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 August 24, 2010
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 Free Software
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
53 @node Top, Preface, (dir), (dir)
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.
185 @section Accepting Contributions
187 @cindex accepting contributions
189 If the program you are working on is copyrighted by the Free Software
190 Foundation, then when someone else sends you a piece of code to add to
191 the program, we need legal papers to use it---just as we asked you to
192 sign papers initially. @emph{Each} person who makes a nontrivial
193 contribution to a program must sign some sort of legal papers in order
194 for us to have clear title to the program; the main author alone is not
197 So, before adding in any contributions from other people, please tell
198 us, so we can arrange to get the papers. Then wait until we tell you
199 that we have received the signed papers, before you actually use the
202 This applies both before you release the program and afterward. If
203 you receive diffs to fix a bug, and they make significant changes, we
204 need legal papers for that change.
206 This also applies to comments and documentation files. For copyright
207 law, comments and code are just text. Copyright applies to all kinds of
208 text, so we need legal papers for all kinds.
210 We know it is frustrating to ask for legal papers; it's frustrating for
211 us as well. But if you don't wait, you are going out on a limb---for
212 example, what if the contributor's employer won't sign a disclaimer?
213 You might have to take that code out again!
215 You don't need papers for changes of a few lines here or there, since
216 they are not significant for copyright purposes. Also, you don't need
217 papers if all you get from the suggestion is some ideas, not actual code
218 which you use. For example, if someone sent you one implementation, but
219 you write a different implementation of the same idea, you don't need to
222 The very worst thing is if you forget to tell us about the other
223 contributor. We could be very embarrassed in court some day as a
226 We have more detailed advice for maintainers of programs; if you have
227 reached the stage of actually maintaining a program for GNU (whether
228 released or not), please ask us for a copy. It is also available
229 online for your perusal: @uref{http://www.gnu.org/prep/maintain/}.
235 Please do not include any trademark acknowledgements in GNU software
236 packages or documentation.
238 Trademark acknowledgements are the statements that such-and-such is a
239 trademark of so-and-so. The GNU Project has no objection to the basic
240 idea of trademarks, but these acknowledgements feel like kowtowing,
241 and there is no legal requirement for them, so we don't use them.
243 What is legally required, as regards other people's trademarks, is to
244 avoid using them in ways which a reader might reasonably understand as
245 naming or labeling our own programs or activities. For example, since
246 ``Objective C'' is (or at least was) a trademark, we made sure to say
247 that we provide a ``compiler for the Objective C language'' rather
248 than an ``Objective C compiler''. The latter would have been meant as
249 a shorter way of saying the former, but it does not explicitly state
250 the relationship, so it could be misinterpreted as using ``Objective
251 C'' as a label for the compiler rather than for the language.
253 Please don't use ``win'' as an abbreviation for Microsoft Windows in
254 GNU software or documentation. In hacker terminology, calling
255 something a ``win'' is a form of praise. If you wish to praise
256 Microsoft Windows when speaking on your own, by all means do so, but
257 not in GNU software. Usually we write the name ``Windows'' in full,
258 but when brevity is very important (as in file names and sometimes
259 symbol names), we abbreviate it to ``w''. For instance, the files and
260 functions in Emacs that deal with Windows start with @samp{w32}.
263 @chapter General Program Design
264 @cindex program design
266 This chapter discusses some of the issues you should take into
267 account when designing your program.
269 @c Standard or ANSI C
271 @c In 1989 the American National Standards Institute (ANSI) standardized
272 @c C as standard X3.159-1989. In December of that year the
273 @c International Standards Organization ISO adopted the ANSI C standard
274 @c making minor changes. In 1990 ANSI then re-adopted ISO standard
275 @c C. This version of C is known as either ANSI C or Standard C.
277 @c A major revision of the C Standard appeared in 1999.
280 * Source Language:: Which languages to use.
281 * Compatibility:: Compatibility with other implementations.
282 * Using Extensions:: Using non-standard features.
283 * Standard C:: Using standard C features.
284 * Conditional Compilation:: Compiling code only if a conditional is true.
287 @node Source Language
288 @section Which Languages to Use
289 @cindex programming languages
291 When you want to use a language that gets compiled and runs at high
292 speed, the best language to use is C. Using another language is like
293 using a non-standard feature: it will cause trouble for users. Even if
294 GCC supports the other language, users may find it inconvenient to have
295 to install the compiler for that other language in order to build your
296 program. For example, if you write your program in C++, people will
297 have to install the GNU C++ compiler in order to compile your program.
299 C has one other advantage over C++ and other compiled languages: more
300 people know C, so more people will find it easy to read and modify the
301 program if it is written in C.
303 So in general it is much better to use C, rather than the
304 comparable alternatives.
306 But there are two exceptions to that conclusion:
310 It is no problem to use another language to write a tool specifically
311 intended for use with that language. That is because the only people
312 who want to build the tool will be those who have installed the other
316 If an application is of interest only to a narrow part of the community,
317 then the question of which language it is written in has less effect on
318 other people, so you may as well please yourself.
321 Many programs are designed to be extensible: they include an interpreter
322 for a language that is higher level than C. Often much of the program
323 is written in that language, too. The Emacs editor pioneered this
327 @cindex GNOME and Guile
328 The standard extensibility interpreter for GNU software is Guile
329 (@uref{http://www.gnu.org/@/software/@/guile/}), which implements the
330 language Scheme (an especially clean and simple dialect of Lisp).
331 Guile also includes bindings for GTK+/GNOME, making it practical to
332 write modern GUI functionality within Guile. We don't reject programs
333 written in other ``scripting languages'' such as Perl and Python, but
334 using Guile is very important for the overall consistency of the GNU
339 @section Compatibility with Other Implementations
340 @cindex compatibility with C and @sc{posix} standards
341 @cindex @sc{posix} compatibility
343 With occasional exceptions, utility programs and libraries for GNU
344 should be upward compatible with those in Berkeley Unix, and upward
345 compatible with Standard C if Standard C specifies their
346 behavior, and upward compatible with @sc{posix} if @sc{posix} specifies
349 When these standards conflict, it is useful to offer compatibility
350 modes for each of them.
352 @cindex options for compatibility
353 Standard C and @sc{posix} prohibit many kinds of extensions. Feel
354 free to make the extensions anyway, and include a @samp{--ansi},
355 @samp{--posix}, or @samp{--compatible} option to turn them off.
356 However, if the extension has a significant chance of breaking any real
357 programs or scripts, then it is not really upward compatible. So you
358 should try to redesign its interface to make it upward compatible.
360 @cindex @code{POSIXLY_CORRECT}, environment variable
361 Many GNU programs suppress extensions that conflict with @sc{posix} if the
362 environment variable @code{POSIXLY_CORRECT} is defined (even if it is
363 defined with a null value). Please make your program recognize this
364 variable if appropriate.
366 When a feature is used only by users (not by programs or command
367 files), and it is done poorly in Unix, feel free to replace it
368 completely with something totally different and better. (For example,
369 @code{vi} is replaced with Emacs.) But it is nice to offer a compatible
370 feature as well. (There is a free @code{vi} clone, so we offer it.)
372 Additional useful features are welcome regardless of whether
373 there is any precedent for them.
375 @node Using Extensions
376 @section Using Non-standard Features
377 @cindex non-standard extensions
379 Many GNU facilities that already exist support a number of convenient
380 extensions over the comparable Unix facilities. Whether to use these
381 extensions in implementing your program is a difficult question.
383 On the one hand, using the extensions can make a cleaner program.
384 On the other hand, people will not be able to build the program
385 unless the other GNU tools are available. This might cause the
386 program to work on fewer kinds of machines.
388 With some extensions, it might be easy to provide both alternatives.
389 For example, you can define functions with a ``keyword'' @code{INLINE}
390 and define that as a macro to expand into either @code{inline} or
391 nothing, depending on the compiler.
393 In general, perhaps it is best not to use the extensions if you can
394 straightforwardly do without them, but to use the extensions if they
395 are a big improvement.
397 An exception to this rule are the large, established programs (such as
398 Emacs) which run on a great variety of systems. Using GNU extensions in
399 such programs would make many users unhappy, so we don't do that.
401 Another exception is for programs that are used as part of compilation:
402 anything that must be compiled with other compilers in order to
403 bootstrap the GNU compilation facilities. If these require the GNU
404 compiler, then no one can compile them without having them installed
405 already. That would be extremely troublesome in certain cases.
408 @section Standard C and Pre-Standard C
409 @cindex @sc{ansi} C standard
411 1989 Standard C is widespread enough now that it is ok to use its
412 features in new programs. There is one exception: do not ever use the
413 ``trigraph'' feature of Standard C.
415 1999 Standard C is not widespread yet, so please do not require its
416 features in programs. It is ok to use its features if they are present.
418 However, it is easy to support pre-standard compilers in most programs,
419 so if you know how to do that, feel free. If a program you are
420 maintaining has such support, you should try to keep it working.
422 @cindex function prototypes
423 To support pre-standard C, instead of writing function definitions in
424 standard prototype form,
433 write the definition in pre-standard style like this,
443 and use a separate declaration to specify the argument prototype:
449 You need such a declaration anyway, in a header file, to get the benefit
450 of prototypes in all the files where the function is called. And once
451 you have the declaration, you normally lose nothing by writing the
452 function definition in the pre-standard style.
454 This technique does not work for integer types narrower than @code{int}.
455 If you think of an argument as being of a type narrower than @code{int},
456 declare it as @code{int} instead.
458 There are a few special cases where this technique is hard to use. For
459 example, if a function argument needs to hold the system type
460 @code{dev_t}, you run into trouble, because @code{dev_t} is shorter than
461 @code{int} on some machines; but you cannot use @code{int} instead,
462 because @code{dev_t} is wider than @code{int} on some machines. There
463 is no type you can safely use on all machines in a non-standard
464 definition. The only way to support non-standard C and pass such an
465 argument is to check the width of @code{dev_t} using Autoconf and choose
466 the argument type accordingly. This may not be worth the trouble.
468 In order to support pre-standard compilers that do not recognize
469 prototypes, you may want to use a preprocessor macro like this:
472 /* Declare the prototype for a general external function. */
473 #if defined (__STDC__) || defined (WINDOWSNT)
474 #define P_(proto) proto
480 @node Conditional Compilation
481 @section Conditional Compilation
483 When supporting configuration options already known when building your
484 program we prefer using @code{if (... )} over conditional compilation,
485 as in the former case the compiler is able to perform more extensive
486 checking of all possible code paths.
488 For example, please write
508 A modern compiler such as GCC will generate exactly the same code in
509 both cases, and we have been using similar techniques with good success
510 in several projects. Of course, the former method assumes that
511 @code{HAS_FOO} is defined as either 0 or 1.
513 While this is not a silver bullet solving all portability problems,
514 and is not always appropriate, following this policy would have saved
515 GCC developers many hours, or even days, per year.
517 In the case of function-like macros like @code{REVERSIBLE_CC_MODE} in
518 GCC which cannot be simply used in @code{if (...)} statements, there is
519 an easy workaround. Simply introduce another macro
520 @code{HAS_REVERSIBLE_CC_MODE} as in the following example:
523 #ifdef REVERSIBLE_CC_MODE
524 #define HAS_REVERSIBLE_CC_MODE 1
526 #define HAS_REVERSIBLE_CC_MODE 0
530 @node Program Behavior
531 @chapter Program Behavior for All Programs
533 This chapter describes conventions for writing robust
534 software. It also describes general standards for error messages, the
535 command line interface, and how libraries should behave.
538 * Non-GNU Standards:: We consider standards such as POSIX;
539 we don't "obey" them.
540 * Semantics:: Writing robust programs.
541 * Libraries:: Library behavior.
542 * Errors:: Formatting error messages.
543 * User Interfaces:: Standards about interfaces generally.
544 * Graphical Interfaces:: Standards for graphical interfaces.
545 * Command-Line Interfaces:: Standards for command line interfaces.
546 * Dynamic Plug-In Interfaces:: Standards for dynamic plug-in interfaces.
547 * Option Table:: Table of long options.
548 * OID Allocations:: Table of OID slots for GNU.
549 * Memory Usage:: When and how to care about memory needs.
550 * File Usage:: Which files to use, and where.
553 @node Non-GNU Standards
554 @section Non-GNU Standards
556 The GNU Project regards standards published by other organizations as
557 suggestions, not orders. We consider those standards, but we do not
558 ``obey'' them. In developing a GNU program, you should implement
559 an outside standard's specifications when that makes the GNU system
560 better overall in an objective sense. When it doesn't, you shouldn't.
562 In most cases, following published standards is convenient for
563 users---it means that their programs or scripts will work more
564 portably. For instance, GCC implements nearly all the features of
565 Standard C as specified by that standard. C program developers would
566 be unhappy if it did not. And GNU utilities mostly follow
567 specifications of POSIX.2; shell script writers and users would be
568 unhappy if our programs were incompatible.
570 But we do not follow either of these specifications rigidly, and there
571 are specific points on which we decided not to follow them, so as to
572 make the GNU system better for users.
574 For instance, Standard C says that nearly all extensions to C are
575 prohibited. How silly! GCC implements many extensions, some of which
576 were later adopted as part of the standard. If you want these
577 constructs to give an error message as ``required'' by the standard,
578 you must specify @samp{--pedantic}, which was implemented only so that
579 we can say ``GCC is a 100% implementation of the standard,'' not
580 because there is any reason to actually use it.
582 POSIX.2 specifies that @samp{df} and @samp{du} must output sizes by
583 default in units of 512 bytes. What users want is units of 1k, so
584 that is what we do by default. If you want the ridiculous behavior
585 ``required'' by POSIX, you must set the environment variable
586 @samp{POSIXLY_CORRECT} (which was originally going to be named
587 @samp{POSIX_ME_HARDER}).
589 GNU utilities also depart from the letter of the POSIX.2 specification
590 when they support long-named command-line options, and intermixing
591 options with ordinary arguments. This minor incompatibility with
592 POSIX is never a problem in practice, and it is very useful.
594 In particular, don't reject a new feature, or remove an old one,
595 merely because a standard says it is ``forbidden'' or ``deprecated.''
598 @section Writing Robust Programs
600 @cindex arbitrary limits on data
601 Avoid arbitrary limits on the length or number of @emph{any} data
602 structure, including file names, lines, files, and symbols, by allocating
603 all data structures dynamically. In most Unix utilities, ``long lines
604 are silently truncated''. This is not acceptable in a GNU utility.
606 @cindex @code{NUL} characters
607 Utilities reading files should not drop NUL characters, or any other
608 nonprinting characters @emph{including those with codes above 0177}.
609 The only sensible exceptions would be utilities specifically intended
610 for interface to certain types of terminals or printers
611 that can't handle those characters.
612 Whenever possible, try to make programs work properly with
613 sequences of bytes that represent multibyte characters, using encodings
614 such as UTF-8 and others.
616 @cindex error messages
617 Check every system call for an error return, unless you know you wish to
618 ignore errors. Include the system error text (from @code{perror} or
619 equivalent) in @emph{every} error message resulting from a failing
620 system call, as well as the name of the file if any and the name of the
621 utility. Just ``cannot open foo.c'' or ``stat failed'' is not
624 @cindex @code{malloc} return value
625 @cindex memory allocation failure
626 Check every call to @code{malloc} or @code{realloc} to see if it
627 returned zero. Check @code{realloc} even if you are making the block
628 smaller; in a system that rounds block sizes to a power of 2,
629 @code{realloc} may get a different block if you ask for less space.
631 In Unix, @code{realloc} can destroy the storage block if it returns
632 zero. GNU @code{realloc} does not have this bug: if it fails, the
633 original block is unchanged. Feel free to assume the bug is fixed. If
634 you wish to run your program on Unix, and wish to avoid lossage in this
635 case, you can use the GNU @code{malloc}.
637 You must expect @code{free} to alter the contents of the block that was
638 freed. Anything you want to fetch from the block, you must fetch before
641 If @code{malloc} fails in a noninteractive program, make that a fatal
642 error. In an interactive program (one that reads commands from the
643 user), it is better to abort the command and return to the command
644 reader loop. This allows the user to kill other processes to free up
645 virtual memory, and then try the command again.
647 @cindex command-line arguments, decoding
648 Use @code{getopt_long} to decode arguments, unless the argument syntax
649 makes this unreasonable.
651 When static storage is to be written in during program execution, use
652 explicit C code to initialize it. Reserve C initialized declarations
653 for data that will not be changed.
656 Try to avoid low-level interfaces to obscure Unix data structures (such
657 as file directories, utmp, or the layout of kernel memory), since these
658 are less likely to work compatibly. If you need to find all the files
659 in a directory, use @code{readdir} or some other high-level interface.
660 These are supported compatibly by GNU.
662 @cindex signal handling
663 The preferred signal handling facilities are the BSD variant of
664 @code{signal}, and the @sc{posix} @code{sigaction} function; the
665 alternative USG @code{signal} interface is an inferior design.
667 Nowadays, using the @sc{posix} signal functions may be the easiest way
668 to make a program portable. If you use @code{signal}, then on GNU/Linux
669 systems running GNU libc version 1, you should include
670 @file{bsd/signal.h} instead of @file{signal.h}, so as to get BSD
671 behavior. It is up to you whether to support systems where
672 @code{signal} has only the USG behavior, or give up on them.
674 @cindex impossible conditions
675 In error checks that detect ``impossible'' conditions, just abort.
676 There is usually no point in printing any message. These checks
677 indicate the existence of bugs. Whoever wants to fix the bugs will have
678 to read the source code and run a debugger. So explain the problem with
679 comments in the source. The relevant data will be in variables, which
680 are easy to examine with the debugger, so there is no point moving them
683 Do not use a count of errors as the exit status for a program.
684 @emph{That does not work}, because exit status values are limited to 8
685 bits (0 through 255). A single run of the program might have 256
686 errors; if you try to return 256 as the exit status, the parent process
687 will see 0 as the status, and it will appear that the program succeeded.
689 @cindex temporary files
690 @cindex @code{TMPDIR} environment variable
691 If you make temporary files, check the @code{TMPDIR} environment
692 variable; if that variable is defined, use the specified directory
693 instead of @file{/tmp}.
695 In addition, be aware that there is a possible security problem when
696 creating temporary files in world-writable directories. In C, you can
697 avoid this problem by creating temporary files in this manner:
700 fd = open (filename, O_WRONLY | O_CREAT | O_EXCL, 0600);
704 or by using the @code{mkstemps} function from libiberty.
706 In bash, use @code{set -C} to avoid this problem.
709 @section Library Behavior
712 Try to make library functions reentrant. If they need to do dynamic
713 storage allocation, at least try to avoid any nonreentrancy aside from
714 that of @code{malloc} itself.
716 Here are certain name conventions for libraries, to avoid name
719 Choose a name prefix for the library, more than two characters long.
720 All external function and variable names should start with this
721 prefix. In addition, there should only be one of these in any given
722 library member. This usually means putting each one in a separate
725 An exception can be made when two external symbols are always used
726 together, so that no reasonable program could use one without the
727 other; then they can both go in the same file.
729 External symbols that are not documented entry points for the user
730 should have names beginning with @samp{_}. The @samp{_} should be
731 followed by the chosen name prefix for the library, to prevent
732 collisions with other libraries. These can go in the same files with
733 user entry points if you like.
735 Static functions and variables can be used as you like and need not
736 fit any naming convention.
739 @section Formatting Error Messages
740 @cindex formatting error messages
741 @cindex error messages, formatting
743 Error messages from compilers should look like this:
746 @var{source-file-name}:@var{lineno}: @var{message}
750 If you want to mention the column number, use one of these formats:
753 @var{source-file-name}:@var{lineno}:@var{column}: @var{message}
754 @var{source-file-name}:@var{lineno}.@var{column}: @var{message}
759 Line numbers should start from 1 at the beginning of the file, and
760 column numbers should start from 1 at the beginning of the line. (Both
761 of these conventions are chosen for compatibility.) Calculate column
762 numbers assuming that space and all ASCII printing characters have
763 equal width, and assuming tab stops every 8 columns.
765 The error message can also give both the starting and ending positions
766 of the erroneous text. There are several formats so that you can
767 avoid redundant information such as a duplicate line number.
768 Here are the possible formats:
771 @var{source-file-name}:@var{lineno-1}.@var{column-1}-@var{lineno-2}.@var{column-2}: @var{message}
772 @var{source-file-name}:@var{lineno-1}.@var{column-1}-@var{column-2}: @var{message}
773 @var{source-file-name}:@var{lineno-1}-@var{lineno-2}: @var{message}
777 When an error is spread over several files, you can use this format:
780 @var{file-1}:@var{lineno-1}.@var{column-1}-@var{file-2}:@var{lineno-2}.@var{column-2}: @var{message}
783 Error messages from other noninteractive programs should look like this:
786 @var{program}:@var{source-file-name}:@var{lineno}: @var{message}
790 when there is an appropriate source file, or like this:
793 @var{program}: @var{message}
797 when there is no relevant source file.
799 If you want to mention the column number, use this format:
802 @var{program}:@var{source-file-name}:@var{lineno}:@var{column}: @var{message}
805 In an interactive program (one that is reading commands from a
806 terminal), it is better not to include the program name in an error
807 message. The place to indicate which program is running is in the
808 prompt or with the screen layout. (When the same program runs with
809 input from a source other than a terminal, it is not interactive and
810 would do best to print error messages using the noninteractive style.)
812 The string @var{message} should not begin with a capital letter when
813 it follows a program name and/or file name, because that isn't the
814 beginning of a sentence. (The sentence conceptually starts at the
815 beginning of the line.) Also, it should not end with a period.
817 Error messages from interactive programs, and other messages such as
818 usage messages, should start with a capital letter. But they should not
821 @node User Interfaces
822 @section Standards for Interfaces Generally
824 @cindex program name and its behavior
825 @cindex behavior, dependent on program's name
826 Please don't make the behavior of a utility depend on the name used
827 to invoke it. It is useful sometimes to make a link to a utility
828 with a different name, and that should not change what it does.
830 Instead, use a run time option or a compilation switch or both
831 to select among the alternate behaviors.
833 @cindex output device and program's behavior
834 Likewise, please don't make the behavior of the program depend on the
835 type of output device it is used with. Device independence is an
836 important principle of the system's design; do not compromise it merely
837 to save someone from typing an option now and then. (Variation in error
838 message syntax when using a terminal is ok, because that is a side issue
839 that people do not depend on.)
841 If you think one behavior is most useful when the output is to a
842 terminal, and another is most useful when the output is a file or a
843 pipe, then it is usually best to make the default behavior the one that
844 is useful with output to a terminal, and have an option for the other
847 Compatibility requires certain programs to depend on the type of output
848 device. It would be disastrous if @code{ls} or @code{sh} did not do so
849 in the way all users expect. In some of these cases, we supplement the
850 program with a preferred alternate version that does not depend on the
851 output device type. For example, we provide a @code{dir} program much
852 like @code{ls} except that its default output format is always
856 @node Graphical Interfaces
857 @section Standards for Graphical Interfaces
858 @cindex graphical user interface
859 @cindex interface styles
860 @cindex user interface styles
863 When you write a program that provides a graphical user interface,
864 please make it work with the X Window System and the GTK+ toolkit
865 unless the functionality specifically requires some alternative (for
866 example, ``displaying jpeg images while in console mode'').
868 In addition, please provide a command-line interface to control the
869 functionality. (In many cases, the graphical user interface can be a
870 separate program which invokes the command-line program.) This is
871 so that the same jobs can be done from scripts.
876 @cindex keyboard interface
877 @cindex library interface
878 Please also consider providing a D-bus interface for use from other
879 running programs, such as within GNOME. (GNOME used to use CORBA
880 for this, but that is being phased out.) In addition, consider
881 providing a library interface (for use from C), and perhaps a
882 keyboard-driven console interface (for use by users from console
883 mode). Once you are doing the work to provide the functionality and
884 the graphical interface, these won't be much extra work.
887 @node Command-Line Interfaces
888 @section Standards for Command Line Interfaces
889 @cindex command-line interface
892 It is a good idea to follow the @sc{posix} guidelines for the
893 command-line options of a program. The easiest way to do this is to use
894 @code{getopt} to parse them. Note that the GNU version of @code{getopt}
895 will normally permit options anywhere among the arguments unless the
896 special argument @samp{--} is used. This is not what @sc{posix}
897 specifies; it is a GNU extension.
899 @cindex long-named options
900 Please define long-named options that are equivalent to the
901 single-letter Unix-style options. We hope to make GNU more user
902 friendly this way. This is easy to do with the GNU function
905 One of the advantages of long-named options is that they can be
906 consistent from program to program. For example, users should be able
907 to expect the ``verbose'' option of any GNU program which has one, to be
908 spelled precisely @samp{--verbose}. To achieve this uniformity, look at
909 the table of common long-option names when you choose the option names
910 for your program (@pxref{Option Table}).
912 It is usually a good idea for file names given as ordinary arguments to
913 be input files only; any output files would be specified using options
914 (preferably @samp{-o} or @samp{--output}). Even if you allow an output
915 file name as an ordinary argument for compatibility, try to provide an
916 option as another way to specify it. This will lead to more consistency
917 among GNU utilities, and fewer idiosyncrasies for users to remember.
919 @cindex standard command-line options
920 @cindex options, standard command-line
921 @cindex CGI programs, standard options for
922 @cindex PATH_INFO, specifying standard options as
923 All programs should support two standard options: @samp{--version}
924 and @samp{--help}. CGI programs should accept these as command-line
925 options, and also if given as the @env{PATH_INFO}; for instance,
926 visiting @url{http://example.org/p.cgi/--help} in a browser should
927 output the same information as invoking @samp{p.cgi --help} from the
931 * --version:: The standard output for --version.
932 * --help:: The standard output for --help.
936 @subsection @option{--version}
938 @cindex @samp{--version} output
940 The standard @code{--version} option should direct the program to
941 print information about its name, version, origin and legal status,
942 all on standard output, and then exit successfully. Other options and
943 arguments should be ignored once this is seen, and the program should
944 not perform its normal function.
946 @cindex canonical name of a program
947 @cindex program's canonical name
948 The first line is meant to be easy for a program to parse; the version
949 number proper starts after the last space. In addition, it contains
950 the canonical name for this program, in this format:
957 The program's name should be a constant string; @emph{don't} compute it
958 from @code{argv[0]}. The idea is to state the standard or canonical
959 name for the program, not its file name. There are other ways to find
960 out the precise file name where a command is found in @code{PATH}.
962 If the program is a subsidiary part of a larger package, mention the
963 package name in parentheses, like this:
966 emacsserver (GNU Emacs) 19.30
970 If the package has a version number which is different from this
971 program's version number, you can mention the package version number
972 just before the close-parenthesis.
974 If you @emph{need} to mention the version numbers of libraries which
975 are distributed separately from the package which contains this program,
976 you can do so by printing an additional line of version info for each
977 library you want to mention. Use the same format for these lines as for
980 Please do not mention all of the libraries that the program uses ``just
981 for completeness''---that would produce a lot of unhelpful clutter.
982 Please mention library version numbers only if you find in practice that
983 they are very important to you in debugging.
985 The following line, after the version number line or lines, should be a
986 copyright notice. If more than one copyright notice is called for, put
987 each on a separate line.
989 Next should follow a line stating the license, preferably using one of
990 abbrevations below, and a brief statement that the program is free
991 software, and that users are free to copy and change it. Also mention
992 that there is no warranty, to the extent permitted by law. See
993 recommended wording below.
995 It is ok to finish the output with a list of the major authors of the
996 program, as a way of giving credit.
998 Here's an example of output that follows these rules:
1002 Copyright (C) 2007 Free Software Foundation, Inc.
1003 License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
1004 This is free software: you are free to change and redistribute it.
1005 There is NO WARRANTY, to the extent permitted by law.
1008 You should adapt this to your program, of course, filling in the proper
1009 year, copyright holder, name of program, and the references to
1010 distribution terms, and changing the rest of the wording as necessary.
1012 This copyright notice only needs to mention the most recent year in
1013 which changes were made---there's no need to list the years for previous
1014 versions' changes. You don't have to mention the name of the program in
1015 these notices, if that is inconvenient, since it appeared in the first
1016 line. (The rules are different for copyright notices in source files;
1017 @pxref{Copyright Notices,,,maintain,Information for GNU Maintainers}.)
1019 Translations of the above lines must preserve the validity of the
1020 copyright notices (@pxref{Internationalization}). If the translation's
1021 character set supports it, the @samp{(C)} should be replaced with the
1022 copyright symbol, as follows:
1025 (the official copyright symbol, which is the letter C in a circle);
1031 Write the word ``Copyright'' exactly like that, in English. Do not
1032 translate it into another language. International treaties recognize
1033 the English word ``Copyright''; translations into other languages do not
1034 have legal significance.
1036 Finally, here is the table of our suggested license abbreviations.
1037 Any abbreviation can be followed by @samp{v@var{version}[+]}, meaning
1038 that particular version, or later versions with the @samp{+}, as shown
1041 In the case of exceptions for extra permissions with the GPL, we use
1042 @samp{/} for a separator; the version number can follow the license
1043 abbreviation as usual, as in the examples below.
1047 GNU General Public License, @url{http://www.gnu.org/@/licenses/@/gpl.html}.
1050 GNU Lesser General Public License, @url{http://www.gnu.org/@/licenses/@/lgpl.html}.
1053 GNU GPL with the exception for Ada.
1056 The Apache Software Foundation license,
1057 @url{http://www.apache.org/@/licenses}.
1060 The Artistic license used for Perl, @url{http://www.perlfoundation.org/@/legal}.
1063 The Expat license, @url{http://www.jclark.com/@/xml/@/copying.txt}.
1066 The Mozilla Public License, @url{http://www.mozilla.org/@/MPL/}.
1069 The original (4-clause) BSD license, incompatible with the GNU GPL
1070 @url{http://www.xfree86.org/@/3.3.6/@/COPYRIGHT2.html#6}.
1073 The license used for PHP, @url{http://www.php.net/@/license/}.
1076 The non-license that is being in the public domain,
1077 @url{http://www.gnu.org/@/licenses/@/license-list.html#PublicDomain}.
1080 The license for Python, @url{http://www.python.org/@/2.0.1/@/license.html}.
1083 The revised (3-clause) BSD, compatible with the GNU GPL,@*
1084 @url{http://www.xfree86.org/@/3.3.6/@/COPYRIGHT2.html#5}.
1087 The simple non-copyleft license used for most versions of the X Window
1088 System, @url{http://www.xfree86.org/@/3.3.6/@/COPYRIGHT2.html#3}.
1091 The license for Zlib, @url{http://www.gzip.org/@/zlib/@/zlib_license.html}.
1095 More information about these licenses and many more are on the GNU
1096 licensing web pages,
1097 @url{http://www.gnu.org/@/licenses/@/license-list.html}.
1101 @subsection @option{--help}
1103 @cindex @samp{--help} output
1105 The standard @code{--help} option should output brief documentation
1106 for how to invoke the program, on standard output, then exit
1107 successfully. Other options and arguments should be ignored once this
1108 is seen, and the program should not perform its normal function.
1110 @cindex address for bug reports
1112 Near the end of the @samp{--help} option's output, please place lines
1113 giving the email address for bug reports, the package's home page
1114 (normally @indicateurl{http://www.gnu.org/software/@var{pkg}}, and the
1115 general page for help using GNU programs. The format should be like this:
1118 Report bugs to: @var{mailing-address}
1119 @var{pkg} home page: <http://www.gnu.org/software/@var{pkg}/>
1120 General help using GNU software: <http://www.gnu.org/gethelp/>
1123 It is ok to mention other appropriate mailing lists and web pages.
1126 @node Dynamic Plug-In Interfaces
1127 @section Standards for Dynamic Plug-in Interfaces
1129 @cindex dynamic plug-ins
1131 Another aspect of keeping free programs free is encouraging
1132 development of free plug-ins, and discouraging development of
1133 proprietary plug-ins. Many GNU programs will not have anything like
1134 plug-ins at all, but those that do should follow these
1137 First, the general plug-in architecture design should closely tie the
1138 plug-in to the original code, such that the plug-in and the base
1139 program are parts of one extended program. For GCC, for example,
1140 plug-ins receive and modify GCC's internal data structures, and so
1141 clearly form an extended program with the base GCC.
1143 @vindex plugin_is_GPL_compatible
1144 Second, you should require plug-in developers to affirm that their
1145 plug-ins are released under an appropriate license. This should be
1146 enforced with a simple programmatic check. For GCC, again for
1147 example, a plug-in must define the global symbol
1148 @code{plugin_is_GPL_compatible}, thus asserting that the plug-in is
1149 released under a GPL-compatible license (@pxref{Plugins,, Plugins,
1150 gccint, GCC Internals}).
1152 By adding this check to your program you are not creating a new legal
1153 requirement. The GPL itself requires plug-ins to be free software,
1154 licensed compatibly. As long as you have followed the first rule above
1155 to keep plug-ins closely tied to your original program, the GPL and AGPL
1156 already require those plug-ins to be released under a compatible
1157 license. The symbol definition in the plug-in---or whatever equivalent
1158 works best in your program---makes it harder for anyone who might
1159 distribute proprietary plug-ins to legally defend themselves. If a case
1160 about this got to court, we can point to that symbol as evidence that
1161 the plug-in developer understood that the license had this requirement.
1165 @section Table of Long Options
1166 @cindex long option names
1167 @cindex table of long options
1169 Here is a table of long options used by GNU programs. It is surely
1170 incomplete, but we aim to list all the options that a new program might
1171 want to be compatible with. If you use names not already in the table,
1172 please send @email{bug-standards@@gnu.org} a list of them, with their
1173 meanings, so we can update the table.
1175 @c Please leave newlines between items in this table; it's much easier
1176 @c to update when it isn't completely squashed together and unreadable.
1177 @c When there is more than one short option for a long option name, put
1178 @c a semicolon between the lists of the programs that use them, not a
1179 @c period. --friedman
1183 @samp{-N} in @code{tar}.
1186 @samp{-a} in @code{du}, @code{ls}, @code{nm}, @code{stty}, @code{uname},
1187 and @code{unexpand}.
1190 @samp{-a} in @code{diff}.
1193 @samp{-A} in @code{ls}.
1196 @samp{-a} in @code{etags}, @code{tee}, @code{time};
1197 @samp{-r} in @code{tar}.
1200 @samp{-a} in @code{cp}.
1203 @samp{-n} in @code{shar}.
1206 @samp{-l} in @code{m4}.
1209 @samp{-a} in @code{diff}.
1212 @samp{-v} in @code{gawk}.
1215 @samp{-W} in @code{make}.
1218 @samp{-o} in @code{make}.
1221 @samp{-a} in @code{recode}.
1224 @samp{-a} in @code{wdiff}.
1226 @item auto-reference
1227 @samp{-A} in @code{ptx}.
1230 @samp{-n} in @code{wdiff}.
1233 For server programs, run in the background.
1235 @item backward-search
1236 @samp{-B} in @code{ctags}.
1239 @samp{-f} in @code{shar}.
1248 @samp{-b} in @code{tac}.
1251 @samp{-b} in @code{cpio} and @code{diff}.
1254 @samp{-b} in @code{shar}.
1257 Used in @code{cpio} and @code{tar}.
1260 @samp{-b} in @code{head} and @code{tail}.
1263 @samp{-b} in @code{ptx}.
1266 Used in various programs to make output shorter.
1269 @samp{-c} in @code{head}, @code{split}, and @code{tail}.
1272 @samp{-C} in @code{etags}.
1275 @samp{-A} in @code{tar}.
1278 Used in various programs to specify the directory to use.
1281 @samp{-c} in @code{chgrp} and @code{chown}.
1284 @samp{-F} in @code{ls}.
1287 @samp{-c} in @code{recode}.
1290 @samp{-c} in @code{su};
1294 @samp{-d} in @code{tar}.
1297 Used in @code{gawk}.
1300 @samp{-Z} in @code{tar} and @code{shar}.
1303 @samp{-A} in @code{tar}.
1306 @samp{-w} in @code{tar}.
1309 Used in @code{diff}.
1312 @samp{-W copyleft} in @code{gawk}.
1315 @samp{-C} in @code{ptx}, @code{recode}, and @code{wdiff};
1316 @samp{-W copyright} in @code{gawk}.
1322 @samp{-q} in @code{who}.
1325 @samp{-l} in @code{du}.
1328 Used in @code{tar} and @code{cpio}.
1331 @samp{-c} in @code{shar}.
1334 @samp{-x} in @code{ctags}.
1337 @samp{-d} in @code{touch}.
1340 @samp{-d} in @code{make} and @code{m4};
1344 @samp{-D} in @code{m4}.
1347 @samp{-d} in Bison and @code{ctags}.
1350 @samp{-D} in @code{tar}.
1353 @samp{-L} in @code{chgrp}, @code{chown}, @code{cpio}, @code{du},
1354 @code{ls}, and @code{tar}.
1356 @item dereference-args
1357 @samp{-D} in @code{du}.
1360 Specify an I/O device (special file name).
1363 @samp{-d} in @code{recode}.
1365 @item dictionary-order
1366 @samp{-d} in @code{look}.
1369 @samp{-d} in @code{tar}.
1372 @samp{-n} in @code{csplit}.
1375 Specify the directory to use, in various programs. In @code{ls}, it
1376 means to show directories themselves rather than their contents. In
1377 @code{rm} and @code{ln}, it means to not treat links to directories
1381 @samp{-x} in @code{strip}.
1383 @item discard-locals
1384 @samp{-X} in @code{strip}.
1387 @samp{-n} in @code{make}.
1390 @samp{-e} in @code{diff}.
1392 @item elide-empty-files
1393 @samp{-z} in @code{csplit}.
1396 @samp{-x} in @code{wdiff}.
1399 @samp{-z} in @code{wdiff}.
1401 @item entire-new-file
1402 @samp{-N} in @code{diff}.
1404 @item environment-overrides
1405 @samp{-e} in @code{make}.
1408 @samp{-e} in @code{xargs}.
1414 Used in @code{makeinfo}.
1417 @samp{-o} in @code{m4}.
1420 @samp{-b} in @code{ls}.
1423 @samp{-X} in @code{tar}.
1429 @samp{-x} in @code{xargs}.
1432 @samp{-e} in @code{unshar}.
1435 @samp{-t} in @code{diff}.
1438 @samp{-e} in @code{sed}.
1441 @samp{-g} in @code{nm}.
1444 @samp{-i} in @code{cpio};
1445 @samp{-x} in @code{tar}.
1448 @samp{-f} in @code{finger}.
1451 @samp{-f} in @code{su}.
1453 @item fatal-warnings
1454 @samp{-E} in @code{m4}.
1457 @samp{-f} in @code{gawk}, @code{info}, @code{make}, @code{mt},
1458 @code{sed}, and @code{tar}.
1460 @item field-separator
1461 @samp{-F} in @code{gawk}.
1467 @samp{-F} in @code{ls}.
1470 @samp{-T} in @code{tar}.
1473 Used in @code{makeinfo}.
1475 @item flag-truncation
1476 @samp{-F} in @code{ptx}.
1478 @item fixed-output-files
1482 @samp{-f} in @code{tail}.
1484 @item footnote-style
1485 Used in @code{makeinfo}.
1488 @samp{-f} in @code{cp}, @code{ln}, @code{mv}, and @code{rm}.
1491 @samp{-F} in @code{shar}.
1494 For server programs, run in the foreground;
1495 in other words, don't do anything special to run the server
1499 Used in @code{ls}, @code{time}, and @code{ptx}.
1502 @samp{-F} in @code{m4}.
1508 @samp{-g} in @code{ptx}.
1511 @samp{-x} in @code{tar}.
1514 @samp{-i} in @code{ul}.
1517 @samp{-g} in @code{recode}.
1520 @samp{-g} in @code{install}.
1523 @samp{-z} in @code{tar} and @code{shar}.
1526 @samp{-H} in @code{m4}.
1529 @samp{-h} in @code{objdump} and @code{recode}
1532 @samp{-H} in @code{who}.
1535 Used to ask for brief usage information.
1537 @item here-delimiter
1538 @samp{-d} in @code{shar}.
1540 @item hide-control-chars
1541 @samp{-q} in @code{ls}.
1544 In @code{makeinfo}, output HTML.
1547 @samp{-u} in @code{who}.
1550 @samp{-D} in @code{diff}.
1553 @samp{-I} in @code{ls};
1554 @samp{-x} in @code{recode}.
1556 @item ignore-all-space
1557 @samp{-w} in @code{diff}.
1559 @item ignore-backups
1560 @samp{-B} in @code{ls}.
1562 @item ignore-blank-lines
1563 @samp{-B} in @code{diff}.
1566 @samp{-f} in @code{look} and @code{ptx};
1567 @samp{-i} in @code{diff} and @code{wdiff}.
1570 @samp{-i} in @code{make}.
1573 @samp{-i} in @code{ptx}.
1575 @item ignore-indentation
1576 @samp{-I} in @code{etags}.
1578 @item ignore-init-file
1581 @item ignore-interrupts
1582 @samp{-i} in @code{tee}.
1584 @item ignore-matching-lines
1585 @samp{-I} in @code{diff}.
1587 @item ignore-space-change
1588 @samp{-b} in @code{diff}.
1591 @samp{-i} in @code{tar}.
1594 @samp{-i} in @code{etags};
1595 @samp{-I} in @code{m4}.
1598 @samp{-I} in @code{make}.
1601 @samp{-G} in @code{tar}.
1604 @samp{-i}, @samp{-l}, and @samp{-m} in Finger.
1607 In some programs, specify the name of the file to read as the user's
1611 @samp{-i} in @code{expand}.
1614 @samp{-T} in @code{diff}.
1617 @samp{-i} in @code{ls}.
1620 @samp{-i} in @code{cp}, @code{ln}, @code{mv}, @code{rm};
1621 @samp{-e} in @code{m4};
1622 @samp{-p} in @code{xargs};
1623 @samp{-w} in @code{tar}.
1626 @samp{-p} in @code{shar}.
1632 @samp{-j} in @code{make}.
1635 @samp{-n} in @code{make}.
1638 @samp{-k} in @code{make}.
1641 @samp{-k} in @code{csplit}.
1644 @samp{-k} in @code{du} and @code{ls}.
1647 @samp{-l} in @code{etags}.
1650 @samp{-l} in @code{wdiff}.
1652 @item level-for-gzip
1653 @samp{-g} in @code{shar}.
1656 @samp{-C} in @code{split}.
1659 Used in @code{split}, @code{head}, and @code{tail}.
1662 @samp{-l} in @code{cpio}.
1666 Used in @code{gawk}.
1669 @samp{-t} in @code{cpio};
1670 @samp{-l} in @code{recode}.
1673 @samp{-t} in @code{tar}.
1676 @samp{-N} in @code{ls}.
1679 @samp{-l} in @code{make}.
1685 Used in @code{uname}.
1688 @samp{-M} in @code{ptx}.
1691 @samp{-m} in @code{hello} and @code{uname}.
1693 @item make-directories
1694 @samp{-d} in @code{cpio}.
1697 @samp{-f} in @code{make}.
1703 @samp{-n} in @code{xargs}.
1706 @samp{-n} in @code{xargs}.
1709 @samp{-l} in @code{xargs}.
1712 @samp{-l} in @code{make}.
1715 @samp{-P} in @code{xargs}.
1718 @samp{-T} in @code{who}.
1721 @samp{-T} in @code{who}.
1724 @samp{-d} in @code{diff}.
1726 @item mixed-uuencode
1727 @samp{-M} in @code{shar}.
1730 @samp{-m} in @code{install}, @code{mkdir}, and @code{mkfifo}.
1732 @item modification-time
1733 @samp{-m} in @code{tar}.
1736 @samp{-M} in @code{tar}.
1742 @samp{-L} in @code{m4}.
1745 @samp{-a} in @code{shar}.
1748 @samp{-W} in @code{make}.
1750 @item no-builtin-rules
1751 @samp{-r} in @code{make}.
1753 @item no-character-count
1754 @samp{-w} in @code{shar}.
1756 @item no-check-existing
1757 @samp{-x} in @code{shar}.
1760 @samp{-3} in @code{wdiff}.
1763 @samp{-c} in @code{touch}.
1766 @samp{-D} in @code{etags}.
1769 @samp{-1} in @code{wdiff}.
1771 @item no-dereference
1772 @samp{-d} in @code{cp}.
1775 @samp{-2} in @code{wdiff}.
1778 @samp{-S} in @code{make}.
1784 @samp{-P} in @code{shar}.
1787 @samp{-e} in @code{gprof}.
1790 @samp{-R} in @code{etags}.
1793 @samp{-p} in @code{nm}.
1796 Don't print a startup splash screen.
1799 Used in @code{makeinfo}.
1802 @samp{-a} in @code{gprof}.
1805 @samp{-E} in @code{gprof}.
1808 @samp{-m} in @code{shar}.
1811 Used in @code{makeinfo}.
1814 Used in @code{emacsclient}.
1817 Used in various programs to inhibit warnings.
1820 @samp{-n} in @code{info}.
1823 @samp{-n} in @code{uname}.
1826 @samp{-f} in @code{cpio}.
1829 @samp{-n} in @code{objdump}.
1832 @samp{-0} in @code{xargs}.
1835 @samp{-n} in @code{cat}.
1837 @item number-nonblank
1838 @samp{-b} in @code{cat}.
1841 @samp{-n} in @code{nm}.
1843 @item numeric-uid-gid
1844 @samp{-n} in @code{cpio} and @code{ls}.
1850 @samp{-o} in @code{tar}.
1853 @samp{-o} in @code{make}.
1855 @item one-file-system
1856 @samp{-l} in @code{tar}, @code{cp}, and @code{du}.
1859 @samp{-o} in @code{ptx}.
1862 @samp{-f} in @code{gprof}.
1865 @samp{-F} in @code{gprof}.
1868 @samp{-o} in @code{getopt}, @code{fdlist}, @code{fdmount},
1869 @code{fdmountd}, and @code{fdumount}.
1872 In various programs, specify the output file name.
1875 @samp{-o} in @code{shar}.
1878 @samp{-o} in @code{rm}.
1881 @samp{-c} in @code{unshar}.
1884 @samp{-o} in @code{install}.
1887 @samp{-l} in @code{diff}.
1889 @item paragraph-indent
1890 Used in @code{makeinfo}.
1893 @samp{-p} in @code{mkdir} and @code{rmdir}.
1896 @samp{-p} in @code{ul}.
1899 @samp{-p} in @code{cpio}.
1902 @samp{-P} in @code{finger}.
1905 @samp{-c} in @code{cpio} and @code{tar}.
1908 Used in @code{gawk}.
1910 @item prefix-builtins
1911 @samp{-P} in @code{m4}.
1914 @samp{-f} in @code{csplit}.
1917 Used in @code{tar} and @code{cp}.
1919 @item preserve-environment
1920 @samp{-p} in @code{su}.
1922 @item preserve-modification-time
1923 @samp{-m} in @code{cpio}.
1925 @item preserve-order
1926 @samp{-s} in @code{tar}.
1928 @item preserve-permissions
1929 @samp{-p} in @code{tar}.
1932 @samp{-l} in @code{diff}.
1935 @samp{-L} in @code{cmp}.
1937 @item print-data-base
1938 @samp{-p} in @code{make}.
1940 @item print-directory
1941 @samp{-w} in @code{make}.
1943 @item print-file-name
1944 @samp{-o} in @code{nm}.
1947 @samp{-s} in @code{nm}.
1950 @samp{-p} in @code{wdiff}.
1953 @samp{-p} in @code{ed}.
1956 Specify an HTTP proxy.
1959 @samp{-X} in @code{shar}.
1962 @samp{-q} in @code{make}.
1965 Used in many programs to inhibit the usual output. Every
1966 program accepting @samp{--quiet} should accept @samp{--silent} as a
1970 @samp{-Q} in @code{shar}
1973 @samp{-Q} in @code{ls}.
1976 @samp{-n} in @code{diff}.
1979 Used in @code{gawk}.
1981 @item read-full-blocks
1982 @samp{-B} in @code{tar}.
1988 @samp{-n} in @code{make}.
1991 @samp{-R} in @code{tar}.
1994 Used in @code{chgrp}, @code{chown}, @code{cp}, @code{ls}, @code{diff},
1998 @samp{-r} in @code{touch}.
2001 @samp{-r} in @code{ptx}.
2004 @samp{-r} in @code{tac} and @code{etags}.
2007 @samp{-r} in @code{uname}.
2010 @samp{-R} in @code{m4}.
2013 @samp{-r} in @code{objdump}.
2016 @samp{-r} in @code{cpio}.
2019 @samp{-i} in @code{xargs}.
2021 @item report-identical-files
2022 @samp{-s} in @code{diff}.
2024 @item reset-access-time
2025 @samp{-a} in @code{cpio}.
2028 @samp{-r} in @code{ls} and @code{nm}.
2031 @samp{-f} in @code{diff}.
2033 @item right-side-defs
2034 @samp{-R} in @code{ptx}.
2037 @samp{-s} in @code{tar}.
2039 @item same-permissions
2040 @samp{-p} in @code{tar}.
2043 @samp{-g} in @code{stty}.
2048 @item sentence-regexp
2049 @samp{-S} in @code{ptx}.
2052 @samp{-S} in @code{du}.
2055 @samp{-s} in @code{tac}.
2058 Used by @code{recode} to chose files or pipes for sequencing passes.
2061 @samp{-s} in @code{su}.
2064 @samp{-A} in @code{cat}.
2066 @item show-c-function
2067 @samp{-p} in @code{diff}.
2070 @samp{-E} in @code{cat}.
2072 @item show-function-line
2073 @samp{-F} in @code{diff}.
2076 @samp{-T} in @code{cat}.
2079 Used in many programs to inhibit the usual output.
2080 Every program accepting
2081 @samp{--silent} should accept @samp{--quiet} as a synonym.
2084 @samp{-s} in @code{ls}.
2087 Specify a file descriptor for a network server to use for its socket,
2088 instead of opening and binding a new socket. This provides a way to
2089 run, in a non-privileged process, a server that normally needs a
2090 reserved port number.
2096 @samp{-W source} in @code{gawk}.
2099 @samp{-S} in @code{tar}.
2101 @item speed-large-files
2102 @samp{-H} in @code{diff}.
2105 @samp{-E} in @code{unshar}.
2107 @item split-size-limit
2108 @samp{-L} in @code{shar}.
2111 @samp{-s} in @code{cat}.
2114 @samp{-w} in @code{wdiff}.
2117 @samp{-y} in @code{wdiff}.
2120 Used in @code{tar} and @code{diff} to specify which file within
2121 a directory to start processing with.
2124 @samp{-s} in @code{wdiff}.
2126 @item stdin-file-list
2127 @samp{-S} in @code{shar}.
2130 @samp{-S} in @code{make}.
2133 @samp{-s} in @code{recode}.
2136 @samp{-s} in @code{install}.
2139 @samp{-s} in @code{strip}.
2142 @samp{-S} in @code{strip}.
2145 @samp{-s} in @code{shar}.
2148 @samp{-S} in @code{cp}, @code{ln}, @code{mv}.
2151 @samp{-b} in @code{csplit}.
2154 @samp{-s} in @code{gprof}.
2157 @samp{-s} in @code{du}.
2160 @samp{-s} in @code{ln}.
2163 Used in GDB and @code{objdump}.
2166 @samp{-s} in @code{m4}.
2169 @samp{-s} in @code{uname}.
2172 @samp{-t} in @code{expand} and @code{unexpand}.
2175 @samp{-T} in @code{ls}.
2178 @samp{-T} in @code{tput} and @code{ul}.
2179 @samp{-t} in @code{wdiff}.
2182 @samp{-a} in @code{diff}.
2185 @samp{-T} in @code{shar}.
2188 Used in @code{ls} and @code{touch}.
2191 Specify how long to wait before giving up on some operation.
2194 @samp{-O} in @code{tar}.
2197 @samp{-c} in @code{du}.
2200 @samp{-t} in @code{make}, @code{ranlib}, and @code{recode}.
2203 @samp{-t} in @code{m4}.
2206 @samp{-t} in @code{hello};
2207 @samp{-W traditional} in @code{gawk};
2208 @samp{-G} in @code{ed}, @code{m4}, and @code{ptx}.
2214 @samp{-t} in @code{ctags}.
2216 @item typedefs-and-c++
2217 @samp{-T} in @code{ctags}.
2220 @samp{-t} in @code{ptx}.
2223 @samp{-z} in @code{tar}.
2226 @samp{-u} in @code{cpio}.
2229 @samp{-U} in @code{m4}.
2231 @item undefined-only
2232 @samp{-u} in @code{nm}.
2235 @samp{-u} in @code{cp}, @code{ctags}, @code{mv}, @code{tar}.
2238 Used in @code{gawk}; same as @samp{--help}.
2241 @samp{-B} in @code{shar}.
2243 @item vanilla-operation
2244 @samp{-V} in @code{shar}.
2247 Print more information about progress. Many programs support this.
2250 @samp{-W} in @code{tar}.
2253 Print the version number.
2255 @item version-control
2256 @samp{-V} in @code{cp}, @code{ln}, @code{mv}.
2259 @samp{-v} in @code{ctags}.
2262 @samp{-V} in @code{tar}.
2265 @samp{-W} in @code{make}.
2267 @item whole-size-limit
2268 @samp{-l} in @code{shar}.
2271 @samp{-w} in @code{ls} and @code{ptx}.
2274 @samp{-W} in @code{ptx}.
2277 @samp{-T} in @code{who}.
2280 @samp{-z} in @code{gprof}.
2283 @node OID Allocations
2284 @section OID Allocations
2285 @cindex OID allocations for GNU
2290 The OID (object identifier) 1.3.6.1.4.1.11591 has been assigned to the
2291 GNU Project (thanks to Werner Koch). These are used for SNMP, LDAP,
2292 X.509 certificates, and so on. The web site
2293 @url{http://www.alvestrand.no/objectid} has a (voluntary) listing of
2294 many OID assignments.
2296 If you need a new slot for your GNU package, write
2297 @email{maintainers@@gnu.org}. Here is a list of arcs currently
2301 @include gnu-oids.texi
2306 @section Memory Usage
2307 @cindex memory usage
2309 If a program typically uses just a few meg of memory, don't bother making any
2310 effort to reduce memory usage. For example, if it is impractical for
2311 other reasons to operate on files more than a few meg long, it is
2312 reasonable to read entire input files into memory to operate on them.
2314 However, for programs such as @code{cat} or @code{tail}, that can
2315 usefully operate on very large files, it is important to avoid using a
2316 technique that would artificially limit the size of files it can handle.
2317 If a program works by lines and could be applied to arbitrary
2318 user-supplied input files, it should keep only a line in memory, because
2319 this is not very hard and users will want to be able to operate on input
2320 files that are bigger than will fit in memory all at once.
2322 If your program creates complicated data structures, just make them in
2323 memory and give a fatal error if @code{malloc} returns zero.
2329 Programs should be prepared to operate when @file{/usr} and @file{/etc}
2330 are read-only file systems. Thus, if the program manages log files,
2331 lock files, backup files, score files, or any other files which are
2332 modified for internal purposes, these files should not be stored in
2333 @file{/usr} or @file{/etc}.
2335 There are two exceptions. @file{/etc} is used to store system
2336 configuration information; it is reasonable for a program to modify
2337 files in @file{/etc} when its job is to update the system configuration.
2338 Also, if the user explicitly asks to modify one file in a directory, it
2339 is reasonable for the program to store other files in the same
2343 @chapter Making The Best Use of C
2345 This chapter provides advice on how best to use the C language
2346 when writing GNU software.
2349 * Formatting:: Formatting your source code.
2350 * Comments:: Commenting your work.
2351 * Syntactic Conventions:: Clean use of C constructs.
2352 * Names:: Naming variables, functions, and files.
2353 * System Portability:: Portability among different operating systems.
2354 * CPU Portability:: Supporting the range of CPU types.
2355 * System Functions:: Portability and ``standard'' library functions.
2356 * Internationalization:: Techniques for internationalization.
2357 * Character Set:: Use ASCII by default.
2358 * Quote Characters:: Use `...' in the C locale.
2359 * Mmap:: How you can safely use @code{mmap}.
2363 @section Formatting Your Source Code
2364 @cindex formatting source code
2367 @cindex braces, in C source
2368 It is important to put the open-brace that starts the body of a C
2369 function in column one, so that they will start a defun. Several
2370 tools look for open-braces in column one to find the beginnings of C
2371 functions. These tools will not work on code not formatted that way.
2373 Avoid putting open-brace, open-parenthesis or open-bracket in column
2374 one when they are inside a function, so that they won't start a defun.
2375 The open-brace that starts a @code{struct} body can go in column one
2376 if you find it useful to treat that definition as a defun.
2378 It is also important for function definitions to start the name of the
2379 function in column one. This helps people to search for function
2380 definitions, and may also help certain tools recognize them. Thus,
2381 using Standard C syntax, the format is this:
2385 concat (char *s1, char *s2)
2392 or, if you want to use traditional C syntax, format the definition like
2397 concat (s1, s2) /* Name starts in column one here */
2399 @{ /* Open brace in column one here */
2404 In Standard C, if the arguments don't fit nicely on one line,
2409 lots_of_args (int an_integer, long a_long, short a_short,
2410 double a_double, float a_float)
2414 The rest of this section gives our recommendations for other aspects of
2415 C formatting style, which is also the default style of the @code{indent}
2416 program in version 1.2 and newer. It corresponds to the options
2419 -nbad -bap -nbc -bbo -bl -bli2 -bls -ncdb -nce -cp1 -cs -di2
2420 -ndj -nfc1 -nfca -hnl -i2 -ip5 -lp -pcs -psl -nsc -nsob
2423 We don't think of these recommendations as requirements, because it
2424 causes no problems for users if two different programs have different
2427 But whatever style you use, please use it consistently, since a mixture
2428 of styles within one program tends to look ugly. If you are
2429 contributing changes to an existing program, please follow the style of
2432 For the body of the function, our recommended style looks like this:
2444 return ++x + bar ();
2448 @cindex spaces before open-paren
2449 We find it easier to read a program when it has spaces before the
2450 open-parentheses and after the commas. Especially after the commas.
2452 When you split an expression into multiple lines, split it
2453 before an operator, not after one. Here is the right way:
2455 @cindex expressions, splitting
2457 if (foo_this_is_long && bar > win (x, y, z)
2458 && remaining_condition)
2461 Try to avoid having two operators of different precedence at the same
2462 level of indentation. For example, don't write this:
2465 mode = (inmode[j] == VOIDmode
2466 || GET_MODE_SIZE (outmode[j]) > GET_MODE_SIZE (inmode[j])
2467 ? outmode[j] : inmode[j]);
2470 Instead, use extra parentheses so that the indentation shows the nesting:
2473 mode = ((inmode[j] == VOIDmode
2474 || (GET_MODE_SIZE (outmode[j]) > GET_MODE_SIZE (inmode[j])))
2475 ? outmode[j] : inmode[j]);
2478 Insert extra parentheses so that Emacs will indent the code properly.
2479 For example, the following indentation looks nice if you do it by hand,
2482 v = rup->ru_utime.tv_sec*1000 + rup->ru_utime.tv_usec/1000
2483 + rup->ru_stime.tv_sec*1000 + rup->ru_stime.tv_usec/1000;
2487 but Emacs would alter it. Adding a set of parentheses produces
2488 something that looks equally nice, and which Emacs will preserve:
2491 v = (rup->ru_utime.tv_sec*1000 + rup->ru_utime.tv_usec/1000
2492 + rup->ru_stime.tv_sec*1000 + rup->ru_stime.tv_usec/1000);
2495 Format do-while statements like this:
2507 Please use formfeed characters (control-L) to divide the program into
2508 pages at logical places (but not within a function). It does not matter
2509 just how long the pages are, since they do not have to fit on a printed
2510 page. The formfeeds should appear alone on lines by themselves.
2513 @section Commenting Your Work
2516 Every program should start with a comment saying briefly what it is for.
2517 Example: @samp{fmt - filter for simple filling of text}. This comment
2518 should be at the top of the source file containing the @samp{main}
2519 function of the program.
2521 Also, please write a brief comment at the start of each source file,
2522 with the file name and a line or two about the overall purpose of the
2525 Please write the comments in a GNU program in English, because English
2526 is the one language that nearly all programmers in all countries can
2527 read. If you do not write English well, please write comments in
2528 English as well as you can, then ask other people to help rewrite them.
2529 If you can't write comments in English, please find someone to work with
2530 you and translate your comments into English.
2532 Please put a comment on each function saying what the function does,
2533 what sorts of arguments it gets, and what the possible values of
2534 arguments mean and are used for. It is not necessary to duplicate in
2535 words the meaning of the C argument declarations, if a C type is being
2536 used in its customary fashion. If there is anything nonstandard about
2537 its use (such as an argument of type @code{char *} which is really the
2538 address of the second character of a string, not the first), or any
2539 possible values that would not work the way one would expect (such as,
2540 that strings containing newlines are not guaranteed to work), be sure
2543 Also explain the significance of the return value, if there is one.
2545 Please put two spaces after the end of a sentence in your comments, so
2546 that the Emacs sentence commands will work. Also, please write
2547 complete sentences and capitalize the first word. If a lower-case
2548 identifier comes at the beginning of a sentence, don't capitalize it!
2549 Changing the spelling makes it a different identifier. If you don't
2550 like starting a sentence with a lower case letter, write the sentence
2551 differently (e.g., ``The identifier lower-case is @dots{}'').
2553 The comment on a function is much clearer if you use the argument
2554 names to speak about the argument values. The variable name itself
2555 should be lower case, but write it in upper case when you are speaking
2556 about the value rather than the variable itself. Thus, ``the inode
2557 number NODE_NUM'' rather than ``an inode''.
2559 There is usually no purpose in restating the name of the function in
2560 the comment before it, because the reader can see that for himself.
2561 There might be an exception when the comment is so long that the function
2562 itself would be off the bottom of the screen.
2564 There should be a comment on each static variable as well, like this:
2567 /* Nonzero means truncate lines in the display;
2568 zero means continue them. */
2572 @cindex conditionals, comments for
2573 @cindex @code{#endif}, commenting
2574 Every @samp{#endif} should have a comment, except in the case of short
2575 conditionals (just a few lines) that are not nested. The comment should
2576 state the condition of the conditional that is ending, @emph{including
2577 its sense}. @samp{#else} should have a comment describing the condition
2578 @emph{and sense} of the code that follows. For example:
2586 #endif /* not foo */
2596 but, by contrast, write the comments this way for a @samp{#ifndef}:
2609 #endif /* not foo */
2613 @node Syntactic Conventions
2614 @section Clean Use of C Constructs
2615 @cindex syntactic conventions
2617 @cindex implicit @code{int}
2618 @cindex function argument, declaring
2619 Please explicitly declare the types of all objects. For example, you
2620 should explicitly declare all arguments to functions, and you should
2621 declare functions to return @code{int} rather than omitting the
2624 @cindex compiler warnings
2625 @cindex @samp{-Wall} compiler option
2626 Some programmers like to use the GCC @samp{-Wall} option, and change the
2627 code whenever it issues a warning. If you want to do this, then do.
2628 Other programmers prefer not to use @samp{-Wall}, because it gives
2629 warnings for valid and legitimate code which they do not want to change.
2630 If you want to do this, then do. The compiler should be your servant,
2633 Declarations of external functions and functions to appear later in the
2634 source file should all go in one place near the beginning of the file
2635 (somewhere before the first function definition in the file), or else
2636 should go in a header file. Don't put @code{extern} declarations inside
2639 @cindex temporary variables
2640 It used to be common practice to use the same local variables (with
2641 names like @code{tem}) over and over for different values within one
2642 function. Instead of doing this, it is better to declare a separate local
2643 variable for each distinct purpose, and give it a name which is
2644 meaningful. This not only makes programs easier to understand, it also
2645 facilitates optimization by good compilers. You can also move the
2646 declaration of each local variable into the smallest scope that includes
2647 all its uses. This makes the program even cleaner.
2649 Don't use local variables or parameters that shadow global identifiers.
2651 @cindex multiple variables in a line
2652 Don't declare multiple variables in one declaration that spans lines.
2653 Start a new declaration on each line, instead. For example, instead
2679 (If they are global variables, each should have a comment preceding it
2682 When you have an @code{if}-@code{else} statement nested in another
2683 @code{if} statement, always put braces around the @code{if}-@code{else}.
2684 Thus, never write like this:
2707 If you have an @code{if} statement nested inside of an @code{else}
2708 statement, either write @code{else if} on one line, like this,
2718 with its @code{then}-part indented like the preceding @code{then}-part,
2719 or write the nested @code{if} within braces like this:
2731 Don't declare both a structure tag and variables or typedefs in the
2732 same declaration. Instead, declare the structure tag separately
2733 and then use it to declare the variables or typedefs.
2735 Try to avoid assignments inside @code{if}-conditions (assignments
2736 inside @code{while}-conditions are ok). For example, don't write
2740 if ((foo = (char *) malloc (sizeof *foo)) == 0)
2741 fatal ("virtual memory exhausted");
2745 instead, write this:
2748 foo = (char *) malloc (sizeof *foo);
2750 fatal ("virtual memory exhausted");
2754 Don't make the program ugly to placate @code{lint}. Please don't insert any
2755 casts to @code{void}. Zero without a cast is perfectly fine as a null
2756 pointer constant, except when calling a varargs function.
2759 @section Naming Variables, Functions, and Files
2761 @cindex names of variables, functions, and files
2762 The names of global variables and functions in a program serve as
2763 comments of a sort. So don't choose terse names---instead, look for
2764 names that give useful information about the meaning of the variable or
2765 function. In a GNU program, names should be English, like other
2768 Local variable names can be shorter, because they are used only within
2769 one context, where (presumably) comments explain their purpose.
2771 Try to limit your use of abbreviations in symbol names. It is ok to
2772 make a few abbreviations, explain what they mean, and then use them
2773 frequently, but don't use lots of obscure abbreviations.
2775 Please use underscores to separate words in a name, so that the Emacs
2776 word commands can be useful within them. Stick to lower case; reserve
2777 upper case for macros and @code{enum} constants, and for name-prefixes
2778 that follow a uniform convention.
2780 For example, you should use names like @code{ignore_space_change_flag};
2781 don't use names like @code{iCantReadThis}.
2783 Variables that indicate whether command-line options have been
2784 specified should be named after the meaning of the option, not after
2785 the option-letter. A comment should state both the exact meaning of
2786 the option and its letter. For example,
2790 /* Ignore changes in horizontal whitespace (-b). */
2791 int ignore_space_change_flag;
2795 When you want to define names with constant integer values, use
2796 @code{enum} rather than @samp{#define}. GDB knows about enumeration
2799 @cindex file-name limitations
2801 You might want to make sure that none of the file names would conflict
2802 if the files were loaded onto an MS-DOS file system which shortens the
2803 names. You can use the program @code{doschk} to test for this.
2805 Some GNU programs were designed to limit themselves to file names of 14
2806 characters or less, to avoid file name conflicts if they are read into
2807 older System V systems. Please preserve this feature in the existing
2808 GNU programs that have it, but there is no need to do this in new GNU
2809 programs. @code{doschk} also reports file names longer than 14
2812 @node System Portability
2813 @section Portability between System Types
2814 @cindex portability, between system types
2816 In the Unix world, ``portability'' refers to porting to different Unix
2817 versions. For a GNU program, this kind of portability is desirable, but
2820 The primary purpose of GNU software is to run on top of the GNU kernel,
2821 compiled with the GNU C compiler, on various types of @sc{cpu}. So the
2822 kinds of portability that are absolutely necessary are quite limited.
2823 But it is important to support Linux-based GNU systems, since they
2824 are the form of GNU that is popular.
2826 Beyond that, it is good to support the other free operating systems
2827 (*BSD), and it is nice to support other Unix-like systems if you want
2828 to. Supporting a variety of Unix-like systems is desirable, although
2829 not paramount. It is usually not too hard, so you may as well do it.
2830 But you don't have to consider it an obligation, if it does turn out to
2834 The easiest way to achieve portability to most Unix-like systems is to
2835 use Autoconf. It's unlikely that your program needs to know more
2836 information about the host platform than Autoconf can provide, simply
2837 because most of the programs that need such knowledge have already been
2840 Avoid using the format of semi-internal data bases (e.g., directories)
2841 when there is a higher-level alternative (@code{readdir}).
2843 @cindex non-@sc{posix} systems, and portability
2844 As for systems that are not like Unix, such as MSDOS, Windows, VMS, MVS,
2845 and older Macintosh systems, supporting them is often a lot of work.
2846 When that is the case, it is better to spend your time adding features
2847 that will be useful on GNU and GNU/Linux, rather than on supporting
2848 other incompatible systems.
2850 If you do support Windows, please do not abbreviate it as ``win''. In
2851 hacker terminology, calling something a ``win'' is a form of praise.
2852 You're free to praise Microsoft Windows on your own if you want, but
2853 please don't do this in GNU packages. Instead of abbreviating
2854 ``Windows'' to ``win'', you can write it in full or abbreviate it to
2855 ``woe'' or ``w''. In GNU Emacs, for instance, we use @samp{w32} in
2856 file names of Windows-specific files, but the macro for Windows
2857 conditionals is called @code{WINDOWSNT}.
2859 It is a good idea to define the ``feature test macro''
2860 @code{_GNU_SOURCE} when compiling your C files. When you compile on GNU
2861 or GNU/Linux, this will enable the declarations of GNU library extension
2862 functions, and that will usually give you a compiler error message if
2863 you define the same function names in some other way in your program.
2864 (You don't have to actually @emph{use} these functions, if you prefer
2865 to make the program more portable to other systems.)
2867 But whether or not you use these GNU extensions, you should avoid
2868 using their names for any other meanings. Doing so would make it hard
2869 to move your code into other GNU programs.
2871 @node CPU Portability
2872 @section Portability between @sc{cpu}s
2874 @cindex data types, and portability
2875 @cindex portability, and data types
2876 Even GNU systems will differ because of differences among @sc{cpu}
2877 types---for example, difference in byte ordering and alignment
2878 requirements. It is absolutely essential to handle these differences.
2879 However, don't make any effort to cater to the possibility that an
2880 @code{int} will be less than 32 bits. We don't support 16-bit machines
2883 Similarly, don't make any effort to cater to the possibility that
2884 @code{long} will be smaller than predefined types like @code{size_t}.
2885 For example, the following code is ok:
2888 printf ("size = %lu\n", (unsigned long) sizeof array);
2889 printf ("diff = %ld\n", (long) (pointer2 - pointer1));
2892 1989 Standard C requires this to work, and we know of only one
2893 counterexample: 64-bit programs on Microsoft Windows. We will
2894 leave it to those who want to port GNU programs to that environment
2895 to figure out how to do it.
2897 Predefined file-size types like @code{off_t} are an exception: they are
2898 longer than @code{long} on many platforms, so code like the above won't
2899 work with them. One way to print an @code{off_t} value portably is to
2900 print its digits yourself, one by one.
2902 Don't assume that the address of an @code{int} object is also the
2903 address of its least-significant byte. This is false on big-endian
2904 machines. Thus, don't make the following mistake:
2909 while ((c = getchar ()) != EOF)
2910 write (file_descriptor, &c, 1);
2913 @noindent Instead, use @code{unsigned char} as follows. (The @code{unsigned}
2914 is for portability to unusual systems where @code{char} is signed and
2915 where there is integer overflow checking.)
2919 while ((c = getchar ()) != EOF)
2921 unsigned char u = c;
2922 write (file_descriptor, &u, 1);
2926 It used to be ok to not worry about the difference between pointers
2927 and integers when passing arguments to functions. However, on most
2928 modern 64-bit machines pointers are wider than @code{int}.
2929 Conversely, integer types like @code{long long int} and @code{off_t}
2930 are wider than pointers on most modern 32-bit machines. Hence it's
2931 often better nowadays to use prototypes to define functions whose
2932 argument types are not trivial.
2934 In particular, if functions accept varying argument counts or types
2935 they should be declared using prototypes containing @samp{...} and
2936 defined using @file{stdarg.h}. For an example of this, please see the
2937 @uref{http://www.gnu.org/software/gnulib/, Gnulib} error module, which
2938 declares and defines the following function:
2941 /* Print a message with `fprintf (stderr, FORMAT, ...)';
2942 if ERRNUM is nonzero, follow it with ": " and strerror (ERRNUM).
2943 If STATUS is nonzero, terminate the program with `exit (STATUS)'. */
2945 void error (int status, int errnum, const char *format, ...);
2948 A simple way to use the Gnulib error module is to obtain the two
2949 source files @file{error.c} and @file{error.h} from the Gnulib library
2950 source code repository at
2951 @uref{http://git.savannah.gnu.org/@/gitweb/@/?p=gnulib.git}.
2952 Here's a sample use:
2959 char *program_name = "myprogram";
2962 xfopen (char const *name)
2964 FILE *fp = fopen (name, "r");
2966 error (1, errno, "cannot read %s", name);
2971 @cindex casting pointers to integers
2972 Avoid casting pointers to integers if you can. Such casts greatly
2973 reduce portability, and in most programs they are easy to avoid. In the
2974 cases where casting pointers to integers is essential---such as, a Lisp
2975 interpreter which stores type information as well as an address in one
2976 word---you'll have to make explicit provisions to handle different word
2977 sizes. You will also need to make provision for systems in which the
2978 normal range of addresses you can get from @code{malloc} starts far away
2981 @node System Functions
2982 @section Calling System Functions
2983 @cindex library functions, and portability
2984 @cindex portability, and library functions
2986 C implementations differ substantially. Standard C reduces but does
2987 not eliminate the incompatibilities; meanwhile, many GNU packages still
2988 support pre-standard compilers because this is not hard to do. This
2989 chapter gives recommendations for how to use the more-or-less standard C
2990 library functions to avoid unnecessary loss of portability.
2994 Don't use the return value of @code{sprintf}. It returns the number of
2995 characters written on some systems, but not on all systems.
2998 Be aware that @code{vfprintf} is not always available.
3001 @code{main} should be declared to return type @code{int}. It should
3002 terminate either by calling @code{exit} or by returning the integer
3003 status code; make sure it cannot ever return an undefined value.
3005 @cindex declaration for system functions
3007 Don't declare system functions explicitly.
3009 Almost any declaration for a system function is wrong on some system.
3010 To minimize conflicts, leave it to the system header files to declare
3011 system functions. If the headers don't declare a function, let it
3014 While it may seem unclean to use a function without declaring it, in
3015 practice this works fine for most system library functions on the
3016 systems where this really happens; thus, the disadvantage is only
3017 theoretical. By contrast, actual declarations have frequently caused
3021 If you must declare a system function, don't specify the argument types.
3022 Use an old-style declaration, not a Standard C prototype. The more you
3023 specify about the function, the more likely a conflict.
3026 In particular, don't unconditionally declare @code{malloc} or
3029 Most GNU programs use those functions just once, in functions
3030 conventionally named @code{xmalloc} and @code{xrealloc}. These
3031 functions call @code{malloc} and @code{realloc}, respectively, and
3034 Because @code{xmalloc} and @code{xrealloc} are defined in your program,
3035 you can declare them in other files without any risk of type conflict.
3037 On most systems, @code{int} is the same length as a pointer; thus, the
3038 calls to @code{malloc} and @code{realloc} work fine. For the few
3039 exceptional systems (mostly 64-bit machines), you can use
3040 @strong{conditionalized} declarations of @code{malloc} and
3041 @code{realloc}---or put these declarations in configuration files
3042 specific to those systems.
3044 @cindex string library functions
3046 The string functions require special treatment. Some Unix systems have
3047 a header file @file{string.h}; others have @file{strings.h}. Neither
3048 file name is portable. There are two things you can do: use Autoconf to
3049 figure out which file to include, or don't include either file.
3052 If you don't include either strings file, you can't get declarations for
3053 the string functions from the header file in the usual way.
3055 That causes less of a problem than you might think. The newer standard
3056 string functions should be avoided anyway because many systems still
3057 don't support them. The string functions you can use are these:
3060 strcpy strncpy strcat strncat
3061 strlen strcmp strncmp
3065 The copy and concatenate functions work fine without a declaration as
3066 long as you don't use their values. Using their values without a
3067 declaration fails on systems where the width of a pointer differs from
3068 the width of @code{int}, and perhaps in other cases. It is trivial to
3069 avoid using their values, so do that.
3071 The compare functions and @code{strlen} work fine without a declaration
3072 on most systems, possibly all the ones that GNU software runs on.
3073 You may find it necessary to declare them @strong{conditionally} on a
3076 The search functions must be declared to return @code{char *}. Luckily,
3077 there is no variation in the data type they return. But there is
3078 variation in their names. Some systems give these functions the names
3079 @code{index} and @code{rindex}; other systems use the names
3080 @code{strchr} and @code{strrchr}. Some systems support both pairs of
3081 names, but neither pair works on all systems.
3083 You should pick a single pair of names and use it throughout your
3084 program. (Nowadays, it is better to choose @code{strchr} and
3085 @code{strrchr} for new programs, since those are the standard
3086 names.) Declare both of those names as functions returning @code{char
3087 *}. On systems which don't support those names, define them as macros
3088 in terms of the other pair. For example, here is what to put at the
3089 beginning of your file (or in a header) if you want to use the names
3090 @code{strchr} and @code{strrchr} throughout:
3094 #define strchr index
3096 #ifndef HAVE_STRRCHR
3097 #define strrchr rindex
3105 Here we assume that @code{HAVE_STRCHR} and @code{HAVE_STRRCHR} are
3106 macros defined in systems where the corresponding functions exist.
3107 One way to get them properly defined is to use Autoconf.
3109 @node Internationalization
3110 @section Internationalization
3111 @cindex internationalization
3114 GNU has a library called GNU gettext that makes it easy to translate the
3115 messages in a program into various languages. You should use this
3116 library in every program. Use English for the messages as they appear
3117 in the program, and let gettext provide the way to translate them into
3120 Using GNU gettext involves putting a call to the @code{gettext} macro
3121 around each string that might need translation---like this:
3124 printf (gettext ("Processing file `%s'..."));
3128 This permits GNU gettext to replace the string @code{"Processing file
3129 `%s'..."} with a translated version.
3131 Once a program uses gettext, please make a point of writing calls to
3132 @code{gettext} when you add new strings that call for translation.
3134 Using GNU gettext in a package involves specifying a @dfn{text domain
3135 name} for the package. The text domain name is used to separate the
3136 translations for this package from the translations for other packages.
3137 Normally, the text domain name should be the same as the name of the
3138 package---for example, @samp{coreutils} for the GNU core utilities.
3140 @cindex message text, and internationalization
3141 To enable gettext to work well, avoid writing code that makes
3142 assumptions about the structure of words or sentences. When you want
3143 the precise text of a sentence to vary depending on the data, use two or
3144 more alternative string constants each containing a complete sentences,
3145 rather than inserting conditionalized words or phrases into a single
3148 Here is an example of what not to do:
3151 printf ("%s is full", capacity > 5000000 ? "disk" : "floppy disk");
3154 If you apply gettext to all strings, like this,
3157 printf (gettext ("%s is full"),
3158 capacity > 5000000 ? gettext ("disk") : gettext ("floppy disk"));
3162 the translator will hardly know that "disk" and "floppy disk" are meant to
3163 be substituted in the other string. Worse, in some languages (like French)
3164 the construction will not work: the translation of the word "full" depends
3165 on the gender of the first part of the sentence; it happens to be not the
3166 same for "disk" as for "floppy disk".
3168 Complete sentences can be translated without problems:
3171 printf (capacity > 5000000 ? gettext ("disk is full")
3172 : gettext ("floppy disk is full"));
3175 A similar problem appears at the level of sentence structure with this
3179 printf ("# Implicit rule search has%s been done.\n",
3180 f->tried_implicit ? "" : " not");
3184 Adding @code{gettext} calls to this code cannot give correct results for
3185 all languages, because negation in some languages requires adding words
3186 at more than one place in the sentence. By contrast, adding
3187 @code{gettext} calls does the job straightforwardly if the code starts
3191 printf (f->tried_implicit
3192 ? "# Implicit rule search has been done.\n",
3193 : "# Implicit rule search has not been done.\n");
3196 Another example is this one:
3199 printf ("%d file%s processed", nfiles,
3200 nfiles != 1 ? "s" : "");
3204 The problem with this example is that it assumes that plurals are made
3205 by adding `s'. If you apply gettext to the format string, like this,
3208 printf (gettext ("%d file%s processed"), nfiles,
3209 nfiles != 1 ? "s" : "");
3213 the message can use different words, but it will still be forced to use
3214 `s' for the plural. Here is a better way, with gettext being applied to
3215 the two strings independently:
3218 printf ((nfiles != 1 ? gettext ("%d files processed")
3219 : gettext ("%d file processed")),
3224 But this still doesn't work for languages like Polish, which has three
3225 plural forms: one for nfiles == 1, one for nfiles == 2, 3, 4, 22, 23, 24, ...
3226 and one for the rest. The GNU @code{ngettext} function solves this problem:
3229 printf (ngettext ("%d files processed", "%d file processed", nfiles),
3235 @section Character Set
3236 @cindex character set
3238 @cindex ASCII characters
3239 @cindex non-ASCII characters
3241 Sticking to the ASCII character set (plain text, 7-bit characters) is
3242 preferred in GNU source code comments, text documents, and other
3243 contexts, unless there is good reason to do something else because of
3244 the application domain. For example, if source code deals with the
3245 French Revolutionary calendar, it is OK if its literal strings contain
3246 accented characters in month names like ``Flor@'eal''. Also, it is OK
3247 to use non-ASCII characters to represent proper names of contributors in
3248 change logs (@pxref{Change Logs}).
3250 If you need to use non-ASCII characters, you should normally stick with
3251 one encoding, as one cannot in general mix encodings reliably.
3254 @node Quote Characters
3255 @section Quote Characters
3256 @cindex quote characters
3257 @cindex locale-specific quote characters
3259 @cindex grave accent
3261 In the C locale, GNU programs should stick to plain ASCII for quotation
3262 characters in messages to users: preferably 0x60 (@samp{`}) for left
3263 quotes and 0x27 (@samp{'}) for right quotes. It is ok, but not
3264 required, to use locale-specific quotes in other locales.
3266 The @uref{http://www.gnu.org/software/gnulib/, Gnulib} @code{quote} and
3267 @code{quotearg} modules provide a reasonably straightforward way to
3268 support locale-specific quote characters, as well as taking care of
3269 other issues, such as quoting a filename that itself contains a quote
3270 character. See the Gnulib documentation for usage details.
3272 In any case, the documentation for your program should clearly specify
3273 how it does quoting, if different than the preferred method of @samp{`}
3274 and @samp{'}. This is especially important if the output of your
3275 program is ever likely to be parsed by another program.
3277 Quotation characters are a difficult area in the computing world at
3278 this time: there are no true left or right quote characters in Latin1;
3279 the @samp{`} character we use was standardized there as a grave
3280 accent. Moreover, Latin1 is still not universally usable.
3282 Unicode contains the unambiguous quote characters required, and its
3283 common encoding UTF-8 is upward compatible with Latin1. However,
3284 Unicode and UTF-8 are not universally well-supported, either.
3286 This may change over the next few years, and then we will revisit
3294 Don't assume that @code{mmap} either works on all files or fails
3295 for all files. It may work on some files and fail on others.
3297 The proper way to use @code{mmap} is to try it on the specific file for
3298 which you want to use it---and if @code{mmap} doesn't work, fall back on
3299 doing the job in another way using @code{read} and @code{write}.
3301 The reason this precaution is needed is that the GNU kernel (the HURD)
3302 provides a user-extensible file system, in which there can be many
3303 different kinds of ``ordinary files.'' Many of them support
3304 @code{mmap}, but some do not. It is important to make programs handle
3305 all these kinds of files.
3308 @chapter Documenting Programs
3309 @cindex documentation
3311 A GNU program should ideally come with full free documentation, adequate
3312 for both reference and tutorial purposes. If the package can be
3313 programmed or extended, the documentation should cover programming or
3314 extending it, as well as just using it.
3317 * GNU Manuals:: Writing proper manuals.
3318 * Doc Strings and Manuals:: Compiling doc strings doesn't make a manual.
3319 * Manual Structure Details:: Specific structure conventions.
3320 * License for Manuals:: Writing the distribution terms for a manual.
3321 * Manual Credits:: Giving credit to documentation contributors.
3322 * Printed Manuals:: Mentioning the printed manual.
3323 * NEWS File:: NEWS files supplement manuals.
3324 * Change Logs:: Recording changes.
3325 * Man Pages:: Man pages are secondary.
3326 * Reading other Manuals:: How far you can go in learning
3331 @section GNU Manuals
3333 The preferred document format for the GNU system is the Texinfo
3334 formatting language. Every GNU package should (ideally) have
3335 documentation in Texinfo both for reference and for learners. Texinfo
3336 makes it possible to produce a good quality formatted book, using
3337 @TeX{}, and to generate an Info file. It is also possible to generate
3338 HTML output from Texinfo source. See the Texinfo manual, either the
3339 hardcopy, or the on-line version available through @code{info} or the
3340 Emacs Info subsystem (@kbd{C-h i}).
3342 Nowadays some other formats such as Docbook and Sgmltexi can be
3343 converted automatically into Texinfo. It is ok to produce the Texinfo
3344 documentation by conversion this way, as long as it gives good results.
3346 Make sure your manual is clear to a reader who knows nothing about the
3347 topic and reads it straight through. This means covering basic topics
3348 at the beginning, and advanced topics only later. This also means
3349 defining every specialized term when it is first used.
3351 Programmers tend to carry over the structure of the program as the
3352 structure for its documentation. But this structure is not
3353 necessarily good for explaining how to use the program; it may be
3354 irrelevant and confusing for a user.
3356 Instead, the right way to structure documentation is according to the
3357 concepts and questions that a user will have in mind when reading it.
3358 This principle applies at every level, from the lowest (ordering
3359 sentences in a paragraph) to the highest (ordering of chapter topics
3360 within the manual). Sometimes this structure of ideas matches the
3361 structure of the implementation of the software being documented---but
3362 often they are different. An important part of learning to write good
3363 documentation is to learn to notice when you have unthinkingly
3364 structured the documentation like the implementation, stop yourself,
3365 and look for better alternatives.
3367 For example, each program in the GNU system probably ought to be
3368 documented in one manual; but this does not mean each program should
3369 have its own manual. That would be following the structure of the
3370 implementation, rather than the structure that helps the user
3373 Instead, each manual should cover a coherent @emph{topic}. For example,
3374 instead of a manual for @code{diff} and a manual for @code{diff3}, we
3375 have one manual for ``comparison of files'' which covers both of those
3376 programs, as well as @code{cmp}. By documenting these programs
3377 together, we can make the whole subject clearer.
3379 The manual which discusses a program should certainly document all of
3380 the program's command-line options and all of its commands. It should
3381 give examples of their use. But don't organize the manual as a list
3382 of features. Instead, organize it logically, by subtopics. Address
3383 the questions that a user will ask when thinking about the job that
3384 the program does. Don't just tell the reader what each feature can
3385 do---say what jobs it is good for, and show how to use it for those
3386 jobs. Explain what is recommended usage, and what kinds of usage
3389 In general, a GNU manual should serve both as tutorial and reference.
3390 It should be set up for convenient access to each topic through Info,
3391 and for reading straight through (appendixes aside). A GNU manual
3392 should give a good introduction to a beginner reading through from the
3393 start, and should also provide all the details that hackers want.
3394 The Bison manual is a good example of this---please take a look at it
3395 to see what we mean.
3397 That is not as hard as it first sounds. Arrange each chapter as a
3398 logical breakdown of its topic, but order the sections, and write their
3399 text, so that reading the chapter straight through makes sense. Do
3400 likewise when structuring the book into chapters, and when structuring a
3401 section into paragraphs. The watchword is, @emph{at each point, address
3402 the most fundamental and important issue raised by the preceding text.}
3404 If necessary, add extra chapters at the beginning of the manual which
3405 are purely tutorial and cover the basics of the subject. These provide
3406 the framework for a beginner to understand the rest of the manual. The
3407 Bison manual provides a good example of how to do this.
3409 To serve as a reference, a manual should have an Index that list all the
3410 functions, variables, options, and important concepts that are part of
3411 the program. One combined Index should do for a short manual, but
3412 sometimes for a complex package it is better to use multiple indices.
3413 The Texinfo manual includes advice on preparing good index entries, see
3414 @ref{Index Entries, , Making Index Entries, texinfo, GNU Texinfo}, and
3415 see @ref{Indexing Commands, , Defining the Entries of an
3416 Index, texinfo, GNU Texinfo}.
3418 Don't use Unix man pages as a model for how to write GNU documentation;
3419 most of them are terse, badly structured, and give inadequate
3420 explanation of the underlying concepts. (There are, of course, some
3421 exceptions.) Also, Unix man pages use a particular format which is
3422 different from what we use in GNU manuals.
3424 Please include an email address in the manual for where to report
3425 bugs @emph{in the text of the manual}.
3427 Please do not use the term ``pathname'' that is used in Unix
3428 documentation; use ``file name'' (two words) instead. We use the term
3429 ``path'' only for search paths, which are lists of directory names.
3431 Please do not use the term ``illegal'' to refer to erroneous input to
3432 a computer program. Please use ``invalid'' for this, and reserve the
3433 term ``illegal'' for activities prohibited by law.
3435 Please do not write @samp{()} after a function name just to indicate
3436 it is a function. @code{foo ()} is not a function, it is a function
3437 call with no arguments.
3439 @node Doc Strings and Manuals
3440 @section Doc Strings and Manuals
3442 Some programming systems, such as Emacs, provide a documentation string
3443 for each function, command or variable. You may be tempted to write a
3444 reference manual by compiling the documentation strings and writing a
3445 little additional text to go around them---but you must not do it. That
3446 approach is a fundamental mistake. The text of well-written
3447 documentation strings will be entirely wrong for a manual.
3449 A documentation string needs to stand alone---when it appears on the
3450 screen, there will be no other text to introduce or explain it.
3451 Meanwhile, it can be rather informal in style.
3453 The text describing a function or variable in a manual must not stand
3454 alone; it appears in the context of a section or subsection. Other text
3455 at the beginning of the section should explain some of the concepts, and
3456 should often make some general points that apply to several functions or
3457 variables. The previous descriptions of functions and variables in the
3458 section will also have given information about the topic. A description
3459 written to stand alone would repeat some of that information; this
3460 redundancy looks bad. Meanwhile, the informality that is acceptable in
3461 a documentation string is totally unacceptable in a manual.
3463 The only good way to use documentation strings in writing a good manual
3464 is to use them as a source of information for writing good text.
3466 @node Manual Structure Details
3467 @section Manual Structure Details
3468 @cindex manual structure
3470 The title page of the manual should state the version of the programs or
3471 packages documented in the manual. The Top node of the manual should
3472 also contain this information. If the manual is changing more
3473 frequently than or independent of the program, also state a version
3474 number for the manual in both of these places.
3476 Each program documented in the manual should have a node named
3477 @samp{@var{program} Invocation} or @samp{Invoking @var{program}}. This
3478 node (together with its subnodes, if any) should describe the program's
3479 command line arguments and how to run it (the sort of information people
3480 would look for in a man page). Start with an @samp{@@example}
3481 containing a template for all the options and arguments that the program
3484 Alternatively, put a menu item in some menu whose item name fits one of
3485 the above patterns. This identifies the node which that item points to
3486 as the node for this purpose, regardless of the node's actual name.
3488 The @samp{--usage} feature of the Info reader looks for such a node
3489 or menu item in order to find the relevant text, so it is essential
3490 for every Texinfo file to have one.
3492 If one manual describes several programs, it should have such a node for
3493 each program described in the manual.
3495 @node License for Manuals
3496 @section License for Manuals
3497 @cindex license for manuals
3499 Please use the GNU Free Documentation License for all GNU manuals that
3500 are more than a few pages long. Likewise for a collection of short
3501 documents---you only need one copy of the GNU FDL for the whole
3502 collection. For a single short document, you can use a very permissive
3503 non-copyleft license, to avoid taking up space with a long license.
3505 See @uref{http://www.gnu.org/copyleft/fdl-howto.html} for more explanation
3506 of how to employ the GFDL.
3508 Note that it is not obligatory to include a copy of the GNU GPL or GNU
3509 LGPL in a manual whose license is neither the GPL nor the LGPL. It can
3510 be a good idea to include the program's license in a large manual; in a
3511 short manual, whose size would be increased considerably by including
3512 the program's license, it is probably better not to include it.
3514 @node Manual Credits
3515 @section Manual Credits
3516 @cindex credits for manuals
3518 Please credit the principal human writers of the manual as the authors,
3519 on the title page of the manual. If a company sponsored the work, thank
3520 the company in a suitable place in the manual, but do not cite the
3521 company as an author.
3523 @node Printed Manuals
3524 @section Printed Manuals
3526 The FSF publishes some GNU manuals in printed form. To encourage sales
3527 of these manuals, the on-line versions of the manual should mention at
3528 the very start that the printed manual is available and should point at
3529 information for getting it---for instance, with a link to the page
3530 @url{http://www.gnu.org/order/order.html}. This should not be included
3531 in the printed manual, though, because there it is redundant.
3533 It is also useful to explain in the on-line forms of the manual how the
3534 user can print out the manual from the sources.
3537 @section The NEWS File
3538 @cindex @file{NEWS} file
3540 In addition to its manual, the package should have a file named
3541 @file{NEWS} which contains a list of user-visible changes worth
3542 mentioning. In each new release, add items to the front of the file and
3543 identify the version they pertain to. Don't discard old items; leave
3544 them in the file after the newer items. This way, a user upgrading from
3545 any previous version can see what is new.
3547 If the @file{NEWS} file gets very long, move some of the older items
3548 into a file named @file{ONEWS} and put a note at the end referring the
3552 @section Change Logs
3555 Keep a change log to describe all the changes made to program source
3556 files. The purpose of this is so that people investigating bugs in the
3557 future will know about the changes that might have introduced the bug.
3558 Often a new bug can be found by looking at what was recently changed.
3559 More importantly, change logs can help you eliminate conceptual
3560 inconsistencies between different parts of a program, by giving you a
3561 history of how the conflicting concepts arose and who they came from.
3564 * Change Log Concepts::
3565 * Style of Change Logs::
3567 * Conditional Changes::
3568 * Indicating the Part Changed::
3571 @node Change Log Concepts
3572 @subsection Change Log Concepts
3574 You can think of the change log as a conceptual ``undo list'' which
3575 explains how earlier versions were different from the current version.
3576 People can see the current version; they don't need the change log
3577 to tell them what is in it. What they want from a change log is a
3578 clear explanation of how the earlier version differed.
3580 The change log file is normally called @file{ChangeLog} and covers an
3581 entire directory. Each directory can have its own change log, or a
3582 directory can use the change log of its parent directory---it's up to
3585 Another alternative is to record change log information with a version
3586 control system such as RCS or CVS. This can be converted automatically
3587 to a @file{ChangeLog} file using @code{rcs2log}; in Emacs, the command
3588 @kbd{C-x v a} (@code{vc-update-change-log}) does the job.
3590 There's no need to describe the full purpose of the changes or how
3591 they work together. However, sometimes it is useful to write one line
3592 to describe the overall purpose of a change or a batch of changes. If
3593 you think that a change calls for explanation, you're probably right.
3594 Please do explain it---but please put the full explanation in comments
3595 in the code, where people will see it whenever they see the code. For
3596 example, ``New function'' is enough for the change log when you add a
3597 function, because there should be a comment before the function
3598 definition to explain what it does.
3600 In the past, we recommended not mentioning changes in non-software
3601 files (manuals, help files, etc.) in change logs. However, we've been
3602 advised that it is a good idea to include them, for the sake of
3605 The easiest way to add an entry to @file{ChangeLog} is with the Emacs
3606 command @kbd{M-x add-change-log-entry}. An entry should have an
3607 asterisk, the name of the changed file, and then in parentheses the name
3608 of the changed functions, variables or whatever, followed by a colon.
3609 Then describe the changes you made to that function or variable.
3611 @node Style of Change Logs
3612 @subsection Style of Change Logs
3613 @cindex change logs, style
3615 Here are some simple examples of change log entries, starting with the
3616 header line that says who made the change and when it was installed,
3617 followed by descriptions of specific changes. (These examples are
3618 drawn from Emacs and GCC.)
3621 1998-08-17 Richard Stallman <rms@@gnu.org>
3623 * register.el (insert-register): Return nil.
3624 (jump-to-register): Likewise.
3626 * sort.el (sort-subr): Return nil.
3628 * tex-mode.el (tex-bibtex-file, tex-file, tex-region):
3629 Restart the tex shell if process is gone or stopped.
3630 (tex-shell-running): New function.
3632 * expr.c (store_one_arg): Round size up for move_block_to_reg.
3633 (expand_call): Round up when emitting USE insns.
3634 * stmt.c (assign_parms): Round size up for move_block_from_reg.
3637 It's important to name the changed function or variable in full. Don't
3638 abbreviate function or variable names, and don't combine them.
3639 Subsequent maintainers will often search for a function name to find all
3640 the change log entries that pertain to it; if you abbreviate the name,
3641 they won't find it when they search.
3643 For example, some people are tempted to abbreviate groups of function
3644 names by writing @samp{* register.el (@{insert,jump-to@}-register)};
3645 this is not a good idea, since searching for @code{jump-to-register} or
3646 @code{insert-register} would not find that entry.
3648 Separate unrelated change log entries with blank lines. When two
3649 entries represent parts of the same change, so that they work together,
3650 then don't put blank lines between them. Then you can omit the file
3651 name and the asterisk when successive entries are in the same file.
3653 Break long lists of function names by closing continued lines with
3654 @samp{)}, rather than @samp{,}, and opening the continuation with
3655 @samp{(} as in this example:
3658 * keyboard.c (menu_bar_items, tool_bar_items)
3659 (Fexecute_extended_command): Deal with `keymap' property.
3662 When you install someone else's changes, put the contributor's name in
3663 the change log entry rather than in the text of the entry. In other
3667 2002-07-14 John Doe <jdoe@@gnu.org>
3669 * sewing.c: Make it sew.
3676 2002-07-14 Usual Maintainer <usual@@gnu.org>
3678 * sewing.c: Make it sew. Patch by jdoe@@gnu.org.
3681 As for the date, that should be the date you applied the change.
3683 @node Simple Changes
3684 @subsection Simple Changes
3686 Certain simple kinds of changes don't need much detail in the change
3689 When you change the calling sequence of a function in a simple fashion,
3690 and you change all the callers of the function to use the new calling
3691 sequence, there is no need to make individual entries for all the
3692 callers that you changed. Just write in the entry for the function
3693 being called, ``All callers changed''---like this:
3696 * keyboard.c (Fcommand_execute): New arg SPECIAL.
3697 All callers changed.
3700 When you change just comments or doc strings, it is enough to write an
3701 entry for the file, without mentioning the functions. Just ``Doc
3702 fixes'' is enough for the change log.
3704 There's no technical need to make change log entries for documentation
3705 files. This is because documentation is not susceptible to bugs that
3706 are hard to fix. Documentation does not consist of parts that must
3707 interact in a precisely engineered fashion. To correct an error, you
3708 need not know the history of the erroneous passage; it is enough to
3709 compare what the documentation says with the way the program actually
3712 However, you should keep change logs for documentation files when the
3713 project gets copyright assignments from its contributors, so as to
3714 make the records of authorship more accurate.
3716 @node Conditional Changes
3717 @subsection Conditional Changes
3718 @cindex conditional changes, and change logs
3719 @cindex change logs, conditional changes
3721 C programs often contain compile-time @code{#if} conditionals. Many
3722 changes are conditional; sometimes you add a new definition which is
3723 entirely contained in a conditional. It is very useful to indicate in
3724 the change log the conditions for which the change applies.
3726 Our convention for indicating conditional changes is to use square
3727 brackets around the name of the condition.
3729 Here is a simple example, describing a change which is conditional but
3730 does not have a function or entity name associated with it:
3733 * xterm.c [SOLARIS2]: Include string.h.
3736 Here is an entry describing a new definition which is entirely
3737 conditional. This new definition for the macro @code{FRAME_WINDOW_P} is
3738 used only when @code{HAVE_X_WINDOWS} is defined:
3741 * frame.h [HAVE_X_WINDOWS] (FRAME_WINDOW_P): Macro defined.
3744 Here is an entry for a change within the function @code{init_display},
3745 whose definition as a whole is unconditional, but the changes themselves
3746 are contained in a @samp{#ifdef HAVE_LIBNCURSES} conditional:
3749 * dispnew.c (init_display) [HAVE_LIBNCURSES]: If X, call tgetent.
3752 Here is an entry for a change that takes affect only when
3753 a certain macro is @emph{not} defined:
3756 (gethostname) [!HAVE_SOCKETS]: Replace with winsock version.
3759 @node Indicating the Part Changed
3760 @subsection Indicating the Part Changed
3762 Indicate the part of a function which changed by using angle brackets
3763 enclosing an indication of what the changed part does. Here is an entry
3764 for a change in the part of the function @code{sh-while-getopts} that
3765 deals with @code{sh} commands:
3768 * progmodes/sh-script.el (sh-while-getopts) <sh>: Handle case that
3769 user-specified option string is empty.
3777 In the GNU project, man pages are secondary. It is not necessary or
3778 expected for every GNU program to have a man page, but some of them do.
3779 It's your choice whether to include a man page in your program.
3781 When you make this decision, consider that supporting a man page
3782 requires continual effort each time the program is changed. The time
3783 you spend on the man page is time taken away from more useful work.
3785 For a simple program which changes little, updating the man page may be
3786 a small job. Then there is little reason not to include a man page, if
3789 For a large program that changes a great deal, updating a man page may
3790 be a substantial burden. If a user offers to donate a man page, you may
3791 find this gift costly to accept. It may be better to refuse the man
3792 page unless the same person agrees to take full responsibility for
3793 maintaining it---so that you can wash your hands of it entirely. If
3794 this volunteer later ceases to do the job, then don't feel obliged to
3795 pick it up yourself; it may be better to withdraw the man page from the
3796 distribution until someone else agrees to update it.
3798 When a program changes only a little, you may feel that the
3799 discrepancies are small enough that the man page remains useful without
3800 updating. If so, put a prominent note near the beginning of the man
3801 page explaining that you don't maintain it and that the Texinfo manual
3802 is more authoritative. The note should say how to access the Texinfo
3805 Be sure that man pages include a copyright statement and free license.
3806 The simple all-permissive license is appropriate for simple man pages
3807 (@pxref{License Notices for Other Files,,,maintain,Information for GNU
3810 For long man pages, with enough explanation and documentation that
3811 they can be considered true manuals, use the GFDL (@pxref{License for
3814 Finally, the GNU help2man program
3815 (@uref{http://www.gnu.org/software/help2man/}) is one way to automate
3816 generation of a man page, in this case from @option{--help} output.
3817 This is sufficient in many cases.
3819 @node Reading other Manuals
3820 @section Reading other Manuals
3822 There may be non-free books or documentation files that describe the
3823 program you are documenting.
3825 It is ok to use these documents for reference, just as the author of a
3826 new algebra textbook can read other books on algebra. A large portion
3827 of any non-fiction book consists of facts, in this case facts about how
3828 a certain program works, and these facts are necessarily the same for
3829 everyone who writes about the subject. But be careful not to copy your
3830 outline structure, wording, tables or examples from preexisting non-free
3831 documentation. Copying from free documentation may be ok; please check
3832 with the FSF about the individual case.
3834 @node Managing Releases
3835 @chapter The Release Process
3838 Making a release is more than just bundling up your source files in a
3839 tar file and putting it up for FTP. You should set up your software so
3840 that it can be configured to run on a variety of systems. Your Makefile
3841 should conform to the GNU standards described below, and your directory
3842 layout should also conform to the standards discussed below. Doing so
3843 makes it easy to include your package into the larger framework of
3847 * Configuration:: How configuration of GNU packages should work.
3848 * Makefile Conventions:: Makefile conventions.
3849 * Releases:: Making releases
3853 @section How Configuration Should Work
3854 @cindex program configuration
3857 Each GNU distribution should come with a shell script named
3858 @code{configure}. This script is given arguments which describe the
3859 kind of machine and system you want to compile the program for.
3860 The @code{configure} script must record the configuration options so
3861 that they affect compilation.
3863 The description here is the specification of the interface for the
3864 @code{configure} script in GNU packages. Many packages implement it
3865 using GNU Autoconf (@pxref{Top,, Introduction, autoconf, Autoconf})
3866 and/or GNU Automake (@pxref{Top,, Introduction, automake, Automake}),
3867 but you do not have to use these tools. You can implement it any way
3868 you like; for instance, by making @code{configure} be a wrapper around
3869 a completely different configuration system.
3871 Another way for the @code{configure} script to operate is to make a
3872 link from a standard name such as @file{config.h} to the proper
3873 configuration file for the chosen system. If you use this technique,
3874 the distribution should @emph{not} contain a file named
3875 @file{config.h}. This is so that people won't be able to build the
3876 program without configuring it first.
3878 Another thing that @code{configure} can do is to edit the Makefile. If
3879 you do this, the distribution should @emph{not} contain a file named
3880 @file{Makefile}. Instead, it should include a file @file{Makefile.in} which
3881 contains the input used for editing. Once again, this is so that people
3882 won't be able to build the program without configuring it first.
3884 If @code{configure} does write the @file{Makefile}, then @file{Makefile}
3885 should have a target named @file{Makefile} which causes @code{configure}
3886 to be rerun, setting up the same configuration that was set up last
3887 time. The files that @code{configure} reads should be listed as
3888 dependencies of @file{Makefile}.
3890 All the files which are output from the @code{configure} script should
3891 have comments at the beginning explaining that they were generated
3892 automatically using @code{configure}. This is so that users won't think
3893 of trying to edit them by hand.
3895 The @code{configure} script should write a file named @file{config.status}
3896 which describes which configuration options were specified when the
3897 program was last configured. This file should be a shell script which,
3898 if run, will recreate the same configuration.
3900 The @code{configure} script should accept an option of the form
3901 @samp{--srcdir=@var{dirname}} to specify the directory where sources are found
3902 (if it is not the current directory). This makes it possible to build
3903 the program in a separate directory, so that the actual source directory
3906 If the user does not specify @samp{--srcdir}, then @code{configure} should
3907 check both @file{.} and @file{..} to see if it can find the sources. If
3908 it finds the sources in one of these places, it should use them from
3909 there. Otherwise, it should report that it cannot find the sources, and
3910 should exit with nonzero status.
3912 Usually the easy way to support @samp{--srcdir} is by editing a
3913 definition of @code{VPATH} into the Makefile. Some rules may need to
3914 refer explicitly to the specified source directory. To make this
3915 possible, @code{configure} can add to the Makefile a variable named
3916 @code{srcdir} whose value is precisely the specified directory.
3918 In addition, the @samp{configure} script should take options
3919 corresponding to most of the standard directory variables
3920 (@pxref{Directory Variables}). Here is the list:
3923 --prefix --exec-prefix --bindir --sbindir --libexecdir --sysconfdir
3924 --sharedstatedir --localstatedir --libdir --includedir --oldincludedir
3925 --datarootdir --datadir --infodir --localedir --mandir --docdir
3926 --htmldir --dvidir --pdfdir --psdir
3929 The @code{configure} script should also take an argument which specifies the
3930 type of system to build the program for. This argument should look like
3934 @var{cpu}-@var{company}-@var{system}
3937 For example, an Athlon-based GNU/Linux system might be
3938 @samp{i686-pc-linux-gnu}.
3940 The @code{configure} script needs to be able to decode all plausible
3941 alternatives for how to describe a machine. Thus,
3942 @samp{athlon-pc-gnu/linux} would be a valid alias. There is a shell
3944 @uref{http://git.savannah.gnu.org/@/gitweb/@/?p=config.git;a=blob_plain;f=config.sub;hb=HEAD,
3945 @file{config.sub}} that you can use as a subroutine to validate system
3946 types and canonicalize aliases.
3948 The @code{configure} script should also take the option
3949 @option{--build=@var{buildtype}}, which should be equivalent to a
3950 plain @var{buildtype} argument. For example, @samp{configure
3951 --build=i686-pc-linux-gnu} is equivalent to @samp{configure
3952 i686-pc-linux-gnu}. When the build type is not specified by an option
3953 or argument, the @code{configure} script should normally guess it using
3955 @uref{http://git.savannah.gnu.org/@/gitweb/@/?p=config.git;a=blob_plain;f=config.guess;hb=HEAD,
3956 @file{config.guess}}.
3958 @cindex optional features, configure-time
3959 Other options are permitted to specify in more detail the software
3960 or hardware present on the machine, to include or exclude optional parts
3961 of the package, or to adjust the name of some tools or arguments to them:
3964 @item --enable-@var{feature}@r{[}=@var{parameter}@r{]}
3965 Configure the package to build and install an optional user-level
3966 facility called @var{feature}. This allows users to choose which
3967 optional features to include. Giving an optional @var{parameter} of
3968 @samp{no} should omit @var{feature}, if it is built by default.
3970 No @samp{--enable} option should @strong{ever} cause one feature to
3971 replace another. No @samp{--enable} option should ever substitute one
3972 useful behavior for another useful behavior. The only proper use for
3973 @samp{--enable} is for questions of whether to build part of the program
3976 @item --with-@var{package}
3977 @c @r{[}=@var{parameter}@r{]}
3978 The package @var{package} will be installed, so configure this package
3979 to work with @var{package}.
3981 @c Giving an optional @var{parameter} of
3982 @c @samp{no} should omit @var{package}, if it is used by default.
3984 Possible values of @var{package} include
3985 @samp{gnu-as} (or @samp{gas}), @samp{gnu-ld}, @samp{gnu-libc},
3991 Do not use a @samp{--with} option to specify the file name to use to
3992 find certain files. That is outside the scope of what @samp{--with}
3995 @item @var{variable}=@var{value}
3996 Set the value of the variable @var{variable} to @var{value}. This is
3997 used to override the default values of commands or arguments in the
3998 build process. For example, the user could issue @samp{configure
3999 CFLAGS=-g CXXFLAGS=-g} to build with debugging information and without
4000 the default optimization.
4002 Specifying variables as arguments to @code{configure}, like this:
4006 is preferable to setting them in environment variables:
4010 as it helps to recreate the same configuration later with
4011 @file{config.status}. However, both methods should be supported.
4014 All @code{configure} scripts should accept all of the ``detail''
4015 options and the variable settings, whether or not they make any
4016 difference to the particular package at hand. In particular, they
4017 should accept any option that starts with @samp{--with-} or
4018 @samp{--enable-}. This is so users will be able to configure an
4019 entire GNU source tree at once with a single set of options.
4021 You will note that the categories @samp{--with-} and @samp{--enable-}
4022 are narrow: they @strong{do not} provide a place for any sort of option
4023 you might think of. That is deliberate. We want to limit the possible
4024 configuration options in GNU software. We do not want GNU programs to
4025 have idiosyncratic configuration options.
4027 Packages that perform part of the compilation process may support
4028 cross-compilation. In such a case, the host and target machines for the
4029 program may be different.
4031 The @code{configure} script should normally treat the specified type of
4032 system as both the host and the target, thus producing a program which
4033 works for the same type of machine that it runs on.
4035 To compile a program to run on a host type that differs from the build
4036 type, use the configure option @option{--host=@var{hosttype}}, where
4037 @var{hosttype} uses the same syntax as @var{buildtype}. The host type
4038 normally defaults to the build type.
4040 To configure a cross-compiler, cross-assembler, or what have you, you
4041 should specify a target different from the host, using the configure
4042 option @samp{--target=@var{targettype}}. The syntax for
4043 @var{targettype} is the same as for the host type. So the command would
4047 ./configure --host=@var{hosttype} --target=@var{targettype}
4050 The target type normally defaults to the host type.
4051 Programs for which cross-operation is not meaningful need not accept the
4052 @samp{--target} option, because configuring an entire operating system for
4053 cross-operation is not a meaningful operation.
4055 Some programs have ways of configuring themselves automatically. If
4056 your program is set up to do this, your @code{configure} script can simply
4057 ignore most of its arguments.
4059 @comment The makefile standards are in a separate file that is also
4060 @comment included by make.texinfo. Done by roland@gnu.ai.mit.edu on 1/6/93.
4061 @comment For this document, turn chapters into sections, etc.
4063 @include make-stds.texi
4067 @section Making Releases
4070 You should identify each release with a pair of version numbers, a
4071 major version and a minor. We have no objection to using more than
4072 two numbers, but it is very unlikely that you really need them.
4074 Package the distribution of @code{Foo version 69.96} up in a gzipped tar
4075 file with the name @file{foo-69.96.tar.gz}. It should unpack into a
4076 subdirectory named @file{foo-69.96}.
4078 Building and installing the program should never modify any of the files
4079 contained in the distribution. This means that all the files that form
4080 part of the program in any way must be classified into @dfn{source
4081 files} and @dfn{non-source files}. Source files are written by humans
4082 and never changed automatically; non-source files are produced from
4083 source files by programs under the control of the Makefile.
4085 @cindex @file{README} file
4086 The distribution should contain a file named @file{README} which gives
4087 the name of the package, and a general description of what it does. It
4088 is also good to explain the purpose of each of the first-level
4089 subdirectories in the package, if there are any. The @file{README} file
4090 should either state the version number of the package, or refer to where
4091 in the package it can be found.
4093 The @file{README} file should refer to the file @file{INSTALL}, which
4094 should contain an explanation of the installation procedure.
4096 The @file{README} file should also refer to the file which contains the
4097 copying conditions. The GNU GPL, if used, should be in a file called
4098 @file{COPYING}. If the GNU LGPL is used, it should be in a file called
4099 @file{COPYING.LESSER}.
4101 Naturally, all the source files must be in the distribution. It is okay
4102 to include non-source files in the distribution, provided they are
4103 up-to-date and machine-independent, so that building the distribution
4104 normally will never modify them. We commonly include non-source files
4105 produced by Bison, @code{lex}, @TeX{}, and @code{makeinfo}; this helps avoid
4106 unnecessary dependencies between our distributions, so that users can
4107 install whichever packages they want to install.
4109 Non-source files that might actually be modified by building and
4110 installing the program should @strong{never} be included in the
4111 distribution. So if you do distribute non-source files, always make
4112 sure they are up to date when you make a new distribution.
4114 Make sure that all the files in the distribution are world-readable, and
4115 that directories are world-readable and world-searchable (octal mode 755).
4116 We used to recommend that all directories in the distribution also be
4117 world-writable (octal mode 777), because ancient versions of @code{tar}
4118 would otherwise not cope when extracting the archive as an unprivileged
4119 user. That can easily lead to security issues when creating the archive,
4120 however, so now we recommend against that.
4122 Don't include any symbolic links in the distribution itself. If the tar
4123 file contains symbolic links, then people cannot even unpack it on
4124 systems that don't support symbolic links. Also, don't use multiple
4125 names for one file in different directories, because certain file
4126 systems cannot handle this and that prevents unpacking the
4129 Try to make sure that all the file names will be unique on MS-DOS. A
4130 name on MS-DOS consists of up to 8 characters, optionally followed by a
4131 period and up to three characters. MS-DOS will truncate extra
4132 characters both before and after the period. Thus,
4133 @file{foobarhacker.c} and @file{foobarhacker.o} are not ambiguous; they
4134 are truncated to @file{foobarha.c} and @file{foobarha.o}, which are
4137 @cindex @file{texinfo.tex}, in a distribution
4138 Include in your distribution a copy of the @file{texinfo.tex} you used
4139 to test print any @file{*.texinfo} or @file{*.texi} files.
4141 Likewise, if your program uses small GNU software packages like regex,
4142 getopt, obstack, or termcap, include them in the distribution file.
4143 Leaving them out would make the distribution file a little smaller at
4144 the expense of possible inconvenience to a user who doesn't know what
4148 @chapter References to Non-Free Software and Documentation
4149 @cindex references to non-free material
4151 A GNU program should not recommend, promote, or grant legitimacy to
4152 the use of any non-free program. Proprietary software is a social and
4153 ethical problem, and our aim is to put an end to that problem. We
4154 can't stop some people from writing proprietary programs, or stop
4155 other people from using them, but we can and should refuse to
4156 advertise them to new potential customers, or to give the public the
4157 idea that their existence is ethical.
4159 The GNU definition of free software is found on the GNU web site at
4160 @url{http://www.gnu.org/@/philosophy/@/free-sw.html}, and the definition
4161 of free documentation is found at
4162 @url{http://www.gnu.org/@/philosophy/@/free-doc.html}. The terms ``free''
4163 and ``non-free'', used in this document, refer to those definitions.
4165 A list of important licenses and whether they qualify as free is in
4166 @url{http://www.gnu.org/@/licenses/@/license-list.html}. If it is not
4167 clear whether a license qualifies as free, please ask the GNU Project
4168 by writing to @email{licensing@@gnu.org}. We will answer, and if the
4169 license is an important one, we will add it to the list.
4171 When a non-free program or system is well known, you can mention it in
4172 passing---that is harmless, since users who might want to use it
4173 probably already know about it. For instance, it is fine to explain
4174 how to build your package on top of some widely used non-free
4175 operating system, or how to use it together with some widely used
4178 However, you should give only the necessary information to help those
4179 who already use the non-free program to use your program with
4180 it---don't give, or refer to, any further information about the
4181 proprietary program, and don't imply that the proprietary program
4182 enhances your program, or that its existence is in any way a good
4183 thing. The goal should be that people already using the proprietary
4184 program will get the advice they need about how to use your free
4185 program with it, while people who don't already use the proprietary
4186 program will not see anything likely to lead them to take an interest
4189 If a non-free program or system is obscure in your program's domain,
4190 your program should not mention or support it at all, since doing so
4191 would tend to popularize the non-free program more than it popularizes
4192 your program. (You cannot hope to find many additional users for your
4193 program among the users of Foobar, if the existence of Foobar is not
4194 generally known among people who might want to use your program.)
4196 Sometimes a program is free software in itself but depends on a
4197 non-free platform in order to run. For instance, many Java programs
4198 depend on some non-free Java libraries. To recommend or promote such
4199 a program is to promote the other programs it needs. This is why we
4200 are careful about listing Java programs in the Free Software
4201 Directory: we don't want to promote the non-free Java libraries.
4203 We hope this particular problem with Java will be gone by and by, as
4204 we replace the remaining non-free standard Java libraries with free
4205 software, but the general principle will remain the same: don't
4206 recommend, promote or legitimize programs that depend on non-free
4209 Some free programs strongly encourage the use of non-free software. A
4210 typical example is @command{mplayer}. It is free software in itself,
4211 and the free code can handle some kinds of files. However,
4212 @command{mplayer} recommends use of non-free codecs for other kinds of
4213 files, and users that install @command{mplayer} are very likely to
4214 install those codecs along with it. To recommend @command{mplayer}
4215 is, in effect, to promote use of the non-free codecs.
4217 Thus, you should not recommend programs that strongly encourage the
4218 use of non-free software. This is why we do not list
4219 @command{mplayer} in the Free Software Directory.
4221 A GNU package should not refer the user to any non-free documentation
4222 for free software. Free documentation that can be included in free
4223 operating systems is essential for completing the GNU system, or any
4224 free operating system, so encouraging it is a priority; to recommend
4225 use of documentation that we are not allowed to include undermines the
4226 impetus for the community to produce documentation that we can
4227 include. So GNU packages should never recommend non-free
4230 By contrast, it is ok to refer to journal articles and textbooks in
4231 the comments of a program for explanation of how it functions, even
4232 though they are non-free. This is because we don't include such
4233 things in the GNU system even if they are free---they are outside the
4234 scope of what a software distribution needs to include.
4236 Referring to a web site that describes or recommends a non-free
4237 program is promoting that program, so please do not make links (or
4238 mention by name) web sites that contain such material. This policy is
4239 relevant particularly for the web pages for a GNU package.
4241 Following links from nearly any web site can lead eventually to
4242 non-free software; this is inherent in the nature of the web. So it
4243 makes no sense to criticize a site for having such links. As long as
4244 the site does not itself recommend a non-free program, there is no
4245 need to consider the question of the sites that it links to for other
4248 Thus, for example, you should not refer to AT&T's web site if that
4249 recommends AT&T's non-free software packages; you should not refer to
4250 a site that links to AT&T's site presenting it as a place to get some
4251 non-free program, because that link recommends and legitimizes the
4252 non-free program. However, that a site contains a link to AT&T's web
4253 site for some other purpose (such as long-distance telephone service)
4254 is not an objection against it.
4256 @node GNU Free Documentation License
4257 @appendix GNU Free Documentation License
4259 @cindex FDL, GNU Free Documentation License
4269 eval: (add-hook 'write-file-hooks 'time-stamp)
4270 time-stamp-start: "@set lastupdate "
4272 time-stamp-format: "%:b %:d, %:y"
4273 compile-command: "cd work.s && make"