1 \input texinfo @c -*-texinfo-*-
3 @setfilename standards.info
4 @settitle GNU Coding Standards
5 @c This date is automagically updated when you save this file:
6 @set lastupdate January 1, 2013
9 @dircategory GNU organization
11 * Standards: (standards). GNU coding standards.
14 @c @setchapternewpage odd
15 @setchapternewpage off
17 @c Put everything in one index (arbitrarily chosen to be the concept index).
23 @c This is used by a cross ref in make-stds.texi
27 The GNU coding standards, last updated @value{lastupdate}.
29 Copyright @copyright{} 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999,
30 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010,
31 2011, 2012, 2013 Free Software Foundation, Inc.
33 Permission is granted to copy, distribute and/or modify this document
34 under the terms of the GNU Free Documentation License, Version 1.3 or
35 any later version published by the Free Software Foundation; with no
36 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
37 Texts. A copy of the license is included in the section entitled
38 ``GNU Free Documentation License''.
42 @title GNU Coding Standards
43 @author Richard Stallman, et al.
44 @author last updated @value{lastupdate}
46 @vskip 0pt plus 1filll
54 @top GNU Coding Standards
60 * Preface:: About the GNU Coding Standards.
61 * Legal Issues:: Keeping free software free.
62 * Design Advice:: General program design.
63 * Program Behavior:: Program behavior for all programs
64 * Writing C:: Making the best use of C.
65 * Documentation:: Documenting programs.
66 * Managing Releases:: The release process.
67 * References:: Mentioning non-free software or documentation.
68 * GNU Free Documentation License:: Copying and sharing this manual.
74 @chapter About the GNU Coding Standards
76 The GNU Coding Standards were written by Richard Stallman and other GNU
77 Project volunteers. Their purpose is to make the GNU system clean,
78 consistent, and easy to install. This document can also be read as a
79 guide to writing portable, robust and reliable programs. It focuses on
80 programs written in C, but many of the rules and principles are useful
81 even if you write in another programming language. The rules often
82 state reasons for writing in a certain way.
84 @cindex where to obtain @code{standards.texi}
85 @cindex downloading this manual
86 If you did not obtain this file directly from the GNU project and
87 recently, please check for a newer version. You can get the GNU
88 Coding Standards from the GNU web server in many
89 different formats, including the Texinfo source, PDF, HTML, DVI, plain
90 text, and more, at: @uref{http://www.gnu.org/prep/standards/}.
92 If you are maintaining an official GNU package, in addition to this
93 document, please read and follow the GNU maintainer information
94 (@pxref{Top, , Contents, maintain, Information for Maintainers of GNU
97 @cindex @code{gnustandards-commit@@gnu.org} mailing list
98 If you want to receive diffs for every change to these GNU documents,
99 join the mailing list @code{gnustandards-commit@@gnu.org}, via the web
101 @url{http://lists.gnu.org/mailman/listinfo/gnustandards-commit}.
102 Archives are also available there.
104 @cindex @code{bug-standards@@gnu.org} email address
105 @cindex Savannah repository for gnustandards
106 @cindex gnustandards project repository
107 Please send corrections or suggestions for this document to
108 @email{bug-standards@@gnu.org}. If you make a suggestion, please
109 include a suggested new wording for it, to help us consider the
110 suggestion efficiently. We prefer a context diff to the Texinfo
111 source, but if that's difficult for you, you can make a context diff
112 for some other version of this document, or propose it in any way that
113 makes it clear. The source repository for this document can be found
114 at @url{http://savannah.gnu.org/projects/gnustandards}.
116 These standards cover the minimum of what is important when writing a
117 GNU package. Likely, the need for additional standards will come up.
118 Sometimes, you might suggest that such standards be added to this
119 document. If you think your standards would be generally useful, please
122 You should also set standards for your package on many questions not
123 addressed or not firmly specified here. The most important point is to
124 be self-consistent---try to stick to the conventions you pick, and try
125 to document them as much as possible. That way, your program will be
126 more maintainable by others.
128 The GNU Hello program serves as an example of how to follow the GNU
129 coding standards for a trivial program.
130 @uref{http://www.gnu.org/software/hello/hello.html}.
132 This release of the GNU Coding Standards was last updated
137 @chapter Keeping Free Software Free
138 @cindex legal aspects
140 This chapter discusses how you can make sure that GNU software
141 avoids legal difficulties, and other related issues.
144 * Reading Non-Free Code:: Referring to proprietary programs.
145 * Contributions:: Accepting contributions.
146 * Trademarks:: How we deal with trademark issues.
149 @node Reading Non-Free Code
150 @section Referring to Proprietary Programs
151 @cindex proprietary programs
152 @cindex avoiding proprietary code
154 Don't in any circumstances refer to Unix source code for or during
155 your work on GNU! (Or to any other proprietary programs.)
157 If you have a vague recollection of the internals of a Unix program,
158 this does not absolutely mean you can't write an imitation of it, but
159 do try to organize the imitation internally along different lines,
160 because this is likely to make the details of the Unix version
161 irrelevant and dissimilar to your results.
163 For example, Unix utilities were generally optimized to minimize
164 memory use; if you go for speed instead, your program will be very
165 different. You could keep the entire input file in memory and scan it
166 there instead of using stdio. Use a smarter algorithm discovered more
167 recently than the Unix program. Eliminate use of temporary files. Do
168 it in one pass instead of two (we did this in the assembler).
170 Or, on the contrary, emphasize simplicity instead of speed. For some
171 applications, the speed of today's computers makes simpler algorithms
174 Or go for generality. For example, Unix programs often have static
175 tables or fixed-size strings, which make for arbitrary limits; use
176 dynamic allocation instead. Make sure your program handles NULs and
177 other funny characters in the input files. Add a programming language
178 for extensibility and write part of the program in that language.
180 Or turn some parts of the program into independently usable libraries.
181 Or use a simple garbage collector instead of tracking precisely when
182 to free memory, or use a new GNU facility such as obstacks.
186 @section Accepting Contributions
188 @cindex accepting contributions
190 If the program you are working on is copyrighted by the Free Software
191 Foundation, then when someone else sends you a piece of code to add to
192 the program, we need legal papers to use it---just as we asked you to
193 sign papers initially. @emph{Each} person who makes a nontrivial
194 contribution to a program must sign some sort of legal papers in order
195 for us to have clear title to the program; the main author alone is not
198 So, before adding in any contributions from other people, please tell
199 us, so we can arrange to get the papers. Then wait until we tell you
200 that we have received the signed papers, before you actually use the
203 This applies both before you release the program and afterward. If
204 you receive diffs to fix a bug, and they make significant changes, we
205 need legal papers for that change.
207 This also applies to comments and documentation files. For copyright
208 law, comments and code are just text. Copyright applies to all kinds of
209 text, so we need legal papers for all kinds.
211 We know it is frustrating to ask for legal papers; it's frustrating for
212 us as well. But if you don't wait, you are going out on a limb---for
213 example, what if the contributor's employer won't sign a disclaimer?
214 You might have to take that code out again!
216 You don't need papers for changes of a few lines here or there, since
217 they are not significant for copyright purposes. Also, you don't need
218 papers if all you get from the suggestion is some ideas, not actual code
219 which you use. For example, if someone sent you one implementation, but
220 you write a different implementation of the same idea, you don't need to
223 The very worst thing is if you forget to tell us about the other
224 contributor. We could be very embarrassed in court some day as a
227 We have more detailed advice for maintainers of GNU packages. If you
228 have reached the stage of maintaining a GNU program (whether released
229 or not), please take a look: @pxref{Legal Matters,,, maintain,
230 Information for GNU Maintainers}.
237 Please do not include any trademark acknowledgements in GNU software
238 packages or documentation.
240 Trademark acknowledgements are the statements that such-and-such is a
241 trademark of so-and-so. The GNU Project has no objection to the basic
242 idea of trademarks, but these acknowledgements feel like kowtowing,
243 and there is no legal requirement for them, so we don't use them.
245 What is legally required, as regards other people's trademarks, is to
246 avoid using them in ways which a reader might reasonably understand as
247 naming or labeling our own programs or activities. For example, since
248 ``Objective C'' is (or at least was) a trademark, we made sure to say
249 that we provide a ``compiler for the Objective C language'' rather
250 than an ``Objective C compiler''. The latter would have been meant as
251 a shorter way of saying the former, but it does not explicitly state
252 the relationship, so it could be misinterpreted as using ``Objective
253 C'' as a label for the compiler rather than for the language.
255 Please don't use ``win'' as an abbreviation for Microsoft Windows in
256 GNU software or documentation. In hacker terminology, calling
257 something a ``win'' is a form of praise. If you wish to praise
258 Microsoft Windows when speaking on your own, by all means do so, but
259 not in GNU software. Usually we write the name ``Windows'' in full,
260 but when brevity is very important (as in file names and sometimes
261 symbol names), we abbreviate it to ``w''. For instance, the files and
262 functions in Emacs that deal with Windows start with @samp{w32}.
265 @chapter General Program Design
266 @cindex program design
268 This chapter discusses some of the issues you should take into
269 account when designing your program.
271 @c Standard or ANSI C
273 @c In 1989 the American National Standards Institute (ANSI) standardized
274 @c C as standard X3.159-1989. In December of that year the
275 @c International Standards Organization ISO adopted the ANSI C standard
276 @c making minor changes. In 1990 ANSI then re-adopted ISO standard
277 @c C. This version of C is known as either ANSI C or Standard C.
279 @c A major revision of the C Standard appeared in 1999.
282 * Source Language:: Which languages to use.
283 * Compatibility:: Compatibility with other implementations.
284 * Using Extensions:: Using non-standard features.
285 * Standard C:: Using standard C features.
286 * Conditional Compilation:: Compiling code only if a conditional is true.
289 @node Source Language
290 @section Which Languages to Use
291 @cindex programming languages
293 When you want to use a language that gets compiled and runs at high
294 speed, the best language to use is C. Using another language is like
295 using a non-standard feature: it will cause trouble for users. Even if
296 GCC supports the other language, users may find it inconvenient to have
297 to install the compiler for that other language in order to build your
298 program. For example, if you write your program in C++, people will
299 have to install the GNU C++ compiler in order to compile your program.
301 C has one other advantage over C++ and other compiled languages: more
302 people know C, so more people will find it easy to read and modify the
303 program if it is written in C.
305 So in general it is much better to use C, rather than the
306 comparable alternatives.
308 But there are two exceptions to that conclusion:
312 It is no problem to use another language to write a tool specifically
313 intended for use with that language. That is because the only people
314 who want to build the tool will be those who have installed the other
318 If an application is of interest only to a narrow part of the community,
319 then the question of which language it is written in has less effect on
320 other people, so you may as well please yourself.
323 Many programs are designed to be extensible: they include an interpreter
324 for a language that is higher level than C. Often much of the program
325 is written in that language, too. The Emacs editor pioneered this
329 @cindex GNOME and Guile
330 The standard extensibility interpreter for GNU software is Guile
331 (@uref{http://www.gnu.org/@/software/@/guile/}), which implements the
332 language Scheme (an especially clean and simple dialect of Lisp).
333 Guile also includes bindings for GTK+/GNOME, making it practical to
334 write modern GUI functionality within Guile. We don't reject programs
335 written in other ``scripting languages'' such as Perl and Python, but
336 using Guile is very important for the overall consistency of the GNU
341 @section Compatibility with Other Implementations
342 @cindex compatibility with C and POSIX standards
343 @cindex C compatibility
344 @cindex POSIX compatibility
346 With occasional exceptions, utility programs and libraries for GNU
347 should be upward compatible with those in Berkeley Unix, and upward
348 compatible with Standard C if Standard C specifies their
349 behavior, and upward compatible with POSIX if POSIX specifies
352 When these standards conflict, it is useful to offer compatibility
353 modes for each of them.
355 @cindex options for compatibility
356 Standard C and POSIX prohibit many kinds of extensions. Feel
357 free to make the extensions anyway, and include a @samp{--ansi},
358 @samp{--posix}, or @samp{--compatible} option to turn them off.
359 However, if the extension has a significant chance of breaking any real
360 programs or scripts, then it is not really upward compatible. So you
361 should try to redesign its interface to make it upward compatible.
363 @cindex @code{POSIXLY_CORRECT}, environment variable
364 Many GNU programs suppress extensions that conflict with POSIX if the
365 environment variable @code{POSIXLY_CORRECT} is defined (even if it is
366 defined with a null value). Please make your program recognize this
367 variable if appropriate.
369 When a feature is used only by users (not by programs or command
370 files), and it is done poorly in Unix, feel free to replace it
371 completely with something totally different and better. (For example,
372 @code{vi} is replaced with Emacs.) But it is nice to offer a compatible
373 feature as well. (There is a free @code{vi} clone, so we offer it.)
375 Additional useful features are welcome regardless of whether
376 there is any precedent for them.
378 @node Using Extensions
379 @section Using Non-standard Features
380 @cindex non-standard extensions
382 Many GNU facilities that already exist support a number of convenient
383 extensions over the comparable Unix facilities. Whether to use these
384 extensions in implementing your program is a difficult question.
386 On the one hand, using the extensions can make a cleaner program.
387 On the other hand, people will not be able to build the program
388 unless the other GNU tools are available. This might cause the
389 program to work on fewer kinds of machines.
391 With some extensions, it might be easy to provide both alternatives.
392 For example, you can define functions with a ``keyword'' @code{INLINE}
393 and define that as a macro to expand into either @code{inline} or
394 nothing, depending on the compiler.
396 In general, perhaps it is best not to use the extensions if you can
397 straightforwardly do without them, but to use the extensions if they
398 are a big improvement.
400 An exception to this rule are the large, established programs (such as
401 Emacs) which run on a great variety of systems. Using GNU extensions in
402 such programs would make many users unhappy, so we don't do that.
404 Another exception is for programs that are used as part of compilation:
405 anything that must be compiled with other compilers in order to
406 bootstrap the GNU compilation facilities. If these require the GNU
407 compiler, then no one can compile them without having them installed
408 already. That would be extremely troublesome in certain cases.
411 @section Standard C and Pre-Standard C
412 @cindex ANSI C standard
414 1989 Standard C is widespread enough now that it is ok to use its
415 features in programs. There is one exception: do not ever use the
416 ``trigraph'' feature of Standard C.
418 The 1999 and 2011 editions of Standard C are not fully supported
419 on all platforms. If you aim to support compilation by
420 compilers other than GCC, you should not require these C
421 features in your programs. It is ok to use these features
422 conditionally when the compiler supports them.
424 If your program is only meant to compile with GCC, then you can
425 use these features if GCC supports them, when they give substantial
428 However, it is easy to support pre-standard compilers in most programs,
429 so if you know how to do that, feel free.
431 @cindex function prototypes
432 To support pre-standard C, instead of writing function definitions in
433 standard prototype form,
442 write the definition in pre-standard style like this,
452 and use a separate declaration to specify the argument prototype:
458 You need such a declaration anyway, in a header file, to get the benefit
459 of prototypes in all the files where the function is called. And once
460 you have the declaration, you normally lose nothing by writing the
461 function definition in the pre-standard style.
463 This technique does not work for integer types narrower than @code{int}.
464 If you think of an argument as being of a type narrower than @code{int},
465 declare it as @code{int} instead.
467 There are a few special cases where this technique is hard to use. For
468 example, if a function argument needs to hold the system type
469 @code{dev_t}, you run into trouble, because @code{dev_t} is shorter than
470 @code{int} on some machines; but you cannot use @code{int} instead,
471 because @code{dev_t} is wider than @code{int} on some machines. There
472 is no type you can safely use on all machines in a non-standard
473 definition. The only way to support non-standard C and pass such an
474 argument is to check the width of @code{dev_t} using Autoconf and choose
475 the argument type accordingly. This may not be worth the trouble.
477 In order to support pre-standard compilers that do not recognize
478 prototypes, you may want to use a preprocessor macro like this:
481 /* Declare the prototype for a general external function. */
482 #if defined (__STDC__) || defined (WINDOWSNT)
483 #define P_(proto) proto
489 @node Conditional Compilation
490 @section Conditional Compilation
492 When supporting configuration options already known when building your
493 program we prefer using @code{if (... )} over conditional compilation,
494 as in the former case the compiler is able to perform more extensive
495 checking of all possible code paths.
497 For example, please write
517 A modern compiler such as GCC will generate exactly the same code in
518 both cases, and we have been using similar techniques with good success
519 in several projects. Of course, the former method assumes that
520 @code{HAS_FOO} is defined as either 0 or 1.
522 While this is not a silver bullet solving all portability problems,
523 and is not always appropriate, following this policy would have saved
524 GCC developers many hours, or even days, per year.
526 In the case of function-like macros like @code{REVERSIBLE_CC_MODE} in
527 GCC which cannot be simply used in @code{if (...)} statements, there is
528 an easy workaround. Simply introduce another macro
529 @code{HAS_REVERSIBLE_CC_MODE} as in the following example:
532 #ifdef REVERSIBLE_CC_MODE
533 #define HAS_REVERSIBLE_CC_MODE 1
535 #define HAS_REVERSIBLE_CC_MODE 0
539 @node Program Behavior
540 @chapter Program Behavior for All Programs
542 This chapter describes conventions for writing robust
543 software. It also describes general standards for error messages, the
544 command line interface, and how libraries should behave.
547 * Non-GNU Standards:: We consider standards such as POSIX;
548 we don't "obey" them.
549 * Semantics:: Writing robust programs.
550 * Libraries:: Library behavior.
551 * Errors:: Formatting error messages.
552 * User Interfaces:: Standards about interfaces generally.
553 * Graphical Interfaces:: Standards for graphical interfaces.
554 * Command-Line Interfaces:: Standards for command line interfaces.
555 * Dynamic Plug-In Interfaces:: Standards for dynamic plug-in interfaces.
556 * Option Table:: Table of long options.
557 * OID Allocations:: Table of OID slots for GNU.
558 * Memory Usage:: When and how to care about memory needs.
559 * File Usage:: Which files to use, and where.
562 @node Non-GNU Standards
563 @section Non-GNU Standards
565 The GNU Project regards standards published by other organizations as
566 suggestions, not orders. We consider those standards, but we do not
567 ``obey'' them. In developing a GNU program, you should implement
568 an outside standard's specifications when that makes the GNU system
569 better overall in an objective sense. When it doesn't, you shouldn't.
571 In most cases, following published standards is convenient for
572 users---it means that their programs or scripts will work more
573 portably. For instance, GCC implements nearly all the features of
574 Standard C as specified by that standard. C program developers would
575 be unhappy if it did not. And GNU utilities mostly follow
576 specifications of POSIX.2; shell script writers and users would be
577 unhappy if our programs were incompatible.
579 But we do not follow either of these specifications rigidly, and there
580 are specific points on which we decided not to follow them, so as to
581 make the GNU system better for users.
583 For instance, Standard C says that nearly all extensions to C are
584 prohibited. How silly! GCC implements many extensions, some of which
585 were later adopted as part of the standard. If you want these
586 constructs to give an error message as ``required'' by the standard,
587 you must specify @samp{--pedantic}, which was implemented only so that
588 we can say ``GCC is a 100% implementation of the standard'', not
589 because there is any reason to actually use it.
591 POSIX.2 specifies that @samp{df} and @samp{du} must output sizes by
592 default in units of 512 bytes. What users want is units of 1k, so
593 that is what we do by default. If you want the ridiculous behavior
594 ``required'' by POSIX, you must set the environment variable
595 @samp{POSIXLY_CORRECT} (which was originally going to be named
596 @samp{POSIX_ME_HARDER}).
598 GNU utilities also depart from the letter of the POSIX.2 specification
599 when they support long-named command-line options, and intermixing
600 options with ordinary arguments. This minor incompatibility with
601 POSIX is never a problem in practice, and it is very useful.
603 In particular, don't reject a new feature, or remove an old one,
604 merely because a standard says it is ``forbidden'' or ``deprecated''.
608 @section Writing Robust Programs
610 @cindex arbitrary limits on data
611 Avoid arbitrary limits on the length or number of @emph{any} data
612 structure, including file names, lines, files, and symbols, by allocating
613 all data structures dynamically. In most Unix utilities, ``long lines
614 are silently truncated''. This is not acceptable in a GNU utility.
616 @cindex @code{NUL} characters
618 Utilities reading files should not drop NUL characters, or any other
619 nonprinting characters @emph{including those with codes above 0177}.
620 The only sensible exceptions would be utilities specifically intended
621 for interface to certain types of terminals or printers that can't
622 handle those characters. Whenever possible, try to make programs work
623 properly with sequences of bytes that represent multibyte characters;
624 UTF-8 is the most important.
626 @cindex error messages
627 Check every system call for an error return, unless you know you wish
628 to ignore errors. Include the system error text (from @code{perror},
629 @code{strerror}, or equivalent) in @emph{every} error message
630 resulting from a failing system call, as well as the name of the file
631 if any and the name of the utility. Just ``cannot open foo.c'' or
632 ``stat failed'' is not sufficient.
634 @cindex @code{malloc} return value
635 @cindex memory allocation failure
636 Check every call to @code{malloc} or @code{realloc} to see if it
637 returned zero. Check @code{realloc} even if you are making the block
638 smaller; in a system that rounds block sizes to a power of 2,
639 @code{realloc} may get a different block if you ask for less space.
641 In Unix, @code{realloc} can destroy the storage block if it returns
642 zero. GNU @code{realloc} does not have this bug: if it fails, the
643 original block is unchanged. Feel free to assume the bug is fixed. If
644 you wish to run your program on Unix, and wish to avoid lossage in this
645 case, you can use the GNU @code{malloc}.
647 You must expect @code{free} to alter the contents of the block that was
648 freed. Anything you want to fetch from the block, you must fetch before
651 If @code{malloc} fails in a noninteractive program, make that a fatal
652 error. In an interactive program (one that reads commands from the
653 user), it is better to abort the command and return to the command
654 reader loop. This allows the user to kill other processes to free up
655 virtual memory, and then try the command again.
657 @cindex command-line arguments, decoding
658 Use @code{getopt_long} to decode arguments, unless the argument syntax
659 makes this unreasonable.
661 When static storage is to be written in during program execution, use
662 explicit C code to initialize it. Reserve C initialized declarations
663 for data that will not be changed.
666 Try to avoid low-level interfaces to obscure Unix data structures (such
667 as file directories, utmp, or the layout of kernel memory), since these
668 are less likely to work compatibly. If you need to find all the files
669 in a directory, use @code{readdir} or some other high-level interface.
670 These are supported compatibly by GNU.
672 @cindex signal handling
673 The preferred signal handling facilities are the BSD variant of
674 @code{signal}, and the POSIX @code{sigaction} function; the
675 alternative USG @code{signal} interface is an inferior design.
677 Nowadays, using the POSIX signal functions may be the easiest way
678 to make a program portable. If you use @code{signal}, then on GNU/Linux
679 systems running GNU libc version 1, you should include
680 @file{bsd/signal.h} instead of @file{signal.h}, so as to get BSD
681 behavior. It is up to you whether to support systems where
682 @code{signal} has only the USG behavior, or give up on them.
684 @cindex impossible conditions
685 In error checks that detect ``impossible'' conditions, just abort.
686 There is usually no point in printing any message. These checks
687 indicate the existence of bugs. Whoever wants to fix the bugs will have
688 to read the source code and run a debugger. So explain the problem with
689 comments in the source. The relevant data will be in variables, which
690 are easy to examine with the debugger, so there is no point moving them
693 Do not use a count of errors as the exit status for a program.
694 @emph{That does not work}, because exit status values are limited to 8
695 bits (0 through 255). A single run of the program might have 256
696 errors; if you try to return 256 as the exit status, the parent process
697 will see 0 as the status, and it will appear that the program succeeded.
699 @cindex temporary files
700 @cindex @code{TMPDIR} environment variable
701 If you make temporary files, check the @code{TMPDIR} environment
702 variable; if that variable is defined, use the specified directory
703 instead of @file{/tmp}.
705 In addition, be aware that there is a possible security problem when
706 creating temporary files in world-writable directories. In C, you can
707 avoid this problem by creating temporary files in this manner:
710 fd = open (filename, O_WRONLY | O_CREAT | O_EXCL, 0600);
714 or by using the @code{mkstemps} function from Gnulib
715 (@pxref{mkstemps,,, gnulib, Gnulib}).
717 In bash, use @code{set -C} (long name @code{noclobber}) to avoid this
718 problem. In addition, the @code{mktemp} utility is a more general
719 solution for creating temporary files from shell scripts
720 (@pxref{mktemp invocation,,, coreutils, GNU Coreutils}).
724 @section Library Behavior
727 Try to make library functions reentrant. If they need to do dynamic
728 storage allocation, at least try to avoid any nonreentrancy aside from
729 that of @code{malloc} itself.
731 Here are certain name conventions for libraries, to avoid name
734 Choose a name prefix for the library, more than two characters long.
735 All external function and variable names should start with this
736 prefix. In addition, there should only be one of these in any given
737 library member. This usually means putting each one in a separate
740 An exception can be made when two external symbols are always used
741 together, so that no reasonable program could use one without the
742 other; then they can both go in the same file.
744 External symbols that are not documented entry points for the user
745 should have names beginning with @samp{_}. The @samp{_} should be
746 followed by the chosen name prefix for the library, to prevent
747 collisions with other libraries. These can go in the same files with
748 user entry points if you like.
750 Static functions and variables can be used as you like and need not
751 fit any naming convention.
754 @section Formatting Error Messages
755 @cindex formatting error messages
756 @cindex error messages, formatting
758 Error messages from compilers should look like this:
761 @var{sourcefile}:@var{lineno}: @var{message}
765 If you want to mention the column number, use one of these formats:
768 @var{sourcefile}:@var{lineno}:@var{column}: @var{message}
769 @var{sourcefile}:@var{lineno}.@var{column}: @var{message}
774 Line numbers should start from 1 at the beginning of the file, and
775 column numbers should start from 1 at the beginning of the line.
776 (Both of these conventions are chosen for compatibility.) Calculate
777 column numbers assuming that space and all ASCII printing characters
778 have equal width, and assuming tab stops every 8 columns. For
779 non-ASCII characters, Unicode character widths should be used when in
780 a UTF-8 locale; GNU libc and GNU gnulib provide suitable
781 @code{wcwidth} functions.
783 The error message can also give both the starting and ending positions
784 of the erroneous text. There are several formats so that you can
785 avoid redundant information such as a duplicate line number.
786 Here are the possible formats:
789 @var{sourcefile}:@var{line1}.@var{column1}-@var{line2}.@var{column2}: @var{message}
790 @var{sourcefile}:@var{line1}.@var{column1}-@var{column2}: @var{message}
791 @var{sourcefile}:@var{line1}-@var{line2}: @var{message}
795 When an error is spread over several files, you can use this format:
798 @var{file1}:@var{line1}.@var{column1}-@var{file2}:@var{line2}.@var{column2}: @var{message}
801 Error messages from other noninteractive programs should look like this:
804 @var{program}:@var{sourcefile}:@var{lineno}: @var{message}
808 when there is an appropriate source file, or like this:
811 @var{program}: @var{message}
815 when there is no relevant source file.
817 If you want to mention the column number, use this format:
820 @var{program}:@var{sourcefile}:@var{lineno}:@var{column}: @var{message}
823 In an interactive program (one that is reading commands from a
824 terminal), it is better not to include the program name in an error
825 message. The place to indicate which program is running is in the
826 prompt or with the screen layout. (When the same program runs with
827 input from a source other than a terminal, it is not interactive and
828 would do best to print error messages using the noninteractive style.)
830 The string @var{message} should not begin with a capital letter when
831 it follows a program name and/or file name, because that isn't the
832 beginning of a sentence. (The sentence conceptually starts at the
833 beginning of the line.) Also, it should not end with a period.
835 Error messages from interactive programs, and other messages such as
836 usage messages, should start with a capital letter. But they should not
839 @node User Interfaces
840 @section Standards for Interfaces Generally
842 @cindex program name and its behavior
843 @cindex behavior, dependent on program's name
844 Please don't make the behavior of a utility depend on the name used
845 to invoke it. It is useful sometimes to make a link to a utility
846 with a different name, and that should not change what it does.
848 Instead, use a run time option or a compilation switch or both
849 to select among the alternate behaviors.
851 @cindex output device and program's behavior
852 Likewise, please don't make the behavior of the program depend on the
853 type of output device it is used with. Device independence is an
854 important principle of the system's design; do not compromise it merely
855 to save someone from typing an option now and then. (Variation in error
856 message syntax when using a terminal is ok, because that is a side issue
857 that people do not depend on.)
859 If you think one behavior is most useful when the output is to a
860 terminal, and another is most useful when the output is a file or a
861 pipe, then it is usually best to make the default behavior the one that
862 is useful with output to a terminal, and have an option for the other
865 Compatibility requires certain programs to depend on the type of output
866 device. It would be disastrous if @code{ls} or @code{sh} did not do so
867 in the way all users expect. In some of these cases, we supplement the
868 program with a preferred alternate version that does not depend on the
869 output device type. For example, we provide a @code{dir} program much
870 like @code{ls} except that its default output format is always
874 @node Graphical Interfaces
875 @section Standards for Graphical Interfaces
876 @cindex graphical user interface
877 @cindex interface styles
878 @cindex user interface styles
881 When you write a program that provides a graphical user interface,
882 please make it work with the X Window System and the GTK+ toolkit
883 unless the functionality specifically requires some alternative (for
884 example, ``displaying jpeg images while in console mode'').
886 In addition, please provide a command-line interface to control the
887 functionality. (In many cases, the graphical user interface can be a
888 separate program which invokes the command-line program.) This is
889 so that the same jobs can be done from scripts.
894 @cindex keyboard interface
895 @cindex library interface
896 Please also consider providing a D-bus interface for use from other
897 running programs, such as within GNOME. (GNOME used to use CORBA
898 for this, but that is being phased out.) In addition, consider
899 providing a library interface (for use from C), and perhaps a
900 keyboard-driven console interface (for use by users from console
901 mode). Once you are doing the work to provide the functionality and
902 the graphical interface, these won't be much extra work.
905 @node Command-Line Interfaces
906 @section Standards for Command Line Interfaces
907 @cindex command-line interface
910 It is a good idea to follow the POSIX guidelines for the
911 command-line options of a program. The easiest way to do this is to use
912 @code{getopt} to parse them. Note that the GNU version of @code{getopt}
913 will normally permit options anywhere among the arguments unless the
914 special argument @samp{--} is used. This is not what POSIX
915 specifies; it is a GNU extension.
917 @cindex long-named options
918 Please define long-named options that are equivalent to the
919 single-letter Unix-style options. We hope to make GNU more user
920 friendly this way. This is easy to do with the GNU function
923 One of the advantages of long-named options is that they can be
924 consistent from program to program. For example, users should be able
925 to expect the ``verbose'' option of any GNU program which has one, to be
926 spelled precisely @samp{--verbose}. To achieve this uniformity, look at
927 the table of common long-option names when you choose the option names
928 for your program (@pxref{Option Table}).
930 It is usually a good idea for file names given as ordinary arguments to
931 be input files only; any output files would be specified using options
932 (preferably @samp{-o} or @samp{--output}). Even if you allow an output
933 file name as an ordinary argument for compatibility, try to provide an
934 option as another way to specify it. This will lead to more consistency
935 among GNU utilities, and fewer idiosyncrasies for users to remember.
937 @cindex standard command-line options
938 @cindex options, standard command-line
939 @cindex CGI programs, standard options for
940 @cindex PATH_INFO, specifying standard options as
941 All programs should support two standard options: @samp{--version}
942 and @samp{--help}. CGI programs should accept these as command-line
943 options, and also if given as the @env{PATH_INFO}; for instance,
944 visiting @url{http://example.org/p.cgi/--help} in a browser should
945 output the same information as invoking @samp{p.cgi --help} from the
949 * --version:: The standard output for --version.
950 * --help:: The standard output for --help.
954 @subsection @option{--version}
956 @cindex @samp{--version} output
958 The standard @code{--version} option should direct the program to
959 print information about its name, version, origin and legal status,
960 all on standard output, and then exit successfully. Other options and
961 arguments should be ignored once this is seen, and the program should
962 not perform its normal function.
964 @cindex canonical name of a program
965 @cindex program's canonical name
966 The first line is meant to be easy for a program to parse; the version
967 number proper starts after the last space. In addition, it contains
968 the canonical name for this program, in this format:
975 The program's name should be a constant string; @emph{don't} compute it
976 from @code{argv[0]}. The idea is to state the standard or canonical
977 name for the program, not its file name. There are other ways to find
978 out the precise file name where a command is found in @code{PATH}.
980 If the program is a subsidiary part of a larger package, mention the
981 package name in parentheses, like this:
984 emacsserver (GNU Emacs) 19.30
988 If the package has a version number which is different from this
989 program's version number, you can mention the package version number
990 just before the close-parenthesis.
992 If you @emph{need} to mention the version numbers of libraries which
993 are distributed separately from the package which contains this program,
994 you can do so by printing an additional line of version info for each
995 library you want to mention. Use the same format for these lines as for
998 Please do not mention all of the libraries that the program uses ``just
999 for completeness''---that would produce a lot of unhelpful clutter.
1000 Please mention library version numbers only if you find in practice that
1001 they are very important to you in debugging.
1003 The following line, after the version number line or lines, should be a
1004 copyright notice. If more than one copyright notice is called for, put
1005 each on a separate line.
1007 Next should follow a line stating the license, preferably using one of
1008 abbreviations below, and a brief statement that the program is free
1009 software, and that users are free to copy and change it. Also mention
1010 that there is no warranty, to the extent permitted by law. See
1011 recommended wording below.
1013 It is ok to finish the output with a list of the major authors of the
1014 program, as a way of giving credit.
1016 Here's an example of output that follows these rules:
1020 Copyright (C) 2007 Free Software Foundation, Inc.
1021 License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
1022 This is free software: you are free to change and redistribute it.
1023 There is NO WARRANTY, to the extent permitted by law.
1026 You should adapt this to your program, of course, filling in the proper
1027 year, copyright holder, name of program, and the references to
1028 distribution terms, and changing the rest of the wording as necessary.
1030 This copyright notice only needs to mention the most recent year in
1031 which changes were made---there's no need to list the years for previous
1032 versions' changes. You don't have to mention the name of the program in
1033 these notices, if that is inconvenient, since it appeared in the first
1034 line. (The rules are different for copyright notices in source files;
1035 @pxref{Copyright Notices,,,maintain,Information for GNU Maintainers}.)
1037 Translations of the above lines must preserve the validity of the
1038 copyright notices (@pxref{Internationalization}). If the translation's
1039 character set supports it, the @samp{(C)} should be replaced with the
1040 copyright symbol, as follows:
1043 (the official copyright symbol, which is the letter C in a circle);
1049 Write the word ``Copyright'' exactly like that, in English. Do not
1050 translate it into another language. International treaties recognize
1051 the English word ``Copyright''; translations into other languages do not
1052 have legal significance.
1054 Finally, here is the table of our suggested license abbreviations.
1055 Any abbreviation can be followed by @samp{v@var{version}[+]}, meaning
1056 that particular version, or later versions with the @samp{+}, as shown
1057 above. In the case of a GNU license, @emph{always} indicate the permitted
1058 versions in this way.
1060 In the case of exceptions for extra permissions with the GPL, we use
1061 @samp{/} for a separator; the version number can follow the license
1062 abbreviation as usual, as in the examples below.
1066 GNU General Public License, @url{http://www.gnu.org/@/licenses/@/gpl.html}.
1069 GNU Lesser General Public License, @url{http://www.gnu.org/@/licenses/@/lgpl.html}.
1072 GNU GPL with the exception for Ada.
1075 The Apache Software Foundation license,
1076 @url{http://www.apache.org/@/licenses}.
1079 The Artistic license used for Perl, @url{http://www.perlfoundation.org/@/legal}.
1082 The Expat license, @url{http://www.jclark.com/@/xml/@/copying.txt}.
1085 The Mozilla Public License, @url{http://www.mozilla.org/@/MPL/}.
1088 The original (4-clause) BSD license, incompatible with the GNU GPL
1089 @url{http://www.xfree86.org/@/3.3.6/@/COPYRIGHT2.html#6}.
1092 The license used for PHP, @url{http://www.php.net/@/license/}.
1095 The non-license that is being in the public domain,
1096 @url{http://www.gnu.org/@/licenses/@/license-list.html#PublicDomain}.
1099 The license for Python, @url{http://www.python.org/@/2.0.1/@/license.html}.
1102 The revised (3-clause) BSD, compatible with the GNU GPL,@*
1103 @url{http://www.xfree86.org/@/3.3.6/@/COPYRIGHT2.html#5}.
1106 The simple non-copyleft license used for most versions of the X Window
1107 System, @url{http://www.xfree86.org/@/3.3.6/@/COPYRIGHT2.html#3}.
1110 The license for Zlib, @url{http://www.gzip.org/@/zlib/@/zlib_license.html}.
1114 More information about these licenses and many more are on the GNU
1115 licensing web pages,
1116 @url{http://www.gnu.org/@/licenses/@/license-list.html}.
1120 @subsection @option{--help}
1122 @cindex @samp{--help} output
1124 The standard @code{--help} option should output brief documentation
1125 for how to invoke the program, on standard output, then exit
1126 successfully. Other options and arguments should be ignored once this
1127 is seen, and the program should not perform its normal function.
1129 @cindex address for bug reports
1131 Near the end of the @samp{--help} option's output, please place lines
1132 giving the email address for bug reports, the package's home page
1133 (normally @indicateurl{http://www.gnu.org/software/@var{pkg}}, and the
1134 general page for help using GNU programs. The format should be like this:
1137 Report bugs to: @var{mailing-address}
1138 @var{pkg} home page: <http://www.gnu.org/software/@var{pkg}/>
1139 General help using GNU software: <http://www.gnu.org/gethelp/>
1142 It is ok to mention other appropriate mailing lists and web pages.
1145 @node Dynamic Plug-In Interfaces
1146 @section Standards for Dynamic Plug-in Interfaces
1148 @cindex dynamic plug-ins
1150 Another aspect of keeping free programs free is encouraging
1151 development of free plug-ins, and discouraging development of
1152 proprietary plug-ins. Many GNU programs will not have anything like
1153 plug-ins at all, but those that do should follow these
1156 First, the general plug-in architecture design should closely tie the
1157 plug-in to the original code, such that the plug-in and the base
1158 program are parts of one extended program. For GCC, for example,
1159 plug-ins receive and modify GCC's internal data structures, and so
1160 clearly form an extended program with the base GCC.
1162 @vindex plugin_is_GPL_compatible
1163 Second, you should require plug-in developers to affirm that their
1164 plug-ins are released under an appropriate license. This should be
1165 enforced with a simple programmatic check. For GCC, again for
1166 example, a plug-in must define the global symbol
1167 @code{plugin_is_GPL_compatible}, thus asserting that the plug-in is
1168 released under a GPL-compatible license (@pxref{Plugins,, Plugins,
1169 gccint, GCC Internals}).
1171 By adding this check to your program you are not creating a new legal
1172 requirement. The GPL itself requires plug-ins to be free software,
1173 licensed compatibly. As long as you have followed the first rule above
1174 to keep plug-ins closely tied to your original program, the GPL and AGPL
1175 already require those plug-ins to be released under a compatible
1176 license. The symbol definition in the plug-in---or whatever equivalent
1177 works best in your program---makes it harder for anyone who might
1178 distribute proprietary plug-ins to legally defend themselves. If a case
1179 about this got to court, we can point to that symbol as evidence that
1180 the plug-in developer understood that the license had this requirement.
1184 @section Table of Long Options
1185 @cindex long option names
1186 @cindex table of long options
1188 Here is a table of long options used by GNU programs. It is surely
1189 incomplete, but we aim to list all the options that a new program might
1190 want to be compatible with. If you use names not already in the table,
1191 please send @email{bug-standards@@gnu.org} a list of them, with their
1192 meanings, so we can update the table.
1194 @c Please leave newlines between items in this table; it's much easier
1195 @c to update when it isn't completely squashed together and unreadable.
1196 @c When there is more than one short option for a long option name, put
1197 @c a semicolon between the lists of the programs that use them, not a
1198 @c period. --friedman
1202 @samp{-N} in @code{tar}.
1205 @samp{-a} in @code{du}, @code{ls}, @code{nm}, @code{stty}, @code{uname},
1206 and @code{unexpand}.
1209 @samp{-a} in @code{diff}.
1212 @samp{-A} in @code{ls}.
1215 @samp{-a} in @code{etags}, @code{tee}, @code{time};
1216 @samp{-r} in @code{tar}.
1219 @samp{-a} in @code{cp}.
1222 @samp{-n} in @code{shar}.
1225 @samp{-l} in @code{m4}.
1228 @samp{-a} in @code{diff}.
1231 @samp{-v} in @code{gawk}.
1234 @samp{-W} in @code{make}.
1237 @samp{-o} in @code{make}.
1240 @samp{-a} in @code{recode}.
1243 @samp{-a} in @code{wdiff}.
1245 @item auto-reference
1246 @samp{-A} in @code{ptx}.
1249 @samp{-n} in @code{wdiff}.
1252 For server programs, run in the background.
1254 @item backward-search
1255 @samp{-B} in @code{ctags}.
1258 @samp{-f} in @code{shar}.
1267 @samp{-b} in @code{tac}.
1270 @samp{-b} in @code{cpio} and @code{diff}.
1273 @samp{-b} in @code{shar}.
1276 Used in @code{cpio} and @code{tar}.
1279 @samp{-b} in @code{head} and @code{tail}.
1282 @samp{-b} in @code{ptx}.
1285 Used in various programs to make output shorter.
1288 @samp{-c} in @code{head}, @code{split}, and @code{tail}.
1291 @samp{-C} in @code{etags}.
1294 @samp{-A} in @code{tar}.
1297 Used in various programs to specify the directory to use.
1300 @samp{-c} in @code{chgrp} and @code{chown}.
1303 @samp{-F} in @code{ls}.
1306 @samp{-c} in @code{recode}.
1309 @samp{-c} in @code{su};
1313 @samp{-d} in @code{tar}.
1316 Used in @code{gawk}.
1319 @samp{-Z} in @code{tar} and @code{shar}.
1322 @samp{-A} in @code{tar}.
1325 @samp{-w} in @code{tar}.
1328 Used in @code{diff}.
1331 @samp{-W copyleft} in @code{gawk}.
1334 @samp{-C} in @code{ptx}, @code{recode}, and @code{wdiff};
1335 @samp{-W copyright} in @code{gawk}.
1341 @samp{-q} in @code{who}.
1344 @samp{-l} in @code{du}.
1347 Used in @code{tar} and @code{cpio}.
1350 @samp{-c} in @code{shar}.
1353 @samp{-x} in @code{ctags}.
1356 @samp{-d} in @code{touch}.
1359 @samp{-d} in @code{make} and @code{m4};
1363 @samp{-D} in @code{m4}.
1366 @samp{-d} in Bison and @code{ctags}.
1369 @samp{-D} in @code{tar}.
1372 @samp{-L} in @code{chgrp}, @code{chown}, @code{cpio}, @code{du},
1373 @code{ls}, and @code{tar}.
1375 @item dereference-args
1376 @samp{-D} in @code{du}.
1379 Specify an I/O device (special file name).
1382 @samp{-d} in @code{recode}.
1384 @item dictionary-order
1385 @samp{-d} in @code{look}.
1388 @samp{-d} in @code{tar}.
1391 @samp{-n} in @code{csplit}.
1394 Specify the directory to use, in various programs. In @code{ls}, it
1395 means to show directories themselves rather than their contents. In
1396 @code{rm} and @code{ln}, it means to not treat links to directories
1400 @samp{-x} in @code{strip}.
1402 @item discard-locals
1403 @samp{-X} in @code{strip}.
1406 @samp{-n} in @code{make}.
1409 @samp{-e} in @code{diff}.
1411 @item elide-empty-files
1412 @samp{-z} in @code{csplit}.
1415 @samp{-x} in @code{wdiff}.
1418 @samp{-z} in @code{wdiff}.
1420 @item entire-new-file
1421 @samp{-N} in @code{diff}.
1423 @item environment-overrides
1424 @samp{-e} in @code{make}.
1427 @samp{-e} in @code{xargs}.
1433 Used in @code{makeinfo}.
1436 @samp{-o} in @code{m4}.
1439 @samp{-b} in @code{ls}.
1442 @samp{-X} in @code{tar}.
1448 @samp{-x} in @code{xargs}.
1451 @samp{-e} in @code{unshar}.
1454 @samp{-t} in @code{diff}.
1457 @samp{-e} in @code{sed}.
1460 @samp{-g} in @code{nm}.
1463 @samp{-i} in @code{cpio};
1464 @samp{-x} in @code{tar}.
1467 @samp{-f} in @code{finger}.
1470 @samp{-f} in @code{su}.
1472 @item fatal-warnings
1473 @samp{-E} in @code{m4}.
1476 @samp{-f} in @code{gawk}, @code{info}, @code{make}, @code{mt},
1477 @code{sed}, and @code{tar}.
1479 @item field-separator
1480 @samp{-F} in @code{gawk}.
1486 @samp{-F} in @code{ls}.
1489 @samp{-T} in @code{tar}.
1492 Used in @code{makeinfo}.
1494 @item flag-truncation
1495 @samp{-F} in @code{ptx}.
1497 @item fixed-output-files
1501 @samp{-f} in @code{tail}.
1503 @item footnote-style
1504 Used in @code{makeinfo}.
1507 @samp{-f} in @code{cp}, @code{ln}, @code{mv}, and @code{rm}.
1510 @samp{-F} in @code{shar}.
1513 For server programs, run in the foreground;
1514 in other words, don't do anything special to run the server
1518 Used in @code{ls}, @code{time}, and @code{ptx}.
1521 @samp{-F} in @code{m4}.
1527 @samp{-g} in @code{ptx}.
1530 @samp{-x} in @code{tar}.
1533 @samp{-i} in @code{ul}.
1536 @samp{-g} in @code{recode}.
1539 @samp{-g} in @code{install}.
1542 @samp{-z} in @code{tar} and @code{shar}.
1545 @samp{-H} in @code{m4}.
1548 @samp{-h} in @code{objdump} and @code{recode}
1551 @samp{-H} in @code{who}.
1554 Used to ask for brief usage information.
1556 @item here-delimiter
1557 @samp{-d} in @code{shar}.
1559 @item hide-control-chars
1560 @samp{-q} in @code{ls}.
1563 In @code{makeinfo}, output HTML.
1566 @samp{-u} in @code{who}.
1569 @samp{-D} in @code{diff}.
1572 @samp{-I} in @code{ls};
1573 @samp{-x} in @code{recode}.
1575 @item ignore-all-space
1576 @samp{-w} in @code{diff}.
1578 @item ignore-backups
1579 @samp{-B} in @code{ls}.
1581 @item ignore-blank-lines
1582 @samp{-B} in @code{diff}.
1585 @samp{-f} in @code{look} and @code{ptx};
1586 @samp{-i} in @code{diff} and @code{wdiff}.
1589 @samp{-i} in @code{make}.
1592 @samp{-i} in @code{ptx}.
1594 @item ignore-indentation
1595 @samp{-I} in @code{etags}.
1597 @item ignore-init-file
1600 @item ignore-interrupts
1601 @samp{-i} in @code{tee}.
1603 @item ignore-matching-lines
1604 @samp{-I} in @code{diff}.
1606 @item ignore-space-change
1607 @samp{-b} in @code{diff}.
1610 @samp{-i} in @code{tar}.
1613 @samp{-i} in @code{etags};
1614 @samp{-I} in @code{m4}.
1617 @samp{-I} in @code{make}.
1620 @samp{-G} in @code{tar}.
1623 @samp{-i}, @samp{-l}, and @samp{-m} in Finger.
1626 In some programs, specify the name of the file to read as the user's
1630 @samp{-i} in @code{expand}.
1633 @samp{-T} in @code{diff}.
1636 @samp{-i} in @code{ls}.
1639 @samp{-i} in @code{cp}, @code{ln}, @code{mv}, @code{rm};
1640 @samp{-e} in @code{m4};
1641 @samp{-p} in @code{xargs};
1642 @samp{-w} in @code{tar}.
1645 @samp{-p} in @code{shar}.
1651 @samp{-j} in @code{make}.
1654 @samp{-n} in @code{make}.
1657 @samp{-k} in @code{make}.
1660 @samp{-k} in @code{csplit}.
1663 @samp{-k} in @code{du} and @code{ls}.
1666 @samp{-l} in @code{etags}.
1669 @samp{-l} in @code{wdiff}.
1671 @item level-for-gzip
1672 @samp{-g} in @code{shar}.
1675 @samp{-C} in @code{split}.
1678 Used in @code{split}, @code{head}, and @code{tail}.
1681 @samp{-l} in @code{cpio}.
1685 Used in @code{gawk}.
1688 @samp{-t} in @code{cpio};
1689 @samp{-l} in @code{recode}.
1692 @samp{-t} in @code{tar}.
1695 @samp{-N} in @code{ls}.
1698 @samp{-l} in @code{make}.
1704 Used in @code{uname}.
1707 @samp{-M} in @code{ptx}.
1710 @samp{-m} in @code{hello} and @code{uname}.
1712 @item make-directories
1713 @samp{-d} in @code{cpio}.
1716 @samp{-f} in @code{make}.
1722 @samp{-n} in @code{xargs}.
1725 @samp{-n} in @code{xargs}.
1728 @samp{-l} in @code{xargs}.
1731 @samp{-l} in @code{make}.
1734 @samp{-P} in @code{xargs}.
1737 @samp{-T} in @code{who}.
1740 @samp{-T} in @code{who}.
1743 @samp{-d} in @code{diff}.
1745 @item mixed-uuencode
1746 @samp{-M} in @code{shar}.
1749 @samp{-m} in @code{install}, @code{mkdir}, and @code{mkfifo}.
1751 @item modification-time
1752 @samp{-m} in @code{tar}.
1755 @samp{-M} in @code{tar}.
1761 @samp{-L} in @code{m4}.
1764 @samp{-a} in @code{shar}.
1767 @samp{-W} in @code{make}.
1769 @item no-builtin-rules
1770 @samp{-r} in @code{make}.
1772 @item no-character-count
1773 @samp{-w} in @code{shar}.
1775 @item no-check-existing
1776 @samp{-x} in @code{shar}.
1779 @samp{-3} in @code{wdiff}.
1782 @samp{-c} in @code{touch}.
1785 @samp{-D} in @code{etags}.
1788 @samp{-1} in @code{wdiff}.
1790 @item no-dereference
1791 @samp{-d} in @code{cp}.
1794 @samp{-2} in @code{wdiff}.
1797 @samp{-S} in @code{make}.
1803 @samp{-P} in @code{shar}.
1806 @samp{-e} in @code{gprof}.
1809 @samp{-R} in @code{etags}.
1812 @samp{-p} in @code{nm}.
1815 Don't print a startup splash screen.
1818 Used in @code{makeinfo}.
1821 @samp{-a} in @code{gprof}.
1824 @samp{-E} in @code{gprof}.
1827 @samp{-m} in @code{shar}.
1830 Used in @code{makeinfo}.
1833 Used in @code{emacsclient}.
1836 Used in various programs to inhibit warnings.
1839 @samp{-n} in @code{info}.
1842 @samp{-n} in @code{uname}.
1845 @samp{-f} in @code{cpio}.
1848 @samp{-n} in @code{objdump}.
1851 @samp{-0} in @code{xargs}.
1854 @samp{-n} in @code{cat}.
1856 @item number-nonblank
1857 @samp{-b} in @code{cat}.
1860 @samp{-n} in @code{nm}.
1862 @item numeric-uid-gid
1863 @samp{-n} in @code{cpio} and @code{ls}.
1869 @samp{-o} in @code{tar}.
1872 @samp{-o} in @code{make}.
1874 @item one-file-system
1875 @samp{-l} in @code{tar}, @code{cp}, and @code{du}.
1878 @samp{-o} in @code{ptx}.
1881 @samp{-f} in @code{gprof}.
1884 @samp{-F} in @code{gprof}.
1887 @samp{-o} in @code{getopt}, @code{fdlist}, @code{fdmount},
1888 @code{fdmountd}, and @code{fdumount}.
1891 In various programs, specify the output file name.
1894 @samp{-o} in @code{shar}.
1897 @samp{-o} in @code{rm}.
1900 @samp{-c} in @code{unshar}.
1903 @samp{-o} in @code{install}.
1906 @samp{-l} in @code{diff}.
1908 @item paragraph-indent
1909 Used in @code{makeinfo}.
1912 @samp{-p} in @code{mkdir} and @code{rmdir}.
1915 @samp{-p} in @code{ul}.
1918 @samp{-p} in @code{cpio}.
1921 @samp{-P} in @code{finger}.
1924 @samp{-c} in @code{cpio} and @code{tar}.
1927 Used in @code{gawk}.
1929 @item prefix-builtins
1930 @samp{-P} in @code{m4}.
1933 @samp{-f} in @code{csplit}.
1936 Used in @code{tar} and @code{cp}.
1938 @item preserve-environment
1939 @samp{-p} in @code{su}.
1941 @item preserve-modification-time
1942 @samp{-m} in @code{cpio}.
1944 @item preserve-order
1945 @samp{-s} in @code{tar}.
1947 @item preserve-permissions
1948 @samp{-p} in @code{tar}.
1951 @samp{-l} in @code{diff}.
1954 @samp{-L} in @code{cmp}.
1956 @item print-data-base
1957 @samp{-p} in @code{make}.
1959 @item print-directory
1960 @samp{-w} in @code{make}.
1962 @item print-file-name
1963 @samp{-o} in @code{nm}.
1966 @samp{-s} in @code{nm}.
1969 @samp{-p} in @code{wdiff}.
1972 @samp{-p} in @code{ed}.
1975 Specify an HTTP proxy.
1978 @samp{-X} in @code{shar}.
1981 @samp{-q} in @code{make}.
1984 Used in many programs to inhibit the usual output. Every
1985 program accepting @samp{--quiet} should accept @samp{--silent} as a
1989 @samp{-Q} in @code{shar}
1992 @samp{-Q} in @code{ls}.
1995 @samp{-n} in @code{diff}.
1998 Used in @code{gawk}.
2000 @item read-full-blocks
2001 @samp{-B} in @code{tar}.
2007 @samp{-n} in @code{make}.
2010 @samp{-R} in @code{tar}.
2013 Used in @code{chgrp}, @code{chown}, @code{cp}, @code{ls}, @code{diff},
2017 @samp{-r} in @code{touch}.
2020 @samp{-r} in @code{ptx}.
2023 @samp{-r} in @code{tac} and @code{etags}.
2026 @samp{-r} in @code{uname}.
2029 @samp{-R} in @code{m4}.
2032 @samp{-r} in @code{objdump}.
2035 @samp{-r} in @code{cpio}.
2038 @samp{-i} in @code{xargs}.
2040 @item report-identical-files
2041 @samp{-s} in @code{diff}.
2043 @item reset-access-time
2044 @samp{-a} in @code{cpio}.
2047 @samp{-r} in @code{ls} and @code{nm}.
2050 @samp{-f} in @code{diff}.
2052 @item right-side-defs
2053 @samp{-R} in @code{ptx}.
2056 @samp{-s} in @code{tar}.
2058 @item same-permissions
2059 @samp{-p} in @code{tar}.
2062 @samp{-g} in @code{stty}.
2067 @item sentence-regexp
2068 @samp{-S} in @code{ptx}.
2071 @samp{-S} in @code{du}.
2074 @samp{-s} in @code{tac}.
2077 Used by @code{recode} to chose files or pipes for sequencing passes.
2080 @samp{-s} in @code{su}.
2083 @samp{-A} in @code{cat}.
2085 @item show-c-function
2086 @samp{-p} in @code{diff}.
2089 @samp{-E} in @code{cat}.
2091 @item show-function-line
2092 @samp{-F} in @code{diff}.
2095 @samp{-T} in @code{cat}.
2098 Used in many programs to inhibit the usual output.
2099 Every program accepting
2100 @samp{--silent} should accept @samp{--quiet} as a synonym.
2103 @samp{-s} in @code{ls}.
2106 Specify a file descriptor for a network server to use for its socket,
2107 instead of opening and binding a new socket. This provides a way to
2108 run, in a non-privileged process, a server that normally needs a
2109 reserved port number.
2115 @samp{-W source} in @code{gawk}.
2118 @samp{-S} in @code{tar}.
2120 @item speed-large-files
2121 @samp{-H} in @code{diff}.
2124 @samp{-E} in @code{unshar}.
2126 @item split-size-limit
2127 @samp{-L} in @code{shar}.
2130 @samp{-s} in @code{cat}.
2133 @samp{-w} in @code{wdiff}.
2136 @samp{-y} in @code{wdiff}.
2139 Used in @code{tar} and @code{diff} to specify which file within
2140 a directory to start processing with.
2143 @samp{-s} in @code{wdiff}.
2145 @item stdin-file-list
2146 @samp{-S} in @code{shar}.
2149 @samp{-S} in @code{make}.
2152 @samp{-s} in @code{recode}.
2155 @samp{-s} in @code{install}.
2158 @samp{-s} in @code{strip}.
2161 @samp{-S} in @code{strip}.
2164 @samp{-s} in @code{shar}.
2167 @samp{-S} in @code{cp}, @code{ln}, @code{mv}.
2170 @samp{-b} in @code{csplit}.
2173 @samp{-s} in @code{gprof}.
2176 @samp{-s} in @code{du}.
2179 @samp{-s} in @code{ln}.
2182 Used in GDB and @code{objdump}.
2185 @samp{-s} in @code{m4}.
2188 @samp{-s} in @code{uname}.
2191 @samp{-t} in @code{expand} and @code{unexpand}.
2194 @samp{-T} in @code{ls}.
2197 @samp{-T} in @code{tput} and @code{ul}.
2198 @samp{-t} in @code{wdiff}.
2201 @samp{-a} in @code{diff}.
2204 @samp{-T} in @code{shar}.
2207 Used in @code{ls} and @code{touch}.
2210 Specify how long to wait before giving up on some operation.
2213 @samp{-O} in @code{tar}.
2216 @samp{-c} in @code{du}.
2219 @samp{-t} in @code{make}, @code{ranlib}, and @code{recode}.
2222 @samp{-t} in @code{m4}.
2225 @samp{-t} in @code{hello};
2226 @samp{-W traditional} in @code{gawk};
2227 @samp{-G} in @code{ed}, @code{m4}, and @code{ptx}.
2233 @samp{-t} in @code{ctags}.
2235 @item typedefs-and-c++
2236 @samp{-T} in @code{ctags}.
2239 @samp{-t} in @code{ptx}.
2242 @samp{-z} in @code{tar}.
2245 @samp{-u} in @code{cpio}.
2248 @samp{-U} in @code{m4}.
2250 @item undefined-only
2251 @samp{-u} in @code{nm}.
2254 @samp{-u} in @code{cp}, @code{ctags}, @code{mv}, @code{tar}.
2257 Used in @code{gawk}; same as @samp{--help}.
2260 @samp{-B} in @code{shar}.
2262 @item vanilla-operation
2263 @samp{-V} in @code{shar}.
2266 Print more information about progress. Many programs support this.
2269 @samp{-W} in @code{tar}.
2272 Print the version number.
2274 @item version-control
2275 @samp{-V} in @code{cp}, @code{ln}, @code{mv}.
2278 @samp{-v} in @code{ctags}.
2281 @samp{-V} in @code{tar}.
2284 @samp{-W} in @code{make}.
2286 @item whole-size-limit
2287 @samp{-l} in @code{shar}.
2290 @samp{-w} in @code{ls} and @code{ptx}.
2293 @samp{-W} in @code{ptx}.
2296 @samp{-T} in @code{who}.
2299 @samp{-z} in @code{gprof}.
2302 @node OID Allocations
2303 @section OID Allocations
2304 @cindex OID allocations for GNU
2309 The OID (object identifier) 1.3.6.1.4.1.11591 has been assigned to the
2310 GNU Project (thanks to Werner Koch). These are used for SNMP, LDAP,
2311 X.509 certificates, and so on. The web site
2312 @url{http://www.alvestrand.no/objectid} has a (voluntary) listing of
2313 many OID assignments.
2315 If you need a new slot for your GNU package, write
2316 @email{maintainers@@gnu.org}. Here is a list of arcs currently
2320 @include gnu-oids.texi
2325 @section Memory Usage
2326 @cindex memory usage
2328 If a program typically uses just a few meg of memory, don't bother making any
2329 effort to reduce memory usage. For example, if it is impractical for
2330 other reasons to operate on files more than a few meg long, it is
2331 reasonable to read entire input files into memory to operate on them.
2333 However, for programs such as @code{cat} or @code{tail}, that can
2334 usefully operate on very large files, it is important to avoid using a
2335 technique that would artificially limit the size of files it can handle.
2336 If a program works by lines and could be applied to arbitrary
2337 user-supplied input files, it should keep only a line in memory, because
2338 this is not very hard and users will want to be able to operate on input
2339 files that are bigger than will fit in memory all at once.
2341 If your program creates complicated data structures, just make them in
2342 memory and give a fatal error if @code{malloc} returns zero.
2346 Memory analysis tools such as @command{valgrind} can be useful, but
2347 don't complicate a program merely to avoid their false alarms. For
2348 example, if memory is used until just before a process exits, don't
2349 free it simply to silence such a tool.
2356 Programs should be prepared to operate when @file{/usr} and @file{/etc}
2357 are read-only file systems. Thus, if the program manages log files,
2358 lock files, backup files, score files, or any other files which are
2359 modified for internal purposes, these files should not be stored in
2360 @file{/usr} or @file{/etc}.
2362 There are two exceptions. @file{/etc} is used to store system
2363 configuration information; it is reasonable for a program to modify
2364 files in @file{/etc} when its job is to update the system configuration.
2365 Also, if the user explicitly asks to modify one file in a directory, it
2366 is reasonable for the program to store other files in the same
2370 @chapter Making The Best Use of C
2372 This chapter provides advice on how best to use the C language
2373 when writing GNU software.
2376 * Formatting:: Formatting your source code.
2377 * Comments:: Commenting your work.
2378 * Syntactic Conventions:: Clean use of C constructs.
2379 * Names:: Naming variables, functions, and files.
2380 * System Portability:: Portability among different operating systems.
2381 * CPU Portability:: Supporting the range of CPU types.
2382 * System Functions:: Portability and ``standard'' library functions.
2383 * Internationalization:: Techniques for internationalization.
2384 * Character Set:: Use ASCII by default.
2385 * Quote Characters:: Use "..." or '...' in the C locale.
2386 * Mmap:: How you can safely use @code{mmap}.
2390 @section Formatting Your Source Code
2391 @cindex formatting source code
2394 @cindex braces, in C source
2395 @cindex function definitions, formatting
2396 It is important to put the open-brace that starts the body of a C
2397 function in column one, so that they will start a defun. Several
2398 tools look for open-braces in column one to find the beginnings of C
2399 functions. These tools will not work on code not formatted that way.
2401 Avoid putting open-brace, open-parenthesis or open-bracket in column
2402 one when they are inside a function, so that they won't start a defun.
2403 The open-brace that starts a @code{struct} body can go in column one
2404 if you find it useful to treat that definition as a defun.
2406 It is also important for function definitions to start the name of the
2407 function in column one. This helps people to search for function
2408 definitions, and may also help certain tools recognize them. Thus,
2409 using Standard C syntax, the format is this:
2413 concat (char *s1, char *s2)
2420 or, if you want to use traditional C syntax, format the definition like
2425 concat (s1, s2) /* Name starts in column one here */
2427 @{ /* Open brace in column one here */
2432 In Standard C, if the arguments don't fit nicely on one line,
2437 lots_of_args (int an_integer, long a_long, short a_short,
2438 double a_double, float a_float)
2442 @cindex @code{struct} types, formatting
2443 @cindex @code{enum} types, formatting
2444 For @code{struct} and @code{enum} types, likewise put the braces in
2445 column one, unless the whole contents fits on one line:
2453 struct foo @{ int a, b; @}
2456 The rest of this section gives our recommendations for other aspects of
2457 C formatting style, which is also the default style of the @code{indent}
2458 program in version 1.2 and newer. It corresponds to the options
2461 -nbad -bap -nbc -bbo -bl -bli2 -bls -ncdb -nce -cp1 -cs -di2
2462 -ndj -nfc1 -nfca -hnl -i2 -ip5 -lp -pcs -psl -nsc -nsob
2465 We don't think of these recommendations as requirements, because it
2466 causes no problems for users if two different programs have different
2469 But whatever style you use, please use it consistently, since a mixture
2470 of styles within one program tends to look ugly. If you are
2471 contributing changes to an existing program, please follow the style of
2474 For the body of the function, our recommended style looks like this:
2486 return ++x + bar ();
2490 @cindex spaces before open-paren
2491 We find it easier to read a program when it has spaces before the
2492 open-parentheses and after the commas. Especially after the commas.
2494 When you split an expression into multiple lines, split it
2495 before an operator, not after one. Here is the right way:
2497 @cindex expressions, splitting
2499 if (foo_this_is_long && bar > win (x, y, z)
2500 && remaining_condition)
2503 Try to avoid having two operators of different precedence at the same
2504 level of indentation. For example, don't write this:
2507 mode = (inmode[j] == VOIDmode
2508 || GET_MODE_SIZE (outmode[j]) > GET_MODE_SIZE (inmode[j])
2509 ? outmode[j] : inmode[j]);
2512 Instead, use extra parentheses so that the indentation shows the nesting:
2515 mode = ((inmode[j] == VOIDmode
2516 || (GET_MODE_SIZE (outmode[j]) > GET_MODE_SIZE (inmode[j])))
2517 ? outmode[j] : inmode[j]);
2520 Insert extra parentheses so that Emacs will indent the code properly.
2521 For example, the following indentation looks nice if you do it by hand,
2524 v = rup->ru_utime.tv_sec*1000 + rup->ru_utime.tv_usec/1000
2525 + rup->ru_stime.tv_sec*1000 + rup->ru_stime.tv_usec/1000;
2529 but Emacs would alter it. Adding a set of parentheses produces
2530 something that looks equally nice, and which Emacs will preserve:
2533 v = (rup->ru_utime.tv_sec*1000 + rup->ru_utime.tv_usec/1000
2534 + rup->ru_stime.tv_sec*1000 + rup->ru_stime.tv_usec/1000);
2537 Format do-while statements like this:
2549 Please use formfeed characters (control-L) to divide the program into
2550 pages at logical places (but not within a function). It does not matter
2551 just how long the pages are, since they do not have to fit on a printed
2552 page. The formfeeds should appear alone on lines by themselves.
2555 @section Commenting Your Work
2558 Every program should start with a comment saying briefly what it is for.
2559 Example: @samp{fmt - filter for simple filling of text}. This comment
2560 should be at the top of the source file containing the @samp{main}
2561 function of the program.
2563 Also, please write a brief comment at the start of each source file,
2564 with the file name and a line or two about the overall purpose of the
2567 Please write the comments in a GNU program in English, because English
2568 is the one language that nearly all programmers in all countries can
2569 read. If you do not write English well, please write comments in
2570 English as well as you can, then ask other people to help rewrite them.
2571 If you can't write comments in English, please find someone to work with
2572 you and translate your comments into English.
2574 Please put a comment on each function saying what the function does,
2575 what sorts of arguments it gets, and what the possible values of
2576 arguments mean and are used for. It is not necessary to duplicate in
2577 words the meaning of the C argument declarations, if a C type is being
2578 used in its customary fashion. If there is anything nonstandard about
2579 its use (such as an argument of type @code{char *} which is really the
2580 address of the second character of a string, not the first), or any
2581 possible values that would not work the way one would expect (such as,
2582 that strings containing newlines are not guaranteed to work), be sure
2585 Also explain the significance of the return value, if there is one.
2587 Please put two spaces after the end of a sentence in your comments, so
2588 that the Emacs sentence commands will work. Also, please write
2589 complete sentences and capitalize the first word. If a lower-case
2590 identifier comes at the beginning of a sentence, don't capitalize it!
2591 Changing the spelling makes it a different identifier. If you don't
2592 like starting a sentence with a lower case letter, write the sentence
2593 differently (e.g., ``The identifier lower-case is @dots{}'').
2595 The comment on a function is much clearer if you use the argument
2596 names to speak about the argument values. The variable name itself
2597 should be lower case, but write it in upper case when you are speaking
2598 about the value rather than the variable itself. Thus, ``the inode
2599 number NODE_NUM'' rather than ``an inode''.
2601 There is usually no purpose in restating the name of the function in
2602 the comment before it, because readers can see that for themselves.
2603 There might be an exception when the comment is so long that the function
2604 itself would be off the bottom of the screen.
2606 There should be a comment on each static variable as well, like this:
2609 /* Nonzero means truncate lines in the display;
2610 zero means continue them. */
2614 @cindex conditionals, comments for
2615 @cindex @code{#endif}, commenting
2616 Every @samp{#endif} should have a comment, except in the case of short
2617 conditionals (just a few lines) that are not nested. The comment should
2618 state the condition of the conditional that is ending, @emph{including
2619 its sense}. @samp{#else} should have a comment describing the condition
2620 @emph{and sense} of the code that follows. For example:
2628 #endif /* not foo */
2638 but, by contrast, write the comments this way for a @samp{#ifndef}:
2651 #endif /* not foo */
2655 @node Syntactic Conventions
2656 @section Clean Use of C Constructs
2657 @cindex syntactic conventions
2659 @cindex implicit @code{int}
2660 @cindex function argument, declaring
2661 Please explicitly declare the types of all objects. For example, you
2662 should explicitly declare all arguments to functions, and you should
2663 declare functions to return @code{int} rather than omitting the
2666 @cindex compiler warnings
2667 @cindex @samp{-Wall} compiler option
2668 Some programmers like to use the GCC @samp{-Wall} option, and change the
2669 code whenever it issues a warning. If you want to do this, then do.
2670 Other programmers prefer not to use @samp{-Wall}, because it gives
2671 warnings for valid and legitimate code which they do not want to change.
2672 If you want to do this, then do. The compiler should be your servant,
2677 Don't make the program ugly just to placate static analysis tools such
2678 as @command{lint}, @command{clang}, and GCC with extra warnings
2679 options such as @option{-Wconversion} and @option{-Wundef}. These
2680 tools can help find bugs and unclear code, but they can also generate
2681 so many false alarms that it hurts readability to silence them with
2682 unnecessary casts, wrappers, and other complications. For example,
2683 please don't insert casts to @code{void} or calls to do-nothing
2684 functions merely to pacify a lint checker.
2686 Declarations of external functions and functions to appear later in the
2687 source file should all go in one place near the beginning of the file
2688 (somewhere before the first function definition in the file), or else
2689 should go in a header file. Don't put @code{extern} declarations inside
2692 @cindex temporary variables
2693 It used to be common practice to use the same local variables (with
2694 names like @code{tem}) over and over for different values within one
2695 function. Instead of doing this, it is better to declare a separate local
2696 variable for each distinct purpose, and give it a name which is
2697 meaningful. This not only makes programs easier to understand, it also
2698 facilitates optimization by good compilers. You can also move the
2699 declaration of each local variable into the smallest scope that includes
2700 all its uses. This makes the program even cleaner.
2702 Don't use local variables or parameters that shadow global identifiers.
2703 GCC's @samp{-Wshadow} option can detect this problem.
2705 @cindex multiple variables in a line
2706 Don't declare multiple variables in one declaration that spans lines.
2707 Start a new declaration on each line, instead. For example, instead
2733 (If they are global variables, each should have a comment preceding it
2736 When you have an @code{if}-@code{else} statement nested in another
2737 @code{if} statement, always put braces around the @code{if}-@code{else}.
2738 Thus, never write like this:
2761 If you have an @code{if} statement nested inside of an @code{else}
2762 statement, either write @code{else if} on one line, like this,
2772 with its @code{then}-part indented like the preceding @code{then}-part,
2773 or write the nested @code{if} within braces like this:
2785 Don't declare both a structure tag and variables or typedefs in the
2786 same declaration. Instead, declare the structure tag separately
2787 and then use it to declare the variables or typedefs.
2789 Try to avoid assignments inside @code{if}-conditions (assignments
2790 inside @code{while}-conditions are ok). For example, don't write
2794 if ((foo = (char *) malloc (sizeof *foo)) == 0)
2795 fatal ("virtual memory exhausted");
2799 instead, write this:
2802 foo = (char *) malloc (sizeof *foo);
2804 fatal ("virtual memory exhausted");
2807 This example uses zero without a cast as a null pointer constant.
2808 This is perfectly fine, except that a cast is needed when calling a
2809 varargs function or when using @code{sizeof}.
2812 @section Naming Variables, Functions, and Files
2814 @cindex names of variables, functions, and files
2815 The names of global variables and functions in a program serve as
2816 comments of a sort. So don't choose terse names---instead, look for
2817 names that give useful information about the meaning of the variable or
2818 function. In a GNU program, names should be English, like other
2821 Local variable names can be shorter, because they are used only within
2822 one context, where (presumably) comments explain their purpose.
2824 Try to limit your use of abbreviations in symbol names. It is ok to
2825 make a few abbreviations, explain what they mean, and then use them
2826 frequently, but don't use lots of obscure abbreviations.
2828 Please use underscores to separate words in a name, so that the Emacs
2829 word commands can be useful within them. Stick to lower case; reserve
2830 upper case for macros and @code{enum} constants, and for name-prefixes
2831 that follow a uniform convention.
2833 For example, you should use names like @code{ignore_space_change_flag};
2834 don't use names like @code{iCantReadThis}.
2836 Variables that indicate whether command-line options have been
2837 specified should be named after the meaning of the option, not after
2838 the option-letter. A comment should state both the exact meaning of
2839 the option and its letter. For example,
2843 /* Ignore changes in horizontal whitespace (-b). */
2844 int ignore_space_change_flag;
2848 When you want to define names with constant integer values, use
2849 @code{enum} rather than @samp{#define}. GDB knows about enumeration
2852 @cindex file-name limitations
2854 You might want to make sure that none of the file names would conflict
2855 if the files were loaded onto an MS-DOS file system which shortens the
2856 names. You can use the program @code{doschk} to test for this.
2858 Some GNU programs were designed to limit themselves to file names of 14
2859 characters or less, to avoid file name conflicts if they are read into
2860 older System V systems. Please preserve this feature in the existing
2861 GNU programs that have it, but there is no need to do this in new GNU
2862 programs. @code{doschk} also reports file names longer than 14
2866 @node System Portability
2867 @section Portability between System Types
2868 @cindex portability, between system types
2870 In the Unix world, ``portability'' refers to porting to different Unix
2871 versions. For a GNU program, this kind of portability is desirable, but
2874 The primary purpose of GNU software is to run on top of the GNU kernel,
2875 compiled with the GNU C compiler, on various types of CPU. So the
2876 kinds of portability that are absolutely necessary are quite limited.
2877 But it is important to support Linux-based GNU systems, since they
2878 are the form of GNU that is popular.
2880 Beyond that, it is good to support the other free operating systems
2881 (*BSD), and it is nice to support other Unix-like systems if you want
2882 to. Supporting a variety of Unix-like systems is desirable, although
2883 not paramount. It is usually not too hard, so you may as well do it.
2884 But you don't have to consider it an obligation, if it does turn out to
2888 The easiest way to achieve portability to most Unix-like systems is to
2889 use Autoconf. It's unlikely that your program needs to know more
2890 information about the host platform than Autoconf can provide, simply
2891 because most of the programs that need such knowledge have already been
2894 Avoid using the format of semi-internal data bases (e.g., directories)
2895 when there is a higher-level alternative (@code{readdir}).
2897 @cindex non-POSIX systems, and portability
2898 As for systems that are not like Unix, such as MSDOS, Windows, VMS, MVS,
2899 and older Macintosh systems, supporting them is often a lot of work.
2900 When that is the case, it is better to spend your time adding features
2901 that will be useful on GNU and GNU/Linux, rather than on supporting
2902 other incompatible systems.
2904 If you do support Windows, please do not abbreviate it as ``win''. In
2905 hacker terminology, calling something a ``win'' is a form of praise.
2906 You're free to praise Microsoft Windows on your own if you want, but
2907 please don't do this in GNU packages. Instead of abbreviating
2908 ``Windows'' to ``win'', you can write it in full or abbreviate it to
2909 ``woe'' or ``w''. In GNU Emacs, for instance, we use @samp{w32} in
2910 file names of Windows-specific files, but the macro for Windows
2911 conditionals is called @code{WINDOWSNT}.
2913 It is a good idea to define the ``feature test macro''
2914 @code{_GNU_SOURCE} when compiling your C files. When you compile on GNU
2915 or GNU/Linux, this will enable the declarations of GNU library extension
2916 functions, and that will usually give you a compiler error message if
2917 you define the same function names in some other way in your program.
2918 (You don't have to actually @emph{use} these functions, if you prefer
2919 to make the program more portable to other systems.)
2921 But whether or not you use these GNU extensions, you should avoid
2922 using their names for any other meanings. Doing so would make it hard
2923 to move your code into other GNU programs.
2925 @node CPU Portability
2926 @section Portability between CPUs
2928 @cindex data types, and portability
2929 @cindex portability, and data types
2930 Even GNU systems will differ because of differences among CPU
2931 types---for example, difference in byte ordering and alignment
2932 requirements. It is absolutely essential to handle these differences.
2933 However, don't make any effort to cater to the possibility that an
2934 @code{int} will be less than 32 bits. We don't support 16-bit machines
2937 Similarly, don't make any effort to cater to the possibility that
2938 @code{long} will be smaller than predefined types like @code{size_t}.
2939 For example, the following code is ok:
2942 printf ("size = %lu\n", (unsigned long) sizeof array);
2943 printf ("diff = %ld\n", (long) (pointer2 - pointer1));
2946 1989 Standard C requires this to work, and we know of only one
2947 counterexample: 64-bit programs on Microsoft Windows. We will leave
2948 it to those who want to port GNU programs to that environment to
2949 figure out how to do it.
2951 Predefined file-size types like @code{off_t} are an exception: they are
2952 longer than @code{long} on many platforms, so code like the above won't
2953 work with them. One way to print an @code{off_t} value portably is to
2954 print its digits yourself, one by one.
2956 Don't assume that the address of an @code{int} object is also the
2957 address of its least-significant byte. This is false on big-endian
2958 machines. Thus, don't make the following mistake:
2963 while ((c = getchar ()) != EOF)
2964 write (file_descriptor, &c, 1);
2967 @noindent Instead, use @code{unsigned char} as follows. (The @code{unsigned}
2968 is for portability to unusual systems where @code{char} is signed and
2969 where there is integer overflow checking.)
2973 while ((c = getchar ()) != EOF)
2975 unsigned char u = c;
2976 write (file_descriptor, &u, 1);
2980 @cindex casting pointers to integers
2981 Avoid casting pointers to integers if you can. Such casts greatly
2982 reduce portability, and in most programs they are easy to avoid. In the
2983 cases where casting pointers to integers is essential---such as, a Lisp
2984 interpreter which stores type information as well as an address in one
2985 word---you'll have to make explicit provisions to handle different word
2986 sizes. You will also need to make provision for systems in which the
2987 normal range of addresses you can get from @code{malloc} starts far away
2991 @node System Functions
2992 @section Calling System Functions
2994 @cindex C library functions, and portability
2995 @cindex POSIX functions, and portability
2996 @cindex library functions, and portability
2997 @cindex portability, and library functions
2999 Historically, C implementations differed substantially, and many
3000 systems lacked a full implementation of ANSI/ISO C89. Nowadays,
3001 however, all practical systems have a C89 compiler and GNU C supports
3002 almost all of C99 and some of C11. Similarly, most systems implement
3003 POSIX.1-2001 libraries and tools, and many have POSIX.1-2008.
3005 Hence, there is little reason to support old C or non-POSIX systems,
3006 and you may want to take advantage of standard C and POSIX to write
3007 clearer, more portable, or faster code. You should use standard
3008 interfaces where possible; but if GNU extensions make your program
3009 more maintainable, powerful, or otherwise better, don't hesitate to
3010 use them. In any case, don't make your own declaration of system
3011 functions; that's a recipe for conflict.
3013 Despite the standards, nearly every library function has some sort of
3014 portability issue on some system or another. Here are some examples:
3018 Names with trailing @code{/}'s are mishandled on many platforms.
3021 @code{long double} may be unimplemented; floating values Infinity and
3022 NaN are often mishandled; output for large precisions may be
3026 May return @code{int} instead of @code{ssize_t}.
3029 On Windows, @code{errno} is not set on failure.
3033 @uref{http://www.gnu.org/software/gnulib/, Gnulib} is a big help in
3034 this regard. Gnulib provides implementations of standard interfaces
3035 on many of the systems that lack them, including portable
3036 implementations of enhanced GNU interfaces, thereby making their use
3037 portable, and of POSIX-1.2008 interfaces, some of which are missing
3038 even on up-to-date GNU systems.
3040 @findex xmalloc, in Gnulib
3041 @findex error messages, in Gnulib
3042 @findex data structures, in Gnulib
3043 Gnulib also provides many useful non-standard interfaces; for example,
3044 C implementations of standard data structures (hash tables, binary
3045 trees), error-checking type-safe wrappers for memory allocation
3046 functions (@code{xmalloc}, @code{xrealloc}), and output of error
3049 Gnulib integrates with GNU Autoconf and Automake to remove much of the
3050 burden of writing portable code from the programmer: Gnulib makes your
3051 configure script automatically determine what features are missing and
3052 use the Gnulib code to supply the missing pieces.
3054 The Gnulib and Autoconf manuals have extensive sections on
3055 portability: @ref{Top,, Introduction, gnulib, Gnulib} and
3056 @pxref{Portable C and C++,,, autoconf, Autoconf}. Please consult them
3057 for many more details.
3060 @node Internationalization
3061 @section Internationalization
3062 @cindex internationalization
3065 GNU has a library called GNU gettext that makes it easy to translate the
3066 messages in a program into various languages. You should use this
3067 library in every program. Use English for the messages as they appear
3068 in the program, and let gettext provide the way to translate them into
3071 Using GNU gettext involves putting a call to the @code{gettext} macro
3072 around each string that might need translation---like this:
3075 printf (gettext ("Processing file '%s'..."), file);
3079 This permits GNU gettext to replace the string @code{"Processing file
3080 '%s'..."} with a translated version.
3082 Once a program uses gettext, please make a point of writing calls to
3083 @code{gettext} when you add new strings that call for translation.
3085 Using GNU gettext in a package involves specifying a @dfn{text domain
3086 name} for the package. The text domain name is used to separate the
3087 translations for this package from the translations for other packages.
3088 Normally, the text domain name should be the same as the name of the
3089 package---for example, @samp{coreutils} for the GNU core utilities.
3091 @cindex message text, and internationalization
3092 To enable gettext to work well, avoid writing code that makes
3093 assumptions about the structure of words or sentences. When you want
3094 the precise text of a sentence to vary depending on the data, use two or
3095 more alternative string constants each containing a complete sentences,
3096 rather than inserting conditionalized words or phrases into a single
3099 Here is an example of what not to do:
3102 printf ("%s is full", capacity > 5000000 ? "disk" : "floppy disk");
3105 If you apply gettext to all strings, like this,
3108 printf (gettext ("%s is full"),
3109 capacity > 5000000 ? gettext ("disk") : gettext ("floppy disk"));
3113 the translator will hardly know that "disk" and "floppy disk" are meant to
3114 be substituted in the other string. Worse, in some languages (like French)
3115 the construction will not work: the translation of the word "full" depends
3116 on the gender of the first part of the sentence; it happens to be not the
3117 same for "disk" as for "floppy disk".
3119 Complete sentences can be translated without problems:
3122 printf (capacity > 5000000 ? gettext ("disk is full")
3123 : gettext ("floppy disk is full"));
3126 A similar problem appears at the level of sentence structure with this
3130 printf ("# Implicit rule search has%s been done.\n",
3131 f->tried_implicit ? "" : " not");
3135 Adding @code{gettext} calls to this code cannot give correct results for
3136 all languages, because negation in some languages requires adding words
3137 at more than one place in the sentence. By contrast, adding
3138 @code{gettext} calls does the job straightforwardly if the code starts
3142 printf (f->tried_implicit
3143 ? "# Implicit rule search has been done.\n",
3144 : "# Implicit rule search has not been done.\n");
3147 Another example is this one:
3150 printf ("%d file%s processed", nfiles,
3151 nfiles != 1 ? "s" : "");
3155 The problem with this example is that it assumes that plurals are made
3156 by adding `s'. If you apply gettext to the format string, like this,
3159 printf (gettext ("%d file%s processed"), nfiles,
3160 nfiles != 1 ? "s" : "");
3164 the message can use different words, but it will still be forced to use
3165 `s' for the plural. Here is a better way, with gettext being applied to
3166 the two strings independently:
3169 printf ((nfiles != 1 ? gettext ("%d files processed")
3170 : gettext ("%d file processed")),
3175 But this still doesn't work for languages like Polish, which has three
3176 plural forms: one for nfiles == 1, one for nfiles == 2, 3, 4, 22, 23, 24, ...
3177 and one for the rest. The GNU @code{ngettext} function solves this problem:
3180 printf (ngettext ("%d files processed", "%d file processed", nfiles),
3186 @section Character Set
3187 @cindex character set
3189 @cindex ASCII characters
3190 @cindex non-ASCII characters
3192 Sticking to the ASCII character set (plain text, 7-bit characters) is
3193 preferred in GNU source code comments, text documents, and other
3194 contexts, unless there is good reason to do something else because of
3195 the application domain. For example, if source code deals with the
3196 French Revolutionary calendar, it is OK if its literal strings contain
3197 accented characters in month names like ``Flor@'eal''. Also, it is OK
3198 (but not required) to use non-ASCII characters to represent proper
3199 names of contributors in change logs (@pxref{Change Logs}).
3201 If you need to use non-ASCII characters, you should normally stick
3202 with one encoding, certainly within a single file. UTF-8 is likely to
3206 @node Quote Characters
3207 @section Quote Characters
3208 @cindex quote characters
3209 @cindex locale-specific quote characters
3212 @cindex opening quote
3213 @cindex single quote
3214 @cindex double quote
3215 @cindex grave accent
3216 @set txicodequoteundirected
3217 @set txicodequotebacktick
3219 In the C locale, the output of GNU programs should stick to plain
3220 ASCII for quotation characters in messages to users: preferably 0x22
3221 (@samp{"}) or 0x27 (@samp{'}) for both opening and closing quotes.
3222 Although GNU programs traditionally used 0x60 (@samp{`}) for opening
3223 and 0x27 (@samp{'}) for closing quotes, nowadays quotes @samp{`like
3224 this'} are typically rendered asymmetrically, so quoting @samp{"like
3225 this"} or @samp{'like this'} typically looks better.
3227 It is ok, but not required, for GNU programs to generate
3228 locale-specific quotes in non-C locales. For example:
3231 printf (gettext ("Processing file '%s'..."), file);
3235 Here, a French translation might cause @code{gettext} to return the
3236 string @code{"Traitement de fichier
3237 @guilsinglleft{}@tie{}%s@tie{}@guilsinglright{}..."}, yielding quotes
3238 more appropriate for a French locale.
3240 Sometimes a program may need to use opening and closing quotes
3241 directly. By convention, @code{gettext} translates the string
3242 @samp{"`"} to the opening quote and the string @samp{"'"} to the
3243 closing quote, and a program can use these translations. Generally,
3244 though, it is better to translate quote characters in the context of
3247 If the output of your program is ever likely to be parsed by another
3248 program, it is good to provide an option that makes this parsing
3249 reliable. For example, you could escape special characters using
3250 conventions from the C language or the Bourne shell. See for example
3251 the option @option{--quoting-style} of GNU @code{ls}.
3253 @clear txicodequoteundirected
3254 @clear txicodequotebacktick
3261 Don't assume that @code{mmap} either works on all files or fails
3262 for all files. It may work on some files and fail on others.
3264 The proper way to use @code{mmap} is to try it on the specific file for
3265 which you want to use it---and if @code{mmap} doesn't work, fall back on
3266 doing the job in another way using @code{read} and @code{write}.
3268 The reason this precaution is needed is that the GNU kernel (the HURD)
3269 provides a user-extensible file system, in which there can be many
3270 different kinds of ``ordinary files''. Many of them support
3271 @code{mmap}, but some do not. It is important to make programs handle
3272 all these kinds of files.
3276 @chapter Documenting Programs
3277 @cindex documentation
3279 A GNU program should ideally come with full free documentation, adequate
3280 for both reference and tutorial purposes. If the package can be
3281 programmed or extended, the documentation should cover programming or
3282 extending it, as well as just using it.
3285 * GNU Manuals:: Writing proper manuals.
3286 * Doc Strings and Manuals:: Compiling doc strings doesn't make a manual.
3287 * Manual Structure Details:: Specific structure conventions.
3288 * License for Manuals:: Writing the distribution terms for a manual.
3289 * Manual Credits:: Giving credit to documentation contributors.
3290 * Printed Manuals:: Mentioning the printed manual.
3291 * NEWS File:: NEWS files supplement manuals.
3292 * Change Logs:: Recording changes.
3293 * Man Pages:: Man pages are secondary.
3294 * Reading other Manuals:: How far you can go in learning
3299 @section GNU Manuals
3301 The preferred document format for the GNU system is the Texinfo
3302 formatting language. Every GNU package should (ideally) have
3303 documentation in Texinfo both for reference and for learners. Texinfo
3304 makes it possible to produce a good quality formatted book, using
3305 @TeX{}, and to generate an Info file. It is also possible to generate
3306 HTML output from Texinfo source. See the Texinfo manual, either the
3307 hardcopy, or the on-line version available through @code{info} or the
3308 Emacs Info subsystem (@kbd{C-h i}).
3310 Nowadays some other formats such as Docbook and Sgmltexi can be
3311 converted automatically into Texinfo. It is ok to produce the Texinfo
3312 documentation by conversion this way, as long as it gives good results.
3314 Make sure your manual is clear to a reader who knows nothing about the
3315 topic and reads it straight through. This means covering basic topics
3316 at the beginning, and advanced topics only later. This also means
3317 defining every specialized term when it is first used.
3319 Programmers tend to carry over the structure of the program as the
3320 structure for its documentation. But this structure is not
3321 necessarily good for explaining how to use the program; it may be
3322 irrelevant and confusing for a user.
3324 Instead, the right way to structure documentation is according to the
3325 concepts and questions that a user will have in mind when reading it.
3326 This principle applies at every level, from the lowest (ordering
3327 sentences in a paragraph) to the highest (ordering of chapter topics
3328 within the manual). Sometimes this structure of ideas matches the
3329 structure of the implementation of the software being documented---but
3330 often they are different. An important part of learning to write good
3331 documentation is to learn to notice when you have unthinkingly
3332 structured the documentation like the implementation, stop yourself,
3333 and look for better alternatives.
3335 For example, each program in the GNU system probably ought to be
3336 documented in one manual; but this does not mean each program should
3337 have its own manual. That would be following the structure of the
3338 implementation, rather than the structure that helps the user
3341 Instead, each manual should cover a coherent @emph{topic}. For example,
3342 instead of a manual for @code{diff} and a manual for @code{diff3}, we
3343 have one manual for ``comparison of files'' which covers both of those
3344 programs, as well as @code{cmp}. By documenting these programs
3345 together, we can make the whole subject clearer.
3347 The manual which discusses a program should certainly document all of
3348 the program's command-line options and all of its commands. It should
3349 give examples of their use. But don't organize the manual as a list
3350 of features. Instead, organize it logically, by subtopics. Address
3351 the questions that a user will ask when thinking about the job that
3352 the program does. Don't just tell the reader what each feature can
3353 do---say what jobs it is good for, and show how to use it for those
3354 jobs. Explain what is recommended usage, and what kinds of usage
3357 In general, a GNU manual should serve both as tutorial and reference.
3358 It should be set up for convenient access to each topic through Info,
3359 and for reading straight through (appendixes aside). A GNU manual
3360 should give a good introduction to a beginner reading through from the
3361 start, and should also provide all the details that hackers want.
3362 The Bison manual is a good example of this---please take a look at it
3363 to see what we mean.
3365 That is not as hard as it first sounds. Arrange each chapter as a
3366 logical breakdown of its topic, but order the sections, and write their
3367 text, so that reading the chapter straight through makes sense. Do
3368 likewise when structuring the book into chapters, and when structuring a
3369 section into paragraphs. The watchword is, @emph{at each point, address
3370 the most fundamental and important issue raised by the preceding text.}
3372 If necessary, add extra chapters at the beginning of the manual which
3373 are purely tutorial and cover the basics of the subject. These provide
3374 the framework for a beginner to understand the rest of the manual. The
3375 Bison manual provides a good example of how to do this.
3377 To serve as a reference, a manual should have an Index that list all the
3378 functions, variables, options, and important concepts that are part of
3379 the program. One combined Index should do for a short manual, but
3380 sometimes for a complex package it is better to use multiple indices.
3381 The Texinfo manual includes advice on preparing good index entries, see
3382 @ref{Index Entries, , Making Index Entries, texinfo, GNU Texinfo}, and
3383 see @ref{Indexing Commands, , Defining the Entries of an
3384 Index, texinfo, GNU Texinfo}.
3386 Don't use Unix man pages as a model for how to write GNU documentation;
3387 most of them are terse, badly structured, and give inadequate
3388 explanation of the underlying concepts. (There are, of course, some
3389 exceptions.) Also, Unix man pages use a particular format which is
3390 different from what we use in GNU manuals.
3392 Please include an email address in the manual for where to report
3393 bugs @emph{in the text of the manual}.
3395 Please do not use the term ``pathname'' that is used in Unix
3396 documentation; use ``file name'' (two words) instead. We use the term
3397 ``path'' only for search paths, which are lists of directory names.
3399 Please do not use the term ``illegal'' to refer to erroneous input to
3400 a computer program. Please use ``invalid'' for this, and reserve the
3401 term ``illegal'' for activities prohibited by law.
3403 Please do not write @samp{()} after a function name just to indicate
3404 it is a function. @code{foo ()} is not a function, it is a function
3405 call with no arguments.
3407 @node Doc Strings and Manuals
3408 @section Doc Strings and Manuals
3410 Some programming systems, such as Emacs, provide a documentation string
3411 for each function, command or variable. You may be tempted to write a
3412 reference manual by compiling the documentation strings and writing a
3413 little additional text to go around them---but you must not do it. That
3414 approach is a fundamental mistake. The text of well-written
3415 documentation strings will be entirely wrong for a manual.
3417 A documentation string needs to stand alone---when it appears on the
3418 screen, there will be no other text to introduce or explain it.
3419 Meanwhile, it can be rather informal in style.
3421 The text describing a function or variable in a manual must not stand
3422 alone; it appears in the context of a section or subsection. Other text
3423 at the beginning of the section should explain some of the concepts, and
3424 should often make some general points that apply to several functions or
3425 variables. The previous descriptions of functions and variables in the
3426 section will also have given information about the topic. A description
3427 written to stand alone would repeat some of that information; this
3428 redundancy looks bad. Meanwhile, the informality that is acceptable in
3429 a documentation string is totally unacceptable in a manual.
3431 The only good way to use documentation strings in writing a good manual
3432 is to use them as a source of information for writing good text.
3434 @node Manual Structure Details
3435 @section Manual Structure Details
3436 @cindex manual structure
3438 The title page of the manual should state the version of the programs or
3439 packages documented in the manual. The Top node of the manual should
3440 also contain this information. If the manual is changing more
3441 frequently than or independent of the program, also state a version
3442 number for the manual in both of these places.
3444 Each program documented in the manual should have a node named
3445 @samp{@var{program} Invocation} or @samp{Invoking @var{program}}. This
3446 node (together with its subnodes, if any) should describe the program's
3447 command line arguments and how to run it (the sort of information people
3448 would look for in a man page). Start with an @samp{@@example}
3449 containing a template for all the options and arguments that the program
3452 Alternatively, put a menu item in some menu whose item name fits one of
3453 the above patterns. This identifies the node which that item points to
3454 as the node for this purpose, regardless of the node's actual name.
3456 The @samp{--usage} feature of the Info reader looks for such a node
3457 or menu item in order to find the relevant text, so it is essential
3458 for every Texinfo file to have one.
3460 If one manual describes several programs, it should have such a node for
3461 each program described in the manual.
3463 @node License for Manuals
3464 @section License for Manuals
3465 @cindex license for manuals
3467 Please use the GNU Free Documentation License for all GNU manuals that
3468 are more than a few pages long. Likewise for a collection of short
3469 documents---you only need one copy of the GNU FDL for the whole
3470 collection. For a single short document, you can use a very permissive
3471 non-copyleft license, to avoid taking up space with a long license.
3473 See @uref{http://www.gnu.org/copyleft/fdl-howto.html} for more explanation
3474 of how to employ the GFDL.
3476 Note that it is not obligatory to include a copy of the GNU GPL or GNU
3477 LGPL in a manual whose license is neither the GPL nor the LGPL. It can
3478 be a good idea to include the program's license in a large manual; in a
3479 short manual, whose size would be increased considerably by including
3480 the program's license, it is probably better not to include it.
3482 @node Manual Credits
3483 @section Manual Credits
3484 @cindex credits for manuals
3486 Please credit the principal human writers of the manual as the authors,
3487 on the title page of the manual. If a company sponsored the work, thank
3488 the company in a suitable place in the manual, but do not cite the
3489 company as an author.
3491 @node Printed Manuals
3492 @section Printed Manuals
3494 The FSF publishes some GNU manuals in printed form. To encourage sales
3495 of these manuals, the on-line versions of the manual should mention at
3496 the very start that the printed manual is available and should point at
3497 information for getting it---for instance, with a link to the page
3498 @url{http://www.gnu.org/order/order.html}. This should not be included
3499 in the printed manual, though, because there it is redundant.
3501 It is also useful to explain in the on-line forms of the manual how the
3502 user can print out the manual from the sources.
3505 @section The NEWS File
3506 @cindex @file{NEWS} file
3508 In addition to its manual, the package should have a file named
3509 @file{NEWS} which contains a list of user-visible changes worth
3510 mentioning. In each new release, add items to the front of the file and
3511 identify the version they pertain to. Don't discard old items; leave
3512 them in the file after the newer items. This way, a user upgrading from
3513 any previous version can see what is new.
3515 If the @file{NEWS} file gets very long, move some of the older items
3516 into a file named @file{ONEWS} and put a note at the end referring the
3520 @section Change Logs
3523 Keep a change log to describe all the changes made to program source
3524 files. The purpose of this is so that people investigating bugs in the
3525 future will know about the changes that might have introduced the bug.
3526 Often a new bug can be found by looking at what was recently changed.
3527 More importantly, change logs can help you eliminate conceptual
3528 inconsistencies between different parts of a program, by giving you a
3529 history of how the conflicting concepts arose and who they came from.
3532 * Change Log Concepts::
3533 * Style of Change Logs::
3535 * Conditional Changes::
3536 * Indicating the Part Changed::
3539 @node Change Log Concepts
3540 @subsection Change Log Concepts
3543 @cindex batch of changes
3544 You can think of the change log as a conceptual ``undo list'' which
3545 explains how earlier versions were different from the current version.
3546 People can see the current version; they don't need the change log to
3547 tell them what is in it. What they want from a change log is a clear
3548 explanation of how the earlier version differed. Each @dfn{entry} in
3549 a change log describes either an individual change or the smallest
3550 batch of changes that belong together, also known as a @dfn{change
3552 @cindex title, change log entry
3553 @cindex description, change log entry
3554 For later reference or for summarizing, sometimes it is useful to
3555 start the entry with a one-line description (sometimes called a
3556 @dfn{title}) to describe its overall purpose.
3558 In the past, we recommended not mentioning changes in non-software
3559 files (manuals, help files, media files, etc.)@: in change logs.
3560 However, we've been advised that it is a good idea to include them,
3561 for the sake of copyright records.
3563 The change log file is normally called @file{ChangeLog} and covers an
3564 entire directory. Each directory can have its own change log, or a
3565 directory can use the change log of its parent directory---it's up to
3568 Another alternative is to record change log information with a version
3569 control system such as RCS or CVS. This can be converted automatically
3570 to a @file{ChangeLog} file using @code{rcs2log}; in Emacs, the command
3571 @kbd{C-x v a} (@code{vc-update-change-log}) does the job.
3573 For changes to code, there's no need to describe the full purpose of
3574 the changes or how they work together. If you think that a change
3575 calls for explanation, you're probably right. Please do explain
3576 it---but please put the full explanation in comments in the code,
3577 where people will see it whenever they see the code. For example,
3578 ``New function'' is enough for the change log when you add a function,
3579 because there should be a comment before the function definition to
3580 explain what it does.
3582 For changes to files that do not support a comment syntax (e.g., media
3583 files), it is ok to include the full explanation in the change log file,
3584 after the title and before the list of individual changes.
3586 The easiest way to add an entry to @file{ChangeLog} is with the Emacs
3587 command @kbd{M-x add-change-log-entry}. An individual change should
3588 have an asterisk, the name of the changed file, and then in
3589 parentheses the name of the changed functions, variables or whatever,
3590 followed by a colon. Then describe the changes you made to that
3591 function or variable.
3594 @node Style of Change Logs
3595 @subsection Style of Change Logs
3596 @cindex change logs, style
3598 Here are some simple examples of change log entries, starting with the
3599 header line that says who made the change and when it was installed,
3600 followed by descriptions of specific changes. (These examples are
3601 drawn from Emacs and GCC.)
3604 1998-08-17 Richard Stallman <rms@@gnu.org>
3606 * register.el (insert-register): Return nil.
3607 (jump-to-register): Likewise.
3609 * sort.el (sort-subr): Return nil.
3611 * tex-mode.el (tex-bibtex-file, tex-file, tex-region):
3612 Restart the tex shell if process is gone or stopped.
3613 (tex-shell-running): New function.
3615 * expr.c (store_one_arg): Round size up for move_block_to_reg.
3616 (expand_call): Round up when emitting USE insns.
3617 * stmt.c (assign_parms): Round size up for move_block_from_reg.
3620 It's important to name the changed function or variable in full. Don't
3621 abbreviate function or variable names, and don't combine them.
3622 Subsequent maintainers will often search for a function name to find all
3623 the change log entries that pertain to it; if you abbreviate the name,
3624 they won't find it when they search.
3626 For example, some people are tempted to abbreviate groups of function
3627 names by writing @samp{* register.el (@{insert,jump-to@}-register)};
3628 this is not a good idea, since searching for @code{jump-to-register} or
3629 @code{insert-register} would not find that entry.
3631 Separate unrelated change log entries with blank lines. Don't put
3632 blank lines between individual changes of an entry. You can omit the
3633 file name and the asterisk when successive individual changes are in
3636 Break long lists of function names by closing continued lines with
3637 @samp{)}, rather than @samp{,}, and opening the continuation with
3638 @samp{(} as in this example:
3641 * keyboard.c (menu_bar_items, tool_bar_items)
3642 (Fexecute_extended_command): Deal with 'keymap' property.
3645 When you install someone else's changes, put the contributor's name in
3646 the change log entry rather than in the text of the entry. In other
3650 2002-07-14 John Doe <jdoe@@gnu.org>
3652 * sewing.c: Make it sew.
3659 2002-07-14 Usual Maintainer <usual@@gnu.org>
3661 * sewing.c: Make it sew. Patch by jdoe@@gnu.org.
3664 As for the date, that should be the date you applied the change.
3666 @node Simple Changes
3667 @subsection Simple Changes
3669 Certain simple kinds of changes don't need much detail in the change
3672 When you change the calling sequence of a function in a simple fashion,
3673 and you change all the callers of the function to use the new calling
3674 sequence, there is no need to make individual entries for all the
3675 callers that you changed. Just write in the entry for the function
3676 being called, ``All callers changed''---like this:
3679 * keyboard.c (Fcommand_execute): New arg SPECIAL.
3680 All callers changed.
3683 When you change just comments or doc strings, it is enough to write an
3684 entry for the file, without mentioning the functions. Just ``Doc
3685 fixes'' is enough for the change log.
3687 There's no technical need to make change log entries for documentation
3688 files. This is because documentation is not susceptible to bugs that
3689 are hard to fix. Documentation does not consist of parts that must
3690 interact in a precisely engineered fashion. To correct an error, you
3691 need not know the history of the erroneous passage; it is enough to
3692 compare what the documentation says with the way the program actually
3695 However, you should keep change logs for documentation files when the
3696 project gets copyright assignments from its contributors, so as to
3697 make the records of authorship more accurate.
3699 @node Conditional Changes
3700 @subsection Conditional Changes
3701 @cindex conditional changes, and change logs
3702 @cindex change logs, conditional changes
3704 Source files can often contain code that is conditional to build-time
3705 or static conditions. For example, C programs can contain
3706 compile-time @code{#if} conditionals; programs implemented in
3707 interpreted languages can contain module imports of function
3708 definitions that are only performed for certain versions of the
3709 interpreter; and Automake @file{Makefile.am} files can contain
3710 variable definitions or target declarations that are only to be
3711 considered if a configure-time Automake conditional is true.
3713 Many changes are conditional as well: sometimes you add a new variable,
3714 or function, or even a new program or library, which is entirely
3715 dependent on a build-time condition. It is useful to indicate
3716 in the change log the conditions for which a change applies.
3718 Our convention for indicating conditional changes is to use
3719 @emph{square brackets around the name of the condition}.
3721 Conditional changes can happen in numerous scenarios and with many
3722 variations, so here are some examples to help clarify. This first
3723 example describes changes in C, Perl, and Python files which are
3724 conditional but do not have an associated function or entity name:
3727 * xterm.c [SOLARIS2]: Include <string.h>.
3728 * FilePath.pm [$^O eq 'VMS']: Import the VMS::Feature module.
3729 * framework.py [sys.version_info < (2, 6)]: Make "with" statement
3730 available by importing it from __future__,
3731 to support also python 2.5.
3734 Our other examples will for simplicity be limited to C, as the minor
3735 changes necessary to adapt them to other languages should be
3738 Next, here is an entry describing a new definition which is entirely
3739 conditional: the C macro @code{FRAME_WINDOW_P} is defined (and used)
3740 only when the macro @code{HAVE_X_WINDOWS} is defined:
3743 * frame.h [HAVE_X_WINDOWS] (FRAME_WINDOW_P): Macro defined.
3746 Next, an entry for a change within the function @code{init_display},
3747 whose definition as a whole is unconditional, but the changes
3748 themselves are contained in a @samp{#ifdef HAVE_LIBNCURSES}
3752 * dispnew.c (init_display) [HAVE_LIBNCURSES]: If X, call tgetent.
3755 Finally, here is an entry for a change that takes effect only when
3756 a certain macro is @emph{not} defined:
3759 * host.c (gethostname) [!HAVE_SOCKETS]: Replace with winsock version.
3763 @node Indicating the Part Changed
3764 @subsection Indicating the Part Changed
3766 Indicate the part of a function which changed by using angle brackets
3767 enclosing an indication of what the changed part does. Here is an entry
3768 for a change in the part of the function @code{sh-while-getopts} that
3769 deals with @code{sh} commands:
3772 * progmodes/sh-script.el (sh-while-getopts) <sh>: Handle case that
3773 user-specified option string is empty.
3781 In the GNU project, man pages are secondary. It is not necessary or
3782 expected for every GNU program to have a man page, but some of them do.
3783 It's your choice whether to include a man page in your program.
3785 When you make this decision, consider that supporting a man page
3786 requires continual effort each time the program is changed. The time
3787 you spend on the man page is time taken away from more useful work.
3789 For a simple program which changes little, updating the man page may be
3790 a small job. Then there is little reason not to include a man page, if
3793 For a large program that changes a great deal, updating a man page may
3794 be a substantial burden. If a user offers to donate a man page, you may
3795 find this gift costly to accept. It may be better to refuse the man
3796 page unless the same person agrees to take full responsibility for
3797 maintaining it---so that you can wash your hands of it entirely. If
3798 this volunteer later ceases to do the job, then don't feel obliged to
3799 pick it up yourself; it may be better to withdraw the man page from the
3800 distribution until someone else agrees to update it.
3802 When a program changes only a little, you may feel that the
3803 discrepancies are small enough that the man page remains useful without
3804 updating. If so, put a prominent note near the beginning of the man
3805 page explaining that you don't maintain it and that the Texinfo manual
3806 is more authoritative. The note should say how to access the Texinfo
3809 Be sure that man pages include a copyright statement and free license.
3810 The simple all-permissive license is appropriate for simple man pages
3811 (@pxref{License Notices for Other Files,,,maintain,Information for GNU
3814 For long man pages, with enough explanation and documentation that
3815 they can be considered true manuals, use the GFDL (@pxref{License for
3818 Finally, the GNU help2man program
3819 (@uref{http://www.gnu.org/software/help2man/}) is one way to automate
3820 generation of a man page, in this case from @option{--help} output.
3821 This is sufficient in many cases.
3823 @node Reading other Manuals
3824 @section Reading other Manuals
3826 There may be non-free books or documentation files that describe the
3827 program you are documenting.
3829 It is ok to use these documents for reference, just as the author of a
3830 new algebra textbook can read other books on algebra. A large portion
3831 of any non-fiction book consists of facts, in this case facts about how
3832 a certain program works, and these facts are necessarily the same for
3833 everyone who writes about the subject. But be careful not to copy your
3834 outline structure, wording, tables or examples from preexisting non-free
3835 documentation. Copying from free documentation may be ok; please check
3836 with the FSF about the individual case.
3838 @node Managing Releases
3839 @chapter The Release Process
3842 Making a release is more than just bundling up your source files in a
3843 tar file and putting it up for FTP. You should set up your software so
3844 that it can be configured to run on a variety of systems. Your Makefile
3845 should conform to the GNU standards described below, and your directory
3846 layout should also conform to the standards discussed below. Doing so
3847 makes it easy to include your package into the larger framework of
3851 * Configuration:: How configuration of GNU packages should work.
3852 * Makefile Conventions:: Makefile conventions.
3853 * Releases:: Making releases
3857 @section How Configuration Should Work
3858 @cindex program configuration
3861 Each GNU distribution should come with a shell script named
3862 @code{configure}. This script is given arguments which describe the
3863 kind of machine and system you want to compile the program for.
3864 The @code{configure} script must record the configuration options so
3865 that they affect compilation.
3867 The description here is the specification of the interface for the
3868 @code{configure} script in GNU packages. Many packages implement it
3869 using GNU Autoconf (@pxref{Top,, Introduction, autoconf, Autoconf})
3870 and/or GNU Automake (@pxref{Top,, Introduction, automake, Automake}),
3871 but you do not have to use these tools. You can implement it any way
3872 you like; for instance, by making @code{configure} be a wrapper around
3873 a completely different configuration system.
3875 Another way for the @code{configure} script to operate is to make a
3876 link from a standard name such as @file{config.h} to the proper
3877 configuration file for the chosen system. If you use this technique,
3878 the distribution should @emph{not} contain a file named
3879 @file{config.h}. This is so that people won't be able to build the
3880 program without configuring it first.
3882 Another thing that @code{configure} can do is to edit the Makefile. If
3883 you do this, the distribution should @emph{not} contain a file named
3884 @file{Makefile}. Instead, it should include a file @file{Makefile.in} which
3885 contains the input used for editing. Once again, this is so that people
3886 won't be able to build the program without configuring it first.
3888 If @code{configure} does write the @file{Makefile}, then @file{Makefile}
3889 should have a target named @file{Makefile} which causes @code{configure}
3890 to be rerun, setting up the same configuration that was set up last
3891 time. The files that @code{configure} reads should be listed as
3892 dependencies of @file{Makefile}.
3894 All the files which are output from the @code{configure} script should
3895 have comments at the beginning explaining that they were generated
3896 automatically using @code{configure}. This is so that users won't think
3897 of trying to edit them by hand.
3899 The @code{configure} script should write a file named @file{config.status}
3900 which describes which configuration options were specified when the
3901 program was last configured. This file should be a shell script which,
3902 if run, will recreate the same configuration.
3904 The @code{configure} script should accept an option of the form
3905 @samp{--srcdir=@var{dirname}} to specify the directory where sources are found
3906 (if it is not the current directory). This makes it possible to build
3907 the program in a separate directory, so that the actual source directory
3910 If the user does not specify @samp{--srcdir}, then @code{configure} should
3911 check both @file{.} and @file{..} to see if it can find the sources. If
3912 it finds the sources in one of these places, it should use them from
3913 there. Otherwise, it should report that it cannot find the sources, and
3914 should exit with nonzero status.
3916 Usually the easy way to support @samp{--srcdir} is by editing a
3917 definition of @code{VPATH} into the Makefile. Some rules may need to
3918 refer explicitly to the specified source directory. To make this
3919 possible, @code{configure} can add to the Makefile a variable named
3920 @code{srcdir} whose value is precisely the specified directory.
3922 In addition, the @samp{configure} script should take options
3923 corresponding to most of the standard directory variables
3924 (@pxref{Directory Variables}). Here is the list:
3927 --prefix --exec-prefix --bindir --sbindir --libexecdir --sysconfdir
3928 --sharedstatedir --localstatedir --libdir --includedir --oldincludedir
3929 --datarootdir --datadir --infodir --localedir --mandir --docdir
3930 --htmldir --dvidir --pdfdir --psdir
3933 The @code{configure} script should also take an argument which specifies the
3934 type of system to build the program for. This argument should look like
3938 @var{cpu}-@var{company}-@var{system}
3941 For example, an Athlon-based GNU/Linux system might be
3942 @samp{i686-pc-linux-gnu}.
3944 The @code{configure} script needs to be able to decode all plausible
3945 alternatives for how to describe a machine. Thus,
3946 @samp{athlon-pc-gnu/linux} would be a valid alias. There is a shell
3948 @uref{http://git.savannah.gnu.org/@/gitweb/@/?p=config.git;a=blob_plain;f=config.sub;hb=HEAD,
3949 @file{config.sub}} that you can use as a subroutine to validate system
3950 types and canonicalize aliases.
3952 The @code{configure} script should also take the option
3953 @option{--build=@var{buildtype}}, which should be equivalent to a
3954 plain @var{buildtype} argument. For example, @samp{configure
3955 --build=i686-pc-linux-gnu} is equivalent to @samp{configure
3956 i686-pc-linux-gnu}. When the build type is not specified by an option
3957 or argument, the @code{configure} script should normally guess it using
3959 @uref{http://git.savannah.gnu.org/@/gitweb/@/?p=config.git;a=blob_plain;f=config.guess;hb=HEAD,
3960 @file{config.guess}}.
3962 @cindex optional features, configure-time
3963 Other options are permitted to specify in more detail the software
3964 or hardware present on the machine, to include or exclude optional parts
3965 of the package, or to adjust the name of some tools or arguments to them:
3968 @item --enable-@var{feature}@r{[}=@var{parameter}@r{]}
3969 Configure the package to build and install an optional user-level
3970 facility called @var{feature}. This allows users to choose which
3971 optional features to include. Giving an optional @var{parameter} of
3972 @samp{no} should omit @var{feature}, if it is built by default.
3974 No @samp{--enable} option should @strong{ever} cause one feature to
3975 replace another. No @samp{--enable} option should ever substitute one
3976 useful behavior for another useful behavior. The only proper use for
3977 @samp{--enable} is for questions of whether to build part of the program
3980 @item --with-@var{package}
3981 @c @r{[}=@var{parameter}@r{]}
3982 The package @var{package} will be installed, so configure this package
3983 to work with @var{package}.
3985 @c Giving an optional @var{parameter} of
3986 @c @samp{no} should omit @var{package}, if it is used by default.
3988 Possible values of @var{package} include
3989 @samp{gnu-as} (or @samp{gas}), @samp{gnu-ld}, @samp{gnu-libc},
3995 Do not use a @samp{--with} option to specify the file name to use to
3996 find certain files. That is outside the scope of what @samp{--with}
3999 @item @var{variable}=@var{value}
4000 Set the value of the variable @var{variable} to @var{value}. This is
4001 used to override the default values of commands or arguments in the
4002 build process. For example, the user could issue @samp{configure
4003 CFLAGS=-g CXXFLAGS=-g} to build with debugging information and without
4004 the default optimization.
4006 Specifying variables as arguments to @code{configure}, like this:
4010 is preferable to setting them in environment variables:
4014 as it helps to recreate the same configuration later with
4015 @file{config.status}. However, both methods should be supported.
4018 All @code{configure} scripts should accept all of the ``detail''
4019 options and the variable settings, whether or not they make any
4020 difference to the particular package at hand. In particular, they
4021 should accept any option that starts with @samp{--with-} or
4022 @samp{--enable-}. This is so users will be able to configure an
4023 entire GNU source tree at once with a single set of options.
4025 You will note that the categories @samp{--with-} and @samp{--enable-}
4026 are narrow: they @strong{do not} provide a place for any sort of option
4027 you might think of. That is deliberate. We want to limit the possible
4028 configuration options in GNU software. We do not want GNU programs to
4029 have idiosyncratic configuration options.
4031 Packages that perform part of the compilation process may support
4032 cross-compilation. In such a case, the host and target machines for the
4033 program may be different.
4035 The @code{configure} script should normally treat the specified type of
4036 system as both the host and the target, thus producing a program which
4037 works for the same type of machine that it runs on.
4039 To compile a program to run on a host type that differs from the build
4040 type, use the configure option @option{--host=@var{hosttype}}, where
4041 @var{hosttype} uses the same syntax as @var{buildtype}. The host type
4042 normally defaults to the build type.
4044 To configure a cross-compiler, cross-assembler, or what have you, you
4045 should specify a target different from the host, using the configure
4046 option @samp{--target=@var{targettype}}. The syntax for
4047 @var{targettype} is the same as for the host type. So the command would
4051 ./configure --host=@var{hosttype} --target=@var{targettype}
4054 The target type normally defaults to the host type.
4055 Programs for which cross-operation is not meaningful need not accept the
4056 @samp{--target} option, because configuring an entire operating system for
4057 cross-operation is not a meaningful operation.
4059 Some programs have ways of configuring themselves automatically. If
4060 your program is set up to do this, your @code{configure} script can simply
4061 ignore most of its arguments.
4063 @comment The makefile standards are in a separate file that is also
4064 @comment included by make.texinfo. Done by roland@gnu.ai.mit.edu on 1/6/93.
4065 @comment For this document, turn chapters into sections, etc.
4067 @include make-stds.texi
4071 @section Making Releases
4074 @cindex version numbers, for releases
4075 You should identify each release with a pair of version numbers, a
4076 major version and a minor. We have no objection to using more than
4077 two numbers, but it is very unlikely that you really need them.
4079 Package the distribution of @code{Foo version 69.96} up in a gzipped tar
4080 file with the name @file{foo-69.96.tar.gz}. It should unpack into a
4081 subdirectory named @file{foo-69.96}.
4083 Building and installing the program should never modify any of the files
4084 contained in the distribution. This means that all the files that form
4085 part of the program in any way must be classified into @dfn{source
4086 files} and @dfn{non-source files}. Source files are written by humans
4087 and never changed automatically; non-source files are produced from
4088 source files by programs under the control of the Makefile.
4090 @cindex @file{README} file
4091 The distribution should contain a file named @file{README} with a
4092 general overview of the package:
4095 @item the name of the package;
4097 @item the version number of the package, or refer to where in the
4098 package the version can be found;
4100 @item a general description of what the package does;
4102 @item a reference to the file @file{INSTALL}, which
4103 should in turn contain an explanation of the installation procedure;
4105 @item a brief explanation of any unusual top-level directories or
4106 files, or other hints for readers to find their way around the source;
4108 @item a reference to the file which contains the copying conditions.
4109 The GNU GPL, if used, should be in a file called @file{COPYING}. If
4110 the GNU LGPL is used, it should be in a file called
4111 @file{COPYING.LESSER}.
4114 Naturally, all the source files must be in the distribution. It is
4115 okay to include non-source files in the distribution along with the
4116 source files they are generated from, provided they are up-to-date
4117 with the source they are made from, and machine-independent, so that
4118 normal building of the distribution will never modify them. We
4119 commonly include non-source files produced by Autoconf, Automake,
4120 Bison, @code{flex}, @TeX{}, and @code{makeinfo}; this helps avoid
4121 unnecessary dependencies between our distributions, so that users can
4122 install whichever versions of whichever packages they like. Do not
4123 induce new dependencies on other software lightly.
4125 Non-source files that might actually be modified by building and
4126 installing the program should @strong{never} be included in the
4127 distribution. So if you do distribute non-source files, always make
4128 sure they are up to date when you make a new distribution.
4130 Make sure that all the files in the distribution are world-readable, and
4131 that directories are world-readable and world-searchable (octal mode 755).
4132 We used to recommend that all directories in the distribution also be
4133 world-writable (octal mode 777), because ancient versions of @code{tar}
4134 would otherwise not cope when extracting the archive as an unprivileged
4135 user. That can easily lead to security issues when creating the archive,
4136 however, so now we recommend against that.
4138 Don't include any symbolic links in the distribution itself. If the tar
4139 file contains symbolic links, then people cannot even unpack it on
4140 systems that don't support symbolic links. Also, don't use multiple
4141 names for one file in different directories, because certain file
4142 systems cannot handle this and that prevents unpacking the
4145 Try to make sure that all the file names will be unique on MS-DOS. A
4146 name on MS-DOS consists of up to 8 characters, optionally followed by a
4147 period and up to three characters. MS-DOS will truncate extra
4148 characters both before and after the period. Thus,
4149 @file{foobarhacker.c} and @file{foobarhacker.o} are not ambiguous; they
4150 are truncated to @file{foobarha.c} and @file{foobarha.o}, which are
4153 @cindex @file{texinfo.tex}, in a distribution
4154 Include in your distribution a copy of the @file{texinfo.tex} you used
4155 to test print any @file{*.texinfo} or @file{*.texi} files.
4157 Likewise, if your program uses small GNU software packages like regex,
4158 getopt, obstack, or termcap, include them in the distribution file.
4159 Leaving them out would make the distribution file a little smaller at
4160 the expense of possible inconvenience to a user who doesn't know what
4164 @chapter References to Non-Free Software and Documentation
4165 @cindex references to non-free material
4167 A GNU program should not recommend, promote, or grant legitimacy to
4168 the use of any non-free program. Proprietary software is a social and
4169 ethical problem, and our aim is to put an end to that problem. We
4170 can't stop some people from writing proprietary programs, or stop
4171 other people from using them, but we can and should refuse to
4172 advertise them to new potential customers, or to give the public the
4173 idea that their existence is ethical.
4175 The GNU definition of free software is found on the GNU web site at
4176 @url{http://www.gnu.org/@/philosophy/@/free-sw.html}, and the definition
4177 of free documentation is found at
4178 @url{http://www.gnu.org/@/philosophy/@/free-doc.html}. The terms ``free''
4179 and ``non-free'', used in this document, refer to those definitions.
4181 A list of important licenses and whether they qualify as free is in
4182 @url{http://www.gnu.org/@/licenses/@/license-list.html}. If it is not
4183 clear whether a license qualifies as free, please ask the GNU Project
4184 by writing to @email{licensing@@gnu.org}. We will answer, and if the
4185 license is an important one, we will add it to the list.
4187 When a non-free program or system is well known, you can mention it in
4188 passing---that is harmless, since users who might want to use it
4189 probably already know about it. For instance, it is fine to explain
4190 how to build your package on top of some widely used non-free
4191 operating system, or how to use it together with some widely used
4194 However, you should give only the necessary information to help those
4195 who already use the non-free program to use your program with
4196 it---don't give, or refer to, any further information about the
4197 proprietary program, and don't imply that the proprietary program
4198 enhances your program, or that its existence is in any way a good
4199 thing. The goal should be that people already using the proprietary
4200 program will get the advice they need about how to use your free
4201 program with it, while people who don't already use the proprietary
4202 program will not see anything likely to lead them to take an interest
4205 If a non-free program or system is obscure in your program's domain,
4206 your program should not mention or support it at all, since doing so
4207 would tend to popularize the non-free program more than it popularizes
4208 your program. (You cannot hope to find many additional users for your
4209 program among the users of Foobar, if the existence of Foobar is not
4210 generally known among people who might want to use your program.)
4212 Sometimes a program is free software in itself but depends on a
4213 non-free platform in order to run. For instance, many Java programs
4214 depend on some non-free Java libraries. To recommend or promote such
4215 a program is to promote the other programs it needs. This is why we
4216 are careful about listing Java programs in the Free Software
4217 Directory: we don't want to promote the non-free Java libraries.
4219 We hope this particular problem with Java will be gone by and by, as
4220 we replace the remaining non-free standard Java libraries with free
4221 software, but the general principle will remain the same: don't
4222 recommend, promote or legitimize programs that depend on non-free
4225 Some free programs strongly encourage the use of non-free software. A
4226 typical example is @command{mplayer}. It is free software in itself,
4227 and the free code can handle some kinds of files. However,
4228 @command{mplayer} recommends use of non-free codecs for other kinds of
4229 files, and users that install @command{mplayer} are very likely to
4230 install those codecs along with it. To recommend @command{mplayer}
4231 is, in effect, to promote use of the non-free codecs.
4233 Thus, you should not recommend programs that strongly encourage the
4234 use of non-free software. This is why we do not list
4235 @command{mplayer} in the Free Software Directory.
4237 A GNU package should not refer the user to any non-free documentation
4238 for free software. Free documentation that can be included in free
4239 operating systems is essential for completing the GNU system, or any
4240 free operating system, so encouraging it is a priority; to recommend
4241 use of documentation that we are not allowed to include undermines the
4242 impetus for the community to produce documentation that we can
4243 include. So GNU packages should never recommend non-free
4246 By contrast, it is ok to refer to journal articles and textbooks in
4247 the comments of a program for explanation of how it functions, even
4248 though they are non-free. This is because we don't include such
4249 things in the GNU system even if they are free---they are outside the
4250 scope of what a software distribution needs to include.
4252 Referring to a web site that describes or recommends a non-free
4253 program is promoting that program, so please do not make links to (or
4254 mention by name) web sites that contain such material. This policy is
4255 relevant particularly for the web pages for a GNU package.
4257 Following links from nearly any web site can lead eventually to
4258 non-free software; this is inherent in the nature of the web. So it
4259 makes no sense to criticize a site for having such links. As long as
4260 the site does not itself recommend a non-free program, there is no
4261 need to consider the question of the sites that it links to for other
4264 Thus, for example, you should not refer to AT&T's web site if that
4265 recommends AT&T's non-free software packages; you should not refer to
4266 a site that links to AT&T's site presenting it as a place to get some
4267 non-free program, because that link recommends and legitimizes the
4268 non-free program. However, that a site contains a link to AT&T's web
4269 site for some other purpose (such as long-distance telephone service)
4270 is not an objection against it.
4272 @node GNU Free Documentation License
4273 @appendix GNU Free Documentation License
4275 @cindex FDL, GNU Free Documentation License
4285 eval: (add-hook 'write-file-hooks 'time-stamp)
4286 time-stamp-start: "@set lastupdate "
4288 time-stamp-format: "%:b %:d, %:y"
4289 compile-command: "cd work.s && make"