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diff --git a/doc/standards.texi b/doc/standards.texi new file mode 100644 index 0000000..69a400e --- /dev/null +++ b/doc/standards.texi @@ -0,0 +1,4256 @@ +\input texinfo @c -*-texinfo-*- +@c %**start of header +@setfilename standards.info +@settitle GNU Coding Standards +@c This date is automagically updated when you save this file: +@set lastupdate April 7, 2012 +@c %**end of header + +@dircategory GNU organization +@direntry +* Standards: (standards). GNU coding standards. +@end direntry + +@c @setchapternewpage odd +@setchapternewpage off + +@c Put everything in one index (arbitrarily chosen to be the concept index). +@syncodeindex fn cp +@syncodeindex ky cp +@syncodeindex pg cp +@syncodeindex vr cp + +@c This is used by a cross ref in make-stds.texi +@set CODESTD 1 + +@copying +The GNU coding standards, last updated @value{lastupdate}. + +Copyright @copyright{} 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, +2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, +2011, 2012 Free Software Foundation, Inc. + +Permission is granted to copy, distribute and/or modify this document +under the terms of the GNU Free Documentation License, Version 1.3 or +any later version published by the Free Software Foundation; with no +Invariant Sections, with no Front-Cover Texts, and with no Back-Cover +Texts. A copy of the license is included in the section entitled +``GNU Free Documentation License''. +@end copying + +@titlepage +@title GNU Coding Standards +@author Richard Stallman, et al. +@author last updated @value{lastupdate} +@page +@vskip 0pt plus 1filll +@insertcopying +@end titlepage + +@contents + +@ifnottex +@node Top +@top GNU Coding Standards + +@insertcopying +@end ifnottex + +@menu +* Preface:: About the GNU Coding Standards. +* Legal Issues:: Keeping free software free. +* Design Advice:: General program design. +* Program Behavior:: Program behavior for all programs +* Writing C:: Making the best use of C. +* Documentation:: Documenting programs. +* Managing Releases:: The release process. +* References:: Mentioning non-free software or documentation. +* GNU Free Documentation License:: Copying and sharing this manual. +* Index:: + +@end menu + +@node Preface +@chapter About the GNU Coding Standards + +The GNU Coding Standards were written by Richard Stallman and other GNU +Project volunteers. Their purpose is to make the GNU system clean, +consistent, and easy to install. This document can also be read as a +guide to writing portable, robust and reliable programs. It focuses on +programs written in C, but many of the rules and principles are useful +even if you write in another programming language. The rules often +state reasons for writing in a certain way. + +@cindex where to obtain @code{standards.texi} +@cindex downloading this manual +If you did not obtain this file directly from the GNU project and +recently, please check for a newer version. You can get the GNU +Coding Standards from the GNU web server in many +different formats, including the Texinfo source, PDF, HTML, DVI, plain +text, and more, at: @uref{http://www.gnu.org/prep/standards/}. + +If you are maintaining an official GNU package, in addition to this +document, please read and follow the GNU maintainer information +(@pxref{Top, , Contents, maintain, Information for Maintainers of GNU +Software}). + +@cindex @code{gnustandards-commit@@gnu.org} mailing list +If you want to receive diffs for every change to these GNU documents, +join the mailing list @code{gnustandards-commit@@gnu.org}, via the web +interface at +@url{http://lists.gnu.org/mailman/listinfo/gnustandards-commit}. +Archives are also available there. + +@cindex @code{bug-standards@@gnu.org} email address +@cindex Savannah repository for gnustandards +@cindex gnustandards project repository +Please send corrections or suggestions for this document to +@email{bug-standards@@gnu.org}. If you make a suggestion, please +include a suggested new wording for it, to help us consider the +suggestion efficiently. We prefer a context diff to the Texinfo +source, but if that's difficult for you, you can make a context diff +for some other version of this document, or propose it in any way that +makes it clear. The source repository for this document can be found +at @url{http://savannah.gnu.org/projects/gnustandards}. + +These standards cover the minimum of what is important when writing a +GNU package. Likely, the need for additional standards will come up. +Sometimes, you might suggest that such standards be added to this +document. If you think your standards would be generally useful, please +do suggest them. + +You should also set standards for your package on many questions not +addressed or not firmly specified here. The most important point is to +be self-consistent---try to stick to the conventions you pick, and try +to document them as much as possible. That way, your program will be +more maintainable by others. + +The GNU Hello program serves as an example of how to follow the GNU +coding standards for a trivial program. +@uref{http://www.gnu.org/software/hello/hello.html}. + +This release of the GNU Coding Standards was last updated +@value{lastupdate}. + + +@node Legal Issues +@chapter Keeping Free Software Free +@cindex legal aspects + +This chapter discusses how you can make sure that GNU software +avoids legal difficulties, and other related issues. + +@menu +* Reading Non-Free Code:: Referring to proprietary programs. +* Contributions:: Accepting contributions. +* Trademarks:: How we deal with trademark issues. +@end menu + +@node Reading Non-Free Code +@section Referring to Proprietary Programs +@cindex proprietary programs +@cindex avoiding proprietary code + +Don't in any circumstances refer to Unix source code for or during +your work on GNU! (Or to any other proprietary programs.) + +If you have a vague recollection of the internals of a Unix program, +this does not absolutely mean you can't write an imitation of it, but +do try to organize the imitation internally along different lines, +because this is likely to make the details of the Unix version +irrelevant and dissimilar to your results. + +For example, Unix utilities were generally optimized to minimize +memory use; if you go for speed instead, your program will be very +different. You could keep the entire input file in memory and scan it +there instead of using stdio. Use a smarter algorithm discovered more +recently than the Unix program. Eliminate use of temporary files. Do +it in one pass instead of two (we did this in the assembler). + +Or, on the contrary, emphasize simplicity instead of speed. For some +applications, the speed of today's computers makes simpler algorithms +adequate. + +Or go for generality. For example, Unix programs often have static +tables or fixed-size strings, which make for arbitrary limits; use +dynamic allocation instead. Make sure your program handles NULs and +other funny characters in the input files. Add a programming language +for extensibility and write part of the program in that language. + +Or turn some parts of the program into independently usable libraries. +Or use a simple garbage collector instead of tracking precisely when +to free memory, or use a new GNU facility such as obstacks. + + +@node Contributions +@section Accepting Contributions +@cindex legal papers +@cindex accepting contributions + +If the program you are working on is copyrighted by the Free Software +Foundation, then when someone else sends you a piece of code to add to +the program, we need legal papers to use it---just as we asked you to +sign papers initially. @emph{Each} person who makes a nontrivial +contribution to a program must sign some sort of legal papers in order +for us to have clear title to the program; the main author alone is not +enough. + +So, before adding in any contributions from other people, please tell +us, so we can arrange to get the papers. Then wait until we tell you +that we have received the signed papers, before you actually use the +contribution. + +This applies both before you release the program and afterward. If +you receive diffs to fix a bug, and they make significant changes, we +need legal papers for that change. + +This also applies to comments and documentation files. For copyright +law, comments and code are just text. Copyright applies to all kinds of +text, so we need legal papers for all kinds. + +We know it is frustrating to ask for legal papers; it's frustrating for +us as well. But if you don't wait, you are going out on a limb---for +example, what if the contributor's employer won't sign a disclaimer? +You might have to take that code out again! + +You don't need papers for changes of a few lines here or there, since +they are not significant for copyright purposes. Also, you don't need +papers if all you get from the suggestion is some ideas, not actual code +which you use. For example, if someone sent you one implementation, but +you write a different implementation of the same idea, you don't need to +get papers. + +The very worst thing is if you forget to tell us about the other +contributor. We could be very embarrassed in court some day as a +result. + +We have more detailed advice for maintainers of GNU packages. If you +have reached the stage of maintaining a GNU program (whether released +or not), please take a look: @pxref{Legal Matters,,, maintain, +Information for GNU Maintainers}. + + +@node Trademarks +@section Trademarks +@cindex trademarks + +Please do not include any trademark acknowledgements in GNU software +packages or documentation. + +Trademark acknowledgements are the statements that such-and-such is a +trademark of so-and-so. The GNU Project has no objection to the basic +idea of trademarks, but these acknowledgements feel like kowtowing, +and there is no legal requirement for them, so we don't use them. + +What is legally required, as regards other people's trademarks, is to +avoid using them in ways which a reader might reasonably understand as +naming or labeling our own programs or activities. For example, since +``Objective C'' is (or at least was) a trademark, we made sure to say +that we provide a ``compiler for the Objective C language'' rather +than an ``Objective C compiler''. The latter would have been meant as +a shorter way of saying the former, but it does not explicitly state +the relationship, so it could be misinterpreted as using ``Objective +C'' as a label for the compiler rather than for the language. + +Please don't use ``win'' as an abbreviation for Microsoft Windows in +GNU software or documentation. In hacker terminology, calling +something a ``win'' is a form of praise. If you wish to praise +Microsoft Windows when speaking on your own, by all means do so, but +not in GNU software. Usually we write the name ``Windows'' in full, +but when brevity is very important (as in file names and sometimes +symbol names), we abbreviate it to ``w''. For instance, the files and +functions in Emacs that deal with Windows start with @samp{w32}. + +@node Design Advice +@chapter General Program Design +@cindex program design + +This chapter discusses some of the issues you should take into +account when designing your program. + +@c Standard or ANSI C +@c +@c In 1989 the American National Standards Institute (ANSI) standardized +@c C as standard X3.159-1989. In December of that year the +@c International Standards Organization ISO adopted the ANSI C standard +@c making minor changes. In 1990 ANSI then re-adopted ISO standard +@c C. This version of C is known as either ANSI C or Standard C. + +@c A major revision of the C Standard appeared in 1999. + +@menu +* Source Language:: Which languages to use. +* Compatibility:: Compatibility with other implementations. +* Using Extensions:: Using non-standard features. +* Standard C:: Using standard C features. +* Conditional Compilation:: Compiling code only if a conditional is true. +@end menu + +@node Source Language +@section Which Languages to Use +@cindex programming languages + +When you want to use a language that gets compiled and runs at high +speed, the best language to use is C. Using another language is like +using a non-standard feature: it will cause trouble for users. Even if +GCC supports the other language, users may find it inconvenient to have +to install the compiler for that other language in order to build your +program. For example, if you write your program in C++, people will +have to install the GNU C++ compiler in order to compile your program. + +C has one other advantage over C++ and other compiled languages: more +people know C, so more people will find it easy to read and modify the +program if it is written in C. + +So in general it is much better to use C, rather than the +comparable alternatives. + +But there are two exceptions to that conclusion: + +@itemize @bullet +@item +It is no problem to use another language to write a tool specifically +intended for use with that language. That is because the only people +who want to build the tool will be those who have installed the other +language anyway. + +@item +If an application is of interest only to a narrow part of the community, +then the question of which language it is written in has less effect on +other people, so you may as well please yourself. +@end itemize + +Many programs are designed to be extensible: they include an interpreter +for a language that is higher level than C. Often much of the program +is written in that language, too. The Emacs editor pioneered this +technique. + +@cindex Guile +@cindex GNOME and Guile +The standard extensibility interpreter for GNU software is Guile +(@uref{http://www.gnu.org/@/software/@/guile/}), which implements the +language Scheme (an especially clean and simple dialect of Lisp). +Guile also includes bindings for GTK+/GNOME, making it practical to +write modern GUI functionality within Guile. We don't reject programs +written in other ``scripting languages'' such as Perl and Python, but +using Guile is very important for the overall consistency of the GNU +system. + + +@node Compatibility +@section Compatibility with Other Implementations +@cindex compatibility with C and @sc{posix} standards +@cindex @sc{posix} compatibility + +With occasional exceptions, utility programs and libraries for GNU +should be upward compatible with those in Berkeley Unix, and upward +compatible with Standard C if Standard C specifies their +behavior, and upward compatible with @sc{posix} if @sc{posix} specifies +their behavior. + +When these standards conflict, it is useful to offer compatibility +modes for each of them. + +@cindex options for compatibility +Standard C and @sc{posix} prohibit many kinds of extensions. Feel +free to make the extensions anyway, and include a @samp{--ansi}, +@samp{--posix}, or @samp{--compatible} option to turn them off. +However, if the extension has a significant chance of breaking any real +programs or scripts, then it is not really upward compatible. So you +should try to redesign its interface to make it upward compatible. + +@cindex @code{POSIXLY_CORRECT}, environment variable +Many GNU programs suppress extensions that conflict with @sc{posix} if the +environment variable @code{POSIXLY_CORRECT} is defined (even if it is +defined with a null value). Please make your program recognize this +variable if appropriate. + +When a feature is used only by users (not by programs or command +files), and it is done poorly in Unix, feel free to replace it +completely with something totally different and better. (For example, +@code{vi} is replaced with Emacs.) But it is nice to offer a compatible +feature as well. (There is a free @code{vi} clone, so we offer it.) + +Additional useful features are welcome regardless of whether +there is any precedent for them. + +@node Using Extensions +@section Using Non-standard Features +@cindex non-standard extensions + +Many GNU facilities that already exist support a number of convenient +extensions over the comparable Unix facilities. Whether to use these +extensions in implementing your program is a difficult question. + +On the one hand, using the extensions can make a cleaner program. +On the other hand, people will not be able to build the program +unless the other GNU tools are available. This might cause the +program to work on fewer kinds of machines. + +With some extensions, it might be easy to provide both alternatives. +For example, you can define functions with a ``keyword'' @code{INLINE} +and define that as a macro to expand into either @code{inline} or +nothing, depending on the compiler. + +In general, perhaps it is best not to use the extensions if you can +straightforwardly do without them, but to use the extensions if they +are a big improvement. + +An exception to this rule are the large, established programs (such as +Emacs) which run on a great variety of systems. Using GNU extensions in +such programs would make many users unhappy, so we don't do that. + +Another exception is for programs that are used as part of compilation: +anything that must be compiled with other compilers in order to +bootstrap the GNU compilation facilities. If these require the GNU +compiler, then no one can compile them without having them installed +already. That would be extremely troublesome in certain cases. + +@node Standard C +@section Standard C and Pre-Standard C +@cindex @sc{ansi} C standard + +1989 Standard C is widespread enough now that it is ok to use its +features in new programs. There is one exception: do not ever use the +``trigraph'' feature of Standard C. + +1999 Standard C is not widespread yet, so please do not require its +features in programs. It is ok to use its features if they are present. + +However, it is easy to support pre-standard compilers in most programs, +so if you know how to do that, feel free. If a program you are +maintaining has such support, you should try to keep it working. + +@cindex function prototypes +To support pre-standard C, instead of writing function definitions in +standard prototype form, + +@example +int +foo (int x, int y) +@dots{} +@end example + +@noindent +write the definition in pre-standard style like this, + +@example +int +foo (x, y) + int x, y; +@dots{} +@end example + +@noindent +and use a separate declaration to specify the argument prototype: + +@example +int foo (int, int); +@end example + +You need such a declaration anyway, in a header file, to get the benefit +of prototypes in all the files where the function is called. And once +you have the declaration, you normally lose nothing by writing the +function definition in the pre-standard style. + +This technique does not work for integer types narrower than @code{int}. +If you think of an argument as being of a type narrower than @code{int}, +declare it as @code{int} instead. + +There are a few special cases where this technique is hard to use. For +example, if a function argument needs to hold the system type +@code{dev_t}, you run into trouble, because @code{dev_t} is shorter than +@code{int} on some machines; but you cannot use @code{int} instead, +because @code{dev_t} is wider than @code{int} on some machines. There +is no type you can safely use on all machines in a non-standard +definition. The only way to support non-standard C and pass such an +argument is to check the width of @code{dev_t} using Autoconf and choose +the argument type accordingly. This may not be worth the trouble. + +In order to support pre-standard compilers that do not recognize +prototypes, you may want to use a preprocessor macro like this: + +@example +/* Declare the prototype for a general external function. */ +#if defined (__STDC__) || defined (WINDOWSNT) +#define P_(proto) proto +#else +#define P_(proto) () +#endif +@end example + +@node Conditional Compilation +@section Conditional Compilation + +When supporting configuration options already known when building your +program we prefer using @code{if (... )} over conditional compilation, +as in the former case the compiler is able to perform more extensive +checking of all possible code paths. + +For example, please write + +@smallexample + if (HAS_FOO) + ... + else + ... +@end smallexample + +@noindent +instead of: + +@smallexample + #ifdef HAS_FOO + ... + #else + ... + #endif +@end smallexample + +A modern compiler such as GCC will generate exactly the same code in +both cases, and we have been using similar techniques with good success +in several projects. Of course, the former method assumes that +@code{HAS_FOO} is defined as either 0 or 1. + +While this is not a silver bullet solving all portability problems, +and is not always appropriate, following this policy would have saved +GCC developers many hours, or even days, per year. + +In the case of function-like macros like @code{REVERSIBLE_CC_MODE} in +GCC which cannot be simply used in @code{if (...)} statements, there is +an easy workaround. Simply introduce another macro +@code{HAS_REVERSIBLE_CC_MODE} as in the following example: + +@smallexample + #ifdef REVERSIBLE_CC_MODE + #define HAS_REVERSIBLE_CC_MODE 1 + #else + #define HAS_REVERSIBLE_CC_MODE 0 + #endif +@end smallexample + +@node Program Behavior +@chapter Program Behavior for All Programs + +This chapter describes conventions for writing robust +software. It also describes general standards for error messages, the +command line interface, and how libraries should behave. + +@menu +* Non-GNU Standards:: We consider standards such as POSIX; + we don't "obey" them. +* Semantics:: Writing robust programs. +* Libraries:: Library behavior. +* Errors:: Formatting error messages. +* User Interfaces:: Standards about interfaces generally. +* Graphical Interfaces:: Standards for graphical interfaces. +* Command-Line Interfaces:: Standards for command line interfaces. +* Dynamic Plug-In Interfaces:: Standards for dynamic plug-in interfaces. +* Option Table:: Table of long options. +* OID Allocations:: Table of OID slots for GNU. +* Memory Usage:: When and how to care about memory needs. +* File Usage:: Which files to use, and where. +@end menu + +@node Non-GNU Standards +@section Non-GNU Standards + +The GNU Project regards standards published by other organizations as +suggestions, not orders. We consider those standards, but we do not +``obey'' them. In developing a GNU program, you should implement +an outside standard's specifications when that makes the GNU system +better overall in an objective sense. When it doesn't, you shouldn't. + +In most cases, following published standards is convenient for +users---it means that their programs or scripts will work more +portably. For instance, GCC implements nearly all the features of +Standard C as specified by that standard. C program developers would +be unhappy if it did not. And GNU utilities mostly follow +specifications of POSIX.2; shell script writers and users would be +unhappy if our programs were incompatible. + +But we do not follow either of these specifications rigidly, and there +are specific points on which we decided not to follow them, so as to +make the GNU system better for users. + +For instance, Standard C says that nearly all extensions to C are +prohibited. How silly! GCC implements many extensions, some of which +were later adopted as part of the standard. If you want these +constructs to give an error message as ``required'' by the standard, +you must specify @samp{--pedantic}, which was implemented only so that +we can say ``GCC is a 100% implementation of the standard'', not +because there is any reason to actually use it. + +POSIX.2 specifies that @samp{df} and @samp{du} must output sizes by +default in units of 512 bytes. What users want is units of 1k, so +that is what we do by default. If you want the ridiculous behavior +``required'' by POSIX, you must set the environment variable +@samp{POSIXLY_CORRECT} (which was originally going to be named +@samp{POSIX_ME_HARDER}). + +GNU utilities also depart from the letter of the POSIX.2 specification +when they support long-named command-line options, and intermixing +options with ordinary arguments. This minor incompatibility with +POSIX is never a problem in practice, and it is very useful. + +In particular, don't reject a new feature, or remove an old one, +merely because a standard says it is ``forbidden'' or ``deprecated''. + + +@node Semantics +@section Writing Robust Programs + +@cindex arbitrary limits on data +Avoid arbitrary limits on the length or number of @emph{any} data +structure, including file names, lines, files, and symbols, by allocating +all data structures dynamically. In most Unix utilities, ``long lines +are silently truncated''. This is not acceptable in a GNU utility. + +@cindex @code{NUL} characters +@findex libiconv +Utilities reading files should not drop NUL characters, or any other +nonprinting characters @emph{including those with codes above 0177}. +The only sensible exceptions would be utilities specifically intended +for interface to certain types of terminals or printers that can't +handle those characters. Whenever possible, try to make programs work +properly with sequences of bytes that represent multibyte characters; +UTF-8 is the most important. + +@cindex error messages +Check every system call for an error return, unless you know you wish +to ignore errors. Include the system error text (from @code{perror}, +@code{strerror}, or equivalent) in @emph{every} error message +resulting from a failing system call, as well as the name of the file +if any and the name of the utility. Just ``cannot open foo.c'' or +``stat failed'' is not sufficient. + +@cindex @code{malloc} return value +@cindex memory allocation failure +Check every call to @code{malloc} or @code{realloc} to see if it +returned zero. Check @code{realloc} even if you are making the block +smaller; in a system that rounds block sizes to a power of 2, +@code{realloc} may get a different block if you ask for less space. + +In Unix, @code{realloc} can destroy the storage block if it returns +zero. GNU @code{realloc} does not have this bug: if it fails, the +original block is unchanged. Feel free to assume the bug is fixed. If +you wish to run your program on Unix, and wish to avoid lossage in this +case, you can use the GNU @code{malloc}. + +You must expect @code{free} to alter the contents of the block that was +freed. Anything you want to fetch from the block, you must fetch before +calling @code{free}. + +If @code{malloc} fails in a noninteractive program, make that a fatal +error. In an interactive program (one that reads commands from the +user), it is better to abort the command and return to the command +reader loop. This allows the user to kill other processes to free up +virtual memory, and then try the command again. + +@cindex command-line arguments, decoding +Use @code{getopt_long} to decode arguments, unless the argument syntax +makes this unreasonable. + +When static storage is to be written in during program execution, use +explicit C code to initialize it. Reserve C initialized declarations +for data that will not be changed. +@c ADR: why? + +Try to avoid low-level interfaces to obscure Unix data structures (such +as file directories, utmp, or the layout of kernel memory), since these +are less likely to work compatibly. If you need to find all the files +in a directory, use @code{readdir} or some other high-level interface. +These are supported compatibly by GNU. + +@cindex signal handling +The preferred signal handling facilities are the BSD variant of +@code{signal}, and the @sc{posix} @code{sigaction} function; the +alternative USG @code{signal} interface is an inferior design. + +Nowadays, using the @sc{posix} signal functions may be the easiest way +to make a program portable. If you use @code{signal}, then on GNU/Linux +systems running GNU libc version 1, you should include +@file{bsd/signal.h} instead of @file{signal.h}, so as to get BSD +behavior. It is up to you whether to support systems where +@code{signal} has only the USG behavior, or give up on them. + +@cindex impossible conditions +In error checks that detect ``impossible'' conditions, just abort. +There is usually no point in printing any message. These checks +indicate the existence of bugs. Whoever wants to fix the bugs will have +to read the source code and run a debugger. So explain the problem with +comments in the source. The relevant data will be in variables, which +are easy to examine with the debugger, so there is no point moving them +elsewhere. + +Do not use a count of errors as the exit status for a program. +@emph{That does not work}, because exit status values are limited to 8 +bits (0 through 255). A single run of the program might have 256 +errors; if you try to return 256 as the exit status, the parent process +will see 0 as the status, and it will appear that the program succeeded. + +@cindex temporary files +@cindex @code{TMPDIR} environment variable +If you make temporary files, check the @code{TMPDIR} environment +variable; if that variable is defined, use the specified directory +instead of @file{/tmp}. + +In addition, be aware that there is a possible security problem when +creating temporary files in world-writable directories. In C, you can +avoid this problem by creating temporary files in this manner: + +@example +fd = open (filename, O_WRONLY | O_CREAT | O_EXCL, 0600); +@end example + +@noindent +or by using the @code{mkstemps} function from Gnulib +(@pxref{mkstemps,,, gnulib, Gnulib}). + +In bash, use @code{set -C} (long name @code{noclobber}) to avoid this +problem. In addition, the @code{mktemp} utility is a more general +solution for creating temporary files from shell scripts +(@pxref{mktemp invocation,,, coreutils, GNU Coreutils}). + + +@node Libraries +@section Library Behavior +@cindex libraries + +Try to make library functions reentrant. If they need to do dynamic +storage allocation, at least try to avoid any nonreentrancy aside from +that of @code{malloc} itself. + +Here are certain name conventions for libraries, to avoid name +conflicts. + +Choose a name prefix for the library, more than two characters long. +All external function and variable names should start with this +prefix. In addition, there should only be one of these in any given +library member. This usually means putting each one in a separate +source file. + +An exception can be made when two external symbols are always used +together, so that no reasonable program could use one without the +other; then they can both go in the same file. + +External symbols that are not documented entry points for the user +should have names beginning with @samp{_}. The @samp{_} should be +followed by the chosen name prefix for the library, to prevent +collisions with other libraries. These can go in the same files with +user entry points if you like. + +Static functions and variables can be used as you like and need not +fit any naming convention. + +@node Errors +@section Formatting Error Messages +@cindex formatting error messages +@cindex error messages, formatting + +Error messages from compilers should look like this: + +@example +@var{sourcefile}:@var{lineno}: @var{message} +@end example + +@noindent +If you want to mention the column number, use one of these formats: + +@example +@var{sourcefile}:@var{lineno}:@var{column}: @var{message} +@var{sourcefile}:@var{lineno}.@var{column}: @var{message} + +@end example + +@noindent +Line numbers should start from 1 at the beginning of the file, and +column numbers should start from 1 at the beginning of the line. +(Both of these conventions are chosen for compatibility.) Calculate +column numbers assuming that space and all ASCII printing characters +have equal width, and assuming tab stops every 8 columns. For +non-ASCII characters, Unicode character widths should be used when in +a UTF-8 locale; GNU libc and GNU gnulib provide suitable +@code{wcwidth} functions. + +The error message can also give both the starting and ending positions +of the erroneous text. There are several formats so that you can +avoid redundant information such as a duplicate line number. +Here are the possible formats: + +@example +@var{sourcefile}:@var{line1}.@var{column1}-@var{line2}.@var{column2}: @var{message} +@var{sourcefile}:@var{line1}.@var{column1}-@var{column2}: @var{message} +@var{sourcefile}:@var{line1}-@var{line2}: @var{message} +@end example + +@noindent +When an error is spread over several files, you can use this format: + +@example +@var{file1}:@var{line1}.@var{column1}-@var{file2}:@var{line2}.@var{column2}: @var{message} +@end example + +Error messages from other noninteractive programs should look like this: + +@example +@var{program}:@var{sourcefile}:@var{lineno}: @var{message} +@end example + +@noindent +when there is an appropriate source file, or like this: + +@example +@var{program}: @var{message} +@end example + +@noindent +when there is no relevant source file. + +If you want to mention the column number, use this format: + +@example +@var{program}:@var{sourcefile}:@var{lineno}:@var{column}: @var{message} +@end example + +In an interactive program (one that is reading commands from a +terminal), it is better not to include the program name in an error +message. The place to indicate which program is running is in the +prompt or with the screen layout. (When the same program runs with +input from a source other than a terminal, it is not interactive and +would do best to print error messages using the noninteractive style.) + +The string @var{message} should not begin with a capital letter when +it follows a program name and/or file name, because that isn't the +beginning of a sentence. (The sentence conceptually starts at the +beginning of the line.) Also, it should not end with a period. + +Error messages from interactive programs, and other messages such as +usage messages, should start with a capital letter. But they should not +end with a period. + +@node User Interfaces +@section Standards for Interfaces Generally + +@cindex program name and its behavior +@cindex behavior, dependent on program's name +Please don't make the behavior of a utility depend on the name used +to invoke it. It is useful sometimes to make a link to a utility +with a different name, and that should not change what it does. + +Instead, use a run time option or a compilation switch or both +to select among the alternate behaviors. + +@cindex output device and program's behavior +Likewise, please don't make the behavior of the program depend on the +type of output device it is used with. Device independence is an +important principle of the system's design; do not compromise it merely +to save someone from typing an option now and then. (Variation in error +message syntax when using a terminal is ok, because that is a side issue +that people do not depend on.) + +If you think one behavior is most useful when the output is to a +terminal, and another is most useful when the output is a file or a +pipe, then it is usually best to make the default behavior the one that +is useful with output to a terminal, and have an option for the other +behavior. + +Compatibility requires certain programs to depend on the type of output +device. It would be disastrous if @code{ls} or @code{sh} did not do so +in the way all users expect. In some of these cases, we supplement the +program with a preferred alternate version that does not depend on the +output device type. For example, we provide a @code{dir} program much +like @code{ls} except that its default output format is always +multi-column format. + + +@node Graphical Interfaces +@section Standards for Graphical Interfaces +@cindex graphical user interface +@cindex interface styles +@cindex user interface styles + +@cindex GTK+ +When you write a program that provides a graphical user interface, +please make it work with the X Window System and the GTK+ toolkit +unless the functionality specifically requires some alternative (for +example, ``displaying jpeg images while in console mode''). + +In addition, please provide a command-line interface to control the +functionality. (In many cases, the graphical user interface can be a +separate program which invokes the command-line program.) This is +so that the same jobs can be done from scripts. + +@cindex CORBA +@cindex GNOME +@cindex D-bus +@cindex keyboard interface +@cindex library interface +Please also consider providing a D-bus interface for use from other +running programs, such as within GNOME. (GNOME used to use CORBA +for this, but that is being phased out.) In addition, consider +providing a library interface (for use from C), and perhaps a +keyboard-driven console interface (for use by users from console +mode). Once you are doing the work to provide the functionality and +the graphical interface, these won't be much extra work. + + +@node Command-Line Interfaces +@section Standards for Command Line Interfaces +@cindex command-line interface + +@findex getopt +It is a good idea to follow the @sc{posix} guidelines for the +command-line options of a program. The easiest way to do this is to use +@code{getopt} to parse them. Note that the GNU version of @code{getopt} +will normally permit options anywhere among the arguments unless the +special argument @samp{--} is used. This is not what @sc{posix} +specifies; it is a GNU extension. + +@cindex long-named options +Please define long-named options that are equivalent to the +single-letter Unix-style options. We hope to make GNU more user +friendly this way. This is easy to do with the GNU function +@code{getopt_long}. + +One of the advantages of long-named options is that they can be +consistent from program to program. For example, users should be able +to expect the ``verbose'' option of any GNU program which has one, to be +spelled precisely @samp{--verbose}. To achieve this uniformity, look at +the table of common long-option names when you choose the option names +for your program (@pxref{Option Table}). + +It is usually a good idea for file names given as ordinary arguments to +be input files only; any output files would be specified using options +(preferably @samp{-o} or @samp{--output}). Even if you allow an output +file name as an ordinary argument for compatibility, try to provide an +option as another way to specify it. This will lead to more consistency +among GNU utilities, and fewer idiosyncrasies for users to remember. + +@cindex standard command-line options +@cindex options, standard command-line +@cindex CGI programs, standard options for +@cindex PATH_INFO, specifying standard options as +All programs should support two standard options: @samp{--version} +and @samp{--help}. CGI programs should accept these as command-line +options, and also if given as the @env{PATH_INFO}; for instance, +visiting @url{http://example.org/p.cgi/--help} in a browser should +output the same information as invoking @samp{p.cgi --help} from the +command line. + +@menu +* --version:: The standard output for --version. +* --help:: The standard output for --help. +@end menu + +@node --version +@subsection @option{--version} + +@cindex @samp{--version} output + +The standard @code{--version} option should direct the program to +print information about its name, version, origin and legal status, +all on standard output, and then exit successfully. Other options and +arguments should be ignored once this is seen, and the program should +not perform its normal function. + +@cindex canonical name of a program +@cindex program's canonical name +The first line is meant to be easy for a program to parse; the version +number proper starts after the last space. In addition, it contains +the canonical name for this program, in this format: + +@example +GNU Emacs 19.30 +@end example + +@noindent +The program's name should be a constant string; @emph{don't} compute it +from @code{argv[0]}. The idea is to state the standard or canonical +name for the program, not its file name. There are other ways to find +out the precise file name where a command is found in @code{PATH}. + +If the program is a subsidiary part of a larger package, mention the +package name in parentheses, like this: + +@example +emacsserver (GNU Emacs) 19.30 +@end example + +@noindent +If the package has a version number which is different from this +program's version number, you can mention the package version number +just before the close-parenthesis. + +If you @emph{need} to mention the version numbers of libraries which +are distributed separately from the package which contains this program, +you can do so by printing an additional line of version info for each +library you want to mention. Use the same format for these lines as for +the first line. + +Please do not mention all of the libraries that the program uses ``just +for completeness''---that would produce a lot of unhelpful clutter. +Please mention library version numbers only if you find in practice that +they are very important to you in debugging. + +The following line, after the version number line or lines, should be a +copyright notice. If more than one copyright notice is called for, put +each on a separate line. + +Next should follow a line stating the license, preferably using one of +abbreviations below, and a brief statement that the program is free +software, and that users are free to copy and change it. Also mention +that there is no warranty, to the extent permitted by law. See +recommended wording below. + +It is ok to finish the output with a list of the major authors of the +program, as a way of giving credit. + +Here's an example of output that follows these rules: + +@smallexample +GNU hello 2.3 +Copyright (C) 2007 Free Software Foundation, Inc. +License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html> +This is free software: you are free to change and redistribute it. +There is NO WARRANTY, to the extent permitted by law. +@end smallexample + +You should adapt this to your program, of course, filling in the proper +year, copyright holder, name of program, and the references to +distribution terms, and changing the rest of the wording as necessary. + +This copyright notice only needs to mention the most recent year in +which changes were made---there's no need to list the years for previous +versions' changes. You don't have to mention the name of the program in +these notices, if that is inconvenient, since it appeared in the first +line. (The rules are different for copyright notices in source files; +@pxref{Copyright Notices,,,maintain,Information for GNU Maintainers}.) + +Translations of the above lines must preserve the validity of the +copyright notices (@pxref{Internationalization}). If the translation's +character set supports it, the @samp{(C)} should be replaced with the +copyright symbol, as follows: + +@ifinfo +(the official copyright symbol, which is the letter C in a circle); +@end ifinfo +@ifnotinfo +@copyright{} +@end ifnotinfo + +Write the word ``Copyright'' exactly like that, in English. Do not +translate it into another language. International treaties recognize +the English word ``Copyright''; translations into other languages do not +have legal significance. + +Finally, here is the table of our suggested license abbreviations. +Any abbreviation can be followed by @samp{v@var{version}[+]}, meaning +that particular version, or later versions with the @samp{+}, as shown +above. + +In the case of exceptions for extra permissions with the GPL, we use +@samp{/} for a separator; the version number can follow the license +abbreviation as usual, as in the examples below. + +@table @asis +@item GPL +GNU General Public License, @url{http://www.gnu.org/@/licenses/@/gpl.html}. + +@item LGPL +GNU Lesser General Public License, @url{http://www.gnu.org/@/licenses/@/lgpl.html}. + +@item GPL/Ada +GNU GPL with the exception for Ada. + +@item Apache +The Apache Software Foundation license, +@url{http://www.apache.org/@/licenses}. + +@item Artistic +The Artistic license used for Perl, @url{http://www.perlfoundation.org/@/legal}. + +@item Expat +The Expat license, @url{http://www.jclark.com/@/xml/@/copying.txt}. + +@item MPL +The Mozilla Public License, @url{http://www.mozilla.org/@/MPL/}. + +@item OBSD +The original (4-clause) BSD license, incompatible with the GNU GPL +@url{http://www.xfree86.org/@/3.3.6/@/COPYRIGHT2.html#6}. + +@item PHP +The license used for PHP, @url{http://www.php.net/@/license/}. + +@item public domain +The non-license that is being in the public domain, +@url{http://www.gnu.org/@/licenses/@/license-list.html#PublicDomain}. + +@item Python +The license for Python, @url{http://www.python.org/@/2.0.1/@/license.html}. + +@item RBSD +The revised (3-clause) BSD, compatible with the GNU GPL,@* +@url{http://www.xfree86.org/@/3.3.6/@/COPYRIGHT2.html#5}. + +@item X11 +The simple non-copyleft license used for most versions of the X Window +System, @url{http://www.xfree86.org/@/3.3.6/@/COPYRIGHT2.html#3}. + +@item Zlib +The license for Zlib, @url{http://www.gzip.org/@/zlib/@/zlib_license.html}. + +@end table + +More information about these licenses and many more are on the GNU +licensing web pages, +@url{http://www.gnu.org/@/licenses/@/license-list.html}. + + +@node --help +@subsection @option{--help} + +@cindex @samp{--help} output + +The standard @code{--help} option should output brief documentation +for how to invoke the program, on standard output, then exit +successfully. Other options and arguments should be ignored once this +is seen, and the program should not perform its normal function. + +@cindex address for bug reports +@cindex bug reports +Near the end of the @samp{--help} option's output, please place lines +giving the email address for bug reports, the package's home page +(normally @indicateurl{http://www.gnu.org/software/@var{pkg}}, and the +general page for help using GNU programs. The format should be like this: + +@example +Report bugs to: @var{mailing-address} +@var{pkg} home page: <http://www.gnu.org/software/@var{pkg}/> +General help using GNU software: <http://www.gnu.org/gethelp/> +@end example + +It is ok to mention other appropriate mailing lists and web pages. + + +@node Dynamic Plug-In Interfaces +@section Standards for Dynamic Plug-in Interfaces +@cindex plug-ins +@cindex dynamic plug-ins + +Another aspect of keeping free programs free is encouraging +development of free plug-ins, and discouraging development of +proprietary plug-ins. Many GNU programs will not have anything like +plug-ins at all, but those that do should follow these +practices. + +First, the general plug-in architecture design should closely tie the +plug-in to the original code, such that the plug-in and the base +program are parts of one extended program. For GCC, for example, +plug-ins receive and modify GCC's internal data structures, and so +clearly form an extended program with the base GCC. + +@vindex plugin_is_GPL_compatible +Second, you should require plug-in developers to affirm that their +plug-ins are released under an appropriate license. This should be +enforced with a simple programmatic check. For GCC, again for +example, a plug-in must define the global symbol +@code{plugin_is_GPL_compatible}, thus asserting that the plug-in is +released under a GPL-compatible license (@pxref{Plugins,, Plugins, +gccint, GCC Internals}). + +By adding this check to your program you are not creating a new legal +requirement. The GPL itself requires plug-ins to be free software, +licensed compatibly. As long as you have followed the first rule above +to keep plug-ins closely tied to your original program, the GPL and AGPL +already require those plug-ins to be released under a compatible +license. The symbol definition in the plug-in---or whatever equivalent +works best in your program---makes it harder for anyone who might +distribute proprietary plug-ins to legally defend themselves. If a case +about this got to court, we can point to that symbol as evidence that +the plug-in developer understood that the license had this requirement. + + +@node Option Table +@section Table of Long Options +@cindex long option names +@cindex table of long options + +Here is a table of long options used by GNU programs. It is surely +incomplete, but we aim to list all the options that a new program might +want to be compatible with. If you use names not already in the table, +please send @email{bug-standards@@gnu.org} a list of them, with their +meanings, so we can update the table. + +@c Please leave newlines between items in this table; it's much easier +@c to update when it isn't completely squashed together and unreadable. +@c When there is more than one short option for a long option name, put +@c a semicolon between the lists of the programs that use them, not a +@c period. --friedman + +@table @samp +@item after-date +@samp{-N} in @code{tar}. + +@item all +@samp{-a} in @code{du}, @code{ls}, @code{nm}, @code{stty}, @code{uname}, +and @code{unexpand}. + +@item all-text +@samp{-a} in @code{diff}. + +@item almost-all +@samp{-A} in @code{ls}. + +@item append +@samp{-a} in @code{etags}, @code{tee}, @code{time}; +@samp{-r} in @code{tar}. + +@item archive +@samp{-a} in @code{cp}. + +@item archive-name +@samp{-n} in @code{shar}. + +@item arglength +@samp{-l} in @code{m4}. + +@item ascii +@samp{-a} in @code{diff}. + +@item assign +@samp{-v} in @code{gawk}. + +@item assume-new +@samp{-W} in @code{make}. + +@item assume-old +@samp{-o} in @code{make}. + +@item auto-check +@samp{-a} in @code{recode}. + +@item auto-pager +@samp{-a} in @code{wdiff}. + +@item auto-reference +@samp{-A} in @code{ptx}. + +@item avoid-wraps +@samp{-n} in @code{wdiff}. + +@item background +For server programs, run in the background. + +@item backward-search +@samp{-B} in @code{ctags}. + +@item basename +@samp{-f} in @code{shar}. + +@item batch +Used in GDB. + +@item baud +Used in GDB. + +@item before +@samp{-b} in @code{tac}. + +@item binary +@samp{-b} in @code{cpio} and @code{diff}. + +@item bits-per-code +@samp{-b} in @code{shar}. + +@item block-size +Used in @code{cpio} and @code{tar}. + +@item blocks +@samp{-b} in @code{head} and @code{tail}. + +@item break-file +@samp{-b} in @code{ptx}. + +@item brief +Used in various programs to make output shorter. + +@item bytes +@samp{-c} in @code{head}, @code{split}, and @code{tail}. + +@item c@t{++} +@samp{-C} in @code{etags}. + +@item catenate +@samp{-A} in @code{tar}. + +@item cd +Used in various programs to specify the directory to use. + +@item changes +@samp{-c} in @code{chgrp} and @code{chown}. + +@item classify +@samp{-F} in @code{ls}. + +@item colons +@samp{-c} in @code{recode}. + +@item command +@samp{-c} in @code{su}; +@samp{-x} in GDB. + +@item compare +@samp{-d} in @code{tar}. + +@item compat +Used in @code{gawk}. + +@item compress +@samp{-Z} in @code{tar} and @code{shar}. + +@item concatenate +@samp{-A} in @code{tar}. + +@item confirmation +@samp{-w} in @code{tar}. + +@item context +Used in @code{diff}. + +@item copyleft +@samp{-W copyleft} in @code{gawk}. + +@item copyright +@samp{-C} in @code{ptx}, @code{recode}, and @code{wdiff}; +@samp{-W copyright} in @code{gawk}. + +@item core +Used in GDB. + +@item count +@samp{-q} in @code{who}. + +@item count-links +@samp{-l} in @code{du}. + +@item create +Used in @code{tar} and @code{cpio}. + +@item cut-mark +@samp{-c} in @code{shar}. + +@item cxref +@samp{-x} in @code{ctags}. + +@item date +@samp{-d} in @code{touch}. + +@item debug +@samp{-d} in @code{make} and @code{m4}; +@samp{-t} in Bison. + +@item define +@samp{-D} in @code{m4}. + +@item defines +@samp{-d} in Bison and @code{ctags}. + +@item delete +@samp{-D} in @code{tar}. + +@item dereference +@samp{-L} in @code{chgrp}, @code{chown}, @code{cpio}, @code{du}, +@code{ls}, and @code{tar}. + +@item dereference-args +@samp{-D} in @code{du}. + +@item device +Specify an I/O device (special file name). + +@item diacritics +@samp{-d} in @code{recode}. + +@item dictionary-order +@samp{-d} in @code{look}. + +@item diff +@samp{-d} in @code{tar}. + +@item digits +@samp{-n} in @code{csplit}. + +@item directory +Specify the directory to use, in various programs. In @code{ls}, it +means to show directories themselves rather than their contents. In +@code{rm} and @code{ln}, it means to not treat links to directories +specially. + +@item discard-all +@samp{-x} in @code{strip}. + +@item discard-locals +@samp{-X} in @code{strip}. + +@item dry-run +@samp{-n} in @code{make}. + +@item ed +@samp{-e} in @code{diff}. + +@item elide-empty-files +@samp{-z} in @code{csplit}. + +@item end-delete +@samp{-x} in @code{wdiff}. + +@item end-insert +@samp{-z} in @code{wdiff}. + +@item entire-new-file +@samp{-N} in @code{diff}. + +@item environment-overrides +@samp{-e} in @code{make}. + +@item eof +@samp{-e} in @code{xargs}. + +@item epoch +Used in GDB. + +@item error-limit +Used in @code{makeinfo}. + +@item error-output +@samp{-o} in @code{m4}. + +@item escape +@samp{-b} in @code{ls}. + +@item exclude-from +@samp{-X} in @code{tar}. + +@item exec +Used in GDB. + +@item exit +@samp{-x} in @code{xargs}. + +@item exit-0 +@samp{-e} in @code{unshar}. + +@item expand-tabs +@samp{-t} in @code{diff}. + +@item expression +@samp{-e} in @code{sed}. + +@item extern-only +@samp{-g} in @code{nm}. + +@item extract +@samp{-i} in @code{cpio}; +@samp{-x} in @code{tar}. + +@item faces +@samp{-f} in @code{finger}. + +@item fast +@samp{-f} in @code{su}. + +@item fatal-warnings +@samp{-E} in @code{m4}. + +@item file +@samp{-f} in @code{gawk}, @code{info}, @code{make}, @code{mt}, +@code{sed}, and @code{tar}. + +@item field-separator +@samp{-F} in @code{gawk}. + +@item file-prefix +@samp{-b} in Bison. + +@item file-type +@samp{-F} in @code{ls}. + +@item files-from +@samp{-T} in @code{tar}. + +@item fill-column +Used in @code{makeinfo}. + +@item flag-truncation +@samp{-F} in @code{ptx}. + +@item fixed-output-files +@samp{-y} in Bison. + +@item follow +@samp{-f} in @code{tail}. + +@item footnote-style +Used in @code{makeinfo}. + +@item force +@samp{-f} in @code{cp}, @code{ln}, @code{mv}, and @code{rm}. + +@item force-prefix +@samp{-F} in @code{shar}. + +@item foreground +For server programs, run in the foreground; +in other words, don't do anything special to run the server +in the background. + +@item format +Used in @code{ls}, @code{time}, and @code{ptx}. + +@item freeze-state +@samp{-F} in @code{m4}. + +@item fullname +Used in GDB. + +@item gap-size +@samp{-g} in @code{ptx}. + +@item get +@samp{-x} in @code{tar}. + +@item graphic +@samp{-i} in @code{ul}. + +@item graphics +@samp{-g} in @code{recode}. + +@item group +@samp{-g} in @code{install}. + +@item gzip +@samp{-z} in @code{tar} and @code{shar}. + +@item hashsize +@samp{-H} in @code{m4}. + +@item header +@samp{-h} in @code{objdump} and @code{recode} + +@item heading +@samp{-H} in @code{who}. + +@item help +Used to ask for brief usage information. + +@item here-delimiter +@samp{-d} in @code{shar}. + +@item hide-control-chars +@samp{-q} in @code{ls}. + +@item html +In @code{makeinfo}, output HTML. + +@item idle +@samp{-u} in @code{who}. + +@item ifdef +@samp{-D} in @code{diff}. + +@item ignore +@samp{-I} in @code{ls}; +@samp{-x} in @code{recode}. + +@item ignore-all-space +@samp{-w} in @code{diff}. + +@item ignore-backups +@samp{-B} in @code{ls}. + +@item ignore-blank-lines +@samp{-B} in @code{diff}. + +@item ignore-case +@samp{-f} in @code{look} and @code{ptx}; +@samp{-i} in @code{diff} and @code{wdiff}. + +@item ignore-errors +@samp{-i} in @code{make}. + +@item ignore-file +@samp{-i} in @code{ptx}. + +@item ignore-indentation +@samp{-I} in @code{etags}. + +@item ignore-init-file +@samp{-f} in Oleo. + +@item ignore-interrupts +@samp{-i} in @code{tee}. + +@item ignore-matching-lines +@samp{-I} in @code{diff}. + +@item ignore-space-change +@samp{-b} in @code{diff}. + +@item ignore-zeros +@samp{-i} in @code{tar}. + +@item include +@samp{-i} in @code{etags}; +@samp{-I} in @code{m4}. + +@item include-dir +@samp{-I} in @code{make}. + +@item incremental +@samp{-G} in @code{tar}. + +@item info +@samp{-i}, @samp{-l}, and @samp{-m} in Finger. + +@item init-file +In some programs, specify the name of the file to read as the user's +init file. + +@item initial +@samp{-i} in @code{expand}. + +@item initial-tab +@samp{-T} in @code{diff}. + +@item inode +@samp{-i} in @code{ls}. + +@item interactive +@samp{-i} in @code{cp}, @code{ln}, @code{mv}, @code{rm}; +@samp{-e} in @code{m4}; +@samp{-p} in @code{xargs}; +@samp{-w} in @code{tar}. + +@item intermix-type +@samp{-p} in @code{shar}. + +@item iso-8601 +Used in @code{date} + +@item jobs +@samp{-j} in @code{make}. + +@item just-print +@samp{-n} in @code{make}. + +@item keep-going +@samp{-k} in @code{make}. + +@item keep-files +@samp{-k} in @code{csplit}. + +@item kilobytes +@samp{-k} in @code{du} and @code{ls}. + +@item language +@samp{-l} in @code{etags}. + +@item less-mode +@samp{-l} in @code{wdiff}. + +@item level-for-gzip +@samp{-g} in @code{shar}. + +@item line-bytes +@samp{-C} in @code{split}. + +@item lines +Used in @code{split}, @code{head}, and @code{tail}. + +@item link +@samp{-l} in @code{cpio}. + +@item lint +@itemx lint-old +Used in @code{gawk}. + +@item list +@samp{-t} in @code{cpio}; +@samp{-l} in @code{recode}. + +@item list +@samp{-t} in @code{tar}. + +@item literal +@samp{-N} in @code{ls}. + +@item load-average +@samp{-l} in @code{make}. + +@item login +Used in @code{su}. + +@item machine +Used in @code{uname}. + +@item macro-name +@samp{-M} in @code{ptx}. + +@item mail +@samp{-m} in @code{hello} and @code{uname}. + +@item make-directories +@samp{-d} in @code{cpio}. + +@item makefile +@samp{-f} in @code{make}. + +@item mapped +Used in GDB. + +@item max-args +@samp{-n} in @code{xargs}. + +@item max-chars +@samp{-n} in @code{xargs}. + +@item max-lines +@samp{-l} in @code{xargs}. + +@item max-load +@samp{-l} in @code{make}. + +@item max-procs +@samp{-P} in @code{xargs}. + +@item mesg +@samp{-T} in @code{who}. + +@item message +@samp{-T} in @code{who}. + +@item minimal +@samp{-d} in @code{diff}. + +@item mixed-uuencode +@samp{-M} in @code{shar}. + +@item mode +@samp{-m} in @code{install}, @code{mkdir}, and @code{mkfifo}. + +@item modification-time +@samp{-m} in @code{tar}. + +@item multi-volume +@samp{-M} in @code{tar}. + +@item name-prefix +@samp{-a} in Bison. + +@item nesting-limit +@samp{-L} in @code{m4}. + +@item net-headers +@samp{-a} in @code{shar}. + +@item new-file +@samp{-W} in @code{make}. + +@item no-builtin-rules +@samp{-r} in @code{make}. + +@item no-character-count +@samp{-w} in @code{shar}. + +@item no-check-existing +@samp{-x} in @code{shar}. + +@item no-common +@samp{-3} in @code{wdiff}. + +@item no-create +@samp{-c} in @code{touch}. + +@item no-defines +@samp{-D} in @code{etags}. + +@item no-deleted +@samp{-1} in @code{wdiff}. + +@item no-dereference +@samp{-d} in @code{cp}. + +@item no-inserted +@samp{-2} in @code{wdiff}. + +@item no-keep-going +@samp{-S} in @code{make}. + +@item no-lines +@samp{-l} in Bison. + +@item no-piping +@samp{-P} in @code{shar}. + +@item no-prof +@samp{-e} in @code{gprof}. + +@item no-regex +@samp{-R} in @code{etags}. + +@item no-sort +@samp{-p} in @code{nm}. + +@item no-splash +Don't print a startup splash screen. + +@item no-split +Used in @code{makeinfo}. + +@item no-static +@samp{-a} in @code{gprof}. + +@item no-time +@samp{-E} in @code{gprof}. + +@item no-timestamp +@samp{-m} in @code{shar}. + +@item no-validate +Used in @code{makeinfo}. + +@item no-wait +Used in @code{emacsclient}. + +@item no-warn +Used in various programs to inhibit warnings. + +@item node +@samp{-n} in @code{info}. + +@item nodename +@samp{-n} in @code{uname}. + +@item nonmatching +@samp{-f} in @code{cpio}. + +@item nstuff +@samp{-n} in @code{objdump}. + +@item null +@samp{-0} in @code{xargs}. + +@item number +@samp{-n} in @code{cat}. + +@item number-nonblank +@samp{-b} in @code{cat}. + +@item numeric-sort +@samp{-n} in @code{nm}. + +@item numeric-uid-gid +@samp{-n} in @code{cpio} and @code{ls}. + +@item nx +Used in GDB. + +@item old-archive +@samp{-o} in @code{tar}. + +@item old-file +@samp{-o} in @code{make}. + +@item one-file-system +@samp{-l} in @code{tar}, @code{cp}, and @code{du}. + +@item only-file +@samp{-o} in @code{ptx}. + +@item only-prof +@samp{-f} in @code{gprof}. + +@item only-time +@samp{-F} in @code{gprof}. + +@item options +@samp{-o} in @code{getopt}, @code{fdlist}, @code{fdmount}, +@code{fdmountd}, and @code{fdumount}. + +@item output +In various programs, specify the output file name. + +@item output-prefix +@samp{-o} in @code{shar}. + +@item override +@samp{-o} in @code{rm}. + +@item overwrite +@samp{-c} in @code{unshar}. + +@item owner +@samp{-o} in @code{install}. + +@item paginate +@samp{-l} in @code{diff}. + +@item paragraph-indent +Used in @code{makeinfo}. + +@item parents +@samp{-p} in @code{mkdir} and @code{rmdir}. + +@item pass-all +@samp{-p} in @code{ul}. + +@item pass-through +@samp{-p} in @code{cpio}. + +@item port +@samp{-P} in @code{finger}. + +@item portability +@samp{-c} in @code{cpio} and @code{tar}. + +@item posix +Used in @code{gawk}. + +@item prefix-builtins +@samp{-P} in @code{m4}. + +@item prefix +@samp{-f} in @code{csplit}. + +@item preserve +Used in @code{tar} and @code{cp}. + +@item preserve-environment +@samp{-p} in @code{su}. + +@item preserve-modification-time +@samp{-m} in @code{cpio}. + +@item preserve-order +@samp{-s} in @code{tar}. + +@item preserve-permissions +@samp{-p} in @code{tar}. + +@item print +@samp{-l} in @code{diff}. + +@item print-chars +@samp{-L} in @code{cmp}. + +@item print-data-base +@samp{-p} in @code{make}. + +@item print-directory +@samp{-w} in @code{make}. + +@item print-file-name +@samp{-o} in @code{nm}. + +@item print-symdefs +@samp{-s} in @code{nm}. + +@item printer +@samp{-p} in @code{wdiff}. + +@item prompt +@samp{-p} in @code{ed}. + +@item proxy +Specify an HTTP proxy. + +@item query-user +@samp{-X} in @code{shar}. + +@item question +@samp{-q} in @code{make}. + +@item quiet +Used in many programs to inhibit the usual output. Every +program accepting @samp{--quiet} should accept @samp{--silent} as a +synonym. + +@item quiet-unshar +@samp{-Q} in @code{shar} + +@item quote-name +@samp{-Q} in @code{ls}. + +@item rcs +@samp{-n} in @code{diff}. + +@item re-interval +Used in @code{gawk}. + +@item read-full-blocks +@samp{-B} in @code{tar}. + +@item readnow +Used in GDB. + +@item recon +@samp{-n} in @code{make}. + +@item record-number +@samp{-R} in @code{tar}. + +@item recursive +Used in @code{chgrp}, @code{chown}, @code{cp}, @code{ls}, @code{diff}, +and @code{rm}. + +@item reference +@samp{-r} in @code{touch}. + +@item references +@samp{-r} in @code{ptx}. + +@item regex +@samp{-r} in @code{tac} and @code{etags}. + +@item release +@samp{-r} in @code{uname}. + +@item reload-state +@samp{-R} in @code{m4}. + +@item relocation +@samp{-r} in @code{objdump}. + +@item rename +@samp{-r} in @code{cpio}. + +@item replace +@samp{-i} in @code{xargs}. + +@item report-identical-files +@samp{-s} in @code{diff}. + +@item reset-access-time +@samp{-a} in @code{cpio}. + +@item reverse +@samp{-r} in @code{ls} and @code{nm}. + +@item reversed-ed +@samp{-f} in @code{diff}. + +@item right-side-defs +@samp{-R} in @code{ptx}. + +@item same-order +@samp{-s} in @code{tar}. + +@item same-permissions +@samp{-p} in @code{tar}. + +@item save +@samp{-g} in @code{stty}. + +@item se +Used in GDB. + +@item sentence-regexp +@samp{-S} in @code{ptx}. + +@item separate-dirs +@samp{-S} in @code{du}. + +@item separator +@samp{-s} in @code{tac}. + +@item sequence +Used by @code{recode} to chose files or pipes for sequencing passes. + +@item shell +@samp{-s} in @code{su}. + +@item show-all +@samp{-A} in @code{cat}. + +@item show-c-function +@samp{-p} in @code{diff}. + +@item show-ends +@samp{-E} in @code{cat}. + +@item show-function-line +@samp{-F} in @code{diff}. + +@item show-tabs +@samp{-T} in @code{cat}. + +@item silent +Used in many programs to inhibit the usual output. +Every program accepting +@samp{--silent} should accept @samp{--quiet} as a synonym. + +@item size +@samp{-s} in @code{ls}. + +@item socket +Specify a file descriptor for a network server to use for its socket, +instead of opening and binding a new socket. This provides a way to +run, in a non-privileged process, a server that normally needs a +reserved port number. + +@item sort +Used in @code{ls}. + +@item source +@samp{-W source} in @code{gawk}. + +@item sparse +@samp{-S} in @code{tar}. + +@item speed-large-files +@samp{-H} in @code{diff}. + +@item split-at +@samp{-E} in @code{unshar}. + +@item split-size-limit +@samp{-L} in @code{shar}. + +@item squeeze-blank +@samp{-s} in @code{cat}. + +@item start-delete +@samp{-w} in @code{wdiff}. + +@item start-insert +@samp{-y} in @code{wdiff}. + +@item starting-file +Used in @code{tar} and @code{diff} to specify which file within +a directory to start processing with. + +@item statistics +@samp{-s} in @code{wdiff}. + +@item stdin-file-list +@samp{-S} in @code{shar}. + +@item stop +@samp{-S} in @code{make}. + +@item strict +@samp{-s} in @code{recode}. + +@item strip +@samp{-s} in @code{install}. + +@item strip-all +@samp{-s} in @code{strip}. + +@item strip-debug +@samp{-S} in @code{strip}. + +@item submitter +@samp{-s} in @code{shar}. + +@item suffix +@samp{-S} in @code{cp}, @code{ln}, @code{mv}. + +@item suffix-format +@samp{-b} in @code{csplit}. + +@item sum +@samp{-s} in @code{gprof}. + +@item summarize +@samp{-s} in @code{du}. + +@item symbolic +@samp{-s} in @code{ln}. + +@item symbols +Used in GDB and @code{objdump}. + +@item synclines +@samp{-s} in @code{m4}. + +@item sysname +@samp{-s} in @code{uname}. + +@item tabs +@samp{-t} in @code{expand} and @code{unexpand}. + +@item tabsize +@samp{-T} in @code{ls}. + +@item terminal +@samp{-T} in @code{tput} and @code{ul}. +@samp{-t} in @code{wdiff}. + +@item text +@samp{-a} in @code{diff}. + +@item text-files +@samp{-T} in @code{shar}. + +@item time +Used in @code{ls} and @code{touch}. + +@item timeout +Specify how long to wait before giving up on some operation. + +@item to-stdout +@samp{-O} in @code{tar}. + +@item total +@samp{-c} in @code{du}. + +@item touch +@samp{-t} in @code{make}, @code{ranlib}, and @code{recode}. + +@item trace +@samp{-t} in @code{m4}. + +@item traditional +@samp{-t} in @code{hello}; +@samp{-W traditional} in @code{gawk}; +@samp{-G} in @code{ed}, @code{m4}, and @code{ptx}. + +@item tty +Used in GDB. + +@item typedefs +@samp{-t} in @code{ctags}. + +@item typedefs-and-c++ +@samp{-T} in @code{ctags}. + +@item typeset-mode +@samp{-t} in @code{ptx}. + +@item uncompress +@samp{-z} in @code{tar}. + +@item unconditional +@samp{-u} in @code{cpio}. + +@item undefine +@samp{-U} in @code{m4}. + +@item undefined-only +@samp{-u} in @code{nm}. + +@item update +@samp{-u} in @code{cp}, @code{ctags}, @code{mv}, @code{tar}. + +@item usage +Used in @code{gawk}; same as @samp{--help}. + +@item uuencode +@samp{-B} in @code{shar}. + +@item vanilla-operation +@samp{-V} in @code{shar}. + +@item verbose +Print more information about progress. Many programs support this. + +@item verify +@samp{-W} in @code{tar}. + +@item version +Print the version number. + +@item version-control +@samp{-V} in @code{cp}, @code{ln}, @code{mv}. + +@item vgrind +@samp{-v} in @code{ctags}. + +@item volume +@samp{-V} in @code{tar}. + +@item what-if +@samp{-W} in @code{make}. + +@item whole-size-limit +@samp{-l} in @code{shar}. + +@item width +@samp{-w} in @code{ls} and @code{ptx}. + +@item word-regexp +@samp{-W} in @code{ptx}. + +@item writable +@samp{-T} in @code{who}. + +@item zeros +@samp{-z} in @code{gprof}. +@end table + +@node OID Allocations +@section OID Allocations +@cindex OID allocations for GNU +@cindex SNMP +@cindex LDAP +@cindex X.509 + +The OID (object identifier) 1.3.6.1.4.1.11591 has been assigned to the +GNU Project (thanks to Werner Koch). These are used for SNMP, LDAP, +X.509 certificates, and so on. The web site +@url{http://www.alvestrand.no/objectid} has a (voluntary) listing of +many OID assignments. + +If you need a new slot for your GNU package, write +@email{maintainers@@gnu.org}. Here is a list of arcs currently +assigned: + +@example +@include gnu-oids.texi +@end example + + +@node Memory Usage +@section Memory Usage +@cindex memory usage + +If a program typically uses just a few meg of memory, don't bother making any +effort to reduce memory usage. For example, if it is impractical for +other reasons to operate on files more than a few meg long, it is +reasonable to read entire input files into memory to operate on them. + +However, for programs such as @code{cat} or @code{tail}, that can +usefully operate on very large files, it is important to avoid using a +technique that would artificially limit the size of files it can handle. +If a program works by lines and could be applied to arbitrary +user-supplied input files, it should keep only a line in memory, because +this is not very hard and users will want to be able to operate on input +files that are bigger than will fit in memory all at once. + +If your program creates complicated data structures, just make them in +memory and give a fatal error if @code{malloc} returns zero. + +@pindex valgrind +@cindex memory leak +Memory analysis tools such as @command{valgrind} can be useful, but +don't complicate a program merely to avoid their false alarms. For +example, if memory is used until just before a process exits, don't +free it simply to silence such a tool. + + +@node File Usage +@section File Usage +@cindex file usage + +Programs should be prepared to operate when @file{/usr} and @file{/etc} +are read-only file systems. Thus, if the program manages log files, +lock files, backup files, score files, or any other files which are +modified for internal purposes, these files should not be stored in +@file{/usr} or @file{/etc}. + +There are two exceptions. @file{/etc} is used to store system +configuration information; it is reasonable for a program to modify +files in @file{/etc} when its job is to update the system configuration. +Also, if the user explicitly asks to modify one file in a directory, it +is reasonable for the program to store other files in the same +directory. + +@node Writing C +@chapter Making The Best Use of C + +This chapter provides advice on how best to use the C language +when writing GNU software. + +@menu +* Formatting:: Formatting your source code. +* Comments:: Commenting your work. +* Syntactic Conventions:: Clean use of C constructs. +* Names:: Naming variables, functions, and files. +* System Portability:: Portability among different operating systems. +* CPU Portability:: Supporting the range of CPU types. +* System Functions:: Portability and ``standard'' library functions. +* Internationalization:: Techniques for internationalization. +* Character Set:: Use ASCII by default. +* Quote Characters:: Use "..." or '...' in the C locale. +* Mmap:: How you can safely use @code{mmap}. +@end menu + +@node Formatting +@section Formatting Your Source Code +@cindex formatting source code + +@cindex open brace +@cindex braces, in C source +@cindex function definitions, formatting +It is important to put the open-brace that starts the body of a C +function in column one, so that they will start a defun. Several +tools look for open-braces in column one to find the beginnings of C +functions. These tools will not work on code not formatted that way. + +Avoid putting open-brace, open-parenthesis or open-bracket in column +one when they are inside a function, so that they won't start a defun. +The open-brace that starts a @code{struct} body can go in column one +if you find it useful to treat that definition as a defun. + +It is also important for function definitions to start the name of the +function in column one. This helps people to search for function +definitions, and may also help certain tools recognize them. Thus, +using Standard C syntax, the format is this: + +@example +static char * +concat (char *s1, char *s2) +@{ + @dots{} +@} +@end example + +@noindent +or, if you want to use traditional C syntax, format the definition like +this: + +@example +static char * +concat (s1, s2) /* Name starts in column one here */ + char *s1, *s2; +@{ /* Open brace in column one here */ + @dots{} +@} +@end example + +In Standard C, if the arguments don't fit nicely on one line, +split it like this: + +@example +int +lots_of_args (int an_integer, long a_long, short a_short, + double a_double, float a_float) +@dots{} +@end example + +@cindex @code{struct} types, formatting +@cindex @code{enum} types, formatting +For @code{struct} and @code{enum} types, likewise put the braces in +column one, unless the whole contents fits on one line: + +@example +struct foo +@{ + int a, b; +@} +@exdent @r{or} +struct foo @{ int a, b; @} +@end example + +The rest of this section gives our recommendations for other aspects of +C formatting style, which is also the default style of the @code{indent} +program in version 1.2 and newer. It corresponds to the options + +@smallexample +-nbad -bap -nbc -bbo -bl -bli2 -bls -ncdb -nce -cp1 -cs -di2 +-ndj -nfc1 -nfca -hnl -i2 -ip5 -lp -pcs -psl -nsc -nsob +@end smallexample + +We don't think of these recommendations as requirements, because it +causes no problems for users if two different programs have different +formatting styles. + +But whatever style you use, please use it consistently, since a mixture +of styles within one program tends to look ugly. If you are +contributing changes to an existing program, please follow the style of +that program. + +For the body of the function, our recommended style looks like this: + +@example +if (x < foo (y, z)) + haha = bar[4] + 5; +else + @{ + while (z) + @{ + haha += foo (z, z); + z--; + @} + return ++x + bar (); + @} +@end example + +@cindex spaces before open-paren +We find it easier to read a program when it has spaces before the +open-parentheses and after the commas. Especially after the commas. + +When you split an expression into multiple lines, split it +before an operator, not after one. Here is the right way: + +@cindex expressions, splitting +@example +if (foo_this_is_long && bar > win (x, y, z) + && remaining_condition) +@end example + +Try to avoid having two operators of different precedence at the same +level of indentation. For example, don't write this: + +@example +mode = (inmode[j] == VOIDmode + || GET_MODE_SIZE (outmode[j]) > GET_MODE_SIZE (inmode[j]) + ? outmode[j] : inmode[j]); +@end example + +Instead, use extra parentheses so that the indentation shows the nesting: + +@example +mode = ((inmode[j] == VOIDmode + || (GET_MODE_SIZE (outmode[j]) > GET_MODE_SIZE (inmode[j]))) + ? outmode[j] : inmode[j]); +@end example + +Insert extra parentheses so that Emacs will indent the code properly. +For example, the following indentation looks nice if you do it by hand, + +@example +v = rup->ru_utime.tv_sec*1000 + rup->ru_utime.tv_usec/1000 + + rup->ru_stime.tv_sec*1000 + rup->ru_stime.tv_usec/1000; +@end example + +@noindent +but Emacs would alter it. Adding a set of parentheses produces +something that looks equally nice, and which Emacs will preserve: + +@example +v = (rup->ru_utime.tv_sec*1000 + rup->ru_utime.tv_usec/1000 + + rup->ru_stime.tv_sec*1000 + rup->ru_stime.tv_usec/1000); +@end example + +Format do-while statements like this: + +@example +do + @{ + a = foo (a); + @} +while (a > 0); +@end example + +@cindex formfeed +@cindex control-L +Please use formfeed characters (control-L) to divide the program into +pages at logical places (but not within a function). It does not matter +just how long the pages are, since they do not have to fit on a printed +page. The formfeeds should appear alone on lines by themselves. + +@node Comments +@section Commenting Your Work +@cindex commenting + +Every program should start with a comment saying briefly what it is for. +Example: @samp{fmt - filter for simple filling of text}. This comment +should be at the top of the source file containing the @samp{main} +function of the program. + +Also, please write a brief comment at the start of each source file, +with the file name and a line or two about the overall purpose of the +file. + +Please write the comments in a GNU program in English, because English +is the one language that nearly all programmers in all countries can +read. If you do not write English well, please write comments in +English as well as you can, then ask other people to help rewrite them. +If you can't write comments in English, please find someone to work with +you and translate your comments into English. + +Please put a comment on each function saying what the function does, +what sorts of arguments it gets, and what the possible values of +arguments mean and are used for. It is not necessary to duplicate in +words the meaning of the C argument declarations, if a C type is being +used in its customary fashion. If there is anything nonstandard about +its use (such as an argument of type @code{char *} which is really the +address of the second character of a string, not the first), or any +possible values that would not work the way one would expect (such as, +that strings containing newlines are not guaranteed to work), be sure +to say so. + +Also explain the significance of the return value, if there is one. + +Please put two spaces after the end of a sentence in your comments, so +that the Emacs sentence commands will work. Also, please write +complete sentences and capitalize the first word. If a lower-case +identifier comes at the beginning of a sentence, don't capitalize it! +Changing the spelling makes it a different identifier. If you don't +like starting a sentence with a lower case letter, write the sentence +differently (e.g., ``The identifier lower-case is @dots{}''). + +The comment on a function is much clearer if you use the argument +names to speak about the argument values. The variable name itself +should be lower case, but write it in upper case when you are speaking +about the value rather than the variable itself. Thus, ``the inode +number NODE_NUM'' rather than ``an inode''. + +There is usually no purpose in restating the name of the function in +the comment before it, because readers can see that for themselves. +There might be an exception when the comment is so long that the function +itself would be off the bottom of the screen. + +There should be a comment on each static variable as well, like this: + +@example +/* Nonzero means truncate lines in the display; + zero means continue them. */ +int truncate_lines; +@end example + +@cindex conditionals, comments for +@cindex @code{#endif}, commenting +Every @samp{#endif} should have a comment, except in the case of short +conditionals (just a few lines) that are not nested. The comment should +state the condition of the conditional that is ending, @emph{including +its sense}. @samp{#else} should have a comment describing the condition +@emph{and sense} of the code that follows. For example: + +@example +@group +#ifdef foo + @dots{} +#else /* not foo */ + @dots{} +#endif /* not foo */ +@end group +@group +#ifdef foo + @dots{} +#endif /* foo */ +@end group +@end example + +@noindent +but, by contrast, write the comments this way for a @samp{#ifndef}: + +@example +@group +#ifndef foo + @dots{} +#else /* foo */ + @dots{} +#endif /* foo */ +@end group +@group +#ifndef foo + @dots{} +#endif /* not foo */ +@end group +@end example + +@node Syntactic Conventions +@section Clean Use of C Constructs +@cindex syntactic conventions + +@cindex implicit @code{int} +@cindex function argument, declaring +Please explicitly declare the types of all objects. For example, you +should explicitly declare all arguments to functions, and you should +declare functions to return @code{int} rather than omitting the +@code{int}. + +@cindex compiler warnings +@cindex @samp{-Wall} compiler option +Some programmers like to use the GCC @samp{-Wall} option, and change the +code whenever it issues a warning. If you want to do this, then do. +Other programmers prefer not to use @samp{-Wall}, because it gives +warnings for valid and legitimate code which they do not want to change. +If you want to do this, then do. The compiler should be your servant, +not your master. + +@pindex clang +@pindex lint +Don't make the program ugly just to placate static analysis tools such +as @command{lint}, @command{clang}, and GCC with extra warnings +options such as @option{-Wconversion} and @option{-Wundef}. These +tools can help find bugs and unclear code, but they can also generate +so many false alarms that it hurts readability to silence them with +unnecessary casts, wrappers, and other complications. For example, +please don't insert casts to @code{void} or calls to do-nothing +functions merely to pacify a lint checker. + +Declarations of external functions and functions to appear later in the +source file should all go in one place near the beginning of the file +(somewhere before the first function definition in the file), or else +should go in a header file. Don't put @code{extern} declarations inside +functions. + +@cindex temporary variables +It used to be common practice to use the same local variables (with +names like @code{tem}) over and over for different values within one +function. Instead of doing this, it is better to declare a separate local +variable for each distinct purpose, and give it a name which is +meaningful. This not only makes programs easier to understand, it also +facilitates optimization by good compilers. You can also move the +declaration of each local variable into the smallest scope that includes +all its uses. This makes the program even cleaner. + +Don't use local variables or parameters that shadow global identifiers. +GCC's @samp{-Wshadow} option can detect this problem. + +@cindex multiple variables in a line +Don't declare multiple variables in one declaration that spans lines. +Start a new declaration on each line, instead. For example, instead +of this: + +@example +@group +int foo, + bar; +@end group +@end example + +@noindent +write either this: + +@example +int foo, bar; +@end example + +@noindent +or this: + +@example +int foo; +int bar; +@end example + +@noindent +(If they are global variables, each should have a comment preceding it +anyway.) + +When you have an @code{if}-@code{else} statement nested in another +@code{if} statement, always put braces around the @code{if}-@code{else}. +Thus, never write like this: + +@example +if (foo) + if (bar) + win (); + else + lose (); +@end example + +@noindent +always like this: + +@example +if (foo) + @{ + if (bar) + win (); + else + lose (); + @} +@end example + +If you have an @code{if} statement nested inside of an @code{else} +statement, either write @code{else if} on one line, like this, + +@example +if (foo) + @dots{} +else if (bar) + @dots{} +@end example + +@noindent +with its @code{then}-part indented like the preceding @code{then}-part, +or write the nested @code{if} within braces like this: + +@example +if (foo) + @dots{} +else + @{ + if (bar) + @dots{} + @} +@end example + +Don't declare both a structure tag and variables or typedefs in the +same declaration. Instead, declare the structure tag separately +and then use it to declare the variables or typedefs. + +Try to avoid assignments inside @code{if}-conditions (assignments +inside @code{while}-conditions are ok). For example, don't write +this: + +@example +if ((foo = (char *) malloc (sizeof *foo)) == 0) + fatal ("virtual memory exhausted"); +@end example + +@noindent +instead, write this: + +@example +foo = (char *) malloc (sizeof *foo); +if (foo == 0) + fatal ("virtual memory exhausted"); +@end example + +This example uses zero without a cast as a null pointer constant. +This is perfectly fine, except that a cast is needed when calling a +varargs function or when using @code{sizeof}. + +@node Names +@section Naming Variables, Functions, and Files + +@cindex names of variables, functions, and files +The names of global variables and functions in a program serve as +comments of a sort. So don't choose terse names---instead, look for +names that give useful information about the meaning of the variable or +function. In a GNU program, names should be English, like other +comments. + +Local variable names can be shorter, because they are used only within +one context, where (presumably) comments explain their purpose. + +Try to limit your use of abbreviations in symbol names. It is ok to +make a few abbreviations, explain what they mean, and then use them +frequently, but don't use lots of obscure abbreviations. + +Please use underscores to separate words in a name, so that the Emacs +word commands can be useful within them. Stick to lower case; reserve +upper case for macros and @code{enum} constants, and for name-prefixes +that follow a uniform convention. + +For example, you should use names like @code{ignore_space_change_flag}; +don't use names like @code{iCantReadThis}. + +Variables that indicate whether command-line options have been +specified should be named after the meaning of the option, not after +the option-letter. A comment should state both the exact meaning of +the option and its letter. For example, + +@example +@group +/* Ignore changes in horizontal whitespace (-b). */ +int ignore_space_change_flag; +@end group +@end example + +When you want to define names with constant integer values, use +@code{enum} rather than @samp{#define}. GDB knows about enumeration +constants. + +@cindex file-name limitations +@pindex doschk +You might want to make sure that none of the file names would conflict +if the files were loaded onto an MS-DOS file system which shortens the +names. You can use the program @code{doschk} to test for this. + +Some GNU programs were designed to limit themselves to file names of 14 +characters or less, to avoid file name conflicts if they are read into +older System V systems. Please preserve this feature in the existing +GNU programs that have it, but there is no need to do this in new GNU +programs. @code{doschk} also reports file names longer than 14 +characters. + + +@node System Portability +@section Portability between System Types +@cindex portability, between system types + +In the Unix world, ``portability'' refers to porting to different Unix +versions. For a GNU program, this kind of portability is desirable, but +not paramount. + +The primary purpose of GNU software is to run on top of the GNU kernel, +compiled with the GNU C compiler, on various types of @sc{cpu}. So the +kinds of portability that are absolutely necessary are quite limited. +But it is important to support Linux-based GNU systems, since they +are the form of GNU that is popular. + +Beyond that, it is good to support the other free operating systems +(*BSD), and it is nice to support other Unix-like systems if you want +to. Supporting a variety of Unix-like systems is desirable, although +not paramount. It is usually not too hard, so you may as well do it. +But you don't have to consider it an obligation, if it does turn out to +be hard. + +@pindex autoconf +The easiest way to achieve portability to most Unix-like systems is to +use Autoconf. It's unlikely that your program needs to know more +information about the host platform than Autoconf can provide, simply +because most of the programs that need such knowledge have already been +written. + +Avoid using the format of semi-internal data bases (e.g., directories) +when there is a higher-level alternative (@code{readdir}). + +@cindex non-@sc{posix} systems, and portability +As for systems that are not like Unix, such as MSDOS, Windows, VMS, MVS, +and older Macintosh systems, supporting them is often a lot of work. +When that is the case, it is better to spend your time adding features +that will be useful on GNU and GNU/Linux, rather than on supporting +other incompatible systems. + +If you do support Windows, please do not abbreviate it as ``win''. In +hacker terminology, calling something a ``win'' is a form of praise. +You're free to praise Microsoft Windows on your own if you want, but +please don't do this in GNU packages. Instead of abbreviating +``Windows'' to ``win'', you can write it in full or abbreviate it to +``woe'' or ``w''. In GNU Emacs, for instance, we use @samp{w32} in +file names of Windows-specific files, but the macro for Windows +conditionals is called @code{WINDOWSNT}. + +It is a good idea to define the ``feature test macro'' +@code{_GNU_SOURCE} when compiling your C files. When you compile on GNU +or GNU/Linux, this will enable the declarations of GNU library extension +functions, and that will usually give you a compiler error message if +you define the same function names in some other way in your program. +(You don't have to actually @emph{use} these functions, if you prefer +to make the program more portable to other systems.) + +But whether or not you use these GNU extensions, you should avoid +using their names for any other meanings. Doing so would make it hard +to move your code into other GNU programs. + +@node CPU Portability +@section Portability between @sc{cpu}s + +@cindex data types, and portability +@cindex portability, and data types +Even GNU systems will differ because of differences among @sc{cpu} +types---for example, difference in byte ordering and alignment +requirements. It is absolutely essential to handle these differences. +However, don't make any effort to cater to the possibility that an +@code{int} will be less than 32 bits. We don't support 16-bit machines +in GNU. + +Similarly, don't make any effort to cater to the possibility that +@code{long} will be smaller than predefined types like @code{size_t}. +For example, the following code is ok: + +@example +printf ("size = %lu\n", (unsigned long) sizeof array); +printf ("diff = %ld\n", (long) (pointer2 - pointer1)); +@end example + +1989 Standard C requires this to work, and we know of only one +counterexample: 64-bit programs on Microsoft Windows. We will leave +it to those who want to port GNU programs to that environment to +figure out how to do it. + +Predefined file-size types like @code{off_t} are an exception: they are +longer than @code{long} on many platforms, so code like the above won't +work with them. One way to print an @code{off_t} value portably is to +print its digits yourself, one by one. + +Don't assume that the address of an @code{int} object is also the +address of its least-significant byte. This is false on big-endian +machines. Thus, don't make the following mistake: + +@example +int c; +@dots{} +while ((c = getchar ()) != EOF) + write (file_descriptor, &c, 1); +@end example + +@noindent Instead, use @code{unsigned char} as follows. (The @code{unsigned} +is for portability to unusual systems where @code{char} is signed and +where there is integer overflow checking.) + +@example +int c; +while ((c = getchar ()) != EOF) + @{ + unsigned char u = c; + write (file_descriptor, &u, 1); + @} +@end example + +@cindex casting pointers to integers +Avoid casting pointers to integers if you can. Such casts greatly +reduce portability, and in most programs they are easy to avoid. In the +cases where casting pointers to integers is essential---such as, a Lisp +interpreter which stores type information as well as an address in one +word---you'll have to make explicit provisions to handle different word +sizes. You will also need to make provision for systems in which the +normal range of addresses you can get from @code{malloc} starts far away +from zero. + + +@node System Functions +@section Calling System Functions + +@cindex C library functions, and portability +@cindex POSIX functions, and portability +@cindex library functions, and portability +@cindex portability, and library functions + +Historically, C implementations differed substantially, and many +systems lacked a full implementation of ANSI/ISO C89. Nowadays, +however, very few systems lack a C89 compiler and GNU C supports +almost all of C99. Similarly, most systems implement POSIX.1-1993 +libraries and tools, and many have POSIX.1-2001. + +Hence, there is little reason to support old C or non-POSIX systems, +and you may want to take advantage of C99 and POSIX-1.2001 to write +clearer, more portable, or faster code. You should use standard +interfaces where possible; but if GNU extensions make your program +more maintainable, powerful, or otherwise better, don't hesitate to +use them. In any case, don't make your own declaration of system +functions; that's a recipe for conflict. + +Despite the standards, nearly every library function has some sort of +portability issue on some system or another. Here are some examples: + +@table @code +@item open +Names with trailing @code{/}'s are mishandled on many platforms. + +@item printf +@code{long double} may be unimplemented; floating values Infinity and +NaN are often mishandled; output for large precisions may be +incorrect. + +@item readlink +May return @code{int} instead of @code{ssize_t}. + +@item scanf +On Windows, @code{errno} is not set on failure. +@end table + +@cindex Gnulib +@uref{http://www.gnu.org/software/gnulib/, Gnulib} is a big help in +this regard. Gnulib provides implementations of standard interfaces +on many of the systems that lack them, including portable +implementations of enhanced GNU interfaces, thereby making their use +portable, and of POSIX-1.2008 interfaces, some of which are missing +even on up-to-date GNU systems. + +@findex xmalloc, in Gnulib +@findex error messages, in Gnulib +@findex data structures, in Gnulib +Gnulib also provides many useful non-standard interfaces; for example, +C implementations of standard data structures (hash tables, binary +trees), error-checking type-safe wrappers for memory allocation +functions (@code{xmalloc}, @code{xrealloc}), and output of error +messages. + +Gnulib integrates with GNU Autoconf and Automake to remove much of the +burden of writing portable code from the programmer: Gnulib makes your +configure script automatically determine what features are missing and +use the Gnulib code to supply the missing pieces. + +The Gnulib and Autoconf manuals have extensive sections on +portability: @ref{Top,, Introduction, gnulib, Gnulib} and +@pxref{Portable C and C++,,, autoconf, Autoconf}. Please consult them +for many more details. + + +@node Internationalization +@section Internationalization +@cindex internationalization + +@pindex gettext +GNU has a library called GNU gettext that makes it easy to translate the +messages in a program into various languages. You should use this +library in every program. Use English for the messages as they appear +in the program, and let gettext provide the way to translate them into +other languages. + +Using GNU gettext involves putting a call to the @code{gettext} macro +around each string that might need translation---like this: + +@example +printf (gettext ("Processing file '%s'..."), file); +@end example + +@noindent +This permits GNU gettext to replace the string @code{"Processing file +'%s'..."} with a translated version. + +Once a program uses gettext, please make a point of writing calls to +@code{gettext} when you add new strings that call for translation. + +Using GNU gettext in a package involves specifying a @dfn{text domain +name} for the package. The text domain name is used to separate the +translations for this package from the translations for other packages. +Normally, the text domain name should be the same as the name of the +package---for example, @samp{coreutils} for the GNU core utilities. + +@cindex message text, and internationalization +To enable gettext to work well, avoid writing code that makes +assumptions about the structure of words or sentences. When you want +the precise text of a sentence to vary depending on the data, use two or +more alternative string constants each containing a complete sentences, +rather than inserting conditionalized words or phrases into a single +sentence framework. + +Here is an example of what not to do: + +@smallexample +printf ("%s is full", capacity > 5000000 ? "disk" : "floppy disk"); +@end smallexample + +If you apply gettext to all strings, like this, + +@smallexample +printf (gettext ("%s is full"), + capacity > 5000000 ? gettext ("disk") : gettext ("floppy disk")); +@end smallexample + +@noindent +the translator will hardly know that "disk" and "floppy disk" are meant to +be substituted in the other string. Worse, in some languages (like French) +the construction will not work: the translation of the word "full" depends +on the gender of the first part of the sentence; it happens to be not the +same for "disk" as for "floppy disk". + +Complete sentences can be translated without problems: + +@example +printf (capacity > 5000000 ? gettext ("disk is full") + : gettext ("floppy disk is full")); +@end example + +A similar problem appears at the level of sentence structure with this +code: + +@example +printf ("# Implicit rule search has%s been done.\n", + f->tried_implicit ? "" : " not"); +@end example + +@noindent +Adding @code{gettext} calls to this code cannot give correct results for +all languages, because negation in some languages requires adding words +at more than one place in the sentence. By contrast, adding +@code{gettext} calls does the job straightforwardly if the code starts +out like this: + +@example +printf (f->tried_implicit + ? "# Implicit rule search has been done.\n", + : "# Implicit rule search has not been done.\n"); +@end example + +Another example is this one: + +@example +printf ("%d file%s processed", nfiles, + nfiles != 1 ? "s" : ""); +@end example + +@noindent +The problem with this example is that it assumes that plurals are made +by adding `s'. If you apply gettext to the format string, like this, + +@example +printf (gettext ("%d file%s processed"), nfiles, + nfiles != 1 ? "s" : ""); +@end example + +@noindent +the message can use different words, but it will still be forced to use +`s' for the plural. Here is a better way, with gettext being applied to +the two strings independently: + +@example +printf ((nfiles != 1 ? gettext ("%d files processed") + : gettext ("%d file processed")), + nfiles); +@end example + +@noindent +But this still doesn't work for languages like Polish, which has three +plural forms: one for nfiles == 1, one for nfiles == 2, 3, 4, 22, 23, 24, ... +and one for the rest. The GNU @code{ngettext} function solves this problem: + +@example +printf (ngettext ("%d files processed", "%d file processed", nfiles), + nfiles); +@end example + + +@node Character Set +@section Character Set +@cindex character set +@cindex encodings +@cindex ASCII characters +@cindex non-ASCII characters + +Sticking to the ASCII character set (plain text, 7-bit characters) is +preferred in GNU source code comments, text documents, and other +contexts, unless there is good reason to do something else because of +the application domain. For example, if source code deals with the +French Revolutionary calendar, it is OK if its literal strings contain +accented characters in month names like ``Flor@'eal''. Also, it is OK +(but not required) to use non-ASCII characters to represent proper +names of contributors in change logs (@pxref{Change Logs}). + +If you need to use non-ASCII characters, you should normally stick +with one encoding, certainly within a single file. UTF-8 is likely to +be the best choice. + + +@node Quote Characters +@section Quote Characters +@cindex quote characters +@cindex locale-specific quote characters +@cindex left quote +@cindex right quote +@cindex opening quote +@cindex single quote +@cindex double quote +@cindex grave accent +@set txicodequoteundirected +@set txicodequotebacktick + +In the C locale, the output of GNU programs should stick to plain +ASCII for quotation characters in messages to users: preferably 0x22 +(@samp{"}) or 0x27 (@samp{'}) for both opening and closing quotes. +Although GNU programs traditionally used 0x60 (@samp{`}) for opening +and 0x27 (@samp{'}) for closing quotes, nowadays quotes @samp{`like +this'} are typically rendered asymmetrically, so quoting @samp{"like +this"} or @samp{'like this'} typically looks better. + +It is ok, but not required, for GNU programs to generate +locale-specific quotes in non-C locales. For example: + +@example +printf (gettext ("Processing file '%s'..."), file); +@end example + +@noindent +Here, a French translation might cause @code{gettext} to return the +string @code{"Traitement de fichier +@guilsinglleft{}@tie{}%s@tie{}@guilsinglright{}..."}, yielding quotes +more appropriate for a French locale. + +Sometimes a program may need to use opening and closing quotes +directly. By convention, @code{gettext} translates the string +@samp{"`"} to the opening quote and the string @samp{"'"} to the +closing quote, and a program can use these translations. Generally, +though, it is better to translate quote characters in the context of +longer strings. + +If the output of your program is ever likely to be parsed by another +program, it is good to provide an option that makes this parsing +reliable. For example, you could escape special characters using +conventions from the C language or the Bourne shell. See for example +the option @option{--quoting-style} of GNU @code{ls}. + +@clear txicodequoteundirected +@clear txicodequotebacktick + + +@node Mmap +@section Mmap +@findex mmap + +Don't assume that @code{mmap} either works on all files or fails +for all files. It may work on some files and fail on others. + +The proper way to use @code{mmap} is to try it on the specific file for +which you want to use it---and if @code{mmap} doesn't work, fall back on +doing the job in another way using @code{read} and @code{write}. + +The reason this precaution is needed is that the GNU kernel (the HURD) +provides a user-extensible file system, in which there can be many +different kinds of ``ordinary files''. Many of them support +@code{mmap}, but some do not. It is important to make programs handle +all these kinds of files. + + +@node Documentation +@chapter Documenting Programs +@cindex documentation + +A GNU program should ideally come with full free documentation, adequate +for both reference and tutorial purposes. If the package can be +programmed or extended, the documentation should cover programming or +extending it, as well as just using it. + +@menu +* GNU Manuals:: Writing proper manuals. +* Doc Strings and Manuals:: Compiling doc strings doesn't make a manual. +* Manual Structure Details:: Specific structure conventions. +* License for Manuals:: Writing the distribution terms for a manual. +* Manual Credits:: Giving credit to documentation contributors. +* Printed Manuals:: Mentioning the printed manual. +* NEWS File:: NEWS files supplement manuals. +* Change Logs:: Recording changes. +* Man Pages:: Man pages are secondary. +* Reading other Manuals:: How far you can go in learning + from other manuals. +@end menu + +@node GNU Manuals +@section GNU Manuals + +The preferred document format for the GNU system is the Texinfo +formatting language. Every GNU package should (ideally) have +documentation in Texinfo both for reference and for learners. Texinfo +makes it possible to produce a good quality formatted book, using +@TeX{}, and to generate an Info file. It is also possible to generate +HTML output from Texinfo source. See the Texinfo manual, either the +hardcopy, or the on-line version available through @code{info} or the +Emacs Info subsystem (@kbd{C-h i}). + +Nowadays some other formats such as Docbook and Sgmltexi can be +converted automatically into Texinfo. It is ok to produce the Texinfo +documentation by conversion this way, as long as it gives good results. + +Make sure your manual is clear to a reader who knows nothing about the +topic and reads it straight through. This means covering basic topics +at the beginning, and advanced topics only later. This also means +defining every specialized term when it is first used. + +Programmers tend to carry over the structure of the program as the +structure for its documentation. But this structure is not +necessarily good for explaining how to use the program; it may be +irrelevant and confusing for a user. + +Instead, the right way to structure documentation is according to the +concepts and questions that a user will have in mind when reading it. +This principle applies at every level, from the lowest (ordering +sentences in a paragraph) to the highest (ordering of chapter topics +within the manual). Sometimes this structure of ideas matches the +structure of the implementation of the software being documented---but +often they are different. An important part of learning to write good +documentation is to learn to notice when you have unthinkingly +structured the documentation like the implementation, stop yourself, +and look for better alternatives. + +For example, each program in the GNU system probably ought to be +documented in one manual; but this does not mean each program should +have its own manual. That would be following the structure of the +implementation, rather than the structure that helps the user +understand. + +Instead, each manual should cover a coherent @emph{topic}. For example, +instead of a manual for @code{diff} and a manual for @code{diff3}, we +have one manual for ``comparison of files'' which covers both of those +programs, as well as @code{cmp}. By documenting these programs +together, we can make the whole subject clearer. + +The manual which discusses a program should certainly document all of +the program's command-line options and all of its commands. It should +give examples of their use. But don't organize the manual as a list +of features. Instead, organize it logically, by subtopics. Address +the questions that a user will ask when thinking about the job that +the program does. Don't just tell the reader what each feature can +do---say what jobs it is good for, and show how to use it for those +jobs. Explain what is recommended usage, and what kinds of usage +users should avoid. + +In general, a GNU manual should serve both as tutorial and reference. +It should be set up for convenient access to each topic through Info, +and for reading straight through (appendixes aside). A GNU manual +should give a good introduction to a beginner reading through from the +start, and should also provide all the details that hackers want. +The Bison manual is a good example of this---please take a look at it +to see what we mean. + +That is not as hard as it first sounds. Arrange each chapter as a +logical breakdown of its topic, but order the sections, and write their +text, so that reading the chapter straight through makes sense. Do +likewise when structuring the book into chapters, and when structuring a +section into paragraphs. The watchword is, @emph{at each point, address +the most fundamental and important issue raised by the preceding text.} + +If necessary, add extra chapters at the beginning of the manual which +are purely tutorial and cover the basics of the subject. These provide +the framework for a beginner to understand the rest of the manual. The +Bison manual provides a good example of how to do this. + +To serve as a reference, a manual should have an Index that list all the +functions, variables, options, and important concepts that are part of +the program. One combined Index should do for a short manual, but +sometimes for a complex package it is better to use multiple indices. +The Texinfo manual includes advice on preparing good index entries, see +@ref{Index Entries, , Making Index Entries, texinfo, GNU Texinfo}, and +see @ref{Indexing Commands, , Defining the Entries of an +Index, texinfo, GNU Texinfo}. + +Don't use Unix man pages as a model for how to write GNU documentation; +most of them are terse, badly structured, and give inadequate +explanation of the underlying concepts. (There are, of course, some +exceptions.) Also, Unix man pages use a particular format which is +different from what we use in GNU manuals. + +Please include an email address in the manual for where to report +bugs @emph{in the text of the manual}. + +Please do not use the term ``pathname'' that is used in Unix +documentation; use ``file name'' (two words) instead. We use the term +``path'' only for search paths, which are lists of directory names. + +Please do not use the term ``illegal'' to refer to erroneous input to +a computer program. Please use ``invalid'' for this, and reserve the +term ``illegal'' for activities prohibited by law. + +Please do not write @samp{()} after a function name just to indicate +it is a function. @code{foo ()} is not a function, it is a function +call with no arguments. + +@node Doc Strings and Manuals +@section Doc Strings and Manuals + +Some programming systems, such as Emacs, provide a documentation string +for each function, command or variable. You may be tempted to write a +reference manual by compiling the documentation strings and writing a +little additional text to go around them---but you must not do it. That +approach is a fundamental mistake. The text of well-written +documentation strings will be entirely wrong for a manual. + +A documentation string needs to stand alone---when it appears on the +screen, there will be no other text to introduce or explain it. +Meanwhile, it can be rather informal in style. + +The text describing a function or variable in a manual must not stand +alone; it appears in the context of a section or subsection. Other text +at the beginning of the section should explain some of the concepts, and +should often make some general points that apply to several functions or +variables. The previous descriptions of functions and variables in the +section will also have given information about the topic. A description +written to stand alone would repeat some of that information; this +redundancy looks bad. Meanwhile, the informality that is acceptable in +a documentation string is totally unacceptable in a manual. + +The only good way to use documentation strings in writing a good manual +is to use them as a source of information for writing good text. + +@node Manual Structure Details +@section Manual Structure Details +@cindex manual structure + +The title page of the manual should state the version of the programs or +packages documented in the manual. The Top node of the manual should +also contain this information. If the manual is changing more +frequently than or independent of the program, also state a version +number for the manual in both of these places. + +Each program documented in the manual should have a node named +@samp{@var{program} Invocation} or @samp{Invoking @var{program}}. This +node (together with its subnodes, if any) should describe the program's +command line arguments and how to run it (the sort of information people +would look for in a man page). Start with an @samp{@@example} +containing a template for all the options and arguments that the program +uses. + +Alternatively, put a menu item in some menu whose item name fits one of +the above patterns. This identifies the node which that item points to +as the node for this purpose, regardless of the node's actual name. + +The @samp{--usage} feature of the Info reader looks for such a node +or menu item in order to find the relevant text, so it is essential +for every Texinfo file to have one. + +If one manual describes several programs, it should have such a node for +each program described in the manual. + +@node License for Manuals +@section License for Manuals +@cindex license for manuals + +Please use the GNU Free Documentation License for all GNU manuals that +are more than a few pages long. Likewise for a collection of short +documents---you only need one copy of the GNU FDL for the whole +collection. For a single short document, you can use a very permissive +non-copyleft license, to avoid taking up space with a long license. + +See @uref{http://www.gnu.org/copyleft/fdl-howto.html} for more explanation +of how to employ the GFDL. + +Note that it is not obligatory to include a copy of the GNU GPL or GNU +LGPL in a manual whose license is neither the GPL nor the LGPL. It can +be a good idea to include the program's license in a large manual; in a +short manual, whose size would be increased considerably by including +the program's license, it is probably better not to include it. + +@node Manual Credits +@section Manual Credits +@cindex credits for manuals + +Please credit the principal human writers of the manual as the authors, +on the title page of the manual. If a company sponsored the work, thank +the company in a suitable place in the manual, but do not cite the +company as an author. + +@node Printed Manuals +@section Printed Manuals + +The FSF publishes some GNU manuals in printed form. To encourage sales +of these manuals, the on-line versions of the manual should mention at +the very start that the printed manual is available and should point at +information for getting it---for instance, with a link to the page +@url{http://www.gnu.org/order/order.html}. This should not be included +in the printed manual, though, because there it is redundant. + +It is also useful to explain in the on-line forms of the manual how the +user can print out the manual from the sources. + +@node NEWS File +@section The NEWS File +@cindex @file{NEWS} file + +In addition to its manual, the package should have a file named +@file{NEWS} which contains a list of user-visible changes worth +mentioning. In each new release, add items to the front of the file and +identify the version they pertain to. Don't discard old items; leave +them in the file after the newer items. This way, a user upgrading from +any previous version can see what is new. + +If the @file{NEWS} file gets very long, move some of the older items +into a file named @file{ONEWS} and put a note at the end referring the +user to that file. + +@node Change Logs +@section Change Logs +@cindex change logs + +Keep a change log to describe all the changes made to program source +files. The purpose of this is so that people investigating bugs in the +future will know about the changes that might have introduced the bug. +Often a new bug can be found by looking at what was recently changed. +More importantly, change logs can help you eliminate conceptual +inconsistencies between different parts of a program, by giving you a +history of how the conflicting concepts arose and who they came from. + +@menu +* Change Log Concepts:: +* Style of Change Logs:: +* Simple Changes:: +* Conditional Changes:: +* Indicating the Part Changed:: +@end menu + +@node Change Log Concepts +@subsection Change Log Concepts + +You can think of the change log as a conceptual ``undo list'' which +explains how earlier versions were different from the current version. +People can see the current version; they don't need the change log +to tell them what is in it. What they want from a change log is a +clear explanation of how the earlier version differed. + +The change log file is normally called @file{ChangeLog} and covers an +entire directory. Each directory can have its own change log, or a +directory can use the change log of its parent directory---it's up to +you. + +Another alternative is to record change log information with a version +control system such as RCS or CVS. This can be converted automatically +to a @file{ChangeLog} file using @code{rcs2log}; in Emacs, the command +@kbd{C-x v a} (@code{vc-update-change-log}) does the job. + +There's no need to describe the full purpose of the changes or how +they work together. However, sometimes it is useful to write one line +to describe the overall purpose of a change or a batch of changes. If +you think that a change calls for explanation, you're probably right. +Please do explain it---but please put the full explanation in comments +in the code, where people will see it whenever they see the code. For +example, ``New function'' is enough for the change log when you add a +function, because there should be a comment before the function +definition to explain what it does. + +In the past, we recommended not mentioning changes in non-software +files (manuals, help files, etc.) in change logs. However, we've been +advised that it is a good idea to include them, for the sake of +copyright records. + +The easiest way to add an entry to @file{ChangeLog} is with the Emacs +command @kbd{M-x add-change-log-entry}. An entry should have an +asterisk, the name of the changed file, and then in parentheses the name +of the changed functions, variables or whatever, followed by a colon. +Then describe the changes you made to that function or variable. + +@node Style of Change Logs +@subsection Style of Change Logs +@cindex change logs, style + +Here are some simple examples of change log entries, starting with the +header line that says who made the change and when it was installed, +followed by descriptions of specific changes. (These examples are +drawn from Emacs and GCC.) + +@example +1998-08-17 Richard Stallman <rms@@gnu.org> + +* register.el (insert-register): Return nil. +(jump-to-register): Likewise. + +* sort.el (sort-subr): Return nil. + +* tex-mode.el (tex-bibtex-file, tex-file, tex-region): +Restart the tex shell if process is gone or stopped. +(tex-shell-running): New function. + +* expr.c (store_one_arg): Round size up for move_block_to_reg. +(expand_call): Round up when emitting USE insns. +* stmt.c (assign_parms): Round size up for move_block_from_reg. +@end example + +It's important to name the changed function or variable in full. Don't +abbreviate function or variable names, and don't combine them. +Subsequent maintainers will often search for a function name to find all +the change log entries that pertain to it; if you abbreviate the name, +they won't find it when they search. + +For example, some people are tempted to abbreviate groups of function +names by writing @samp{* register.el (@{insert,jump-to@}-register)}; +this is not a good idea, since searching for @code{jump-to-register} or +@code{insert-register} would not find that entry. + +Separate unrelated change log entries with blank lines. When two +entries represent parts of the same change, so that they work together, +then don't put blank lines between them. Then you can omit the file +name and the asterisk when successive entries are in the same file. + +Break long lists of function names by closing continued lines with +@samp{)}, rather than @samp{,}, and opening the continuation with +@samp{(} as in this example: + +@example +* keyboard.c (menu_bar_items, tool_bar_items) +(Fexecute_extended_command): Deal with 'keymap' property. +@end example + +When you install someone else's changes, put the contributor's name in +the change log entry rather than in the text of the entry. In other +words, write this: + +@example +2002-07-14 John Doe <jdoe@@gnu.org> + + * sewing.c: Make it sew. +@end example + +@noindent +rather than this: + +@example +2002-07-14 Usual Maintainer <usual@@gnu.org> + + * sewing.c: Make it sew. Patch by jdoe@@gnu.org. +@end example + +As for the date, that should be the date you applied the change. + +@node Simple Changes +@subsection Simple Changes + +Certain simple kinds of changes don't need much detail in the change +log. + +When you change the calling sequence of a function in a simple fashion, +and you change all the callers of the function to use the new calling +sequence, there is no need to make individual entries for all the +callers that you changed. Just write in the entry for the function +being called, ``All callers changed''---like this: + +@example +* keyboard.c (Fcommand_execute): New arg SPECIAL. +All callers changed. +@end example + +When you change just comments or doc strings, it is enough to write an +entry for the file, without mentioning the functions. Just ``Doc +fixes'' is enough for the change log. + +There's no technical need to make change log entries for documentation +files. This is because documentation is not susceptible to bugs that +are hard to fix. Documentation does not consist of parts that must +interact in a precisely engineered fashion. To correct an error, you +need not know the history of the erroneous passage; it is enough to +compare what the documentation says with the way the program actually +works. + +However, you should keep change logs for documentation files when the +project gets copyright assignments from its contributors, so as to +make the records of authorship more accurate. + +@node Conditional Changes +@subsection Conditional Changes +@cindex conditional changes, and change logs +@cindex change logs, conditional changes + +Source files can often contain code that is conditional to build-time +or static conditions. For example, C programs can contain +compile-time @code{#if} conditionals; programs implemented in +interpreted languages can contain module imports of function +definitions that are only performed for certain versions of the +interpreter; and Automake @file{Makefile.am} files can contain +variable definitions or target declarations that are only to be +considered if a configure-time Automake conditional is true. + +Many changes are conditional as well: sometimes you add a new variable, +or function, or even a new program or library, which is entirely +dependent on a build-time condition. It is useful to indicate +in the change log the conditions for which a change applies. + +Our convention for indicating conditional changes is to use +@emph{square brackets around the name of the condition}. + +Conditional changes can happen in numerous scenarios and with many +variations, so here are some examples to help clarify. This first +example describes changes in C, Perl, and Python files which are +conditional but do not have an associated function or entity name: + +@example +* xterm.c [SOLARIS2]: Include <string.h>. +* FilePath.pm [$^O eq 'VMS']: Import the VMS::Feature module. +* framework.py [sys.version_info < (2, 6)]: Make "with" statement + available by importing it from __future__, + to support also python 2.5. +@end example + +Our other examples will for simplicity be limited to C, as the minor +changes necessary to adapt them to other languages should be +self-evident. + +Next, here is an entry describing a new definition which is entirely +conditional: the C macro @code{FRAME_WINDOW_P} is defined (and used) +only when the macro @code{HAVE_X_WINDOWS} is defined: + +@example +* frame.h [HAVE_X_WINDOWS] (FRAME_WINDOW_P): Macro defined. +@end example + +Next, an entry for a change within the function @code{init_display}, +whose definition as a whole is unconditional, but the changes +themselves are contained in a @samp{#ifdef HAVE_LIBNCURSES} +conditional: + +@example +* dispnew.c (init_display) [HAVE_LIBNCURSES]: If X, call tgetent. +@end example + +Finally, here is an entry for a change that takes effect only when +a certain macro is @emph{not} defined: + +@example +(gethostname) [!HAVE_SOCKETS]: Replace with winsock version. +@end example + +@node Indicating the Part Changed +@subsection Indicating the Part Changed + +Indicate the part of a function which changed by using angle brackets +enclosing an indication of what the changed part does. Here is an entry +for a change in the part of the function @code{sh-while-getopts} that +deals with @code{sh} commands: + +@example +* progmodes/sh-script.el (sh-while-getopts) <sh>: Handle case that +user-specified option string is empty. +@end example + + +@node Man Pages +@section Man Pages +@cindex man pages + +In the GNU project, man pages are secondary. It is not necessary or +expected for every GNU program to have a man page, but some of them do. +It's your choice whether to include a man page in your program. + +When you make this decision, consider that supporting a man page +requires continual effort each time the program is changed. The time +you spend on the man page is time taken away from more useful work. + +For a simple program which changes little, updating the man page may be +a small job. Then there is little reason not to include a man page, if +you have one. + +For a large program that changes a great deal, updating a man page may +be a substantial burden. If a user offers to donate a man page, you may +find this gift costly to accept. It may be better to refuse the man +page unless the same person agrees to take full responsibility for +maintaining it---so that you can wash your hands of it entirely. If +this volunteer later ceases to do the job, then don't feel obliged to +pick it up yourself; it may be better to withdraw the man page from the +distribution until someone else agrees to update it. + +When a program changes only a little, you may feel that the +discrepancies are small enough that the man page remains useful without +updating. If so, put a prominent note near the beginning of the man +page explaining that you don't maintain it and that the Texinfo manual +is more authoritative. The note should say how to access the Texinfo +documentation. + +Be sure that man pages include a copyright statement and free license. +The simple all-permissive license is appropriate for simple man pages +(@pxref{License Notices for Other Files,,,maintain,Information for GNU +Maintainers}). + +For long man pages, with enough explanation and documentation that +they can be considered true manuals, use the GFDL (@pxref{License for +Manuals}). + +Finally, the GNU help2man program +(@uref{http://www.gnu.org/software/help2man/}) is one way to automate +generation of a man page, in this case from @option{--help} output. +This is sufficient in many cases. + +@node Reading other Manuals +@section Reading other Manuals + +There may be non-free books or documentation files that describe the +program you are documenting. + +It is ok to use these documents for reference, just as the author of a +new algebra textbook can read other books on algebra. A large portion +of any non-fiction book consists of facts, in this case facts about how +a certain program works, and these facts are necessarily the same for +everyone who writes about the subject. But be careful not to copy your +outline structure, wording, tables or examples from preexisting non-free +documentation. Copying from free documentation may be ok; please check +with the FSF about the individual case. + +@node Managing Releases +@chapter The Release Process +@cindex releasing + +Making a release is more than just bundling up your source files in a +tar file and putting it up for FTP. You should set up your software so +that it can be configured to run on a variety of systems. Your Makefile +should conform to the GNU standards described below, and your directory +layout should also conform to the standards discussed below. Doing so +makes it easy to include your package into the larger framework of +all GNU software. + +@menu +* Configuration:: How configuration of GNU packages should work. +* Makefile Conventions:: Makefile conventions. +* Releases:: Making releases +@end menu + +@node Configuration +@section How Configuration Should Work +@cindex program configuration + +@pindex configure +Each GNU distribution should come with a shell script named +@code{configure}. This script is given arguments which describe the +kind of machine and system you want to compile the program for. +The @code{configure} script must record the configuration options so +that they affect compilation. + +The description here is the specification of the interface for the +@code{configure} script in GNU packages. Many packages implement it +using GNU Autoconf (@pxref{Top,, Introduction, autoconf, Autoconf}) +and/or GNU Automake (@pxref{Top,, Introduction, automake, Automake}), +but you do not have to use these tools. You can implement it any way +you like; for instance, by making @code{configure} be a wrapper around +a completely different configuration system. + +Another way for the @code{configure} script to operate is to make a +link from a standard name such as @file{config.h} to the proper +configuration file for the chosen system. If you use this technique, +the distribution should @emph{not} contain a file named +@file{config.h}. This is so that people won't be able to build the +program without configuring it first. + +Another thing that @code{configure} can do is to edit the Makefile. If +you do this, the distribution should @emph{not} contain a file named +@file{Makefile}. Instead, it should include a file @file{Makefile.in} which +contains the input used for editing. Once again, this is so that people +won't be able to build the program without configuring it first. + +If @code{configure} does write the @file{Makefile}, then @file{Makefile} +should have a target named @file{Makefile} which causes @code{configure} +to be rerun, setting up the same configuration that was set up last +time. The files that @code{configure} reads should be listed as +dependencies of @file{Makefile}. + +All the files which are output from the @code{configure} script should +have comments at the beginning explaining that they were generated +automatically using @code{configure}. This is so that users won't think +of trying to edit them by hand. + +The @code{configure} script should write a file named @file{config.status} +which describes which configuration options were specified when the +program was last configured. This file should be a shell script which, +if run, will recreate the same configuration. + +The @code{configure} script should accept an option of the form +@samp{--srcdir=@var{dirname}} to specify the directory where sources are found +(if it is not the current directory). This makes it possible to build +the program in a separate directory, so that the actual source directory +is not modified. + +If the user does not specify @samp{--srcdir}, then @code{configure} should +check both @file{.} and @file{..} to see if it can find the sources. If +it finds the sources in one of these places, it should use them from +there. Otherwise, it should report that it cannot find the sources, and +should exit with nonzero status. + +Usually the easy way to support @samp{--srcdir} is by editing a +definition of @code{VPATH} into the Makefile. Some rules may need to +refer explicitly to the specified source directory. To make this +possible, @code{configure} can add to the Makefile a variable named +@code{srcdir} whose value is precisely the specified directory. + +In addition, the @samp{configure} script should take options +corresponding to most of the standard directory variables +(@pxref{Directory Variables}). Here is the list: + +@example +--prefix --exec-prefix --bindir --sbindir --libexecdir --sysconfdir +--sharedstatedir --localstatedir --libdir --includedir --oldincludedir +--datarootdir --datadir --infodir --localedir --mandir --docdir +--htmldir --dvidir --pdfdir --psdir +@end example + +The @code{configure} script should also take an argument which specifies the +type of system to build the program for. This argument should look like +this: + +@example +@var{cpu}-@var{company}-@var{system} +@end example + +For example, an Athlon-based GNU/Linux system might be +@samp{i686-pc-linux-gnu}. + +The @code{configure} script needs to be able to decode all plausible +alternatives for how to describe a machine. Thus, +@samp{athlon-pc-gnu/linux} would be a valid alias. There is a shell +script called +@uref{http://git.savannah.gnu.org/@/gitweb/@/?p=config.git;a=blob_plain;f=config.sub;hb=HEAD, +@file{config.sub}} that you can use as a subroutine to validate system +types and canonicalize aliases. + +The @code{configure} script should also take the option +@option{--build=@var{buildtype}}, which should be equivalent to a +plain @var{buildtype} argument. For example, @samp{configure +--build=i686-pc-linux-gnu} is equivalent to @samp{configure +i686-pc-linux-gnu}. When the build type is not specified by an option +or argument, the @code{configure} script should normally guess it using +the shell script +@uref{http://git.savannah.gnu.org/@/gitweb/@/?p=config.git;a=blob_plain;f=config.guess;hb=HEAD, +@file{config.guess}}. + +@cindex optional features, configure-time +Other options are permitted to specify in more detail the software +or hardware present on the machine, to include or exclude optional parts +of the package, or to adjust the name of some tools or arguments to them: + +@table @samp +@item --enable-@var{feature}@r{[}=@var{parameter}@r{]} +Configure the package to build and install an optional user-level +facility called @var{feature}. This allows users to choose which +optional features to include. Giving an optional @var{parameter} of +@samp{no} should omit @var{feature}, if it is built by default. + +No @samp{--enable} option should @strong{ever} cause one feature to +replace another. No @samp{--enable} option should ever substitute one +useful behavior for another useful behavior. The only proper use for +@samp{--enable} is for questions of whether to build part of the program +or exclude it. + +@item --with-@var{package} +@c @r{[}=@var{parameter}@r{]} +The package @var{package} will be installed, so configure this package +to work with @var{package}. + +@c Giving an optional @var{parameter} of +@c @samp{no} should omit @var{package}, if it is used by default. + +Possible values of @var{package} include +@samp{gnu-as} (or @samp{gas}), @samp{gnu-ld}, @samp{gnu-libc}, +@samp{gdb}, +@samp{x}, +and +@samp{x-toolkit}. + +Do not use a @samp{--with} option to specify the file name to use to +find certain files. That is outside the scope of what @samp{--with} +options are for. + +@item @var{variable}=@var{value} +Set the value of the variable @var{variable} to @var{value}. This is +used to override the default values of commands or arguments in the +build process. For example, the user could issue @samp{configure +CFLAGS=-g CXXFLAGS=-g} to build with debugging information and without +the default optimization. + +Specifying variables as arguments to @code{configure}, like this: +@example +./configure CC=gcc +@end example +is preferable to setting them in environment variables: +@example +CC=gcc ./configure +@end example +as it helps to recreate the same configuration later with +@file{config.status}. However, both methods should be supported. +@end table + +All @code{configure} scripts should accept all of the ``detail'' +options and the variable settings, whether or not they make any +difference to the particular package at hand. In particular, they +should accept any option that starts with @samp{--with-} or +@samp{--enable-}. This is so users will be able to configure an +entire GNU source tree at once with a single set of options. + +You will note that the categories @samp{--with-} and @samp{--enable-} +are narrow: they @strong{do not} provide a place for any sort of option +you might think of. That is deliberate. We want to limit the possible +configuration options in GNU software. We do not want GNU programs to +have idiosyncratic configuration options. + +Packages that perform part of the compilation process may support +cross-compilation. In such a case, the host and target machines for the +program may be different. + +The @code{configure} script should normally treat the specified type of +system as both the host and the target, thus producing a program which +works for the same type of machine that it runs on. + +To compile a program to run on a host type that differs from the build +type, use the configure option @option{--host=@var{hosttype}}, where +@var{hosttype} uses the same syntax as @var{buildtype}. The host type +normally defaults to the build type. + +To configure a cross-compiler, cross-assembler, or what have you, you +should specify a target different from the host, using the configure +option @samp{--target=@var{targettype}}. The syntax for +@var{targettype} is the same as for the host type. So the command would +look like this: + +@example +./configure --host=@var{hosttype} --target=@var{targettype} +@end example + +The target type normally defaults to the host type. +Programs for which cross-operation is not meaningful need not accept the +@samp{--target} option, because configuring an entire operating system for +cross-operation is not a meaningful operation. + +Some programs have ways of configuring themselves automatically. If +your program is set up to do this, your @code{configure} script can simply +ignore most of its arguments. + +@comment The makefile standards are in a separate file that is also +@comment included by make.texinfo. Done by roland@gnu.ai.mit.edu on 1/6/93. +@comment For this document, turn chapters into sections, etc. +@lowersections +@include make-stds.texi +@raisesections + +@node Releases +@section Making Releases +@cindex packaging + +You should identify each release with a pair of version numbers, a +major version and a minor. We have no objection to using more than +two numbers, but it is very unlikely that you really need them. + +Package the distribution of @code{Foo version 69.96} up in a gzipped tar +file with the name @file{foo-69.96.tar.gz}. It should unpack into a +subdirectory named @file{foo-69.96}. + +Building and installing the program should never modify any of the files +contained in the distribution. This means that all the files that form +part of the program in any way must be classified into @dfn{source +files} and @dfn{non-source files}. Source files are written by humans +and never changed automatically; non-source files are produced from +source files by programs under the control of the Makefile. + +@cindex @file{README} file +The distribution should contain a file named @file{README} which gives +the name of the package, and a general description of what it does. It +is also good to explain the purpose of each of the first-level +subdirectories in the package, if there are any. The @file{README} file +should either state the version number of the package, or refer to where +in the package it can be found. + +The @file{README} file should refer to the file @file{INSTALL}, which +should contain an explanation of the installation procedure. + +The @file{README} file should also refer to the file which contains the +copying conditions. The GNU GPL, if used, should be in a file called +@file{COPYING}. If the GNU LGPL is used, it should be in a file called +@file{COPYING.LESSER}. + +Naturally, all the source files must be in the distribution. It is +okay to include non-source files in the distribution along with the +source files they are generated from, provided they are up-to-date +with the source they are made from, and machine-independent, so that +normal building of the distribution will never modify them. We +commonly include non-source files produced by Autoconf, Automake, +Bison, @code{lex}, @TeX{}, and @code{makeinfo}; this helps avoid +unnecessary dependencies between our distributions, so that users can +install whichever packages they want to install. + +Non-source files that might actually be modified by building and +installing the program should @strong{never} be included in the +distribution. So if you do distribute non-source files, always make +sure they are up to date when you make a new distribution. + +Make sure that all the files in the distribution are world-readable, and +that directories are world-readable and world-searchable (octal mode 755). +We used to recommend that all directories in the distribution also be +world-writable (octal mode 777), because ancient versions of @code{tar} +would otherwise not cope when extracting the archive as an unprivileged +user. That can easily lead to security issues when creating the archive, +however, so now we recommend against that. + +Don't include any symbolic links in the distribution itself. If the tar +file contains symbolic links, then people cannot even unpack it on +systems that don't support symbolic links. Also, don't use multiple +names for one file in different directories, because certain file +systems cannot handle this and that prevents unpacking the +distribution. + +Try to make sure that all the file names will be unique on MS-DOS. A +name on MS-DOS consists of up to 8 characters, optionally followed by a +period and up to three characters. MS-DOS will truncate extra +characters both before and after the period. Thus, +@file{foobarhacker.c} and @file{foobarhacker.o} are not ambiguous; they +are truncated to @file{foobarha.c} and @file{foobarha.o}, which are +distinct. + +@cindex @file{texinfo.tex}, in a distribution +Include in your distribution a copy of the @file{texinfo.tex} you used +to test print any @file{*.texinfo} or @file{*.texi} files. + +Likewise, if your program uses small GNU software packages like regex, +getopt, obstack, or termcap, include them in the distribution file. +Leaving them out would make the distribution file a little smaller at +the expense of possible inconvenience to a user who doesn't know what +other files to get. + +@node References +@chapter References to Non-Free Software and Documentation +@cindex references to non-free material + +A GNU program should not recommend, promote, or grant legitimacy to +the use of any non-free program. Proprietary software is a social and +ethical problem, and our aim is to put an end to that problem. We +can't stop some people from writing proprietary programs, or stop +other people from using them, but we can and should refuse to +advertise them to new potential customers, or to give the public the +idea that their existence is ethical. + +The GNU definition of free software is found on the GNU web site at +@url{http://www.gnu.org/@/philosophy/@/free-sw.html}, and the definition +of free documentation is found at +@url{http://www.gnu.org/@/philosophy/@/free-doc.html}. The terms ``free'' +and ``non-free'', used in this document, refer to those definitions. + +A list of important licenses and whether they qualify as free is in +@url{http://www.gnu.org/@/licenses/@/license-list.html}. If it is not +clear whether a license qualifies as free, please ask the GNU Project +by writing to @email{licensing@@gnu.org}. We will answer, and if the +license is an important one, we will add it to the list. + +When a non-free program or system is well known, you can mention it in +passing---that is harmless, since users who might want to use it +probably already know about it. For instance, it is fine to explain +how to build your package on top of some widely used non-free +operating system, or how to use it together with some widely used +non-free program. + +However, you should give only the necessary information to help those +who already use the non-free program to use your program with +it---don't give, or refer to, any further information about the +proprietary program, and don't imply that the proprietary program +enhances your program, or that its existence is in any way a good +thing. The goal should be that people already using the proprietary +program will get the advice they need about how to use your free +program with it, while people who don't already use the proprietary +program will not see anything likely to lead them to take an interest +in it. + +If a non-free program or system is obscure in your program's domain, +your program should not mention or support it at all, since doing so +would tend to popularize the non-free program more than it popularizes +your program. (You cannot hope to find many additional users for your +program among the users of Foobar, if the existence of Foobar is not +generally known among people who might want to use your program.) + +Sometimes a program is free software in itself but depends on a +non-free platform in order to run. For instance, many Java programs +depend on some non-free Java libraries. To recommend or promote such +a program is to promote the other programs it needs. This is why we +are careful about listing Java programs in the Free Software +Directory: we don't want to promote the non-free Java libraries. + +We hope this particular problem with Java will be gone by and by, as +we replace the remaining non-free standard Java libraries with free +software, but the general principle will remain the same: don't +recommend, promote or legitimize programs that depend on non-free +software to run. + +Some free programs strongly encourage the use of non-free software. A +typical example is @command{mplayer}. It is free software in itself, +and the free code can handle some kinds of files. However, +@command{mplayer} recommends use of non-free codecs for other kinds of +files, and users that install @command{mplayer} are very likely to +install those codecs along with it. To recommend @command{mplayer} +is, in effect, to promote use of the non-free codecs. + +Thus, you should not recommend programs that strongly encourage the +use of non-free software. This is why we do not list +@command{mplayer} in the Free Software Directory. + +A GNU package should not refer the user to any non-free documentation +for free software. Free documentation that can be included in free +operating systems is essential for completing the GNU system, or any +free operating system, so encouraging it is a priority; to recommend +use of documentation that we are not allowed to include undermines the +impetus for the community to produce documentation that we can +include. So GNU packages should never recommend non-free +documentation. + +By contrast, it is ok to refer to journal articles and textbooks in +the comments of a program for explanation of how it functions, even +though they are non-free. This is because we don't include such +things in the GNU system even if they are free---they are outside the +scope of what a software distribution needs to include. + +Referring to a web site that describes or recommends a non-free +program is promoting that program, so please do not make links to (or +mention by name) web sites that contain such material. This policy is +relevant particularly for the web pages for a GNU package. + +Following links from nearly any web site can lead eventually to +non-free software; this is inherent in the nature of the web. So it +makes no sense to criticize a site for having such links. As long as +the site does not itself recommend a non-free program, there is no +need to consider the question of the sites that it links to for other +reasons. + +Thus, for example, you should not refer to AT&T's web site if that +recommends AT&T's non-free software packages; you should not refer to +a site that links to AT&T's site presenting it as a place to get some +non-free program, because that link recommends and legitimizes the +non-free program. However, that a site contains a link to AT&T's web +site for some other purpose (such as long-distance telephone service) +is not an objection against it. + +@node GNU Free Documentation License +@appendix GNU Free Documentation License + +@cindex FDL, GNU Free Documentation License +@include fdl.texi + +@node Index +@unnumbered Index +@printindex cp + +@bye + +Local variables: +eval: (add-hook 'write-file-hooks 'time-stamp) +time-stamp-start: "@set lastupdate " +time-stamp-end: "$" +time-stamp-format: "%:b %:d, %:y" +compile-command: "cd work.s && make" +End: |