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 May 5, 2011
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, 2011
31 Free Software Foundation, Inc.
33 Permission is granted to copy, distribute and/or modify this document
34 under the terms of the GNU Free Documentation License, Version 1.3 or
35 any later version published by the Free Software Foundation; with no
36 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
37 Texts. A copy of the license is included in the section entitled
38 ``GNU Free Documentation License''.
42 @title GNU Coding Standards
43 @author Richard Stallman, et al.
44 @author last updated @value{lastupdate}
46 @vskip 0pt plus 1filll
54 @top GNU Coding Standards
60 * Preface:: About the GNU Coding Standards.
61 * Legal Issues:: Keeping free software free.
62 * Design Advice:: General program design.
63 * Program Behavior:: Program behavior for all programs
64 * Writing C:: Making the best use of C.
65 * Documentation:: Documenting programs.
66 * Managing Releases:: The release process.
67 * References:: Mentioning non-free software or documentation.
68 * GNU Free Documentation License:: Copying and sharing this manual.
74 @chapter About the GNU Coding Standards
76 The GNU Coding Standards were written by Richard Stallman and other GNU
77 Project volunteers. Their purpose is to make the GNU system clean,
78 consistent, and easy to install. This document can also be read as a
79 guide to writing portable, robust and reliable programs. It focuses on
80 programs written in C, but many of the rules and principles are useful
81 even if you write in another programming language. The rules often
82 state reasons for writing in a certain way.
84 @cindex where to obtain @code{standards.texi}
85 @cindex downloading this manual
86 If you did not obtain this file directly from the GNU project and
87 recently, please check for a newer version. You can get the GNU
88 Coding Standards from the GNU web server in many
89 different formats, including the Texinfo source, PDF, HTML, DVI, plain
90 text, and more, at: @uref{http://www.gnu.org/prep/standards/}.
92 If you are maintaining an official GNU package, in addition to this
93 document, please read and follow the GNU maintainer information
94 (@pxref{Top, , Contents, maintain, Information for Maintainers of GNU
97 @cindex @code{gnustandards-commit@@gnu.org} mailing list
98 If you want to receive diffs for every change to these GNU documents,
99 join the mailing list @code{gnustandards-commit@@gnu.org}, via the web
101 @url{http://lists.gnu.org/mailman/listinfo/gnustandards-commit}.
102 Archives are also available there.
104 @cindex @code{bug-standards@@gnu.org} email address
105 @cindex Savannah repository for gnustandards
106 @cindex gnustandards project repository
107 Please send corrections or suggestions for this document to
108 @email{bug-standards@@gnu.org}. If you make a suggestion, please
109 include a suggested new wording for it, to help us consider the
110 suggestion efficiently. We prefer a context diff to the Texinfo
111 source, but if that's difficult for you, you can make a context diff
112 for some other version of this document, or propose it in any way that
113 makes it clear. The source repository for this document can be found
114 at @url{http://savannah.gnu.org/projects/gnustandards}.
116 These standards cover the minimum of what is important when writing a
117 GNU package. Likely, the need for additional standards will come up.
118 Sometimes, you might suggest that such standards be added to this
119 document. If you think your standards would be generally useful, please
122 You should also set standards for your package on many questions not
123 addressed or not firmly specified here. The most important point is to
124 be self-consistent---try to stick to the conventions you pick, and try
125 to document them as much as possible. That way, your program will be
126 more maintainable by others.
128 The GNU Hello program serves as an example of how to follow the GNU
129 coding standards for a trivial program.
130 @uref{http://www.gnu.org/software/hello/hello.html}.
132 This release of the GNU Coding Standards was last updated
137 @chapter Keeping Free Software Free
138 @cindex legal aspects
140 This chapter discusses how you can make sure that GNU software
141 avoids legal difficulties, and other related issues.
144 * Reading Non-Free Code:: Referring to proprietary programs.
145 * Contributions:: Accepting contributions.
146 * Trademarks:: How we deal with trademark issues.
149 @node Reading Non-Free Code
150 @section Referring to Proprietary Programs
151 @cindex proprietary programs
152 @cindex avoiding proprietary code
154 Don't in any circumstances refer to Unix source code for or during
155 your work on GNU! (Or to any other proprietary programs.)
157 If you have a vague recollection of the internals of a Unix program,
158 this does not absolutely mean you can't write an imitation of it, but
159 do try to organize the imitation internally along different lines,
160 because this is likely to make the details of the Unix version
161 irrelevant and dissimilar to your results.
163 For example, Unix utilities were generally optimized to minimize
164 memory use; if you go for speed instead, your program will be very
165 different. You could keep the entire input file in memory and scan it
166 there instead of using stdio. Use a smarter algorithm discovered more
167 recently than the Unix program. Eliminate use of temporary files. Do
168 it in one pass instead of two (we did this in the assembler).
170 Or, on the contrary, emphasize simplicity instead of speed. For some
171 applications, the speed of today's computers makes simpler algorithms
174 Or go for generality. For example, Unix programs often have static
175 tables or fixed-size strings, which make for arbitrary limits; use
176 dynamic allocation instead. Make sure your program handles NULs and
177 other funny characters in the input files. Add a programming language
178 for extensibility and write part of the program in that language.
180 Or turn some parts of the program into independently usable libraries.
181 Or use a simple garbage collector instead of tracking precisely when
182 to free memory, or use a new GNU facility such as obstacks.
186 @section Accepting Contributions
188 @cindex accepting contributions
190 If the program you are working on is copyrighted by the Free Software
191 Foundation, then when someone else sends you a piece of code to add to
192 the program, we need legal papers to use it---just as we asked you to
193 sign papers initially. @emph{Each} person who makes a nontrivial
194 contribution to a program must sign some sort of legal papers in order
195 for us to have clear title to the program; the main author alone is not
198 So, before adding in any contributions from other people, please tell
199 us, so we can arrange to get the papers. Then wait until we tell you
200 that we have received the signed papers, before you actually use the
203 This applies both before you release the program and afterward. If
204 you receive diffs to fix a bug, and they make significant changes, we
205 need legal papers for that change.
207 This also applies to comments and documentation files. For copyright
208 law, comments and code are just text. Copyright applies to all kinds of
209 text, so we need legal papers for all kinds.
211 We know it is frustrating to ask for legal papers; it's frustrating for
212 us as well. But if you don't wait, you are going out on a limb---for
213 example, what if the contributor's employer won't sign a disclaimer?
214 You might have to take that code out again!
216 You don't need papers for changes of a few lines here or there, since
217 they are not significant for copyright purposes. Also, you don't need
218 papers if all you get from the suggestion is some ideas, not actual code
219 which you use. For example, if someone sent you one implementation, but
220 you write a different implementation of the same idea, you don't need to
223 The very worst thing is if you forget to tell us about the other
224 contributor. We could be very embarrassed in court some day as a
227 We have more detailed advice for maintainers of GNU packages. If you
228 have reached the stage of maintaining a GNU program (whether released
229 or not), please take a look: @pxref{Legal Matters,,, maintain,
230 Information for GNU Maintainers}.
237 Please do not include any trademark acknowledgements in GNU software
238 packages or documentation.
240 Trademark acknowledgements are the statements that such-and-such is a
241 trademark of so-and-so. The GNU Project has no objection to the basic
242 idea of trademarks, but these acknowledgements feel like kowtowing,
243 and there is no legal requirement for them, so we don't use them.
245 What is legally required, as regards other people's trademarks, is to
246 avoid using them in ways which a reader might reasonably understand as
247 naming or labeling our own programs or activities. For example, since
248 ``Objective C'' is (or at least was) a trademark, we made sure to say
249 that we provide a ``compiler for the Objective C language'' rather
250 than an ``Objective C compiler''. The latter would have been meant as
251 a shorter way of saying the former, but it does not explicitly state
252 the relationship, so it could be misinterpreted as using ``Objective
253 C'' as a label for the compiler rather than for the language.
255 Please don't use ``win'' as an abbreviation for Microsoft Windows in
256 GNU software or documentation. In hacker terminology, calling
257 something a ``win'' is a form of praise. If you wish to praise
258 Microsoft Windows when speaking on your own, by all means do so, but
259 not in GNU software. Usually we write the name ``Windows'' in full,
260 but when brevity is very important (as in file names and sometimes
261 symbol names), we abbreviate it to ``w''. For instance, the files and
262 functions in Emacs that deal with Windows start with @samp{w32}.
265 @chapter General Program Design
266 @cindex program design
268 This chapter discusses some of the issues you should take into
269 account when designing your program.
271 @c Standard or ANSI C
273 @c In 1989 the American National Standards Institute (ANSI) standardized
274 @c C as standard X3.159-1989. In December of that year the
275 @c International Standards Organization ISO adopted the ANSI C standard
276 @c making minor changes. In 1990 ANSI then re-adopted ISO standard
277 @c C. This version of C is known as either ANSI C or Standard C.
279 @c A major revision of the C Standard appeared in 1999.
282 * Source Language:: Which languages to use.
283 * Compatibility:: Compatibility with other implementations.
284 * Using Extensions:: Using non-standard features.
285 * Standard C:: Using standard C features.
286 * Conditional Compilation:: Compiling code only if a conditional is true.
289 @node Source Language
290 @section Which Languages to Use
291 @cindex programming languages
293 When you want to use a language that gets compiled and runs at high
294 speed, the best language to use is C. Using another language is like
295 using a non-standard feature: it will cause trouble for users. Even if
296 GCC supports the other language, users may find it inconvenient to have
297 to install the compiler for that other language in order to build your
298 program. For example, if you write your program in C++, people will
299 have to install the GNU C++ compiler in order to compile your program.
301 C has one other advantage over C++ and other compiled languages: more
302 people know C, so more people will find it easy to read and modify the
303 program if it is written in C.
305 So in general it is much better to use C, rather than the
306 comparable alternatives.
308 But there are two exceptions to that conclusion:
312 It is no problem to use another language to write a tool specifically
313 intended for use with that language. That is because the only people
314 who want to build the tool will be those who have installed the other
318 If an application is of interest only to a narrow part of the community,
319 then the question of which language it is written in has less effect on
320 other people, so you may as well please yourself.
323 Many programs are designed to be extensible: they include an interpreter
324 for a language that is higher level than C. Often much of the program
325 is written in that language, too. The Emacs editor pioneered this
329 @cindex GNOME and Guile
330 The standard extensibility interpreter for GNU software is Guile
331 (@uref{http://www.gnu.org/@/software/@/guile/}), which implements the
332 language Scheme (an especially clean and simple dialect of Lisp).
333 Guile also includes bindings for GTK+/GNOME, making it practical to
334 write modern GUI functionality within Guile. We don't reject programs
335 written in other ``scripting languages'' such as Perl and Python, but
336 using Guile is very important for the overall consistency of the GNU
341 @section Compatibility with Other Implementations
342 @cindex compatibility with C and @sc{posix} standards
343 @cindex @sc{posix} compatibility
345 With occasional exceptions, utility programs and libraries for GNU
346 should be upward compatible with those in Berkeley Unix, and upward
347 compatible with Standard C if Standard C specifies their
348 behavior, and upward compatible with @sc{posix} if @sc{posix} specifies
351 When these standards conflict, it is useful to offer compatibility
352 modes for each of them.
354 @cindex options for compatibility
355 Standard C and @sc{posix} prohibit many kinds of extensions. Feel
356 free to make the extensions anyway, and include a @samp{--ansi},
357 @samp{--posix}, or @samp{--compatible} option to turn them off.
358 However, if the extension has a significant chance of breaking any real
359 programs or scripts, then it is not really upward compatible. So you
360 should try to redesign its interface to make it upward compatible.
362 @cindex @code{POSIXLY_CORRECT}, environment variable
363 Many GNU programs suppress extensions that conflict with @sc{posix} if the
364 environment variable @code{POSIXLY_CORRECT} is defined (even if it is
365 defined with a null value). Please make your program recognize this
366 variable if appropriate.
368 When a feature is used only by users (not by programs or command
369 files), and it is done poorly in Unix, feel free to replace it
370 completely with something totally different and better. (For example,
371 @code{vi} is replaced with Emacs.) But it is nice to offer a compatible
372 feature as well. (There is a free @code{vi} clone, so we offer it.)
374 Additional useful features are welcome regardless of whether
375 there is any precedent for them.
377 @node Using Extensions
378 @section Using Non-standard Features
379 @cindex non-standard extensions
381 Many GNU facilities that already exist support a number of convenient
382 extensions over the comparable Unix facilities. Whether to use these
383 extensions in implementing your program is a difficult question.
385 On the one hand, using the extensions can make a cleaner program.
386 On the other hand, people will not be able to build the program
387 unless the other GNU tools are available. This might cause the
388 program to work on fewer kinds of machines.
390 With some extensions, it might be easy to provide both alternatives.
391 For example, you can define functions with a ``keyword'' @code{INLINE}
392 and define that as a macro to expand into either @code{inline} or
393 nothing, depending on the compiler.
395 In general, perhaps it is best not to use the extensions if you can
396 straightforwardly do without them, but to use the extensions if they
397 are a big improvement.
399 An exception to this rule are the large, established programs (such as
400 Emacs) which run on a great variety of systems. Using GNU extensions in
401 such programs would make many users unhappy, so we don't do that.
403 Another exception is for programs that are used as part of compilation:
404 anything that must be compiled with other compilers in order to
405 bootstrap the GNU compilation facilities. If these require the GNU
406 compiler, then no one can compile them without having them installed
407 already. That would be extremely troublesome in certain cases.
410 @section Standard C and Pre-Standard C
411 @cindex @sc{ansi} C standard
413 1989 Standard C is widespread enough now that it is ok to use its
414 features in new programs. There is one exception: do not ever use the
415 ``trigraph'' feature of Standard C.
417 1999 Standard C is not widespread yet, so please do not require its
418 features in programs. It is ok to use its features if they are present.
420 However, it is easy to support pre-standard compilers in most programs,
421 so if you know how to do that, feel free. If a program you are
422 maintaining has such support, you should try to keep it working.
424 @cindex function prototypes
425 To support pre-standard C, instead of writing function definitions in
426 standard prototype form,
435 write the definition in pre-standard style like this,
445 and use a separate declaration to specify the argument prototype:
451 You need such a declaration anyway, in a header file, to get the benefit
452 of prototypes in all the files where the function is called. And once
453 you have the declaration, you normally lose nothing by writing the
454 function definition in the pre-standard style.
456 This technique does not work for integer types narrower than @code{int}.
457 If you think of an argument as being of a type narrower than @code{int},
458 declare it as @code{int} instead.
460 There are a few special cases where this technique is hard to use. For
461 example, if a function argument needs to hold the system type
462 @code{dev_t}, you run into trouble, because @code{dev_t} is shorter than
463 @code{int} on some machines; but you cannot use @code{int} instead,
464 because @code{dev_t} is wider than @code{int} on some machines. There
465 is no type you can safely use on all machines in a non-standard
466 definition. The only way to support non-standard C and pass such an
467 argument is to check the width of @code{dev_t} using Autoconf and choose
468 the argument type accordingly. This may not be worth the trouble.
470 In order to support pre-standard compilers that do not recognize
471 prototypes, you may want to use a preprocessor macro like this:
474 /* Declare the prototype for a general external function. */
475 #if defined (__STDC__) || defined (WINDOWSNT)
476 #define P_(proto) proto
482 @node Conditional Compilation
483 @section Conditional Compilation
485 When supporting configuration options already known when building your
486 program we prefer using @code{if (... )} over conditional compilation,
487 as in the former case the compiler is able to perform more extensive
488 checking of all possible code paths.
490 For example, please write
510 A modern compiler such as GCC will generate exactly the same code in
511 both cases, and we have been using similar techniques with good success
512 in several projects. Of course, the former method assumes that
513 @code{HAS_FOO} is defined as either 0 or 1.
515 While this is not a silver bullet solving all portability problems,
516 and is not always appropriate, following this policy would have saved
517 GCC developers many hours, or even days, per year.
519 In the case of function-like macros like @code{REVERSIBLE_CC_MODE} in
520 GCC which cannot be simply used in @code{if (...)} statements, there is
521 an easy workaround. Simply introduce another macro
522 @code{HAS_REVERSIBLE_CC_MODE} as in the following example:
525 #ifdef REVERSIBLE_CC_MODE
526 #define HAS_REVERSIBLE_CC_MODE 1
528 #define HAS_REVERSIBLE_CC_MODE 0
532 @node Program Behavior
533 @chapter Program Behavior for All Programs
535 This chapter describes conventions for writing robust
536 software. It also describes general standards for error messages, the
537 command line interface, and how libraries should behave.
540 * Non-GNU Standards:: We consider standards such as POSIX;
541 we don't "obey" them.
542 * Semantics:: Writing robust programs.
543 * Libraries:: Library behavior.
544 * Errors:: Formatting error messages.
545 * User Interfaces:: Standards about interfaces generally.
546 * Graphical Interfaces:: Standards for graphical interfaces.
547 * Command-Line Interfaces:: Standards for command line interfaces.
548 * Dynamic Plug-In Interfaces:: Standards for dynamic plug-in interfaces.
549 * Option Table:: Table of long options.
550 * OID Allocations:: Table of OID slots for GNU.
551 * Memory Usage:: When and how to care about memory needs.
552 * File Usage:: Which files to use, and where.
555 @node Non-GNU Standards
556 @section Non-GNU Standards
558 The GNU Project regards standards published by other organizations as
559 suggestions, not orders. We consider those standards, but we do not
560 ``obey'' them. In developing a GNU program, you should implement
561 an outside standard's specifications when that makes the GNU system
562 better overall in an objective sense. When it doesn't, you shouldn't.
564 In most cases, following published standards is convenient for
565 users---it means that their programs or scripts will work more
566 portably. For instance, GCC implements nearly all the features of
567 Standard C as specified by that standard. C program developers would
568 be unhappy if it did not. And GNU utilities mostly follow
569 specifications of POSIX.2; shell script writers and users would be
570 unhappy if our programs were incompatible.
572 But we do not follow either of these specifications rigidly, and there
573 are specific points on which we decided not to follow them, so as to
574 make the GNU system better for users.
576 For instance, Standard C says that nearly all extensions to C are
577 prohibited. How silly! GCC implements many extensions, some of which
578 were later adopted as part of the standard. If you want these
579 constructs to give an error message as ``required'' by the standard,
580 you must specify @samp{--pedantic}, which was implemented only so that
581 we can say ``GCC is a 100% implementation of the standard,'' not
582 because there is any reason to actually use it.
584 POSIX.2 specifies that @samp{df} and @samp{du} must output sizes by
585 default in units of 512 bytes. What users want is units of 1k, so
586 that is what we do by default. If you want the ridiculous behavior
587 ``required'' by POSIX, you must set the environment variable
588 @samp{POSIXLY_CORRECT} (which was originally going to be named
589 @samp{POSIX_ME_HARDER}).
591 GNU utilities also depart from the letter of the POSIX.2 specification
592 when they support long-named command-line options, and intermixing
593 options with ordinary arguments. This minor incompatibility with
594 POSIX is never a problem in practice, and it is very useful.
596 In particular, don't reject a new feature, or remove an old one,
597 merely because a standard says it is ``forbidden'' or ``deprecated.''
601 @section Writing Robust Programs
603 @cindex arbitrary limits on data
604 Avoid arbitrary limits on the length or number of @emph{any} data
605 structure, including file names, lines, files, and symbols, by allocating
606 all data structures dynamically. In most Unix utilities, ``long lines
607 are silently truncated''. This is not acceptable in a GNU utility.
609 @cindex @code{NUL} characters
611 Utilities reading files should not drop NUL characters, or any other
612 nonprinting characters @emph{including those with codes above 0177}.
613 The only sensible exceptions would be utilities specifically intended
614 for interface to certain types of terminals or printers
615 that can't handle those characters.
616 Whenever possible, try to make programs work properly with
617 sequences of bytes that represent multibyte characters, using encodings
618 such as UTF-8 and others. You can use libiconv to deal with a wide
621 @cindex error messages
622 Check every system call for an error return, unless you know you wish
623 to ignore errors. Include the system error text (from @code{perror},
624 @code{strerror}, or equivalent) in @emph{every} error message
625 resulting from a failing system call, as well as the name of the file
626 if any and the name of the utility. Just ``cannot open foo.c'' or
627 ``stat failed'' is not sufficient.
629 @cindex @code{malloc} return value
630 @cindex memory allocation failure
631 Check every call to @code{malloc} or @code{realloc} to see if it
632 returned zero. Check @code{realloc} even if you are making the block
633 smaller; in a system that rounds block sizes to a power of 2,
634 @code{realloc} may get a different block if you ask for less space.
636 In Unix, @code{realloc} can destroy the storage block if it returns
637 zero. GNU @code{realloc} does not have this bug: if it fails, the
638 original block is unchanged. Feel free to assume the bug is fixed. If
639 you wish to run your program on Unix, and wish to avoid lossage in this
640 case, you can use the GNU @code{malloc}.
642 You must expect @code{free} to alter the contents of the block that was
643 freed. Anything you want to fetch from the block, you must fetch before
646 If @code{malloc} fails in a noninteractive program, make that a fatal
647 error. In an interactive program (one that reads commands from the
648 user), it is better to abort the command and return to the command
649 reader loop. This allows the user to kill other processes to free up
650 virtual memory, and then try the command again.
652 @cindex command-line arguments, decoding
653 Use @code{getopt_long} to decode arguments, unless the argument syntax
654 makes this unreasonable.
656 When static storage is to be written in during program execution, use
657 explicit C code to initialize it. Reserve C initialized declarations
658 for data that will not be changed.
661 Try to avoid low-level interfaces to obscure Unix data structures (such
662 as file directories, utmp, or the layout of kernel memory), since these
663 are less likely to work compatibly. If you need to find all the files
664 in a directory, use @code{readdir} or some other high-level interface.
665 These are supported compatibly by GNU.
667 @cindex signal handling
668 The preferred signal handling facilities are the BSD variant of
669 @code{signal}, and the @sc{posix} @code{sigaction} function; the
670 alternative USG @code{signal} interface is an inferior design.
672 Nowadays, using the @sc{posix} signal functions may be the easiest way
673 to make a program portable. If you use @code{signal}, then on GNU/Linux
674 systems running GNU libc version 1, you should include
675 @file{bsd/signal.h} instead of @file{signal.h}, so as to get BSD
676 behavior. It is up to you whether to support systems where
677 @code{signal} has only the USG behavior, or give up on them.
679 @cindex impossible conditions
680 In error checks that detect ``impossible'' conditions, just abort.
681 There is usually no point in printing any message. These checks
682 indicate the existence of bugs. Whoever wants to fix the bugs will have
683 to read the source code and run a debugger. So explain the problem with
684 comments in the source. The relevant data will be in variables, which
685 are easy to examine with the debugger, so there is no point moving them
688 Do not use a count of errors as the exit status for a program.
689 @emph{That does not work}, because exit status values are limited to 8
690 bits (0 through 255). A single run of the program might have 256
691 errors; if you try to return 256 as the exit status, the parent process
692 will see 0 as the status, and it will appear that the program succeeded.
694 @cindex temporary files
695 @cindex @code{TMPDIR} environment variable
696 If you make temporary files, check the @code{TMPDIR} environment
697 variable; if that variable is defined, use the specified directory
698 instead of @file{/tmp}.
700 In addition, be aware that there is a possible security problem when
701 creating temporary files in world-writable directories. In C, you can
702 avoid this problem by creating temporary files in this manner:
705 fd = open (filename, O_WRONLY | O_CREAT | O_EXCL, 0600);
709 or by using the @code{mkstemps} function from Gnulib
710 (@pxref{mkstemps,,, gnulib, Gnulib}).
712 In bash, use @code{set -C} (long name @code{noclobber}) to avoid this
713 problem. In addition, the @code{mktemp} utility is a more general
714 solution for creating temporary files from shell scripts
715 (@pxref{mktemp invocation,,, coreutils, GNU Coreutils}).
719 @section Library Behavior
722 Try to make library functions reentrant. If they need to do dynamic
723 storage allocation, at least try to avoid any nonreentrancy aside from
724 that of @code{malloc} itself.
726 Here are certain name conventions for libraries, to avoid name
729 Choose a name prefix for the library, more than two characters long.
730 All external function and variable names should start with this
731 prefix. In addition, there should only be one of these in any given
732 library member. This usually means putting each one in a separate
735 An exception can be made when two external symbols are always used
736 together, so that no reasonable program could use one without the
737 other; then they can both go in the same file.
739 External symbols that are not documented entry points for the user
740 should have names beginning with @samp{_}. The @samp{_} should be
741 followed by the chosen name prefix for the library, to prevent
742 collisions with other libraries. These can go in the same files with
743 user entry points if you like.
745 Static functions and variables can be used as you like and need not
746 fit any naming convention.
749 @section Formatting Error Messages
750 @cindex formatting error messages
751 @cindex error messages, formatting
753 Error messages from compilers should look like this:
756 @var{source-file-name}:@var{lineno}: @var{message}
760 If you want to mention the column number, use one of these formats:
763 @var{source-file-name}:@var{lineno}:@var{column}: @var{message}
764 @var{source-file-name}:@var{lineno}.@var{column}: @var{message}
769 Line numbers should start from 1 at the beginning of the file, and
770 column numbers should start from 1 at the beginning of the line. (Both
771 of these conventions are chosen for compatibility.) Calculate column
772 numbers assuming that space and all ASCII printing characters have
773 equal width, and assuming tab stops every 8 columns.
775 The error message can also give both the starting and ending positions
776 of the erroneous text. There are several formats so that you can
777 avoid redundant information such as a duplicate line number.
778 Here are the possible formats:
781 @var{source-file-name}:@var{lineno-1}.@var{column-1}-@var{lineno-2}.@var{column-2}: @var{message}
782 @var{source-file-name}:@var{lineno-1}.@var{column-1}-@var{column-2}: @var{message}
783 @var{source-file-name}:@var{lineno-1}-@var{lineno-2}: @var{message}
787 When an error is spread over several files, you can use this format:
790 @var{file-1}:@var{lineno-1}.@var{column-1}-@var{file-2}:@var{lineno-2}.@var{column-2}: @var{message}
793 Error messages from other noninteractive programs should look like this:
796 @var{program}:@var{source-file-name}:@var{lineno}: @var{message}
800 when there is an appropriate source file, or like this:
803 @var{program}: @var{message}
807 when there is no relevant source file.
809 If you want to mention the column number, use this format:
812 @var{program}:@var{source-file-name}:@var{lineno}:@var{column}: @var{message}
815 In an interactive program (one that is reading commands from a
816 terminal), it is better not to include the program name in an error
817 message. The place to indicate which program is running is in the
818 prompt or with the screen layout. (When the same program runs with
819 input from a source other than a terminal, it is not interactive and
820 would do best to print error messages using the noninteractive style.)
822 The string @var{message} should not begin with a capital letter when
823 it follows a program name and/or file name, because that isn't the
824 beginning of a sentence. (The sentence conceptually starts at the
825 beginning of the line.) Also, it should not end with a period.
827 Error messages from interactive programs, and other messages such as
828 usage messages, should start with a capital letter. But they should not
831 @node User Interfaces
832 @section Standards for Interfaces Generally
834 @cindex program name and its behavior
835 @cindex behavior, dependent on program's name
836 Please don't make the behavior of a utility depend on the name used
837 to invoke it. It is useful sometimes to make a link to a utility
838 with a different name, and that should not change what it does.
840 Instead, use a run time option or a compilation switch or both
841 to select among the alternate behaviors.
843 @cindex output device and program's behavior
844 Likewise, please don't make the behavior of the program depend on the
845 type of output device it is used with. Device independence is an
846 important principle of the system's design; do not compromise it merely
847 to save someone from typing an option now and then. (Variation in error
848 message syntax when using a terminal is ok, because that is a side issue
849 that people do not depend on.)
851 If you think one behavior is most useful when the output is to a
852 terminal, and another is most useful when the output is a file or a
853 pipe, then it is usually best to make the default behavior the one that
854 is useful with output to a terminal, and have an option for the other
857 Compatibility requires certain programs to depend on the type of output
858 device. It would be disastrous if @code{ls} or @code{sh} did not do so
859 in the way all users expect. In some of these cases, we supplement the
860 program with a preferred alternate version that does not depend on the
861 output device type. For example, we provide a @code{dir} program much
862 like @code{ls} except that its default output format is always
866 @node Graphical Interfaces
867 @section Standards for Graphical Interfaces
868 @cindex graphical user interface
869 @cindex interface styles
870 @cindex user interface styles
873 When you write a program that provides a graphical user interface,
874 please make it work with the X Window System and the GTK+ toolkit
875 unless the functionality specifically requires some alternative (for
876 example, ``displaying jpeg images while in console mode'').
878 In addition, please provide a command-line interface to control the
879 functionality. (In many cases, the graphical user interface can be a
880 separate program which invokes the command-line program.) This is
881 so that the same jobs can be done from scripts.
886 @cindex keyboard interface
887 @cindex library interface
888 Please also consider providing a D-bus interface for use from other
889 running programs, such as within GNOME. (GNOME used to use CORBA
890 for this, but that is being phased out.) In addition, consider
891 providing a library interface (for use from C), and perhaps a
892 keyboard-driven console interface (for use by users from console
893 mode). Once you are doing the work to provide the functionality and
894 the graphical interface, these won't be much extra work.
897 @node Command-Line Interfaces
898 @section Standards for Command Line Interfaces
899 @cindex command-line interface
902 It is a good idea to follow the @sc{posix} guidelines for the
903 command-line options of a program. The easiest way to do this is to use
904 @code{getopt} to parse them. Note that the GNU version of @code{getopt}
905 will normally permit options anywhere among the arguments unless the
906 special argument @samp{--} is used. This is not what @sc{posix}
907 specifies; it is a GNU extension.
909 @cindex long-named options
910 Please define long-named options that are equivalent to the
911 single-letter Unix-style options. We hope to make GNU more user
912 friendly this way. This is easy to do with the GNU function
915 One of the advantages of long-named options is that they can be
916 consistent from program to program. For example, users should be able
917 to expect the ``verbose'' option of any GNU program which has one, to be
918 spelled precisely @samp{--verbose}. To achieve this uniformity, look at
919 the table of common long-option names when you choose the option names
920 for your program (@pxref{Option Table}).
922 It is usually a good idea for file names given as ordinary arguments to
923 be input files only; any output files would be specified using options
924 (preferably @samp{-o} or @samp{--output}). Even if you allow an output
925 file name as an ordinary argument for compatibility, try to provide an
926 option as another way to specify it. This will lead to more consistency
927 among GNU utilities, and fewer idiosyncrasies for users to remember.
929 @cindex standard command-line options
930 @cindex options, standard command-line
931 @cindex CGI programs, standard options for
932 @cindex PATH_INFO, specifying standard options as
933 All programs should support two standard options: @samp{--version}
934 and @samp{--help}. CGI programs should accept these as command-line
935 options, and also if given as the @env{PATH_INFO}; for instance,
936 visiting @url{http://example.org/p.cgi/--help} in a browser should
937 output the same information as invoking @samp{p.cgi --help} from the
941 * --version:: The standard output for --version.
942 * --help:: The standard output for --help.
946 @subsection @option{--version}
948 @cindex @samp{--version} output
950 The standard @code{--version} option should direct the program to
951 print information about its name, version, origin and legal status,
952 all on standard output, and then exit successfully. Other options and
953 arguments should be ignored once this is seen, and the program should
954 not perform its normal function.
956 @cindex canonical name of a program
957 @cindex program's canonical name
958 The first line is meant to be easy for a program to parse; the version
959 number proper starts after the last space. In addition, it contains
960 the canonical name for this program, in this format:
967 The program's name should be a constant string; @emph{don't} compute it
968 from @code{argv[0]}. The idea is to state the standard or canonical
969 name for the program, not its file name. There are other ways to find
970 out the precise file name where a command is found in @code{PATH}.
972 If the program is a subsidiary part of a larger package, mention the
973 package name in parentheses, like this:
976 emacsserver (GNU Emacs) 19.30
980 If the package has a version number which is different from this
981 program's version number, you can mention the package version number
982 just before the close-parenthesis.
984 If you @emph{need} to mention the version numbers of libraries which
985 are distributed separately from the package which contains this program,
986 you can do so by printing an additional line of version info for each
987 library you want to mention. Use the same format for these lines as for
990 Please do not mention all of the libraries that the program uses ``just
991 for completeness''---that would produce a lot of unhelpful clutter.
992 Please mention library version numbers only if you find in practice that
993 they are very important to you in debugging.
995 The following line, after the version number line or lines, should be a
996 copyright notice. If more than one copyright notice is called for, put
997 each on a separate line.
999 Next should follow a line stating the license, preferably using one of
1000 abbrevations below, and a brief statement that the program is free
1001 software, and that users are free to copy and change it. Also mention
1002 that there is no warranty, to the extent permitted by law. See
1003 recommended wording below.
1005 It is ok to finish the output with a list of the major authors of the
1006 program, as a way of giving credit.
1008 Here's an example of output that follows these rules:
1012 Copyright (C) 2007 Free Software Foundation, Inc.
1013 License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
1014 This is free software: you are free to change and redistribute it.
1015 There is NO WARRANTY, to the extent permitted by law.
1018 You should adapt this to your program, of course, filling in the proper
1019 year, copyright holder, name of program, and the references to
1020 distribution terms, and changing the rest of the wording as necessary.
1022 This copyright notice only needs to mention the most recent year in
1023 which changes were made---there's no need to list the years for previous
1024 versions' changes. You don't have to mention the name of the program in
1025 these notices, if that is inconvenient, since it appeared in the first
1026 line. (The rules are different for copyright notices in source files;
1027 @pxref{Copyright Notices,,,maintain,Information for GNU Maintainers}.)
1029 Translations of the above lines must preserve the validity of the
1030 copyright notices (@pxref{Internationalization}). If the translation's
1031 character set supports it, the @samp{(C)} should be replaced with the
1032 copyright symbol, as follows:
1035 (the official copyright symbol, which is the letter C in a circle);
1041 Write the word ``Copyright'' exactly like that, in English. Do not
1042 translate it into another language. International treaties recognize
1043 the English word ``Copyright''; translations into other languages do not
1044 have legal significance.
1046 Finally, here is the table of our suggested license abbreviations.
1047 Any abbreviation can be followed by @samp{v@var{version}[+]}, meaning
1048 that particular version, or later versions with the @samp{+}, as shown
1051 In the case of exceptions for extra permissions with the GPL, we use
1052 @samp{/} for a separator; the version number can follow the license
1053 abbreviation as usual, as in the examples below.
1057 GNU General Public License, @url{http://www.gnu.org/@/licenses/@/gpl.html}.
1060 GNU Lesser General Public License, @url{http://www.gnu.org/@/licenses/@/lgpl.html}.
1063 GNU GPL with the exception for Ada.
1066 The Apache Software Foundation license,
1067 @url{http://www.apache.org/@/licenses}.
1070 The Artistic license used for Perl, @url{http://www.perlfoundation.org/@/legal}.
1073 The Expat license, @url{http://www.jclark.com/@/xml/@/copying.txt}.
1076 The Mozilla Public License, @url{http://www.mozilla.org/@/MPL/}.
1079 The original (4-clause) BSD license, incompatible with the GNU GPL
1080 @url{http://www.xfree86.org/@/3.3.6/@/COPYRIGHT2.html#6}.
1083 The license used for PHP, @url{http://www.php.net/@/license/}.
1086 The non-license that is being in the public domain,
1087 @url{http://www.gnu.org/@/licenses/@/license-list.html#PublicDomain}.
1090 The license for Python, @url{http://www.python.org/@/2.0.1/@/license.html}.
1093 The revised (3-clause) BSD, compatible with the GNU GPL,@*
1094 @url{http://www.xfree86.org/@/3.3.6/@/COPYRIGHT2.html#5}.
1097 The simple non-copyleft license used for most versions of the X Window
1098 System, @url{http://www.xfree86.org/@/3.3.6/@/COPYRIGHT2.html#3}.
1101 The license for Zlib, @url{http://www.gzip.org/@/zlib/@/zlib_license.html}.
1105 More information about these licenses and many more are on the GNU
1106 licensing web pages,
1107 @url{http://www.gnu.org/@/licenses/@/license-list.html}.
1111 @subsection @option{--help}
1113 @cindex @samp{--help} output
1115 The standard @code{--help} option should output brief documentation
1116 for how to invoke the program, on standard output, then exit
1117 successfully. Other options and arguments should be ignored once this
1118 is seen, and the program should not perform its normal function.
1120 @cindex address for bug reports
1122 Near the end of the @samp{--help} option's output, please place lines
1123 giving the email address for bug reports, the package's home page
1124 (normally @indicateurl{http://www.gnu.org/software/@var{pkg}}, and the
1125 general page for help using GNU programs. The format should be like this:
1128 Report bugs to: @var{mailing-address}
1129 @var{pkg} home page: <http://www.gnu.org/software/@var{pkg}/>
1130 General help using GNU software: <http://www.gnu.org/gethelp/>
1133 It is ok to mention other appropriate mailing lists and web pages.
1136 @node Dynamic Plug-In Interfaces
1137 @section Standards for Dynamic Plug-in Interfaces
1139 @cindex dynamic plug-ins
1141 Another aspect of keeping free programs free is encouraging
1142 development of free plug-ins, and discouraging development of
1143 proprietary plug-ins. Many GNU programs will not have anything like
1144 plug-ins at all, but those that do should follow these
1147 First, the general plug-in architecture design should closely tie the
1148 plug-in to the original code, such that the plug-in and the base
1149 program are parts of one extended program. For GCC, for example,
1150 plug-ins receive and modify GCC's internal data structures, and so
1151 clearly form an extended program with the base GCC.
1153 @vindex plugin_is_GPL_compatible
1154 Second, you should require plug-in developers to affirm that their
1155 plug-ins are released under an appropriate license. This should be
1156 enforced with a simple programmatic check. For GCC, again for
1157 example, a plug-in must define the global symbol
1158 @code{plugin_is_GPL_compatible}, thus asserting that the plug-in is
1159 released under a GPL-compatible license (@pxref{Plugins,, Plugins,
1160 gccint, GCC Internals}).
1162 By adding this check to your program you are not creating a new legal
1163 requirement. The GPL itself requires plug-ins to be free software,
1164 licensed compatibly. As long as you have followed the first rule above
1165 to keep plug-ins closely tied to your original program, the GPL and AGPL
1166 already require those plug-ins to be released under a compatible
1167 license. The symbol definition in the plug-in---or whatever equivalent
1168 works best in your program---makes it harder for anyone who might
1169 distribute proprietary plug-ins to legally defend themselves. If a case
1170 about this got to court, we can point to that symbol as evidence that
1171 the plug-in developer understood that the license had this requirement.
1175 @section Table of Long Options
1176 @cindex long option names
1177 @cindex table of long options
1179 Here is a table of long options used by GNU programs. It is surely
1180 incomplete, but we aim to list all the options that a new program might
1181 want to be compatible with. If you use names not already in the table,
1182 please send @email{bug-standards@@gnu.org} a list of them, with their
1183 meanings, so we can update the table.
1185 @c Please leave newlines between items in this table; it's much easier
1186 @c to update when it isn't completely squashed together and unreadable.
1187 @c When there is more than one short option for a long option name, put
1188 @c a semicolon between the lists of the programs that use them, not a
1189 @c period. --friedman
1193 @samp{-N} in @code{tar}.
1196 @samp{-a} in @code{du}, @code{ls}, @code{nm}, @code{stty}, @code{uname},
1197 and @code{unexpand}.
1200 @samp{-a} in @code{diff}.
1203 @samp{-A} in @code{ls}.
1206 @samp{-a} in @code{etags}, @code{tee}, @code{time};
1207 @samp{-r} in @code{tar}.
1210 @samp{-a} in @code{cp}.
1213 @samp{-n} in @code{shar}.
1216 @samp{-l} in @code{m4}.
1219 @samp{-a} in @code{diff}.
1222 @samp{-v} in @code{gawk}.
1225 @samp{-W} in @code{make}.
1228 @samp{-o} in @code{make}.
1231 @samp{-a} in @code{recode}.
1234 @samp{-a} in @code{wdiff}.
1236 @item auto-reference
1237 @samp{-A} in @code{ptx}.
1240 @samp{-n} in @code{wdiff}.
1243 For server programs, run in the background.
1245 @item backward-search
1246 @samp{-B} in @code{ctags}.
1249 @samp{-f} in @code{shar}.
1258 @samp{-b} in @code{tac}.
1261 @samp{-b} in @code{cpio} and @code{diff}.
1264 @samp{-b} in @code{shar}.
1267 Used in @code{cpio} and @code{tar}.
1270 @samp{-b} in @code{head} and @code{tail}.
1273 @samp{-b} in @code{ptx}.
1276 Used in various programs to make output shorter.
1279 @samp{-c} in @code{head}, @code{split}, and @code{tail}.
1282 @samp{-C} in @code{etags}.
1285 @samp{-A} in @code{tar}.
1288 Used in various programs to specify the directory to use.
1291 @samp{-c} in @code{chgrp} and @code{chown}.
1294 @samp{-F} in @code{ls}.
1297 @samp{-c} in @code{recode}.
1300 @samp{-c} in @code{su};
1304 @samp{-d} in @code{tar}.
1307 Used in @code{gawk}.
1310 @samp{-Z} in @code{tar} and @code{shar}.
1313 @samp{-A} in @code{tar}.
1316 @samp{-w} in @code{tar}.
1319 Used in @code{diff}.
1322 @samp{-W copyleft} in @code{gawk}.
1325 @samp{-C} in @code{ptx}, @code{recode}, and @code{wdiff};
1326 @samp{-W copyright} in @code{gawk}.
1332 @samp{-q} in @code{who}.
1335 @samp{-l} in @code{du}.
1338 Used in @code{tar} and @code{cpio}.
1341 @samp{-c} in @code{shar}.
1344 @samp{-x} in @code{ctags}.
1347 @samp{-d} in @code{touch}.
1350 @samp{-d} in @code{make} and @code{m4};
1354 @samp{-D} in @code{m4}.
1357 @samp{-d} in Bison and @code{ctags}.
1360 @samp{-D} in @code{tar}.
1363 @samp{-L} in @code{chgrp}, @code{chown}, @code{cpio}, @code{du},
1364 @code{ls}, and @code{tar}.
1366 @item dereference-args
1367 @samp{-D} in @code{du}.
1370 Specify an I/O device (special file name).
1373 @samp{-d} in @code{recode}.
1375 @item dictionary-order
1376 @samp{-d} in @code{look}.
1379 @samp{-d} in @code{tar}.
1382 @samp{-n} in @code{csplit}.
1385 Specify the directory to use, in various programs. In @code{ls}, it
1386 means to show directories themselves rather than their contents. In
1387 @code{rm} and @code{ln}, it means to not treat links to directories
1391 @samp{-x} in @code{strip}.
1393 @item discard-locals
1394 @samp{-X} in @code{strip}.
1397 @samp{-n} in @code{make}.
1400 @samp{-e} in @code{diff}.
1402 @item elide-empty-files
1403 @samp{-z} in @code{csplit}.
1406 @samp{-x} in @code{wdiff}.
1409 @samp{-z} in @code{wdiff}.
1411 @item entire-new-file
1412 @samp{-N} in @code{diff}.
1414 @item environment-overrides
1415 @samp{-e} in @code{make}.
1418 @samp{-e} in @code{xargs}.
1424 Used in @code{makeinfo}.
1427 @samp{-o} in @code{m4}.
1430 @samp{-b} in @code{ls}.
1433 @samp{-X} in @code{tar}.
1439 @samp{-x} in @code{xargs}.
1442 @samp{-e} in @code{unshar}.
1445 @samp{-t} in @code{diff}.
1448 @samp{-e} in @code{sed}.
1451 @samp{-g} in @code{nm}.
1454 @samp{-i} in @code{cpio};
1455 @samp{-x} in @code{tar}.
1458 @samp{-f} in @code{finger}.
1461 @samp{-f} in @code{su}.
1463 @item fatal-warnings
1464 @samp{-E} in @code{m4}.
1467 @samp{-f} in @code{gawk}, @code{info}, @code{make}, @code{mt},
1468 @code{sed}, and @code{tar}.
1470 @item field-separator
1471 @samp{-F} in @code{gawk}.
1477 @samp{-F} in @code{ls}.
1480 @samp{-T} in @code{tar}.
1483 Used in @code{makeinfo}.
1485 @item flag-truncation
1486 @samp{-F} in @code{ptx}.
1488 @item fixed-output-files
1492 @samp{-f} in @code{tail}.
1494 @item footnote-style
1495 Used in @code{makeinfo}.
1498 @samp{-f} in @code{cp}, @code{ln}, @code{mv}, and @code{rm}.
1501 @samp{-F} in @code{shar}.
1504 For server programs, run in the foreground;
1505 in other words, don't do anything special to run the server
1509 Used in @code{ls}, @code{time}, and @code{ptx}.
1512 @samp{-F} in @code{m4}.
1518 @samp{-g} in @code{ptx}.
1521 @samp{-x} in @code{tar}.
1524 @samp{-i} in @code{ul}.
1527 @samp{-g} in @code{recode}.
1530 @samp{-g} in @code{install}.
1533 @samp{-z} in @code{tar} and @code{shar}.
1536 @samp{-H} in @code{m4}.
1539 @samp{-h} in @code{objdump} and @code{recode}
1542 @samp{-H} in @code{who}.
1545 Used to ask for brief usage information.
1547 @item here-delimiter
1548 @samp{-d} in @code{shar}.
1550 @item hide-control-chars
1551 @samp{-q} in @code{ls}.
1554 In @code{makeinfo}, output HTML.
1557 @samp{-u} in @code{who}.
1560 @samp{-D} in @code{diff}.
1563 @samp{-I} in @code{ls};
1564 @samp{-x} in @code{recode}.
1566 @item ignore-all-space
1567 @samp{-w} in @code{diff}.
1569 @item ignore-backups
1570 @samp{-B} in @code{ls}.
1572 @item ignore-blank-lines
1573 @samp{-B} in @code{diff}.
1576 @samp{-f} in @code{look} and @code{ptx};
1577 @samp{-i} in @code{diff} and @code{wdiff}.
1580 @samp{-i} in @code{make}.
1583 @samp{-i} in @code{ptx}.
1585 @item ignore-indentation
1586 @samp{-I} in @code{etags}.
1588 @item ignore-init-file
1591 @item ignore-interrupts
1592 @samp{-i} in @code{tee}.
1594 @item ignore-matching-lines
1595 @samp{-I} in @code{diff}.
1597 @item ignore-space-change
1598 @samp{-b} in @code{diff}.
1601 @samp{-i} in @code{tar}.
1604 @samp{-i} in @code{etags};
1605 @samp{-I} in @code{m4}.
1608 @samp{-I} in @code{make}.
1611 @samp{-G} in @code{tar}.
1614 @samp{-i}, @samp{-l}, and @samp{-m} in Finger.
1617 In some programs, specify the name of the file to read as the user's
1621 @samp{-i} in @code{expand}.
1624 @samp{-T} in @code{diff}.
1627 @samp{-i} in @code{ls}.
1630 @samp{-i} in @code{cp}, @code{ln}, @code{mv}, @code{rm};
1631 @samp{-e} in @code{m4};
1632 @samp{-p} in @code{xargs};
1633 @samp{-w} in @code{tar}.
1636 @samp{-p} in @code{shar}.
1642 @samp{-j} in @code{make}.
1645 @samp{-n} in @code{make}.
1648 @samp{-k} in @code{make}.
1651 @samp{-k} in @code{csplit}.
1654 @samp{-k} in @code{du} and @code{ls}.
1657 @samp{-l} in @code{etags}.
1660 @samp{-l} in @code{wdiff}.
1662 @item level-for-gzip
1663 @samp{-g} in @code{shar}.
1666 @samp{-C} in @code{split}.
1669 Used in @code{split}, @code{head}, and @code{tail}.
1672 @samp{-l} in @code{cpio}.
1676 Used in @code{gawk}.
1679 @samp{-t} in @code{cpio};
1680 @samp{-l} in @code{recode}.
1683 @samp{-t} in @code{tar}.
1686 @samp{-N} in @code{ls}.
1689 @samp{-l} in @code{make}.
1695 Used in @code{uname}.
1698 @samp{-M} in @code{ptx}.
1701 @samp{-m} in @code{hello} and @code{uname}.
1703 @item make-directories
1704 @samp{-d} in @code{cpio}.
1707 @samp{-f} in @code{make}.
1713 @samp{-n} in @code{xargs}.
1716 @samp{-n} in @code{xargs}.
1719 @samp{-l} in @code{xargs}.
1722 @samp{-l} in @code{make}.
1725 @samp{-P} in @code{xargs}.
1728 @samp{-T} in @code{who}.
1731 @samp{-T} in @code{who}.
1734 @samp{-d} in @code{diff}.
1736 @item mixed-uuencode
1737 @samp{-M} in @code{shar}.
1740 @samp{-m} in @code{install}, @code{mkdir}, and @code{mkfifo}.
1742 @item modification-time
1743 @samp{-m} in @code{tar}.
1746 @samp{-M} in @code{tar}.
1752 @samp{-L} in @code{m4}.
1755 @samp{-a} in @code{shar}.
1758 @samp{-W} in @code{make}.
1760 @item no-builtin-rules
1761 @samp{-r} in @code{make}.
1763 @item no-character-count
1764 @samp{-w} in @code{shar}.
1766 @item no-check-existing
1767 @samp{-x} in @code{shar}.
1770 @samp{-3} in @code{wdiff}.
1773 @samp{-c} in @code{touch}.
1776 @samp{-D} in @code{etags}.
1779 @samp{-1} in @code{wdiff}.
1781 @item no-dereference
1782 @samp{-d} in @code{cp}.
1785 @samp{-2} in @code{wdiff}.
1788 @samp{-S} in @code{make}.
1794 @samp{-P} in @code{shar}.
1797 @samp{-e} in @code{gprof}.
1800 @samp{-R} in @code{etags}.
1803 @samp{-p} in @code{nm}.
1806 Don't print a startup splash screen.
1809 Used in @code{makeinfo}.
1812 @samp{-a} in @code{gprof}.
1815 @samp{-E} in @code{gprof}.
1818 @samp{-m} in @code{shar}.
1821 Used in @code{makeinfo}.
1824 Used in @code{emacsclient}.
1827 Used in various programs to inhibit warnings.
1830 @samp{-n} in @code{info}.
1833 @samp{-n} in @code{uname}.
1836 @samp{-f} in @code{cpio}.
1839 @samp{-n} in @code{objdump}.
1842 @samp{-0} in @code{xargs}.
1845 @samp{-n} in @code{cat}.
1847 @item number-nonblank
1848 @samp{-b} in @code{cat}.
1851 @samp{-n} in @code{nm}.
1853 @item numeric-uid-gid
1854 @samp{-n} in @code{cpio} and @code{ls}.
1860 @samp{-o} in @code{tar}.
1863 @samp{-o} in @code{make}.
1865 @item one-file-system
1866 @samp{-l} in @code{tar}, @code{cp}, and @code{du}.
1869 @samp{-o} in @code{ptx}.
1872 @samp{-f} in @code{gprof}.
1875 @samp{-F} in @code{gprof}.
1878 @samp{-o} in @code{getopt}, @code{fdlist}, @code{fdmount},
1879 @code{fdmountd}, and @code{fdumount}.
1882 In various programs, specify the output file name.
1885 @samp{-o} in @code{shar}.
1888 @samp{-o} in @code{rm}.
1891 @samp{-c} in @code{unshar}.
1894 @samp{-o} in @code{install}.
1897 @samp{-l} in @code{diff}.
1899 @item paragraph-indent
1900 Used in @code{makeinfo}.
1903 @samp{-p} in @code{mkdir} and @code{rmdir}.
1906 @samp{-p} in @code{ul}.
1909 @samp{-p} in @code{cpio}.
1912 @samp{-P} in @code{finger}.
1915 @samp{-c} in @code{cpio} and @code{tar}.
1918 Used in @code{gawk}.
1920 @item prefix-builtins
1921 @samp{-P} in @code{m4}.
1924 @samp{-f} in @code{csplit}.
1927 Used in @code{tar} and @code{cp}.
1929 @item preserve-environment
1930 @samp{-p} in @code{su}.
1932 @item preserve-modification-time
1933 @samp{-m} in @code{cpio}.
1935 @item preserve-order
1936 @samp{-s} in @code{tar}.
1938 @item preserve-permissions
1939 @samp{-p} in @code{tar}.
1942 @samp{-l} in @code{diff}.
1945 @samp{-L} in @code{cmp}.
1947 @item print-data-base
1948 @samp{-p} in @code{make}.
1950 @item print-directory
1951 @samp{-w} in @code{make}.
1953 @item print-file-name
1954 @samp{-o} in @code{nm}.
1957 @samp{-s} in @code{nm}.
1960 @samp{-p} in @code{wdiff}.
1963 @samp{-p} in @code{ed}.
1966 Specify an HTTP proxy.
1969 @samp{-X} in @code{shar}.
1972 @samp{-q} in @code{make}.
1975 Used in many programs to inhibit the usual output. Every
1976 program accepting @samp{--quiet} should accept @samp{--silent} as a
1980 @samp{-Q} in @code{shar}
1983 @samp{-Q} in @code{ls}.
1986 @samp{-n} in @code{diff}.
1989 Used in @code{gawk}.
1991 @item read-full-blocks
1992 @samp{-B} in @code{tar}.
1998 @samp{-n} in @code{make}.
2001 @samp{-R} in @code{tar}.
2004 Used in @code{chgrp}, @code{chown}, @code{cp}, @code{ls}, @code{diff},
2008 @samp{-r} in @code{touch}.
2011 @samp{-r} in @code{ptx}.
2014 @samp{-r} in @code{tac} and @code{etags}.
2017 @samp{-r} in @code{uname}.
2020 @samp{-R} in @code{m4}.
2023 @samp{-r} in @code{objdump}.
2026 @samp{-r} in @code{cpio}.
2029 @samp{-i} in @code{xargs}.
2031 @item report-identical-files
2032 @samp{-s} in @code{diff}.
2034 @item reset-access-time
2035 @samp{-a} in @code{cpio}.
2038 @samp{-r} in @code{ls} and @code{nm}.
2041 @samp{-f} in @code{diff}.
2043 @item right-side-defs
2044 @samp{-R} in @code{ptx}.
2047 @samp{-s} in @code{tar}.
2049 @item same-permissions
2050 @samp{-p} in @code{tar}.
2053 @samp{-g} in @code{stty}.
2058 @item sentence-regexp
2059 @samp{-S} in @code{ptx}.
2062 @samp{-S} in @code{du}.
2065 @samp{-s} in @code{tac}.
2068 Used by @code{recode} to chose files or pipes for sequencing passes.
2071 @samp{-s} in @code{su}.
2074 @samp{-A} in @code{cat}.
2076 @item show-c-function
2077 @samp{-p} in @code{diff}.
2080 @samp{-E} in @code{cat}.
2082 @item show-function-line
2083 @samp{-F} in @code{diff}.
2086 @samp{-T} in @code{cat}.
2089 Used in many programs to inhibit the usual output.
2090 Every program accepting
2091 @samp{--silent} should accept @samp{--quiet} as a synonym.
2094 @samp{-s} in @code{ls}.
2097 Specify a file descriptor for a network server to use for its socket,
2098 instead of opening and binding a new socket. This provides a way to
2099 run, in a non-privileged process, a server that normally needs a
2100 reserved port number.
2106 @samp{-W source} in @code{gawk}.
2109 @samp{-S} in @code{tar}.
2111 @item speed-large-files
2112 @samp{-H} in @code{diff}.
2115 @samp{-E} in @code{unshar}.
2117 @item split-size-limit
2118 @samp{-L} in @code{shar}.
2121 @samp{-s} in @code{cat}.
2124 @samp{-w} in @code{wdiff}.
2127 @samp{-y} in @code{wdiff}.
2130 Used in @code{tar} and @code{diff} to specify which file within
2131 a directory to start processing with.
2134 @samp{-s} in @code{wdiff}.
2136 @item stdin-file-list
2137 @samp{-S} in @code{shar}.
2140 @samp{-S} in @code{make}.
2143 @samp{-s} in @code{recode}.
2146 @samp{-s} in @code{install}.
2149 @samp{-s} in @code{strip}.
2152 @samp{-S} in @code{strip}.
2155 @samp{-s} in @code{shar}.
2158 @samp{-S} in @code{cp}, @code{ln}, @code{mv}.
2161 @samp{-b} in @code{csplit}.
2164 @samp{-s} in @code{gprof}.
2167 @samp{-s} in @code{du}.
2170 @samp{-s} in @code{ln}.
2173 Used in GDB and @code{objdump}.
2176 @samp{-s} in @code{m4}.
2179 @samp{-s} in @code{uname}.
2182 @samp{-t} in @code{expand} and @code{unexpand}.
2185 @samp{-T} in @code{ls}.
2188 @samp{-T} in @code{tput} and @code{ul}.
2189 @samp{-t} in @code{wdiff}.
2192 @samp{-a} in @code{diff}.
2195 @samp{-T} in @code{shar}.
2198 Used in @code{ls} and @code{touch}.
2201 Specify how long to wait before giving up on some operation.
2204 @samp{-O} in @code{tar}.
2207 @samp{-c} in @code{du}.
2210 @samp{-t} in @code{make}, @code{ranlib}, and @code{recode}.
2213 @samp{-t} in @code{m4}.
2216 @samp{-t} in @code{hello};
2217 @samp{-W traditional} in @code{gawk};
2218 @samp{-G} in @code{ed}, @code{m4}, and @code{ptx}.
2224 @samp{-t} in @code{ctags}.
2226 @item typedefs-and-c++
2227 @samp{-T} in @code{ctags}.
2230 @samp{-t} in @code{ptx}.
2233 @samp{-z} in @code{tar}.
2236 @samp{-u} in @code{cpio}.
2239 @samp{-U} in @code{m4}.
2241 @item undefined-only
2242 @samp{-u} in @code{nm}.
2245 @samp{-u} in @code{cp}, @code{ctags}, @code{mv}, @code{tar}.
2248 Used in @code{gawk}; same as @samp{--help}.
2251 @samp{-B} in @code{shar}.
2253 @item vanilla-operation
2254 @samp{-V} in @code{shar}.
2257 Print more information about progress. Many programs support this.
2260 @samp{-W} in @code{tar}.
2263 Print the version number.
2265 @item version-control
2266 @samp{-V} in @code{cp}, @code{ln}, @code{mv}.
2269 @samp{-v} in @code{ctags}.
2272 @samp{-V} in @code{tar}.
2275 @samp{-W} in @code{make}.
2277 @item whole-size-limit
2278 @samp{-l} in @code{shar}.
2281 @samp{-w} in @code{ls} and @code{ptx}.
2284 @samp{-W} in @code{ptx}.
2287 @samp{-T} in @code{who}.
2290 @samp{-z} in @code{gprof}.
2293 @node OID Allocations
2294 @section OID Allocations
2295 @cindex OID allocations for GNU
2300 The OID (object identifier) 1.3.6.1.4.1.11591 has been assigned to the
2301 GNU Project (thanks to Werner Koch). These are used for SNMP, LDAP,
2302 X.509 certificates, and so on. The web site
2303 @url{http://www.alvestrand.no/objectid} has a (voluntary) listing of
2304 many OID assignments.
2306 If you need a new slot for your GNU package, write
2307 @email{maintainers@@gnu.org}. Here is a list of arcs currently
2311 @include gnu-oids.texi
2316 @section Memory Usage
2317 @cindex memory usage
2319 If a program typically uses just a few meg of memory, don't bother making any
2320 effort to reduce memory usage. For example, if it is impractical for
2321 other reasons to operate on files more than a few meg long, it is
2322 reasonable to read entire input files into memory to operate on them.
2324 However, for programs such as @code{cat} or @code{tail}, that can
2325 usefully operate on very large files, it is important to avoid using a
2326 technique that would artificially limit the size of files it can handle.
2327 If a program works by lines and could be applied to arbitrary
2328 user-supplied input files, it should keep only a line in memory, because
2329 this is not very hard and users will want to be able to operate on input
2330 files that are bigger than will fit in memory all at once.
2332 If your program creates complicated data structures, just make them in
2333 memory and give a fatal error if @code{malloc} returns zero.
2337 Memory analysis tools such as @command{valgrind} can be useful, but
2338 don't complicate a program merely to avoid their false alarms. For
2339 example, if memory is used until just before a process exits, don't
2340 free it simply to silence such a tool.
2347 Programs should be prepared to operate when @file{/usr} and @file{/etc}
2348 are read-only file systems. Thus, if the program manages log files,
2349 lock files, backup files, score files, or any other files which are
2350 modified for internal purposes, these files should not be stored in
2351 @file{/usr} or @file{/etc}.
2353 There are two exceptions. @file{/etc} is used to store system
2354 configuration information; it is reasonable for a program to modify
2355 files in @file{/etc} when its job is to update the system configuration.
2356 Also, if the user explicitly asks to modify one file in a directory, it
2357 is reasonable for the program to store other files in the same
2361 @chapter Making The Best Use of C
2363 This chapter provides advice on how best to use the C language
2364 when writing GNU software.
2367 * Formatting:: Formatting your source code.
2368 * Comments:: Commenting your work.
2369 * Syntactic Conventions:: Clean use of C constructs.
2370 * Names:: Naming variables, functions, and files.
2371 * System Portability:: Portability among different operating systems.
2372 * CPU Portability:: Supporting the range of CPU types.
2373 * System Functions:: Portability and ``standard'' library functions.
2374 * Internationalization:: Techniques for internationalization.
2375 * Character Set:: Use ASCII by default.
2376 * Quote Characters:: Use `...' in the C locale.
2377 * Mmap:: How you can safely use @code{mmap}.
2381 @section Formatting Your Source Code
2382 @cindex formatting source code
2385 @cindex braces, in C source
2386 It is important to put the open-brace that starts the body of a C
2387 function in column one, so that they will start a defun. Several
2388 tools look for open-braces in column one to find the beginnings of C
2389 functions. These tools will not work on code not formatted that way.
2391 Avoid putting open-brace, open-parenthesis or open-bracket in column
2392 one when they are inside a function, so that they won't start a defun.
2393 The open-brace that starts a @code{struct} body can go in column one
2394 if you find it useful to treat that definition as a defun.
2396 It is also important for function definitions to start the name of the
2397 function in column one. This helps people to search for function
2398 definitions, and may also help certain tools recognize them. Thus,
2399 using Standard C syntax, the format is this:
2403 concat (char *s1, char *s2)
2410 or, if you want to use traditional C syntax, format the definition like
2415 concat (s1, s2) /* Name starts in column one here */
2417 @{ /* Open brace in column one here */
2422 In Standard C, if the arguments don't fit nicely on one line,
2427 lots_of_args (int an_integer, long a_long, short a_short,
2428 double a_double, float a_float)
2432 The rest of this section gives our recommendations for other aspects of
2433 C formatting style, which is also the default style of the @code{indent}
2434 program in version 1.2 and newer. It corresponds to the options
2437 -nbad -bap -nbc -bbo -bl -bli2 -bls -ncdb -nce -cp1 -cs -di2
2438 -ndj -nfc1 -nfca -hnl -i2 -ip5 -lp -pcs -psl -nsc -nsob
2441 We don't think of these recommendations as requirements, because it
2442 causes no problems for users if two different programs have different
2445 But whatever style you use, please use it consistently, since a mixture
2446 of styles within one program tends to look ugly. If you are
2447 contributing changes to an existing program, please follow the style of
2450 For the body of the function, our recommended style looks like this:
2462 return ++x + bar ();
2466 @cindex spaces before open-paren
2467 We find it easier to read a program when it has spaces before the
2468 open-parentheses and after the commas. Especially after the commas.
2470 When you split an expression into multiple lines, split it
2471 before an operator, not after one. Here is the right way:
2473 @cindex expressions, splitting
2475 if (foo_this_is_long && bar > win (x, y, z)
2476 && remaining_condition)
2479 Try to avoid having two operators of different precedence at the same
2480 level of indentation. For example, don't write this:
2483 mode = (inmode[j] == VOIDmode
2484 || GET_MODE_SIZE (outmode[j]) > GET_MODE_SIZE (inmode[j])
2485 ? outmode[j] : inmode[j]);
2488 Instead, use extra parentheses so that the indentation shows the nesting:
2491 mode = ((inmode[j] == VOIDmode
2492 || (GET_MODE_SIZE (outmode[j]) > GET_MODE_SIZE (inmode[j])))
2493 ? outmode[j] : inmode[j]);
2496 Insert extra parentheses so that Emacs will indent the code properly.
2497 For example, the following indentation looks nice if you do it by hand,
2500 v = rup->ru_utime.tv_sec*1000 + rup->ru_utime.tv_usec/1000
2501 + rup->ru_stime.tv_sec*1000 + rup->ru_stime.tv_usec/1000;
2505 but Emacs would alter it. Adding a set of parentheses produces
2506 something that looks equally nice, and which Emacs will preserve:
2509 v = (rup->ru_utime.tv_sec*1000 + rup->ru_utime.tv_usec/1000
2510 + rup->ru_stime.tv_sec*1000 + rup->ru_stime.tv_usec/1000);
2513 Format do-while statements like this:
2525 Please use formfeed characters (control-L) to divide the program into
2526 pages at logical places (but not within a function). It does not matter
2527 just how long the pages are, since they do not have to fit on a printed
2528 page. The formfeeds should appear alone on lines by themselves.
2531 @section Commenting Your Work
2534 Every program should start with a comment saying briefly what it is for.
2535 Example: @samp{fmt - filter for simple filling of text}. This comment
2536 should be at the top of the source file containing the @samp{main}
2537 function of the program.
2539 Also, please write a brief comment at the start of each source file,
2540 with the file name and a line or two about the overall purpose of the
2543 Please write the comments in a GNU program in English, because English
2544 is the one language that nearly all programmers in all countries can
2545 read. If you do not write English well, please write comments in
2546 English as well as you can, then ask other people to help rewrite them.
2547 If you can't write comments in English, please find someone to work with
2548 you and translate your comments into English.
2550 Please put a comment on each function saying what the function does,
2551 what sorts of arguments it gets, and what the possible values of
2552 arguments mean and are used for. It is not necessary to duplicate in
2553 words the meaning of the C argument declarations, if a C type is being
2554 used in its customary fashion. If there is anything nonstandard about
2555 its use (such as an argument of type @code{char *} which is really the
2556 address of the second character of a string, not the first), or any
2557 possible values that would not work the way one would expect (such as,
2558 that strings containing newlines are not guaranteed to work), be sure
2561 Also explain the significance of the return value, if there is one.
2563 Please put two spaces after the end of a sentence in your comments, so
2564 that the Emacs sentence commands will work. Also, please write
2565 complete sentences and capitalize the first word. If a lower-case
2566 identifier comes at the beginning of a sentence, don't capitalize it!
2567 Changing the spelling makes it a different identifier. If you don't
2568 like starting a sentence with a lower case letter, write the sentence
2569 differently (e.g., ``The identifier lower-case is @dots{}'').
2571 The comment on a function is much clearer if you use the argument
2572 names to speak about the argument values. The variable name itself
2573 should be lower case, but write it in upper case when you are speaking
2574 about the value rather than the variable itself. Thus, ``the inode
2575 number NODE_NUM'' rather than ``an inode''.
2577 There is usually no purpose in restating the name of the function in
2578 the comment before it, because the reader can see that for himself.
2579 There might be an exception when the comment is so long that the function
2580 itself would be off the bottom of the screen.
2582 There should be a comment on each static variable as well, like this:
2585 /* Nonzero means truncate lines in the display;
2586 zero means continue them. */
2590 @cindex conditionals, comments for
2591 @cindex @code{#endif}, commenting
2592 Every @samp{#endif} should have a comment, except in the case of short
2593 conditionals (just a few lines) that are not nested. The comment should
2594 state the condition of the conditional that is ending, @emph{including
2595 its sense}. @samp{#else} should have a comment describing the condition
2596 @emph{and sense} of the code that follows. For example:
2604 #endif /* not foo */
2614 but, by contrast, write the comments this way for a @samp{#ifndef}:
2627 #endif /* not foo */
2631 @node Syntactic Conventions
2632 @section Clean Use of C Constructs
2633 @cindex syntactic conventions
2635 @cindex implicit @code{int}
2636 @cindex function argument, declaring
2637 Please explicitly declare the types of all objects. For example, you
2638 should explicitly declare all arguments to functions, and you should
2639 declare functions to return @code{int} rather than omitting the
2642 @cindex compiler warnings
2643 @cindex @samp{-Wall} compiler option
2644 Some programmers like to use the GCC @samp{-Wall} option, and change the
2645 code whenever it issues a warning. If you want to do this, then do.
2646 Other programmers prefer not to use @samp{-Wall}, because it gives
2647 warnings for valid and legitimate code which they do not want to change.
2648 If you want to do this, then do. The compiler should be your servant,
2653 Don't make the program ugly just to placate static analysis tools such
2654 as @command{lint}, @command{clang}, and GCC with extra warnings
2655 options such as @option{-Wconversion} and @option{-Wundef}. These
2656 tools can help find bugs and unclear code, but they can also generate
2657 so many false alarms that that it hurts readability to silence them
2658 with unnecessary casts, wrappers, and other complications. For
2659 example, please don't insert casts to @code{void} or calls to
2660 do-nothing functions merely to pacify a lint checker.
2662 Declarations of external functions and functions to appear later in the
2663 source file should all go in one place near the beginning of the file
2664 (somewhere before the first function definition in the file), or else
2665 should go in a header file. Don't put @code{extern} declarations inside
2668 @cindex temporary variables
2669 It used to be common practice to use the same local variables (with
2670 names like @code{tem}) over and over for different values within one
2671 function. Instead of doing this, it is better to declare a separate local
2672 variable for each distinct purpose, and give it a name which is
2673 meaningful. This not only makes programs easier to understand, it also
2674 facilitates optimization by good compilers. You can also move the
2675 declaration of each local variable into the smallest scope that includes
2676 all its uses. This makes the program even cleaner.
2678 Don't use local variables or parameters that shadow global identifiers.
2679 GCC's @samp{-Wshadow} option can detect this problem.
2681 @cindex multiple variables in a line
2682 Don't declare multiple variables in one declaration that spans lines.
2683 Start a new declaration on each line, instead. For example, instead
2709 (If they are global variables, each should have a comment preceding it
2712 When you have an @code{if}-@code{else} statement nested in another
2713 @code{if} statement, always put braces around the @code{if}-@code{else}.
2714 Thus, never write like this:
2737 If you have an @code{if} statement nested inside of an @code{else}
2738 statement, either write @code{else if} on one line, like this,
2748 with its @code{then}-part indented like the preceding @code{then}-part,
2749 or write the nested @code{if} within braces like this:
2761 Don't declare both a structure tag and variables or typedefs in the
2762 same declaration. Instead, declare the structure tag separately
2763 and then use it to declare the variables or typedefs.
2765 Try to avoid assignments inside @code{if}-conditions (assignments
2766 inside @code{while}-conditions are ok). For example, don't write
2770 if ((foo = (char *) malloc (sizeof *foo)) == 0)
2771 fatal ("virtual memory exhausted");
2775 instead, write this:
2778 foo = (char *) malloc (sizeof *foo);
2780 fatal ("virtual memory exhausted");
2783 This example uses zero without a cast as a null pointer constant.
2784 This is perfectly fine, except that a cast is needed when calling a
2785 varargs function or when using @code{sizeof}.
2788 @section Naming Variables, Functions, and Files
2790 @cindex names of variables, functions, and files
2791 The names of global variables and functions in a program serve as
2792 comments of a sort. So don't choose terse names---instead, look for
2793 names that give useful information about the meaning of the variable or
2794 function. In a GNU program, names should be English, like other
2797 Local variable names can be shorter, because they are used only within
2798 one context, where (presumably) comments explain their purpose.
2800 Try to limit your use of abbreviations in symbol names. It is ok to
2801 make a few abbreviations, explain what they mean, and then use them
2802 frequently, but don't use lots of obscure abbreviations.
2804 Please use underscores to separate words in a name, so that the Emacs
2805 word commands can be useful within them. Stick to lower case; reserve
2806 upper case for macros and @code{enum} constants, and for name-prefixes
2807 that follow a uniform convention.
2809 For example, you should use names like @code{ignore_space_change_flag};
2810 don't use names like @code{iCantReadThis}.
2812 Variables that indicate whether command-line options have been
2813 specified should be named after the meaning of the option, not after
2814 the option-letter. A comment should state both the exact meaning of
2815 the option and its letter. For example,
2819 /* Ignore changes in horizontal whitespace (-b). */
2820 int ignore_space_change_flag;
2824 When you want to define names with constant integer values, use
2825 @code{enum} rather than @samp{#define}. GDB knows about enumeration
2828 @cindex file-name limitations
2830 You might want to make sure that none of the file names would conflict
2831 if the files were loaded onto an MS-DOS file system which shortens the
2832 names. You can use the program @code{doschk} to test for this.
2834 Some GNU programs were designed to limit themselves to file names of 14
2835 characters or less, to avoid file name conflicts if they are read into
2836 older System V systems. Please preserve this feature in the existing
2837 GNU programs that have it, but there is no need to do this in new GNU
2838 programs. @code{doschk} also reports file names longer than 14
2842 @node System Portability
2843 @section Portability between System Types
2844 @cindex portability, between system types
2846 In the Unix world, ``portability'' refers to porting to different Unix
2847 versions. For a GNU program, this kind of portability is desirable, but
2850 The primary purpose of GNU software is to run on top of the GNU kernel,
2851 compiled with the GNU C compiler, on various types of @sc{cpu}. So the
2852 kinds of portability that are absolutely necessary are quite limited.
2853 But it is important to support Linux-based GNU systems, since they
2854 are the form of GNU that is popular.
2856 Beyond that, it is good to support the other free operating systems
2857 (*BSD), and it is nice to support other Unix-like systems if you want
2858 to. Supporting a variety of Unix-like systems is desirable, although
2859 not paramount. It is usually not too hard, so you may as well do it.
2860 But you don't have to consider it an obligation, if it does turn out to
2864 The easiest way to achieve portability to most Unix-like systems is to
2865 use Autoconf. It's unlikely that your program needs to know more
2866 information about the host platform than Autoconf can provide, simply
2867 because most of the programs that need such knowledge have already been
2870 Avoid using the format of semi-internal data bases (e.g., directories)
2871 when there is a higher-level alternative (@code{readdir}).
2873 @cindex non-@sc{posix} systems, and portability
2874 As for systems that are not like Unix, such as MSDOS, Windows, VMS, MVS,
2875 and older Macintosh systems, supporting them is often a lot of work.
2876 When that is the case, it is better to spend your time adding features
2877 that will be useful on GNU and GNU/Linux, rather than on supporting
2878 other incompatible systems.
2880 If you do support Windows, please do not abbreviate it as ``win''. In
2881 hacker terminology, calling something a ``win'' is a form of praise.
2882 You're free to praise Microsoft Windows on your own if you want, but
2883 please don't do this in GNU packages. Instead of abbreviating
2884 ``Windows'' to ``win'', you can write it in full or abbreviate it to
2885 ``woe'' or ``w''. In GNU Emacs, for instance, we use @samp{w32} in
2886 file names of Windows-specific files, but the macro for Windows
2887 conditionals is called @code{WINDOWSNT}.
2889 It is a good idea to define the ``feature test macro''
2890 @code{_GNU_SOURCE} when compiling your C files. When you compile on GNU
2891 or GNU/Linux, this will enable the declarations of GNU library extension
2892 functions, and that will usually give you a compiler error message if
2893 you define the same function names in some other way in your program.
2894 (You don't have to actually @emph{use} these functions, if you prefer
2895 to make the program more portable to other systems.)
2897 But whether or not you use these GNU extensions, you should avoid
2898 using their names for any other meanings. Doing so would make it hard
2899 to move your code into other GNU programs.
2901 @node CPU Portability
2902 @section Portability between @sc{cpu}s
2904 @cindex data types, and portability
2905 @cindex portability, and data types
2906 Even GNU systems will differ because of differences among @sc{cpu}
2907 types---for example, difference in byte ordering and alignment
2908 requirements. It is absolutely essential to handle these differences.
2909 However, don't make any effort to cater to the possibility that an
2910 @code{int} will be less than 32 bits. We don't support 16-bit machines
2913 Similarly, don't make any effort to cater to the possibility that
2914 @code{long} will be smaller than predefined types like @code{size_t}.
2915 For example, the following code is ok:
2918 printf ("size = %lu\n", (unsigned long) sizeof array);
2919 printf ("diff = %ld\n", (long) (pointer2 - pointer1));
2922 1989 Standard C requires this to work, and we know of only one
2923 counterexample: 64-bit programs on Microsoft Windows. We will leave
2924 it to those who want to port GNU programs to that environment to
2925 figure out how to do it.
2927 Predefined file-size types like @code{off_t} are an exception: they are
2928 longer than @code{long} on many platforms, so code like the above won't
2929 work with them. One way to print an @code{off_t} value portably is to
2930 print its digits yourself, one by one.
2932 Don't assume that the address of an @code{int} object is also the
2933 address of its least-significant byte. This is false on big-endian
2934 machines. Thus, don't make the following mistake:
2939 while ((c = getchar ()) != EOF)
2940 write (file_descriptor, &c, 1);
2943 @noindent Instead, use @code{unsigned char} as follows. (The @code{unsigned}
2944 is for portability to unusual systems where @code{char} is signed and
2945 where there is integer overflow checking.)
2949 while ((c = getchar ()) != EOF)
2951 unsigned char u = c;
2952 write (file_descriptor, &u, 1);
2956 @cindex casting pointers to integers
2957 Avoid casting pointers to integers if you can. Such casts greatly
2958 reduce portability, and in most programs they are easy to avoid. In the
2959 cases where casting pointers to integers is essential---such as, a Lisp
2960 interpreter which stores type information as well as an address in one
2961 word---you'll have to make explicit provisions to handle different word
2962 sizes. You will also need to make provision for systems in which the
2963 normal range of addresses you can get from @code{malloc} starts far away
2967 @node System Functions
2968 @section Calling System Functions
2970 @cindex C library functions, and portability
2971 @cindex POSIX functions, and portability
2972 @cindex library functions, and portability
2973 @cindex portability, and library functions
2975 Historically, C implementations differed substantially, and many
2976 systems lacked a full implementation of ANSI/ISO C89. Nowadays,
2977 however, very few systems lack a C89 compiler and GNU C supports
2978 almost all of C99. Similarly, most systems implement POSIX.1-1993
2979 libraries and tools, and many have POSIX.1-2001.
2981 Hence, there is little reason to support old C or non-POSIX systems,
2982 and you may want to take advantage of C99 and POSIX-1.2001 to write
2983 clearer, more portable, or faster code. You should use standard
2984 interfaces where possible; but if GNU extensions make your program
2985 more maintainable, powerful, or otherwise better, don't hesitate to
2986 use them. In any case, don't make your own declaration of system
2987 functions; that's a recipe for conflict.
2989 Despite the standards, nearly every library function has some sort of
2990 portability issue on some system or another. Here are some examples:
2994 Names with trailing @code{/}'s are mishandled on many platforms.
2997 @code{long double} may be unimplemented; floating values Infinity and
2998 NaN are often mishandled; output for large precisions may be
3002 May return @code{int} instead of @code{ssize_t}.
3005 On Windows, @code{errno} is not set on failure.
3009 @uref{http://www.gnu.org/software/gnulib/, Gnulib} is a big help in
3010 this regard. Gnulib provides implementations of standard interfaces
3011 on many of the systems that lack them, including portable
3012 implementations of enhanced GNU interfaces, thereby making their use
3013 portable, and of POSIX-1.2008 interfaces, some of which are missing
3014 even on up-to-date GNU systems.
3016 @findex xmalloc, in Gnulib
3017 @findex error messages, in Gnulib
3018 @findex data structures, in Gnulib
3019 Gnulib also provides many useful non-standard interfaces; for example,
3020 C implementations of standard data structures (hash tables, binary
3021 trees), error-checking type-safe wrappers for memory allocation
3022 functions (@code{xmalloc}, @code{xrealloc}), and output of error
3025 Gnulib integrates with GNU Autoconf and Automake to remove much of the
3026 burden of writing portable code from the programmer: Gnulib makes your
3027 configure script automatically determine what features are missing and
3028 use the Gnulib code to supply the missing pieces.
3030 The Gnulib and Autoconf manuals have extensive sections on
3031 portability: @ref{Top,, Introduction, gnulib, Gnulib} and
3032 @pxref{Portable C and C++,,, autoconf, Autoconf}. Please consult them
3033 for many more details.
3036 @node Internationalization
3037 @section Internationalization
3038 @cindex internationalization
3041 GNU has a library called GNU gettext that makes it easy to translate the
3042 messages in a program into various languages. You should use this
3043 library in every program. Use English for the messages as they appear
3044 in the program, and let gettext provide the way to translate them into
3047 Using GNU gettext involves putting a call to the @code{gettext} macro
3048 around each string that might need translation---like this:
3051 printf (gettext ("Processing file `%s'..."));
3055 This permits GNU gettext to replace the string @code{"Processing file
3056 `%s'..."} with a translated version.
3058 Once a program uses gettext, please make a point of writing calls to
3059 @code{gettext} when you add new strings that call for translation.
3061 Using GNU gettext in a package involves specifying a @dfn{text domain
3062 name} for the package. The text domain name is used to separate the
3063 translations for this package from the translations for other packages.
3064 Normally, the text domain name should be the same as the name of the
3065 package---for example, @samp{coreutils} for the GNU core utilities.
3067 @cindex message text, and internationalization
3068 To enable gettext to work well, avoid writing code that makes
3069 assumptions about the structure of words or sentences. When you want
3070 the precise text of a sentence to vary depending on the data, use two or
3071 more alternative string constants each containing a complete sentences,
3072 rather than inserting conditionalized words or phrases into a single
3075 Here is an example of what not to do:
3078 printf ("%s is full", capacity > 5000000 ? "disk" : "floppy disk");
3081 If you apply gettext to all strings, like this,
3084 printf (gettext ("%s is full"),
3085 capacity > 5000000 ? gettext ("disk") : gettext ("floppy disk"));
3089 the translator will hardly know that "disk" and "floppy disk" are meant to
3090 be substituted in the other string. Worse, in some languages (like French)
3091 the construction will not work: the translation of the word "full" depends
3092 on the gender of the first part of the sentence; it happens to be not the
3093 same for "disk" as for "floppy disk".
3095 Complete sentences can be translated without problems:
3098 printf (capacity > 5000000 ? gettext ("disk is full")
3099 : gettext ("floppy disk is full"));
3102 A similar problem appears at the level of sentence structure with this
3106 printf ("# Implicit rule search has%s been done.\n",
3107 f->tried_implicit ? "" : " not");
3111 Adding @code{gettext} calls to this code cannot give correct results for
3112 all languages, because negation in some languages requires adding words
3113 at more than one place in the sentence. By contrast, adding
3114 @code{gettext} calls does the job straightforwardly if the code starts
3118 printf (f->tried_implicit
3119 ? "# Implicit rule search has been done.\n",
3120 : "# Implicit rule search has not been done.\n");
3123 Another example is this one:
3126 printf ("%d file%s processed", nfiles,
3127 nfiles != 1 ? "s" : "");
3131 The problem with this example is that it assumes that plurals are made
3132 by adding `s'. If you apply gettext to the format string, like this,
3135 printf (gettext ("%d file%s processed"), nfiles,
3136 nfiles != 1 ? "s" : "");
3140 the message can use different words, but it will still be forced to use
3141 `s' for the plural. Here is a better way, with gettext being applied to
3142 the two strings independently:
3145 printf ((nfiles != 1 ? gettext ("%d files processed")
3146 : gettext ("%d file processed")),
3151 But this still doesn't work for languages like Polish, which has three
3152 plural forms: one for nfiles == 1, one for nfiles == 2, 3, 4, 22, 23, 24, ...
3153 and one for the rest. The GNU @code{ngettext} function solves this problem:
3156 printf (ngettext ("%d files processed", "%d file processed", nfiles),
3162 @section Character Set
3163 @cindex character set
3165 @cindex ASCII characters
3166 @cindex non-ASCII characters
3168 Sticking to the ASCII character set (plain text, 7-bit characters) is
3169 preferred in GNU source code comments, text documents, and other
3170 contexts, unless there is good reason to do something else because of
3171 the application domain. For example, if source code deals with the
3172 French Revolutionary calendar, it is OK if its literal strings contain
3173 accented characters in month names like ``Flor@'eal''. Also, it is OK
3174 to use non-ASCII characters to represent proper names of contributors in
3175 change logs (@pxref{Change Logs}).
3177 If you need to use non-ASCII characters, you should normally stick with
3178 one encoding, as one cannot in general mix encodings reliably.
3181 @node Quote Characters
3182 @section Quote Characters
3183 @cindex quote characters
3184 @cindex locale-specific quote characters
3186 @cindex grave accent
3188 In the C locale, GNU programs should stick to plain ASCII for quotation
3189 characters in messages to users: preferably 0x60 (@samp{`}) for left
3190 quotes and 0x27 (@samp{'}) for right quotes. It is ok, but not
3191 required, to use locale-specific quotes in other locales.
3193 The @uref{http://www.gnu.org/software/gnulib/, Gnulib} @code{quote} and
3194 @code{quotearg} modules provide a reasonably straightforward way to
3195 support locale-specific quote characters, as well as taking care of
3196 other issues, such as quoting a filename that itself contains a quote
3197 character. See the Gnulib documentation for usage details.
3199 In any case, the documentation for your program should clearly specify
3200 how it does quoting, if different than the preferred method of @samp{`}
3201 and @samp{'}. This is especially important if the output of your
3202 program is ever likely to be parsed by another program.
3204 Quotation characters are a difficult area in the computing world at
3205 this time: there are no true left or right quote characters in Latin1;
3206 the @samp{`} character we use was standardized there as a grave
3207 accent. Moreover, Latin1 is still not universally usable.
3209 Unicode contains the unambiguous quote characters required, and its
3210 common encoding UTF-8 is upward compatible with Latin1. However,
3211 Unicode and UTF-8 are not universally well-supported, either.
3213 This may change over the next few years, and then we will revisit
3221 Don't assume that @code{mmap} either works on all files or fails
3222 for all files. It may work on some files and fail on others.
3224 The proper way to use @code{mmap} is to try it on the specific file for
3225 which you want to use it---and if @code{mmap} doesn't work, fall back on
3226 doing the job in another way using @code{read} and @code{write}.
3228 The reason this precaution is needed is that the GNU kernel (the HURD)
3229 provides a user-extensible file system, in which there can be many
3230 different kinds of ``ordinary files.'' Many of them support
3231 @code{mmap}, but some do not. It is important to make programs handle
3232 all these kinds of files.
3235 @chapter Documenting Programs
3236 @cindex documentation
3238 A GNU program should ideally come with full free documentation, adequate
3239 for both reference and tutorial purposes. If the package can be
3240 programmed or extended, the documentation should cover programming or
3241 extending it, as well as just using it.
3244 * GNU Manuals:: Writing proper manuals.
3245 * Doc Strings and Manuals:: Compiling doc strings doesn't make a manual.
3246 * Manual Structure Details:: Specific structure conventions.
3247 * License for Manuals:: Writing the distribution terms for a manual.
3248 * Manual Credits:: Giving credit to documentation contributors.
3249 * Printed Manuals:: Mentioning the printed manual.
3250 * NEWS File:: NEWS files supplement manuals.
3251 * Change Logs:: Recording changes.
3252 * Man Pages:: Man pages are secondary.
3253 * Reading other Manuals:: How far you can go in learning
3258 @section GNU Manuals
3260 The preferred document format for the GNU system is the Texinfo
3261 formatting language. Every GNU package should (ideally) have
3262 documentation in Texinfo both for reference and for learners. Texinfo
3263 makes it possible to produce a good quality formatted book, using
3264 @TeX{}, and to generate an Info file. It is also possible to generate
3265 HTML output from Texinfo source. See the Texinfo manual, either the
3266 hardcopy, or the on-line version available through @code{info} or the
3267 Emacs Info subsystem (@kbd{C-h i}).
3269 Nowadays some other formats such as Docbook and Sgmltexi can be
3270 converted automatically into Texinfo. It is ok to produce the Texinfo
3271 documentation by conversion this way, as long as it gives good results.
3273 Make sure your manual is clear to a reader who knows nothing about the
3274 topic and reads it straight through. This means covering basic topics
3275 at the beginning, and advanced topics only later. This also means
3276 defining every specialized term when it is first used.
3278 Programmers tend to carry over the structure of the program as the
3279 structure for its documentation. But this structure is not
3280 necessarily good for explaining how to use the program; it may be
3281 irrelevant and confusing for a user.
3283 Instead, the right way to structure documentation is according to the
3284 concepts and questions that a user will have in mind when reading it.
3285 This principle applies at every level, from the lowest (ordering
3286 sentences in a paragraph) to the highest (ordering of chapter topics
3287 within the manual). Sometimes this structure of ideas matches the
3288 structure of the implementation of the software being documented---but
3289 often they are different. An important part of learning to write good
3290 documentation is to learn to notice when you have unthinkingly
3291 structured the documentation like the implementation, stop yourself,
3292 and look for better alternatives.
3294 For example, each program in the GNU system probably ought to be
3295 documented in one manual; but this does not mean each program should
3296 have its own manual. That would be following the structure of the
3297 implementation, rather than the structure that helps the user
3300 Instead, each manual should cover a coherent @emph{topic}. For example,
3301 instead of a manual for @code{diff} and a manual for @code{diff3}, we
3302 have one manual for ``comparison of files'' which covers both of those
3303 programs, as well as @code{cmp}. By documenting these programs
3304 together, we can make the whole subject clearer.
3306 The manual which discusses a program should certainly document all of
3307 the program's command-line options and all of its commands. It should
3308 give examples of their use. But don't organize the manual as a list
3309 of features. Instead, organize it logically, by subtopics. Address
3310 the questions that a user will ask when thinking about the job that
3311 the program does. Don't just tell the reader what each feature can
3312 do---say what jobs it is good for, and show how to use it for those
3313 jobs. Explain what is recommended usage, and what kinds of usage
3316 In general, a GNU manual should serve both as tutorial and reference.
3317 It should be set up for convenient access to each topic through Info,
3318 and for reading straight through (appendixes aside). A GNU manual
3319 should give a good introduction to a beginner reading through from the
3320 start, and should also provide all the details that hackers want.
3321 The Bison manual is a good example of this---please take a look at it
3322 to see what we mean.
3324 That is not as hard as it first sounds. Arrange each chapter as a
3325 logical breakdown of its topic, but order the sections, and write their
3326 text, so that reading the chapter straight through makes sense. Do
3327 likewise when structuring the book into chapters, and when structuring a
3328 section into paragraphs. The watchword is, @emph{at each point, address
3329 the most fundamental and important issue raised by the preceding text.}
3331 If necessary, add extra chapters at the beginning of the manual which
3332 are purely tutorial and cover the basics of the subject. These provide
3333 the framework for a beginner to understand the rest of the manual. The
3334 Bison manual provides a good example of how to do this.
3336 To serve as a reference, a manual should have an Index that list all the
3337 functions, variables, options, and important concepts that are part of
3338 the program. One combined Index should do for a short manual, but
3339 sometimes for a complex package it is better to use multiple indices.
3340 The Texinfo manual includes advice on preparing good index entries, see
3341 @ref{Index Entries, , Making Index Entries, texinfo, GNU Texinfo}, and
3342 see @ref{Indexing Commands, , Defining the Entries of an
3343 Index, texinfo, GNU Texinfo}.
3345 Don't use Unix man pages as a model for how to write GNU documentation;
3346 most of them are terse, badly structured, and give inadequate
3347 explanation of the underlying concepts. (There are, of course, some
3348 exceptions.) Also, Unix man pages use a particular format which is
3349 different from what we use in GNU manuals.
3351 Please include an email address in the manual for where to report
3352 bugs @emph{in the text of the manual}.
3354 Please do not use the term ``pathname'' that is used in Unix
3355 documentation; use ``file name'' (two words) instead. We use the term
3356 ``path'' only for search paths, which are lists of directory names.
3358 Please do not use the term ``illegal'' to refer to erroneous input to
3359 a computer program. Please use ``invalid'' for this, and reserve the
3360 term ``illegal'' for activities prohibited by law.
3362 Please do not write @samp{()} after a function name just to indicate
3363 it is a function. @code{foo ()} is not a function, it is a function
3364 call with no arguments.
3366 @node Doc Strings and Manuals
3367 @section Doc Strings and Manuals
3369 Some programming systems, such as Emacs, provide a documentation string
3370 for each function, command or variable. You may be tempted to write a
3371 reference manual by compiling the documentation strings and writing a
3372 little additional text to go around them---but you must not do it. That
3373 approach is a fundamental mistake. The text of well-written
3374 documentation strings will be entirely wrong for a manual.
3376 A documentation string needs to stand alone---when it appears on the
3377 screen, there will be no other text to introduce or explain it.
3378 Meanwhile, it can be rather informal in style.
3380 The text describing a function or variable in a manual must not stand
3381 alone; it appears in the context of a section or subsection. Other text
3382 at the beginning of the section should explain some of the concepts, and
3383 should often make some general points that apply to several functions or
3384 variables. The previous descriptions of functions and variables in the
3385 section will also have given information about the topic. A description
3386 written to stand alone would repeat some of that information; this
3387 redundancy looks bad. Meanwhile, the informality that is acceptable in
3388 a documentation string is totally unacceptable in a manual.
3390 The only good way to use documentation strings in writing a good manual
3391 is to use them as a source of information for writing good text.
3393 @node Manual Structure Details
3394 @section Manual Structure Details
3395 @cindex manual structure
3397 The title page of the manual should state the version of the programs or
3398 packages documented in the manual. The Top node of the manual should
3399 also contain this information. If the manual is changing more
3400 frequently than or independent of the program, also state a version
3401 number for the manual in both of these places.
3403 Each program documented in the manual should have a node named
3404 @samp{@var{program} Invocation} or @samp{Invoking @var{program}}. This
3405 node (together with its subnodes, if any) should describe the program's
3406 command line arguments and how to run it (the sort of information people
3407 would look for in a man page). Start with an @samp{@@example}
3408 containing a template for all the options and arguments that the program
3411 Alternatively, put a menu item in some menu whose item name fits one of
3412 the above patterns. This identifies the node which that item points to
3413 as the node for this purpose, regardless of the node's actual name.
3415 The @samp{--usage} feature of the Info reader looks for such a node
3416 or menu item in order to find the relevant text, so it is essential
3417 for every Texinfo file to have one.
3419 If one manual describes several programs, it should have such a node for
3420 each program described in the manual.
3422 @node License for Manuals
3423 @section License for Manuals
3424 @cindex license for manuals
3426 Please use the GNU Free Documentation License for all GNU manuals that
3427 are more than a few pages long. Likewise for a collection of short
3428 documents---you only need one copy of the GNU FDL for the whole
3429 collection. For a single short document, you can use a very permissive
3430 non-copyleft license, to avoid taking up space with a long license.
3432 See @uref{http://www.gnu.org/copyleft/fdl-howto.html} for more explanation
3433 of how to employ the GFDL.
3435 Note that it is not obligatory to include a copy of the GNU GPL or GNU
3436 LGPL in a manual whose license is neither the GPL nor the LGPL. It can
3437 be a good idea to include the program's license in a large manual; in a
3438 short manual, whose size would be increased considerably by including
3439 the program's license, it is probably better not to include it.
3441 @node Manual Credits
3442 @section Manual Credits
3443 @cindex credits for manuals
3445 Please credit the principal human writers of the manual as the authors,
3446 on the title page of the manual. If a company sponsored the work, thank
3447 the company in a suitable place in the manual, but do not cite the
3448 company as an author.
3450 @node Printed Manuals
3451 @section Printed Manuals
3453 The FSF publishes some GNU manuals in printed form. To encourage sales
3454 of these manuals, the on-line versions of the manual should mention at
3455 the very start that the printed manual is available and should point at
3456 information for getting it---for instance, with a link to the page
3457 @url{http://www.gnu.org/order/order.html}. This should not be included
3458 in the printed manual, though, because there it is redundant.
3460 It is also useful to explain in the on-line forms of the manual how the
3461 user can print out the manual from the sources.
3464 @section The NEWS File
3465 @cindex @file{NEWS} file
3467 In addition to its manual, the package should have a file named
3468 @file{NEWS} which contains a list of user-visible changes worth
3469 mentioning. In each new release, add items to the front of the file and
3470 identify the version they pertain to. Don't discard old items; leave
3471 them in the file after the newer items. This way, a user upgrading from
3472 any previous version can see what is new.
3474 If the @file{NEWS} file gets very long, move some of the older items
3475 into a file named @file{ONEWS} and put a note at the end referring the
3479 @section Change Logs
3482 Keep a change log to describe all the changes made to program source
3483 files. The purpose of this is so that people investigating bugs in the
3484 future will know about the changes that might have introduced the bug.
3485 Often a new bug can be found by looking at what was recently changed.
3486 More importantly, change logs can help you eliminate conceptual
3487 inconsistencies between different parts of a program, by giving you a
3488 history of how the conflicting concepts arose and who they came from.
3491 * Change Log Concepts::
3492 * Style of Change Logs::
3494 * Conditional Changes::
3495 * Indicating the Part Changed::
3498 @node Change Log Concepts
3499 @subsection Change Log Concepts
3501 You can think of the change log as a conceptual ``undo list'' which
3502 explains how earlier versions were different from the current version.
3503 People can see the current version; they don't need the change log
3504 to tell them what is in it. What they want from a change log is a
3505 clear explanation of how the earlier version differed.
3507 The change log file is normally called @file{ChangeLog} and covers an
3508 entire directory. Each directory can have its own change log, or a
3509 directory can use the change log of its parent directory---it's up to
3512 Another alternative is to record change log information with a version
3513 control system such as RCS or CVS. This can be converted automatically
3514 to a @file{ChangeLog} file using @code{rcs2log}; in Emacs, the command
3515 @kbd{C-x v a} (@code{vc-update-change-log}) does the job.
3517 There's no need to describe the full purpose of the changes or how
3518 they work together. However, sometimes it is useful to write one line
3519 to describe the overall purpose of a change or a batch of changes. If
3520 you think that a change calls for explanation, you're probably right.
3521 Please do explain it---but please put the full explanation in comments
3522 in the code, where people will see it whenever they see the code. For
3523 example, ``New function'' is enough for the change log when you add a
3524 function, because there should be a comment before the function
3525 definition to explain what it does.
3527 In the past, we recommended not mentioning changes in non-software
3528 files (manuals, help files, etc.) in change logs. However, we've been
3529 advised that it is a good idea to include them, for the sake of
3532 The easiest way to add an entry to @file{ChangeLog} is with the Emacs
3533 command @kbd{M-x add-change-log-entry}. An entry should have an
3534 asterisk, the name of the changed file, and then in parentheses the name
3535 of the changed functions, variables or whatever, followed by a colon.
3536 Then describe the changes you made to that function or variable.
3538 @node Style of Change Logs
3539 @subsection Style of Change Logs
3540 @cindex change logs, style
3542 Here are some simple examples of change log entries, starting with the
3543 header line that says who made the change and when it was installed,
3544 followed by descriptions of specific changes. (These examples are
3545 drawn from Emacs and GCC.)
3548 1998-08-17 Richard Stallman <rms@@gnu.org>
3550 * register.el (insert-register): Return nil.
3551 (jump-to-register): Likewise.
3553 * sort.el (sort-subr): Return nil.
3555 * tex-mode.el (tex-bibtex-file, tex-file, tex-region):
3556 Restart the tex shell if process is gone or stopped.
3557 (tex-shell-running): New function.
3559 * expr.c (store_one_arg): Round size up for move_block_to_reg.
3560 (expand_call): Round up when emitting USE insns.
3561 * stmt.c (assign_parms): Round size up for move_block_from_reg.
3564 It's important to name the changed function or variable in full. Don't
3565 abbreviate function or variable names, and don't combine them.
3566 Subsequent maintainers will often search for a function name to find all
3567 the change log entries that pertain to it; if you abbreviate the name,
3568 they won't find it when they search.
3570 For example, some people are tempted to abbreviate groups of function
3571 names by writing @samp{* register.el (@{insert,jump-to@}-register)};
3572 this is not a good idea, since searching for @code{jump-to-register} or
3573 @code{insert-register} would not find that entry.
3575 Separate unrelated change log entries with blank lines. When two
3576 entries represent parts of the same change, so that they work together,
3577 then don't put blank lines between them. Then you can omit the file
3578 name and the asterisk when successive entries are in the same file.
3580 Break long lists of function names by closing continued lines with
3581 @samp{)}, rather than @samp{,}, and opening the continuation with
3582 @samp{(} as in this example:
3585 * keyboard.c (menu_bar_items, tool_bar_items)
3586 (Fexecute_extended_command): Deal with `keymap' property.
3589 When you install someone else's changes, put the contributor's name in
3590 the change log entry rather than in the text of the entry. In other
3594 2002-07-14 John Doe <jdoe@@gnu.org>
3596 * sewing.c: Make it sew.
3603 2002-07-14 Usual Maintainer <usual@@gnu.org>
3605 * sewing.c: Make it sew. Patch by jdoe@@gnu.org.
3608 As for the date, that should be the date you applied the change.
3610 @node Simple Changes
3611 @subsection Simple Changes
3613 Certain simple kinds of changes don't need much detail in the change
3616 When you change the calling sequence of a function in a simple fashion,
3617 and you change all the callers of the function to use the new calling
3618 sequence, there is no need to make individual entries for all the
3619 callers that you changed. Just write in the entry for the function
3620 being called, ``All callers changed''---like this:
3623 * keyboard.c (Fcommand_execute): New arg SPECIAL.
3624 All callers changed.
3627 When you change just comments or doc strings, it is enough to write an
3628 entry for the file, without mentioning the functions. Just ``Doc
3629 fixes'' is enough for the change log.
3631 There's no technical need to make change log entries for documentation
3632 files. This is because documentation is not susceptible to bugs that
3633 are hard to fix. Documentation does not consist of parts that must
3634 interact in a precisely engineered fashion. To correct an error, you
3635 need not know the history of the erroneous passage; it is enough to
3636 compare what the documentation says with the way the program actually
3639 However, you should keep change logs for documentation files when the
3640 project gets copyright assignments from its contributors, so as to
3641 make the records of authorship more accurate.
3643 @node Conditional Changes
3644 @subsection Conditional Changes
3645 @cindex conditional changes, and change logs
3646 @cindex change logs, conditional changes
3648 C programs often contain compile-time @code{#if} conditionals. Many
3649 changes are conditional; sometimes you add a new definition which is
3650 entirely contained in a conditional. It is very useful to indicate in
3651 the change log the conditions for which the change applies.
3653 Our convention for indicating conditional changes is to use square
3654 brackets around the name of the condition.
3656 Here is a simple example, describing a change which is conditional but
3657 does not have a function or entity name associated with it:
3660 * xterm.c [SOLARIS2]: Include string.h.
3663 Here is an entry describing a new definition which is entirely
3664 conditional. This new definition for the macro @code{FRAME_WINDOW_P} is
3665 used only when @code{HAVE_X_WINDOWS} is defined:
3668 * frame.h [HAVE_X_WINDOWS] (FRAME_WINDOW_P): Macro defined.
3671 Here is an entry for a change within the function @code{init_display},
3672 whose definition as a whole is unconditional, but the changes themselves
3673 are contained in a @samp{#ifdef HAVE_LIBNCURSES} conditional:
3676 * dispnew.c (init_display) [HAVE_LIBNCURSES]: If X, call tgetent.
3679 Here is an entry for a change that takes affect only when
3680 a certain macro is @emph{not} defined:
3683 (gethostname) [!HAVE_SOCKETS]: Replace with winsock version.
3686 @node Indicating the Part Changed
3687 @subsection Indicating the Part Changed
3689 Indicate the part of a function which changed by using angle brackets
3690 enclosing an indication of what the changed part does. Here is an entry
3691 for a change in the part of the function @code{sh-while-getopts} that
3692 deals with @code{sh} commands:
3695 * progmodes/sh-script.el (sh-while-getopts) <sh>: Handle case that
3696 user-specified option string is empty.
3704 In the GNU project, man pages are secondary. It is not necessary or
3705 expected for every GNU program to have a man page, but some of them do.
3706 It's your choice whether to include a man page in your program.
3708 When you make this decision, consider that supporting a man page
3709 requires continual effort each time the program is changed. The time
3710 you spend on the man page is time taken away from more useful work.
3712 For a simple program which changes little, updating the man page may be
3713 a small job. Then there is little reason not to include a man page, if
3716 For a large program that changes a great deal, updating a man page may
3717 be a substantial burden. If a user offers to donate a man page, you may
3718 find this gift costly to accept. It may be better to refuse the man
3719 page unless the same person agrees to take full responsibility for
3720 maintaining it---so that you can wash your hands of it entirely. If
3721 this volunteer later ceases to do the job, then don't feel obliged to
3722 pick it up yourself; it may be better to withdraw the man page from the
3723 distribution until someone else agrees to update it.
3725 When a program changes only a little, you may feel that the
3726 discrepancies are small enough that the man page remains useful without
3727 updating. If so, put a prominent note near the beginning of the man
3728 page explaining that you don't maintain it and that the Texinfo manual
3729 is more authoritative. The note should say how to access the Texinfo
3732 Be sure that man pages include a copyright statement and free license.
3733 The simple all-permissive license is appropriate for simple man pages
3734 (@pxref{License Notices for Other Files,,,maintain,Information for GNU
3737 For long man pages, with enough explanation and documentation that
3738 they can be considered true manuals, use the GFDL (@pxref{License for
3741 Finally, the GNU help2man program
3742 (@uref{http://www.gnu.org/software/help2man/}) is one way to automate
3743 generation of a man page, in this case from @option{--help} output.
3744 This is sufficient in many cases.
3746 @node Reading other Manuals
3747 @section Reading other Manuals
3749 There may be non-free books or documentation files that describe the
3750 program you are documenting.
3752 It is ok to use these documents for reference, just as the author of a
3753 new algebra textbook can read other books on algebra. A large portion
3754 of any non-fiction book consists of facts, in this case facts about how
3755 a certain program works, and these facts are necessarily the same for
3756 everyone who writes about the subject. But be careful not to copy your
3757 outline structure, wording, tables or examples from preexisting non-free
3758 documentation. Copying from free documentation may be ok; please check
3759 with the FSF about the individual case.
3761 @node Managing Releases
3762 @chapter The Release Process
3765 Making a release is more than just bundling up your source files in a
3766 tar file and putting it up for FTP. You should set up your software so
3767 that it can be configured to run on a variety of systems. Your Makefile
3768 should conform to the GNU standards described below, and your directory
3769 layout should also conform to the standards discussed below. Doing so
3770 makes it easy to include your package into the larger framework of
3774 * Configuration:: How configuration of GNU packages should work.
3775 * Makefile Conventions:: Makefile conventions.
3776 * Releases:: Making releases
3780 @section How Configuration Should Work
3781 @cindex program configuration
3784 Each GNU distribution should come with a shell script named
3785 @code{configure}. This script is given arguments which describe the
3786 kind of machine and system you want to compile the program for.
3787 The @code{configure} script must record the configuration options so
3788 that they affect compilation.
3790 The description here is the specification of the interface for the
3791 @code{configure} script in GNU packages. Many packages implement it
3792 using GNU Autoconf (@pxref{Top,, Introduction, autoconf, Autoconf})
3793 and/or GNU Automake (@pxref{Top,, Introduction, automake, Automake}),
3794 but you do not have to use these tools. You can implement it any way
3795 you like; for instance, by making @code{configure} be a wrapper around
3796 a completely different configuration system.
3798 Another way for the @code{configure} script to operate is to make a
3799 link from a standard name such as @file{config.h} to the proper
3800 configuration file for the chosen system. If you use this technique,
3801 the distribution should @emph{not} contain a file named
3802 @file{config.h}. This is so that people won't be able to build the
3803 program without configuring it first.
3805 Another thing that @code{configure} can do is to edit the Makefile. If
3806 you do this, the distribution should @emph{not} contain a file named
3807 @file{Makefile}. Instead, it should include a file @file{Makefile.in} which
3808 contains the input used for editing. Once again, this is so that people
3809 won't be able to build the program without configuring it first.
3811 If @code{configure} does write the @file{Makefile}, then @file{Makefile}
3812 should have a target named @file{Makefile} which causes @code{configure}
3813 to be rerun, setting up the same configuration that was set up last
3814 time. The files that @code{configure} reads should be listed as
3815 dependencies of @file{Makefile}.
3817 All the files which are output from the @code{configure} script should
3818 have comments at the beginning explaining that they were generated
3819 automatically using @code{configure}. This is so that users won't think
3820 of trying to edit them by hand.
3822 The @code{configure} script should write a file named @file{config.status}
3823 which describes which configuration options were specified when the
3824 program was last configured. This file should be a shell script which,
3825 if run, will recreate the same configuration.
3827 The @code{configure} script should accept an option of the form
3828 @samp{--srcdir=@var{dirname}} to specify the directory where sources are found
3829 (if it is not the current directory). This makes it possible to build
3830 the program in a separate directory, so that the actual source directory
3833 If the user does not specify @samp{--srcdir}, then @code{configure} should
3834 check both @file{.} and @file{..} to see if it can find the sources. If
3835 it finds the sources in one of these places, it should use them from
3836 there. Otherwise, it should report that it cannot find the sources, and
3837 should exit with nonzero status.
3839 Usually the easy way to support @samp{--srcdir} is by editing a
3840 definition of @code{VPATH} into the Makefile. Some rules may need to
3841 refer explicitly to the specified source directory. To make this
3842 possible, @code{configure} can add to the Makefile a variable named
3843 @code{srcdir} whose value is precisely the specified directory.
3845 In addition, the @samp{configure} script should take options
3846 corresponding to most of the standard directory variables
3847 (@pxref{Directory Variables}). Here is the list:
3850 --prefix --exec-prefix --bindir --sbindir --libexecdir --sysconfdir
3851 --sharedstatedir --localstatedir --libdir --includedir --oldincludedir
3852 --datarootdir --datadir --infodir --localedir --mandir --docdir
3853 --htmldir --dvidir --pdfdir --psdir
3856 The @code{configure} script should also take an argument which specifies the
3857 type of system to build the program for. This argument should look like
3861 @var{cpu}-@var{company}-@var{system}
3864 For example, an Athlon-based GNU/Linux system might be
3865 @samp{i686-pc-linux-gnu}.
3867 The @code{configure} script needs to be able to decode all plausible
3868 alternatives for how to describe a machine. Thus,
3869 @samp{athlon-pc-gnu/linux} would be a valid alias. There is a shell
3871 @uref{http://git.savannah.gnu.org/@/gitweb/@/?p=config.git;a=blob_plain;f=config.sub;hb=HEAD,
3872 @file{config.sub}} that you can use as a subroutine to validate system
3873 types and canonicalize aliases.
3875 The @code{configure} script should also take the option
3876 @option{--build=@var{buildtype}}, which should be equivalent to a
3877 plain @var{buildtype} argument. For example, @samp{configure
3878 --build=i686-pc-linux-gnu} is equivalent to @samp{configure
3879 i686-pc-linux-gnu}. When the build type is not specified by an option
3880 or argument, the @code{configure} script should normally guess it using
3882 @uref{http://git.savannah.gnu.org/@/gitweb/@/?p=config.git;a=blob_plain;f=config.guess;hb=HEAD,
3883 @file{config.guess}}.
3885 @cindex optional features, configure-time
3886 Other options are permitted to specify in more detail the software
3887 or hardware present on the machine, to include or exclude optional parts
3888 of the package, or to adjust the name of some tools or arguments to them:
3891 @item --enable-@var{feature}@r{[}=@var{parameter}@r{]}
3892 Configure the package to build and install an optional user-level
3893 facility called @var{feature}. This allows users to choose which
3894 optional features to include. Giving an optional @var{parameter} of
3895 @samp{no} should omit @var{feature}, if it is built by default.
3897 No @samp{--enable} option should @strong{ever} cause one feature to
3898 replace another. No @samp{--enable} option should ever substitute one
3899 useful behavior for another useful behavior. The only proper use for
3900 @samp{--enable} is for questions of whether to build part of the program
3903 @item --with-@var{package}
3904 @c @r{[}=@var{parameter}@r{]}
3905 The package @var{package} will be installed, so configure this package
3906 to work with @var{package}.
3908 @c Giving an optional @var{parameter} of
3909 @c @samp{no} should omit @var{package}, if it is used by default.
3911 Possible values of @var{package} include
3912 @samp{gnu-as} (or @samp{gas}), @samp{gnu-ld}, @samp{gnu-libc},
3918 Do not use a @samp{--with} option to specify the file name to use to
3919 find certain files. That is outside the scope of what @samp{--with}
3922 @item @var{variable}=@var{value}
3923 Set the value of the variable @var{variable} to @var{value}. This is
3924 used to override the default values of commands or arguments in the
3925 build process. For example, the user could issue @samp{configure
3926 CFLAGS=-g CXXFLAGS=-g} to build with debugging information and without
3927 the default optimization.
3929 Specifying variables as arguments to @code{configure}, like this:
3933 is preferable to setting them in environment variables:
3937 as it helps to recreate the same configuration later with
3938 @file{config.status}. However, both methods should be supported.
3941 All @code{configure} scripts should accept all of the ``detail''
3942 options and the variable settings, whether or not they make any
3943 difference to the particular package at hand. In particular, they
3944 should accept any option that starts with @samp{--with-} or
3945 @samp{--enable-}. This is so users will be able to configure an
3946 entire GNU source tree at once with a single set of options.
3948 You will note that the categories @samp{--with-} and @samp{--enable-}
3949 are narrow: they @strong{do not} provide a place for any sort of option
3950 you might think of. That is deliberate. We want to limit the possible
3951 configuration options in GNU software. We do not want GNU programs to
3952 have idiosyncratic configuration options.
3954 Packages that perform part of the compilation process may support
3955 cross-compilation. In such a case, the host and target machines for the
3956 program may be different.
3958 The @code{configure} script should normally treat the specified type of
3959 system as both the host and the target, thus producing a program which
3960 works for the same type of machine that it runs on.
3962 To compile a program to run on a host type that differs from the build
3963 type, use the configure option @option{--host=@var{hosttype}}, where
3964 @var{hosttype} uses the same syntax as @var{buildtype}. The host type
3965 normally defaults to the build type.
3967 To configure a cross-compiler, cross-assembler, or what have you, you
3968 should specify a target different from the host, using the configure
3969 option @samp{--target=@var{targettype}}. The syntax for
3970 @var{targettype} is the same as for the host type. So the command would
3974 ./configure --host=@var{hosttype} --target=@var{targettype}
3977 The target type normally defaults to the host type.
3978 Programs for which cross-operation is not meaningful need not accept the
3979 @samp{--target} option, because configuring an entire operating system for
3980 cross-operation is not a meaningful operation.
3982 Some programs have ways of configuring themselves automatically. If
3983 your program is set up to do this, your @code{configure} script can simply
3984 ignore most of its arguments.
3986 @comment The makefile standards are in a separate file that is also
3987 @comment included by make.texinfo. Done by roland@gnu.ai.mit.edu on 1/6/93.
3988 @comment For this document, turn chapters into sections, etc.
3990 @include make-stds.texi
3994 @section Making Releases
3997 You should identify each release with a pair of version numbers, a
3998 major version and a minor. We have no objection to using more than
3999 two numbers, but it is very unlikely that you really need them.
4001 Package the distribution of @code{Foo version 69.96} up in a gzipped tar
4002 file with the name @file{foo-69.96.tar.gz}. It should unpack into a
4003 subdirectory named @file{foo-69.96}.
4005 Building and installing the program should never modify any of the files
4006 contained in the distribution. This means that all the files that form
4007 part of the program in any way must be classified into @dfn{source
4008 files} and @dfn{non-source files}. Source files are written by humans
4009 and never changed automatically; non-source files are produced from
4010 source files by programs under the control of the Makefile.
4012 @cindex @file{README} file
4013 The distribution should contain a file named @file{README} which gives
4014 the name of the package, and a general description of what it does. It
4015 is also good to explain the purpose of each of the first-level
4016 subdirectories in the package, if there are any. The @file{README} file
4017 should either state the version number of the package, or refer to where
4018 in the package it can be found.
4020 The @file{README} file should refer to the file @file{INSTALL}, which
4021 should contain an explanation of the installation procedure.
4023 The @file{README} file should also refer to the file which contains the
4024 copying conditions. The GNU GPL, if used, should be in a file called
4025 @file{COPYING}. If the GNU LGPL is used, it should be in a file called
4026 @file{COPYING.LESSER}.
4028 Naturally, all the source files must be in the distribution. It is okay
4029 to include non-source files in the distribution, provided they are
4030 up-to-date and machine-independent, so that building the distribution
4031 normally will never modify them. We commonly include non-source files
4032 produced by Bison, @code{lex}, @TeX{}, and @code{makeinfo}; this helps avoid
4033 unnecessary dependencies between our distributions, so that users can
4034 install whichever packages they want to install.
4036 Non-source files that might actually be modified by building and
4037 installing the program should @strong{never} be included in the
4038 distribution. So if you do distribute non-source files, always make
4039 sure they are up to date when you make a new distribution.
4041 Make sure that all the files in the distribution are world-readable, and
4042 that directories are world-readable and world-searchable (octal mode 755).
4043 We used to recommend that all directories in the distribution also be
4044 world-writable (octal mode 777), because ancient versions of @code{tar}
4045 would otherwise not cope when extracting the archive as an unprivileged
4046 user. That can easily lead to security issues when creating the archive,
4047 however, so now we recommend against that.
4049 Don't include any symbolic links in the distribution itself. If the tar
4050 file contains symbolic links, then people cannot even unpack it on
4051 systems that don't support symbolic links. Also, don't use multiple
4052 names for one file in different directories, because certain file
4053 systems cannot handle this and that prevents unpacking the
4056 Try to make sure that all the file names will be unique on MS-DOS. A
4057 name on MS-DOS consists of up to 8 characters, optionally followed by a
4058 period and up to three characters. MS-DOS will truncate extra
4059 characters both before and after the period. Thus,
4060 @file{foobarhacker.c} and @file{foobarhacker.o} are not ambiguous; they
4061 are truncated to @file{foobarha.c} and @file{foobarha.o}, which are
4064 @cindex @file{texinfo.tex}, in a distribution
4065 Include in your distribution a copy of the @file{texinfo.tex} you used
4066 to test print any @file{*.texinfo} or @file{*.texi} files.
4068 Likewise, if your program uses small GNU software packages like regex,
4069 getopt, obstack, or termcap, include them in the distribution file.
4070 Leaving them out would make the distribution file a little smaller at
4071 the expense of possible inconvenience to a user who doesn't know what
4075 @chapter References to Non-Free Software and Documentation
4076 @cindex references to non-free material
4078 A GNU program should not recommend, promote, or grant legitimacy to
4079 the use of any non-free program. Proprietary software is a social and
4080 ethical problem, and our aim is to put an end to that problem. We
4081 can't stop some people from writing proprietary programs, or stop
4082 other people from using them, but we can and should refuse to
4083 advertise them to new potential customers, or to give the public the
4084 idea that their existence is ethical.
4086 The GNU definition of free software is found on the GNU web site at
4087 @url{http://www.gnu.org/@/philosophy/@/free-sw.html}, and the definition
4088 of free documentation is found at
4089 @url{http://www.gnu.org/@/philosophy/@/free-doc.html}. The terms ``free''
4090 and ``non-free'', used in this document, refer to those definitions.
4092 A list of important licenses and whether they qualify as free is in
4093 @url{http://www.gnu.org/@/licenses/@/license-list.html}. If it is not
4094 clear whether a license qualifies as free, please ask the GNU Project
4095 by writing to @email{licensing@@gnu.org}. We will answer, and if the
4096 license is an important one, we will add it to the list.
4098 When a non-free program or system is well known, you can mention it in
4099 passing---that is harmless, since users who might want to use it
4100 probably already know about it. For instance, it is fine to explain
4101 how to build your package on top of some widely used non-free
4102 operating system, or how to use it together with some widely used
4105 However, you should give only the necessary information to help those
4106 who already use the non-free program to use your program with
4107 it---don't give, or refer to, any further information about the
4108 proprietary program, and don't imply that the proprietary program
4109 enhances your program, or that its existence is in any way a good
4110 thing. The goal should be that people already using the proprietary
4111 program will get the advice they need about how to use your free
4112 program with it, while people who don't already use the proprietary
4113 program will not see anything likely to lead them to take an interest
4116 If a non-free program or system is obscure in your program's domain,
4117 your program should not mention or support it at all, since doing so
4118 would tend to popularize the non-free program more than it popularizes
4119 your program. (You cannot hope to find many additional users for your
4120 program among the users of Foobar, if the existence of Foobar is not
4121 generally known among people who might want to use your program.)
4123 Sometimes a program is free software in itself but depends on a
4124 non-free platform in order to run. For instance, many Java programs
4125 depend on some non-free Java libraries. To recommend or promote such
4126 a program is to promote the other programs it needs. This is why we
4127 are careful about listing Java programs in the Free Software
4128 Directory: we don't want to promote the non-free Java libraries.
4130 We hope this particular problem with Java will be gone by and by, as
4131 we replace the remaining non-free standard Java libraries with free
4132 software, but the general principle will remain the same: don't
4133 recommend, promote or legitimize programs that depend on non-free
4136 Some free programs strongly encourage the use of non-free software. A
4137 typical example is @command{mplayer}. It is free software in itself,
4138 and the free code can handle some kinds of files. However,
4139 @command{mplayer} recommends use of non-free codecs for other kinds of
4140 files, and users that install @command{mplayer} are very likely to
4141 install those codecs along with it. To recommend @command{mplayer}
4142 is, in effect, to promote use of the non-free codecs.
4144 Thus, you should not recommend programs that strongly encourage the
4145 use of non-free software. This is why we do not list
4146 @command{mplayer} in the Free Software Directory.
4148 A GNU package should not refer the user to any non-free documentation
4149 for free software. Free documentation that can be included in free
4150 operating systems is essential for completing the GNU system, or any
4151 free operating system, so encouraging it is a priority; to recommend
4152 use of documentation that we are not allowed to include undermines the
4153 impetus for the community to produce documentation that we can
4154 include. So GNU packages should never recommend non-free
4157 By contrast, it is ok to refer to journal articles and textbooks in
4158 the comments of a program for explanation of how it functions, even
4159 though they are non-free. This is because we don't include such
4160 things in the GNU system even if they are free---they are outside the
4161 scope of what a software distribution needs to include.
4163 Referring to a web site that describes or recommends a non-free
4164 program is promoting that program, so please do not make links (or
4165 mention by name) web sites that contain such material. This policy is
4166 relevant particularly for the web pages for a GNU package.
4168 Following links from nearly any web site can lead eventually to
4169 non-free software; this is inherent in the nature of the web. So it
4170 makes no sense to criticize a site for having such links. As long as
4171 the site does not itself recommend a non-free program, there is no
4172 need to consider the question of the sites that it links to for other
4175 Thus, for example, you should not refer to AT&T's web site if that
4176 recommends AT&T's non-free software packages; you should not refer to
4177 a site that links to AT&T's site presenting it as a place to get some
4178 non-free program, because that link recommends and legitimizes the
4179 non-free program. However, that a site contains a link to AT&T's web
4180 site for some other purpose (such as long-distance telephone service)
4181 is not an objection against it.
4183 @node GNU Free Documentation License
4184 @appendix GNU Free Documentation License
4186 @cindex FDL, GNU Free Documentation License
4196 eval: (add-hook 'write-file-hooks 'time-stamp)
4197 time-stamp-start: "@set lastupdate "
4199 time-stamp-format: "%:b %:d, %:y"
4200 compile-command: "cd work.s && make"