path: root/doc/autoconf.texi

\input texinfo @c -*-texinfo-*-
@c %**start of header
@setfilename autoconf.info
@settitle Autoconf

@finalout
@setchapternewpage odd
@setcontentsaftertitlepage

@include version.texi

@c A simple macro for optional variables.
@macro ovar{varname}
@r{[}@var{\varname\}@r{]}
@end macro

@c I don't like the way URL are displayed in TeX with @uref.
@ifhtml
@macro href{url, title}
@uref{\url\, \title\}
@end macro
@end ifhtml
@ifnothtml
@macro href{url, title}
\title\@footnote{\title\, @url{\url\}.}
@end macro
@end ifnothtml


@dircategory GNU admin
@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 the conditionals in source code
* autoconf: (autoconf)autoconf Invocation.
                                How to create configuration scripts
* autoreconf: (autoconf)autoreconf Invocation.
                                Remaking multiple @code{configure} scripts
* configure: (autoconf)configure Invocation.
                                Configuring a package
* config.status: (autoconf)config.status Invocation.
                                Recreating a configuration
@end direntry

@ifinfo
Autoconf: Creating Automatic Configuration Scripts, by David MacKenzie.

This file documents the GNU Autoconf package for creating scripts to
configure source code packages using templates and an @code{m4} macro
package.

Copyright 1992, 1993, 1994, 1995, 1996, 1998, 1999, 2000, 2001 Free
Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

@ignore
Permission is granted to process this file through TeX and print the
results, provided the printed document carries copying permission notice
identical to this one except for the removal of this paragraph (this
paragraph not being relevant to the printed manual).

@end ignore
Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions,
except that this permission notice may be stated in a translation
approved by the Foundation.
@end ifinfo

@titlepage
@title Autoconf
@subtitle Creating Automatic Configuration Scripts
@subtitle Edition @value{EDITION}, for Autoconf version @value{VERSION}
@subtitle @value{UPDATED}
@author David MacKenzie and Ben Elliston
@c I think I've rewritten all of Noah and Roland's contributions by now.

@page
@vskip 0pt plus 1filll
Copyright @copyright{} 1992, 1993, 1994, 1995, 1996, 1998, 1999, 2000,
2001 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions,
except that this permission notice may be stated in a translation
approved by the Foundation.
@end titlepage

@c Define an environment variable index.
@defcodeindex ev
@c Define an output variable index.
@defcodeindex ov
@c Define a CPP variable index.
@defcodeindex cv
@c Define an Autoconf macro index that @defmac doesn't write to.
@defcodeindex ma
@c Define an M4sugar macro index that @defmac doesn't write to.
@defcodeindex ms

@node Top, Introduction, (dir), (dir)
@comment  node-name,  next,  previous,  up

@ifinfo
This file documents the GNU Autoconf package for creating scripts to
configure source code packages using templates and the GNU M4 macro
package.  This is edition @value{EDITION}, for Autoconf version
@value{VERSION}.

@end ifinfo

@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
* Writing Autoconf Macros::     Adding new macros to Autoconf
* Portable Shell::              Shell script portability pitfalls
* Manual Configuration::        Selecting features that can't be guessed
* Site Configuration::          Local defaults for @code{configure}
* Running configure scripts::   How to use the Autoconf output
* config.status Invocation::    Recreating a configuration
* Obsolete Constructs::         Kept for backward compatibility
* Questions::                   Questions about Autoconf, with answers
* History::                     History of Autoconf
* 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
* Autoconf Macro Index::        Index of Autoconf macros
* M4 Macro Index::              Index of M4, M4sugar, and M4sh macros
* Concept Index::               General index

@detailmenu --- The Detailed Node Listing ---

The GNU build system

* Automake::                    Escaping Makefile hell
* Libtool::                     Building libraries portably
* Pointers::                    More info on the GNU build system

Making @code{configure} Scripts

* Writing configure.ac::        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 @code{configure} scripts

Writing @file{configure.ac}

* Shell Script Compiler::       Autoconf as solution of a problem
* Autoconf Language::           Programming in Autoconf
* configure.ac Layout::         Standard organization of configure.ac

Initialization and Output Files

* Notices::                     Copyright, version numbers in @code{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 @file{Makefile}s
* Configuration Headers::       Creating a configuration header file
* Configuration Commands::      Running arbitrary instantiation commands
* Configuration Links::         Links depending from 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
* 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
* 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
* C Compiler::                  Checking its characteristics
* Fortran 77 Compiler::         Checking its characteristics
* System Services::             Operating system services
* UNIX Variants::               Special kludges for specific UNIX variants

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

* 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

Writing Tests

* Examining Declarations::      Detecting header files and declarations
* Examining Syntax::            Detecting language syntax features
* Examining Libraries::         Detecting functions and global variables
* Run Time::                    Testing for run-time features
* Systemology::                 A zoology of operating systems
* Multiple Cases::              Tests for several possible values
* Language Choice::             Selecting which language to use for testing

Checking Run Time Behavior

* Test Programs::               Running test programs
* Guidelines::                  General rules for writing test programs
* Test Functions::              Avoiding pitfalls in test programs

Results of Tests

* Defining Symbols::            Defining C preprocessor symbols
* Setting Output Variables::    Replacing variables in output files
* Caching Results::             Speeding up subsequent @code{configure} runs
* Printing Messages::           Notifying @code{configure} users

Caching Results

* Cache Variable Names::        Shell variables used in caches
* Cache Files::                 Files @code{configure} uses for caching
* Cache Checkpointing::         Loading and saving the cache file

Programming in M4

* M4 Quotation::                Protecting macros from unwanted expansion
* Programming in M4sugar::      Convenient pure M4 macros

M4 Quotation

* Active Characters::           Characters that change the behavior of m4
* One Macro Call::              Quotation and one macro call
* Quotation and Nested Macros::  Macros calling macros
* Quadrigraphs::                Another way to escape special characters
* Quotation Rule Of Thumb::     One parenthesis, one quote

Programming in M4sugar

* Redefined M4 Macros::         M4 builtins changed in M4sugar
* Forbidden Patterns::          Catching unexpanded macros

Writing Autoconf Macros

* Macro Definitions::           Basic format of an Autoconf macro
* Macro Names::                 What to call your new macros
* Reporting Messages::          Notifying @code{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

Portable Shell Programming

* Shellology::                  A zoology of shells
* Here-Documents::              Quirks and tricks
* File Descriptors::            FDs and redirections
* File System Conventions::     File- and pathnames
* Shell Substitutions::         Variable and command expansions
* Assignments::                 Varying side effects of assignments
* Special Shell Variables::     Variables you should not change
* Limitations of Builtins::     Portable use of not so portable /bin/sh
* Limitations of Usual Tools::  Portable use of portable tools
* Limitations of Make::         Portable Makefiles

Manual Configuration

* Specifying Names::            Specifying the system type
* Canonicalizing::              Getting the canonical system type
* Using System Type::           What to do with the system type

Site Configuration

* External Software::           Working with other optional software
* Package Options::             Selecting optional features
* Pretty Help Strings::         Formating help string
* Site Details::                Configuring site details
* Transforming Names::          Changing program names when installing
* Site Defaults::               Giving @code{configure} local defaults

Transforming Program Names When Installing

* Transformation Options::      @code{configure} options to transform names
* Transformation Examples::     Sample uses of transforming names
* Transformation Rules::        @file{Makefile} uses of transforming names

Running @code{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
* System Type::                 Specifying the system type
* Sharing Defaults::            Setting site-wide defaults for @code{configure}
* Environment Variables::       Defining environment variables.
* configure Invocation::        Changing how @code{configure} runs

Obsolete Constructs

* Obsolete config.status Use::  Different calling convention
* acconfig.h::                  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

Questions About Autoconf

* Distributing::                Distributing @code{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 @code{configure} instead of Imake

History of Autoconf

* Genesis::                     Prehistory and naming of @code{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 detailmenu
@end menu

@c ============================================================= Introduction.

@node Introduction, The GNU build system, Top, Top
@chapter 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@dots{}'' ``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@dots{}'' 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
@sc{unix}-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 @sc{unix} variants.  There is
no need to maintain files that list the features supported by each
release of each variant of @sc{unix}.

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.

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 @file{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 @sc{gnu} M4 in order to generate the scripts.  It uses
features that some @sc{unix} versions of M4, including @sc{gnu} M4 1.3,
do not have.  You must use version 1.4 or later of @sc{gnu} M4.

@xref{Autoconf 1}, for information about upgrading from version 1.
@xref{History}, for the story of Autoconf's development.
@xref{Questions}, for answers to some common questions about Autoconf.


See the @href{http://www.gnu.org/software/autoconf/autoconf.html,
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}.

Bug reports should be preferably submitted to the
@href{http://sources.redhat.com/cgi-bin/gnatsweb.pl?database=autoconf,
Autoconf Gnats database}, or sent to @email{bug-autoconf@@gnu.org, the
Autoconf Bugs mailing list}.  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 @sc{cvs}; see the Autoconf
web page for details.  There is also a
@href{http://subversions.gnu.org/cgi-bin/cvsweb/autoconf/, @sc{cvs}web
interface to the Autoconf development tree}.  Patches relative to the
current @sc{cvs} version can be sent for review to the
@email{autoconf-patches@@gnu.org, Autoconf Patches mailing list}.

Because of its mission, Autoconf 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
@href{http://www.gnu.org/software/ac-archive/, Autoconf Macro
Archive}, which is kindly run by @email{simons@@computer.org,
Peter Simons}.


@c ================================================= The GNU build system

@node The GNU build system, Making configure Scripts, Introduction, Top
@chapter The 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
* Libtool::                     Building libraries portably
* Pointers::                    More info on the GNU build system
@end menu

@node Automake, Libtool, The GNU build system, The GNU build system
@section Automake

The ubiquity of @code{make} means that a @code{Makefile} is almost the
only viable way to distribute automatic build rules for software, but
one quickly runs into @code{make}'s numerous limitations.  Its lack of
support for automatic dependency tracking, recursive builds in
subdirectories, reliable timestamps (e.g. for network filesystems), 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 @code{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 @code{Makefile.in} to
recognize @code{@@CC@@}, @code{@@CFLAGS@@}, and other substitutions
provided by @code{configure}.  Into this mess steps @dfn{Automake}.
@cindex Automake

Automake allows you to specify your build needs in a @code{Makefile.am}
file with a vastly simpler and more powerful syntax than that of a plain
@code{Makefile}, and then generates a portable @code{Makefile.in} for
use with Autoconf.  For example, the @code{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 @code{Makefile.in} (~400 lines) automatically supports all
the standard targets, the substitutions provided by Autoconf, automatic
dependency tracking, @code{VPATH} building, and so on.  @code{make} will
build the @code{hello} program, and @code{make install} will install it
in @file{/usr/local/bin} (or whatever prefix was given to
@code{configure}, if not @file{/usr/local}).

Automake may require that additional tools be present on the
@emph{developer's} machine.  For example, the @code{Makefile.in} that
the developer works with may not be portable (e.g. it might use special
features of your compiler to automatically generate dependency
information).  Running @code{make dist}, however, produces a
@file{hello-1.0.tar.gz} package (or whatever the program/version is)
with a @code{Makefile.in} that will work on any system.

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@dots{}

@node Libtool, Pointers, Automake, The GNU build system
@section Libtool

Very 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 @code{Makefile} rules with the variable suffixes of
shared libraries, linking reliably to 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 on its own, 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,  , Libtool, The GNU build system
@section Pointers

Developers who are used to the simplicity of @code{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 will 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
will be 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 home pages for
@href{http://www.gnu.org/software/autoconf/,Autoconf},
@href{http://www.gnu.org/software/automake/,Automake}, and
@href{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. New Riders, 2000, ISBN 1578701902.}
describes the complete GNU build environment.  You can also find the
entire book on-line at @href{http://sources.redhat.com/autobook/,``The
Goat Book'' home page}.

@item Tutorials and Examples

The @href{http://sources.redhat.com/autoconf/,Autoconf Developer Page}
maintains links to a number of Autoconf/Automake tutorials online, and
also links to the @href{http://www.gnu.org/software/ac-archive/,
Autoconf Macro Archive}.

@end itemize

@c ================================================= Making configure Scripts.

@node Making configure Scripts, Setup, The GNU build system, Top
@chapter Making @code{configure} Scripts
@cindex @file{aclocal.m4}
@cindex @code{configure}

The configuration scripts that Autoconf produces are by convention
called @code{configure}.  When run, @code{configure} creates several
files, replacing configuration parameters in them with appropriate
values.  The files that @code{configure} creates are:

@itemize @minus
@item
one or more @file{Makefile} files, 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, will recreate
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 @code{configure} makes a mistake.
@end itemize

@cindex @file{configure.in}
@cindex @file{configure.ac}
To create a @code{configure} script with Autoconf, you need to write an
Autoconf input file @file{configure.ac} (or @file{configure.in}) and run
@code{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
@code{autoheader}, and you will 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{[]}).
@code{autoconf} and @code{autoheader} also read the installed Autoconf
macro files (by reading @file{autoconf.m4}).

@noindent
Files used in preparing a software package for distribution:
@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 -------------------------------> Makefile.in
@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 configure.ac::        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 @code{configure} scripts
@end menu

@node Writing configure.ac, autoscan Invocation, Making configure Scripts, Making configure Scripts
@section Writing @file{configure.ac}

To produce a @code{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}.  The
@code{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 produce 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 @code{configure}'').  Using @file{configure.ac} is now
preferred.

@menu
* Shell Script Compiler::       Autoconf as solution of a problem
* Autoconf Language::           Programming in Autoconf
* configure.ac Layout::         Standard organization of configure.ac
@end menu

@node Shell Script Compiler, Autoconf Language, Writing configure.ac, Writing configure.ac
@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: @code{configure}
must run on all those systems, and thus @code{configure} must limit itself
to their lowest common denominator of features.

Naturally, you might then think of shell scripts; who needs
@code{autoconf}?  A set of properly written shell functions is enough to
make it easy to write @code{configure} scripts by hand.  Sigh!
Unfortunately, shell functions do not belong to the least common
denominator; therefore, where you would like to define a function and
use it ten times, you would instead need to copy its body ten times.

So, what is really needed is some kind of compiler, @code{autoconf},
that takes an Autoconf program, @file{configure.ac}, and transforms it
into a portable shell script, @code{configure}.

How does @code{autoconf} perform this task?

There are two obvious possibilities: creating a brand new language or
extending an existing one.  The former option is very 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 @code{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, configure.ac Layout, Shell Script Compiler, Writing configure.ac
@subsection The Autoconf Language
@cindex quotation

The Autoconf language is very different from many other computer
languages because it treats actual code the same as plain text.  Whereas
in C, for instance, data and instructions have very 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 blank
space between the macro name and the open parenthesis.  Arguments should
be enclosed within the M4 quote characters @samp{[} and @samp{]}, and be
separated by commas.  Any leading spaces in arguments are ignored,
unless they are quoted.  You may safely leave out the quotes when the
argument is simple text, but @emph{always} quote complex arguments such
as other macro calls.  This rule applies recursively for every macro
call, including macros called from other macros.

For instance:

@example
AC_CHECK_HEADER([stdio.h],
                [AC_DEFINE([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)],
                [AC_MSG_ERROR([Sorry, can't do anything for you])])
@end example

@noindent
Notice that the argument of @code{AC_MSG_ERROR} is still quoted;
otherwise, its comma would have been interpreted as an argument separator.

The following example is wrong and dangerous, as it is underquoted:

@example
AC_CHECK_HEADER(stdio.h,
                AC_DEFINE(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:

@example
echo "Hard rock was here!  --[AC_DC]"
@end example

@noindent
which will result in

@example
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}:

@example
AC_MSG_WARN([[AC_DC stinks  --Iron Maiden]])
@end example

You are now able to understand one of the constructs of Autoconf that
has been continually misunderstood@dots{}  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([char b[10];],, [AC_MSG_ERROR([you lose])])
@end example

@noindent
is incorrect: here, the first argument of @code{AC_COMPILE_IFELSE} is
@samp{char b[10];} and will be expanded once, which results in
@samp{char b10;}.  (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:

@example
AC_COMPILE_IFELSE([[char b[10];]],, [AC_MSG_ERROR([you lose])])
@end example

@noindent
Voil@`a, you actually produce @samp{char b[10];} this time!

The careful reader will notice that, according to these guidelines, the
``properly'' quoted @code{AC_CHECK_HEADER} example above is actually
lacking three pairs of quotes!  Nevertheless, for the sake of readability,
double quotation of literals is used only where needed in this manual.

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 @code{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 configure.ac Layout,  , Autoconf Language, Writing configure.ac
@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 @code{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, ifnames Invocation, Writing configure.ac, Making configure Scripts
@section Using @code{autoscan} to Create @file{configure.ac}
@cindex @code{autoscan}

The @code{autoscan} program can help you create a @file{configure.ac}
file for a software package.  @code{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.

You should manually examine @file{configure.scan} before renaming it to
@file{configure.ac}; it will probably need some adjustments.
Occasionally, @code{autoscan} outputs a macro in the wrong order relative
to another macro, so that @code{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).

@code{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, whitespace, and the Autoconf macro to
output if that symbol is encountered.  Lines starting with @samp{#} are
comments.

@code{autoscan} is only installed if you already have Perl installed.
@code{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 --autoconf-dir=@var{dir}
@itemx -A @var{dir}
@evindex AC_MACRODIR
Override the location where the installed Autoconf data files are looked
for.  You can also set the @code{AC_MACRODIR} environment variable to a
directory; this option overrides the environment variable.

This option is rarely needed and dangerous; it is only used when one
plays with different versions of Autoconf simultaneously.
@end table

@node ifnames Invocation, autoconf Invocation, autoscan Invocation, Making configure Scripts
@section Using @code{ifnames} to List Conditionals
@cindex @code{ifnames}

@code{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, @code{ifnames} can thus help you figure out what its
@code{configure} needs to check for.  It may help fill in some gaps in a
@file{configure.ac} generated by @code{autoscan} (@pxref{autoscan
Invocation}).

@code{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
@code{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, autoreconf Invocation, ifnames Invocation, Making configure Scripts
@section Using @code{autoconf} to Create @code{configure}
@cindex @code{autoconf}

To create @code{configure} from @file{configure.ac}, run the
@code{autoconf} program with no arguments.  @code{autoconf} processes
@file{configure.ac} with the @code{m4} macro processor, using the
Autoconf macros.  If you give @code{autoconf} an argument, it reads that
file instead of @file{configure.ac} and writes the configuration script
to the standard output instead of to @code{configure}.  If you give
@code{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; @code{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 @code{autoconf} reads, the last
definition it reads overrides the earlier ones.

@code{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 --autoconf-dir=@var{dir}
@itemx -A @var{dir}
@evindex AC_MACRODIR
Override the location where the installed Autoconf data files are looked
for.  You can also set the @code{AC_MACRODIR} environment variable to a
directory; this option overrides the environment variable.

This option is rarely needed and dangerous; it is only used when one
plays with different versions of Autoconf simultaneously.

@item --localdir=@var{dir}
@itemx -l @var{dir}
Look for the package file @file{aclocal.m4} in directory @var{dir}
instead of in the current directory.

@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 @code{WARNINGS}, a comma separated list of categories, is
honored. @command{autoconf -W @var{category}} will actually
behave as if you had run:

@example
autoconf --warnings=syntax,$WARNINGS,@var{category}
@end example

@noindent
If you want to disable @command{autoconf}'s defaults and @code{WARNINGS},
but (for example) enable the warnings about obsolete constructs, you
would use @option{-W none,obsolete}.

@cindex Back trace
@cindex Macro invocation stack
@command{autoconf} displays a back trace for errors, but not for
warnings; if you want them, just pass @option{-W error}.  For instance,
on this @file{configure.ac}:

@example
AC_DEFUN([INNER],
[AC_TRY_RUN([true])])

AC_DEFUN([OUTER],
[INNER])

AC_INIT
OUTER
@end example

@noindent
you get:

@example
$ autoconf -Wcross
configure.ac:8: warning: AC_TRY_RUN called without default \
to allow cross compiling
$ autoconf -Wcross,error
configure.ac:8: error: AC_TRY_RUN called without default \
to allow cross compiling
acgeneral.m4:3044: AC_TRY_RUN is expanded from...
configure.ac:2: INNER is expanded from...
configure.ac:5: OUTER is expanded from...
configure.ac:8: the top level
@end example

@item --trace=@var{macro}[:@var{format}]
@itemx -t @var{macro}[:@var{format}]
Do not create the @code{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
below 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}.

The @var{format} of @option{--trace} can use the following special
escapes:

@table @samp
@item $$
The character @samp{$}.

@item $f
The filename 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

For instance, to find the list of variables that are substituted, use:

@example
@group
$ 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 @strong{$%}.

@example
@group
$ cat configure.ac
AC_DEFINE(This, is, [an
[example]])
$ 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
$ 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 its parsing (for instance when no single
character is suitable as a separator)):

@example
@group
$ 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,  , autoconf Invocation, Making configure Scripts
@section Using @code{autoreconf} to Update @code{configure} Scripts
@cindex @code{autoreconf}

If you have a lot of Autoconf-generated @code{configure} scripts, the
@code{autoreconf} program can save you some work.  It runs
@code{autoconf} (and @code{autoheader}, where appropriate) repeatedly to
remake the Autoconf @code{configure} scripts and configuration header
templates in the directory tree rooted at the current directory.  By
default, it only remakes those files that are older than their
@file{configure.ac} or (if present) @file{aclocal.m4}.  Since
@code{autoheader} does not change the timestamp of its output file if
the file wouldn't be changing, this is not necessarily the minimum
amount of work.  If you install a new version of Autoconf, you can make
@code{autoreconf} remake @emph{all} of the files by giving it the
@option{--force} option.

If you give @code{autoreconf} the @option{--autoconf-dir=@var{dir}} or
@option{--localdir=@var{dir}} options, it passes them down to
@code{autoconf} and @code{autoheader} (with relative paths adjusted
properly).

@code{autoreconf} does not support having, in the same directory tree,
both directories that are parts of a larger package (sharing
@file{aclocal.m4} and @file{acconfig.h}) and directories that are
independent packages (each with their own @file{aclocal.m4} and
@file{acconfig.h}).  It assumes that they are all part of the same
package if you use @option{--localdir}, or that each directory is a
separate package if you don't use it.  This restriction may be removed
in the future.

@xref{Automatic Remaking}, for @file{Makefile} rules to automatically
remake @code{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}}.

@noindent
@code{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
Print the name of each directory where @code{autoreconf} runs
@code{autoconf} (and @code{autoheader}, if appropriate).

@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
Copy missing auxiliary files.  This option is similar to the option
@code{--add-missing} in @code{automake}.

@item --symlink
@itemx -s
Instead of copying missing auxiliary files, install symbolic links.

@item --localdir=@var{dir}
@itemx -l @var{dir}
Have @code{autoconf} and @code{autoheader} look for the package files
@file{aclocal.m4} and (@code{autoheader} only) @file{acconfig.h} (but
not @file{@var{file}.top} and @file{@var{file}.bot}) in directory
@var{dir} instead of in the directory containing each @file{configure.ac}.

@item --autoconf-dir=@var{dir}
@itemx -A @var{dir}
@evindex AC_MACRODIR
Override the location where the installed Autoconf data files are looked
for.  You can also set the @code{AC_MACRODIR} environment variable to a
directory; this option overrides the environment variable.

This option is rarely needed and dangerous; it is only used when one
plays with different versions of Autoconf simultaneously.

@item --m4dir=@var{dir}
@itemx -M @var{dir}
Specify location of additional macro files (@file{m4} by default).
@end table

Additionally, the following options are recognized and passed to
@code{automake}:

@table @option
@item --cygnus
Assume program is part of Cygnus-style tree.

@item --foreign
Set strictness to foreign.

@item --gnits
Set strictness to gnits.

@item --gnu
Set strictness to gnu.

@item --include-deps
Include generated dependencies in @file{Makefile.in}.
@end table


@c ========================================= Initialization and Output Files.

@node Setup, Existing Tests, Making configure Scripts, Top
@chapter Initialization and Output Files

Autoconf-generated @code{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
initialization and the creation of output files.

@menu
* Notices::                     Copyright, version numbers in @code{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 @file{Makefile}s
* Configuration Headers::       Creating a configuration header file
* Configuration Commands::      Running arbitrary instantiation commands
* Configuration Links::         Links depending from the configuration
* Subdirectories::              Configuring independent packages together
* Default Prefix::              Changing the default installation prefix
@end menu

@node Notices, Input, Setup, Setup
@section Notices in @code{configure}

The following macros manage version numbers for @code{configure}
scripts.  Using them is optional.

@c FIXME: AC_PREREQ should not be here, but where should it go?
@defmac AC_PREREQ (@var{version})
@maindex PREREQ
@cindex Version
Ensure that a recent enough version of Autoconf is being used.  If the
version of Autoconf being used to create @code{configure} is earlier
than @var{version}, print an error message to the standard error output
and do not create @code{configure}.  For example:

@example
AC_PREREQ(@value{VERSION})
@end example

This macro is the only macro that may be used before @code{AC_INIT}, but
for consistency, you are invited not to do so.
@end defmac

@defmac AC_COPYRIGHT (@var{copyright-notice})
@maindex COPYRIGHT
@cindex Copyright Notice
State that, in addition to the Free Software Foundation's copyright on
the Autoconf macros, parts of your @code{configure} are covered by the
@var{copyright-notice}.

The @var{copyright-notice} will show up in both the head of
@code{configure} and in @samp{configure --version}.
@end defmac


@defmac AC_REVISION (@var{revision-info})
@maindex REVISION
@cindex Revision
Copy revision stamp @var{revision-info} into the @code{configure}
script, with any dollar signs or double-quotes removed.  This macro lets
you put a revision stamp from @file{configure.ac} into @code{configure}
without @sc{rcs} or @code{cvs} changing it when you check in
@code{configure}.  That way, you can determine easily which revision of
@file{configure.ac} a particular @code{configure} corresponds to.

For example, this line in @file{configure.ac}:

@c The asis prevents RCS from changing the example in the manual.
@example
AC_REVISION($@asis{Revision: 1.30 }$)
@end example

@noindent
produces this in @code{configure}:

@example
#! /bin/sh
# From configure.ac Revision: 1.30
@end example
@end defmac


@node Input, Output, Notices, Setup
@section Finding @code{configure} Input

Every @code{configure} script must call @code{AC_INIT} before doing
anything else.  The only other required macro is @code{AC_OUTPUT}
(@pxref{Output}).

@defmac AC_INIT (@var{package}, @var{version}, @ovar{bug-report-address})
@maindex INIT
Process any command-line arguments and perform various initializations
and verifications.  Set the name of the @var{package} and its
@var{version}.  The optional argument @var{bug-report-address} should be
the email to which users should send bug reports.
@end defmac

@defmac AC_CONFIG_SRCDIR (@var{unique-file-in-source-dir})
@maindex CONFIG_SRCDIR
@var{unique-file-in-source-dir} is some file that is in the package's
source directory; @code{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 @maindex 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 filenames or shell globbing patterns is expected.  The
@c files will be 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 @code{install} program
might need to tell @code{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})
@maindex CONFIG_AUX_DIR
Use the auxiliary build tools (e.g., @file{install-sh},
@file{config.sub}, @file{config.guess}, Cygnus @code{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 @file{Makefile}.
@end defmac


@node Output, Configuration Actions, Input, Setup
@section Outputting Files

Every Autoconf-generated @code{configure} script must finish by calling
@code{AC_OUTPUT}.  It is the macro that generates @file{config.status},
which will create the @file{Makefile}s and any other files resulting
from configuration.  The only other required macro is @code{AC_INIT}
(@pxref{Input}).

@defmac AC_OUTPUT
@maindex OUTPUT
@cindex Instantiation
Generate @file{config.status} and launch it.  Call this macro once, at
the end of @file{configure.ac}.

@file{config.status} will take 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.
@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 @code{make} on subdirectories, you should run it using the
@code{make} variable @code{MAKE}.  Most versions of @code{make} set
@code{MAKE} to the name of the @code{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 @code{make} do not set this variable.  The
following macro allows you to use it even with those versions.

@defmac AC_PROG_MAKE_SET
@maindex PROG_MAKE_SET
@ovindex SET_MAKE
If @code{make} predefines the variable @code{MAKE}, define output
variable @code{SET_MAKE} to be empty.  Otherwise, define @code{SET_MAKE}
to contain @samp{MAKE=make}.  Calls @code{AC_SUBST} for @code{SET_MAKE}.
@end defmac

To use this macro, place a line like this in each @file{Makefile.in}
that runs @code{MAKE} on other directories:

@example
@@SET_MAKE@@
@end example



@node Configuration Actions, Configuration Files, Output, Setup
@section Taking 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.

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 Can't use @ovar here, Texinfo 4.0 goes lunatic and emits something
@c awful.
@example
AC_CONFIG_FOOS(@var{tag}@dots{}, [@var{commands}], [@var{init-cmds}])
@end example

@noindent
where the arguments are:

@table @var
@item @var{tag}@dots{}
A whitespace-separated list of tags, which are typically the names of
the files to instantiate.

@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 the 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.

@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} will be 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}.
@end table

All these macros can be called multiple times, with different
@var{tag}s, of course!

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_FOOS($my_foos)
@end example

@noindent
and use this instead:

@example
@dots{} && AC_CONFIG_FOOS(fooo)
@dots{} && AC_CONFIG_FOOS(foooo)
@end example

The macro @code{AC_CONFIG_FILES} and @code{AC_CONFIG_HEADERS} use
specials @var{tag}s: 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} if specified, defaulting to
@samp{@var{output}.in}.

For instance
@samp{AC_CONFIG_FILES(Makefile:boiler/top.mk:boiler/bot.mk)} asks for
the creation of @file{Makefile} that will be 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 filenames.  In the latter
case they are first looked for in the build tree, and then in the source
tree.


@node Configuration Files, Makefile Substitutions, Configuration Actions, Setup
@section Creating Configuration Files

Be sure to read the previous section, @ref{Configuration Actions}.

@defmac AC_CONFIG_FILES (@var{file}@dots{}, @ovar{cmds}, @ovar{init-cmds})
@maindex 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 FIXME: Before we used to have this feature, which was later rejected
@c because it complicates the write 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, @file{Makefile}s 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 MS-DOS, or
to prepend and/or append boilerplate to the file.
@end defmac



@node Makefile Substitutions, Configuration Headers, Configuration Files, Setup
@section Substitutions in Makefiles

Each subdirectory in a distribution that contains something to be
compiled or installed should come with a file @file{Makefile.in}, from
which @code{configure} will create a @file{Makefile} in that directory.
To create a @file{Makefile}, @code{configure} performs a simple variable
substitution, replacing occurrences of @samp{@@@var{variable}@@} in
@file{Makefile.in} with the value that @code{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 @code{configure}.  To make @code{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 @code{configure} script should be
distributed with a file @file{Makefile.in}, but no @file{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
@file{Makefile}s.

@menu
* Preset Output Variables::     Output variables that are always set
* Installation Directory Variables::  Other preset output variables
* Build Directories::           Supporting multiple concurrent compiles
* Automatic Remaking::          Makefile rules for configuring
@end menu

@node Preset Output Variables, Installation Directory Variables, Makefile Substitutions, Makefile Substitutions
@subsection Preset 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.  They all are
precious variables (@pxref{Setting Output Variables},
@code{AC_ARG_VAR}).

@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
@ovindex CFLAGS
Debugging and optimization options for the C compiler.  If it is not set
in the environment when @code{configure} runs, the default value is set
when you call @code{AC_PROG_CC} (or empty if you don't).  @code{configure}
uses this variable when compiling programs to test for C features.
@end defvar

@defvar configure_input
@ovindex configure_input
A comment saying that the file was generated automatically by
@code{configure} and giving the name of the input file.
@code{AC_OUTPUT} adds a comment line containing this variable to the top
of every @file{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 @code{configure} in order to be used.
@end defvar

@defvar CPPFLAGS
@ovindex CPPFLAGS
Header file search directory (@option{-I@var{dir}}) and any other
miscellaneous options for the C and C++ preprocessors and compilers.  If
it is not set in the environment when @code{configure} runs, the default
value is empty.  @code{configure} uses this variable when compiling or
preprocessing programs to test for C and C++ features.
@end defvar

@defvar CXXFLAGS
@ovindex CXXFLAGS
Debugging and optimization options for the C++ compiler.  If it is not
set in the environment when @code{configure} runs, the default value is
set when you call @code{AC_PROG_CXX} (or empty if you don't).
@code{configure} uses this variable when compiling programs to test for
C++ features.
@end defvar

@defvar DEFS
@ovindex DEFS
@option{-D} options to pass to the C compiler.  If @code{AC_CONFIG_HEADERS}
is called, @code{configure} replaces @samp{@@DEFS@@} with
@option{-DHAVE_CONFIG_H} instead (@pxref{Configuration Headers}).  This
variable is not defined while @code{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 @code{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 @code{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 FFLAGS
@ovindex FFLAGS
Debugging and optimization options for the Fortran 77 compiler.  If it
is not set in the environment when @code{configure} runs, the default
value is set when you call @code{AC_PROG_F77} (or empty if you don't).
@code{configure} uses this variable when compiling programs to test for
Fortran 77 features.
@end defvar

@defvar LDFLAGS
@ovindex LDFLAGS
Stripping (@option{-s}), path (@option{-L}), and any other miscellaneous
options for the linker.  Don't use this variable to pass library names
(@option{-l}) to the linker, use @code{LIBS} instead.  If it is not set
in the environment when @code{configure} runs, the default value is empty.
@code{configure} uses this variable when linking programs to test for
C, C++ and Fortran 77 features.
@end defvar

@defvar 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}.  @code{configure} uses this variable when linking
programs to test for C, C++ and Fortran 77 features.
@end defvar

@defvar srcdir
@ovindex srcdir
The directory that contains the source code for that @file{Makefile}.
@end defvar

@defvar top_srcdir
@ovindex top_srcdir
The top-level source code directory for the package.  In the top-level
directory, this is the same as @code{srcdir}.
@end defvar

@node Installation Directory Variables, Build Directories, Preset Output Variables, Makefile Substitutions
@subsection Installation Directory Variables

The following variables specify the directories where the package will
be installed, see @ref{Directory Variables,, Variables for Installation
Directories, standards, The GNU Coding Standards}, for more information.
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 read-only architecture-independent data.
@end defvar

@defvar exec_prefix
@ovindex exec_prefix
The installation prefix for architecture-dependent files.  By default
it's the same as @var{prefix}.  You should avoid installing anything
directly to @var{exec_prefix}.  However, the default value for
directories containing architecture-dependent files should be relative
to @var{exec_prefix}.
@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 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 prefix
@ovindex prefix
The common installation prefix for all files. If @var{exec_prefix}
is defined to a different value, @var{prefix} is used only for
architecture-independent files.
@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 on purpose that the directory output
variables keep them unexpanded: typically @samp{@@datadir@@} will be
replaced by @samp{$@{prefix@}/share}, not @samp{/usr/local/share}.

This behavior is mandated by the @sc{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 shall hard
code dependencies to her late desires.

@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{datadir}
remains being defined as @samp{$@{prefix@}/share} to depend upon the
current value of @code{prefix}.

A corollary is that you should not use these variables but in Makefiles.
For instance, instead of trying to evaluate @code{datadir} in
@file{configure} and hardcoding it in Makefiles using
e.g. @samp{AC_DEFINE_UNQUOTED(DATADIR, "$datadir")}, you should add
@samp{-DDATADIR="$(datadir)"} to your @code{CPPFLAGS}.

Similarly you should not rely on @code{AC_OUTPUT_FILES} to replace
@code{datadir} and friends in your shell scripts and other files, rather
let @command{make} manage their replacement.  For instance Autoconf
ships templates of its shell scripts ending with @samp{.sh}, and uses
this Makefile snippet:

@example
.sh:
        rm -f $@@ $@@.tmp
        sed 's,@@datadir\@@,$(pkgdatadir),g' $< >$@@.tmp
        chmod +x $@@.tmp
        mv $@@.tmp $@@
@end example

Three things are noteworthy:

@table @samp
@item @@datadir\@@
The backslash prevents @command{configure} from replacing
@samp{@@datadir@@} in the sed expression itself.

@item $(pkgdatadir)
Don't use @samp{@@pkgdatadir@@}!  Use the matching makefile variable
instead.

@item ,
Don't use @samp{/} in the sed expression(s) since most probably the
variables you use, such as @samp{$(pkgdatadir)}, will contain
some.
@end table


@node Build Directories, Automatic Remaking, Installation Directory Variables, Makefile Substitutions
@subsection Build Directories

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, @code{make} uses the @code{VPATH} variable to
find the files that are in the source directory.  @sc{gnu} @code{make}
and most other recent @code{make} programs can do this.  Older
@code{make} programs do not support @code{VPATH}; when using them, the
source code must be in the same directory as the object files.

To support @code{VPATH}, each @file{Makefile.in} should contain two
lines that look like:

@example
srcdir = @@srcdir@@
VPATH = @@srcdir@@
@end example

Do not set @code{VPATH} to the value of another variable, for example
@samp{VPATH = $(srcdir)}, because some versions of @code{make} do not do
variable substitutions on the value of @code{VPATH}.

@code{configure} substitutes in the correct value for @code{srcdir} when
it produces @file{Makefile}.

Do not use the @code{make} variable @code{$<}, which expands to the
file name of the file in the source directory (found with @code{VPATH}),
except in implicit rules.  (An implicit rule is one such as @samp{.c.o},
which tells how to create a @file{.o} file from a @file{.c} file.)  Some
versions of @code{make} do not set @code{$<} in explicit rules; they
expand it to an empty value.

Instead, @file{Makefile} 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,  , Build Directories, Makefile Substitutions
@subsection Automatic Remaking

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} will not be changed if remaking
them does not change their contents.  This feature avoids unnecessary
recompilation.  You should include the file @file{stamp-h.in} your
package's distribution, so @command{make} will consider
@file{config.h.in} up to date.  Don't use @command{touch}
(@pxref{Limitations of Usual Tools}), rather use @command{echo} (using
@command{date} would cause needless differences, hence @sc{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
will need to convert the indented lines to start with the tab character.)

In addition, you should use @samp{AC_CONFIG_FILES(stamp-h, echo
timestamp > stamp-h)} so @file{config.status} will ensure 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, Configuration Commands, Makefile Substitutions, Setup
@section Configuration Header Files
@cindex Configuration Header
@cindex @file{config.h}

When a package tests more than a few 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 @code{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, @code{configure} scripts can
create a C header file containing @samp{#define} directives.  The
@code{AC_CONFIG_HEADERS} macro selects this kind of output.  It should
be called 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}).  Use @samp{#include <config.h>}
instead of @samp{#include "config.h"}, and pass the C compiler a
@option{-I.} option (or @option{-I..}; whichever directory contains
@file{config.h}).  That way, even if the source directory is configured
itself (perhaps to make a distribution), other build directories can
also be configured without finding the @file{config.h} from the source
directory.

@defmac AC_CONFIG_HEADERS (@var{header} @dots{}, @ovar{cmds}, @ovar{init-cmds})
@maindex 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
whitespace-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
some changes in 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.  Examples:

@example
AC_CONFIG_HEADERS(config.h:config.hin)
AC_CONFIG_HEADERS(defines.h:defs.pre:defines.h.in:defs.post)
@end example

@noindent
Doing this allows you to keep your file names acceptable to MS-DOS, or
to prepend and/or append boilerplate to the file.
@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, autoheader Invocation, Configuration Headers, Configuration Headers
@subsection Configuration Header Templates
@cindex Configuration Header Template
@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}.  On
systems that have @file{unistd.h}, @code{configure} will @samp{#define}
@samp{HAVE_UNISTD_H} to 1.  On other systems, the whole line will be
commented out (in case the system predefines that symbol).

@example
@group
/* Define as 1 if you have unistd.h.  */
#undef HAVE_UNISTD_H
@end group
@end example

You can then decode the configuration header using the preprocessor
directives:

@example
@group
#include <conf.h>

#if HAVE_UNISTD_H
# include <unistd.h>
#else
/* We are in trouble. */
#endif
@end group
@end example

The use of old form templates, with @samp{#define} instead of
@samp{#undef} is strongly discouraged.

Since it is a tedious task to keep a template header up to date, you may
use @code{autoheader} to generate it, see @ref{autoheader Invocation}.


@node autoheader Invocation, Autoheader Macros, Header Templates, Configuration Headers
@subsection Using @code{autoheader} to Create @file{config.h.in}
@cindex @code{autoheader}

The @command{autoheader} program can create a template file of C
@samp{#define} statements for @code{configure} to use.  If
@file{configure.ac} invokes @code{AC_CONFIG_HEADERS(@var{file})},
@command{autoheader} creates @file{@var{file}.in}; if multiple file
arguments are given, the first one is used.  Otherwise,
@command{autoheader} creates @file{config.h.in}.

In order to do its job, @command{autoheader} needs you to document all
of the symbols that you might use; i.e., there must be at least one
@code{AC_DEFINE} or one @code{AC_DEFINE_UNQUOTED} using its third
argument for each symbol (@pxref{Defining Symbols}).  An additional
constraint is that the first argument of @code{AC_DEFINE} must be a
literal.  Note that all symbols defined by Autoconf's built-in tests are
already documented properly; you only need to document 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 @code{#define}'d (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: @command{autoheader}'s invocation@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.

@code{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 --debug
@itemx -d
Don't remove the temporary files.

@item --verbose
@itemx -v
Report processing steps.

@item --autoconf-dir=@var{dir}
@itemx -A @var{dir}
@evindex AC_MACRODIR
Override the location where the installed Autoconf data files are looked
for.  You can also set the @code{AC_MACRODIR} environment variable to a
directory; this option overrides the environment variable.

This option is rarely needed and dangerous; it is only used when one
plays with different versions of Autoconf simultaneously.

@item --localdir=@var{dir}
@itemx -l @var{dir}
Look for the package files @file{aclocal.m4} and @file{acconfig.h} (but
not @file{@var{file}.top} and @file{@var{file}.bot}) in directory
@var{dir} instead of in the current directory.

@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,  , autoheader Invocation, Configuration Headers
@subsection Autoheader Macros

@code{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, @code{autoheader} fails with an error message.

The simplest way to create a template for a @var{symbol} is to supply
the @var{description} argument to an @samp{AC_DEFINE(@var{symbol})}; see
@ref{Defining Symbols}.  You may also use one of the following macros.

@defmac AH_VERBATIM (@var{key}, @var{template})
@maindex AH_VERBATIM
@maindex VERBATIM
Tell @code{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 the symbol that can be @code{AC_DEFINE}'d.

For example:

@example
AH_VERBATIM([_GNU_SOURCE],
[/* Enable GNU extensions on systems that have them.  */
#ifndef _GNU_SOURCE
# define _GNU_SOURCE
#endif])
@end example
@end defmac


@defmac AH_TEMPLATE (@var{key}, @var{description})
@maindex AH_TEMPLATE
@maindex TEMPLATE
Tell @code{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
will generate 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_TOP (@var{text})
@maindex AH_TOP
@maindex TOP
Include @var{text} at the top of the header template file.
@end defmac


@defmac AH_BOTTOM (@var{text})
@maindex AH_BOTTOM
@maindex BOTTOM
Include @var{text} at the bottom of the header template file.
@end defmac


@node Configuration Commands, Configuration Links, Configuration Headers, Setup
@section Running Arbitrary Configuration Commands

You execute arbitrary commands either 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.

@defmac AC_CONFIG_COMMANDS (@var{tag}@dots{}, @ovar{cmds}, @ovar{init-cmds})
@maindex CONFIG_COMMANDS
Specify additional shell commands to run at the end of
@file{config.status}, and shell commands to initialize any variables
from @code{configure}.  Associate the commands to the @var{tag}.  Since
typically the @var{cmds} create a file, @var{tag} should naturally be
the name of that file.  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(time-stamp, [date >time-stamp])
@end example
@end defmac

@defmac AC_CONFIG_COMMANDS_PRE (@var{cmds})
@maindex OUTPUT_COMMANDS_PRE
Execute the @var{cmds} right before creating @file{config.status}.  A
typical use is computing values derived from variables built during the
execution of @code{configure}:

@example
AC_CONFIG_COMMANDS_PRE(
[LTLIBOBJS=`echo $LIBOBJS | sed 's/\.o/\.lo/g'`
AC_SUBST(LTLIBOBJS)])
@end example
@end defmac

@defmac AC_CONFIG_COMMANDS_POST (@var{cmds})
@maindex OUTPUT_COMMANDS_POST
Execute the @var{cmds} right after creating @file{config.status}.
@end defmac




@node Configuration Links, Subdirectories, Configuration Commands, Setup
@section Creating Configuration Links

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 another directory than its sources.

@defmac AC_CONFIG_LINKS (@var{dest}:@var{source}@dots{}, @ovar{cmds}, @ovar{init-cmds})
@maindex 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.  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, Default Prefix, Configuration Links, Setup
@section Configuring Other Packages in Subdirectories

In most situations, calling @code{AC_OUTPUT} is sufficient to produce
@file{Makefile}s in subdirectories.  However, @code{configure} scripts
that control more than one independent package can use
@code{AC_CONFIG_SUBDIRS} to run @code{configure} scripts for other
packages in subdirectories.

@defmac AC_CONFIG_SUBDIRS (@var{dir} @dots{})
@maindex CONFIG_SUBDIRS
@ovindex subdirs
Make @code{AC_OUTPUT} run @code{configure} in each subdirectory
@var{dir} in the given whitespace-separated list.  Each @var{dir} should
be a literal, i.e., please do not use:

@example
if test "$package_foo_enabled" = yes; 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}.  Rather, you should
write:

@example
if test "$package_foo_enabled" = yes then;
  AC_CONFIG_SUBDIRS(foo)
fi
@end example

If a given @var{dir} is not found, no error is reported, so a
@code{configure} script can configure whichever parts of a large source
tree are present.  If a given @var{dir} contains @code{configure.gnu},
it is run instead of @code{configure}. This is for packages that might
use a non-autoconf script @code{Configure}, which can't be called
through a wrapper @code{configure} since it would be the same file on
case-insensitive filesystems. Likewise, if a @var{dir} contains
@file{configure.ac} but no @code{configure}, the Cygnus @code{configure}
script found by @code{AC_CONFIG_AUXDIR} is used.

The subdirectory @code{configure} scripts are given the same command
line options that were given to this @code{configure} script, with minor
changes if needed (e.g., to adjust a relative path for the cache file or
source directory).  This macro also sets the output variable
@code{subdirs} to the list of directories @samp{@var{dir} @dots{}}.
@file{Makefile} rules can use this variable to determine which
subdirectories to recurse into.  This macro may be called multiple
times.
@end defmac

@node Default Prefix,  , Subdirectories, Setup
@section Default Prefix

By default, @code{configure} sets the prefix for files it installs to
@file{/usr/local}.  The user of @code{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
@code{configure}, and when running it.

Some software packages might want to install in a directory besides
@file{/usr/local} by default.  To accomplish that, use the
@code{AC_PREFIX_DEFAULT} macro.

@defmac AC_PREFIX_DEFAULT (@var{prefix})
@maindex 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 @code{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}.

@defmac AC_PREFIX_PROGRAM (@var{program})
@maindex 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 @code{PATH}, the way the shell does.  If @var{program}
is found, set the prefix to the parent of the directory containing
@var{program}; otherwise leave the prefix specified in
@file{Makefile.in} unchanged.  For example, if @var{program} is
@code{gcc} and the @code{PATH} contains @file{/usr/local/gnu/bin/gcc},
set the prefix to @file{/usr/local/gnu}.
@end defmac



@c ======================================================== Existing tests

@node Existing Tests, Writing Tests, Setup, Top
@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
@code{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 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
* 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
* C Compiler::                  Checking its characteristics
* Fortran 77 Compiler::         Checking its characteristics
* System Services::             Operating system services
* UNIX Variants::               Special kludges for specific UNIX variants
@end menu

@node Common Behavior, Alternative Programs, Existing Tests, Existing Tests
@section Common 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, Default Includes, Common Behavior, Common Behavior
@subsection Standard Symbols

All the generic macros that @code{AC_DEFINE} a symbol as a result of
their test transform their @var{argument}s 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
will define the symbol @samp{HAVE_STRUCT__EXPENSIVEP} if the check succeeds.


@node Default Includes,  , Standard Symbols, Common Behavior
@subsection 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
#if TIME_WITH_SYS_TIME
# include <sys/time.h>
# include <time.h>
#else
# if 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 provide the following default set of includes:

@example
@group
#include <stdio.h>
#if HAVE_SYS_TYPES_H
# include <sys/types.h>
#endif
#if HAVE_SYS_STAT_H
# include <sys/stat.h>
#endif
#if STDC_HEADERS
# include <stdlib.h>
# include <stddef.h>
#else
# if HAVE_STDLIB_H
#  include <stdlib.h>
# endif
#endif
#if HAVE_STRING_H
# if !STDC_HEADERS && HAVE_MEMORY_H
#  include <memory.h>
# endif
# include <string.h>
#else
# if HAVE_STRINGS_H
#  include <strings.h>
# endif
#endif
#if HAVE_INTTYPES_H
# include <inttypes.h>
#else
# if HAVE_STDINT_H
#  include <stdint.h>
# endif
#endif
#if HAVE_UNISTD_H
# include <unistd.h>
#endif
@end group
@end example

If the default includes are used, then Autoconf will automatically check
for the presence of these headers, i.e., you don't need to run
@code{AC_HEADERS_STDC}, nor check for @file{stdlib.h} etc.

@node Alternative Programs, Libraries, Common Behavior, Existing Tests
@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, Generic Programs, Alternative Programs, Alternative 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
@maindex PROG_AWK
@ovindex AWK
Check for @code{mawk}, @code{gawk}, @code{nawk}, and @code{awk}, in that
order, and set output variable @code{AWK} to the first one that is found.
It tries @code{mawk} first because that is reported to be the
fastest implementation.
@end defmac


@defmac AC_PROG_INSTALL
@maindex PROG_INSTALL
@ovindex INSTALL
@ovindex INSTALL_PROGRAM
@ovindex INSTALL_DATA
@ovindex INSTALL_SCRIPT
Set output variable @code{INSTALL} to the path of a @sc{bsd} compatible
@code{install} program, if one is found in the current @code{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}.

This macro screens out various instances of @code{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 @code{make} programs have a rule
that creates @file{install} from it if there is no @file{Makefile}.

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, or
@code{configure} will produce an error message saying it can't find
them---even if the system you're on has a good @code{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 @sc{bsd}-compatible @code{install} program.

If you need to use your own installation program because it has features
not found in standard @code{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.
@end defmac

@defmac AC_PROG_LEX
@maindex PROG_LEX
@ovindex LEX
@ovindex LEXLIB
@cvindex YYTEXT_POINTER
@ovindex LEX_OUTPUT_ROOT
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}.

Define @code{YYTEXT_POINTER} if @code{yytext} is a @samp{char *} instead
of a @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 "$LEX" != flex; then
  LEX="$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.

To ensure backward compatibility, Automake's @code{AM_PROG_LEX} invokes
(indirectly) this macro twice, which will cause 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.
@end defmac

@defmac AC_PROG_LN_S
@maindex 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 -p}.

If you make a link 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
@maindex PROG_RANLIB
@ovindex 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_YACC
@maindex PROG_YACC
@ovindex 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}.
@end defmac

@node Generic Programs,  , Particular Programs, Alternative 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 @code{PATH}.  If you need to check for a program that might not
be in the user's @code{PATH}, you can pass a modified path to use
instead, like this:

@example
AC_PATH_PROG(INETD, inetd, /usr/libexec/inetd,
  $PATH:/usr/libexec:/usr/sbin:/usr/etc: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.

@defmac AC_CHECK_PROG (@var{variable}, @var{prog-to-check-for}, @var{value-if-found}, @ovar{value-if-not-found}, @ovar{path},  @ovar{reject})
@maindex CHECK_PROG
Check whether program @var{prog-to-check-for} exists in @code{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}.
@end defmac

@defmac AC_CHECK_PROGS (@var{variable}, @var{progs-to-check-for}, @ovar{value-if-not-found}, @ovar{path})
@maindex CHECK_PROGS
Check for each program in the whitespace-separated list
@var{progs-to-check-for} exists on the @code{PATH}.  If it 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}.
@end defmac

@defmac AC_CHECK_TOOL (@var{variable}, @var{prog-to-check-for}, @ovar{value-if-not-found}, @ovar{path})
@maindex CHECK_TOOL
Like @code{AC_CHECK_PROG}, but first looks for @var{prog-to-check-for}
with a prefix of the host type as determined by
@code{AC_CANONICAL_HOST}, followed by a dash (@pxref{Canonicalizing}).
For example, if the user runs @samp{configure --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
@code{PATH}, or otherwise to @samp{ranlib} if that program exists in
@code{PATH}, or to @samp{:} if neither program exists.
@end defmac

@defmac AC_CHECK_TOOLS (@var{variable}, @var{progs-to-check-for}, @ovar{value-if-not-found}, @ovar{path})
@maindex 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}.
@end defmac

@defmac AC_PATH_PROG (@var{variable}, @var{prog-to-check-for}, @ovar{value-if-not-found}, @ovar{path})
@maindex PATH_PROG
Like @code{AC_CHECK_PROG}, but set @var{variable} to the entire
path of @var{prog-to-check-for} if found.
@end defmac

@defmac AC_PATH_PROGS (@var{variable}, @var{progs-to-check-for}, @ovar{value-if-not-found}, @ovar{path})
@maindex PATH_PROGS
Like @code{AC_CHECK_PROGS}, but if any of @var{progs-to-check-for}
are found, set @var{variable} to the entire path of the program
found.
@end defmac

@defmac AC_PATH_TOOL (@var{variable}, @var{prog-to-check-for}, @ovar{value-if-not-found}, @ovar{path})
@maindex PATH_TOOL
Like @code{AC_CHECK_TOOL}, but set @var{variable} to the entire
path of the program if it is found.
@end defmac


@c FIXME: Not the proper place for these guys.

You might also need to check for the existence of files.  Before using
these macros, ask yourself whether a run time 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})
@maindex CHECK_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.
@end defmac

@defmac AC_CHECK_FILES (@var{files}, @ovar{action-if-found}, @ovar{action-if-not-found})
@maindex CHECK_FILES
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.
@end defmac


@node Libraries, Library Functions, Alternative Programs, Existing Tests
@section Library Files
@cindex Library, checking

The following macros check for the presence of certain C, C++ or Fortran
77 library archive files.

@defmac AC_CHECK_LIB (@var{library}, @var{function}, @ovar{action-if-found}, @ovar{action-if-not-found}, @ovar{other-libraries})
@maindex CHECK_LIB
Depending on the current language(@pxref{Language Choice}), try to
ensure that the C, C++, or Fortran 77 function @var{function} is
available by checking whether a test program can be linked with the
library @var{library} to get the function.  @var{library} is 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 will prepend @option{-l@var{library}} to
@code{LIBS} and define @samp{HAVE_LIB@var{library}} (in all
capitals). This macro is intended to support building of @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. Some linkers are very 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 will fail to detect
that @var{library} is present, because linking the test program will
always 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}.
@end defmac


@defmac AC_SEARCH_LIBS (@var{function}, @var{search-libs}, @ovar{action-if-found}, @ovar{action-if-not-found}, @ovar{other-libraries})
@maindex SEARCH_LIBS
Search for a library defining @var{function} if it's not already
available.  This equates to calling @code{AC_TRY_LINK_FUNC} first
with no libraries, then for each library listed in @var{search-libs}.

Add @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 will fail to detect
that @var{function} is present, because linking the test program will
always fail with unresolved symbols.
@end defmac



@node Library Functions, Header Files, Libraries, Existing Tests
@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, Particular Functions, Library Functions, Library Functions
@subsection Portability of Classical Functions

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 will always require
additions, please help us keeping it as complete as possible

@table @code

@item unlink
The @sc{posix} spec says that @code{unlink} causes the given files to be
removed only after there are no more open file handles for it.  Not all
OS's support this behaviour though.  So even on systems that provide
@code{unlink}, you cannot portably assume it is OK to call it on files
that are open.  For example, on Windows 9x and ME, such a call would fail;
on DOS it could even lead to file system corruption, as the file might end
up being written to after the OS has removed it.

@end table


@node Particular Functions, Generic Functions, Function Portability, Library 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.

@defmac AC_FUNC_ALLOCA
@maindex FUNC_ALLOCA
@cvindex C_ALLOCA
@cvindex HAVE_ALLOCA_H
@ovindex ALLOCA
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{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}.

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.  In some versions of AIX,
the declaration of @code{alloca} must precede everything else except for
comments and preprocessor directives.  The @code{#pragma} directive is
indented so that pre-@sc{ansi} C compilers will ignore it, rather than
choke on it.

@example
@group
/* AIX requires this to be the first thing in the file.  */
#ifndef __GNUC__
# if HAVE_ALLOCA_H
#  include <alloca.h>
# else
#  ifdef _AIX
 #pragma alloca
#  else
#   ifndef alloca /* predefined by HP cc +Olibcalls */
char *alloca ();
#   endif
#  endif
# endif
#endif
@end group
@end example
@end defmac

@defmac AC_FUNC_CHOWN
@maindex FUNC_CHOWN
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}.
@end defmac


@defmac AC_FUNC_CLOSEDIR_VOID
@maindex FUNC_CLOSEDIR_VOID
@cvindex 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.
@end defmac

@defmac AC_FUNC_ERROR_AT_LINE
@maindex FUNC_ERROR_AT_LINE
If the @code{error_at_line} function is not found, require an
@code{AC_LIBOBJ} replacement of @samp{error}.
@end defmac

@defmac AC_FUNC_FNMATCH
@maindex FUNC_FNMATCH
If the @code{fnmatch} function is available and works (unlike the one on
Solaris 2.4), define @code{HAVE_FNMATCH}.
@end defmac

@defmac AC_FUNC_FORK
@maindex FUNC_FORK
@cvindex HAVE_VFORK_H
@cvindex HAVE_WORKING_FORK
@cvindex HAVE_WORKING_VFORK
@cvindex vfork
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
#if !HAVE_WORKING_VFORK
# define vfork fork
#endif
@end group
@end example
@end defmac

@defmac AC_FUNC_FSEEKO
@maindex FUNC_FSEEKO
@cvindex _LARGEFILE_SOURCE
If the @code{fseeko} function is available, define @code{HAVE_FSEEKO}.
Define @code{_LARGEFILE_SOURCE} if necessary.
@end defmac

@defmac AC_FUNC_GETGROUPS
@maindex FUNC_GETGROUPS
@ovindex GETGROUPS_LIBS
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

@defmac AC_FUNC_GETLOADAVG
@maindex FUNC_GETLOADAVG
@cvindex SVR4
@cvindex DGUX
@cvindex UMAX
@cvindex UMAX4_3
@cvindex NLIST_STRUCT
@cvindex NLIST_NAME_UNION
@cvindex GETLODAVG_PRIVILEGED
@cvindex NEED_SETGID
@cvindex C_GETLOADAVG
@ovindex LIBOBJS
@ovindex NEED_SETGID
@ovindex KMEM_GROUP
@ovindex GETLOADAVG_LIBS
Check how to get the system load averages.  If the system has the
@code{getloadavg} function, define @code{HAVE_GETLOADAVG}, and set
@code{GETLOADAVG_LIBS} to any libraries needed to get that function.
Also add @code{GETLOADAVG_LIBS} to @code{LIBS}.

Otherwise, require an @code{AC_LIBOBJ} replacement (@file{getloadavg.c})
of @samp{getloadavg}, 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
If @file{nlist.h} is found, define @code{NLIST_STRUCT}.

@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 setgid (or setuid) 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
@end defmac

@defmac AC_FUNC_GETMNTENT
@maindex FUNC_GETMNTENT
@cvindex HAVE_GETMNTENT
Check for @code{getmntent} in the @file{sun}, @file{seq}, and @file{gen}
libraries, for Irix 4, PTX, and Unixware, respectively.  Then, if
@code{getmntent} is available, define @code{HAVE_GETMNTENT}.
@end defmac

@defmac AC_FUNC_GETPGRP
@maindex FUNC_GETPGRP
@cvindex GETPGRP_VOID
If @code{getpgrp} takes no argument (the @sc{posix.1} version), define
@code{GETPGRP_VOID}.  Otherwise, it is the @sc{bsd} version, which takes
a process ID as an argument.  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}.
@end defmac

@defmac AC_FUNC_LSTAT_FOLLOWS_SLASHED_SYMLINK
@maindex FUNC_LSTAT_FOLLOWS_SLASHED_SYMLINK
@cvindex LSTAT_FOLLOWS_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} and @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}.
@end defmac

@defmac AC_FUNC_MALLOC
@maindex FUNC_MALLOC
If the @code{malloc} works correctly (@samp{malloc (0)} returns a valid
pointer), define @code{HAVE_MALLOC}.
@end defmac

@defmac AC_FUNC_MEMCMP
@maindex FUNC_MEMCMP
@ovindex LIBOBJS
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}.
@end defmac

@defmac AC_FUNC_MKTIME
@maindex FUNC_MKTIME
@ovindex LIBOBJS
If the @code{mktime} function is not available, or does not work
correctly, require an @code{AC_LIBOBJ} replacement for @samp{mktime}.
@end defmac

@defmac AC_FUNC_MMAP
@maindex FUNC_MMAP
@cvindex HAVE_MMAP
If the @code{mmap} function exists and works correctly, define
@code{HAVE_MMAP}.  Only checks private fixed mapping of already-mapped
memory.
@end defmac

@defmac AC_FUNC_OBSTACK
@maindex FUNC_OBSTACK
@cvindex HAVE_OBSTACK
@cindex obstack
If the obstacks are found, define @code{HAVE_OBSTACK}, else require an
@code{AC_LIBOBJ} replacement for @samp{obstack}.
@end defmac

@defmac AC_FUNC_SELECT_ARGTYPES
@maindex FUNC_SELECT_ARGTYPES
@cvindex SELECT_TYPE_ARG1
@cvindex SELECT_TYPE_ARG234
@cvindex SELECT_TYPE_ARG5
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 *}.
@end defmac

@defmac AC_FUNC_SETPGRP
@maindex FUNC_SETPGRP
@cvindex SETPGRP_VOID
If @code{setpgrp} takes no argument (the @sc{posix.1} version), define
@code{SETPGRP_VOID}.  Otherwise, it is the @sc{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}.
@end defmac

@defmac AC_FUNC_STAT
@defmacx AC_FUNC_LSTAT
@maindex FUNC_STAT
@maindex FUNC_LSTAT
@cvindex HAVE_STAT_EMPTY_STRING_BUG
@cvindex HAVE_LSTAT_EMPTY_STRING_BUG
Determine whether @code{stat} or @code{lstat} have the bug that it
succeeds when given the zero-length file name 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.
@end defmac

@defmac AC_FUNC_SETVBUF_REVERSED
@maindex FUNC_SETVBUF_REVERSED
@cvindex SETVBUF_REVERSED
If @code{setvbuf} takes the buffering type as its second argument and
the buffer pointer as the third, instead of the other way around, define
@code{SETVBUF_REVERSED}.
@end defmac

@defmac AC_FUNC_STRCOLL
@maindex FUNC_STRCOLL
@cvindex HAVE_STRCOLL
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.
@end defmac

@defmac AC_FUNC_STRTOD
@maindex FUNC_STRTOD
@ovindex POW_LIB
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.
@end defmac

@defmac AC_FUNC_STRERROR_R
@maindex FUNC_STRERROR_R
@cvindex HAVE_STRERROR_R
@cvindex HAVE_WORKING_STRERROR_R
If @code{strerror_r} is available, define @code{HAVE_STRERROR_R}.  If
its implementation correctly returns a @code{char *}, define
@code{HAVE_WORKING_STRERROR_R}.  On at least DEC UNIX 4.0[A-D] and HP-UX
B.10.20, @code{strerror_r} returns @code{int}.  Actually, this tests
only whether it returns a scalar or an array, but that should be enough.
This is used by the common @file{error.c}.
@end defmac

@defmac AC_FUNC_STRFTIME
@maindex FUNC_STRFTIME
@cvindex HAVE_STRFTIME
Check for @code{strftime} in the @file{intl} library, for SCO @sc{unix}.
Then, if @code{strftime} is available, define @code{HAVE_STRFTIME}.
@end defmac

@defmac AC_FUNC_UTIME_NULL
@maindex FUNC_UTIME_NULL
@cvindex HAVE_UTIME_NULL
If @samp{utime(@var{file}, NULL)} sets @var{file}'s timestamp to
the present, define @code{HAVE_UTIME_NULL}.
@end defmac

@defmac AC_FUNC_VPRINTF
@maindex FUNC_VPRINTF
@cvindex HAVE_VPRINTF
@cvindex HAVE_DOPRNT
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.)
@end defmac

@node Generic Functions,  , Particular Functions, Library 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}).

@defmac AC_CHECK_FUNC (@var{function}, @ovar{action-if-found}, @ovar{action-if-not-found})
@maindex CHECK_FUNC
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.)
@end defmac

@defmac AC_CHECK_FUNCS (@var{function}@dots{}, @ovar{action-if-found}, @ovar{action-if-not-found})
@maindex CHECK_FUNCS
@cvindex HAVE_@var{function}
For each @var{function} in the whitespace-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.
@end defmac

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 @sc{posix}
environment.  Some functions may be missing or unfixable, and your
package must be ready to replace them.

Use the first three of the following macros to specify a function to be
replaced, and the last one (@code{AC_REPLACE_FUNCS}) to check for and
replace the function if needed.

@defmac AC_LIBOBJ (@var{function})
@maindex 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}.

Technically, it adds @samp{@var{function}.$ac_objext} to the output
variable @code{LIBOBJS} 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})
@maindex 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})
@maindex 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_REPLACE_FUNCS (@var{function}@dots{})
@maindex REPLACE_FUNCS
@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{#if !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, Declarations, Library Functions, Existing Tests
@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
* Particular Headers::          Special handling to find certain headers
* Generic Headers::             How to find other headers
@end menu

@node Particular Headers, Generic Headers, Header Files, Header Files
@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_HEADER_DIRENT
@maindex HEADER_DIRENT
@cvindex HAVE_DIRENT_H
@cvindex HAVE_NDIR_H
@cvindex HAVE_SYS_DIR_H
@cvindex HAVE_SYS_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
#if HAVE_DIRENT_H
# include <dirent.h>
# define NAMLEN(dirent) strlen((dirent)->d_name)
#else
# define dirent direct
# define NAMLEN(dirent) (dirent)->d_namlen
# if HAVE_SYS_NDIR_H
#  include <sys/ndir.h>
# endif
# if HAVE_SYS_DIR_H
#  include <sys/dir.h>
# endif
# if 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.
@end defmac

@defmac AC_HEADER_MAJOR
@maindex HEADER_MAJOR
@cvindex MAJOR_IN_MKDEV
@cvindex MAJOR_IN_SYSMACROS
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_STAT
@maindex HEADER_STAT
@maindex STAT_MACROS_BROKEN
If the macros @code{S_ISDIR}, @code{S_ISREG} et al. 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.
@end defmac


@defmac AC_HEADER_STDC
@maindex HEADER_STDC
@cvindex STDC_HEADERS
Define @code{STDC_HEADERS} if the system has @sc{ansi} C header files.
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 @sc{ansi} C 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 @sc{ansi} C requires.

Use @code{STDC_HEADERS} instead of @code{__STDC__} to determine whether
the system has @sc{ansi}-compliant header files (and probably C library
functions) because many systems that have GCC do not have @sc{ansi} C
header files.

On systems without @sc{ansi} C 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 systems contain
a mix of functions @sc{ansi} and @sc{bsd}; some are mostly @sc{ansi} but
lack @samp{memmove}; some define the @sc{bsd} functions as macros in
@file{string.h} or @file{strings.h}; some have only the @sc{bsd}
functions but @file{string.h}; some declare 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 has the @sc{ansi} versions of those then it probably has
most of the others.  If you put the following in @file{configure.ac}:

@example
AC_HEADER_STDC
AC_CHECK_FUNCS(strchr memcpy)
@end example

@noindent
then, in your code, you can put declarations like this:

@example
@group
#if STDC_HEADERS
# include <string.h>
#else
# if !HAVE_STRCHR
#  define strchr index
#  define strrchr rindex
# endif
char *strchr (), *strrchr ();
# if !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 @sc{bsd} equivalent, then macros won't
suffice; 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
@maindex HEADER_SYS_WAIT
@cvindex HAVE_SYS_WAIT_H
If @file{sys/wait.h} exists and is compatible with @sc{posix.1}, define
@code{HAVE_SYS_WAIT_H}.  Incompatibility can occur if @file{sys/wait.h}
does not exist, or if it uses the old @sc{bsd} @code{union wait} instead
of @code{int} to store a status value.  If @file{sys/wait.h} is not
@sc{posix.1} compatible, then instead of including it, define the
@sc{posix.1} macros with their usual interpretations.  Here is an
example:

@example
@group
#include <sys/types.h>
#if HAVE_SYS_WAIT_H
# include <sys/wait.h>
#endif
#ifndef WEXITSTATUS
# define WEXITSTATUS(stat_val) ((unsigned)(stat_val) >> 8)
#endif
#ifndef WIFEXITED
# define WIFEXITED(stat_val) (((stat_val) & 255) == 0)
#endif
@end group
@end example
@end defmac

@cvindex _POSIX_VERSION
@code{_POSIX_VERSION} is defined when @file{unistd.h} is included on
@sc{posix.1} systems.  If there is no @file{unistd.h}, it is definitely
not a @sc{posix.1} system.  However, some non-@sc{posix.1} systems do
have @file{unistd.h}.

The way to check if the system supports @sc{posix.1} is:

@example
@group
#if HAVE_UNISTD_H
# include <sys/types.h>
# include <unistd.h>
#endif

#ifdef _POSIX_VERSION
/* Code for POSIX.1 systems.  */
#endif
@end group
@end example

@defmac AC_HEADER_TIME
@maindex HEADER_TIME
@cvindex TIME_WITH_SYS_TIME
If a program may include both @file{time.h} and @file{sys/time.h},
define @code{TIME_WITH_SYS_TIME}.  On some older systems,
@file{sys/time.h} includes @file{time.h}, but @file{time.h} is not
protected against multiple inclusion, so programs should not explicitly
include both files.  This macro is useful in programs that use, for
example, @code{struct timeval} or @code{struct timezone} 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
#if TIME_WITH_SYS_TIME
# include <sys/time.h>
# include <time.h>
#else
# if HAVE_SYS_TIME_H
#  include <sys/time.h>
# else
#  include <time.h>
# endif
#endif
@end group
@end example
@end defmac


@defmac AC_HEADER_TIOCGWINSZ
@maindex HEADER_TIOCGWINSZ
@cvindex GWINSZ_IN_SYS_IOCTL
@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
#if HAVE_TERMIOS_H
# include <termios.h>
#endif

#if GWINSZ_IN_SYS_IOCTL
# include <sys/ioctl.h>
#endif
@end group
@end example
@end defmac

@node Generic Headers,  , Particular Headers, Header Files
@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}).

@defmac AC_CHECK_HEADER (@var{header-file}, @ovar{action-if-found}, @ovar{action-if-not-found}, @ovar{includes})
@maindex CHECK_HEADER
If the system header file @var{header-file} exists, 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.

This macro actually checks whether @var{header-file} can be included
without error.  If @var{includes} is specified, they are included before
@var{header-file}.  Note that the @var{includes} are @emph{not}
defaulted.  They might be in future releases.
@end defmac

@defmac AC_CHECK_HEADERS (@var{header-file}@dots{}, @ovar{action-if-found}, @ovar{action-if-not-found}, @ovar{includes})
@maindex CHECK_HEADERS
@cvindex HAVE_@var{header}
For each given system header file @var{header-file} in the
whitespace-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.

This macro actually checks whether @var{header-file} can be included
without error.  If @var{includes} is specified, they are included before
@var{header-file}.  Note that the @var{includes} are @emph{not}
defaulted.  They might be in future releases.
@end defmac

@node Declarations, Structures, Header Files, Existing Tests
@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_TRY_COMPILE} (@pxref{Examining Syntax}).

@menu
* Particular Declarations::     Macros to check for certain declarations
* Generic Declarations::        How to find other declarations
@end menu

@node Particular Declarations, Generic Declarations, Declarations, Declarations
@subsection Particular Declaration Checks

The following macros check for certain declarations.

@defmac AC_DECL_SYS_SIGLIST
@maindex DECL_SYS_SIGLIST
@cvindex SYS_SIGLIST_DECLARED
Define @code{SYS_SIGLIST_DECLARED} if the variable @code{sys_siglist}
is declared in a system header file, either @file{signal.h} or
@file{unistd.h}.
@end defmac

@node Generic Declarations,  , Particular Declarations, 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}, @ovar{includes})
@maindex CHECK_DECL
If @var{symbol} (a function or a variable) 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}.  If no
@var{includes} are specified, the default includes are used
(@pxref{Default Includes}).

This macro actually tests whether it is valid to use @var{symbol} as an
r-value, not if it is really declared, because it is much safer to avoid
introducing extra declarations when they are not needed.
@end defmac

@defmac AC_CHECK_DECLS (@var{symbols}, @ovar{action-if-found}, @ovar{action-if-not-found}, @ovar{includes})
@maindex CHECK_DECLS
@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, 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.

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])
@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}} just like any other result of Autoconf:

@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
char *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.
@end defmac


@node Structures, Types, Declarations, Existing Tests
@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 macro
(@pxref{Generic Structures}) or, for more complex tests, you may use
@code{AC_TRY_COMPILE} (@pxref{Examining Syntax}).

@menu
* Particular Structures::       Macros to check for certain structure members
* Generic Structures::          How to find other structure members
@end menu

@node Particular Structures, Generic Structures, Structures, Structures
@subsection Particular Structure Checks

The following macros check for certain structures or structure members.

@defmac AC_STRUCT_ST_BLKSIZE
@maindex 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
@end defmac

@defmac AC_STRUCT_ST_BLOCKS
@maindex STRUCT_ST_BLOCKS
@cvindex HAVE_STRUCT_STAT_ST_BLOCKS
@cvindex HAVE_ST_BLOCKS
@ovindex LIBOBJS
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.
@end defmac

@defmac AC_STRUCT_ST_RDEV
@maindex 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
@end defmac

@defmac AC_STRUCT_TM
@maindex STRUCT_TM
@cvindex TM_IN_SYS_TIME
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}.
@end defmac

@defmac AC_STRUCT_TIMEZONE
@maindex STRUCT_TIMEZONE
@cvindex HAVE_TM_ZONE
@cvindex HAVE_TZNAME
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}.
@end defmac

@node Generic Structures,  , Particular Structures, 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}, @ovar{includes})
@maindex CHECK_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 sub-members:

@example
AC_CHECK_MEMBER(struct top.middle.bot)
@end example
@end defmac

@defmac AC_CHECK_MEMBERS (@var{members}, @ovar{action-if-found}, @ovar{action-if-not-found}, @ovar{includes})
@maindex CHECK_MEMBERS
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).

This macro uses m4 lists:
@example
AC_CHECK_MEMBERS([struct stat.st_rdev, struct stat.st_blksize])
@end example
@end defmac


@node Types, Compilers and Preprocessors, Structures, Existing Tests
@section 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, Generic Types, Types, Types
@subsection Particular Type Checks

These macros check for particular C types in @file{sys/types.h},
@file{stdlib.h} and others, if they exist.

@defmac AC_TYPE_GETGROUPS
@maindex TYPE_GETGROUPS
@cvindex GETGROUPS_T
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}.
@end defmac

@defmac AC_TYPE_MODE_T
@maindex TYPE_MODE_T
@cvindex mode_t
Equivalent to @samp{AC_CHECK_TYPE(mode_t, int)}.
@end defmac

@defmac AC_TYPE_OFF_T
@maindex TYPE_OFF_T
@cvindex off_t
Equivalent to @samp{AC_CHECK_TYPE(off_t, long)}.
@end defmac

@defmac AC_TYPE_PID_T
@maindex TYPE_PID_T
@cvindex pid_t
Equivalent to @samp{AC_CHECK_TYPE(pid_t, int)}.
@end defmac

@defmac AC_TYPE_SIGNAL
@maindex TYPE_SIGNAL
@cvindex RETSIGTYPE
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}.

Define signal handlers as returning type @code{RETSIGTYPE}:

@example
@group
RETSIGTYPE
hup_handler ()
@{
@dots{}
@}
@end group
@end example
@end defmac

@defmac AC_TYPE_SIZE_T
@maindex TYPE_SIZE_T
@cvindex size_t
Equivalent to @samp{AC_CHECK_TYPE(size_t, unsigned)}.
@end defmac

@defmac AC_TYPE_UID_T
@maindex TYPE_UID_T
@cvindex uid_t
@cvindex gid_t
If @code{uid_t} is not defined, define @code{uid_t} to be @code{int} and
@code{gid_t} to be @code{int}.
@end defmac

@node Generic Types,  , Particular Types, 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}, @ovar{includes})
@maindex CHECK_TYPE
Check whether @var{type} is defined.  It may be a compiler builtin type
or defined by the @ovar{includes} (@pxref{Default Includes}).
@end defmac


@defmac AC_CHECK_TYPES (@var{types}, @ovar{action-if-found}, @ovar{action-if-not-found}, @ovar{includes})
@maindex CHECK_TYPES
For each @var{type} of the @var{types} that is defined, define
@code{HAVE_@var{type}} (in all capitals).  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, uintmax_t])
@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 heuristics, 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, C Compiler, Types, Existing Tests
@section Compilers and 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 Unix and
@samp{.exe} if Win32 or OS/2.

@ovindex OBJEXT
They also define the output variable @code{OBJEXT} based on the
output of the compiler, after .c files have been excluded, typically
to @samp{o} if Unix, @samp{obj} if Win32.

If the compiler being used does not produce executables, they 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.

@defmac AC_PROG_CC (@ovar{compiler-search-list})
@maindex PROG_CC
@ovindex CC
@ovindex CFLAGS
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 space 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(cl egcs gcc cc)
@end example

If using the @sc{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 @sc{gnu} C compiler (@option{-O2} on systems
where GCC does not accept @option{-g}), or @option{-g} for other compilers.
@end defmac

@defmac AC_PROG_CC_C_O
@maindex PROG_CC_C_O
@cvindex NO_MINUS_C_MINUS_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 @sc{gnu} Make to choose the default C compilation
rule.
@end defmac

@defmac AC_PROG_CC_STDC
@maindex PROG_CC_STDC
@ovindex CC
If the C compiler is not in @sc{ansi} C mode by default, try to add an
option to output variable @code{CC} to make it so.  This macro tries
various options that select @sc{ansi} C on some system or another.  It
considers the compiler to be in @sc{ansi} C mode if it handles function
prototypes correctly.

If you use this macro, you should check after calling it whether the C
compiler has been set to accept @sc{ansi} C; if not, the shell variable
@code{ac_cv_prog_cc_stdc} is set to @samp{no}.  If you wrote your source
code in @sc{ansi} C, you can make an un-@sc{ansi}fied copy of it by
using the program @code{ansi2knr}, which comes with Automake.
@end defmac


@defmac AC_PROG_CPP
@maindex PROG_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.

If the current language is C (@pxref{Language Choice}), many of the
specific test macros use the value of @code{CPP} indirectly by calling
@code{AC_TRY_CPP}, @code{AC_CHECK_HEADER}, @code{AC_EGREP_HEADER}, or
@code{AC_EGREP_CPP}.

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.
@end defmac

@defmac AC_PROG_CXX (@ovar{compiler-search-list})
@maindex PROG_CXX
@ovindex CXX
@ovindex CXXFLAGS
Determine a C++ compiler to use.  Check if 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{gcc}.

This macro may, however, be invoked with an optional first argument
which, if specified, must be a space 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(cl KCC CC cxx cc++ xlC aCC c++ g++ egcs gcc)
@end example

If using the @sc{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 @sc{gnu} C++ compiler (@option{-O2} on
systems where G++ does not accept @option{-g}), or @option{-g} for other
compilers.
@end defmac

@defmac AC_PROG_CXXCPP
@maindex PROG_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 only portable to run @code{CXXCPP} on files with a @file{.c},
@file{.C}, or @file{.cc} extension.

If the current language is C++ (@pxref{Language Choice}), many of the
specific test macros use the value of @code{CXXCPP} indirectly by
calling @code{AC_TRY_CPP}, @code{AC_CHECK_HEADER},
@code{AC_EGREP_HEADER}, or @code{AC_EGREP_CPP}.

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_F77 (@ovar{compiler-search-list})
@maindex PROG_FORTRAN
@ovindex F77
@ovindex FFLAGS
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 space 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 cf77 g77 f90 xlf90)
@end example

If using @code{g77} (the @sc{gnu} Fortran 77 compiler), then
@code{AC_PROG_F77} will 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.
@end defmac

@defmac AC_PROG_F77_C_O
@maindex PROG_F77_C_O
@cvindex F77_NO_MINUS_C_MINUS_O
Test if the Fortran 77 compiler accepts the options @option{-c} and
@option{-o} simultaneously, and define @code{F77_NO_MINUS_C_MINUS_O} if it
does not.
@end defmac

@defmac AC_PROG_GCC_TRADITIONAL
@maindex PROG_GCC_TRADITIONAL
@ovindex CC
Add @option{-traditional} to output variable @code{CC} if using the
@sc{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.  Since recent versions of the
@sc{gnu} C compiler fix the header files automatically when installed,
this is becoming a less prevalent problem.
@end defmac


@node C Compiler, Fortran 77 Compiler, Compilers and Preprocessors, Existing Tests
@section C Compiler Characteristics

The following macros check for C compiler or machine architecture
features.  To check for characteristics not listed here, use
@code{AC_TRY_COMPILE} (@pxref{Examining Syntax}) or @code{AC_TRY_RUN}
(@pxref{Run Time})

@defmac AC_C_BIGENDIAN
@maindex C_BIGENDIAN
@cvindex WORDS_BIGENDIAN
@cindex Endianness
If words are stored with the most significant byte first (like Motorola
and SPARC, but not Intel and VAX, CPUs), define @code{WORDS_BIGENDIAN}.
@end defmac


@defmac AC_C_CONST
@maindex C_CONST
@cvindex const
If the C compiler does not fully support the @sc{ansi} C qualifier
@code{const}, 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 @file{Makefile} or configuration header file will
define 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 @code{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.
@end defmac

@defmac AC_C_VOLATILE
@maindex 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 @file{Makefile} or configuration header will define 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 will compile, when it wouldn't before.

In general, the @code{volatile} keyword is a feature of @sc{ansi} C, 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 defined @code{__STDC__}.
@end defmac

@defmac AC_C_INLINE
@maindex 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

@defmac AC_C_CHAR_UNSIGNED
@maindex C_CHAR_UNSIGNED
@cvindex __CHAR_UNSIGNED__
If the C type @code{char} is unsigned, define @code{__CHAR_UNSIGNED__},
unless the C compiler predefines it.
@end defmac

@defmac AC_C_LONG_DOUBLE
@maindex C_LONG_DOUBLE
@cvindex HAVE_LONG_DOUBLE
If the C compiler supports the @code{long double} type, define
@code{HAVE_LONG_DOUBLE}.  Some C compilers that do not define
@code{__STDC__} do support the @code{long double} type; some compilers
that define @code{__STDC__} do not support @code{long double}.
@end defmac

@defmac AC_C_STRINGIZE
@maindex 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
@end defmac

@defmac AC_C_PROTOTYPES
@maindex C_PROTOTYPES
@cvindex PROTOTYPES
@cvindex PARAMS
Check to see if function prototypes are understood by the compiler.  If
so, define @samp{PROTOTYPES}.  In the case the compiler does not handle
prototypes, you should use @code{ansi2knr}, which comes with the
Automake distribution, to unprotoize function definitions.  For
function prototypes, you should first define @code{PARAMS}:

@example
#ifndef PARAMS
# if PROTOTYPES
#  define PARAMS(protos) protos
# else /* no PROTOTYPES */
#  define PARAMS(protos) ()
# endif /* no PROTOTYPES */
#endif
@end example

@noindent
then use it this way:

@example
size_t my_strlen PARAMS ((const char *));
@end example
@end defmac

@c FIXME: What the heck is this macro doing here?  Move it out of
@c the way, in its proper section!!!
@defmac AC_CHECK_SIZEOF (@var{type}, @ovar{unused}, @ovar{includes})
@maindex CHECK_SIZEOF
Define @code{SIZEOF_@var{type}} (@pxref{Standard Symbols}) to be the
size in bytes of @var{type}.  If @samp{type} is unknown, it gets a size
of 0.  If no @var{includes} are specified, the default includes are used
(@pxref{Default Includes}).  If you provide @var{include}, make sure to
include @file{stdio.h} which is required for this macro to run.

This macro now works even when cross-compiling.  The @var{unused}
argument was used when cross-compiling.

For example, the call

@example
AC_CHECK_SIZEOF(int *)
@end example

@noindent
defines @code{SIZEOF_INT_P} to be 8 on DEC Alpha AXP systems.
@end defmac



@node Fortran 77 Compiler, System Services, C Compiler, Existing Tests
@section Fortran 77 Compiler Characteristics

The following macros check for Fortran 77 compiler characteristics.  To
check for characteristics not listed here, use @code{AC_TRY_COMPILE}
(@pxref{Examining Syntax}) or @code{AC_TRY_RUN} (@pxref{Run Time}),
making sure to first set the current language to Fortran 77
@code{AC_LANG(Fortran 77)} (@pxref{Language Choice}).

@defmac AC_F77_LIBRARY_LDFLAGS
@maindex F77_LIBRARY_LDFLAGS
@ovindex FLIBS
Determine the linker flags (e.g. @option{-L} and @option{-l}) for the
@dfn{Fortran 77 intrinsic and run-time libraries} that are required to
successfully link a Fortran 77 program or shared library.  The output
variable @code{FLIBS} is set to these flags.

This macro is intended to be used in those situations when it is
necessary to mix, e.g. C++ and Fortran 77 source code into 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 77 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 77 intrinsic and run-time 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, the macro
@code{AC_F77_LIBRARY_LDFLAGS} was created to determine these Fortran 77
libraries.

The macro @code{AC_F77_DUMMY_MAIN} or @code{AC_F77_MAIN} will probably
also be necessary to link C/C++ with Fortran; see below.
@end defmac

@defmac AC_F77_DUMMY_MAIN(@ovar{ACTION-IF-FAIL}, @ovar{ACTION-IF-NONE}, @ovar{ACTION-IF-FOUND})
@maindex F77_DUMMY_MAIN
@cvindex F77_DUMMY_MAIN
With many compilers, the Fortran libraries detected by
@code{AC_F77_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 e.g. @code{MAIN__} to run the
user's program.  The @code{AC_F77_DUMMY_MAIN} or @code{AC_F77_MAIN}
macros figure out how to deal with this interaction.

When using Fortran only for purely numerical functions (no I/O,
etcetera), users often prefer to provide their 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} detects
whether any such routine is @emph{required} for linking, and what its
name is.

If it cannot figure out how to link successfully, @code{ACTION-IF-FAIL}
is executed, with the default action being to exit with an error
message.  @code{ACTION-IF-NONE} is executed if no dummy main is needed
(default: no action).  @code{ACTION-IF-FOUND} is executed if a dummy
main is required; the default action is to define @code{F77_DUMMY_MAIN}
to the name of this required routine (e.g. @code{MAIN__}).

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
#ifdef F77_DUMMY_MAIN
#  ifdef __cplusplus
     extern "C"
#  endif
   int F77_DUMMY_MAIN() @{ return 1; @}
#endif
@end example

Note that @code{AC_F77_DUMMY_MAIN} is called automatically from
@code{AC_F77_WRAPPERS}; there is generally no need to call it explicitly
unless one wants to change the default actions.
@end defmac

@defmac AC_F77_MAIN
@maindex F77_MAIN
@cvindex F77_MAIN
As discussed above for @code{AC_F77_DUMMY_MAIN}, many Fortran libraries
allow you to provide an entry point called e.g. @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} macro detects whether it is @emph{possible} to
utilize such an alternate main function, and defines @code{F77_MAIN} to
the name of the function.  (If no alternate main function name is found,
@code{F77_MAIN} is simply defined to @code{main}.)

Thus, when calling Fortran routines from C that perform things like I/O,
one should use this macro and name the "main" function @code{F77_MAIN}
instead of @code{main}.
@end defmac

@defmac AC_F77_WRAPPERS
@maindex F77_WRAPPERS
@cvindex F77_FUNC
@cvindex F77_FUNC_
Defines C macros @code{F77_FUNC(name,NAME)} and
@code{F77_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 77 compiler.

Fortran 77 is case-insensitive, and in order to achieve this the Fortran
77 compiler converts all identifiers into a canonical case and format.
To call a Fortran 77 subroutine from C or to write a C function that is
callable from Fortran 77, the C program must explicitly use identifiers
in the format expected by the Fortran 77 compiler.  In order to do this,
one simply wraps all C identifiers in one of the macros provided by
@code{AC_F77_WRAPPERS}.  For example, suppose you have the following
Fortran 77 subroutine:

@example
      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
#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}).

Although Autoconf tries to be intelligent about detecting the
name-mangling scheme of the Fortran 77 compiler, there may be Fortran 77
compilers that it doesn't support yet.  In this case, the above code
will generate a compile-time error, but some other behavior
(e.g. disabling Fortran-related features) can be induced by checking
whether the @code{F77_FUNC} macro is defined.

Now, to call that routine from a C program, we would do something like:

@example
@{
    double x = 2.7183, y;
    FOOBAR_F77(&x, &y);
@}
@end example

If the Fortran 77 identifier contains an underscore
(e.g. @code{foo_bar}), you should use @code{F77_FUNC_} instead of
@code{F77_FUNC} (with the same arguments).  This is because some Fortran
77 compilers mangle names differently if they contain an underscore.
@end defmac

@defmac AC_F77_FUNC (@var{name}, @ovar{shellvar})
@maindex F77_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 77 linker (see also @code{AC_F77_WRAPPERS}).  @var{shellvar} is
optional; if it is not supplied, the shell variable will be 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

@node System Services, UNIX Variants, Fortran 77 Compiler, Existing Tests
@section System Services

The following macros check for operating system services or capabilities.

@defmac AC_PATH_X
@maindex PATH_X
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} on a
trivial @file{Imakefile} and examining the @file{Makefile} that it
produces.  If that fails (such as if @code{xmkmf} is not present), look
for them 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

@defmac AC_PATH_XTRA
@maindex 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

@defmac AC_SYS_INTERPRETER
@maindex SYS_INTERPRETER
Check whether the system supports starting scripts with a line of the
form @samp{#! /bin/csh} to select the interpreter to use for the script.
After running this macro, shell code in @code{configure.ac} can check
the shell variable @code{interpval}; it will be set to @samp{yes}
if the system supports @samp{#!}, @samp{no} if not.
@end defmac

@defmac AC_SYS_LARGEFILE
@maindex SYS_LARGEFILE
@cvindex _FILE_OFFSET_BITS
@cvindex _LARGE_FILES
@ovindex CC
Arrange for
@href{http://www.sas.com/standards/large.file/x_open.20Mar96.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 longer than @code{long}, 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) X)}.
@end defmac

@defmac AC_SYS_LONG_FILE_NAMES
@maindex 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
@maindex SYS_POSIX_TERMIOS
@cindex POSIX termios headers
@cindex termios POSIX headers
Check to see if POSIX termios headers and functions are available on the
system.  If so, set the shell variable @code{am_cv_sys_posix_termios} to
@samp{yes}.  If not, set the variable to @samp{no}.
@end defmac

@node UNIX Variants,  , System Services, Existing Tests
@section UNIX Variants

The following macros check for certain operating systems that need
special treatment for some programs, due to exceptional oddities in
their header files or libraries.  These macros are warts; they will be
replaced by a more systematic approach, based on the functions they make
available or the environments they provide.

@defmac AC_AIX
@maindex AIX
@cvindex _ALL_SOURCE
If on AIX, define @code{_ALL_SOURCE}.  Allows the use of some @sc{bsd}
functions.  Should be called before any macros that run the C compiler.
@end defmac

@defmac AC_ISC_POSIX
@maindex ISC_POSIX
@cvindex _POSIX_SOURCE
@ovindex CC
If on a POSIXized ISC @sc{unix}, define @code{_POSIX_SOURCE} and add
@option{-posix} (for the @sc{gnu} C compiler) or @option{-Xp} (for other C
compilers) to output variable @code{CC}.  This allows the use of
@sc{posix} facilities.  Must be called after @code{AC_PROG_CC} and
before any other macros that run the C compiler.
@end defmac

@defmac AC_MINIX
@maindex MINIX
@cvindex _MINIX
@cvindex _POSIX_SOURCE
@cvindex _POSIX_1_SOURCE
If on Minix, define @code{_MINIX} and @code{_POSIX_SOURCE} and define
@code{_POSIX_1_SOURCE} to be 2.  This allows the use of @sc{posix}
facilities.  Should be called before any macros that run the C compiler.
@end defmac




@c ========================================================= Writing Tests

@node Writing Tests, Results, Existing Tests, Top
@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 C compiler system.  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 C
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
* Examining Declarations::      Detecting header files and declarations
* Examining Syntax::            Detecting language syntax features
* Examining Libraries::         Detecting functions and global variables
* Run Time::                    Testing for run-time features
* Systemology::                 A zoology of operating systems
* Multiple Cases::              Tests for several possible values
* Language Choice::             Selecting which language to use for testing
@end menu

@node Examining Declarations, Examining Syntax, Writing Tests, Writing Tests
@section Examining Declarations

The macro @code{AC_TRY_CPP} is used to check whether particular header
files exist.  You can check for one at a time, or more than one if you
need several header files to all exist for some purpose.

@defmac AC_TRY_CPP (@var{includes}, @ovar{action-if-true}, @ovar{action-if-false})
@maindex TRY_CPP
@var{includes} is C or C++ @code{#include} statements and declarations,
on which shell variable, back quote, and backslash substitutions are
performed.  (Actually, it can be any C program, but other statements are
probably not useful.)  If the preprocessor produces no error messages
while processing it, run shell commands @var{action-if-true}.  Otherwise
run shell commands @var{action-if-false}.

This macro uses @code{CPPFLAGS}, but not @code{CFLAGS}, because
@option{-g}, @option{-O}, etc. are not valid options to many C
preprocessors.
@end defmac

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 @samp{#include}s.

@defmac AC_EGREP_HEADER (@var{pattern}, @var{header-file}, @var{action-if-found}, @ovar{action-if-not-found})
@maindex EGREP_HEADER
If the output of running the preprocessor on the system header file
@var{header-file} matches the @code{egrep} regular expression
@var{pattern}, execute shell commands @var{action-if-found}, otherwise
execute @var{action-if-not-found}.
@end defmac

To check for C preprocessor symbols, either defined by header files or
predefined by the C preprocessor, use @code{AC_EGREP_CPP}.  Here is an
example of the latter:

@example
AC_EGREP_CPP(yes,
[#ifdef _AIX
  yes
#endif
], is_aix=yes, is_aix=no)
@end example

@defmac AC_EGREP_CPP (@var{pattern}, @var{program}, @ovar{action-if-found}, @ovar{action-if-not-found})
@maindex 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
@code{egrep} regular expression @var{pattern}, execute shell commands
@var{action-if-found}, otherwise execute @var{action-if-not-found}.

This macro calls @code{AC_PROG_CPP} or @code{AC_PROG_CXXCPP} (depending
on which language is current, @pxref{Language Choice}), if it hasn't
been called already.
@end defmac

@node Examining Syntax, Examining Libraries, Examining Declarations, Writing Tests
@section Examining Syntax

To check for a syntax feature of the C, C++ or Fortran 77 compiler, such
as whether it recognizes a certain keyword, use @code{AC_TRY_COMPILE} to
try to compile a small program that uses that feature.  You can also use
it to check for structures and structure members that are not present on
all systems.

@defmac AC_TRY_COMPILE (@var{includes}, @var{function-body}, @ovar{action-if-found}, @ovar{action-if-not-found})
@maindex TRY_COMPILE
Create a C, C++ or Fortran 77 test program (depending on which language
is current, @pxref{Language Choice}), to see whether a function whose
body consists of @var{function-body} can be compiled.

For C and C++, @var{includes} is any @code{#include} statements needed
by the code in @var{function-body} (@var{includes} will be ignored if
the currently selected language is Fortran 77).  This macro also uses
@code{CFLAGS} or @code{CXXFLAGS} if either C or C++ is the currently
selected language, as well as @code{CPPFLAGS}, when compiling.  If
Fortran 77 is the currently selected language then @code{FFLAGS} will be
used when compiling.

If the file compiles successfully, run shell commands
@var{action-if-found}, otherwise run @var{action-if-not-found}.

This macro does not try to link; use @code{AC_TRY_LINK} if you need to
do that (@pxref{Examining Libraries}).
@end defmac

@node Examining Libraries, Run Time, Examining Syntax, Writing Tests
@section Examining Libraries

To check for a library, a function, or a global variable, Autoconf
@code{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.

A few systems have linkers that do not return a failure exit status when
there are unresolved functions in the link.  This bug makes the
configuration scripts produced by Autoconf unusable on those systems.
However, some of them can be given options that make the exit status
correct.  This is a problem that Autoconf does not currently handle
automatically.  If users encounter this problem, they might be able to
solve it by setting @code{LDFLAGS} in the environment to pass whatever
options the linker needs (for example, @option{-Wl,-dn} on @sc{mips
risc/os}).

@code{AC_TRY_LINK} 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.

@defmac AC_TRY_LINK (@var{includes}, @var{function-body}, @ovar{action-if-found}, @ovar{action-if-not-found})
@maindex TRY_LINK
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.

For C and C++, @var{includes} is any @code{#include} statements needed
by the code in @var{function-body} (@var{includes} will be ignored if
the currently selected language is Fortran 77).  This macro also uses
@code{CFLAGS} or @code{CXXFLAGS} if either C or C++ is the currently
selected language, as well as @code{CPPFLAGS}, when compiling.  If
Fortran 77 is the currently selected language then @code{FFLAGS} will be
used when compiling.  However, both @code{LDFLAGS} and @code{LIBS} will
be used during linking in all cases.

If the file compiles and links successfully, run shell commands
@var{action-if-found}, otherwise run @var{action-if-not-found}.
@end defmac

@defmac AC_TRY_LINK_FUNC (@var{function}, @ovar{action-if-found}, @ovar{action-if-not-found})
@maindex TRY_LINK_FUNC
Depending on the current language (@pxref{Language Choice}), create a
test program to see whether a program whose body consists of
a prototype of and a call to @var{function} 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}.
@end defmac



@node Run Time, Systemology, Examining Libraries, Writing Tests
@section Checking Run Time Behavior

Sometimes you need to find out how a system performs at run time, 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 run-time behavior while configuring,
you can write a test program to determine the result, and compile and
run it using @code{AC_TRY_RUN}.  Avoid running test programs if
possible, because this prevents people from configuring your package for
cross-compiling.

@menu
* Test Programs::               Running test programs
* Guidelines::                  General rules for writing test programs
* Test Functions::              Avoiding pitfalls in test programs
@end menu

@node Test Programs, Guidelines, Run Time, Run Time
@subsection Running Test Programs

Use the following macro if you need to test run-time behavior of the
system while configuring.

@defmac AC_TRY_RUN (@var{program}, @ovar{action-if-true}, @ovar{action-if-false}, @ovar{action-if-cross-compiling})
@maindex TRY_RUN
@var{program} is the text of a C program, on which shell variable and
back quote substitutions are performed.  If it compiles and links
successfully and returns an exit status of 0 when executed, run shell
commands @var{action-if-true}.  Otherwise, run shell commands
@var{action-if-false}; the exit status of the program is available in
the shell variable @samp{$?}.  This macro uses @code{CFLAGS} or
@code{CXXFLAGS}, @code{CPPFLAGS}, @code{LDFLAGS}, and @code{LIBS} when
compiling.

If the C compiler being used does not produce executables that run on
the system where @code{configure} is being run, then the test program is
not run.  If the optional shell commands @var{action-if-cross-compiling}
are given, they are run instead.  Otherwise, @code{configure} prints
an error message and exits.
@end defmac

Try to provide a pessimistic default value to use when cross-compiling
makes run-time tests impossible.  You do this by passing the optional
last argument to @code{AC_TRY_RUN}.  @code{autoconf} prints a warning
message when creating @code{configure} each time it encounters a call to
@code{AC_TRY_RUN} with no @var{action-if-cross-compiling} argument
given.  You may ignore the warning, though users will not be able to
configure your package for cross-compiling.  A few of the macros
distributed with Autoconf produce this warning message.

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}).

To provide a default for calls of @code{AC_TRY_RUN} that are embedded in
other macros, including a few of the ones that come with Autoconf, you
can call @code{AC_PROG_CC} before running them.  Then, if the shell
variable @code{cross_compiling} is set to @samp{yes}, use an alternate
method to get the results instead of calling the macros.


@node Guidelines, Test Functions, Test Programs, Run Time
@subsection Guidelines for Test Programs

Test programs should not write anything to the standard output.  They
should return 0 if the test succeeds, nonzero 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.  Test programs should @code{exit}, not @code{return}, from
@code{main}, because on some systems (old Suns, at least) the argument
to @code{return} in @code{main} is ignored.

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_STDC}, then later on in
@file{configure.ac} you can have a test program that includes an
@sc{ansi} C header file conditionally:

@example
@group
#if STDC_HEADERS
# include <stdlib.h>
#endif
@end group
@end example

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
@code{configure} script cleans up by running @samp{rm -rf conftest*}
after running test programs and if the script is interrupted.

@node Test Functions,  , Guidelines, Run Time
@subsection Test Functions

Function declarations in test programs should have a prototype
conditionalized for C++.  In practice, though, test programs rarely need
functions that take arguments.

@example
#ifdef __cplusplus
foo (int i)
#else
foo (i) int i;
#endif
@end example

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" void *malloc (size_t);
#else
char *malloc ();
#endif
@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, make 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 Systemology, Multiple Cases, Run Time, Writing Tests
@section Systemology

This section aims at presenting some systems and pointers to
documentation.  It may help you addressing particular problems reported
by users.

@table @asis
@item @sc{qnx 4.25}
@cindex @sc{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.
@sc{qnx} is a realtime operating system running on Intel architecture
meant to be scalable from the small embedded systems to hundred
processor super-computer.  It claims to be @sc{posix} certified. More
information is available on the @href{www.qnx.com, @sc{qnx} home page},
including the @href{http://support.qnx.com/support/docs/qnx4/, @sc{qnx}
man pages}.
@end table


@node Multiple Cases, Language Choice, Systemology, Writing Tests
@section Multiple Cases

Some operations are accomplished in several possible ways, depending on
the @sc{unix} 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.

@example
@group
AC_MSG_CHECKING([how to get file system type])
fstype=no
# The order of these tests is important.
AC_TRY_CPP([#include <sys/statvfs.h>
#include <sys/fstyp.h>],
           [AC_DEFINE(FSTYPE_STATVFS) fstype=SVR4])
if test $fstype = no; then
  AC_TRY_CPP([#include <sys/statfs.h>
#include <sys/fstyp.h>],
             [AC_DEFINE(FSTYPE_USG_STATFS) fstype=SVR3])
fi
if test $fstype = no; then
  AC_TRY_CPP([#include <sys/statfs.h>
#include <sys/vmount.h>],
             [AC_DEFINE(FSTYPE_AIX_STATFS) fstype=AIX])
fi
# (more cases omitted here)
AC_MSG_RESULT([$fstype])
@end group
@end example

@node Language Choice,  , Multiple Cases, Writing Tests
@section Language Choice
@cindex Language

Autoconf-generated @code{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}.

@defmac AC_LANG (@var{language})
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.

@item C++
Do compilation tests using @code{CXX} and @code{CXXCPP} and use
extension @file{.C} for test programs.

@item Fortran 77
Do compilation tests using @code{F77} and use extension @file{.f} for
test programs.
@end table
@end defmac

@defmac AC_LANG_PUSH (@var{language})
@maindex 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})
@maindex 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 will detect inconsistencies.

@example
AC_LANG_PUSH(Fortran 77)
# Perform some tests on Fortran 77.
# ...
AC_LANG_POP(Fortran 77)
@end example
@end defmac

@defmac AC_REQUIRE_CPP
@maindex 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



@c ====================================================== Results of Tests.

@node Results, Programming in M4, Writing Tests, Top
@chapter Results of Tests

Once @code{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 @code{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
* Caching Results::             Speeding up subsequent @code{configure} runs
* Printing Messages::           Notifying @code{configure} users
@end menu

@node Defining Symbols, Setting Output Variables, Results, Results
@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
@code{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)])
if test "$ac_cv_func_vprintf" != yes; then
  AC_CHECK_FUNC(_doprnt, [AC_DEFINE(HAVE_DOPRNT)])
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}, @ovar{value}, @ovar{description})
@maindex DEFINE
Define C preprocessor variable @var{variable}.  If @var{value} is given,
set @var{variable} to that value (verbatim), otherwise set it to 1.
@var{value} should not contain literal newlines, and if you are not
using @code{AC_CONFIG_HEADERS} it should not contain any @samp{#}
characters, as @code{make} tends to eat them.  To use a shell variable
(which you need to do in order to define a value containing the M4 quote
characters @samp{[} or @samp{]}), 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")
@end example
@end defmac

@defmac AC_DEFINE_UNQUOTED (@var{variable}, @ovar{value}, @ovar{description})
@maindex DEFINE_UNQUOTED
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{\}).  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")
AC_DEFINE_UNQUOTED(GETGROUPS_T, $ac_cv_type_getgroups)
AC_DEFINE_UNQUOTED($ac_tr_hdr)
@end example
@end defmac

Due to the 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 @code{configure} script.  Use either spaces or
newlines.  That is, do this:

@example
AC_CHECK_HEADER(elf.h, [AC_DEFINE(SVR4) LIBS="$LIBS -lelf"])
@end example

@noindent
or this:

@example
AC_CHECK_HEADER(elf.h,
 [AC_DEFINE(SVR4)
  LIBS="$LIBS -lelf"])
@end example

@noindent
instead of this:

@example
AC_CHECK_HEADER(elf.h, [AC_DEFINE(SVR4); LIBS="$LIBS -lelf"])
@end example

@node Setting Output Variables, Caching Results, Defining Symbols, Results
@section Setting 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 @code{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})
@maindex 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 @file{Makefile}s).  This means that @code{AC_OUTPUT} will
replace instances of @samp{@@@var{variable}@@} in input files with the
value that the shell variable @var{variable} has when @code{AC_OUTPUT}
is called.  This value of @var{variable} should not contain literal
newlines.

If @var{value} is given, in addition assign it to @samp{variable}.
@end defmac

@defmac AC_SUBST_FILE (@var{variable})
@maindex 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} will replace 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 macro is useful for inserting @file{Makefile} fragments containing
special dependencies or other @code{make} directives for particular host
or target types into @file{Makefile}s.  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
@end defmac

@cindex Previous Variable
@cindex Variable, Precious
Running @command{configure} in different 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 will 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.

Such variables are named @dfn{precious variables}, and can be declared
as such by @code{AC_ARG_VAR}.

@defmac AC_ARG_VAR (@var{variable}, @var{description})
@maindex 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 @code{AC_SUBST}'d.

@item
@var{variable} is kept in the cache including if it was not specified on
the @samp{./configure} command line.  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.

@item
@var{variable} is checked for consistency between two
@command{configure} runs.  For instance:

@example
$ ./configure --silent --config-cache
$ 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.


@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
$ CC=/usr/bin/cc ./configure undeclared_var=raboof --silent
$ ./config.status --recheck
running /bin/sh ./configure undeclared_var=raboof --silent \
  CC=/usr/bin/cc  --no-create --no-recursion
@end example
@end itemize
@end defmac


@node Caching Results, Printing Messages, Setting Output Variables, Results
@section Caching Results
@cindex Cache

To avoid checking for the same features repeatedly in various
@code{configure} scripts (or in repeated runs of one script),
@code{configure} can optionally save the results of many checks in a
@dfn{cache file} (@pxref{Cache Files}).  If a @code{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, @code{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})
@maindex 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 @code{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 will be
saved in the cache file just before @code{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})
@maindex 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 very 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
@group
AC_DEFUN([AC_SHELL_TRUE],
[AC_CACHE_CHECK([whether true(1) works], [ac_cv_shell_true_works],
                [ac_cv_shell_true_works=no
                 true && ac_cv_shell_true_works=yes
                 if test $ac_cv_shell_true_works = yes; 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
@group
AC_DEFUN([AC_SHELL_TRUE],
[AC_CACHE_CHECK([whether true(1) works], [ac_cv_shell_true_works],
                [ac_cv_shell_true_works=no
                 true && ac_cv_shell_true_works=yes])
 if test $ac_cv_shell_true_works = yes; 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 @code{configure} uses for caching
* Cache Checkpointing::         Loading and saving the cache file
@end menu

@node Cache Variable Names, Cache Files, Caching Results, Caching Results
@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 will be boolean (@samp{yes} or @samp{no}) or the
names of files or functions; so this is not an important restriction.

@node Cache Files, Cache Checkpointing, Cache Variable Names, Caching Results
@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.

By default, @code{configure} uses no cache file (technically, it uses
@option{--cache-file=/dev/null}), to avoid problems caused by accidental
use of stale cache files.

To enable caching, @code{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 @code{configure} calls @code{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
@code{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 will gradually accumulate information whenever someone runs a new
@code{configure} script.  (Running @code{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.

@node Cache Checkpointing,  , Cache Files, Caching Results
@subsection Cache Checkpointing

If your configure script, or a macro called from 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 will
reduce the amount of time it takes to re-run 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
@maindex 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
@maindex 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 configure.ac.
@end defmac

For instance:

@example
@r{ @dots{} AC_INIT, etc. @dots{}}
@group
# Checks for programs.
AC_PROG_CC
AC_PROG_GCC_TRADITIONAL
@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...
AM_PATH_GTK(1.0.2,, (exit 1); exit)
AM_PATH_GTKMM(0.9.5,, (exit 1); exit)
@end group
@r{ @dots{} AC_OUTPUT, etc. @dots{}}
@end example

@node Printing Messages,  , Caching Results, Results
@section Printing Messages
@cindex Messages, from @code{configure}

@code{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 @code{echo} shell command.
@code{configure} scripts should rarely need to run @code{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 of the callers will change
automatically.

To diagnose static issues, i.e., when @code{autoconf} is run, see
@ref{Reporting Messages}.

@defmac AC_MSG_CHECKING (@var{feature-description})
@maindex MSG_CHECKING
Notify the user that @code{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 @code{configure} is run with the
@option{--quiet} or @option{--silent} option.
@end defmac

@defmac AC_MSG_RESULT (@var{result-description})
@maindex 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 @code{configure} is run with the
@option{--quiet} or @option{--silent} option.
@end defmac

@defmac AC_MSG_NOTICE (@var{message})
@maindex 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 @code{configure} is run with the
@option{--quiet} or @option{--silent} option.
@end defmac

@defmac AC_MSG_ERROR (@var{error-description}, @ovar{exit-status})
@maindex MSG_ERROR
Notify the user of an error that prevents @code{configure} from
completing.  This macro prints an error message to the standard error
output and exits @code{configure} with @var{exit-status} (1 by default).
@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_WARN (@var{problem-description})
@maindex MSG_WARN
Notify the @code{configure} user of a possible problem.  This macro
prints the message to the standard error output; @code{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, Writing Autoconf Macros, Results, Top
@chapter Programming in 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 are still experimental,
and their 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
* Programming in M4sugar::      Convenient pure M4 macros
@end menu

@node M4 Quotation, Programming in M4sugar, Programming in M4, Programming in M4
@section M4 Quotation
@cindex quotation

@c FIXME: Grmph, yet another quoting myth: quotation has *never*
@c prevented `expansion' of $1.  Unless it refers to the expansion
@c of the value of $1?  Anyway, we need a rewrite here@dots{}

The most common brokenness of 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
* Quotation and Nested Macros::  Macros calling macros
* Quadrigraphs::                Another way to escape special characters
* Quotation Rule Of Thumb::     One parenthesis, one quote
@end menu

@node Active Characters, One Macro Call, M4 Quotation, M4 Quotation
@subsection Active Characters

To fully understand where proper quotation is important, you first need
to know what are the special characters in Autoconf: @samp{#} introduces
a comment inside which no macro expansion is performed, @samp{,}
separates arguments, @samp{[} and @samp{]} are the quotes themselves,
and finally @samp{(} and @samp{)} (which @code{m4} tries to match by
pairs).

In order to understand the delicate case of macro calls, we first have
to present some obvious failures.  Below they are ``obvious-ified'',
although 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 will 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, Quotation and Nested Macros, Active Characters, M4 Quotation
@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, it is
equivalent to write:

@example
define(car, $1)
@end example

@noindent
But, while it is acceptable for a @file{configure.ac} to avoid unneeded
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 possible quotings: 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

The reader will easily understand the following examples:

@example
car(foo, bar)
@result{}foo
car([foo, bar])
@result{}foo, bar
car((foo, bar))
@result{}(foo, bar)
car([(foo], [bar)])
@result{}(foo
car([], [])
@result{}
car([[]], [[]])
@result{}[]
@end example

With this in mind, we can explore the cases where macros invoke
macros@dots{}


@node Quotation and Nested Macros, Quadrigraphs, One Macro Call, M4 Quotation
@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 @code{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
@code{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 arguments are literal strings,
if the user wants to use the results of expansions as arguments, she has
to use an @emph{unquoted} macro call:

@example
qar(active)
@result{}ACT
@end example

@noindent
where she wanted 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 @code{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 Quadrigraphs, Quotation Rule Of Thumb, Quotation and Nested Macros, M4 Quotation
@subsection Quadrigraphs
@cindex quadrigraphs
@cindex @samp{@@<:@@}
@cindex @samp{@@:>@@}
@cindex @samp{@@S|@@}
@cindex @samp{@@%:@@}

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.

You can work around this problem by using one of the following
@dfn{quadrigraphs}:

@table @samp
@item @@<:@@
@samp{[}
@item @@:>@@
@samp{]}
@item @@S|@@
@samp{$}
@item @@%:@@
@samp{#}
@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{[^@@<:@@]}, if properly quoted, will
appear as @samp{[^[]} in the @code{configure} script.


@node Quotation Rule Of Thumb,  , Quadrigraphs, M4 Quotation
@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 will note that these two examples are rigorously
equivalent, since @code{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

See @xref{Quadrigraphs}, for what to do if you run into a hopeless case
where quoting does not suffice.

When you create a @code{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 @code{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 @code{configure} script will contain
unexpanded macros.  The @code{autoconf} program checks for this problem
by doing @samp{grep AC_ configure}.


@node Programming in M4sugar,  , M4 Quotation, Programming in M4
@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''.

@menu
* Redefined M4 Macros::         M4 builtins changed in M4sugar
* Forbidden Patterns::          Catching unexpanded macros
@end menu

@node Redefined M4 Macros, Forbidden Patterns, Programming in M4sugar, Programming in M4sugar
@subsection Redefined M4 Macros

All the M4 native macros are moved in the @samp{m4_} pseudo-namespace,
e.g., M4sugar renames @code{define} as @code{m4_define} etc.  There is
one exception: @code{dnl} kept its original name, and no @code{m4_dnl}
is defined.

M4sugar redefines some M4 macros, and made them slightly incompatible
with their native equivalent.

@defmac m4_defn (@var{macro})
@msindex defn
Contrary to the M4 builtin, this macro fails if @var{macro} is not
defined.  See @code{m4_undefine}.
@end defmac

@defmac m4_undefine (@var{macro})
@msindex undefine
Contrary to the M4 builtin, this macro fails if @var{macro} is not
defined.  Use

@example
m4_ifdef([@var{macro}], [m4_undefine([@var{macro}])])
@end example

@noindent
to recover the behavior of the builtin.
@end defmac

@defmac m4_popdef (@var{macro})
@msindex defn
Contrary to the M4 builtin, this macro fails if @var{macro} is not
defined.  See @code{m4_undefine}.
@end defmac

@node Forbidden Patterns,  , Redefined M4 Macros, Programming in M4sugar
@subsection Forbidden Patterns

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 over quotation).

M4sugar forbids all the tokens matching @samp{^m4_} and @samp{^dnl$}.

@defmac m4_pattern_forbid (@var{pattern})
@msindex pattern_forbid
Declare 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 makes sense to
the author of this documentation, as @samp{#}-comments should document
the output, not the input, documented vy @samp{dnl}-comments).
@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

@c=================================================== Writing Autoconf Macros.

@node Writing Autoconf Macros, Portable Shell, Programming in M4, Top
@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 @code{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, Macro Names, Writing Autoconf Macros, Writing Autoconf Macros
@section Macro Definitions

@maindex DEFUN
Autoconf macros are defined using the @code{AC_DEFUN} macro, which is
similar to the M4 builtin @code{define} macro.  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 (@pxref{Prerequisite Macros}).

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.

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])
# --------------------------------------
define([AC_MSG_ERROR],
[@{ _AC_ECHO([configure: error: $1], 2); exit m4_default([$2], 1); @}])
@end example

Comments about the macro should be left in the header comment.  Most
other comments will make their way into @file{configure}, so just keep
using @samp{#} to introduce comments.

@cindex @code{dnl}
If you have some very 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 @code{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}.


@node Macro Names, Reporting Messages, Macro Definitions, Writing Autoconf Macros
@section Macro Names

All of the Autoconf macros have all-uppercase names starting with
@samp{AC_} to prevent them from accidentally conflicting with other
text.  All shell variables that they use for internal purposes have
mostly-lowercase names starting with @samp{ac_}.  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.

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 @samp{AC_} usually tells the category
of 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
@sc{unix} group owners of files.
@item HEADER
Header files.
@item LIB
C libraries.
@item PATH
The full path names to files, including programs.
@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_FUNC_UTIME_NULL} checks the
behavior of the @code{utime} function when called with a @code{NULL}
pointer.

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, Dependencies Between Macros, Macro Names, Writing Autoconf Macros
@section Reporting Messages
@cindex Messages, from @code{autoconf}

When macros statically diagnose abnormal situations, benign or fatal,
they should report them using these macros.  For dynamic issues, i.e.,
when @code{configure} is run, see @ref{Printing Messages}.

@defmac AC_DIAGNOSE (@var{category}, @var{message})
@maindex DIAGNOSE
Report @var{message} as a warning (or as an error if requested by the
user) if it falls into the @var{category}.  You are encouraged to use
standard categories, which currently include:

@table @samp
@item all
messages that don't fall into one of the following category.  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

@defmac AC_WARNING (@var{message})
@maindex WARNING
Equivalent to @samp{AC_DIAGNOSE([syntax], @var{message})}, but you are
strongly encouraged to use a finer grained category.
@end defmac

@defmac AC_FATAL (@var{message})
@maindex FATAL
Report a severe error @var{message}, and have @code{autoconf} die.
@end defmac

When the user runs @samp{autoconf -W error}, warnings from
@code{AC_DIAGNOSE} and @code{AC_WARNING} are reported as error, see
@ref{autoconf Invocation}.

@node Dependencies Between Macros, Obsoleting Macros, Reporting Messages, Writing Autoconf Macros
@section 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
@end menu

@node Prerequisite Macros, Suggested Ordering, Dependencies Between Macros, Dependencies Between Macros
@subsection Prerequisite Macros

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})
@maindex 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 an @code{AC_DEFUN}'d macro; it
must not be called from the top level.
@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 will be expanded @emph{before} the body of the
former.  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 "$body_temparature_in_celsius" -gt "38" &&
  dance_floor=occupied])
AC_DEFUN([NEWTON_JOHN],
[test "$hair_style" = "curly" &&
  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
RESERVE_DANCE_FLOOR
if test "$dance_floor" = occupied; then
  AC_MSG_ERROR([cannot pick up here, let's move])
fi
@end example

@noindent
will 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 "$body_temperature_in_Celsius" -gt "38" &&
  dance_floor=occupied
test "$hair_style" = "curly" &&
  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


You are encouraged to put all @code{AC_REQUIRE}s at the beginning of a
macro.  You can use @code{dnl} to avoid the empty lines they leave.

@node Suggested Ordering,  , Prerequisite Macros, Dependencies Between Macros
@subsection Suggested Ordering

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
@code{configure} from @file{configure.ac}, not when running
@code{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})
@maindex BEFORE
Make @code{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 Obsoleting Macros, Coding Style, Dependencies Between Macros, Writing Autoconf Macros
@section Obsoleting Macros

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, @code{autoupdate} should substitute the old macro calls
with their modern implementation.

Autoconf provides a simple means to obsolete a macro.

@defmac AU_DEFUN (@var{old-macro}, @var{implementation}, @ovar{message})
@maindex DEFUN
@maindex AU_DEFUN
Define @var{old-macro} as @var{implementation}.  The only difference
with @code{AC_DEFUN} is that the user will be warned that
@var{old-macro} is now obsolete.

If she then uses @code{autoupdate}, the call to @var{old-macro} will be
replaced by the modern @var{implementation}.  The additional
@var{message} is then printed.
@end defmac

@node Coding Style,  , Obsoleting Macros, Writing Autoconf Macros
@section 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 via 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}s 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)@dots{}or better yet, high-level
macros such as @code{AC_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 spaces 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_TRY_RUN} 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
patsubst([$1], [$"])
@end example

@noindent
use

@example
patsubst([$1], [$""])
@end example

@noindent
so that Emacsen do not open a 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 unneeded.  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
purpose.  With the @code{s} command, the preferred separator is @samp{/}
unless @samp{/} itself is used in the command, in which case you should
use @samp{,}.

@xref{Macro Definitions}, for details on how to define a macro.  If a
macro doesn't use @code{AC_REQUIRE} and it is expected to never be the
object of an @code{AC_REQUIRE} directive, then use @code{define}.  In
case of doubt, use @code{AC_DEFUN}.  All the @code{AC_REQUIRE}
statements should be at the beginning of the macro, @code{dnl}'ed.

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
@code{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 "$ac_cv_emxos2" = yes && EMXOS2=yes])
@end example

@noindent
and the new way:

@example
# _AC_EMXOS2
# ----------
# Check for EMX on OS/2.
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 "$ac_cv_emxos2" = yes && EMXOS2=yes[]dnl
])# _AC_EMXOS2
@end example




@c ============================================= Portable Shell Programming

@node Portable Shell, Manual Configuration, Writing Autoconf Macros, Top
@chapter 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, but to prevent trouble, do not take
advantage of features that were added after @sc{unix} version 7, circa
1977.  You should not use shell functions, aliases, negated character
classes, or other features that are not found in all Bourne-compatible
shells; restrict yourself to the lowest common denominator.  Even
@code{unset} is not supported by all shells!  Also, include a space
after the exclamation point in interpreter specifications, like this:

@example
#! /usr/bin/perl
@end example

@noindent
If you omit the space before the path, then 4.2@sc{bsd} based systems
(such as Sequent DYNIX) will ignore the line, because they interpret
@samp{#! /} as a 4-byte magic number.

The set of external programs you should run in a @code{configure} script
is fairly small.  @xref{Utilities in Makefiles,, Utilities in
Makefiles, standards, 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}.

@menu
* Shellology::                  A zoology of shells
* Here-Documents::              Quirks and tricks
* File Descriptors::            FDs and redirections
* File System Conventions::     File- and pathnames
* Shell Substitutions::         Variable and command expansions
* Assignments::                 Varying side effects of assignments
* Special Shell Variables::     Variables you should not change
* Limitations of Builtins::     Portable use of not so portable /bin/sh
* Limitations of Usual Tools::  Portable use of portable tools
* Limitations of Make::         Portable Makefiles
@end menu

@node Shellology, Here-Documents, Portable Shell, Portable Shell
@section 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.

Below we describe some of the members of the Bourne shell family.

@table @asis
@item Ash
@cindex Ash
@command{ash} is often used on @sc{gnu}/Linux and @sc{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 workaround
them, since version 0.2 is still shipped with many @sc{gnu}/Linux
distributions.

To be compatible with Ash 0.2:

@itemize @minus
@item
don't use @samp{$?} after expanding empty or unset variables:

@example
foo=
false
$foo
echo "Don't 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 sub
shell, 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 @command{bash}, test if
@code{BASH_VERSION} is set.  To disable its extensions and require
@sc{posix} compatibility, run @samp{set -o posix}. @xref{Bash POSIX
Mode,, Bash @sc{posix} Mode, bash, The GNU Bash Reference Manual}, for
details.

@item @command{/usr/xpg4/bin/sh} on Solaris
@cindex @command{/usr/xpg4/bin/sh} on Solaris
The @sc{posix}-compliant Bourne shell on a Solaris system is
@command{/usr/xpg4/bin/sh} and is part of an extra optional package.
There is no extra charge for this package, but it is also not part of a
minimal OS install and therefore some folks may not have it.

@item Zsh
@cindex Zsh
To detect whether you are running @command{zsh}, test if
@code{ZSH_VERSION} is set.  By default @command{zsh} is @emph{not}
compatible with the Bourne shell: you have to run @samp{emulate sh} and
set @code{NULLCMD} to @samp{:}. @xref{Compatibility,, Compatibility,
zsh, The Z Shell Manual}, for details.

Zsh 3.0.8 is the native @command{/bin/sh} on Mac OS X 10.0.3.
@end table

The following discussion between Russ Allbery and Robert Lipe is worth
reading:

@noindent
Russ Allbery:

@quotation
The @sc{gnu} assumption that @command{/bin/sh} is the one and only shell
leads to a permanent deadlock.  Vendors don't want to break user's
existant shell scripts, and there are some corner cases in the Bourne
shell that are not completely compatible with a @sc{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
@sc{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 @sc{posix} shell.

So while most modern systems do have a shell _somewhere_ that meets the
@sc{posix} standard, the challenge is to find it.
@end quotation

@node Here-Documents, File Descriptors, Shellology, Portable Shell
@section 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{/bin/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{/bin/sh}

@example
@group
$ cat <<EOF
> \" \\
> EOF
" \
@end group
@end example

@noindent
and with Bash:

@example
@group
bash-2.04$ cat <<EOF
> \" \\
> EOF
\" \
@end group
@end example


Many older shells (including the Bourne shell) implement here-documents
inefficiently.  Users can generally speed things up by using a faster
shell, e.g., by using the command @samp{bash ./configure} rather than
plain @samp{./configure}.

Some shells can be extremely inefficient when there are a lot of
here-documents inside a single statement.  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 the shell can take 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 run-time, and we end up not
executing the macro at all.

@node File Descriptors, File System Conventions, Here-Documents, Portable Shell
@section File Descriptors

Some file descriptors shall not be used, since some systems, admittedly
arcane, use them for special purpose:

@table @asis
@item 3
some systems may open it to @samp{/dev/tty}.

@item 4
used on the Kubota Titan.
@end table

Don't redirect several times the same file descriptor, 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)
$ eval 'echo matter >fullness' >void
illegal io
$ eval '(echo matter >fullness)' >void
illegal io
$ (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.

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.

It is worth noting that Zsh (but not Ash nor Bash) makes it possible
in assignments though: @samp{foo=`cd /zorglub` 2>/dev/null}.

Most shells, if not all (including Bash, Zsh, Ash), output traces on
stderr, even for sub-shells.  This might result in undesired content
if you meant to capture the standard-error output of the inner command:

@example
$ ash -x -c '(eval "echo foo >&2") 2>stderr'
$ cat stderr
+ eval echo foo >&2
+ echo foo
foo
$ bash -x -c '(eval "echo foo >&2") 2>stderr'
$ cat stderr
+ eval 'echo foo >&2'
++ echo foo
foo
$ zsh -x -c '(eval "echo foo >&2") 2>stderr'
@i{# Traces on startup files deleted here.}
$ cat stderr
+zsh:1> eval echo foo >&2
+zsh:1> echo foo
foo
@end example

@noindent
You'll appreciate the various levels of detail@dots{}

One workaround is to grep out uninteresting lines, hoping not to remove
good ones@dots{}

@node File System Conventions, Shell Substitutions, File Descriptors, Portable Shell
@section File System Conventions

While @command{autoconf} and friends will usually be run on some Unix
variety, it can and will be used on other systems, most notably @sc{dos}
variants.  This impacts several assumptions regarding file and
path names.

@noindent
For example, the following code:

@example
case $foo_dir in
  /*) # Absolute
     ;;
  *)
     foo_dir=$dots$foo_dir ;;
esac
@end example

@noindent
will fail to properly detect absolute paths on those systems, because
they can use a drivespec, and will usually use a backslash as directory
separator.  The canonical way to check for absolute paths is:

@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{Limitations of 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, use
@code{$ac_path_separator} instead (or the @code{PATH_SEPARATOR} output
variable).  @command{autoconf} sets this to the appropriate value (@samp{:}
or @samp{;}) when it starts up.

File names need extra care as well.  While @sc{dos}-based environments
that are Unixy enough to run @command{autoconf} (such as DJGPP) will
usually be 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
@href{ftp://ftp.gnu.org/gnu/non-gnu/doschk/doschk-1.1.tar.gz, doschk}
package.

A short overview follows; problems are marked with @sc{sfn}/@sc{lfn} to
indicate where they apply: @sc{sfn} means the issues are only relevant to
plain @sc{dos}, not to @sc{dos} boxes under Windows, while @sc{lfn}
identifies problems that exist even under Windows.

@table @asis
@item No multiple dots (@sc{sfn})
@sc{dos} cannot handle multiple dots in filenames.  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 Unices:

@example
AC_CONFIG_HEADER(config.h)
AC_CONFIG_FILES([source.c foo.bar])
AC_OUTPUT
@end example

@noindent
but it causes problems on @sc{dos}, as it requires @samp{config.h.in},
@samp{source.c.in} and @samp{foo.bar.in}.  To make your package more portable
to @sc{dos}-based environments, you should use this instead:

@example
AC_CONFIG_HEADER(config.h:config.hin)
AC_CONFIG_FILES([source.c:source.cin foo.bar:foobar.in])
AC_OUTPUT
@end example

@item No leading dot (@sc{sfn})
@sc{dos} cannot handle filenames that start with a dot.  This is usually
not a very important issue for @command{autoconf}.

@item Case insensitivity (@sc{lfn})
@sc{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, @command{make install} will do nothing (unless the
@samp{install} target is marked as PHONY).

@item The 8+3 limit (@sc{sfn})
Because the @sc{dos} file system only stores the first 8 characters of
the filename 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 filename
(@file{FOOBAR-P.C}).  The same goes for @file{foo.bar} and
@file{foo.bartender}.

Note: This is not usually a problem under Windows, as it uses numeric
tails in the short version of filenames to make them unique.  However, a
registry setting can turn this behaviour off.  While this makes it
possible to share file trees containing long file names between @sc{sfn}
and @sc{lfn} environments, it also means the above problem applies there
as well.

@item Invalid characters
Some characters are invalid in @sc{dos} filenames, and should therefore
be avoided. In a @sc{lfn} environment, these are @samp{/}, @samp{\},
@samp{?}, @samp{*}, @samp{:}, @samp{<}, @samp{>}, @samp{|} and @samp{"}.
In a @sc{sfn} environment, other characters are also invalid.  These
include @samp{+}, @samp{,}, @samp{[} and @samp{]}.
@end table

@node Shell Substitutions, Assignments, File System Conventions, Portable Shell
@section Shell Substitutions

Contrary to a persistent urban legend, the Bourne shell does not
systematically split variables and backquoted 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{}`"}.
Worse yet, not all shells understand @code{"`@dots{}\"@dots{}\"@dots{}`"}
the same way.  There is just no portable way to use double-quoted
strings inside double-quoted backquoted expressions (pfew!).

@table @code
@item $@@
@cindex @samp{"$@@"}
One of the most famous shell-portability issues is related to
@samp{"$@@"}: when there are no positional arguments, it is supposed to
be equivalent to nothing.  But some shells, for instance under Digital
Unix 4.0 and 5.0, will then replace it with an empty argument.  To be
portable, use @samp{$@{1+"$@@"@}}.

@item $@{@var{var}:-@var{value}@}
@cindex $@{@var{var}:-@var{value}@}
Old @sc{bsd} shells, including the Ultrix @code{sh}, don't accept the
colon for any shell substitution, and complain and die.

@item $@{@var{var}=@var{literal}@}
@cindex $@{@var{var}=@var{literal}@}
Be sure to quote:

@example
: $@{var='Some words'@}
@end example

@noindent
otherwise some shells, such as on Digital Unix V 5.0, will die because
of a ``bad substitution''.

Solaris' @command{/bin/sh} has a frightening bug in its interpretation
of this.  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
$ unset foo
$ foo=$@{foo='@}'@}
$ echo $foo
@}
$ foo=$@{foo='@}'   # no error; this hints to what the bug is
$ echo $foo
@}
$ foo=$@{foo='@}'@}
$ 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.

@item $@{@var{var}=@var{expanded-value}@}
@cindex $@{@var{var}=@var{expanded-value}@}
On Ultrix,
running

@example
default="yu,yaa"
: $@{var="$default"@}
@end example

@noindent
will set @var{var} to @samp{M-yM-uM-,M-yM-aM-a}, i.e., the 8th bit of
each char will be set. You won'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
$ cat -v <<EOF
$var
EOF
@end example

@noindent
and

@example
$ 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 @sc{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{commands}`
@cindex `@var{commands}`
@cindex Command Substitution
While in general it makes no sense, do not substitute a single builtin
with side effects as Ash 0.2, trying to optimize, does not fork a
sub-shell 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
$ pwd
/tmp
$ test -n "`cd /`" && pwd
/
@end example

@noindent
The result of @samp{foo=`exit 1`} is left as an exercise to the reader.


@item $(@var{commands})
@cindex $(@var{commands})
This construct is meant to replace @samp{`@var{commands}`}; they can be
nested while this is impossible to do portably with back quotes.
Unfortunately it is not yet widely supported.  Most notably, even recent
releases of Solaris don't support it:

@example
$ showrev -c /bin/sh | grep version
Command version: SunOS 5.8 Generic 109324-02 February 2001
$ echo $(echo blah)
syntax error: `(' unexpected
@end example

@noindent
nor does @sc{irix} 6.5's Bourne shell:
@example
$ uname -a
IRIX firebird-image 6.5 07151432 IP22
$ echo $(echo blah)
$(echo blah)
@end example
@end table


@node Assignments, Special Shell Variables, Shell Substitutions, Portable Shell
@section 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 sh on Solaris, 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 exit status of an assignment: Ash 0.2 does not change
the status and propagates that of the last statement:

@example
$ false || foo=bar; echo $?
1
$ false || foo=`:`; echo $?
0
@end example

@noindent
and to make things even worse, @sc{qnx 4.25} just sets the exit status
to 0 in any case:

@example
$ 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 will never 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 will 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 "$@{var+set@}" = set || var='$@{indirection@}'
@end example
@end enumerate

In most cases @samp{var=$@{var="$default"@}} is fine, but in case of
doubt, just use the latter.  @xref{Shell Substitutions}, items
@samp{$@{@var{var}:-@var{value}@}} and @samp{$@{@var{var}=@var{value}@}}
for the rationale.


@node Special Shell Variables, Limitations of Builtins, Assignments, Portable Shell
@section 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, but
@command{unset} is not portable (@pxref{Limitations of Builtins}) and a
fallback value is needed.  We list these values below.

@c Alphabetical order, case insensitive, `A' before `a'.
@table @code
@item CDPATH
@evindex CDPATH
When this variable is set @code{cd} is verbose, so idioms such as
@samp{abs=`cd $rel && pwd`} break because @code{abs} receives the path
twice.

@c FIXME: Which shells?  How do they behave?
Setting @code{CDPATH} to the empty value is not enough for most shells.
A simple colon is enough except for @code{zsh}, which prefers a leading
dot:

@example
zsh-3.1.6 % mkdir foo && (CDPATH=: cd foo)
/tmp/foo
zsh-3.1.6 % (CDPATH=:. cd foo)
/tmp/foo
zsh-3.1.6 % (CDPATH=.: cd foo)
zsh-3.1.6 %
@end example

@noindent
(of course we could just @code{unset} @code{CDPATH}, since it also
behaves properly if set to the empty string).

Life wouldn't be so much fun if @command{bash} and @command{zsh} had the
same behavior:

@example
bash-2.02 % (CDPATH=:. cd foo)
bash-2.02 % (CDPATH=.: cd foo)
/tmp/foo
@end example

Therefore, a portable solution to neutralize @samp{CDPATH} is

@example
CDPATH=$@{ZSH_VERSION+.@}:
@end example

@noindent
Note that since @command{zsh} supports @command{unset}, you may unset
@samp{CDPATH} using @samp{:} as a fallback, see
@ref{Limitations of Builtins}.

@item IFS
@evindex IFS
Don't set the first character of @code{IFS} to backslash.  Indeed,
Bourne shells use the first character (backslash) when joining the
components in @samp{"$@@"} and some shells then re-interpret (!) the
backslash escapes, so you can end up with backspace and other strange
characters.

@item LANG
@itemx LC_ALL
@itemx LC_TIME
@itemx LC_CTYPE
@itemx LANGUAGE
@itemx LC_COLLATE
@itemx LC_NUMERIC
@itemx LC_MESSAGES
@evindex LANG
@evindex LC_ALL
@evindex LC_TIME
@evindex LC_CTYPE
@evindex LANGUAGE
@evindex LC_COLLATE
@evindex LC_NUMERIC
@evindex LC_MESSAGES

These must not be set unconditionally because not all systems understand
e.g. @samp{LANG=C} (notably SCO).  Fixing @env{LC_MESSAGES} prevents
Solaris @command{sh} from translating var values in @code{set}!  Non-C
@env{LC_CTYPE} values break the ctype check.  Fixing @env{LC_COLLATE}
makes scripts more portable in some cases.  For example, it causes the
regular expression @samp{[a-z]} to match only lower-case letters on
@sc{ascii} platforms.  However, @samp{[a-z]} does not work in general
even when @env{LC_COLLATE} is fixed; for example, it does not work for
@sc{ebcdic} platforms.  For maximum portability, you should use regular
expressions like @samp{[abcdefghijklmnopqrstuvwxyz]} that list
characters explicitly instead of relying on ranges.

@emph{If} one of these variables is set, you should try to unset it,
using @samp{C} as a fall back value. see @ref{Limitations of Builtins},
builtin @command{unset}, for more details.

@item NULLCMD
@evindex NULLCMD
When executing the command @samp{>foo}, @command{zsh} executes
@samp{$NULLCMD >foo}.  The Bourne shell considers @code{NULLCMD} is
@samp{:}, while @command{zsh}, even in Bourne shell compatibility mode,
sets @code{NULLCMD} to @samp{cat}.  If you forgot to set @code{NULLCMD},
your script might be suspended waiting for data on its standard input.

@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.

@item PATH_SEPARATOR
@evindex PATH_SEPARATOR
On DJGPP systems, the @code{PATH_SEPARATOR} variable can be set to
either @samp{:} or @samp{;} to control the path separator @command{bash}
uses to set up certain environment variables (such as
@code{PATH}). Since this only works inside bash, you want autoconf to
detect the regular @sc{dos} path separator @samp{;}, so it can be safely
substituted in files that may not support @samp{;} as path separator. So
either unset this variable or set it to @samp{;}.

@item RANDOM
@evindex RANDOM
Many shells provide @code{RANDOM}, a variable that returns a different
integer when used.  Most of the time, its value does not change when it
is not used, but on @sc{irix 6.5} the value changes all the time.  This
can be observed by using @command{set}.
@end table


@node Limitations of Builtins, Limitations of Usual Tools, Special Shell Variables, Portable Shell
@section Limitations of Shell Builtins

No, no, we are serious: some shells do have limitations! :)

You should always keep in mind that any built-in or command may support
options, and therefore have a very different behavior with arguments
starting with a dash.  For instance, 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.

@table @asis
@item @command{!}
@cindex @command{!}
You can't use @command{!}, you'll have to rewrite your code.


@item @command{break}
@c ------------------
@cindex @command{break}
The use of @samp{break 2}, etcetera, is safe.


@item @command{case}
@c -----------------
@cindex @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.

Because of a bug in its @code{fnmatch}, @command{bash} fails to properly
handle backslashes in character classes:

@example
bash-2.02$ 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 @sc{unix} or @sc{ms-dos} absolute paths.  To work around this
bug, always put the backslash first:

@example
bash-2.02$ case '\TMP' in [\\/]*) echo OK;; esac
OK
bash-2.02$ case /tmp in [\\/]*) echo OK;; esac
OK
@end example


@item @command{echo}
@c -----------------
@cindex @command{echo}
The simple @code{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.  New applications
which are not aiming at portability should use @samp{printf} instead of
@samp{echo}.

Don't expect any option.  @xref{Preset Output Variables}, @code{ECHO_N}
etc. for a means to simulate @option{-c}.

Do not use backslashes in the arguments, as there is no consensus on
their handling.  On @samp{echo '\n' | wc -l}, the @command{sh} of
Digital Unix 4.0, @sc{mips risc/os} 4.52, answer 2, but the Solaris'
@command{sh}, Bash and Zsh (in @command{sh} emulation mode) report 1.
Please note that the problem is truly @command{echo}: all the shells
understand @samp{'\n'} as the string composed of a backslash and an
@samp{n}.

Because of these problems, do not pass a string containing arbitrary
characters to @command{echo}.  For example, @samp{echo "$foo"} is safe
if you know that @var{foo}'s value cannot contain backslashes and cannot
start with @samp{-}, but otherwise you should use a here-document like
this:

@example
cat <<EOF
$foo
EOF
@end example


@item @command{exit}
@c -----------------
@cindex @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$ foo=`exit 1` || echo fail
fail
bash-2.04$ foo=`(exit 1)` || echo fail
fail
bash-2.04$ 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 8 @command{sh}, an exit
trap ignores the @code{exit} command's status.  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.


@item @command{export}
@c -------------------
@cindex @command{export}
The builtin @command{export} dubs @dfn{environment variable} a shell
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 2.5, IRIX 6.3, IRIX 5.2, AIX 4.1.5
and DU 4.0, forget to @command{export} the environment variables they
receive.  As a result, two variables are coexisting: 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 will print
alternately @samp{foo} and @samp{bar}, although it should only print
@samp{foo} and then a sequence of @samp{bar}s.

Therefore you should @command{export} again each environment variable
that you update.


@item @command{false}
@c ------------------
@cindex @command{false}
Don't expect @command{false} to exit with status 1: in the native Bourne
shell of Solaris 8, it exits with status 255.


@item @command{for}
@c ----------------
@cindex @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

@item @command{if}
@c ---------------
@cindex @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

There are shells that do not reset the exit status from an @command{if}:

@example
$ if (exit 42); then true; fi; echo $?
42
@end example

@noindent
whereas a proper shell should have printed @samp{0}.  This is especially
bad in Makefiles since it produces false failures.  This is why properly
written Makefiles, such as Automake's, have such hairy constructs:

@example
if test -f "$file"; then
  install "$file" "$dest"
else
  :
fi
@end example


@item @command{set}
@c ----------------
@cindex @command{set}
This 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 parameters,
even @samp{-x} for instance), but most shells simply stop the 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

@item @command{shift}
@c ------------------
@cindex @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 @sc{mips
risc/os} 4.52 refuses to do it.

@item @command{test}
@c -----------------
@cindex @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.

If you need to make multiple checks using @code{test}, combine them with
the shell operators @samp{&&} and @samp{||} instead of using the
@code{test} operators @option{-a} and @option{-o}.  On System V, the
precedence of @option{-a} and @option{-o} is wrong relative to the unary
operators; consequently, @sc{posix} does not specify them, so using them
is nonportable.  If you combine @samp{&&} and @samp{||} in the same
statement, keep in mind that they have equal precedence.

You may use @samp{!} with @command{test}, but not with @command{if}:
@samp{test ! -r foo || exit 1}.

@item @command{test} (files)
@c -------------------------
To enable @code{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} or
@samp{test -r}.  Do not use @samp{test -x}, because @sc{4.3bsd} does not
have it.  Do not use @samp{test -e} either, because Solaris 2.5 does not
have it.

@item @command{test} (strings)
@c ---------------------------
Avoid @samp{test "@var{string}"}, in particular if @var{string} might
start with a dash, since @code{test} might interpret its argument as an
option (e.g., @samp{@var{string} = "-n"}).

Contrary to a common belief, @samp{test -n @var{string}} and @samp{test
-z @var{string}} @strong{are} portable, nevertheless many shells (such
as Solaris 2.5, AIX 3.2, UNICOS 10.0.0.6, Digital Unix 4 etc.) have
bizarre precedence and may be confused if @var{string} looks like an
operator:

@example
$ test -n =
test: argument expected
@end example

If there are risks, use @samp{test "x@var{string}" = x} or @samp{test
"x@var{string}" != x} instead.

It is frequent 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 always be avoided by using:

@example
echo "$ac_feature" | grep '[^-a-zA-Z0-9_]' >/dev/null 2>&1 &&
  @var{action}
@end example

@noindent
Use @code{case} where possible since it is faster, being a shell builtin:


@example
case $ac_feature in
  *[!-a-zA-Z0-9_]*) @var{action};;
esac
@end example

Alas, negated character classes are probably not portable, although no
shell is known to not support the @sc{posix.2} syntax @samp{[!@dots{}]}
(when in interactive mode, @command{zsh} is confused by the
@samp{[!@dots{}]} syntax and looks for an event in its history because of
@samp{!}).  Many shells do not support the alternative syntax
@samp{[^@dots{}]} (Solaris, Digital Unix, etc.).

One solution can be:

@example
expr "$ac_feature" : '.*[^-a-zA-Z0-9_]' >/dev/null &&
  @var{action}
@end example

@noindent
or better yet

@example
expr "x$ac_feature" : '.*[^-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.


@item @command{trap}
@c -----------------
@cindex @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).

Although @sc{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 8 @command{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
$ cat trap.sh
trap 'echo $?' 0
(exit 42); exit 0
$ zsh trap.sh
42
$ 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.

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
$ trap 'false

echo $?' 0
$ exit
0
@end example

@noindent
Fortunately, this bug only affects @command{trap}.

@item @command{true}
@c -----------------
@cindex @command{true}
@cindex @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


@item @command{unset}
@c ------------------
@cindex @command{unset}
You cannot assume the support of @command{unset}, nevertheless, because
it is extremely useful to disable embarrassing variables such as
@code{CDPATH} or @code{LANG}, you can test for its existence and use
it @emph{provided} you give a neutralizing value when @command{unset} is
not supported:

@example
if (unset FOO) >/dev/null 2>&1; then
  unset=unset
else
  unset=false
fi
$unset CDPATH || CDPATH=:
@end example

@xref{Special Shell Variables}, for some neutralizing values. Also, see
@ref{Limitations of Builtins}, documentation of @command{export}, for
the case of environment variables.
@end table

@node Limitations of Usual Tools, Limitations of Make, Limitations of Builtins, Portable Shell
@section 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.

@table @asis
@item @command{awk}
@c ----------------
@cindex @command{awk}
Don't leave white spaces before the parentheses in user functions calls,
@sc{gnu} awk will reject it:

@example
$ gawk 'function die () @{ print "Aaaaarg!"  @}
        BEGIN @{ die () @}'
gawk: cmd. line:2:         BEGIN @{ die () @}
gawk: cmd. line:2:                      ^ parse error
$ gawk 'function die () @{ print "Aaaaarg!"  @}
        BEGIN @{ die() @}'
Aaaaarg!
@end example

If you want your program to be deterministic, don't depend on @code{for}
on arrays:

@example
$ cat for.awk
END @{
  arr["foo"] = 1
  arr["bar"] = 1
  for (i in arr)
    print i
@}
$ gawk -f for.awk </dev/null
foo
bar
$ nawk -f for.awk </dev/null
bar
foo
@end example


@item @command{cat}
@c ----------------
@cindex @command{cat}
Don't rely on any option.  The option @option{-v}, which displays
non-printing characters, @emph{seems} portable, though.


@item @command{cmp}
@c ----------------
@cindex @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 ---------------
@cindex @command{cp}
@c This is thanks to Ian.
SunOS @command{cp} does not support @option{-f}, although its
@command{mv} does.  It's possible to deduce why @command{mv} and
@command{cp} are different with respect to @option{-f}.  @command{mv}
prompts by default before overwriting a read-only file.  @command{cp}
does not.  Therefore, @command{mv} requires a @option{-f} option, but
@command{cp} does not.  @command{mv} and @command{cp} behave differently
with respect to read-only files because the simplest form of
@command{cp} cannot overwrite a read-only file, but the simplest form of
@command{mv} can.  This is because @command{cp} opens the target for
write access, whereas @command{mv} simply calls @code{link} (or, in
newer systems, @code{rename}).
@c Ian said: ``I don't think -p or -r are portable''!!! How can you live
@c without -r???


@item @command{diff}
@c -----------------
@cindex @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 --------------------
@cindex @command{dirname}
Not all hosts have @command{dirname}, but it is reasonably easy to
emulate, e.g.:

@example
dir=`expr "x$file" : 'x\(.*\)/[^/]*' \|
          '.'      : '.'
@end example

@noindent
But there are a few subtilities, e.g., under UN*X, should @samp{//1}
give @samp{/}?  Paul Eggert answers:

@quotation
No, under some older flavors of Unix, leading @samp{//} is a special
path name: it refers to a ``super-root'' and is used to access other
machines' files.  Leading @samp{///}, @samp{////}, etc. are equivalent
to @samp{/}; but leading @samp{//} is special.  I think this tradition
started with Apollo Domain/OS, an OS that is still in use on some older
hosts.

POSIX.2 allows but does not require the special treatment for @samp{//}.
It says that the behavior of dirname on path names of the form
@samp{//([^/]+/*)?}  is implementation defined.  In these cases, GNU
@command{dirname} returns @samp{/}, but it's more portable to return
@samp{//} as this works even on those older flavors of Unix.

I have heard rumors that this special treatment of @samp{//} may be
dropped in future versions of POSIX, but for now it's still the
standard.
@end quotation


@item @command{egrep}
@c ------------------
@cindex @command{egrep}
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}.


@item @command{expr}
@c -----------------
@cindex @command{expr}
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{|})
@cindex @command{expr} (@samp{|})
You can use @samp{|}.  Although @sc{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

@sc{gnu}/Linux and @sc{posix.2-1992} return the empty string for this
case, but traditional Unix returns @samp{0} (Solaris is one such
example).  In the latest @sc{posix} draft, the specification has been
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 ----------------------------
@cindex @command{expr}
Don't use @samp{\?}, @samp{\+} and @samp{\|} in patterns, they are
not supported on Solaris.

The @sc{posix.2-1992} standard is ambiguous as to whether @samp{expr a :
b} (and @samp{expr 'a' : '\(b\)'}) output @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 @sc{qnx} 4.25 native
@command{expr} returns @samp{0}.

You may believe that one means 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
@samp{@command{expr} (@samp{:})} entry for more information.

Older @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.

Don't leave, there is some more!

The @sc{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
$ val=`expr 'a' : 'a'`; echo "$?: $val"
0: 1
$ val=`expr 'a' : 'b'`; echo "$?: $val"
1: 0

$ val=`expr 'a' : '\(a\)'`; echo "?: $val"
1: a
$ 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
$ expr 'a' : '\(a\)' || echo 'a' | sed 's/^\(a\)$/\1/'
@end example

@noindent
will output @samp{a} on most hosts, but @samp{aa} on @sc{qnx} 4.25.  A
simple work around consists in testing @command{expr} and use a variable
set to @command{expr} or to @command{false} according to the result.


@item @command{find}
@c -----------------
The option @option{-maxdepth} seems to be GNU specific. Tru64 v5.1,
NetBSD 1.5 and Solaris 2.5 @command{find} commands do not understand it.


@item @command{grep}
@c -----------------
@cindex @command{grep}
Don't use @samp{grep -s} to suppress output, because @samp{grep -s} on
System V does not suppress output, only error messages.  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.

Don't use multiple regexps with @option{-e}, as some @code{grep} will only
honor the last pattern (eg., IRIX 6.5 and Solaris 2.5.1).  Anyway,
Stardent Vistra SVR4 @code{grep} lacks @option{-e}@dots{}  Instead, use
alternation and @code{egrep}.


@item @command{ln}
@c ---------------
@cindex @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} as a fall back.

For versions of the DJGPP before 2.04, @command{ln} emulates soft links
for executables by generating a stub that in turn calls the real
program.  This feature also works with nonexistent files like in the
Unix spec. So @samp{ln -s file link} will generate @file{link.exe},
which will attempt 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 symlink support.


@item @command{mv}
@c ---------------
@cindex @command{mv}
The only portable options are @option{-f} and @option{-i}.

Moving individual files between file systems is portable (it was in V6),
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.

Moving directories across mount points is not portable, use @command{cp}
and @command{rm}.


@item @command{sed}
@c ----------------
@cindex @command{sed}
Patterns should not include the separator (unless escaped), even as part
of a character class.  In conformance with @sc{posix}, the Cray
@command{sed} will reject @samp{s/[^/]*$//}: use @samp{s,[^/]*$,,}.

Sed scripts should not use branch labels longer than 8 characters and
should not contain comments.

Don't include extra @samp{;}, as some @command{sed}, such as NetBSD
1.4.2's, try to interpret the second as a command:

@example
$ echo a | sed 's/x/x/;;s/x/x/'
sed: 1: "s/x/x/;;s/x/x/": invalid command code ;
@end example

Input should have reasonably long lines, since some @command{sed} have
an input buffer limited to 4000 bytes.

Alternation, @samp{\|}, is common but not portable.
@c FIXME: I know Solaris is guilty, but I don't remember how.
Anchors (@samp{^} and @samp{$}) inside groups are not portable.

Nested groups are extremely portable, but there is at least one
@command{sed} (System V/68 Base Operating System R3V7.1) that does not
support it.

Of course the option @option{-e} is portable, but it is not needed.  No
valid Sed program can start with a dash, so it does not help
disambiguating.  Its sole usefulness is helping enforcing indenting as
in:

@example
sed -e @var{instruction-1} \
    -e @var{instruction-2}
@end example

@noindent
as opposed to

@example
sed @var{instruction-1};@var{instruction-2}
@end example

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''.


@item @command{sed} (@samp{t})
@c ---------------------------
@cindex @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 @sc{mips
risc/os}, and on @sc{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 1, a matches, therefore sets the t flag, b jumps to
d, and the output is produced.  When processing line 2, the t flag is
still set (this is the bug).  Line a fails to match, but @command{sed}
is not supposed to clear the t flag when a substitution fails.  Line 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 3 t
is clear, a matches, so the flag is set, hence b clears the flags and
jumps.  Finally, since the flag is clear, 4 is processed properly.

There are two things one should remind 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; : clear} to reset the t flag where
indeed.

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{touch}
@c ------------------
@cindex @command{touch}
On some old @sc{bsd} systems, @command{touch} or any command that
results in an empty file does not update the timestamps, so use a
command like @code{echo} as a workaround.

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 @sc{nfs}-mounted 4.2 volume.

@end table


@node Limitations of Make,  , Limitations of Usual Tools, Portable Shell
@section Limitations of Make

Make itself suffers a great number of limitations, only a few of which
being listed here.  First of all, remember that since commands are
executed by the shell, all its weaknesses are inherited@dots{}

@table @asis
@item @code{VPATH}
@cindex @code{VPATH}
Don't use it!  For instance any assignment to @code{VPATH} causes Sun
@command{make} to only execute the first set of double-colon rules.
@end table




@c ================================================== Manual Configuration

@node Manual Configuration, Site Configuration, Portable Shell, Top
@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
@code{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 Names::            Specifying the system type
* Canonicalizing::              Getting the canonical system type
* Using System Type::           What to do with the system type
@end menu

@node Specifying Names, Canonicalizing, Manual Configuration, Manual Configuration
@section Specifying the System Type

Like other @sc{gnu} @code{configure} scripts, Autoconf-generated
@code{configure} scripts can make decisions based on a canonical name
for the system type, 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}}

@code{configure} can usually guess the canonical name for the type of
system it's running on.  To do so it runs a script called
@code{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 @code{configure}.  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@footnote{For
backward compatibility, @code{configure} will accept a system type as an
option by itself.  Such an option will override the defaults for build,
host and target system types.  The following configure statement will
configure a cross toolchain that will run on NetBSD/alpha but generate
code for GNU Hurd/sparc, which is also the build platform.

@example
./configure --host=alpha-netbsd sparc-gnu
@end example
}:

@table @option
@item --build=@var{build-type}
the type of system on which the package is being configured and
compiled.

@item --host=@var{host-type}
@ovindex cross_compiling
the type of system on which the package will run.

@item --target=@var{target-type}
the type of system for which any compiler tools in the package will
produce code (rarely needed).  By default, it is the same as host.
@end table

They all default to the result of running @code{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, @code{configure} will enter cross compilation mode,
so it won't run any tests that require execution.

Hint: if you mean to override the result of @code{config.guess}, prefer
@option{--build} over @option{--host}.  In the future, @option{--host} will
not override the name of the build system type.  Also, if you specify
@option{--host}, but not @option{--build}, when @code{configure} performs
the first compiler test it will try to run an executable produced by the
compiler.  If the execution fails, it will enter cross-compilation mode.
Note, however, that it won't guess the build-system type, since this may
require running test programs.  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
@code{--host}, be sure to specify @code{--build} too.

@example
./configure --build=i686-pc-linux-gnu --host=m68k-coff
@end example

@noindent
will enter cross-compilation mode, but @code{configure} will fail 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

@code{configure} recognizes short aliases for many system types; for
example, @samp{decstation} can be used instead of
@samp{mips-dec-ultrix4.2}.  @code{configure} runs a script called
@code{config.sub} to canonicalize system type aliases.



@node Canonicalizing, Using System Type, Specifying Names, Manual Configuration
@section Getting the Canonical System Type

The following macros make the system type available to @code{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 @code{config.guess} and
@code{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 @code{configure} looks for those scripts.


@defmac AC_CANONICAL_BUILD
@maindex 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 @code{config.guess}.
@end defmac

@defmac AC_CANONICAL_HOST
@maindex 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}.

For temporary backward-compatibility, when @option{--host} is specified
by @option{--build} isn't, the build system will be assumed to be the
same as @option{--host}, and @samp{build_alias} will be set to that
value.  Eventually, this historically incorrect behavior will go away.

@end defmac

@defmac AC_CANONICAL_TARGET
@maindex 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


@node Using System Type,  , Canonicalizing, Manual Configuration
@section Using the System Type

How do you use a canonical system type?  Usually, you use it in one or
more @code{case} statements in @file{configure.ac} to select
system-specific C files.  Then, using @code{AC_CONFIG_LINKS}, link those
files which have names based on the system name, to generic names, such
as @file{host.h} or @file{target.c} (@pxref{Configuration Links}).  The
@code{case} statement patterns can use shell wild cards to group several
cases together, like in this fragment:

@example
case "$target" in
i386-*-mach* | i386-*-gnu*)
             obj_format=aout emulation=mach bfd_gas=yes ;;
i960-*-bout) obj_format=bout ;;
esac
@end example

@noindent
and in @file{configure.ac}, use:

@example
AC_CONFIG_LINKS(host.h:config/$machine.h
                object.h:config/$obj_format.h)
@end example

You can also use the host system type to find cross-compilation tools.
@xref{Generic Programs}, for information about the @code{AC_CHECK_TOOL}
macro which does that.


@c ===================================================== Site Configuration.

@node Site Configuration, Running configure scripts, Manual Configuration, Top
@chapter Site Configuration

@code{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 @code{configure}
options.

@menu
* External Software::           Working with other optional software
* Package Options::             Selecting optional features
* Pretty Help Strings::         Formating help string
* Site Details::                Configuring site details
* Transforming Names::          Changing program names when installing
* Site Defaults::               Giving @code{configure} local defaults
@end menu

@node External Software, Package Options, Site Configuration, Site Configuration
@section Working With External Software

Some packages require, or can optionally use, other software packages
that are already installed.  The user can give @code{configure}
command line options to specify which such external software to use.
The options have one of these forms:

@example
--with-@var{package}=@ovar{arg}
--without-@var{package}
@end example

For example, @option{--with-gnu-ld} means work with the @sc{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}.

@code{configure} scripts do not complain about
@option{--with-@var{package}} options that they do not support.  This
behavior permits configuring a source tree containing multiple packages
with a top-level @code{configure} script when the packages support
different options, without spurious error messages about options that
some of the packages support.  An unfortunate side effect is that option
spelling errors are not diagnosed.  No better approach to this problem
has been suggested so far.

For each external software package that may be used, @file{configure.ac}
should call @code{AC_ARG_WITH} to detect whether the @code{configure}
user asked to use it.  Whether each package is used or not by default,
and which arguments are valid, is up to you.

@defmac AC_ARG_WITH (@var{package}, @var{help-string}, @ovar{action-if-given}, @ovar{action-if-not-given})
@maindex ARG_WITH
If the user gave @code{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 and dashes.

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 @code{with_@var{package}},
with any @option{-} characters 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.  You'll need to enclose it in @samp{[}
and @samp{]} in order to produce the leading spaces.

You should format your @var{help-string} with the macro
@code{AC_HELP_STRING} (@pxref{Pretty Help Strings}).
@end defmac

@defmac AC_WITH (@var{package}, @var{action-if-given}, @ovar{action-if-not-given})
@maindex WITH
This is an obsolete version of @code{AC_ARG_WITH} that does not
support providing a help string.
@end defmac

@node Package Options, Pretty Help Strings, External Software, Site Configuration
@section Choosing Package Options

If a software package has optional compile-time features, the user can
give @code{configure} command line options to specify whether to
compile them.  The options have one of these forms:

@example
--enable-@var{feature}=@ovar{arg}
--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}.

@code{configure} scripts do not complain about
@option{--enable-@var{feature}} options that they do not support.
This behavior permits configuring a source tree containing multiple
packages with a top-level @code{configure} script when the packages
support different options, without spurious error messages about options
that some of the packages support.
An unfortunate side effect is that option spelling errors are not diagnosed.
No better approach to this problem has been suggested so far.

For each optional feature, @file{configure.ac} should call
@code{AC_ARG_ENABLE} to detect whether the @code{configure} user asked
to include it.  Whether each feature is included or not by default, and
which arguments are valid, is up to you.

@defmac AC_ARG_ENABLE (@var{feature}, @var{help-string}, @ovar{action-if-given}, @ovar{action-if-not-given})
@maindex ARG_ENABLE
If the user gave @code{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 and dashes.

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
@code{enable_@var{feature}}, with any @option{-} characters 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{AC_HELP_STRING} (@pxref{Pretty Help Strings}).
@end defmac

@defmac AC_ENABLE (@var{feature}, @var{action-if-given}, @ovar{action-if-not-given})
@maindex ENABLE
This is an obsolete version of @code{AC_ARG_ENABLE} that does not
support providing a help string.
@end defmac


@node Pretty Help Strings, Site Details, Package Options, Site Configuration
@section Making Your Help Strings Look Pretty

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{AC_HELP_STRING} macro.

@defmac AC_HELP_STRING (@var{left-hand-side}, @var{right-hand-side})
@maindex HELP_STRING

Expands into an 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 will make this clearer.

@example
AC_DEFUN(TEST_MACRO,
[AC_ARG_WITH(foo,
             AC_HELP_STRING([--with-foo],
                            [use foo (default is NO)]),
             ac_cv_use_foo=$withval, ac_cv_use_foo=no),
AC_CACHE_CHECK(whether to use foo,
               ac_cv_use_foo, ac_cv_use_foo=no)])
@end example

Please note that the call to @code{AC_HELP_STRING} is @strong{unquoted}.
Then the last few lines of @samp{configure --help} will appear like
this:

@example
--enable and --with options recognized:
  --with-foo              use foo (default is NO)
@end example

The @code{AC_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.

@example
AC_DEFUN(MY_ARG_WITH,
[AC_ARG_WITH([$1],
             AC_HELP_STRING([--with-$1], [use $1 (default is $2)]),
             ac_cv_use_$1=$withval, ac_cv_use_$1=no),
AC_CACHE_CHECK(whether to use $1, ac_cv_use_$1, ac_cv_use_$1=$2)])
@end example
@end defmac


@node Site Details, Transforming Names, Pretty Help Strings, Site Configuration
@section Configuring 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
run time, rather than at compile time.  Run time 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, GNU Coding
Standards}, for more information on where to put data files.

@node Transforming Names, Site Defaults, Site Details, Site Configuration
@section Transforming Program Names When Installing

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
@maindex 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 @code{configure},
program names are transformed accordingly.  Otherwise, if
@code{AC_CANONICAL_TARGET} has been called and a @option{--target} value
is given that differs from the host type (specified with @option{--host}),
the target type followed by a dash is used as a prefix.  Otherwise, no
program name transformation is done.
@end defmac

@menu
* Transformation Options::      @code{configure} options to transform names
* Transformation Examples::     Sample uses of transforming names
* Transformation Rules::        @file{Makefile} uses of transforming names
@end menu

@node Transformation Options, Transformation Examples, Transforming Names, Transforming Names
@subsection Transformation Options

You can specify name transformations by giving @code{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, Transformation Rules, Transformation Options, Transforming Names
@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
@sc{gnu} programs installed on your system to shadow other programs with
the same name.  For example, if you configure @sc{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 @sc{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,  , Transformation Examples, Transforming Names
@subsection Transformation Rules

Here is how to use the variable @code{program_transform_name} in a
@file{Makefile.in}:

@example
transform = @@program_transform_name@@
install: all
        $(INSTALL_PROGRAM) myprog $(bindir)/`echo myprog | \
                                               sed '$(transform)'`

uninstall:
        rm -f $(bindir)/`echo myprog | sed '$(transform)'`
@end example

@noindent
If you have more than one program to install, you can do it in a loop:

@example
PROGRAMS = cp ls rm
install:
        for p in $(PROGRAMS); do \
          $(INSTALL_PROGRAM) $$p $(bindir)/`echo $$p | \
                                              sed '$(transform)'`; \
        done

uninstall:
        for p in $(PROGRAMS); do \
          rm -f $(bindir)/`echo $$p | sed '$(transform)'`; \
        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,  , Transforming Names, Site Configuration
@section Setting Site Defaults

Autoconf-generated @code{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, @code{configure}
uses its value as the name of a shell script to read.  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 @code{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
@code{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
@code{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, so 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, look for shell variables with @samp{_cv_} in
their names in the affected @code{configure} scripts, or in the Autoconf
M4 source code for those macros.

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
@code{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
# 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 ============================================== Running configure Scripts.

@node Running configure scripts, config.status Invocation, Site Configuration, Top
@chapter Running @code{configure} Scripts
@cindex @code{configure}

Below are instructions on how to configure a package that uses a
@code{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
* System Type::                 Specifying the system type
* Sharing Defaults::            Setting site-wide defaults for @code{configure}
* Environment Variables::       Defining environment variables.
* configure Invocation::        Changing how @code{configure} runs
@end menu

@include install.texi


@c ============================================== Recreating a Configuration

@node config.status Invocation, Obsolete Constructs, Running configure scripts, Top
@chapter Recreating a Configuration
@cindex @code{config.status}

The @code{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 @var{option}@dots{} [@var{file}@dots{}]
@end example

It configures the @var{files}, if none are specified, all the templates
are instantiated.  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 @var{option}s 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 exit.

@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} filename 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 @code{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.

@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 @code{configure}, so that the
results of some tests might be different from the previous run.  The
@option{--recheck} option re-runs @code{configure} with the same arguments
you used before, plus the @option{--no-create} option, which prevents
@code{configure} from running @file{config.status} and creating
@file{Makefile} and other files, and the @option{--no-recursion} option,
which prevents @code{configure} from running other @code{configure}
scripts in subdirectories.  (This is so other @file{Makefile} 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:

@defvar CONFIG_SHELL
@evindex CONFIG_SHELL
The shell with which to run @code{configure} for the @option{--recheck}
option.  It must be Bourne-compatible.  The default is @file{/bin/sh}.
@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 @code{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, Questions, config.status Invocation, Top
@chapter Obsolete Constructs

Autoconf changes, and throughout the years some constructs are obsoleted.
Most of the changes involve the macros, but 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::  Different calling convention
* acconfig.h::                  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::
@end menu

@node Obsolete config.status Use, acconfig.h, Obsolete Constructs, Obsolete Constructs
@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 @code{make} rules, equally
for @code{CONFIG_COMMANDS} etc.)


@node acconfig.h, autoupdate Invocation, Obsolete config.status Use, Obsolete Constructs
@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.  @code{autoheader} copies 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@@},
@code{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@@},
@code{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 jurasik 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, @code{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

Use only the @code{AH_} macros, @file{configure.ac} should be
self-contained, and should not depend upon @file{acconfig.h} etc.


@node autoupdate Invocation, Obsolete Macros, acconfig.h, Obsolete Constructs
@section Using @code{autoupdate} to Modernize @file{configure.ac}
@cindex @code{autoupdate}

The @code{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, @code{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 @code{autoupdate} an argument, it reads that file
instead of @file{configure.ac} and writes the updated file to the
standard output.

@noindent
@code{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 --autoconf-dir=@var{dir}
@itemx -A @var{dir}
@evindex AC_MACRODIR
Override the location where the installed Autoconf data files are looked
for.  You can also set the @code{AC_MACRODIR} environment variable to a
directory; this option overrides the environment variable.

This option is rarely needed and dangerous; it is only used when one
plays with different versions of Autoconf simultaneously.

@item --localdir=@var{dir}
@itemx -l @var{dir}
Look for the package file @file{aclocal.m4} in directory @var{dir}
instead of in the current directory.
@end table

@node Obsolete Macros, Autoconf 1, autoupdate Invocation, Obsolete Constructs
@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, 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_ALLOCA
@maindex ALLOCA
@code{AC_FUNC_ALLOCA}
@end defmac

@defmac AC_ARG_ARRAY
@maindex ARG_ARRAY
removed because of limited usefulness
@end defmac

@defmac AC_C_CROSS
@maindex C_CROSS
This macro is obsolete; it does nothing.
@end defmac

@defmac AC_CANONICAL_SYSTEM
@maindex 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.
@end defmac

@defmac AC_CHAR_UNSIGNED
@maindex CHAR_UNSIGNED
@code{AC_C_CHAR_UNSIGNED}
@end defmac

@defmac AC_CHECK_TYPE (@var{type}, @var{default})
@maindex CHECK_TYPE
Autoconf, up to 2.13, used to provide this version of
@code{AC_CHECK_TYPE}, deprecated because of its flaws.  Firstly, although
it is a member of the @code{CHECK} clan, singular sub-family, it does
more than just checking.  Second, missing types are not
@code{typedef}'d, they are @code{#define}'d, which can lead to
incompatible code 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} or @samp{unsigned}.

This macro is equivalent to:

@example
AC_CHECK_TYPE([@var{type}],
              [AC_DEFINE([@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 by a simple heuristics:

@enumerate
@item
If there are three or four arguments, the modern version is used.

@item
If the second argument is a C or C++ @emph{builtin} type, then the
obsolete version is used.  Because many people have used @samp{off_t}
and @samp{size_t} as replacement types, they are recognized too.

@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
#if !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})
@maindex CHECKING
Same as @samp{AC_MSG_NOTICE([checking @var{feature-description}@dots{}]}.
@end defmac

@defmac AC_COMPILE_CHECK (@var{echo-text}, @var{includes}, @var{function-body}, @var{action-if-found}, @ovar{action-if-not-found})
@maindex COMPILE_CHECK
This is an obsolete version of @code{AC_TRY_LINK} (@pxref{Examining
Libraries}), 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
@maindex CONST
@code{AC_C_CONST}
@end defmac

@defmac AC_CROSS_CHECK
@maindex CROSS_CHECK
Same as @code{AC_C_CROSS}, which is obsolete too, and does nothing
@code{:-)}.
@end defmac

@defmac AC_CYGWIN
@maindex 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}.  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 @code{CYGWIN} has a very special meaning when
running CygWin32, and should not be changed.  That's yet another reason
not to use this macro.
@end defmac

@defmac AC_DECL_YYTEXT
@maindex DECL_YYTEXT
Does nothing, now integrated in @code{AC_PROG_LEX}.
@end defmac

@defmac AC_DIR_HEADER
@maindex DIR_HEADER
@cvindex DIRENT
@cvindex SYSNDIR
@cvindex SYSDIR
@cvindex NDIR
Like calling @code{AC_FUNC_CLOSEDIR_VOID} and@code{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
@maindex DYNIX_SEQ
If on Dynix/PTX (Sequent @sc{unix}), 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}.
@end defmac

@defmac AC_EXEEXT
@maindex 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 Unix and @samp{.exe} if Win32 or OS/2.
@end defmac

@defmac AC_EMXOS2
@maindex EMXOS2
Similar to @code{AC_CYGWIN} but checks for the EMX environment on OS/2
and sets @code{EMXOS2}.
@end defmac

@defmac AC_ERROR
@maindex ERROR
@code{AC_MSG_ERROR}
@end defmac

@defmac AC_FIND_X
@maindex FIND_X
@code{AC_PATH_X}
@end defmac

@defmac AC_FIND_XTRA
@maindex FIND_XTRA
@code{AC_PATH_XTRA}
@end defmac

@defmac AC_FUNC_CHECK
@maindex FUNC_CHECK
@code{AC_CHECK_FUNC}
@end defmac

@defmac AC_FUNC_WAIT3
@maindex FUNC_WAIT3
@cvindex HAVE_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} is being removed from the Open Group
standards, and will not appear in the next revision of POSIX.
@end defmac

@defmac AC_GCC_TRADITIONAL
@maindex GCC_TRADITIONAL
@code{AC_PROG_GCC_TRADITIONAL}
@end defmac

@defmac AC_GETGROUPS_T
@maindex GETGROUPS_T
@code{AC_TYPE_GETGROUPS}
@end defmac

@defmac AC_GETLOADAVG
@maindex GETLOADAVG
@code{AC_FUNC_GETLOADAVG}
@end defmac

@defmac AC_HAVE_FUNCS
@maindex HAVE_FUNCS
@code{AC_CHECK_FUNCS}
@end defmac

@defmac AC_HAVE_HEADERS
@maindex HAVE_HEADERS
@code{AC_CHECK_HEADERS}
@end defmac

@defmac AC_HAVE_LIBRARY (@var{library}, @ovar{action-if-found}, @ovar{action-if-not-found}, @ovar{other-libraries})
@maindex 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.
@end defmac

@defmac AC_HAVE_POUNDBANG
@maindex HAVE_POUNDBANG
@code{AC_SYS_INTERPRETER} (different calling convention)
@end defmac

@defmac AC_HEADER_CHECK
@maindex HEADER_CHECK
@code{AC_CHECK_HEADER}
@end defmac

@defmac AC_HEADER_EGREP
@maindex HEADER_EGREP
@code{AC_EGREP_HEADER}
@end defmac

@defmac AC_INIT (@var{unique-file-in-source-dir})
@maindex 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
@end defmac

@defmac AC_INLINE
@maindex INLINE
@code{AC_C_INLINE}
@end defmac

@defmac AC_INT_16_BITS
@maindex 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.
@end defmac

@defmac AC_IRIX_SUN
@maindex IRIX_SUN
If on IRIX (Silicon Graphics @sc{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
@end defmac

@defmac AC_LANG_C
@maindex LANG_C
Same as @samp{AC_LANG(C)}.
@end defmac

@defmac AC_LANG_CPLUSPLUS
@maindex LANG_CPLUSPLUS
Same as @samp{AC_LANG(C++)}.
@end defmac

@defmac AC_LANG_FORTRAN77
@maindex LANG_FORTRAN77
Same as @samp{AC_LANG(Fortran 77)}.
@end defmac

@defmac AC_LANG_RESTORE
@maindex 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})}.
@end defmac

@defmac AC_LANG_SAVE
@maindex 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.
@end defmac

@defmac AC_LINK_FILES (@var{source}@dots{}, @var{dest}@dots{})
@maindex LINK_FILES
This is an obsolete version of @code{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
@maindex LN_S
@code{AC_PROG_LN_S}
@end defmac

@defmac AC_LONG_64_BITS
@maindex 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.
@end defmac

@defmac AC_LONG_DOUBLE
@maindex LONG_DOUBLE
@code{AC_C_LONG_DOUBLE}
@end defmac

@defmac AC_LONG_FILE_NAMES
@maindex LONG_FILE_NAMES
@code{AC_SYS_LONG_FILE_NAMES}
@end defmac

@defmac AC_MAJOR_HEADER
@maindex MAJOR_HEADER
@code{AC_HEADER_MAJOR}
@end defmac

@defmac AC_MEMORY_H
@maindex 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)}.  Adjust your code to depend upon
@code{HAVE_MEMORY_H}, not @code{NEED_MEMORY_H}, see @xref{Standard
Symbols}.
@end defmac

@defmac AC_MINGW32
@maindex MINGW32
Similar to @code{AC_CYGWIN} but checks for the MingW32 compiler
environment and sets @code{MINGW32}.
@end defmac

@defmac AC_MINUS_C_MINUS_O
@maindex MINUS_C_MINUS_O
@code{AC_PROG_CC_C_O}
@end defmac

@defmac AC_MMAP
@maindex MMAP
@code{AC_FUNC_MMAP}
@end defmac

@defmac AC_MODE_T
@maindex MODE_T
@code{AC_TYPE_MODE_T}
@end defmac

@defmac AC_OBJEXT
@maindex 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 Unix, @samp{obj} if Win32.  Now the compiler checking macros handle
this automatically.
@end defmac

@defmac AC_OBSOLETE (@var{this-macro-name}, @ovar{suggestion})
@maindex OBSOLETE
Make @code{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

You are encouraged to use @code{AU_DEFUN} instead, since it gives better
services to the user.
@end defmac

@defmac AC_OFF_T
@maindex OFF_T
@code{AC_TYPE_OFF_T}
@end defmac

@defmac AC_OUTPUT (@ovar{file}@dots{}, @ovar{extra-cmds}, @ovar{init-cmds})
@maindex OUTPUT
The use of @code{AC_OUTPUT} with argument 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
@end defmac

@defmac AC_OUTPUT_COMMANDS (@var{extra-cmds}, @ovar{init-cmds})
@maindex OUTPUT_COMMANDS
Specify additional shell commands to run at the end of
@file{config.status}, and shell commands to initialize any variables
from @code{configure}.  This macro may be called multiple times.  It is
obsolete, replaced by @code{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, while
@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
@maindex PID_T
@code{AC_TYPE_PID_T}
@end defmac

@defmac AC_PREFIX
@maindex PREFIX
@code{AC_PREFIX_PROGRAM}
@end defmac

@defmac AC_PROGRAMS_CHECK
@maindex PROGRAMS_CHECK
@code{AC_CHECK_PROGS}
@end defmac

@defmac AC_PROGRAMS_PATH
@maindex PROGRAMS_PATH
@code{AC_PATH_PROGS}
@end defmac

@defmac AC_PROGRAM_CHECK
@maindex PROGRAM_CHECK
@code{AC_CHECK_PROG}
@end defmac

@defmac AC_PROGRAM_EGREP
@maindex PROGRAM_EGREP
@code{AC_EGREP_CPP}
@end defmac

@defmac AC_PROGRAM_PATH
@maindex PROGRAM_PATH
@code{AC_PATH_PROG}
@end defmac

@defmac AC_REMOTE_TAPE
@maindex REMOTE_TAPE
removed because of limited usefulness
@end defmac

@defmac AC_RESTARTABLE_SYSCALLS
@maindex RESTARTABLE_SYSCALLS
@code{AC_SYS_RESTARTABLE_SYSCALLS}
@end defmac

@defmac AC_RETSIGTYPE
@maindex RETSIGTYPE
@code{AC_TYPE_SIGNAL}
@end defmac

@defmac AC_RSH
@maindex RSH
Removed because of limited usefulness.
@end defmac

@defmac AC_SCO_INTL
@maindex SCO_INTL
@ovindex LIBS
If on SCO UNIX, add @option{-lintl} to output variable @code{LIBS}.  This
macro used to

@example
AC_CHECK_LIB(intl, strftime, LIBS="-lintl $LIBS")
@end example

@noindent
now it just calls @code{AC_FUNC_STRFTIME} instead.
@end defmac

@defmac AC_SETVBUF_REVERSED
@maindex SETVBUF_REVERSED
@code{AC_FUNC_SETVBUF_REVERSED}
@end defmac

@defmac AC_SET_MAKE
@maindex SET_MAKE
@code{AC_PROG_MAKE_SET}
@end defmac

@defmac AC_SIZEOF_TYPE
@maindex SIZEOF_TYPE
@code{AC_CHECK_SIZEOF}
@end defmac

@defmac AC_SIZE_T
@maindex SIZE_T
@code{AC_TYPE_SIZE_T}
@end defmac

@defmac AC_STAT_MACROS_BROKEN
@maindex STAT_MACROS_BROKEN
@code{AC_HEADER_STAT}
@end defmac

@defmac AC_STDC_HEADERS
@maindex STDC_HEADERS
@code{AC_HEADER_STDC}
@end defmac

@defmac AC_STRCOLL
@maindex STRCOLL
@code{AC_FUNC_STRCOLL}
@end defmac

@defmac AC_ST_BLKSIZE
@maindex ST_BLKSIZE
@code{AC_STRUCT_ST_BLKSIZE}
@end defmac

@defmac AC_ST_BLOCKS
@maindex ST_BLOCKS
@code{AC_STRUCT_ST_BLOCKS}
@end defmac

@defmac AC_ST_RDEV
@maindex ST_RDEV
@code{AC_STRUCT_ST_RDEV}
@end defmac

@defmac AC_SYS_RESTARTABLE_SYSCALLS
@maindex 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 if system calls are restarted in general--it tests whether a
signal handler installed with @code{signal} (but not @code{sigaction})
causes system calls to be restarted. It does not test if 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
@maindex SYS_SIGLIST_DECLARED
@code{AC_DECL_SYS_SIGLIST}
@end defmac

@defmac AC_TEST_CPP
@maindex TEST_CPP
@code{AC_TRY_CPP}
@end defmac

@defmac AC_TEST_PROGRAM
@maindex TEST_PROGRAM
@code{AC_TRY_RUN}
@end defmac

@defmac AC_TIMEZONE
@maindex TIMEZONE
@code{AC_STRUCT_TIMEZONE}
@end defmac

@defmac AC_TIME_WITH_SYS_TIME
@maindex TIME_WITH_SYS_TIME
@code{AC_HEADER_TIME}
@end defmac

@defmac AC_UID_T
@maindex UID_T
@code{AC_TYPE_UID_T}
@end defmac

@defmac AC_UNISTD_H
@maindex UNISTD_H
Same as @samp{AC_CHECK_HEADERS(unistd.h)}.
@end defmac

@defmac AC_USG
@maindex USG
@cvindex USG
Define @code{USG} if the @sc{bsd} string functions are defined in
@file{strings.h}.  You should no longer depend upon @code{USG}, but on
@code{HAVE_STRING_H}, see @xref{Standard Symbols}.
@end defmac

@defmac AC_UTIME_NULL
@maindex UTIME_NULL
@code{AC_FUNC_UTIME_NULL}
@end defmac

@defmac AC_VALIDATE_CACHED_SYSTEM_TUPLE (@ovar{cmd})
@maindex 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})
@maindex VERBOSE
@code{AC_MSG_RESULT}.
@end defmac

@defmac AC_VFORK
@maindex VFORK
@code{AC_FUNC_VFORK}
@end defmac

@defmac AC_VPRINTF
@maindex VPRINTF
@code{AC_FUNC_VPRINTF}
@end defmac

@defmac AC_WAIT3
@maindex WAIT3
@code{AC_FUNC_WAIT3}
@end defmac

@defmac AC_WARN
@maindex WARN
@code{AC_MSG_WARN}
@end defmac

@defmac AC_WORDS_BIGENDIAN
@maindex WORDS_BIGENDIAN
@code{AC_C_BIGENDIAN}
@end defmac

@defmac AC_XENIX_DIR
@maindex 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 @sc{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
@end defmac

@defmac AC_YYTEXT_POINTER
@maindex YYTEXT_POINTER
@code{AC_DECL_YYTEXT}
@end defmac

@node Autoconf 1, Autoconf 2.13, Obsolete Macros, Obsolete Constructs
@section Upgrading From Version 1

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
@code{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, Changed Makefiles, Autoconf 1, Autoconf 1
@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 @code{make} builtin rules won'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 will have less clutter if you
use the @code{AH_} macros.  @xref{Autoheader Macros}.

@node Changed Makefiles, Changed Macros, Changed File Names, Autoconf 1
@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 @code{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 will contain a comment saying
they were produced by @code{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, Changed Results, Changed Makefiles, Autoconf 1
@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 @code{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 @code{autoconf}, you may safely ignore them, but
your @code{configure} script will generally work better if you follow
the advice it prints about what to replace the obsolete macros with.  In
particular, the mechanism for reporting the results of tests has
changed.  If you were using @code{echo} or @code{AC_VERBOSE} (perhaps
via @code{AC_COMPILE_CHECK}), your @code{configure} script's output will
look 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, Changed Macro Writing, Changed Macros, Autoconf 1
@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
@code{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$lib)
    LIBS="$saved_LIBS -l$lib"
    AC_HAVE_FUNCS(syslog)
    case "$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 $ac_cv_func_syslog = no; then
  # syslog is not in the default libraries.  See if it's in some other.
  for lib in bsd socket inet; do
    AC_CHECK_LIB($lib, syslog, [AC_DEFINE(HAVE_SYSLOG)
      LIBS="$LIBS -l$lib"; 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,  , Changed Results, Autoconf 1
@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 into macros that you can share.


@node Autoconf 2.13,  , Autoconf 1, Obsolete Constructs
@section Upgrading From Version 2.13

The introduction of the previous section (@pxref{Autoconf 1}) perfectly
suits this section...

@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 @code{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
@end menu

@node Changed Quotation, New Macros, Autoconf 2.13, Autoconf 2.13
@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
$ autoconf-2.13; ./configure --silent
creating cache ./config.cache
configure: error: cannot find foo.h
$
@end example

@noindent
while Autoconf 2.50 will produce a broken @file{configure}:

@example
$ 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
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... @code{AC_DEFUN} itself!

@example
$ cat configure.in
AC_DEFUN([AC_PROG_INSTALL],
[# My own much better version
])
AC_INIT
AC_PROG_INSTALL
AC_OUTPUT
$ 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
$ autoconf-2.50
$
@end example


@node New Macros,  , Changed Quotation, Autoconf 2.13
@subsection New Macros

@cindex @code{undefined macro: _m4_divert_diversion}

Because Autoconf has been dormant for years, Automake provided
Autoconf-like macros for a while.  Autoconf 2.50 now provides better
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 Automake did not quote the name of these macros!
Therefore, when @command{m4} find in @file{aclocal.m4} something like
@samp{AC_DEFUN(AM_TYPE_PTRDIFF_T, ...)}, @code{AM_TYPE_PTRDIFF_T} is
expanded, replaced with its Autoconf definition.

Fortunately Autoconf catches pre-@code{AC_INIT} expansions, and will
complain, in its own words:

@example
$ cat configure.in
AC_INIT
AM_TYPE_PTRDIFF_T
$ aclocal-1.4
$ autoconf
./aclocal.m4:17: error: m4_defn: undefined macro: _m4_divert_diversion
actypes.m4:289: AM_TYPE_PTRDIFF_T is expanded from...
./aclocal.m4:17: the top level
$
@end example

Future versions of Automake will simply no longer define most of these
macros, and will properly quote the names of the remaining macros.
But you don't have to wait for it to happen to do the right thing right
now: do not depend upon macros from Automake as it is simply not its job
to provide macros (but the one it requires by itself):

@example
$ cat configure.in
AC_INIT
AM_TYPE_PTRDIFF_T
$ rm aclocal.m4
$ autoupdate
autoupdate: `configure.in' is updated
$ cat configure.in
AC_INIT
AC_CHECK_TYPES([ptrdiff_t])
$ aclocal-1.4
$ autoconf
$
@end example

@c ================================================ Questions About Autoconf.

@node Questions, History, Obsolete Constructs, Top
@chapter Questions About Autoconf

Several questions about Autoconf come up occasionally.  Here some of them
are addressed.

@menu
* Distributing::                Distributing @code{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 @code{configure} instead of Imake
@end menu

@node Distributing, Why GNU m4, Questions, Questions
@section Distributing @code{configure} Scripts

@display
What are the restrictions on distributing @code{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 @sc{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 @code{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
@code{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, Bootstrapping, Distributing, Questions
@section Why Require GNU M4?

@display
Why does Autoconf require @sc{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
builtin
indir
patsubst
__file__
__line__
@end example

Autoconf requires version 1.4 or above of @sc{gnu} M4 because it uses
frozen state files.

Since only software maintainers need to use Autoconf, and since @sc{gnu}
M4 is simple to configure and install, it seems reasonable to require
@sc{gnu} M4 to be installed also.  Many maintainers of @sc{gnu} and
other free software already have most of the @sc{gnu} utilities
installed, since they prefer them.

@node Bootstrapping, Why Not Imake, Why GNU m4, Questions
@section How Can I Bootstrap?

@display
If Autoconf requires @sc{gnu} M4 and @sc{gnu} M4 has an Autoconf
@code{configure} script, how do I bootstrap?  It seems like a chicken
and egg problem!
@end display

This is a misunderstanding.  Although @sc{gnu} M4 does come with a
@code{configure} script produced by Autoconf, Autoconf is not required
in order to run the script and install @sc{gnu} M4.  Autoconf is only
required if you want to change the M4 @code{configure} script, which few
people have to do (mainly its maintainer).

@node Why Not Imake,  , Bootstrapping, Questions
@section Why Not Imake?

@display
Why not use Imake instead of @code{configure} scripts?
@end display

Several people have written addressing this question, so I include
adaptations of their explanations 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.

@sc{gnu} tools are not released this way.  Each @sc{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 @sc{gnu} suite as a collection of native tools, then the
problems are similar.  But the @sc{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 @sc{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 easy to generate large
Makefiles using @code{cpp}'s @samp{#include} and macro mechanisms.
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
@code{make} and the installer's C preprocessor.  The fundamental problem
here is that the C preprocessor was designed to preprocess C programs,
not @file{Makefile}s.  This is much less of a problem with Autoconf,
which uses the general-purpose preprocessor @code{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 @code{configure}:
@file{Imakefile}s tend to be much shorter (likewise, less redundant)
than @file{Makefile.in}s.  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} @file{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 @code{configure} setups.
@end quotation




@c ===================================================== History of Autoconf.

@node History, Environment Variable Index, Questions, Top
@chapter History of Autoconf

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@dots{}

@menu
* Genesis::                     Prehistory and naming of @code{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, Exodus, History, History
@section Genesis

In June 1991 I was maintaining many of the @sc{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 @file{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 @code{configure} script worked well enough that
the next month I adapted it (by hand) to create similar @code{configure}
scripts for several other @sc{gnu} utilities packages.  Brian Berliner
also adapted one of my scripts for his @sc{cvs} revision control system.

Later that summer, I learned that Richard Stallman and Richard Pixley
were developing similar scripts to use in the @sc{gnu} compiler tools;
so I adapted my @code{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, Leviticus, Genesis, History
@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 @sc{gnu} utilities packages to use
@code{configure} scripts, updating them all by hand became impractical.
Rich Murphey, the maintainer of the @sc{gnu} graphics utilities, sent me
mail saying that the @code{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 @code{configure} scripts to the
abundance and ease of Autoconf began.

Cygnus @code{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
@code{configure}, where there is a single @code{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 @code{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 @code{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 @code{Configure} scripts I had
seen didn't work on many modern systems (such as System V R4 and NeXT);
it wasn't very 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 @code{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 @sc{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, Numbers, Exodus, History
@section Leviticus

Since my @code{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 @sc{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@,cois Pinard, who came up
with the idea of making an @file{autoconf} shell script to run @code{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
@file{Makefile}, so he could use Autoconf for his @sc{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, Deuteronomy, Leviticus, History
@section Numbers

In July 1992, after months of alpha testing, I released Autoconf 1.0,
and converted many @sc{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 @sc{gnu} Project (such as TCL, FSP, and Kerberos V5).
Autoconf continued to improve rapidly, as many people using the
@code{configure} scripts reported problems they encountered.

Autoconf turned out to be a good torture test for M4 implementations.
@sc{unix} @code{m4} started to dump core because of the length of the
macros that Autoconf defined, and several bugs showed up in @sc{gnu}
@code{m4} as well.  Eventually, we realized that we needed to use some
features that only @sc{gnu} M4 has.  4.3@sc{bsd} @code{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@,cois Pinard made it diagnose
invalid arguments.  Jim Blandy bravely coerced it into configuring
@sc{gnu} Emacs, laying the groundwork for several later improvements.
Roland McGrath got it to configure the @sc{gnu} C Library, wrote the
@code{autoheader} script to automate the creation of C header file
templates, and added a @option{--verbose} option to @code{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,  , Numbers, History
@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
@code{configure} that Autoconf had lacked, largely by adapting the
relevant parts of Cygnus @code{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 @code{configure}
scripts in subdirectories.  Adding these features enabled Ken to convert
@sc{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 @code{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
@code{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@,cois Pinard, I made
the macros not interrupt each others' messages.  (That feature revealed
some performance bottlenecks in @sc{gnu} @code{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@,cois 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 Environment Variable Index, Output Variable Index, History, Top
@unnumbered Environment Variable Index

This is an alphabetical list of the environment variables that Autoconf
checks.

@printindex ev

@node Output Variable Index, Preprocessor Symbol Index, Environment Variable Index, Top
@unnumbered Output Variable Index

This is an alphabetical list of the variables that Autoconf can
substitute into files that it creates, typically one or more
@file{Makefile}s.  @xref{Setting Output Variables}, for more information
on how this is done.

@printindex ov

@node Preprocessor Symbol Index, Autoconf Macro Index, Output Variable Index, Top
@unnumbered 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} directives.

@printindex cv

@node Autoconf Macro Index, M4 Macro Index, Preprocessor Symbol Index, Top
@unnumbered Autoconf Macro Index

This is an alphabetical list of the Autoconf macros.  To make the list
easier to use, the macros are listed without their preceding @samp{AC_}.

@printindex ma

@node M4 Macro Index, Concept Index, Autoconf Macro Index, Top
@unnumbered M4 Macro Index

This is an alphabetical list of the M4, M4sugar, and M4sh macros.  To
make the list easier to use, the macros are listed without their
preceding @samp{m4_} or @samp{AS_}.

@printindex ms

@node Concept Index,  , M4 Macro Index, Top
@unnumbered Concept Index

This is an alphabetical list of the files, tools, and concepts
introduced in this document.

@printindex cp

@contents
@bye

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