\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 August 17, 2021 @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, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020 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, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled ``GNU Free Documentation License''. @end 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{https://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{https://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{https://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{https://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 acknowledgments in GNU software packages or documentation. Trademark acknowledgments 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 acknowledgments 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. You're free to praise Microsoft Windows on your own if you want, but please don't do so in GNU packages. Please write ``Windows'' in full, or abbreviate it to ``w.'' @xref{System Portability}. @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@. C++ is ok too, but please don't make heavy use of templates. So is Java, if you compile it. When highest efficiency is not required, other languages commonly used in the free software community, such as Lisp, Scheme, Python, Ruby, and Java, are OK too. Scheme, as implemented by GNU@tie{}Guile, plays a particular role in the GNU System: it is the preferred language to extend programs written in C/C++, and also a fine language for a wide range of applications. The more GNU components use Guile and Scheme, the more users are able to extend and combine them (@pxref{The Emacs Thesis,,, guile, GNU Guile Reference Manual}). 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{https://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 the path that will lead to overall consistency of the GNU system. @node Compatibility @section Compatibility with Other Implementations @cindex compatibility with C and POSIX standards @cindex C compatibility @cindex 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 POSIX if 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 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 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 ANSI C standard 1989 Standard C is widespread enough now that it is ok to use its features in programs. There is one exception: do not ever use the ``trigraph'' feature of Standard C. The 1999 and 2011 editions of Standard C are not fully supported on all platforms. If you aim to support compilation by compilers other than GCC, you should not require these C features in your programs. It is ok to use these features conditionally when the compiler supports them. If your program is only meant to compile with GCC, then you can use these features if GCC supports them, when they give substantial benefit. However, it is easy to support pre-standard compilers in most programs, so if you know how to do that, feel free. @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. * Finding Program Files:: How to find the program's executable and other files that go with it. * 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. Programs should work properly with multibyte character encodings, such as UTF-8. You can use libiconv to deal with a range of encodings. @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{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 @code{NULL}. 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. 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. This way, restarting the program (without reloading it), or part of it, will reinitialize those variables. 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 POSIX @code{sigaction} function; the alternative USG @code{signal} interface is an inferior design. Nowadays, using the 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. Thus, if you make @file{foo} a link to @file{ls}, the program should behave the same regardless of which of those names is used to invoke it. Instead, use a run time option or a compilation switch or both to select among the alternate behaviors. You can also build two versions of the program, with different default behaviors, and install them under two different names. @cindex output device and program's behavior Likewise, please don't make the behavior of a command-line program depend on the type of output device it gets as standard output or standard input. 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. You can also build two different versions of the program with different names. There is an exception for programs whose output in certain cases is binary data. Sending such output to a terminal is useless and can cause trouble. If such a program normally sends its output to stdout, it should detect, in these cases, when the output is a terminal and give an error message instead. The @code{-f} option should override this exception, thus permitting the output to go to the terminal. 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 Finding Program Files @section Finding the Program's Executable and Associated Files A program may need to find the executable file it was started with, so as to relaunch the same program. It may need to find associated files, either source files or files constructed by building, that it uses at run time. The way to find them starts with looking at @code{argv[0]}. If that string contains a slash, it is by convention the file name of the executable and its directory part is the directory that contained the executable. This is the case when the program was not found through @env{PATH}, which normally means it was built but not installed, and run from the build directory. The program can use the @code{argv[0]} file name to relaunch itself, and can look in its directory part for associated files. If that file name is not absolute, then it is relative to the working directory in which the program started. If @code{argv[0]} does not contain a slash, it is a command name whose executable was found via @env{PATH}. The program should search for that name in the directories in @env{PATH}, interpreting @file{.} as the working directory that was current when the program started. If this procedure finds the executable, we call the directory it was found in the @dfn{invocation directory}. The program should check for the presence in that directory of the associated files it needs. If the program's executable is normally built in a subdirectory of the main build directory, and the main build directory contains associated files (perhaps including subdirectories), the program should look at the parent of the invocation directory, checking for the associated files and subdirectories the main build directory should contain. If the invocation directory doesn't contain what's needed, but the executable file name is a symbolic link, the program should try using the link target's containing directory as the invocation directory. If this procedure doesn't come up with an invocation directory that is valid---normally the case for an installed program that was found via @env{PATH}---the program should look for the associated files in the directories where the program's makefile installs them. @xref{Directory Variables}. Providing valid information in @code{argv[0]} is a convention, not guaranteed. Well-behaved programs that launch other programs, such as shells, follow the convention; your code should follow it too, when launching other programs. But it is always possible to launch the program and give a nonsensical value in @code{argv[0]}. Therefore, any program that needs to know the location of its executable, or that of of other associated files, should offer the user environment variables to specify those locations explicitly. @strong{Don't give special privilege, such as with the @code{setuid} bit, to programs that will search heuristically for associated files or for their own executables when invoked that way.} Limit that privilege to programs that find associated files in hard-coded installed locations such as under @file{/usr} and @file{/etc}. @c ??? Is even that safe, in a setuid program? @xref{Bourne Shell Variables,,, bash, Bash Reference Manual}, for more information about @env{PATH}. @node Graphical Interfaces @section Standards for Graphical Interfaces @cindex graphical user interface @cindex interface styles @cindex user interface styles @cindex GTK+ @cindex GNUstep When you write a program that provides a graphical user interface, please make it work with the X Window System, using the GTK+ toolkit or the GNUstep 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. Please make your program interoperate with access technology such as screen readers (see @url{https://www.gnu.org/accessibility/accessibility.html}). This should be automatic if you use GTK+. @node Command-Line Interfaces @section Standards for Command Line Interfaces @cindex command-line interface @findex getopt It is a good idea to follow the 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 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 @indicateurl{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 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 a GNU license, @emph{always} indicate the permitted versions in this way. 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{https://www.gnu.org/@/licenses/@/gpl.html}. @item LGPL GNU Lesser General Public License, @url{https://www.gnu.org/@/licenses/@/lgpl.html}. @item GPL/Ada GNU GPL with the exception for Ada. @item Apache The Apache Software Foundation license, @url{https://directory.fsf.org/@/wiki/@/License:Apache2.0}. @item Artistic The Artistic license used for Perl, @url{https://directory.fsf.org/@/wiki/@/License:ArtisticLicense2.0}. @item Expat The Expat license, @url{https://directory.fsf.org/@/wiki/@/License:Expat}. @item MPL The Mozilla Public License, @url{https://directory.fsf.org/@/wiki/@/License:MPLv2.0}. @item OBSD The original (4-clause) BSD license, incompatible with the GNU GPL,@* @url{https://directory.fsf.org/@/wiki/@/License:BSD_4Clause}. @item PHP The license used for PHP, @url{https://directory.fsf.org/@/wiki/@/License:PHPv3.01}. @item public domain The non-license that is being in the public domain,@* @url{https://www.gnu.org/@/licenses/@/license-list.html#PublicDomain}. @item Python The license for Python, @url{https://directory.fsf.org/@/wiki/@/License:Python2.0.1}. @item RBSD The revised (3-clause) BSD, compatible with the GNU GPL,@* @url{https://directory.fsf.org/@/wiki/@/License:BSD_3Clause}. @item X11 The simple non-copyleft license used for most versions of the X Window System, @url{https://directory.fsf.org/@/wiki/@/License:X11}. @item Zlib The license for Zlib, @url{https://directory.fsf.org/@/wiki/@/License:Zlib}. @end table More information about these licenses and many more are on the GNU licensing web pages, @url{https://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{https://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: General help using GNU software: @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 Sergey Poznyakoff). These are used for SNMP, LDAP, X.509 certificates, and so on. The web site @url{https://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 @code{NULL}. @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 line length @cindex length of source lines Please keep the length of source lines to 79 characters or less, for maximum readability in the widest range of environments. @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)) == NULL) fatal ("virtual memory exhausted"); @end example @noindent instead, write this: @example foo = (char *) malloc (sizeof *foo); if (foo == NULL) fatal ("virtual memory exhausted"); @end example @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 as part of the GNU operating system, compiled with GNU compilers, on various types of hardware. So the kinds of portability that are absolutely necessary are quite limited. It is important to support Linux-based GNU systems, since they are the form of GNU that people mainly use. Making a GNU program operate on operating systems other than the GNU system is not part of the core goal of developing a GNU package. You don't ever have to do that. However, users will ask you to do that, and cooperating with those requests is useful---as long as you don't let it dominate the project or impede the primary goal. It is good to support the other free or nearly free operating systems (for instance, *BSD). 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. For the most part it is good to port the program to more platforms, but you should not let take up so much of your time that it hinders you from improving the program in more central ways. If it starts to do that, please tell users that you don't want to spend any more time on this---someone else must write that code, debug it, document it, etc., and then you can install it. You can reject porting patches for technical reasons too, as with any other patch that users submit. It is up to you. @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-POSIX systems, and portability As for systems that are not like Unix, such as MS-DOS, 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''. @xref{Trademarks}. Usually we write the name ``Windows'' in full, but when brevity is very important (as in file names and some symbol names), we abbreviate it to ``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}. In principle there could also be @samp{w64}. 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 CPUs @cindex data types, and portability @cindex portability, and data types Even GNU systems will differ because of differences among 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. You need not cater to the possibility that @code{long} will be smaller than pointers and @code{size_t}. We know of one such platform: 64-bit programs on Microsoft Windows. If you care about making your package run on Windows using Mingw64, you would need to deal with 8-byte pointers and 4-byte @code{long}, which would break this code: @example printf ("size = %lu\n", (unsigned long) sizeof array); printf ("diff = %ld\n", (long) (pointer2 - pointer1)); @end example Whether to support Mingw64, and Windows in general, in your package is your choice. The GNU Project doesn't say you have any responsibility to do so. Our goal is to replace proprietary systems, including Windows, not to enhance them. If people pressure you to make your program run on Windows, and you are not interested, you can respond with, ``Switch to GNU/Linux --- your freedom depends on 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, all practical systems have a C89 compiler and GNU C supports almost all of C99 and some of C11. Similarly, most systems implement POSIX.1-2001 libraries and tools, and many have POSIX.1-2008. Hence, there is little reason to support old C or non-POSIX systems, and you may want to take advantage of standard C and POSIX 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{https://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 If you use @code{mmap} to read or write files, don't assume it 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. Remember that the audience for a GNU manual (and other GNU documentation) is global, and that it will be used for years, maybe decades. This means that the reader could have very different cultural reference points. Decades from now, all but old folks will have very different cultural reference points; many things that "everyone knows about" today may be mostly forgotten. For this reason, try to avoid writing in a way that depends on cultural reference points for proper understanding, or that refers to them in ways that would impede reading for someone that doesn't recognize them. Likewise, be conservative in your choice of words (aside from technical terms), linguistic constructs, and spelling: aim to make them intelligible to readers from ten years ago. In any contest for trendiness, GNU writing should not even qualify to enter. It is ok to refer once in a rare while to spatially or temporally localized reference points or facts, if it is directly pertinent or as an aside. Changing these few things (which in any case stand out) when they no longer make sense will not be a lot of work. By contrast, it is always proper to refer to concepts of GNU and the free software movement, when they are pertinent. These are a central part of our message, so we should take advantage of opportunities to mention them. They are fundamental moral positions, so they will rarely if ever change. 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 lists 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. Whenever possible, please stick to the active voice, avoiding the passive, and use the present tense, not the future tense. For instance, write ``The function @code{foo} returns a list containing @var{a} and @var{b}'' rather than ``A list containing @var{a} and @var{b} will be returned.'' One advantage of the active voice is it requires you to state the subject of the sentence; with the passive voice, you might omit the subject, which leads to vagueness. It is proper to use the future tense when grammar demands it, as in, ``If you type @kbd{x}, the computer will self-destruct in 10 seconds.'' @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{https://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{https://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, who they came from, and why the conflicting changes were made. @cindex software forensics, and change logs Therefore, change logs should be detailed enough and accurate enough to provide the information commonly required for such @dfn{software forensics}. Specifically, change logs should make finding answers to the following questions easy: @itemize @bullet @item What changes affected a particular source file? @item Was a particular source file renamed or moved, and if so, as part of what change? @item What changes affected a given function or macro or definition of a data structure? @item Was a function (or a macro or the definition of a data structure) renamed or moved from another file, and if so, as part of which change? @item What changes deleted a function (or macro or data structure)? @item What was the rationale for a given change, and what were its main ideas? @item Is there any additional information regarding the change, and if so, where can it be found? @end itemize @cindex VCS @cindex version control system, for keeping change logs Historically, change logs were maintained on specially formatted files. Nowadays, projects commonly keep their source files under a @dfn{version control system} (VCS), such as Git, Subversion, or Mercurial. If the VCS repository is publicly accessible, and changes are committed to it separately (one commit for each logical changeset) and record the authors of each change, then the information recorded by the VCS can be used to produce the change logs out of VCS logs, and to answer the above questions by using the suitable VCS commands. (However, the VCS log messages still need to provide some supporting information, as described below.) Projects that maintain such VCS repositories can decide not to maintain separate change log files, and instead rely on the VCS to keep the change logs. If you decide not to maintain separate change log files, you should still consider providing them in the release tarballs, for the benefit of users who'd like to review the change logs without accessing the project's VCS repository. Scripts exist that can produce @file{ChangeLog} files from the VCS logs; for example, the @file{gitlog-to-changelog} script, which is part of Gnulib, can do that for Git repositories. In Emacs, the command @kbd{C-x v a} (@code{vc-update-change-log}) does the job of incrementally updating a @file{ChangeLog} file from the VCS logs. If separate change log files @emph{are} maintained, they are normally called @file{ChangeLog}, and each such file covers an entire directory. Each directory can have its own change log file, or a directory can use the change log of its parent directory---it's up to you. @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 and Conventions @cindex changeset, in a change log @cindex batch of changes, in a change log You can think of the change log as a conceptual ``undo list'' which states 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. Each @dfn{entry} in a change log describes either an individual change or the smallest batch of changes that belong together, also known as a @dfn{changeset}. @cindex title, change log entry @cindex header line, change log entry It is a good idea to start the change log entry with a @dfn{header line}: a single line that is a complete sentence which summarizes the changeset. If you keep the change log in a VCS, this should be a requirement, as VCS commands that show the change log in abbreviated form, such as @kbd{git log --oneline}, treat the header line specially. (In a @file{ChangeLog} file, the header line follows a line that says who was the author of the change and when it was installed.) @cindex description, change log entry Follow the change log entry's header line with a description of the overall change. This should be as long as needed to give a clear description. Pay special attention to aspects of the changeset not easily gleaned from the diffs or from the names of modified files and functions: the overall idea of the change and the need for it, and the relations, if any, between changes made to different files/functions. If the change or its reasons were discussed on some public forum, such as the project's issue tracker or mailing list, it is a good idea to summarize the main points of that discussion in the change's description, and include a pointer to that discussion or the issue ID for those who'd like to read it in full. The best place to explain how parts of the new code work with other code is in comments in the code, not in the change log. If you think that a change calls for explanation of @emph{why} the change was needed---that is, what problem the old code had such that it required this change---you're probably right. Please put the explanation in comments in the code, where people will see it whenever they see the code. An example of such an explanation is, ``This function used to be iterative, but that failed when MUMBLE was a tree.'' (Though such a simple reason would not need this kind of explanation.) The best place for other kinds of explanation of the change is in the change log entry. In particular, comments usually will not say why some code was deleted or moved to another place---that belongs to the description of the change which did that. Following the free-text description of the change, it is a good idea to give a list of names of the entities or definitions that you changed, according to the files they are in, and what was changed in each one. @xref{Style of Change Logs}. If a project uses a modern VCS to keep the change log information, as described in @ref{Change Logs}, explicitly listing the files and functions that were changed is not strictly necessary, and in some cases (like identical mechanical changes in many places) even tedious. It is up to you to decide whether to allow your project's developers to omit the list of changed files and functions from the log entries, and whether to allow such omissions under some specific conditions. However, while making this decision, please consider the following benefits of providing the list of changed entities with each change: @itemize @bullet @item Generation of useful @file{ChangeLog} files from VCS logs becomes more difficult if the change log entries don't list the modified functions/macros, because VCS commands cannot reliably reproduce their names from the commit information alone. For example, when there is a change in the header part of a function definition, the heading of the diff hunk as shown in the VCS log commands will name the wrong function as being modified (usually, the function defined before the one being modified), so using those diffs to glean the names of the modified functions will produce inaccurate results. You will need to use specialized scripts, such as gnulib's @file{vcs-to-changelog.py}, mentioned below, to solve these difficulties, and make sure it supports the source languages used by your project. @item While modern VCS commands, such as Git's @kbd{git log -L} and @kbd{git log -G}, provide powerful means for finding changes that affected a certain function or macro or data structure (and thus might make @file{ChangeLog} files unnecessary if you have the repository available), they can sometimes fail. For example, @kbd{git log -L} doesn't support syntax of some programming languages out of the box. Mentioning the modified functions/macros explicitly allows finding the related changes simply and reliably. @item Some VCS commands have difficulties or limitations when tracking changes across file moves or renames. Again, if the entities are mentioned explicitly, those difficulties can be overcome. @item Users that review changes using the generated @file{ChangeLog} files may not have the repository and the VCS commands available to them. Naming the modified entities alleviates that problem. @end itemize @noindent For these reasons, providing lists of modified files and functions with each change makes the change logs more useful, and we therefore recommend to include them whenever possible and practical. It is also possible to generate the lists naming the modified entities by running a script. One such script is @file{mklog.py} (written in Python 3); it is used by the @code{GCC} project. Gnulib provides another variant of such a script, called @file{vcs-to-changelog.py}, part of the @code{vcs-to-changelog} module. Note that these scripts currently support fewer programming languages than the manual commands provided by Emacs (@pxref{Style of Change Logs}). Therefore, the above mentioned method of generating the @code{ChangeLog} file from the VCS commit history, for instance via the @code{gitlog-to-changelog} script, usually gives better results---provided that the contributors stick to providing good commit messages. @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.) Keep in mind that the line which shows the date of the change and the author's name and email address is needed only in a separate @file{ChangeLog} file, not when the change logs are kept in a VCS. @example 2019-08-29 Noam Postavsky Handle completely undecoded input in term (Bug#29918) * lisp/term.el (term-emulate-terminal): Avoid errors if the whole decoded string is eight-bit characters. Don't attempt to save the string for next iteration in that case. * test/lisp/term-tests.el (term-decode-partial) (term-undecodable-input): New tests. 2019-06-15 Paul Eggert Port to platforms where tputs is in libtinfow * configure.ac (tputs_library): Also try tinfow, ncursesw (Bug#33977). 2019-02-08 Eli Zaretskii Improve documentation of 'date-to-time' and 'parse-time-string' * doc/lispref/os.texi (Time Parsing): Document 'parse-time-string', and refer to it for the description of the argument of 'date-to-time'. * lisp/calendar/time-date.el (date-to-time): Refer in the doc string to 'parse-time-string' for more information about the format of the DATE argument. (Bug#34303) @end example If you mention the names of the modified functions or variables, it's important to name them 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. Don't put blank lines between individual changes of an entry. You can omit the file name and the asterisk when successive individual changes 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{(}. This makes highlighting in Emacs work better. Here is an example: @example * src/keyboard.c (menu_bar_items, tool_bar_items) (Fexecute_extended_command): Deal with 'keymap' property. @end example The easiest way to add an entry to @file{ChangeLog} is with the Emacs command @kbd{M-x add-change-log-entry}, or its variant @kbd{C-x 4 a} (@code{add-change-log-entry-other-window}). This automatically collects the name of the changed file and the changed function or variable, and formats a change log entry according to the conventions described above, leaving it up to you to describe the changes you made to that function or variable. 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 * sewing.c: Make it sew. @end example @noindent rather than this: @example 2002-07-14 Usual Maintainer * sewing.c: Make it sew. Patch by jdoe@@gnu.org. @end example When committing someone else's changes into a VCS, use the VCS features to specify the author. For example, with Git, use @kbd{git commit --author=@var{author}}. As for the date, that should be the date you applied the change. (With a VCS, use the appropriate command-line switches, e.g., @kbd{git commit --date=@var{date}}.) Modern VCS have commands to apply changes sent via email (e.g., Git has @kbd{git am}); in that case the author of the changeset and the date it was made will be automatically gleaned from the email message and recorded in the repository. If the patches are prepared with suitable VCS commands, such as @kbd{git format-patch}, the email message body will also have the original author of the changeset, so resending or forwarding the message will not interfere with attributing the changes to their author. Thus, we recommend that you request your contributors to use commands such as @kbd{git format-patch} to prepare the patches. @node Simple Changes @subsection Simple Changes Certain simple kinds of changes don't need much detail in the change log. If the description of the change is short enough, it can serve as its own header line: @example 2019-08-29 Eli Zaretskii * lisp/simple.el (kill-do-not-save-duplicates): Doc fix. (Bug#36827) @end example 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. When you make changes in many files that follow mechanically from one underlying change, it is enough to describe the underlying change. Here's an example of a change that affects all of the files in the repository: @example 2019-01-07 Paul Eggert Update copyright year to 2019 Run 'TZ=UTC0 admin/update-copyright $(git ls-files)'. @end example Test suite files are part of the software, so we recommend treating them as code for change-log purposes. There's no technical need to make change log entries for non-software files (manuals, help files, media files, etc.). This is because they are not susceptible to bugs that are hard to understand. To correct an error, you need not know the history of the erroneous passage; it is enough to compare what the file says with the actual facts. However, you should keep change logs for non-software files when the project gets copyright assignments from its contributors, so as to make the records of authorship more accurate. For that reason, we recommend to keep change logs for Texinfo sources of your project's manuals. @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 . * 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 * host.c (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) : 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 stating 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{https://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 stating 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 --runstatedir --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{https://git.savannah.gnu.org/cgit/config.git/plain/config.sub, @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{https://git.savannah.gnu.org/cgit/config.git/plain/config.guess, @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 @cindex version numbers, for releases 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} with a general overview of the package: @itemize @item the name of the package; @item the version number of the package, or refer to where in the package the version can be found; @item a general description of what the package does; @item a reference to the file @file{INSTALL}, which should in turn contain an explanation of the installation procedure; @item a brief explanation of any unusual top-level directories or files, or other hints for readers to find their way around the source; @item a reference 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}. @end itemize 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{flex}, @TeX{}, and @code{makeinfo}; this helps avoid unnecessary dependencies between our distributions, so that users can install whichever versions of whichever packages they like. Do not induce new dependencies on other software lightly. 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 impression that their existence is legitimate. The GNU definition of free software is found on the GNU web site at @url{https://www.gnu.org/@/philosophy/@/free-sw.html}, and the definition of free documentation is found at @url{https://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{https://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, after first explaining how to use it on the GNU system. 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. You shouldn't recommend any non-free add-ons for the non-free program, but it is ok to mention free add-ons that help it to work with your program, and how to install the free add-ons even if that requires running some non-free program. 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, it used to be the case that many Java programs depended on some non-free Java libraries. (See @uref{https://www.gnu.org/philosophy/java-trap.html}.) To recommend or promote such a program is to promote the other programs it needs; therefore, judge mentions of the former as if they were mentions of the latter. For this reason, we were careful about listing Java programs in the Free Software Directory: we wanted to avoid promoting the non-free Java libraries. Java no longer has this problem, 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. What about chains of links? Following links from nearly any web site can lead eventually to promotion of non-free software; this is inherent in the nature of the web. Here's how we treat that. 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 page @var{p} that links to AT&T's site presenting it as a place to get some non-free program, because that part of the page @var{p} itself recommends and legitimizes the non-free program. However, if @var{p} contains a link to AT&T's web site for some other purpose (such as long-distance telephone service), that is no reason you should not link to @var{p}. A web page recommends a program in an implicit but particularly strong way if it requires users to run that program in order to use the page. Many pages contain JavaScript code which they recommend in this way. This JavaScript code may be free or non-free, but non-free is the usual case. If the purpose for which you would refer to the page cannot be carried out without running non-free JavaScript code, then you should not refer to it. Thus, if the purpose of referring to the page is for people to view a video, or subscribing to a mailing list, and the viewing or subscribing fail to work if the user's browser blocks the non-free JavaScript code, then don't refer to that page. The extreme case is that of web sites which depend on non-free JavaScript code even to @emph{see} the contents of the pages. Any site hosted on @indicateurl{wix.com} has this problem, and so do some other sites. Referring people to such pages to read their contents is, in effect, urging them to run those non-free programs---so please don't refer to those pages. (Such pages also break the Web, so they deserve condemnation for two reasons.) Instead, please quote excerpts from the page to make your point, or find another place to refer to that information. @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 'before-save-hook 'time-stamp) time-stamp-start: "@set lastupdate " time-stamp-end: "$" time-stamp-format: "%:b %:d, %:y" compile-command: "cd work.s && make" End: