two +m4_foreach([number], m4_make_list(zero, [one], [[two]]), [ number]) +@result{} 0 1 two +@end example +@end defmac + +@c m4_noquote is too dangerous to document - it invokes macros that +@c probably rely on @samp{[]} nested quoting for proper operation. The +@c user should generally prefer m4_unquote instead. + +@defmac m4_quote (@var{arg}, @dots{}) +@msindex{quote} +Return the arguments as a single entity, i.e., wrap them into a pair of +quotes. This effectively collapses multiple arguments into one, +although it loses whitespace after unquoted commas in the process. +@end defmac + +@defmac m4_reverse (@var{arg}, @dots{}) +@msindex{reverse} +Outputs each argument with the same level of quoting, but in reverse +order, and with space following each comma for readability. + +@example +m4_define([active], [ACT,IVE]) +@result{} +m4_reverse(active, [active]) +@result{}active, IVE, ACT +@end example +@end defmac + +@defmac m4_unquote (@var{arg}, @dots{}) +@msindex{unquote} +This macro was introduced in Autoconf 2.62. Expand each argument, +separated by commas. For a single @var{arg}, this effectively removes a +layer of quoting, and @code{m4_unquote([@var{arg}])} is more efficient +than the equivalent @code{m4_do([@var{arg}])}. For multiple arguments, +this results in an unquoted list of expansions. This is commonly used +with @code{m4_split}, in order to convert a single quoted list into a +series of quoted elements. +@end defmac + +The following example aims at emphasizing the difference between several +scenarios: not using these macros, using @code{m4_defn}, using +@code{m4_quote}, using @code{m4_dquote}, and using @code{m4_expand}. + +@example +$ @kbd{cat example.m4} +dnl Overquote, so that quotes are visible. +m4_define([show], [$[]1 = [$1], $[]@@ = [$@@]]) +m4_define([a], [A]) +m4_define([mkargs], [1, 2[,] 3]) +m4_define([arg1], [[$1]]) +m4_divert([0])dnl +show(a, b) +show([a, b]) +show(m4_quote(a, b)) +show(m4_dquote(a, b)) +show(m4_expand([a, b])) + +arg1(mkargs) +arg1([mkargs]) +arg1(m4_defn([mkargs])) +arg1(m4_quote(mkargs)) +arg1(m4_dquote(mkargs)) +arg1(m4_expand([mkargs])) +$ @kbd{autom4te -l m4sugar example.m4} +$1 = A, $@@ = [A],[b] +$1 = a, b, $@@ = [a, b] +$1 = A,b, $@@ = [A,b] +$1 = [A],[b], $@@ = [[A],[b]] +$1 = A, b, $@@ = [A, b] + +1 +mkargs +1, 2[,] 3 +1,2, 3 +[1],[2, 3] +1, 2, 3 +@end example + + +@node Text processing Macros +@subsection String manipulation in M4 + +The following macros may be used to manipulate strings in M4. Many of +the macros in this section intentionally result in quoted strings as +output, rather than subjecting the arguments to further expansions. As +a result, if you are manipulating text that contains active M4 +characters, the arguments are passed with single quoting rather than +double. + +@defmac m4_append (@var{macro-name}, @var{string}, @ovar{separator}) +@defmacx m4_append_uniq (@var{macro-name}, @var{string}, @ovar{separator} @ + @ovar{if-uniq}, @ovar{if-duplicate}) +@msindex{append} +@msindex{append_uniq} +Redefine @var{macro-name} to its former contents with @var{separator} +and @var{string} added at the end. If @var{macro-name} was undefined +before (but not if it was defined but empty), then no @var{separator} is +added. As of Autoconf 2.62, neither @var{string} nor @var{separator} +are expanded during this macro; instead, they are expanded when +@var{macro-name} is invoked. + +@code{m4_append} can be used to grow strings, and @code{m4_append_uniq} +to grow strings without duplicating substrings. Additionally, +@code{m4_append_uniq} takes two optional parameters as of Autoconf 2.62; +@var{if-uniq} is expanded if @var{string} was appended, and +@var{if-duplicate} is expanded if @var{string} was already present. +Also, @code{m4_append_uniq} warns if @var{separator} is not empty, but +occurs within @var{string}, since that can lead to duplicates. + +Note that @code{m4_append} can scale linearly in the length of the final +string, depending on the quality of the underlying M4 implementation, +while @code{m4_append_uniq} has an inherent quadratic scaling factor. +If an algorithm can tolerate duplicates in the final string, use the +former for speed. If duplicates must be avoided, consider using +@code{m4_set_add} instead (@pxref{Set manipulation Macros}). + +@example +m4_define([active], [ACTIVE])dnl +m4_append([sentence], [This is an])dnl +m4_append([sentence], [ active ])dnl +m4_append([sentence], [symbol.])dnl +sentence +@result{}This is an ACTIVE symbol. +m4_undefine([active])dnl +@result{}This is an active symbol. +m4_append_uniq([list], [one], [, ], [new], [existing]) +@result{}new +m4_append_uniq([list], [one], [, ], [new], [existing]) +@result{}existing +m4_append_uniq([list], [two], [, ], [new], [existing]) +@result{}new +m4_append_uniq([list], [three], [, ], [new], [existing]) +@result{}new +m4_append_uniq([list], [two], [, ], [new], [existing]) +@result{}existing +list +@result{}one, two, three +m4_dquote(list) +@result{}[one],[two],[three] +m4_append([list2], [one], [[, ]])dnl +m4_append_uniq([list2], [two], [[, ]])dnl +m4_append([list2], [three], [[, ]])dnl +list2 +@result{}one, two, three +m4_dquote(list2) +@result{}[one, two, three] +@end example +@end defmac + +@defmac m4_append_uniq_w (@var{macro-name}, @var{strings}) +@msindex{append_uniq_w} +This macro was introduced in Autoconf 2.62. It is similar to +@code{m4_append_uniq}, but treats @var{strings} as a whitespace +separated list of words to append, and only appends unique words. +@var{macro-name} is updated with a single space between new words. +@example +m4_append_uniq_w([numbers], [1 1 2])dnl +m4_append_uniq_w([numbers], [ 2 3 ])dnl +numbers +@result{}1 2 3 +@end example +@end defmac + +@defmac m4_chomp (@var{string}) +@defmacx m4_chomp_all (@var{string}) +@msindex{chomp} +@msindex{chomp_all} +Output @var{string} in quotes, but without a trailing newline. The +macro @code{m4_chomp} is slightly faster, and removes at most one +newline; the macro @code{m4_chomp_all} removes all consecutive trailing +newlines. Unlike @code{m4_flatten}, embedded newlines are left intact, +and backslash does not influence the result. +@end defmac + +@defmac m4_combine (@ovar{separator}, @var{prefix-list}, @ovar{infix}, @ + @var{suffix-1}, @ovar{suffix-2}, @dots{}) +@msindex{combine} +This macro produces a quoted string containing the pairwise combination +of every element of the quoted, comma-separated @var{prefix-list}, and +every element from the @var{suffix} arguments. Each pairwise +combination is joined with @var{infix} in the middle, and successive +pairs are joined by @var{separator}. No expansion occurs on any of the +arguments. No output occurs if either the @var{prefix} or @var{suffix} +list is empty, but the lists can contain empty elements. +@example +m4_define([a], [oops])dnl +m4_combine([, ], [[a], [b], [c]], [-], [1], [2], [3]) +@result{}a-1, a-2, a-3, b-1, b-2, b-3, c-1, c-2, c-3 +m4_combine([, ], [[a], [b]], [-]) +@result{} +m4_combine([, ], [[a], [b]], [-], []) +@result{}a-, b- +m4_combine([, ], [], [-], [1], [2]) +@result{} +m4_combine([, ], [[]], [-], [1], [2]) +@result{}-1, -2 +@end example +@end defmac + +@defmac m4_escape (@var{string}) +@msindex{escape} +Convert all instances of @samp{[}, @samp{]}, @samp{#}, and @samp{$} +within @var{string} into their respective quadrigraphs. The result is +still a quoted string. +@end defmac + +@defmac m4_flatten (@var{string}) +@msindex{flatten} +Flatten @var{string} into a single line. Delete all backslash-newline +pairs, and replace all remaining newlines with a space. The result is +still a quoted string. +@end defmac + +@defmac m4_join (@ovar{separator}, @var{args}@dots{}) +@defmacx m4_joinall (@ovar{separator}, @var{args}@dots{}) +@msindex{join} +@msindex{joinall} +Concatenate each @var{arg}, separated by @var{separator}. +@code{joinall} uses every argument, while @code{join} omits empty +arguments so that there are no back-to-back separators in the output. +The result is a quoted string. +@example +m4_define([active], [ACTIVE])dnl +m4_join([|], [one], [], [active], [two]) +@result{}one|active|two +m4_joinall([|], [one], [], [active], [two]) +@result{}one||active|two +@end example + +Note that if all you intend to do is join @var{args} with commas between +them, to form a quoted list suitable for @code{m4_foreach}, it is more +efficient to use @code{m4_dquote}. +@end defmac + +@defmac m4_newline (@ovar{text}) +@msindex{newline} +This macro was introduced in Autoconf 2.62, and expands to a newline, +followed by any @var{text}. +It is primarily useful for maintaining macro formatting, and ensuring +that M4 does not discard leading whitespace during argument collection. +@end defmac + +@defmac m4_normalize (@var{string}) +@msindex{normalize} +Remove leading and trailing spaces and tabs, sequences of +backslash-then-newline, and replace multiple spaces, tabs, and newlines +with a single space. This is a combination of @code{m4_flatten} and +@code{m4_strip}. To determine if @var{string} consists only of bytes +that would be removed by @code{m4_normalize}, you can use +@code{m4_ifblank}. +@end defmac + +@defmac m4_re_escape (@var{string}) +@msindex{re_escape} +Backslash-escape all characters in @var{string} that are active in +regexps. +@end defmac + +@c We cannot use @dvar because the macro expansion mistreats backslashes. +@defmac m4_split (@var{string}, @r{[}@var{regexp} = @samp{[\t ]+}@r{]}) +@msindex{split} +Split @var{string} into an M4 list of elements quoted by @samp{[} and +@samp{]}, while keeping white space at the beginning and at the end. +If @var{regexp} is given, use it instead of @samp{[\t ]+} for splitting. +If @var{string} is empty, the result is an empty list. +@end defmac + +@defmac m4_strip (@var{string}) +@msindex{strip} +Strip whitespace from @var{string}. Sequences of spaces and tabs are +reduced to a single space, then leading and trailing spaces are removed. +The result is still a quoted string. Note that this does not interfere +with newlines; if you want newlines stripped as well, consider +@code{m4_flatten}, or do it all at once with @code{m4_normalize}. To +quickly test if @var{string} has only whitespace, use @code{m4_ifblank}. +@end defmac + +@defmac m4_text_box (@var{message}, @dvar{frame, -}) +@msindex{text_box} +Add a text box around @var{message}, using @var{frame} as the border +character above and below the message. The @var{frame} argument must be +a single byte, and does not support quadrigraphs. +The frame correctly accounts for +the subsequent expansion of @var{message}. For example: +@example +m4_define([macro], [abc])dnl +m4_text_box([macro]) +@result{}## --- ## +@result{}## abc ## +@result{}## --- ## +@end example + +The @var{message} must contain balanced quotes and parentheses, although +quadrigraphs can be used to work around this. +@end defmac + +@defmac m4_text_wrap (@var{string}, @ovar{prefix}, @ + @dvar{prefix1, @var{prefix}}, @dvar{width, 79}) +@msindex{text_wrap} +Break @var{string} into a series of whitespace-separated words, then +output those words separated by spaces, and wrapping lines any time the +output would exceed @var{width} columns. If given, @var{prefix1} begins +the first line, and @var{prefix} begins all wrapped lines. If +@var{prefix1} is longer than @var{prefix}, then the first line consists +of just @var{prefix1}. If @var{prefix} is longer than @var{prefix1}, +padding is inserted so that the first word of @var{string} begins at the +same indentation as all wrapped lines. Note that using literal tab +characters in any of the arguments will interfere with the calculation +of width. No expansions occur on @var{prefix}, @var{prefix1}, or the +words of @var{string}, although quadrigraphs are recognized. + +For some examples: +@example +m4_text_wrap([Short string */], [ ], [/* ], [20]) +@result{}/* Short string */ +m4_text_wrap([Much longer string */], [ ], [/* ], [20]) +@result{}/* Much longer +@result{} string */ +m4_text_wrap([Short doc.], [ ], [ --short ], [30]) +@result{} --short Short doc. +m4_text_wrap([Short doc.], [ ], [ --too-wide ], [30]) +@result{} --too-wide +@result{} Short doc. +m4_text_wrap([Super long documentation.], [ ], + [ --too-wide ], 30) +@result{} --too-wide +@result{} Super long +@result{} documentation. +@end example +@end defmac + +@defmac m4_tolower (@var{string}) +@defmacx m4_toupper (@var{string}) +@msindex{tolower} +@msindex{toupper} +Return @var{string} with letters converted to upper or lower case, +respectively. +@end defmac + +@node Number processing Macros +@subsection Arithmetic computation in M4 + +The following macros facilitate integer arithmetic operations. +Where a parameter is documented as taking an arithmetic expression, you +can use anything that can be parsed by @code{m4_eval}. + +@defmac m4_cmp (@var{expr-1}, @var{expr-2}) +@msindex{cmp} +Compare the arithmetic expressions @var{expr-1} and @var{expr-2}, and +expand to @samp{-1} if @var{expr-1} is smaller, @samp{0} if they are +equal, and @samp{1} if @var{expr-1} is larger. +@end defmac + +@defmac m4_list_cmp (@var{list-1}, @var{list-2}) +@msindex{list_cmp} +Compare the two M4 lists consisting of comma-separated arithmetic +expressions, left to right. Expand to @samp{-1} for the first element +pairing where the value from @var{list-1} is smaller, @samp{1} where the +value from @var{list-2} is smaller, or @samp{0} if both lists have the +same values. If one list is shorter than the other, the remaining +elements of the longer list are compared against zero. +@example +m4_list_cmp([1, 0], [1]) +@result{}0 +m4_list_cmp([1, [1 * 0]], [1, 0]) +@result{}0 +m4_list_cmp([1, 2], [1, 0]) +@result{}1 +m4_list_cmp([1, [1+1], 3],[1, 2]) +@result{}1 +m4_list_cmp([1, 2, -3], [1, 2]) +@result{}-1 +m4_list_cmp([1, 0], [1, 2]) +@result{}-1 +m4_list_cmp([1], [1, 2]) +@result{}-1 +@end example +@end defmac + +@defmac m4_max (@var{arg}, @dots{}) +@msindex{max} +This macro was introduced in Autoconf 2.62. Expand to the decimal value +of the maximum arithmetic expression among all the arguments. +@end defmac + +@defmac m4_min (@var{arg}, @dots{}) +@msindex{min} +This macro was introduced in Autoconf 2.62. Expand to the decimal value +of the minimum arithmetic expression among all the arguments. +@end defmac + +@defmac m4_sign (@var{expr}) +@msindex{sign} +Expand to @samp{-1} if the arithmetic expression @var{expr} is negative, +@samp{1} if it is positive, and @samp{0} if it is zero. +@end defmac + +@anchor{m4_version_compare} +@defmac m4_version_compare (@var{version-1}, @var{version-2}) +@msindex{version_compare} +This macro was introduced in Autoconf 2.53, but had a number of +usability limitations that were not lifted until Autoconf 2.62. Compare +the version strings @var{version-1} and @var{version-2}, and expand to +@samp{-1} if @var{version-1} is smaller, @samp{0} if they are the same, +or @samp{1} @var{version-2} is smaller. Version strings must be a list +of elements separated by @samp{.}, @samp{,} or @samp{-}, where each +element is a number along with optional case-insensitive letters +designating beta releases. The comparison stops at the leftmost element +that contains a difference, although a 0 element compares equal to a +missing element. + +It is permissible to include commit identifiers in @var{version}, such +as an abbreviated SHA1 of the commit, provided there is still a +monotonically increasing prefix to allow for accurate version-based +comparisons. For example, this paragraph was written when the +development snapshot of autoconf claimed to be at version +@samp{2.61a-248-dc51}, or 248 commits after the 2.61a release, with an +abbreviated commit identification of @samp{dc51}. + +@example +m4_version_compare([1.1], [2.0]) +@result{}-1 +m4_version_compare([2.0b], [2.0a]) +@result{}1 +m4_version_compare([1.1.1], [1.1.1a]) +@result{}-1 +m4_version_compare([1.2], [1.1.1a]) +@result{}1 +m4_version_compare([1.0], [1]) +@result{}0 +m4_version_compare([1.1pre], [1.1PRE]) +@result{}0 +m4_version_compare([1.1a], [1,10]) +@result{}-1 +m4_version_compare([2.61a], [2.61a-248-dc51]) +@result{}-1 +m4_version_compare([2.61b], [2.61a-248-dc51]) +@result{}1 +@end example +@end defmac + +@defmac m4_version_prereq (@var{version}, @ovar{if-new-enough}, @ + @dvar{if-old, m4_fatal}) +@msindex{version_prereq} +Compares @var{version} against the version of Autoconf currently +running. If the running version is at @var{version} or newer, expand +@var{if-new-enough}, but if @var{version} is larger than the version +currently executing, expand @var{if-old}, which defaults to printing an +error message and exiting m4sugar with status 63. When given only one +argument, this behaves like @code{AC_PREREQ} (@pxref{Versioning}). +Remember that the autoconf philosophy favors feature checks over version +checks. +@end defmac + +@node Set manipulation Macros +@subsection Set manipulation in M4 +@cindex Set manipulation +@cindex Data structure, set +@cindex Unordered set manipulation + +Sometimes, it is necessary to track a set of data, where the order does +not matter and where there are no duplicates in the set. The following +macros facilitate set manipulations. Each set is an opaque object, +which can only be accessed via these basic operations. The underlying +implementation guarantees linear scaling for set creation, which is more +efficient than using the quadratic @code{m4_append_uniq}. Both set +names and values can be arbitrary strings, except for unbalanced quotes. +This implementation ties up memory for removed elements until the next +operation that must traverse all the elements of a set; and although +that may slow down some operations until the memory for removed elements +is pruned, it still guarantees linear performance. + +@defmac m4_set_add (@var{set}, @var{value}, @ovar{if-uniq}, @ovar{if-dup}) +@msindex{set_add} +Adds the string @var{value} as a member of set @var{set}. Expand +@var{if-uniq} if the element was added, or @var{if-dup} if it was +previously in the set. Operates in amortized constant time, so that set +creation scales linearly. +@end defmac + +@defmac m4_set_add_all (@var{set}, @var{value}@dots{}) +@msindex{set_add_all} +Adds each @var{value} to the set @var{set}. This is slightly more +efficient than repeatedly invoking @code{m4_set_add}. +@end defmac + +@defmac m4_set_contains (@var{set}, @var{value}, @ovar{if-present}, @ + @ovar{if-absent}) +@msindex{set_contains} +Expands @var{if-present} if the string @var{value} is a member of +@var{set}, otherwise @var{if-absent}. + +@example +m4_set_contains([a], [1], [yes], [no]) +@result{}no +m4_set_add([a], [1], [added], [dup]) +@result{}added +m4_set_add([a], [1], [added], [dup]) +@result{}dup +m4_set_contains([a], [1], [yes], [no]) +@result{}yes +m4_set_remove([a], [1], [removed], [missing]) +@result{}removed +m4_set_contains([a], [1], [yes], [no]) +@result{}no +m4_set_remove([a], [1], [removed], [missing]) +@result{}missing +@end example +@end defmac + +@defmac m4_set_contents (@var{set}, @ovar{sep}) +@defmacx m4_set_dump (@var{set}, @ovar{sep}) +@msindex{set_contents} +@msindex{set_dump} +Expands to a single string consisting of all the members of the set +@var{set}, each separated by @var{sep}, which is not expanded. +@code{m4_set_contents} leaves the elements in @var{set} but reclaims any +memory occupied by removed elements, while @code{m4_set_dump} is a +faster one-shot action that also deletes the set. No provision is made +for disambiguating members that contain a non-empty @var{sep} as a +substring; use @code{m4_set_empty} to distinguish between an empty set +and the set containing only the empty string. The order of the output +is unspecified; in the current implementation, part of the speed of +@code{m4_set_dump} results from using a different output order than +@code{m4_set_contents}. These macros scale linearly in the size of the +set before memory pruning, and @code{m4_set_contents([@var{set}], +[@var{sep}])} is faster than +@code{m4_joinall([@var{sep}]m4_set_listc([@var{set}]))}. + +@example +m4_set_add_all([a], [1], [2], [3]) +@result{} +m4_set_contents([a], [-]) +@result{}1-2-3 +m4_joinall([-]m4_set_listc([a])) +@result{}1-2-3 +m4_set_dump([a], [-]) +@result{}3-2-1 +m4_set_contents([a]) +@result{} +m4_set_add([a], []) +@result{} +m4_set_contents([a], [-]) +@result{} +@end example +@end defmac + +@defmac m4_set_delete (@var{set}) +@msindex{set_delete} +Delete all elements and memory associated with @var{set}. This is +linear in the set size, and faster than removing one element at a time. +@end defmac + +@defmac m4_set_difference (@var{seta}, @var{setb}) +@defmacx m4_set_intersection (@var{seta}, @var{setb}) +@defmacx m4_set_union (@var{seta}, @var{setb}) +@msindex{set_difference} +@msindex{set_intersection} +@msindex{set_union} +Compute the relation between @var{seta} and @var{setb}, and output the +result as a list of quoted arguments without duplicates and with a +leading comma. Set difference selects the elements in @var{seta} but +not @var{setb}, intersection selects only elements in both sets, and +union selects elements in either set. These actions are linear in the +sum of the set sizes. The leading comma is necessary to distinguish +between no elements and the empty string as the only element. + +@example +m4_set_add_all([a], [1], [2], [3]) +@result{} +m4_set_add_all([b], [3], [], [4]) +@result{} +m4_set_difference([a], [b]) +@result{},1,2 +m4_set_difference([b], [a]) +@result{},,4 +m4_set_intersection([a], [b]) +@result{},3 +m4_set_union([a], [b]) +@result{},1,2,3,,4 +@end example +@end defmac + +@defmac m4_set_empty (@var{set}, @ovar{if-empty}, @ovar{if-elements}) +@msindex{set_empty} +Expand @var{if-empty} if the set @var{set} has no elements, otherwise +expand @var{if-elements}. This macro operates in constant time. Using +this macro can help disambiguate output from @code{m4_set_contents} or +@code{m4_set_list}. +@end defmac + +@defmac m4_set_foreach (@var{set}, @var{variable}, @var{action}) +@msindex{set_foreach} +For each element in the set @var{set}, expand @var{action} with the +macro @var{variable} defined as the set element. Behavior is +unspecified if @var{action} recursively lists the contents of @var{set} +(although listing other sets is acceptable), or if it modifies the set +in any way other than removing the element currently contained in +@var{variable}. This macro is faster than the corresponding +@code{m4_foreach([@var{variable}], +m4_indir([m4_dquote]m4_set_listc([@var{set}])), [@var{action}])}, +although @code{m4_set_map} might be faster still. + +@example +m4_set_add_all([a]m4_for([i], [1], [5], [], [,i])) +@result{} +m4_set_contents([a]) +@result{}12345 +m4_set_foreach([a], [i], + [m4_if(m4_eval(i&1), [0], [m4_set_remove([a], i, [i])])]) +@result{}24 +m4_set_contents([a]) +@result{}135 +@end example +@end defmac + +@defmac m4_set_list (@var{set}) +@defmacx m4_set_listc (@var{set}) +@msindex{set_list} +@msindex{set_listc} +Produce a list of arguments, where each argument is a quoted element +from the set @var{set}. The variant @code{m4_set_listc} is unambiguous, +by adding a leading comma if there are any set elements, whereas the +variant @code{m4_set_list} cannot distinguish between an empty set and a +set containing only the empty string. These can be directly used in +macros that take multiple arguments, such as @code{m4_join} or +@code{m4_set_add_all}, or wrapped by @code{m4_dquote} for macros that +take a quoted list, such as @code{m4_map} or @code{m4_foreach}. Any +memory occupied by removed elements is reclaimed during these macros. + +@example +m4_set_add_all([a], [1], [2], [3]) +@result{} +m4_set_list([a]) +@result{}1,2,3 +m4_set_list([b]) +@result{} +m4_set_listc([b]) +@result{} +m4_count(m4_set_list([b])) +@result{}1 +m4_set_empty([b], [0], [m4_count(m4_set_list([b]))]) +@result{}0 +m4_set_add([b], []) +@result{} +m4_set_list([b]) +@result{} +m4_set_listc([b]) +@result{}, +m4_count(m4_set_list([b])) +@result{}1 +m4_set_empty([b], [0], [m4_count(m4_set_list([b]))]) +@result{}1 +@end example +@end defmac + +@defmac m4_set_map (@var{set}, @var{action}) +@msindex{set_map} +For each element in the set @var{set}, expand @var{action} with a single +argument of the set element. Behavior is unspecified if @var{action} +recursively lists the contents of @var{set} (although listing other sets +is acceptable), or if it modifies the set in any way other than removing +the element passed as an argument. This macro is faster than either +corresponding counterpart of +@code{m4_map_args([@var{action}]m4_set_listc([@var{set}]))} or +@code{m4_set_foreach([@var{set}], [var], +[@var{action}(m4_defn([var]))])}. It is possible to use @code{m4_curry} +if more than one argument is needed for @var{action}, although it is +more efficient to use @code{m4_set_map_sep} in that case. +@end defmac + +@defmac m4_set_map_sep (@var{set}, @ovar{pre}, @ovar{post}, @ovar{sep}) +@msindex{set_map_sep} +For each element in the set @var{set}, expand +@code{@var{pre}[element]@var{post}}, additionally expanding @var{sep} +between elements. Behavior is unspecified if the expansion recursively +lists the contents of @var{set} (although listing other sets +is acceptable), or if it modifies the set in any way other than removing +the element visited by the expansion. This macro provides the most +efficient means for non-destructively visiting the elements of a set; in +particular, @code{m4_set_map([@var{set}], [@var{action}])} is equivalent +to @code{m4_set_map_sep([@var{set}], [@var{action}(], [)])}. +@end defmac + +@defmac m4_set_remove (@var{set}, @var{value}, @ovar{if-present}, @ + @ovar{if-absent}) +@msindex{set_remove} +If @var{value} is an element in the set @var{set}, then remove it and +expand @var{if-present}. Otherwise expand @var{if-absent}. This macro +operates in constant time so that multiple removals will scale linearly +rather than quadratically; but when used outside of +@code{m4_set_foreach} or @code{m4_set_map}, it leaves memory occupied +until the set is later +compacted by @code{m4_set_contents} or @code{m4_set_list}. Several +other set operations are then less efficient between the time of element +removal and subsequent memory compaction, but still maintain their +guaranteed scaling performance. +@end defmac + +@defmac m4_set_size (@var{set}) +@msindex{set_size} +Expand to the size of the set @var{set}. This implementation operates +in constant time, and is thus more efficient than +@code{m4_eval(m4_count(m4_set_listc([set])) - 1)}. +@end defmac + + +@node Forbidden Patterns +@subsection Forbidden Patterns +@cindex Forbidden patterns +@cindex Patterns, forbidden + +M4sugar provides a means to define suspicious patterns, patterns +describing tokens which should not be found in the output. For +instance, if an Autoconf @file{configure} script includes tokens such as +@samp{AC_DEFINE}, or @samp{dnl}, then most probably something went +wrong (typically a macro was not evaluated because of overquotation). + +M4sugar forbids all the tokens matching @samp{^_?m4_} and @samp{^dnl$}. +Additional layers, such as M4sh and Autoconf, add additional forbidden +patterns to the list. + +@defmac m4_pattern_forbid (@var{pattern}) +@msindex{pattern_forbid} +Declare that no token matching @var{pattern} must be found in the output. +Comments are not checked; this can be a problem if, for instance, you +have some macro left unexpanded after an @samp{#include}. No consensus +is currently found in the Autoconf community, as some people consider it +should be valid to name macros in comments (which doesn't make sense to +the authors of this documentation: input, such as macros, should be +documented by @samp{dnl} comments; reserving @samp{#}-comments to +document the output). +@end defmac + +Of course, you might encounter exceptions to these generic rules, for +instance you might have to refer to @samp{$m4_flags}. + +@defmac m4_pattern_allow (@var{pattern}) +@msindex{pattern_allow} +Any token matching @var{pattern} is allowed, including if it matches an +@code{m4_pattern_forbid} pattern. +@end defmac + +@node Debugging via autom4te +@section Debugging via autom4te +@cindex debugging tips +@cindex autom4te debugging tips +@cindex m4sugar debugging tips +At times, it is desirable to see what was happening inside m4, to see +why output was not matching expectations. However, post-processing done +by @command{autom4te} means that directly using the m4 builtin +@code{m4_traceon} is likely to interfere with operation. Also, frequent +diversion changes and the concept of forbidden tokens make it difficult +to use @code{m4_defn} to generate inline comments in the final output. + +There are a couple of tools to help with this. One is the use of the +@option{--trace} option provided by @command{autom4te} (as well as each +of the programs that wrap @command{autom4te}, such as +@command{autoconf}), in order to inspect when a macro is called and with +which arguments. For example, when this paragraph was written, the +autoconf version could be found by: + +@example +$ @kbd{autoconf --trace=AC_INIT} +configure.ac:23:AC_INIT:GNU Autoconf:2.63b.95-3963:bug-autoconf@@gnu.org +$ @kbd{autoconf --trace='AC_INIT:version is $2'} +version is 2.63b.95-3963 +@end example + +Another trick is to print out the expansion of various m4 expressions to +standard error or to an independent file, with no further m4 expansion, +and without interfering with diversion changes or the post-processing +done to standard output. @code{m4_errprintn} shows a given expression +on standard error. For example, if you want to see the expansion of an +autoconf primitive or of one of your autoconf macros, you can do it like +this: + +@example +$ @kbd{cat <<\EOF > configure.ac} +AC_INIT +m4_errprintn([The definition of AC_DEFINE_UNQUOTED:]) +m4_errprintn(m4_defn([AC_DEFINE_UNQUOTED])) +AC_OUTPUT +EOF +$ @kbd{autoconf} +@error{}The definition of AC_DEFINE_UNQUOTED: +@error{}_AC_DEFINE_Q([], $@@) +@end example + +@node Programming in M4sh +@chapter Programming in M4sh + +M4sh, pronounced ``mash'', is aiming at producing portable Bourne shell +scripts. This name was coined by Lars J. Aas, who notes that, +according to the Webster's Revised Unabridged Dictionary (1913): + +@quotation +Mash \Mash\, n. [Akin to G. meisch, maisch, meische, maische, mash, +wash, and prob.@: to AS. miscian to mix. See ``Mix''.] + +@enumerate 1 +@item +A mass of mixed ingredients reduced to a soft pulpy state by beating or +pressure@enddots{} + +@item +A mixture of meal or bran and water fed to animals. + +@item +A mess; trouble. [Obs.] --Beau.@: & Fl. +@end enumerate +@end quotation + +M4sh reserves the M4 macro namespace @samp{^_AS_} for internal use, and +the namespace @samp{^AS_} for M4sh macros. It also reserves the shell +and environment variable namespace @samp{^as_}, and the here-document +delimiter namespace @samp{^_AS[A-Z]} in the output file. You should not +define your own macros or output shell code that conflicts with these +namespaces. + +@menu +* Common Shell Constructs:: Portability layer for common shell constructs +* Polymorphic Variables:: Support for indirect variable names +* Initialization Macros:: Macros to establish a sane shell environment +* File Descriptor Macros:: File descriptor macros for input and output +@end menu + +@node Common Shell Constructs +@section Common Shell Constructs + +M4sh provides portable alternatives for some common shell constructs +that unfortunately are not portable in practice. + +@c Deprecated, to be replaced by a better API +@ignore +@defmac AS_BASENAME (@var{file-name}) +@asindex{BASENAME} +Output the non-directory portion of @var{file-name}. For example, +if @code{$file} is @samp{/one/two/three}, the command +@code{base=`AS_BASENAME(["$file"])`} sets @code{base} to @samp{three}. +@end defmac +@end ignore + +@defmac AS_BOX (@var{text}, @dvar{char, -}) +@asindex{BOX} +Expand into shell code that will output @var{text} surrounded by a box +with @var{char} in the top and bottom border. @var{text} should not +contain a newline, but may contain shell expansions valid for unquoted +here-documents. @var{char} defaults to @samp{-}, but can be any +character except @samp{/}, @samp{'}, @samp{"}, @samp{\}, +@samp{&}, or @samp{`}. This is useful for outputting a comment box into +log files to separate distinct phases of script operation. +@end defmac + +@defmac AS_CASE (@var{word}, @ovar{pattern1}, @ovar{if-matched1}, @ + @dots{}, @ovar{default}) +@asindex{CASE} +Expand into a shell @samp{case} statement, where @var{word} is matched +against one or more patterns. @var{if-matched} is run if the +corresponding pattern matched @var{word}, else @var{default} is run. +Avoids several portability issues (@pxref{case, , Limitations of Shell +Builtins}). +@end defmac + +@c Deprecated, to be replaced by a better API +@defmac AS_DIRNAME (@var{file-name}) +@asindex{DIRNAME} +Output the directory portion of @var{file-name}. For example, +if @code{$file} is @samp{/one/two/three}, the command +@code{dir=`AS_DIRNAME(["$file"])`} sets @code{dir} to @samp{/one/two}. + +This interface may be improved in the future to avoid forks and losing +trailing newlines. +@end defmac + +@defmac AS_ECHO (@var{word}) +@asindex{ECHO} +Emits @var{word} to the standard output, followed by a newline. @var{word} +must be a single shell word (typically a quoted string). The bytes of +@var{word} are output as-is, even if it starts with "-" or contains "\". +Redirections can be placed outside the macro invocation. This is much +more portable than using @command{echo} (@pxref{echo, , Limitations of +Shell Builtins}). +@end defmac + +@defmac AS_ECHO_N (@var{word}) +@asindex{ECHO_N} +Emits @var{word} to the standard output, without a following newline. +@var{word} must be a single shell word (typically a quoted string) and, +for portability, should not include more than one newline. The bytes of +@var{word} are output as-is, even if it starts with "-" or contains "\". +Redirections can be placed outside the macro invocation. +@end defmac + +@c We cannot use @dvar because the macro expansion mistreats backslashes. +@defmac AS_ESCAPE (@var{string}, @r{[}@var{chars} = @samp{`\"$}@r{]}) +@asindex{ESCAPE} +Expands to @var{string}, with any characters in @var{chars} escaped with +a backslash (@samp{\}). @var{chars} should be at most four bytes long, +and only contain characters from the set @samp{`\"$}; however, +characters may be safely listed more than once in @var{chars} for the +sake of syntax highlighting editors. The current implementation expands +@var{string} after adding escapes; if @var{string} contains macro calls +that in turn expand to text needing shell quoting, you can use +@code{AS_ESCAPE(m4_dquote(m4_expand([string])))}. + +The default for @var{chars} (@samp{\"$`}) is the set of characters +needing escapes when @var{string} will be used literally within double +quotes. One common variant is the set of characters to protect when +@var{string} will be used literally within back-ticks or an unquoted +here-document (@samp{\$`}). Another common variant is @samp{""}, which can +be used to form a double-quoted string containing the same expansions +that would have occurred if @var{string} were expanded in an unquoted +here-document; however, when using this variant, care must be taken that +@var{string} does not use double quotes within complex variable +expansions (such as @samp{$@{foo-`echo "hi"`@}}) that would be broken +with improper escapes. + +This macro is often used with @code{AS_ECHO}. For an example, observe +the output generated by the shell code generated from this snippet: + +@example +foo=bar +AS_ECHO(["AS_ESCAPE(["$foo" = ])AS_ESCAPE(["$foo"], [""])"]) +@result{}"$foo" = "bar" +m4_define([macro], [a, [\b]]) +AS_ECHO(["AS_ESCAPE([[macro]])"]) +@result{}macro +AS_ECHO(["AS_ESCAPE([macro])"]) +@result{}a, b +AS_ECHO(["AS_ESCAPE(m4_dquote(m4_expand([macro])))"]) +@result{}a, \b +@end example + +@comment Should we add AS_ESCAPE_SINGLE? If we do, we can optimize in +@comment the case of @var{string} that does not contain '. +To escape a string that will be placed within single quotes, use: + +@example +m4_bpatsubst([[@var{string}]], ['], ['\\'']) +@end example +@end defmac + +@defmac AS_EXECUTABLE_P (@var{file}) +@asindex{EXECUTABLE_P} +Emit code to probe whether @var{file} is a regular file with executable +permissions (and not a directory with search permissions). The caller +is responsible for quoting @var{file}. +@end defmac + +@defmac AS_EXIT (@dvar{status, $?}) +@asindex{EXIT} +Emit code to exit the shell with @var{status}, defaulting to @samp{$?}. +This macro +works around shells that see the exit status of the command prior to +@code{exit} inside a @samp{trap 0} handler (@pxref{trap, , Limitations +of Shell Builtins}). +@end defmac + +@defmac AS_IF (@var{test1}, @ovar{run-if-true1}, @dots{}, @ovar{run-if-false}) +@asindex{IF} +Run shell code @var{test1}. If @var{test1} exits with a zero status then +run shell code @var{run-if-true1}, else examine further tests. If no test +exits with a zero status, run shell code @var{run-if-false}, with +simplifications if either @var{run-if-true1} or @var{run-if-false} +is empty. For example, + +@example +AS_IF([test "x$foo" = xyes], [HANDLE_FOO([yes])], + [test "x$foo" != xno], [HANDLE_FOO([maybe])], + [echo foo not specified]) +@end example + +@noindent +ensures any required macros of @code{HANDLE_FOO} +are expanded before the first test. +@end defmac + +@defmac AS_MKDIR_P (@var{file-name}) +@asindex{MKDIR_P} +Make the directory @var{file-name}, including intervening directories +as necessary. This is equivalent to @samp{mkdir -p -- @var{file-name}}, +except that it is portable to older versions of @command{mkdir} that +lack support for the @option{-p} option or for the @option{--} +delimiter (@pxref{mkdir, , Limitations of Usual Tools}). Also, +@code{AS_MKDIR_P} +succeeds if @var{file-name} is a symbolic link to an existing directory, +even though Posix is unclear whether @samp{mkdir -p} should +succeed in that case. If creation of @var{file-name} fails, exit the +script. + +Also see the @code{AC_PROG_MKDIR_P} macro (@pxref{Particular Programs}). +@end defmac + +@defmac AS_SET_STATUS (@var{status}) +@asindex{SET_STATUS} +Emit shell code to set the value of @samp{$?} to @var{status}, as +efficiently as possible. However, this is not guaranteed to abort a +shell running with @code{set -e} (@pxref{set, , Limitations of Shell +Builtins}). This should also be used at the end of a complex shell +function instead of @samp{return} (@pxref{Shell Functions}) to avoid +a DJGPP shell bug. +@end defmac + +@defmac AS_TR_CPP (@var{expression}) +@asindex{TR_CPP} +Transform @var{expression} into a valid right-hand side for a C @code{#define}. +For example: + +@example +# This outputs "#define HAVE_CHAR_P 1". +# Notice the m4 quoting around #, to prevent an m4 comment +type="char *" +echo "[#]define AS_TR_CPP([HAVE_$type]) 1" +@end example +@end defmac + +@defmac AS_TR_SH (@var{expression}) +@asindex{TR_SH} +Transform @var{expression} into shell code that generates a valid shell +variable name. The result is literal when possible at m4 time, but must +be used with @code{eval} if @var{expression} causes shell indirections. +For example: + +@example +# This outputs "Have it!". +header="sys/some file.h" +eval AS_TR_SH([HAVE_$header])=yes +if test "x$HAVE_sys_some_file_h" = xyes; then echo "Have it!"; fi +@end example +@end defmac + +@defmac AS_SET_CATFILE (@var{var}, @var{dir}, @var{file}) +@asindex{SET_CATFILE} +Set the polymorphic shell variable @var{var} to @var{dir}/@var{file}, +but optimizing the common cases (@var{dir} or @var{file} is @samp{.}, +@var{file} is absolute, etc.). +@end defmac + +@defmac AS_UNSET (@var{var}) +@asindex{UNSET} +Unsets the shell variable @var{var}, working around bugs in older +shells (@pxref{unset, , Limitations of Shell +Builtins}). @var{var} can be a literal or indirect variable name. +@end defmac + +@defmac AS_VERSION_COMPARE (@var{version-1}, @var{version-2}, @ + @ovar{action-if-less}, @ovar{action-if-equal}, @ovar{action-if-greater}) +@asindex{VERSION_COMPARE} +Compare two strings @var{version-1} and @var{version-2}, possibly +containing shell variables, as version strings, and expand +@var{action-if-less}, @var{action-if-equal}, or @var{action-if-greater} +depending upon the result. +The algorithm to compare is similar to the one used by strverscmp in +glibc (@pxref{String/Array Comparison, , String/Array Comparison, libc, +The GNU C Library}). +@end defmac + +@node Polymorphic Variables +@section Support for indirect variable names +@cindex variable name indirection +@cindex polymorphic variable name +@cindex indirection, variable name + +Often, it is convenient to write a macro that will emit shell code +operating on a shell variable. The simplest case is when the variable +name is known. But a more powerful idiom is writing shell code that can +work through an indirection, where another variable or command +substitution produces the name of the variable to actually manipulate. +M4sh supports the notion of polymorphic shell variables, making it easy +to write a macro that can deal with either literal or indirect variable +names and output shell code appropriate for both use cases. Behavior is +undefined if expansion of an indirect variable does not result in a +literal variable name. + +@defmac AS_LITERAL_IF (@var{expression}, @ovar{if-literal}, @ovar{if-not}, @ + @dvar{if-simple-ref, @var{if-not}}) +@defmacx AS_LITERAL_WORD_IF (@var{expression}, @ovar{if-literal}, @ + @ovar{if-not}, @dvar{if-simple-ref, @var{if-not}}) +@asindex{LITERAL_IF} +@asindex{LITERAL_WORD_IF} +If the expansion of @var{expression} is definitely a shell literal, +expand @var{if-literal}. If the expansion of @var{expression} looks +like it might contain shell indirections (such as @code{$var} or +@code{`expr`}), then @var{if-not} is expanded. Sometimes, it is +possible to output optimized code if @var{expression} consists only of +shell variable expansions (such as @code{$@{var@}}), in which case +@var{if-simple-ref} can be provided; but defaulting to @var{if-not} +should always be safe. @code{AS_LITERAL_WORD_IF} only expands +@var{if-literal} if @var{expression} looks like a single shell word, +containing no whitespace; while @code{AS_LITERAL_IF} allows whitespace +in @var{expression}. + +In order to reduce the time spent recognizing whether an +@var{expression} qualifies as a literal or a simple indirection, the +implementation is somewhat conservative: @var{expression} must be a +single shell word (possibly after stripping whitespace), consisting only +of bytes that would have the same meaning whether unquoted or enclosed +in double quotes (for example, @samp{a.b} results in @var{if-literal}, +even though it is not a valid shell variable name; while both @samp{'a'} +and @samp{[$]} result in @var{if-not}, because they behave differently +than @samp{"'a'"} and @samp{"[$]"}). This macro can be used in contexts +for recognizing portable file names (such as in the implementation of +@code{AC_LIBSOURCE}), or coupled with some transliterations for forming +valid variable names (such as in the implementation of @code{AS_TR_SH}, +which uses an additional @code{m4_translit} to convert @samp{.} to +@samp{_}). + +This example shows how to read the contents of the shell variable +@code{bar}, exercising all three arguments to @code{AS_LITERAL_IF}. It +results in a script that will output the line @samp{hello} three times. + +@example +AC_DEFUN([MY_ACTION], +[AS_LITERAL_IF([$1], + [echo "$$1"], +@c $$ + [AS_VAR_COPY([var], [$1]) + echo "$var"], + [eval 'echo "$'"$1"\"])]) +foo=bar bar=hello +MY_ACTION([bar]) +MY_ACTION([`echo bar`]) +MY_ACTION([$foo]) +@end example +@end defmac + +@defmac AS_VAR_APPEND (@var{var}, @var{text}) +@asindex{VAR_APPEND} +Emit shell code to append the shell expansion of @var{text} to the end +of the current contents of the polymorphic shell variable @var{var}, +taking advantage of shells that provide the @samp{+=} extension for more +efficient scaling. + +For situations where the final contents of @var{var} are relatively +short (less than 256 bytes), it is more efficient to use the simpler +code sequence of @code{@var{var}=$@{@var{var}@}@var{text}} (or its +polymorphic equivalent of @code{AS_VAR_COPY([t], [@var{var}])} and +@code{AS_VAR_SET([@var{var}], ["$t"@var{text}])}). But in the case +when the script will be repeatedly appending text into @code{var}, +issues of scaling start to become apparent. A naive implementation +requires execution time linear to the length of the current contents of +@var{var} as well as the length of @var{text} for a single append, for +an overall quadratic scaling with multiple appends. This macro takes +advantage of shells which provide the extension +@code{@var{var}+=@var{text}}, which can provide amortized constant time +for a single append, for an overall linear scaling with multiple +appends. Note that unlike @code{AS_VAR_SET}, this macro requires that +@var{text} be quoted properly to avoid field splitting and file name +expansion. +@end defmac + +@defmac AS_VAR_ARITH (@var{var}, @var{expression}) +@asindex{VAR_ARITH} +Emit shell code to compute the arithmetic expansion of @var{expression}, +assigning the result as the contents of the polymorphic shell variable +@var{var}. The code takes advantage of shells that provide @samp{$(())} +for fewer forks, but uses @command{expr} as a fallback. Therefore, the +syntax for a valid @var{expression} is rather limited: all operators +must occur as separate shell arguments and with proper quoting, there is +no portable equality operator, all variables containing numeric values +must be expanded prior to the computation, all numeric values must be +provided in decimal without leading zeroes, and the first shell argument +should not be a negative number. In the following example, this snippet +will print @samp{(2+3)*4 == 20}. + +@example +bar=3 +AS_VAR_ARITH([foo], [\( 2 + $bar \) \* 4]) +echo "(2+$bar)*4 == $foo" +@end example +@end defmac + +@defmac AS_VAR_COPY (@var{dest}, @var{source}) +@asindex{VAR_COPY} +Emit shell code to assign the contents of the polymorphic shell variable +@var{source} to the polymorphic shell variable @var{dest}. For example, +executing this M4sh snippet will output @samp{bar hi}: + +@example +foo=bar bar=hi +AS_VAR_COPY([a], [foo]) +AS_VAR_COPY([b], [$foo]) +echo "$a $b" +@end example + +When it is necessary to access the contents of an indirect variable +inside a shell double-quoted context, the recommended idiom is to first +copy the contents into a temporary literal shell variable. + +@smallexample +for header in stdint_h inttypes_h ; do + AS_VAR_COPY([var], [ac_cv_header_$header]) + echo "$header detected: $var" +done +@end smallexample +@end defmac + +@comment AS_VAR_GET is intentionally undocumented; it can't handle +@comment trailing newlines uniformly, and forks too much. + +@defmac AS_VAR_IF (@var{var}, @ovar{word}, @ovar{if-equal}, @ + @ovar{if-not-equal}) +@asindex{VAR_IF} +Output a shell conditional statement. If the contents of the +polymorphic shell variable @var{var} match the string @var{word}, +execute @var{if-equal}; otherwise execute @var{if-not-equal}. @var{word} +must be a single shell word (typically a quoted string). Avoids +shell bugs if an interrupt signal arrives while a command substitution +in @var{var} is being expanded. +@end defmac + +@defmac AS_VAR_PUSHDEF (@var{m4-name}, @var{value}) +@defmacx AS_VAR_POPDEF (@var{m4-name}) +@asindex{VAR_PUSHDEF} +@asindex{VAR_POPDEF} +@cindex composing variable names +@cindex variable names, composing +A common M4sh idiom involves composing shell variable names from an m4 +argument (for example, writing a macro that uses a cache variable). +@var{value} can be an arbitrary string, which will be transliterated +into a valid shell name by @code{AS_TR_SH}. In order to access the +composed variable name based on @var{value}, it is easier to declare a +temporary m4 macro @var{m4-name} with @code{AS_VAR_PUSHDEF}, then use +that macro as the argument to subsequent @code{AS_VAR} macros as a +polymorphic variable name, and finally free the temporary macro with +@code{AS_VAR_POPDEF}. These macros are often followed with @code{dnl}, +to avoid excess newlines in the output. + +Here is an involved example, that shows the power of writing macros that +can handle composed shell variable names: + +@example +m4_define([MY_CHECK_HEADER], +[AS_VAR_PUSHDEF([my_Header], [ac_cv_header_$1])dnl +AS_VAR_IF([my_Header], [yes], [echo "header $1 detected"])dnl +AS_VAR_POPDEF([my_Header])dnl +]) +MY_CHECK_HEADER([stdint.h]) +for header in inttypes.h stdlib.h ; do + MY_CHECK_HEADER([$header]) +done +@end example + +@noindent +In the above example, @code{MY_CHECK_HEADER} can operate on polymorphic +variable names. In the first invocation, the m4 argument is +@code{stdint.h}, which transliterates into a literal @code{stdint_h}. +As a result, the temporary macro @code{my_Header} expands to the literal +shell name @samp{ac_cv_header_stdint_h}. In the second invocation, the +m4 argument to @code{MY_CHECK_HEADER} is @code{$header}, and the +temporary macro @code{my_Header} expands to the indirect shell name +@samp{$as_my_Header}. During the shell execution of the for loop, when +@samp{$header} contains @samp{inttypes.h}, then @samp{$as_my_Header} +contains @samp{ac_cv_header_inttypes_h}. If this script is then run on a +platform where all three headers have been previously detected, the +output of the script will include: + +@smallexample +header stdint.h detected +header inttypes.h detected +header stdlib.h detected +@end smallexample +@end defmac + +@defmac AS_VAR_SET (@var{var}, @ovar{value}) +@asindex{VAR_SET} +Emit shell code to assign the contents of the polymorphic shell variable +@var{var} to the shell expansion of @var{value}. @var{value} is not +subject to field splitting or file name expansion, so if command +substitution is used, it may be done with @samp{`""`} rather than using +an intermediate variable (@pxref{Shell Substitutions}). However, +@var{value} does undergo rescanning for additional macro names; behavior +is unspecified if late expansion results in any shell meta-characters. +@end defmac + +@defmac AS_VAR_SET_IF (@var{var}, @ovar{if-set}, @ovar{if-undef}) +@asindex{VAR_SET_IF} +Emit a shell conditional statement, which executes @var{if-set} if the +polymorphic shell variable @code{var} is set to any value, and +@var{if-undef} otherwise. +@end defmac + +@defmac AS_VAR_TEST_SET (@var{var}) +@asindex{VAR_TEST_SET} +Emit a shell statement that results in a successful exit status only if +the polymorphic shell variable @code{var} is set. +@end defmac + +@node Initialization Macros +@section Initialization Macros + +@defmac AS_BOURNE_COMPATIBLE +@asindex{BOURNE_COMPATIBLE} +Set up the shell to be more compatible with the Bourne shell as +standardized by Posix, if possible. This may involve setting +environment variables, or setting options, or similar +implementation-specific actions. This macro is deprecated, since +@code{AS_INIT} already invokes it. +@end defmac + +@defmac AS_INIT +@asindex{INIT} +@evindex LC_ALL +@evindex SHELL +Initialize the M4sh environment. This macro calls @code{m4_init}, then +outputs the @code{#! /bin/sh} line, a notice about where the output was +generated from, and code to sanitize the environment for the rest of the +script. Among other initializations, this sets @env{SHELL} to the shell +chosen to run the script (@pxref{CONFIG_SHELL}), and @env{LC_ALL} to +ensure the C locale. Finally, it changes the current diversion to +@code{BODY}. @code{AS_INIT} is called automatically by @code{AC_INIT} +and @code{AT_INIT}, so shell code in @file{configure}, +@file{config.status}, and @file{testsuite} all benefit from a sanitized +shell environment. +@end defmac + +@defmac AS_INIT_GENERATED (@var{file}, @ovar{comment}) +@asindex{INIT_GENERATED} +Emit shell code to start the creation of a subsidiary shell script in +@var{file}, including changing @var{file} to be executable. This macro +populates the child script with information learned from the parent +(thus, the emitted code is equivalent in effect, but more efficient, +than the code output by @code{AS_INIT}, @code{AS_BOURNE_COMPATIBLE}, and +@code{AS_SHELL_SANITIZE}). If present, @var{comment} is output near the +beginning of the child, prior to the shell initialization code, and is +subject to parameter expansion, command substitution, and backslash +quote removal. The +parent script should check the exit status after this macro, in case +@var{file} could not be properly created (for example, if the disk was +full). If successfully created, the parent script can then proceed to +append additional M4sh constructs into the child script. + +Note that the child script starts life without a log file open, so if +the parent script uses logging (@pxref{AS_MESSAGE_LOG_FD}), you +must temporarily disable any attempts to use the log file until after +emitting code to open a log within the child. On the other hand, if the +parent script has @code{AS_MESSAGE_FD} redirected somewhere besides +@samp{1}, then the child script already has code that copies stdout to +that descriptor. Currently, the suggested +idiom for writing a M4sh shell script from within another script is: + +@example +AS_INIT_GENERATED([@var{file}], [[# My child script. +]]) || @{ AS_ECHO(["Failed to create child script"]); AS_EXIT; @} +m4_pushdef([AS_MESSAGE_LOG_FD])dnl +cat >> "@var{file}" <<\__EOF__ +# Code to initialize AS_MESSAGE_LOG_FD +m4_popdef([AS_MESSAGE_LOG_FD])dnl +# Additional code +__EOF__ +@end example + +This, however, may change in the future as the M4sh interface is +stabilized further. + +Also, be aware that use of @env{LINENO} within the child script may +report line numbers relative to their location in the parent script, +even when using @code{AS_LINENO_PREPARE}, if the parent script was +unable to locate a shell with working @env{LINENO} support. +@end defmac + +@defmac AS_LINENO_PREPARE +@asindex{LINENO_PREPARE} +@evindex LINENO +Find a shell that supports the special variable @env{LINENO}, which +contains the number of the currently executing line. This macro is +automatically invoked by @code{AC_INIT} in configure scripts. +@end defmac + +@defmac AS_ME_PREPARE +@asindex{ME_PREPARE} +Set up variable @env{as_me} to be the basename of the currently executing +script. This macro is automatically invoked by @code{AC_INIT} in +configure scripts. +@end defmac + +@defmac AS_TMPDIR (@var{prefix}, @dvar{dir, $@{TMPDIR:=/tmp@}}) +@asindex{TMPDIR} +@evindex TMPDIR +@ovindex tmp +Create, as safely as possible, a temporary sub-directory within +@var{dir} with a name starting with @var{prefix}. @var{prefix} should +be 2-4 characters, to make it slightly easier to identify the owner of +the directory. If @var{dir} is omitted, then the value of @env{TMPDIR} +will be used (defaulting to @samp{/tmp}). On success, the name of the +newly created directory is stored in the shell variable @code{tmp}. On +error, the script is aborted. + +Typically, this macro is coupled with some exit traps to delete the created +directory and its contents on exit or interrupt. However, there is a +slight window between when the directory is created and when the name is +actually known to the shell, so an interrupt at the right moment might +leave the temporary directory behind. Hence it is important to use a +@var{prefix} that makes it easier to determine if a leftover temporary +directory from an interrupted script is safe to delete. + +The use of the output variable @samp{$tmp} rather than something in the +@samp{as_} namespace is historical; it has the unfortunate consequence +that reusing this otherwise common name for any other purpose inside +your script has the potential to break any cleanup traps designed to +remove the temporary directory. +@end defmac + +@defmac AS_SHELL_SANITIZE +@asindex{SHELL_SANITIZE} +Initialize the shell suitably for @command{configure} scripts. This has +the effect of @code{AS_BOURNE_COMPATIBLE}, and sets some other +environment variables for predictable results from configuration tests. +For example, it sets @env{LC_ALL} to change to the default C locale. +@xref{Special Shell Variables}. This macro is deprecated, since +@code{AS_INIT} already invokes it. +@end defmac + + +@node File Descriptor Macros +@section File Descriptor Macros +@cindex input +@cindex standard input +@cindex file descriptors +@cindex descriptors +@cindex low-level output +@cindex output, low-level + +The following macros define file descriptors used to output messages +(or input values) from @file{configure} scripts. +For example: + +@example +echo "$wombats found" >&AS_MESSAGE_LOG_FD +echo 'Enter desired kangaroo count:' >&AS_MESSAGE_FD +read kangaroos <&AS_ORIGINAL_STDIN_FD` +@end example + +@noindent +However doing so is seldom needed, because Autoconf provides higher +level macros as described below. + +@defmac AS_MESSAGE_FD +@asindex{MESSAGE_FD} +The file descriptor for @samp{checking for...} messages and results. +By default, @code{AS_INIT} sets this to @samp{1} for standalone M4sh +clients. However, @code{AC_INIT} shuffles things around to another file +descriptor, in order to allow the @option{-q} option of +@command{configure} to choose whether messages should go to the script's +standard output or be discarded. + +If you want to display some messages, consider using one of the printing +macros (@pxref{Printing Messages}) instead. Copies of messages output +via these macros are also recorded in @file{config.log}. +@end defmac + +@anchor{AS_MESSAGE_LOG_FD} +@defmac AS_MESSAGE_LOG_FD +@asindex{MESSAGE_LOG_FD} +This must either be empty, or expand to a file descriptor for log +messages. By default, @code{AS_INIT} sets this macro to the empty +string for standalone M4sh clients, thus disabling logging. However, +@code{AC_INIT} shuffles things around so that both @command{configure} +and @command{config.status} use @file{config.log} for log messages. +Macros that run tools, like @code{AC_COMPILE_IFELSE} (@pxref{Running the +Compiler}), redirect all output to this descriptor. You may want to do +so if you develop such a low-level macro. +@end defmac + +@defmac AS_ORIGINAL_STDIN_FD +@asindex{ORIGINAL_STDIN_FD} +This must expand to a file descriptor for the original standard input. +By default, @code{AS_INIT} sets this macro to @samp{0} for standalone +M4sh clients. However, @code{AC_INIT} shuffles things around for +safety. + +When @command{configure} runs, it may accidentally execute an +interactive command that has the same name as the non-interactive meant +to be used or checked. If the standard input was the terminal, such +interactive programs would cause @command{configure} to stop, pending +some user input. Therefore @command{configure} redirects its standard +input from @file{/dev/null} during its initialization. This is not +normally a problem, since @command{configure} normally does not need +user input. + +In the extreme case where your @file{configure} script really needs to +obtain some values from the original standard input, you can read them +explicitly from @code{AS_ORIGINAL_STDIN_FD}. +@end defmac + + +@c =================================================== Writing Autoconf Macros. + +@node Writing Autoconf Macros +@chapter Writing Autoconf Macros + +When you write a feature test that could be applicable to more than one +software package, the best thing to do is encapsulate it in a new macro. +Here are some instructions and guidelines for writing Autoconf macros. + +@menu +* Macro Definitions:: Basic format of an Autoconf macro +* Macro Names:: What to call your new macros +* Reporting Messages:: Notifying @command{autoconf} users +* Dependencies Between Macros:: What to do when macros depend on other macros +* Obsoleting Macros:: Warning about old ways of doing things +* Coding Style:: Writing Autoconf macros @`a la Autoconf +@end menu + +@node Macro Definitions +@section Macro Definitions + +@defmac AC_DEFUN (@var{name}, @ovar{body}) +@acindex{DEFUN} +Autoconf macros are defined using the @code{AC_DEFUN} macro, which is +similar to the M4 builtin @code{m4_define} macro; this creates a macro +named @var{name} and with @var{body} as its expansion. In addition to +defining a macro, @code{AC_DEFUN} adds to it some code that is used to +constrain the order in which macros are called, while avoiding redundant +output (@pxref{Prerequisite Macros}). +@end defmac + +An Autoconf macro definition looks like this: + +@example +AC_DEFUN(@var{macro-name}, @var{macro-body}) +@end example + +You can refer to any arguments passed to the macro as @samp{$1}, +@samp{$2}, etc. @xref{Definitions, , How to define new macros, m4.info, +GNU M4}, for more complete information on writing M4 macros. + +Most macros fall in one of two general categories. The first category +includes macros which take arguments, in order to generate output +parameterized by those arguments. Macros in this category are designed +to be directly expanded, often multiple times, and should not be used as +the argument to @code{AC_REQUIRE}. The other category includes macros +which are shorthand for a fixed block of text, and therefore do not take +arguments. For this category of macros, directly expanding the macro +multiple times results in redundant output, so it is more common to use +the macro as the argument to @code{AC_REQUIRE}, or to declare the macro +with @code{AC_DEFUN_ONCE} (@pxref{One-Shot Macros}). + +Be sure to properly quote both the @var{macro-body} @emph{and} the +@var{macro-name} to avoid any problems if the macro happens to have +been previously defined. + +Each macro should have a header comment that gives its prototype, and a +brief description. When arguments have default values, display them in +the prototype. For example: + +@example +# AC_MSG_ERROR(ERROR, [EXIT-STATUS = 1]) +# -------------------------------------- +m4_define([AC_MSG_ERROR], + [@{ AS_MESSAGE([error: $1], [2]) + exit m4_default([$2], [1]); @}]) +@end example + +Comments about the macro should be left in the header comment. Most +other comments make their way into @file{configure}, so just keep +using @samp{#} to introduce comments. + +@cindex @code{dnl} +If you have some special comments about pure M4 code, comments +that make no sense in @file{configure} and in the header comment, then +use the builtin @code{dnl}: it causes M4 to discard the text +through the next newline. + +Keep in mind that @code{dnl} is rarely needed to introduce comments; +@code{dnl} is more useful to get rid of the newlines following macros +that produce no output, such as @code{AC_REQUIRE}. + +Public third-party macros need to use @code{AC_DEFUN}, and not +@code{m4_define}, in order to be found by @command{aclocal} +(@pxref{Extending aclocal,,, automake, GNU Automake}). +Additionally, if it is ever determined that a macro should be made +obsolete, it is easy to convert from @code{AC_DEFUN} to @code{AU_DEFUN} +in order to have @command{autoupdate} assist the user in choosing a +better alternative, but there is no corresponding way to make +@code{m4_define} issue an upgrade notice (@pxref{AU_DEFUN}). + +There is another subtle, but important, difference between using +@code{m4_define} and @code{AC_DEFUN}: only the former is unaffected by +@code{AC_REQUIRE}. When writing a file, it is always safe to replace a +block of text with a @code{m4_define} macro that will expand to the same +text. But replacing a block of text with an @code{AC_DEFUN} macro with +the same content does not necessarily give the same results, because it +changes the location where any embedded but unsatisfied +@code{AC_REQUIRE} invocations within the block will be expanded. For an +example of this, see @ref{Expanded Before Required}. + +@node Macro Names +@section Macro Names + +All of the public Autoconf macros have all-uppercase names in the +namespace @samp{^AC_} to prevent them from accidentally conflicting with +other text; Autoconf also reserves the namespace @samp{^_AC_} for +internal macros. All shell variables that they use for internal +purposes have mostly-lowercase names starting with @samp{ac_}. Autoconf +also uses here-document delimiters in the namespace @samp{^_AC[A-Z]}. During +@command{configure}, files produced by Autoconf make heavy use of the +file system namespace @samp{^conf}. + +Since Autoconf is built on top of M4sugar (@pxref{Programming in +M4sugar}) and M4sh (@pxref{Programming in M4sh}), you must also be aware +of those namespaces (@samp{^_?\(m4\|AS\)_}). And since +@file{configure.ac} is also designed to be scanned by Autoheader, +Autoscan, Autoupdate, and Automake, you should be aware of the +@samp{^_?A[HNUM]_} namespaces. In general, you @emph{should not use} +the namespace of a package that does not own the macro or shell code you +are writing. + +To ensure that your macros don't conflict with present or future +Autoconf macros, you should prefix your own macro names and any shell +variables they use with some other sequence. Possibilities include your +initials, or an abbreviation for the name of your organization or +software package. Historically, people have not always followed the +rule of using a namespace appropriate for their package, and this has +made it difficult for determining the origin of a macro (and where to +report bugs about that macro), as well as difficult for the true +namespace owner to add new macros without interference from pre-existing +uses of third-party macros. Perhaps the best example of this confusion +is the @code{AM_GNU_GETTEXT} macro, which belongs, not to Automake, but +to Gettext. + +Most of the Autoconf macros' names follow a structured naming convention +that indicates the kind of feature check by the name. The macro names +consist of several words, separated by underscores, going from most +general to most specific. The names of their cache variables use the +same convention (@pxref{Cache Variable Names}, for more information on +them). + +The first word of the name after the namespace initials (such as +@samp{AC_}) usually tells the category +of the feature being tested. Here are the categories used in Autoconf for +specific test macros, the kind of macro that you are more likely to +write. They are also used for cache variables, in all-lowercase. Use +them where applicable; where they're not, invent your own categories. + +@table @code +@item C +C language builtin features. +@item DECL +Declarations of C variables in header files. +@item FUNC +Functions in libraries. +@item GROUP +Posix group owners of files. +@item HEADER +Header files. +@item LIB +C libraries. +@item PROG +The base names of programs. +@item MEMBER +Members of aggregates. +@item SYS +Operating system features. +@item TYPE +C builtin or declared types. +@item VAR +C variables in libraries. +@end table + +After the category comes the name of the particular feature being +tested. Any further words in the macro name indicate particular aspects +of the feature. For example, @code{AC_PROG_CC_STDC} checks whether the +C compiler supports ISO Standard C. + +An internal macro should have a name that starts with an underscore; +Autoconf internals should therefore start with @samp{_AC_}. +Additionally, a macro that is an internal subroutine of another macro +should have a name that starts with an underscore and the name of that +other macro, followed by one or more words saying what the internal +macro does. For example, @code{AC_PATH_X} has internal macros +@code{_AC_PATH_X_XMKMF} and @code{_AC_PATH_X_DIRECT}. + +@node Reporting Messages +@section Reporting Messages +@cindex Messages, from @command{autoconf} + +When macros statically diagnose abnormal situations, benign or fatal, it +is possible to make @command{autoconf} detect the problem, and refuse to +create @file{configure} in the case of an error. The macros in this +section are considered obsolescent, and new code should use M4sugar +macros for this purpose, see @ref{Diagnostic Macros}. + +On the other hand, it is possible to want to detect errors when +@command{configure} is run, which are dependent on the environment of +the user rather than the maintainer. For dynamic diagnostics, see +@ref{Printing Messages}. + +@defmac AC_DIAGNOSE (@var{category}, @var{message}) +@acindex{DIAGNOSE} +Report @var{message} as a warning (or as an error if requested by the +user) if warnings of the @var{category} are turned on. This macro is +obsolescent; you are encouraged to use: +@example +m4_warn([@var{category}], [@var{message}]) +@end example +@noindent +instead. @xref{m4_warn}, for more details, including valid +@var{category} names. +@end defmac + +@defmac AC_WARNING (@var{message}) +@acindex{WARNING} +Report @var{message} as a syntax warning. This macro is obsolescent; +you are encouraged to use: +@example +m4_warn([syntax], [@var{message}]) +@end example +@noindent +instead. @xref{m4_warn}, for more details, as well as better +finer-grained categories of warnings (not all problems have to do with +syntax). +@end defmac + +@defmac AC_FATAL (@var{message}) +@acindex{FATAL} +Report a severe error @var{message}, and have @command{autoconf} die. +This macro is obsolescent; you are encouraged to use: +@example +m4_fatal([@var{message}]) +@end example +@noindent +instead. @xref{m4_fatal}, for more details. +@end defmac + +When the user runs @samp{autoconf -W error}, warnings from +@code{m4_warn} (including those issued through @code{AC_DIAGNOSE} and +@code{AC_WARNING}) are reported as errors, see @ref{autoconf Invocation}. + +@node Dependencies Between Macros +@section Dependencies Between Macros +@cindex Dependencies between macros + +Some Autoconf macros depend on other macros having been called first in +order to work correctly. Autoconf provides a way to ensure that certain +macros are called if needed and a way to warn the user if macros are +called in an order that might cause incorrect operation. + +@menu +* Prerequisite Macros:: Ensuring required information +* Suggested Ordering:: Warning about possible ordering problems +* One-Shot Macros:: Ensuring a macro is called only once +@end menu + +@node Prerequisite Macros +@subsection Prerequisite Macros +@cindex Prerequisite macros +@cindex Macros, prerequisites + +A macro that you write might need to use values that have previously +been computed by other macros. For example, @code{AC_DECL_YYTEXT} +examines the output of @code{flex} or @code{lex}, so it depends on +@code{AC_PROG_LEX} having been called first to set the shell variable +@code{LEX}. + +Rather than forcing the user of the macros to keep track of the +dependencies between them, you can use the @code{AC_REQUIRE} macro to do +it automatically. @code{AC_REQUIRE} can ensure that a macro is only +called if it is needed, and only called once. + +@defmac AC_REQUIRE (@var{macro-name}) +@acindex{REQUIRE} +If the M4 macro @var{macro-name} has not already been called, call it +(without any arguments). Make sure to quote @var{macro-name} with +square brackets. @var{macro-name} must have been defined using +@code{AC_DEFUN} or else contain a call to @code{AC_PROVIDE} to indicate +that it has been called. + +@code{AC_REQUIRE} must be used inside a macro defined by @code{AC_DEFUN}; it +must not be called from the top level. Also, it does not make sense to +require a macro that takes parameters. +@end defmac + +@code{AC_REQUIRE} is often misunderstood. It really implements +dependencies between macros in the sense that if one macro depends upon +another, the latter is expanded @emph{before} the body of the +former. To be more precise, the required macro is expanded before +the outermost defined macro in the current expansion stack. +In particular, @samp{AC_REQUIRE([FOO])} is not replaced with the body of +@code{FOO}. For instance, this definition of macros: + +@example +@group +AC_DEFUN([TRAVOLTA], +[test "$body_temperature_in_celsius" -gt "38" && + dance_floor=occupied]) +AC_DEFUN([NEWTON_JOHN], +[test "x$hair_style" = xcurly && + dance_floor=occupied]) +@end group + +@group +AC_DEFUN([RESERVE_DANCE_FLOOR], +[if date | grep '^Sat.*pm' >/dev/null 2>&1; then + AC_REQUIRE([TRAVOLTA]) + AC_REQUIRE([NEWTON_JOHN]) +fi]) +@end group +@end example + +@noindent +with this @file{configure.ac} + +@example +AC_INIT([Dance Manager], [1.0], [bug-dance@@example.org]) +RESERVE_DANCE_FLOOR +if test "x$dance_floor" = xoccupied; then + AC_MSG_ERROR([cannot pick up here, let's move]) +fi +@end example + +@noindent +does not leave you with a better chance to meet a kindred soul at +other times than Saturday night since it expands into: + +@example +@group +test "$body_temperature_in_Celsius" -gt "38" && + dance_floor=occupied +test "x$hair_style" = xcurly && + dance_floor=occupied +fi +if date | grep '^Sat.*pm' >/dev/null 2>&1; then + + +fi +@end group +@end example + +This behavior was chosen on purpose: (i) it prevents messages in +required macros from interrupting the messages in the requiring macros; +(ii) it avoids bad surprises when shell conditionals are used, as in: + +@example +@group +if @dots{}; then + AC_REQUIRE([SOME_CHECK]) +fi +@dots{} +SOME_CHECK +@end group +@end example + +However, this implementation can lead to another class of problems. +Consider the case where an outer macro first expands, then indirectly +requires, an inner macro: + +@example +AC_DEFUN([TESTA], [[echo in A +if test -n "$SEEN_A" ; then echo duplicate ; fi +SEEN_A=:]]) +AC_DEFUN([TESTB], [AC_REQUIRE([TESTA])[echo in B +if test -z "$SEEN_A" ; then echo bug ; fi]]) +AC_DEFUN([TESTC], [AC_REQUIRE([TESTB])[echo in C]]) +AC_DEFUN([OUTER], [[echo in OUTER] +TESTA +TESTC]) +OUTER +@end example + +@noindent +Prior to Autoconf 2.64, the implementation of @code{AC_REQUIRE} +recognized that @code{TESTB} needed to be hoisted prior to the expansion +of @code{OUTER}, but because @code{TESTA} had already been directly +expanded, it failed to hoist @code{TESTA}. Therefore, the expansion of +@code{TESTB} occurs prior to its prerequisites, leading to the following +output: + +@example +in B +bug +in OUTER +in A +in C +@end example + +@noindent +Newer Autoconf is smart enough to recognize this situation, and hoists +@code{TESTA} even though it has already been expanded, but issues a +syntax warning in the process. This is because the hoisted expansion of +@code{TESTA} defeats the purpose of using @code{AC_REQUIRE} to avoid +redundant code, and causes its own set of problems if the hoisted macro +is not idempotent: + +@example +in A +in B +in OUTER +in A +duplicate +in C +@end example + +The bug is not in Autoconf, but in the macro definitions. If you ever +pass a particular macro name to @code{AC_REQUIRE}, then you are implying +that the macro only needs to be expanded once. But to enforce this, +either the macro must be declared with @code{AC_DEFUN_ONCE} (although +this only helps in Autoconf 2.64 or newer), or all +uses of that macro should be through @code{AC_REQUIRE}; directly +expanding the macro defeats the point of using @code{AC_REQUIRE} to +eliminate redundant expansion. In the example, this rule of thumb was +violated because @code{TESTB} requires @code{TESTA} while @code{OUTER} +directly expands it. One way of fixing the bug is to factor +@code{TESTA} into two macros, the portion designed for direct and +repeated use (here, named @code{TESTA}), and the portion designed for +one-shot output and used only inside @code{AC_REQUIRE} (here, named +@code{TESTA_PREREQ}). Then, by fixing all clients to use the correct +calling convention according to their needs: + +@example +AC_DEFUN([TESTA], [AC_REQUIRE([TESTA_PREREQ])[echo in A]]) +AC_DEFUN([TESTA_PREREQ], [[echo in A_PREREQ +if test -n "$SEEN_A" ; then echo duplicate ; fi +SEEN_A=:]]) +AC_DEFUN([TESTB], [AC_REQUIRE([TESTA_PREREQ])[echo in B +if test -z "$SEEN_A" ; then echo bug ; fi]]) +AC_DEFUN([TESTC], [AC_REQUIRE([TESTB])[echo in C]]) +AC_DEFUN([OUTER], [[echo in OUTER] +TESTA +TESTC]) +OUTER +@end example + +@noindent +the resulting output will then obey all dependency rules and avoid any +syntax warnings, whether the script is built with old or new Autoconf +versions: + +@example +in A_PREREQ +in B +in OUTER +in A +in C +@end example + +The helper macros @code{AS_IF} and @code{AS_CASE} may be used to +enforce expansion of required macros outside of shell conditional +constructs. You are furthermore encouraged, although not required, to +put all @code{AC_REQUIRE} calls +at the beginning of a macro. You can use @code{dnl} to avoid the empty +lines they leave. + +@node Suggested Ordering +@subsection Suggested Ordering +@cindex Macros, ordering +@cindex Ordering macros + +Some macros should be run before another macro if both are called, but +neither @emph{requires} that the other be called. For example, a macro +that changes the behavior of the C compiler should be called before any +macros that run the C compiler. Many of these dependencies are noted in +the documentation. + +Autoconf provides the @code{AC_BEFORE} macro to warn users when macros +with this kind of dependency appear out of order in a +@file{configure.ac} file. The warning occurs when creating +@command{configure} from @file{configure.ac}, not when running +@command{configure}. + +For example, @code{AC_PROG_CPP} checks whether the C compiler +can run the C preprocessor when given the @option{-E} option. It should +therefore be called after any macros that change which C compiler is +being used, such as @code{AC_PROG_CC}. So @code{AC_PROG_CC} contains: + +@example +AC_BEFORE([$0], [AC_PROG_CPP])dnl +@end example + +@noindent +This warns the user if a call to @code{AC_PROG_CPP} has already occurred +when @code{AC_PROG_CC} is called. + +@defmac AC_BEFORE (@var{this-macro-name}, @var{called-macro-name}) +@acindex{BEFORE} +Make M4 print a warning message to the standard error output if +@var{called-macro-name} has already been called. @var{this-macro-name} +should be the name of the macro that is calling @code{AC_BEFORE}. The +macro @var{called-macro-name} must have been defined using +@code{AC_DEFUN} or else contain a call to @code{AC_PROVIDE} to indicate +that it has been called. +@end defmac + +@node One-Shot Macros +@subsection One-Shot Macros +@cindex One-shot macros +@cindex Macros, called once + +Some macros should be called only once, either because calling them +multiple time is unsafe, or because it is bad style. For instance +Autoconf ensures that @code{AC_CANONICAL_BUILD} and cousins +(@pxref{Canonicalizing}) are evaluated only once, because it makes no +sense to run these expensive checks more than once. Such one-shot +macros can be defined using @code{AC_DEFUN_ONCE}. + +@defmac AC_DEFUN_ONCE (@var{macro-name}, @var{macro-body}) +@acindex{DEFUN_ONCE} +Declare macro @var{macro-name} like @code{AC_DEFUN} would (@pxref{Macro +Definitions}), but add additional logic that guarantees that only the +first use of the macro (whether by direct expansion or +@code{AC_REQUIRE}) causes an expansion of @var{macro-body}; the +expansion will occur before the start of any enclosing macro defined by +@code{AC_DEFUN}. Subsequent expansions are silently ignored. +Generally, it does not make sense for @var{macro-body} to use parameters +such as @code{$1}. +@end defmac + +Prior to Autoconf 2.64, a macro defined by @code{AC_DEFUN_ONCE} would +emit a warning if it was directly expanded a second time, so for +portability, it is better to use @code{AC_REQUIRE} than direct +invocation of @var{macro-name} inside a macro defined by @code{AC_DEFUN} +(@pxref{Prerequisite Macros}). + +@node Obsoleting Macros +@section Obsoleting Macros +@cindex Obsoleting macros +@cindex Macros, obsoleting + +Configuration and portability technology has evolved over the years. +Often better ways of solving a particular problem are developed, or +ad-hoc approaches are systematized. This process has occurred in many +parts of Autoconf. One result is that some of the macros are now +considered @dfn{obsolete}; they still work, but are no longer considered +the best thing to do, hence they should be replaced with more modern +macros. Ideally, @command{autoupdate} should replace the old macro calls +with their modern implementation. + +Autoconf provides a simple means to obsolete a macro. + +@anchor{AU_DEFUN} +@defmac AU_DEFUN (@var{old-macro}, @var{implementation}, @ovar{message}) +@auindex{DEFUN} +Define @var{old-macro} as @var{implementation}. The only difference +with @code{AC_DEFUN} is that the user is warned that +@var{old-macro} is now obsolete. + +If she then uses @command{autoupdate}, the call to @var{old-macro} is +replaced by the modern @var{implementation}. @var{message} should +include information on what to do after running @command{autoupdate}; +@command{autoupdate} prints it as a warning, and includes it +in the updated @file{configure.ac} file. + +The details of this macro are hairy: if @command{autoconf} encounters an +@code{AU_DEFUN}ed macro, all macros inside its second argument are expanded +as usual. However, when @command{autoupdate} is run, only M4 and M4sugar +macros are expanded here, while all other macros are disabled and +appear literally in the updated @file{configure.ac}. +@end defmac + +@defmac AU_ALIAS (@var{old-name}, @var{new-name}) +@auindex{ALIAS} +Used if the @var{old-name} is to be replaced by a call to @var{new-macro} +with the same parameters. This happens for example if the macro was renamed. +@end defmac + +@node Coding Style +@section Coding Style +@cindex Coding style + +The Autoconf macros follow a strict coding style. You are encouraged to +follow this style, especially if you intend to distribute your macro, +either by contributing it to Autoconf itself or the +@uref{http://@/www.gnu.org/@/software/@/autoconf-archive/, Autoconf Macro +Archive}, or by other means. + +The first requirement is to pay great attention to the quotation. For +more details, see @ref{Autoconf Language}, and @ref{M4 Quotation}. + +Do not try to invent new interfaces. It is likely that there is a macro +in Autoconf that resembles the macro you are defining: try to stick to +this existing interface (order of arguments, default values, etc.). We +@emph{are} conscious that some of these interfaces are not perfect; +nevertheless, when harmless, homogeneity should be preferred over +creativity. + +Be careful about clashes both between M4 symbols and between shell +variables. + +If you stick to the suggested M4 naming scheme (@pxref{Macro Names}), +you are unlikely to generate conflicts. Nevertheless, when you need to +set a special value, @emph{avoid using a regular macro name}; rather, +use an ``impossible'' name. For instance, up to version 2.13, the macro +@code{AC_SUBST} used to remember what @var{symbol} macros were already defined +by setting @code{AC_SUBST_@var{symbol}}, which is a regular macro name. +But since there is a macro named @code{AC_SUBST_FILE}, it was just +impossible to @samp{AC_SUBST(FILE)}! In this case, +@code{AC_SUBST(@var{symbol})} or @code{_AC_SUBST(@var{symbol})} should +have been used (yes, with the parentheses). +@c or better yet, high-level macros such as @code{m4_expand_once} + +No Autoconf macro should ever enter the user-variable name space; i.e., +except for the variables that are the actual result of running the +macro, all shell variables should start with @code{ac_}. In +addition, small macros or any macro that is likely to be embedded in +other macros should be careful not to use obvious names. + +@cindex @code{dnl} +Do not use @code{dnl} to introduce comments: most of the comments you +are likely to write are either header comments which are not output +anyway, or comments that should make their way into @file{configure}. +There are exceptional cases where you do want to comment special M4 +constructs, in which case @code{dnl} is right, but keep in mind that it +is unlikely. + +M4 ignores the leading blanks and newlines before each argument. +Use this feature to +indent in such a way that arguments are (more or less) aligned with the +opening parenthesis of the macro being called. For instance, instead of + +@example +AC_CACHE_CHECK(for EMX OS/2 environment, +ac_cv_emxos2, +[AC_COMPILE_IFELSE([AC_LANG_PROGRAM(, [return __EMX__;])], +[ac_cv_emxos2=yes], [ac_cv_emxos2=no])]) +@end example + +@noindent +write + +@example +AC_CACHE_CHECK([for EMX OS/2 environment], [ac_cv_emxos2], +[AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [return __EMX__;])], + [ac_cv_emxos2=yes], + [ac_cv_emxos2=no])]) +@end example + +@noindent +or even + +@example +AC_CACHE_CHECK([for EMX OS/2 environment], + [ac_cv_emxos2], + [AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], + [return __EMX__;])], + [ac_cv_emxos2=yes], + [ac_cv_emxos2=no])]) +@end example + +When using @code{AC_RUN_IFELSE} or any macro that cannot work when +cross-compiling, provide a pessimistic value (typically @samp{no}). + +Feel free to use various tricks to prevent auxiliary tools, such as +syntax-highlighting editors, from behaving improperly. For instance, +instead of: + +@example +m4_bpatsubst([$1], [$"]) +@end example + +@noindent +use + +@example +m4_bpatsubst([$1], [$""]) +@end example + +@noindent +so that Emacsen do not open an endless ``string'' at the first quote. +For the same reasons, avoid: + +@example +test $[#] != 0 +@end example + +@noindent +and use: + +@example +test $[@@%:@@] != 0 +@end example + +@noindent +Otherwise, the closing bracket would be hidden inside a @samp{#}-comment, +breaking the bracket-matching highlighting from Emacsen. Note the +preferred style to escape from M4: @samp{$[1]}, @samp{$[@@]}, etc. Do +not escape when it is unnecessary. Common examples of useless quotation +are @samp{[$]$1} (write @samp{$$1}), @samp{[$]var} (use @samp{$var}), +etc. If you add portability issues to the picture, you'll prefer +@samp{$@{1+"$[@@]"@}} to @samp{"[$]@@"}, and you'll prefer do something +better than hacking Autoconf @code{:-)}. + +When using @command{sed}, don't use @option{-e} except for indenting +purposes. With the @code{s} and @code{y} commands, the preferred +separator is @samp{/} unless @samp{/} itself might appear in the pattern +or replacement, in which case you should use @samp{|}, or optionally +@samp{,} if you know the pattern and replacement cannot contain a file +name. If none of these characters will do, choose a printable character +that cannot appear in the pattern or replacement. Characters from the +set @samp{"#$&'()*;<=>?`|~} are good choices if the pattern or +replacement might contain a file name, since they have special meaning +to the shell and are less likely to occur in file names. + +@xref{Macro Definitions}, for details on how to define a macro. If a +macro doesn't use @code{AC_REQUIRE}, is expected to never be the object +of an @code{AC_REQUIRE} directive, and macros required by other macros +inside arguments do not need to be expanded before this macro, then +use @code{m4_define}. In case of doubt, use @code{AC_DEFUN}. +Also take into account that public third-party macros need to use +@code{AC_DEFUN} in order to be found by @command{aclocal} +(@pxref{Extending aclocal,,, automake, GNU Automake}). +All the @code{AC_REQUIRE} statements should be at the beginning of the +macro, and each statement should be followed by @code{dnl}. + +You should not rely on the number of arguments: instead of checking +whether an argument is missing, test that it is not empty. It provides +both a simpler and a more predictable interface to the user, and saves +room for further arguments. + +Unless the macro is short, try to leave the closing @samp{])} at the +beginning of a line, followed by a comment that repeats the name of the +macro being defined. This introduces an additional newline in +@command{configure}; normally, that is not a problem, but if you want to +remove it you can use @samp{[]dnl} on the last line. You can similarly +use @samp{[]dnl} after a macro call to remove its newline. @samp{[]dnl} +is recommended instead of @samp{dnl} to ensure that M4 does not +interpret the @samp{dnl} as being attached to the preceding text or +macro output. For example, instead of: + +@example +AC_DEFUN([AC_PATH_X], +[AC_MSG_CHECKING([for X]) +AC_REQUIRE_CPP() +@r{# @dots{}omitted@dots{}} + AC_MSG_RESULT([libraries $x_libraries, headers $x_includes]) +fi]) +@end example + +@noindent +you would write: + +@example +AC_DEFUN([AC_PATH_X], +[AC_REQUIRE_CPP()[]dnl +AC_MSG_CHECKING([for X]) +@r{# @dots{}omitted@dots{}} + AC_MSG_RESULT([libraries $x_libraries, headers $x_includes]) +fi[]dnl +])# AC_PATH_X +@end example + +If the macro is long, try to split it into logical chunks. Typically, +macros that check for a bug in a function and prepare its +@code{AC_LIBOBJ} replacement should have an auxiliary macro to perform +this setup. Do not hesitate to introduce auxiliary macros to factor +your code. + +In order to highlight the recommended coding style, here is a macro +written the old way: + +@example +dnl Check for EMX on OS/2. +dnl _AC_EMXOS2 +AC_DEFUN(_AC_EMXOS2, +[AC_CACHE_CHECK(for EMX OS/2 environment, ac_cv_emxos2, +[AC_COMPILE_IFELSE([AC_LANG_PROGRAM(, return __EMX__;)], +ac_cv_emxos2=yes, ac_cv_emxos2=no)]) +test "x$ac_cv_emxos2" = xyes && EMXOS2=yes]) +@end example + +@noindent +and the new way: + +@example +# _AC_EMXOS2 +# ---------- +# Check for EMX on OS/2. +m4_define([_AC_EMXOS2], +[AC_CACHE_CHECK([for EMX OS/2 environment], [ac_cv_emxos2], +[AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [return __EMX__;])], + [ac_cv_emxos2=yes], + [ac_cv_emxos2=no])]) +test "x$ac_cv_emxos2" = xyes && EMXOS2=yes[]dnl +])# _AC_EMXOS2 +@end example + + + + +@c ============================================= Portable Shell Programming + +@node Portable Shell +@chapter Portable Shell Programming +@cindex Portable shell programming + +When writing your own checks, there are some shell-script programming +techniques you should avoid in order to make your code portable. The +Bourne shell and upward-compatible shells like the Korn shell and Bash +have evolved over the years, and many features added to the original +System7 shell are now supported on all interesting porting targets. +However, the following discussion between Russ Allbery and Robert Lipe +is worth reading: + +@noindent +Russ Allbery: + +@quotation +The GNU assumption that @command{/bin/sh} is the one and only shell +leads to a permanent deadlock. Vendors don't want to break users' +existing shell scripts, and there are some corner cases in the Bourne +shell that are not completely compatible with a Posix shell. Thus, +vendors who have taken this route will @emph{never} (OK@dots{}``never say +never'') replace the Bourne shell (as @command{/bin/sh}) with a +Posix shell. +@end quotation + +@noindent +Robert Lipe: + +@quotation +This is exactly the problem. While most (at least most System V's) do +have a Bourne shell that accepts shell functions most vendor +@command{/bin/sh} programs are not the Posix shell. + +So while most modern systems do have a shell @emph{somewhere} that meets the +Posix standard, the challenge is to find it. +@end quotation + +For this reason, part of the job of M4sh (@pxref{Programming in M4sh}) +is to find such a shell. But to prevent trouble, if you're not using +M4sh you should not take advantage of features that were added after Unix +version 7, circa 1977 (@pxref{Systemology}); you should not use aliases, +negated character classes, or even @command{unset}. @code{#} comments, +while not in Unix version 7, were retrofitted in the original Bourne +shell and can be assumed to be part of the least common denominator. + +On the other hand, if you're using M4sh you can assume that the shell +has the features that were added in SVR2 (circa 1984), including shell +functions, +@command{return}, @command{unset}, and I/O redirection for builtins. For +more information, refer to @uref{http://@/www.in-ulm.de/@/~mascheck/@/bourne/}. +However, some pitfalls have to be avoided for portable use of these +constructs; these will be documented in the rest of this chapter. +See in particular @ref{Shell Functions} and @ref{Limitations of +Builtins, , Limitations of Shell Builtins}. + +Some ancient systems have quite +small limits on the length of the @samp{#!} line; for instance, 32 +bytes (not including the newline) on SunOS 4. +However, these ancient systems are no longer of practical concern. + +The set of external programs you should run in a @command{configure} script +is fairly small. @xref{Utilities in Makefiles, , Utilities in +Makefiles, standards, The GNU Coding Standards}, for the list. This +restriction allows users to start out with a fairly small set of +programs and build the rest, avoiding too many interdependencies between +packages. + +Some of these external utilities have a portable subset of features; see +@ref{Limitations of Usual Tools}. + +There are other sources of documentation about shells. The +specification for the Posix +@uref{http://@/www.opengroup.org/@/susv3/@/utilities/@/xcu_chap02@/.html, Shell +Command Language}, though more generous than the restrictive shell +subset described above, is fairly portable nowadays. Also please see +@uref{http://@/www.faqs.org/@/faqs/@/unix-faq/@/shell/, the Shell FAQs}. + +@menu +* Shellology:: A zoology of shells +* Invoking the Shell:: Invoking the shell as a command +* Here-Documents:: Quirks and tricks +* File Descriptors:: FDs and redirections +* Signal Handling:: Shells, signals, and headaches +* File System Conventions:: File names +* Shell Pattern Matching:: Pattern matching +* Shell Substitutions:: Variable and command expansions +* Assignments:: Varying side effects of assignments +* Parentheses:: Parentheses in shell scripts +* Slashes:: Slashes in shell scripts +* Special Shell Variables:: Variables you should not change +* Shell Functions:: What to look out for if you use them +* Limitations of Builtins:: Portable use of not so portable /bin/sh +* Limitations of Usual Tools:: Portable use of portable tools +@end menu + +@node Shellology +@section Shellology +@cindex Shellology + +There are several families of shells, most prominently the Bourne family +and the C shell family which are deeply incompatible. If you want to +write portable shell scripts, avoid members of the C shell family. The +@uref{http://@/www.faqs.org/@/faqs/@/unix-faq/@/shell/@/shell-differences/, the +Shell difference FAQ} includes a small history of Posix shells, and a +comparison between several of them. + +Below we describe some of the members of the Bourne shell family. + +@table @asis +@item Ash +@cindex Ash +Ash is often used on GNU/Linux and BSD +systems as a light-weight Bourne-compatible shell. Ash 0.2 has some +bugs that are fixed in the 0.3.x series, but portable shell scripts +should work around them, since version 0.2 is still shipped with many +GNU/Linux distributions. + +To be compatible with Ash 0.2: + +@itemize @minus +@item +don't use @samp{$?} after expanding empty or unset variables, +or at the start of an @command{eval}: + +@example +foo= +false +$foo +echo "Do not use it: $?" +false +eval 'echo "Do not use it: $?"' +@end example + +@item +don't use command substitution within variable expansion: + +@example +cat $@{FOO=`bar`@} +@end example + +@item +beware that single builtin substitutions are not performed by a +subshell, hence their effect applies to the current shell! @xref{Shell +Substitutions}, item ``Command Substitution''. +@end itemize + +@item Bash +@cindex Bash +To detect whether you are running Bash, test whether +@code{BASH_VERSION} is set. To require +Posix compatibility, run @samp{set -o posix}. @xref{Bash POSIX +Mode, , Bash Posix Mode, bash, The GNU Bash Reference +Manual}, for details. + +@item Bash 2.05 and later +@cindex Bash 2.05 and later +Versions 2.05 and later of Bash use a different format for the +output of the @command{set} builtin, designed to make evaluating its +output easier. However, this output is not compatible with earlier +versions of Bash (or with many other shells, probably). So if +you use Bash 2.05 or higher to execute @command{configure}, +you'll need to use Bash 2.05 for all other build tasks as well. + +@item Ksh +@cindex Ksh +@cindex Korn shell +@prindex @samp{ksh} +@prindex @samp{ksh88} +@prindex @samp{ksh93} +The Korn shell is compatible with the Bourne family and it mostly +conforms to Posix. It has two major variants commonly +called @samp{ksh88} and @samp{ksh93}, named after the years of initial +release. It is usually called @command{ksh}, but is called @command{sh} +on some hosts if you set your path appropriately. + +Solaris systems have three variants: +@prindex @command{/usr/bin/ksh} on Solaris +@command{/usr/bin/ksh} is @samp{ksh88}; it is +standard on Solaris 2.0 and later. +@prindex @command{/usr/xpg4/bin/sh} on Solaris +@command{/usr/xpg4/bin/sh} is a Posix-compliant variant of +@samp{ksh88}; it is standard on Solaris 9 and later. +@prindex @command{/usr/dt/bin/dtksh} on Solaris +@command{/usr/dt/bin/dtksh} is @samp{ksh93}. +Variants that are not standard may be parts of optional +packages. There is no extra charge for these packages, but they are +not part of a minimal OS install and therefore some installations may +not have it. + +Starting with Tru64 Version 4.0, the Korn shell @command{/usr/bin/ksh} +is also available as @command{/usr/bin/posix/sh}. If the environment +variable @env{BIN_SH} is set to @code{xpg4}, subsidiary invocations of +the standard shell conform to Posix. + +@item Pdksh +@prindex @samp{pdksh} +A public-domain clone of the Korn shell called @command{pdksh} is widely +available: it has most of the @samp{ksh88} features along with a few of +its own. It usually sets @code{KSH_VERSION}, except if invoked as +@command{/bin/sh} on OpenBSD, and similarly to Bash you can require +Posix compatibility by running @samp{set -o posix}. Unfortunately, with +@command{pdksh} 5.2.14 (the latest stable version as of January 2007) +Posix mode is buggy and causes @command{pdksh} to depart from Posix in +at least one respect, see @ref{Shell Substitutions}. + +@item Zsh +@cindex Zsh +To detect whether you are running @command{zsh}, test whether +@code{ZSH_VERSION} is set. By default @command{zsh} is @emph{not} +compatible with the Bourne shell: you must execute @samp{emulate sh}, +and for @command{zsh} versions before 3.1.6-dev-18 you must also +set @code{NULLCMD} to @samp{:}. @xref{Compatibility, , Compatibility, +zsh, The Z Shell Manual}, for details. + +The default Mac OS X @command{sh} was originally Zsh; it was changed to +Bash in Mac OS X 10.2. +@end table + +@node Invoking the Shell +@section Invoking the Shell +@cindex invoking the shell +@cindex shell invocation + +The Korn shell (up to at least version M-12/28/93d) has a bug when +invoked on a file whose name does not contain a slash. It first +searches for the file's name in @env{PATH}, and if found it executes +that rather than the original file. For example, assuming there is a +binary executable @file{/usr/bin/script} in your @env{PATH}, the last +command in the following example fails because the Korn shell finds +@file{/usr/bin/script} and refuses to execute it as a shell script: + +@example +$ @kbd{touch xxyzzyz script} +$ @kbd{ksh xxyzzyz} +$ @kbd{ksh ./script} +$ @kbd{ksh script} +ksh: script: cannot execute +@end example + +Bash 2.03 has a bug when invoked with the @option{-c} option: if the +option-argument ends in backslash-newline, Bash incorrectly reports a +syntax error. The problem does not occur if a character follows the +backslash: + +@example +$ @kbd{$ bash -c 'echo foo \} +> @kbd{'} +bash: -c: line 2: syntax error: unexpected end of file +$ @kbd{bash -c 'echo foo \} +> @kbd{ '} +foo +@end example + +@noindent +@xref{Backslash-Newline-Empty}, for how this can cause problems in makefiles. + +@node Here-Documents +@section Here-Documents +@cindex Here-documents +@cindex Shell here-documents + +Don't rely on @samp{\} being preserved just because it has no special +meaning together with the next symbol. In the native @command{sh} +on OpenBSD 2.7 @samp{\"} expands to @samp{"} in here-documents with +unquoted delimiter. As a general rule, if @samp{\\} expands to @samp{\} +use @samp{\\} to get @samp{\}. + +With OpenBSD 2.7's @command{sh} + +@example +@group +$ @kbd{cat <<EOF +> \" \\ +> EOF} +" \ +@end group +@end example + +@noindent +and with Bash: + +@example +@group +bash-2.04$ @kbd{cat <<EOF +> \" \\ +> EOF} +\" \ +@end group +@end example + +Using command substitutions in a here-document that is fed to a shell +function is not portable. For example, with Solaris 10 @command{/bin/sh}: + +@example +$ @kbd{kitty () @{ cat; @}} +$ @kbd{kitty <<EOF +> `echo ok` +> EOF} +/tmp/sh199886: cannot open +$ @kbd{echo $?} +1 +@end example + +Some shells mishandle large here-documents: for example, +Solaris 10 @command{dtksh} and the UnixWare 7.1.1 Posix shell, which are +derived from Korn shell version M-12/28/93d, mishandle braced variable +expansion that crosses a 1024- or 4096-byte buffer boundary +within a here-document. Only the part of the variable name after the boundary +is used. For example, @code{$@{variable@}} could be replaced by the expansion +of @code{$@{ble@}}. If the end of the variable name is aligned with the block +boundary, the shell reports an error, as if you used @code{$@{@}}. +Instead of @code{$@{variable-default@}}, the shell may expand +@code{$@{riable-default@}}, or even @code{$@{fault@}}. This bug can often +be worked around by omitting the braces: @code{$variable}. The bug was +fixed in +@samp{ksh93g} (1998-04-30) but as of 2006 many operating systems were +still shipping older versions with the bug. + +Empty here-documents are not portable either; with the following code, +@command{zsh} up to at least version 4.3.10 creates a file with a single +newline, whereas other shells create an empty file: + +@example +cat >file <<EOF +EOF +@end example + +Many shells (including the Bourne shell) implement here-documents +inefficiently. In particular, some shells can be extremely inefficient when +a single statement contains many here-documents. For instance if your +@file{configure.ac} includes something like: + +@example +@group +if <cross_compiling>; then + assume this and that +else + check this + check that + check something else + @dots{} + on and on forever + @dots{} +fi +@end group +@end example + +A shell parses the whole @code{if}/@code{fi} construct, creating +temporary files for each here-document in it. Some shells create links +for such here-documents on every @code{fork}, so that the clean-up code +they had installed correctly removes them. It is creating the links +that can take the shell forever. + +Moving the tests out of the @code{if}/@code{fi}, or creating multiple +@code{if}/@code{fi} constructs, would improve the performance +significantly. Anyway, this kind of construct is not exactly the +typical use of Autoconf. In fact, it's even not recommended, because M4 +macros can't look into shell conditionals, so we may fail to expand a +macro when it was expanded before in a conditional path, and the +condition turned out to be false at runtime, and we end up not +executing the macro at all. + +Be careful with the use of @samp{<<-} to unindent here-documents. The +behavior is only portable for stripping leading @key{TAB}s, and things +can silently break if an overzealous editor converts to using leading +spaces (not all shells are nice enough to warn about unterminated +here-documents). + +@example +$ @kbd{printf 'cat <<-x\n\t1\n\t 2\n\tx\n' | bash && echo done} +1 + 2 +done +$ @kbd{printf 'cat <<-x\n 1\n 2\n x\n' | bash-3.2 && echo done} + 1 + 2 + x +done +@end example + +@node File Descriptors +@section File Descriptors +@cindex Descriptors +@cindex File descriptors +@cindex Shell file descriptors + +Most shells, if not all (including Bash, Zsh, Ash), output traces on +stderr, even for subshells. This might result in undesirable content +if you meant to capture the standard-error output of the inner command: + +@example +$ @kbd{ash -x -c '(eval "echo foo >&2") 2>stderr'} +$ @kbd{cat stderr} ++ eval echo foo >&2 ++ echo foo +foo +$ @kbd{bash -x -c '(eval "echo foo >&2") 2>stderr'} +$ @kbd{cat stderr} ++ eval 'echo foo >&2' +++ echo foo +foo +$ @kbd{zsh -x -c '(eval "echo foo >&2") 2>stderr'} +@i{# Traces on startup files deleted here.} +$ @kbd{cat stderr} ++zsh:1> eval echo foo >&2 ++zsh:1> echo foo +foo +@end example + +@noindent +One workaround is to grep out uninteresting lines, hoping not to remove +good ones. + +If you intend to redirect both standard error and standard output, +redirect standard output first. This works better with HP-UX, +since its shell mishandles tracing if standard error is redirected +first: + +@example +$ @kbd{sh -x -c ': 2>err >out'} ++ : ++ 2> err $ @kbd{cat err} +1> out +@end example + +Don't try to redirect the standard error of a command substitution. It +must be done @emph{inside} the command substitution. When running +@samp{: `cd /zorglub` 2>/dev/null} expect the error message to +escape, while @samp{: `cd /zorglub 2>/dev/null`} works properly. + +On the other hand, some shells, such as Solaris or FreeBSD +@command{/bin/sh}, warn about missing programs before performing +redirections. Therefore, to silently check whether a program exists, it +is necessary to perform redirections on a subshell or brace group: +@example +$ @kbd{/bin/sh -c 'nosuch 2>/dev/null'} +nosuch: not found +$ @kbd{/bin/sh -c '(nosuch) 2>/dev/null'} +$ @kbd{/bin/sh -c '@{ nosuch; @} 2>/dev/null'} +$ @kbd{bash -c 'nosuch 2>/dev/null'} +@end example + +FreeBSD 6.2 sh may mix the trace output lines from the statements in a +shell pipeline. + +It is worth noting that Zsh (but not Ash nor Bash) makes it possible +in assignments though: @samp{foo=`cd /zorglub` 2>/dev/null}. + +Some shells, like @command{ash}, don't recognize bi-directional +redirection (@samp{<>}). And even on shells that recognize it, it is +not portable to use on fifos: Posix does not require read-write support +for named pipes, and Cygwin does not support it: + +@example +$ @kbd{mkfifo fifo} +$ @kbd{exec 5<>fifo} +$ @kbd{echo hi >&5} +bash: echo: write error: Communication error on send +@end example + +@noindent +Furthermore, versions of @command{dash} before 0.5.6 mistakenly truncate +regular files when using @samp{<>}: + +@example +$ @kbd{echo a > file} +$ @kbd{bash -c ': 1<>file'; cat file} +a +$ @kbd{dash -c ': 1<>file'; cat file} +$ rm a +@end example + +When catering to old systems, don't redirect the same file descriptor +several times, as you are doomed to failure under Ultrix. + +@example +ULTRIX V4.4 (Rev. 69) System #31: Thu Aug 10 19:42:23 GMT 1995 +UWS V4.4 (Rev. 11) +$ @kbd{eval 'echo matter >fullness' >void} +illegal io +$ @kbd{eval '(echo matter >fullness)' >void} +illegal io +$ @kbd{(eval '(echo matter >fullness)') >void} +Ambiguous output redirect. +@end example + +@noindent +In each case the expected result is of course @file{fullness} containing +@samp{matter} and @file{void} being empty. However, this bug is +probably not of practical concern to modern platforms. + +Solaris 10 @command{sh} will try to optimize away a @command{:} command +(even if it is redirected) in a loop after the first iteration, or in a +shell function after the first call: + +@example +$ @kbd{for i in 1 2 3 ; do : >x$i; done} +$ @kbd{ls x*} +x1 +$ @kbd{f () @{ : >$1; @}; f y1; f y2; f y3;} +$ @kbd{ls y*} +y1 +@end example + +@noindent +As a workaround, @command{echo} or @command{eval} can be used. + +Don't rely on file descriptors 0, 1, and 2 remaining closed in a +subsidiary program. If any of these descriptors is closed, the +operating system may open an unspecified file for the descriptor in the +new process image. Posix 2008 says this may be done only if the +subsidiary program is set-user-ID or set-group-ID, but HP-UX 11.23 does +it even for ordinary programs, and the next version of Posix will allow +HP-UX behavior. + +If you want a file descriptor above 2 to be inherited into a child +process, then you must use redirections specific to that command or a +containing subshell or command group, rather than relying on +@command{exec} in the shell. In @command{ksh} as well as HP-UX +@command{sh}, file descriptors above 2 which are opened using +@samp{exec @var{n}>file} are closed by a subsequent @samp{exec} (such as +that involved in the fork-and-exec which runs a program or script): + +@example +$ @kbd{echo 'echo hello >&5' >k} +$ @kbd{/bin/sh -c 'exec 5>t; ksh ./k; exec 5>&-; cat t} +hello +$ @kbd{bash -c 'exec 5>t; ksh ./k; exec 5>&-; cat t} +hello +$ @kbd{ksh -c 'exec 5>t; ksh ./k; exec 5>&-; cat t} +./k[1]: 5: cannot open [Bad file number] +$ @kbd{ksh -c '(ksh ./k) 5>t; cat t'} +hello +$ @kbd{ksh -c '@{ ksh ./k; @} 5>t; cat t'} +hello +$ @kbd{ksh -c '5>t ksh ./k; cat t} +hello +@end example + +Don't rely on duplicating a closed file descriptor to cause an +error. With Solaris @command{/bin/sh}, failed duplication is silently +ignored, which can cause unintended leaks to the original file +descriptor. In this example, observe the leak to standard output: + +@example +$ @kbd{bash -c 'echo hi >&3' 3>&-; echo $?} +bash: 3: Bad file descriptor +1 +$ @kbd{/bin/sh -c 'echo hi >&3' 3>&-; echo $?} +hi +0 +@end example + +Fortunately, an attempt to close an already closed file descriptor will +portably succeed. Likewise, it is safe to use either style of +@samp{@var{n}<&-} or @samp{@var{n}>&-} for closing a file descriptor, +even if it doesn't match the read/write mode that the file descriptor +was opened with. + +DOS variants cannot rename or remove open files, such as in +@samp{mv foo bar >foo} or @samp{rm foo >foo}, even though this is +perfectly portable among Posix hosts. + +A few ancient systems reserved some file descriptors. By convention, +file descriptor 3 was opened to @file{/dev/tty} when you logged into +Eighth Edition (1985) through Tenth Edition Unix (1989). File +descriptor 4 had a special use on the Stardent/Kubota Titan (circa +1990), though we don't now remember what it was. Both these systems are +obsolete, so it's now safe to treat file descriptors 3 and 4 like any +other file descriptors. + +On the other hand, you can't portably use multi-digit file descriptors. +Solaris @command{ksh} doesn't understand any file descriptor larger than +@samp{9}: + +@example +$ @kbd{bash -c 'exec 10>&-'; echo $?} +0 +$ @kbd{ksh -c 'exec 9>&-'; echo $?} +0 +$ @kbd{ksh -c 'exec 10>&-'; echo $?} +ksh[1]: exec: 10: not found +127 +@end example + +@c <http://lists.gnu.org/archive/html/bug-autoconf/2011-09/msg00004.html> +@node Signal Handling +@section Signal Handling +@cindex Signal handling in the shell +@cindex Signals, shells and + +Portable handling of signals within the shell is another major source of +headaches. This is worsened by the fact that various different, mutually +incompatible approaches are possible in this area, each with its +distinctive merits and demerits. A detailed description of these possible +approaches, as well as of their pros and cons, can be found in +@uref{http://www.cons.org/cracauer/sigint.html, this article}. + +Solaris 10 @command{/bin/sh} automatically traps most signals by default; +@c See: <http://dbaspot.com/shell/396118-bourne-shell-exit-code-term.html> +the shell still exits with error upon termination by one of those signals, +but in such a case the exit status might be somewhat unexpected (even if +allowed by POSIX, strictly speaking): + +@example +$ @kbd{bash -c 'kill -1 $$'; echo $?} # Will exit 128 + (signal number). +Hangup +129 +$ @kbd{/bin/ksh -c 'kill -15 $$'; echo $?} # Likewise. +Terminated +143 +$ @kbd{for sig in 1 2 3 15; do} +> @kbd{ echo $sig:} +> @kbd{ /bin/sh -c "kill -$s \$\$"; echo $?} +> @kbd{done} +signal 1: +Hangup +129 +signal 2: +208 +signal 3: +208 +signal 15: +208 +@end example + +This gets even worse if one is using the POSIX `wait' interface to get +details about the shell process terminations: it will result in the shell +having exited normally, rather than by receiving a signal. + +@example +$ @kbd{cat > foo.c <<'END'} +#include <stdio.h> /* for printf */ +#include <stdlib.h> /* for system */ +#include <sys/wait.h> /* for WIF* macros */ +int main(void) +@{ + int status = system ("kill -15 $$"); + printf ("Terminated by signal: %s\n", + WIFSIGNALED (status) ? "yes" : "no"); + printf ("Exited normally: %s\n", + WIFEXITED (status) ? "yes" : "no"); + return 0; +@} +END +@c $$ font-lock +$ @kbd{cc -o foo foo.c} +$ @kbd{./a.out} # On GNU/Linux +Terminated by signal: no +Exited normally: yes +$ @kbd{./a.out} # On Solaris 10 +Terminated by signal: yes +Exited normally: no +@end example + +Various shells seem to handle @code{SIGQUIT} specially: they ignore it even +if it is not blocked, and even if the shell is not running interactively +(in fact, even if the shell has no attached tty); among these shells +are at least Bash (from version 2 onwards), Zsh 4.3.12, Solaris 10 +@code{/bin/ksh} and @code{/usr/xpg4/bin/sh}, and AT&T @code{ksh93} (2011). +Still, @code{SIGQUIT} seems to be trappable quite portably within all +these shells. OTOH, some other shells doesn't special-case the handling +of @code{SIGQUIT}; among these shells are at least @code{pdksh} 5.2.14, +Solaris 10 and NetBSD 5.1 @code{/bin/sh}, and the Almquist Shell 0.5.5.1. + +@c See: <http://mail.opensolaris.org/pipermail/ksh93-integration-discuss/2009-February/004121.html> +Some shells (especially Korn shells and derivatives) might try to +propagate to themselves a signal that has killed a child process; this is +not a bug, but a conscious design choice (although its overall value might +be debatable). The exact details of how this is attained vary from shell +to shell. For example, upon running @code{perl -e 'kill 2, $$'}, after +the perl process has been interrupted AT&T @code{ksh93} (2011) will +proceed to send itself a @code{SIGINT}, while Solaris 10 @code{/bin/ksh} +and @code{/usr/xpg4/bin/sh} will proceed to exit with status 130 (i.e., +128 + 2). In any case, if there is an active trap associated with +@code{SIGINT}, those shells will correctly execute it. + +@c See: <http://www.austingroupbugs.net/view.php?id=51> +Some Korn shells, when a child process die due receiving a signal with +signal number @var{n}, can leave in @samp{$?} an exit status of +256+@var{n} instead of the more common 128+@var{n}. Observe the +difference between AT&T @code{ksh93} (2011) and @code{bash} 4.1.5 on +Debian: + +@example +$ @kbd{/bin/ksh -c 'sh -c "kill -1 \$\$"; echo $?'} +/bin/ksh: line 1: 7837: Hangup +257 +$ @kbd{/bin/bash -c 'sh -c "kill -1 \$\$"; echo $?'} +/bin/bash: line 1: 7861 Hangup (sh -c "kill -1 \$\$") +129 +@end example + +@noindent +This @command{ksh} behavior is allowed by POSIX, if implemented with +due care; see this @uref{http://www.austingroupbugs.net/view.php?id=51, +Austin Group discussion} for more background. However, if it is not +implemented with proper care, such a behavior might cause problems +in some corner cases. To see why, assume we have a ``wrapper'' script +like this: + +@example +#!/bin/sh +# Ignore some signals in the shell only, not in its child processes. +trap : 1 2 13 15 +wrapped_command "$@@" +ret=$? +other_command +exit $ret +@end example + +@noindent +If @command{wrapped_command} is interrupted by a @code{SIGHUP} (which +has signal number 1), @code{ret} will be set to 257. Unless the +@command{exit} shell builtin is smart enough to understand that such +a value can only have originated from a signal, and adjust the final +wait status of the shell appropriately, the value 257 will just get +truncated to 1 by the closing @code{exit} call, so that a caller of +the script will have no way to determine that termination by a signal +was involved. Observe the different behavior of AT&T @code{ksh93} +(2011) and @code{bash} 4.1.5 on Debian: + +@example +$ @kbd{cat foo.sh} +#!/bin/sh +sh -c 'kill -1 $$' +ret=$? +echo $ret +exit $ret +$ @kbd{/bin/ksh foo.sh; echo $?} +foo.sh: line 2: 12479: Hangup +257 +1 +$ @kbd{/bin/bash foo.sh; echo $?} +foo.sh: line 2: 12487 Hangup (sh -c 'kill -1 $$') +129 +129 +@end example + +@node File System Conventions +@section File System Conventions +@cindex File system conventions + +Autoconf uses shell-script processing extensively, so the file names +that it processes should not contain characters that are special to the +shell. Special characters include space, tab, newline, NUL, and +the following: + +@example +" # $ & ' ( ) * ; < = > ? [ \ ` | +@end example + +Also, file names should not begin with @samp{~} or @samp{-}, and should +contain neither @samp{-} immediately after @samp{/} nor @samp{~} +immediately after @samp{:}. On Posix-like platforms, directory names +should not contain @samp{:}, as this runs afoul of @samp{:} used as the +path separator. + +These restrictions apply not only to the files that you distribute, but +also to the absolute file names of your source, build, and destination +directories. + +On some Posix-like platforms, @samp{!} and @samp{^} are special too, so +they should be avoided. + +Posix lets implementations treat leading @file{//} specially, but +requires leading @file{///} and beyond to be equivalent to @file{/}. +Most Unix variants treat @file{//} like @file{/}. However, some treat +@file{//} as a ``super-root'' that can provide access to files that are +not otherwise reachable from @file{/}. The super-root tradition began +with Apollo Domain/OS, which died out long ago, but unfortunately Cygwin +has revived it. + +While @command{autoconf} and friends are usually run on some Posix +variety, they can be used on other systems, most notably DOS +variants. This impacts several assumptions regarding file names. + +@noindent +For example, the following code: + +@example +case $foo_dir in + /*) # Absolute + ;; + *) + foo_dir=$dots$foo_dir ;; +esac +@end example + +@noindent +fails to properly detect absolute file names on those systems, because +they can use a drivespec, and usually use a backslash as directory +separator. If you want to be portable to DOS variants (at the +price of rejecting valid but oddball Posix file names like @file{a:\b}), +you can check for absolute file names like this: + +@cindex absolute file names, detect +@example +case $foo_dir in + [\\/]* | ?:[\\/]* ) # Absolute + ;; + *) + foo_dir=$dots$foo_dir ;; +esac +@end example + +@noindent +Make sure you quote the brackets if appropriate and keep the backslash as +first character (@pxref{case, , Limitations of Shell Builtins}). + +Also, because the colon is used as part of a drivespec, these systems don't +use it as path separator. When creating or accessing paths, you can use the +@code{PATH_SEPARATOR} output variable instead. @command{configure} sets this +to the appropriate value for the build system (@samp{:} or @samp{;}) when it +starts up. + +File names need extra care as well. While DOS variants +that are Posixy enough to run @command{autoconf} (such as DJGPP) +are usually able to handle long file names properly, there are still +limitations that can seriously break packages. Several of these issues +can be easily detected by the +@uref{ftp://@/ftp.gnu.org/@/gnu/@/non-gnu/@/doschk/@/doschk-1.1.tar.gz, doschk} +package. + +A short overview follows; problems are marked with SFN/LFN to +indicate where they apply: SFN means the issues are only relevant to +plain DOS, not to DOS under Microsoft Windows +variants, while LFN identifies problems that exist even under +Microsoft Windows variants. + +@table @asis +@item No multiple dots (SFN) +DOS cannot handle multiple dots in file names. This is an especially +important thing to remember when building a portable configure script, +as @command{autoconf} uses a .in suffix for template files. + +This is perfectly OK on Posix variants: + +@example +AC_CONFIG_HEADERS([config.h]) +AC_CONFIG_FILES([source.c foo.bar]) +AC_OUTPUT +@end example + +@noindent +but it causes problems on DOS, as it requires @samp{config.h.in}, +@samp{source.c.in} and @samp{foo.bar.in}. To make your package more portable +to DOS-based environments, you should use this instead: + +@example +AC_CONFIG_HEADERS([config.h:config.hin]) +AC_CONFIG_FILES([source.c:source.cin foo.bar:foobar.in]) +AC_OUTPUT +@end example + +@item No leading dot (SFN) +DOS cannot handle file names that start with a dot. This is usually +not important for @command{autoconf}. + +@item Case insensitivity (LFN) +DOS is case insensitive, so you cannot, for example, have both a +file called @samp{INSTALL} and a directory called @samp{install}. This +also affects @command{make}; if there's a file called @samp{INSTALL} in +the directory, @samp{make install} does nothing (unless the +@samp{install} target is marked as PHONY). + +@item The 8+3 limit (SFN) +Because the DOS file system only stores the first 8 characters of +the file name and the first 3 of the extension, those must be unique. +That means that @file{foobar-part1.c}, @file{foobar-part2.c} and +@file{foobar-prettybird.c} all resolve to the same file name +(@file{FOOBAR-P.C}). The same goes for @file{foo.bar} and +@file{foo.bartender}. + +The 8+3 limit is not usually a problem under Microsoft Windows, as it +uses numeric +tails in the short version of file names to make them unique. However, a +registry setting can turn this behavior off. While this makes it +possible to share file trees containing long file names between SFN +and LFN environments, it also means the above problem applies there +as well. + +@item Invalid characters (LFN) +Some characters are invalid in DOS file names, and should therefore +be avoided. In a LFN environment, these are @samp{/}, @samp{\}, +@samp{?}, @samp{*}, @samp{:}, @samp{<}, @samp{>}, @samp{|} and @samp{"}. +In a SFN environment, other characters are also invalid. These +include @samp{+}, @samp{,}, @samp{[} and @samp{]}. + +@item Invalid names (LFN) +Some DOS file names are reserved, and cause problems if you +try to use files with those names. These names include @file{CON}, +@file{AUX}, @file{COM1}, @file{COM2}, @file{COM3}, @file{COM4}, +@file{LPT1}, @file{LPT2}, @file{LPT3}, @file{NUL}, and @file{PRN}. +File names are case insensitive, so even names like +@file{aux/config.guess} are disallowed. + +@end table + +@node Shell Pattern Matching +@section Shell Pattern Matching +@cindex Shell pattern matching + +Nowadays portable patterns can use negated character classes like +@samp{[!-aeiou]}. The older syntax @samp{[^-aeiou]} is supported by +some shells but not others; hence portable scripts should never use +@samp{^} as the first character of a bracket pattern. + +Outside the C locale, patterns like @samp{[a-z]} are problematic since +they may match characters that are not lower-case letters. + +@node Shell Substitutions +@section Shell Substitutions +@cindex Shell substitutions + +Contrary to a persistent urban legend, the Bourne shell does not +systematically split variables and back-quoted expressions, in particular +on the right-hand side of assignments and in the argument of @code{case}. +For instance, the following code: + +@example +case "$given_srcdir" in +.) top_srcdir="`echo "$dots" | sed 's|/$||'`" ;; +*) top_srcdir="$dots$given_srcdir" ;; +esac +@end example + +@noindent +is more readable when written as: + +@example +case $given_srcdir in +.) top_srcdir=`echo "$dots" | sed 's|/$||'` ;; +*) top_srcdir=$dots$given_srcdir ;; +esac +@end example + +@noindent +and in fact it is even @emph{more} portable: in the first case of the +first attempt, the computation of @code{top_srcdir} is not portable, +since not all shells properly understand @code{"`@dots{}"@dots{}"@dots{}`"}, +for example Solaris 10 ksh: + +@example +$ @kbd{foo="`echo " bar" | sed 's, ,,'`"} +ksh: : cannot execute +ksh: bar | sed 's, ,,': cannot execute +@end example + +@noindent +Posix does not specify behavior for this sequence. On the other hand, +behavior for @code{"`@dots{}\"@dots{}\"@dots{}`"} is specified by Posix, +but in practice, not all shells understand it the same way: pdksh 5.2.14 +prints spurious quotes when in Posix mode: + +@example +$ @kbd{echo "`echo \"hello\"`"} +hello +$ @kbd{set -o posix} +$ @kbd{echo "`echo \"hello\"`"} +"hello" +@end example + +@noindent +There is just no portable way to use double-quoted strings inside +double-quoted back-quoted expressions (pfew!). + +Bash 4.1 has a bug where quoted empty strings adjacent to unquoted +parameter expansions are elided during word splitting. Meanwhile, zsh +does not perform word splitting except when in Bourne compatibility +mode. In the example below, the correct behavior is to have five +arguments to the function, and exactly two spaces on either side of the +middle @samp{-}, since word splitting collapses multiple spaces in +@samp{$f} but leaves empty arguments intact. + +@example +$ @kbd{bash -c 'n() @{ echo "$#$@@"; @}; f=" - "; n - ""$f"" -'} +3- - - +$ @kbd{ksh -c 'n() @{ echo "$#$@@"; @}; f=" - "; n - ""$f"" -'} +5- - - +$ @kbd{zsh -c 'n() @{ echo "$#$@@"; @}; f=" - "; n - ""$f"" -'} +3- - - +$ @kbd{zsh -c 'emulate sh;} +> @kbd{n() @{ echo "$#$@@"; @}; f=" - "; n - ""$f"" -'} +5- - - +@end example + +@noindent +You can work around this by doing manual word splitting, such as using +@samp{"$str" $list} rather than @samp{"$str"$list}. + +There are also portability pitfalls with particular expansions: + +@table @code +@item $@@ +@cindex @code{"$@@"} +One of the most famous shell-portability issues is related to +@samp{"$@@"}. When there are no positional arguments, Posix says +that @samp{"$@@"} is supposed to be equivalent to nothing, but the +original Unix version 7 Bourne shell treated it as equivalent to +@samp{""} instead, and this behavior survives in later implementations +like Digital Unix 5.0. + +The traditional way to work around this portability problem is to use +@samp{$@{1+"$@@"@}}. Unfortunately this method does not work with +Zsh (3.x and 4.x), which is used on Mac OS X@. When emulating +the Bourne shell, Zsh performs word splitting on @samp{$@{1+"$@@"@}}: + +@example +zsh $ @kbd{emulate sh} +zsh $ @kbd{for i in "$@@"; do echo $i; done} +Hello World +! +zsh $ @kbd{for i in $@{1+"$@@"@}; do echo $i; done} +Hello +World +! +@end example + +@noindent +Zsh handles plain @samp{"$@@"} properly, but we can't use plain +@samp{"$@@"} because of the portability problems mentioned above. +One workaround relies on Zsh's ``global aliases'' to convert +@samp{$@{1+"$@@"@}} into @samp{"$@@"} by itself: + +@example +test "$@{ZSH_VERSION+set@}" = set && alias -g '$@{1+"$@@"@}'='"$@@"' +@end example + +Zsh only recognizes this alias when a shell word matches it exactly; +@samp{"foo"$@{1+"$@@"@}} remains subject to word splitting. Since this +case always yields at least one shell word, use plain @samp{"$@@"}. + +A more conservative workaround is to avoid @samp{"$@@"} if it is +possible that there may be no positional arguments. For example, +instead of: + +@example +cat conftest.c "$@@" +@end example + +you can use this instead: + +@example +case $# in +0) cat conftest.c;; +*) cat conftest.c "$@@";; +esac +@end example + +Autoconf macros often use the @command{set} command to update +@samp{$@@}, so if you are writing shell code intended for +@command{configure} you should not assume that the value of @samp{$@@} +persists for any length of time. + + +@item $@{10@} +@cindex positional parameters +The 10th, 11th, @dots{} positional parameters can be accessed only after +a @code{shift}. The 7th Edition shell reported an error if given +@code{$@{10@}}, and +Solaris 10 @command{/bin/sh} still acts that way: + +@example +$ @kbd{set 1 2 3 4 5 6 7 8 9 10} +$ @kbd{echo $@{10@}} +bad substitution +@end example + +Conversely, not all shells obey the Posix rule that when braces are +omitted, multiple digits beyond a @samp{$} imply the single-digit +positional parameter expansion concatenated with the remaining literal +digits. To work around the issue, you must use braces. + +@example +$ @kbd{bash -c 'set a b c d e f g h i j; echo $10 $@{1@}0'} +a0 a0 +$ @kbd{dash -c 'set a b c d e f g h i j; echo $10 $@{1@}0'} +j a0 +@end example + +@item $@{@var{var}:-@var{value}@} +@c Info cannot handle `:' in index entries. +@ifnotinfo +@cindex @code{$@{@var{var}:-@var{value}@}} +@end ifnotinfo +@cindex @code{$@{@var{var}-@var{value}@}} +Old BSD shells, including the Ultrix @code{sh}, don't accept the +colon for any shell substitution, and complain and die. +Similarly for $@{@var{var}:=@var{value}@}, $@{@var{var}:?@var{value}@}, etc. +However, all shells that support functions allow the use of colon in +shell substitution, and since m4sh requires functions, you can portably +use null variable substitution patterns in configure scripts. + +@item $@{@var{var}+@var{value}@} +@cindex @code{$@{@var{var}+@var{value}@}} +When using @samp{$@{@var{var}-@var{value}@}} or +@samp{$@{@var{var}-@var{value}@}} for providing alternate substitutions, +@var{value} must either be a single shell word, quoted, or in the +context of an unquoted here-document. Solaris +@command{/bin/sh} complains otherwise. + +@example +$ @kbd{/bin/sh -c 'echo $@{a-b c@}'} +/bin/sh: bad substitution +$ @kbd{/bin/sh -c 'echo $@{a-'\''b c'\''@}'} +b c +$ @kbd{/bin/sh -c 'echo "$@{a-b c@}"'} +b c +$ @kbd{/bin/sh -c 'cat <<EOF +$@{a-b c@} +EOF} +b c +@end example + +According to Posix, if an expansion occurs inside double quotes, then +the use of unquoted double quotes within @var{value} is unspecified, and +any single quotes become literal characters; in that case, escaping must +be done with backslash. Likewise, the use of unquoted here-documents is +a case where double quotes have unspecified results: + +@example +$ @kbd{/bin/sh -c 'echo "$@{a-"b c"@}"'} +/bin/sh: bad substitution +$ @kbd{ksh -c 'echo "$@{a-"b c"@}"'} +b c +$ @kbd{bash -c 'echo "$@{a-"b c"@}"'} +b c +$ @kbd{/bin/sh -c 'a=; echo $@{a+'\''b c'\''@}'} +b c +$ @kbd{/bin/sh -c 'a=; echo "$@{a+'\''b c'\''@}"'} +'b c' +$ @kbd{/bin/sh -c 'a=; echo "$@{a+\"b c\"@}"'} +"b c" +$ @kbd{/bin/sh -c 'a=; echo "$@{a+b c@}"'} +b c +$ @kbd{/bin/sh -c 'cat <<EOF +$@{a-"b c"@} +EOF'} +"b c" +$ @kbd{/bin/sh -c 'cat <<EOF +$@{a-'b c'@} +EOF'} +'b c' +$ @kbd{bash -c 'cat <<EOF +$@{a-"b c"@} +EOF'} +b c +$ @kbd{bash -c 'cat <<EOF +$@{a-'b c'@} +EOF'} +'b c' +@end example + +Perhaps the easiest way to work around quoting issues in a manner +portable to all shells is to place the results in a temporary variable, +then use @samp{$t} as the @var{value}, rather than trying to inline +the expression needing quoting. + +@example +$ @kbd{/bin/sh -c 't="b c\"'\''@}\\"; echo "$@{a-$t@}"'} +b c"'@}\ +$ @kbd{ksh -c 't="b c\"'\''@}\\"; echo "$@{a-$t@}"'} +b c"'@}\ +$ @kbd{bash -c 't="b c\"'\''@}\\"; echo "$@{a-$t@}"'} +b c"'@}\ +@end example + +@item $@{@var{var}=@var{value}@} +@cindex @code{$@{@var{var}=@var{value}@}} +When using @samp{$@{@var{var}=@var{value}@}} to assign a default value +to @var{var}, remember that even though the assignment to @var{var} does +not undergo file name expansion, the result of the variable expansion +does unless the expansion occurred within double quotes. In particular, +when using @command{:} followed by unquoted variable expansion for the +side effect of setting a default value, if the final value of +@samp{$var} contains any globbing characters (either from @var{value} or +from prior contents), the shell has to spend time performing file name +expansion and field splitting even though those results will not be +used. Therefore, it is a good idea to consider double quotes when performing +default initialization; while remembering how this impacts any quoting +characters appearing in @var{value}. + +@example +$ @kbd{time bash -c ': "$@{a=/usr/bin/*@}"; echo "$a"'} +/usr/bin/* + +real 0m0.005s +user 0m0.002s +sys 0m0.003s +$ @kbd{time bash -c ': $@{a=/usr/bin/*@}; echo "$a"'} +/usr/bin/* + +real 0m0.039s +user 0m0.026s +sys 0m0.009s +$ @kbd{time bash -c 'a=/usr/bin/*; : $@{a=noglob@}; echo "$a"'} +/usr/bin/* + +real 0m0.031s +user 0m0.020s +sys 0m0.010s + +$ @kbd{time bash -c 'a=/usr/bin/*; : "$@{a=noglob@}"; echo "$a"'} +/usr/bin/* + +real 0m0.006s +user 0m0.002s +sys 0m0.003s +@end example + +As with @samp{+} and @samp{-}, you must use quotes when using @samp{=} +if the @var{value} contains more than one shell word; either single +quotes for just the @var{value}, or double quotes around the entire +expansion: + +@example +$ @kbd{: $@{var1='Some words'@}} +$ @kbd{: "$@{var2=like this@}"} +$ @kbd{echo $var1 $var2} +Some words like this +@end example + +@noindent +otherwise some shells, such as Solaris @command{/bin/sh} or on Digital +Unix V 5.0, die because of a ``bad substitution''. Meanwhile, Posix +requires that with @samp{=}, quote removal happens prior to the +assignment, and the expansion be the final contents of @var{var} without +quoting (and thus subject to field splitting), in contrast to the +behavior with @samp{-} passing the quoting through to the final +expansion. However, @command{bash} 4.1 does not obey this rule. + +@example +$ @kbd{ksh -c 'echo $@{var-a\ \ b@}'} +a b +$ @kbd{ksh -c 'echo $@{var=a\ \ b@}'} +a b +$ @kbd{bash -c 'echo $@{var=a\ \ b@}'} +a b +@end example + +Finally, Posix states that when mixing @samp{$@{a=b@}} with regular +commands, it is unspecified whether the assignments affect the parent +shell environment. It is best to perform assignments independently from +commands, to avoid the problems demonstrated in this example: + +@example +$ @kbd{bash -c 'x= y=$@{x:=b@} sh -c "echo +\$x+\$y+";echo -$x-'} ++b+b+ +-b- +$ @kbd{/bin/sh -c 'x= y=$@{x:=b@} sh -c "echo +\$x+\$y+";echo -$x-'} +++b+ +-- +$ @kbd{ksh -c 'x= y=$@{x:=b@} sh -c "echo +\$x+\$y+";echo -$x-'} ++b+b+ +-- +@end example + +@item $@{@var{var}=@var{value}@} +@cindex @code{$@{@var{var}=@var{literal}@}} +Solaris @command{/bin/sh} has a frightening bug in its handling of +literal assignments. Imagine you need set a variable to a string containing +@samp{@}}. This @samp{@}} character confuses Solaris @command{/bin/sh} +when the affected variable was already set. This bug can be exercised +by running: + +@example +$ @kbd{unset foo} +$ @kbd{foo=$@{foo='@}'@}} +$ @kbd{echo $foo} +@} +$ @kbd{foo=$@{foo='@}' # no error; this hints to what the bug is} +$ @kbd{echo $foo} +@} +$ @kbd{foo=$@{foo='@}'@}} +$ @kbd{echo $foo} +@}@} + ^ ugh! +@end example + +It seems that @samp{@}} is interpreted as matching @samp{$@{}, even +though it is enclosed in single quotes. The problem doesn't happen +using double quotes, or when using a temporary variable holding the +problematic string. + +@item $@{@var{var}=@var{expanded-value}@} +@cindex @code{$@{@var{var}=@var{expanded-value}@}} +On Ultrix, +running + +@example +default="yu,yaa" +: $@{var="$default"@} +@end example + +@noindent +sets @var{var} to @samp{M-yM-uM-,M-yM-aM-a}, i.e., the 8th bit of +each char is set. You don't observe the phenomenon using a simple +@samp{echo $var} since apparently the shell resets the 8th bit when it +expands $var. Here are two means to make this shell confess its sins: + +@example +$ @kbd{cat -v <<EOF +$var +EOF} +@end example + +@noindent +and + +@example +$ @kbd{set | grep '^var=' | cat -v} +@end example + +One classic incarnation of this bug is: + +@example +default="a b c" +: $@{list="$default"@} +for c in $list; do + echo $c +done +@end example + +@noindent +You'll get @samp{a b c} on a single line. Why? Because there are no +spaces in @samp{$list}: there are @samp{M- }, i.e., spaces with the 8th +bit set, hence no IFS splitting is performed!!! + +One piece of good news is that Ultrix works fine with @samp{: +$@{list=$default@}}; i.e., if you @emph{don't} quote. The bad news is +then that QNX 4.25 then sets @var{list} to the @emph{last} item of +@var{default}! + +The portable way out consists in using a double assignment, to switch +the 8th bit twice on Ultrix: + +@example +list=$@{list="$default"@} +@end example + +@noindent +@dots{}but beware of the @samp{@}} bug from Solaris (see above). For safety, +use: + +@example +test "$@{var+set@}" = set || var=@var{@{value@}} +@end example + +@item $@{#@var{var}@} +@itemx $@{@var{var}%@var{word}@} +@itemx $@{@var{var}%%@var{word}@} +@itemx $@{@var{var}#@var{word}@} +@itemx $@{@var{var}##@var{word}@} +@cindex @code{$@{#@var{var}@}} +@cindex @code{$@{@var{var}%@var{word}@}} +@cindex @code{$@{@var{var}%%@var{word}@}} +@cindex @code{$@{@var{var}#@var{word}@}} +@cindex @code{$@{@var{var}##@var{word}@}} +Posix requires support for these usages, but they do not work with many +traditional shells, e.g., Solaris 10 @command{/bin/sh}. + +Also, @command{pdksh} 5.2.14 mishandles some @var{word} forms. For +example if @samp{$1} is @samp{a/b} and @samp{$2} is @samp{a}, then +@samp{$@{1#$2@}} should yield @samp{/b}, but with @command{pdksh} it +yields the empty string. + + +@item `@var{commands}` +@cindex @code{`@var{commands}`} +@cindex Command Substitution +Posix requires shells to trim all trailing newlines from command +output before substituting it, so assignments like +@samp{dir=`echo "$file" | tr a A`} do not work as expected if +@samp{$file} ends in a newline. + +While in general it makes no sense, do not substitute a single builtin +with side effects, because Ash 0.2, trying to optimize, does not fork a +subshell to perform the command. + +For instance, if you wanted to check that @command{cd} is silent, do not +use @samp{test -z "`cd /`"} because the following can happen: + +@example +$ @kbd{pwd} +/tmp +$ @kbd{test -z "`cd /`" && pwd} +/ +@end example + +@noindent +The result of @samp{foo=`exit 1`} is left as an exercise to the reader. + +The MSYS shell leaves a stray byte in the expansion of a double-quoted +command substitution of a native program, if the end of the substitution +is not aligned with the end of the double quote. This may be worked +around by inserting another pair of quotes: + +@example +$ @kbd{echo "`printf 'foo\r\n'` bar" > broken} +$ @kbd{echo "`printf 'foo\r\n'`"" bar" | cmp - broken} +- broken differ: char 4, line 1 +@end example + +Upon interrupt or SIGTERM, some shells may abort a command substitution, +replace it with a null string, and wrongly evaluate the enclosing +command before entering the trap or ending the script. This can lead to +spurious errors: + +@example +$ @kbd{sh -c 'if test `sleep 5; echo hi` = hi; then echo yes; fi'} +$ @kbd{^C} +sh: test: hi: unexpected operator/operand +@end example + +@noindent +You can avoid this by assigning the command substitution to a temporary +variable: + +@example +$ @kbd{sh -c 'res=`sleep 5; echo hi` + if test "x$res" = xhi; then echo yes; fi'} +$ @kbd{^C} +@end example + +@item $(@var{commands}) +@cindex @code{$(@var{commands})} +This construct is meant to replace @samp{`@var{commands}`}, +and it has most of the problems listed under @code{`@var{commands}`}. + +This construct can be +nested while this is impossible to do portably with back quotes. +Unfortunately it is not yet universally supported. Most notably, even recent +releases of Solaris don't support it: + +@example +$ @kbd{showrev -c /bin/sh | grep version} +Command version: SunOS 5.10 Generic 121005-03 Oct 2006 +$ @kbd{echo $(echo blah)} +syntax error: `(' unexpected +@end example + +@noindent +nor does IRIX 6.5's Bourne shell: +@example +$ @kbd{uname -a} +IRIX firebird-image 6.5 07151432 IP22 +$ @kbd{echo $(echo blah)} +$(echo blah) +@end example + +If you do use @samp{$(@var{commands})}, make sure that the commands +do not start with a parenthesis, as that would cause confusion with +a different notation @samp{$((@var{expression}))} that in modern +shells is an arithmetic expression not a command. To avoid the +confusion, insert a space between the two opening parentheses. + +Avoid @var{commands} that contain unbalanced parentheses in +here-documents, comments, or case statement patterns, as many shells +mishandle them. For example, Bash 3.1, @samp{ksh88}, @command{pdksh} +5.2.14, and Zsh 4.2.6 all mishandle the following valid command: + +@example +echo $(case x in x) echo hello;; esac) +@end example + + +@item $((@var{expression})) +@cindex @code{$((@var{expression}))} +Arithmetic expansion is not portable as some shells (most +notably Solaris 10 @command{/bin/sh}) don't support it. + +Among shells that do support @samp{$(( ))}, not all of them obey the +Posix rule that octal and hexadecimal constants must be recognized: + +@example +$ @kbd{bash -c 'echo $(( 010 + 0x10 ))'} +24 +$ @kbd{zsh -c 'echo $(( 010 + 0x10 ))'} +26 +$ @kbd{zsh -c 'emulate sh; echo $(( 010 + 0x10 ))'} +24 +$ @kbd{pdksh -c 'echo $(( 010 + 0x10 ))'} +pdksh: 010 + 0x10 : bad number `0x10' +$ @kbd{pdksh -c 'echo $(( 010 ))'} +10 +@end example + +When it is available, using arithmetic expansion provides a noticeable +speedup in script execution; but testing for support requires +@command{eval} to avoid syntax errors. The following construct is used +by @code{AS_VAR_ARITH} to provide arithmetic computation when all +arguments are provided in decimal and without a leading zero, and all +operators are properly quoted and appear as distinct arguments: + +@example +if ( eval 'test $(( 1 + 1 )) = 2' ) 2>/dev/null; then + eval 'func_arith () + @{ + func_arith_result=$(( $* )) + @}' +else + func_arith () + @{ + func_arith_result=`expr "$@@"` + @} +fi +func_arith 1 + 1 +foo=$func_arith_result +@end example + + +@item ^ +@cindex @code{^} quoting +Always quote @samp{^}, otherwise traditional shells such as +@command{/bin/sh} on Solaris 10 treat this like @samp{|}. + +@end table + + +@node Assignments +@section Assignments +@cindex Shell assignments + +When setting several variables in a row, be aware that the order of the +evaluation is undefined. For instance @samp{foo=1 foo=2; echo $foo} +gives @samp{1} with Solaris @command{/bin/sh}, but @samp{2} with Bash. +You must use +@samp{;} to enforce the order: @samp{foo=1; foo=2; echo $foo}. + +Don't rely on the following to find @file{subdir/program}: + +@example +PATH=subdir$PATH_SEPARATOR$PATH program +@end example + +@noindent +as this does not work with Zsh 3.0.6. Use something like this +instead: + +@example +(PATH=subdir$PATH_SEPARATOR$PATH; export PATH; exec program) +@end example + +Don't rely on the exit status of an assignment: Ash 0.2 does not change +the status and propagates that of the last statement: + +@example +$ @kbd{false || foo=bar; echo $?} +1 +$ @kbd{false || foo=`:`; echo $?} +0 +@end example + +@noindent +and to make things even worse, QNX 4.25 just sets the exit status +to 0 in any case: + +@example +$ @kbd{foo=`exit 1`; echo $?} +0 +@end example + +To assign default values, follow this algorithm: + +@enumerate +@item +If the default value is a literal and does not contain any closing +brace, use: + +@example +: "$@{var='my literal'@}" +@end example + +@item +If the default value contains no closing brace, has to be expanded, and +the variable being initialized is not intended to be IFS-split +(i.e., it's not a list), then use: + +@example +: $@{var="$default"@} +@end example + +@item +If the default value contains no closing brace, has to be expanded, and +the variable being initialized is intended to be IFS-split (i.e., it's a list), +then use: + +@example +var=$@{var="$default"@} +@end example + +@item +If the default value contains a closing brace, then use: + +@example +test "$@{var+set@}" = set || var="has a '@}'" +@end example +@end enumerate + +In most cases @samp{var=$@{var="$default"@}} is fine, but in case of +doubt, just use the last form. @xref{Shell Substitutions}, items +@samp{$@{@var{var}:-@var{value}@}} and @samp{$@{@var{var}=@var{value}@}} +for the rationale. + +@node Parentheses +@section Parentheses in Shell Scripts +@cindex Shell parentheses + +Beware of two opening parentheses in a row, as many shell +implementations treat them specially, and Posix says that a portable +script cannot use @samp{((} outside the @samp{$((} form used for shell +arithmetic. In traditional shells, @samp{((cat))} behaves like +@samp{(cat)}; but many shells, including +Bash and the Korn shell, treat @samp{((cat))} as an arithmetic +expression equivalent to @samp{let "cat"}, and may or may not report an +error when they detect that @samp{cat} is not a number. As another +example, @samp{pdksh} 5.2.14 does not treat the following code +as a traditional shell would: + +@example +if ((true) || false); then + echo ok +fi +@end example + +@noindent +To work around this problem, insert a space between the two opening +parentheses. There is a similar problem and workaround with +@samp{$((}; see @ref{Shell Substitutions}. + +@node Slashes +@section Slashes in Shell Scripts +@cindex Shell slashes + +Unpatched Tru64 5.1 @command{sh} omits the last slash of command-line +arguments that contain two trailing slashes: + +@example +$ @kbd{echo / // /// //// .// //.} +/ / // /// ./ //. +$ @kbd{x=//} +$ @kbd{eval "echo \$x"} +/ +$ @kbd{set -x} +$ @kbd{echo abc | tr -t ab //} ++ echo abc ++ tr -t ab / +/bc +@end example + +Unpatched Tru64 4.0 @command{sh} adds a slash after @samp{"$var"} if the +variable is empty and the second double-quote is followed by a word that +begins and ends with slash: + +@example +$ @kbd{sh -xc 'p=; echo "$p"/ouch/'} +p= ++ echo //ouch/ +//ouch/ +@end example + +However, our understanding is that patches are available, so perhaps +it's not worth worrying about working around these horrendous bugs. + +@node Special Shell Variables +@section Special Shell Variables +@cindex Shell variables +@cindex Special shell variables + +Some shell variables should not be used, since they can have a deep +influence on the behavior of the shell. In order to recover a sane +behavior from the shell, some variables should be unset; M4sh takes +care of this and provides fallback values, whenever needed, to cater +for a very old @file{/bin/sh} that does not support @command{unset}. +(@pxref{Portable Shell, , Portable Shell Programming}). + +As a general rule, shell variable names containing a lower-case letter +are safe; you can define and use these variables without worrying about +their effect on the underlying system, and without worrying about +whether the shell changes them unexpectedly. (The exception is the +shell variable @code{status}, as described below.) + +Here is a list of names that are known to cause trouble. This list is +not exhaustive, but you should be safe if you avoid the name +@code{status} and names containing only upper-case letters and +underscores. + +@c Alphabetical order, case insensitive, `A' before `a'. +@table @code +@item ? +Not all shells correctly reset @samp{$?} after conditionals (@pxref{if, +, Limitations of Shell Builtins}). Not all shells manage @samp{$?} +correctly in shell functions (@pxref{Shell Functions}) or in traps +(@pxref{trap, , Limitations of Shell Builtins}). Not all shells reset +@samp{$?} to zero after an empty command. + +@example +$ @kbd{bash -c 'false; $empty; echo $?'} +0 +$ @kbd{zsh -c 'false; $empty; echo $?'} +1 +@end example + +@item _ +@evindex _ +Many shells reserve @samp{$_} for various purposes, e.g., the name of +the last command executed. + +@item BIN_SH +@evindex BIN_SH +In Tru64, if @env{BIN_SH} is set to @code{xpg4}, subsidiary invocations of +the standard shell conform to Posix. + +@item CDPATH +@evindex CDPATH +When this variable is set it specifies a list of directories to search +when invoking @code{cd} with a relative file name that did not start +with @samp{./} or @samp{../}. Posix +1003.1-2001 says that if a nonempty directory name from @env{CDPATH} +is used successfully, @code{cd} prints the resulting absolute +file name. Unfortunately this output can break idioms like +@samp{abs=`cd src && pwd`} because @code{abs} receives the name twice. +Also, many shells do not conform to this part of Posix; for +example, @command{zsh} prints the result only if a directory name +other than @file{.} was chosen from @env{CDPATH}. + +In practice the shells that have this problem also support +@command{unset}, so you can work around the problem as follows: + +@example +(unset CDPATH) >/dev/null 2>&1 && unset CDPATH +@end example + +You can also avoid output by ensuring that your directory name is +absolute or anchored at @samp{./}, as in @samp{abs=`cd ./src && pwd`}. + +Configure scripts use M4sh, which automatically unsets @env{CDPATH} if +possible, so you need not worry about this problem in those scripts. + +@item CLICOLOR_FORCE +@evindex CLICOLOR_FORCE +When this variable is set, some implementations of tools like +@command{ls} attempt to add color to their output via terminal escape +sequences, even when the output is not directed to a terminal, and can +thus cause spurious failures in scripts. Configure scripts use M4sh, +which automatically unsets this variable. + +@item DUALCASE +@evindex DUALCASE +In the MKS shell, case statements and file name generation are +case-insensitive unless @env{DUALCASE} is nonzero. +Autoconf-generated scripts export this variable when they start up. + +@item ENV +@itemx MAIL +@itemx MAILPATH +@itemx PS1 +@itemx PS2 +@itemx PS4 +@evindex ENV +@evindex MAIL +@evindex MAILPATH +@evindex PS1 +@evindex PS2 +@evindex PS4 +These variables should not matter for shell scripts, since they are +supposed to affect only interactive shells. However, at least one +shell (the pre-3.0 UWIN Korn shell) gets confused about +whether it is interactive, which means that (for example) a @env{PS1} +with a side effect can unexpectedly modify @samp{$?}. To work around +this bug, M4sh scripts (including @file{configure} scripts) do something +like this: + +@example +(unset ENV) >/dev/null 2>&1 && unset ENV MAIL MAILPATH +PS1='$ ' +PS2='> ' +PS4='+ ' +@end example + +@noindent +(actually, there is some complication due to bugs in @command{unset}; +@pxref{unset, , Limitations of Shell Builtins}). + +@item FPATH +@evindex FPATH +The Korn shell uses @env{FPATH} to find shell functions, so avoid +@env{FPATH} in portable scripts. @env{FPATH} is consulted after +@env{PATH}, but you still need to be wary of tests that use @env{PATH} +to find whether a command exists, since they might report the wrong +result if @env{FPATH} is also set. + +@item GREP_OPTIONS +@evindex GREP_OPTIONS +When this variable is set, some implementations of @command{grep} honor +these options, even if the options include direction to enable colored +output via terminal escape sequences, and the result can cause spurious +failures when the output is not directed to a terminal. Configure +scripts use M4sh, which automatically unsets this variable. + +@item IFS +@evindex IFS +Long ago, shell scripts inherited @env{IFS} from the environment, +but this caused many problems so modern shells ignore any environment +settings for @env{IFS}. + +Don't set the first character of @env{IFS} to backslash. Indeed, +Bourne shells use the first character (backslash) when joining the +components in @samp{"$@@"} and some shells then reinterpret (!)@: the +backslash escapes, so you can end up with backspace and other strange +characters. + +The proper value for @env{IFS} (in regular code, not when performing +splits) is @samp{@key{SPC}@key{TAB}@key{RET}}. The first character is +especially important, as it is used to join the arguments in @samp{$*}; +however, note that traditional shells, but also bash-2.04, fail to adhere +to this and join with a space anyway. + +M4sh guarantees that @env{IFS} will have the default value at the +beginning of a script, and many macros within autoconf rely on this +setting. It is okay to use blocks of shell code that temporarily change +the value of @env{IFS} in order to split on another character, but +remember to restore it before expanding further macros. + +Unsetting @code{IFS} instead of resetting it to the default sequence +is not suggested, since code that tries to save and restore the +variable's value will incorrectly reset it to an empty value, thus +disabling field splitting: + +@example +unset IFS +# default separators used for field splitting + +save_IFS=$IFS +IFS=: +# ... +IFS=$save_IFS +# no field splitting performed +@end example + +@item LANG +@itemx LC_ALL +@itemx LC_COLLATE +@itemx LC_CTYPE +@itemx LC_MESSAGES +@itemx LC_MONETARY +@itemx LC_NUMERIC +@itemx LC_TIME +@evindex LANG +@evindex LC_ALL +@evindex LC_COLLATE +@evindex LC_CTYPE +@evindex LC_MESSAGES +@evindex LC_MONETARY +@evindex LC_NUMERIC +@evindex LC_TIME + +You should set all these variables to @samp{C} because so much +configuration code assumes the C locale and Posix requires that locale +environment variables be set to @samp{C} if the C locale is desired; +@file{configure} scripts and M4sh do that for you. +Export these variables after setting them. + +@c However, some older, nonstandard +@c systems (notably SCO) break if locale environment variables +@c are set to @samp{C}, so when running on these systems +@c Autoconf-generated scripts unset the variables instead. + +@item LANGUAGE +@evindex LANGUAGE + +@env{LANGUAGE} is not specified by Posix, but it is a GNU +extension that overrides @env{LC_ALL} in some cases, so you (or M4sh) +should set it too. + +@item LC_ADDRESS +@itemx LC_IDENTIFICATION +@itemx LC_MEASUREMENT +@itemx LC_NAME +@itemx LC_PAPER +@itemx LC_TELEPHONE +@evindex LC_ADDRESS +@evindex LC_IDENTIFICATION +@evindex LC_MEASUREMENT +@evindex LC_NAME +@evindex LC_PAPER +@evindex LC_TELEPHONE + +These locale environment variables are GNU extensions. They +are treated like their Posix brethren (@env{LC_COLLATE}, +etc.)@: as described above. + +@item LINENO +@evindex LINENO +Most modern shells provide the current line number in @code{LINENO}. +Its value is the line number of the beginning of the current command. +M4sh, and hence Autoconf, attempts to execute @command{configure} with +a shell that supports @code{LINENO}. If no such shell is available, it +attempts to implement @code{LINENO} with a Sed prepass that replaces each +instance of the string @code{$LINENO} (not followed by an alphanumeric +character) with the line's number. In M4sh scripts you should execute +@code{AS_LINENO_PREPARE} so that these workarounds are included in +your script; configure scripts do this automatically in @code{AC_INIT}. + +You should not rely on @code{LINENO} within @command{eval} or shell +functions, as the behavior differs in practice. The presence of a +quoted newline within simple commands can alter which line number is +used as the starting point for @code{$LINENO} substitutions within that +command. Also, the possibility of the Sed prepass means that you should +not rely on @code{$LINENO} when quoted, when in here-documents, or when +line continuations are used. Subshells should be OK, though. In the +following example, lines 1, 9, and 14 are portable, but the other +instances of @code{$LINENO} do not have deterministic values: + +@example +@group +$ @kbd{cat lineno} +echo 1. $LINENO +echo "2. $LINENO +3. $LINENO" +cat <<EOF +5. $LINENO +6. $LINENO +7. \$LINENO +EOF +( echo 9. $LINENO ) +eval 'echo 10. $LINENO' +eval 'echo 11. $LINENO +echo 12. $LINENO' +echo 13. '$LINENO' +echo 14. $LINENO ' +15.' $LINENO +f () @{ echo $1 $LINENO; +echo $1 $LINENO @} +f 18. +echo 19. \ +$LINENO +@end group +@group +$ @kbd{bash-3.2 ./lineno} +1. 1 +2. 3 +3. 3 +5. 4 +6. 4 +7. $LINENO +9. 9 +10. 10 +11. 12 +12. 13 +13. $LINENO +14. 14 +15. 14 +18. 16 +18. 17 +19. 19 +@end group +@group +$ @kbd{zsh-4.3.4 ./lineno} +1. 1 +2. 2 +3. 2 +5. 4 +6. 4 +7. $LINENO +9. 9 +10. 1 +11. 1 +12. 2 +13. $LINENO +14. 14 +15. 14 +18. 0 +18. 1 +19. 19 +@end group +@group +$ @kbd{pdksh-5.2.14 ./lineno} +1. 1 +2. 2 +3. 2 +5. 4 +6. 4 +7. $LINENO +9. 9 +10. 0 +11. 0 +12. 0 +13. $LINENO +14. 14 +15. 14 +18. 16 +18. 17 +19. 19 +@end group +@group +$ @kbd{sed '=' <lineno |} +> @kbd{ sed '} +> @kbd{ N} +> @kbd{ s,$,-,} +> @kbd{ t loop} +> @kbd{ :loop} +> @kbd{ s,^\([0-9]*\)\(.*\)[$]LINENO\([^a-zA-Z0-9_]\),\1\2\1\3,} +> @kbd{ t loop} +> @kbd{ s,-$,,} +> @kbd{ s,^[0-9]*\n,,} +> @kbd{ ' |} +> @kbd{ sh} +1. 1 +2. 2 +3. 3 +5. 5 +6. 6 +7. \7 +9. 9 +10. 10 +11. 11 +12. 12 +13. 13 +14. 14 +15. 15 +18. 16 +18. 17 +19. 20 +@end group +@end example + +In particular, note that @file{config.status} (and any other subsidiary +script created by @code{AS_INIT_GENERATED}) might report line numbers +relative to the parent script as a result of the potential Sed pass. + +@item NULLCMD +@evindex NULLCMD +When executing the command @samp{>foo}, @command{zsh} executes +@samp{$NULLCMD >foo} unless it is operating in Bourne shell +compatibility mode and the @command{zsh} version is newer +than 3.1.6-dev-18. If you are using an older @command{zsh} +and forget to set @env{NULLCMD}, +your script might be suspended waiting for data on its standard input. + +@item options +@evindex options +For @command{zsh} 4.3.10, @env{options} is treated as an associative +array even after @code{emulate sh}, so it should not be used. + +@item PATH_SEPARATOR +@evindex PATH_SEPARATOR +On DJGPP systems, the @env{PATH_SEPARATOR} environment +variable can be set to either @samp{:} or @samp{;} to control the path +separator Bash uses to set up certain environment variables (such as +@env{PATH}). You can set this variable to @samp{;} if you want +@command{configure} to use @samp{;} as a separator; this might be useful +if you plan to use non-Posix shells to execute files. @xref{File System +Conventions}, for more information about @code{PATH_SEPARATOR}. + +@item POSIXLY_CORRECT +@evindex POSIXLY_CORRECT +In the GNU environment, exporting @env{POSIXLY_CORRECT} with any value +(even empty) causes programs to try harder to conform to Posix. +Autoconf does not directly manipulate this variable, but @command{bash} +ties the shell variable @env{POSIXLY_CORRECT} to whether the script is +running in Posix mode. Therefore, take care when exporting or unsetting +this variable, so as not to change whether @command{bash} is in Posix +mode. + +@example +$ @kbd{bash --posix -c 'set -o | grep posix} +> @kbd{unset POSIXLY_CORRECT} +> @kbd{set -o | grep posix'} +posix on +posix off +@end example + +@item PWD +@evindex PWD +Posix 1003.1-2001 requires that @command{cd} and +@command{pwd} must update the @env{PWD} environment variable to point +to the logical name of the current directory, but traditional shells +do not support this. This can cause confusion if one shell instance +maintains @env{PWD} but a subsidiary and different shell does not know +about @env{PWD} and executes @command{cd}; in this case @env{PWD} +points to the wrong directory. Use @samp{`pwd`} rather than +@samp{$PWD}. + +@item RANDOM +@evindex RANDOM +Many shells provide @code{RANDOM}, a variable that returns a different +integer each time it is used. Most of the time, its value does not +change when it is not used, but on IRIX 6.5 the value changes all +the time. This can be observed by using @command{set}. It is common +practice to use @code{$RANDOM} as part of a file name, but code +shouldn't rely on @code{$RANDOM} expanding to a nonempty string. + +@item status +@evindex status +This variable is an alias to @samp{$?} for @code{zsh} (at least 3.1.6), +hence read-only. Do not use it. +@end table + +@node Shell Functions +@section Shell Functions +@cindex Shell Functions + +Nowadays, it is difficult to find a shell that does not support +shell functions at all. However, some differences should be expected. + +When declaring a shell function, you must include whitespace between the +@samp{)} after the function name and the start of the compound +expression, to avoid upsetting @command{ksh}. While it is possible to +use any compound command, most scripts use @samp{@{@dots{}@}}. + +@example +$ @kbd{/bin/sh -c 'a()@{ echo hi;@}; a'} +hi +$ @kbd{ksh -c 'a()@{ echo hi;@}; a'} +ksh: syntax error at line 1: `@}' unexpected +$ @kbd{ksh -c 'a() @{ echo hi;@}; a'} +hi +@end example + +Inside a shell function, you should not rely on the error status of a +subshell if the last command of that subshell was @code{exit} or +@code{trap}, as this triggers bugs in zsh 4.x; while Autoconf tries to +find a shell that does not exhibit the bug, zsh might be the only shell +present on the user's machine. + +Likewise, the state of @samp{$?} is not reliable when entering a shell +function. This has the effect that using a function as the first +command in a @command{trap} handler can cause problems. + +@example +$ @kbd{bash -c 'foo() @{ echo $?; @}; trap foo 0; (exit 2); exit 2'; echo $?} +2 +2 +$ @kbd{ash -c 'foo() @{ echo $?; @}; trap foo 0; (exit 2); exit 2'; echo $?} +0 +2 +@end example + +DJGPP bash 2.04 has a bug in that @command{return} from a +shell function which also used a command substitution causes a +segmentation fault. To work around the issue, you can use +@command{return} from a subshell, or @samp{AS_SET_STATUS} as last command +in the execution flow of the function (@pxref{Common Shell Constructs}). + +Not all shells treat shell functions as simple commands impacted by +@samp{set -e}, for example with Solaris 10 @command{/bin/sh}: + +@example +$ @kbd{bash -c 'f() @{ return 1; @}; set -e; f; echo oops'} +$ @kbd{/bin/sh -c 'f() @{ return 1; @}; set -e; f; echo oops'} +oops +@end example + +Shell variables and functions may share the same namespace, for example +with Solaris 10 @command{/bin/sh}: + +@example +$ @kbd{f () @{ :; @}; f=; f} +f: not found +@end example + +@noindent +For this reason, Autoconf (actually M4sh, @pxref{Programming in M4sh}) +uses the prefix @samp{as_fn_} for its functions. + +Handling of positional parameters and shell options varies among shells. +For example, Korn shells reset and restore trace output (@samp{set -x}) +and other options upon function entry and exit. Inside a function, +IRIX sh sets @samp{$0} to the function name. + +It is not portable to pass temporary environment variables to shell +functions. Solaris @command{/bin/sh} does not see the variable. +Meanwhile, not all shells follow the Posix rule that the assignment must +affect the current environment in the same manner as special built-ins. + +@example +$ @kbd{/bin/sh -c 'func() @{ echo $a;@}; a=1 func; echo $a'} +@result{} +@result{} +$ @kbd{ash -c 'func() @{ echo $a;@}; a=1 func; echo $a'} +@result{}1 +@result{} +$ @kbd{bash -c 'set -o posix; func() @{ echo $a;@}; a=1 func; echo $a'} +@result{}1 +@result{}1 +@end example + +Some ancient Bourne shell variants with function support did not reset +@samp{$@var{i}, @var{i} >= 0}, upon function exit, so effectively the +arguments of the script were lost after the first function invocation. +It is probably not worth worrying about these shells any more. + +With AIX sh, a @command{trap} on 0 installed in a shell function +triggers at function exit rather than at script exit. @xref{trap, , +Limitations of Shell Builtins}. + +@node Limitations of Builtins +@section Limitations of Shell Builtins +@cindex Shell builtins +@cindex Limitations of shell builtins + +No, no, we are serious: some shells do have limitations! :) + +You should always keep in mind that any builtin or command may support +options, and therefore differ in behavior with arguments +starting with a dash. For instance, even the innocent @samp{echo "$word"} +can give unexpected results when @code{word} starts with a dash. It is +often possible to avoid this problem using @samp{echo "x$word"}, taking +the @samp{x} into account later in the pipe. Many of these limitations +can be worked around using M4sh (@pxref{Programming in M4sh}). + +@c This table includes things like `@command{test} (files)', so we can't +@c use @table @command. +@table @asis +@item @command{.} +@c -------------- +@prindex @command{.} +Use @command{.} only with regular files (use @samp{test -f}). Bash +2.03, for instance, chokes on @samp{. /dev/null}. Remember that +@command{.} uses @env{PATH} if its argument contains no slashes. Also, +some shells, including bash 3.2, implicitly append the current directory +to this @env{PATH} search, even though Posix forbids it. So if you want +to use @command{.} on a file @file{foo} in the current directory, you +must use @samp{. ./foo}. + +Not all shells gracefully handle syntax errors within a sourced file. +On one extreme, some non-interactive shells abort the entire script. On +the other, @command{zsh} 4.3.10 has a bug where it fails to react to the +syntax error. + +@example +$ @kbd{echo 'fi' > syntax} +$ @kbd{bash -c '. ./syntax; echo $?'} +./syntax: line 1: syntax error near unexpected token `fi' +./syntax: line 1: `fi' +1 +$ @kbd{ash -c '. ./syntax; echo $?'} +./syntax: 1: Syntax error: "fi" unexpected +$ @kbd{zsh -c '. ./syntax; echo $?'} +./syntax:1: parse error near `fi' +0 +@end example + +@item @command{!} +@c -------------- +@prindex @command{!} +The Unix version 7 shell did not support +negating the exit status of commands with @command{!}, and this feature +is still absent from some shells (e.g., Solaris @command{/bin/sh}). +Other shells, such as FreeBSD @command{/bin/sh} or @command{ash}, have +bugs when using @command{!}: + +@example +$ @kbd{sh -c '! : | :'; echo $?} +1 +$ @kbd{ash -c '! : | :'; echo $?} +0 +$ @kbd{sh -c '! @{ :; @}'; echo $?} +1 +$ @kbd{ash -c '! @{ :; @}'; echo $?} +@{: not found +Syntax error: "@}" unexpected +2 +@end example + +Shell code like this: + +@example +if ! cmp file1 file2 >/dev/null 2>&1; then + echo files differ or trouble +fi +@end example + +is therefore not portable in practice. Typically it is easy to rewrite +such code, e.g.: + +@example +cmp file1 file2 >/dev/null 2>&1 || + echo files differ or trouble +@end example + +More generally, one can always rewrite @samp{! @var{command}} as: + +@example +if @var{command}; then (exit 1); else :; fi +@end example + + +@item @command{@{...@}} +@c -------------------- +@prindex @command{@{...@}} +Bash 3.2 (and earlier versions) sometimes does not properly set +@samp{$?} when failing to write redirected output of a compound command. +This problem is most commonly observed with @samp{@{@dots{}@}}; it does +not occur with @samp{(@dots{})}. For example: + +@example +$ @kbd{bash -c '@{ echo foo; @} >/bad; echo $?'} +bash: line 1: /bad: Permission denied +0 +$ @kbd{bash -c 'while :; do echo; done >/bad; echo $?'} +bash: line 1: /bad: Permission denied +0 +@end example + +To work around the bug, prepend @samp{:;}: + +@example +$ @kbd{bash -c ':;@{ echo foo; @} >/bad; echo $?'} +bash: line 1: /bad: Permission denied +1 +@end example + +Posix requires a syntax error if a brace list has no contents. However, +not all shells obey this rule; and on shells where empty lists are +permitted, the effect on @samp{$?} is inconsistent. To avoid problems, +ensure that a brace list is never empty. + +@example +$ @kbd{bash -c 'false; @{ @}; echo $?' || echo $?} +bash: line 1: syntax error near unexpected token `@}' +bash: line 1: `false; @{ @}; echo $?' +2 +$ @kbd{zsh -c 'false; @{ @}; echo $?' || echo $?} +1 +$ @kbd{pdksh -c 'false; @{ @}; echo $?' || echo $?} +0 +@end example + + +@item @command{break} +@c ------------------ +@prindex @command{break} +The use of @samp{break 2} etc.@: is safe. + + +@anchor{case} +@item @command{case} +@c ----------------- +@prindex @command{case} +You don't need to quote the argument; no splitting is performed. + +You don't need the final @samp{;;}, but you should use it. + +Posix requires support for @code{case} patterns with opening +parentheses like this: + +@example +case $file_name in + (*.c) echo "C source code";; +esac +@end example + +@noindent +but the @code{(} in this example is not portable to many Bourne +shell implementations, which is a pity for those of us using tools that +rely on balanced parentheses. For instance, with Solaris +@command{/bin/sh}: + +@example +$ @kbd{case foo in (foo) echo foo;; esac} +@error{}syntax error: `(' unexpected +@end example + +@noindent +The leading @samp{(} can be omitted safely. Unfortunately, there are +contexts where unbalanced parentheses cause other problems, such as when +using a syntax-highlighting editor that searches for the balancing +counterpart, or more importantly, when using a case statement as an +underquoted argument to an Autoconf macro. @xref{Balancing +Parentheses}, for tradeoffs involved in various styles of dealing with +unbalanced @samp{)}. + +Zsh handles pattern fragments derived from parameter expansions or +command substitutions as though quoted: + +@example +$ pat=\?; case aa in ?$pat) echo match;; esac +$ pat=\?; case a? in ?$pat) echo match;; esac +match +@end example + +@noindent +Because of a bug in its @code{fnmatch}, Bash fails to properly +handle backslashes in character classes: + +@example +bash-2.02$ @kbd{case /tmp in [/\\]*) echo OK;; esac} +bash-2.02$ +@end example + +@noindent +This is extremely unfortunate, since you are likely to use this code to +handle Posix or MS-DOS absolute file names. To work around this +bug, always put the backslash first: + +@example +bash-2.02$ @kbd{case '\TMP' in [\\/]*) echo OK;; esac} +OK +bash-2.02$ @kbd{case /tmp in [\\/]*) echo OK;; esac} +OK +@end example + +Many Bourne shells cannot handle closing brackets in character classes +correctly. + +Some shells also have problems with backslash escaping in case you do not want +to match the backslash: both a backslash and the escaped character match this +pattern. To work around this, specify the character class in a variable, so +that quote removal does not apply afterwards, and the special characters don't +have to be backslash-escaped: + +@example +$ @kbd{case '\' in [\<]) echo OK;; esac} +OK +$ @kbd{scanset='[<]'; case '\' in $scanset) echo OK;; esac} +$ +@end example + +Even with this, Solaris @command{ksh} matches a backslash if the set +contains any +of the characters @samp{|}, @samp{&}, @samp{(}, or @samp{)}. + +Conversely, Tru64 @command{ksh} (circa 2003) erroneously always matches +a closing parenthesis if not specified in a character class: + +@example +$ @kbd{case foo in *\)*) echo fail ;; esac} +fail +$ @kbd{case foo in *')'*) echo fail ;; esac} +fail +@end example + +Some shells, such as Ash 0.3.8, are confused by an empty +@code{case}/@code{esac}: + +@example +ash-0.3.8 $ @kbd{case foo in esac;} +@error{}Syntax error: ";" unexpected (expecting ")") +@end example + +Posix requires @command{case} to give an exit status of 0 if no cases +match. However, @command{/bin/sh} in Solaris 10 does not obey this +rule. Meanwhile, it is unclear whether a case that matches, but +contains no statements, must also change the exit status to 0. The M4sh +macro @code{AS_CASE} works around these inconsistencies. + +@example +$ @kbd{bash -c 'case `false` in ?) ;; esac; echo $?'} +0 +$ @kbd{/bin/sh -c 'case `false` in ?) ;; esac; echo $?'} +255 +@end example + + +@item @command{cd} +@c --------------- +@prindex @command{cd} +Posix 1003.1-2001 requires that @command{cd} must support +the @option{-L} (``logical'') and @option{-P} (``physical'') options, +with @option{-L} being the default. However, traditional shells do +not support these options, and their @command{cd} command has the +@option{-P} behavior. + +Portable scripts should assume neither option is supported, and should +assume neither behavior is the default. This can be a bit tricky, +since the Posix default behavior means that, for example, +@samp{ls ..} and @samp{cd ..} may refer to different directories if +the current logical directory is a symbolic link. It is safe to use +@code{cd @var{dir}} if @var{dir} contains no @file{..} components. +Also, Autoconf-generated scripts check for this problem when computing +variables like @code{ac_top_srcdir} (@pxref{Configuration Actions}), +so it is safe to @command{cd} to these variables. + +Posix states that behavior is undefined if @command{cd} is given an +explicit empty argument. Some shells do nothing, some change to the +first entry in @env{CDPATH}, some change to @env{HOME}, and some exit +the shell rather than returning an error. Unfortunately, this means +that if @samp{$var} is empty, then @samp{cd "$var"} is less predictable +than @samp{cd $var} (at least the latter is well-behaved in all shells +at changing to @env{HOME}, although this is probably not what you wanted +in a script). You should check that a directory name was supplied +before trying to change locations. + +@xref{Special Shell Variables}, for portability problems involving +@command{cd} and the @env{CDPATH} environment variable. +Also please see the discussion of the @command{pwd} command. + + +@anchor{echo} +@item @command{echo} +@c ----------------- +@prindex @command{echo} +The simple @command{echo} is probably the most surprising source of +portability troubles. It is not possible to use @samp{echo} portably +unless both options and escape sequences are omitted. Don't expect any +option. + +Do not use backslashes in the arguments, as there is no consensus on +their handling. For @samp{echo '\n' | wc -l}, the @command{sh} of +Solaris outputs 2, but Bash and Zsh (in @command{sh} emulation mode) output 1. +The problem is truly @command{echo}: all the shells +understand @samp{'\n'} as the string composed of a backslash and an +@samp{n}. Within a command substitution, @samp{echo 'string\c'} will +mess up the internal state of ksh88 on AIX 6.1 so that it will print +the first character @samp{s} only, followed by a newline, and then +entirely drop the output of the next echo in a command substitution. + +Because of these problems, do not pass a string containing arbitrary +characters to @command{echo}. For example, @samp{echo "$foo"} is safe +only if you know that @var{foo}'s value cannot contain backslashes and +cannot start with @samp{-}. + +If this may not be true, @command{printf} is in general safer and +easier to use than @command{echo} and @command{echo -n}. Thus, scripts +where portability is not a major concern should use @command{printf +'%s\n'} whenever @command{echo} could fail, and similarly use +@command{printf %s} instead of @command{echo -n}. For portable shell +scripts, instead, it is suggested to use a here-document like this: + +@example +cat <<EOF +$foo +EOF +@end example + +Alternatively, M4sh provides @code{AS_ECHO} and @code{AS_ECHO_N} macros +which choose between various portable implementations: @samp{echo} +or @samp{print} where they work, @command{printf} if it is available, +or else other creative tricks in order to work around the above problems. + + +@item @command{eval} +@c ----------------- +@prindex @command{eval} +The @command{eval} command is useful in limited circumstances, e.g., +using commands like @samp{eval table_$key=\$value} and @samp{eval +value=table_$key} to simulate a hash table when the key is known to be +alphanumeric. + +You should also be wary of common bugs in @command{eval} implementations. +In some shell implementations (e.g., older @command{ash}, OpenBSD 3.8 +@command{sh}, @command{pdksh} v5.2.14 99/07/13.2, and @command{zsh} +4.2.5), the arguments of @samp{eval} are evaluated in a context where +@samp{$?} is 0, so they exhibit behavior like this: + +@example +$ @kbd{false; eval 'echo $?'} +0 +@end example + +The correct behavior here is to output a nonzero value, +but portable scripts should not rely on this. + +You should not rely on @code{LINENO} within @command{eval}. +@xref{Special Shell Variables}. + +Note that, even though these bugs are easily avoided, +@command{eval} is tricky to use on arbitrary arguments. +It is obviously unwise to use @samp{eval $cmd} if the string value of +@samp{cmd} was derived from an untrustworthy source. But even if the +string value is valid, @samp{eval $cmd} might not work as intended, +since it causes field splitting and file name expansion to occur twice, +once for the @command{eval} and once for the command itself. It is +therefore safer to use @samp{eval "$cmd"}. For example, if @var{cmd} +has the value @samp{cat test?.c}, @samp{eval $cmd} might expand to the +equivalent of @samp{cat test;.c} if there happens to be a file named +@file{test;.c} in the current directory; and this in turn +mistakenly attempts to invoke @command{cat} on the file @file{test} and +then execute the command @command{.c}. To avoid this problem, use +@samp{eval "$cmd"} rather than @samp{eval $cmd}. + +However, suppose that you want to output the text of the evaluated +command just before executing it. Assuming the previous example, +@samp{echo "Executing: $cmd"} outputs @samp{Executing: cat test?.c}, but +this output doesn't show the user that @samp{test;.c} is the actual name +of the copied file. Conversely, @samp{eval "echo Executing: $cmd"} +works on this example, but it fails with @samp{cmd='cat foo >bar'}, +since it mistakenly replaces the contents of @file{bar} by the +string @samp{cat foo}. No simple, general, and portable solution to +this problem is known. + +@item @command{exec} +@c ----------------- +@prindex @command{exec} +Posix describes several categories of shell built-ins. Special +built-ins (such as @command{exit}) must impact the environment of the +current shell, and need not be available through @command{exec}. All +other built-ins are regular, and must not propagate variable assignments +to the environment of the current shell. However, the group of regular +built-ins is further distinguished by commands that do not require a +@env{PATH} search (such as @command{cd}), in contrast to built-ins that +are offered as a more efficient version of something that must still be +found in a @env{PATH} search (such as @command{echo}). Posix is not +clear on whether @command{exec} must work with the list of 17 utilities +that are invoked without a @env{PATH} search, and many platforms lack an +executable for some of those built-ins: + +@example +$ @kbd{sh -c 'exec cd /tmp'} +sh: line 0: exec: cd: not found +@end example + +All other built-ins that provide utilities specified by Posix must have +a counterpart executable that exists on @env{PATH}, although Posix +allows @command{exec} to use the built-in instead of the executable. +For example, contrast @command{bash} 3.2 and @command{pdksh} 5.2.14: + +@example +$ @kbd{bash -c 'pwd --version' | head -n1} +bash: line 0: pwd: --: invalid option +pwd: usage: pwd [-LP] +$ @kbd{bash -c 'exec pwd --version' | head -n1} +pwd (GNU coreutils) 6.10 +$ @kbd{pdksh -c 'exec pwd --version' | head -n1} +pdksh: pwd: --: unknown option +@end example + +When it is desired to avoid a regular shell built-in, the workaround is +to use some other forwarding command, such as @command{env} or +@command{nice}, that will ensure a path search: + +@example +$ @kbd{pdksh -c 'exec true --version' | head -n1} +$ @kbd{pdksh -c 'nice true --version' | head -n1} +true (GNU coreutils) 6.10 +$ @kbd{pdksh -c 'env true --version' | head -n1} +true (GNU coreutils) 6.10 +@end example + +@item @command{exit} +@c ----------------- +@prindex @command{exit} +The default value of @command{exit} is supposed to be @code{$?}; +unfortunately, some shells, such as the DJGPP port of Bash 2.04, just +perform @samp{exit 0}. + +@example +bash-2.04$ @kbd{foo=`exit 1` || echo fail} +fail +bash-2.04$ @kbd{foo=`(exit 1)` || echo fail} +fail +bash-2.04$ @kbd{foo=`(exit 1); exit` || echo fail} +bash-2.04$ +@end example + +Using @samp{exit $?} restores the expected behavior. + +Some shell scripts, such as those generated by @command{autoconf}, use a +trap to clean up before exiting. If the last shell command exited with +nonzero status, the trap also exits with nonzero status so that the +invoker can tell that an error occurred. + +Unfortunately, in some shells, such as Solaris @command{/bin/sh}, an exit +trap ignores the @code{exit} command's argument. In these shells, a trap +cannot determine whether it was invoked by plain @code{exit} or by +@code{exit 1}. Instead of calling @code{exit} directly, use the +@code{AC_MSG_ERROR} macro that has a workaround for this problem. + + +@anchor{export} +@item @command{export} +@c ------------------- +@prindex @command{export} +The builtin @command{export} dubs a shell variable @dfn{environment +variable}. Each update of exported variables corresponds to an update +of the environment variables. Conversely, each environment variable +received by the shell when it is launched should be imported as a shell +variable marked as exported. + +Alas, many shells, such as Solaris @command{/bin/sh}, +IRIX 6.3, IRIX 5.2, +AIX 4.1.5, and Digital Unix 4.0, forget to +@command{export} the environment variables they receive. As a result, +two variables coexist: the environment variable and the shell +variable. The following code demonstrates this failure: + +@example +#!/bin/sh +echo $FOO +FOO=bar +echo $FOO +exec /bin/sh $0 +@end example + +@noindent +when run with @samp{FOO=foo} in the environment, these shells print +alternately @samp{foo} and @samp{bar}, although they should print only +@samp{foo} and then a sequence of @samp{bar}s. + +Therefore you should @command{export} again each environment variable +that you update; the export can occur before or after the assignment. + +Posix is not clear on whether the @command{export} of an undefined +variable causes the variable to be defined with the value of an empty +string, or merely marks any future definition of a variable by that name +for export. Various shells behave differently in this regard: + +@example +$ @kbd{sh -c 'export foo; env | grep foo'} +$ @kbd{ash -c 'export foo; env | grep foo'} +foo= +@end example + +Posix requires @command{export} to honor assignments made as arguments, +but older shells do not support this, including @command{/bin/sh} in +Solaris 10. Portable scripts should separate assignments and exports +into different statements. + +@example +$ @kbd{bash -c 'export foo=bar; echo $foo'} +bar +$ @kbd{/bin/sh -c 'export foo=bar; echo $foo'} +/bin/sh: foo=bar: is not an identifier +$ @kbd{/bin/sh -c 'export foo; foo=bar; echo $foo'} +bar +@end example + +@item @command{false} +@c ------------------ +@prindex @command{false} +Don't expect @command{false} to exit with status 1: in native +Solaris @file{/bin/false} exits with status 255. + + +@item @command{for} +@c ---------------- +@prindex @command{for} +To loop over positional arguments, use: + +@example +for arg +do + echo "$arg" +done +@end example + +@noindent +You may @emph{not} leave the @code{do} on the same line as @code{for}, +since some shells improperly grok: + +@example +for arg; do + echo "$arg" +done +@end example + +If you want to explicitly refer to the positional arguments, given the +@samp{$@@} bug (@pxref{Shell Substitutions}), use: + +@example +for arg in $@{1+"$@@"@}; do + echo "$arg" +done +@end example + +@noindent +But keep in mind that Zsh, even in Bourne shell emulation mode, performs +word splitting on @samp{$@{1+"$@@"@}}; see @ref{Shell Substitutions}, +item @samp{$@@}, for more. + +In Solaris @command{/bin/sh}, when the list of arguments of a +@command{for} loop starts with @emph{unquoted} tokens looking like +variable assignments, the loop is not executed on those tokens: + +@example +$ @kbd{/bin/sh -c 'for v in a=b c=d x e=f; do echo $v; done'} +x +e=f +@end example + +@noindent +Thankfully, quoting the assignment-like tokens, or starting the list +with other tokens (including unquoted variable expansion that results in +an assignment-like result), avoids the problem, so it is easy to work +around: + +@example +$ @kbd{/bin/sh -c 'for v in "a=b"; do echo $v; done'} +a=b +$ @kbd{/bin/sh -c 'x=a=b; for v in $x c=d; do echo $v; done'} +a=b +c=d +@end example + +@anchor{if} +@item @command{if} +@c --------------- +@prindex @command{if} +Using @samp{!} is not portable. Instead of: + +@example +if ! cmp -s file file.new; then + mv file.new file +fi +@end example + +@noindent +use: + +@example +if cmp -s file file.new; then :; else + mv file.new file +fi +@end example + +@noindent +Or, especially if the @dfn{else} branch is short, you can use @code{||}. +In M4sh, the @code{AS_IF} macro provides an easy way to write these kinds +of conditionals: + +@example +AS_IF([cmp -s file file.new], [], [mv file.new file]) +@end example + +This is especially useful in other M4 macros, where the @dfn{then} and +@dfn{else} branches might be macro arguments. + +Some very old shells did not reset the exit status from an @command{if} +with no @command{else}: + +@example +$ @kbd{if (exit 42); then true; fi; echo $?} +42 +@end example + +@noindent +whereas a proper shell should have printed @samp{0}. But this is no +longer a portability problem; any shell that supports functions gets it +correct. However, it explains why some makefiles have lengthy +constructs: + +@example +if test -f "$file"; then + install "$file" "$dest" +else + : +fi +@end example + + +@item @command{printf} +@c ------------------ +@prindex @command{printf} +A format string starting with a @samp{-} can cause problems. +Bash interprets it as an option and +gives an error. And @samp{--} to mark the end of options is not good +in the NetBSD Almquist shell (e.g., 0.4.6) which takes that +literally as the format string. Putting the @samp{-} in a @samp{%c} +or @samp{%s} is probably easiest: + +@example +printf %s -foo +@end example + +Bash 2.03 mishandles an escape sequence that happens to evaluate to @samp{%}: + +@example +$ @kbd{printf '\045'} +bash: printf: `%': missing format character +@end example + +Large outputs may cause trouble. On Solaris 2.5.1 through 10, for +example, @file{/usr/bin/printf} is buggy, so when using +@command{/bin/sh} the command @samp{printf %010000x 123} normally dumps +core. + +Since @command{printf} is not always a shell builtin, there is a +potential speed penalty for using @code{printf '%s\n'} as a replacement +for an @command{echo} that does not interpret @samp{\} or leading +@samp{-}. With Solaris @command{ksh}, it is possible to use @code{print +-r --} for this role instead. + +@xref{echo, , Limitations of Shell Builtins} for a discussion of +portable alternatives to both @command{printf} and @command{echo}. + + +@item @command{pwd} +@c ---------------- +@prindex @command{pwd} +With modern shells, plain @command{pwd} outputs a ``logical'' +directory name, some of whose components may be symbolic links. These +directory names are in contrast to ``physical'' directory names, whose +components are all directories. + +Posix 1003.1-2001 requires that @command{pwd} must support +the @option{-L} (``logical'') and @option{-P} (``physical'') options, +with @option{-L} being the default. However, traditional shells do +not support these options, and their @command{pwd} command has the +@option{-P} behavior. + +Portable scripts should assume neither option is supported, and should +assume neither behavior is the default. Also, on many hosts +@samp{/bin/pwd} is equivalent to @samp{pwd -P}, but Posix +does not require this behavior and portable scripts should not rely on +it. + +Typically it's best to use plain @command{pwd}. On modern hosts this +outputs logical directory names, which have the following advantages: + +@itemize @bullet +@item +Logical names are what the user specified. +@item +Physical names may not be portable from one installation +host to another due to network file system gymnastics. +@item +On modern hosts @samp{pwd -P} may fail due to lack of permissions to +some parent directory, but plain @command{pwd} cannot fail for this +reason. +@end itemize + +Also please see the discussion of the @command{cd} command. + + +@item @command{read} +@c ----------------- +@prindex @command{read} +No options are portable, not even support @option{-r} (Solaris +@command{/bin/sh} for example). Tru64/OSF 5.1 @command{sh} treats +@command{read} as a special built-in, so it may exit if input is +redirected from a non-existent or unreadable file. + + +@anchor{set} +@item @command{set} +@c ---------------- +@prindex @command{set} +With the FreeBSD 6.0 shell, the @command{set} command (without +any options) does not sort its output. + +The @command{set} builtin faces the usual problem with arguments +starting with a +dash. Modern shells such as Bash or Zsh understand @option{--} to specify +the end of the options (any argument after @option{--} is a parameter, +even @samp{-x} for instance), but many traditional shells (e.g., Solaris +10 @command{/bin/sh}) simply stop option +processing as soon as a non-option argument is found. Therefore, use +@samp{dummy} or simply @samp{x} to end the option processing, and use +@command{shift} to pop it out: + +@example +set x $my_list; shift +@end example + +Avoid @samp{set -}, e.g., @samp{set - $my_list}. Posix no +longer requires support for this command, and in traditional shells +@samp{set - $my_list} resets the @option{-v} and @option{-x} options, which +makes scripts harder to debug. + +Some nonstandard shells do not recognize more than one option +(e.g., @samp{set -e -x} assigns @samp{-x} to the command line). It is +better to combine them: + +@example +set -ex +@end example + +@cindex @command{set -e} +The option @option{-e} has historically been underspecified, with enough +ambiguities to cause numerous differences across various shell +implementations; see for example +@uref{http://www.in-ulm.de/@/~mascheck/@/various/@/set-e/, this overview}, +or @uref{http://www.austingroupbugs.net/@/view.php?id=52, this link}, +documenting a change to Posix 2008 to match @command{ksh88} behavior. +Note that mixing @code{set -e} and shell functions is asking for surprises: + +@example +set -e +doit() +@{ + rm file + echo one +@} +doit || echo two +@end example + +@noindent +According to the recommendation, @samp{one} should always be output +regardless of whether the @command{rm} failed, because it occurs within +the body of the shell function @samp{doit} invoked on the left side of +@samp{||}, where the effects of @samp{set -e} are not enforced. +Likewise, @samp{two} should never be printed, since the failure of +@command{rm} does not abort the function, such that the status of +@samp{doit} is 0. + +The BSD shell has had several problems with the @option{-e} +option. Older versions of the BSD +shell (circa 1990) mishandled @samp{&&}, @samp{||}, @samp{if}, and +@samp{case} when @option{-e} was in effect, causing the shell to exit +unexpectedly in some cases. This was particularly a problem with +makefiles, and led to circumlocutions like @samp{sh -c 'test -f file || +touch file'}, where the seemingly-unnecessary @samp{sh -c '@dots{}'} +wrapper works around the bug (@pxref{Failure in Make Rules}). + +Even relatively-recent versions of the BSD shell (e.g., OpenBSD 3.4) +wrongly exit with @option{-e} if the last command within a compound +statement fails and is guarded by an @samp{&&} only. For example: + +@example +#! /bin/sh +set -e +foo='' +test -n "$foo" && exit 1 +echo one +if :; then + test -n "$foo" && exit 1 + echo two + test -n "$foo" && exit 1 +fi +echo three +@end example + +@noindent +does not print @samp{three}. One workaround is to change the last +instance of @samp{test -n "$foo" && exit 1} to be @samp{if test -n +"$foo"; then exit 1; fi} instead. Another possibility is to warn BSD +users not to use @samp{sh -e}. + +When @samp{set -e} is in effect, a failed command substitution in +Solaris @command{/bin/sh} cannot be ignored, even with @samp{||}. + +@example +$ @kbd{/bin/sh -c 'set -e; foo=`false` || echo foo; echo bar'} +$ @kbd{bash -c 'set -e; foo=`false` || echo foo; echo bar'} +foo +bar +@end example + +@noindent +Moreover, a command substitution, successful or not, causes this shell to +exit from a failing outer command even in presence of an @samp{&&} list: + +@example +$ @kbd{bash -c 'set -e; false `true` && echo notreached; echo ok'} +ok +$ @kbd{sh -c 'set -e; false `true` && echo notreached; echo ok'} +$ +@end example + +Portable scripts should not use @samp{set -e} if @command{trap} is used +to install an exit handler. This is because Tru64/OSF 5.1 @command{sh} +sometimes enters the trap handler with the exit status of the command +prior to the one that triggered the errexit handler: + +@example +$ @kbd{sh -ec 'trap '\''echo $?'\'' 0; false'} +0 +$ @kbd{sh -c 'set -e; trap '\''echo $?'\'' 0; false'} +1 +@end example + +@noindent +Thus, when writing a script in M4sh, rather than trying to rely on +@samp{set -e}, it is better to append @samp{|| AS_EXIT} to any +statement where it is desirable to abort on failure. + +@cindex @command{set -b} +@cindex @command{set -m} +Job control is not provided by all shells, so the use of @samp{set -m} +or @samp{set -b} must be done with care. When using @command{zsh} in +native mode, asynchronous notification (@samp{set -b}) is enabled by +default, and using @samp{emulate sh} to switch to Posix mode does not +clear this setting (although asynchronous notification has no impact +unless job monitoring is also enabled). Also, @command{zsh} 4.3.10 and +earlier have a bug where job control can be manipulated in interactive +shells, but not in subshells or scripts. Furthermore, some shells, like +@command{pdksh}, fail to treat subshells as interactive, even though the +parent shell was. + +@example +$ @kbd{echo $ZSH_VERSION} +4.3.10 +$ @kbd{set -m; echo $?} +0 +$ @kbd{zsh -c 'set -m; echo $?'} +set: can't change option: -m +$ @kbd{(set -m); echo $?} +set: can't change option: -m +1 +$ @kbd{pdksh -ci 'echo $-; (echo $-)'} +cim +c +@end example + +@cindex @command{set -n} +Use of @command{set -n} (typically via @command{sh -n script}) to +validate a script is not foolproof. Modern @command{ksh93} tries to be +helpful by informing you about better syntax, but switching the script +to use the suggested syntax in order to silence the warnings would +render the script no longer portable to older shells: + +@example +$ @kbd{ksh -nc '``'} +ksh: warning: line 1: `...` obsolete, use $(...) +0 +@end example + +Furthermore, on ancient hosts, such as SunOS 4, @command{sh -n} could go +into an infinite loop; even with that bug fixed, Solaris 8 +@command{/bin/sh} takes extremely long to parse large scripts. Autoconf +itself uses @command{sh -n} within its testsuite to check that correct +scripts were generated, but only after first probing for other shell +features (such as @code{test -n "$@{BASH_VERSION+set@}"}) that indicate +a reasonably fast and working implementation. + +@item @command{shift} +@c ------------------ +@prindex @command{shift} +Not only is @command{shift}ing a bad idea when there is nothing left to +shift, but in addition it is not portable: the shell of MIPS +RISC/OS 4.52 refuses to do it. + +Don't use @samp{shift 2} etc.; while it in the SVR1 shell (1983), +it is also absent in many pre-Posix shells. + + +@item @command{source} +@c ------------------- +@prindex @command{source} +This command is not portable, as Posix does not require it; use +@command{.} instead. + + +@item @command{test} +@c ----------------- +@prindex @command{test} +The @code{test} program is the way to perform many file and string +tests. It is often invoked by the alternate name @samp{[}, but using +that name in Autoconf code is asking for trouble since it is an M4 quote +character. + +The @option{-a}, @option{-o}, @samp{(}, and @samp{)} operands are not +present in all implementations, and have been marked obsolete by Posix +2008. This is because there are inherent ambiguities in using them. +For example, @samp{test "$1" -a "$2"} looks like a binary operator to +check whether two strings are both non-empty, but if @samp{$1} is the +literal @samp{!}, then some implementations of @command{test} treat it +as a negation of the unary operator @option{-a}. + +Thus, portable uses of @command{test} should never have more than four +arguments, and scripts should use shell constructs like @samp{&&} and +@samp{||} instead. If you combine @samp{&&} and @samp{||} in the same +statement, keep in mind that they have equal precedence, so it is often +better to parenthesize even when this is redundant. For example: + +@smallexample +# Not portable: +test "X$a" = "X$b" -a \ + '(' "X$c" != "X$d" -o "X$e" = "X$f" ')' + +# Portable: +test "X$a" = "X$b" && + @{ test "X$c" != "X$d" || test "X$e" = "X$f"; @} +@end smallexample + +@command{test} does not process options like most other commands do; for +example, it does not recognize the @option{--} argument as marking the +end of options. + +It is safe to use @samp{!} as a @command{test} operator. For example, +@samp{if test ! -d foo; @dots{}} is portable even though @samp{if ! test +-d foo; @dots{}} is not. + + +@item @command{test} (files) +@c ------------------------- +To enable @command{configure} scripts to support cross-compilation, they +shouldn't do anything that tests features of the build system instead of +the host system. But occasionally you may find it necessary to check +whether some arbitrary file exists. To do so, use @samp{test -f}, +@samp{test -r}, or @samp{test -x}. Do not use @samp{test -e}, because +Solaris 10 @command{/bin/sh} +lacks it. To test for symbolic links on systems that have them, use +@samp{test -h} rather than @samp{test -L}; either form conforms to +Posix 1003.1-2001, but older shells like Solaris 8 +@code{/bin/sh} support only @option{-h}. + +For historical reasons, Posix reluctantly allows implementations of +@samp{test -x} that will succeed for the root user, even if no execute +permissions are present. Furthermore, shells do not all agree on +whether Access Control Lists should affect @samp{test -r}, @samp{test +-w}, and @samp{test -x}; some shells base test results strictly on the +current user id compared to file owner and mode, as if by +@code{stat(2)}; while other shells base test results on whether the +current user has the given right, even if that right is only granted by +an ACL, as if by @code{faccessat(2)}. Furthermore, there is a classic +time of check to time of use race between any use of @command{test} +followed by operating on the just-checked file. Therefore, it is a good +idea to write scripts that actually attempt an operation, and are +prepared for the resulting failure if permission is denied, rather than +trying to avoid an operation based solely on whether @command{test} +guessed that it might not be permitted. + +@item @command{test} (strings) +@c --------------------------- +Posix says that @samp{test "@var{string}"} succeeds if @var{string} is +not null, but this usage is not portable to traditional platforms like +Solaris 10 @command{/bin/sh}, which mishandle strings like @samp{!} and +@samp{-n}. + +Posix also says that @samp{test ! "@var{string}"}, +@samp{test -n "@var{string}"} and +@samp{test -z "@var{string}"} work with any string, but many +shells (such as Solaris, AIX 3.2, UNICOS 10.0.0.6, +Digital Unix 4, etc.)@: get confused if +@var{string} looks like an operator: + +@example +$ @kbd{test -n =} +test: argument expected +$ @kbd{test ! -n} +test: argument expected +$ @kbd{test -z ")"; echo $?} +0 +@end example + +Similarly, Posix says that both @samp{test "@var{string1}" = "@var{string2"}} +and @samp{test "@var{string1}" != "@var{string2"}} work for any pairs of +strings, but in practice this is not true for troublesome strings that +look like operators or parentheses, or that begin with @samp{-}. + +It is best to protect such strings with a leading @samp{X}, e.g., +@samp{test "X@var{string}" != X} rather than @samp{test -n +"@var{string}"} or @samp{test ! "@var{string}"}. + +It is common to find variations of the following idiom: + +@example +test -n "`echo $ac_feature | sed 's/[-a-zA-Z0-9_]//g'`" && + @var{action} +@end example + +@noindent +to take an action when a token matches a given pattern. Such constructs +should be avoided by using: + +@example +case $ac_feature in + *[!-a-zA-Z0-9_]*) @var{action};; +esac +@end example + +If the pattern is a complicated regular expression that cannot be +expressed as a shell pattern, use something like this instead: + +@example +expr "X$ac_feature" : 'X.*[^-a-zA-Z0-9_]' >/dev/null && + @var{action} +@end example + +@samp{expr "X@var{foo}" : "X@var{bar}"} is more robust than @samp{echo +"X@var{foo}" | grep "^X@var{bar}"}, because it avoids problems when +@samp{@var{foo}} contains backslashes. + + +@anchor{trap} +@item @command{trap} +@c ----------------- +@prindex @command{trap} +It is safe to trap at least the signals 1, 2, 13, and 15. You can also +trap 0, i.e., have the @command{trap} run when the script ends (either via an +explicit @command{exit}, or the end of the script). The trap for 0 should be +installed outside of a shell function, or AIX 5.3 @command{/bin/sh} +will invoke the trap at the end of this function. + +Posix says that @samp{trap - 1 2 13 15} resets the traps for the +specified signals to their default values, but many common shells (e.g., +Solaris @command{/bin/sh}) misinterpret this and attempt to execute a +``command'' named @command{-} when the specified conditions arise. +Posix 2008 also added a requirement to support @samp{trap 1 2 13 15} to +reset traps, as this is supported by a larger set of shells, but there +are still shells like @command{dash} that mistakenly try to execute +@command{1} instead of resetting the traps. Therefore, there is no +portable workaround, except for @samp{trap - 0}, for which +@samp{trap '' 0} is a portable substitute. + +Although Posix is not absolutely clear on this point, it is widely +admitted that when entering the trap @samp{$?} should be set to the exit +status of the last command run before the trap. The ambiguity can be +summarized as: ``when the trap is launched by an @command{exit}, what is +the @emph{last} command run: that before @command{exit}, or +@command{exit} itself?'' + +Bash considers @command{exit} to be the last command, while Zsh and +Solaris @command{/bin/sh} consider that when the trap is run it is +@emph{still} in the @command{exit}, hence it is the previous exit status +that the trap receives: + +@example +$ @kbd{cat trap.sh} +trap 'echo $?' 0 +(exit 42); exit 0 +$ @kbd{zsh trap.sh} +42 +$ @kbd{bash trap.sh} +0 +@end example + +The portable solution is then simple: when you want to @samp{exit 42}, +run @samp{(exit 42); exit 42}, the first @command{exit} being used to +set the exit status to 42 for Zsh, and the second to trigger the trap +and pass 42 as exit status for Bash. In M4sh, this is covered by using +@code{AS_EXIT}. + +The shell in FreeBSD 4.0 has the following bug: @samp{$?} is +reset to 0 by empty lines if the code is inside @command{trap}. + +@example +$ @kbd{trap 'false} + +echo $?' 0 +$ @kbd{exit} +0 +@end example + +@noindent +Fortunately, this bug only affects @command{trap}. + +Several shells fail to execute an exit trap that is defined inside a +subshell, when the last command of that subshell is not a builtin. A +workaround is to use @samp{exit $?} as the shell builtin. + +@example +$ @kbd{bash -c '(trap "echo hi" 0; /bin/true)'} +hi +$ @kbd{/bin/sh -c '(trap "echo hi" 0; /bin/true)'} +$ @kbd{/bin/sh -c '(trap "echo hi" 0; /bin/true; exit $?)'} +hi +@end example + +@noindent +Likewise, older implementations of @command{bash} failed to preserve +@samp{$?} across an exit trap consisting of a single cleanup command. + +@example +$ @kbd{bash -c 'trap "/bin/true" 0; exit 2'; echo $?} +2 +$ @kbd{bash-2.05b -c 'trap "/bin/true" 0; exit 2'; echo $?} +0 +$ @kbd{bash-2.05b -c 'trap ":; /bin/true" 0; exit 2'; echo $?} +2 +@end example + +@item @command{true} +@c ----------------- +@prindex @command{true} +@c Info cannot handle `:' in index entries. +@c @prindex @command{:} +Don't worry: as far as we know @command{true} is portable. +Nevertheless, it's not always a builtin (e.g., Bash 1.x), and the +portable shell community tends to prefer using @command{:}. This has a +funny side effect: when asked whether @command{false} is more portable +than @command{true} Alexandre Oliva answered: + +@quotation +In a sense, yes, because if it doesn't exist, the shell will produce an +exit status of failure, which is correct for @command{false}, but not +for @command{true}. +@end quotation + +Remember that even though @samp{:} ignores its arguments, it still takes +time to compute those arguments. It is a good idea to use double quotes +around any arguments to @samp{:} to avoid time spent in field splitting +and file name expansion. + + +@anchor{unset} +@item @command{unset} +@c ------------------ +@prindex @command{unset} +In some nonconforming shells (e.g., Solaris 10 @command{/bin/ksh} and +@command{/usr/xpg4/bin/sh}, NetBSD 5.99.43 sh, or Bash 2.05a), +@code{unset FOO} fails when @code{FOO} is not set. This can interfere +with @code{set -e} operation. You can use + +@smallexample +FOO=; unset FOO +@end smallexample + +@noindent +if you are not sure that @code{FOO} is set. + +A few ancient shells lack @command{unset} entirely. For some variables +such as @code{PS1}, you can use a neutralizing value instead: + +@smallexample +PS1='$ ' +@end smallexample + +Usually, shells that do not support @command{unset} need less effort to +make the environment sane, so for example is not a problem if you cannot +unset @command{CDPATH} on those shells. However, Bash 2.01 mishandles +@code{unset MAIL} and @code{unset MAILPATH} in some cases and dumps core. +So, you should do something like + +@smallexample +( (unset MAIL) || exit 1) >/dev/null 2>&1 && unset MAIL || : +@end smallexample + +@noindent +@xref{Special Shell Variables}, for some neutralizing values. Also, see +@ref{export, , Limitations of Builtins}, for +the case of environment variables. + +@item @command{wait} +@c ----------------- +@prindex @command{wait} +The exit status of @command{wait} is not always reliable. +@end table + +@node Limitations of Usual Tools +@section Limitations of Usual Tools +@cindex Limitations of usual tools + +The small set of tools you can expect to find on any machine can still +include some limitations you should be aware of. + +@comment Between this list and the list of builtins above, we should +@comment mention all the tools in GNU Coding Standards ``Utilities in +@comment Makefiles''. + +@c This table includes things like `@command{expr} (|)', so we can't +@c use @table @command. +@table @asis +@anchor{awk} +@item @command{awk} +@c ---------------- +@prindex @command{awk} +Don't leave white space before the opening parenthesis in a user function call. +Posix does not allow this and GNU Awk rejects it: + +@example +$ @kbd{gawk 'function die () @{ print "Aaaaarg!" @} + BEGIN @{ die () @}'} +gawk: cmd. line:2: BEGIN @{ die () @} +gawk: cmd. line:2: ^ parse error +$ @kbd{gawk 'function die () @{ print "Aaaaarg!" @} + BEGIN @{ die() @}'} +Aaaaarg! +@end example + +Posix says that if a program contains only @samp{BEGIN} actions, and +contains no instances of @code{getline}, then the program merely +executes the actions without reading input. However, traditional Awk +implementations (such as Solaris 10 @command{awk}) read and discard +input in this case. Portable scripts can redirect input from +@file{/dev/null} to work around the problem. For example: + +@example +awk 'BEGIN @{print "hello world"@}' </dev/null +@end example + +Posix says that in an @samp{END} action, @samp{$NF} (and presumably, +@samp{$1}) retain their value from the last record read, if no +intervening @samp{getline} occurred. However, some implementations +(such as Solaris 10 @samp{/usr/bin/awk}, @samp{nawk}, or Darwin +@samp{awk}) reset these variables. A workaround is to use an +intermediate variable prior to the @samp{END} block. For example: + +@example +$ @kbd{cat end.awk} +@{ tmp = $1 @} +END @{ print "a", $1, $NF, "b", tmp @} +$ @kbd{echo 1 | awk -f end.awk} +a b 1 +$ @kbd{echo 1 | gawk -f end.awk} +a 1 1 b 1 +@end example + +If you want your program to be deterministic, don't depend on @code{for} +on arrays: + +@example +$ @kbd{cat for.awk} +END @{ + arr["foo"] = 1 + arr["bar"] = 1 + for (i in arr) + print i +@} +$ @kbd{gawk -f for.awk </dev/null} +foo +bar +$ @kbd{nawk -f for.awk </dev/null} +bar +foo +@end example + +Some Awk implementations, such as HP-UX 11.0's native one, +mishandle anchors: + +@example +$ @kbd{echo xfoo | $AWK '/foo|^bar/ @{ print @}'} +$ @kbd{echo bar | $AWK '/foo|^bar/ @{ print @}'} +bar +$ @kbd{echo xfoo | $AWK '/^bar|foo/ @{ print @}'} +xfoo +$ @kbd{echo bar | $AWK '/^bar|foo/ @{ print @}'} +bar +@end example + +@noindent +Either do not depend on such patterns (i.e., use @samp{/^(.*foo|bar)/}, +or use a simple test to reject such implementations. + +On @samp{ia64-hp-hpux11.23}, Awk mishandles @code{printf} conversions +after @code{%u}: + +@example +$ @kbd{awk 'BEGIN @{ printf "%u %d\n", 0, -1 @}'} +0 0 +@end example + +AIX version 5.2 has an arbitrary limit of 399 on the +length of regular expressions and literal strings in an Awk program. + +Traditional Awk implementations derived from Unix version 7, such as +Solaris @command{/bin/awk}, have many limitations and do not +conform to Posix. Nowadays @code{AC_PROG_AWK} (@pxref{Particular +Programs}) finds you an Awk that doesn't have these problems, but if +for some reason you prefer not to use @code{AC_PROG_AWK} you may need to +address them. For more detailed descriptions, see @ref{Language +History, , @command{awk} language history, gawk, GNU Awk User's Guide}. + +Traditional Awk does not support multidimensional arrays or user-defined +functions. + +Traditional Awk does not support the @option{-v} option. You can use +assignments after the program instead, e.g., @code{$AWK '@{print v +$1@}' v=x}; however, don't forget that such assignments are not +evaluated until they are encountered (e.g., after any @code{BEGIN} +action). + +Traditional Awk does not support the keywords @code{delete} or @code{do}. + +Traditional Awk does not support the expressions +@code{@var{a}?@var{b}:@var{c}}, @code{!@var{a}}, @code{@var{a}^@var{b}}, +or @code{@var{a}^=@var{b}}. + +Traditional Awk does not support the predefined @code{CONVFMT} or +@code{ENVIRON} variables. + +Traditional Awk supports only the predefined functions @code{exp}, @code{index}, +@code{int}, @code{length}, @code{log}, @code{split}, @code{sprintf}, +@code{sqrt}, and @code{substr}. + +Traditional Awk @code{getline} is not at all compatible with Posix; +avoid it. + +Traditional Awk has @code{for (i in a) @dots{}} but no other uses of the +@code{in} keyword. For example, it lacks @code{if (i in a) @dots{}}. + +In code portable to both traditional and modern Awk, @code{FS} must be a +string containing just one ordinary character, and similarly for the +field-separator argument to @code{split}. + +Traditional Awk has a limit of 99 fields in a record. Since some Awk +implementations, like Tru64's, split the input even if you don't refer +to any field in the script, to circumvent this problem, set @samp{FS} +to an unusual character and use @code{split}. + +Traditional Awk has a limit of at most 99 bytes in a number formatted by +@code{OFMT}; for example, @code{OFMT="%.300e"; print 0.1;} typically +dumps core. + +The original version of Awk had a limit of at most 99 bytes per +@code{split} field, 99 bytes per @code{substr} substring, and 99 bytes +per run of non-special characters in a @code{printf} format, but these +bugs have been fixed on all practical hosts that we know of. + +HP-UX 11.00 and IRIX 6.5 Awk require that input files have a line length +of at most 3070 bytes. + +@item @command{basename} +@c --------------------- +@prindex @command{basename} +Not all hosts have a working @command{basename}. +You can use @command{expr} instead. + +@c AS_BASENAME is to be replaced by a better API. +@ignore +Not all hosts have a working @command{basename}, and you should instead +use @code{AS_BASENAME} (@pxref{Programming in M4sh}), followed by +@command{expr} if you need to strip a suffix. For example: + +@example +a=`basename "$aname"` # This is not portable. +a=`AS_BASENAME(["$aname"])` # This is more portable. + +# This is not portable. +c=`basename "$cname" .c` + +# This is more portable. +c=`AS_BASENAME(["$cname"])` +case $c in +?*.c) c=`expr "X$c" : 'X\(.*\)\.c'`;; +esac +@end example +@end ignore + + +@item @command{cat} +@c ---------------- +@prindex @command{cat} +Don't rely on any option. + + +@item @command{cc} +@c --------------- +@prindex @command{cc} +The command @samp{cc -c foo.c} traditionally produces an object file +named @file{foo.o}. Most compilers allow @option{-c} to be combined +with @option{-o} to specify a different object file name, but +Posix does not require this combination and a few compilers +lack support for it. @xref{C Compiler}, for how GNU Make +tests for this feature with @code{AC_PROG_CC_C_O}. + +When a compilation such as @samp{cc -o foo foo.c} fails, some compilers +(such as CDS on Reliant Unix) leave a @file{foo.o}. + +HP-UX @command{cc} doesn't accept @file{.S} files to preprocess and +assemble. @samp{cc -c foo.S} appears to succeed, but in fact does +nothing. + +The default executable, produced by @samp{cc foo.c}, can be + +@itemize +@item @file{a.out} --- usual Posix convention. +@item @file{b.out} --- i960 compilers (including @command{gcc}). +@item @file{a.exe} --- DJGPP port of @command{gcc}. +@item @file{a_out.exe} --- GNV @command{cc} wrapper for DEC C on OpenVMS. +@item @file{foo.exe} --- various MS-DOS compilers. +@end itemize + +The C compiler's traditional name is @command{cc}, but other names like +@command{gcc} are common. Posix 1003.1-2001 specifies the +name @command{c99}, but older Posix editions specified +@command{c89} and anyway these standard names are rarely used in +practice. Typically the C compiler is invoked from makefiles that use +@samp{$(CC)}, so the value of the @samp{CC} make variable selects the +compiler name. + +@item @command{chgrp} +@itemx @command{chown} +@c ------------------- +@prindex @command{chgrp} +@prindex @command{chown} +It is not portable to change a file's group to a group that the owner +does not belong to. + +@item @command{chmod} +@c ------------------ +@prindex @command{chmod} +Avoid usages like @samp{chmod -w file}; use @samp{chmod a-w file} +instead, for two reasons. First, plain @option{-w} does not necessarily +make the file unwritable, since it does not affect mode bits that +correspond to bits in the file mode creation mask. Second, +Posix says that the @option{-w} might be interpreted as an +implementation-specific option, not as a mode; Posix suggests +using @samp{chmod -- -w file} to avoid this confusion, but unfortunately +@samp{--} does not work on some older hosts. + + +@item @command{cmp} +@c ---------------- +@prindex @command{cmp} +@command{cmp} performs a raw data comparison of two files, while +@command{diff} compares two text files. Therefore, if you might compare +DOS files, even if only checking whether two files are different, use +@command{diff} to avoid spurious differences due to differences of +newline encoding. + + +@item @command{cp} +@c --------------- +@prindex @command{cp} +Avoid the @option{-r} option, since Posix 1003.1-2004 marks it as +obsolescent and its behavior on special files is implementation-defined. +Use @option{-R} instead. On GNU hosts the two options +are equivalent, but on Solaris hosts (for example) @code{cp -r} +reads from pipes instead of replicating them. AIX 5.3 @code{cp -R} may +corrupt its own memory with some directory hierarchies and error out or +dump core: + +@example +@kbd{mkdir -p 12345678/12345678/12345678/12345678} +@kbd{touch 12345678/12345678/x} +@kbd{cp -R 12345678 t} +cp: 0653-440 12345678/12345678/: name too long. +@end example + +Some @command{cp} implementations (e.g., BSD/OS 4.2) do not allow +trailing slashes at the end of nonexistent destination directories. To +avoid this problem, omit the trailing slashes. For example, use +@samp{cp -R source /tmp/newdir} rather than @samp{cp -R source +/tmp/newdir/} if @file{/tmp/newdir} does not exist. + +@c This is thanks to Ian. +The ancient SunOS 4 @command{cp} does not support @option{-f}, although +its @command{mv} does. + +@cindex timestamp resolution +Traditionally, file timestamps had 1-second resolution, and @samp{cp +-p} copied the timestamps exactly. However, many modern file systems +have timestamps with 1-nanosecond resolution. Unfortunately, some older +@samp{cp -p} implementations truncate timestamps when copying files, +which can cause the destination file to appear to be older than the +source. The exact amount of truncation depends on the resolution of +the system calls that @command{cp} uses. Traditionally this was +@code{utime}, which has 1-second resolution. Less-ancient @command{cp} +implementations such as GNU Core Utilities 5.0.91 (2003) use +@code{utimes}, which has 1-microsecond resolution. Modern +implementations such as GNU Core Utilities 6.12 (2008) can set timestamps to +the full nanosecond resolution, using the modern system calls +@code{futimens} and @code{utimensat} when they are available. As of +2011, though, many platforms do not yet fully support these new system +calls. + +Bob Proulx notes that @samp{cp -p} always @emph{tries} to copy +ownerships. But whether it actually does copy ownerships or not is a +system dependent policy decision implemented by the kernel. If the +kernel allows it then it happens. If the kernel does not allow it then +it does not happen. It is not something @command{cp} itself has control +over. + +In Unix System V any user can chown files to any other user, and System +V also has a non-sticky @file{/tmp}. That probably derives from the +heritage of System V in a business environment without hostile users. +BSD changed this +to be a more secure model where only root can @command{chown} files and +a sticky @file{/tmp} is used. That undoubtedly derives from the heritage +of BSD in a campus environment. + +GNU/Linux and Solaris by default follow BSD, but +can be configured to allow a System V style @command{chown}. On the +other hand, HP-UX follows System V, but can +be configured to use the modern security model and disallow +@command{chown}. Since it is an administrator-configurable parameter +you can't use the name of the kernel as an indicator of the behavior. + + + +@item @command{date} +@c ----------------- +@prindex @command{date} +Some versions of @command{date} do not recognize special @samp{%} directives, +and unfortunately, instead of complaining, they just pass them through, +and exit with success: + +@example +$ @kbd{uname -a} +OSF1 medusa.sis.pasteur.fr V5.1 732 alpha +$ @kbd{date "+%s"} +%s +@end example + + +@item @command{diff} +@c ----------------- +@prindex @command{diff} +Option @option{-u} is nonportable. + +Some implementations, such as Tru64's, fail when comparing to +@file{/dev/null}. Use an empty file instead. + + +@item @command{dirname} +@c -------------------- +@prindex @command{dirname} +Not all hosts have a working @command{dirname}, and you should instead +use @code{AS_DIRNAME} (@pxref{Programming in M4sh}). For example: + +@example +dir=`dirname "$file"` # This is not portable. +dir=`AS_DIRNAME(["$file"])` # This is more portable. +@end example + + +@item @command{egrep} +@c ------------------ +@prindex @command{egrep} +Posix 1003.1-2001 no longer requires @command{egrep}, +but many hosts do not yet support the Posix +replacement @code{grep -E}. Also, some traditional implementations do +not work on long input lines. To work around these problems, invoke +@code{AC_PROG_EGREP} and then use @code{$EGREP}. + +Portable extended regular expressions should use @samp{\} only to escape +characters in the string @samp{$()*+.?[\^@{|}. For example, @samp{\@}} +is not portable, even though it typically matches @samp{@}}. + +The empty alternative is not portable. Use @samp{?} instead. For +instance with Digital Unix v5.0: + +@example +> printf "foo\n|foo\n" | $EGREP '^(|foo|bar)$' +|foo +> printf "bar\nbar|\n" | $EGREP '^(foo|bar|)$' +bar| +> printf "foo\nfoo|\n|bar\nbar\n" | $EGREP '^(foo||bar)$' +foo +|bar +@end example + +@command{$EGREP} also suffers the limitations of @command{grep} +(@pxref{grep, , Limitations of Usual Tools}). + +@item @command{expr} +@c ----------------- +@prindex @command{expr} +Not all implementations obey the Posix rule that @samp{--} separates +options from arguments; likewise, not all implementations provide the +extension to Posix that the first argument can be treated as part of a +valid expression rather than an invalid option if it begins with +@samp{-}. When performing arithmetic, use @samp{expr 0 + $var} if +@samp{$var} might be a negative number, to keep @command{expr} from +interpreting it as an option. + +No @command{expr} keyword starts with @samp{X}, so use @samp{expr +X"@var{word}" : 'X@var{regex}'} to keep @command{expr} from +misinterpreting @var{word}. + +Don't use @code{length}, @code{substr}, @code{match} and @code{index}. + +@item @command{expr} (@samp{|}) +@prindex @command{expr} (@samp{|}) +You can use @samp{|}. Although Posix does require that @samp{expr +''} return the empty string, it does not specify the result when you +@samp{|} together the empty string (or zero) with the empty string. For +example: + +@example +expr '' \| '' +@end example + +Posix 1003.2-1992 returns the empty string +for this case, but traditional Unix returns @samp{0} (Solaris is +one such example). In Posix 1003.1-2001, the specification was +changed to match traditional Unix's behavior (which is +bizarre, but it's too late to fix this). Please note that the same +problem does arise when the empty string results from a computation, +as in: + +@example +expr bar : foo \| foo : bar +@end example + +@noindent +Avoid this portability problem by avoiding the empty string. + + +@item @command{expr} (@samp{:}) +@c ---------------------------- +@prindex @command{expr} +Portable @command{expr} regular expressions should use @samp{\} to +escape only characters in the string @samp{$()*.0123456789[\^n@{@}}. +For example, alternation, @samp{\|}, is common but Posix does not +require its support, so it should be avoided in portable scripts. +Similarly, @samp{\+} and @samp{\?} should be avoided. + +Portable @command{expr} regular expressions should not begin with +@samp{^}. Patterns are automatically anchored so leading @samp{^} is +not needed anyway. + +On the other hand, the behavior of the @samp{$} anchor is not portable +on multi-line strings. Posix is ambiguous whether the anchor applies to +each line, as was done in older versions of the GNU Core Utilities, or +whether it applies only to the end of the overall string, as in +Coreutils 6.0 and most other implementations. + +@example +$ @kbd{baz='foo} +> @kbd{bar'} +$ @kbd{expr "X$baz" : 'X\(foo\)$'} + +$ @kbd{expr-5.97 "X$baz" : 'X\(foo\)$'} +foo +@end example + +The Posix standard is ambiguous as to whether +@samp{expr 'a' : '\(b\)'} outputs @samp{0} or the empty string. +In practice, it outputs the empty string on most platforms, but portable +scripts should not assume this. For instance, the QNX 4.25 native +@command{expr} returns @samp{0}. + +One might think that a way to get a uniform behavior would be to use +the empty string as a default value: + +@example +expr a : '\(b\)' \| '' +@end example + +@noindent +Unfortunately this behaves exactly as the original expression; see the +@command{expr} (@samp{|}) entry for more information. + +Some ancient @command{expr} implementations (e.g., SunOS 4 @command{expr} and +Solaris 8 @command{/usr/ucb/expr}) have a silly length limit that causes +@command{expr} to fail if the matched substring is longer than 120 +bytes. In this case, you might want to fall back on @samp{echo|sed} if +@command{expr} fails. Nowadays this is of practical importance only for +the rare installer who mistakenly puts @file{/usr/ucb} before +@file{/usr/bin} in @env{PATH}. + +On Mac OS X 10.4, @command{expr} mishandles the pattern @samp{[^-]} in +some cases. For example, the command +@example +expr Xpowerpc-apple-darwin8.1.0 : 'X[^-]*-[^-]*-\(.*\)' +@end example + +@noindent +outputs @samp{apple-darwin8.1.0} rather than the correct @samp{darwin8.1.0}. +This particular case can be worked around by substituting @samp{[^--]} +for @samp{[^-]}. + +Don't leave, there is some more! + +The QNX 4.25 @command{expr}, in addition of preferring @samp{0} to +the empty string, has a funny behavior in its exit status: it's always 1 +when parentheses are used! + +@example +$ @kbd{val=`expr 'a' : 'a'`; echo "$?: $val"} +0: 1 +$ @kbd{val=`expr 'a' : 'b'`; echo "$?: $val"} +1: 0 + +$ @kbd{val=`expr 'a' : '\(a\)'`; echo "?: $val"} +1: a +$ @kbd{val=`expr 'a' : '\(b\)'`; echo "?: $val"} +1: 0 +@end example + +@noindent +In practice this can be a big problem if you are ready to catch failures +of @command{expr} programs with some other method (such as using +@command{sed}), since you may get twice the result. For instance + +@example +$ @kbd{expr 'a' : '\(a\)' || echo 'a' | sed 's/^\(a\)$/\1/'} +@end example + +@noindent +outputs @samp{a} on most hosts, but @samp{aa} on QNX 4.25. A +simple workaround consists of testing @command{expr} and using a variable +set to @command{expr} or to @command{false} according to the result. + +Tru64 @command{expr} incorrectly treats the result as a number, if it +can be interpreted that way: + +@example +$ @kbd{expr 00001 : '.*\(...\)'} +1 +@end example + +On HP-UX 11, @command{expr} only supports a single +sub-expression. + +@example +$ @kbd{expr 'Xfoo' : 'X\(f\(oo\)*\)$'} +expr: More than one '\(' was used. +@end example + + +@item @command{fgrep} +@c ------------------ +@prindex @command{fgrep} +Posix 1003.1-2001 no longer requires @command{fgrep}, +but many hosts do not yet support the Posix +replacement @code{grep -F}. Also, some traditional implementations do +not work on long input lines. To work around these problems, invoke +@code{AC_PROG_FGREP} and then use @code{$FGREP}. + +Tru64/OSF 5.1 @command{fgrep} does not match an empty pattern. + + +@item @command{find} +@c ----------------- +@prindex @command{find} +The option @option{-maxdepth} seems to be GNU specific. +Tru64 v5.1, NetBSD 1.5 and Solaris @command{find} +commands do not understand it. + +The replacement of @samp{@{@}} is guaranteed only if the argument is +exactly @emph{@{@}}, not if it's only a part of an argument. For +instance on DU, and HP-UX 10.20 and HP-UX 11: + +@example +$ @kbd{touch foo} +$ @kbd{find . -name foo -exec echo "@{@}-@{@}" \;} +@{@}-@{@} +@end example + +@noindent +while GNU @command{find} reports @samp{./foo-./foo}. + + +@anchor{grep} +@item @command{grep} +@c ----------------- +@prindex @command{grep} +Portable scripts can rely on the @command{grep} options @option{-c}, +@option{-l}, @option{-n}, and @option{-v}, but should avoid other +options. For example, don't use @option{-w}, as Posix does not require +it and Irix 6.5.16m's @command{grep} does not support it. Also, +portable scripts should not combine @option{-c} with @option{-l}, +as Posix does not allow this. + +Some of the options required by Posix are not portable in practice. +Don't use @samp{grep -q} to suppress output, because many @command{grep} +implementations (e.g., Solaris) do not support @option{-q}. +Don't use @samp{grep -s} to suppress output either, because Posix +says @option{-s} does not suppress output, only some error messages; +also, the @option{-s} option of traditional @command{grep} behaved +like @option{-q} does in most modern implementations. Instead, +redirect the standard output and standard error (in case the file +doesn't exist) of @code{grep} to @file{/dev/null}. Check the exit +status of @code{grep} to determine whether it found a match. + +The QNX4 implementation fails to count lines with @code{grep -c '$'}, +but works with @code{grep -c '^'}. Other alternatives for counting +lines are to use @code{sed -n '$='} or @code{wc -l}. + +Some traditional @command{grep} implementations do not work on long +input lines. On AIX the default @code{grep} silently truncates long +lines on the input before matching. + +Also, many implementations do not support multiple regexps +with @option{-e}: they either reject @option{-e} entirely (e.g., Solaris) +or honor only the last pattern (e.g., IRIX 6.5 and NeXT). To +work around these problems, invoke @code{AC_PROG_GREP} and then use +@code{$GREP}. + +Another possible workaround for the multiple @option{-e} problem is to +separate the patterns by newlines, for example: + +@example +grep 'foo +bar' in.txt +@end example + +@noindent +except that this fails with traditional @command{grep} +implementations and with OpenBSD 3.8 @command{grep}. + +Traditional @command{grep} implementations (e.g., Solaris) do not +support the @option{-E} or @option{-F} options. To work around these +problems, invoke @code{AC_PROG_EGREP} and then use @code{$EGREP}, and +similarly for @code{AC_PROG_FGREP} and @code{$FGREP}. Even if you are +willing to require support for Posix @command{grep}, your script should +not use both @option{-E} and @option{-F}, since Posix does not allow +this combination. + +Portable @command{grep} regular expressions should use @samp{\} only to +escape characters in the string @samp{$()*.0123456789[\^@{@}}. For example, +alternation, @samp{\|}, is common but Posix does not require its +support in basic regular expressions, so it should be avoided in +portable scripts. Solaris and HP-UX @command{grep} do not support it. +Similarly, the following escape sequences should also be avoided: +@samp{\<}, @samp{\>}, @samp{\+}, @samp{\?}, @samp{\`}, @samp{\'}, +@samp{\B}, @samp{\b}, @samp{\S}, @samp{\s}, @samp{\W}, and @samp{\w}. + +Posix does not specify the behavior of @command{grep} on binary files. +An example where this matters is using BSD @command{grep} to +search text that includes embedded ANSI escape sequences for +colored output to terminals (@samp{\033[m} is the sequence to restore +normal output); the behavior depends on whether input is seekable: + +@example +$ @kbd{printf 'esc\033[mape\n' > sample} +$ @kbd{grep . sample} +Binary file sample matches +$ @kbd{cat sample | grep .} +escape +@end example + + +@item @command{join} +@c ----------------- +@prindex @command{join} +Solaris 8 @command{join} has bugs when the second operand is standard +input, and when standard input is a pipe. For example, the following +shell script causes Solaris 8 @command{join} to loop forever: + +@example +cat >file <<'EOF' +1 x +2 y +EOF +cat file | join file - +@end example + +Use @samp{join - file} instead. + +On NetBSD, @command{join -a 1 file1 file2} mistakenly behaves like +@command{join -a 1 -a 2 1 file1 file2}, resulting in a usage warning; +the workaround is to use @command{join -a1 file1 file2} instead. + +@item @command{ln} +@c --------------- +@prindex @command{ln} +@cindex Symbolic links +Don't rely on @command{ln} having a @option{-f} option. Symbolic links +are not available on old systems; use @samp{$(LN_S)} as a portable substitute. + +For versions of the DJGPP before 2.04, +@command{ln} emulates symbolic links +to executables by generating a stub that in turn calls the real +program. This feature also works with nonexistent files like in the +Posix spec. So @samp{ln -s file link} generates @file{link.exe}, +which attempts to call @file{file.exe} if run. But this feature only +works for executables, so @samp{cp -p} is used instead for these +systems. DJGPP versions 2.04 and later have full support +for symbolic links. + + +@item @command{ls} +@c --------------- +@prindex @command{ls} +@cindex Listing directories +The portable options are @option{-acdilrtu}. Current practice is for +@option{-l} to output both owner and group, even though ancient versions +of @command{ls} omitted the group. + +On ancient hosts, @samp{ls foo} sent the diagnostic @samp{foo not found} +to standard output if @file{foo} did not exist. Hence a shell command +like @samp{sources=`ls *.c 2>/dev/null`} did not always work, since it +was equivalent to @samp{sources='*.c not found'} in the absence of +@samp{.c} files. This is no longer a practical problem, since current +@command{ls} implementations send diagnostics to standard error. + +The behavior of @command{ls} on a directory that is being concurrently +modified is not always predictable, because of a data race where cached +information returned by @code{readdir} does not match the current +directory state. In fact, MacOS 10.5 has an intermittent bug where +@code{readdir}, and thus @command{ls}, sometimes lists a file more than +once if other files were added or removed from the directory immediately +prior to the @command{ls} call. Since @command{ls} already sorts its +output, the duplicate entries can be avoided by piping the results +through @code{uniq}. + +@anchor{mkdir} +@item @command{mkdir} +@c ------------------ +@prindex @command{mkdir} +@cindex Making directories +No @command{mkdir} option is portable to older systems. Instead of +@samp{mkdir -p @var{file-name}}, you should use +@code{AS_MKDIR_P(@var{file-name})} (@pxref{Programming in M4sh}) +or @code{AC_PROG_MKDIR_P} (@pxref{Particular Programs}). + +Combining the @option{-m} and @option{-p} options, as in @samp{mkdir -m +go-w -p @var{dir}}, often leads to trouble. FreeBSD +@command{mkdir} incorrectly attempts to change the permissions of +@var{dir} even if it already exists. HP-UX 11.23 and +IRIX 6.5 @command{mkdir} often assign the wrong permissions to +any newly-created parents of @var{dir}. + +Posix does not clearly specify whether @samp{mkdir -p foo} +should succeed when @file{foo} is a symbolic link to an already-existing +directory. The GNU Core Utilities 5.1.0 @command{mkdir} +succeeds, but Solaris @command{mkdir} fails. + +Traditional @code{mkdir -p} implementations suffer from race conditions. +For example, if you invoke @code{mkdir -p a/b} and @code{mkdir -p a/c} +at the same time, both processes might detect that @file{a} is missing, +one might create @file{a}, then the other might try to create @file{a} +and fail with a @code{File exists} diagnostic. The GNU Core +Utilities (@samp{fileutils} version 4.1), FreeBSD 5.0, +NetBSD 2.0.2, and OpenBSD 2.4 are known to be +race-free when two processes invoke @code{mkdir -p} simultaneously, but +earlier versions are vulnerable. Solaris @command{mkdir} is still +vulnerable as of Solaris 10, and other traditional Unix systems are +probably vulnerable too. This possible race is harmful in parallel +builds when several Make rules call @code{mkdir -p} to +construct directories. You may use +@code{install-sh -d} as a safe replacement, provided this script is +recent enough; the copy shipped with Autoconf 2.60 and Automake 1.10 is +OK, but copies from older versions are vulnerable. + + +@item @command{mkfifo} +@itemx @command{mknod} +@c ------------------- +@prindex @command{mkfifo} +@prindex @command{mknod} +The GNU Coding Standards state that @command{mknod} is safe to use on +platforms where it has been tested to exist; but it is generally portable +only for creating named FIFOs, since device numbers are +platform-specific. Autotest uses @command{mkfifo} to implement parallel +testsuites. Posix states that behavior is unspecified when opening a +named FIFO for both reading and writing; on at least Cygwin, this +results in failure on any attempt to read or write to that file +descriptor. + +@item @command{mktemp} +@c ------------------- +@prindex @command{mktemp} +@cindex Creating temporary files +Shell scripts can use temporary files safely with @command{mktemp}, but +it does not exist on all systems. A portable way to create a safe +temporary file name is to create a temporary directory with mode 700 and +use a file inside this directory. Both methods prevent attackers from +gaining control, though @command{mktemp} is far less likely to fail +gratuitously under attack. + +Here is sample code to create a new temporary directory @samp{$dir} safely: + +@example +# Create a temporary directory $dir in $TMPDIR (default /tmp). +# Use mktemp if possible; otherwise fall back on mkdir, +# with $RANDOM to make collisions less likely. +: "$@{TMPDIR:=/tmp@}" +@{ + dir=` + (umask 077 && mktemp -d "$TMPDIR/fooXXXXXX") 2>/dev/null + ` && + test -d "$dir" +@} || @{ + dir=$TMPDIR/foo$$-$RANDOM +@c $$ restore font-lock + (umask 077 && mkdir "$dir") +@} || exit $? +@end example + + +@item @command{mv} +@c --------------- +@prindex @command{mv} +@cindex Moving open files +The only portable options are @option{-f} and @option{-i}. + +Moving individual files between file systems is portable (it was in Unix +version 6), +but it is not always atomic: when doing @samp{mv new existing}, there's +a critical section where neither the old nor the new version of +@file{existing} actually exists. + +On some systems moving files from @file{/tmp} can sometimes cause +undesirable (but perfectly valid) warnings, even if you created these +files. This is because @file{/tmp} belongs to a group that ordinary +users are not members of, and files created in @file{/tmp} inherit +the group of @file{/tmp}. When the file is copied, @command{mv} issues +a diagnostic without failing: + +@smallexample +$ @kbd{touch /tmp/foo} +$ @kbd{mv /tmp/foo .} +@error{}mv: ./foo: set owner/group (was: 100/0): Operation not permitted +$ @kbd{echo $?} +0 +$ @kbd{ls foo} +foo +@end smallexample + +@noindent +This annoying behavior conforms to Posix, unfortunately. + +Moving directories across mount points is not portable, use @command{cp} +and @command{rm}. + +DOS variants cannot rename or remove open files, and do not +support commands like @samp{mv foo bar >foo}, even though this is +perfectly portable among Posix hosts. + + +@item @command{od} +@c --------------- +@prindex @command{od} + +In Mac OS X 10.3, @command{od} does not support the +standard Posix options @option{-A}, @option{-j}, @option{-N}, or +@option{-t}, or the XSI option @option{-s}. The only +supported Posix option is @option{-v}, and the only supported +XSI options are those in @option{-bcdox}. The BSD +@command{hexdump} program can be used instead. + +This problem no longer exists in Mac OS X 10.4.3. + + +@item @command{rm} +@c --------------- +@prindex @command{rm} +The @option{-f} and @option{-r} options are portable. + +It is not portable to invoke @command{rm} without options or operands. +On the other hand, Posix now requires @command{rm -f} to silently +succeed when there are no operands (useful for constructs like +@command{rm -rf $filelist} without first checking if @samp{$filelist} +was empty). But this was not always portable; at least NetBSD +@command{rm} built before 2008 would fail with a diagnostic. + +A file might not be removed even if its parent directory is writable +and searchable. Many Posix hosts cannot remove a mount point, a named +stream, a working directory, or a last link to a file that is being +executed. + +DOS variants cannot rename or remove open files, and do not +support commands like @samp{rm foo >foo}, even though this is +perfectly portable among Posix hosts. + +@item @command{rmdir} +@c ------------------ +@prindex @command{rmdir} +Just as with @command{rm}, some platforms refuse to remove a working +directory. + +@anchor{sed} +@item @command{sed} +@c ---------------- +@prindex @command{sed} +Patterns should not include the separator (unless escaped), even as part +of a character class. In conformance with Posix, the Cray +@command{sed} rejects @samp{s/[^/]*$//}: use @samp{s%[^/]*$%%}. +Even when escaped, patterns should not include separators that are also +used as @command{sed} metacharacters. For example, GNU sed 4.0.9 rejects +@samp{s,x\@{1\,\@},,}, while sed 4.1 strips the backslash before the comma +before evaluating the basic regular expression. + +Avoid empty patterns within parentheses (i.e., @samp{\(\)}). Posix does +not require support for empty patterns, and Unicos 9 @command{sed} rejects +them. + +Unicos 9 @command{sed} loops endlessly on patterns like @samp{.*\n.*}. + +Sed scripts should not use branch labels longer than 7 characters and +should not contain comments; AIX 5.3 @command{sed} rejects indented comments. +HP-UX sed has a limit of 99 commands (not counting @samp{:} commands) and +48 labels, which cannot be circumvented by using more than one script +file. It can execute up to 19 reads with the @samp{r} command per cycle. +Solaris @command{/usr/ucb/sed} rejects usages that exceed a limit of +about 6000 bytes for the internal representation of commands. + +Avoid redundant @samp{;}, as some @command{sed} implementations, such as +NetBSD 1.4.2's, incorrectly try to interpret the second +@samp{;} as a command: + +@example +$ @kbd{echo a | sed 's/x/x/;;s/x/x/'} +sed: 1: "s/x/x/;;s/x/x/": invalid command code ; +@end example + +Some @command{sed} implementations have a buffer limited to 4000 bytes, +and this limits the size of input lines, output lines, and internal +buffers that can be processed portably. Likewise, +not all @command{sed} implementations can handle embedded @code{NUL} or +a missing trailing newline. + +Remember that ranges within a bracket expression of a regular expression +are only well-defined in the @samp{C} (or @samp{POSIX}) locale. +Meanwhile, support for character classes like @samp{[[:upper:]]} is not +yet universal, so if you cannot guarantee the setting of @env{LC_ALL}, +it is better to spell out a range @samp{[ABCDEFGHIJKLMNOPQRSTUVWXYZ]} +than to rely on @samp{[A-Z]}. + +Additionally, Posix states that regular expressions are only +well-defined on characters. Unfortunately, there exist platforms such +as MacOS X 10.5 where not all 8-bit byte values are valid characters, +even though that platform has a single-byte @samp{C} locale. And Posix +allows the existence of a multi-byte @samp{C} locale, although that does +not yet appear to be a common implementation. At any rate, it means +that not all bytes will be matched by the regular expression @samp{.}: + +@example +$ @kbd{printf '\200\n' | LC_ALL=C sed -n /./p | wc -l} +0 +$ @kbd{printf '\200\n' | LC_ALL=en_US.ISO8859-1 sed -n /./p | wc -l} +1 +@end example + +Portable @command{sed} regular expressions should use @samp{\} only to escape +characters in the string @samp{$()*.0123456789[\^n@{@}}. For example, +alternation, @samp{\|}, is common but Posix does not require its +support, so it should be avoided in portable scripts. Solaris +@command{sed} does not support alternation; e.g., @samp{sed '/a\|b/d'} +deletes only lines that contain the literal string @samp{a|b}. +Similarly, @samp{\+} and @samp{\?} should be avoided. + +Anchors (@samp{^} and @samp{$}) inside groups are not portable. + +Nested parentheses in patterns (e.g., @samp{\(\(a*\)b*)\)}) are +quite portable to current hosts, but was not supported by some ancient +@command{sed} implementations like SVR3. + +Some @command{sed} implementations, e.g., Solaris, restrict the special +role of the asterisk @samp{*} to one-character regular expressions and +back-references, and the special role of interval expressions +@samp{\@{@var{m}\@}}, @samp{\@{@var{m},\@}}, or @samp{\@{@var{m},@var{n}\@}} +to one-character regular expressions. This may lead to unexpected behavior: + +@example +$ @kbd{echo '1*23*4' | /usr/bin/sed 's/\(.\)*/x/g'} +x2x4 +$ @kbd{echo '1*23*4' | /usr/xpg4/bin/sed 's/\(.\)*/x/g'} +x +@end example + +The @option{-e} option is mostly portable. +However, its argument +cannot start with @samp{a}, @samp{c}, or @samp{i}, +as this runs afoul of a Tru64 5.1 bug. +Also, its argument cannot be empty, as this fails on AIX 5.3. +Some people prefer to use @samp{-e}: + +@example +sed -e '@var{command-1}' \ + -e '@var{command-2}' +@end example + +@noindent +as opposed to the equivalent: + +@example +sed ' + @var{command-1} + @var{command-2} +' +@end example + +@noindent +The following usage is sometimes equivalent: + +@example +sed '@var{command-1};@var{command-2}' +@end example + +but Posix says that this use of a semicolon has undefined effect if +@var{command-1}'s verb is @samp{@{}, @samp{a}, @samp{b}, @samp{c}, +@samp{i}, @samp{r}, @samp{t}, @samp{w}, @samp{:}, or @samp{#}, so you +should use semicolon only with simple scripts that do not use these +verbs. + +Posix up to the 2008 revision requires the argument of the @option{-e} +option to be a syntactically complete script. GNU @command{sed} allows +to pass multiple script fragments, each as argument of a separate +@option{-e} option, that are then combined, with newlines between the +fragments, and a future Posix revision may allow this as well. This +approach is not portable with script fragments ending in backslash; for +example, the @command{sed} programs on Solaris 10, HP-UX 11, and AIX +don't allow splitting in this case: + +@example +$ @kbd{echo a | sed -n -e 'i\} +@kbd{0'} +0 +$ @kbd{echo a | sed -n -e 'i\' -e 0} +Unrecognized command: 0 +@end example + +@noindent +In practice, however, this technique of joining fragments +through @option{-e} works for multiple @command{sed} functions within +@samp{@{} and @samp{@}}, even if that is not specified by Posix: + +@example +@c The quote around the closing brace silences interactive zsh. +$ @kbd{echo a | sed -n -e '/a/@{' -e s/a/b/ -e p -e '@}'} +b +@end example + +Commands inside @{ @} brackets are further restricted. Posix 2008 says that +they cannot be preceded by addresses, @samp{!}, or @samp{;}, and that +each command must be followed immediately by a newline, without any +intervening blanks or semicolons. The closing bracket must be alone on +a line, other than white space preceding or following it. However, a +future version of Posix may standardize the use of addresses within brackets. + +Contrary to yet another urban legend, you may portably use @samp{&} in +the replacement part of the @code{s} command to mean ``what was +matched''. All descendants of Unix version 7 @command{sed} +(at least; we +don't have first hand experience with older @command{sed} implementations) have +supported it. + +Posix requires that you must not have any white space between +@samp{!} and the following command. It is OK to have blanks between +the address and the @samp{!}. For instance, on Solaris: + +@example +$ @kbd{echo "foo" | sed -n '/bar/ ! p'} +@error{}Unrecognized command: /bar/ ! p +$ @kbd{echo "foo" | sed -n '/bar/! p'} +@error{}Unrecognized command: /bar/! p +$ @kbd{echo "foo" | sed -n '/bar/ !p'} +foo +@end example + +Posix also says that you should not combine @samp{!} and @samp{;}. If +you use @samp{!}, it is best to put it on a command that is delimited by +newlines rather than @samp{;}. + +Also note that Posix requires that the @samp{b}, @samp{t}, @samp{r}, and +@samp{w} commands be followed by exactly one space before their argument. +On the other hand, no white space is allowed between @samp{:} and the +subsequent label name. + +If a sed script is specified on the command line and ends in an +@samp{a}, @samp{c}, or @samp{i} command, the last line of inserted text +should be followed by a newline. Otherwise some @command{sed} +implementations (e.g., OpenBSD 3.9) do not append a newline to the +inserted text. + +Many @command{sed} implementations (e.g., MacOS X 10.4, +OpenBSD 3.9, Solaris 10 +@command{/usr/ucb/sed}) strip leading white space from the text of +@samp{a}, @samp{c}, and @samp{i} commands. Prepend a backslash to +work around this incompatibility with Posix: + +@example +$ @kbd{echo flushleft | sed 'a\} +> @kbd{ indented} +> @kbd{'} +flushleft +indented +$ @kbd{echo foo | sed 'a\} +> @kbd{\ indented} +> @kbd{'} +flushleft + indented +@end example + +Posix requires that with an empty regular expression, the last non-empty +regular expression from either an address specification or substitution +command is applied. However, busybox 1.6.1 complains when using a +substitution command with a replacement containing a back-reference to +an empty regular expression; the workaround is repeating the regular +expression. + +@example +$ @kbd{echo abc | busybox sed '/a\(b\)c/ s//\1/'} +sed: No previous regexp. +$ @kbd{echo abc | busybox sed '/a\(b\)c/ s/a\(b\)c/\1/'} +b +@end example + + +@item @command{sed} (@samp{t}) +@c --------------------------- +@prindex @command{sed} (@samp{t}) +Some old systems have @command{sed} that ``forget'' to reset their +@samp{t} flag when starting a new cycle. For instance on MIPS +RISC/OS, and on IRIX 5.3, if you run the following @command{sed} +script (the line numbers are not actual part of the texts): + +@example +s/keep me/kept/g # a +t end # b +s/.*/deleted/g # c +:end # d +@end example + +@noindent +on + +@example +delete me # 1 +delete me # 2 +keep me # 3 +delete me # 4 +@end example + +@noindent +you get + +@example +deleted +delete me +kept +deleted +@end example + +@noindent +instead of + +@example +deleted +deleted +kept +deleted +@end example + +Why? When processing line 1, (c) matches, therefore sets the @samp{t} +flag, and the output is produced. When processing +line 2, the @samp{t} flag is still set (this is the bug). Command (a) +fails to match, but @command{sed} is not supposed to clear the @samp{t} +flag when a substitution fails. Command (b) sees that the flag is set, +therefore it clears it, and jumps to (d), hence you get @samp{delete me} +instead of @samp{deleted}. When processing line (3), @samp{t} is clear, +(a) matches, so the flag is set, hence (b) clears the flags and jumps. +Finally, since the flag is clear, line 4 is processed properly. + +There are two things one should remember about @samp{t} in @command{sed}. +Firstly, always remember that @samp{t} jumps if @emph{some} substitution +succeeded, not only the immediately preceding substitution. Therefore, +always use a fake @samp{t clear} followed by a @samp{:clear} on the next +line, to reset the @samp{t} flag where needed. + +Secondly, you cannot rely on @command{sed} to clear the flag at each new +cycle. + +One portable implementation of the script above is: + +@example +t clear +:clear +s/keep me/kept/g +t end +s/.*/deleted/g +:end +@end example + +@item @command{sleep} +@c ------------------ +@prindex @command{sleep} +Using @command{sleep} is generally portable. However, remember that +adding a @command{sleep} to work around timestamp issues, with a minimum +granularity of one second, doesn't scale well for parallel builds on +modern machines with sub-second process completion. + +@item @command{sort} +@c ----------------- +@prindex @command{sort} +Remember that sort order is influenced by the current locale. Inside +@file{configure}, the C locale is in effect, but in Makefile snippets, +you may need to specify @code{LC_ALL=C sort}. + +@item @command{tar} +@c ---------------- +@prindex @command{tar} +There are multiple file formats for @command{tar}; if you use Automake, +the macro @code{AM_INIT_AUTOMAKE} has some options controlling which +level of portability to use. + +@anchor{touch} +@item @command{touch} +@c ------------------ +@prindex @command{touch} +@cindex timestamp resolution +If you specify the desired timestamp (e.g., with the @option{-r} +option), older @command{touch} implementations use the @code{utime} or +@code{utimes} system call, which can result in the same kind of +timestamp truncation problems that @samp{cp -p} has. + +On ancient BSD systems, @command{touch} or any command that +results in an empty file does not update the timestamps, so use a +command like @command{echo} as a workaround. +Also, +GNU @command{touch} 3.16r (and presumably all before that) +fails to work on SunOS 4.1.3 when the empty file is on an +NFS-mounted 4.2 volume. +However, these problems are no longer of practical concern. + +@item @command{tr} +@c --------------- +@prindex @command{tr} +@cindex carriage return, deleting +@cindex newline, deleting +@cindex deleting carriage return +Not all versions of @command{tr} handle all backslash character escapes. +For example, Solaris 10 @command{/usr/ucb/tr} falls over, even though +Solaris contains more modern @command{tr} in other locations. +Using octal escapes is more portable for carriage returns, since +@samp{\015} is the same for both ASCII and EBCDIC, and since use of +literal carriage returns in scripts causes a number of other problems. +But for other characters, like newline, using octal escapes ties the +operation to ASCII, so it is better to use literal characters. + +@example +$ @kbd{@{ echo moon; echo light; @} | /usr/ucb/tr -d '\n' ; echo} +moo +light +$ @kbd{@{ echo moon; echo light; @} | /usr/bin/tr -d '\n' ; echo} +moonlight +$ @kbd{@{ echo moon; echo light; @} | /usr/ucb/tr -d '\012' ; echo} +moonlight +$ @kbd{nl='} +@kbd{'; @{ echo moon; echo light; @} | /usr/ucb/tr -d "$nl" ; echo} +moonlight +@end example + +Not all versions of @command{tr} recognize direct ranges of characters: at +least Solaris @command{/usr/bin/tr} still fails to do so. But you can +use @command{/usr/xpg4/bin/tr} instead, or add brackets (which in Posix +transliterate to themselves). + +@example +$ @kbd{echo "Hazy Fantazy" | LC_ALL=C /usr/bin/tr a-z A-Z} +HAZy FAntAZy +$ @kbd{echo "Hazy Fantazy" | LC_ALL=C /usr/bin/tr '[a-z]' '[A-Z]'} +HAZY FANTAZY +$ @kbd{echo "Hazy Fantazy" | LC_ALL=C /usr/xpg4/bin/tr a-z A-Z} +HAZY FANTAZY +@end example + +When providing two arguments, be sure the second string is at least as +long as the first. + +@example +$ @kbd{echo abc | /usr/xpg4/bin/tr bc d} +adc +$ @kbd{echo abc | coreutils/tr bc d} +add +@end example + +Posix requires @command{tr} to operate on binary files. But at least +Solaris @command{/usr/ucb/tr} and @command{/usr/bin/tr} silently discard +@code{NUL} in the input prior to doing any translation. When using +@command{tr} to process a binary file that may contain @code{NUL} bytes, +it is necessary to use @command{/usr/xpg4/bin/tr} instead, or +@command{/usr/xpg6/bin/tr} if that is available. + +@example +$ @kbd{printf 'a\0b' | /usr/ucb/tr x x | od -An -tx1} + 61 62 +$ @kbd{printf 'a\0b' | /usr/bin/tr x x | od -An -tx1} + 61 62 +$ @kbd{printf 'a\0b' | /usr/xpg4/bin/tr x x | od -An -tx1} + 61 00 62 +@end example + +Solaris @command{/usr/ucb/tr} additionally fails to handle @samp{\0} as the +octal escape for @code{NUL}. + +@example +$ @kbd{printf 'abc' | /usr/ucb/tr 'bc' '\0d' | od -An -tx1} + 61 62 63 +$ @kbd{printf 'abc' | /usr/bin/tr 'bc' '\0d' | od -An -tx1} + 61 00 64 +$ @kbd{printf 'abc' | /usr/xpg4/bin/tr 'bc' '\0d' | od -An -tx1} + 61 00 64 +@end example + +@end table + + +@node Portable Make +@chapter Portable Make Programming +@prindex @command{make} +@cindex Limitations of @command{make} + +Writing portable makefiles is an art. Since a makefile's commands are +executed by the shell, you must consider the shell portability issues +already mentioned. However, other issues are specific to @command{make} +itself. + +@menu +* $< in Ordinary Make Rules:: $< in ordinary rules +* Failure in Make Rules:: Failing portably in rules +* Special Chars in Names:: Special Characters in Macro Names +* Backslash-Newline-Empty:: Empty lines after backslash-newline +* Backslash-Newline Comments:: Spanning comments across line boundaries +* Long Lines in Makefiles:: Line length limitations +* Macros and Submakes:: @code{make macro=value} and submakes +* The Make Macro MAKEFLAGS:: @code{$(MAKEFLAGS)} portability issues +* The Make Macro SHELL:: @code{$(SHELL)} portability issues +* Parallel Make:: Parallel @command{make} quirks +* Comments in Make Rules:: Other problems with Make comments +* Newlines in Make Rules:: Using literal newlines in rules +* Comments in Make Macros:: Other problems with Make comments in macros +* Trailing whitespace in Make Macros:: Macro substitution problems +* Command-line Macros and whitespace:: Whitespace trimming of values +* obj/ and Make:: Don't name a subdirectory @file{obj} +* make -k Status:: Exit status of @samp{make -k} +* VPATH and Make:: @code{VPATH} woes +* Single Suffix Rules:: Single suffix rules and separated dependencies +* Timestamps and Make:: Subsecond timestamp resolution +@end menu + +@node $< in Ordinary Make Rules +@section @code{$<} in Ordinary Make Rules + +Posix says that the @samp{$<} construct in makefiles can be +used only in inference rules and in the @samp{.DEFAULT} rule; its +meaning in ordinary rules is unspecified. Solaris @command{make} +for instance replaces it with the empty string. OpenBSD (3.0 and +later) @command{make} diagnoses these uses and errors out. + +@node Failure in Make Rules +@section Failure in Make Rules + +Posix 2008 requires that @command{make} must invoke each command with +the equivalent of a @samp{sh -e -c} subshell, which causes the +subshell to exit immediately if a subsidiary simple-command fails, +although not all @command{make} implementations have historically +followed this rule. For +example, the command @samp{touch T; rm -f U} may attempt to +remove @file{U} even if the @command{touch} fails, although this is not +permitted with Posix make. One way to work around failures in simple +commands is to reword them so that they always succeed, e.g., @samp{touch +T || :; rm -f U}. +However, even this approach can run into common bugs in BSD +implementations of the @option{-e} option of @command{sh} and +@command{set} (@pxref{set, , Limitations of Shell Builtins}), so if you +are worried +about porting to buggy BSD shells it may be simpler to migrate +complicated @command{make} actions into separate scripts. + +@node Special Chars in Names +@section Special Characters in Make Macro Names + +Posix limits macro names to nonempty strings containing only +ASCII letters and digits, @samp{.}, and @samp{_}. Many +@command{make} implementations allow a wider variety of characters, but +portable makefiles should avoid them. It is portable to start a name +with a special character, e.g., @samp{$(.FOO)}. + +Some ancient @command{make} implementations don't support leading +underscores in macro names. An example is NEWS-OS 4.2R. + +@example +$ @kbd{cat Makefile} +_am_include = # +_am_quote = +all:; @@echo this is test +$ @kbd{make} +Make: Must be a separator on rules line 2. Stop. +$ @kbd{cat Makefile2} +am_include = # +am_quote = +all:; @@echo this is test +$ @kbd{make -f Makefile2} +this is test +@end example + +@noindent +However, this problem is no longer of practical concern. + +@node Backslash-Newline-Empty +@section Backslash-Newline Before Empty Lines + +A bug in Bash 2.03 can cause problems if a Make rule contains a +backslash-newline followed by line that expands to nothing. +For example, on Solaris 8: + +@example +SHELL = /bin/bash +EMPTY = +foo: + touch foo \ + $(EMPTY) +@end example + +@noindent +executes + +@example +/bin/bash -c 'touch foo \ +' +@end example + +@noindent +which fails with a syntax error, due to the Bash bug. To avoid this +problem, avoid nullable macros in the last line of a multiline command. + +@c This has been seen on ia64 hpux 11.20, and on one hppa hpux 10.20, +@c but another hppa hpux 10.20 didn't have it. Bob Proulx +@c <bob@proulx.com> thinks it was in hpux 8.0 too. +On some versions of HP-UX, @command{make} reads multiple newlines +following a backslash, continuing to the next non-empty line. For +example, + +@example +FOO = one \ + +BAR = two + +test: + : FOO is "$(FOO)" + : BAR is "$(BAR)" +@end example + +@noindent +shows @code{FOO} equal to @code{one BAR = two}. Other implementations +sensibly let a backslash continue only to the immediately following +line. + +@node Backslash-Newline Comments +@section Backslash-Newline in Make Comments + +According to Posix, Make comments start with @code{#} +and continue until an unescaped newline is reached. + +@example +$ @kbd{cat Makefile} +# A = foo \ + bar \ + baz + +all: + @@echo ok +$ @kbd{make} # GNU make +ok +@end example + +@noindent +However this is not always the case. Some implementations +discard everything from @code{#} through the end of the line, ignoring any +trailing backslash. + +@example +$ @kbd{pmake} # BSD make +"Makefile", line 3: Need an operator +Fatal errors encountered -- cannot continue +@end example + +@noindent +Therefore, if you want to comment out a multi-line definition, prefix each +line with @code{#}, not only the first. + +@example +# A = foo \ +# bar \ +# baz +@end example + +@node Long Lines in Makefiles +@section Long Lines in Makefiles + +Tru64 5.1's @command{make} has been reported to crash when given a +makefile with lines longer than around 20 kB. Earlier versions are +reported to exit with @code{Line too long} diagnostics. + +@node Macros and Submakes +@section @code{make macro=value} and Submakes + +A command-line variable definition such as @code{foo=bar} overrides any +definition of @code{foo} in a makefile. Some @command{make} +implementations (such as GNU @command{make}) propagate this +override to subsidiary invocations of @command{make}. Some other +implementations do not pass the substitution along to submakes. + +@example +$ @kbd{cat Makefile} +foo = foo +one: + @@echo $(foo) + $(MAKE) two +two: + @@echo $(foo) +$ @kbd{make foo=bar} # GNU make 3.79.1 +bar +make two +make[1]: Entering directory `/home/adl' +bar +make[1]: Leaving directory `/home/adl' +$ @kbd{pmake foo=bar} # BSD make +bar +pmake two +foo +@end example + +You have a few possibilities if you do want the @code{foo=bar} override +to propagate to submakes. One is to use the @option{-e} +option, which causes all environment variables to have precedence over +the makefile macro definitions, and declare foo as an environment +variable: + +@example +$ @kbd{env foo=bar make -e} +@end example + +The @option{-e} option is propagated to submakes automatically, +and since the environment is inherited between @command{make} +invocations, the @code{foo} macro is overridden in +submakes as expected. + +This syntax (@code{foo=bar make -e}) is portable only when used +outside of a makefile, for instance from a script or from the +command line. When run inside a @command{make} rule, GNU +@command{make} 3.80 and prior versions forget to propagate the +@option{-e} option to submakes. + +Moreover, using @option{-e} could have unexpected side effects if your +environment contains some other macros usually defined by the +makefile. (See also the note about @code{make -e} and @code{SHELL} +below.) + +If you can foresee all macros that a user might want to override, then +you can propagate them to submakes manually, from your makefile: + +@example +foo = foo +one: + @@echo $(foo) + $(MAKE) foo=$(foo) two +two: + @@echo $(foo) +@end example + +Another way to propagate a variable to submakes in a portable way is to +expand an extra variable in every invocation of @samp{$(MAKE)} within +your makefile: + +@example +foo = foo +one: + @@echo $(foo) + $(MAKE) $(SUBMAKEFLAGS) two +two: + @@echo $(foo) +@end example + +Users must be aware that this technique is in use to take advantage of +it, e.g.@: with @code{make foo=bar SUBMAKEFLAGS='foo=bar'}, but it +allows any macro to be overridden. Makefiles generated by +@command{automake} use this technique, expanding @code{$(AM_MAKEFLAGS)} +on the command lines of submakes (@pxref{Subdirectories, , Automake, +automake, GNU Automake}). + +@node The Make Macro MAKEFLAGS +@section The Make Macro MAKEFLAGS +@cindex @code{MAKEFLAGS} and @command{make} +@cindex @command{make} and @code{MAKEFLAGS} + +Posix requires @command{make} to use @code{MAKEFLAGS} to affect the +current and recursive invocations of make, but allows implementations +several formats for the variable. It is tricky to parse +@code{$MAKEFLAGS} to determine whether @option{-s} for silent execution +or @option{-k} for continued execution are in effect. For example, you +cannot assume that the first space-separated word in @code{$MAKEFLAGS} +contains single-letter options, since in the Cygwin version of +GNU @command{make} it is either @option{--unix} or +@option{--win32} with the second word containing single-letter options. + +@example +$ @kbd{cat Makefile} +all: + @@echo MAKEFLAGS = $(MAKEFLAGS) +$ @kbd{make} +MAKEFLAGS = --unix +$ @kbd{make -k} +MAKEFLAGS = --unix -k +@end example + +@node The Make Macro SHELL +@section The Make Macro @code{SHELL} +@cindex @code{SHELL} and @command{make} +@cindex @command{make} and @code{SHELL} + +Posix-compliant @command{make} internally uses the @code{$(SHELL)} +macro to spawn shell processes and execute Make rules. This +is a builtin macro supplied by @command{make}, but it can be modified +by a makefile or by a command-line argument. + +Not all @command{make} implementations define this @code{SHELL} macro. +Tru64 +@command{make} is an example; this implementation always uses +@code{/bin/sh}. So it's a good idea to always define @code{SHELL} in +your makefiles. If you use Autoconf, do + +@example +SHELL = @@SHELL@@ +@end example + +@noindent +If you use Automake, this is done for you. + +Do not force @code{SHELL = /bin/sh} because that is not correct +everywhere. Remember, @file{/bin/sh} is not Posix compliant on many +systems, such as FreeBSD 4, NetBSD 3, AIX 3, Solaris 10, or Tru64. +Additionally, DJGPP lacks @code{/bin/sh}, and when its +GNU @command{make} port sees such a setting it enters a +special emulation mode where features like pipes and redirections are +emulated on top of DOS's @command{command.com}. Unfortunately this +emulation is incomplete; for instance it does not handle command +substitutions. Using @code{@@SHELL@@} means that your makefile will +benefit from the same improved shell, such as @command{bash} or +@command{ksh}, that was discovered during @command{configure}, so that +you aren't fighting two different sets of shell bugs between the two +contexts. + +Posix-compliant @command{make} should never acquire the value of +$(SHELL) from the environment, even when @code{make -e} is used +(otherwise, think about what would happen to your rules if +@code{SHELL=/bin/tcsh}). + +However not all @command{make} implementations have this exception. +For instance it's not surprising that Tru64 @command{make} doesn't +protect @code{SHELL}, since it doesn't use it. + +@example +$ @kbd{cat Makefile} +SHELL = /bin/sh +FOO = foo +all: + @@echo $(SHELL) + @@echo $(FOO) +$ @kbd{env SHELL=/bin/tcsh FOO=bar make -e} # Tru64 Make +/bin/tcsh +bar +$ @kbd{env SHELL=/bin/tcsh FOO=bar gmake -e} # GNU make +/bin/sh +bar +@end example + +Conversely, @command{make} is not supposed to export any changes to the +macro @code{SHELL} to child processes. Again, many implementations +break this rule: + +@example +$ @kbd{cat Makefile} +all: + @@echo $(SHELL) + @@printenv SHELL +$ @kbd{env SHELL=sh make -e SHELL=/bin/ksh} # BSD Make, GNU make 3.80 +/bin/ksh +/bin/ksh +$ @kbd{env SHELL=sh gmake -e SHELL=/bin/ksh} # GNU make 3.81 +/bin/ksh +sh +@end example + +@node Parallel Make +@section Parallel Make +@cindex Parallel @command{make} + +Support for parallel execution in @command{make} implementation varies. +Generally, using GNU make is your best bet. + +When NetBSD or FreeBSD @command{make} are run in parallel mode, they will +reuse the same shell for multiple commands within one recipe. This can +have various unexpected consequences. For example, changes of directories +or variables persist between recipes, so that: + +@example +all: + @@var=value; cd /; pwd; echo $$var; echo $$$$ + @@pwd; echo $$var; echo $$$$ +@end example + +@noindent +may output the following with @code{make -j1}, at least on NetBSD up to +5.1 and FreeBSD up to 8.2: + +@example +/ +value +32235 +/ +value +32235 +@end example + +@noindent +while without @option{-j1}, or with @option{-B}, the output looks less +surprising: + +@example +/ +value +32238 +/tmp + +32239 +@end example + +@noindent +Another consequence is that, if one command in a recipe uses @code{exit 0} +to indicate a successful exit, the shell will be gone and the remaining +commands of this recipe will not be executed. + +The BSD @command{make} implementations, when run in parallel mode, +will also pass the @command{Makefile} recipes to the shell through +its standard input, thus making it unusable from the recipes: + +@example +$ @kbd{cat Makefile} +read: + @@read line; echo LINE: $$line +@c $$ @c restore font-lock +$ @kbd{echo foo | make read} +LINE: foo +$ @kbd{echo foo | make -j1 read} # NetBSD 5.1 and FreeBSD 8.2 +LINE: +@end example + +@noindent +Moreover, when FreeBSD @command{make} (up at least to 8.2) is run in +parallel mode, it implements the @code{@@} and @code{-} ``recipe +modifiers'' by dynamically modifying the active shell flags. This +behavior has the effects of potentially clobbering the exit status +of recipes silenced with the @code{@@} modifier if they also unset +the @option{errexit} shell flag, and of mangling the output in +unexpected ways: + +@example +$ @kbd{cat Makefile} +a: + @@echo $$-; set +e; false +b: + -echo $$-; false; echo set - +$ @kbd{make a; echo status: $?} +ehBc +*** Error code 1 +status: 1 +$ @kbd{make -j1 a; echo status: $?} +ehB +status: 0 +$ @kbd{make b} +echo $-; echo set - +hBc +set - +$ @kbd{make -j1 b} +echo $-; echo hvB +@end example + +@noindent +You can avoid all these issues by using the @option{-B} option to enable +compatibility semantics. However, that will effectively also disable +all parallelism as that will cause prerequisites to be updated in the +order they are listed in a rule. + +Some make implementations (among them, FreeBSD @command{make}, NetBSD +@command{make}, and Solaris @command{dmake}), when invoked with a +@option{-j@var{N}} option, connect the standard output and standard +error of all their child processes to pipes or temporary regular +files. This can lead to subtly different semantics in the behavior +of the spawned processes. For example, even if the @command{make} +standard output is connected to a tty, the recipe command will not be: + +@example +$ @kbd{cat Makefile} +all: + @@test -t 1 && echo "Is a tty" || echo "Is not a tty" +$ @kbd{make -j 2} # FreeBSD 8.2 make +Is not a tty +$ @kbd{make -j 2} # NetBSD 5.1 make +--- all --- +Is not a tty +$ @kbd{dmake -j 2} # Solaris 10 dmake +@var{hostname} --> 1 job +@var{hostname} --> Job output +Is not a tty +@end example + +@noindent +On the other hand: + +@example +$ @kbd{make -j 2} # GNU make, Heirloom make +Is a tty +@end example + +@noindent +The above examples also show additional status output produced in parallel +mode for targets being updated by Solaris @command{dmake} and NetBSD +@command{make} (but @emph{not} by FreeBSD @command{make}). + +Furthermore, parallel runs of those @command{make} implementations will +route standard error from commands that they spawn into their own +standard output, and may remove leading whitespace from output lines. + + +@node Comments in Make Rules +@section Comments in Make Rules +@cindex Comments in @file{Makefile} rules +@cindex @file{Makefile} rules and comments + +Never put comments in a rule. + +Some @command{make} treat anything starting with a tab as a command for +the current rule, even if the tab is immediately followed by a @code{#}. +The @command{make} from Tru64 Unix V5.1 is one of them. The following +makefile runs @code{# foo} through the shell. + +@example +all: + # foo +@end example + +As a workaround, you can use the @command{:} no-op command with a string +argument that gets ignored: + +@example +all: + : "foo" +@end example + +Conversely, if you want to use the @samp{#} character in some command, +you can only do so by expanding it inside a rule (@pxref{Comments in +Make Macros}). So for example, if @samp{COMMENT_CHAR} is substituted by +@command{config.status} as @samp{#}, then the following substitutes +@samp{@@COMMENT_CHAR@@} in a generated header: + +@example +foo.h: foo.h.in + sed -e 's|@@''COMMENT_CHAR''@@|@@COMMENT_CHAR@@|g' \ + $(srcdir)/foo.h.in > $@@ +@end example + +The funny shell quoting avoids a substitution at @command{config.status} +run time of the left-hand side of the @command{sed} @samp{s} command. + +@node Newlines in Make Rules +@section Newlines in Make Rules +@cindex Newlines in @file{Makefile} rules +@cindex @file{Makefile} rules and newlines + +In shell scripts, newlines can be used inside string literals. But in +the shell statements of @file{Makefile} rules, this is not possible: +A newline not preceded by a backslash is a separator between shell +statements. Whereas a newline that is preceded by a backslash becomes +part of the shell statement according to POSIX, but gets replaced, +together with the backslash that precedes it, by a space in GNU +@command{make} 3.80 and older. So, how can a newline be used in a string +literal? + +The trick is to set up a shell variable that contains a newline: + +@example +nlinit=`echo 'nl="'; echo '"'`; eval "$$nlinit" +@end example + +For example, in order to create a multiline @samp{sed} expression that +inserts a blank line after every line of a file, this code can be used: + +@example +nlinit=`echo 'nl="'; echo '"'`; eval "$$nlinit"; \ +sed -e "s/\$$/\\$$@{nl@}/" < input > output +@end example + +@node Comments in Make Macros +@section Comments in Make Macros +@cindex Comments in @file{Makefile} macros +@cindex @file{Makefile} macros and comments + +Avoid putting comments in macro values as far as possible. Posix +specifies that the text starting from the @samp{#} sign until the end of +the line is to be ignored, which has the unfortunate effect of +disallowing them even within quotes. Thus, the following might lead to +a syntax error at compile time: + +@example +CPPFLAGS = "-DCOMMENT_CHAR='#'" +@end example + +@noindent +as @samp{CPPFLAGS} may be expanded to @samp{"-DCOMMENT_CHAR='}. + +Most @command{make} implementations disregard this and treat single and +double quotes specially here. Also, GNU @command{make} lets you put +@samp{#} into a macro value by escaping it with a backslash, i.e., +@samp{\#}. However, neither of these usages are portable. +@xref{Comments in Make Rules}, for a portable alternative. + +Even without quoting involved, comments can have surprising effects, +because the whitespace before them is part of the variable value: + +@example +foo = bar # trailing comment +print: ; @@echo "$(foo)." +@end example + +@noindent +prints @samp{bar .}, which is usually not intended, and can expose +@command{make} bugs as described below. + +@node Trailing whitespace in Make Macros +@section Trailing whitespace in Make Macros +@cindex whitespace in @file{Makefile} macros +@cindex @file{Makefile} macros and whitespace + +GNU @command{make} 3.80 mistreats trailing whitespace in macro +substitutions and appends another spurious suffix: + +@example +empty = +foo = bar $(empty) +print: ; @@echo $(foo:=.test) +@end example + +@noindent +prints @samp{bar.test .test}. + +BSD and Solaris @command{make} implementations do not honor trailing +whitespace in macro definitions as Posix requires: + +@example +foo = bar # Note the space after "bar". +print: ; @@echo $(foo)t +@end example + +@noindent +prints @samp{bart} instead of @samp{bar t}. To work around this, you +can use a helper macro as in the previous example. + + +@node Command-line Macros and whitespace +@section Command-line Macros and whitespace +@cindex whitespace in command-line macros +@cindex command-line, macros set on +@cindex environment, macros set from + +Some @command{make} implementations may strip trailing whitespace off +of macros set on the command line in addition to leading whitespace. +Further, some may strip leading whitespace off of macros set from +environment variables: + +@example +$ @kbd{echo 'print: ; @@echo "x$(foo)x$(bar)x"' | + foo=' f f ' make -f - bar=' b b '} +x f f xb b x # AIX, BSD, GNU make +xf f xb b x # HP-UX, IRIX, Tru64/OSF make +x f f xb bx # Solaris make +@end example + + +@node obj/ and Make +@section The @file{obj/} Subdirectory and Make +@cindex @file{obj/}, subdirectory +@cindex BSD @command{make} and @file{obj/} + +Never name one of your subdirectories @file{obj/} if you don't like +surprises. + +If an @file{obj/} directory exists, BSD @command{make} enters it +before reading the makefile. Hence the makefile in the +current directory is not read. + +@example +$ @kbd{cat Makefile} +all: + echo Hello +$ @kbd{cat obj/Makefile} +all: + echo World +$ @kbd{make} # GNU make +echo Hello +Hello +$ @kbd{pmake} # BSD make +echo World +World +@end example + +@node make -k Status +@section Exit Status of @code{make -k} +@cindex @code{make -k} + +Do not rely on the exit status of @code{make -k}. Some implementations +reflect whether they encountered an error in their exit status; other +implementations always succeed. + +@example +$ @kbd{cat Makefile} +all: + false +$ @kbd{make -k; echo exit status: $?} # GNU make +false +make: *** [all] Error 1 +exit status: 2 +$ @kbd{pmake -k; echo exit status: $?} # BSD make +false +*** Error code 1 (continuing) +exit status: 0 +@end example + +@node VPATH and Make +@section @code{VPATH} and Make +@cindex @code{VPATH} + +Posix does not specify the semantics of @code{VPATH}. Typically, +@command{make} supports @code{VPATH}, but its implementation is not +consistent. + +Autoconf and Automake support makefiles whose usages of @code{VPATH} are +portable to recent-enough popular implementations of @command{make}, but +to keep the resulting makefiles portable, a package's makefile +prototypes must take the following issues into account. These issues +are complicated and are often poorly understood, and installers who use +@code{VPATH} should expect to find many bugs in this area. If you use +@code{VPATH}, the simplest way to avoid these portability bugs is to +stick with GNU @command{make}, since it is the most +commonly-used @command{make} among Autoconf users. + +Here are some known issues with some @code{VPATH} +implementations. + +@menu +* Variables listed in VPATH:: @code{VPATH} must be literal on ancient hosts +* VPATH and Double-colon:: Problems with @samp{::} on ancient hosts +* $< in Explicit Rules:: @code{$<} does not work in ordinary rules +* Automatic Rule Rewriting:: @code{VPATH} goes wild on Solaris +* Tru64 Directory Magic:: @command{mkdir} goes wild on Tru64 +* Make Target Lookup:: More details about @code{VPATH} lookup +@end menu + +@node Variables listed in VPATH +@subsection Variables listed in @code{VPATH} +@cindex @code{VPATH} and variables +@cindex variables and @code{VPATH} + +Do not set @code{VPATH} to the value of another variable, for example +@samp{VPATH = $(srcdir)}, because some ancient versions of +@command{make} do not do variable substitutions on the value of +@code{VPATH}. For example, use this + +@example +srcdir = @@srcdir@@ +VPATH = @@srcdir@@ +@end example + +@noindent +rather than @samp{VPATH = $(srcdir)}. Note that with GNU +Automake, there is no need to set this yourself. + +@node VPATH and Double-colon +@subsection @code{VPATH} and Double-colon Rules +@cindex @code{VPATH} and double-colon rules +@cindex double-colon rules and @code{VPATH} + +With ancient versions of Sun @command{make}, +any assignment to @code{VPATH} causes @command{make} to execute only +the first set of double-colon rules. +However, this problem is no longer of practical concern. + +@node $< in Explicit Rules +@subsection @code{$<} Not Supported in Explicit Rules +@cindex explicit rules, @code{$<}, and @code{VPATH} +@cindex @code{$<}, explicit rules, and @code{VPATH} +@cindex @code{VPATH}, explicit rules, and @code{$<} + +Using @code{$<} in explicit rules is not portable. +The prerequisite file must be named explicitly in the rule. If you want +to find the prerequisite via a @code{VPATH} search, you have to code the +whole thing manually. @xref{Build Directories}. + +@node Automatic Rule Rewriting +@subsection Automatic Rule Rewriting +@cindex @code{VPATH} and automatic rule rewriting +@cindex automatic rule rewriting and @code{VPATH} + +Some @command{make} implementations, such as Solaris and Tru64, +search for prerequisites in @code{VPATH} and +then rewrite each occurrence as a plain word in the rule. +For instance: + +@example +# This isn't portable to GNU make. +VPATH = ../pkg/src +f.c: if.c + cp if.c f.c +@end example + +@noindent +executes @code{cp ../pkg/src/if.c f.c} if @file{if.c} is +found in @file{../pkg/src}. + +However, this rule leads to real problems in practice. For example, if +the source directory contains an ordinary file named @file{test} that is +used in a dependency, Solaris @command{make} rewrites commands like +@samp{if test -r foo; @dots{}} to @samp{if ../pkg/src/test -r foo; +@dots{}}, which is typically undesirable. In fact, @command{make} is +completely unaware of shell syntax used in the rules, so the VPATH +rewrite can potentially apply to @emph{any} whitespace-separated word +in a rule, including shell variables, functions, and keywords. + +@example +$ @kbd{mkdir build} +$ @kbd{cd build} +$ @kbd{cat > Makefile <<'END'} +VPATH = .. +all: arg func for echo + func () @{ for arg in "$$@@"; do echo $$arg; done; @}; \ + func "hello world" +END +$ @kbd{touch ../arg ../func ../for ../echo} +$ @kbd{make} +../func () @{ ../for ../arg in "$@@"; do ../echo $arg; done; @}; \ +../func "hello world" +sh: syntax error at line 1: `do' unexpected +*** Error code 2 +@end example + +@noindent +To avoid this problem, portable makefiles should never mention a source +file or dependency whose name is that of a shell keyword like @file{for} +or @file{until}, a shell command like @command{cat} or @command{gcc} or +@command{test}, or a shell function or variable used in the corresponding +@command{Makefile} recipe. + +Because of these problems GNU @command{make} and many other @command{make} +implementations do not rewrite commands, so portable makefiles should +search @code{VPATH} manually. It is tempting to write this: + +@smallexample +# This isn't portable to Solaris make. +VPATH = ../pkg/src +f.c: if.c + cp `test -f if.c || echo $(VPATH)/`if.c f.c +@end smallexample + +@noindent +However, the ``prerequisite rewriting'' still applies here. So if +@file{if.c} is in @file{../pkg/src}, Solaris and Tru64 @command{make} +execute + +@smallexample +cp `test -f ../pkg/src/if.c || echo ../pkg/src/`if.c f.c +@end smallexample + +@noindent +which reduces to + +@example +cp if.c f.c +@end example + +@noindent +and thus fails. Oops. + +A simple workaround, and good practice anyway, is to use @samp{$?} and +@samp{$@@} when possible: + +@smallexample +VPATH = ../pkg/src +f.c: if.c + cp $? $@@ +@end smallexample + +@noindent +but this does not generalize well to commands with multiple +prerequisites. A more general workaround is to rewrite the rule so that +the prerequisite @file{if.c} never appears as a plain word. For +example, these three rules would be safe, assuming @file{if.c} is in +@file{../pkg/src} and the other files are in the working directory: + +@smallexample +VPATH = ../pkg/src +f.c: if.c f1.c + cat `test -f ./if.c || echo $(VPATH)/`if.c f1.c >$@@ +g.c: if.c g1.c + cat `test -f 'if.c' || echo $(VPATH)/`if.c g1.c >$@@ +h.c: if.c h1.c + cat `test -f "if.c" || echo $(VPATH)/`if.c h1.c >$@@ +@end smallexample + +Things get worse when your prerequisites are in a macro. + +@example +VPATH = ../pkg/src +HEADERS = f.h g.h h.h +install-HEADERS: $(HEADERS) + for i in $(HEADERS); do \ + $(INSTALL) -m 644 \ + `test -f $$i || echo $(VPATH)/`$$i \ + $(DESTDIR)$(includedir)/$$i; \ +@c $$ restore font-lock + done +@end example + +The above @code{install-HEADERS} rule is not Solaris-proof because @code{for +i in $(HEADERS);} is expanded to @code{for i in f.h g.h h.h;} +where @code{f.h} and @code{g.h} are plain words and are hence +subject to @code{VPATH} adjustments. + +If the three files are in @file{../pkg/src}, the rule is run as: + +@example +for i in ../pkg/src/f.h ../pkg/src/g.h h.h; do \ + install -m 644 \ + `test -f $i || echo ../pkg/src/`$i \ + /usr/local/include/$i; \ +done +@end example + +where the two first @command{install} calls fail. For instance, +consider the @code{f.h} installation: + +@example +install -m 644 \ + `test -f ../pkg/src/f.h || \ + echo ../pkg/src/ \ + `../pkg/src/f.h \ + /usr/local/include/../pkg/src/f.h; +@end example + +@noindent +It reduces to: + +@example +install -m 644 \ + ../pkg/src/f.h \ + /usr/local/include/../pkg/src/f.h; +@end example + +Note that the manual @code{VPATH} search did not cause any problems here; +however this command installs @file{f.h} in an incorrect directory. + +Trying to quote @code{$(HEADERS)} in some way, as we did for +@code{foo.c} a few makefiles ago, does not help: + +@example +install-HEADERS: $(HEADERS) + headers='$(HEADERS)'; \ + for i in $$headers; do \ + $(INSTALL) -m 644 \ + `test -f $$i || echo $(VPATH)/`$$i \ + $(DESTDIR)$(includedir)/$$i; \ + done +@end example + +Now, @code{headers='$(HEADERS)'} macro-expands to: + +@example +headers='f.h g.h h.h' +@end example + +@noindent +but @code{g.h} is still a plain word. (As an aside, the idiom +@code{headers='$(HEADERS)'; for i in $$headers;} is a good +idea if @code{$(HEADERS)} can be empty, because some shells diagnose a +syntax error on @code{for i in;}.) + +One workaround is to strip this unwanted @file{../pkg/src/} prefix manually: + +@example +VPATH = ../pkg/src +HEADERS = f.h g.h h.h +install-HEADERS: $(HEADERS) + headers='$(HEADERS)'; \ + for i in $$headers; do \ + i=`expr "$$i" : '$(VPATH)/\(.*\)'`; + $(INSTALL) -m 644 \ + `test -f $$i || echo $(VPATH)/`$$i \ + $(DESTDIR)$(includedir)/$$i; \ +@c $$ restore font-lock + done +@end example + +Automake does something similar. However the above hack works only if +the files listed in @code{HEADERS} are in the current directory or a +subdirectory; they should not be in an enclosing directory. If we had +@code{HEADERS = ../f.h}, the above fragment would fail in a VPATH +build with Tru64 @command{make}. The reason is that not only does +Tru64 @command{make} rewrite dependencies, but it also simplifies +them. Hence @code{../f.h} becomes @code{../pkg/f.h} instead of +@code{../pkg/src/../f.h}. This obviously defeats any attempt to strip +a leading @file{../pkg/src/} component. + +The following example makes the behavior of Tru64 @command{make} +more apparent. + +@example +$ @kbd{cat Makefile} +VPATH = sub +all: ../foo + echo ../foo +$ @kbd{ls} +Makefile foo +$ @kbd{make} +echo foo +foo +@end example + +@noindent +Dependency @file{../foo} was found in @file{sub/../foo}, but Tru64 +@command{make} simplified it as @file{foo}. (Note that the @file{sub/} +directory does not even exist, this just means that the simplification +occurred before the file was checked for.) + +For the record here is how SunOS 4 @command{make} behaves on this +example. + +@smallexample +$ @kbd{make} +make: Fatal error: Don't know how to make target `../foo' +$ @kbd{mkdir sub} +$ @kbd{make} +echo sub/../foo +sub/../foo +@end smallexample + + +@node Tru64 Directory Magic +@subsection Tru64 @command{make} Creates Prerequisite Directories Magically +@cindex @code{VPATH} and prerequisite directories +@cindex prerequisite directories and @code{VPATH} + +When a prerequisite is a subdirectory of @code{VPATH}, Tru64 +@command{make} creates it in the current directory. + +@example +$ @kbd{mkdir -p foo/bar build} +$ @kbd{cd build} +$ @kbd{cat >Makefile <<END +VPATH = .. +all: foo/bar +END} +$ @kbd{make} +mkdir foo +mkdir foo/bar +@end example + +This can yield unexpected results if a rule uses a manual @code{VPATH} +search as presented before. + +@example +VPATH = .. +all : foo/bar + command `test -d foo/bar || echo ../`foo/bar +@end example + +The above @command{command} is run on the empty @file{foo/bar} +directory that was created in the current directory. + +@node Make Target Lookup +@subsection Make Target Lookup +@cindex @code{VPATH}, resolving target pathnames + +GNU @command{make} uses a complex algorithm to decide when it +should use files found via a @code{VPATH} search. @xref{Search +Algorithm, , How Directory Searches are Performed, make, The GNU Make +Manual}. + +If a target needs to be rebuilt, GNU @command{make} discards the +file name found during the @code{VPATH} search for this target, and +builds the file locally using the file name given in the makefile. +If a target does not need to be rebuilt, GNU @command{make} uses the +file name found during the @code{VPATH} search. + +Other @command{make} implementations, like NetBSD @command{make}, are +easier to describe: the file name found during the @code{VPATH} search +is used whether the target needs to be rebuilt or not. Therefore +new files are created locally, but existing files are updated at their +@code{VPATH} location. + +OpenBSD and FreeBSD @command{make}, however, +never perform a +@code{VPATH} search for a dependency that has an explicit rule. +This is extremely annoying. + +When attempting a @code{VPATH} build for an autoconfiscated package +(e.g., @code{mkdir build && cd build && ../configure}), this means +GNU +@command{make} builds everything locally in the @file{build} +directory, while BSD @command{make} builds new files locally and +updates existing files in the source directory. + +@example +$ @kbd{cat Makefile} +VPATH = .. +all: foo.x bar.x +foo.x bar.x: newer.x + @@echo Building $@@ +$ @kbd{touch ../bar.x} +$ @kbd{touch ../newer.x} +$ @kbd{make} # GNU make +Building foo.x +Building bar.x +$ @kbd{pmake} # NetBSD make +Building foo.x +Building ../bar.x +$ @kbd{fmake} # FreeBSD make, OpenBSD make +Building foo.x +Building bar.x +$ @kbd{tmake} # Tru64 make +Building foo.x +Building bar.x +$ @kbd{touch ../bar.x} +$ @kbd{make} # GNU make +Building foo.x +$ @kbd{pmake} # NetBSD make +Building foo.x +$ @kbd{fmake} # FreeBSD make, OpenBSD make +Building foo.x +Building bar.x +$ @kbd{tmake} # Tru64 make +Building foo.x +Building bar.x +@end example + +Note how NetBSD @command{make} updates @file{../bar.x} in its +VPATH location, and how FreeBSD, OpenBSD, and Tru64 +@command{make} always +update @file{bar.x}, even when @file{../bar.x} is up to date. + +Another point worth mentioning is that once GNU @command{make} has +decided to ignore a @code{VPATH} file name (e.g., it ignored +@file{../bar.x} in the above example) it continues to ignore it when +the target occurs as a prerequisite of another rule. + +The following example shows that GNU @command{make} does not look up +@file{bar.x} in @code{VPATH} before performing the @code{.x.y} rule, +because it ignored the @code{VPATH} result of @file{bar.x} while running +the @code{bar.x: newer.x} rule. + +@example +$ @kbd{cat Makefile} +VPATH = .. +all: bar.y +bar.x: newer.x + @@echo Building $@@ +.SUFFIXES: .x .y +.x.y: + cp $< $@@ +$ @kbd{touch ../bar.x} +$ @kbd{touch ../newer.x} +$ @kbd{make} # GNU make +Building bar.x +cp bar.x bar.y +cp: cannot stat `bar.x': No such file or directory +make: *** [bar.y] Error 1 +$ @kbd{pmake} # NetBSD make +Building ../bar.x +cp ../bar.x bar.y +$ @kbd{rm bar.y} +$ @kbd{fmake} # FreeBSD make, OpenBSD make +echo Building bar.x +cp bar.x bar.y +cp: cannot stat `bar.x': No such file or directory +*** Error code 1 +$ @kbd{tmake} # Tru64 make +Building bar.x +cp: bar.x: No such file or directory +*** Exit 1 +@end example + +Note that if you drop away the command from the @code{bar.x: newer.x} +rule, GNU @command{make} magically starts to work: it +knows that @code{bar.x} hasn't been updated, therefore it doesn't +discard the result from @code{VPATH} (@file{../bar.x}) in succeeding +uses. Tru64 also works, but FreeBSD and OpenBSD +still don't. + +@example +$ @kbd{cat Makefile} +VPATH = .. +all: bar.y +bar.x: newer.x +.SUFFIXES: .x .y +.x.y: + cp $< $@@ +$ @kbd{touch ../bar.x} +$ @kbd{touch ../newer.x} +$ @kbd{make} # GNU make +cp ../bar.x bar.y +$ @kbd{rm bar.y} +$ @kbd{pmake} # NetBSD make +cp ../bar.x bar.y +$ @kbd{rm bar.y} +$ @kbd{fmake} # FreeBSD make, OpenBSD make +cp bar.x bar.y +cp: cannot stat `bar.x': No such file or directory +*** Error code 1 +$ @kbd{tmake} # Tru64 make +cp ../bar.x bar.y +@end example + +It seems the sole solution that would please every @command{make} +implementation is to never rely on @code{VPATH} searches for targets. +In other words, @code{VPATH} should be reserved to unbuilt sources. + + +@node Single Suffix Rules +@section Single Suffix Rules and Separated Dependencies +@cindex Single Suffix Inference Rule +@cindex Rule, Single Suffix Inference +A @dfn{Single Suffix Rule} is basically a usual suffix (inference) rule +(@samp{.from.to:}), but which @emph{destination} suffix is empty +(@samp{.from:}). + +@cindex Separated Dependencies +@dfn{Separated dependencies} simply refers to listing the prerequisite +of a target, without defining a rule. Usually one can list on the one +hand side, the rules, and on the other hand side, the dependencies. + +Solaris @command{make} does not support separated dependencies for +targets defined by single suffix rules: + +@example +$ @kbd{cat Makefile} +.SUFFIXES: .in +foo: foo.in +.in: + cp $< $@@ +$ @kbd{touch foo.in} +$ @kbd{make} +$ @kbd{ls} +Makefile foo.in +@end example + +@noindent +while GNU Make does: + +@example +$ @kbd{gmake} +cp foo.in foo +$ @kbd{ls} +Makefile foo foo.in +@end example + +Note it works without the @samp{foo: foo.in} dependency. + +@example +$ @kbd{cat Makefile} +.SUFFIXES: .in +.in: + cp $< $@@ +$ @kbd{make foo} +cp foo.in foo +@end example + +@noindent +and it works with double suffix inference rules: + +@example +$ @kbd{cat Makefile} +foo.out: foo.in +.SUFFIXES: .in .out +.in.out: + cp $< $@@ +$ @kbd{make} +cp foo.in foo.out +@end example + +As a result, in such a case, you have to write target rules. + +@node Timestamps and Make +@section Timestamp Resolution and Make +@cindex timestamp resolution +Traditionally, file timestamps had 1-second resolution, and +@command{make} used those timestamps to determine whether one file was +newer than the other. However, many modern file systems have +timestamps with 1-nanosecond resolution. Some @command{make} +implementations look at the entire timestamp; others ignore the +fractional part, which can lead to incorrect results. Normally this +is not a problem, but in some extreme cases you may need to use tricks +like @samp{sleep 1} to work around timestamp truncation bugs. + +Commands like @samp{cp -p} and @samp{touch -r} typically do not copy +file timestamps to their full resolutions (@pxref{touch, , Limitations of Usual +Tools}). Hence you should be wary of rules like this: + +@example +dest: src + cp -p src dest +@end example + +as @file{dest} often appears to be older than @file{src} after the +timestamp is truncated, and this can cause @command{make} to do +needless rework the next time it is invoked. To work around this +problem, you can use a timestamp file, e.g.: + +@example +dest-stamp: src + cp -p src dest + date >dest-stamp +@end example + +Apart from timestamp resolution, there are also differences in handling +equal timestamps. HP-UX @command{make} updates targets if it has the +same time stamp as one of its prerequisites, in violation of Posix rules. + +This can cause spurious rebuilds for repeated runs of @command{make}. +This in turn can cause @command{make} to fail if it tries to rebuild +generated files in a possibly read-only source tree with tools not +present on the end-user machine. Use GNU @command{make} instead. + + + +@c ======================================== Portable C and C++ Programming + +@node Portable C and C++ +@chapter Portable C and C++ Programming +@cindex Portable C and C++ programming + +C and C++ programs often use low-level features of the underlying +system, and therefore are often more difficult to make portable to other +platforms. + +Several standards have been developed to help make your programs more +portable. If you write programs with these standards in mind, you can +have greater confidence that your programs work on a wide variety +of systems. +@ifhtml +@uref{http://@/gcc.gnu.org/@/onlinedocs/@/gcc/@/Standards.html, Language +Standards Supported by GCC} +@end ifhtml +@ifnothtml +@xref{Standards, , Language Standards Supported by +GCC, gcc, Using the GNU Compiler Collection +(GCC)}, +@end ifnothtml +for a list of C-related standards. Many programs also assume the +@uref{http://@/www.opengroup.org/@/susv3, Posix standard}. + +Some old code is written to be portable to K&R C, which predates any C +standard. K&R C compilers are no longer of practical interest, though, +and the rest of section assumes at least C89, the first C standard. + +Program portability is a huge topic, and this section can only briefly +introduce common pitfalls. @xref{System Portability, , Portability +between System Types, standards, The GNU Coding Standards}, for +more information. + +@menu +* Varieties of Unportability:: How to make your programs unportable +* Integer Overflow:: When integers get too large +* Preprocessor Arithmetic:: @code{#if} expression problems +* Null Pointers:: Properties of null pointers +* Buffer Overruns:: Subscript errors and the like +* Volatile Objects:: @code{volatile} and signals +* Floating Point Portability:: Portable floating-point arithmetic +* Exiting Portably:: Exiting and the exit status +@end menu + +@node Varieties of Unportability +@section Varieties of Unportability +@cindex portability + +Autoconf tests and ordinary programs often need to test what is allowed +on a system, and therefore they may need to deliberately exceed the +boundaries of what the standards allow, if only to see whether an +optional feature is present. When you write such a program, you should +keep in mind the difference between constraints, unspecified behavior, +and undefined behavior. + +In C, a @dfn{constraint} is a rule that the compiler must enforce. An +example constraint is that C programs must not declare a bit-field with +negative width. Tests can therefore reliably assume that programs with +negative-width bit-fields are rejected by a compiler that conforms +to the standard. + +@dfn{Unspecified behavior} is valid behavior, where the standard allows +multiple possibilities. For example, the order of evaluation of +function arguments is unspecified. Some unspecified behavior is +@dfn{implementation-defined}, i.e., documented by the implementation, +but since Autoconf tests cannot read the documentation they cannot +distinguish between implementation-defined and other unspecified +behavior. It is common for Autoconf tests to probe implementations to +determine otherwise-unspecified behavior. + +@dfn{Undefined behavior} is invalid behavior, where the standard allows +the implementation to do anything it pleases. For example, +dereferencing a null pointer leads to undefined behavior. If possible, +test programs should avoid undefined behavior, since a program with +undefined behavior might succeed on a test that should fail. + +The above rules apply to programs that are intended to conform to the +standard. However, strictly-conforming programs are quite rare, since +the standards are so limiting. A major goal of Autoconf is to support +programs that use implementation features not described by the standard, +and it is fairly common for test programs to violate the above rules, if +the programs work well enough in practice. + +@node Integer Overflow +@section Integer Overflow +@cindex integer overflow +@cindex overflow, signed integer +@cindex signed integer overflow +@cindex wraparound arithmetic + +In practice many portable C programs assume that signed integer overflow wraps +around reliably using two's complement arithmetic. Yet the C standard +says that program behavior is undefined on overflow, and in a few cases +C programs do not work on some modern implementations because their +overflows do not wrap around as their authors expected. Conversely, in +signed integer remainder, the C standard requires overflow +behavior that is commonly not implemented. + +@menu +* Integer Overflow Basics:: Why integer overflow is a problem +* Signed Overflow Examples:: Examples of code assuming wraparound +* Optimization and Wraparound:: Optimizations that break uses of wraparound +* Signed Overflow Advice:: Practical advice for signed overflow issues +* Signed Integer Division:: @code{INT_MIN / -1} and @code{INT_MIN % -1} +@end menu + +@node Integer Overflow Basics +@subsection Basics of Integer Overflow +@cindex integer overflow +@cindex overflow, signed integer +@cindex signed integer overflow +@cindex wraparound arithmetic + +In languages like C, unsigned integer overflow reliably wraps around; +e.g., @code{UINT_MAX + 1} yields zero. +This is guaranteed by the C standard and is +portable in practice, unless you specify aggressive, +nonstandard optimization options +suitable only for special applications. + +In contrast, the C standard says that signed integer overflow leads to +undefined behavior where a program can do anything, including dumping +core or overrunning a buffer. The misbehavior can even precede the +overflow. Such an overflow can occur during addition, subtraction, +multiplication, division, and left shift. + +Despite this requirement of the standard, many C programs and Autoconf +tests assume that signed integer overflow silently wraps around modulo a +power of two, using two's complement arithmetic, so long as you cast the +resulting value to a signed integer type or store it into a signed +integer variable. If you use conservative optimization flags, such +programs are generally portable to the vast majority of modern +platforms, with a few exceptions discussed later. + +For historical reasons the C standard also allows implementations with +ones' complement or signed magnitude arithmetic, but it is safe to +assume two's complement nowadays. + +Also, overflow can occur when converting an out-of-range value to a +signed integer type. Here a standard implementation must define what +happens, but this might include raising an exception. In practice all +known implementations support silent wraparound in this case, so you need +not worry about other possibilities. + +@node Signed Overflow Examples +@subsection Examples of Code Assuming Wraparound Overflow +@cindex integer overflow +@cindex overflow, signed integer +@cindex signed integer overflow +@cindex wraparound arithmetic + +There has long been a tension between what the C standard requires for +signed integer overflow, and what C programs commonly assume. The +standard allows aggressive optimizations based on assumptions that +overflow never occurs, but many practical C programs rely on overflow +wrapping around. These programs do not conform to the standard, but +they commonly work in practice because compiler writers are +understandably reluctant to implement optimizations that would break +many programs, unless perhaps a user specifies aggressive optimization. + +The C Standard says that if a program has signed integer overflow its +behavior is undefined, and the undefined behavior can even precede the +overflow. To take an extreme example: + +@c Inspired by Robert Dewar's example in +@c <http://gcc.gnu.org/ml/gcc/2007-01/msg00038.html> (2007-01-01). +@example +if (password == expected_password) + allow_superuser_privileges (); +else if (counter++ == INT_MAX) + abort (); +else + printf ("%d password mismatches\n", counter); +@end example + +@noindent +If the @code{int} variable @code{counter} equals @code{INT_MAX}, +@code{counter++} must overflow and the behavior is undefined, so the C +standard allows the compiler to optimize away the test against +@code{INT_MAX} and the @code{abort} call. +Worse, if an earlier bug in the program lets the compiler deduce that +@code{counter == INT_MAX} or that @code{counter} previously overflowed, +the C standard allows the compiler to optimize away the password test +and generate code that allows superuser privileges unconditionally. + +Despite this requirement by the standard, it has long been common for C +code to assume wraparound arithmetic after signed overflow, and all +known practical C implementations support some C idioms that assume +wraparound signed arithmetic, even if the idioms do not conform +strictly to the standard. If your code looks like the following +examples it will almost surely work with real-world compilers. + +Here is an example derived from the 7th Edition Unix implementation of +@code{atoi} (1979-01-10): + +@example +char *p; +int f, n; +@dots{} +while (*p >= '0' && *p <= '9') + n = n * 10 + *p++ - '0'; +return (f ? -n : n); +@end example + +@noindent +Even if the input string is in range, on most modern machines this has +signed overflow when computing the most negative integer (the @code{-n} +overflows) or a value near an extreme integer (the first @code{+} +overflows). + +Here is another example, derived from the 7th Edition implementation of +@code{rand} (1979-01-10). Here the programmer expects both +multiplication and addition to wrap on overflow: + +@example +static long int randx = 1; +@dots{} +randx = randx * 1103515245 + 12345; +return (randx >> 16) & 077777; +@end example + +In the following example, derived from the GNU C Library 2.5 +implementation of @code{mktime} (2006-09-09), the code assumes +wraparound arithmetic in @code{+} to detect signed overflow: + +@example +time_t t, t1, t2; +int sec_requested, sec_adjustment; +@dots{} +t1 = t + sec_requested; +t2 = t1 + sec_adjustment; +if (((t1 < t) != (sec_requested < 0)) + | ((t2 < t1) != (sec_adjustment < 0))) + return -1; +@end example + +If your code looks like these examples, it is probably safe even though +it does not strictly conform to the C standard. This might lead one to +believe that one can generally assume wraparound on overflow, but that +is not always true, as can be seen in the next section. + +@node Optimization and Wraparound +@subsection Optimizations That Break Wraparound Arithmetic +@cindex loop induction + +Compilers sometimes generate code that is incompatible with wraparound +integer arithmetic. A simple example is an algebraic simplification: a +compiler might translate @code{(i * 2000) / 1000} to @code{i * 2} +because it assumes that @code{i * 2000} does not overflow. The +translation is not equivalent to the original when overflow occurs: +e.g., in the typical case of 32-bit signed two's complement wraparound +@code{int}, if @code{i} has type @code{int} and value @code{1073742}, +the original expression returns @minus{}2147483 but the optimized +version returns the mathematically correct value 2147484. + +More subtly, loop induction optimizations often exploit the undefined +behavior of signed overflow. Consider the following contrived function +@code{sumc}: + +@example +int +sumc (int lo, int hi) +@{ + int sum = 0; + int i; + for (i = lo; i <= hi; i++) + sum ^= i * 53; + return sum; +@} +@end example + +@noindent +To avoid multiplying by 53 each time through the loop, an optimizing +compiler might internally transform @code{sumc} to the equivalent of the +following: + +@example +int +transformed_sumc (int lo, int hi) +@{ + int sum = 0; + int hic = hi * 53; + int ic; + for (ic = lo * 53; ic <= hic; ic += 53) + sum ^= ic; + return sum; +@} +@end example + +@noindent +This transformation is allowed by the C standard, but it is invalid for +wraparound arithmetic when @code{INT_MAX / 53 < hi}, because then the +overflow in computing expressions like @code{hi * 53} can cause the +expression @code{i <= hi} to yield a different value from the +transformed expression @code{ic <= hic}. + +For this reason, compilers that use loop induction and similar +techniques often do not support reliable wraparound arithmetic when a +loop induction variable like @code{ic} is involved. Since loop +induction variables are generated by the compiler, and are not visible +in the source code, it is not always trivial to say whether the problem +affects your code. + +Hardly any code actually depends on wraparound arithmetic in cases like +these, so in practice these loop induction optimizations are almost +always useful. However, edge cases in this area can cause problems. +For example: + +@example +int j; +for (j = 1; 0 < j; j *= 2) + test (j); +@end example + +@noindent +Here, the loop attempts to iterate through all powers of 2 that +@code{int} can represent, but the C standard allows a compiler to +optimize away the comparison and generate an infinite loop, +under the argument that behavior is undefined on overflow. As of this +writing this optimization is not done by any production version of +GCC with @option{-O2}, but it might be performed by other +compilers, or by more aggressive GCC optimization options, +and the GCC developers have not decided whether it will +continue to work with GCC and @option{-O2}. + +@node Signed Overflow Advice +@subsection Practical Advice for Signed Overflow Issues +@cindex integer overflow +@cindex overflow, signed integer +@cindex signed integer overflow +@cindex wraparound arithmetic + +Ideally the safest approach is to avoid signed integer overflow +entirely. For example, instead of multiplying two signed integers, you +can convert them to unsigned integers, multiply the unsigned values, +then test whether the result is in signed range. + +Rewriting code in this way will be inconvenient, though, particularly if +the signed values might be negative. Also, it may hurt +performance. Using unsigned arithmetic to check for overflow is +particularly painful to do portably and efficiently when dealing with an +integer type like @code{uid_t} whose width and signedness vary from +platform to platform. + +Furthermore, many C applications pervasively assume wraparound behavior +and typically it is not easy to find and remove all these assumptions. +Hence it is often useful to maintain nonstandard code that assumes +wraparound on overflow, instead of rewriting the code. The rest of this +section attempts to give practical advice for this situation. + +If your code wants to detect signed integer overflow in @code{sum = a + +b}, it is generally safe to use an expression like @code{(sum < a) != (b +< 0)}. + +If your code uses a signed loop index, make sure that the index cannot +overflow, along with all signed expressions derived from the index. +Here is a contrived example of problematic code with two instances of +overflow. + +@example +for (i = INT_MAX - 10; i <= INT_MAX; i++) + if (i + 1 < 0) + @{ + report_overflow (); + break; + @} +@end example + +@noindent +Because of the two overflows, a compiler might optimize away or +transform the two comparisons in a way that is incompatible with the +wraparound assumption. + +If your code uses an expression like @code{(i * 2000) / 1000} and you +actually want the multiplication to wrap around on overflow, use +unsigned arithmetic +to do it, e.g., @code{((int) (i * 2000u)) / 1000}. + +If your code assumes wraparound behavior and you want to insulate it +against any GCC optimizations that would fail to support that +behavior, you should use GCC's @option{-fwrapv} option, which +causes signed overflow to wrap around reliably (except for division and +remainder, as discussed in the next section). + +If you need to port to platforms where signed integer overflow does not +reliably wrap around (e.g., due to hardware overflow checking, or to +highly aggressive optimizations), you should consider debugging with +GCC's @option{-ftrapv} option, which causes signed overflow to +raise an exception. + +@node Signed Integer Division +@subsection Signed Integer Division and Integer Overflow +@cindex division, integer + +Overflow in signed +integer division is not always harmless: for example, on CPUs of the +i386 family, dividing @code{INT_MIN} by @code{-1} yields a SIGFPE signal +which by default terminates the program. Worse, taking the remainder +of these two values typically yields the same signal on these CPUs, +even though the C standard requires @code{INT_MIN % -1} to yield zero +because the expression does not overflow. + +@node Preprocessor Arithmetic +@section Preprocessor Arithmetic +@cindex preprocessor arithmetic + +In C99, preprocessor arithmetic, used for @code{#if} expressions, must +be evaluated as if all signed values are of type @code{intmax_t} and all +unsigned values of type @code{uintmax_t}. Many compilers are buggy in +this area, though. For example, as of 2007, Sun C mishandles @code{#if +LLONG_MIN < 0} on a platform with 32-bit @code{long int} and 64-bit +@code{long long int}. Also, some older preprocessors mishandle +constants ending in @code{LL}. To work around these problems, you can +compute the value of expressions like @code{LONG_MAX < LLONG_MAX} at +@code{configure}-time rather than at @code{#if}-time. + +@node Null Pointers +@section Properties of Null Pointers +@cindex null pointers + +Most modern hosts reliably fail when you attempt to dereference a null +pointer. + +On almost all modern hosts, null pointers use an all-bits-zero internal +representation, so you can reliably use @code{memset} with 0 to set all +the pointers in an array to null values. + +If @code{p} is a null pointer to an object type, the C expression +@code{p + 0} always evaluates to @code{p} on modern hosts, even though +the standard says that it has undefined behavior. + +@node Buffer Overruns +@section Buffer Overruns and Subscript Errors +@cindex buffer overruns + +Buffer overruns and subscript errors are the most common dangerous +errors in C programs. They result in undefined behavior because storing +outside an array typically modifies storage that is used by some other +object, and most modern systems lack runtime checks to catch these +errors. Programs should not rely on buffer overruns being caught. + +There is one exception to the usual rule that a portable program cannot +address outside an array. In C, it is valid to compute the address just +past an object, e.g., @code{&a[N]} where @code{a} has @code{N} elements, +so long as you do not dereference the resulting pointer. But it is not +valid to compute the address just before an object, e.g., @code{&a[-1]}; +nor is it valid to compute two past the end, e.g., @code{&a[N+1]}. On +most platforms @code{&a[-1] < &a[0] && &a[N] < &a[N+1]}, but this is not +reliable in general, and it is usually easy enough to avoid the +potential portability problem, e.g., by allocating an extra unused array +element at the start or end. + +@uref{http://@/valgrind.org/, Valgrind} can catch many overruns. +GCC +users might also consider using the @option{-fmudflap} option to catch +overruns. + +Buffer overruns are usually caused by off-by-one errors, but there are +more subtle ways to get them. + +Using @code{int} values to index into an array or compute array sizes +causes problems on typical 64-bit hosts where an array index might +be @math{2^{31}} or larger. Index values of type @code{size_t} avoid this +problem, but cannot be negative. Index values of type @code{ptrdiff_t} +are signed, and are wide enough in practice. + +If you add or multiply two numbers to calculate an array size, e.g., +@code{malloc (x * sizeof y + z)}, havoc ensues if the addition or +multiplication overflows. + +Many implementations of the @code{alloca} function silently misbehave +and can generate buffer overflows if given sizes that are too large. +The size limits are implementation dependent, but are at least 4000 +bytes on all platforms that we know about. + +The standard functions @code{asctime}, @code{asctime_r}, @code{ctime}, +@code{ctime_r}, and @code{gets} are prone to buffer overflows, and +portable code should not use them unless the inputs are known to be +within certain limits. The time-related functions can overflow their +buffers if given timestamps out of range (e.g., a year less than -999 +or greater than 9999). Time-related buffer overflows cannot happen with +recent-enough versions of the GNU C library, but are possible +with other +implementations. The @code{gets} function is the worst, since it almost +invariably overflows its buffer when presented with an input line larger +than the buffer. + +@node Volatile Objects +@section Volatile Objects +@cindex volatile objects + +The keyword @code{volatile} is often misunderstood in portable code. +Its use inhibits some memory-access optimizations, but programmers often +wish that it had a different meaning than it actually does. + +@code{volatile} was designed for code that accesses special objects like +memory-mapped device registers whose contents spontaneously change. +Such code is inherently low-level, and it is difficult to specify +portably what @code{volatile} means in these cases. The C standard +says, ``What constitutes an access to an object that has +volatile-qualified type is implementation-defined,'' so in theory each +implementation is supposed to fill in the gap by documenting what +@code{volatile} means for that implementation. In practice, though, +this documentation is usually absent or incomplete. + +One area of confusion is the distinction between objects defined with +volatile types, and volatile lvalues. From the C standard's point of +view, an object defined with a volatile type has externally visible +behavior. You can think of such objects as having little oscilloscope +probes attached to them, so that the user can observe some properties of +accesses to them, just as the user can observe data written to output +files. However, the standard does not make it clear whether users can +observe accesses by volatile lvalues to ordinary objects. For example: + +@example +/* Declare and access a volatile object. + Accesses to X are "visible" to users. */ +static int volatile x; +x = 1; + +/* Access two ordinary objects via a volatile lvalue. + It's not clear whether accesses to *P are "visible". */ +int y; +int *z = malloc (sizeof (int)); +int volatile *p; +p = &y; +*p = 1; +p = z; +*p = 1; +@end example + +Programmers often wish that @code{volatile} meant ``Perform the memory +access here and now, without merging several memory accesses, without +changing the memory word size, and without reordering.'' But the C +standard does not require this. For objects defined with a volatile +type, accesses must be done before the next sequence point; but +otherwise merging, reordering, and word-size change is allowed. Worse, +it is not clear from the standard whether volatile lvalues provide more +guarantees in general than nonvolatile lvalues, if the underlying +objects are ordinary. + +Even when accessing objects defined with a volatile type, +the C standard allows only +extremely limited signal handlers: the behavior is undefined if a signal +handler reads any nonlocal object, or writes to any nonlocal object +whose type is not @code{sig_atomic_t volatile}, or calls any standard +library function other than @code{abort}, @code{signal}, and (if C99) +@code{_Exit}. Hence C compilers need not worry about a signal handler +disturbing ordinary computation, unless the computation accesses a +@code{sig_atomic_t volatile} lvalue that is not a local variable. +(There is an obscure exception for accesses via a pointer to a volatile +character, since it may point into part of a @code{sig_atomic_t +volatile} object.) Posix +adds to the list of library functions callable from a portable signal +handler, but otherwise is like the C standard in this area. + +Some C implementations allow memory-access optimizations within each +translation unit, such that actual behavior agrees with the behavior +required by the standard only when calling a function in some other +translation unit, and a signal handler acts like it was called from a +different translation unit. The C standard hints that in these +implementations, objects referred to by signal handlers ``would require +explicit specification of @code{volatile} storage, as well as other +implementation-defined restrictions.'' But unfortunately even for this +special case these other restrictions are often not documented well. +@xref{Volatiles, , When is a Volatile Object Accessed?, gcc, Using the +GNU Compiler Collection (GCC)}, for some +restrictions imposed by GCC. @xref{Defining Handlers, , +Defining Signal Handlers, libc, The GNU C Library}, for some +restrictions imposed by the GNU C library. Restrictions +differ on other platforms. + +If possible, it is best to use a signal handler that fits within the +limits imposed by the C and Posix standards. + +If this is not practical, you can try the following rules of thumb. A +signal handler should access only volatile lvalues, preferably lvalues +that refer to objects defined with a volatile type, and should not +assume that the accessed objects have an internally consistent state +if they are larger than a machine word. Furthermore, installers +should employ compilers and compiler options that are commonly used +for building operating system kernels, because kernels often need more +from @code{volatile} than the C Standard requires, and installers who +compile an application in a similar environment can sometimes benefit +from the extra constraints imposed by kernels on compilers. +Admittedly we are handwaving somewhat here, as there are few +guarantees in this area; the rules of thumb may help to fix some bugs +but there is a good chance that they will not fix them all. + +For @code{volatile}, C++ has the same problems that C does. +Multithreaded applications have even more problems with @code{volatile}, +but they are beyond the scope of this section. + +The bottom line is that using @code{volatile} typically hurts +performance but should not hurt correctness. In some cases its use +does help correctness, but these cases are often so poorly understood +that all too often adding @code{volatile} to a data structure merely +alleviates some symptoms of a bug while not fixing the bug in general. + +@node Floating Point Portability +@section Floating Point Portability +@cindex floating point + +Almost all modern systems use IEEE-754 floating point, and it is safe to +assume IEEE-754 in most portable code these days. For more information, +please see David Goldberg's classic paper +@uref{http://@/www.validlab.com/@/goldberg/@/paper.pdf, What Every Computer +Scientist Should Know About Floating-Point Arithmetic}. + +@node Exiting Portably +@section Exiting Portably +@cindex exiting portably + +A C or C++ program can exit with status @var{N} by returning +@var{N} from the @code{main} function. Portable programs are supposed +to exit either with status 0 or @code{EXIT_SUCCESS} to succeed, or with +status @code{EXIT_FAILURE} to fail, but in practice it is portable to +fail by exiting with status 1, and test programs that assume Posix can +fail by exiting with status values from 1 through 255. Programs on +SunOS 2.0 (1985) through 3.5.2 (1988) incorrectly exited with zero +status when @code{main} returned nonzero, but ancient systems like these +are no longer of practical concern. + +A program can also exit with status @var{N} by passing @var{N} to the +@code{exit} function, and a program can fail by calling the @code{abort} +function. If a program is specialized to just some platforms, it can fail +by calling functions specific to those platforms, e.g., @code{_exit} +(Posix) and @code{_Exit} (C99). However, like other functions, an exit +function should be declared, typically by including a header. For +example, if a C program calls @code{exit}, it should include @file{stdlib.h} +either directly or via the default includes (@pxref{Default Includes}). + +A program can fail due to undefined behavior such as dereferencing a null +pointer, but this is not recommended as undefined behavior allows an +implementation to do whatever it pleases and this includes exiting +successfully. + + +@c ================================================== Manual Configuration + +@node Manual Configuration +@chapter Manual Configuration + +A few kinds of features can't be guessed automatically by running test +programs. For example, the details of the object-file format, or +special options that need to be passed to the compiler or linker. You +can check for such features using ad-hoc means, such as having +@command{configure} check the output of the @code{uname} program, or +looking for libraries that are unique to particular systems. However, +Autoconf provides a uniform method for handling unguessable features. + +@menu +* Specifying Target Triplets:: Specifying target triplets +* Canonicalizing:: Getting the canonical system type +* Using System Type:: What to do with the system type +@end menu + +@node Specifying Target Triplets +@section Specifying target triplets +@cindex System type +@cindex Target triplet +@c This node used to be named Specifying Names. The @anchor allows old +@c links to still work. +@anchor{Specifying Names} + +Autoconf-generated +@command{configure} scripts can make decisions based on a canonical name +for the system type, or @dfn{target triplet}, which has the form: +@samp{@var{cpu}-@var{vendor}-@var{os}}, where @var{os} can be +@samp{@var{system}} or @samp{@var{kernel}-@var{system}} + +@command{configure} can usually guess the canonical name for the type of +system it's running on. To do so it runs a script called +@command{config.guess}, which infers the name using the @code{uname} +command or symbols predefined by the C preprocessor. + +Alternately, the user can specify the system type with command line +arguments to @command{configure} (@pxref{System Type}. Doing so is +necessary when +cross-compiling. In the most complex case of cross-compiling, three +system types are involved. The options to specify them are: + +@table @option +@item --build=@var{build-type} +the type of system on which the package is being configured and +compiled. It defaults to the result of running @command{config.guess}. +Specifying a @var{build-type} that differs from @var{host-type} enables +cross-compilation mode. + +@item --host=@var{host-type} +the type of system on which the package runs. By default it is the +same as the build machine. Specifying a @var{host-type} that differs +from @var{build-type}, when @var{build-type} was also explicitly +specified, enables cross-compilation mode. + +@item --target=@var{target-type} +the type of system for which any compiler tools in the package +produce code (rarely needed). By default, it is the same as host. +@end table + +If you mean to override the result of @command{config.guess}, use +@option{--build}, not @option{--host}, since the latter enables +cross-compilation. For historical reasons, +whenever you specify @option{--host}, +be sure to specify @option{--build} too; this will be fixed in the +future. So, to enter cross-compilation mode, use a command like this + +@example +./configure --build=i686-pc-linux-gnu --host=m68k-coff +@end example + +@noindent +Note that if you do not specify @option{--host}, @command{configure} +fails if it can't run the code generated by the specified compiler. For +example, configuring as follows fails: + +@example +./configure CC=m68k-coff-gcc +@end example + +When cross-compiling, @command{configure} will warn about any tools +(compilers, linkers, assemblers) whose name is not prefixed with the +host type. This is an aid to users performing cross-compilation. +Continuing the example above, if a cross-compiler named @command{cc} is +used with a native @command{pkg-config}, then libraries found by +@command{pkg-config} will likely cause subtle build failures; but using +the names @command{m68k-coff-cc} and @command{m68k-coff-pkg-config} +avoids any confusion. Avoiding the warning is as simple as creating the +correct symlinks naming the cross tools. + +@cindex @command{config.sub} +@command{configure} recognizes short aliases for many system types; for +example, @samp{decstation} can be used instead of +@samp{mips-dec-ultrix4.2}. @command{configure} runs a script called +@command{config.sub} to canonicalize system type aliases. + +This section deliberately omits the description of the obsolete +interface; see @ref{Hosts and Cross-Compilation}. + + +@node Canonicalizing +@section Getting the Canonical System Type +@cindex System type +@cindex Canonical system type + +The following macros make the system type available to @command{configure} +scripts. + +@ovindex build_alias +@ovindex host_alias +@ovindex target_alias + +The variables @samp{build_alias}, @samp{host_alias}, and +@samp{target_alias} are always exactly the arguments of @option{--build}, +@option{--host}, and @option{--target}; in particular, they are left empty +if the user did not use them, even if the corresponding +@code{AC_CANONICAL} macro was run. Any configure script may use these +variables anywhere. These are the variables that should be used when in +interaction with the user. + +If you need to recognize some special environments based on their system +type, run the following macros to get canonical system names. These +variables are not set before the macro call. + +If you use these macros, you must distribute @command{config.guess} and +@command{config.sub} along with your source code. @xref{Output}, for +information about the @code{AC_CONFIG_AUX_DIR} macro which you can use +to control in which directory @command{configure} looks for those scripts. + + +@defmac AC_CANONICAL_BUILD +@acindex{CANONICAL_BUILD} +@ovindex build +@ovindex build_cpu +@ovindex build_vendor +@ovindex build_os +Compute the canonical build-system type variable, @code{build}, and its +three individual parts @code{build_cpu}, @code{build_vendor}, and +@code{build_os}. + +If @option{--build} was specified, then @code{build} is the +canonicalization of @code{build_alias} by @command{config.sub}, +otherwise it is determined by the shell script @command{config.guess}. +@end defmac + +@defmac AC_CANONICAL_HOST +@acindex{CANONICAL_HOST} +@ovindex host +@ovindex host_cpu +@ovindex host_vendor +@ovindex host_os +Compute the canonical host-system type variable, @code{host}, and its +three individual parts @code{host_cpu}, @code{host_vendor}, and +@code{host_os}. + +If @option{--host} was specified, then @code{host} is the +canonicalization of @code{host_alias} by @command{config.sub}, +otherwise it defaults to @code{build}. +@end defmac + +@defmac AC_CANONICAL_TARGET +@acindex{CANONICAL_TARGET} +@ovindex target +@ovindex target_cpu +@ovindex target_vendor +@ovindex target_os +Compute the canonical target-system type variable, @code{target}, and its +three individual parts @code{target_cpu}, @code{target_vendor}, and +@code{target_os}. + +If @option{--target} was specified, then @code{target} is the +canonicalization of @code{target_alias} by @command{config.sub}, +otherwise it defaults to @code{host}. +@end defmac + +Note that there can be artifacts due to the backward compatibility +code. @xref{Hosts and Cross-Compilation}, for more. + +@node Using System Type +@section Using the System Type + +In @file{configure.ac} the system type is generally used by one or more +@code{case} statements to select system-specifics. Shell wildcards can +be used to match a group of system types. + +For example, an extra assembler code object file could be chosen, giving +access to a CPU cycle counter register. @code{$(CYCLE_OBJ)} in the +following would be used in a makefile to add the object to a +program or library. + +@example +AS_CASE([$host], + [alpha*-*-*], [CYCLE_OBJ=rpcc.o], + [i?86-*-*], [CYCLE_OBJ=rdtsc.o], + [CYCLE_OBJ=""] +) +AC_SUBST([CYCLE_OBJ]) +@end example + +@code{AC_CONFIG_LINKS} (@pxref{Configuration Links}) is another good way +to select variant source files, for example optimized code for some +CPUs. The configured CPU type doesn't always indicate exact CPU types, +so some runtime capability checks may be necessary too. + +@example +case $host in + alpha*-*-*) AC_CONFIG_LINKS([dither.c:alpha/dither.c]) ;; + powerpc*-*-*) AC_CONFIG_LINKS([dither.c:powerpc/dither.c]) ;; + *-*-*) AC_CONFIG_LINKS([dither.c:generic/dither.c]) ;; +esac +@end example + +The host system type can also be used to find cross-compilation tools +with @code{AC_CHECK_TOOL} (@pxref{Generic Programs}). + +The above examples all show @samp{$host}, since this is where the code +is going to run. Only rarely is it necessary to test @samp{$build} +(which is where the build is being done). + +Whenever you're tempted to use @samp{$host} it's worth considering +whether some sort of probe would be better. New system types come along +periodically or previously missing features are added. Well-written +probes can adapt themselves to such things, but hard-coded lists of +names can't. Here are some guidelines, + +@itemize @bullet +@item +Availability of libraries and library functions should always be checked +by probing. +@item +Variant behavior of system calls is best identified with runtime tests +if possible, but bug workarounds or obscure difficulties might have to +be driven from @samp{$host}. +@item +Assembler code is inevitably highly CPU-specific and is best selected +according to @samp{$host_cpu}. +@item +Assembler variations like underscore prefix on globals or ELF versus +COFF type directives are however best determined by probing, perhaps +even examining the compiler output. +@end itemize + +@samp{$target} is for use by a package creating a compiler or similar. +For ordinary packages it's meaningless and should not be used. It +indicates what the created compiler should generate code for, if it can +cross-compile. @samp{$target} generally selects various hard-coded CPU +and system conventions, since usually the compiler or tools under +construction themselves determine how the target works. + + +@c ===================================================== Site Configuration. + +@node Site Configuration +@chapter Site Configuration + +@command{configure} scripts support several kinds of local configuration +decisions. There are ways for users to specify where external software +packages are, include or exclude optional features, install programs +under modified names, and set default values for @command{configure} +options. + +@menu +* Help Formatting:: Customizing @samp{configure --help} +* External Software:: Working with other optional software +* Package Options:: Selecting optional features +* Pretty Help Strings:: Formatting help string +* Option Checking:: Controlling checking of @command{configure} options +* Site Details:: Configuring site details +* Transforming Names:: Changing program names when installing +* Site Defaults:: Giving @command{configure} local defaults +@end menu + +@node Help Formatting +@section Controlling Help Output + +Users consult @samp{configure --help} to learn of configuration +decisions specific to your package. By default, @command{configure} +breaks this output into sections for each type of option; within each +section, help strings appear in the order @file{configure.ac} defines +them: + +@example +Optional Features: + @dots{} + --enable-bar include bar + +Optional Packages: + @dots{} + --with-foo use foo +@end example + +@defmac AC_PRESERVE_HELP_ORDER +@acindex{PRESERVE_HELP_ORDER} + +Request an alternate @option{--help} format, in which options of all +types appear together, in the order defined. Call this macro before any +@code{AC_ARG_ENABLE} or @code{AC_ARG_WITH}. + +@example +Optional Features and Packages: + @dots{} + --enable-bar include bar + --with-foo use foo +@end example + +@end defmac + +@node External Software +@section Working With External Software +@cindex External software + +Some packages require, or can optionally use, other software packages +that are already installed. The user can give @command{configure} +command line options to specify which such external software to use. +The options have one of these forms: + +@c FIXME: Can't use @ovar here, Texinfo 4.0 goes lunatic and emits something +@c awful. +@example +--with-@var{package}@r{[}=@var{arg}@r{]} +--without-@var{package} +@end example + +For example, @option{--with-gnu-ld} means work with the GNU linker +instead of some other linker. @option{--with-x} means work with The X +Window System. + +The user can give an argument by following the package name with +@samp{=} and the argument. Giving an argument of @samp{no} is for +packages that are used by default; it says to @emph{not} use the +package. An argument that is neither @samp{yes} nor @samp{no} could +include a name or number of a version of the other package, to specify +more precisely which other package this program is supposed to work +with. If no argument is given, it defaults to @samp{yes}. +@option{--without-@var{package}} is equivalent to +@option{--with-@var{package}=no}. + +Normally @command{configure} scripts complain about +@option{--with-@var{package}} options that they do not support. +@xref{Option Checking}, for details, and for how to override the +defaults. + +For each external software package that may be used, @file{configure.ac} +should call @code{AC_ARG_WITH} to detect whether the @command{configure} +user asked to use it. Whether each package is used or not by default, +and which arguments are valid, is up to you. + +@anchor{AC_ARG_WITH} +@defmac AC_ARG_WITH (@var{package}, @var{help-string}, @ + @ovar{action-if-given}, @ovar{action-if-not-given}) +@acindex{ARG_WITH} +If the user gave @command{configure} the option @option{--with-@var{package}} +or @option{--without-@var{package}}, run shell commands +@var{action-if-given}. If neither option was given, run shell commands +@var{action-if-not-given}. The name @var{package} indicates another +software package that this program should work with. It should consist +only of alphanumeric characters, dashes, plus signs, and dots. + +The option's argument is available to the shell commands +@var{action-if-given} in the shell variable @code{withval}, which is +actually just the value of the shell variable named +@code{with_@var{package}}, with any non-alphanumeric characters in +@var{package} changed into @samp{_}. You may use that variable instead, +if you wish. + +The argument @var{help-string} is a description of the option that +looks like this: +@example + --with-readline support fancy command line editing +@end example + +@noindent +@var{help-string} may be more than one line long, if more detail is +needed. Just make sure the columns line up in @samp{configure +--help}. Avoid tabs in the help string. The easiest way to provide the +proper leading whitespace is to format your @var{help-string} with the macro +@code{AS_HELP_STRING} (@pxref{Pretty Help Strings}). + +The following example shows how to use the @code{AC_ARG_WITH} macro in +a common situation. You want to let the user decide whether to enable +support for an external library (e.g., the readline library); if the user +specified neither @option{--with-readline} nor @option{--without-readline}, +you want to enable support for readline only if the library is available +on the system. + +@c FIXME: Remove AS_IF when the problem of AC_REQUIRE within `if' is solved. +@example +AC_ARG_WITH([readline], + [AS_HELP_STRING([--with-readline], + [support fancy command line editing @@<:@@default=check@@:>@@])], + [], + [with_readline=check]) + +LIBREADLINE= +AS_IF([test "x$with_readline" != xno], + [AC_CHECK_LIB([readline], [main], + [AC_SUBST([LIBREADLINE], ["-lreadline -lncurses"]) + AC_DEFINE([HAVE_LIBREADLINE], [1], + [Define if you have libreadline]) + ], + [if test "x$with_readline" != xcheck; then + AC_MSG_FAILURE( + [--with-readline was given, but test for readline failed]) + fi + ], -lncurses)]) +@end example + +The next example shows how to use @code{AC_ARG_WITH} to give the user the +possibility to enable support for the readline library, in case it is still +experimental and not well tested, and is therefore disabled by default. + +@c FIXME: Remove AS_IF when the problem of AC_REQUIRE within `if' is solved. +@example +AC_ARG_WITH([readline], + [AS_HELP_STRING([--with-readline], + [enable experimental support for readline])], + [], + [with_readline=no]) + +LIBREADLINE= +AS_IF([test "x$with_readline" != xno], + [AC_CHECK_LIB([readline], [main], + [AC_SUBST([LIBREADLINE], ["-lreadline -lncurses"]) + AC_DEFINE([HAVE_LIBREADLINE], [1], + [Define if you have libreadline]) + ], + [AC_MSG_FAILURE( + [--with-readline was given, but test for readline failed])], + [-lncurses])]) +@end example + +The last example shows how to use @code{AC_ARG_WITH} to give the user the +possibility to disable support for the readline library, given that it is +an important feature and that it should be enabled by default. + +@c FIXME: Remove AS_IF when the problem of AC_REQUIRE within `if' is solved. +@example +AC_ARG_WITH([readline], + [AS_HELP_STRING([--without-readline], + [disable support for readline])], + [], + [with_readline=yes]) + +LIBREADLINE= +AS_IF([test "x$with_readline" != xno], + [AC_CHECK_LIB([readline], [main], + [AC_SUBST([LIBREADLINE], ["-lreadline -lncurses"]) + AC_DEFINE([HAVE_LIBREADLINE], [1], + [Define if you have libreadline]) + ], + [AC_MSG_FAILURE( + [readline test failed (--without-readline to disable)])], + [-lncurses])]) +@end example + +These three examples can be easily adapted to the case where +@code{AC_ARG_ENABLE} should be preferred to @code{AC_ARG_WITH} (see +@ref{Package Options}). +@end defmac + +@node Package Options +@section Choosing Package Options +@cindex Package options +@cindex Options, package + +If a software package has optional compile-time features, the user can +give @command{configure} command line options to specify whether to +compile them. The options have one of these forms: + +@c FIXME: Can't use @ovar here, Texinfo 4.0 goes lunatic and emits something +@c awful. +@example +--enable-@var{feature}@r{[}=@var{arg}@r{]} +--disable-@var{feature} +@end example + +These options allow users to choose which optional features to build and +install. @option{--enable-@var{feature}} options should never make a +feature behave differently or cause one feature to replace another. +They should only cause parts of the program to be built rather than left +out. + +The user can give an argument by following the feature name with +@samp{=} and the argument. Giving an argument of @samp{no} requests +that the feature @emph{not} be made available. A feature with an +argument looks like @option{--enable-debug=stabs}. If no argument is +given, it defaults to @samp{yes}. @option{--disable-@var{feature}} is +equivalent to @option{--enable-@var{feature}=no}. + +Normally @command{configure} scripts complain about +@option{--enable-@var{package}} options that they do not support. +@xref{Option Checking}, for details, and for how to override the +defaults. + +For each optional feature, @file{configure.ac} should call +@code{AC_ARG_ENABLE} to detect whether the @command{configure} user asked +to include it. Whether each feature is included or not by default, and +which arguments are valid, is up to you. + +@anchor{AC_ARG_ENABLE} +@defmac AC_ARG_ENABLE (@var{feature}, @var{help-string}, @ + @ovar{action-if-given}, @ovar{action-if-not-given}) +@acindex{ARG_ENABLE} +If the user gave @command{configure} the option +@option{--enable-@var{feature}} or @option{--disable-@var{feature}}, run +shell commands @var{action-if-given}. If neither option was given, run +shell commands @var{action-if-not-given}. The name @var{feature} +indicates an optional user-level facility. It should consist only of +alphanumeric characters, dashes, plus signs, and dots. + +The option's argument is available to the shell commands +@var{action-if-given} in the shell variable @code{enableval}, which is +actually just the value of the shell variable named +@code{enable_@var{feature}}, with any non-alphanumeric characters in +@var{feature} changed into @samp{_}. You may use that variable instead, +if you wish. The @var{help-string} argument is like that of +@code{AC_ARG_WITH} (@pxref{External Software}). + +You should format your @var{help-string} with the macro +@code{AS_HELP_STRING} (@pxref{Pretty Help Strings}). + +See the examples suggested with the definition of @code{AC_ARG_WITH} +(@pxref{External Software}) to get an idea of possible applications of +@code{AC_ARG_ENABLE}. +@end defmac + +@node Pretty Help Strings +@section Making Your Help Strings Look Pretty +@cindex Help strings + +Properly formatting the @samp{help strings} which are used in +@code{AC_ARG_WITH} (@pxref{External Software}) and @code{AC_ARG_ENABLE} +(@pxref{Package Options}) can be challenging. Specifically, you want +your own @samp{help strings} to line up in the appropriate columns of +@samp{configure --help} just like the standard Autoconf @samp{help +strings} do. This is the purpose of the @code{AS_HELP_STRING} macro. + +@anchor{AS_HELP_STRING} +@defmac AS_HELP_STRING (@var{left-hand-side}, @var{right-hand-side} @ + @dvar{indent-column, 26}, @dvar{wrap-column, 79}) +@asindex{HELP_STRING} + +Expands into a help string that looks pretty when the user executes +@samp{configure --help}. It is typically used in @code{AC_ARG_WITH} +(@pxref{External Software}) or @code{AC_ARG_ENABLE} (@pxref{Package +Options}). The following example makes this clearer. + +@example +AC_ARG_WITH([foo], + [AS_HELP_STRING([--with-foo], + [use foo (default is no)])], + [use_foo=$withval], + [use_foo=no]) +@end example + +Then the last few lines of @samp{configure --help} appear like +this: + +@example +--enable and --with options recognized: + --with-foo use foo (default is no) +@end example + +Macro expansion is performed on the first argument. However, the second +argument of @code{AS_HELP_STRING} is treated as a whitespace separated +list of text to be reformatted, and is not subject to macro expansion. +Since it is not expanded, it should not be double quoted. +@xref{Autoconf Language}, for a more detailed explanation. + +The @code{AS_HELP_STRING} macro is particularly helpful when the +@var{left-hand-side} and/or @var{right-hand-side} are composed of macro +arguments, as shown in the following example. Be aware that +@var{left-hand-side} may not expand to unbalanced quotes, +although quadrigraphs can be used. + +@example +AC_DEFUN([MY_ARG_WITH], + [AC_ARG_WITH(m4_translit([[$1]], [_], [-]), + [AS_HELP_STRING([--with-m4_translit([$1], [_], [-])], + [use $1 (default is $2)])], + [use_[]$1=$withval], + [use_[]$1=$2])]) +MY_ARG_WITH([a_b], [no]) +@end example +@noindent +Here, the last few lines of @samp{configure --help} will include: + +@example +--enable and --with options recognized: + --with-a-b use a_b (default is no) +@end example + +The parameters @var{indent-column} and @var{wrap-column} were introduced +in Autoconf 2.62. Generally, they should not be specified; they exist +for fine-tuning of the wrapping. +@example +AS_HELP_STRING([--option], [description of option]) +@result{} --option description of option +AS_HELP_STRING([--option], [description of option], [15], [30]) +@result{} --option description of +@result{} option +@end example +@end defmac + + +@node Option Checking +@section Controlling Checking of @command{configure} Options +@cindex Options, Package + +The @command{configure} script checks its command-line options against a +list of known options, like @option{--help} or @option{--config-cache}. +An unknown option ordinarily indicates a mistake by the user and +@command{configure} halts with an error. However, by default unknown +@option{--with-@var{package}} and @option{--enable-@var{feature}} +options elicit only a warning, to support configuring entire source +trees. + +Source trees often contain multiple packages with a top-level +@command{configure} script that uses the @code{AC_CONFIG_SUBDIRS} macro +(@pxref{Subdirectories}). Because the packages generally support +different @option{--with-@var{package}} and +@option{--enable-@var{feature}} options, the GNU Coding +Standards say they must accept unrecognized options without halting. +Even a warning message is undesirable here, so @code{AC_CONFIG_SUBDIRS} +automatically disables the warnings. + +This default behavior may be modified in two ways. First, the installer +can invoke @code{configure --disable-option-checking} to disable +these warnings, or invoke @code{configure --enable-option-checking=fatal} +options to turn them into fatal errors, respectively. Second, the +maintainer can use @code{AC_DISABLE_OPTION_CHECKING}. + +@defmac AC_DISABLE_OPTION_CHECKING +@acindex{DISABLE_OPTION_CHECKING} + +By default, disable warnings related to any unrecognized +@option{--with-@var{package}} or @option{--enable-@var{feature}} +options. This is implied by @code{AC_CONFIG_SUBDIRS}. + +The installer can override this behavior by passing +@option{--enable-option-checking} (enable warnings) or +@option{--enable-option-checking=fatal} (enable errors) to +@command{configure}. +@end defmac + + +@node Site Details +@section Configuring Site Details +@cindex Site details + +Some software packages require complex site-specific information. Some +examples are host names to use for certain services, company names, and +email addresses to contact. Since some configuration scripts generated +by Metaconfig ask for such information interactively, people sometimes +wonder how to get that information in Autoconf-generated configuration +scripts, which aren't interactive. + +Such site configuration information should be put in a file that is +edited @emph{only by users}, not by programs. The location of the file +can either be based on the @code{prefix} variable, or be a standard +location such as the user's home directory. It could even be specified +by an environment variable. The programs should examine that file at +runtime, rather than at compile time. Runtime configuration is more +convenient for users and makes the configuration process simpler than +getting the information while configuring. @xref{Directory Variables, , +Variables for Installation Directories, standards, The GNU Coding +Standards}, for more information on where to put data files. + +@node Transforming Names +@section Transforming Program Names When Installing +@cindex Transforming program names +@cindex Program names, transforming + +Autoconf supports changing the names of programs when installing them. +In order to use these transformations, @file{configure.ac} must call the +macro @code{AC_ARG_PROGRAM}. + +@defmac AC_ARG_PROGRAM +@acindex{ARG_PROGRAM} +@ovindex program_transform_name +Place in output variable @code{program_transform_name} a sequence of +@code{sed} commands for changing the names of installed programs. + +If any of the options described below are given to @command{configure}, +program names are transformed accordingly. Otherwise, if +@code{AC_CANONICAL_TARGET} has been called and a @option{--target} value +is given, the target type followed by a dash is used as a prefix. +Otherwise, no program name transformation is done. +@end defmac + +@menu +* Transformation Options:: @command{configure} options to transform names +* Transformation Examples:: Sample uses of transforming names +* Transformation Rules:: Makefile uses of transforming names +@end menu + +@node Transformation Options +@subsection Transformation Options + +You can specify name transformations by giving @command{configure} these +command line options: + +@table @option +@item --program-prefix=@var{prefix} +prepend @var{prefix} to the names; + +@item --program-suffix=@var{suffix} +append @var{suffix} to the names; + +@item --program-transform-name=@var{expression} +perform @code{sed} substitution @var{expression} on the names. +@end table + +@node Transformation Examples +@subsection Transformation Examples + +These transformations are useful with programs that can be part of a +cross-compilation development environment. For example, a +cross-assembler running on a Sun 4 configured with +@option{--target=i960-vxworks} is normally installed as +@file{i960-vxworks-as}, rather than @file{as}, which could be confused +with a native Sun 4 assembler. + +You can force a program name to begin with @file{g}, if you don't want +GNU programs installed on your system to shadow other programs with +the same name. For example, if you configure GNU @code{diff} with +@option{--program-prefix=g}, then when you run @samp{make install} it is +installed as @file{/usr/local/bin/gdiff}. + +As a more sophisticated example, you could use + +@example +--program-transform-name='s/^/g/; s/^gg/g/; s/^gless/less/' +@end example +@noindent + +to prepend @samp{g} to most of the program names in a source tree, +excepting those like @code{gdb} that already have one and those like +@code{less} and @code{lesskey} that aren't GNU programs. (That is +assuming that you have a source tree containing those programs that is +set up to use this feature.) + +One way to install multiple versions of some programs simultaneously is +to append a version number to the name of one or both. For example, if +you want to keep Autoconf version 1 around for awhile, you can configure +Autoconf version 2 using @option{--program-suffix=2} to install the +programs as @file{/usr/local/bin/autoconf2}, +@file{/usr/local/bin/autoheader2}, etc. Nevertheless, pay attention +that only the binaries are renamed, therefore you'd have problems with +the library files which might overlap. + +@node Transformation Rules +@subsection Transformation Rules + +Here is how to use the variable @code{program_transform_name} in a +@file{Makefile.in}: + +@example +PROGRAMS = cp ls rm +transform = @@program_transform_name@@ +install: + for p in $(PROGRAMS); do \ + $(INSTALL_PROGRAM) $$p $(DESTDIR)$(bindir)/`echo $$p | \ + sed '$(transform)'`; \ + done + +uninstall: + for p in $(PROGRAMS); do \ + rm -f $(DESTDIR)$(bindir)/`echo $$p | sed '$(transform)'`; \ +@c $$ restore font-lock + done +@end example + +It is guaranteed that @code{program_transform_name} is never empty, and +that there are no useless separators. Therefore you may safely embed +@code{program_transform_name} within a sed program using @samp{;}: + +@example +transform = @@program_transform_name@@ +transform_exe = s/$(EXEEXT)$$//;$(transform);s/$$/$(EXEEXT)/ +@end example + +Whether to do the transformations on documentation files (Texinfo or +@code{man}) is a tricky question; there seems to be no perfect answer, +due to the several reasons for name transforming. Documentation is not +usually particular to a specific architecture, and Texinfo files do not +conflict with system documentation. But they might conflict with +earlier versions of the same files, and @code{man} pages sometimes do +conflict with system documentation. As a compromise, it is probably +best to do name transformations on @code{man} pages but not on Texinfo +manuals. + +@node Site Defaults +@section Setting Site Defaults +@cindex Site defaults +@cindex config.site + +Autoconf-generated @command{configure} scripts allow your site to provide +default values for some configuration values. You do this by creating +site- and system-wide initialization files. + +@evindex CONFIG_SITE +If the environment variable @code{CONFIG_SITE} is set, @command{configure} +uses its value as the name of a shell script to read; it is recommended +that this be an absolute file name. Otherwise, it +reads the shell script @file{@var{prefix}/share/config.site} if it exists, +then @file{@var{prefix}/etc/config.site} if it exists. Thus, +settings in machine-specific files override those in machine-independent +ones in case of conflict. + +Site files can be arbitrary shell scripts, but only certain kinds of +code are really appropriate to be in them. Because @command{configure} +reads any cache file after it has read any site files, a site file can +define a default cache file to be shared between all Autoconf-generated +@command{configure} scripts run on that system (@pxref{Cache Files}). If +you set a default cache file in a site file, it is a good idea to also +set the output variable @code{CC} in that site file, because the cache +file is only valid for a particular compiler, but many systems have +several available. + +You can examine or override the value set by a command line option to +@command{configure} in a site file; options set shell variables that have +the same names as the options, with any dashes turned into underscores. +The exceptions are that @option{--without-} and @option{--disable-} options +are like giving the corresponding @option{--with-} or @option{--enable-} +option and the value @samp{no}. Thus, @option{--cache-file=localcache} +sets the variable @code{cache_file} to the value @samp{localcache}; +@option{--enable-warnings=no} or @option{--disable-warnings} sets the variable +@code{enable_warnings} to the value @samp{no}; @option{--prefix=/usr} sets the +variable @code{prefix} to the value @samp{/usr}; etc. + +Site files are also good places to set default values for other output +variables, such as @code{CFLAGS}, if you need to give them non-default +values: anything you would normally do, repetitively, on the command +line. If you use non-default values for @var{prefix} or +@var{exec_prefix} (wherever you locate the site file), you can set them +in the site file if you specify it with the @code{CONFIG_SITE} +environment variable. + +You can set some cache values in the site file itself. Doing this is +useful if you are cross-compiling, where it is impossible to check features +that require running a test program. You could ``prime the cache'' by +setting those values correctly for that system in +@file{@var{prefix}/etc/config.site}. To find out the names of the cache +variables you need to set, see the documentation of the respective +Autoconf macro. If the variables or their semantics are undocumented, +you may need to look for shell variables with @samp{_cv_} in their names +in the affected @command{configure} scripts, or in the Autoconf M4 +source code for those macros; but in that case, their name or semantics +may change in a future Autoconf version. + +The cache file is careful to not override any variables set in the site +files. Similarly, you should not override command-line options in the +site files. Your code should check that variables such as @code{prefix} +and @code{cache_file} have their default values (as set near the top of +@command{configure}) before changing them. + +Here is a sample file @file{/usr/share/local/@/gnu/share/@/config.site}. The +command @samp{configure --prefix=/usr/share/local/gnu} would read this +file (if @code{CONFIG_SITE} is not set to a different file). + +@example +# /usr/share/local/gnu/share/config.site for configure +# +# Change some defaults. +test "$prefix" = NONE && prefix=/usr/share/local/gnu +test "$exec_prefix" = NONE && exec_prefix=/usr/local/gnu +test "$sharedstatedir" = '$@{prefix@}/com' && sharedstatedir=/var +test "$localstatedir" = '$@{prefix@}/var' && localstatedir=/var + +# Give Autoconf 2.x generated configure scripts a shared default +# cache file for feature test results, architecture-specific. +if test "$cache_file" = /dev/null; then + cache_file="$prefix/var/config.cache" + # A cache file is only valid for one C compiler. + CC=gcc +fi +@end example + +@c Leave this use of ``File system'' rendered as one word, but +@c slightly obfuscated so as not to trigger the syntax-check prohibition. +@cindex File@/system Hierarchy Standard +@cindex FHS + +Another use of @file{config.site} is for priming the directory variables +@c ``File system'', but slightly obfuscated, as above. +in a manner consistent with the File@/system Hierarchy Standard +(FHS). Once the following file is installed at +@file{/usr/share/config.site}, a user can execute simply +@code{./configure --prefix=/usr} to get all the directories chosen in +the locations recommended by FHS. + +@example +# /usr/share/config.site for FHS defaults when installing below /usr, +# and the respective settings were not changed on the command line. +if test "$prefix" = /usr; then + test "$sysconfdir" = '$@{prefix@}/etc' && sysconfdir=/etc + test "$sharedstatedir" = '$@{prefix@}/com' && sharedstatedir=/var + test "$localstatedir" = '$@{prefix@}/var' && localstatedir=/var +fi +@end example + +@cindex @file{lib64} +@cindex 64-bit libraries +Likewise, on platforms where 64-bit libraries are built by default, then +installed in @file{/usr/local/@/lib64} instead of @file{/usr/local/@/lib}, +it is appropriate to install @file{/usr/local/@/share/config.site}: + +@example +# /usr/local/share/config.site for platforms that prefer +# the directory /usr/local/lib64 over /usr/local/lib. +test "$libdir" = '$@{exec_prefix@}/lib' && libdir='$@{exec_prefix@}/lib64' +@end example + + +@c ============================================== Running configure Scripts. + +@node Running configure Scripts +@chapter Running @command{configure} Scripts +@cindex @command{configure} + +Below are instructions on how to configure a package that uses a +@command{configure} script, suitable for inclusion as an @file{INSTALL} +file in the package. A plain-text version of @file{INSTALL} which you +may use comes with Autoconf. + +@menu +* Basic Installation:: Instructions for typical cases +* Compilers and Options:: Selecting compilers and optimization +* Multiple Architectures:: Compiling for multiple architectures at once +* Installation Names:: Installing in different directories +* Optional Features:: Selecting optional features +* Particular Systems:: Particular systems +* System Type:: Specifying the system type +* Sharing Defaults:: Setting site-wide defaults for @command{configure} +* Defining Variables:: Specifying the compiler etc. +* configure Invocation:: Changing how @command{configure} runs +@end menu + +@set autoconf +@include install.texi + + +@c ============================================== config.status Invocation + +@node config.status Invocation +@chapter config.status Invocation +@cindex @command{config.status} + +The @command{configure} script creates a file named @file{config.status}, +which actually configures, @dfn{instantiates}, the template files. It +also records the configuration options that were specified when the +package was last configured in case reconfiguring is needed. + +Synopsis: +@example +./config.status @ovar{option}@dots{} @ovar{tag}@dots{} +@end example + +It configures each @var{tag}; if none are specified, all the templates +are instantiated. A @var{tag} refers to a file or other tag associated +with a configuration action, as specified by an @code{AC_CONFIG_@var{ITEMS}} +macro (@pxref{Configuration Actions}). The files must be specified +without their dependencies, as in + +@example +./config.status foobar +@end example + +@noindent +not + +@example +./config.status foobar:foo.in:bar.in +@end example + +The supported options are: + +@table @option +@item --help +@itemx -h +Print a summary of the command line options, the list of the template +files, and exit. + +@item --version +@itemx -V +Print the version number of Autoconf and the configuration settings, +and exit. + +@item --config +Print the configuration settings in reusable way, quoted for the shell, +and exit. For example, for a debugging build that otherwise reuses the +configuration from a different build directory @var{build-dir} of a +package in @var{src-dir}, you could use the following: + +@example +args=`@var{build-dir}/config.status --config` +eval @var{src-dir}/configure "$args" CFLAGS=-g --srcdir=@var{src-dir} +@end example + +@noindent +Note that it may be necessary to override a @option{--srcdir} setting +that was saved in the configuration, if the arguments are used in a +different build directory. + +@item --silent +@itemx --quiet +@itemx -q +Do not print progress messages. + +@item --debug +@itemx -d +Don't remove the temporary files. + +@item --file=@var{file}[:@var{template}] +Require that @var{file} be instantiated as if +@samp{AC_CONFIG_FILES(@var{file}:@var{template})} was used. Both +@var{file} and @var{template} may be @samp{-} in which case the standard +output and/or standard input, respectively, is used. If a +@var{template} file name is relative, it is first looked for in the build +tree, and then in the source tree. @xref{Configuration Actions}, for +more details. + +This option and the following ones provide one way for separately +distributed packages to share the values computed by @command{configure}. +Doing so can be useful if some of the packages need a superset of the +features that one of them, perhaps a common library, does. These +options allow a @file{config.status} file to create files other than the +ones that its @file{configure.ac} specifies, so it can be used for a +different package, or for extracting a subset of values. For example, + +@example +echo '@@CC@@' | ./config.status --file=- +@end example + +@noindent +provides the value of @code{@@CC@@} on standard output. + +@item --header=@var{file}[:@var{template}] +Same as @option{--file} above, but with @samp{AC_CONFIG_HEADERS}. + +@item --recheck +Ask @file{config.status} to update itself and exit (no instantiation). +This option is useful if you change @command{configure}, so that the +results of some tests might be different from the previous run. The +@option{--recheck} option reruns @command{configure} with the same arguments +you used before, plus the @option{--no-create} option, which prevents +@command{configure} from running @file{config.status} and creating +@file{Makefile} and other files, and the @option{--no-recursion} option, +which prevents @command{configure} from running other @command{configure} +scripts in subdirectories. (This is so other Make rules can +run @file{config.status} when it changes; @pxref{Automatic Remaking}, +for an example). +@end table + +@file{config.status} checks several optional environment variables that +can alter its behavior: + +@anchor{CONFIG_SHELL} +@defvar CONFIG_SHELL +@evindex CONFIG_SHELL +The shell with which to run @command{configure}. It must be +Bourne-compatible, and the absolute name of the shell should be passed. +The default is a shell that supports @code{LINENO} if available, and +@file{/bin/sh} otherwise. +@end defvar + +@defvar CONFIG_STATUS +@evindex CONFIG_STATUS +The file name to use for the shell script that records the +configuration. The default is @file{./config.status}. This variable is +useful when one package uses parts of another and the @command{configure} +scripts shouldn't be merged because they are maintained separately. +@end defvar + +You can use @file{./config.status} in your makefiles. For example, in +the dependencies given above (@pxref{Automatic Remaking}), +@file{config.status} is run twice when @file{configure.ac} has changed. +If that bothers you, you can make each run only regenerate the files for +that rule: +@example +@group +config.h: stamp-h +stamp-h: config.h.in config.status + ./config.status config.h + echo > stamp-h + +Makefile: Makefile.in config.status + ./config.status Makefile +@end group +@end example + +The calling convention of @file{config.status} has changed; see +@ref{Obsolete config.status Use}, for details. + + +@c =================================================== Obsolete Constructs + +@node Obsolete Constructs +@chapter Obsolete Constructs +@cindex Obsolete constructs + +Autoconf changes, and throughout the years some constructs have been +obsoleted. Most of the changes involve the macros, but in some cases +the tools themselves, or even some concepts, are now considered +obsolete. + +You may completely skip this chapter if you are new to Autoconf. Its +intention is mainly to help maintainers updating their packages by +understanding how to move to more modern constructs. + +@menu +* Obsolete config.status Use:: Obsolete convention for @command{config.status} +* acconfig Header:: Additional entries in @file{config.h.in} +* autoupdate Invocation:: Automatic update of @file{configure.ac} +* Obsolete Macros:: Backward compatibility macros +* Autoconf 1:: Tips for upgrading your files +* Autoconf 2.13:: Some fresher tips +@end menu + +@node Obsolete config.status Use +@section Obsolete @file{config.status} Invocation + +@file{config.status} now supports arguments to specify the files to +instantiate; see @ref{config.status Invocation}, for more details. +Before, environment variables had to be used. + +@defvar CONFIG_COMMANDS +@evindex CONFIG_COMMANDS +The tags of the commands to execute. The default is the arguments given +to @code{AC_OUTPUT} and @code{AC_CONFIG_COMMANDS} in +@file{configure.ac}. +@end defvar + +@defvar CONFIG_FILES +@evindex CONFIG_FILES +The files in which to perform @samp{@@@var{variable}@@} substitutions. +The default is the arguments given to @code{AC_OUTPUT} and +@code{AC_CONFIG_FILES} in @file{configure.ac}. +@end defvar + +@defvar CONFIG_HEADERS +@evindex CONFIG_HEADERS +The files in which to substitute C @code{#define} statements. The +default is the arguments given to @code{AC_CONFIG_HEADERS}; if that +macro was not called, @file{config.status} ignores this variable. +@end defvar + +@defvar CONFIG_LINKS +@evindex CONFIG_LINKS +The symbolic links to establish. The default is the arguments given to +@code{AC_CONFIG_LINKS}; if that macro was not called, +@file{config.status} ignores this variable. +@end defvar + +In @ref{config.status Invocation}, using this old interface, the example +would be: + +@example +@group +config.h: stamp-h +stamp-h: config.h.in config.status + CONFIG_COMMANDS= CONFIG_LINKS= CONFIG_FILES= \ + CONFIG_HEADERS=config.h ./config.status + echo > stamp-h + +Makefile: Makefile.in config.status + CONFIG_COMMANDS= CONFIG_LINKS= CONFIG_HEADERS= \ + CONFIG_FILES=Makefile ./config.status +@end group +@end example + +@noindent +(If @file{configure.ac} does not call @code{AC_CONFIG_HEADERS}, there is +no need to set @code{CONFIG_HEADERS} in the @command{make} rules. Equally +for @code{CONFIG_COMMANDS}, etc.) + + +@node acconfig Header +@section @file{acconfig.h} + +@cindex @file{acconfig.h} +@cindex @file{config.h.top} +@cindex @file{config.h.bot} + +In order to produce @file{config.h.in}, @command{autoheader} needs to +build or to find templates for each symbol. Modern releases of Autoconf +use @code{AH_VERBATIM} and @code{AH_TEMPLATE} (@pxref{Autoheader +Macros}), but in older releases a file, @file{acconfig.h}, contained the +list of needed templates. @command{autoheader} copied comments and +@code{#define} and @code{#undef} statements from @file{acconfig.h} in +the current directory, if present. This file used to be mandatory if +you @code{AC_DEFINE} any additional symbols. + +Modern releases of Autoconf also provide @code{AH_TOP} and +@code{AH_BOTTOM} if you need to prepend/append some information to +@file{config.h.in}. Ancient versions of Autoconf had a similar feature: +if @file{./acconfig.h} contains the string @samp{@@TOP@@}, +@command{autoheader} copies the lines before the line containing +@samp{@@TOP@@} into the top of the file that it generates. Similarly, +if @file{./acconfig.h} contains the string @samp{@@BOTTOM@@}, +@command{autoheader} copies the lines after that line to the end of the +file it generates. Either or both of those strings may be omitted. An +even older alternate way to produce the same effect in ancient versions +of Autoconf is to create the files @file{@var{file}.top} (typically +@file{config.h.top}) and/or @file{@var{file}.bot} in the current +directory. If they exist, @command{autoheader} copies them to the +beginning and end, respectively, of its output. + +In former versions of Autoconf, the files used in preparing a software +package for distribution were: +@example +@group +configure.ac --. .------> autoconf* -----> configure + +---+ +[aclocal.m4] --+ `---. +[acsite.m4] ---' | + +--> [autoheader*] -> [config.h.in] +[acconfig.h] ----. | + +-----' +[config.h.top] --+ +[config.h.bot] --' +@end group +@end example + +Using only the @code{AH_} macros, @file{configure.ac} should be +self-contained, and should not depend upon @file{acconfig.h} etc. + + +@node autoupdate Invocation +@section Using @command{autoupdate} to Modernize @file{configure.ac} +@cindex @command{autoupdate} + +The @command{autoupdate} program updates a @file{configure.ac} file that +calls Autoconf macros by their old names to use the current macro names. +In version 2 of Autoconf, most of the macros were renamed to use a more +uniform and descriptive naming scheme. @xref{Macro Names}, for a +description of the new scheme. Although the old names still work +(@pxref{Obsolete Macros}, for a list of the old macros and the corresponding +new names), you can make your @file{configure.ac} files more readable +and make it easier to use the current Autoconf documentation if you +update them to use the new macro names. + +@evindex SIMPLE_BACKUP_SUFFIX +If given no arguments, @command{autoupdate} updates @file{configure.ac}, +backing up the original version with the suffix @file{~} (or the value +of the environment variable @code{SIMPLE_BACKUP_SUFFIX}, if that is +set). If you give @command{autoupdate} an argument, it reads that file +instead of @file{configure.ac} and writes the updated file to the +standard output. + +@noindent +@command{autoupdate} accepts the following options: + +@table @option +@item --help +@itemx -h +Print a summary of the command line options and exit. + +@item --version +@itemx -V +Print the version number of Autoconf and exit. + +@item --verbose +@itemx -v +Report processing steps. + +@item --debug +@itemx -d +Don't remove the temporary files. + +@item --force +@itemx -f +Force the update even if the file has not changed. Disregard the cache. + +@item --include=@var{dir} +@itemx -I @var{dir} +Also look for input files in @var{dir}. Multiple invocations accumulate. +Directories are browsed from last to first. + +@item --prepend-include=@var{dir} +@itemx -B @var{dir} +Prepend directory @var{dir} to the search path. This is used to include +the language-specific files before any third-party macros. +@end table + +@node Obsolete Macros +@section Obsolete Macros + +Several macros are obsoleted in Autoconf, for various reasons (typically +they failed to quote properly, couldn't be extended for more recent +issues, etc.). They are still supported, but deprecated: their use +should be avoided. + +During the jump from Autoconf version 1 to version 2, most of the +macros were renamed to use a more uniform and descriptive naming scheme, +but their signature did not change. @xref{Macro Names}, for a +description of the new naming scheme. Below, if there is just the mapping +from old names to new names for these macros, the reader is invited to +refer to the definition of the new macro for the signature and the +description. + +@defmac AC_AIX +@acindex{AIX} +@cvindex _ALL_SOURCE +This macro is a platform-specific subset of +@code{AC_USE_SYSTEM_EXTENSIONS} (@pxref{AC_USE_SYSTEM_EXTENSIONS}). +@end defmac + +@defmac AC_ALLOCA +@acindex{ALLOCA} +Replaced by @code{AC_FUNC_ALLOCA} (@pxref{AC_FUNC_ALLOCA}). +@end defmac + +@defmac AC_ARG_ARRAY +@acindex{ARG_ARRAY} +Removed because of limited usefulness. +@end defmac + +@defmac AC_C_CROSS +@acindex{C_CROSS} +This macro is obsolete; it does nothing. +@end defmac + +@defmac AC_C_LONG_DOUBLE +@acindex{C_LONG_DOUBLE} +@cvindex HAVE_LONG_DOUBLE +If the C compiler supports a working @code{long double} type with more +range or precision than the @code{double} type, define +@code{HAVE_LONG_DOUBLE}. + +You should use @code{AC_TYPE_LONG_DOUBLE} or +@code{AC_TYPE_LONG_DOUBLE_WIDER} instead. @xref{Particular Types}. +@end defmac + +@defmac AC_CANONICAL_SYSTEM +@acindex{CANONICAL_SYSTEM} +Determine the system type and set output variables to the names of the +canonical system types. @xref{Canonicalizing}, for details about the +variables this macro sets. + +The user is encouraged to use either @code{AC_CANONICAL_BUILD}, or +@code{AC_CANONICAL_HOST}, or @code{AC_CANONICAL_TARGET}, depending on +the needs. Using @code{AC_CANONICAL_TARGET} is enough to run the two +other macros (@pxref{Canonicalizing}). +@end defmac + +@defmac AC_CHAR_UNSIGNED +@acindex{CHAR_UNSIGNED} +Replaced by @code{AC_C_CHAR_UNSIGNED} (@pxref{AC_C_CHAR_UNSIGNED}). +@end defmac + +@defmac AC_CHECK_TYPE (@var{type}, @var{default}) +@acindex{CHECK_TYPE} +Autoconf, up to 2.13, used to provide this version of +@code{AC_CHECK_TYPE}, deprecated because of its flaws. First, although +it is a member of the @code{CHECK} clan, it does +more than just checking. Secondly, missing types are defined +using @code{#define}, not @code{typedef}, and this can lead to +problems in the case of pointer types. + +This use of @code{AC_CHECK_TYPE} is obsolete and discouraged; see +@ref{Generic Types}, for the description of the current macro. + +If the type @var{type} is not defined, define it to be the C (or C++) +builtin type @var{default}, e.g., @samp{short int} or @samp{unsigned int}. + +This macro is equivalent to: + +@example +AC_CHECK_TYPE([@var{type}], [], + [AC_DEFINE_UNQUOTED([@var{type}], [@var{default}], + [Define to `@var{default}' + if <sys/types.h> does not define.])]) +@end example + +In order to keep backward compatibility, the two versions of +@code{AC_CHECK_TYPE} are implemented, selected using these heuristics: + +@enumerate +@item +If there are three or four arguments, the modern version is used. + +@item +If the second argument appears to be a C or C++ type, then the +obsolete version is used. This happens if the argument is a C or C++ +@emph{builtin} type or a C identifier ending in @samp{_t}, optionally +followed by one of @samp{[(* } and then by a string of zero or more +characters taken from the set @samp{[]()* _a-zA-Z0-9}. + +@item +If the second argument is spelled with the alphabet of valid C and C++ +types, the user is warned and the modern version is used. + +@item +Otherwise, the modern version is used. +@end enumerate + +@noindent +You are encouraged either to use a valid builtin type, or to use the +equivalent modern code (see above), or better yet, to use +@code{AC_CHECK_TYPES} together with + +@example +#ifndef HAVE_LOFF_T +typedef loff_t off_t; +#endif +@end example +@end defmac +@c end of AC_CHECK_TYPE + +@defmac AC_CHECKING (@var{feature-description}) +@acindex{CHECKING} +Same as + +@example +AC_MSG_NOTICE([checking @var{feature-description}@dots{}] +@end example + +@noindent +@xref{AC_MSG_NOTICE}. +@end defmac + +@defmac AC_COMPILE_CHECK (@var{echo-text}, @var{includes}, @ + @var{function-body}, @var{action-if-true}, @ovar{action-if-false}) +@acindex{COMPILE_CHECK} +This is an obsolete version of @code{AC_TRY_COMPILE} itself replaced by +@code{AC_COMPILE_IFELSE} (@pxref{Running the Compiler}), with the +addition that it prints @samp{checking for @var{echo-text}} to the +standard output first, if @var{echo-text} is non-empty. Use +@code{AC_MSG_CHECKING} and @code{AC_MSG_RESULT} instead to print +messages (@pxref{Printing Messages}). +@end defmac + +@defmac AC_CONST +@acindex{CONST} +Replaced by @code{AC_C_CONST} (@pxref{AC_C_CONST}). +@end defmac + +@defmac AC_CROSS_CHECK +@acindex{CROSS_CHECK} +Same as @code{AC_C_CROSS}, which is obsolete too, and does nothing +@code{:-)}. +@end defmac + +@defmac AC_CYGWIN +@acindex{CYGWIN} +@evindex CYGWIN +Check for the Cygwin environment in which case the shell variable +@code{CYGWIN} is set to @samp{yes}. Don't use this macro, the dignified +means to check the nature of the host is using @code{AC_CANONICAL_HOST} +(@pxref{Canonicalizing}). As a matter of fact this macro is defined as: + +@example +AC_REQUIRE([AC_CANONICAL_HOST])[]dnl +case $host_os in + *cygwin* ) CYGWIN=yes;; + * ) CYGWIN=no;; +esac +@end example + +Beware that the variable @env{CYGWIN} has a special meaning when +running Cygwin, and should not be changed. That's yet another reason +not to use this macro. +@end defmac + +@defmac AC_DECL_SYS_SIGLIST +@acindex{DECL_SYS_SIGLIST} +@cvindex SYS_SIGLIST_DECLARED +Same as: + +@example +AC_CHECK_DECLS([sys_siglist], [], [], +[#include <signal.h> +/* NetBSD declares sys_siglist in unistd.h. */ +#ifdef HAVE_UNISTD_H +# include <unistd.h> +#endif +]) +@end example + +@noindent +@xref{AC_CHECK_DECLS}. +@end defmac + +@defmac AC_DECL_YYTEXT +@acindex{DECL_YYTEXT} +Does nothing, now integrated in @code{AC_PROG_LEX} (@pxref{AC_PROG_LEX}). +@end defmac + +@defmac AC_DIR_HEADER +@acindex{DIR_HEADER} +@cvindex DIRENT +@cvindex SYSNDIR +@cvindex SYSDIR +@cvindex NDIR +Like calling @code{AC_FUNC_CLOSEDIR_VOID} +(@pxref{AC_FUNC_CLOSEDIR_VOID}) and @code{AC_HEADER_DIRENT} +(@pxref{AC_HEADER_DIRENT}), +but defines a different set of C preprocessor macros to indicate which +header file is found: + +@multitable {@file{sys/ndir.h}} {Old Symbol} {@code{HAVE_SYS_NDIR_H}} +@item Header @tab Old Symbol @tab New Symbol +@item @file{dirent.h} @tab @code{DIRENT} @tab @code{HAVE_DIRENT_H} +@item @file{sys/ndir.h} @tab @code{SYSNDIR} @tab @code{HAVE_SYS_NDIR_H} +@item @file{sys/dir.h} @tab @code{SYSDIR} @tab @code{HAVE_SYS_DIR_H} +@item @file{ndir.h} @tab @code{NDIR} @tab @code{HAVE_NDIR_H} +@end multitable +@end defmac + +@defmac AC_DYNIX_SEQ +@acindex{DYNIX_SEQ} +If on DYNIX/ptx, add @option{-lseq} to output variable +@code{LIBS}. This macro used to be defined as + +@example +AC_CHECK_LIB([seq], [getmntent], [LIBS="-lseq $LIBS"]) +@end example + +@noindent +now it is just @code{AC_FUNC_GETMNTENT} (@pxref{AC_FUNC_GETMNTENT}). +@end defmac + +@defmac AC_EXEEXT +@acindex{EXEEXT} +@ovindex EXEEXT +Defined the output variable @code{EXEEXT} based on the output of the +compiler, which is now done automatically. Typically set to empty +string if Posix and @samp{.exe} if a DOS variant. +@end defmac + +@defmac AC_EMXOS2 +@acindex{EMXOS2} +Similar to @code{AC_CYGWIN} but checks for the EMX environment on OS/2 +and sets @code{EMXOS2}. Don't use this macro, the dignified means to +check the nature of the host is using @code{AC_CANONICAL_HOST} +(@pxref{Canonicalizing}). +@end defmac + +@defmac AC_ENABLE (@var{feature}, @var{action-if-given}, @ + @ovar{action-if-not-given}) +@acindex{ENABLE} +This is an obsolete version of @code{AC_ARG_ENABLE} that does not +support providing a help string (@pxref{AC_ARG_ENABLE}). +@end defmac + +@defmac AC_ERROR +@acindex{ERROR} +Replaced by @code{AC_MSG_ERROR} (@pxref{AC_MSG_ERROR}). +@end defmac + +@defmac AC_FIND_X +@acindex{FIND_X} +Replaced by @code{AC_PATH_X} (@pxref{AC_PATH_X}). +@end defmac + +@defmac AC_FIND_XTRA +@acindex{FIND_XTRA} +Replaced by @code{AC_PATH_XTRA} (@pxref{AC_PATH_XTRA}). +@end defmac + +@defmac AC_FOREACH +@acindex{FOREACH} +Replaced by @code{m4_foreach_w} (@pxref{m4_foreach_w}). +@end defmac + +@defmac AC_FUNC_CHECK +@acindex{FUNC_CHECK} +Replaced by @code{AC_CHECK_FUNC} (@pxref{AC_CHECK_FUNC}). +@end defmac + +@anchor{AC_FUNC_SETVBUF_REVERSED} +@defmac AC_FUNC_SETVBUF_REVERSED +@acindex{FUNC_SETVBUF_REVERSED} +@cvindex SETVBUF_REVERSED +@c @fuindex setvbuf +@prindex @code{setvbuf} +Do nothing. Formerly, this macro checked whether @code{setvbuf} takes +the buffering type as its second argument and the buffer pointer as the +third, instead of the other way around, and defined +@code{SETVBUF_REVERSED}. However, the last systems to have the problem +were those based on SVR2, which became obsolete in 1987, and the macro +is no longer needed. +@end defmac + +@defmac AC_FUNC_WAIT3 +@acindex{FUNC_WAIT3} +@cvindex HAVE_WAIT3 +@c @fuindex wait3 +@prindex @code{wait3} +If @code{wait3} is found and fills in the contents of its third argument +(a @samp{struct rusage *}), which HP-UX does not do, define +@code{HAVE_WAIT3}. + +These days portable programs should use @code{waitpid}, not +@code{wait3}, as @code{wait3} has been removed from Posix. +@end defmac + +@defmac AC_GCC_TRADITIONAL +@acindex{GCC_TRADITIONAL} +Replaced by @code{AC_PROG_GCC_TRADITIONAL} (@pxref{AC_PROG_GCC_TRADITIONAL}). +@end defmac + +@defmac AC_GETGROUPS_T +@acindex{GETGROUPS_T} +Replaced by @code{AC_TYPE_GETGROUPS} (@pxref{AC_TYPE_GETGROUPS}). +@end defmac + +@defmac AC_GETLOADAVG +@acindex{GETLOADAVG} +Replaced by @code{AC_FUNC_GETLOADAVG} (@pxref{AC_FUNC_GETLOADAVG}). +@end defmac + +@defmac AC_GNU_SOURCE +@acindex{GNU_SOURCE} +@cvindex _GNU_SOURCE +This macro is a platform-specific subset of +@code{AC_USE_SYSTEM_EXTENSIONS} (@pxref{AC_USE_SYSTEM_EXTENSIONS}). +@end defmac + +@defmac AC_HAVE_FUNCS +@acindex{HAVE_FUNCS} +Replaced by @code{AC_CHECK_FUNCS} (@pxref{AC_CHECK_FUNCS}). +@end defmac + +@defmac AC_HAVE_HEADERS +@acindex{HAVE_HEADERS} +Replaced by @code{AC_CHECK_HEADERS} (@pxref{AC_CHECK_HEADERS}). +@end defmac + +@defmac AC_HAVE_LIBRARY (@var{library}, @ovar{action-if-found}, @ + @ovar{action-if-not-found}, @ovar{other-libraries}) +@acindex{HAVE_LIBRARY} +This macro is equivalent to calling @code{AC_CHECK_LIB} with a +@var{function} argument of @code{main}. In addition, @var{library} can +be written as any of @samp{foo}, @option{-lfoo}, or @samp{libfoo.a}. In +all of those cases, the compiler is passed @option{-lfoo}. However, +@var{library} cannot be a shell variable; it must be a literal name. +@xref{AC_CHECK_LIB}. +@end defmac + +@defmac AC_HAVE_POUNDBANG +@acindex{HAVE_POUNDBANG} +Replaced by @code{AC_SYS_INTERPRETER} (@pxref{AC_SYS_INTERPRETER}). +@end defmac + +@defmac AC_HEADER_CHECK +@acindex{HEADER_CHECK} +Replaced by @code{AC_CHECK_HEADER} (@pxref{AC_CHECK_HEADER}). +@end defmac + +@defmac AC_HEADER_EGREP +@acindex{HEADER_EGREP} +Replaced by @code{AC_EGREP_HEADER} (@pxref{AC_EGREP_HEADER}). +@end defmac + +@defmac AC_HELP_STRING +@acindex{HELP_STRING} +Replaced by @code{AS_HELP_STRING} (@pxref{AS_HELP_STRING}). +@end defmac + +@defmac AC_INIT (@var{unique-file-in-source-dir}) +@acindex{INIT} +Formerly @code{AC_INIT} used to have a single argument, and was +equivalent to: + +@example +AC_INIT +AC_CONFIG_SRCDIR(@var{unique-file-in-source-dir}) +@end example +See @ref{AC_INIT} and @ref{AC_CONFIG_SRCDIR}. +@end defmac + +@defmac AC_INLINE +@acindex{INLINE} +Replaced by @code{AC_C_INLINE} (@pxref{AC_C_INLINE}). +@end defmac + +@defmac AC_INT_16_BITS +@acindex{INT_16_BITS} +@cvindex INT_16_BITS +If the C type @code{int} is 16 bits wide, define @code{INT_16_BITS}. +Use @samp{AC_CHECK_SIZEOF(int)} instead (@pxref{AC_CHECK_SIZEOF}). +@end defmac + +@defmac AC_IRIX_SUN +@acindex{IRIX_SUN} +If on IRIX (Silicon Graphics Unix), add @option{-lsun} to output +@code{LIBS}. If you were using it to get @code{getmntent}, use +@code{AC_FUNC_GETMNTENT} instead. If you used it for the NIS versions +of the password and group functions, use @samp{AC_CHECK_LIB(sun, +getpwnam)}. Up to Autoconf 2.13, it used to be + +@example +AC_CHECK_LIB([sun], [getmntent], [LIBS="-lsun $LIBS"]) +@end example + +@noindent +now it is defined as + +@example +AC_FUNC_GETMNTENT +AC_CHECK_LIB([sun], [getpwnam]) +@end example + +@noindent +See @ref{AC_FUNC_GETMNTENT} and @ref{AC_CHECK_LIB}. +@end defmac + +@defmac AC_ISC_POSIX +@acindex{ISC_POSIX} +@ovindex LIBS +This macro adds @option{-lcposix} to output variable @code{LIBS} if +necessary for Posix facilities. Sun dropped support for the obsolete +INTERACTIVE Systems Corporation Unix on 2006-07-23. New programs +need not use this macro. It is implemented as +@code{AC_SEARCH_LIBS([strerror], [cposix])} (@pxref{AC_SEARCH_LIBS}). +@end defmac + +@defmac AC_LANG_C +@acindex{LANG_C} +Same as @samp{AC_LANG([C])} (@pxref{AC_LANG}). +@end defmac + +@defmac AC_LANG_CPLUSPLUS +@acindex{LANG_CPLUSPLUS} +Same as @samp{AC_LANG([C++])} (@pxref{AC_LANG}). +@end defmac + +@defmac AC_LANG_FORTRAN77 +@acindex{LANG_FORTRAN77} +Same as @samp{AC_LANG([Fortran 77])} (@pxref{AC_LANG}). +@end defmac + +@defmac AC_LANG_RESTORE +@acindex{LANG_RESTORE} +Select the @var{language} that is saved on the top of the stack, as set +by @code{AC_LANG_SAVE}, remove it from the stack, and call +@code{AC_LANG(@var{language})}. @xref{Language Choice}, for the +preferred way to change languages. +@end defmac + +@defmac AC_LANG_SAVE +@acindex{LANG_SAVE} +Remember the current language (as set by @code{AC_LANG}) on a stack. +The current language does not change. @code{AC_LANG_PUSH} is preferred +(@pxref{AC_LANG_PUSH}). +@end defmac + +@defmac AC_LINK_FILES (@var{source}@dots{}, @var{dest}@dots{}) +@acindex{LINK_FILES} +This is an obsolete version of @code{AC_CONFIG_LINKS} +(@pxref{AC_CONFIG_LINKS}. An updated version of: + +@example +AC_LINK_FILES(config/$machine.h config/$obj_format.h, + host.h object.h) +@end example + +@noindent +is: + +@example +AC_CONFIG_LINKS([host.h:config/$machine.h + object.h:config/$obj_format.h]) +@end example +@end defmac + +@defmac AC_LN_S +@acindex{LN_S} +Replaced by @code{AC_PROG_LN_S} (@pxref{AC_PROG_LN_S}). +@end defmac + +@defmac AC_LONG_64_BITS +@acindex{LONG_64_BITS} +@cvindex LONG_64_BITS +Define @code{LONG_64_BITS} if the C type @code{long int} is 64 bits wide. +Use the generic macro @samp{AC_CHECK_SIZEOF([long int])} instead +(@pxref{AC_CHECK_SIZEOF}). +@end defmac + +@defmac AC_LONG_DOUBLE +@acindex{LONG_DOUBLE} +If the C compiler supports a working @code{long double} type with more +range or precision than the @code{double} type, define +@code{HAVE_LONG_DOUBLE}. + +You should use @code{AC_TYPE_LONG_DOUBLE} or +@code{AC_TYPE_LONG_DOUBLE_WIDER} instead. @xref{Particular Types}. +@end defmac + +@defmac AC_LONG_FILE_NAMES +@acindex{LONG_FILE_NAMES} +Replaced by +@example +AC_SYS_LONG_FILE_NAMES +@end example +@noindent +@xref{AC_SYS_LONG_FILE_NAMES}. +@end defmac + +@defmac AC_MAJOR_HEADER +@acindex{MAJOR_HEADER} +Replaced by @code{AC_HEADER_MAJOR} (@pxref{AC_HEADER_MAJOR}). +@end defmac + +@defmac AC_MEMORY_H +@acindex{MEMORY_H} +@cvindex NEED_MEMORY_H +Used to define @code{NEED_MEMORY_H} if the @code{mem} functions were +defined in @file{memory.h}. Today it is equivalent to +@samp{AC_CHECK_HEADERS([memory.h])} (@pxref{AC_CHECK_HEADERS}). Adjust +your code to depend upon +@code{HAVE_MEMORY_H}, not @code{NEED_MEMORY_H}; see @ref{Standard +Symbols}. +@end defmac + +@defmac AC_MINGW32 +@acindex{MINGW32} +Similar to @code{AC_CYGWIN} but checks for the MinGW compiler +environment and sets @code{MINGW32}. Don't use this macro, the +dignified means to check the nature of the host is using +@code{AC_CANONICAL_HOST} (@pxref{Canonicalizing}). +@end defmac + +@defmac AC_MINIX +@acindex{MINIX} +@cvindex _MINIX +@cvindex _POSIX_SOURCE +@cvindex _POSIX_1_SOURCE +This macro is a platform-specific subset of +@code{AC_USE_SYSTEM_EXTENSIONS} (@pxref{AC_USE_SYSTEM_EXTENSIONS}). +@end defmac + +@defmac AC_MINUS_C_MINUS_O +@acindex{MINUS_C_MINUS_O} +Replaced by @code{AC_PROG_CC_C_O} (@pxref{AC_PROG_CC_C_O}). +@end defmac + +@defmac AC_MMAP +@acindex{MMAP} +Replaced by @code{AC_FUNC_MMAP} (@pxref{AC_FUNC_MMAP}). +@end defmac + +@defmac AC_MODE_T +@acindex{MODE_T} +Replaced by @code{AC_TYPE_MODE_T} (@pxref{AC_TYPE_MODE_T}). +@end defmac + +@defmac AC_OBJEXT +@acindex{OBJEXT} +@ovindex OBJEXT +Defined the output variable @code{OBJEXT} based on the output of the +compiler, after .c files have been excluded. Typically set to @samp{o} +if Posix, @samp{obj} if a DOS variant. +Now the compiler checking macros handle +this automatically. +@end defmac + +@defmac AC_OBSOLETE (@var{this-macro-name}, @ovar{suggestion}) +@acindex{OBSOLETE} +Make M4 print a message to the standard error output warning that +@var{this-macro-name} is obsolete, and giving the file and line number +where it was called. @var{this-macro-name} should be the name of the +macro that is calling @code{AC_OBSOLETE}. If @var{suggestion} is given, +it is printed at the end of the warning message; for example, it can be +a suggestion for what to use instead of @var{this-macro-name}. + +For instance + +@example +AC_OBSOLETE([$0], [; use AC_CHECK_HEADERS(unistd.h) instead])dnl +@end example + +@noindent +You are encouraged to use @code{AU_DEFUN} instead, since it gives better +services to the user (@pxref{AU_DEFUN}). +@end defmac + +@defmac AC_OFF_T +@acindex{OFF_T} +Replaced by @code{AC_TYPE_OFF_T} (@pxref{AC_TYPE_OFF_T}). +@end defmac + +@defmac AC_OUTPUT (@ovar{file}@dots{}, @ovar{extra-cmds}, @ovar{init-cmds}) +@acindex{OUTPUT} +The use of @code{AC_OUTPUT} with arguments is deprecated. This obsoleted +interface is equivalent to: + +@example +@group +AC_CONFIG_FILES(@var{file}@dots{}) +AC_CONFIG_COMMANDS([default], + @var{extra-cmds}, @var{init-cmds}) +AC_OUTPUT +@end group +@end example + +@noindent +See @ref{AC_CONFIG_FILES}, @ref{AC_CONFIG_COMMANDS}, and @ref{AC_OUTPUT}. +@end defmac + +@defmac AC_OUTPUT_COMMANDS (@var{extra-cmds}, @ovar{init-cmds}) +@acindex{OUTPUT_COMMANDS} +Specify additional shell commands to run at the end of +@file{config.status}, and shell commands to initialize any variables +from @command{configure}. This macro may be called multiple times. It is +obsolete, replaced by @code{AC_CONFIG_COMMANDS} (@pxref{AC_CONFIG_COMMANDS}). + +Here is an unrealistic example: + +@example +fubar=27 +AC_OUTPUT_COMMANDS([echo this is extra $fubar, and so on.], + [fubar=$fubar]) +AC_OUTPUT_COMMANDS([echo this is another, extra, bit], + [echo init bit]) +@end example + +Aside from the fact that @code{AC_CONFIG_COMMANDS} requires an +additional key, an important difference is that +@code{AC_OUTPUT_COMMANDS} is quoting its arguments twice, unlike +@code{AC_CONFIG_COMMANDS}. This means that @code{AC_CONFIG_COMMANDS} +can safely be given macro calls as arguments: + +@example +AC_CONFIG_COMMANDS(foo, [my_FOO()]) +@end example + +@noindent +Conversely, where one level of quoting was enough for literal strings +with @code{AC_OUTPUT_COMMANDS}, you need two with +@code{AC_CONFIG_COMMANDS}. The following lines are equivalent: + +@example +@group +AC_OUTPUT_COMMANDS([echo "Square brackets: []"]) +AC_CONFIG_COMMANDS([default], [[echo "Square brackets: []"]]) +@end group +@end example +@end defmac + +@defmac AC_PID_T +@acindex{PID_T} +Replaced by @code{AC_TYPE_PID_T} (@pxref{AC_TYPE_PID_T}). +@end defmac + +@defmac AC_PREFIX +@acindex{PREFIX} +Replaced by @code{AC_PREFIX_PROGRAM} (@pxref{AC_PREFIX_PROGRAM}). +@end defmac + +@defmac AC_PROGRAMS_CHECK +@acindex{PROGRAMS_CHECK} +Replaced by @code{AC_CHECK_PROGS} (@pxref{AC_CHECK_PROGS}). +@end defmac + +@defmac AC_PROGRAMS_PATH +@acindex{PROGRAMS_PATH} +Replaced by @code{AC_PATH_PROGS} (@pxref{AC_PATH_PROGS}). +@end defmac + +@defmac AC_PROGRAM_CHECK +@acindex{PROGRAM_CHECK} +Replaced by @code{AC_CHECK_PROG} (@pxref{AC_CHECK_PROG}). +@end defmac + +@defmac AC_PROGRAM_EGREP +@acindex{PROGRAM_EGREP} +Replaced by @code{AC_EGREP_CPP} (@pxref{AC_EGREP_CPP}). +@end defmac + +@defmac AC_PROGRAM_PATH +@acindex{PROGRAM_PATH} +Replaced by @code{AC_PATH_PROG} (@pxref{AC_PATH_PROG}). +@end defmac + +@defmac AC_REMOTE_TAPE +@acindex{REMOTE_TAPE} +Removed because of limited usefulness. +@end defmac + +@defmac AC_RESTARTABLE_SYSCALLS +@acindex{RESTARTABLE_SYSCALLS} +This macro was renamed @code{AC_SYS_RESTARTABLE_SYSCALLS}. However, +these days portable programs should use @code{sigaction} with +@code{SA_RESTART} if they want restartable system calls. They should +not rely on @code{HAVE_RESTARTABLE_SYSCALLS}, since nowadays whether a +system call is restartable is a dynamic issue, not a configuration-time +issue. +@end defmac + +@defmac AC_RETSIGTYPE +@acindex{RETSIGTYPE} +Replaced by @code{AC_TYPE_SIGNAL} (@pxref{AC_TYPE_SIGNAL}), which itself +is obsolete when assuming C89 or better. +@end defmac + +@defmac AC_RSH +@acindex{RSH} +Removed because of limited usefulness. +@end defmac + +@defmac AC_SCO_INTL +@acindex{SCO_INTL} +@ovindex LIBS +If on SCO Unix, add @option{-lintl} to output variable @code{LIBS}. This +macro used to do this: + +@example +AC_CHECK_LIB([intl], [strftime], [LIBS="-lintl $LIBS"]) +@end example + +@noindent +Now it just calls @code{AC_FUNC_STRFTIME} instead (@pxref{AC_FUNC_STRFTIME}). +@end defmac + +@defmac AC_SETVBUF_REVERSED +@acindex{SETVBUF_REVERSED} +Replaced by +@example +AC_FUNC_SETVBUF_REVERSED +@end example +@noindent +@xref{AC_FUNC_SETVBUF_REVERSED}. +@end defmac + +@defmac AC_SET_MAKE +@acindex{SET_MAKE} +Replaced by @code{AC_PROG_MAKE_SET} (@pxref{AC_PROG_MAKE_SET}). +@end defmac + +@defmac AC_SIZEOF_TYPE +@acindex{SIZEOF_TYPE} +Replaced by @code{AC_CHECK_SIZEOF} (@pxref{AC_CHECK_SIZEOF}). +@end defmac + +@defmac AC_SIZE_T +@acindex{SIZE_T} +Replaced by @code{AC_TYPE_SIZE_T} (@pxref{AC_TYPE_SIZE_T}). +@end defmac + +@defmac AC_STAT_MACROS_BROKEN +@acindex{STAT_MACROS_BROKEN} +Replaced by @code{AC_HEADER_STAT} (@pxref{AC_HEADER_STAT}). +@end defmac + +@defmac AC_STDC_HEADERS +@acindex{STDC_HEADERS} +Replaced by @code{AC_HEADER_STDC} (@pxref{AC_HEADER_STDC}). +@end defmac + +@defmac AC_STRCOLL +@acindex{STRCOLL} +Replaced by @code{AC_FUNC_STRCOLL} (@pxref{AC_FUNC_STRCOLL}). +@end defmac + +@defmac AC_STRUCT_ST_BLKSIZE +@acindex{STRUCT_ST_BLKSIZE} +@cvindex HAVE_STRUCT_STAT_ST_BLKSIZE +@cvindex HAVE_ST_BLKSIZE +If @code{struct stat} contains an @code{st_blksize} member, define +@code{HAVE_STRUCT_STAT_ST_BLKSIZE}. The former name, +@code{HAVE_ST_BLKSIZE} is to be avoided, as its support will cease in +the future. This macro is obsoleted, and should be replaced by + +@example +AC_CHECK_MEMBERS([struct stat.st_blksize]) +@end example +@noindent +@xref{AC_CHECK_MEMBERS}. +@end defmac + +@defmac AC_STRUCT_ST_RDEV +@acindex{STRUCT_ST_RDEV} +@cvindex HAVE_ST_RDEV +@cvindex HAVE_STRUCT_STAT_ST_RDEV +If @code{struct stat} contains an @code{st_rdev} member, define +@code{HAVE_STRUCT_STAT_ST_RDEV}. The former name for this macro, +@code{HAVE_ST_RDEV}, is to be avoided as it will cease to be supported +in the future. Actually, even the new macro is obsolete and should be +replaced by: +@example +AC_CHECK_MEMBERS([struct stat.st_rdev]) +@end example +@noindent +@xref{AC_CHECK_MEMBERS}. +@end defmac + +@defmac AC_ST_BLKSIZE +@acindex{ST_BLKSIZE} +Replaced by @code{AC_CHECK_MEMBERS} (@pxref{AC_CHECK_MEMBERS}). +@end defmac + +@defmac AC_ST_BLOCKS +@acindex{ST_BLOCKS} +Replaced by @code{AC_STRUCT_ST_BLOCKS} (@pxref{AC_STRUCT_ST_BLOCKS}). +@end defmac + +@defmac AC_ST_RDEV +@acindex{ST_RDEV} +Replaced by @code{AC_CHECK_MEMBERS} (@pxref{AC_CHECK_MEMBERS}). +@end defmac + +@defmac AC_SYS_RESTARTABLE_SYSCALLS +@acindex{SYS_RESTARTABLE_SYSCALLS} +@cvindex HAVE_RESTARTABLE_SYSCALLS +If the system automatically restarts a system call that is interrupted +by a signal, define @code{HAVE_RESTARTABLE_SYSCALLS}. This macro does +not check whether system calls are restarted in general---it checks whether a +signal handler installed with @code{signal} (but not @code{sigaction}) +causes system calls to be restarted. It does not check whether system calls +can be restarted when interrupted by signals that have no handler. + +These days portable programs should use @code{sigaction} with +@code{SA_RESTART} if they want restartable system calls. They should +not rely on @code{HAVE_RESTARTABLE_SYSCALLS}, since nowadays whether a +system call is restartable is a dynamic issue, not a configuration-time +issue. +@end defmac + +@defmac AC_SYS_SIGLIST_DECLARED +@acindex{SYS_SIGLIST_DECLARED} +This macro was renamed @code{AC_DECL_SYS_SIGLIST}. However, even that +name is obsolete, as the same functionality is now achieved via +@code{AC_CHECK_DECLS} (@pxref{AC_CHECK_DECLS}). +@end defmac + +@defmac AC_TEST_CPP +@acindex{TEST_CPP} +This macro was renamed @code{AC_TRY_CPP}, which in turn was replaced by +@code{AC_PREPROC_IFELSE} (@pxref{AC_PREPROC_IFELSE}). +@end defmac + +@defmac AC_TEST_PROGRAM +@acindex{TEST_PROGRAM} +This macro was renamed @code{AC_TRY_RUN}, which in turn was replaced by +@code{AC_RUN_IFELSE} (@pxref{AC_RUN_IFELSE}). +@end defmac + +@defmac AC_TIMEZONE +@acindex{TIMEZONE} +Replaced by @code{AC_STRUCT_TIMEZONE} (@pxref{AC_STRUCT_TIMEZONE}). +@end defmac + +@defmac AC_TIME_WITH_SYS_TIME +@acindex{TIME_WITH_SYS_TIME} +Replaced by @code{AC_HEADER_TIME} (@pxref{AC_HEADER_TIME}). +@end defmac + +@defmac AC_TRY_COMPILE (@var{includes}, @var{function-body}, @ + @ovar{action-if-true}, @ovar{action-if-false}) +@acindex{TRY_COMPILE} +Same as: + +@example +AC_COMPILE_IFELSE( + [AC_LANG_PROGRAM([[@var{includes}]], + [[@var{function-body}]])], + [@var{action-if-true}], + [@var{action-if-false}]) +@end example + +@noindent +@xref{Running the Compiler}. + +This macro double quotes both @var{includes} and @var{function-body}. + +For C and C++, @var{includes} is any @code{#include} statements needed +by the code in @var{function-body} (@var{includes} is ignored if +the currently selected language is Fortran or Fortran 77). The compiler +and compilation flags are determined by the current language +(@pxref{Language Choice}). +@end defmac + +@defmac AC_TRY_CPP (@var{input}, @ovar{action-if-true}, @ovar{action-if-false}) +@acindex{TRY_CPP} +Same as: + +@example +AC_PREPROC_IFELSE( + [AC_LANG_SOURCE([[@var{input}]])], + [@var{action-if-true}], + [@var{action-if-false}]) +@end example + +@noindent +@xref{Running the Preprocessor}. + +This macro double quotes the @var{input}. +@end defmac + +@defmac AC_TRY_LINK (@var{includes}, @var{function-body}, @ + @ovar{action-if-true}, @ovar{action-if-false}) +@acindex{TRY_LINK} +Same as: + +@example +AC_LINK_IFELSE( + [AC_LANG_PROGRAM([[@var{includes}]], + [[@var{function-body}]])], + [@var{action-if-true}], + [@var{action-if-false}]) +@end example + +@noindent +@xref{Running the Compiler}. + +This macro double quotes both @var{includes} and @var{function-body}. + +Depending on the current language (@pxref{Language Choice}), create a +test program to see whether a function whose body consists of +@var{function-body} can be compiled and linked. If the file compiles +and links successfully, run shell commands @var{action-if-found}, +otherwise run @var{action-if-not-found}. + +This macro double quotes both @var{includes} and @var{function-body}. + +For C and C++, @var{includes} is any @code{#include} statements needed +by the code in @var{function-body} (@var{includes} is ignored if +the currently selected language is Fortran or Fortran 77). The compiler +and compilation flags are determined by the current language +(@pxref{Language Choice}), and in addition @code{LDFLAGS} and +@code{LIBS} are used for linking. +@end defmac + +@defmac AC_TRY_LINK_FUNC (@var{function}, @ovar{action-if-found}, @ + @ovar{action-if-not-found}) +@acindex{TRY_LINK_FUNC} +This macro is equivalent to +@example +AC_LINK_IFELSE([AC_LANG_CALL([], [@var{function}])], + [@var{action-if-found}], [@var{action-if-not-found}]) +@end example +@noindent +@xref{AC_LINK_IFELSE}. +@end defmac + +@defmac AC_TRY_RUN (@var{program}, @ovar{action-if-true}, @ + @ovar{action-if-false}, @dvar{action-if-cross-compiling, AC_MSG_FAILURE}) +@acindex{TRY_RUN} +Same as: + +@example +AC_RUN_IFELSE( + [AC_LANG_SOURCE([[@var{program}]])], + [@var{action-if-true}], + [@var{action-if-false}], + [@var{action-if-cross-compiling}]) +@end example + +@noindent +@xref{Runtime}. +@end defmac + +@anchor{AC_TYPE_SIGNAL} +@defmac AC_TYPE_SIGNAL +@acindex{TYPE_SIGNAL} +@cvindex RETSIGTYPE +@hdrindex{signal.h} +If @file{signal.h} declares @code{signal} as returning a pointer to a +function returning @code{void}, define @code{RETSIGTYPE} to be +@code{void}; otherwise, define it to be @code{int}. These days, it is +portable to assume C89, and that signal handlers return @code{void}, +without needing to use this macro or @code{RETSIGTYPE}. + +When targeting older K&R C, it is possible to define signal handlers as +returning type @code{RETSIGTYPE}, and omit a return statement: + +@example +@group +RETSIGTYPE +hup_handler () +@{ +@dots{} +@} +@end group +@end example +@end defmac + +@defmac AC_UID_T +@acindex{UID_T} +Replaced by @code{AC_TYPE_UID_T} (@pxref{AC_TYPE_UID_T}). +@end defmac + +@defmac AC_UNISTD_H +@acindex{UNISTD_H} +Same as @samp{AC_CHECK_HEADERS([unistd.h])} (@pxref{AC_CHECK_HEADERS}). +@end defmac + +@defmac AC_USG +@acindex{USG} +@cvindex USG +Define @code{USG} if the BSD string functions are defined in +@file{strings.h}. You should no longer depend upon @code{USG}, but on +@code{HAVE_STRING_H}; see @ref{Standard Symbols}. +@end defmac + +@defmac AC_UTIME_NULL +@acindex{UTIME_NULL} +Replaced by @code{AC_FUNC_UTIME_NULL} (@pxref{AC_FUNC_UTIME_NULL}). +@end defmac + +@defmac AC_VALIDATE_CACHED_SYSTEM_TUPLE (@ovar{cmd}) +@acindex{VALIDATE_CACHED_SYSTEM_TUPLE} +If the cache file is inconsistent with the current host, target and +build system types, it used to execute @var{cmd} or print a default +error message. This is now handled by default. +@end defmac + +@defmac AC_VERBOSE (@var{result-description}) +@acindex{VERBOSE} +Replaced by @code{AC_MSG_RESULT} (@pxref{AC_MSG_RESULT}). +@end defmac + +@defmac AC_VFORK +@acindex{VFORK} +Replaced by @code{AC_FUNC_FORK} (@pxref{AC_FUNC_FORK}). +@end defmac + +@defmac AC_VPRINTF +@acindex{VPRINTF} +Replaced by @code{AC_FUNC_VPRINTF} (@pxref{AC_FUNC_VPRINTF}). +@end defmac + +@defmac AC_WAIT3 +@acindex{WAIT3} +This macro was renamed @code{AC_FUNC_WAIT3}. However, these days +portable programs should use @code{waitpid}, not @code{wait3}, as +@code{wait3} has been removed from Posix. +@end defmac + +@defmac AC_WARN +@acindex{WARN} +Replaced by @code{AC_MSG_WARN} (@pxref{AC_MSG_WARN}). +@end defmac + +@defmac AC_WITH (@var{package}, @var{action-if-given}, @ + @ovar{action-if-not-given}) +@acindex{WITH} +This is an obsolete version of @code{AC_ARG_WITH} that does not +support providing a help string (@pxref{AC_ARG_WITH}). +@end defmac + +@defmac AC_WORDS_BIGENDIAN +@acindex{WORDS_BIGENDIAN} +Replaced by @code{AC_C_BIGENDIAN} (@pxref{AC_C_BIGENDIAN}). +@end defmac + +@defmac AC_XENIX_DIR +@acindex{XENIX_DIR} +@ovindex LIBS +This macro used to add @option{-lx} to output variable @code{LIBS} if on +Xenix. Also, if @file{dirent.h} is being checked for, added +@option{-ldir} to @code{LIBS}. Now it is merely an alias of +@code{AC_HEADER_DIRENT} instead, plus some code to detect whether +running XENIX on which you should not depend: + +@example +AC_MSG_CHECKING([for Xenix]) +AC_EGREP_CPP([yes], +[#if defined M_XENIX && !defined M_UNIX + yes +#endif], + [AC_MSG_RESULT([yes]); XENIX=yes], + [AC_MSG_RESULT([no]); XENIX=]) +@end example +@noindent +Don't use this macro, the dignified means to check the nature of the +host is using @code{AC_CANONICAL_HOST} (@pxref{Canonicalizing}). +@end defmac + +@defmac AC_YYTEXT_POINTER +@acindex{YYTEXT_POINTER} +This macro was renamed @code{AC_DECL_YYTEXT}, which in turn was +integrated into @code{AC_PROG_LEX} (@pxref{AC_PROG_LEX}). +@end defmac + +@node Autoconf 1 +@section Upgrading From Version 1 +@cindex Upgrading autoconf +@cindex Autoconf upgrading + +Autoconf version 2 is mostly backward compatible with version 1. +However, it introduces better ways to do some things, and doesn't +support some of the ugly things in version 1. So, depending on how +sophisticated your @file{configure.ac} files are, you might have to do +some manual work in order to upgrade to version 2. This chapter points +out some problems to watch for when upgrading. Also, perhaps your +@command{configure} scripts could benefit from some of the new features in +version 2; the changes are summarized in the file @file{NEWS} in the +Autoconf distribution. + +@menu +* Changed File Names:: Files you might rename +* Changed Makefiles:: New things to put in @file{Makefile.in} +* Changed Macros:: Macro calls you might replace +* Changed Results:: Changes in how to check test results +* Changed Macro Writing:: Better ways to write your own macros +@end menu + +@node Changed File Names +@subsection Changed File Names + +If you have an @file{aclocal.m4} installed with Autoconf (as opposed to +in a particular package's source directory), you must rename it to +@file{acsite.m4}. @xref{autoconf Invocation}. + +If you distribute @file{install.sh} with your package, rename it to +@file{install-sh} so @command{make} builtin rules don't inadvertently +create a file called @file{install} from it. @code{AC_PROG_INSTALL} +looks for the script under both names, but it is best to use the new name. + +If you were using @file{config.h.top}, @file{config.h.bot}, or +@file{acconfig.h}, you still can, but you have less clutter if you +use the @code{AH_} macros. @xref{Autoheader Macros}. + +@node Changed Makefiles +@subsection Changed Makefiles + +Add @samp{@@CFLAGS@@}, @samp{@@CPPFLAGS@@}, and @samp{@@LDFLAGS@@} in +your @file{Makefile.in} files, so they can take advantage of the values +of those variables in the environment when @command{configure} is run. +Doing this isn't necessary, but it's a convenience for users. + +Also add @samp{@@configure_input@@} in a comment to each input file for +@code{AC_OUTPUT}, so that the output files contain a comment saying +they were produced by @command{configure}. Automatically selecting the +right comment syntax for all the kinds of files that people call +@code{AC_OUTPUT} on became too much work. + +Add @file{config.log} and @file{config.cache} to the list of files you +remove in @code{distclean} targets. + +If you have the following in @file{Makefile.in}: + +@example +prefix = /usr/local +exec_prefix = $(prefix) +@end example + +@noindent +you must change it to: + +@example +prefix = @@prefix@@ +exec_prefix = @@exec_prefix@@ +@end example + +@noindent +The old behavior of replacing those variables without @samp{@@} +characters around them has been removed. + +@node Changed Macros +@subsection Changed Macros + +Many of the macros were renamed in Autoconf version 2. You can still +use the old names, but the new ones are clearer, and it's easier to find +the documentation for them. @xref{Obsolete Macros}, for a table showing the +new names for the old macros. Use the @command{autoupdate} program to +convert your @file{configure.ac} to using the new macro names. +@xref{autoupdate Invocation}. + +Some macros have been superseded by similar ones that do the job better, +but are not call-compatible. If you get warnings about calling obsolete +macros while running @command{autoconf}, you may safely ignore them, but +your @command{configure} script generally works better if you follow +the advice that is printed about what to replace the obsolete macros with. In +particular, the mechanism for reporting the results of tests has +changed. If you were using @command{echo} or @code{AC_VERBOSE} (perhaps +via @code{AC_COMPILE_CHECK}), your @command{configure} script's output +looks better if you switch to @code{AC_MSG_CHECKING} and +@code{AC_MSG_RESULT}. @xref{Printing Messages}. Those macros work best +in conjunction with cache variables. @xref{Caching Results}. + + + +@node Changed Results +@subsection Changed Results + +If you were checking the results of previous tests by examining the +shell variable @code{DEFS}, you need to switch to checking the values of +the cache variables for those tests. @code{DEFS} no longer exists while +@command{configure} is running; it is only created when generating output +files. This difference from version 1 is because properly quoting the +contents of that variable turned out to be too cumbersome and +inefficient to do every time @code{AC_DEFINE} is called. @xref{Cache +Variable Names}. + +For example, here is a @file{configure.ac} fragment written for Autoconf +version 1: + +@example +AC_HAVE_FUNCS(syslog) +case "$DEFS" in +*-DHAVE_SYSLOG*) ;; +*) # syslog is not in the default libraries. See if it's in some other. + saved_LIBS="$LIBS" + for lib in bsd socket inet; do + AC_CHECKING(for syslog in -l$lib) + LIBS="-l$lib $saved_LIBS" + AC_HAVE_FUNCS(syslog) + case "$DEFS" in + *-DHAVE_SYSLOG*) break ;; + *) ;; + esac + LIBS="$saved_LIBS" + done ;; +esac +@end example + +Here is a way to write it for version 2: + +@example +AC_CHECK_FUNCS([syslog]) +if test "x$ac_cv_func_syslog" = xno; then + # syslog is not in the default libraries. See if it's in some other. + for lib in bsd socket inet; do + AC_CHECK_LIB([$lib], [syslog], [AC_DEFINE([HAVE_SYSLOG]) + LIBS="-l$lib $LIBS"; break]) + done +fi +@end example + +If you were working around bugs in @code{AC_DEFINE_UNQUOTED} by adding +backslashes before quotes, you need to remove them. It now works +predictably, and does not treat quotes (except back quotes) specially. +@xref{Setting Output Variables}. + +All of the Boolean shell variables set by Autoconf macros now use +@samp{yes} for the true value. Most of them use @samp{no} for false, +though for backward compatibility some use the empty string instead. If +you were relying on a shell variable being set to something like 1 or +@samp{t} for true, you need to change your tests. + +@node Changed Macro Writing +@subsection Changed Macro Writing + +When defining your own macros, you should now use @code{AC_DEFUN} +instead of @code{define}. @code{AC_DEFUN} automatically calls +@code{AC_PROVIDE} and ensures that macros called via @code{AC_REQUIRE} +do not interrupt other macros, to prevent nested @samp{checking@dots{}} +messages on the screen. There's no actual harm in continuing to use the +older way, but it's less convenient and attractive. @xref{Macro +Definitions}. + +You probably looked at the macros that came with Autoconf as a guide for +how to do things. It would be a good idea to take a look at the new +versions of them, as the style is somewhat improved and they take +advantage of some new features. + +If you were doing tricky things with undocumented Autoconf internals +(macros, variables, diversions), check whether you need to change +anything to account for changes that have been made. Perhaps you can +even use an officially supported technique in version 2 instead of +kludging. Or perhaps not. + +To speed up your locally written feature tests, add caching to them. +See whether any of your tests are of general enough usefulness to +encapsulate them into macros that you can share. + + +@node Autoconf 2.13 +@section Upgrading From Version 2.13 +@cindex Upgrading autoconf +@cindex Autoconf upgrading + +The introduction of the previous section (@pxref{Autoconf 1}) perfectly +suits this section@enddots{} + +@quotation +Autoconf version 2.50 is mostly backward compatible with version 2.13. +However, it introduces better ways to do some things, and doesn't +support some of the ugly things in version 2.13. So, depending on how +sophisticated your @file{configure.ac} files are, you might have to do +some manual work in order to upgrade to version 2.50. This chapter +points out some problems to watch for when upgrading. Also, perhaps +your @command{configure} scripts could benefit from some of the new +features in version 2.50; the changes are summarized in the file +@file{NEWS} in the Autoconf distribution. +@end quotation + +@menu +* Changed Quotation:: Broken code which used to work +* New Macros:: Interaction with foreign macros +* Hosts and Cross-Compilation:: Bugward compatibility kludges +* AC_LIBOBJ vs LIBOBJS:: LIBOBJS is a forbidden token +* AC_ACT_IFELSE vs AC_TRY_ACT:: A more generic scheme for testing sources +@end menu + +@node Changed Quotation +@subsection Changed Quotation + +The most important changes are invisible to you: the implementation of +most macros have completely changed. This allowed more factorization of +the code, better error messages, a higher uniformity of the user's +interface etc. Unfortunately, as a side effect, some construct which +used to (miraculously) work might break starting with Autoconf 2.50. +The most common culprit is bad quotation. + +For instance, in the following example, the message is not properly +quoted: + +@example +AC_INIT +AC_CHECK_HEADERS(foo.h, , + AC_MSG_ERROR(cannot find foo.h, bailing out)) +AC_OUTPUT +@end example + +@noindent +Autoconf 2.13 simply ignores it: + +@example +$ @kbd{autoconf-2.13; ./configure --silent} +creating cache ./config.cache +configure: error: cannot find foo.h +$ +@end example + +@noindent +while Autoconf 2.50 produces a broken @file{configure}: + +@example +$ @kbd{autoconf-2.50; ./configure --silent} +configure: error: cannot find foo.h +./configure: exit: bad non-numeric arg `bailing' +./configure: exit: bad non-numeric arg `bailing' +$ +@end example + +The message needs to be quoted, and the @code{AC_MSG_ERROR} invocation +too! + +@example +AC_INIT([Example], [1.0], [bug-example@@example.org]) +AC_CHECK_HEADERS([foo.h], [], + [AC_MSG_ERROR([cannot find foo.h, bailing out])]) +AC_OUTPUT +@end example + +Many many (and many more) Autoconf macros were lacking proper quotation, +including no less than@dots{} @code{AC_DEFUN} itself! + +@example +$ @kbd{cat configure.in} +AC_DEFUN([AC_PROG_INSTALL], +[# My own much better version +]) +AC_INIT +AC_PROG_INSTALL +AC_OUTPUT +$ @kbd{autoconf-2.13} +autoconf: Undefined macros: +***BUG in Autoconf--please report*** AC_FD_MSG +***BUG in Autoconf--please report*** AC_EPI +configure.in:1:AC_DEFUN([AC_PROG_INSTALL], +configure.in:5:AC_PROG_INSTALL +$ @kbd{autoconf-2.50} +$ +@end example + + +@node New Macros +@subsection New Macros + +@cindex undefined macro +@cindex @code{_m4_divert_diversion} + +While Autoconf was relatively dormant in the late 1990s, Automake +provided Autoconf-like macros for a while. Starting with Autoconf 2.50 +in 2001, Autoconf provided +versions of these macros, integrated in the @code{AC_} namespace, +instead of @code{AM_}. But in order to ease the upgrading via +@command{autoupdate}, bindings to such @code{AM_} macros are provided. + +Unfortunately older versions of Automake (e.g., Automake 1.4) +did not quote the names of these macros. +Therefore, when @command{m4} finds something like +@samp{AC_DEFUN(AM_TYPE_PTRDIFF_T, @dots{})} in @file{aclocal.m4}, +@code{AM_TYPE_PTRDIFF_T} is +expanded, replaced with its Autoconf definition. + +Fortunately Autoconf catches pre-@code{AC_INIT} expansions, and +complains, in its own words: + +@example +$ @kbd{cat configure.ac} +AC_INIT([Example], [1.0], [bug-example@@example.org]) +AM_TYPE_PTRDIFF_T +$ @kbd{aclocal-1.4} +$ @kbd{autoconf} +aclocal.m4:17: error: m4_defn: undefined macro: _m4_divert_diversion +aclocal.m4:17: the top level +autom4te: m4 failed with exit status: 1 +$ +@end example + +Modern versions of Automake no longer define most of these +macros, and properly quote the names of the remaining macros. +If you must use an old Automake, do not depend upon macros from Automake +as it is simply not its job +to provide macros (but the one it requires itself): + +@example +$ @kbd{cat configure.ac} +AC_INIT([Example], [1.0], [bug-example@@example.org]) +AM_TYPE_PTRDIFF_T +$ @kbd{rm aclocal.m4} +$ @kbd{autoupdate} +autoupdate: `configure.ac' is updated +$ @kbd{cat configure.ac} +AC_INIT([Example], [1.0], [bug-example@@example.org]) +AC_CHECK_TYPES([ptrdiff_t]) +$ @kbd{aclocal-1.4} +$ @kbd{autoconf} +$ +@end example + + +@node Hosts and Cross-Compilation +@subsection Hosts and Cross-Compilation +@cindex Cross compilation + +Based on the experience of compiler writers, and after long public +debates, many aspects of the cross-compilation chain have changed: + +@itemize @minus +@item +the relationship between the build, host, and target architecture types, + +@item +the command line interface for specifying them to @command{configure}, + +@item +the variables defined in @command{configure}, + +@item +the enabling of cross-compilation mode. +@end itemize + +@sp 1 + +The relationship between build, host, and target have been cleaned up: +the chain of default is now simply: target defaults to host, host to +build, and build to the result of @command{config.guess}. Nevertheless, +in order to ease the transition from 2.13 to 2.50, the following +transition scheme is implemented. @emph{Do not rely on it}, as it will +be completely disabled in a couple of releases (we cannot keep it, as it +proves to cause more problems than it cures). + +They all default to the result of running @command{config.guess}, unless +you specify either @option{--build} or @option{--host}. In this case, +the default becomes the system type you specified. If you specify both, +and they're different, @command{configure} enters cross compilation +mode, so it doesn't run any tests that require execution. + +Hint: if you mean to override the result of @command{config.guess}, +prefer @option{--build} over @option{--host}. + +@sp 1 + +For backward compatibility, @command{configure} accepts a system +type as an option by itself. Such an option overrides the +defaults for build, host, and target system types. The following +configure statement configures a cross toolchain that runs on +NetBSD/alpha but generates code for GNU Hurd/sparc, +which is also the build platform. + +@example +./configure --host=alpha-netbsd sparc-gnu +@end example + +@sp 1 + +In Autoconf 2.13 and before, the variables @code{build}, @code{host}, +and @code{target} had a different semantics before and after the +invocation of @code{AC_CANONICAL_BUILD} etc. Now, the argument of +@option{--build} is strictly copied into @code{build_alias}, and is left +empty otherwise. After the @code{AC_CANONICAL_BUILD}, @code{build} is +set to the canonicalized build type. To ease the transition, before, +its contents is the same as that of @code{build_alias}. Do @emph{not} +rely on this broken feature. + +For consistency with the backward compatibility scheme exposed above, +when @option{--host} is specified but @option{--build} isn't, the build +system is assumed to be the same as @option{--host}, and +@samp{build_alias} is set to that value. Eventually, this +historically incorrect behavior will go away. + +@sp 1 + +The former scheme to enable cross-compilation proved to cause more harm +than good, in particular, it used to be triggered too easily, leaving +regular end users puzzled in front of cryptic error messages. +@command{configure} could even enter cross-compilation mode only +because the compiler was not functional. This is mainly because +@command{configure} used to try to detect cross-compilation, instead of +waiting for an explicit flag from the user. + +Now, @command{configure} enters cross-compilation mode if and only if +@option{--host} is passed. + +That's the short documentation. To ease the transition between 2.13 and +its successors, a more complicated scheme is implemented. @emph{Do not +rely on the following}, as it will be removed in the near future. + +If you specify @option{--host}, but not @option{--build}, when +@command{configure} performs the first compiler test it tries to run +an executable produced by the compiler. If the execution fails, it +enters cross-compilation mode. This is fragile. Moreover, by the time +the compiler test is performed, it may be too late to modify the +build-system type: other tests may have already been performed. +Therefore, whenever you specify @option{--host}, be sure to specify +@option{--build} too. + +@example +./configure --build=i686-pc-linux-gnu --host=m68k-coff +@end example + +@noindent +enters cross-compilation mode. The former interface, which +consisted in setting the compiler to a cross-compiler without informing +@command{configure} is obsolete. For instance, @command{configure} +fails if it can't run the code generated by the specified compiler if you +configure as follows: + +@example +./configure CC=m68k-coff-gcc +@end example + + +@node AC_LIBOBJ vs LIBOBJS +@subsection @code{AC_LIBOBJ} vs.@: @code{LIBOBJS} + +Up to Autoconf 2.13, the replacement of functions was triggered via the +variable @code{LIBOBJS}. Since Autoconf 2.50, the macro +@code{AC_LIBOBJ} should be used instead (@pxref{Generic Functions}). +Starting at Autoconf 2.53, the use of @code{LIBOBJS} is an error. + +This change is mandated by the unification of the GNU Build System +components. In particular, the various fragile techniques used to parse +a @file{configure.ac} are all replaced with the use of traces. As a +consequence, any action must be traceable, which obsoletes critical +variable assignments. Fortunately, @code{LIBOBJS} was the only problem, +and it can even be handled gracefully (read, ``without your having to +change something''). + +There were two typical uses of @code{LIBOBJS}: asking for a replacement +function, and adjusting @code{LIBOBJS} for Automake and/or Libtool. + +@sp 1 + +As for function replacement, the fix is immediate: use +@code{AC_LIBOBJ}. For instance: + +@example +LIBOBJS="$LIBOBJS fnmatch.o" +LIBOBJS="$LIBOBJS malloc.$ac_objext" +@end example + +@noindent +should be replaced with: + +@example +AC_LIBOBJ([fnmatch]) +AC_LIBOBJ([malloc]) +@end example + +@sp 1 + +@ovindex LIBOBJDIR +When used with Automake 1.10 or newer, a suitable value for +@code{LIBOBJDIR} is set so that the @code{LIBOBJS} and @code{LTLIBOBJS} +can be referenced from any @file{Makefile.am}. Even without Automake, +arranging for @code{LIBOBJDIR} to be set correctly enables +referencing @code{LIBOBJS} and @code{LTLIBOBJS} in another directory. +The @code{LIBOBJDIR} feature is experimental. + + +@node AC_ACT_IFELSE vs AC_TRY_ACT +@subsection @code{AC_@var{ACT}_IFELSE} vs.@: @code{AC_TRY_@var{ACT}} +@c the anchor keeps the old node name, to try to avoid breaking links +@anchor{AC_FOO_IFELSE vs AC_TRY_FOO} + +@acindex{@var{ACT}_IFELSE} +@acindex{TRY_@var{ACT}} +Since Autoconf 2.50, internal codes uses @code{AC_PREPROC_IFELSE}, +@code{AC_COMPILE_IFELSE}, @code{AC_LINK_IFELSE}, and +@code{AC_RUN_IFELSE} on one hand and @code{AC_LANG_SOURCE}, +and @code{AC_LANG_PROGRAM} on the other hand instead of the deprecated +@code{AC_TRY_CPP}, @code{AC_TRY_COMPILE}, @code{AC_TRY_LINK}, and +@code{AC_TRY_RUN}. The motivations where: +@itemize @minus +@item +a more consistent interface: @code{AC_TRY_COMPILE} etc.@: were double +quoting their arguments; + +@item +the combinatoric explosion is solved by decomposing on the one hand the +generation of sources, and on the other hand executing the program; + +@item +this scheme helps supporting more languages than plain C and C++. +@end itemize + +In addition to the change of syntax, the philosophy has changed too: +while emphasis was put on speed at the expense of accuracy, today's +Autoconf promotes accuracy of the testing framework at, ahem@dots{}, the +expense of speed. + + +As a perfect example of what is @emph{not} to be done, here is how to +find out whether a header file contains a particular declaration, such +as a typedef, a structure, a structure member, or a function. Use +@code{AC_EGREP_HEADER} instead of running @code{grep} directly on the +header file; on some systems the symbol might be defined in another +header file that the file you are checking includes. + +As a (bad) example, here is how you should not check for C preprocessor +symbols, either defined by header files or predefined by the C +preprocessor: using @code{AC_EGREP_CPP}: + +@example +@group +AC_EGREP_CPP(yes, +[#ifdef _AIX + yes +#endif +], is_aix=yes, is_aix=no) +@end group +@end example + +The above example, properly written would (i) use +@code{AC_LANG_PROGRAM}, and (ii) run the compiler: + +@example +@group +AC_COMPILE_IFELSE([AC_LANG_PROGRAM( +[[#ifndef _AIX + error: This isn't AIX! +#endif +]])], + [is_aix=yes], + [is_aix=no]) +@end group +@end example + + +@c ============================= Generating Test Suites with Autotest + +@node Using Autotest +@chapter Generating Test Suites with Autotest + +@cindex Autotest + +@display +@strong{N.B.: This section describes a feature which is still +stabilizing. Although we believe that Autotest is useful as-is, this +documentation describes an interface which might change in the future: +do not depend upon Autotest without subscribing to the Autoconf mailing +lists.} +@end display + +It is paradoxical that portable projects depend on nonportable tools +to run their test suite. Autoconf by itself is the paragon of this +problem: although it aims at perfectly portability, up to 2.13 its +test suite was using DejaGNU, a rich and complex testing +framework, but which is far from being standard on Posix systems. +Worse yet, it was likely to be missing on the most fragile platforms, +the very platforms that are most likely to torture Autoconf and +exhibit deficiencies. + +To circumvent this problem, many package maintainers have developed their +own testing framework, based on simple shell scripts whose sole outputs +are exit status values describing whether the test succeeded. Most of +these tests share common patterns, and this can result in lots of +duplicated code and tedious maintenance. + +Following exactly the same reasoning that yielded to the inception of +Autoconf, Autotest provides a test suite generation framework, based on +M4 macros building a portable shell script. The suite itself is +equipped with automatic logging and tracing facilities which greatly +diminish the interaction with bug reporters, and simple timing reports. + +Autoconf itself has been using Autotest for years, and we do attest that +it has considerably improved the strength of the test suite and the +quality of bug reports. Other projects are known to use some generation +of Autotest, such as Bison, Free Recode, Free Wdiff, GNU Tar, each of +them with different needs, and this usage has validated Autotest as a general +testing framework. + +Nonetheless, compared to DejaGNU, Autotest is inadequate for +interactive tool testing, which is probably its main limitation. + +@menu +* Using an Autotest Test Suite:: Autotest and the user +* Writing Testsuites:: Autotest macros +* testsuite Invocation:: Running @command{testsuite} scripts +* Making testsuite Scripts:: Using autom4te to create @command{testsuite} +@end menu + +@node Using an Autotest Test Suite +@section Using an Autotest Test Suite + +@menu +* testsuite Scripts:: The concepts of Autotest +* Autotest Logs:: Their contents +@end menu + +@node testsuite Scripts +@subsection @command{testsuite} Scripts + +@cindex @command{testsuite} + +Generating testing or validation suites using Autotest is rather easy. +The whole validation suite is held in a file to be processed through +@command{autom4te}, itself using GNU M4 under the hood, to +produce a stand-alone Bourne shell script which then gets distributed. +Neither @command{autom4te} nor GNU M4 are needed at +the installer's end. + +@cindex test group +Each test of the validation suite should be part of some test group. A +@dfn{test group} is a sequence of interwoven tests that ought to be +executed together, usually because one test in the group creates data +files that a later test in the same group needs to read. Complex test +groups make later debugging more tedious. It is much better to +keep only a few tests per test group. Ideally there is only one test +per test group. + +For all but the simplest packages, some file such as @file{testsuite.at} +does not fully hold all test sources, as these are often easier to +maintain in separate files. Each of these separate files holds a single +test group, or a sequence of test groups all addressing some common +functionality in the package. In such cases, @file{testsuite.at} +merely initializes the validation suite, and sometimes does elementary +health checking, before listing include statements for all other test +files. The special file @file{package.m4}, containing the +identification of the package, is automatically included if found. + +A convenient alternative consists in moving all the global issues +(local Autotest macros, elementary health checking, and @code{AT_INIT} +invocation) into the file @code{local.at}, and making +@file{testsuite.at} be a simple list of @code{m4_include}s of sub test +suites. In such case, generating the whole test suite or pieces of it +is only a matter of choosing the @command{autom4te} command line +arguments. + +The validation scripts that Autotest produces are by convention called +@command{testsuite}. When run, @command{testsuite} executes each test +group in turn, producing only one summary line per test to say if that +particular test succeeded or failed. At end of all tests, summarizing +counters get printed. One debugging directory is left for each test +group which failed, if any: such directories are named +@file{testsuite.dir/@var{nn}}, where @var{nn} is the sequence number of +the test group, and they include: + +@itemize @bullet +@item a debugging script named @file{run} which reruns the test in +@dfn{debug mode} (@pxref{testsuite Invocation}). The automatic generation +of debugging scripts has the purpose of easing the chase for bugs. + +@item all the files created with @code{AT_DATA} + +@item all the Erlang source code files created with @code{AT_CHECK_EUNIT} + +@item a log of the run, named @file{testsuite.log} +@end itemize + +In the ideal situation, none of the tests fail, and consequently no +debugging directory is left behind for validation. + +It often happens in practice that individual tests in the validation +suite need to get information coming out of the configuration process. +Some of this information, common for all validation suites, is provided +through the file @file{atconfig}, automatically created by +@code{AC_CONFIG_TESTDIR}. For configuration information which your +testing environment specifically needs, you might prepare an optional +file named @file{atlocal.in}, instantiated by @code{AC_CONFIG_FILES}. +The configuration process produces @file{atconfig} and @file{atlocal} +out of these two input files, and these two produced files are +automatically read by the @file{testsuite} script. + +Here is a diagram showing the relationship between files. + +@noindent +Files used in preparing a software package for distribution: + +@example + [package.m4] -->. + \ +subfile-1.at ->. [local.at] ---->+ + ... \ \ +subfile-i.at ---->-- testsuite.at -->-- autom4te* -->testsuite + ... / +subfile-n.at ->' +@end example + +@noindent +Files used in configuring a software package: + +@example + .--> atconfig + / +[atlocal.in] --> config.status* --< + \ + `--> [atlocal] +@end example + +@noindent +Files created during test suite execution: + +@example +atconfig -->. .--> testsuite.log + \ / + >-- testsuite* --< + / \ +[atlocal] ->' `--> [testsuite.dir] +@end example + + +@node Autotest Logs +@subsection Autotest Logs + +When run, the test suite creates a log file named after itself, e.g., a +test suite named @command{testsuite} creates @file{testsuite.log}. It +contains a lot of information, usually more than maintainers actually +need, but therefore most of the time it contains all that is needed: + +@table @asis +@item command line arguments +A bad but unfortunately widespread habit consists of +setting environment variables before the command, such as in +@samp{CC=my-home-grown-cc ./testsuite}. The test suite does not +know this change, hence (i) it cannot report it to you, and (ii) +it cannot preserve the value of @code{CC} for subsequent runs. +Autoconf faced exactly the same problem, and solved it by asking +users to pass the variable definitions as command line arguments. +Autotest requires this rule, too, but has no means to enforce it; the log +then contains a trace of the variables that were changed by the user. + +@item @file{ChangeLog} excerpts +The topmost lines of all the @file{ChangeLog} files found in the source +hierarchy. This is especially useful when bugs are reported against +development versions of the package, since the version string does not +provide sufficient information to know the exact state of the sources +the user compiled. Of course, this relies on the use of a +@file{ChangeLog}. + +@item build machine +Running a test suite in a cross-compile environment is not an easy task, +since it would mean having the test suite run on a machine @var{build}, +while running programs on a machine @var{host}. It is much simpler to +run both the test suite and the programs on @var{host}, but then, from +the point of view of the test suite, there remains a single environment, +@var{host} = @var{build}. The log contains relevant information on the +state of the @var{build} machine, including some important environment +variables. +@c FIXME: How about having an M4sh macro to say `hey, log the value +@c of `@dots{}'? This would help both Autoconf and Autotest. + +@item tested programs +The absolute file name and answers to @option{--version} of the tested +programs (see @ref{Writing Testsuites}, @code{AT_TESTED}). + +@item configuration log +The contents of @file{config.log}, as created by @command{configure}, +are appended. It contains the configuration flags and a detailed report +on the configuration itself. +@end table + + +@node Writing Testsuites +@section Writing @file{testsuite.at} + +The @file{testsuite.at} is a Bourne shell script making use of special +Autotest M4 macros. It often contains a call to @code{AT_INIT} near +its beginning followed by one call to @code{m4_include} per source file +for tests. Each such included file, or the remainder of +@file{testsuite.at} if include files are not used, contain a sequence of +test groups. Each test group begins with a call to @code{AT_SETUP}, +then an arbitrary number of shell commands or calls to @code{AT_CHECK}, +and then completes with a call to @code{AT_CLEANUP}. Multiple test +groups can be categorized by a call to @code{AT_BANNER}. + +All of the public Autotest macros have all-uppercase names in the +namespace @samp{^AT_} to prevent them from accidentally conflicting with +other text; Autoconf also reserves the namespace @samp{^_AT_} for +internal macros. All shell variables used in the testsuite for internal +purposes have mostly-lowercase names starting with @samp{at_}. Autotest +also uses here-document delimiters in the namespace @samp{^_AT[A-Z]}, and +makes use of the file system namespace @samp{^at-}. + +Since Autoconf is built on top of M4sugar (@pxref{Programming in +M4sugar}) and M4sh (@pxref{Programming in M4sh}), you must also be aware +of those namespaces (@samp{^_?\(m4\|AS\)_}). In general, you +@emph{should not use} the namespace of a package that does not own the +macro or shell code you are writing. + +@defmac AT_INIT (@ovar{name}) +@atindex{INIT} +@c FIXME: Not clear, plus duplication of the information. +Initialize Autotest. Giving a @var{name} to the test suite is +encouraged if your package includes several test suites. Before this +macro is called, @code{AT_PACKAGE_STRING} and +@code{AT_PACKAGE_BUGREPORT} must be defined, which are used to display +information about the testsuite to the user. Typically, these macros +are provided by a file @file{package.m4} built by @command{make} +(@pxref{Making testsuite Scripts}), in order to inherit the package +name, version, and bug reporting address from @file{configure.ac}. +@end defmac + +@defmac AT_COPYRIGHT (@var{copyright-notice}) +@atindex{COPYRIGHT} +@cindex Copyright Notice +State that, in addition to the Free Software Foundation's copyright on +the Autotest macros, parts of your test suite are covered by +@var{copyright-notice}. + +The @var{copyright-notice} shows up in both the head of +@command{testsuite} and in @samp{testsuite --version}. +@end defmac + +@defmac AT_ARG_OPTION (@var{options}, @var{help-text}, @ + @ovar{action-if-given}, @ovar{action-if-not-given}) +@atindex{ARG_OPTION} +@vrindex at_arg_@var{option} +Accept options from the space-separated list @var{options}, a list that +has leading dashes removed from the options. Long options will be +prefixed with @samp{--}, single-character options with @samp{-}. The +first word in this list is the primary @var{option}, any others are +assumed to be short-hand aliases. The variable associated with it +is @code{at_arg_@var{option}}, with any dashes in @var{option} replaced +with underscores. + +If the user passes @option{--@var{option}} to the @command{testsuite}, +the variable will be set to @samp{:}. If the user does not pass the +option, or passes @option{--no-@var{option}}, then the variable will be +set to @samp{false}. + +@vrindex at_optarg +@vrindex at_optarg_@var{option} +@var{action-if-given} is run each time the option is encountered; here, +the variable @code{at_optarg} will be set to @samp{:} or @samp{false} as +appropriate. @code{at_optarg} is actually just a copy of +@code{at_arg_@var{option}}. + +@var{action-if-not-given} will be run once after option parsing is +complete and if no option from @var{options} was used. + +@var{help-text} is added to the end of the list of options shown in +@command{testsuite --help} (@pxref{AS_HELP_STRING}). + +It is recommended that you use a package-specific prefix to @var{options} +names in order to avoid clashes with future Autotest built-in options. +@end defmac + +@defmac AT_ARG_OPTION_ARG (@var{options}, @var{help-text}, @ + @ovar{action-if-given}, @ovar{action-if-not-given}) +@atindex{ARG_OPTION_ARG} +@vrindex at_arg_@var{option} +Accept options with arguments from the space-separated list +@var{options}, a list that has leading dashes removed from the options. +Long options will be prefixed with @samp{--}, single-character options +with @samp{-}. The first word in this list is the primary @var{option}, +any others are assumed to be short-hand aliases. The variable associated +with it is @code{at_arg_@var{option}}, with any dashes in @var{option} +replaced with underscores. + +If the user passes @option{--@var{option}=@var{arg}} or +@option{--@var{option} @var{arg}} to the @command{testsuite}, the +variable will be set to @samp{@var{arg}}. + +@vrindex at_optarg +@var{action-if-given} is run each time the option is encountered; here, +the variable @code{at_optarg} will be set to @samp{@var{arg}}. +@code{at_optarg} is actually just a copy of @code{at_arg_@var{option}}. + +@var{action-if-not-given} will be run once after option parsing is +complete and if no option from @var{options} was used. + +@var{help-text} is added to the end of the list of options shown in +@command{testsuite --help} (@pxref{AS_HELP_STRING}). + +It is recommended that you use a package-specific prefix to @var{options} +names in order to avoid clashes with future Autotest built-in options. +@end defmac + +@defmac AT_COLOR_TESTS +@atindex{COLOR_TESTS} +Enable colored test results by default when the output is connected to +a terminal. +@end defmac + +@defmac AT_TESTED (@var{executables}) +@atindex{TESTED} +Log the file name and answer to @option{--version} of each program in +space-separated list @var{executables}. Several invocations register +new executables, in other words, don't fear registering one program +several times. + +Autotest test suites rely on @env{PATH} to find the tested program. +This avoids the need to generate absolute names of the various tools, and +makes it possible to test installed programs. Therefore, knowing which +programs are being exercised is crucial to understanding problems in +the test suite itself, or its occasional misuses. It is a good idea to +also subscribe foreign programs you depend upon, to avoid incompatible +diagnostics. +@end defmac + +@sp 1 + +@defmac AT_BANNER (@var{test-category-name}) +@atindex{BANNER} +This macro identifies the start of a category of related test groups. +When the resulting @file{testsuite} is invoked with more than one test +group to run, its output will include a banner containing +@var{test-category-name} prior to any tests run from that category. The +banner should be no more than about 40 or 50 characters. A blank banner +indicates uncategorized tests; an empty line will be inserted after +tests from an earlier category, effectively ending that category. +@end defmac + +@defmac AT_SETUP (@var{test-group-name}) +@atindex{SETUP} +This macro starts a group of related tests, all to be executed in the +same subshell. It accepts a single argument, which holds a few words +(no more than about 30 or 40 characters) quickly describing the purpose +of the test group being started. @var{test-group-name} must not expand +to unbalanced quotes, although quadrigraphs can be used. +@end defmac + +@defmac AT_KEYWORDS (@var{keywords}) +@atindex{KEYWORDS} +Associate the space-separated list of @var{keywords} to the enclosing +test group. This makes it possible to run ``slices'' of the test suite. +For instance, if some of your test groups exercise some @samp{foo} +feature, then using @samp{AT_KEYWORDS(foo)} lets you run +@samp{./testsuite -k foo} to run exclusively these test groups. The +@var{test-group-name} of the test group is automatically recorded to +@code{AT_KEYWORDS}. + +Several invocations within a test group accumulate new keywords. In +other words, don't fear registering the same keyword several times in a +test group. +@end defmac + +@defmac AT_CAPTURE_FILE (@var{file}) +@atindex{CAPTURE_FILE} +If the current test group fails, log the contents of @var{file}. +Several identical calls within one test group have no additional effect. +@end defmac + +@defmac AT_FAIL_IF (@var{shell-condition}) +@atindex{FAIL_IF} +Make the test group fail and skip the rest of its execution, if +@var{shell-condition} is true. @var{shell-condition} is a shell expression +such as a @code{test} command. Tests before @command{AT_FAIL_IF} +will be executed and may still cause the test group to be skipped. +You can instantiate this macro many times from within the same test group. + +You should use this macro only for very simple failure conditions. If the +@var{shell-condition} could emit any kind of output you should instead +use @command{AT_CHECK} like +@example +AT_CHECK([if @var{shell-condition}; then exit 99; fi]) +@end example +@noindent +so that such output is properly recorded in the @file{testsuite.log} +file. +@end defmac + +@defmac AT_SKIP_IF (@var{shell-condition}) +@atindex{SKIP_IF} +Determine whether the test should be skipped because it requires +features that are unsupported on the machine under test. +@var{shell-condition} is a shell expression such as a @code{test} +command. Tests before @command{AT_SKIP_IF} will be executed +and may still cause the test group to fail. You can instantiate this +macro many times from within the same test group. + +You should use this macro only for very simple skip conditions. If the +@var{shell-condition} could emit any kind of output you should instead +use @command{AT_CHECK} like +@example +AT_CHECK([if @var{shell-condition}; then exit 77; fi]) +@end example +@noindent +so that such output is properly recorded in the @file{testsuite.log} +file. +@end defmac + +@defmac AT_XFAIL_IF (@var{shell-condition}) +@atindex{XFAIL_IF} +Determine whether the test is expected to fail because it is a known +bug (for unsupported features, you should skip the test). +@var{shell-condition} is a shell expression such as a @code{test} +command; you can instantiate this macro many times from within the +same test group, and one of the conditions is enough to turn +the test into an expected failure. +@end defmac + +@defmac AT_CLEANUP +@atindex{CLEANUP} +End the current test group. +@end defmac + +@sp 1 + +@defmac AT_DATA (@var{file}, @var{contents}) +@atindex{DATA} +Initialize an input data @var{file} with given @var{contents}. Of +course, the @var{contents} have to be properly quoted between square +brackets to protect against included commas or spurious M4 +expansion. @var{contents} must be empty or end with a newline. +@var{file} must +be a single shell word that expands into a single file name. +@end defmac + +@defmac AT_CHECK (@var{commands}, @dvar{status, 0}, @ovar{stdout}, @ + @ovar{stderr}, @ovar{run-if-fail}, @ovar{run-if-pass}) +@defmacx AT_CHECK_UNQUOTED (@var{commands}, @dvar{status, 0}, @ovar{stdout}, @ + @ovar{stderr}, @ovar{run-if-fail}, @ovar{run-if-pass}) +@atindex{CHECK} +@atindex{CHECK_UNQUOTED} +@vrindex at_status +Execute a test by performing given shell @var{commands} in a subshell. +@var{commands} is output as-is, so shell expansions are honored. These +commands should normally exit with @var{status}, while producing expected +@var{stdout} and @var{stderr} contents. If @var{commands} exit with +unexpected status 77, then the rest of the test group is skipped. If +@var{commands} exit with unexpected status 99, then the test group is +immediately failed. Otherwise, if this test fails, run shell commands +@var{run-if-fail} or, if this test passes, run shell commands +@var{run-if-pass}, both inside the current shell execution environment. +At the beginning of @var{run-if-fail} and @var{run-if-pass}, the status of +@var{commands} is available in the @code{at_status} shell variable. + +This macro must be invoked in between @code{AT_SETUP} and @code{AT_CLEANUP}. + +If @var{status} is the literal @samp{ignore}, then the corresponding +exit status is not checked, except for the special cases of 77 (skip) +and 99 (hard failure). The existence of hard failures allows one to +mark a test as an expected failure with @code{AT_XFAIL_IF} because a +feature has not yet been implemented, but to still distinguish between +gracefully handling the missing feature and dumping core. A hard +failure also inhibits post-test actions in @var{run-if-fail}. + +If the value of the @var{stdout} or @var{stderr} parameter is one of the +literals in the following table, then the test treats the output +according to the rules of that literal. Otherwise, the value of the +parameter is treated as text that must exactly match the output given by +@var{commands} on standard output and standard error (including an empty +parameter for no output); any differences are captured in the testsuite +log and the test is failed (unless an unexpected exit status of 77 +skipped the test instead). The difference between @code{AT_CHECK} and +@code{AT_CHECK_UNQUOTED} is that only the latter performs shell variable +expansion (@samp{$}), command substitution (@samp{`}), and backslash +escaping (@samp{\}) on comparison text given in the @var{stdout} and +@var{stderr} arguments; if the text includes a trailing newline, this +would be the same as if it were specified via an unquoted +here-document. (However, there is no difference in the interpretation +of @var{commands}). + +@table @samp +@item ignore +The content of the output is ignored, but still captured in the test +group log (if the testsuite is run with option @option{-v}, the test +group log is displayed as the test is run; if the test group later +fails, the test group log is also copied into the overall testsuite +log). This action is valid for both @var{stdout} and @var{stderr}. + +@item ignore-nolog +The content of the output is ignored, and nothing is captured in the log +files. If @var{commands} are likely to produce binary output (including +long lines) or large amounts of output, then logging the output can make +it harder to locate details related to subsequent tests within the +group, and could potentially corrupt terminal display of a user running +@command{testsuite -v}. + +@item stdout +For the @var{stdout} parameter, capture the content of standard output +to both the file @file{stdout} and the test group log. Subsequent +commands in the test group can then post-process the file. This action +is often used when it is desired to use @command{grep} to look for a +substring in the output, or when the output must be post-processed to +normalize error messages into a common form. + +@item stderr +Like @samp{stdout}, except that it only works for the @var{stderr} +parameter, and the standard error capture file will be named +@file{stderr}. + +@item stdout-nolog +@itemx stderr-nolog +Like @samp{stdout} or @samp{stderr}, except that the captured output is +not duplicated into the test group log. This action is particularly +useful for an intermediate check that produces large amounts of data, +which will be followed by another check that filters down to the +relevant data, as it makes it easier to locate details in the log. + +@item expout +For the @var{stdout} parameter, compare standard output contents with +the previously created file @file{expout}, and list any differences in +the testsuite log. + +@item experr +Like @samp{expout}, except that it only works for the @var{stderr} +parameter, and the standard error contents are compared with +@file{experr}. +@end table +@end defmac + +@defmac AT_CHECK_EUNIT (@var{module}, @var{test-spec}, @ovar{erlflags}, @ + @ovar{run-if-fail}, @ovar{run-if-pass}) +@atindex{CHECK_EUNIT} +Initialize and execute an Erlang module named @var{module} that performs +tests following the @var{test-spec} EUnit test specification. +@var{test-spec} must be a valid EUnit test specification, as defined in +the @uref{http://@/erlang.org/@/doc/@/apps/@/eunit/@/index.html, EUnit +Reference Manual}. @var{erlflags} are optional command-line options +passed to the Erlang interpreter to execute the test Erlang module. +Typically, @var{erlflags} defines at least the paths to directories +containing the compiled Erlang modules under test, as @samp{-pa path1 +path2 ...}. + +For example, the unit tests associated with Erlang module @samp{testme}, +which compiled code is in subdirectory @file{src}, can be performed +with: + +@example +AT_CHECK_EUNIT([testme_testsuite], [@{module, testme@}], + [-pa "$@{abs_top_builddir@}/src"]) +@end example + +This macro must be invoked in between @code{AT_SETUP} and @code{AT_CLEANUP}. + +Variables @code{ERL}, @code{ERLC}, and (optionally) @code{ERLCFLAGS} +must be defined as the path of the Erlang interpreter, the path of the +Erlang compiler, and the command-line flags to pass to the compiler, +respectively. Those variables should be configured in +@file{configure.ac} using the @command{AC_ERLANG_PATH_ERL} and +@command{AC_ERLANG_PATH_ERLC} macros, and the configured values of those +variables are automatically defined in the testsuite. If @code{ERL} or +@code{ERLC} is not defined, the test group is skipped. + +If the EUnit library cannot be found, i.e. if module @code{eunit} cannot +be loaded, the test group is skipped. Otherwise, if @var{test-spec} is +an invalid EUnit test specification, the test group fails. Otherwise, +if the EUnit test passes, shell commands @var{run-if-pass} are executed +or, if the EUnit test fails, shell commands @var{run-if-fail} are +executed and the test group fails. + +Only the generated test Erlang module is automatically compiled and +executed. If @var{test-spec} involves testing other Erlang modules, +e.g. module @samp{testme} in the example above, those modules must be +already compiled. + +If the testsuite is run in verbose mode, with option @option{--verbose}, +EUnit is also run in verbose mode to output more details about +individual unit tests. +@end defmac + + +@node testsuite Invocation +@section Running @command{testsuite} Scripts +@cindex @command{testsuite} + +Autotest test suites support the following options: + +@table @option +@item --help +@itemx -h +Display the list of options and exit successfully. + +@item --version +@itemx -V +Display the version of the test suite and exit successfully. + +@item --directory=@var{dir} +@itemx -C @var{dir} +Change the current directory to @var{dir} before creating any files. +Useful for running the testsuite in a subdirectory from a top-level +Makefile. + +@item --jobs@r{[}=@var{n}@r{]} +@itemx -j@ovar{n} +Run @var{n} tests in parallel, if possible. If @var{n} is not given, +run all given tests in parallel. Note that there should be no space +before the argument to @option{-j}, as @option{-j @var{number}} denotes +the separate arguments @option{-j} and @option{@var{number}}, see below. + +In parallel mode, the standard input device of the testsuite script is +not available to commands inside a test group. Furthermore, banner +lines are not printed, and the summary line for each test group is +output after the test group completes. Summary lines may appear +unordered. If verbose and trace output are enabled (see below), they +may appear intermixed from concurrently running tests. + +Parallel mode requires the @command{mkfifo} command to work, and will be +silently disabled otherwise. + +@item --clean +@itemx -c +Remove all the files the test suite might have created and exit. Meant +for @code{clean} Make targets. + +@item --list +@itemx -l +List all the tests (or only the selection), including their possible +keywords. +@end table + +@sp 1 + +By default all tests are performed (or described with @option{--list}) +silently in the default environment, but the environment, set of tests, +and verbosity level can be tuned: + +@table @samp +@item @var{variable}=@var{value} +Set the environment @var{variable} to @var{value}. Use this rather +than @samp{FOO=foo ./testsuite} as debugging scripts would then run in a +different environment. + +@cindex @code{AUTOTEST_PATH} +The variable @code{AUTOTEST_PATH} specifies the testing path to prepend +to @env{PATH}. Relative directory names (not starting with +@samp{/}) are considered to be relative to the top level of the +package being built. All directories are made absolute, first +starting from the top level @emph{build} tree, then from the +@emph{source} tree. For instance @samp{./testsuite +AUTOTEST_PATH=tests:bin} for a @file{/src/foo-1.0} source package built +in @file{/tmp/foo} results in @samp{/tmp/foo/tests:/tmp/foo/bin} and +then @samp{/src/foo-1.0/tests:/src/foo-1.0/bin} being prepended to +@env{PATH}. + +@item @var{number} +@itemx @var{number}-@var{number} +@itemx @var{number}- +@itemx -@var{number} +Add the corresponding test groups, with obvious semantics, to the +selection. + +@item --keywords=@var{keywords} +@itemx -k @var{keywords} +Add to the selection the test groups with title or keywords (arguments +to @code{AT_SETUP} or @code{AT_KEYWORDS}) that match @emph{all} keywords +of the comma separated list @var{keywords}, case-insensitively. Use +@samp{!} immediately before the keyword to invert the selection for this +keyword. By default, the keywords match whole words; enclose them in +@samp{.*} to also match parts of words. + +For example, running + +@example +@kbd{./testsuite -k 'autoupdate,.*FUNC.*'} +@end example + +@noindent +selects all tests tagged @samp{autoupdate} @emph{and} with tags +containing @samp{FUNC} (as in @samp{AC_CHECK_FUNC}, @samp{AC_FUNC_ALLOCA}, +etc.), while + +@example +@kbd{./testsuite -k '!autoupdate' -k '.*FUNC.*'} +@end example + +@noindent +selects all tests not tagged @samp{autoupdate} @emph{or} with tags +containing @samp{FUNC}. + +@item --errexit +@itemx -e +If any test fails, immediately abort testing. This implies +@option{--debug}: post test group clean up, and top-level logging +are inhibited. This option is meant for the full test +suite, it is not really useful for generated debugging scripts. +If the testsuite is run in parallel mode using @option{--jobs}, +then concurrently running tests will finish before exiting. + +@item --verbose +@itemx -v +Force more verbosity in the detailed output of what is being done. This +is the default for debugging scripts. + +@item --color +@itemx --color@r{[}=never@r{|}auto@r{|}always@r{]} +Enable colored test results. Without an argument, or with @samp{always}, +test results will be colored. With @samp{never}, color mode is turned +off. Otherwise, if either the macro @code{AT_COLOR_TESTS} is used by +the testsuite author, or the argument @samp{auto} is given, then test +results are colored if standard output is connected to a terminal. + +@item --debug +@itemx -d +Do not remove the files after a test group was performed---but they are +still removed @emph{before}, therefore using this option is sane when +running several test groups. Create debugging scripts. Do not +overwrite the top-level +log (in order to preserve a supposedly existing full log file). This is +the default for debugging scripts, but it can also be useful to debug +the testsuite itself. + +@item --recheck +Add to the selection all test groups that failed or passed unexpectedly +during the last non-debugging test run. + +@item --trace +@itemx -x +Trigger shell tracing of the test groups. +@end table + +Besides these options accepted by every Autotest testsuite, the +testsuite author might have added package-specific options +via the @code{AT_ARG_OPTION} and @code{AT_ARG_OPTION_ARG} macros +(@pxref{Writing Testsuites}); refer to @command{testsuite --help} and +the package documentation for details. + + +@node Making testsuite Scripts +@section Making @command{testsuite} Scripts + +For putting Autotest into movement, you need some configuration and +makefile machinery. We recommend, at least if your package uses deep or +shallow hierarchies, that you use @file{tests/} as the name of the +directory holding all your tests and their makefile. Here is a +check list of things to do. + +@itemize @minus + +@item +@cindex @file{package.m4} +@atindex{PACKAGE_STRING} +@atindex{PACKAGE_BUGREPORT} +@atindex{PACKAGE_NAME} +@atindex{PACKAGE_TARNAME} +@atindex{PACKAGE_VERSION} +@atindex{PACKAGE_URL} +Make sure to create the file @file{package.m4}, which defines the +identity of the package. It must define @code{AT_PACKAGE_STRING}, the +full signature of the package, and @code{AT_PACKAGE_BUGREPORT}, the +address to which bug reports should be sent. For sake of completeness, +we suggest that you also define @code{AT_PACKAGE_NAME}, +@code{AT_PACKAGE_TARNAME}, @code{AT_PACKAGE_VERSION}, and +@code{AT_PACKAGE_URL}. +@xref{Initializing configure}, for a description of these variables. +Be sure to distribute @file{package.m4} and to put it into the source +hierarchy: the test suite ought to be shipped! See below for an example +@file{Makefile} excerpt. + +@item +Invoke @code{AC_CONFIG_TESTDIR}. + +@defmac AC_CONFIG_TESTDIR (@var{directory}, @dvar{test-path, directory}) +@acindex{CONFIG_TESTDIR} +An Autotest test suite is to be configured in @var{directory}. This +macro causes @file{@var{directory}/atconfig} to be created by +@command{config.status} and sets the default @code{AUTOTEST_PATH} to +@var{test-path} (@pxref{testsuite Invocation}). +@end defmac + +@item +Still within @file{configure.ac}, as appropriate, ensure that some +@code{AC_CONFIG_FILES} command includes substitution for +@file{tests/atlocal}. + +@item +The appropriate @file{Makefile} should be modified so the validation in +your package is triggered by @samp{make check}. An example is provided +below. +@end itemize + +With Automake, here is a minimal example for inclusion in +@file{tests/Makefile.am}, in order to link @samp{make check} with a +validation suite. + +@example +# The `:;' works around a Bash 3.2 bug when the output is not writable. +$(srcdir)/package.m4: $(top_srcdir)/configure.ac + :;@{ \ + echo '# Signature of the current package.' && \ + echo 'm4_define([AT_PACKAGE_NAME],' && \ + echo ' [$(PACKAGE_NAME)])' && \ + echo 'm4_define([AT_PACKAGE_TARNAME],' && \ + echo ' [$(PACKAGE_TARNAME)])' && \ + echo 'm4_define([AT_PACKAGE_VERSION],' && \ + echo ' [$(PACKAGE_VERSION)])' && \ + echo 'm4_define([AT_PACKAGE_STRING],' && \ + echo ' [$(PACKAGE_STRING)])' && \ + echo 'm4_define([AT_PACKAGE_BUGREPORT],' && \ + echo ' [$(PACKAGE_BUGREPORT)])'; \ + echo 'm4_define([AT_PACKAGE_URL],' && \ + echo ' [$(PACKAGE_URL)])'; \ + @} >'$(srcdir)/package.m4' + +EXTRA_DIST = testsuite.at $(srcdir)/package.m4 $(TESTSUITE) atlocal.in +TESTSUITE = $(srcdir)/testsuite + +check-local: atconfig atlocal $(TESTSUITE) + $(SHELL) '$(TESTSUITE)' $(TESTSUITEFLAGS) + +installcheck-local: atconfig atlocal $(TESTSUITE) + $(SHELL) '$(TESTSUITE)' AUTOTEST_PATH='$(bindir)' \ + $(TESTSUITEFLAGS) + +clean-local: + test ! -f '$(TESTSUITE)' || \ + $(SHELL) '$(TESTSUITE)' --clean + +AUTOM4TE = $(SHELL) $(srcdir)/build-aux/missing --run autom4te +AUTOTEST = $(AUTOM4TE) --language=autotest +$(TESTSUITE): $(srcdir)/testsuite.at $(srcdir)/package.m4 + $(AUTOTEST) -I '$(srcdir)' -o $@@.tmp $@@.at + mv $@@.tmp $@@ +@end example + +Note that the built testsuite is distributed; this is necessary because +users might not have Autoconf installed, and thus would not be able to +rebuild it. Likewise, the use of @file{missing} provides the user with +a nicer error message if they modify a source file to the testsuite, and +accidentally trigger the rebuild rules. + +You might want to list explicitly the dependencies, i.e., the list of +the files @file{testsuite.at} includes. + +If you don't use Automake, you should include the above example in +@file{tests/@/Makefile.in}, along with additional lines inspired from +the following: + +@example +subdir = tests +PACKAGE_NAME = @@PACKAGE_NAME@@ +PACKAGE_TARNAME = @@PACKAGE_TARNAME@@ +PACKAGE_VERSION = @@PACKAGE_VERSION@@ +PACKAGE_STRING = @@PACKAGE_STRING@@ +PACKAGE_BUGREPORT = @@PACKAGE_BUGREPORT@@ +PACKAGE_URL = @@PACKAGE_URL@@ + +atconfig: $(top_builddir)/config.status + cd $(top_builddir) && \ + $(SHELL) ./config.status $(subdir)/$@@ + +atlocal: $(srcdir)/atlocal.in $(top_builddir)/config.status + cd $(top_builddir) && \ + $(SHELL) ./config.status $(subdir)/$@@ +@end example + +@noindent +and manage to have @code{$(EXTRA_DIST)} distributed. You will also want +to distribute the file @file{build-aux/@/missing} from the Automake +project; a copy of this file resides in the Autoconf source tree. + +With all this in place, and if you have not initialized @samp{TESTSUITEFLAGS} +within your makefile, you can fine-tune test suite execution with this +variable, for example: + +@example +make check TESTSUITEFLAGS='-v -d -x 75 -k AC_PROG_CC CFLAGS=-g' +@end example + + + +@c =============================== Frequent Autoconf Questions, with answers + +@node FAQ +@chapter Frequent Autoconf Questions, with answers + +Several questions about Autoconf come up occasionally. Here some of them +are addressed. + +@menu +* Distributing:: Distributing @command{configure} scripts +* Why GNU M4:: Why not use the standard M4? +* Bootstrapping:: Autoconf and GNU M4 require each other? +* Why Not Imake:: Why GNU uses @command{configure} instead of Imake +* Defining Directories:: Passing @code{datadir} to program +* Autom4te Cache:: What is it? Can I remove it? +* Present But Cannot Be Compiled:: Compiler and Preprocessor Disagree +* Expanded Before Required:: Expanded Before Required +* Debugging:: Debugging @command{configure} scripts +@end menu + +@node Distributing +@section Distributing @command{configure} Scripts +@cindex License + +@display +What are the restrictions on distributing @command{configure} +scripts that Autoconf generates? How does that affect my +programs that use them? +@end display + +There are no restrictions on how the configuration scripts that Autoconf +produces may be distributed or used. In Autoconf version 1, they were +covered by the GNU General Public License. We still encourage +software authors to distribute their work under terms like those of the +GPL, but doing so is not required to use Autoconf. + +Of the other files that might be used with @command{configure}, +@file{config.h.in} is under whatever copyright you use for your +@file{configure.ac}. @file{config.sub} and @file{config.guess} have an +exception to the GPL when they are used with an Autoconf-generated +@command{configure} script, which permits you to distribute them under the +same terms as the rest of your package. @file{install-sh} is from the X +Consortium and is not copyrighted. + +@node Why GNU M4 +@section Why Require GNU M4? + +@display +Why does Autoconf require GNU M4? +@end display + +Many M4 implementations have hard-coded limitations on the size and +number of macros that Autoconf exceeds. They also lack several +builtin macros that it would be difficult to get along without in a +sophisticated application like Autoconf, including: + +@example +m4_builtin +m4_indir +m4_bpatsubst +__file__ +__line__ +@end example + +Autoconf requires version 1.4.6 or later of GNU M4. + +Since only software maintainers need to use Autoconf, and since GNU +M4 is simple to configure and install, it seems reasonable to require +GNU M4 to be installed also. Many maintainers of GNU and +other free software already have most of the GNU utilities +installed, since they prefer them. + +@node Bootstrapping +@section How Can I Bootstrap? +@cindex Bootstrap + +@display +If Autoconf requires GNU M4 and GNU M4 has an Autoconf +@command{configure} script, how do I bootstrap? It seems like a chicken +and egg problem! +@end display + +This is a misunderstanding. Although GNU M4 does come with a +@command{configure} script produced by Autoconf, Autoconf is not required +in order to run the script and install GNU M4. Autoconf is only +required if you want to change the M4 @command{configure} script, which few +people have to do (mainly its maintainer). + +@node Why Not Imake +@section Why Not Imake? +@cindex Imake + +@display +Why not use Imake instead of @command{configure} scripts? +@end display + +Several people have written addressing this question, so +adaptations of their explanations are included here. + +The following answer is based on one written by Richard Pixley: + +@quotation +Autoconf generated scripts frequently work on machines that it has +never been set up to handle before. That is, it does a good job of +inferring a configuration for a new system. Imake cannot do this. + +Imake uses a common database of host specific data. For X11, this makes +sense because the distribution is made as a collection of tools, by one +central authority who has control over the database. + +GNU tools are not released this way. Each GNU tool has a +maintainer; these maintainers are scattered across the world. Using a +common database would be a maintenance nightmare. Autoconf may appear +to be this kind of database, but in fact it is not. Instead of listing +host dependencies, it lists program requirements. + +If you view the GNU suite as a collection of native tools, then the +problems are similar. But the GNU development tools can be +configured as cross tools in almost any host+target permutation. All of +these configurations can be installed concurrently. They can even be +configured to share host independent files across hosts. Imake doesn't +address these issues. + +Imake templates are a form of standardization. The GNU coding +standards address the same issues without necessarily imposing the same +restrictions. +@end quotation + + +Here is some further explanation, written by Per Bothner: + +@quotation +One of the advantages of Imake is that it is easy to generate large +makefiles using the @samp{#include} and macro mechanisms of @command{cpp}. +However, @code{cpp} is not programmable: it has limited conditional +facilities, and no looping. And @code{cpp} cannot inspect its +environment. + +All of these problems are solved by using @code{sh} instead of +@code{cpp}. The shell is fully programmable, has macro substitution, +can execute (or source) other shell scripts, and can inspect its +environment. +@end quotation + + +Paul Eggert elaborates more: + +@quotation +With Autoconf, installers need not assume that Imake itself is already +installed and working well. This may not seem like much of an advantage +to people who are accustomed to Imake. But on many hosts Imake is not +installed or the default installation is not working well, and requiring +Imake to install a package hinders the acceptance of that package on +those hosts. For example, the Imake template and configuration files +might not be installed properly on a host, or the Imake build procedure +might wrongly assume that all source files are in one big directory +tree, or the Imake configuration might assume one compiler whereas the +package or the installer needs to use another, or there might be a +version mismatch between the Imake expected by the package and the Imake +supported by the host. These problems are much rarer with Autoconf, +where each package comes with its own independent configuration +processor. + +Also, Imake often suffers from unexpected interactions between +@command{make} and the installer's C preprocessor. The fundamental problem +here is that the C preprocessor was designed to preprocess C programs, +not makefiles. This is much less of a problem with Autoconf, +which uses the general-purpose preprocessor M4, and where the +package's author (rather than the installer) does the preprocessing in a +standard way. +@end quotation + + +Finally, Mark Eichin notes: + +@quotation +Imake isn't all that extensible, either. In order to add new features to +Imake, you need to provide your own project template, and duplicate most +of the features of the existing one. This means that for a sophisticated +project, using the vendor-provided Imake templates fails to provide any +leverage---since they don't cover anything that your own project needs +(unless it is an X11 program). + +On the other side, though: + +The one advantage that Imake has over @command{configure}: +@file{Imakefile} files tend to be much shorter (likewise, less redundant) +than @file{Makefile.in} files. There is a fix to this, however---at least +for the Kerberos V5 tree, we've modified things to call in common +@file{post.in} and @file{pre.in} makefile fragments for the +entire tree. This means that a lot of common things don't have to be +duplicated, even though they normally are in @command{configure} setups. +@end quotation + + +@node Defining Directories +@section How Do I @code{#define} Installation Directories? + +@display +My program needs library files, installed in @code{datadir} and +similar. If I use + +@example +AC_DEFINE_UNQUOTED([DATADIR], [$datadir], + [Define to the read-only architecture-independent + data directory.]) +@end example + +@noindent +I get + +@example +#define DATADIR "$@{prefix@}/share" +@end example +@end display + +As already explained, this behavior is on purpose, mandated by the +GNU Coding Standards, see @ref{Installation Directory +Variables}. There are several means to achieve a similar goal: + +@itemize @minus +@item +Do not use @code{AC_DEFINE} but use your makefile to pass the +actual value of @code{datadir} via compilation flags. +@xref{Installation Directory Variables}, for the details. + +@item +This solution can be simplified when compiling a program: you may either +extend the @code{CPPFLAGS}: + +@example +CPPFLAGS = -DDATADIR='"$(datadir)"' @@CPPFLAGS@@ +@end example + +@noindent +If you are using Automake, you should use @code{AM_CPPFLAGS} instead: + +@example +AM_CPPFLAGS = -DDATADIR='"$(datadir)"' +@end example + +@noindent +Alternatively, create a dedicated header file: + +@example +DISTCLEANFILES = myprog-paths.h +myprog-paths.h: Makefile + echo '#define DATADIR "$(datadir)"' >$@@ +@end example + +@noindent +The gnulib module @samp{configmake} provides such a header with all the +standard directory variables defined, @pxref{configmake,,, gnulib, GNU +Gnulib}. + +@item +Use @code{AC_DEFINE} but have @command{configure} compute the literal +value of @code{datadir} and others. Many people have wrapped macros to +automate this task; for an example, see the macro @code{AC_DEFINE_DIR} from +the @uref{http://@/www.gnu.org/@/software/@/autoconf-archive/, Autoconf Macro +Archive}. + +This solution does not conform to the GNU Coding Standards. + +@item +Note that all the previous solutions hard wire the absolute name of +these directories in the executables, which is not a good property. You +may try to compute the names relative to @code{prefix}, and try to +find @code{prefix} at runtime, this way your package is relocatable. +@end itemize + + +@node Autom4te Cache +@section What is @file{autom4te.cache}? + +@display +What is this directory @file{autom4te.cache}? Can I safely remove it? +@end display + +In the GNU Build System, @file{configure.ac} plays a central +role and is read by many tools: @command{autoconf} to create +@file{configure}, @command{autoheader} to create @file{config.h.in}, +@command{automake} to create @file{Makefile.in}, @command{autoscan} to +check the completeness of @file{configure.ac}, @command{autoreconf} to +check the GNU Build System components that are used. To +``read @file{configure.ac}'' actually means to compile it with M4, +which can be a long process for complex @file{configure.ac}. + +This is why all these tools, instead of running directly M4, invoke +@command{autom4te} (@pxref{autom4te Invocation}) which, while answering to +a specific demand, stores additional information in +@file{autom4te.cache} for future runs. For instance, if you run +@command{autoconf}, behind the scenes, @command{autom4te} also +stores information for the other tools, so that when you invoke +@command{autoheader} or @command{automake} etc., reprocessing +@file{configure.ac} is not needed. The speed up is frequently 30%, +and is increasing with the size of @file{configure.ac}. + +But it is and remains being simply a cache: you can safely remove it. + +@sp 1 + +@display +Can I permanently get rid of it? +@end display + +The creation of this cache can be disabled from +@file{~/.autom4te.cfg}, see @ref{Customizing autom4te}, for more +details. You should be aware that disabling the cache slows down the +Autoconf test suite by 40%. The more GNU Build System +components are used, the more the cache is useful; for instance +running @samp{autoreconf -f} on the Core Utilities is twice slower without +the cache @emph{although @option{--force} implies that the cache is +not fully exploited}, and eight times slower than without +@option{--force}. + + +@node Present But Cannot Be Compiled +@section Header Present But Cannot Be Compiled + +The most important guideline to bear in mind when checking for +features is to mimic as much as possible the intended use. +Unfortunately, old versions of @code{AC_CHECK_HEADER} and +@code{AC_CHECK_HEADERS} failed to follow this idea, and called +the preprocessor, instead of the compiler, to check for headers. As a +result, incompatibilities between headers went unnoticed during +configuration, and maintainers finally had to deal with this issue +elsewhere. + +The transition began with Autoconf 2.56. As of Autoconf 2.64 both +checks are performed, and @command{configure} complains loudly if the +compiler and the preprocessor do not agree. However, only the compiler +result is considered. + +Consider the following example: + +@smallexample +$ @kbd{cat number.h} +typedef int number; +$ @kbd{cat pi.h} +const number pi = 3; +$ @kbd{cat configure.ac} +AC_INIT([Example], [1.0], [bug-example@@example.org]) +AC_CHECK_HEADERS([pi.h]) +$ @kbd{autoconf -Wall} +$ @kbd{./configure} +checking for gcc... gcc +checking for C compiler default output file name... a.out +checking whether the C compiler works... yes +checking whether we are cross compiling... no +checking for suffix of executables... +checking for suffix of object files... o +checking whether we are using the GNU C compiler... yes +checking whether gcc accepts -g... yes +checking for gcc option to accept ISO C89... none needed +checking how to run the C preprocessor... gcc -E +checking for grep that handles long lines and -e... grep +checking for egrep... grep -E +checking for ANSI C header files... yes +checking for sys/types.h... yes +checking for sys/stat.h... yes +checking for stdlib.h... yes +checking for string.h... yes +checking for memory.h... yes +checking for strings.h... yes +checking for inttypes.h... yes +checking for stdint.h... yes +checking for unistd.h... yes +checking pi.h usability... no +checking pi.h presence... yes +configure: WARNING: pi.h: present but cannot be compiled +configure: WARNING: pi.h: check for missing prerequisite headers? +configure: WARNING: pi.h: see the Autoconf documentation +configure: WARNING: pi.h: section "Present But Cannot Be Compiled" +configure: WARNING: pi.h: proceeding with the compiler's result +configure: WARNING: ## -------------------------------------- ## +configure: WARNING: ## Report this to bug-example@@example.org ## +configure: WARNING: ## -------------------------------------- ## +checking for pi.h... yes +@end smallexample + +@noindent +The proper way the handle this case is using the fourth argument +(@pxref{Generic Headers}): + +@example +$ @kbd{cat configure.ac} +AC_INIT([Example], [1.0], [bug-example@@example.org]) +AC_CHECK_HEADERS([number.h pi.h], [], [], +[[#ifdef HAVE_NUMBER_H +# include <number.h> +#endif +]]) +$ @kbd{autoconf -Wall} +$ @kbd{./configure} +checking for gcc... gcc +checking for C compiler default output... a.out +checking whether the C compiler works... yes +checking whether we are cross compiling... no +checking for suffix of executables... +checking for suffix of object files... o +checking whether we are using the GNU C compiler... yes +checking whether gcc accepts -g... yes +checking for gcc option to accept ANSI C... none needed +checking for number.h... yes +checking for pi.h... yes +@end example + +See @ref{Particular Headers}, for a list of headers with their +prerequisites. + +@node Expanded Before Required +@section Expanded Before Required + +@cindex expanded before required +Older versions of Autoconf silently built files with incorrect ordering +between dependent macros if an outer macro first expanded, then later +indirectly required, an inner macro. Starting with Autoconf 2.64, this +situation no longer generates out-of-order code, but results in +duplicate output and a syntax warning: + +@example +$ @kbd{cat configure.ac} +@result{}AC_DEFUN([TESTA], [[echo in A +@result{}if test -n "$SEEN_A" ; then echo duplicate ; fi +@result{}SEEN_A=:]]) +@result{}AC_DEFUN([TESTB], [AC_REQUIRE([TESTA])[echo in B +@result{}if test -z "$SEEN_A" ; then echo bug ; fi]]) +@result{}AC_DEFUN([TESTC], [AC_REQUIRE([TESTB])[echo in C]]) +@result{}AC_DEFUN([OUTER], [[echo in OUTER] +@result{}TESTA +@result{}TESTC]) +@result{}AC_INIT +@result{}OUTER +@result{}AC_OUTPUT +$ @kbd{autoconf} +@result{}configure.ac:11: warning: AC_REQUIRE: +@result{} `TESTA' was expanded before it was required +@result{}configure.ac:4: TESTB is expanded from... +@result{}configure.ac:6: TESTC is expanded from... +@result{}configure.ac:7: OUTER is expanded from... +@result{}configure.ac:11: the top level +@end example + +@noindent +To avoid this warning, decide what purpose the macro in question serves. +If it only needs to be expanded once (for example, if it provides +initialization text used by later macros), then the simplest fix is to +change the macro to be declared with @code{AC_DEFUN_ONCE} +(@pxref{One-Shot Macros}), although this only works in Autoconf 2.64 and +newer. A more portable fix is to change all +instances of direct calls to instead go through @code{AC_REQUIRE} +(@pxref{Prerequisite Macros}). If, instead, the macro is parameterized +by arguments or by the current definition of other macros in the m4 +environment, then the macro should always be directly expanded instead +of required. + +For another case study, consider this example trimmed down from an +actual package. Originally, the package contained shell code and +multiple macro invocations at the top level of @file{configure.ac}: + +@example +AC_DEFUN([FOO], [AC_COMPILE_IFELSE([@dots{}])]) +foobar= +AC_PROG_CC +FOO +@end example + +@noindent +but that was getting complex, so the author wanted to offload some of +the text into a new macro in another file included via +@file{aclocal.m4}. The na@"ive approach merely wraps the text in a new +macro: + +@example +AC_DEFUN([FOO], [AC_COMPILE_IFELSE([@dots{}])]) +AC_DEFUN([BAR], [ +foobar= +AC_PROG_CC +FOO +]) +BAR +@end example + +@noindent +With older versions of Autoconf, the setting of @samp{foobar=} occurs +before the single compiler check, as the author intended. But with +Autoconf 2.64, this issues the ``expanded before it was required'' +warning for @code{AC_PROG_CC}, and outputs two copies of the compiler +check, one before @samp{foobar=}, and one after. To understand why this +is happening, remember that the use of @code{AC_COMPILE_IFELSE} includes +a call to @code{AC_REQUIRE([AC_PROG_CC])} under the hood. According to +the documented semantics of @code{AC_REQUIRE}, this means that +@code{AC_PROG_CC} @emph{must} occur before the body of the outermost +@code{AC_DEFUN}, which in this case is @code{BAR}, thus preceding the +use of @samp{foobar=}. The older versions of Autoconf were broken with +regards to the rules of @code{AC_REQUIRE}, which explains why the code +changed from one over to two copies of @code{AC_PROG_CC} when upgrading +autoconf. In other words, the author was unknowingly relying on a bug +exploit to get the desired results, and that exploit broke once the bug +was fixed. + +So, what recourse does the author have, to restore their intended +semantics of setting @samp{foobar=} prior to a single compiler check, +regardless of whether Autoconf 2.63 or 2.64 is used? One idea is to +remember that only @code{AC_DEFUN} is impacted by @code{AC_REQUIRE}; +there is always the possibility of using the lower-level +@code{m4_define}: + +@example +AC_DEFUN([FOO], [AC_COMPILE_IFELSE([@dots{}])]) +m4_define([BAR], [ +foobar= +AC_PROG_CC +FOO +]) +BAR +@end example + +@noindent +This works great if everything is in the same file. However, it does +not help in the case where the author wants to have @command{aclocal} +find the definition of @code{BAR} from its own file, since +@command{aclocal} requires the use of @code{AC_DEFUN}. In this case, a +better fix is to recognize that if @code{BAR} also uses +@code{AC_REQUIRE}, then there will no longer be direct expansion prior +to a subsequent require. Then, by creating yet another helper macro, +the author can once again guarantee a single invocation of +@code{AC_PROG_CC}, which will still occur after @code{foobar=}. The +author can also use @code{AC_BEFORE} to make sure no other macro +appearing before @code{BAR} has triggered an unwanted expansion of +@code{AC_PROG_CC}. + +@example +AC_DEFUN([FOO], [AC_COMPILE_IFELSE([@dots{}])]) +AC_DEFUN([BEFORE_CC], [ +foobar= +]) +AC_DEFUN([BAR], [ +AC_BEFORE([$0], [AC_PROG_CC])dnl +AC_REQUIRE([BEFORE_CC])dnl +AC_REQUIRE([AC_PROG_CC])dnl +FOO +]) +BAR +@end example + + +@node Debugging +@section Debugging @command{configure} scripts + +While in general, @command{configure} scripts generated by Autoconf +strive to be fairly portable to various systems, compilers, shells, and +other tools, it may still be necessary to debug a failing test, broken +script or makefile, or fix or override an incomplete, faulty, or erroneous +test, especially during macro development. Failures can occur at all levels, +in M4 syntax or semantics, shell script issues, or due to bugs in the +test or the tools invoked by @command{configure}. Together with the +rather arcane error message that @command{m4} and @command{make} may +produce when their input contains syntax errors, this can make debugging +rather painful. + +Nevertheless, here is a list of hints and strategies that may help: + +@itemize +@item +When @command{autoconf} fails, common causes for error include: + +@itemize +@item +mismatched or unbalanced parentheses or braces (@pxref{Balancing +Parentheses}), + +@item under- or overquoted macro arguments (@pxref{Autoconf +Language}, @pxref{Quoting and Parameters}, @pxref{Quotation and Nested +Macros}), + +@item spaces between macro name and opening parenthesis (@pxref{Autoconf +Language}). +@end itemize + +Typically, it helps to go back to the last working version of the input +and compare the differences for each of these errors. Another +possibility is to sprinkle pairs of @code{m4_traceon} and +@code{m4_traceoff} judiciously in the code, either without a parameter +or listing some macro names and watch @command{m4} expand its input +verbosely (@pxref{Debugging via autom4te}). + +@item +Sometimes @command{autoconf} succeeds but the generated +@command{configure} script has invalid shell syntax. You can detect this +case by running @samp{bash -n configure} or @samp{sh -n configure}. +If this command fails, the same tips apply, as if @command{autoconf} had +failed. + +@item +Debugging @command{configure} script execution may be done by sprinkling +pairs of @code{set -x} and @code{set +x} into the shell script before +and after the region that contains a bug. Running the whole script with +@samp{@var{shell} -vx ./configure 2>&1 | tee @var{log-file}} with a decent +@var{shell} may work, but produces lots of output. Here, it can help to +search for markers like @samp{checking for} a particular test in the +@var{log-file}. + +@item +Alternatively, you might use a shell with debugging capabilities like +@uref{http://bashdb.sourceforge.net/, bashdb}. + +@item +When @command{configure} tests produce invalid results for your system, +it may be necessary to override them: + +@itemize +@item +For programs, tools or libraries variables, preprocessor, compiler, or +linker flags, it is often sufficient to override them at @command{make} +run time with some care (@pxref{Macros and Submakes}). Since this +normally won't cause @command{configure} to be run again with these +changed settings, it may fail if the changed variable would have caused +different test results from @command{configure}, so this may work only +for simple differences. + +@item +Most tests which produce their result in a substituted variable allow to +override the test by setting the variable on the @command{configure} +command line (@pxref{Compilers and Options}, @pxref{Defining Variables}, +@pxref{Particular Systems}). + +@item +Many tests store their result in a cache variable (@pxref{Caching +Results}). This lets you override them either on the +@command{configure} command line as above, or through a primed cache or +site file (@pxref{Cache Files}, @pxref{Site Defaults}). The name of a +cache variable is documented with a test macro or may be inferred from +@ref{Cache Variable Names}; the precise semantics of undocumented +variables are often internal details, subject to change. +@end itemize + +@item +Alternatively, @command{configure} may produce invalid results because +of uncaught programming errors, in your package or in an upstream +library package. For example, when @code{AC_CHECK_LIB} fails to find a +library with a specified function, always check @file{config.log}. This +will reveal the exact error that produced the failing result: the +library linked by @code{AC_CHECK_LIB} probably has a fatal bug. +@end itemize + +Conversely, as macro author, you can make it easier for users of your +macro: + +@itemize +@item +by minimizing dependencies between tests and between test results as far +as possible, + +@item +by using @command{make} variables to factorize and allow +override of settings at @command{make} run time, + +@item +by honoring the GNU Coding Standards and not overriding flags +reserved for the user except temporarily during @command{configure} +tests, + +@item +by not requiring users of your macro to use the cache variables. +Instead, expose the result of the test via @var{run-if-true} and +@var{run-if-false} parameters. If the result is not a boolean, +then provide it through documented shell variables. +@end itemize + + +@c ===================================================== History of Autoconf. + +@node History +@chapter History of Autoconf +@cindex History of autoconf + +@emph{This chapter was written by the original author, David MacKenzie.} + +You may be wondering, Why was Autoconf originally written? How did it +get into its present form? (Why does it look like gorilla spit?) If +you're not wondering, then this chapter contains no information useful +to you, and you might as well skip it. If you @emph{are} wondering, +then let there be light@enddots{} + +@menu +* Genesis:: Prehistory and naming of @command{configure} +* Exodus:: The plagues of M4 and Perl +* Leviticus:: The priestly code of portability arrives +* Numbers:: Growth and contributors +* Deuteronomy:: Approaching the promises of easy configuration +@end menu + +@node Genesis +@section Genesis + +In June 1991 I was maintaining many of the GNU utilities for the +Free Software Foundation. As they were ported to more platforms and +more programs were added, the number of @option{-D} options that users +had to select in the makefile (around 20) became burdensome. +Especially for me---I had to test each new release on a bunch of +different systems. So I wrote a little shell script to guess some of +the correct settings for the fileutils package, and released it as part +of fileutils 2.0. That @command{configure} script worked well enough that +the next month I adapted it (by hand) to create similar @command{configure} +scripts for several other GNU utilities packages. Brian Berliner +also adapted one of my scripts for his CVS revision control system. + +Later that summer, I learned that Richard Stallman and Richard Pixley +were developing similar scripts to use in the GNU compiler tools; +so I adapted my @command{configure} scripts to support their evolving +interface: using the file name @file{Makefile.in} as the templates; +adding @samp{+srcdir}, the first option (of many); and creating +@file{config.status} files. + +@node Exodus +@section Exodus + +As I got feedback from users, I incorporated many improvements, using +Emacs to search and replace, cut and paste, similar changes in each of +the scripts. As I adapted more GNU utilities packages to use +@command{configure} scripts, updating them all by hand became impractical. +Rich Murphey, the maintainer of the GNU graphics utilities, sent me +mail saying that the @command{configure} scripts were great, and asking if +I had a tool for generating them that I could send him. No, I thought, +but I should! So I started to work out how to generate them. And the +journey from the slavery of hand-written @command{configure} scripts to the +abundance and ease of Autoconf began. + +Cygnus @command{configure}, which was being developed at around that time, +is table driven; it is meant to deal mainly with a discrete number of +system types with a small number of mainly unguessable features (such as +details of the object file format). The automatic configuration system +that Brian Fox had developed for Bash takes a similar approach. For +general use, it seems to me a hopeless cause to try to maintain an +up-to-date database of which features each variant of each operating +system has. It's easier and more reliable to check for most features on +the fly---especially on hybrid systems that people have hacked on +locally or that have patches from vendors installed. + +I considered using an architecture similar to that of Cygnus +@command{configure}, where there is a single @command{configure} script that +reads pieces of @file{configure.in} when run. But I didn't want to have +to distribute all of the feature tests with every package, so I settled +on having a different @command{configure} made from each +@file{configure.in} by a preprocessor. That approach also offered more +control and flexibility. + +I looked briefly into using the Metaconfig package, by Larry Wall, +Harlan Stenn, and Raphael Manfredi, but I decided not to for several +reasons. The @command{Configure} scripts it produces are interactive, +which I find quite inconvenient; I didn't like the ways it checked for +some features (such as library functions); I didn't know that it was +still being maintained, and the @command{Configure} scripts I had +seen didn't work on many modern systems (such as System V R4 and NeXT); +it wasn't flexible in what it could do in response to a feature's +presence or absence; I found it confusing to learn; and it was too big +and complex for my needs (I didn't realize then how much Autoconf would +eventually have to grow). + +I considered using Perl to generate my style of @command{configure} +scripts, but decided that M4 was better suited to the job of simple +textual substitutions: it gets in the way less, because output is +implicit. Plus, everyone already has it. (Initially I didn't rely on +the GNU extensions to M4.) Also, some of my friends at the +University of Maryland had recently been putting M4 front ends on +several programs, including @code{tvtwm}, and I was interested in trying +out a new language. + +@node Leviticus +@section Leviticus + +Since my @command{configure} scripts determine the system's capabilities +automatically, with no interactive user intervention, I decided to call +the program that generates them Autoconfig. But with a version number +tacked on, that name would be too long for old Unix file systems, +so I shortened it to Autoconf. + +In the fall of 1991 I called together a group of fellow questers after +the Holy Grail of portability (er, that is, alpha testers) to give me +feedback as I encapsulated pieces of my handwritten scripts in M4 macros +and continued to add features and improve the techniques used in the +checks. Prominent among the testers were Fran@,{c}ois Pinard, who came up +with the idea of making an Autoconf shell script to run M4 +and check for unresolved macro calls; Richard Pixley, who suggested +running the compiler instead of searching the file system to find +include files and symbols, for more accurate results; Karl Berry, who +got Autoconf to configure @TeX{} and added the macro index to the +documentation; and Ian Lance Taylor, who added support for creating a C +header file as an alternative to putting @option{-D} options in a +makefile, so he could use Autoconf for his UUCP package. +The alpha testers cheerfully adjusted their files again and again as the +names and calling conventions of the Autoconf macros changed from +release to release. They all contributed many specific checks, great +ideas, and bug fixes. + +@node Numbers +@section Numbers + +In July 1992, after months of alpha testing, I released Autoconf 1.0, +and converted many GNU packages to use it. I was surprised by how +positive the reaction to it was. More people started using it than I +could keep track of, including people working on software that wasn't +part of the GNU Project (such as TCL, FSP, and Kerberos V5). +Autoconf continued to improve rapidly, as many people using the +@command{configure} scripts reported problems they encountered. + +Autoconf turned out to be a good torture test for M4 implementations. +Unix M4 started to dump core because of the length of the +macros that Autoconf defined, and several bugs showed up in GNU +M4 as well. Eventually, we realized that we needed to use some +features that only GNU M4 has. 4.3BSD M4, in +particular, has an impoverished set of builtin macros; the System V +version is better, but still doesn't provide everything we need. + +More development occurred as people put Autoconf under more stresses +(and to uses I hadn't anticipated). Karl Berry added checks for X11. +david zuhn contributed C++ support. Fran@,{c}ois Pinard made it diagnose +invalid arguments. Jim Blandy bravely coerced it into configuring +GNU Emacs, laying the groundwork for several later improvements. +Roland McGrath got it to configure the GNU C Library, wrote the +@command{autoheader} script to automate the creation of C header file +templates, and added a @option{--verbose} option to @command{configure}. +Noah Friedman added the @option{--autoconf-dir} option and +@code{AC_MACRODIR} environment variable. (He also coined the term +@dfn{autoconfiscate} to mean ``adapt a software package to use +Autoconf''.) Roland and Noah improved the quoting protection in +@code{AC_DEFINE} and fixed many bugs, especially when I got sick of +dealing with portability problems from February through June, 1993. + +@node Deuteronomy +@section Deuteronomy + +A long wish list for major features had accumulated, and the effect of +several years of patching by various people had left some residual +cruft. In April 1994, while working for Cygnus Support, I began a major +revision of Autoconf. I added most of the features of the Cygnus +@command{configure} that Autoconf had lacked, largely by adapting the +relevant parts of Cygnus @command{configure} with the help of david zuhn +and Ken Raeburn. These features include support for using +@file{config.sub}, @file{config.guess}, @option{--host}, and +@option{--target}; making links to files; and running @command{configure} +scripts in subdirectories. Adding these features enabled Ken to convert +GNU @code{as}, and Rob Savoye to convert DejaGNU, to using +Autoconf. + +I added more features in response to other peoples' requests. Many +people had asked for @command{configure} scripts to share the results of +the checks between runs, because (particularly when configuring a large +source tree, like Cygnus does) they were frustratingly slow. Mike +Haertel suggested adding site-specific initialization scripts. People +distributing software that had to unpack on MS-DOS asked for a way to +override the @file{.in} extension on the file names, which produced file +names like @file{config.h.in} containing two dots. Jim Avera did an +extensive examination of the problems with quoting in @code{AC_DEFINE} +and @code{AC_SUBST}; his insights led to significant improvements. +Richard Stallman asked that compiler output be sent to @file{config.log} +instead of @file{/dev/null}, to help people debug the Emacs +@command{configure} script. + +I made some other changes because of my dissatisfaction with the quality +of the program. I made the messages showing results of the checks less +ambiguous, always printing a result. I regularized the names of the +macros and cleaned up coding style inconsistencies. I added some +auxiliary utilities that I had developed to help convert source code +packages to use Autoconf. With the help of Fran@,{c}ois Pinard, I made +the macros not interrupt each others' messages. (That feature revealed +some performance bottlenecks in GNU M4, which he hastily +corrected!) I reorganized the documentation around problems people want +to solve. And I began a test suite, because experience had shown that +Autoconf has a pronounced tendency to regress when we change it. + +Again, several alpha testers gave invaluable feedback, especially +Fran@,{c}ois Pinard, Jim Meyering, Karl Berry, Rob Savoye, Ken Raeburn, +and Mark Eichin. + +Finally, version 2.0 was ready. And there was much rejoicing. (And I +have free time again. I think. Yeah, right.) + + +@c ========================================================== Appendices + + +@node GNU Free Documentation License +@appendix GNU Free Documentation License + +@include fdl.texi + +@node Indices +@appendix Indices + +@menu +* Environment Variable Index:: Index of environment variables used +* Output Variable Index:: Index of variables set in output files +* Preprocessor Symbol Index:: Index of C preprocessor symbols defined +* Cache Variable Index:: Index of documented cache variables +* Autoconf Macro Index:: Index of Autoconf macros +* M4 Macro Index:: Index of M4, M4sugar, and M4sh macros +* Autotest Macro Index:: Index of Autotest macros +* Program & Function Index:: Index of those with portability problems +* Concept Index:: General index +@end menu + +@node Environment Variable Index +@appendixsec Environment Variable Index + +This is an alphabetical list of the environment variables that might +influence Autoconf checks. + +@printindex ev + +@node Output Variable Index +@appendixsec Output Variable Index + +This is an alphabetical list of the variables that Autoconf can +substitute into files that it creates, typically one or more +makefiles. @xref{Setting Output Variables}, for more information +on how this is done. + +@printindex ov + +@node Preprocessor Symbol Index +@appendixsec Preprocessor Symbol Index + +This is an alphabetical list of the C preprocessor symbols that the +Autoconf macros define. To work with Autoconf, C source code needs to +use these names in @code{#if} or @code{#ifdef} directives. + +@printindex cv + +@node Cache Variable Index +@appendixsec Cache Variable Index + +This is an alphabetical list of documented cache variables used +by macros defined in Autoconf. Autoconf macros may use additional cache +variables internally. +@ifset shortindexflag +To make the list easier to use, the variables are listed without their +preceding @samp{ac_cv_}. +@end ifset + +@printindex CA + +@node Autoconf Macro Index +@appendixsec Autoconf Macro Index + +This is an alphabetical list of the Autoconf macros. +@ifset shortindexflag +To make the list easier to use, the macros are listed without their +preceding @samp{AC_}. +@end ifset + +@printindex AC + +@node M4 Macro Index +@appendixsec M4 Macro Index + +This is an alphabetical list of the M4, M4sugar, and M4sh macros. +@ifset shortindexflag +To make the list easier to use, the macros are listed without their +preceding @samp{m4_} or @samp{AS_}. The prefix is @samp{m4_} for +all-lowercase macro names and @samp{AS_} for all-uppercase macro +names. +@end ifset + +@printindex MS + +@node Autotest Macro Index +@appendixsec Autotest Macro Index + +This is an alphabetical list of the Autotest macros. +@ifset shortindexflag +To make the list easier to use, the macros are listed without their +preceding @samp{AT_}. +@end ifset + +@printindex AT + +@node Program & Function Index +@appendixsec Program and Function Index + +This is an alphabetical list of the programs and functions whose +portability is discussed in this document. + +@printindex pr + +@node Concept Index +@appendixsec Concept Index + +This is an alphabetical list of the files, tools, and concepts +introduced in this document. + +@printindex cp + +@bye + +@c LocalWords: texinfo setfilename autoconf texi settitle setchapternewpage +@c LocalWords: setcontentsaftertitlepage finalout ARG ovar varname dvar acx +@c LocalWords: makeinfo dvi defcodeindex ev ov CPP cv Autotest mv 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