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 March 25, 2010
9 @dircategory GNU organization
11 * Standards: (standards). GNU coding standards.
14 @c @setchapternewpage odd
15 @setchapternewpage off
17 @c Put everything in one index (arbitrarily chosen to be the concept index).
23 @c This is used by a cross ref in make-stds.texi
27 The GNU coding standards, last updated @value{lastupdate}.
29 Copyright @copyright{} 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999,
30 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010 Free Software
33 Permission is granted to copy, distribute and/or modify this document
34 under the terms of the GNU Free Documentation License, Version 1.3 or
35 any later version published by the Free Software Foundation; with no
36 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
37 Texts. A copy of the license is included in the section entitled
38 ``GNU Free Documentation License''.
42 @title GNU Coding Standards
43 @author Richard Stallman, et al.
44 @author last updated @value{lastupdate}
46 @vskip 0pt plus 1filll
53 @node Top, Preface, (dir), (dir)
60 * Preface:: About the GNU Coding Standards.
61 * Legal Issues:: Keeping free software free.
62 * Design Advice:: General program design.
63 * Program Behavior:: Program behavior for all programs
64 * Writing C:: Making the best use of C.
65 * Documentation:: Documenting programs.
66 * Managing Releases:: The release process.
67 * References:: Mentioning non-free software or documentation.
68 * GNU Free Documentation License:: Copying and sharing this manual.
74 @chapter About the GNU Coding Standards
76 The GNU Coding Standards were written by Richard Stallman and other GNU
77 Project volunteers. Their purpose is to make the GNU system clean,
78 consistent, and easy to install. This document can also be read as a
79 guide to writing portable, robust and reliable programs. It focuses on
80 programs written in C, but many of the rules and principles are useful
81 even if you write in another programming language. The rules often
82 state reasons for writing in a certain way.
84 @cindex where to obtain @code{standards.texi}
85 @cindex downloading this manual
86 If you did not obtain this file directly from the GNU project and
87 recently, please check for a newer version. You can get the GNU
88 Coding Standards from the GNU web server in many
89 different formats, including the Texinfo source, PDF, HTML, DVI, plain
90 text, and more, at: @uref{http://www.gnu.org/prep/standards/}.
92 If you are maintaining an official GNU package, in addition to this
93 document, please read and follow the GNU maintainer information
94 (@pxref{Top, , Contents, maintain, Information for Maintainers of GNU
97 @cindex @code{gnustandards-commit@@gnu.org} mailing list
98 If you want to receive diffs for every change to these GNU documents,
99 join the mailing list @code{gnustandards-commit@@gnu.org}, via the web
101 @url{http://lists.gnu.org/mailman/listinfo/gnustandards-commit}.
102 Archives are also available there.
104 @cindex @code{bug-standards@@gnu.org} email address
105 @cindex Savannah repository for gnustandards
106 @cindex gnustandards project repository
107 Please send corrections or suggestions for this document to
108 @email{bug-standards@@gnu.org}. If you make a suggestion, please
109 include a suggested new wording for it, to help us consider the
110 suggestion efficiently. We prefer a context diff to the Texinfo
111 source, but if that's difficult for you, you can make a context diff
112 for some other version of this document, or propose it in any way that
113 makes it clear. The source repository for this document can be found
114 at @url{http://savannah.gnu.org/projects/gnustandards}.
116 These standards cover the minimum of what is important when writing a
117 GNU package. Likely, the need for additional standards will come up.
118 Sometimes, you might suggest that such standards be added to this
119 document. If you think your standards would be generally useful, please
122 You should also set standards for your package on many questions not
123 addressed or not firmly specified here. The most important point is to
124 be self-consistent---try to stick to the conventions you pick, and try
125 to document them as much as possible. That way, your program will be
126 more maintainable by others.
128 The GNU Hello program serves as an example of how to follow the GNU
129 coding standards for a trivial program.
130 @uref{http://www.gnu.org/software/hello/hello.html}.
132 This release of the GNU Coding Standards was last updated
137 @chapter Keeping Free Software Free
138 @cindex legal aspects
140 This chapter discusses how you can make sure that GNU software
141 avoids legal difficulties, and other related issues.
144 * Reading Non-Free Code:: Referring to proprietary programs.
145 * Contributions:: Accepting contributions.
146 * Trademarks:: How we deal with trademark issues.
149 @node Reading Non-Free Code
150 @section Referring to Proprietary Programs
151 @cindex proprietary programs
152 @cindex avoiding proprietary code
154 Don't in any circumstances refer to Unix source code for or during
155 your work on GNU! (Or to any other proprietary programs.)
157 If you have a vague recollection of the internals of a Unix program,
158 this does not absolutely mean you can't write an imitation of it, but
159 do try to organize the imitation internally along different lines,
160 because this is likely to make the details of the Unix version
161 irrelevant and dissimilar to your results.
163 For example, Unix utilities were generally optimized to minimize
164 memory use; if you go for speed instead, your program will be very
165 different. You could keep the entire input file in memory and scan it
166 there instead of using stdio. Use a smarter algorithm discovered more
167 recently than the Unix program. Eliminate use of temporary files. Do
168 it in one pass instead of two (we did this in the assembler).
170 Or, on the contrary, emphasize simplicity instead of speed. For some
171 applications, the speed of today's computers makes simpler algorithms
174 Or go for generality. For example, Unix programs often have static
175 tables or fixed-size strings, which make for arbitrary limits; use
176 dynamic allocation instead. Make sure your program handles NULs and
177 other funny characters in the input files. Add a programming language
178 for extensibility and write part of the program in that language.
180 Or turn some parts of the program into independently usable libraries.
181 Or use a simple garbage collector instead of tracking precisely when
182 to free memory, or use a new GNU facility such as obstacks.
185 @section Accepting Contributions
187 @cindex accepting contributions
189 If the program you are working on is copyrighted by the Free Software
190 Foundation, then when someone else sends you a piece of code to add to
191 the program, we need legal papers to use it---just as we asked you to
192 sign papers initially. @emph{Each} person who makes a nontrivial
193 contribution to a program must sign some sort of legal papers in order
194 for us to have clear title to the program; the main author alone is not
197 So, before adding in any contributions from other people, please tell
198 us, so we can arrange to get the papers. Then wait until we tell you
199 that we have received the signed papers, before you actually use the
202 This applies both before you release the program and afterward. If
203 you receive diffs to fix a bug, and they make significant changes, we
204 need legal papers for that change.
206 This also applies to comments and documentation files. For copyright
207 law, comments and code are just text. Copyright applies to all kinds of
208 text, so we need legal papers for all kinds.
210 We know it is frustrating to ask for legal papers; it's frustrating for
211 us as well. But if you don't wait, you are going out on a limb---for
212 example, what if the contributor's employer won't sign a disclaimer?
213 You might have to take that code out again!
215 You don't need papers for changes of a few lines here or there, since
216 they are not significant for copyright purposes. Also, you don't need
217 papers if all you get from the suggestion is some ideas, not actual code
218 which you use. For example, if someone sent you one implementation, but
219 you write a different implementation of the same idea, you don't need to
222 The very worst thing is if you forget to tell us about the other
223 contributor. We could be very embarrassed in court some day as a
226 We have more detailed advice for maintainers of programs; if you have
227 reached the stage of actually maintaining a program for GNU (whether
228 released or not), please ask us for a copy. It is also available
229 online for your perusal: @uref{http://www.gnu.org/prep/maintain/}.
235 Please do not include any trademark acknowledgements in GNU software
236 packages or documentation.
238 Trademark acknowledgements are the statements that such-and-such is a
239 trademark of so-and-so. The GNU Project has no objection to the basic
240 idea of trademarks, but these acknowledgements feel like kowtowing,
241 and there is no legal requirement for them, so we don't use them.
243 What is legally required, as regards other people's trademarks, is to
244 avoid using them in ways which a reader might reasonably understand as
245 naming or labeling our own programs or activities. For example, since
246 ``Objective C'' is (or at least was) a trademark, we made sure to say
247 that we provide a ``compiler for the Objective C language'' rather
248 than an ``Objective C compiler''. The latter would have been meant as
249 a shorter way of saying the former, but it does not explicitly state
250 the relationship, so it could be misinterpreted as using ``Objective
251 C'' as a label for the compiler rather than for the language.
253 Please don't use ``win'' as an abbreviation for Microsoft Windows in
254 GNU software or documentation. In hacker terminology, calling
255 something a ``win'' is a form of praise. If you wish to praise
256 Microsoft Windows when speaking on your own, by all means do so, but
257 not in GNU software. Usually we write the name ``Windows'' in full,
258 but when brevity is very important (as in file names and sometimes
259 symbol names), we abbreviate it to ``w''. For instance, the files and
260 functions in Emacs that deal with Windows start with @samp{w32}.
263 @chapter General Program Design
264 @cindex program design
266 This chapter discusses some of the issues you should take into
267 account when designing your program.
269 @c Standard or ANSI C
271 @c In 1989 the American National Standards Institute (ANSI) standardized
272 @c C as standard X3.159-1989. In December of that year the
273 @c International Standards Organization ISO adopted the ANSI C standard
274 @c making minor changes. In 1990 ANSI then re-adopted ISO standard
275 @c C. This version of C is known as either ANSI C or Standard C.
277 @c A major revision of the C Standard appeared in 1999.
280 * Source Language:: Which languages to use.
281 * Compatibility:: Compatibility with other implementations.
282 * Using Extensions:: Using non-standard features.
283 * Standard C:: Using standard C features.
284 * Conditional Compilation:: Compiling code only if a conditional is true.
287 @node Source Language
288 @section Which Languages to Use
289 @cindex programming languages
291 When you want to use a language that gets compiled and runs at high
292 speed, the best language to use is C. Using another language is like
293 using a non-standard feature: it will cause trouble for users. Even if
294 GCC supports the other language, users may find it inconvenient to have
295 to install the compiler for that other language in order to build your
296 program. For example, if you write your program in C++, people will
297 have to install the GNU C++ compiler in order to compile your program.
299 C has one other advantage over C++ and other compiled languages: more
300 people know C, so more people will find it easy to read and modify the
301 program if it is written in C.
303 So in general it is much better to use C, rather than the
304 comparable alternatives.
306 But there are two exceptions to that conclusion:
310 It is no problem to use another language to write a tool specifically
311 intended for use with that language. That is because the only people
312 who want to build the tool will be those who have installed the other
316 If an application is of interest only to a narrow part of the community,
317 then the question of which language it is written in has less effect on
318 other people, so you may as well please yourself.
321 Many programs are designed to be extensible: they include an interpreter
322 for a language that is higher level than C. Often much of the program
323 is written in that language, too. The Emacs editor pioneered this
327 @cindex GNOME and Guile
328 The standard extensibility interpreter for GNU software is Guile
329 (@uref{http://www.gnu.org/@/software/@/guile/}), which implements the
330 language Scheme (an especially clean and simple dialect of Lisp).
331 Guile also includes bindings for GTK+/GNOME, making it practical to
332 write modern GUI functionality within Guile. We don't reject programs
333 written in other ``scripting languages'' such as Perl and Python, but
334 using Guile is very important for the overall consistency of the GNU
339 @section Compatibility with Other Implementations
340 @cindex compatibility with C and @sc{posix} standards
341 @cindex @sc{posix} compatibility
343 With occasional exceptions, utility programs and libraries for GNU
344 should be upward compatible with those in Berkeley Unix, and upward
345 compatible with Standard C if Standard C specifies their
346 behavior, and upward compatible with @sc{posix} if @sc{posix} specifies
349 When these standards conflict, it is useful to offer compatibility
350 modes for each of them.
352 @cindex options for compatibility
353 Standard C and @sc{posix} prohibit many kinds of extensions. Feel
354 free to make the extensions anyway, and include a @samp{--ansi},
355 @samp{--posix}, or @samp{--compatible} option to turn them off.
356 However, if the extension has a significant chance of breaking any real
357 programs or scripts, then it is not really upward compatible. So you
358 should try to redesign its interface to make it upward compatible.
360 @cindex @code{POSIXLY_CORRECT}, environment variable
361 Many GNU programs suppress extensions that conflict with @sc{posix} if the
362 environment variable @code{POSIXLY_CORRECT} is defined (even if it is
363 defined with a null value). Please make your program recognize this
364 variable if appropriate.
366 When a feature is used only by users (not by programs or command
367 files), and it is done poorly in Unix, feel free to replace it
368 completely with something totally different and better. (For example,
369 @code{vi} is replaced with Emacs.) But it is nice to offer a compatible
370 feature as well. (There is a free @code{vi} clone, so we offer it.)
372 Additional useful features are welcome regardless of whether
373 there is any precedent for them.
375 @node Using Extensions
376 @section Using Non-standard Features
377 @cindex non-standard extensions
379 Many GNU facilities that already exist support a number of convenient
380 extensions over the comparable Unix facilities. Whether to use these
381 extensions in implementing your program is a difficult question.
383 On the one hand, using the extensions can make a cleaner program.
384 On the other hand, people will not be able to build the program
385 unless the other GNU tools are available. This might cause the
386 program to work on fewer kinds of machines.
388 With some extensions, it might be easy to provide both alternatives.
389 For example, you can define functions with a ``keyword'' @code{INLINE}
390 and define that as a macro to expand into either @code{inline} or
391 nothing, depending on the compiler.
393 In general, perhaps it is best not to use the extensions if you can
394 straightforwardly do without them, but to use the extensions if they
395 are a big improvement.
397 An exception to this rule are the large, established programs (such as
398 Emacs) which run on a great variety of systems. Using GNU extensions in
399 such programs would make many users unhappy, so we don't do that.
401 Another exception is for programs that are used as part of compilation:
402 anything that must be compiled with other compilers in order to
403 bootstrap the GNU compilation facilities. If these require the GNU
404 compiler, then no one can compile them without having them installed
405 already. That would be extremely troublesome in certain cases.
408 @section Standard C and Pre-Standard C
409 @cindex @sc{ansi} C standard
411 1989 Standard C is widespread enough now that it is ok to use its
412 features in new programs. There is one exception: do not ever use the
413 ``trigraph'' feature of Standard C.
415 1999 Standard C is not widespread yet, so please do not require its
416 features in programs. It is ok to use its features if they are present.
418 However, it is easy to support pre-standard compilers in most programs,
419 so if you know how to do that, feel free. If a program you are
420 maintaining has such support, you should try to keep it working.
422 @cindex function prototypes
423 To support pre-standard C, instead of writing function definitions in
424 standard prototype form,
433 write the definition in pre-standard style like this,
443 and use a separate declaration to specify the argument prototype:
449 You need such a declaration anyway, in a header file, to get the benefit
450 of prototypes in all the files where the function is called. And once
451 you have the declaration, you normally lose nothing by writing the
452 function definition in the pre-standard style.
454 This technique does not work for integer types narrower than @code{int}.
455 If you think of an argument as being of a type narrower than @code{int},
456 declare it as @code{int} instead.
458 There are a few special cases where this technique is hard to use. For
459 example, if a function argument needs to hold the system type
460 @code{dev_t}, you run into trouble, because @code{dev_t} is shorter than
461 @code{int} on some machines; but you cannot use @code{int} instead,
462 because @code{dev_t} is wider than @code{int} on some machines. There
463 is no type you can safely use on all machines in a non-standard
464 definition. The only way to support non-standard C and pass such an
465 argument is to check the width of @code{dev_t} using Autoconf and choose
466 the argument type accordingly. This may not be worth the trouble.
468 In order to support pre-standard compilers that do not recognize
469 prototypes, you may want to use a preprocessor macro like this:
472 /* Declare the prototype for a general external function. */
473 #if defined (__STDC__) || defined (WINDOWSNT)
474 #define P_(proto) proto
480 @node Conditional Compilation
481 @section Conditional Compilation
483 When supporting configuration options already known when building your
484 program we prefer using @code{if (... )} over conditional compilation,
485 as in the former case the compiler is able to perform more extensive
486 checking of all possible code paths.
488 For example, please write
508 A modern compiler such as GCC will generate exactly the same code in
509 both cases, and we have been using similar techniques with good success
510 in several projects. Of course, the former method assumes that
511 @code{HAS_FOO} is defined as either 0 or 1.
513 While this is not a silver bullet solving all portability problems,
514 and is not always appropriate, following this policy would have saved
515 GCC developers many hours, or even days, per year.
517 In the case of function-like macros like @code{REVERSIBLE_CC_MODE} in
518 GCC which cannot be simply used in @code{if (...)} statements, there is
519 an easy workaround. Simply introduce another macro
520 @code{HAS_REVERSIBLE_CC_MODE} as in the following example:
523 #ifdef REVERSIBLE_CC_MODE
524 #define HAS_REVERSIBLE_CC_MODE 1
526 #define HAS_REVERSIBLE_CC_MODE 0
530 @node Program Behavior
531 @chapter Program Behavior for All Programs
533 This chapter describes conventions for writing robust
534 software. It also describes general standards for error messages, the
535 command line interface, and how libraries should behave.
538 * Non-GNU Standards:: We consider standards such as POSIX;
539 we don't "obey" them.
540 * Semantics:: Writing robust programs.
541 * Libraries:: Library behavior.
542 * Errors:: Formatting error messages.
543 * User Interfaces:: Standards about interfaces generally.
544 * Graphical Interfaces:: Standards for graphical interfaces.
545 * Command-Line Interfaces:: Standards for command line interfaces.
546 * Option Table:: Table of long options.
547 * OID Allocations:: Table of OID slots for GNU.
548 * Memory Usage:: When and how to care about memory needs.
549 * File Usage:: Which files to use, and where.
552 @node Non-GNU Standards
553 @section Non-GNU Standards
555 The GNU Project regards standards published by other organizations as
556 suggestions, not orders. We consider those standards, but we do not
557 ``obey'' them. In developing a GNU program, you should implement
558 an outside standard's specifications when that makes the GNU system
559 better overall in an objective sense. When it doesn't, you shouldn't.
561 In most cases, following published standards is convenient for
562 users---it means that their programs or scripts will work more
563 portably. For instance, GCC implements nearly all the features of
564 Standard C as specified by that standard. C program developers would
565 be unhappy if it did not. And GNU utilities mostly follow
566 specifications of POSIX.2; shell script writers and users would be
567 unhappy if our programs were incompatible.
569 But we do not follow either of these specifications rigidly, and there
570 are specific points on which we decided not to follow them, so as to
571 make the GNU system better for users.
573 For instance, Standard C says that nearly all extensions to C are
574 prohibited. How silly! GCC implements many extensions, some of which
575 were later adopted as part of the standard. If you want these
576 constructs to give an error message as ``required'' by the standard,
577 you must specify @samp{--pedantic}, which was implemented only so that
578 we can say ``GCC is a 100% implementation of the standard,'' not
579 because there is any reason to actually use it.
581 POSIX.2 specifies that @samp{df} and @samp{du} must output sizes by
582 default in units of 512 bytes. What users want is units of 1k, so
583 that is what we do by default. If you want the ridiculous behavior
584 ``required'' by POSIX, you must set the environment variable
585 @samp{POSIXLY_CORRECT} (which was originally going to be named
586 @samp{POSIX_ME_HARDER}).
588 GNU utilities also depart from the letter of the POSIX.2 specification
589 when they support long-named command-line options, and intermixing
590 options with ordinary arguments. This minor incompatibility with
591 POSIX is never a problem in practice, and it is very useful.
593 In particular, don't reject a new feature, or remove an old one,
594 merely because a standard says it is ``forbidden'' or ``deprecated.''
597 @section Writing Robust Programs
599 @cindex arbitrary limits on data
600 Avoid arbitrary limits on the length or number of @emph{any} data
601 structure, including file names, lines, files, and symbols, by allocating
602 all data structures dynamically. In most Unix utilities, ``long lines
603 are silently truncated''. This is not acceptable in a GNU utility.
605 @cindex @code{NUL} characters
606 Utilities reading files should not drop NUL characters, or any other
607 nonprinting characters @emph{including those with codes above 0177}.
608 The only sensible exceptions would be utilities specifically intended
609 for interface to certain types of terminals or printers
610 that can't handle those characters.
611 Whenever possible, try to make programs work properly with
612 sequences of bytes that represent multibyte characters, using encodings
613 such as UTF-8 and others.
615 @cindex error messages
616 Check every system call for an error return, unless you know you wish to
617 ignore errors. Include the system error text (from @code{perror} or
618 equivalent) in @emph{every} error message resulting from a failing
619 system call, as well as the name of the file if any and the name of the
620 utility. Just ``cannot open foo.c'' or ``stat failed'' is not
623 @cindex @code{malloc} return value
624 @cindex memory allocation failure
625 Check every call to @code{malloc} or @code{realloc} to see if it
626 returned zero. Check @code{realloc} even if you are making the block
627 smaller; in a system that rounds block sizes to a power of 2,
628 @code{realloc} may get a different block if you ask for less space.
630 In Unix, @code{realloc} can destroy the storage block if it returns
631 zero. GNU @code{realloc} does not have this bug: if it fails, the
632 original block is unchanged. Feel free to assume the bug is fixed. If
633 you wish to run your program on Unix, and wish to avoid lossage in this
634 case, you can use the GNU @code{malloc}.
636 You must expect @code{free} to alter the contents of the block that was
637 freed. Anything you want to fetch from the block, you must fetch before
640 If @code{malloc} fails in a noninteractive program, make that a fatal
641 error. In an interactive program (one that reads commands from the
642 user), it is better to abort the command and return to the command
643 reader loop. This allows the user to kill other processes to free up
644 virtual memory, and then try the command again.
646 @cindex command-line arguments, decoding
647 Use @code{getopt_long} to decode arguments, unless the argument syntax
648 makes this unreasonable.
650 When static storage is to be written in during program execution, use
651 explicit C code to initialize it. Reserve C initialized declarations
652 for data that will not be changed.
655 Try to avoid low-level interfaces to obscure Unix data structures (such
656 as file directories, utmp, or the layout of kernel memory), since these
657 are less likely to work compatibly. If you need to find all the files
658 in a directory, use @code{readdir} or some other high-level interface.
659 These are supported compatibly by GNU.
661 @cindex signal handling
662 The preferred signal handling facilities are the BSD variant of
663 @code{signal}, and the @sc{posix} @code{sigaction} function; the
664 alternative USG @code{signal} interface is an inferior design.
666 Nowadays, using the @sc{posix} signal functions may be the easiest way
667 to make a program portable. If you use @code{signal}, then on GNU/Linux
668 systems running GNU libc version 1, you should include
669 @file{bsd/signal.h} instead of @file{signal.h}, so as to get BSD
670 behavior. It is up to you whether to support systems where
671 @code{signal} has only the USG behavior, or give up on them.
673 @cindex impossible conditions
674 In error checks that detect ``impossible'' conditions, just abort.
675 There is usually no point in printing any message. These checks
676 indicate the existence of bugs. Whoever wants to fix the bugs will have
677 to read the source code and run a debugger. So explain the problem with
678 comments in the source. The relevant data will be in variables, which
679 are easy to examine with the debugger, so there is no point moving them
682 Do not use a count of errors as the exit status for a program.
683 @emph{That does not work}, because exit status values are limited to 8
684 bits (0 through 255). A single run of the program might have 256
685 errors; if you try to return 256 as the exit status, the parent process
686 will see 0 as the status, and it will appear that the program succeeded.
688 @cindex temporary files
689 @cindex @code{TMPDIR} environment variable
690 If you make temporary files, check the @code{TMPDIR} environment
691 variable; if that variable is defined, use the specified directory
692 instead of @file{/tmp}.
694 In addition, be aware that there is a possible security problem when
695 creating temporary files in world-writable directories. In C, you can
696 avoid this problem by creating temporary files in this manner:
699 fd = open (filename, O_WRONLY | O_CREAT | O_EXCL, 0600);
703 or by using the @code{mkstemps} function from libiberty.
705 In bash, use @code{set -C} to avoid this problem.
708 @section Library Behavior
711 Try to make library functions reentrant. If they need to do dynamic
712 storage allocation, at least try to avoid any nonreentrancy aside from
713 that of @code{malloc} itself.
715 Here are certain name conventions for libraries, to avoid name
718 Choose a name prefix for the library, more than two characters long.
719 All external function and variable names should start with this
720 prefix. In addition, there should only be one of these in any given
721 library member. This usually means putting each one in a separate
724 An exception can be made when two external symbols are always used
725 together, so that no reasonable program could use one without the
726 other; then they can both go in the same file.
728 External symbols that are not documented entry points for the user
729 should have names beginning with @samp{_}. The @samp{_} should be
730 followed by the chosen name prefix for the library, to prevent
731 collisions with other libraries. These can go in the same files with
732 user entry points if you like.
734 Static functions and variables can be used as you like and need not
735 fit any naming convention.
738 @section Formatting Error Messages
739 @cindex formatting error messages
740 @cindex error messages, formatting
742 Error messages from compilers should look like this:
745 @var{source-file-name}:@var{lineno}: @var{message}
749 If you want to mention the column number, use one of these formats:
752 @var{source-file-name}:@var{lineno}:@var{column}: @var{message}
753 @var{source-file-name}:@var{lineno}.@var{column}: @var{message}
758 Line numbers should start from 1 at the beginning of the file, and
759 column numbers should start from 1 at the beginning of the line. (Both
760 of these conventions are chosen for compatibility.) Calculate column
761 numbers assuming that space and all ASCII printing characters have
762 equal width, and assuming tab stops every 8 columns.
764 The error message can also give both the starting and ending positions
765 of the erroneous text. There are several formats so that you can
766 avoid redundant information such as a duplicate line number.
767 Here are the possible formats:
770 @var{source-file-name}:@var{lineno-1}.@var{column-1}-@var{lineno-2}.@var{column-2}: @var{message}
771 @var{source-file-name}:@var{lineno-1}.@var{column-1}-@var{column-2}: @var{message}
772 @var{source-file-name}:@var{lineno-1}-@var{lineno-2}: @var{message}
776 When an error is spread over several files, you can use this format:
779 @var{file-1}:@var{lineno-1}.@var{column-1}-@var{file-2}:@var{lineno-2}.@var{column-2}: @var{message}
782 Error messages from other noninteractive programs should look like this:
785 @var{program}:@var{source-file-name}:@var{lineno}: @var{message}
789 when there is an appropriate source file, or like this:
792 @var{program}: @var{message}
796 when there is no relevant source file.
798 If you want to mention the column number, use this format:
801 @var{program}:@var{source-file-name}:@var{lineno}:@var{column}: @var{message}
804 In an interactive program (one that is reading commands from a
805 terminal), it is better not to include the program name in an error
806 message. The place to indicate which program is running is in the
807 prompt or with the screen layout. (When the same program runs with
808 input from a source other than a terminal, it is not interactive and
809 would do best to print error messages using the noninteractive style.)
811 The string @var{message} should not begin with a capital letter when
812 it follows a program name and/or file name, because that isn't the
813 beginning of a sentence. (The sentence conceptually starts at the
814 beginning of the line.) Also, it should not end with a period.
816 Error messages from interactive programs, and other messages such as
817 usage messages, should start with a capital letter. But they should not
820 @node User Interfaces
821 @section Standards for Interfaces Generally
823 @cindex program name and its behavior
824 @cindex behavior, dependent on program's name
825 Please don't make the behavior of a utility depend on the name used
826 to invoke it. It is useful sometimes to make a link to a utility
827 with a different name, and that should not change what it does.
829 Instead, use a run time option or a compilation switch or both
830 to select among the alternate behaviors.
832 @cindex output device and program's behavior
833 Likewise, please don't make the behavior of the program depend on the
834 type of output device it is used with. Device independence is an
835 important principle of the system's design; do not compromise it merely
836 to save someone from typing an option now and then. (Variation in error
837 message syntax when using a terminal is ok, because that is a side issue
838 that people do not depend on.)
840 If you think one behavior is most useful when the output is to a
841 terminal, and another is most useful when the output is a file or a
842 pipe, then it is usually best to make the default behavior the one that
843 is useful with output to a terminal, and have an option for the other
846 Compatibility requires certain programs to depend on the type of output
847 device. It would be disastrous if @code{ls} or @code{sh} did not do so
848 in the way all users expect. In some of these cases, we supplement the
849 program with a preferred alternate version that does not depend on the
850 output device type. For example, we provide a @code{dir} program much
851 like @code{ls} except that its default output format is always
855 @node Graphical Interfaces
856 @section Standards for Graphical Interfaces
857 @cindex graphical user interface
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.
872 Please also consider providing a CORBA interface (for use from GNOME), a
873 library interface (for use from C), and perhaps a keyboard-driven
874 console interface (for use by users from console mode). Once you are
875 doing the work to provide the functionality and the graphical interface,
876 these won't be much extra work.
879 @node Command-Line Interfaces
880 @section Standards for Command Line Interfaces
881 @cindex command-line interface
884 It is a good idea to follow the @sc{posix} guidelines for the
885 command-line options of a program. The easiest way to do this is to use
886 @code{getopt} to parse them. Note that the GNU version of @code{getopt}
887 will normally permit options anywhere among the arguments unless the
888 special argument @samp{--} is used. This is not what @sc{posix}
889 specifies; it is a GNU extension.
891 @cindex long-named options
892 Please define long-named options that are equivalent to the
893 single-letter Unix-style options. We hope to make GNU more user
894 friendly this way. This is easy to do with the GNU function
897 One of the advantages of long-named options is that they can be
898 consistent from program to program. For example, users should be able
899 to expect the ``verbose'' option of any GNU program which has one, to be
900 spelled precisely @samp{--verbose}. To achieve this uniformity, look at
901 the table of common long-option names when you choose the option names
902 for your program (@pxref{Option Table}).
904 It is usually a good idea for file names given as ordinary arguments to
905 be input files only; any output files would be specified using options
906 (preferably @samp{-o} or @samp{--output}). Even if you allow an output
907 file name as an ordinary argument for compatibility, try to provide an
908 option as another way to specify it. This will lead to more consistency
909 among GNU utilities, and fewer idiosyncrasies for users to remember.
911 @cindex standard command-line options
912 @cindex options, standard command-line
913 @cindex CGI programs, standard options for
914 @cindex PATH_INFO, specifying standard options as
915 All programs should support two standard options: @samp{--version}
916 and @samp{--help}. CGI programs should accept these as command-line
917 options, and also if given as the @env{PATH_INFO}; for instance,
918 visiting @url{http://example.org/p.cgi/--help} in a browser should
919 output the same information as invoking @samp{p.cgi --help} from the
923 * --version:: The standard output for --version.
924 * --help:: The standard output for --help.
928 @subsection @option{--version}
930 @cindex @samp{--version} output
932 The standard @code{--version} option should direct the program to
933 print information about its name, version, origin and legal status,
934 all on standard output, and then exit successfully. Other options and
935 arguments should be ignored once this is seen, and the program should
936 not perform its normal function.
938 @cindex canonical name of a program
939 @cindex program's canonical name
940 The first line is meant to be easy for a program to parse; the version
941 number proper starts after the last space. In addition, it contains
942 the canonical name for this program, in this format:
949 The program's name should be a constant string; @emph{don't} compute it
950 from @code{argv[0]}. The idea is to state the standard or canonical
951 name for the program, not its file name. There are other ways to find
952 out the precise file name where a command is found in @code{PATH}.
954 If the program is a subsidiary part of a larger package, mention the
955 package name in parentheses, like this:
958 emacsserver (GNU Emacs) 19.30
962 If the package has a version number which is different from this
963 program's version number, you can mention the package version number
964 just before the close-parenthesis.
966 If you @emph{need} to mention the version numbers of libraries which
967 are distributed separately from the package which contains this program,
968 you can do so by printing an additional line of version info for each
969 library you want to mention. Use the same format for these lines as for
972 Please do not mention all of the libraries that the program uses ``just
973 for completeness''---that would produce a lot of unhelpful clutter.
974 Please mention library version numbers only if you find in practice that
975 they are very important to you in debugging.
977 The following line, after the version number line or lines, should be a
978 copyright notice. If more than one copyright notice is called for, put
979 each on a separate line.
981 Next should follow a line stating the license, preferably using one of
982 abbrevations below, and a brief statement that the program is free
983 software, and that users are free to copy and change it. Also mention
984 that there is no warranty, to the extent permitted by law. See
985 recommended wording below.
987 It is ok to finish the output with a list of the major authors of the
988 program, as a way of giving credit.
990 Here's an example of output that follows these rules:
994 Copyright (C) 2007 Free Software Foundation, Inc.
995 License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
996 This is free software: you are free to change and redistribute it.
997 There is NO WARRANTY, to the extent permitted by law.
1000 You should adapt this to your program, of course, filling in the proper
1001 year, copyright holder, name of program, and the references to
1002 distribution terms, and changing the rest of the wording as necessary.
1004 This copyright notice only needs to mention the most recent year in
1005 which changes were made---there's no need to list the years for previous
1006 versions' changes. You don't have to mention the name of the program in
1007 these notices, if that is inconvenient, since it appeared in the first
1008 line. (The rules are different for copyright notices in source files;
1009 @pxref{Copyright Notices,,,maintain,Information for GNU Maintainers}.)
1011 Translations of the above lines must preserve the validity of the
1012 copyright notices (@pxref{Internationalization}). If the translation's
1013 character set supports it, the @samp{(C)} should be replaced with the
1014 copyright symbol, as follows:
1017 (the official copyright symbol, which is the letter C in a circle);
1023 Write the word ``Copyright'' exactly like that, in English. Do not
1024 translate it into another language. International treaties recognize
1025 the English word ``Copyright''; translations into other languages do not
1026 have legal significance.
1028 Finally, here is the table of our suggested license abbreviations.
1029 Any abbreviation can be followed by @samp{v@var{version}[+]}, meaning
1030 that particular version, or later versions with the @samp{+}, as shown
1033 In the case of exceptions for extra permissions with the GPL, we use
1034 @samp{/} for a separator; the version number can follow the license
1035 abbreviation as usual, as in the examples below.
1039 GNU General Public License, @url{http://www.gnu.org/@/licenses/@/gpl.html}.
1042 GNU Lesser General Public License, @url{http://www.gnu.org/@/licenses/@/lgpl.html}.
1045 GNU GPL with the exception for Ada.
1048 The Apache Software Foundation license,
1049 @url{http://www.apache.org/@/licenses}.
1052 The Artistic license used for Perl, @url{http://www.perlfoundation.org/@/legal}.
1055 The Expat license, @url{http://www.jclark.com/@/xml/@/copying.txt}.
1058 The Mozilla Public License, @url{http://www.mozilla.org/@/MPL/}.
1061 The original (4-clause) BSD license, incompatible with the GNU GPL
1062 @url{http://www.xfree86.org/@/3.3.6/@/COPYRIGHT2.html#6}.
1065 The license used for PHP, @url{http://www.php.net/@/license/}.
1068 The non-license that is being in the public domain,
1069 @url{http://www.gnu.org/@/licenses/@/license-list.html#PublicDomain}.
1072 The license for Python, @url{http://www.python.org/@/2.0.1/@/license.html}.
1075 The revised (3-clause) BSD, compatible with the GNU GPL,
1076 @url{http://www.xfree86.org/@/3.3.6/@/COPYRIGHT2.html#5}.
1079 The simple non-copyleft license used for most versions of the X Window
1080 System, @url{http://www.xfree86.org/@/3.3.6/@/COPYRIGHT2.html#3}.
1083 The license for Zlib, @url{http://www.gzip.org/@/zlib/@/zlib_license.html}.
1087 More information about these licenses and many more are on the GNU
1088 licensing web pages,
1089 @url{http://www.gnu.org/@/licenses/@/license-list.html}.
1093 @subsection @option{--help}
1095 @cindex @samp{--help} output
1097 The standard @code{--help} option should output brief documentation
1098 for how to invoke the program, on standard output, then exit
1099 successfully. Other options and arguments should be ignored once this
1100 is seen, and the program should not perform its normal function.
1102 @cindex address for bug reports
1104 Near the end of the @samp{--help} option's output, please place lines
1105 giving the email address for bug reports, the package's home page
1106 (normally @indicateurl{http://www.gnu.org/software/@var{pkg}}, and the
1107 general page for help using GNU programs. The format should be like this:
1110 Report bugs to: @var{mailing-address}
1111 @var{pkg} home page: <http://www.gnu.org/software/@var{pkg}/>
1112 General help using GNU software: <http://www.gnu.org/gethelp/>
1115 It is ok to mention other appropriate mailing lists and web pages.
1119 @section Table of Long Options
1120 @cindex long option names
1121 @cindex table of long options
1123 Here is a table of long options used by GNU programs. It is surely
1124 incomplete, but we aim to list all the options that a new program might
1125 want to be compatible with. If you use names not already in the table,
1126 please send @email{bug-standards@@gnu.org} a list of them, with their
1127 meanings, so we can update the table.
1129 @c Please leave newlines between items in this table; it's much easier
1130 @c to update when it isn't completely squashed together and unreadable.
1131 @c When there is more than one short option for a long option name, put
1132 @c a semicolon between the lists of the programs that use them, not a
1133 @c period. --friedman
1137 @samp{-N} in @code{tar}.
1140 @samp{-a} in @code{du}, @code{ls}, @code{nm}, @code{stty}, @code{uname},
1141 and @code{unexpand}.
1144 @samp{-a} in @code{diff}.
1147 @samp{-A} in @code{ls}.
1150 @samp{-a} in @code{etags}, @code{tee}, @code{time};
1151 @samp{-r} in @code{tar}.
1154 @samp{-a} in @code{cp}.
1157 @samp{-n} in @code{shar}.
1160 @samp{-l} in @code{m4}.
1163 @samp{-a} in @code{diff}.
1166 @samp{-v} in @code{gawk}.
1169 @samp{-W} in @code{make}.
1172 @samp{-o} in @code{make}.
1175 @samp{-a} in @code{recode}.
1178 @samp{-a} in @code{wdiff}.
1180 @item auto-reference
1181 @samp{-A} in @code{ptx}.
1184 @samp{-n} in @code{wdiff}.
1187 For server programs, run in the background.
1189 @item backward-search
1190 @samp{-B} in @code{ctags}.
1193 @samp{-f} in @code{shar}.
1202 @samp{-b} in @code{tac}.
1205 @samp{-b} in @code{cpio} and @code{diff}.
1208 @samp{-b} in @code{shar}.
1211 Used in @code{cpio} and @code{tar}.
1214 @samp{-b} in @code{head} and @code{tail}.
1217 @samp{-b} in @code{ptx}.
1220 Used in various programs to make output shorter.
1223 @samp{-c} in @code{head}, @code{split}, and @code{tail}.
1226 @samp{-C} in @code{etags}.
1229 @samp{-A} in @code{tar}.
1232 Used in various programs to specify the directory to use.
1235 @samp{-c} in @code{chgrp} and @code{chown}.
1238 @samp{-F} in @code{ls}.
1241 @samp{-c} in @code{recode}.
1244 @samp{-c} in @code{su};
1248 @samp{-d} in @code{tar}.
1251 Used in @code{gawk}.
1254 @samp{-Z} in @code{tar} and @code{shar}.
1257 @samp{-A} in @code{tar}.
1260 @samp{-w} in @code{tar}.
1263 Used in @code{diff}.
1266 @samp{-W copyleft} in @code{gawk}.
1269 @samp{-C} in @code{ptx}, @code{recode}, and @code{wdiff};
1270 @samp{-W copyright} in @code{gawk}.
1276 @samp{-q} in @code{who}.
1279 @samp{-l} in @code{du}.
1282 Used in @code{tar} and @code{cpio}.
1285 @samp{-c} in @code{shar}.
1288 @samp{-x} in @code{ctags}.
1291 @samp{-d} in @code{touch}.
1294 @samp{-d} in @code{make} and @code{m4};
1298 @samp{-D} in @code{m4}.
1301 @samp{-d} in Bison and @code{ctags}.
1304 @samp{-D} in @code{tar}.
1307 @samp{-L} in @code{chgrp}, @code{chown}, @code{cpio}, @code{du},
1308 @code{ls}, and @code{tar}.
1310 @item dereference-args
1311 @samp{-D} in @code{du}.
1314 Specify an I/O device (special file name).
1317 @samp{-d} in @code{recode}.
1319 @item dictionary-order
1320 @samp{-d} in @code{look}.
1323 @samp{-d} in @code{tar}.
1326 @samp{-n} in @code{csplit}.
1329 Specify the directory to use, in various programs. In @code{ls}, it
1330 means to show directories themselves rather than their contents. In
1331 @code{rm} and @code{ln}, it means to not treat links to directories
1335 @samp{-x} in @code{strip}.
1337 @item discard-locals
1338 @samp{-X} in @code{strip}.
1341 @samp{-n} in @code{make}.
1344 @samp{-e} in @code{diff}.
1346 @item elide-empty-files
1347 @samp{-z} in @code{csplit}.
1350 @samp{-x} in @code{wdiff}.
1353 @samp{-z} in @code{wdiff}.
1355 @item entire-new-file
1356 @samp{-N} in @code{diff}.
1358 @item environment-overrides
1359 @samp{-e} in @code{make}.
1362 @samp{-e} in @code{xargs}.
1368 Used in @code{makeinfo}.
1371 @samp{-o} in @code{m4}.
1374 @samp{-b} in @code{ls}.
1377 @samp{-X} in @code{tar}.
1383 @samp{-x} in @code{xargs}.
1386 @samp{-e} in @code{unshar}.
1389 @samp{-t} in @code{diff}.
1392 @samp{-e} in @code{sed}.
1395 @samp{-g} in @code{nm}.
1398 @samp{-i} in @code{cpio};
1399 @samp{-x} in @code{tar}.
1402 @samp{-f} in @code{finger}.
1405 @samp{-f} in @code{su}.
1407 @item fatal-warnings
1408 @samp{-E} in @code{m4}.
1411 @samp{-f} in @code{gawk}, @code{info}, @code{make}, @code{mt},
1412 @code{sed}, and @code{tar}.
1414 @item field-separator
1415 @samp{-F} in @code{gawk}.
1421 @samp{-F} in @code{ls}.
1424 @samp{-T} in @code{tar}.
1427 Used in @code{makeinfo}.
1429 @item flag-truncation
1430 @samp{-F} in @code{ptx}.
1432 @item fixed-output-files
1436 @samp{-f} in @code{tail}.
1438 @item footnote-style
1439 Used in @code{makeinfo}.
1442 @samp{-f} in @code{cp}, @code{ln}, @code{mv}, and @code{rm}.
1445 @samp{-F} in @code{shar}.
1448 For server programs, run in the foreground;
1449 in other words, don't do anything special to run the server
1453 Used in @code{ls}, @code{time}, and @code{ptx}.
1456 @samp{-F} in @code{m4}.
1462 @samp{-g} in @code{ptx}.
1465 @samp{-x} in @code{tar}.
1468 @samp{-i} in @code{ul}.
1471 @samp{-g} in @code{recode}.
1474 @samp{-g} in @code{install}.
1477 @samp{-z} in @code{tar} and @code{shar}.
1480 @samp{-H} in @code{m4}.
1483 @samp{-h} in @code{objdump} and @code{recode}
1486 @samp{-H} in @code{who}.
1489 Used to ask for brief usage information.
1491 @item here-delimiter
1492 @samp{-d} in @code{shar}.
1494 @item hide-control-chars
1495 @samp{-q} in @code{ls}.
1498 In @code{makeinfo}, output HTML.
1501 @samp{-u} in @code{who}.
1504 @samp{-D} in @code{diff}.
1507 @samp{-I} in @code{ls};
1508 @samp{-x} in @code{recode}.
1510 @item ignore-all-space
1511 @samp{-w} in @code{diff}.
1513 @item ignore-backups
1514 @samp{-B} in @code{ls}.
1516 @item ignore-blank-lines
1517 @samp{-B} in @code{diff}.
1520 @samp{-f} in @code{look} and @code{ptx};
1521 @samp{-i} in @code{diff} and @code{wdiff}.
1524 @samp{-i} in @code{make}.
1527 @samp{-i} in @code{ptx}.
1529 @item ignore-indentation
1530 @samp{-I} in @code{etags}.
1532 @item ignore-init-file
1535 @item ignore-interrupts
1536 @samp{-i} in @code{tee}.
1538 @item ignore-matching-lines
1539 @samp{-I} in @code{diff}.
1541 @item ignore-space-change
1542 @samp{-b} in @code{diff}.
1545 @samp{-i} in @code{tar}.
1548 @samp{-i} in @code{etags};
1549 @samp{-I} in @code{m4}.
1552 @samp{-I} in @code{make}.
1555 @samp{-G} in @code{tar}.
1558 @samp{-i}, @samp{-l}, and @samp{-m} in Finger.
1561 In some programs, specify the name of the file to read as the user's
1565 @samp{-i} in @code{expand}.
1568 @samp{-T} in @code{diff}.
1571 @samp{-i} in @code{ls}.
1574 @samp{-i} in @code{cp}, @code{ln}, @code{mv}, @code{rm};
1575 @samp{-e} in @code{m4};
1576 @samp{-p} in @code{xargs};
1577 @samp{-w} in @code{tar}.
1580 @samp{-p} in @code{shar}.
1586 @samp{-j} in @code{make}.
1589 @samp{-n} in @code{make}.
1592 @samp{-k} in @code{make}.
1595 @samp{-k} in @code{csplit}.
1598 @samp{-k} in @code{du} and @code{ls}.
1601 @samp{-l} in @code{etags}.
1604 @samp{-l} in @code{wdiff}.
1606 @item level-for-gzip
1607 @samp{-g} in @code{shar}.
1610 @samp{-C} in @code{split}.
1613 Used in @code{split}, @code{head}, and @code{tail}.
1616 @samp{-l} in @code{cpio}.
1620 Used in @code{gawk}.
1623 @samp{-t} in @code{cpio};
1624 @samp{-l} in @code{recode}.
1627 @samp{-t} in @code{tar}.
1630 @samp{-N} in @code{ls}.
1633 @samp{-l} in @code{make}.
1639 Used in @code{uname}.
1642 @samp{-M} in @code{ptx}.
1645 @samp{-m} in @code{hello} and @code{uname}.
1647 @item make-directories
1648 @samp{-d} in @code{cpio}.
1651 @samp{-f} in @code{make}.
1657 @samp{-n} in @code{xargs}.
1660 @samp{-n} in @code{xargs}.
1663 @samp{-l} in @code{xargs}.
1666 @samp{-l} in @code{make}.
1669 @samp{-P} in @code{xargs}.
1672 @samp{-T} in @code{who}.
1675 @samp{-T} in @code{who}.
1678 @samp{-d} in @code{diff}.
1680 @item mixed-uuencode
1681 @samp{-M} in @code{shar}.
1684 @samp{-m} in @code{install}, @code{mkdir}, and @code{mkfifo}.
1686 @item modification-time
1687 @samp{-m} in @code{tar}.
1690 @samp{-M} in @code{tar}.
1696 @samp{-L} in @code{m4}.
1699 @samp{-a} in @code{shar}.
1702 @samp{-W} in @code{make}.
1704 @item no-builtin-rules
1705 @samp{-r} in @code{make}.
1707 @item no-character-count
1708 @samp{-w} in @code{shar}.
1710 @item no-check-existing
1711 @samp{-x} in @code{shar}.
1714 @samp{-3} in @code{wdiff}.
1717 @samp{-c} in @code{touch}.
1720 @samp{-D} in @code{etags}.
1723 @samp{-1} in @code{wdiff}.
1725 @item no-dereference
1726 @samp{-d} in @code{cp}.
1729 @samp{-2} in @code{wdiff}.
1732 @samp{-S} in @code{make}.
1738 @samp{-P} in @code{shar}.
1741 @samp{-e} in @code{gprof}.
1744 @samp{-R} in @code{etags}.
1747 @samp{-p} in @code{nm}.
1750 Don't print a startup splash screen.
1753 Used in @code{makeinfo}.
1756 @samp{-a} in @code{gprof}.
1759 @samp{-E} in @code{gprof}.
1762 @samp{-m} in @code{shar}.
1765 Used in @code{makeinfo}.
1768 Used in @code{emacsclient}.
1771 Used in various programs to inhibit warnings.
1774 @samp{-n} in @code{info}.
1777 @samp{-n} in @code{uname}.
1780 @samp{-f} in @code{cpio}.
1783 @samp{-n} in @code{objdump}.
1786 @samp{-0} in @code{xargs}.
1789 @samp{-n} in @code{cat}.
1791 @item number-nonblank
1792 @samp{-b} in @code{cat}.
1795 @samp{-n} in @code{nm}.
1797 @item numeric-uid-gid
1798 @samp{-n} in @code{cpio} and @code{ls}.
1804 @samp{-o} in @code{tar}.
1807 @samp{-o} in @code{make}.
1809 @item one-file-system
1810 @samp{-l} in @code{tar}, @code{cp}, and @code{du}.
1813 @samp{-o} in @code{ptx}.
1816 @samp{-f} in @code{gprof}.
1819 @samp{-F} in @code{gprof}.
1822 @samp{-o} in @code{getopt}, @code{fdlist}, @code{fdmount},
1823 @code{fdmountd}, and @code{fdumount}.
1826 In various programs, specify the output file name.
1829 @samp{-o} in @code{shar}.
1832 @samp{-o} in @code{rm}.
1835 @samp{-c} in @code{unshar}.
1838 @samp{-o} in @code{install}.
1841 @samp{-l} in @code{diff}.
1843 @item paragraph-indent
1844 Used in @code{makeinfo}.
1847 @samp{-p} in @code{mkdir} and @code{rmdir}.
1850 @samp{-p} in @code{ul}.
1853 @samp{-p} in @code{cpio}.
1856 @samp{-P} in @code{finger}.
1859 @samp{-c} in @code{cpio} and @code{tar}.
1862 Used in @code{gawk}.
1864 @item prefix-builtins
1865 @samp{-P} in @code{m4}.
1868 @samp{-f} in @code{csplit}.
1871 Used in @code{tar} and @code{cp}.
1873 @item preserve-environment
1874 @samp{-p} in @code{su}.
1876 @item preserve-modification-time
1877 @samp{-m} in @code{cpio}.
1879 @item preserve-order
1880 @samp{-s} in @code{tar}.
1882 @item preserve-permissions
1883 @samp{-p} in @code{tar}.
1886 @samp{-l} in @code{diff}.
1889 @samp{-L} in @code{cmp}.
1891 @item print-data-base
1892 @samp{-p} in @code{make}.
1894 @item print-directory
1895 @samp{-w} in @code{make}.
1897 @item print-file-name
1898 @samp{-o} in @code{nm}.
1901 @samp{-s} in @code{nm}.
1904 @samp{-p} in @code{wdiff}.
1907 @samp{-p} in @code{ed}.
1910 Specify an HTTP proxy.
1913 @samp{-X} in @code{shar}.
1916 @samp{-q} in @code{make}.
1919 Used in many programs to inhibit the usual output. Every
1920 program accepting @samp{--quiet} should accept @samp{--silent} as a
1924 @samp{-Q} in @code{shar}
1927 @samp{-Q} in @code{ls}.
1930 @samp{-n} in @code{diff}.
1933 Used in @code{gawk}.
1935 @item read-full-blocks
1936 @samp{-B} in @code{tar}.
1942 @samp{-n} in @code{make}.
1945 @samp{-R} in @code{tar}.
1948 Used in @code{chgrp}, @code{chown}, @code{cp}, @code{ls}, @code{diff},
1952 @samp{-r} in @code{touch}.
1955 @samp{-r} in @code{ptx}.
1958 @samp{-r} in @code{tac} and @code{etags}.
1961 @samp{-r} in @code{uname}.
1964 @samp{-R} in @code{m4}.
1967 @samp{-r} in @code{objdump}.
1970 @samp{-r} in @code{cpio}.
1973 @samp{-i} in @code{xargs}.
1975 @item report-identical-files
1976 @samp{-s} in @code{diff}.
1978 @item reset-access-time
1979 @samp{-a} in @code{cpio}.
1982 @samp{-r} in @code{ls} and @code{nm}.
1985 @samp{-f} in @code{diff}.
1987 @item right-side-defs
1988 @samp{-R} in @code{ptx}.
1991 @samp{-s} in @code{tar}.
1993 @item same-permissions
1994 @samp{-p} in @code{tar}.
1997 @samp{-g} in @code{stty}.
2002 @item sentence-regexp
2003 @samp{-S} in @code{ptx}.
2006 @samp{-S} in @code{du}.
2009 @samp{-s} in @code{tac}.
2012 Used by @code{recode} to chose files or pipes for sequencing passes.
2015 @samp{-s} in @code{su}.
2018 @samp{-A} in @code{cat}.
2020 @item show-c-function
2021 @samp{-p} in @code{diff}.
2024 @samp{-E} in @code{cat}.
2026 @item show-function-line
2027 @samp{-F} in @code{diff}.
2030 @samp{-T} in @code{cat}.
2033 Used in many programs to inhibit the usual output.
2034 Every program accepting
2035 @samp{--silent} should accept @samp{--quiet} as a synonym.
2038 @samp{-s} in @code{ls}.
2041 Specify a file descriptor for a network server to use for its socket,
2042 instead of opening and binding a new socket. This provides a way to
2043 run, in a non-privileged process, a server that normally needs a
2044 reserved port number.
2050 @samp{-W source} in @code{gawk}.
2053 @samp{-S} in @code{tar}.
2055 @item speed-large-files
2056 @samp{-H} in @code{diff}.
2059 @samp{-E} in @code{unshar}.
2061 @item split-size-limit
2062 @samp{-L} in @code{shar}.
2065 @samp{-s} in @code{cat}.
2068 @samp{-w} in @code{wdiff}.
2071 @samp{-y} in @code{wdiff}.
2074 Used in @code{tar} and @code{diff} to specify which file within
2075 a directory to start processing with.
2078 @samp{-s} in @code{wdiff}.
2080 @item stdin-file-list
2081 @samp{-S} in @code{shar}.
2084 @samp{-S} in @code{make}.
2087 @samp{-s} in @code{recode}.
2090 @samp{-s} in @code{install}.
2093 @samp{-s} in @code{strip}.
2096 @samp{-S} in @code{strip}.
2099 @samp{-s} in @code{shar}.
2102 @samp{-S} in @code{cp}, @code{ln}, @code{mv}.
2105 @samp{-b} in @code{csplit}.
2108 @samp{-s} in @code{gprof}.
2111 @samp{-s} in @code{du}.
2114 @samp{-s} in @code{ln}.
2117 Used in GDB and @code{objdump}.
2120 @samp{-s} in @code{m4}.
2123 @samp{-s} in @code{uname}.
2126 @samp{-t} in @code{expand} and @code{unexpand}.
2129 @samp{-T} in @code{ls}.
2132 @samp{-T} in @code{tput} and @code{ul}.
2133 @samp{-t} in @code{wdiff}.
2136 @samp{-a} in @code{diff}.
2139 @samp{-T} in @code{shar}.
2142 Used in @code{ls} and @code{touch}.
2145 Specify how long to wait before giving up on some operation.
2148 @samp{-O} in @code{tar}.
2151 @samp{-c} in @code{du}.
2154 @samp{-t} in @code{make}, @code{ranlib}, and @code{recode}.
2157 @samp{-t} in @code{m4}.
2160 @samp{-t} in @code{hello};
2161 @samp{-W traditional} in @code{gawk};
2162 @samp{-G} in @code{ed}, @code{m4}, and @code{ptx}.
2168 @samp{-t} in @code{ctags}.
2170 @item typedefs-and-c++
2171 @samp{-T} in @code{ctags}.
2174 @samp{-t} in @code{ptx}.
2177 @samp{-z} in @code{tar}.
2180 @samp{-u} in @code{cpio}.
2183 @samp{-U} in @code{m4}.
2185 @item undefined-only
2186 @samp{-u} in @code{nm}.
2189 @samp{-u} in @code{cp}, @code{ctags}, @code{mv}, @code{tar}.
2192 Used in @code{gawk}; same as @samp{--help}.
2195 @samp{-B} in @code{shar}.
2197 @item vanilla-operation
2198 @samp{-V} in @code{shar}.
2201 Print more information about progress. Many programs support this.
2204 @samp{-W} in @code{tar}.
2207 Print the version number.
2209 @item version-control
2210 @samp{-V} in @code{cp}, @code{ln}, @code{mv}.
2213 @samp{-v} in @code{ctags}.
2216 @samp{-V} in @code{tar}.
2219 @samp{-W} in @code{make}.
2221 @item whole-size-limit
2222 @samp{-l} in @code{shar}.
2225 @samp{-w} in @code{ls} and @code{ptx}.
2228 @samp{-W} in @code{ptx}.
2231 @samp{-T} in @code{who}.
2234 @samp{-z} in @code{gprof}.
2237 @node OID Allocations
2238 @section OID Allocations
2239 @cindex OID allocations for GNU
2244 The OID (object identifier) 1.3.6.1.4.1.11591 has been assigned to the
2245 GNU Project (thanks to Werner Koch). These are used for SNMP, LDAP,
2246 X.509 certificates, and so on. The web site
2247 @url{http://www.alvestrand.no/objectid} has a (voluntary) listing of
2248 many OID assignments.
2250 If you need a new slot for your GNU package, write
2251 @email{maintainers@@gnu.org}. Here is a list of arcs currently
2255 @include gnu-oids.texi
2260 @section Memory Usage
2261 @cindex memory usage
2263 If a program typically uses just a few meg of memory, don't bother making any
2264 effort to reduce memory usage. For example, if it is impractical for
2265 other reasons to operate on files more than a few meg long, it is
2266 reasonable to read entire input files into memory to operate on them.
2268 However, for programs such as @code{cat} or @code{tail}, that can
2269 usefully operate on very large files, it is important to avoid using a
2270 technique that would artificially limit the size of files it can handle.
2271 If a program works by lines and could be applied to arbitrary
2272 user-supplied input files, it should keep only a line in memory, because
2273 this is not very hard and users will want to be able to operate on input
2274 files that are bigger than will fit in memory all at once.
2276 If your program creates complicated data structures, just make them in
2277 memory and give a fatal error if @code{malloc} returns zero.
2283 Programs should be prepared to operate when @file{/usr} and @file{/etc}
2284 are read-only file systems. Thus, if the program manages log files,
2285 lock files, backup files, score files, or any other files which are
2286 modified for internal purposes, these files should not be stored in
2287 @file{/usr} or @file{/etc}.
2289 There are two exceptions. @file{/etc} is used to store system
2290 configuration information; it is reasonable for a program to modify
2291 files in @file{/etc} when its job is to update the system configuration.
2292 Also, if the user explicitly asks to modify one file in a directory, it
2293 is reasonable for the program to store other files in the same
2297 @chapter Making The Best Use of C
2299 This chapter provides advice on how best to use the C language
2300 when writing GNU software.
2303 * Formatting:: Formatting your source code.
2304 * Comments:: Commenting your work.
2305 * Syntactic Conventions:: Clean use of C constructs.
2306 * Names:: Naming variables, functions, and files.
2307 * System Portability:: Portability among different operating systems.
2308 * CPU Portability:: Supporting the range of CPU types.
2309 * System Functions:: Portability and ``standard'' library functions.
2310 * Internationalization:: Techniques for internationalization.
2311 * Character Set:: Use ASCII by default.
2312 * Quote Characters:: Use `...' in the C locale.
2313 * Mmap:: How you can safely use @code{mmap}.
2317 @section Formatting Your Source Code
2318 @cindex formatting source code
2321 @cindex braces, in C source
2322 It is important to put the open-brace that starts the body of a C
2323 function in column one, so that they will start a defun. Several
2324 tools look for open-braces in column one to find the beginnings of C
2325 functions. These tools will not work on code not formatted that way.
2327 Avoid putting open-brace, open-parenthesis or open-bracket in column
2328 one when they are inside a function, so that they won't start a defun.
2329 The open-brace that starts a @code{struct} body can go in column one
2330 if you find it useful to treat that definition as a defun.
2332 It is also important for function definitions to start the name of the
2333 function in column one. This helps people to search for function
2334 definitions, and may also help certain tools recognize them. Thus,
2335 using Standard C syntax, the format is this:
2339 concat (char *s1, char *s2)
2346 or, if you want to use traditional C syntax, format the definition like
2351 concat (s1, s2) /* Name starts in column one here */
2353 @{ /* Open brace in column one here */
2358 In Standard C, if the arguments don't fit nicely on one line,
2363 lots_of_args (int an_integer, long a_long, short a_short,
2364 double a_double, float a_float)
2368 The rest of this section gives our recommendations for other aspects of
2369 C formatting style, which is also the default style of the @code{indent}
2370 program in version 1.2 and newer. It corresponds to the options
2373 -nbad -bap -nbc -bbo -bl -bli2 -bls -ncdb -nce -cp1 -cs -di2
2374 -ndj -nfc1 -nfca -hnl -i2 -ip5 -lp -pcs -psl -nsc -nsob
2377 We don't think of these recommendations as requirements, because it
2378 causes no problems for users if two different programs have different
2381 But whatever style you use, please use it consistently, since a mixture
2382 of styles within one program tends to look ugly. If you are
2383 contributing changes to an existing program, please follow the style of
2386 For the body of the function, our recommended style looks like this:
2398 return ++x + bar ();
2402 @cindex spaces before open-paren
2403 We find it easier to read a program when it has spaces before the
2404 open-parentheses and after the commas. Especially after the commas.
2406 When you split an expression into multiple lines, split it
2407 before an operator, not after one. Here is the right way:
2409 @cindex expressions, splitting
2411 if (foo_this_is_long && bar > win (x, y, z)
2412 && remaining_condition)
2415 Try to avoid having two operators of different precedence at the same
2416 level of indentation. For example, don't write this:
2419 mode = (inmode[j] == VOIDmode
2420 || GET_MODE_SIZE (outmode[j]) > GET_MODE_SIZE (inmode[j])
2421 ? outmode[j] : inmode[j]);
2424 Instead, use extra parentheses so that the indentation shows the nesting:
2427 mode = ((inmode[j] == VOIDmode
2428 || (GET_MODE_SIZE (outmode[j]) > GET_MODE_SIZE (inmode[j])))
2429 ? outmode[j] : inmode[j]);
2432 Insert extra parentheses so that Emacs will indent the code properly.
2433 For example, the following indentation looks nice if you do it by hand,
2436 v = rup->ru_utime.tv_sec*1000 + rup->ru_utime.tv_usec/1000
2437 + rup->ru_stime.tv_sec*1000 + rup->ru_stime.tv_usec/1000;
2441 but Emacs would alter it. Adding a set of parentheses produces
2442 something that looks equally nice, and which Emacs will preserve:
2445 v = (rup->ru_utime.tv_sec*1000 + rup->ru_utime.tv_usec/1000
2446 + rup->ru_stime.tv_sec*1000 + rup->ru_stime.tv_usec/1000);
2449 Format do-while statements like this:
2461 Please use formfeed characters (control-L) to divide the program into
2462 pages at logical places (but not within a function). It does not matter
2463 just how long the pages are, since they do not have to fit on a printed
2464 page. The formfeeds should appear alone on lines by themselves.
2467 @section Commenting Your Work
2470 Every program should start with a comment saying briefly what it is for.
2471 Example: @samp{fmt - filter for simple filling of text}. This comment
2472 should be at the top of the source file containing the @samp{main}
2473 function of the program.
2475 Also, please write a brief comment at the start of each source file,
2476 with the file name and a line or two about the overall purpose of the
2479 Please write the comments in a GNU program in English, because English
2480 is the one language that nearly all programmers in all countries can
2481 read. If you do not write English well, please write comments in
2482 English as well as you can, then ask other people to help rewrite them.
2483 If you can't write comments in English, please find someone to work with
2484 you and translate your comments into English.
2486 Please put a comment on each function saying what the function does,
2487 what sorts of arguments it gets, and what the possible values of
2488 arguments mean and are used for. It is not necessary to duplicate in
2489 words the meaning of the C argument declarations, if a C type is being
2490 used in its customary fashion. If there is anything nonstandard about
2491 its use (such as an argument of type @code{char *} which is really the
2492 address of the second character of a string, not the first), or any
2493 possible values that would not work the way one would expect (such as,
2494 that strings containing newlines are not guaranteed to work), be sure
2497 Also explain the significance of the return value, if there is one.
2499 Please put two spaces after the end of a sentence in your comments, so
2500 that the Emacs sentence commands will work. Also, please write
2501 complete sentences and capitalize the first word. If a lower-case
2502 identifier comes at the beginning of a sentence, don't capitalize it!
2503 Changing the spelling makes it a different identifier. If you don't
2504 like starting a sentence with a lower case letter, write the sentence
2505 differently (e.g., ``The identifier lower-case is @dots{}'').
2507 The comment on a function is much clearer if you use the argument
2508 names to speak about the argument values. The variable name itself
2509 should be lower case, but write it in upper case when you are speaking
2510 about the value rather than the variable itself. Thus, ``the inode
2511 number NODE_NUM'' rather than ``an inode''.
2513 There is usually no purpose in restating the name of the function in
2514 the comment before it, because the reader can see that for himself.
2515 There might be an exception when the comment is so long that the function
2516 itself would be off the bottom of the screen.
2518 There should be a comment on each static variable as well, like this:
2521 /* Nonzero means truncate lines in the display;
2522 zero means continue them. */
2526 @cindex conditionals, comments for
2527 @cindex @code{#endif}, commenting
2528 Every @samp{#endif} should have a comment, except in the case of short
2529 conditionals (just a few lines) that are not nested. The comment should
2530 state the condition of the conditional that is ending, @emph{including
2531 its sense}. @samp{#else} should have a comment describing the condition
2532 @emph{and sense} of the code that follows. For example:
2540 #endif /* not foo */
2550 but, by contrast, write the comments this way for a @samp{#ifndef}:
2563 #endif /* not foo */
2567 @node Syntactic Conventions
2568 @section Clean Use of C Constructs
2569 @cindex syntactic conventions
2571 @cindex implicit @code{int}
2572 @cindex function argument, declaring
2573 Please explicitly declare the types of all objects. For example, you
2574 should explicitly declare all arguments to functions, and you should
2575 declare functions to return @code{int} rather than omitting the
2578 @cindex compiler warnings
2579 @cindex @samp{-Wall} compiler option
2580 Some programmers like to use the GCC @samp{-Wall} option, and change the
2581 code whenever it issues a warning. If you want to do this, then do.
2582 Other programmers prefer not to use @samp{-Wall}, because it gives
2583 warnings for valid and legitimate code which they do not want to change.
2584 If you want to do this, then do. The compiler should be your servant,
2587 Declarations of external functions and functions to appear later in the
2588 source file should all go in one place near the beginning of the file
2589 (somewhere before the first function definition in the file), or else
2590 should go in a header file. Don't put @code{extern} declarations inside
2593 @cindex temporary variables
2594 It used to be common practice to use the same local variables (with
2595 names like @code{tem}) over and over for different values within one
2596 function. Instead of doing this, it is better to declare a separate local
2597 variable for each distinct purpose, and give it a name which is
2598 meaningful. This not only makes programs easier to understand, it also
2599 facilitates optimization by good compilers. You can also move the
2600 declaration of each local variable into the smallest scope that includes
2601 all its uses. This makes the program even cleaner.
2603 Don't use local variables or parameters that shadow global identifiers.
2605 @cindex multiple variables in a line
2606 Don't declare multiple variables in one declaration that spans lines.
2607 Start a new declaration on each line, instead. For example, instead
2633 (If they are global variables, each should have a comment preceding it
2636 When you have an @code{if}-@code{else} statement nested in another
2637 @code{if} statement, always put braces around the @code{if}-@code{else}.
2638 Thus, never write like this:
2661 If you have an @code{if} statement nested inside of an @code{else}
2662 statement, either write @code{else if} on one line, like this,
2672 with its @code{then}-part indented like the preceding @code{then}-part,
2673 or write the nested @code{if} within braces like this:
2685 Don't declare both a structure tag and variables or typedefs in the
2686 same declaration. Instead, declare the structure tag separately
2687 and then use it to declare the variables or typedefs.
2689 Try to avoid assignments inside @code{if}-conditions (assignments
2690 inside @code{while}-conditions are ok). For example, don't write
2694 if ((foo = (char *) malloc (sizeof *foo)) == 0)
2695 fatal ("virtual memory exhausted");
2699 instead, write this:
2702 foo = (char *) malloc (sizeof *foo);
2704 fatal ("virtual memory exhausted");
2708 Don't make the program ugly to placate @code{lint}. Please don't insert any
2709 casts to @code{void}. Zero without a cast is perfectly fine as a null
2710 pointer constant, except when calling a varargs function.
2713 @section Naming Variables, Functions, and Files
2715 @cindex names of variables, functions, and files
2716 The names of global variables and functions in a program serve as
2717 comments of a sort. So don't choose terse names---instead, look for
2718 names that give useful information about the meaning of the variable or
2719 function. In a GNU program, names should be English, like other
2722 Local variable names can be shorter, because they are used only within
2723 one context, where (presumably) comments explain their purpose.
2725 Try to limit your use of abbreviations in symbol names. It is ok to
2726 make a few abbreviations, explain what they mean, and then use them
2727 frequently, but don't use lots of obscure abbreviations.
2729 Please use underscores to separate words in a name, so that the Emacs
2730 word commands can be useful within them. Stick to lower case; reserve
2731 upper case for macros and @code{enum} constants, and for name-prefixes
2732 that follow a uniform convention.
2734 For example, you should use names like @code{ignore_space_change_flag};
2735 don't use names like @code{iCantReadThis}.
2737 Variables that indicate whether command-line options have been
2738 specified should be named after the meaning of the option, not after
2739 the option-letter. A comment should state both the exact meaning of
2740 the option and its letter. For example,
2744 /* Ignore changes in horizontal whitespace (-b). */
2745 int ignore_space_change_flag;
2749 When you want to define names with constant integer values, use
2750 @code{enum} rather than @samp{#define}. GDB knows about enumeration
2753 @cindex file-name limitations
2755 You might want to make sure that none of the file names would conflict
2756 if the files were loaded onto an MS-DOS file system which shortens the
2757 names. You can use the program @code{doschk} to test for this.
2759 Some GNU programs were designed to limit themselves to file names of 14
2760 characters or less, to avoid file name conflicts if they are read into
2761 older System V systems. Please preserve this feature in the existing
2762 GNU programs that have it, but there is no need to do this in new GNU
2763 programs. @code{doschk} also reports file names longer than 14
2766 @node System Portability
2767 @section Portability between System Types
2768 @cindex portability, between system types
2770 In the Unix world, ``portability'' refers to porting to different Unix
2771 versions. For a GNU program, this kind of portability is desirable, but
2774 The primary purpose of GNU software is to run on top of the GNU kernel,
2775 compiled with the GNU C compiler, on various types of @sc{cpu}. So the
2776 kinds of portability that are absolutely necessary are quite limited.
2777 But it is important to support Linux-based GNU systems, since they
2778 are the form of GNU that is popular.
2780 Beyond that, it is good to support the other free operating systems
2781 (*BSD), and it is nice to support other Unix-like systems if you want
2782 to. Supporting a variety of Unix-like systems is desirable, although
2783 not paramount. It is usually not too hard, so you may as well do it.
2784 But you don't have to consider it an obligation, if it does turn out to
2788 The easiest way to achieve portability to most Unix-like systems is to
2789 use Autoconf. It's unlikely that your program needs to know more
2790 information about the host platform than Autoconf can provide, simply
2791 because most of the programs that need such knowledge have already been
2794 Avoid using the format of semi-internal data bases (e.g., directories)
2795 when there is a higher-level alternative (@code{readdir}).
2797 @cindex non-@sc{posix} systems, and portability
2798 As for systems that are not like Unix, such as MSDOS, Windows, VMS, MVS,
2799 and older Macintosh systems, supporting them is often a lot of work.
2800 When that is the case, it is better to spend your time adding features
2801 that will be useful on GNU and GNU/Linux, rather than on supporting
2802 other incompatible systems.
2804 If you do support Windows, please do not abbreviate it as ``win''. In
2805 hacker terminology, calling something a ``win'' is a form of praise.
2806 You're free to praise Microsoft Windows on your own if you want, but
2807 please don't do this in GNU packages. Instead of abbreviating
2808 ``Windows'' to ``win'', you can write it in full or abbreviate it to
2809 ``woe'' or ``w''. In GNU Emacs, for instance, we use @samp{w32} in
2810 file names of Windows-specific files, but the macro for Windows
2811 conditionals is called @code{WINDOWSNT}.
2813 It is a good idea to define the ``feature test macro''
2814 @code{_GNU_SOURCE} when compiling your C files. When you compile on GNU
2815 or GNU/Linux, this will enable the declarations of GNU library extension
2816 functions, and that will usually give you a compiler error message if
2817 you define the same function names in some other way in your program.
2818 (You don't have to actually @emph{use} these functions, if you prefer
2819 to make the program more portable to other systems.)
2821 But whether or not you use these GNU extensions, you should avoid
2822 using their names for any other meanings. Doing so would make it hard
2823 to move your code into other GNU programs.
2825 @node CPU Portability
2826 @section Portability between @sc{cpu}s
2828 @cindex data types, and portability
2829 @cindex portability, and data types
2830 Even GNU systems will differ because of differences among @sc{cpu}
2831 types---for example, difference in byte ordering and alignment
2832 requirements. It is absolutely essential to handle these differences.
2833 However, don't make any effort to cater to the possibility that an
2834 @code{int} will be less than 32 bits. We don't support 16-bit machines
2837 Similarly, don't make any effort to cater to the possibility that
2838 @code{long} will be smaller than predefined types like @code{size_t}.
2839 For example, the following code is ok:
2842 printf ("size = %lu\n", (unsigned long) sizeof array);
2843 printf ("diff = %ld\n", (long) (pointer2 - pointer1));
2846 1989 Standard C requires this to work, and we know of only one
2847 counterexample: 64-bit programs on Microsoft Windows. We will
2848 leave it to those who want to port GNU programs to that environment
2849 to figure out how to do it.
2851 Predefined file-size types like @code{off_t} are an exception: they are
2852 longer than @code{long} on many platforms, so code like the above won't
2853 work with them. One way to print an @code{off_t} value portably is to
2854 print its digits yourself, one by one.
2856 Don't assume that the address of an @code{int} object is also the
2857 address of its least-significant byte. This is false on big-endian
2858 machines. Thus, don't make the following mistake:
2863 while ((c = getchar ()) != EOF)
2864 write (file_descriptor, &c, 1);
2867 @noindent Instead, use @code{unsigned char} as follows. (The @code{unsigned}
2868 is for portability to unusual systems where @code{char} is signed and
2869 where there is integer overflow checking.)
2873 while ((c = getchar ()) != EOF)
2875 unsigned char u = c;
2876 write (file_descriptor, &u, 1);
2880 It used to be ok to not worry about the difference between pointers
2881 and integers when passing arguments to functions. However, on most
2882 modern 64-bit machines pointers are wider than @code{int}.
2883 Conversely, integer types like @code{long long int} and @code{off_t}
2884 are wider than pointers on most modern 32-bit machines. Hence it's
2885 often better nowadays to use prototypes to define functions whose
2886 argument types are not trivial.
2888 In particular, if functions accept varying argument counts or types
2889 they should be declared using prototypes containing @samp{...} and
2890 defined using @file{stdarg.h}. For an example of this, please see the
2891 @uref{http://www.gnu.org/software/gnulib/, Gnulib} error module, which
2892 declares and defines the following function:
2895 /* Print a message with `fprintf (stderr, FORMAT, ...)';
2896 if ERRNUM is nonzero, follow it with ": " and strerror (ERRNUM).
2897 If STATUS is nonzero, terminate the program with `exit (STATUS)'. */
2899 void error (int status, int errnum, const char *format, ...);
2902 A simple way to use the Gnulib error module is to obtain the two
2903 source files @file{error.c} and @file{error.h} from the Gnulib library
2904 source code repository at
2905 @uref{http://git.savannah.gnu.org/@/gitweb/@/?p=gnulib.git}.
2906 Here's a sample use:
2913 char *program_name = "myprogram";
2916 xfopen (char const *name)
2918 FILE *fp = fopen (name, "r");
2920 error (1, errno, "cannot read %s", name);
2925 @cindex casting pointers to integers
2926 Avoid casting pointers to integers if you can. Such casts greatly
2927 reduce portability, and in most programs they are easy to avoid. In the
2928 cases where casting pointers to integers is essential---such as, a Lisp
2929 interpreter which stores type information as well as an address in one
2930 word---you'll have to make explicit provisions to handle different word
2931 sizes. You will also need to make provision for systems in which the
2932 normal range of addresses you can get from @code{malloc} starts far away
2935 @node System Functions
2936 @section Calling System Functions
2937 @cindex library functions, and portability
2938 @cindex portability, and library functions
2940 C implementations differ substantially. Standard C reduces but does
2941 not eliminate the incompatibilities; meanwhile, many GNU packages still
2942 support pre-standard compilers because this is not hard to do. This
2943 chapter gives recommendations for how to use the more-or-less standard C
2944 library functions to avoid unnecessary loss of portability.
2948 Don't use the return value of @code{sprintf}. It returns the number of
2949 characters written on some systems, but not on all systems.
2952 Be aware that @code{vfprintf} is not always available.
2955 @code{main} should be declared to return type @code{int}. It should
2956 terminate either by calling @code{exit} or by returning the integer
2957 status code; make sure it cannot ever return an undefined value.
2959 @cindex declaration for system functions
2961 Don't declare system functions explicitly.
2963 Almost any declaration for a system function is wrong on some system.
2964 To minimize conflicts, leave it to the system header files to declare
2965 system functions. If the headers don't declare a function, let it
2968 While it may seem unclean to use a function without declaring it, in
2969 practice this works fine for most system library functions on the
2970 systems where this really happens; thus, the disadvantage is only
2971 theoretical. By contrast, actual declarations have frequently caused
2975 If you must declare a system function, don't specify the argument types.
2976 Use an old-style declaration, not a Standard C prototype. The more you
2977 specify about the function, the more likely a conflict.
2980 In particular, don't unconditionally declare @code{malloc} or
2983 Most GNU programs use those functions just once, in functions
2984 conventionally named @code{xmalloc} and @code{xrealloc}. These
2985 functions call @code{malloc} and @code{realloc}, respectively, and
2988 Because @code{xmalloc} and @code{xrealloc} are defined in your program,
2989 you can declare them in other files without any risk of type conflict.
2991 On most systems, @code{int} is the same length as a pointer; thus, the
2992 calls to @code{malloc} and @code{realloc} work fine. For the few
2993 exceptional systems (mostly 64-bit machines), you can use
2994 @strong{conditionalized} declarations of @code{malloc} and
2995 @code{realloc}---or put these declarations in configuration files
2996 specific to those systems.
2998 @cindex string library functions
3000 The string functions require special treatment. Some Unix systems have
3001 a header file @file{string.h}; others have @file{strings.h}. Neither
3002 file name is portable. There are two things you can do: use Autoconf to
3003 figure out which file to include, or don't include either file.
3006 If you don't include either strings file, you can't get declarations for
3007 the string functions from the header file in the usual way.
3009 That causes less of a problem than you might think. The newer standard
3010 string functions should be avoided anyway because many systems still
3011 don't support them. The string functions you can use are these:
3014 strcpy strncpy strcat strncat
3015 strlen strcmp strncmp
3019 The copy and concatenate functions work fine without a declaration as
3020 long as you don't use their values. Using their values without a
3021 declaration fails on systems where the width of a pointer differs from
3022 the width of @code{int}, and perhaps in other cases. It is trivial to
3023 avoid using their values, so do that.
3025 The compare functions and @code{strlen} work fine without a declaration
3026 on most systems, possibly all the ones that GNU software runs on.
3027 You may find it necessary to declare them @strong{conditionally} on a
3030 The search functions must be declared to return @code{char *}. Luckily,
3031 there is no variation in the data type they return. But there is
3032 variation in their names. Some systems give these functions the names
3033 @code{index} and @code{rindex}; other systems use the names
3034 @code{strchr} and @code{strrchr}. Some systems support both pairs of
3035 names, but neither pair works on all systems.
3037 You should pick a single pair of names and use it throughout your
3038 program. (Nowadays, it is better to choose @code{strchr} and
3039 @code{strrchr} for new programs, since those are the standard
3040 names.) Declare both of those names as functions returning @code{char
3041 *}. On systems which don't support those names, define them as macros
3042 in terms of the other pair. For example, here is what to put at the
3043 beginning of your file (or in a header) if you want to use the names
3044 @code{strchr} and @code{strrchr} throughout:
3048 #define strchr index
3050 #ifndef HAVE_STRRCHR
3051 #define strrchr rindex
3059 Here we assume that @code{HAVE_STRCHR} and @code{HAVE_STRRCHR} are
3060 macros defined in systems where the corresponding functions exist.
3061 One way to get them properly defined is to use Autoconf.
3063 @node Internationalization
3064 @section Internationalization
3065 @cindex internationalization
3068 GNU has a library called GNU gettext that makes it easy to translate the
3069 messages in a program into various languages. You should use this
3070 library in every program. Use English for the messages as they appear
3071 in the program, and let gettext provide the way to translate them into
3074 Using GNU gettext involves putting a call to the @code{gettext} macro
3075 around each string that might need translation---like this:
3078 printf (gettext ("Processing file `%s'..."));
3082 This permits GNU gettext to replace the string @code{"Processing file
3083 `%s'..."} with a translated version.
3085 Once a program uses gettext, please make a point of writing calls to
3086 @code{gettext} when you add new strings that call for translation.
3088 Using GNU gettext in a package involves specifying a @dfn{text domain
3089 name} for the package. The text domain name is used to separate the
3090 translations for this package from the translations for other packages.
3091 Normally, the text domain name should be the same as the name of the
3092 package---for example, @samp{coreutils} for the GNU core utilities.
3094 @cindex message text, and internationalization
3095 To enable gettext to work well, avoid writing code that makes
3096 assumptions about the structure of words or sentences. When you want
3097 the precise text of a sentence to vary depending on the data, use two or
3098 more alternative string constants each containing a complete sentences,
3099 rather than inserting conditionalized words or phrases into a single
3102 Here is an example of what not to do:
3105 printf ("%s is full", capacity > 5000000 ? "disk" : "floppy disk");
3108 If you apply gettext to all strings, like this,
3111 printf (gettext ("%s is full"),
3112 capacity > 5000000 ? gettext ("disk") : gettext ("floppy disk"));
3116 the translator will hardly know that "disk" and "floppy disk" are meant to
3117 be substituted in the other string. Worse, in some languages (like French)
3118 the construction will not work: the translation of the word "full" depends
3119 on the gender of the first part of the sentence; it happens to be not the
3120 same for "disk" as for "floppy disk".
3122 Complete sentences can be translated without problems:
3125 printf (capacity > 5000000 ? gettext ("disk is full")
3126 : gettext ("floppy disk is full"));
3129 A similar problem appears at the level of sentence structure with this
3133 printf ("# Implicit rule search has%s been done.\n",
3134 f->tried_implicit ? "" : " not");
3138 Adding @code{gettext} calls to this code cannot give correct results for
3139 all languages, because negation in some languages requires adding words
3140 at more than one place in the sentence. By contrast, adding
3141 @code{gettext} calls does the job straightforwardly if the code starts
3145 printf (f->tried_implicit
3146 ? "# Implicit rule search has been done.\n",
3147 : "# Implicit rule search has not been done.\n");
3150 Another example is this one:
3153 printf ("%d file%s processed", nfiles,
3154 nfiles != 1 ? "s" : "");
3158 The problem with this example is that it assumes that plurals are made
3159 by adding `s'. If you apply gettext to the format string, like this,
3162 printf (gettext ("%d file%s processed"), nfiles,
3163 nfiles != 1 ? "s" : "");
3167 the message can use different words, but it will still be forced to use
3168 `s' for the plural. Here is a better way, with gettext being applied to
3169 the two strings independently:
3172 printf ((nfiles != 1 ? gettext ("%d files processed")
3173 : gettext ("%d file processed")),
3178 But this still doesn't work for languages like Polish, which has three
3179 plural forms: one for nfiles == 1, one for nfiles == 2, 3, 4, 22, 23, 24, ...
3180 and one for the rest. The GNU @code{ngettext} function solves this problem:
3183 printf (ngettext ("%d files processed", "%d file processed", nfiles),
3189 @section Character Set
3190 @cindex character set
3192 @cindex ASCII characters
3193 @cindex non-ASCII characters
3195 Sticking to the ASCII character set (plain text, 7-bit characters) is
3196 preferred in GNU source code comments, text documents, and other
3197 contexts, unless there is good reason to do something else because of
3198 the application domain. For example, if source code deals with the
3199 French Revolutionary calendar, it is OK if its literal strings contain
3200 accented characters in month names like ``Flor@'eal''. Also, it is OK
3201 to use non-ASCII characters to represent proper names of contributors in
3202 change logs (@pxref{Change Logs}).
3204 If you need to use non-ASCII characters, you should normally stick with
3205 one encoding, as one cannot in general mix encodings reliably.
3208 @node Quote Characters
3209 @section Quote Characters
3210 @cindex quote characters
3211 @cindex locale-specific quote characters
3213 @cindex grave accent
3215 In the C locale, GNU programs should stick to plain ASCII for quotation
3216 characters in messages to users: preferably 0x60 (@samp{`}) for left
3217 quotes and 0x27 (@samp{'}) for right quotes. It is ok, but not
3218 required, to use locale-specific quotes in other locales.
3220 The @uref{http://www.gnu.org/software/gnulib/, Gnulib} @code{quote} and
3221 @code{quotearg} modules provide a reasonably straightforward way to
3222 support locale-specific quote characters, as well as taking care of
3223 other issues, such as quoting a filename that itself contains a quote
3224 character. See the Gnulib documentation for usage details.
3226 In any case, the documentation for your program should clearly specify
3227 how it does quoting, if different than the preferred method of @samp{`}
3228 and @samp{'}. This is especially important if the output of your
3229 program is ever likely to be parsed by another program.
3231 Quotation characters are a difficult area in the computing world at
3232 this time: there are no true left or right quote characters in Latin1;
3233 the @samp{`} character we use was standardized there as a grave
3234 accent. Moreover, Latin1 is still not universally usable.
3236 Unicode contains the unambiguous quote characters required, and its
3237 common encoding UTF-8 is upward compatible with Latin1. However,
3238 Unicode and UTF-8 are not universally well-supported, either.
3240 This may change over the next few years, and then we will revisit
3248 Don't assume that @code{mmap} either works on all files or fails
3249 for all files. It may work on some files and fail on others.
3251 The proper way to use @code{mmap} is to try it on the specific file for
3252 which you want to use it---and if @code{mmap} doesn't work, fall back on
3253 doing the job in another way using @code{read} and @code{write}.
3255 The reason this precaution is needed is that the GNU kernel (the HURD)
3256 provides a user-extensible file system, in which there can be many
3257 different kinds of ``ordinary files.'' Many of them support
3258 @code{mmap}, but some do not. It is important to make programs handle
3259 all these kinds of files.
3262 @chapter Documenting Programs
3263 @cindex documentation
3265 A GNU program should ideally come with full free documentation, adequate
3266 for both reference and tutorial purposes. If the package can be
3267 programmed or extended, the documentation should cover programming or
3268 extending it, as well as just using it.
3271 * GNU Manuals:: Writing proper manuals.
3272 * Doc Strings and Manuals:: Compiling doc strings doesn't make a manual.
3273 * Manual Structure Details:: Specific structure conventions.
3274 * License for Manuals:: Writing the distribution terms for a manual.
3275 * Manual Credits:: Giving credit to documentation contributors.
3276 * Printed Manuals:: Mentioning the printed manual.
3277 * NEWS File:: NEWS files supplement manuals.
3278 * Change Logs:: Recording changes.
3279 * Man Pages:: Man pages are secondary.
3280 * Reading other Manuals:: How far you can go in learning
3285 @section GNU Manuals
3287 The preferred document format for the GNU system is the Texinfo
3288 formatting language. Every GNU package should (ideally) have
3289 documentation in Texinfo both for reference and for learners. Texinfo
3290 makes it possible to produce a good quality formatted book, using
3291 @TeX{}, and to generate an Info file. It is also possible to generate
3292 HTML output from Texinfo source. See the Texinfo manual, either the
3293 hardcopy, or the on-line version available through @code{info} or the
3294 Emacs Info subsystem (@kbd{C-h i}).
3296 Nowadays some other formats such as Docbook and Sgmltexi can be
3297 converted automatically into Texinfo. It is ok to produce the Texinfo
3298 documentation by conversion this way, as long as it gives good results.
3300 Make sure your manual is clear to a reader who knows nothing about the
3301 topic and reads it straight through. This means covering basic topics
3302 at the beginning, and advanced topics only later. This also means
3303 defining every specialized term when it is first used.
3305 Programmers tend to carry over the structure of the program as the
3306 structure for its documentation. But this structure is not
3307 necessarily good for explaining how to use the program; it may be
3308 irrelevant and confusing for a user.
3310 Instead, the right way to structure documentation is according to the
3311 concepts and questions that a user will have in mind when reading it.
3312 This principle applies at every level, from the lowest (ordering
3313 sentences in a paragraph) to the highest (ordering of chapter topics
3314 within the manual). Sometimes this structure of ideas matches the
3315 structure of the implementation of the software being documented---but
3316 often they are different. An important part of learning to write good
3317 documentation is to learn to notice when you have unthinkingly
3318 structured the documentation like the implementation, stop yourself,
3319 and look for better alternatives.
3321 For example, each program in the GNU system probably ought to be
3322 documented in one manual; but this does not mean each program should
3323 have its own manual. That would be following the structure of the
3324 implementation, rather than the structure that helps the user
3327 Instead, each manual should cover a coherent @emph{topic}. For example,
3328 instead of a manual for @code{diff} and a manual for @code{diff3}, we
3329 have one manual for ``comparison of files'' which covers both of those
3330 programs, as well as @code{cmp}. By documenting these programs
3331 together, we can make the whole subject clearer.
3333 The manual which discusses a program should certainly document all of
3334 the program's command-line options and all of its commands. It should
3335 give examples of their use. But don't organize the manual as a list
3336 of features. Instead, organize it logically, by subtopics. Address
3337 the questions that a user will ask when thinking about the job that
3338 the program does. Don't just tell the reader what each feature can
3339 do---say what jobs it is good for, and show how to use it for those
3340 jobs. Explain what is recommended usage, and what kinds of usage
3343 In general, a GNU manual should serve both as tutorial and reference.
3344 It should be set up for convenient access to each topic through Info,
3345 and for reading straight through (appendixes aside). A GNU manual
3346 should give a good introduction to a beginner reading through from the
3347 start, and should also provide all the details that hackers want.
3348 The Bison manual is a good example of this---please take a look at it
3349 to see what we mean.
3351 That is not as hard as it first sounds. Arrange each chapter as a
3352 logical breakdown of its topic, but order the sections, and write their
3353 text, so that reading the chapter straight through makes sense. Do
3354 likewise when structuring the book into chapters, and when structuring a
3355 section into paragraphs. The watchword is, @emph{at each point, address
3356 the most fundamental and important issue raised by the preceding text.}
3358 If necessary, add extra chapters at the beginning of the manual which
3359 are purely tutorial and cover the basics of the subject. These provide
3360 the framework for a beginner to understand the rest of the manual. The
3361 Bison manual provides a good example of how to do this.
3363 To serve as a reference, a manual should have an Index that list all the
3364 functions, variables, options, and important concepts that are part of
3365 the program. One combined Index should do for a short manual, but
3366 sometimes for a complex package it is better to use multiple indices.
3367 The Texinfo manual includes advice on preparing good index entries, see
3368 @ref{Index Entries, , Making Index Entries, texinfo, GNU Texinfo}, and
3369 see @ref{Indexing Commands, , Defining the Entries of an
3370 Index, texinfo, GNU Texinfo}.
3372 Don't use Unix man pages as a model for how to write GNU documentation;
3373 most of them are terse, badly structured, and give inadequate
3374 explanation of the underlying concepts. (There are, of course, some
3375 exceptions.) Also, Unix man pages use a particular format which is
3376 different from what we use in GNU manuals.
3378 Please include an email address in the manual for where to report
3379 bugs @emph{in the text of the manual}.
3381 Please do not use the term ``pathname'' that is used in Unix
3382 documentation; use ``file name'' (two words) instead. We use the term
3383 ``path'' only for search paths, which are lists of directory names.
3385 Please do not use the term ``illegal'' to refer to erroneous input to
3386 a computer program. Please use ``invalid'' for this, and reserve the
3387 term ``illegal'' for activities prohibited by law.
3389 Please do not write @samp{()} after a function name just to indicate
3390 it is a function. @code{foo ()} is not a function, it is a function
3391 call with no arguments.
3393 @node Doc Strings and Manuals
3394 @section Doc Strings and Manuals
3396 Some programming systems, such as Emacs, provide a documentation string
3397 for each function, command or variable. You may be tempted to write a
3398 reference manual by compiling the documentation strings and writing a
3399 little additional text to go around them---but you must not do it. That
3400 approach is a fundamental mistake. The text of well-written
3401 documentation strings will be entirely wrong for a manual.
3403 A documentation string needs to stand alone---when it appears on the
3404 screen, there will be no other text to introduce or explain it.
3405 Meanwhile, it can be rather informal in style.
3407 The text describing a function or variable in a manual must not stand
3408 alone; it appears in the context of a section or subsection. Other text
3409 at the beginning of the section should explain some of the concepts, and
3410 should often make some general points that apply to several functions or
3411 variables. The previous descriptions of functions and variables in the
3412 section will also have given information about the topic. A description
3413 written to stand alone would repeat some of that information; this
3414 redundancy looks bad. Meanwhile, the informality that is acceptable in
3415 a documentation string is totally unacceptable in a manual.
3417 The only good way to use documentation strings in writing a good manual
3418 is to use them as a source of information for writing good text.
3420 @node Manual Structure Details
3421 @section Manual Structure Details
3422 @cindex manual structure
3424 The title page of the manual should state the version of the programs or
3425 packages documented in the manual. The Top node of the manual should
3426 also contain this information. If the manual is changing more
3427 frequently than or independent of the program, also state a version
3428 number for the manual in both of these places.
3430 Each program documented in the manual should have a node named
3431 @samp{@var{program} Invocation} or @samp{Invoking @var{program}}. This
3432 node (together with its subnodes, if any) should describe the program's
3433 command line arguments and how to run it (the sort of information people
3434 would look for in a man page). Start with an @samp{@@example}
3435 containing a template for all the options and arguments that the program
3438 Alternatively, put a menu item in some menu whose item name fits one of
3439 the above patterns. This identifies the node which that item points to
3440 as the node for this purpose, regardless of the node's actual name.
3442 The @samp{--usage} feature of the Info reader looks for such a node
3443 or menu item in order to find the relevant text, so it is essential
3444 for every Texinfo file to have one.
3446 If one manual describes several programs, it should have such a node for
3447 each program described in the manual.
3449 @node License for Manuals
3450 @section License for Manuals
3451 @cindex license for manuals
3453 Please use the GNU Free Documentation License for all GNU manuals that
3454 are more than a few pages long. Likewise for a collection of short
3455 documents---you only need one copy of the GNU FDL for the whole
3456 collection. For a single short document, you can use a very permissive
3457 non-copyleft license, to avoid taking up space with a long license.
3459 See @uref{http://www.gnu.org/copyleft/fdl-howto.html} for more explanation
3460 of how to employ the GFDL.
3462 Note that it is not obligatory to include a copy of the GNU GPL or GNU
3463 LGPL in a manual whose license is neither the GPL nor the LGPL. It can
3464 be a good idea to include the program's license in a large manual; in a
3465 short manual, whose size would be increased considerably by including
3466 the program's license, it is probably better not to include it.
3468 @node Manual Credits
3469 @section Manual Credits
3470 @cindex credits for manuals
3472 Please credit the principal human writers of the manual as the authors,
3473 on the title page of the manual. If a company sponsored the work, thank
3474 the company in a suitable place in the manual, but do not cite the
3475 company as an author.
3477 @node Printed Manuals
3478 @section Printed Manuals
3480 The FSF publishes some GNU manuals in printed form. To encourage sales
3481 of these manuals, the on-line versions of the manual should mention at
3482 the very start that the printed manual is available and should point at
3483 information for getting it---for instance, with a link to the page
3484 @url{http://www.gnu.org/order/order.html}. This should not be included
3485 in the printed manual, though, because there it is redundant.
3487 It is also useful to explain in the on-line forms of the manual how the
3488 user can print out the manual from the sources.
3491 @section The NEWS File
3492 @cindex @file{NEWS} file
3494 In addition to its manual, the package should have a file named
3495 @file{NEWS} which contains a list of user-visible changes worth
3496 mentioning. In each new release, add items to the front of the file and
3497 identify the version they pertain to. Don't discard old items; leave
3498 them in the file after the newer items. This way, a user upgrading from
3499 any previous version can see what is new.
3501 If the @file{NEWS} file gets very long, move some of the older items
3502 into a file named @file{ONEWS} and put a note at the end referring the
3506 @section Change Logs
3509 Keep a change log to describe all the changes made to program source
3510 files. The purpose of this is so that people investigating bugs in the
3511 future will know about the changes that might have introduced the bug.
3512 Often a new bug can be found by looking at what was recently changed.
3513 More importantly, change logs can help you eliminate conceptual
3514 inconsistencies between different parts of a program, by giving you a
3515 history of how the conflicting concepts arose and who they came from.
3518 * Change Log Concepts::
3519 * Style of Change Logs::
3521 * Conditional Changes::
3522 * Indicating the Part Changed::
3525 @node Change Log Concepts
3526 @subsection Change Log Concepts
3528 You can think of the change log as a conceptual ``undo list'' which
3529 explains how earlier versions were different from the current version.
3530 People can see the current version; they don't need the change log
3531 to tell them what is in it. What they want from a change log is a
3532 clear explanation of how the earlier version differed.
3534 The change log file is normally called @file{ChangeLog} and covers an
3535 entire directory. Each directory can have its own change log, or a
3536 directory can use the change log of its parent directory---it's up to
3539 Another alternative is to record change log information with a version
3540 control system such as RCS or CVS. This can be converted automatically
3541 to a @file{ChangeLog} file using @code{rcs2log}; in Emacs, the command
3542 @kbd{C-x v a} (@code{vc-update-change-log}) does the job.
3544 There's no need to describe the full purpose of the changes or how
3545 they work together. However, sometimes it is useful to write one line
3546 to describe the overall purpose of a change or a batch of changes. If
3547 you think that a change calls for explanation, you're probably right.
3548 Please do explain it---but please put the full explanation in comments
3549 in the code, where people will see it whenever they see the code. For
3550 example, ``New function'' is enough for the change log when you add a
3551 function, because there should be a comment before the function
3552 definition to explain what it does.
3554 In the past, we recommended not mentioning changes in non-software
3555 files (manuals, help files, etc.) in change logs. However, we've been
3556 advised that it is a good idea to include them, for the sake of
3559 The easiest way to add an entry to @file{ChangeLog} is with the Emacs
3560 command @kbd{M-x add-change-log-entry}. An entry should have an
3561 asterisk, the name of the changed file, and then in parentheses the name
3562 of the changed functions, variables or whatever, followed by a colon.
3563 Then describe the changes you made to that function or variable.
3565 @node Style of Change Logs
3566 @subsection Style of Change Logs
3567 @cindex change logs, style
3569 Here are some simple examples of change log entries, starting with the
3570 header line that says who made the change and when it was installed,
3571 followed by descriptions of specific changes. (These examples are
3572 drawn from Emacs and GCC.)
3575 1998-08-17 Richard Stallman <rms@@gnu.org>
3577 * register.el (insert-register): Return nil.
3578 (jump-to-register): Likewise.
3580 * sort.el (sort-subr): Return nil.
3582 * tex-mode.el (tex-bibtex-file, tex-file, tex-region):
3583 Restart the tex shell if process is gone or stopped.
3584 (tex-shell-running): New function.
3586 * expr.c (store_one_arg): Round size up for move_block_to_reg.
3587 (expand_call): Round up when emitting USE insns.
3588 * stmt.c (assign_parms): Round size up for move_block_from_reg.
3591 It's important to name the changed function or variable in full. Don't
3592 abbreviate function or variable names, and don't combine them.
3593 Subsequent maintainers will often search for a function name to find all
3594 the change log entries that pertain to it; if you abbreviate the name,
3595 they won't find it when they search.
3597 For example, some people are tempted to abbreviate groups of function
3598 names by writing @samp{* register.el (@{insert,jump-to@}-register)};
3599 this is not a good idea, since searching for @code{jump-to-register} or
3600 @code{insert-register} would not find that entry.
3602 Separate unrelated change log entries with blank lines. When two
3603 entries represent parts of the same change, so that they work together,
3604 then don't put blank lines between them. Then you can omit the file
3605 name and the asterisk when successive entries are in the same file.
3607 Break long lists of function names by closing continued lines with
3608 @samp{)}, rather than @samp{,}, and opening the continuation with
3609 @samp{(} as in this example:
3612 * keyboard.c (menu_bar_items, tool_bar_items)
3613 (Fexecute_extended_command): Deal with `keymap' property.
3616 When you install someone else's changes, put the contributor's name in
3617 the change log entry rather than in the text of the entry. In other
3621 2002-07-14 John Doe <jdoe@@gnu.org>
3623 * sewing.c: Make it sew.
3630 2002-07-14 Usual Maintainer <usual@@gnu.org>
3632 * sewing.c: Make it sew. Patch by jdoe@@gnu.org.
3635 As for the date, that should be the date you applied the change.
3637 @node Simple Changes
3638 @subsection Simple Changes
3640 Certain simple kinds of changes don't need much detail in the change
3643 When you change the calling sequence of a function in a simple fashion,
3644 and you change all the callers of the function to use the new calling
3645 sequence, there is no need to make individual entries for all the
3646 callers that you changed. Just write in the entry for the function
3647 being called, ``All callers changed''---like this:
3650 * keyboard.c (Fcommand_execute): New arg SPECIAL.
3651 All callers changed.
3654 When you change just comments or doc strings, it is enough to write an
3655 entry for the file, without mentioning the functions. Just ``Doc
3656 fixes'' is enough for the change log.
3658 There's no technical need to make change log entries for documentation
3659 files. This is because documentation is not susceptible to bugs that
3660 are hard to fix. Documentation does not consist of parts that must
3661 interact in a precisely engineered fashion. To correct an error, you
3662 need not know the history of the erroneous passage; it is enough to
3663 compare what the documentation says with the way the program actually
3666 However, you should keep change logs for documentation files when the
3667 project gets copyright assignments from its contributors, so as to
3668 make the records of authorship more accurate.
3670 @node Conditional Changes
3671 @subsection Conditional Changes
3672 @cindex conditional changes, and change logs
3673 @cindex change logs, conditional changes
3675 C programs often contain compile-time @code{#if} conditionals. Many
3676 changes are conditional; sometimes you add a new definition which is
3677 entirely contained in a conditional. It is very useful to indicate in
3678 the change log the conditions for which the change applies.
3680 Our convention for indicating conditional changes is to use square
3681 brackets around the name of the condition.
3683 Here is a simple example, describing a change which is conditional but
3684 does not have a function or entity name associated with it:
3687 * xterm.c [SOLARIS2]: Include string.h.
3690 Here is an entry describing a new definition which is entirely
3691 conditional. This new definition for the macro @code{FRAME_WINDOW_P} is
3692 used only when @code{HAVE_X_WINDOWS} is defined:
3695 * frame.h [HAVE_X_WINDOWS] (FRAME_WINDOW_P): Macro defined.
3698 Here is an entry for a change within the function @code{init_display},
3699 whose definition as a whole is unconditional, but the changes themselves
3700 are contained in a @samp{#ifdef HAVE_LIBNCURSES} conditional:
3703 * dispnew.c (init_display) [HAVE_LIBNCURSES]: If X, call tgetent.
3706 Here is an entry for a change that takes affect only when
3707 a certain macro is @emph{not} defined:
3710 (gethostname) [!HAVE_SOCKETS]: Replace with winsock version.
3713 @node Indicating the Part Changed
3714 @subsection Indicating the Part Changed
3716 Indicate the part of a function which changed by using angle brackets
3717 enclosing an indication of what the changed part does. Here is an entry
3718 for a change in the part of the function @code{sh-while-getopts} that
3719 deals with @code{sh} commands:
3722 * progmodes/sh-script.el (sh-while-getopts) <sh>: Handle case that
3723 user-specified option string is empty.
3731 In the GNU project, man pages are secondary. It is not necessary or
3732 expected for every GNU program to have a man page, but some of them do.
3733 It's your choice whether to include a man page in your program.
3735 When you make this decision, consider that supporting a man page
3736 requires continual effort each time the program is changed. The time
3737 you spend on the man page is time taken away from more useful work.
3739 For a simple program which changes little, updating the man page may be
3740 a small job. Then there is little reason not to include a man page, if
3743 For a large program that changes a great deal, updating a man page may
3744 be a substantial burden. If a user offers to donate a man page, you may
3745 find this gift costly to accept. It may be better to refuse the man
3746 page unless the same person agrees to take full responsibility for
3747 maintaining it---so that you can wash your hands of it entirely. If
3748 this volunteer later ceases to do the job, then don't feel obliged to
3749 pick it up yourself; it may be better to withdraw the man page from the
3750 distribution until someone else agrees to update it.
3752 When a program changes only a little, you may feel that the
3753 discrepancies are small enough that the man page remains useful without
3754 updating. If so, put a prominent note near the beginning of the man
3755 page explaining that you don't maintain it and that the Texinfo manual
3756 is more authoritative. The note should say how to access the Texinfo
3759 Be sure that man pages include a copyright statement and free license.
3760 The simple all-permissive license is appropriate for simple man pages
3761 (@pxref{License Notices for Other Files,,,maintain,Information for GNU
3764 For long man pages, with enough explanation and documentation that
3765 they can be considered true manuals, use the GFDL (@pxref{License for
3768 Finally, the GNU help2man program
3769 (@uref{http://www.gnu.org/software/help2man/}) is one way to automate
3770 generation of a man page, in this case from @option{--help} output.
3771 This is sufficient in many cases.
3773 @node Reading other Manuals
3774 @section Reading other Manuals
3776 There may be non-free books or documentation files that describe the
3777 program you are documenting.
3779 It is ok to use these documents for reference, just as the author of a
3780 new algebra textbook can read other books on algebra. A large portion
3781 of any non-fiction book consists of facts, in this case facts about how
3782 a certain program works, and these facts are necessarily the same for
3783 everyone who writes about the subject. But be careful not to copy your
3784 outline structure, wording, tables or examples from preexisting non-free
3785 documentation. Copying from free documentation may be ok; please check
3786 with the FSF about the individual case.
3788 @node Managing Releases
3789 @chapter The Release Process
3792 Making a release is more than just bundling up your source files in a
3793 tar file and putting it up for FTP. You should set up your software so
3794 that it can be configured to run on a variety of systems. Your Makefile
3795 should conform to the GNU standards described below, and your directory
3796 layout should also conform to the standards discussed below. Doing so
3797 makes it easy to include your package into the larger framework of
3801 * Configuration:: How configuration of GNU packages should work.
3802 * Makefile Conventions:: Makefile conventions.
3803 * Releases:: Making releases
3807 @section How Configuration Should Work
3808 @cindex program configuration
3811 Each GNU distribution should come with a shell script named
3812 @code{configure}. This script is given arguments which describe the
3813 kind of machine and system you want to compile the program for.
3814 The @code{configure} script must record the configuration options so
3815 that they affect compilation.
3817 The description here is the specification of the interface for the
3818 @code{configure} script in GNU packages. Many packages implement it
3819 using GNU Autoconf (@pxref{Top,, Introduction, autoconf, Autoconf})
3820 and/or GNU Automake (@pxref{Top,, Introduction, automake, Automake}),
3821 but you do not have to use these tools. You can implement it any way
3822 you like; for instance, by making @code{configure} be a wrapper around
3823 a completely different configuration system.
3825 Another way for the @code{configure} script to operate is to make a
3826 link from a standard name such as @file{config.h} to the proper
3827 configuration file for the chosen system. If you use this technique,
3828 the distribution should @emph{not} contain a file named
3829 @file{config.h}. This is so that people won't be able to build the
3830 program without configuring it first.
3832 Another thing that @code{configure} can do is to edit the Makefile. If
3833 you do this, the distribution should @emph{not} contain a file named
3834 @file{Makefile}. Instead, it should include a file @file{Makefile.in} which
3835 contains the input used for editing. Once again, this is so that people
3836 won't be able to build the program without configuring it first.
3838 If @code{configure} does write the @file{Makefile}, then @file{Makefile}
3839 should have a target named @file{Makefile} which causes @code{configure}
3840 to be rerun, setting up the same configuration that was set up last
3841 time. The files that @code{configure} reads should be listed as
3842 dependencies of @file{Makefile}.
3844 All the files which are output from the @code{configure} script should
3845 have comments at the beginning explaining that they were generated
3846 automatically using @code{configure}. This is so that users won't think
3847 of trying to edit them by hand.
3849 The @code{configure} script should write a file named @file{config.status}
3850 which describes which configuration options were specified when the
3851 program was last configured. This file should be a shell script which,
3852 if run, will recreate the same configuration.
3854 The @code{configure} script should accept an option of the form
3855 @samp{--srcdir=@var{dirname}} to specify the directory where sources are found
3856 (if it is not the current directory). This makes it possible to build
3857 the program in a separate directory, so that the actual source directory
3860 If the user does not specify @samp{--srcdir}, then @code{configure} should
3861 check both @file{.} and @file{..} to see if it can find the sources. If
3862 it finds the sources in one of these places, it should use them from
3863 there. Otherwise, it should report that it cannot find the sources, and
3864 should exit with nonzero status.
3866 Usually the easy way to support @samp{--srcdir} is by editing a
3867 definition of @code{VPATH} into the Makefile. Some rules may need to
3868 refer explicitly to the specified source directory. To make this
3869 possible, @code{configure} can add to the Makefile a variable named
3870 @code{srcdir} whose value is precisely the specified directory.
3872 In addition, the @samp{configure} script should take options
3873 corresponding to most of the standard directory variables
3874 (@pxref{Directory Variables}). Here is the list:
3877 --prefix --exec-prefix --bindir --sbindir --libexecdir --sysconfdir
3878 --sharedstatedir --localstatedir --libdir --includedir --oldincludedir
3879 --datarootdir --datadir --infodir --localedir --mandir --docdir
3880 --htmldir --dvidir --pdfdir --psdir
3883 The @code{configure} script should also take an argument which specifies the
3884 type of system to build the program for. This argument should look like
3888 @var{cpu}-@var{company}-@var{system}
3891 For example, an Athlon-based GNU/Linux system might be
3892 @samp{i686-pc-linux-gnu}.
3894 The @code{configure} script needs to be able to decode all plausible
3895 alternatives for how to describe a machine. Thus,
3896 @samp{athlon-pc-gnu/linux} would be a valid alias. There is a shell
3898 @uref{http://git.savannah.gnu.org/@/gitweb/@/?p=config.git;a=blob_plain;f=config.sub;hb=HEAD,
3899 @file{config.sub}} that you can use as a subroutine to validate system
3900 types and canonicalize aliases.
3902 The @code{configure} script should also take the option
3903 @option{--build=@var{buildtype}}, which should be equivalent to a
3904 plain @var{buildtype} argument. For example, @samp{configure
3905 --build=i686-pc-linux-gnu} is equivalent to @samp{configure
3906 i686-pc-linux-gnu}. When the build type is not specified by an option
3907 or argument, the @code{configure} script should normally guess it using
3909 @uref{http://git.savannah.gnu.org/@/gitweb/@/?p=config.git;a=blob_plain;f=config.guess;hb=HEAD,
3910 @file{config.guess}}.
3912 @cindex optional features, configure-time
3913 Other options are permitted to specify in more detail the software
3914 or hardware present on the machine, to include or exclude optional parts
3915 of the package, or to adjust the name of some tools or arguments to them:
3918 @item --enable-@var{feature}@r{[}=@var{parameter}@r{]}
3919 Configure the package to build and install an optional user-level
3920 facility called @var{feature}. This allows users to choose which
3921 optional features to include. Giving an optional @var{parameter} of
3922 @samp{no} should omit @var{feature}, if it is built by default.
3924 No @samp{--enable} option should @strong{ever} cause one feature to
3925 replace another. No @samp{--enable} option should ever substitute one
3926 useful behavior for another useful behavior. The only proper use for
3927 @samp{--enable} is for questions of whether to build part of the program
3930 @item --with-@var{package}
3931 @c @r{[}=@var{parameter}@r{]}
3932 The package @var{package} will be installed, so configure this package
3933 to work with @var{package}.
3935 @c Giving an optional @var{parameter} of
3936 @c @samp{no} should omit @var{package}, if it is used by default.
3938 Possible values of @var{package} include
3939 @samp{gnu-as} (or @samp{gas}), @samp{gnu-ld}, @samp{gnu-libc},
3945 Do not use a @samp{--with} option to specify the file name to use to
3946 find certain files. That is outside the scope of what @samp{--with}
3949 @item @var{variable}=@var{value}
3950 Set the value of the variable @var{variable} to @var{value}. This is
3951 used to override the default values of commands or arguments in the
3952 build process. For example, the user could issue @samp{configure
3953 CFLAGS=-g CXXFLAGS=-g} to build with debugging information and without
3954 the default optimization.
3956 Specifying variables as arguments to @code{configure}, like this:
3960 is preferable to setting them in environment variables:
3964 as it helps to recreate the same configuration later with
3965 @file{config.status}. However, both methods should be supported.
3968 All @code{configure} scripts should accept all of the ``detail''
3969 options and the variable settings, whether or not they make any
3970 difference to the particular package at hand. In particular, they
3971 should accept any option that starts with @samp{--with-} or
3972 @samp{--enable-}. This is so users will be able to configure an
3973 entire GNU source tree at once with a single set of options.
3975 You will note that the categories @samp{--with-} and @samp{--enable-}
3976 are narrow: they @strong{do not} provide a place for any sort of option
3977 you might think of. That is deliberate. We want to limit the possible
3978 configuration options in GNU software. We do not want GNU programs to
3979 have idiosyncratic configuration options.
3981 Packages that perform part of the compilation process may support
3982 cross-compilation. In such a case, the host and target machines for the
3983 program may be different.
3985 The @code{configure} script should normally treat the specified type of
3986 system as both the host and the target, thus producing a program which
3987 works for the same type of machine that it runs on.
3989 To compile a program to run on a host type that differs from the build
3990 type, use the configure option @option{--host=@var{hosttype}}, where
3991 @var{hosttype} uses the same syntax as @var{buildtype}. The host type
3992 normally defaults to the build type.
3994 To configure a cross-compiler, cross-assembler, or what have you, you
3995 should specify a target different from the host, using the configure
3996 option @samp{--target=@var{targettype}}. The syntax for
3997 @var{targettype} is the same as for the host type. So the command would
4001 ./configure --host=@var{hosttype} --target=@var{targettype}
4004 The target type normally defaults to the host type.
4005 Programs for which cross-operation is not meaningful need not accept the
4006 @samp{--target} option, because configuring an entire operating system for
4007 cross-operation is not a meaningful operation.
4009 Some programs have ways of configuring themselves automatically. If
4010 your program is set up to do this, your @code{configure} script can simply
4011 ignore most of its arguments.
4013 @comment The makefile standards are in a separate file that is also
4014 @comment included by make.texinfo. Done by roland@gnu.ai.mit.edu on 1/6/93.
4015 @comment For this document, turn chapters into sections, etc.
4017 @include make-stds.texi
4021 @section Making Releases
4024 You should identify each release with a pair of version numbers, a
4025 major version and a minor. We have no objection to using more than
4026 two numbers, but it is very unlikely that you really need them.
4028 Package the distribution of @code{Foo version 69.96} up in a gzipped tar
4029 file with the name @file{foo-69.96.tar.gz}. It should unpack into a
4030 subdirectory named @file{foo-69.96}.
4032 Building and installing the program should never modify any of the files
4033 contained in the distribution. This means that all the files that form
4034 part of the program in any way must be classified into @dfn{source
4035 files} and @dfn{non-source files}. Source files are written by humans
4036 and never changed automatically; non-source files are produced from
4037 source files by programs under the control of the Makefile.
4039 @cindex @file{README} file
4040 The distribution should contain a file named @file{README} which gives
4041 the name of the package, and a general description of what it does. It
4042 is also good to explain the purpose of each of the first-level
4043 subdirectories in the package, if there are any. The @file{README} file
4044 should either state the version number of the package, or refer to where
4045 in the package it can be found.
4047 The @file{README} file should refer to the file @file{INSTALL}, which
4048 should contain an explanation of the installation procedure.
4050 The @file{README} file should also refer to the file which contains the
4051 copying conditions. The GNU GPL, if used, should be in a file called
4052 @file{COPYING}. If the GNU LGPL is used, it should be in a file called
4053 @file{COPYING.LESSER}.
4055 Naturally, all the source files must be in the distribution. It is okay
4056 to include non-source files in the distribution, provided they are
4057 up-to-date and machine-independent, so that building the distribution
4058 normally will never modify them. We commonly include non-source files
4059 produced by Bison, @code{lex}, @TeX{}, and @code{makeinfo}; this helps avoid
4060 unnecessary dependencies between our distributions, so that users can
4061 install whichever packages they want to install.
4063 Non-source files that might actually be modified by building and
4064 installing the program should @strong{never} be included in the
4065 distribution. So if you do distribute non-source files, always make
4066 sure they are up to date when you make a new distribution.
4068 Make sure that all the files in the distribution are world-readable, and
4069 that directories are world-readable and world-searchable (octal mode 755).
4070 We used to recommend that all directories in the distribution also be
4071 world-writable (octal mode 777), because ancient versions of @code{tar}
4072 would otherwise not cope when extracting the archive as an unprivileged
4073 user. That can easily lead to security issues when creating the archive,
4074 however, so now we recommend against that.
4076 Don't include any symbolic links in the distribution itself. If the tar
4077 file contains symbolic links, then people cannot even unpack it on
4078 systems that don't support symbolic links. Also, don't use multiple
4079 names for one file in different directories, because certain file
4080 systems cannot handle this and that prevents unpacking the
4083 Try to make sure that all the file names will be unique on MS-DOS. A
4084 name on MS-DOS consists of up to 8 characters, optionally followed by a
4085 period and up to three characters. MS-DOS will truncate extra
4086 characters both before and after the period. Thus,
4087 @file{foobarhacker.c} and @file{foobarhacker.o} are not ambiguous; they
4088 are truncated to @file{foobarha.c} and @file{foobarha.o}, which are
4091 @cindex @file{texinfo.tex}, in a distribution
4092 Include in your distribution a copy of the @file{texinfo.tex} you used
4093 to test print any @file{*.texinfo} or @file{*.texi} files.
4095 Likewise, if your program uses small GNU software packages like regex,
4096 getopt, obstack, or termcap, include them in the distribution file.
4097 Leaving them out would make the distribution file a little smaller at
4098 the expense of possible inconvenience to a user who doesn't know what
4102 @chapter References to Non-Free Software and Documentation
4103 @cindex references to non-free material
4105 A GNU program should not recommend, promote, or grant legitimacy to
4106 the use of any non-free program. Proprietary software is a social and
4107 ethical problem, and our aim is to put an end to that problem. We
4108 can't stop some people from writing proprietary programs, or stop
4109 other people from using them, but we can and should refuse to
4110 advertise them to new potential customers, or to give the public the
4111 idea that their existence is ethical.
4113 The GNU definition of free software is found on the GNU web site at
4114 @url{http://www.gnu.org/@/philosophy/@/free-sw.html}, and the definition
4115 of free documentation is found at
4116 @url{http://www.gnu.org/@/philosophy/@/free-doc.html}. The terms ``free''
4117 and ``non-free'', used in this document, refer to those definitions.
4119 A list of important licenses and whether they qualify as free is in
4120 @url{http://www.gnu.org/@/licenses/@/license-list.html}. If it is not
4121 clear whether a license qualifies as free, please ask the GNU Project
4122 by writing to @email{licensing@@gnu.org}. We will answer, and if the
4123 license is an important one, we will add it to the list.
4125 When a non-free program or system is well known, you can mention it in
4126 passing---that is harmless, since users who might want to use it
4127 probably already know about it. For instance, it is fine to explain
4128 how to build your package on top of some widely used non-free
4129 operating system, or how to use it together with some widely used
4132 However, you should give only the necessary information to help those
4133 who already use the non-free program to use your program with
4134 it---don't give, or refer to, any further information about the
4135 proprietary program, and don't imply that the proprietary program
4136 enhances your program, or that its existence is in any way a good
4137 thing. The goal should be that people already using the proprietary
4138 program will get the advice they need about how to use your free
4139 program with it, while people who don't already use the proprietary
4140 program will not see anything likely to lead them to take an interest
4143 If a non-free program or system is obscure in your program's domain,
4144 your program should not mention or support it at all, since doing so
4145 would tend to popularize the non-free program more than it popularizes
4146 your program. (You cannot hope to find many additional users for your
4147 program among the users of Foobar, if the existence of Foobar is not
4148 generally known among people who might want to use your program.)
4150 Sometimes a program is free software in itself but depends on a
4151 non-free platform in order to run. For instance, many Java programs
4152 depend on some non-free Java libraries. To recommend or promote such
4153 a program is to promote the other programs it needs. This is why we
4154 are careful about listing Java programs in the Free Software
4155 Directory: we don't want to promote the non-free Java libraries.
4157 We hope this particular problem with Java will be gone by and by, as
4158 we replace the remaining non-free standard Java libraries with free
4159 software, but the general principle will remain the same: don't
4160 recommend, promote or legitimize programs that depend on non-free
4163 Some free programs strongly encourage the use of non-free software. A
4164 typical example is @command{mplayer}. It is free software in itself,
4165 and the free code can handle some kinds of files. However,
4166 @command{mplayer} recommends use of non-free codecs for other kinds of
4167 files, and users that install @command{mplayer} are very likely to
4168 install those codecs along with it. To recommend @command{mplayer}
4169 is, in effect, to promote use of the non-free codecs.
4171 Thus, you should not recommend programs that strongly encourage the
4172 use of non-free software. This is why we do not list
4173 @command{mplayer} in the Free Software Directory.
4175 A GNU package should not refer the user to any non-free documentation
4176 for free software. Free documentation that can be included in free
4177 operating systems is essential for completing the GNU system, or any
4178 free operating system, so encouraging it is a priority; to recommend
4179 use of documentation that we are not allowed to include undermines the
4180 impetus for the community to produce documentation that we can
4181 include. So GNU packages should never recommend non-free
4184 By contrast, it is ok to refer to journal articles and textbooks in
4185 the comments of a program for explanation of how it functions, even
4186 though they are non-free. This is because we don't include such
4187 things in the GNU system even they are free---they are outside the
4188 scope of what a software distribution needs to include.
4190 Referring to a web site that describes or recommends a non-free
4191 program is promoting that program, so please do not make links (or
4192 mention by name) web sites that contain such material. This policy is
4193 relevant particularly for the web pages for a GNU package.
4195 Following links from nearly any web site can lead eventually to
4196 non-free software; this is inherent in the nature of the web. So it
4197 makes no sense to criticize a site for having such links. As long as
4198 the site does not itself recommend a non-free program, there is no
4199 need to consider the question of the sites that it links to for other
4202 Thus, for example, you should not refer to AT&T's web site if that
4203 recommends AT&T's non-free software packages; you should not refer to
4204 a site that links to AT&T's site presenting it as a place to get some
4205 non-free program, because that link recommends and legitimizes the
4206 non-free program. However, that a site contains a link to AT&T's web
4207 site for some other purpose (such as long-distance telephone service)
4208 is not an objection against it.
4210 @node GNU Free Documentation License
4211 @appendix GNU Free Documentation License
4213 @cindex FDL, GNU Free Documentation License
4223 eval: (add-hook 'write-file-hooks 'time-stamp)
4224 time-stamp-start: "@set lastupdate "
4226 time-stamp-format: "%:b %:d, %:y"
4227 compile-command: "cd work.s && make"