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 July 20, 2013
9 @dircategory GNU organization
11 * Standards: (standards). GNU coding standards.
14 @c @setchapternewpage odd
15 @setchapternewpage off
17 @c Put everything in one index (arbitrarily chosen to be the concept index).
23 @c This is used by a cross ref in make-stds.texi
27 The GNU coding standards, last updated @value{lastupdate}.
29 Copyright @copyright{} 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999,
30 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010,
31 2011, 2012, 2013 Free Software Foundation, Inc.
33 Permission is granted to copy, distribute and/or modify this document
34 under the terms of the GNU Free Documentation License, Version 1.3 or
35 any later version published by the Free Software Foundation; with no
36 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
37 Texts. A copy of the license is included in the section entitled
38 ``GNU Free Documentation License''.
42 @title GNU Coding Standards
43 @author Richard Stallman, et al.
44 @author last updated @value{lastupdate}
46 @vskip 0pt plus 1filll
54 @top GNU Coding Standards
60 * Preface:: About the GNU Coding Standards.
61 * Legal Issues:: Keeping free software free.
62 * Design Advice:: General program design.
63 * Program Behavior:: Program behavior for all programs
64 * Writing C:: Making the best use of C.
65 * Documentation:: Documenting programs.
66 * Managing Releases:: The release process.
67 * References:: Mentioning non-free software or documentation.
68 * GNU Free Documentation License:: Copying and sharing this manual.
74 @chapter About the GNU Coding Standards
76 The GNU Coding Standards were written by Richard Stallman and other GNU
77 Project volunteers. Their purpose is to make the GNU system clean,
78 consistent, and easy to install. This document can also be read as a
79 guide to writing portable, robust and reliable programs. It focuses on
80 programs written in C, but many of the rules and principles are useful
81 even if you write in another programming language. The rules often
82 state reasons for writing in a certain way.
84 @cindex where to obtain @code{standards.texi}
85 @cindex downloading this manual
86 If you did not obtain this file directly from the GNU project and
87 recently, please check for a newer version. You can get the GNU
88 Coding Standards from the GNU web server in many
89 different formats, including the Texinfo source, PDF, HTML, DVI, plain
90 text, and more, at: @uref{http://www.gnu.org/prep/standards/}.
92 If you are maintaining an official GNU package, in addition to this
93 document, please read and follow the GNU maintainer information
94 (@pxref{Top, , Contents, maintain, Information for Maintainers of GNU
97 @cindex @code{gnustandards-commit@@gnu.org} mailing list
98 If you want to receive diffs for every change to these GNU documents,
99 join the mailing list @code{gnustandards-commit@@gnu.org}, via the web
101 @url{http://lists.gnu.org/mailman/listinfo/gnustandards-commit}.
102 Archives are also available there.
104 @cindex @code{bug-standards@@gnu.org} email address
105 @cindex Savannah repository for gnustandards
106 @cindex gnustandards project repository
107 Please send corrections or suggestions for this document to
108 @email{bug-standards@@gnu.org}. If you make a suggestion, please
109 include a suggested new wording for it, to help us consider the
110 suggestion efficiently. We prefer a context diff to the Texinfo
111 source, but if that's difficult for you, you can make a context diff
112 for some other version of this document, or propose it in any way that
113 makes it clear. The source repository for this document can be found
114 at @url{http://savannah.gnu.org/projects/gnustandards}.
116 These standards cover the minimum of what is important when writing a
117 GNU package. Likely, the need for additional standards will come up.
118 Sometimes, you might suggest that such standards be added to this
119 document. If you think your standards would be generally useful, please
122 You should also set standards for your package on many questions not
123 addressed or not firmly specified here. The most important point is to
124 be self-consistent---try to stick to the conventions you pick, and try
125 to document them as much as possible. That way, your program will be
126 more maintainable by others.
128 The GNU Hello program serves as an example of how to follow the GNU
129 coding standards for a trivial program.
130 @uref{http://www.gnu.org/software/hello/hello.html}.
132 This release of the GNU Coding Standards was last updated
137 @chapter Keeping Free Software Free
138 @cindex legal aspects
140 This chapter discusses how you can make sure that GNU software
141 avoids legal difficulties, and other related issues.
144 * Reading Non-Free Code:: Referring to proprietary programs.
145 * Contributions:: Accepting contributions.
146 * Trademarks:: How we deal with trademark issues.
149 @node Reading Non-Free Code
150 @section Referring to Proprietary Programs
151 @cindex proprietary programs
152 @cindex avoiding proprietary code
154 Don't in any circumstances refer to Unix source code for or during
155 your work on GNU! (Or to any other proprietary programs.)
157 If you have a vague recollection of the internals of a Unix program,
158 this does not absolutely mean you can't write an imitation of it, but
159 do try to organize the imitation internally along different lines,
160 because this is likely to make the details of the Unix version
161 irrelevant and dissimilar to your results.
163 For example, Unix utilities were generally optimized to minimize
164 memory use; if you go for speed instead, your program will be very
165 different. You could keep the entire input file in memory and scan it
166 there instead of using stdio. Use a smarter algorithm discovered more
167 recently than the Unix program. Eliminate use of temporary files. Do
168 it in one pass instead of two (we did this in the assembler).
170 Or, on the contrary, emphasize simplicity instead of speed. For some
171 applications, the speed of today's computers makes simpler algorithms
174 Or go for generality. For example, Unix programs often have static
175 tables or fixed-size strings, which make for arbitrary limits; use
176 dynamic allocation instead. Make sure your program handles NULs and
177 other funny characters in the input files. Add a programming language
178 for extensibility and write part of the program in that language.
180 Or turn some parts of the program into independently usable libraries.
181 Or use a simple garbage collector instead of tracking precisely when
182 to free memory, or use a new GNU facility such as obstacks.
186 @section Accepting Contributions
188 @cindex accepting contributions
190 If the program you are working on is copyrighted by the Free Software
191 Foundation, then when someone else sends you a piece of code to add to
192 the program, we need legal papers to use it---just as we asked you to
193 sign papers initially. @emph{Each} person who makes a nontrivial
194 contribution to a program must sign some sort of legal papers in order
195 for us to have clear title to the program; the main author alone is not
198 So, before adding in any contributions from other people, please tell
199 us, so we can arrange to get the papers. Then wait until we tell you
200 that we have received the signed papers, before you actually use the
203 This applies both before you release the program and afterward. If
204 you receive diffs to fix a bug, and they make significant changes, we
205 need legal papers for that change.
207 This also applies to comments and documentation files. For copyright
208 law, comments and code are just text. Copyright applies to all kinds of
209 text, so we need legal papers for all kinds.
211 We know it is frustrating to ask for legal papers; it's frustrating for
212 us as well. But if you don't wait, you are going out on a limb---for
213 example, what if the contributor's employer won't sign a disclaimer?
214 You might have to take that code out again!
216 You don't need papers for changes of a few lines here or there, since
217 they are not significant for copyright purposes. Also, you don't need
218 papers if all you get from the suggestion is some ideas, not actual code
219 which you use. For example, if someone sent you one implementation, but
220 you write a different implementation of the same idea, you don't need to
223 The very worst thing is if you forget to tell us about the other
224 contributor. We could be very embarrassed in court some day as a
227 We have more detailed advice for maintainers of GNU packages. If you
228 have reached the stage of maintaining a GNU program (whether released
229 or not), please take a look: @pxref{Legal Matters,,, maintain,
230 Information for GNU Maintainers}.
237 Please do not include any trademark acknowledgements in GNU software
238 packages or documentation.
240 Trademark acknowledgements are the statements that such-and-such is a
241 trademark of so-and-so. The GNU Project has no objection to the basic
242 idea of trademarks, but these acknowledgements feel like kowtowing,
243 and there is no legal requirement for them, so we don't use them.
245 What is legally required, as regards other people's trademarks, is to
246 avoid using them in ways which a reader might reasonably understand as
247 naming or labeling our own programs or activities. For example, since
248 ``Objective C'' is (or at least was) a trademark, we made sure to say
249 that we provide a ``compiler for the Objective C language'' rather
250 than an ``Objective C compiler''. The latter would have been meant as
251 a shorter way of saying the former, but it does not explicitly state
252 the relationship, so it could be misinterpreted as using ``Objective
253 C'' as a label for the compiler rather than for the language.
255 Please don't use ``win'' as an abbreviation for Microsoft Windows in
256 GNU software or documentation. In hacker terminology, calling
257 something a ``win'' is a form of praise. If you wish to praise
258 Microsoft Windows when speaking on your own, by all means do so, but
259 not in GNU software. Usually we write the name ``Windows'' in full,
260 but when brevity is very important (as in file names and sometimes
261 symbol names), we abbreviate it to ``w''. For instance, the files and
262 functions in Emacs that deal with Windows start with @samp{w32}.
265 @chapter General Program Design
266 @cindex program design
268 This chapter discusses some of the issues you should take into
269 account when designing your program.
271 @c Standard or ANSI C
273 @c In 1989 the American National Standards Institute (ANSI) standardized
274 @c C as standard X3.159-1989. In December of that year the
275 @c International Standards Organization ISO adopted the ANSI C standard
276 @c making minor changes. In 1990 ANSI then re-adopted ISO standard
277 @c C. This version of C is known as either ANSI C or Standard C.
279 @c A major revision of the C Standard appeared in 1999.
282 * Source Language:: Which languages to use.
283 * Compatibility:: Compatibility with other implementations.
284 * Using Extensions:: Using non-standard features.
285 * Standard C:: Using standard C features.
286 * Conditional Compilation:: Compiling code only if a conditional is true.
289 @node Source Language
290 @section Which Languages to Use
291 @cindex programming languages
293 When you want to use a language that gets compiled and runs at high
294 speed, the best language to use is C. C++ is ok too, but please don't
295 make heavy use of templates. So is Java, if you compile it.
297 When highest efficiency is not required, other languages commonly used
298 in the free software community, such as Python and Ruby, are ok too.
299 We encourage use of Lisp and Scheme as well. Please implement the GNU
300 configure and make interface no matter which language you use.
302 Many programs are designed to be extensible: they include an interpreter
303 for a language that is higher level than C. Often much of the program
304 is written in that language, too. The Emacs editor pioneered this
308 @cindex GNOME and Guile
309 The standard extensibility interpreter for GNU software is Guile
310 (@uref{http://www.gnu.org/@/software/@/guile/}), which implements the
311 language Scheme (an especially clean and simple dialect of Lisp).
312 Guile also includes bindings for GTK+/GNOME, making it practical to
313 write modern GUI functionality within Guile. We don't reject programs
314 written in other ``scripting languages'' such as Perl and Python, but
315 using Guile is the path that will lead to overall consistency of the
320 @section Compatibility with Other Implementations
321 @cindex compatibility with C and POSIX standards
322 @cindex C compatibility
323 @cindex POSIX compatibility
325 With occasional exceptions, utility programs and libraries for GNU
326 should be upward compatible with those in Berkeley Unix, and upward
327 compatible with Standard C if Standard C specifies their
328 behavior, and upward compatible with POSIX if POSIX specifies
331 When these standards conflict, it is useful to offer compatibility
332 modes for each of them.
334 @cindex options for compatibility
335 Standard C and POSIX prohibit many kinds of extensions. Feel
336 free to make the extensions anyway, and include a @samp{--ansi},
337 @samp{--posix}, or @samp{--compatible} option to turn them off.
338 However, if the extension has a significant chance of breaking any real
339 programs or scripts, then it is not really upward compatible. So you
340 should try to redesign its interface to make it upward compatible.
342 @cindex @code{POSIXLY_CORRECT}, environment variable
343 Many GNU programs suppress extensions that conflict with POSIX if the
344 environment variable @code{POSIXLY_CORRECT} is defined (even if it is
345 defined with a null value). Please make your program recognize this
346 variable if appropriate.
348 When a feature is used only by users (not by programs or command
349 files), and it is done poorly in Unix, feel free to replace it
350 completely with something totally different and better. (For example,
351 @code{vi} is replaced with Emacs.) But it is nice to offer a compatible
352 feature as well. (There is a free @code{vi} clone, so we offer it.)
354 Additional useful features are welcome regardless of whether
355 there is any precedent for them.
357 @node Using Extensions
358 @section Using Non-standard Features
359 @cindex non-standard extensions
361 Many GNU facilities that already exist support a number of convenient
362 extensions over the comparable Unix facilities. Whether to use these
363 extensions in implementing your program is a difficult question.
365 On the one hand, using the extensions can make a cleaner program.
366 On the other hand, people will not be able to build the program
367 unless the other GNU tools are available. This might cause the
368 program to work on fewer kinds of machines.
370 With some extensions, it might be easy to provide both alternatives.
371 For example, you can define functions with a ``keyword'' @code{INLINE}
372 and define that as a macro to expand into either @code{inline} or
373 nothing, depending on the compiler.
375 In general, perhaps it is best not to use the extensions if you can
376 straightforwardly do without them, but to use the extensions if they
377 are a big improvement.
379 An exception to this rule are the large, established programs (such as
380 Emacs) which run on a great variety of systems. Using GNU extensions in
381 such programs would make many users unhappy, so we don't do that.
383 Another exception is for programs that are used as part of compilation:
384 anything that must be compiled with other compilers in order to
385 bootstrap the GNU compilation facilities. If these require the GNU
386 compiler, then no one can compile them without having them installed
387 already. That would be extremely troublesome in certain cases.
390 @section Standard C and Pre-Standard C
391 @cindex ANSI C standard
393 1989 Standard C is widespread enough now that it is ok to use its
394 features in programs. There is one exception: do not ever use the
395 ``trigraph'' feature of Standard C.
397 The 1999 and 2011 editions of Standard C are not fully supported
398 on all platforms. If you aim to support compilation by
399 compilers other than GCC, you should not require these C
400 features in your programs. It is ok to use these features
401 conditionally when the compiler supports them.
403 If your program is only meant to compile with GCC, then you can
404 use these features if GCC supports them, when they give substantial
407 However, it is easy to support pre-standard compilers in most programs,
408 so if you know how to do that, feel free.
410 @cindex function prototypes
411 To support pre-standard C, instead of writing function definitions in
412 standard prototype form,
421 write the definition in pre-standard style like this,
431 and use a separate declaration to specify the argument prototype:
437 You need such a declaration anyway, in a header file, to get the benefit
438 of prototypes in all the files where the function is called. And once
439 you have the declaration, you normally lose nothing by writing the
440 function definition in the pre-standard style.
442 This technique does not work for integer types narrower than @code{int}.
443 If you think of an argument as being of a type narrower than @code{int},
444 declare it as @code{int} instead.
446 There are a few special cases where this technique is hard to use. For
447 example, if a function argument needs to hold the system type
448 @code{dev_t}, you run into trouble, because @code{dev_t} is shorter than
449 @code{int} on some machines; but you cannot use @code{int} instead,
450 because @code{dev_t} is wider than @code{int} on some machines. There
451 is no type you can safely use on all machines in a non-standard
452 definition. The only way to support non-standard C and pass such an
453 argument is to check the width of @code{dev_t} using Autoconf and choose
454 the argument type accordingly. This may not be worth the trouble.
456 In order to support pre-standard compilers that do not recognize
457 prototypes, you may want to use a preprocessor macro like this:
460 /* Declare the prototype for a general external function. */
461 #if defined (__STDC__) || defined (WINDOWSNT)
462 #define P_(proto) proto
468 @node Conditional Compilation
469 @section Conditional Compilation
471 When supporting configuration options already known when building your
472 program we prefer using @code{if (... )} over conditional compilation,
473 as in the former case the compiler is able to perform more extensive
474 checking of all possible code paths.
476 For example, please write
496 A modern compiler such as GCC will generate exactly the same code in
497 both cases, and we have been using similar techniques with good success
498 in several projects. Of course, the former method assumes that
499 @code{HAS_FOO} is defined as either 0 or 1.
501 While this is not a silver bullet solving all portability problems,
502 and is not always appropriate, following this policy would have saved
503 GCC developers many hours, or even days, per year.
505 In the case of function-like macros like @code{REVERSIBLE_CC_MODE} in
506 GCC which cannot be simply used in @code{if (...)} statements, there is
507 an easy workaround. Simply introduce another macro
508 @code{HAS_REVERSIBLE_CC_MODE} as in the following example:
511 #ifdef REVERSIBLE_CC_MODE
512 #define HAS_REVERSIBLE_CC_MODE 1
514 #define HAS_REVERSIBLE_CC_MODE 0
518 @node Program Behavior
519 @chapter Program Behavior for All Programs
521 This chapter describes conventions for writing robust
522 software. It also describes general standards for error messages, the
523 command line interface, and how libraries should behave.
526 * Non-GNU Standards:: We consider standards such as POSIX;
527 we don't "obey" them.
528 * Semantics:: Writing robust programs.
529 * Libraries:: Library behavior.
530 * Errors:: Formatting error messages.
531 * User Interfaces:: Standards about interfaces generally.
532 * Graphical Interfaces:: Standards for graphical interfaces.
533 * Command-Line Interfaces:: Standards for command line interfaces.
534 * Dynamic Plug-In Interfaces:: Standards for dynamic plug-in interfaces.
535 * Option Table:: Table of long options.
536 * OID Allocations:: Table of OID slots for GNU.
537 * Memory Usage:: When and how to care about memory needs.
538 * File Usage:: Which files to use, and where.
541 @node Non-GNU Standards
542 @section Non-GNU Standards
544 The GNU Project regards standards published by other organizations as
545 suggestions, not orders. We consider those standards, but we do not
546 ``obey'' them. In developing a GNU program, you should implement
547 an outside standard's specifications when that makes the GNU system
548 better overall in an objective sense. When it doesn't, you shouldn't.
550 In most cases, following published standards is convenient for
551 users---it means that their programs or scripts will work more
552 portably. For instance, GCC implements nearly all the features of
553 Standard C as specified by that standard. C program developers would
554 be unhappy if it did not. And GNU utilities mostly follow
555 specifications of POSIX.2; shell script writers and users would be
556 unhappy if our programs were incompatible.
558 But we do not follow either of these specifications rigidly, and there
559 are specific points on which we decided not to follow them, so as to
560 make the GNU system better for users.
562 For instance, Standard C says that nearly all extensions to C are
563 prohibited. How silly! GCC implements many extensions, some of which
564 were later adopted as part of the standard. If you want these
565 constructs to give an error message as ``required'' by the standard,
566 you must specify @samp{--pedantic}, which was implemented only so that
567 we can say ``GCC is a 100% implementation of the standard'', not
568 because there is any reason to actually use it.
570 POSIX.2 specifies that @samp{df} and @samp{du} must output sizes by
571 default in units of 512 bytes. What users want is units of 1k, so
572 that is what we do by default. If you want the ridiculous behavior
573 ``required'' by POSIX, you must set the environment variable
574 @samp{POSIXLY_CORRECT} (which was originally going to be named
575 @samp{POSIX_ME_HARDER}).
577 GNU utilities also depart from the letter of the POSIX.2 specification
578 when they support long-named command-line options, and intermixing
579 options with ordinary arguments. This minor incompatibility with
580 POSIX is never a problem in practice, and it is very useful.
582 In particular, don't reject a new feature, or remove an old one,
583 merely because a standard says it is ``forbidden'' or ``deprecated''.
587 @section Writing Robust Programs
589 @cindex arbitrary limits on data
590 Avoid arbitrary limits on the length or number of @emph{any} data
591 structure, including file names, lines, files, and symbols, by allocating
592 all data structures dynamically. In most Unix utilities, ``long lines
593 are silently truncated''. This is not acceptable in a GNU utility.
595 @cindex @code{NUL} characters
597 Utilities reading files should not drop NUL characters, or any other
598 nonprinting characters @emph{including those with codes above 0177}.
599 The only sensible exceptions would be utilities specifically intended
600 for interface to certain types of terminals or printers that can't
601 handle those characters. Whenever possible, try to make programs work
602 properly with sequences of bytes that represent multibyte characters;
603 UTF-8 is the most important.
605 @cindex error messages
606 Check every system call for an error return, unless you know you wish
607 to ignore errors. Include the system error text (from @code{perror},
608 @code{strerror}, or equivalent) in @emph{every} error message
609 resulting from a failing system call, as well as the name of the file
610 if any and the name of the utility. Just ``cannot open foo.c'' or
611 ``stat failed'' is not sufficient.
613 @cindex @code{malloc} return value
614 @cindex memory allocation failure
615 Check every call to @code{malloc} or @code{realloc} to see if it
616 returned zero. Check @code{realloc} even if you are making the block
617 smaller; in a system that rounds block sizes to a power of 2,
618 @code{realloc} may get a different block if you ask for less space.
620 In Unix, @code{realloc} can destroy the storage block if it returns
621 zero. GNU @code{realloc} does not have this bug: if it fails, the
622 original block is unchanged. Feel free to assume the bug is fixed. If
623 you wish to run your program on Unix, and wish to avoid lossage in this
624 case, you can use the GNU @code{malloc}.
626 You must expect @code{free} to alter the contents of the block that was
627 freed. Anything you want to fetch from the block, you must fetch before
630 If @code{malloc} fails in a noninteractive program, make that a fatal
631 error. In an interactive program (one that reads commands from the
632 user), it is better to abort the command and return to the command
633 reader loop. This allows the user to kill other processes to free up
634 virtual memory, and then try the command again.
636 @cindex command-line arguments, decoding
637 Use @code{getopt_long} to decode arguments, unless the argument syntax
638 makes this unreasonable.
640 When static storage is to be written in during program execution, use
641 explicit C code to initialize it. Reserve C initialized declarations
642 for data that will not be changed.
645 Try to avoid low-level interfaces to obscure Unix data structures (such
646 as file directories, utmp, or the layout of kernel memory), since these
647 are less likely to work compatibly. If you need to find all the files
648 in a directory, use @code{readdir} or some other high-level interface.
649 These are supported compatibly by GNU.
651 @cindex signal handling
652 The preferred signal handling facilities are the BSD variant of
653 @code{signal}, and the POSIX @code{sigaction} function; the
654 alternative USG @code{signal} interface is an inferior design.
656 Nowadays, using the POSIX signal functions may be the easiest way
657 to make a program portable. If you use @code{signal}, then on GNU/Linux
658 systems running GNU libc version 1, you should include
659 @file{bsd/signal.h} instead of @file{signal.h}, so as to get BSD
660 behavior. It is up to you whether to support systems where
661 @code{signal} has only the USG behavior, or give up on them.
663 @cindex impossible conditions
664 In error checks that detect ``impossible'' conditions, just abort.
665 There is usually no point in printing any message. These checks
666 indicate the existence of bugs. Whoever wants to fix the bugs will have
667 to read the source code and run a debugger. So explain the problem with
668 comments in the source. The relevant data will be in variables, which
669 are easy to examine with the debugger, so there is no point moving them
672 Do not use a count of errors as the exit status for a program.
673 @emph{That does not work}, because exit status values are limited to 8
674 bits (0 through 255). A single run of the program might have 256
675 errors; if you try to return 256 as the exit status, the parent process
676 will see 0 as the status, and it will appear that the program succeeded.
678 @cindex temporary files
679 @cindex @code{TMPDIR} environment variable
680 If you make temporary files, check the @code{TMPDIR} environment
681 variable; if that variable is defined, use the specified directory
682 instead of @file{/tmp}.
684 In addition, be aware that there is a possible security problem when
685 creating temporary files in world-writable directories. In C, you can
686 avoid this problem by creating temporary files in this manner:
689 fd = open (filename, O_WRONLY | O_CREAT | O_EXCL, 0600);
693 or by using the @code{mkstemps} function from Gnulib
694 (@pxref{mkstemps,,, gnulib, Gnulib}).
696 In bash, use @code{set -C} (long name @code{noclobber}) to avoid this
697 problem. In addition, the @code{mktemp} utility is a more general
698 solution for creating temporary files from shell scripts
699 (@pxref{mktemp invocation,,, coreutils, GNU Coreutils}).
703 @section Library Behavior
706 Try to make library functions reentrant. If they need to do dynamic
707 storage allocation, at least try to avoid any nonreentrancy aside from
708 that of @code{malloc} itself.
710 Here are certain name conventions for libraries, to avoid name
713 Choose a name prefix for the library, more than two characters long.
714 All external function and variable names should start with this
715 prefix. In addition, there should only be one of these in any given
716 library member. This usually means putting each one in a separate
719 An exception can be made when two external symbols are always used
720 together, so that no reasonable program could use one without the
721 other; then they can both go in the same file.
723 External symbols that are not documented entry points for the user
724 should have names beginning with @samp{_}. The @samp{_} should be
725 followed by the chosen name prefix for the library, to prevent
726 collisions with other libraries. These can go in the same files with
727 user entry points if you like.
729 Static functions and variables can be used as you like and need not
730 fit any naming convention.
733 @section Formatting Error Messages
734 @cindex formatting error messages
735 @cindex error messages, formatting
737 Error messages from compilers should look like this:
740 @var{sourcefile}:@var{lineno}: @var{message}
744 If you want to mention the column number, use one of these formats:
747 @var{sourcefile}:@var{lineno}:@var{column}: @var{message}
748 @var{sourcefile}:@var{lineno}.@var{column}: @var{message}
753 Line numbers should start from 1 at the beginning of the file, and
754 column numbers should start from 1 at the beginning of the line.
755 (Both of these conventions are chosen for compatibility.) Calculate
756 column numbers assuming that space and all ASCII printing characters
757 have equal width, and assuming tab stops every 8 columns. For
758 non-ASCII characters, Unicode character widths should be used when in
759 a UTF-8 locale; GNU libc and GNU gnulib provide suitable
760 @code{wcwidth} functions.
762 The error message can also give both the starting and ending positions
763 of the erroneous text. There are several formats so that you can
764 avoid redundant information such as a duplicate line number.
765 Here are the possible formats:
768 @var{sourcefile}:@var{line1}.@var{column1}-@var{line2}.@var{column2}: @var{message}
769 @var{sourcefile}:@var{line1}.@var{column1}-@var{column2}: @var{message}
770 @var{sourcefile}:@var{line1}-@var{line2}: @var{message}
774 When an error is spread over several files, you can use this format:
777 @var{file1}:@var{line1}.@var{column1}-@var{file2}:@var{line2}.@var{column2}: @var{message}
780 Error messages from other noninteractive programs should look like this:
783 @var{program}:@var{sourcefile}:@var{lineno}: @var{message}
787 when there is an appropriate source file, or like this:
790 @var{program}: @var{message}
794 when there is no relevant source file.
796 If you want to mention the column number, use this format:
799 @var{program}:@var{sourcefile}:@var{lineno}:@var{column}: @var{message}
802 In an interactive program (one that is reading commands from a
803 terminal), it is better not to include the program name in an error
804 message. The place to indicate which program is running is in the
805 prompt or with the screen layout. (When the same program runs with
806 input from a source other than a terminal, it is not interactive and
807 would do best to print error messages using the noninteractive style.)
809 The string @var{message} should not begin with a capital letter when
810 it follows a program name and/or file name, because that isn't the
811 beginning of a sentence. (The sentence conceptually starts at the
812 beginning of the line.) Also, it should not end with a period.
814 Error messages from interactive programs, and other messages such as
815 usage messages, should start with a capital letter. But they should not
818 @node User Interfaces
819 @section Standards for Interfaces Generally
821 @cindex program name and its behavior
822 @cindex behavior, dependent on program's name
823 Please don't make the behavior of a utility depend on the name used
824 to invoke it. It is useful sometimes to make a link to a utility
825 with a different name, and that should not change what it does.
827 Instead, use a run time option or a compilation switch or both
828 to select among the alternate behaviors.
830 @cindex output device and program's behavior
831 Likewise, please don't make the behavior of the program depend on the
832 type of output device it is used with. Device independence is an
833 important principle of the system's design; do not compromise it merely
834 to save someone from typing an option now and then. (Variation in error
835 message syntax when using a terminal is ok, because that is a side issue
836 that people do not depend on.)
838 If you think one behavior is most useful when the output is to a
839 terminal, and another is most useful when the output is a file or a
840 pipe, then it is usually best to make the default behavior the one that
841 is useful with output to a terminal, and have an option for the other
844 Compatibility requires certain programs to depend on the type of output
845 device. It would be disastrous if @code{ls} or @code{sh} did not do so
846 in the way all users expect. In some of these cases, we supplement the
847 program with a preferred alternate version that does not depend on the
848 output device type. For example, we provide a @code{dir} program much
849 like @code{ls} except that its default output format is always
853 @node Graphical Interfaces
854 @section Standards for Graphical Interfaces
855 @cindex graphical user interface
856 @cindex interface styles
857 @cindex user interface styles
860 When you write a program that provides a graphical user interface,
861 please make it work with the X Window System and the GTK+ toolkit
862 unless the functionality specifically requires some alternative (for
863 example, ``displaying jpeg images while in console mode'').
865 In addition, please provide a command-line interface to control the
866 functionality. (In many cases, the graphical user interface can be a
867 separate program which invokes the command-line program.) This is
868 so that the same jobs can be done from scripts.
873 @cindex keyboard interface
874 @cindex library interface
875 Please also consider providing a D-bus interface for use from other
876 running programs, such as within GNOME. (GNOME used to use CORBA
877 for this, but that is being phased out.) In addition, consider
878 providing a library interface (for use from C), and perhaps a
879 keyboard-driven console interface (for use by users from console
880 mode). Once you are doing the work to provide the functionality and
881 the graphical interface, these won't be much extra work.
883 @node Command-Line Interfaces
884 @section Standards for Command Line Interfaces
885 @cindex command-line interface
888 It is a good idea to follow the POSIX guidelines for the
889 command-line options of a program. The easiest way to do this is to use
890 @code{getopt} to parse them. Note that the GNU version of @code{getopt}
891 will normally permit options anywhere among the arguments unless the
892 special argument @samp{--} is used. This is not what POSIX
893 specifies; it is a GNU extension.
895 @cindex long-named options
896 Please define long-named options that are equivalent to the
897 single-letter Unix-style options. We hope to make GNU more user
898 friendly this way. This is easy to do with the GNU function
901 One of the advantages of long-named options is that they can be
902 consistent from program to program. For example, users should be able
903 to expect the ``verbose'' option of any GNU program which has one, to be
904 spelled precisely @samp{--verbose}. To achieve this uniformity, look at
905 the table of common long-option names when you choose the option names
906 for your program (@pxref{Option Table}).
908 It is usually a good idea for file names given as ordinary arguments to
909 be input files only; any output files would be specified using options
910 (preferably @samp{-o} or @samp{--output}). Even if you allow an output
911 file name as an ordinary argument for compatibility, try to provide an
912 option as another way to specify it. This will lead to more consistency
913 among GNU utilities, and fewer idiosyncrasies for users to remember.
915 @cindex standard command-line options
916 @cindex options, standard command-line
917 @cindex CGI programs, standard options for
918 @cindex PATH_INFO, specifying standard options as
919 All programs should support two standard options: @samp{--version}
920 and @samp{--help}. CGI programs should accept these as command-line
921 options, and also if given as the @env{PATH_INFO}; for instance,
922 visiting @url{http://example.org/p.cgi/--help} in a browser should
923 output the same information as invoking @samp{p.cgi --help} from the
927 * --version:: The standard output for --version.
928 * --help:: The standard output for --help.
932 @subsection @option{--version}
934 @cindex @samp{--version} output
936 The standard @code{--version} option should direct the program to
937 print information about its name, version, origin and legal status,
938 all on standard output, and then exit successfully. Other options and
939 arguments should be ignored once this is seen, and the program should
940 not perform its normal function.
942 @cindex canonical name of a program
943 @cindex program's canonical name
944 The first line is meant to be easy for a program to parse; the version
945 number proper starts after the last space. In addition, it contains
946 the canonical name for this program, in this format:
953 The program's name should be a constant string; @emph{don't} compute it
954 from @code{argv[0]}. The idea is to state the standard or canonical
955 name for the program, not its file name. There are other ways to find
956 out the precise file name where a command is found in @code{PATH}.
958 If the program is a subsidiary part of a larger package, mention the
959 package name in parentheses, like this:
962 emacsserver (GNU Emacs) 19.30
966 If the package has a version number which is different from this
967 program's version number, you can mention the package version number
968 just before the close-parenthesis.
970 If you @emph{need} to mention the version numbers of libraries which
971 are distributed separately from the package which contains this program,
972 you can do so by printing an additional line of version info for each
973 library you want to mention. Use the same format for these lines as for
976 Please do not mention all of the libraries that the program uses ``just
977 for completeness''---that would produce a lot of unhelpful clutter.
978 Please mention library version numbers only if you find in practice that
979 they are very important to you in debugging.
981 The following line, after the version number line or lines, should be a
982 copyright notice. If more than one copyright notice is called for, put
983 each on a separate line.
985 Next should follow a line stating the license, preferably using one of
986 abbreviations below, and a brief statement that the program is free
987 software, and that users are free to copy and change it. Also mention
988 that there is no warranty, to the extent permitted by law. See
989 recommended wording below.
991 It is ok to finish the output with a list of the major authors of the
992 program, as a way of giving credit.
994 Here's an example of output that follows these rules:
998 Copyright (C) 2007 Free Software Foundation, Inc.
999 License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
1000 This is free software: you are free to change and redistribute it.
1001 There is NO WARRANTY, to the extent permitted by law.
1004 You should adapt this to your program, of course, filling in the proper
1005 year, copyright holder, name of program, and the references to
1006 distribution terms, and changing the rest of the wording as necessary.
1008 This copyright notice only needs to mention the most recent year in
1009 which changes were made---there's no need to list the years for previous
1010 versions' changes. You don't have to mention the name of the program in
1011 these notices, if that is inconvenient, since it appeared in the first
1012 line. (The rules are different for copyright notices in source files;
1013 @pxref{Copyright Notices,,,maintain,Information for GNU Maintainers}.)
1015 Translations of the above lines must preserve the validity of the
1016 copyright notices (@pxref{Internationalization}). If the translation's
1017 character set supports it, the @samp{(C)} should be replaced with the
1018 copyright symbol, as follows:
1021 (the official copyright symbol, which is the letter C in a circle);
1027 Write the word ``Copyright'' exactly like that, in English. Do not
1028 translate it into another language. International treaties recognize
1029 the English word ``Copyright''; translations into other languages do not
1030 have legal significance.
1032 Finally, here is the table of our suggested license abbreviations.
1033 Any abbreviation can be followed by @samp{v@var{version}[+]}, meaning
1034 that particular version, or later versions with the @samp{+}, as shown
1035 above. In the case of a GNU license, @emph{always} indicate the permitted
1036 versions in this way.
1038 In the case of exceptions for extra permissions with the GPL, we use
1039 @samp{/} for a separator; the version number can follow the license
1040 abbreviation as usual, as in the examples below.
1044 GNU General Public License, @url{http://www.gnu.org/@/licenses/@/gpl.html}.
1047 GNU Lesser General Public License, @url{http://www.gnu.org/@/licenses/@/lgpl.html}.
1050 GNU GPL with the exception for Ada.
1053 The Apache Software Foundation license,
1054 @url{http://www.apache.org/@/licenses}.
1057 The Artistic license used for Perl, @url{http://dev.perl.org/licenses/artistic.html}.
1060 The Expat license, @url{http://www.jclark.com/@/xml/@/copying.txt}.
1063 The Mozilla Public License, @url{http://www.mozilla.org/@/MPL/}.
1066 The original (4-clause) BSD license, incompatible with the GNU GPL
1067 @url{http://www.xfree86.org/@/3.3.6/@/COPYRIGHT2.html#6}.
1070 The license used for PHP, @url{http://www.php.net/@/license/}.
1073 The non-license that is being in the public domain,
1074 @url{http://www.gnu.org/@/licenses/@/license-list.html#PublicDomain}.
1077 The license for Python, @url{http://www.python.org/@/2.0.1/@/license.html}.
1080 The revised (3-clause) BSD, compatible with the GNU GPL,@*
1081 @url{http://www.xfree86.org/@/3.3.6/@/COPYRIGHT2.html#5}.
1084 The simple non-copyleft license used for most versions of the X Window
1085 System, @url{http://www.xfree86.org/@/3.3.6/@/COPYRIGHT2.html#3}.
1088 The license for Zlib, @url{http://www.gzip.org/@/zlib/@/zlib_license.html}.
1092 More information about these licenses and many more are on the GNU
1093 licensing web pages,
1094 @url{http://www.gnu.org/@/licenses/@/license-list.html}.
1098 @subsection @option{--help}
1100 @cindex @samp{--help} output
1102 The standard @code{--help} option should output brief documentation
1103 for how to invoke the program, on standard output, then exit
1104 successfully. Other options and arguments should be ignored once this
1105 is seen, and the program should not perform its normal function.
1107 @cindex address for bug reports
1109 Near the end of the @samp{--help} option's output, please place lines
1110 giving the email address for bug reports, the package's home page
1111 (normally @indicateurl{http://www.gnu.org/software/@var{pkg}}, and the
1112 general page for help using GNU programs. The format should be like this:
1115 Report bugs to: @var{mailing-address}
1116 @var{pkg} home page: <http://www.gnu.org/software/@var{pkg}/>
1117 General help using GNU software: <http://www.gnu.org/gethelp/>
1120 It is ok to mention other appropriate mailing lists and web pages.
1123 @node Dynamic Plug-In Interfaces
1124 @section Standards for Dynamic Plug-in Interfaces
1126 @cindex dynamic plug-ins
1128 Another aspect of keeping free programs free is encouraging
1129 development of free plug-ins, and discouraging development of
1130 proprietary plug-ins. Many GNU programs will not have anything like
1131 plug-ins at all, but those that do should follow these
1134 First, the general plug-in architecture design should closely tie the
1135 plug-in to the original code, such that the plug-in and the base
1136 program are parts of one extended program. For GCC, for example,
1137 plug-ins receive and modify GCC's internal data structures, and so
1138 clearly form an extended program with the base GCC.
1140 @vindex plugin_is_GPL_compatible
1141 Second, you should require plug-in developers to affirm that their
1142 plug-ins are released under an appropriate license. This should be
1143 enforced with a simple programmatic check. For GCC, again for
1144 example, a plug-in must define the global symbol
1145 @code{plugin_is_GPL_compatible}, thus asserting that the plug-in is
1146 released under a GPL-compatible license (@pxref{Plugins,, Plugins,
1147 gccint, GCC Internals}).
1149 By adding this check to your program you are not creating a new legal
1150 requirement. The GPL itself requires plug-ins to be free software,
1151 licensed compatibly. As long as you have followed the first rule above
1152 to keep plug-ins closely tied to your original program, the GPL and AGPL
1153 already require those plug-ins to be released under a compatible
1154 license. The symbol definition in the plug-in---or whatever equivalent
1155 works best in your program---makes it harder for anyone who might
1156 distribute proprietary plug-ins to legally defend themselves. If a case
1157 about this got to court, we can point to that symbol as evidence that
1158 the plug-in developer understood that the license had this requirement.
1162 @section Table of Long Options
1163 @cindex long option names
1164 @cindex table of long options
1166 Here is a table of long options used by GNU programs. It is surely
1167 incomplete, but we aim to list all the options that a new program might
1168 want to be compatible with. If you use names not already in the table,
1169 please send @email{bug-standards@@gnu.org} a list of them, with their
1170 meanings, so we can update the table.
1172 @c Please leave newlines between items in this table; it's much easier
1173 @c to update when it isn't completely squashed together and unreadable.
1174 @c When there is more than one short option for a long option name, put
1175 @c a semicolon between the lists of the programs that use them, not a
1176 @c period. --friedman
1180 @samp{-N} in @code{tar}.
1183 @samp{-a} in @code{du}, @code{ls}, @code{nm}, @code{stty}, @code{uname},
1184 and @code{unexpand}.
1187 @samp{-a} in @code{diff}.
1190 @samp{-A} in @code{ls}.
1193 @samp{-a} in @code{etags}, @code{tee}, @code{time};
1194 @samp{-r} in @code{tar}.
1197 @samp{-a} in @code{cp}.
1200 @samp{-n} in @code{shar}.
1203 @samp{-l} in @code{m4}.
1206 @samp{-a} in @code{diff}.
1209 @samp{-v} in @code{gawk}.
1212 @samp{-W} in @code{make}.
1215 @samp{-o} in @code{make}.
1218 @samp{-a} in @code{recode}.
1221 @samp{-a} in @code{wdiff}.
1223 @item auto-reference
1224 @samp{-A} in @code{ptx}.
1227 @samp{-n} in @code{wdiff}.
1230 For server programs, run in the background.
1232 @item backward-search
1233 @samp{-B} in @code{ctags}.
1236 @samp{-f} in @code{shar}.
1245 @samp{-b} in @code{tac}.
1248 @samp{-b} in @code{cpio} and @code{diff}.
1251 @samp{-b} in @code{shar}.
1254 Used in @code{cpio} and @code{tar}.
1257 @samp{-b} in @code{head} and @code{tail}.
1260 @samp{-b} in @code{ptx}.
1263 Used in various programs to make output shorter.
1266 @samp{-c} in @code{head}, @code{split}, and @code{tail}.
1269 @samp{-C} in @code{etags}.
1272 @samp{-A} in @code{tar}.
1275 Used in various programs to specify the directory to use.
1278 @samp{-c} in @code{chgrp} and @code{chown}.
1281 @samp{-F} in @code{ls}.
1284 @samp{-c} in @code{recode}.
1287 @samp{-c} in @code{su};
1291 @samp{-d} in @code{tar}.
1294 Used in @code{gawk}.
1297 @samp{-Z} in @code{tar} and @code{shar}.
1300 @samp{-A} in @code{tar}.
1303 @samp{-w} in @code{tar}.
1306 Used in @code{diff}.
1309 @samp{-W copyleft} in @code{gawk}.
1312 @samp{-C} in @code{ptx}, @code{recode}, and @code{wdiff};
1313 @samp{-W copyright} in @code{gawk}.
1319 @samp{-q} in @code{who}.
1322 @samp{-l} in @code{du}.
1325 Used in @code{tar} and @code{cpio}.
1328 @samp{-c} in @code{shar}.
1331 @samp{-x} in @code{ctags}.
1334 @samp{-d} in @code{touch}.
1337 @samp{-d} in @code{make} and @code{m4};
1341 @samp{-D} in @code{m4}.
1344 @samp{-d} in Bison and @code{ctags}.
1347 @samp{-D} in @code{tar}.
1350 @samp{-L} in @code{chgrp}, @code{chown}, @code{cpio}, @code{du},
1351 @code{ls}, and @code{tar}.
1353 @item dereference-args
1354 @samp{-D} in @code{du}.
1357 Specify an I/O device (special file name).
1360 @samp{-d} in @code{recode}.
1362 @item dictionary-order
1363 @samp{-d} in @code{look}.
1366 @samp{-d} in @code{tar}.
1369 @samp{-n} in @code{csplit}.
1372 Specify the directory to use, in various programs. In @code{ls}, it
1373 means to show directories themselves rather than their contents. In
1374 @code{rm} and @code{ln}, it means to not treat links to directories
1378 @samp{-x} in @code{strip}.
1380 @item discard-locals
1381 @samp{-X} in @code{strip}.
1384 @samp{-n} in @code{make}.
1387 @samp{-e} in @code{diff}.
1389 @item elide-empty-files
1390 @samp{-z} in @code{csplit}.
1393 @samp{-x} in @code{wdiff}.
1396 @samp{-z} in @code{wdiff}.
1398 @item entire-new-file
1399 @samp{-N} in @code{diff}.
1401 @item environment-overrides
1402 @samp{-e} in @code{make}.
1405 @samp{-e} in @code{xargs}.
1411 Used in @code{makeinfo}.
1414 @samp{-o} in @code{m4}.
1417 @samp{-b} in @code{ls}.
1420 @samp{-X} in @code{tar}.
1426 @samp{-x} in @code{xargs}.
1429 @samp{-e} in @code{unshar}.
1432 @samp{-t} in @code{diff}.
1435 @samp{-e} in @code{sed}.
1438 @samp{-g} in @code{nm}.
1441 @samp{-i} in @code{cpio};
1442 @samp{-x} in @code{tar}.
1445 @samp{-f} in @code{finger}.
1448 @samp{-f} in @code{su}.
1450 @item fatal-warnings
1451 @samp{-E} in @code{m4}.
1454 @samp{-f} in @code{gawk}, @code{info}, @code{make}, @code{mt},
1455 @code{sed}, and @code{tar}.
1457 @item field-separator
1458 @samp{-F} in @code{gawk}.
1464 @samp{-F} in @code{ls}.
1467 @samp{-T} in @code{tar}.
1470 Used in @code{makeinfo}.
1472 @item flag-truncation
1473 @samp{-F} in @code{ptx}.
1475 @item fixed-output-files
1479 @samp{-f} in @code{tail}.
1481 @item footnote-style
1482 Used in @code{makeinfo}.
1485 @samp{-f} in @code{cp}, @code{ln}, @code{mv}, and @code{rm}.
1488 @samp{-F} in @code{shar}.
1491 For server programs, run in the foreground;
1492 in other words, don't do anything special to run the server
1496 Used in @code{ls}, @code{time}, and @code{ptx}.
1499 @samp{-F} in @code{m4}.
1505 @samp{-g} in @code{ptx}.
1508 @samp{-x} in @code{tar}.
1511 @samp{-i} in @code{ul}.
1514 @samp{-g} in @code{recode}.
1517 @samp{-g} in @code{install}.
1520 @samp{-z} in @code{tar} and @code{shar}.
1523 @samp{-H} in @code{m4}.
1526 @samp{-h} in @code{objdump} and @code{recode}
1529 @samp{-H} in @code{who}.
1532 Used to ask for brief usage information.
1534 @item here-delimiter
1535 @samp{-d} in @code{shar}.
1537 @item hide-control-chars
1538 @samp{-q} in @code{ls}.
1541 In @code{makeinfo}, output HTML.
1544 @samp{-u} in @code{who}.
1547 @samp{-D} in @code{diff}.
1550 @samp{-I} in @code{ls};
1551 @samp{-x} in @code{recode}.
1553 @item ignore-all-space
1554 @samp{-w} in @code{diff}.
1556 @item ignore-backups
1557 @samp{-B} in @code{ls}.
1559 @item ignore-blank-lines
1560 @samp{-B} in @code{diff}.
1563 @samp{-f} in @code{look} and @code{ptx};
1564 @samp{-i} in @code{diff} and @code{wdiff}.
1567 @samp{-i} in @code{make}.
1570 @samp{-i} in @code{ptx}.
1572 @item ignore-indentation
1573 @samp{-I} in @code{etags}.
1575 @item ignore-init-file
1578 @item ignore-interrupts
1579 @samp{-i} in @code{tee}.
1581 @item ignore-matching-lines
1582 @samp{-I} in @code{diff}.
1584 @item ignore-space-change
1585 @samp{-b} in @code{diff}.
1588 @samp{-i} in @code{tar}.
1591 @samp{-i} in @code{etags};
1592 @samp{-I} in @code{m4}.
1595 @samp{-I} in @code{make}.
1598 @samp{-G} in @code{tar}.
1601 @samp{-i}, @samp{-l}, and @samp{-m} in Finger.
1604 In some programs, specify the name of the file to read as the user's
1608 @samp{-i} in @code{expand}.
1611 @samp{-T} in @code{diff}.
1614 @samp{-i} in @code{ls}.
1617 @samp{-i} in @code{cp}, @code{ln}, @code{mv}, @code{rm};
1618 @samp{-e} in @code{m4};
1619 @samp{-p} in @code{xargs};
1620 @samp{-w} in @code{tar}.
1623 @samp{-p} in @code{shar}.
1629 @samp{-j} in @code{make}.
1632 @samp{-n} in @code{make}.
1635 @samp{-k} in @code{make}.
1638 @samp{-k} in @code{csplit}.
1641 @samp{-k} in @code{du} and @code{ls}.
1644 @samp{-l} in @code{etags}.
1647 @samp{-l} in @code{wdiff}.
1649 @item level-for-gzip
1650 @samp{-g} in @code{shar}.
1653 @samp{-C} in @code{split}.
1656 Used in @code{split}, @code{head}, and @code{tail}.
1659 @samp{-l} in @code{cpio}.
1663 Used in @code{gawk}.
1666 @samp{-t} in @code{cpio};
1667 @samp{-l} in @code{recode}.
1670 @samp{-t} in @code{tar}.
1673 @samp{-N} in @code{ls}.
1676 @samp{-l} in @code{make}.
1682 Used in @code{uname}.
1685 @samp{-M} in @code{ptx}.
1688 @samp{-m} in @code{hello} and @code{uname}.
1690 @item make-directories
1691 @samp{-d} in @code{cpio}.
1694 @samp{-f} in @code{make}.
1700 @samp{-n} in @code{xargs}.
1703 @samp{-n} in @code{xargs}.
1706 @samp{-l} in @code{xargs}.
1709 @samp{-l} in @code{make}.
1712 @samp{-P} in @code{xargs}.
1715 @samp{-T} in @code{who}.
1718 @samp{-T} in @code{who}.
1721 @samp{-d} in @code{diff}.
1723 @item mixed-uuencode
1724 @samp{-M} in @code{shar}.
1727 @samp{-m} in @code{install}, @code{mkdir}, and @code{mkfifo}.
1729 @item modification-time
1730 @samp{-m} in @code{tar}.
1733 @samp{-M} in @code{tar}.
1739 @samp{-L} in @code{m4}.
1742 @samp{-a} in @code{shar}.
1745 @samp{-W} in @code{make}.
1747 @item no-builtin-rules
1748 @samp{-r} in @code{make}.
1750 @item no-character-count
1751 @samp{-w} in @code{shar}.
1753 @item no-check-existing
1754 @samp{-x} in @code{shar}.
1757 @samp{-3} in @code{wdiff}.
1760 @samp{-c} in @code{touch}.
1763 @samp{-D} in @code{etags}.
1766 @samp{-1} in @code{wdiff}.
1768 @item no-dereference
1769 @samp{-d} in @code{cp}.
1772 @samp{-2} in @code{wdiff}.
1775 @samp{-S} in @code{make}.
1781 @samp{-P} in @code{shar}.
1784 @samp{-e} in @code{gprof}.
1787 @samp{-R} in @code{etags}.
1790 @samp{-p} in @code{nm}.
1793 Don't print a startup splash screen.
1796 Used in @code{makeinfo}.
1799 @samp{-a} in @code{gprof}.
1802 @samp{-E} in @code{gprof}.
1805 @samp{-m} in @code{shar}.
1808 Used in @code{makeinfo}.
1811 Used in @code{emacsclient}.
1814 Used in various programs to inhibit warnings.
1817 @samp{-n} in @code{info}.
1820 @samp{-n} in @code{uname}.
1823 @samp{-f} in @code{cpio}.
1826 @samp{-n} in @code{objdump}.
1829 @samp{-0} in @code{xargs}.
1832 @samp{-n} in @code{cat}.
1834 @item number-nonblank
1835 @samp{-b} in @code{cat}.
1838 @samp{-n} in @code{nm}.
1840 @item numeric-uid-gid
1841 @samp{-n} in @code{cpio} and @code{ls}.
1847 @samp{-o} in @code{tar}.
1850 @samp{-o} in @code{make}.
1852 @item one-file-system
1853 @samp{-l} in @code{tar}, @code{cp}, and @code{du}.
1856 @samp{-o} in @code{ptx}.
1859 @samp{-f} in @code{gprof}.
1862 @samp{-F} in @code{gprof}.
1865 @samp{-o} in @code{getopt}, @code{fdlist}, @code{fdmount},
1866 @code{fdmountd}, and @code{fdumount}.
1869 In various programs, specify the output file name.
1872 @samp{-o} in @code{shar}.
1875 @samp{-o} in @code{rm}.
1878 @samp{-c} in @code{unshar}.
1881 @samp{-o} in @code{install}.
1884 @samp{-l} in @code{diff}.
1886 @item paragraph-indent
1887 Used in @code{makeinfo}.
1890 @samp{-p} in @code{mkdir} and @code{rmdir}.
1893 @samp{-p} in @code{ul}.
1896 @samp{-p} in @code{cpio}.
1899 @samp{-P} in @code{finger}.
1902 @samp{-c} in @code{cpio} and @code{tar}.
1905 Used in @code{gawk}.
1907 @item prefix-builtins
1908 @samp{-P} in @code{m4}.
1911 @samp{-f} in @code{csplit}.
1914 Used in @code{tar} and @code{cp}.
1916 @item preserve-environment
1917 @samp{-p} in @code{su}.
1919 @item preserve-modification-time
1920 @samp{-m} in @code{cpio}.
1922 @item preserve-order
1923 @samp{-s} in @code{tar}.
1925 @item preserve-permissions
1926 @samp{-p} in @code{tar}.
1929 @samp{-l} in @code{diff}.
1932 @samp{-L} in @code{cmp}.
1934 @item print-data-base
1935 @samp{-p} in @code{make}.
1937 @item print-directory
1938 @samp{-w} in @code{make}.
1940 @item print-file-name
1941 @samp{-o} in @code{nm}.
1944 @samp{-s} in @code{nm}.
1947 @samp{-p} in @code{wdiff}.
1950 @samp{-p} in @code{ed}.
1953 Specify an HTTP proxy.
1956 @samp{-X} in @code{shar}.
1959 @samp{-q} in @code{make}.
1962 Used in many programs to inhibit the usual output. Every
1963 program accepting @samp{--quiet} should accept @samp{--silent} as a
1967 @samp{-Q} in @code{shar}
1970 @samp{-Q} in @code{ls}.
1973 @samp{-n} in @code{diff}.
1976 Used in @code{gawk}.
1978 @item read-full-blocks
1979 @samp{-B} in @code{tar}.
1985 @samp{-n} in @code{make}.
1988 @samp{-R} in @code{tar}.
1991 Used in @code{chgrp}, @code{chown}, @code{cp}, @code{ls}, @code{diff},
1995 @samp{-r} in @code{touch}.
1998 @samp{-r} in @code{ptx}.
2001 @samp{-r} in @code{tac} and @code{etags}.
2004 @samp{-r} in @code{uname}.
2007 @samp{-R} in @code{m4}.
2010 @samp{-r} in @code{objdump}.
2013 @samp{-r} in @code{cpio}.
2016 @samp{-i} in @code{xargs}.
2018 @item report-identical-files
2019 @samp{-s} in @code{diff}.
2021 @item reset-access-time
2022 @samp{-a} in @code{cpio}.
2025 @samp{-r} in @code{ls} and @code{nm}.
2028 @samp{-f} in @code{diff}.
2030 @item right-side-defs
2031 @samp{-R} in @code{ptx}.
2034 @samp{-s} in @code{tar}.
2036 @item same-permissions
2037 @samp{-p} in @code{tar}.
2040 @samp{-g} in @code{stty}.
2045 @item sentence-regexp
2046 @samp{-S} in @code{ptx}.
2049 @samp{-S} in @code{du}.
2052 @samp{-s} in @code{tac}.
2055 Used by @code{recode} to chose files or pipes for sequencing passes.
2058 @samp{-s} in @code{su}.
2061 @samp{-A} in @code{cat}.
2063 @item show-c-function
2064 @samp{-p} in @code{diff}.
2067 @samp{-E} in @code{cat}.
2069 @item show-function-line
2070 @samp{-F} in @code{diff}.
2073 @samp{-T} in @code{cat}.
2076 Used in many programs to inhibit the usual output.
2077 Every program accepting
2078 @samp{--silent} should accept @samp{--quiet} as a synonym.
2081 @samp{-s} in @code{ls}.
2084 Specify a file descriptor for a network server to use for its socket,
2085 instead of opening and binding a new socket. This provides a way to
2086 run, in a non-privileged process, a server that normally needs a
2087 reserved port number.
2093 @samp{-W source} in @code{gawk}.
2096 @samp{-S} in @code{tar}.
2098 @item speed-large-files
2099 @samp{-H} in @code{diff}.
2102 @samp{-E} in @code{unshar}.
2104 @item split-size-limit
2105 @samp{-L} in @code{shar}.
2108 @samp{-s} in @code{cat}.
2111 @samp{-w} in @code{wdiff}.
2114 @samp{-y} in @code{wdiff}.
2117 Used in @code{tar} and @code{diff} to specify which file within
2118 a directory to start processing with.
2121 @samp{-s} in @code{wdiff}.
2123 @item stdin-file-list
2124 @samp{-S} in @code{shar}.
2127 @samp{-S} in @code{make}.
2130 @samp{-s} in @code{recode}.
2133 @samp{-s} in @code{install}.
2136 @samp{-s} in @code{strip}.
2139 @samp{-S} in @code{strip}.
2142 @samp{-s} in @code{shar}.
2145 @samp{-S} in @code{cp}, @code{ln}, @code{mv}.
2148 @samp{-b} in @code{csplit}.
2151 @samp{-s} in @code{gprof}.
2154 @samp{-s} in @code{du}.
2157 @samp{-s} in @code{ln}.
2160 Used in GDB and @code{objdump}.
2163 @samp{-s} in @code{m4}.
2166 @samp{-s} in @code{uname}.
2169 @samp{-t} in @code{expand} and @code{unexpand}.
2172 @samp{-T} in @code{ls}.
2175 @samp{-T} in @code{tput} and @code{ul}.
2176 @samp{-t} in @code{wdiff}.
2179 @samp{-a} in @code{diff}.
2182 @samp{-T} in @code{shar}.
2185 Used in @code{ls} and @code{touch}.
2188 Specify how long to wait before giving up on some operation.
2191 @samp{-O} in @code{tar}.
2194 @samp{-c} in @code{du}.
2197 @samp{-t} in @code{make}, @code{ranlib}, and @code{recode}.
2200 @samp{-t} in @code{m4}.
2203 @samp{-t} in @code{hello};
2204 @samp{-W traditional} in @code{gawk};
2205 @samp{-G} in @code{ed}, @code{m4}, and @code{ptx}.
2211 @samp{-t} in @code{ctags}.
2213 @item typedefs-and-c++
2214 @samp{-T} in @code{ctags}.
2217 @samp{-t} in @code{ptx}.
2220 @samp{-z} in @code{tar}.
2223 @samp{-u} in @code{cpio}.
2226 @samp{-U} in @code{m4}.
2228 @item undefined-only
2229 @samp{-u} in @code{nm}.
2232 @samp{-u} in @code{cp}, @code{ctags}, @code{mv}, @code{tar}.
2235 Used in @code{gawk}; same as @samp{--help}.
2238 @samp{-B} in @code{shar}.
2240 @item vanilla-operation
2241 @samp{-V} in @code{shar}.
2244 Print more information about progress. Many programs support this.
2247 @samp{-W} in @code{tar}.
2250 Print the version number.
2252 @item version-control
2253 @samp{-V} in @code{cp}, @code{ln}, @code{mv}.
2256 @samp{-v} in @code{ctags}.
2259 @samp{-V} in @code{tar}.
2262 @samp{-W} in @code{make}.
2264 @item whole-size-limit
2265 @samp{-l} in @code{shar}.
2268 @samp{-w} in @code{ls} and @code{ptx}.
2271 @samp{-W} in @code{ptx}.
2274 @samp{-T} in @code{who}.
2277 @samp{-z} in @code{gprof}.
2280 @node OID Allocations
2281 @section OID Allocations
2282 @cindex OID allocations for GNU
2287 The OID (object identifier) 1.3.6.1.4.1.11591 has been assigned to the
2288 GNU Project (thanks to Werner Koch). These are used for SNMP, LDAP,
2289 X.509 certificates, and so on. The web site
2290 @url{http://www.alvestrand.no/objectid} has a (voluntary) listing of
2291 many OID assignments.
2293 If you need a new slot for your GNU package, write
2294 @email{maintainers@@gnu.org}. Here is a list of arcs currently
2298 @include gnu-oids.texi
2303 @section Memory Usage
2304 @cindex memory usage
2306 If a program typically uses just a few meg of memory, don't bother making any
2307 effort to reduce memory usage. For example, if it is impractical for
2308 other reasons to operate on files more than a few meg long, it is
2309 reasonable to read entire input files into memory to operate on them.
2311 However, for programs such as @code{cat} or @code{tail}, that can
2312 usefully operate on very large files, it is important to avoid using a
2313 technique that would artificially limit the size of files it can handle.
2314 If a program works by lines and could be applied to arbitrary
2315 user-supplied input files, it should keep only a line in memory, because
2316 this is not very hard and users will want to be able to operate on input
2317 files that are bigger than will fit in memory all at once.
2319 If your program creates complicated data structures, just make them in
2320 memory and give a fatal error if @code{malloc} returns zero.
2324 Memory analysis tools such as @command{valgrind} can be useful, but
2325 don't complicate a program merely to avoid their false alarms. For
2326 example, if memory is used until just before a process exits, don't
2327 free it simply to silence such a tool.
2334 Programs should be prepared to operate when @file{/usr} and @file{/etc}
2335 are read-only file systems. Thus, if the program manages log files,
2336 lock files, backup files, score files, or any other files which are
2337 modified for internal purposes, these files should not be stored in
2338 @file{/usr} or @file{/etc}.
2340 There are two exceptions. @file{/etc} is used to store system
2341 configuration information; it is reasonable for a program to modify
2342 files in @file{/etc} when its job is to update the system configuration.
2343 Also, if the user explicitly asks to modify one file in a directory, it
2344 is reasonable for the program to store other files in the same
2348 @chapter Making The Best Use of C
2350 This chapter provides advice on how best to use the C language
2351 when writing GNU software.
2354 * Formatting:: Formatting your source code.
2355 * Comments:: Commenting your work.
2356 * Syntactic Conventions:: Clean use of C constructs.
2357 * Names:: Naming variables, functions, and files.
2358 * System Portability:: Portability among different operating systems.
2359 * CPU Portability:: Supporting the range of CPU types.
2360 * System Functions:: Portability and ``standard'' library functions.
2361 * Internationalization:: Techniques for internationalization.
2362 * Character Set:: Use ASCII by default.
2363 * Quote Characters:: Use "..." or '...' in the C locale.
2364 * Mmap:: How you can safely use @code{mmap}.
2368 @section Formatting Your Source Code
2369 @cindex formatting source code
2372 @cindex braces, in C source
2373 @cindex function definitions, formatting
2374 It is important to put the open-brace that starts the body of a C
2375 function in column one, so that they will start a defun. Several
2376 tools look for open-braces in column one to find the beginnings of C
2377 functions. These tools will not work on code not formatted that way.
2379 Avoid putting open-brace, open-parenthesis or open-bracket in column
2380 one when they are inside a function, so that they won't start a defun.
2381 The open-brace that starts a @code{struct} body can go in column one
2382 if you find it useful to treat that definition as a defun.
2384 It is also important for function definitions to start the name of the
2385 function in column one. This helps people to search for function
2386 definitions, and may also help certain tools recognize them. Thus,
2387 using Standard C syntax, the format is this:
2391 concat (char *s1, char *s2)
2398 or, if you want to use traditional C syntax, format the definition like
2403 concat (s1, s2) /* Name starts in column one here */
2405 @{ /* Open brace in column one here */
2410 In Standard C, if the arguments don't fit nicely on one line,
2415 lots_of_args (int an_integer, long a_long, short a_short,
2416 double a_double, float a_float)
2420 @cindex @code{struct} types, formatting
2421 @cindex @code{enum} types, formatting
2422 For @code{struct} and @code{enum} types, likewise put the braces in
2423 column one, unless the whole contents fits on one line:
2431 struct foo @{ int a, b; @}
2434 The rest of this section gives our recommendations for other aspects of
2435 C formatting style, which is also the default style of the @code{indent}
2436 program in version 1.2 and newer. It corresponds to the options
2439 -nbad -bap -nbc -bbo -bl -bli2 -bls -ncdb -nce -cp1 -cs -di2
2440 -ndj -nfc1 -nfca -hnl -i2 -ip5 -lp -pcs -psl -nsc -nsob
2443 We don't think of these recommendations as requirements, because it
2444 causes no problems for users if two different programs have different
2447 But whatever style you use, please use it consistently, since a mixture
2448 of styles within one program tends to look ugly. If you are
2449 contributing changes to an existing program, please follow the style of
2452 For the body of the function, our recommended style looks like this:
2464 return ++x + bar ();
2468 @cindex spaces before open-paren
2469 We find it easier to read a program when it has spaces before the
2470 open-parentheses and after the commas. Especially after the commas.
2472 When you split an expression into multiple lines, split it
2473 before an operator, not after one. Here is the right way:
2475 @cindex expressions, splitting
2477 if (foo_this_is_long && bar > win (x, y, z)
2478 && remaining_condition)
2481 Try to avoid having two operators of different precedence at the same
2482 level of indentation. For example, don't write this:
2485 mode = (inmode[j] == VOIDmode
2486 || GET_MODE_SIZE (outmode[j]) > GET_MODE_SIZE (inmode[j])
2487 ? outmode[j] : inmode[j]);
2490 Instead, use extra parentheses so that the indentation shows the nesting:
2493 mode = ((inmode[j] == VOIDmode
2494 || (GET_MODE_SIZE (outmode[j]) > GET_MODE_SIZE (inmode[j])))
2495 ? outmode[j] : inmode[j]);
2498 Insert extra parentheses so that Emacs will indent the code properly.
2499 For example, the following indentation looks nice if you do it by hand,
2502 v = rup->ru_utime.tv_sec*1000 + rup->ru_utime.tv_usec/1000
2503 + rup->ru_stime.tv_sec*1000 + rup->ru_stime.tv_usec/1000;
2507 but Emacs would alter it. Adding a set of parentheses produces
2508 something that looks equally nice, and which Emacs will preserve:
2511 v = (rup->ru_utime.tv_sec*1000 + rup->ru_utime.tv_usec/1000
2512 + rup->ru_stime.tv_sec*1000 + rup->ru_stime.tv_usec/1000);
2515 Format do-while statements like this:
2527 Please use formfeed characters (control-L) to divide the program into
2528 pages at logical places (but not within a function). It does not matter
2529 just how long the pages are, since they do not have to fit on a printed
2530 page. The formfeeds should appear alone on lines by themselves.
2533 @section Commenting Your Work
2536 Every program should start with a comment saying briefly what it is for.
2537 Example: @samp{fmt - filter for simple filling of text}. This comment
2538 should be at the top of the source file containing the @samp{main}
2539 function of the program.
2541 Also, please write a brief comment at the start of each source file,
2542 with the file name and a line or two about the overall purpose of the
2545 Please write the comments in a GNU program in English, because English
2546 is the one language that nearly all programmers in all countries can
2547 read. If you do not write English well, please write comments in
2548 English as well as you can, then ask other people to help rewrite them.
2549 If you can't write comments in English, please find someone to work with
2550 you and translate your comments into English.
2552 Please put a comment on each function saying what the function does,
2553 what sorts of arguments it gets, and what the possible values of
2554 arguments mean and are used for. It is not necessary to duplicate in
2555 words the meaning of the C argument declarations, if a C type is being
2556 used in its customary fashion. If there is anything nonstandard about
2557 its use (such as an argument of type @code{char *} which is really the
2558 address of the second character of a string, not the first), or any
2559 possible values that would not work the way one would expect (such as,
2560 that strings containing newlines are not guaranteed to work), be sure
2563 Also explain the significance of the return value, if there is one.
2565 Please put two spaces after the end of a sentence in your comments, so
2566 that the Emacs sentence commands will work. Also, please write
2567 complete sentences and capitalize the first word. If a lower-case
2568 identifier comes at the beginning of a sentence, don't capitalize it!
2569 Changing the spelling makes it a different identifier. If you don't
2570 like starting a sentence with a lower case letter, write the sentence
2571 differently (e.g., ``The identifier lower-case is @dots{}'').
2573 The comment on a function is much clearer if you use the argument
2574 names to speak about the argument values. The variable name itself
2575 should be lower case, but write it in upper case when you are speaking
2576 about the value rather than the variable itself. Thus, ``the inode
2577 number NODE_NUM'' rather than ``an inode''.
2579 There is usually no purpose in restating the name of the function in
2580 the comment before it, because readers can see that for themselves.
2581 There might be an exception when the comment is so long that the function
2582 itself would be off the bottom of the screen.
2584 There should be a comment on each static variable as well, like this:
2587 /* Nonzero means truncate lines in the display;
2588 zero means continue them. */
2592 @cindex conditionals, comments for
2593 @cindex @code{#endif}, commenting
2594 Every @samp{#endif} should have a comment, except in the case of short
2595 conditionals (just a few lines) that are not nested. The comment should
2596 state the condition of the conditional that is ending, @emph{including
2597 its sense}. @samp{#else} should have a comment describing the condition
2598 @emph{and sense} of the code that follows. For example:
2606 #endif /* not foo */
2616 but, by contrast, write the comments this way for a @samp{#ifndef}:
2629 #endif /* not foo */
2633 @node Syntactic Conventions
2634 @section Clean Use of C Constructs
2635 @cindex syntactic conventions
2637 @cindex implicit @code{int}
2638 @cindex function argument, declaring
2639 Please explicitly declare the types of all objects. For example, you
2640 should explicitly declare all arguments to functions, and you should
2641 declare functions to return @code{int} rather than omitting the
2644 @cindex compiler warnings
2645 @cindex @samp{-Wall} compiler option
2646 Some programmers like to use the GCC @samp{-Wall} option, and change the
2647 code whenever it issues a warning. If you want to do this, then do.
2648 Other programmers prefer not to use @samp{-Wall}, because it gives
2649 warnings for valid and legitimate code which they do not want to change.
2650 If you want to do this, then do. The compiler should be your servant,
2655 Don't make the program ugly just to placate static analysis tools such
2656 as @command{lint}, @command{clang}, and GCC with extra warnings
2657 options such as @option{-Wconversion} and @option{-Wundef}. These
2658 tools can help find bugs and unclear code, but they can also generate
2659 so many false alarms that it hurts readability to silence them with
2660 unnecessary casts, wrappers, and other complications. For example,
2661 please don't insert casts to @code{void} or calls to do-nothing
2662 functions merely to pacify a lint checker.
2664 Declarations of external functions and functions to appear later in the
2665 source file should all go in one place near the beginning of the file
2666 (somewhere before the first function definition in the file), or else
2667 should go in a header file. Don't put @code{extern} declarations inside
2670 @cindex temporary variables
2671 It used to be common practice to use the same local variables (with
2672 names like @code{tem}) over and over for different values within one
2673 function. Instead of doing this, it is better to declare a separate local
2674 variable for each distinct purpose, and give it a name which is
2675 meaningful. This not only makes programs easier to understand, it also
2676 facilitates optimization by good compilers. You can also move the
2677 declaration of each local variable into the smallest scope that includes
2678 all its uses. This makes the program even cleaner.
2680 Don't use local variables or parameters that shadow global identifiers.
2681 GCC's @samp{-Wshadow} option can detect this problem.
2683 @cindex multiple variables in a line
2684 Don't declare multiple variables in one declaration that spans lines.
2685 Start a new declaration on each line, instead. For example, instead
2711 (If they are global variables, each should have a comment preceding it
2714 When you have an @code{if}-@code{else} statement nested in another
2715 @code{if} statement, always put braces around the @code{if}-@code{else}.
2716 Thus, never write like this:
2739 If you have an @code{if} statement nested inside of an @code{else}
2740 statement, either write @code{else if} on one line, like this,
2750 with its @code{then}-part indented like the preceding @code{then}-part,
2751 or write the nested @code{if} within braces like this:
2763 Don't declare both a structure tag and variables or typedefs in the
2764 same declaration. Instead, declare the structure tag separately
2765 and then use it to declare the variables or typedefs.
2767 Try to avoid assignments inside @code{if}-conditions (assignments
2768 inside @code{while}-conditions are ok). For example, don't write
2772 if ((foo = (char *) malloc (sizeof *foo)) == 0)
2773 fatal ("virtual memory exhausted");
2777 instead, write this:
2780 foo = (char *) malloc (sizeof *foo);
2782 fatal ("virtual memory exhausted");
2785 This example uses zero without a cast as a null pointer constant.
2786 This is perfectly fine, except that a cast is needed when calling a
2787 varargs function or when using @code{sizeof}.
2790 @section Naming Variables, Functions, and Files
2792 @cindex names of variables, functions, and files
2793 The names of global variables and functions in a program serve as
2794 comments of a sort. So don't choose terse names---instead, look for
2795 names that give useful information about the meaning of the variable or
2796 function. In a GNU program, names should be English, like other
2799 Local variable names can be shorter, because they are used only within
2800 one context, where (presumably) comments explain their purpose.
2802 Try to limit your use of abbreviations in symbol names. It is ok to
2803 make a few abbreviations, explain what they mean, and then use them
2804 frequently, but don't use lots of obscure abbreviations.
2806 Please use underscores to separate words in a name, so that the Emacs
2807 word commands can be useful within them. Stick to lower case; reserve
2808 upper case for macros and @code{enum} constants, and for name-prefixes
2809 that follow a uniform convention.
2811 For example, you should use names like @code{ignore_space_change_flag};
2812 don't use names like @code{iCantReadThis}.
2814 Variables that indicate whether command-line options have been
2815 specified should be named after the meaning of the option, not after
2816 the option-letter. A comment should state both the exact meaning of
2817 the option and its letter. For example,
2821 /* Ignore changes in horizontal whitespace (-b). */
2822 int ignore_space_change_flag;
2826 When you want to define names with constant integer values, use
2827 @code{enum} rather than @samp{#define}. GDB knows about enumeration
2830 @cindex file-name limitations
2832 You might want to make sure that none of the file names would conflict
2833 if the files were loaded onto an MS-DOS file system which shortens the
2834 names. You can use the program @code{doschk} to test for this.
2836 Some GNU programs were designed to limit themselves to file names of 14
2837 characters or less, to avoid file name conflicts if they are read into
2838 older System V systems. Please preserve this feature in the existing
2839 GNU programs that have it, but there is no need to do this in new GNU
2840 programs. @code{doschk} also reports file names longer than 14
2844 @node System Portability
2845 @section Portability between System Types
2846 @cindex portability, between system types
2848 In the Unix world, ``portability'' refers to porting to different Unix
2849 versions. For a GNU program, this kind of portability is desirable, but
2852 The primary purpose of GNU software is to run on top of the GNU kernel,
2853 compiled with the GNU C compiler, on various types of CPU. So the
2854 kinds of portability that are absolutely necessary are quite limited.
2855 But it is important to support Linux-based GNU systems, since they
2856 are the form of GNU that is popular.
2858 Beyond that, it is good to support the other free operating systems
2859 (*BSD), and it is nice to support other Unix-like systems if you want
2860 to. Supporting a variety of Unix-like systems is desirable, although
2861 not paramount. It is usually not too hard, so you may as well do it.
2862 But you don't have to consider it an obligation, if it does turn out to
2866 The easiest way to achieve portability to most Unix-like systems is to
2867 use Autoconf. It's unlikely that your program needs to know more
2868 information about the host platform than Autoconf can provide, simply
2869 because most of the programs that need such knowledge have already been
2872 Avoid using the format of semi-internal data bases (e.g., directories)
2873 when there is a higher-level alternative (@code{readdir}).
2875 @cindex non-POSIX systems, and portability
2876 As for systems that are not like Unix, such as MSDOS, Windows, VMS, MVS,
2877 and older Macintosh systems, supporting them is often a lot of work.
2878 When that is the case, it is better to spend your time adding features
2879 that will be useful on GNU and GNU/Linux, rather than on supporting
2880 other incompatible systems.
2882 If you do support Windows, please do not abbreviate it as ``win''. In
2883 hacker terminology, calling something a ``win'' is a form of praise.
2884 You're free to praise Microsoft Windows on your own if you want, but
2885 please don't do this in GNU packages. Instead of abbreviating
2886 ``Windows'' to ``win'', you can write it in full or abbreviate it to
2887 ``woe'' or ``w''. In GNU Emacs, for instance, we use @samp{w32} in
2888 file names of Windows-specific files, but the macro for Windows
2889 conditionals is called @code{WINDOWSNT}.
2891 It is a good idea to define the ``feature test macro''
2892 @code{_GNU_SOURCE} when compiling your C files. When you compile on GNU
2893 or GNU/Linux, this will enable the declarations of GNU library extension
2894 functions, and that will usually give you a compiler error message if
2895 you define the same function names in some other way in your program.
2896 (You don't have to actually @emph{use} these functions, if you prefer
2897 to make the program more portable to other systems.)
2899 But whether or not you use these GNU extensions, you should avoid
2900 using their names for any other meanings. Doing so would make it hard
2901 to move your code into other GNU programs.
2903 @node CPU Portability
2904 @section Portability between CPUs
2906 @cindex data types, and portability
2907 @cindex portability, and data types
2908 Even GNU systems will differ because of differences among CPU
2909 types---for example, difference in byte ordering and alignment
2910 requirements. It is absolutely essential to handle these differences.
2911 However, don't make any effort to cater to the possibility that an
2912 @code{int} will be less than 32 bits. We don't support 16-bit machines
2915 Similarly, don't make any effort to cater to the possibility that
2916 @code{long} will be smaller than predefined types like @code{size_t}.
2917 For example, the following code is ok:
2920 printf ("size = %lu\n", (unsigned long) sizeof array);
2921 printf ("diff = %ld\n", (long) (pointer2 - pointer1));
2924 1989 Standard C requires this to work, and we know of only one
2925 counterexample: 64-bit programs on Microsoft Windows. We will leave
2926 it to those who want to port GNU programs to that environment to
2927 figure out how to do it.
2929 Predefined file-size types like @code{off_t} are an exception: they are
2930 longer than @code{long} on many platforms, so code like the above won't
2931 work with them. One way to print an @code{off_t} value portably is to
2932 print its digits yourself, one by one.
2934 Don't assume that the address of an @code{int} object is also the
2935 address of its least-significant byte. This is false on big-endian
2936 machines. Thus, don't make the following mistake:
2941 while ((c = getchar ()) != EOF)
2942 write (file_descriptor, &c, 1);
2945 @noindent Instead, use @code{unsigned char} as follows. (The @code{unsigned}
2946 is for portability to unusual systems where @code{char} is signed and
2947 where there is integer overflow checking.)
2951 while ((c = getchar ()) != EOF)
2953 unsigned char u = c;
2954 write (file_descriptor, &u, 1);
2958 @cindex casting pointers to integers
2959 Avoid casting pointers to integers if you can. Such casts greatly
2960 reduce portability, and in most programs they are easy to avoid. In the
2961 cases where casting pointers to integers is essential---such as, a Lisp
2962 interpreter which stores type information as well as an address in one
2963 word---you'll have to make explicit provisions to handle different word
2964 sizes. You will also need to make provision for systems in which the
2965 normal range of addresses you can get from @code{malloc} starts far away
2969 @node System Functions
2970 @section Calling System Functions
2972 @cindex C library functions, and portability
2973 @cindex POSIX functions, and portability
2974 @cindex library functions, and portability
2975 @cindex portability, and library functions
2977 Historically, C implementations differed substantially, and many
2978 systems lacked a full implementation of ANSI/ISO C89. Nowadays,
2979 however, all practical systems have a C89 compiler and GNU C supports
2980 almost all of C99 and some of C11. Similarly, most systems implement
2981 POSIX.1-2001 libraries and tools, and many have POSIX.1-2008.
2983 Hence, there is little reason to support old C or non-POSIX systems,
2984 and you may want to take advantage of standard C and POSIX to write
2985 clearer, more portable, or faster code. You should use standard
2986 interfaces where possible; but if GNU extensions make your program
2987 more maintainable, powerful, or otherwise better, don't hesitate to
2988 use them. In any case, don't make your own declaration of system
2989 functions; that's a recipe for conflict.
2991 Despite the standards, nearly every library function has some sort of
2992 portability issue on some system or another. Here are some examples:
2996 Names with trailing @code{/}'s are mishandled on many platforms.
2999 @code{long double} may be unimplemented; floating values Infinity and
3000 NaN are often mishandled; output for large precisions may be
3004 May return @code{int} instead of @code{ssize_t}.
3007 On Windows, @code{errno} is not set on failure.
3011 @uref{http://www.gnu.org/software/gnulib/, Gnulib} is a big help in
3012 this regard. Gnulib provides implementations of standard interfaces
3013 on many of the systems that lack them, including portable
3014 implementations of enhanced GNU interfaces, thereby making their use
3015 portable, and of POSIX-1.2008 interfaces, some of which are missing
3016 even on up-to-date GNU systems.
3018 @findex xmalloc, in Gnulib
3019 @findex error messages, in Gnulib
3020 @findex data structures, in Gnulib
3021 Gnulib also provides many useful non-standard interfaces; for example,
3022 C implementations of standard data structures (hash tables, binary
3023 trees), error-checking type-safe wrappers for memory allocation
3024 functions (@code{xmalloc}, @code{xrealloc}), and output of error
3027 Gnulib integrates with GNU Autoconf and Automake to remove much of the
3028 burden of writing portable code from the programmer: Gnulib makes your
3029 configure script automatically determine what features are missing and
3030 use the Gnulib code to supply the missing pieces.
3032 The Gnulib and Autoconf manuals have extensive sections on
3033 portability: @ref{Top,, Introduction, gnulib, Gnulib} and
3034 @pxref{Portable C and C++,,, autoconf, Autoconf}. Please consult them
3035 for many more details.
3038 @node Internationalization
3039 @section Internationalization
3040 @cindex internationalization
3043 GNU has a library called GNU gettext that makes it easy to translate the
3044 messages in a program into various languages. You should use this
3045 library in every program. Use English for the messages as they appear
3046 in the program, and let gettext provide the way to translate them into
3049 Using GNU gettext involves putting a call to the @code{gettext} macro
3050 around each string that might need translation---like this:
3053 printf (gettext ("Processing file '%s'..."), file);
3057 This permits GNU gettext to replace the string @code{"Processing file
3058 '%s'..."} with a translated version.
3060 Once a program uses gettext, please make a point of writing calls to
3061 @code{gettext} when you add new strings that call for translation.
3063 Using GNU gettext in a package involves specifying a @dfn{text domain
3064 name} for the package. The text domain name is used to separate the
3065 translations for this package from the translations for other packages.
3066 Normally, the text domain name should be the same as the name of the
3067 package---for example, @samp{coreutils} for the GNU core utilities.
3069 @cindex message text, and internationalization
3070 To enable gettext to work well, avoid writing code that makes
3071 assumptions about the structure of words or sentences. When you want
3072 the precise text of a sentence to vary depending on the data, use two or
3073 more alternative string constants each containing a complete sentences,
3074 rather than inserting conditionalized words or phrases into a single
3077 Here is an example of what not to do:
3080 printf ("%s is full", capacity > 5000000 ? "disk" : "floppy disk");
3083 If you apply gettext to all strings, like this,
3086 printf (gettext ("%s is full"),
3087 capacity > 5000000 ? gettext ("disk") : gettext ("floppy disk"));
3091 the translator will hardly know that "disk" and "floppy disk" are meant to
3092 be substituted in the other string. Worse, in some languages (like French)
3093 the construction will not work: the translation of the word "full" depends
3094 on the gender of the first part of the sentence; it happens to be not the
3095 same for "disk" as for "floppy disk".
3097 Complete sentences can be translated without problems:
3100 printf (capacity > 5000000 ? gettext ("disk is full")
3101 : gettext ("floppy disk is full"));
3104 A similar problem appears at the level of sentence structure with this
3108 printf ("# Implicit rule search has%s been done.\n",
3109 f->tried_implicit ? "" : " not");
3113 Adding @code{gettext} calls to this code cannot give correct results for
3114 all languages, because negation in some languages requires adding words
3115 at more than one place in the sentence. By contrast, adding
3116 @code{gettext} calls does the job straightforwardly if the code starts
3120 printf (f->tried_implicit
3121 ? "# Implicit rule search has been done.\n",
3122 : "# Implicit rule search has not been done.\n");
3125 Another example is this one:
3128 printf ("%d file%s processed", nfiles,
3129 nfiles != 1 ? "s" : "");
3133 The problem with this example is that it assumes that plurals are made
3134 by adding `s'. If you apply gettext to the format string, like this,
3137 printf (gettext ("%d file%s processed"), nfiles,
3138 nfiles != 1 ? "s" : "");
3142 the message can use different words, but it will still be forced to use
3143 `s' for the plural. Here is a better way, with gettext being applied to
3144 the two strings independently:
3147 printf ((nfiles != 1 ? gettext ("%d files processed")
3148 : gettext ("%d file processed")),
3153 But this still doesn't work for languages like Polish, which has three
3154 plural forms: one for nfiles == 1, one for nfiles == 2, 3, 4, 22, 23, 24, ...
3155 and one for the rest. The GNU @code{ngettext} function solves this problem:
3158 printf (ngettext ("%d files processed", "%d file processed", nfiles),
3164 @section Character Set
3165 @cindex character set
3167 @cindex ASCII characters
3168 @cindex non-ASCII characters
3170 Sticking to the ASCII character set (plain text, 7-bit characters) is
3171 preferred in GNU source code comments, text documents, and other
3172 contexts, unless there is good reason to do something else because of
3173 the application domain. For example, if source code deals with the
3174 French Revolutionary calendar, it is OK if its literal strings contain
3175 accented characters in month names like ``Flor@'eal''. Also, it is OK
3176 (but not required) to use non-ASCII characters to represent proper
3177 names of contributors in change logs (@pxref{Change Logs}).
3179 If you need to use non-ASCII characters, you should normally stick
3180 with one encoding, certainly within a single file. UTF-8 is likely to
3184 @node Quote Characters
3185 @section Quote Characters
3186 @cindex quote characters
3187 @cindex locale-specific quote characters
3190 @cindex opening quote
3191 @cindex single quote
3192 @cindex double quote
3193 @cindex grave accent
3194 @set txicodequoteundirected
3195 @set txicodequotebacktick
3197 In the C locale, the output of GNU programs should stick to plain
3198 ASCII for quotation characters in messages to users: preferably 0x22
3199 (@samp{"}) or 0x27 (@samp{'}) for both opening and closing quotes.
3200 Although GNU programs traditionally used 0x60 (@samp{`}) for opening
3201 and 0x27 (@samp{'}) for closing quotes, nowadays quotes @samp{`like
3202 this'} are typically rendered asymmetrically, so quoting @samp{"like
3203 this"} or @samp{'like this'} typically looks better.
3205 It is ok, but not required, for GNU programs to generate
3206 locale-specific quotes in non-C locales. For example:
3209 printf (gettext ("Processing file '%s'..."), file);
3213 Here, a French translation might cause @code{gettext} to return the
3214 string @code{"Traitement de fichier
3215 @guilsinglleft{}@tie{}%s@tie{}@guilsinglright{}..."}, yielding quotes
3216 more appropriate for a French locale.
3218 Sometimes a program may need to use opening and closing quotes
3219 directly. By convention, @code{gettext} translates the string
3220 @samp{"`"} to the opening quote and the string @samp{"'"} to the
3221 closing quote, and a program can use these translations. Generally,
3222 though, it is better to translate quote characters in the context of
3225 If the output of your program is ever likely to be parsed by another
3226 program, it is good to provide an option that makes this parsing
3227 reliable. For example, you could escape special characters using
3228 conventions from the C language or the Bourne shell. See for example
3229 the option @option{--quoting-style} of GNU @code{ls}.
3231 @clear txicodequoteundirected
3232 @clear txicodequotebacktick
3239 Don't assume that @code{mmap} either works on all files or fails
3240 for all files. It may work on some files and fail on others.
3242 The proper way to use @code{mmap} is to try it on the specific file for
3243 which you want to use it---and if @code{mmap} doesn't work, fall back on
3244 doing the job in another way using @code{read} and @code{write}.
3246 The reason this precaution is needed is that the GNU kernel (the HURD)
3247 provides a user-extensible file system, in which there can be many
3248 different kinds of ``ordinary files''. Many of them support
3249 @code{mmap}, but some do not. It is important to make programs handle
3250 all these kinds of files.
3254 @chapter Documenting Programs
3255 @cindex documentation
3257 A GNU program should ideally come with full free documentation, adequate
3258 for both reference and tutorial purposes. If the package can be
3259 programmed or extended, the documentation should cover programming or
3260 extending it, as well as just using it.
3263 * GNU Manuals:: Writing proper manuals.
3264 * Doc Strings and Manuals:: Compiling doc strings doesn't make a manual.
3265 * Manual Structure Details:: Specific structure conventions.
3266 * License for Manuals:: Writing the distribution terms for a manual.
3267 * Manual Credits:: Giving credit to documentation contributors.
3268 * Printed Manuals:: Mentioning the printed manual.
3269 * NEWS File:: NEWS files supplement manuals.
3270 * Change Logs:: Recording changes.
3271 * Man Pages:: Man pages are secondary.
3272 * Reading other Manuals:: How far you can go in learning
3277 @section GNU Manuals
3279 The preferred document format for the GNU system is the Texinfo
3280 formatting language. Every GNU package should (ideally) have
3281 documentation in Texinfo both for reference and for learners. Texinfo
3282 makes it possible to produce a good quality formatted book, using
3283 @TeX{}, and to generate an Info file. It is also possible to generate
3284 HTML output from Texinfo source. See the Texinfo manual, either the
3285 hardcopy, or the on-line version available through @code{info} or the
3286 Emacs Info subsystem (@kbd{C-h i}).
3288 Nowadays some other formats such as Docbook and Sgmltexi can be
3289 converted automatically into Texinfo. It is ok to produce the Texinfo
3290 documentation by conversion this way, as long as it gives good results.
3292 Make sure your manual is clear to a reader who knows nothing about the
3293 topic and reads it straight through. This means covering basic topics
3294 at the beginning, and advanced topics only later. This also means
3295 defining every specialized term when it is first used.
3297 Programmers tend to carry over the structure of the program as the
3298 structure for its documentation. But this structure is not
3299 necessarily good for explaining how to use the program; it may be
3300 irrelevant and confusing for a user.
3302 Instead, the right way to structure documentation is according to the
3303 concepts and questions that a user will have in mind when reading it.
3304 This principle applies at every level, from the lowest (ordering
3305 sentences in a paragraph) to the highest (ordering of chapter topics
3306 within the manual). Sometimes this structure of ideas matches the
3307 structure of the implementation of the software being documented---but
3308 often they are different. An important part of learning to write good
3309 documentation is to learn to notice when you have unthinkingly
3310 structured the documentation like the implementation, stop yourself,
3311 and look for better alternatives.
3313 For example, each program in the GNU system probably ought to be
3314 documented in one manual; but this does not mean each program should
3315 have its own manual. That would be following the structure of the
3316 implementation, rather than the structure that helps the user
3319 Instead, each manual should cover a coherent @emph{topic}. For example,
3320 instead of a manual for @code{diff} and a manual for @code{diff3}, we
3321 have one manual for ``comparison of files'' which covers both of those
3322 programs, as well as @code{cmp}. By documenting these programs
3323 together, we can make the whole subject clearer.
3325 The manual which discusses a program should certainly document all of
3326 the program's command-line options and all of its commands. It should
3327 give examples of their use. But don't organize the manual as a list
3328 of features. Instead, organize it logically, by subtopics. Address
3329 the questions that a user will ask when thinking about the job that
3330 the program does. Don't just tell the reader what each feature can
3331 do---say what jobs it is good for, and show how to use it for those
3332 jobs. Explain what is recommended usage, and what kinds of usage
3335 In general, a GNU manual should serve both as tutorial and reference.
3336 It should be set up for convenient access to each topic through Info,
3337 and for reading straight through (appendixes aside). A GNU manual
3338 should give a good introduction to a beginner reading through from the
3339 start, and should also provide all the details that hackers want.
3340 The Bison manual is a good example of this---please take a look at it
3341 to see what we mean.
3343 That is not as hard as it first sounds. Arrange each chapter as a
3344 logical breakdown of its topic, but order the sections, and write their
3345 text, so that reading the chapter straight through makes sense. Do
3346 likewise when structuring the book into chapters, and when structuring a
3347 section into paragraphs. The watchword is, @emph{at each point, address
3348 the most fundamental and important issue raised by the preceding text.}
3350 If necessary, add extra chapters at the beginning of the manual which
3351 are purely tutorial and cover the basics of the subject. These provide
3352 the framework for a beginner to understand the rest of the manual. The
3353 Bison manual provides a good example of how to do this.
3355 To serve as a reference, a manual should have an Index that lists all
3356 the functions, variables, options, and important concepts that are
3357 part of the program. One combined Index should do for a short manual,
3358 but sometimes for a complex package it is better to use multiple
3359 indices. The Texinfo manual includes advice on preparing good index
3360 entries, see @ref{Index Entries, , Making Index Entries, texinfo, GNU
3361 Texinfo}, and see @ref{Indexing Commands, , Defining the Entries of an
3362 Index, texinfo, GNU Texinfo}.
3364 Don't use Unix man pages as a model for how to write GNU documentation;
3365 most of them are terse, badly structured, and give inadequate
3366 explanation of the underlying concepts. (There are, of course, some
3367 exceptions.) Also, Unix man pages use a particular format which is
3368 different from what we use in GNU manuals.
3370 Please include an email address in the manual for where to report
3371 bugs @emph{in the text of the manual}.
3373 Please do not use the term ``pathname'' that is used in Unix
3374 documentation; use ``file name'' (two words) instead. We use the term
3375 ``path'' only for search paths, which are lists of directory names.
3377 Please do not use the term ``illegal'' to refer to erroneous input to
3378 a computer program. Please use ``invalid'' for this, and reserve the
3379 term ``illegal'' for activities prohibited by law.
3381 Please do not write @samp{()} after a function name just to indicate
3382 it is a function. @code{foo ()} is not a function, it is a function
3383 call with no arguments.
3385 @node Doc Strings and Manuals
3386 @section Doc Strings and Manuals
3388 Some programming systems, such as Emacs, provide a documentation string
3389 for each function, command or variable. You may be tempted to write a
3390 reference manual by compiling the documentation strings and writing a
3391 little additional text to go around them---but you must not do it. That
3392 approach is a fundamental mistake. The text of well-written
3393 documentation strings will be entirely wrong for a manual.
3395 A documentation string needs to stand alone---when it appears on the
3396 screen, there will be no other text to introduce or explain it.
3397 Meanwhile, it can be rather informal in style.
3399 The text describing a function or variable in a manual must not stand
3400 alone; it appears in the context of a section or subsection. Other text
3401 at the beginning of the section should explain some of the concepts, and
3402 should often make some general points that apply to several functions or
3403 variables. The previous descriptions of functions and variables in the
3404 section will also have given information about the topic. A description
3405 written to stand alone would repeat some of that information; this
3406 redundancy looks bad. Meanwhile, the informality that is acceptable in
3407 a documentation string is totally unacceptable in a manual.
3409 The only good way to use documentation strings in writing a good manual
3410 is to use them as a source of information for writing good text.
3412 @node Manual Structure Details
3413 @section Manual Structure Details
3414 @cindex manual structure
3416 The title page of the manual should state the version of the programs or
3417 packages documented in the manual. The Top node of the manual should
3418 also contain this information. If the manual is changing more
3419 frequently than or independent of the program, also state a version
3420 number for the manual in both of these places.
3422 Each program documented in the manual should have a node named
3423 @samp{@var{program} Invocation} or @samp{Invoking @var{program}}. This
3424 node (together with its subnodes, if any) should describe the program's
3425 command line arguments and how to run it (the sort of information people
3426 would look for in a man page). Start with an @samp{@@example}
3427 containing a template for all the options and arguments that the program
3430 Alternatively, put a menu item in some menu whose item name fits one of
3431 the above patterns. This identifies the node which that item points to
3432 as the node for this purpose, regardless of the node's actual name.
3434 The @samp{--usage} feature of the Info reader looks for such a node
3435 or menu item in order to find the relevant text, so it is essential
3436 for every Texinfo file to have one.
3438 If one manual describes several programs, it should have such a node for
3439 each program described in the manual.
3441 @node License for Manuals
3442 @section License for Manuals
3443 @cindex license for manuals
3445 Please use the GNU Free Documentation License for all GNU manuals that
3446 are more than a few pages long. Likewise for a collection of short
3447 documents---you only need one copy of the GNU FDL for the whole
3448 collection. For a single short document, you can use a very permissive
3449 non-copyleft license, to avoid taking up space with a long license.
3451 See @uref{http://www.gnu.org/copyleft/fdl-howto.html} for more explanation
3452 of how to employ the GFDL.
3454 Note that it is not obligatory to include a copy of the GNU GPL or GNU
3455 LGPL in a manual whose license is neither the GPL nor the LGPL. It can
3456 be a good idea to include the program's license in a large manual; in a
3457 short manual, whose size would be increased considerably by including
3458 the program's license, it is probably better not to include it.
3460 @node Manual Credits
3461 @section Manual Credits
3462 @cindex credits for manuals
3464 Please credit the principal human writers of the manual as the authors,
3465 on the title page of the manual. If a company sponsored the work, thank
3466 the company in a suitable place in the manual, but do not cite the
3467 company as an author.
3469 @node Printed Manuals
3470 @section Printed Manuals
3472 The FSF publishes some GNU manuals in printed form. To encourage sales
3473 of these manuals, the on-line versions of the manual should mention at
3474 the very start that the printed manual is available and should point at
3475 information for getting it---for instance, with a link to the page
3476 @url{http://www.gnu.org/order/order.html}. This should not be included
3477 in the printed manual, though, because there it is redundant.
3479 It is also useful to explain in the on-line forms of the manual how the
3480 user can print out the manual from the sources.
3483 @section The NEWS File
3484 @cindex @file{NEWS} file
3486 In addition to its manual, the package should have a file named
3487 @file{NEWS} which contains a list of user-visible changes worth
3488 mentioning. In each new release, add items to the front of the file and
3489 identify the version they pertain to. Don't discard old items; leave
3490 them in the file after the newer items. This way, a user upgrading from
3491 any previous version can see what is new.
3493 If the @file{NEWS} file gets very long, move some of the older items
3494 into a file named @file{ONEWS} and put a note at the end referring the
3498 @section Change Logs
3501 Keep a change log to describe all the changes made to program source
3502 files. The purpose of this is so that people investigating bugs in the
3503 future will know about the changes that might have introduced the bug.
3504 Often a new bug can be found by looking at what was recently changed.
3505 More importantly, change logs can help you eliminate conceptual
3506 inconsistencies between different parts of a program, by giving you a
3507 history of how the conflicting concepts arose and who they came from.
3510 * Change Log Concepts::
3511 * Style of Change Logs::
3513 * Conditional Changes::
3514 * Indicating the Part Changed::
3517 @node Change Log Concepts
3518 @subsection Change Log Concepts
3521 @cindex batch of changes
3522 You can think of the change log as a conceptual ``undo list'' which
3523 explains how earlier versions were different from the current version.
3524 People can see the current version; they don't need the change log to
3525 tell them what is in it. What they want from a change log is a clear
3526 explanation of how the earlier version differed. Each @dfn{entry} in
3527 a change log describes either an individual change or the smallest
3528 batch of changes that belong together, also known as a @dfn{change
3530 @cindex title, change log entry
3531 @cindex description, change log entry
3532 For later reference or for summarizing, sometimes it is useful to
3533 start the entry with a one-line description (sometimes called a
3534 @dfn{title}) to describe its overall purpose.
3536 In the past, we recommended not mentioning changes in non-software
3537 files (manuals, help files, media files, etc.)@: in change logs.
3538 However, we've been advised that it is a good idea to include them,
3539 for the sake of copyright records.
3541 The change log file is normally called @file{ChangeLog} and covers an
3542 entire directory. Each directory can have its own change log, or a
3543 directory can use the change log of its parent directory---it's up to
3546 Another alternative is to record change log information with a version
3547 control system such as RCS or CVS. This can be converted automatically
3548 to a @file{ChangeLog} file using @code{rcs2log}; in Emacs, the command
3549 @kbd{C-x v a} (@code{vc-update-change-log}) does the job.
3551 For changes to code, there's no need to describe the full purpose of
3552 the changes or how they work together. If you think that a change
3553 calls for explanation, you're probably right. Please do explain
3554 it---but please put the full explanation in comments in the code,
3555 where people will see it whenever they see the code. For example,
3556 ``New function'' is enough for the change log when you add a function,
3557 because there should be a comment before the function definition to
3558 explain what it does.
3560 For changes to files that do not support a comment syntax (e.g., media
3561 files), it is ok to include the full explanation in the change log file,
3562 after the title and before the list of individual changes.
3564 The easiest way to add an entry to @file{ChangeLog} is with the Emacs
3565 command @kbd{M-x add-change-log-entry}. An individual change should
3566 have an asterisk, the name of the changed file, and then in
3567 parentheses the name of the changed functions, variables or whatever,
3568 followed by a colon. Then describe the changes you made to that
3569 function or variable.
3572 @node Style of Change Logs
3573 @subsection Style of Change Logs
3574 @cindex change logs, style
3576 Here are some simple examples of change log entries, starting with the
3577 header line that says who made the change and when it was installed,
3578 followed by descriptions of specific changes. (These examples are
3579 drawn from Emacs and GCC.)
3582 1998-08-17 Richard Stallman <rms@@gnu.org>
3584 * register.el (insert-register): Return nil.
3585 (jump-to-register): Likewise.
3587 * sort.el (sort-subr): Return nil.
3589 * tex-mode.el (tex-bibtex-file, tex-file, tex-region):
3590 Restart the tex shell if process is gone or stopped.
3591 (tex-shell-running): New function.
3593 * expr.c (store_one_arg): Round size up for move_block_to_reg.
3594 (expand_call): Round up when emitting USE insns.
3595 * stmt.c (assign_parms): Round size up for move_block_from_reg.
3598 It's important to name the changed function or variable in full. Don't
3599 abbreviate function or variable names, and don't combine them.
3600 Subsequent maintainers will often search for a function name to find all
3601 the change log entries that pertain to it; if you abbreviate the name,
3602 they won't find it when they search.
3604 For example, some people are tempted to abbreviate groups of function
3605 names by writing @samp{* register.el (@{insert,jump-to@}-register)};
3606 this is not a good idea, since searching for @code{jump-to-register} or
3607 @code{insert-register} would not find that entry.
3609 Separate unrelated change log entries with blank lines. Don't put
3610 blank lines between individual changes of an entry. You can omit the
3611 file name and the asterisk when successive individual changes are in
3614 Break long lists of function names by closing continued lines with
3615 @samp{)}, rather than @samp{,}, and opening the continuation with
3616 @samp{(} as in this example:
3619 * keyboard.c (menu_bar_items, tool_bar_items)
3620 (Fexecute_extended_command): Deal with 'keymap' property.
3623 When you install someone else's changes, put the contributor's name in
3624 the change log entry rather than in the text of the entry. In other
3628 2002-07-14 John Doe <jdoe@@gnu.org>
3630 * sewing.c: Make it sew.
3637 2002-07-14 Usual Maintainer <usual@@gnu.org>
3639 * sewing.c: Make it sew. Patch by jdoe@@gnu.org.
3642 As for the date, that should be the date you applied the change.
3644 @node Simple Changes
3645 @subsection Simple Changes
3647 Certain simple kinds of changes don't need much detail in the change
3650 When you change the calling sequence of a function in a simple fashion,
3651 and you change all the callers of the function to use the new calling
3652 sequence, there is no need to make individual entries for all the
3653 callers that you changed. Just write in the entry for the function
3654 being called, ``All callers changed''---like this:
3657 * keyboard.c (Fcommand_execute): New arg SPECIAL.
3658 All callers changed.
3661 When you change just comments or doc strings, it is enough to write an
3662 entry for the file, without mentioning the functions. Just ``Doc
3663 fixes'' is enough for the change log.
3665 There's no technical need to make change log entries for documentation
3666 files. This is because documentation is not susceptible to bugs that
3667 are hard to fix. Documentation does not consist of parts that must
3668 interact in a precisely engineered fashion. To correct an error, you
3669 need not know the history of the erroneous passage; it is enough to
3670 compare what the documentation says with the way the program actually
3673 However, you should keep change logs for documentation files when the
3674 project gets copyright assignments from its contributors, so as to
3675 make the records of authorship more accurate.
3677 @node Conditional Changes
3678 @subsection Conditional Changes
3679 @cindex conditional changes, and change logs
3680 @cindex change logs, conditional changes
3682 Source files can often contain code that is conditional to build-time
3683 or static conditions. For example, C programs can contain
3684 compile-time @code{#if} conditionals; programs implemented in
3685 interpreted languages can contain module imports of function
3686 definitions that are only performed for certain versions of the
3687 interpreter; and Automake @file{Makefile.am} files can contain
3688 variable definitions or target declarations that are only to be
3689 considered if a configure-time Automake conditional is true.
3691 Many changes are conditional as well: sometimes you add a new variable,
3692 or function, or even a new program or library, which is entirely
3693 dependent on a build-time condition. It is useful to indicate
3694 in the change log the conditions for which a change applies.
3696 Our convention for indicating conditional changes is to use
3697 @emph{square brackets around the name of the condition}.
3699 Conditional changes can happen in numerous scenarios and with many
3700 variations, so here are some examples to help clarify. This first
3701 example describes changes in C, Perl, and Python files which are
3702 conditional but do not have an associated function or entity name:
3705 * xterm.c [SOLARIS2]: Include <string.h>.
3706 * FilePath.pm [$^O eq 'VMS']: Import the VMS::Feature module.
3707 * framework.py [sys.version_info < (2, 6)]: Make "with" statement
3708 available by importing it from __future__,
3709 to support also python 2.5.
3712 Our other examples will for simplicity be limited to C, as the minor
3713 changes necessary to adapt them to other languages should be
3716 Next, here is an entry describing a new definition which is entirely
3717 conditional: the C macro @code{FRAME_WINDOW_P} is defined (and used)
3718 only when the macro @code{HAVE_X_WINDOWS} is defined:
3721 * frame.h [HAVE_X_WINDOWS] (FRAME_WINDOW_P): Macro defined.
3724 Next, an entry for a change within the function @code{init_display},
3725 whose definition as a whole is unconditional, but the changes
3726 themselves are contained in a @samp{#ifdef HAVE_LIBNCURSES}
3730 * dispnew.c (init_display) [HAVE_LIBNCURSES]: If X, call tgetent.
3733 Finally, here is an entry for a change that takes effect only when
3734 a certain macro is @emph{not} defined:
3737 * host.c (gethostname) [!HAVE_SOCKETS]: Replace with winsock version.
3741 @node Indicating the Part Changed
3742 @subsection Indicating the Part Changed
3744 Indicate the part of a function which changed by using angle brackets
3745 enclosing an indication of what the changed part does. Here is an entry
3746 for a change in the part of the function @code{sh-while-getopts} that
3747 deals with @code{sh} commands:
3750 * progmodes/sh-script.el (sh-while-getopts) <sh>: Handle case that
3751 user-specified option string is empty.
3759 In the GNU project, man pages are secondary. It is not necessary or
3760 expected for every GNU program to have a man page, but some of them do.
3761 It's your choice whether to include a man page in your program.
3763 When you make this decision, consider that supporting a man page
3764 requires continual effort each time the program is changed. The time
3765 you spend on the man page is time taken away from more useful work.
3767 For a simple program which changes little, updating the man page may be
3768 a small job. Then there is little reason not to include a man page, if
3771 For a large program that changes a great deal, updating a man page may
3772 be a substantial burden. If a user offers to donate a man page, you may
3773 find this gift costly to accept. It may be better to refuse the man
3774 page unless the same person agrees to take full responsibility for
3775 maintaining it---so that you can wash your hands of it entirely. If
3776 this volunteer later ceases to do the job, then don't feel obliged to
3777 pick it up yourself; it may be better to withdraw the man page from the
3778 distribution until someone else agrees to update it.
3780 When a program changes only a little, you may feel that the
3781 discrepancies are small enough that the man page remains useful without
3782 updating. If so, put a prominent note near the beginning of the man
3783 page explaining that you don't maintain it and that the Texinfo manual
3784 is more authoritative. The note should say how to access the Texinfo
3787 Be sure that man pages include a copyright statement and free license.
3788 The simple all-permissive license is appropriate for simple man pages
3789 (@pxref{License Notices for Other Files,,,maintain,Information for GNU
3792 For long man pages, with enough explanation and documentation that
3793 they can be considered true manuals, use the GFDL (@pxref{License for
3796 Finally, the GNU help2man program
3797 (@uref{http://www.gnu.org/software/help2man/}) is one way to automate
3798 generation of a man page, in this case from @option{--help} output.
3799 This is sufficient in many cases.
3801 @node Reading other Manuals
3802 @section Reading other Manuals
3804 There may be non-free books or documentation files that describe the
3805 program you are documenting.
3807 It is ok to use these documents for reference, just as the author of a
3808 new algebra textbook can read other books on algebra. A large portion
3809 of any non-fiction book consists of facts, in this case facts about how
3810 a certain program works, and these facts are necessarily the same for
3811 everyone who writes about the subject. But be careful not to copy your
3812 outline structure, wording, tables or examples from preexisting non-free
3813 documentation. Copying from free documentation may be ok; please check
3814 with the FSF about the individual case.
3816 @node Managing Releases
3817 @chapter The Release Process
3820 Making a release is more than just bundling up your source files in a
3821 tar file and putting it up for FTP. You should set up your software so
3822 that it can be configured to run on a variety of systems. Your Makefile
3823 should conform to the GNU standards described below, and your directory
3824 layout should also conform to the standards discussed below. Doing so
3825 makes it easy to include your package into the larger framework of
3829 * Configuration:: How configuration of GNU packages should work.
3830 * Makefile Conventions:: Makefile conventions.
3831 * Releases:: Making releases
3835 @section How Configuration Should Work
3836 @cindex program configuration
3839 Each GNU distribution should come with a shell script named
3840 @code{configure}. This script is given arguments which describe the
3841 kind of machine and system you want to compile the program for.
3842 The @code{configure} script must record the configuration options so
3843 that they affect compilation.
3845 The description here is the specification of the interface for the
3846 @code{configure} script in GNU packages. Many packages implement it
3847 using GNU Autoconf (@pxref{Top,, Introduction, autoconf, Autoconf})
3848 and/or GNU Automake (@pxref{Top,, Introduction, automake, Automake}),
3849 but you do not have to use these tools. You can implement it any way
3850 you like; for instance, by making @code{configure} be a wrapper around
3851 a completely different configuration system.
3853 Another way for the @code{configure} script to operate is to make a
3854 link from a standard name such as @file{config.h} to the proper
3855 configuration file for the chosen system. If you use this technique,
3856 the distribution should @emph{not} contain a file named
3857 @file{config.h}. This is so that people won't be able to build the
3858 program without configuring it first.
3860 Another thing that @code{configure} can do is to edit the Makefile. If
3861 you do this, the distribution should @emph{not} contain a file named
3862 @file{Makefile}. Instead, it should include a file @file{Makefile.in} which
3863 contains the input used for editing. Once again, this is so that people
3864 won't be able to build the program without configuring it first.
3866 If @code{configure} does write the @file{Makefile}, then @file{Makefile}
3867 should have a target named @file{Makefile} which causes @code{configure}
3868 to be rerun, setting up the same configuration that was set up last
3869 time. The files that @code{configure} reads should be listed as
3870 dependencies of @file{Makefile}.
3872 All the files which are output from the @code{configure} script should
3873 have comments at the beginning explaining that they were generated
3874 automatically using @code{configure}. This is so that users won't think
3875 of trying to edit them by hand.
3877 The @code{configure} script should write a file named @file{config.status}
3878 which describes which configuration options were specified when the
3879 program was last configured. This file should be a shell script which,
3880 if run, will recreate the same configuration.
3882 The @code{configure} script should accept an option of the form
3883 @samp{--srcdir=@var{dirname}} to specify the directory where sources are found
3884 (if it is not the current directory). This makes it possible to build
3885 the program in a separate directory, so that the actual source directory
3888 If the user does not specify @samp{--srcdir}, then @code{configure} should
3889 check both @file{.} and @file{..} to see if it can find the sources. If
3890 it finds the sources in one of these places, it should use them from
3891 there. Otherwise, it should report that it cannot find the sources, and
3892 should exit with nonzero status.
3894 Usually the easy way to support @samp{--srcdir} is by editing a
3895 definition of @code{VPATH} into the Makefile. Some rules may need to
3896 refer explicitly to the specified source directory. To make this
3897 possible, @code{configure} can add to the Makefile a variable named
3898 @code{srcdir} whose value is precisely the specified directory.
3900 In addition, the @samp{configure} script should take options
3901 corresponding to most of the standard directory variables
3902 (@pxref{Directory Variables}). Here is the list:
3905 --prefix --exec-prefix --bindir --sbindir --libexecdir --sysconfdir
3906 --sharedstatedir --localstatedir --libdir --includedir --oldincludedir
3907 --datarootdir --datadir --infodir --localedir --mandir --docdir
3908 --htmldir --dvidir --pdfdir --psdir
3911 The @code{configure} script should also take an argument which specifies the
3912 type of system to build the program for. This argument should look like
3916 @var{cpu}-@var{company}-@var{system}
3919 For example, an Athlon-based GNU/Linux system might be
3920 @samp{i686-pc-linux-gnu}.
3922 The @code{configure} script needs to be able to decode all plausible
3923 alternatives for how to describe a machine. Thus,
3924 @samp{athlon-pc-gnu/linux} would be a valid alias. There is a shell
3926 @uref{http://git.savannah.gnu.org/@/gitweb/@/?p=config.git;a=blob_plain;f=config.sub;hb=HEAD,
3927 @file{config.sub}} that you can use as a subroutine to validate system
3928 types and canonicalize aliases.
3930 The @code{configure} script should also take the option
3931 @option{--build=@var{buildtype}}, which should be equivalent to a
3932 plain @var{buildtype} argument. For example, @samp{configure
3933 --build=i686-pc-linux-gnu} is equivalent to @samp{configure
3934 i686-pc-linux-gnu}. When the build type is not specified by an option
3935 or argument, the @code{configure} script should normally guess it using
3937 @uref{http://git.savannah.gnu.org/@/gitweb/@/?p=config.git;a=blob_plain;f=config.guess;hb=HEAD,
3938 @file{config.guess}}.
3940 @cindex optional features, configure-time
3941 Other options are permitted to specify in more detail the software
3942 or hardware present on the machine, to include or exclude optional parts
3943 of the package, or to adjust the name of some tools or arguments to them:
3946 @item --enable-@var{feature}@r{[}=@var{parameter}@r{]}
3947 Configure the package to build and install an optional user-level
3948 facility called @var{feature}. This allows users to choose which
3949 optional features to include. Giving an optional @var{parameter} of
3950 @samp{no} should omit @var{feature}, if it is built by default.
3952 No @samp{--enable} option should @strong{ever} cause one feature to
3953 replace another. No @samp{--enable} option should ever substitute one
3954 useful behavior for another useful behavior. The only proper use for
3955 @samp{--enable} is for questions of whether to build part of the program
3958 @item --with-@var{package}
3959 @c @r{[}=@var{parameter}@r{]}
3960 The package @var{package} will be installed, so configure this package
3961 to work with @var{package}.
3963 @c Giving an optional @var{parameter} of
3964 @c @samp{no} should omit @var{package}, if it is used by default.
3966 Possible values of @var{package} include
3967 @samp{gnu-as} (or @samp{gas}), @samp{gnu-ld}, @samp{gnu-libc},
3973 Do not use a @samp{--with} option to specify the file name to use to
3974 find certain files. That is outside the scope of what @samp{--with}
3977 @item @var{variable}=@var{value}
3978 Set the value of the variable @var{variable} to @var{value}. This is
3979 used to override the default values of commands or arguments in the
3980 build process. For example, the user could issue @samp{configure
3981 CFLAGS=-g CXXFLAGS=-g} to build with debugging information and without
3982 the default optimization.
3984 Specifying variables as arguments to @code{configure}, like this:
3988 is preferable to setting them in environment variables:
3992 as it helps to recreate the same configuration later with
3993 @file{config.status}. However, both methods should be supported.
3996 All @code{configure} scripts should accept all of the ``detail''
3997 options and the variable settings, whether or not they make any
3998 difference to the particular package at hand. In particular, they
3999 should accept any option that starts with @samp{--with-} or
4000 @samp{--enable-}. This is so users will be able to configure an
4001 entire GNU source tree at once with a single set of options.
4003 You will note that the categories @samp{--with-} and @samp{--enable-}
4004 are narrow: they @strong{do not} provide a place for any sort of option
4005 you might think of. That is deliberate. We want to limit the possible
4006 configuration options in GNU software. We do not want GNU programs to
4007 have idiosyncratic configuration options.
4009 Packages that perform part of the compilation process may support
4010 cross-compilation. In such a case, the host and target machines for the
4011 program may be different.
4013 The @code{configure} script should normally treat the specified type of
4014 system as both the host and the target, thus producing a program which
4015 works for the same type of machine that it runs on.
4017 To compile a program to run on a host type that differs from the build
4018 type, use the configure option @option{--host=@var{hosttype}}, where
4019 @var{hosttype} uses the same syntax as @var{buildtype}. The host type
4020 normally defaults to the build type.
4022 To configure a cross-compiler, cross-assembler, or what have you, you
4023 should specify a target different from the host, using the configure
4024 option @samp{--target=@var{targettype}}. The syntax for
4025 @var{targettype} is the same as for the host type. So the command would
4029 ./configure --host=@var{hosttype} --target=@var{targettype}
4032 The target type normally defaults to the host type.
4033 Programs for which cross-operation is not meaningful need not accept the
4034 @samp{--target} option, because configuring an entire operating system for
4035 cross-operation is not a meaningful operation.
4037 Some programs have ways of configuring themselves automatically. If
4038 your program is set up to do this, your @code{configure} script can simply
4039 ignore most of its arguments.
4041 @comment The makefile standards are in a separate file that is also
4042 @comment included by make.texinfo. Done by roland@gnu.ai.mit.edu on 1/6/93.
4043 @comment For this document, turn chapters into sections, etc.
4045 @include make-stds.texi
4049 @section Making Releases
4052 @cindex version numbers, for releases
4053 You should identify each release with a pair of version numbers, a
4054 major version and a minor. We have no objection to using more than
4055 two numbers, but it is very unlikely that you really need them.
4057 Package the distribution of @code{Foo version 69.96} up in a gzipped tar
4058 file with the name @file{foo-69.96.tar.gz}. It should unpack into a
4059 subdirectory named @file{foo-69.96}.
4061 Building and installing the program should never modify any of the files
4062 contained in the distribution. This means that all the files that form
4063 part of the program in any way must be classified into @dfn{source
4064 files} and @dfn{non-source files}. Source files are written by humans
4065 and never changed automatically; non-source files are produced from
4066 source files by programs under the control of the Makefile.
4068 @cindex @file{README} file
4069 The distribution should contain a file named @file{README} with a
4070 general overview of the package:
4073 @item the name of the package;
4075 @item the version number of the package, or refer to where in the
4076 package the version can be found;
4078 @item a general description of what the package does;
4080 @item a reference to the file @file{INSTALL}, which
4081 should in turn contain an explanation of the installation procedure;
4083 @item a brief explanation of any unusual top-level directories or
4084 files, or other hints for readers to find their way around the source;
4086 @item a reference to the file which contains the copying conditions.
4087 The GNU GPL, if used, should be in a file called @file{COPYING}. If
4088 the GNU LGPL is used, it should be in a file called
4089 @file{COPYING.LESSER}.
4092 Naturally, all the source files must be in the distribution. It is
4093 okay to include non-source files in the distribution along with the
4094 source files they are generated from, provided they are up-to-date
4095 with the source they are made from, and machine-independent, so that
4096 normal building of the distribution will never modify them. We
4097 commonly include non-source files produced by Autoconf, Automake,
4098 Bison, @code{flex}, @TeX{}, and @code{makeinfo}; this helps avoid
4099 unnecessary dependencies between our distributions, so that users can
4100 install whichever versions of whichever packages they like. Do not
4101 induce new dependencies on other software lightly.
4103 Non-source files that might actually be modified by building and
4104 installing the program should @strong{never} be included in the
4105 distribution. So if you do distribute non-source files, always make
4106 sure they are up to date when you make a new distribution.
4108 Make sure that all the files in the distribution are world-readable, and
4109 that directories are world-readable and world-searchable (octal mode 755).
4110 We used to recommend that all directories in the distribution also be
4111 world-writable (octal mode 777), because ancient versions of @code{tar}
4112 would otherwise not cope when extracting the archive as an unprivileged
4113 user. That can easily lead to security issues when creating the archive,
4114 however, so now we recommend against that.
4116 Don't include any symbolic links in the distribution itself. If the tar
4117 file contains symbolic links, then people cannot even unpack it on
4118 systems that don't support symbolic links. Also, don't use multiple
4119 names for one file in different directories, because certain file
4120 systems cannot handle this and that prevents unpacking the
4123 Try to make sure that all the file names will be unique on MS-DOS. A
4124 name on MS-DOS consists of up to 8 characters, optionally followed by a
4125 period and up to three characters. MS-DOS will truncate extra
4126 characters both before and after the period. Thus,
4127 @file{foobarhacker.c} and @file{foobarhacker.o} are not ambiguous; they
4128 are truncated to @file{foobarha.c} and @file{foobarha.o}, which are
4131 @cindex @file{texinfo.tex}, in a distribution
4132 Include in your distribution a copy of the @file{texinfo.tex} you used
4133 to test print any @file{*.texinfo} or @file{*.texi} files.
4135 Likewise, if your program uses small GNU software packages like regex,
4136 getopt, obstack, or termcap, include them in the distribution file.
4137 Leaving them out would make the distribution file a little smaller at
4138 the expense of possible inconvenience to a user who doesn't know what
4142 @chapter References to Non-Free Software and Documentation
4143 @cindex references to non-free material
4145 A GNU program should not recommend, promote, or grant legitimacy to
4146 the use of any non-free program. Proprietary software is a social and
4147 ethical problem, and our aim is to put an end to that problem. We
4148 can't stop some people from writing proprietary programs, or stop
4149 other people from using them, but we can and should refuse to
4150 advertise them to new potential customers, or to give the public the
4151 idea that their existence is ethical.
4153 The GNU definition of free software is found on the GNU web site at
4154 @url{http://www.gnu.org/@/philosophy/@/free-sw.html}, and the definition
4155 of free documentation is found at
4156 @url{http://www.gnu.org/@/philosophy/@/free-doc.html}. The terms ``free''
4157 and ``non-free'', used in this document, refer to those definitions.
4159 A list of important licenses and whether they qualify as free is in
4160 @url{http://www.gnu.org/@/licenses/@/license-list.html}. If it is not
4161 clear whether a license qualifies as free, please ask the GNU Project
4162 by writing to @email{licensing@@gnu.org}. We will answer, and if the
4163 license is an important one, we will add it to the list.
4165 When a non-free program or system is well known, you can mention it in
4166 passing---that is harmless, since users who might want to use it
4167 probably already know about it. For instance, it is fine to explain
4168 how to build your package on top of some widely used non-free
4169 operating system, or how to use it together with some widely used
4172 However, you should give only the necessary information to help those
4173 who already use the non-free program to use your program with
4174 it---don't give, or refer to, any further information about the
4175 proprietary program, and don't imply that the proprietary program
4176 enhances your program, or that its existence is in any way a good
4177 thing. The goal should be that people already using the proprietary
4178 program will get the advice they need about how to use your free
4179 program with it, while people who don't already use the proprietary
4180 program will not see anything likely to lead them to take an interest
4183 If a non-free program or system is obscure in your program's domain,
4184 your program should not mention or support it at all, since doing so
4185 would tend to popularize the non-free program more than it popularizes
4186 your program. (You cannot hope to find many additional users for your
4187 program among the users of Foobar, if the existence of Foobar is not
4188 generally known among people who might want to use your program.)
4190 Sometimes a program is free software in itself but depends on a
4191 non-free platform in order to run. For instance, many Java programs
4192 depend on some non-free Java libraries. To recommend or promote such
4193 a program is to promote the other programs it needs. This is why we
4194 are careful about listing Java programs in the Free Software
4195 Directory: we don't want to promote the non-free Java libraries.
4197 We hope this particular problem with Java will be gone by and by, as
4198 we replace the remaining non-free standard Java libraries with free
4199 software, but the general principle will remain the same: don't
4200 recommend, promote or legitimize programs that depend on non-free
4203 Some free programs strongly encourage the use of non-free software. A
4204 typical example is @command{mplayer}. It is free software in itself,
4205 and the free code can handle some kinds of files. However,
4206 @command{mplayer} recommends use of non-free codecs for other kinds of
4207 files, and users that install @command{mplayer} are very likely to
4208 install those codecs along with it. To recommend @command{mplayer}
4209 is, in effect, to promote use of the non-free codecs.
4211 Thus, you should not recommend programs that strongly encourage the
4212 use of non-free software. This is why we do not list
4213 @command{mplayer} in the Free Software Directory.
4215 A GNU package should not refer the user to any non-free documentation
4216 for free software. Free documentation that can be included in free
4217 operating systems is essential for completing the GNU system, or any
4218 free operating system, so encouraging it is a priority; to recommend
4219 use of documentation that we are not allowed to include undermines the
4220 impetus for the community to produce documentation that we can
4221 include. So GNU packages should never recommend non-free
4224 By contrast, it is ok to refer to journal articles and textbooks in
4225 the comments of a program for explanation of how it functions, even
4226 though they are non-free. This is because we don't include such
4227 things in the GNU system even if they are free---they are outside the
4228 scope of what a software distribution needs to include.
4230 Referring to a web site that describes or recommends a non-free
4231 program is promoting that program, so please do not make links to (or
4232 mention by name) web sites that contain such material. This policy is
4233 relevant particularly for the web pages for a GNU package.
4235 Following links from nearly any web site can lead eventually to
4236 non-free software; this is inherent in the nature of the web. So it
4237 makes no sense to criticize a site for having such links. As long as
4238 the site does not itself recommend a non-free program, there is no
4239 need to consider the question of the sites that it links to for other
4242 Thus, for example, you should not refer to AT&T's web site if that
4243 recommends AT&T's non-free software packages; you should not refer to
4244 a site that links to AT&T's site presenting it as a place to get some
4245 non-free program, because that link recommends and legitimizes the
4246 non-free program. However, that a site contains a link to AT&T's web
4247 site for some other purpose (such as long-distance telephone service)
4248 is not an objection against it.
4250 @node GNU Free Documentation License
4251 @appendix GNU Free Documentation License
4253 @cindex FDL, GNU Free Documentation License
4263 eval: (add-hook 'write-file-hooks 'time-stamp)
4264 time-stamp-start: "@set lastupdate "
4266 time-stamp-format: "%:b %:d, %:y"
4267 compile-command: "cd work.s && make"