1 \input texinfo @c -*-texinfo-*-
3 @setfilename standards.info
4 @settitle GNU Coding Standards
5 @c This date is automagically updated when you save this file:
6 @set lastupdate July 19, 2013
9 @dircategory GNU organization
11 * Standards: (standards). GNU coding standards.
14 @c @setchapternewpage odd
15 @setchapternewpage off
17 @c Put everything in one index (arbitrarily chosen to be the concept index).
23 @c This is used by a cross ref in make-stds.texi
27 The GNU coding standards, last updated @value{lastupdate}.
29 Copyright @copyright{} 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999,
30 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010,
31 2011, 2012, 2013 Free Software Foundation, Inc.
33 Permission is granted to copy, distribute and/or modify this document
34 under the terms of the GNU Free Documentation License, Version 1.3 or
35 any later version published by the Free Software Foundation; with no
36 Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
37 Texts. A copy of the license is included in the section entitled
38 ``GNU Free Documentation License''.
42 @title GNU Coding Standards
43 @author Richard Stallman, et al.
44 @author last updated @value{lastupdate}
46 @vskip 0pt plus 1filll
54 @top GNU Coding Standards
60 * Preface:: About the GNU Coding Standards.
61 * Legal Issues:: Keeping free software free.
62 * Design Advice:: General program design.
63 * Program Behavior:: Program behavior for all programs
64 * Writing C:: Making the best use of C.
65 * Documentation:: Documenting programs.
66 * Managing Releases:: The release process.
67 * References:: Mentioning non-free software or documentation.
68 * GNU Free Documentation License:: Copying and sharing this manual.
74 @chapter About the GNU Coding Standards
76 The GNU Coding Standards were written by Richard Stallman and other GNU
77 Project volunteers. Their purpose is to make the GNU system clean,
78 consistent, and easy to install. This document can also be read as a
79 guide to writing portable, robust and reliable programs. It focuses on
80 programs written in C, but many of the rules and principles are useful
81 even if you write in another programming language. The rules often
82 state reasons for writing in a certain way.
84 @cindex where to obtain @code{standards.texi}
85 @cindex downloading this manual
86 If you did not obtain this file directly from the GNU project and
87 recently, please check for a newer version. You can get the GNU
88 Coding Standards from the GNU web server in many
89 different formats, including the Texinfo source, PDF, HTML, DVI, plain
90 text, and more, at: @uref{http://www.gnu.org/prep/standards/}.
92 If you are maintaining an official GNU package, in addition to this
93 document, please read and follow the GNU maintainer information
94 (@pxref{Top, , Contents, maintain, Information for Maintainers of GNU
97 @cindex @code{gnustandards-commit@@gnu.org} mailing list
98 If you want to receive diffs for every change to these GNU documents,
99 join the mailing list @code{gnustandards-commit@@gnu.org}, via the web
101 @url{http://lists.gnu.org/mailman/listinfo/gnustandards-commit}.
102 Archives are also available there.
104 @cindex @code{bug-standards@@gnu.org} email address
105 @cindex Savannah repository for gnustandards
106 @cindex gnustandards project repository
107 Please send corrections or suggestions for this document to
108 @email{bug-standards@@gnu.org}. If you make a suggestion, please
109 include a suggested new wording for it, to help us consider the
110 suggestion efficiently. We prefer a context diff to the Texinfo
111 source, but if that's difficult for you, you can make a context diff
112 for some other version of this document, or propose it in any way that
113 makes it clear. The source repository for this document can be found
114 at @url{http://savannah.gnu.org/projects/gnustandards}.
116 These standards cover the minimum of what is important when writing a
117 GNU package. Likely, the need for additional standards will come up.
118 Sometimes, you might suggest that such standards be added to this
119 document. If you think your standards would be generally useful, please
122 You should also set standards for your package on many questions not
123 addressed or not firmly specified here. The most important point is to
124 be self-consistent---try to stick to the conventions you pick, and try
125 to document them as much as possible. That way, your program will be
126 more maintainable by others.
128 The GNU Hello program serves as an example of how to follow the GNU
129 coding standards for a trivial program.
130 @uref{http://www.gnu.org/software/hello/hello.html}.
132 This release of the GNU Coding Standards was last updated
137 @chapter Keeping Free Software Free
138 @cindex legal aspects
140 This chapter discusses how you can make sure that GNU software
141 avoids legal difficulties, and other related issues.
144 * Reading Non-Free Code:: Referring to proprietary programs.
145 * Contributions:: Accepting contributions.
146 * Trademarks:: How we deal with trademark issues.
149 @node Reading Non-Free Code
150 @section Referring to Proprietary Programs
151 @cindex proprietary programs
152 @cindex avoiding proprietary code
154 Don't in any circumstances refer to Unix source code for or during
155 your work on GNU! (Or to any other proprietary programs.)
157 If you have a vague recollection of the internals of a Unix program,
158 this does not absolutely mean you can't write an imitation of it, but
159 do try to organize the imitation internally along different lines,
160 because this is likely to make the details of the Unix version
161 irrelevant and dissimilar to your results.
163 For example, Unix utilities were generally optimized to minimize
164 memory use; if you go for speed instead, your program will be very
165 different. You could keep the entire input file in memory and scan it
166 there instead of using stdio. Use a smarter algorithm discovered more
167 recently than the Unix program. Eliminate use of temporary files. Do
168 it in one pass instead of two (we did this in the assembler).
170 Or, on the contrary, emphasize simplicity instead of speed. For some
171 applications, the speed of today's computers makes simpler algorithms
174 Or go for generality. For example, Unix programs often have static
175 tables or fixed-size strings, which make for arbitrary limits; use
176 dynamic allocation instead. Make sure your program handles NULs and
177 other funny characters in the input files. Add a programming language
178 for extensibility and write part of the program in that language.
180 Or turn some parts of the program into independently usable libraries.
181 Or use a simple garbage collector instead of tracking precisely when
182 to free memory, or use a new GNU facility such as obstacks.
186 @section Accepting Contributions
188 @cindex accepting contributions
190 If the program you are working on is copyrighted by the Free Software
191 Foundation, then when someone else sends you a piece of code to add to
192 the program, we need legal papers to use it---just as we asked you to
193 sign papers initially. @emph{Each} person who makes a nontrivial
194 contribution to a program must sign some sort of legal papers in order
195 for us to have clear title to the program; the main author alone is not
198 So, before adding in any contributions from other people, please tell
199 us, so we can arrange to get the papers. Then wait until we tell you
200 that we have received the signed papers, before you actually use the
203 This applies both before you release the program and afterward. If
204 you receive diffs to fix a bug, and they make significant changes, we
205 need legal papers for that change.
207 This also applies to comments and documentation files. For copyright
208 law, comments and code are just text. Copyright applies to all kinds of
209 text, so we need legal papers for all kinds.
211 We know it is frustrating to ask for legal papers; it's frustrating for
212 us as well. But if you don't wait, you are going out on a limb---for
213 example, what if the contributor's employer won't sign a disclaimer?
214 You might have to take that code out again!
216 You don't need papers for changes of a few lines here or there, since
217 they are not significant for copyright purposes. Also, you don't need
218 papers if all you get from the suggestion is some ideas, not actual code
219 which you use. For example, if someone sent you one implementation, but
220 you write a different implementation of the same idea, you don't need to
223 The very worst thing is if you forget to tell us about the other
224 contributor. We could be very embarrassed in court some day as a
227 We have more detailed advice for maintainers of GNU packages. If you
228 have reached the stage of maintaining a GNU program (whether released
229 or not), please take a look: @pxref{Legal Matters,,, maintain,
230 Information for GNU Maintainers}.
237 Please do not include any trademark acknowledgements in GNU software
238 packages or documentation.
240 Trademark acknowledgements are the statements that such-and-such is a
241 trademark of so-and-so. The GNU Project has no objection to the basic
242 idea of trademarks, but these acknowledgements feel like kowtowing,
243 and there is no legal requirement for them, so we don't use them.
245 What is legally required, as regards other people's trademarks, is to
246 avoid using them in ways which a reader might reasonably understand as
247 naming or labeling our own programs or activities. For example, since
248 ``Objective C'' is (or at least was) a trademark, we made sure to say
249 that we provide a ``compiler for the Objective C language'' rather
250 than an ``Objective C compiler''. The latter would have been meant as
251 a shorter way of saying the former, but it does not explicitly state
252 the relationship, so it could be misinterpreted as using ``Objective
253 C'' as a label for the compiler rather than for the language.
255 Please don't use ``win'' as an abbreviation for Microsoft Windows in
256 GNU software or documentation. In hacker terminology, calling
257 something a ``win'' is a form of praise. If you wish to praise
258 Microsoft Windows when speaking on your own, by all means do so, but
259 not in GNU software. Usually we write the name ``Windows'' in full,
260 but when brevity is very important (as in file names and sometimes
261 symbol names), we abbreviate it to ``w''. For instance, the files and
262 functions in Emacs that deal with Windows start with @samp{w32}.
265 @chapter General Program Design
266 @cindex program design
268 This chapter discusses some of the issues you should take into
269 account when designing your program.
271 @c Standard or ANSI C
273 @c In 1989 the American National Standards Institute (ANSI) standardized
274 @c C as standard X3.159-1989. In December of that year the
275 @c International Standards Organization ISO adopted the ANSI C standard
276 @c making minor changes. In 1990 ANSI then re-adopted ISO standard
277 @c C. This version of C is known as either ANSI C or Standard C.
279 @c A major revision of the C Standard appeared in 1999.
282 * Source Language:: Which languages to use.
283 * Compatibility:: Compatibility with other implementations.
284 * Using Extensions:: Using non-standard features.
285 * Standard C:: Using standard C features.
286 * Conditional Compilation:: Compiling code only if a conditional is true.
289 @node Source Language
290 @section Which Languages to Use
291 @cindex programming languages
293 When you want to use a language that gets compiled and runs at high
294 speed, the best language to use is C. C++ is ok too, but please don't
295 make heavy use of templates. Other languages commonly used in the
296 free software community, such as Java, Python and Ruby, are ok too.
297 Please implement the GNU configure and make interface no matter which
300 Many programs are designed to be extensible: they include an interpreter
301 for a language that is higher level than C. Often much of the program
302 is written in that language, too. The Emacs editor pioneered this
306 @cindex GNOME and Guile
307 The standard extensibility interpreter for GNU software is Guile
308 (@uref{http://www.gnu.org/@/software/@/guile/}), which implements the
309 language Scheme (an especially clean and simple dialect of Lisp).
310 Guile also includes bindings for GTK+/GNOME, making it practical to
311 write modern GUI functionality within Guile. We don't reject programs
312 written in other ``scripting languages'' such as Perl and Python, but
313 using Guile is the path that will lead to overall consistency of the
318 @section Compatibility with Other Implementations
319 @cindex compatibility with C and POSIX standards
320 @cindex C compatibility
321 @cindex POSIX compatibility
323 With occasional exceptions, utility programs and libraries for GNU
324 should be upward compatible with those in Berkeley Unix, and upward
325 compatible with Standard C if Standard C specifies their
326 behavior, and upward compatible with POSIX if POSIX specifies
329 When these standards conflict, it is useful to offer compatibility
330 modes for each of them.
332 @cindex options for compatibility
333 Standard C and POSIX prohibit many kinds of extensions. Feel
334 free to make the extensions anyway, and include a @samp{--ansi},
335 @samp{--posix}, or @samp{--compatible} option to turn them off.
336 However, if the extension has a significant chance of breaking any real
337 programs or scripts, then it is not really upward compatible. So you
338 should try to redesign its interface to make it upward compatible.
340 @cindex @code{POSIXLY_CORRECT}, environment variable
341 Many GNU programs suppress extensions that conflict with POSIX if the
342 environment variable @code{POSIXLY_CORRECT} is defined (even if it is
343 defined with a null value). Please make your program recognize this
344 variable if appropriate.
346 When a feature is used only by users (not by programs or command
347 files), and it is done poorly in Unix, feel free to replace it
348 completely with something totally different and better. (For example,
349 @code{vi} is replaced with Emacs.) But it is nice to offer a compatible
350 feature as well. (There is a free @code{vi} clone, so we offer it.)
352 Additional useful features are welcome regardless of whether
353 there is any precedent for them.
355 @node Using Extensions
356 @section Using Non-standard Features
357 @cindex non-standard extensions
359 Many GNU facilities that already exist support a number of convenient
360 extensions over the comparable Unix facilities. Whether to use these
361 extensions in implementing your program is a difficult question.
363 On the one hand, using the extensions can make a cleaner program.
364 On the other hand, people will not be able to build the program
365 unless the other GNU tools are available. This might cause the
366 program to work on fewer kinds of machines.
368 With some extensions, it might be easy to provide both alternatives.
369 For example, you can define functions with a ``keyword'' @code{INLINE}
370 and define that as a macro to expand into either @code{inline} or
371 nothing, depending on the compiler.
373 In general, perhaps it is best not to use the extensions if you can
374 straightforwardly do without them, but to use the extensions if they
375 are a big improvement.
377 An exception to this rule are the large, established programs (such as
378 Emacs) which run on a great variety of systems. Using GNU extensions in
379 such programs would make many users unhappy, so we don't do that.
381 Another exception is for programs that are used as part of compilation:
382 anything that must be compiled with other compilers in order to
383 bootstrap the GNU compilation facilities. If these require the GNU
384 compiler, then no one can compile them without having them installed
385 already. That would be extremely troublesome in certain cases.
388 @section Standard C and Pre-Standard C
389 @cindex ANSI C standard
391 1989 Standard C is widespread enough now that it is ok to use its
392 features in programs. There is one exception: do not ever use the
393 ``trigraph'' feature of Standard C.
395 The 1999 and 2011 editions of Standard C are not fully supported
396 on all platforms. If you aim to support compilation by
397 compilers other than GCC, you should not require these C
398 features in your programs. It is ok to use these features
399 conditionally when the compiler supports them.
401 If your program is only meant to compile with GCC, then you can
402 use these features if GCC supports them, when they give substantial
405 However, it is easy to support pre-standard compilers in most programs,
406 so if you know how to do that, feel free.
408 @cindex function prototypes
409 To support pre-standard C, instead of writing function definitions in
410 standard prototype form,
419 write the definition in pre-standard style like this,
429 and use a separate declaration to specify the argument prototype:
435 You need such a declaration anyway, in a header file, to get the benefit
436 of prototypes in all the files where the function is called. And once
437 you have the declaration, you normally lose nothing by writing the
438 function definition in the pre-standard style.
440 This technique does not work for integer types narrower than @code{int}.
441 If you think of an argument as being of a type narrower than @code{int},
442 declare it as @code{int} instead.
444 There are a few special cases where this technique is hard to use. For
445 example, if a function argument needs to hold the system type
446 @code{dev_t}, you run into trouble, because @code{dev_t} is shorter than
447 @code{int} on some machines; but you cannot use @code{int} instead,
448 because @code{dev_t} is wider than @code{int} on some machines. There
449 is no type you can safely use on all machines in a non-standard
450 definition. The only way to support non-standard C and pass such an
451 argument is to check the width of @code{dev_t} using Autoconf and choose
452 the argument type accordingly. This may not be worth the trouble.
454 In order to support pre-standard compilers that do not recognize
455 prototypes, you may want to use a preprocessor macro like this:
458 /* Declare the prototype for a general external function. */
459 #if defined (__STDC__) || defined (WINDOWSNT)
460 #define P_(proto) proto
466 @node Conditional Compilation
467 @section Conditional Compilation
469 When supporting configuration options already known when building your
470 program we prefer using @code{if (... )} over conditional compilation,
471 as in the former case the compiler is able to perform more extensive
472 checking of all possible code paths.
474 For example, please write
494 A modern compiler such as GCC will generate exactly the same code in
495 both cases, and we have been using similar techniques with good success
496 in several projects. Of course, the former method assumes that
497 @code{HAS_FOO} is defined as either 0 or 1.
499 While this is not a silver bullet solving all portability problems,
500 and is not always appropriate, following this policy would have saved
501 GCC developers many hours, or even days, per year.
503 In the case of function-like macros like @code{REVERSIBLE_CC_MODE} in
504 GCC which cannot be simply used in @code{if (...)} statements, there is
505 an easy workaround. Simply introduce another macro
506 @code{HAS_REVERSIBLE_CC_MODE} as in the following example:
509 #ifdef REVERSIBLE_CC_MODE
510 #define HAS_REVERSIBLE_CC_MODE 1
512 #define HAS_REVERSIBLE_CC_MODE 0
516 @node Program Behavior
517 @chapter Program Behavior for All Programs
519 This chapter describes conventions for writing robust
520 software. It also describes general standards for error messages, the
521 command line interface, and how libraries should behave.
524 * Non-GNU Standards:: We consider standards such as POSIX;
525 we don't "obey" them.
526 * Semantics:: Writing robust programs.
527 * Libraries:: Library behavior.
528 * Errors:: Formatting error messages.
529 * User Interfaces:: Standards about interfaces generally.
530 * Graphical Interfaces:: Standards for graphical interfaces.
531 * Command-Line Interfaces:: Standards for command line interfaces.
532 * Dynamic Plug-In Interfaces:: Standards for dynamic plug-in interfaces.
533 * Option Table:: Table of long options.
534 * OID Allocations:: Table of OID slots for GNU.
535 * Memory Usage:: When and how to care about memory needs.
536 * File Usage:: Which files to use, and where.
539 @node Non-GNU Standards
540 @section Non-GNU Standards
542 The GNU Project regards standards published by other organizations as
543 suggestions, not orders. We consider those standards, but we do not
544 ``obey'' them. In developing a GNU program, you should implement
545 an outside standard's specifications when that makes the GNU system
546 better overall in an objective sense. When it doesn't, you shouldn't.
548 In most cases, following published standards is convenient for
549 users---it means that their programs or scripts will work more
550 portably. For instance, GCC implements nearly all the features of
551 Standard C as specified by that standard. C program developers would
552 be unhappy if it did not. And GNU utilities mostly follow
553 specifications of POSIX.2; shell script writers and users would be
554 unhappy if our programs were incompatible.
556 But we do not follow either of these specifications rigidly, and there
557 are specific points on which we decided not to follow them, so as to
558 make the GNU system better for users.
560 For instance, Standard C says that nearly all extensions to C are
561 prohibited. How silly! GCC implements many extensions, some of which
562 were later adopted as part of the standard. If you want these
563 constructs to give an error message as ``required'' by the standard,
564 you must specify @samp{--pedantic}, which was implemented only so that
565 we can say ``GCC is a 100% implementation of the standard'', not
566 because there is any reason to actually use it.
568 POSIX.2 specifies that @samp{df} and @samp{du} must output sizes by
569 default in units of 512 bytes. What users want is units of 1k, so
570 that is what we do by default. If you want the ridiculous behavior
571 ``required'' by POSIX, you must set the environment variable
572 @samp{POSIXLY_CORRECT} (which was originally going to be named
573 @samp{POSIX_ME_HARDER}).
575 GNU utilities also depart from the letter of the POSIX.2 specification
576 when they support long-named command-line options, and intermixing
577 options with ordinary arguments. This minor incompatibility with
578 POSIX is never a problem in practice, and it is very useful.
580 In particular, don't reject a new feature, or remove an old one,
581 merely because a standard says it is ``forbidden'' or ``deprecated''.
585 @section Writing Robust Programs
587 @cindex arbitrary limits on data
588 Avoid arbitrary limits on the length or number of @emph{any} data
589 structure, including file names, lines, files, and symbols, by allocating
590 all data structures dynamically. In most Unix utilities, ``long lines
591 are silently truncated''. This is not acceptable in a GNU utility.
593 @cindex @code{NUL} characters
595 Utilities reading files should not drop NUL characters, or any other
596 nonprinting characters @emph{including those with codes above 0177}.
597 The only sensible exceptions would be utilities specifically intended
598 for interface to certain types of terminals or printers that can't
599 handle those characters. Whenever possible, try to make programs work
600 properly with sequences of bytes that represent multibyte characters;
601 UTF-8 is the most important.
603 @cindex error messages
604 Check every system call for an error return, unless you know you wish
605 to ignore errors. Include the system error text (from @code{perror},
606 @code{strerror}, or equivalent) in @emph{every} error message
607 resulting from a failing system call, as well as the name of the file
608 if any and the name of the utility. Just ``cannot open foo.c'' or
609 ``stat failed'' is not sufficient.
611 @cindex @code{malloc} return value
612 @cindex memory allocation failure
613 Check every call to @code{malloc} or @code{realloc} to see if it
614 returned zero. Check @code{realloc} even if you are making the block
615 smaller; in a system that rounds block sizes to a power of 2,
616 @code{realloc} may get a different block if you ask for less space.
618 In Unix, @code{realloc} can destroy the storage block if it returns
619 zero. GNU @code{realloc} does not have this bug: if it fails, the
620 original block is unchanged. Feel free to assume the bug is fixed. If
621 you wish to run your program on Unix, and wish to avoid lossage in this
622 case, you can use the GNU @code{malloc}.
624 You must expect @code{free} to alter the contents of the block that was
625 freed. Anything you want to fetch from the block, you must fetch before
628 If @code{malloc} fails in a noninteractive program, make that a fatal
629 error. In an interactive program (one that reads commands from the
630 user), it is better to abort the command and return to the command
631 reader loop. This allows the user to kill other processes to free up
632 virtual memory, and then try the command again.
634 @cindex command-line arguments, decoding
635 Use @code{getopt_long} to decode arguments, unless the argument syntax
636 makes this unreasonable.
638 When static storage is to be written in during program execution, use
639 explicit C code to initialize it. Reserve C initialized declarations
640 for data that will not be changed.
643 Try to avoid low-level interfaces to obscure Unix data structures (such
644 as file directories, utmp, or the layout of kernel memory), since these
645 are less likely to work compatibly. If you need to find all the files
646 in a directory, use @code{readdir} or some other high-level interface.
647 These are supported compatibly by GNU.
649 @cindex signal handling
650 The preferred signal handling facilities are the BSD variant of
651 @code{signal}, and the POSIX @code{sigaction} function; the
652 alternative USG @code{signal} interface is an inferior design.
654 Nowadays, using the POSIX signal functions may be the easiest way
655 to make a program portable. If you use @code{signal}, then on GNU/Linux
656 systems running GNU libc version 1, you should include
657 @file{bsd/signal.h} instead of @file{signal.h}, so as to get BSD
658 behavior. It is up to you whether to support systems where
659 @code{signal} has only the USG behavior, or give up on them.
661 @cindex impossible conditions
662 In error checks that detect ``impossible'' conditions, just abort.
663 There is usually no point in printing any message. These checks
664 indicate the existence of bugs. Whoever wants to fix the bugs will have
665 to read the source code and run a debugger. So explain the problem with
666 comments in the source. The relevant data will be in variables, which
667 are easy to examine with the debugger, so there is no point moving them
670 Do not use a count of errors as the exit status for a program.
671 @emph{That does not work}, because exit status values are limited to 8
672 bits (0 through 255). A single run of the program might have 256
673 errors; if you try to return 256 as the exit status, the parent process
674 will see 0 as the status, and it will appear that the program succeeded.
676 @cindex temporary files
677 @cindex @code{TMPDIR} environment variable
678 If you make temporary files, check the @code{TMPDIR} environment
679 variable; if that variable is defined, use the specified directory
680 instead of @file{/tmp}.
682 In addition, be aware that there is a possible security problem when
683 creating temporary files in world-writable directories. In C, you can
684 avoid this problem by creating temporary files in this manner:
687 fd = open (filename, O_WRONLY | O_CREAT | O_EXCL, 0600);
691 or by using the @code{mkstemps} function from Gnulib
692 (@pxref{mkstemps,,, gnulib, Gnulib}).
694 In bash, use @code{set -C} (long name @code{noclobber}) to avoid this
695 problem. In addition, the @code{mktemp} utility is a more general
696 solution for creating temporary files from shell scripts
697 (@pxref{mktemp invocation,,, coreutils, GNU Coreutils}).
701 @section Library Behavior
704 Try to make library functions reentrant. If they need to do dynamic
705 storage allocation, at least try to avoid any nonreentrancy aside from
706 that of @code{malloc} itself.
708 Here are certain name conventions for libraries, to avoid name
711 Choose a name prefix for the library, more than two characters long.
712 All external function and variable names should start with this
713 prefix. In addition, there should only be one of these in any given
714 library member. This usually means putting each one in a separate
717 An exception can be made when two external symbols are always used
718 together, so that no reasonable program could use one without the
719 other; then they can both go in the same file.
721 External symbols that are not documented entry points for the user
722 should have names beginning with @samp{_}. The @samp{_} should be
723 followed by the chosen name prefix for the library, to prevent
724 collisions with other libraries. These can go in the same files with
725 user entry points if you like.
727 Static functions and variables can be used as you like and need not
728 fit any naming convention.
731 @section Formatting Error Messages
732 @cindex formatting error messages
733 @cindex error messages, formatting
735 Error messages from compilers should look like this:
738 @var{sourcefile}:@var{lineno}: @var{message}
742 If you want to mention the column number, use one of these formats:
745 @var{sourcefile}:@var{lineno}:@var{column}: @var{message}
746 @var{sourcefile}:@var{lineno}.@var{column}: @var{message}
751 Line numbers should start from 1 at the beginning of the file, and
752 column numbers should start from 1 at the beginning of the line.
753 (Both of these conventions are chosen for compatibility.) Calculate
754 column numbers assuming that space and all ASCII printing characters
755 have equal width, and assuming tab stops every 8 columns. For
756 non-ASCII characters, Unicode character widths should be used when in
757 a UTF-8 locale; GNU libc and GNU gnulib provide suitable
758 @code{wcwidth} functions.
760 The error message can also give both the starting and ending positions
761 of the erroneous text. There are several formats so that you can
762 avoid redundant information such as a duplicate line number.
763 Here are the possible formats:
766 @var{sourcefile}:@var{line1}.@var{column1}-@var{line2}.@var{column2}: @var{message}
767 @var{sourcefile}:@var{line1}.@var{column1}-@var{column2}: @var{message}
768 @var{sourcefile}:@var{line1}-@var{line2}: @var{message}
772 When an error is spread over several files, you can use this format:
775 @var{file1}:@var{line1}.@var{column1}-@var{file2}:@var{line2}.@var{column2}: @var{message}
778 Error messages from other noninteractive programs should look like this:
781 @var{program}:@var{sourcefile}:@var{lineno}: @var{message}
785 when there is an appropriate source file, or like this:
788 @var{program}: @var{message}
792 when there is no relevant source file.
794 If you want to mention the column number, use this format:
797 @var{program}:@var{sourcefile}:@var{lineno}:@var{column}: @var{message}
800 In an interactive program (one that is reading commands from a
801 terminal), it is better not to include the program name in an error
802 message. The place to indicate which program is running is in the
803 prompt or with the screen layout. (When the same program runs with
804 input from a source other than a terminal, it is not interactive and
805 would do best to print error messages using the noninteractive style.)
807 The string @var{message} should not begin with a capital letter when
808 it follows a program name and/or file name, because that isn't the
809 beginning of a sentence. (The sentence conceptually starts at the
810 beginning of the line.) Also, it should not end with a period.
812 Error messages from interactive programs, and other messages such as
813 usage messages, should start with a capital letter. But they should not
816 @node User Interfaces
817 @section Standards for Interfaces Generally
819 @cindex program name and its behavior
820 @cindex behavior, dependent on program's name
821 Please don't make the behavior of a utility depend on the name used
822 to invoke it. It is useful sometimes to make a link to a utility
823 with a different name, and that should not change what it does.
825 Instead, use a run time option or a compilation switch or both
826 to select among the alternate behaviors.
828 @cindex output device and program's behavior
829 Likewise, please don't make the behavior of the program depend on the
830 type of output device it is used with. Device independence is an
831 important principle of the system's design; do not compromise it merely
832 to save someone from typing an option now and then. (Variation in error
833 message syntax when using a terminal is ok, because that is a side issue
834 that people do not depend on.)
836 If you think one behavior is most useful when the output is to a
837 terminal, and another is most useful when the output is a file or a
838 pipe, then it is usually best to make the default behavior the one that
839 is useful with output to a terminal, and have an option for the other
842 Compatibility requires certain programs to depend on the type of output
843 device. It would be disastrous if @code{ls} or @code{sh} did not do so
844 in the way all users expect. In some of these cases, we supplement the
845 program with a preferred alternate version that does not depend on the
846 output device type. For example, we provide a @code{dir} program much
847 like @code{ls} except that its default output format is always
851 @node Graphical Interfaces
852 @section Standards for Graphical Interfaces
853 @cindex graphical user interface
854 @cindex interface styles
855 @cindex user interface styles
858 When you write a program that provides a graphical user interface,
859 please make it work with the X Window System and the GTK+ toolkit
860 unless the functionality specifically requires some alternative (for
861 example, ``displaying jpeg images while in console mode'').
863 In addition, please provide a command-line interface to control the
864 functionality. (In many cases, the graphical user interface can be a
865 separate program which invokes the command-line program.) This is
866 so that the same jobs can be done from scripts.
871 @cindex keyboard interface
872 @cindex library interface
873 Please also consider providing a D-bus interface for use from other
874 running programs, such as within GNOME. (GNOME used to use CORBA
875 for this, but that is being phased out.) In addition, consider
876 providing a library interface (for use from C), and perhaps a
877 keyboard-driven console interface (for use by users from console
878 mode). Once you are doing the work to provide the functionality and
879 the graphical interface, these won't be much extra work.
881 @node Command-Line Interfaces
882 @section Standards for Command Line Interfaces
883 @cindex command-line interface
886 It is a good idea to follow the POSIX guidelines for the
887 command-line options of a program. The easiest way to do this is to use
888 @code{getopt} to parse them. Note that the GNU version of @code{getopt}
889 will normally permit options anywhere among the arguments unless the
890 special argument @samp{--} is used. This is not what POSIX
891 specifies; it is a GNU extension.
893 @cindex long-named options
894 Please define long-named options that are equivalent to the
895 single-letter Unix-style options. We hope to make GNU more user
896 friendly this way. This is easy to do with the GNU function
899 One of the advantages of long-named options is that they can be
900 consistent from program to program. For example, users should be able
901 to expect the ``verbose'' option of any GNU program which has one, to be
902 spelled precisely @samp{--verbose}. To achieve this uniformity, look at
903 the table of common long-option names when you choose the option names
904 for your program (@pxref{Option Table}).
906 It is usually a good idea for file names given as ordinary arguments to
907 be input files only; any output files would be specified using options
908 (preferably @samp{-o} or @samp{--output}). Even if you allow an output
909 file name as an ordinary argument for compatibility, try to provide an
910 option as another way to specify it. This will lead to more consistency
911 among GNU utilities, and fewer idiosyncrasies for users to remember.
913 @cindex standard command-line options
914 @cindex options, standard command-line
915 @cindex CGI programs, standard options for
916 @cindex PATH_INFO, specifying standard options as
917 All programs should support two standard options: @samp{--version}
918 and @samp{--help}. CGI programs should accept these as command-line
919 options, and also if given as the @env{PATH_INFO}; for instance,
920 visiting @url{http://example.org/p.cgi/--help} in a browser should
921 output the same information as invoking @samp{p.cgi --help} from the
925 * --version:: The standard output for --version.
926 * --help:: The standard output for --help.
930 @subsection @option{--version}
932 @cindex @samp{--version} output
934 The standard @code{--version} option should direct the program to
935 print information about its name, version, origin and legal status,
936 all on standard output, and then exit successfully. Other options and
937 arguments should be ignored once this is seen, and the program should
938 not perform its normal function.
940 @cindex canonical name of a program
941 @cindex program's canonical name
942 The first line is meant to be easy for a program to parse; the version
943 number proper starts after the last space. In addition, it contains
944 the canonical name for this program, in this format:
951 The program's name should be a constant string; @emph{don't} compute it
952 from @code{argv[0]}. The idea is to state the standard or canonical
953 name for the program, not its file name. There are other ways to find
954 out the precise file name where a command is found in @code{PATH}.
956 If the program is a subsidiary part of a larger package, mention the
957 package name in parentheses, like this:
960 emacsserver (GNU Emacs) 19.30
964 If the package has a version number which is different from this
965 program's version number, you can mention the package version number
966 just before the close-parenthesis.
968 If you @emph{need} to mention the version numbers of libraries which
969 are distributed separately from the package which contains this program,
970 you can do so by printing an additional line of version info for each
971 library you want to mention. Use the same format for these lines as for
974 Please do not mention all of the libraries that the program uses ``just
975 for completeness''---that would produce a lot of unhelpful clutter.
976 Please mention library version numbers only if you find in practice that
977 they are very important to you in debugging.
979 The following line, after the version number line or lines, should be a
980 copyright notice. If more than one copyright notice is called for, put
981 each on a separate line.
983 Next should follow a line stating the license, preferably using one of
984 abbreviations below, and a brief statement that the program is free
985 software, and that users are free to copy and change it. Also mention
986 that there is no warranty, to the extent permitted by law. See
987 recommended wording below.
989 It is ok to finish the output with a list of the major authors of the
990 program, as a way of giving credit.
992 Here's an example of output that follows these rules:
996 Copyright (C) 2007 Free Software Foundation, Inc.
997 License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
998 This is free software: you are free to change and redistribute it.
999 There is NO WARRANTY, to the extent permitted by law.
1002 You should adapt this to your program, of course, filling in the proper
1003 year, copyright holder, name of program, and the references to
1004 distribution terms, and changing the rest of the wording as necessary.
1006 This copyright notice only needs to mention the most recent year in
1007 which changes were made---there's no need to list the years for previous
1008 versions' changes. You don't have to mention the name of the program in
1009 these notices, if that is inconvenient, since it appeared in the first
1010 line. (The rules are different for copyright notices in source files;
1011 @pxref{Copyright Notices,,,maintain,Information for GNU Maintainers}.)
1013 Translations of the above lines must preserve the validity of the
1014 copyright notices (@pxref{Internationalization}). If the translation's
1015 character set supports it, the @samp{(C)} should be replaced with the
1016 copyright symbol, as follows:
1019 (the official copyright symbol, which is the letter C in a circle);
1025 Write the word ``Copyright'' exactly like that, in English. Do not
1026 translate it into another language. International treaties recognize
1027 the English word ``Copyright''; translations into other languages do not
1028 have legal significance.
1030 Finally, here is the table of our suggested license abbreviations.
1031 Any abbreviation can be followed by @samp{v@var{version}[+]}, meaning
1032 that particular version, or later versions with the @samp{+}, as shown
1033 above. In the case of a GNU license, @emph{always} indicate the permitted
1034 versions in this way.
1036 In the case of exceptions for extra permissions with the GPL, we use
1037 @samp{/} for a separator; the version number can follow the license
1038 abbreviation as usual, as in the examples below.
1042 GNU General Public License, @url{http://www.gnu.org/@/licenses/@/gpl.html}.
1045 GNU Lesser General Public License, @url{http://www.gnu.org/@/licenses/@/lgpl.html}.
1048 GNU GPL with the exception for Ada.
1051 The Apache Software Foundation license,
1052 @url{http://www.apache.org/@/licenses}.
1055 The Artistic license used for Perl, @url{http://dev.perl.org/licenses/artistic.html}.
1058 The Expat license, @url{http://www.jclark.com/@/xml/@/copying.txt}.
1061 The Mozilla Public License, @url{http://www.mozilla.org/@/MPL/}.
1064 The original (4-clause) BSD license, incompatible with the GNU GPL
1065 @url{http://www.xfree86.org/@/3.3.6/@/COPYRIGHT2.html#6}.
1068 The license used for PHP, @url{http://www.php.net/@/license/}.
1071 The non-license that is being in the public domain,
1072 @url{http://www.gnu.org/@/licenses/@/license-list.html#PublicDomain}.
1075 The license for Python, @url{http://www.python.org/@/2.0.1/@/license.html}.
1078 The revised (3-clause) BSD, compatible with the GNU GPL,@*
1079 @url{http://www.xfree86.org/@/3.3.6/@/COPYRIGHT2.html#5}.
1082 The simple non-copyleft license used for most versions of the X Window
1083 System, @url{http://www.xfree86.org/@/3.3.6/@/COPYRIGHT2.html#3}.
1086 The license for Zlib, @url{http://www.gzip.org/@/zlib/@/zlib_license.html}.
1090 More information about these licenses and many more are on the GNU
1091 licensing web pages,
1092 @url{http://www.gnu.org/@/licenses/@/license-list.html}.
1096 @subsection @option{--help}
1098 @cindex @samp{--help} output
1100 The standard @code{--help} option should output brief documentation
1101 for how to invoke the program, on standard output, then exit
1102 successfully. Other options and arguments should be ignored once this
1103 is seen, and the program should not perform its normal function.
1105 @cindex address for bug reports
1107 Near the end of the @samp{--help} option's output, please place lines
1108 giving the email address for bug reports, the package's home page
1109 (normally @indicateurl{http://www.gnu.org/software/@var{pkg}}, and the
1110 general page for help using GNU programs. The format should be like this:
1113 Report bugs to: @var{mailing-address}
1114 @var{pkg} home page: <http://www.gnu.org/software/@var{pkg}/>
1115 General help using GNU software: <http://www.gnu.org/gethelp/>
1118 It is ok to mention other appropriate mailing lists and web pages.
1121 @node Dynamic Plug-In Interfaces
1122 @section Standards for Dynamic Plug-in Interfaces
1124 @cindex dynamic plug-ins
1126 Another aspect of keeping free programs free is encouraging
1127 development of free plug-ins, and discouraging development of
1128 proprietary plug-ins. Many GNU programs will not have anything like
1129 plug-ins at all, but those that do should follow these
1132 First, the general plug-in architecture design should closely tie the
1133 plug-in to the original code, such that the plug-in and the base
1134 program are parts of one extended program. For GCC, for example,
1135 plug-ins receive and modify GCC's internal data structures, and so
1136 clearly form an extended program with the base GCC.
1138 @vindex plugin_is_GPL_compatible
1139 Second, you should require plug-in developers to affirm that their
1140 plug-ins are released under an appropriate license. This should be
1141 enforced with a simple programmatic check. For GCC, again for
1142 example, a plug-in must define the global symbol
1143 @code{plugin_is_GPL_compatible}, thus asserting that the plug-in is
1144 released under a GPL-compatible license (@pxref{Plugins,, Plugins,
1145 gccint, GCC Internals}).
1147 By adding this check to your program you are not creating a new legal
1148 requirement. The GPL itself requires plug-ins to be free software,
1149 licensed compatibly. As long as you have followed the first rule above
1150 to keep plug-ins closely tied to your original program, the GPL and AGPL
1151 already require those plug-ins to be released under a compatible
1152 license. The symbol definition in the plug-in---or whatever equivalent
1153 works best in your program---makes it harder for anyone who might
1154 distribute proprietary plug-ins to legally defend themselves. If a case
1155 about this got to court, we can point to that symbol as evidence that
1156 the plug-in developer understood that the license had this requirement.
1160 @section Table of Long Options
1161 @cindex long option names
1162 @cindex table of long options
1164 Here is a table of long options used by GNU programs. It is surely
1165 incomplete, but we aim to list all the options that a new program might
1166 want to be compatible with. If you use names not already in the table,
1167 please send @email{bug-standards@@gnu.org} a list of them, with their
1168 meanings, so we can update the table.
1170 @c Please leave newlines between items in this table; it's much easier
1171 @c to update when it isn't completely squashed together and unreadable.
1172 @c When there is more than one short option for a long option name, put
1173 @c a semicolon between the lists of the programs that use them, not a
1174 @c period. --friedman
1178 @samp{-N} in @code{tar}.
1181 @samp{-a} in @code{du}, @code{ls}, @code{nm}, @code{stty}, @code{uname},
1182 and @code{unexpand}.
1185 @samp{-a} in @code{diff}.
1188 @samp{-A} in @code{ls}.
1191 @samp{-a} in @code{etags}, @code{tee}, @code{time};
1192 @samp{-r} in @code{tar}.
1195 @samp{-a} in @code{cp}.
1198 @samp{-n} in @code{shar}.
1201 @samp{-l} in @code{m4}.
1204 @samp{-a} in @code{diff}.
1207 @samp{-v} in @code{gawk}.
1210 @samp{-W} in @code{make}.
1213 @samp{-o} in @code{make}.
1216 @samp{-a} in @code{recode}.
1219 @samp{-a} in @code{wdiff}.
1221 @item auto-reference
1222 @samp{-A} in @code{ptx}.
1225 @samp{-n} in @code{wdiff}.
1228 For server programs, run in the background.
1230 @item backward-search
1231 @samp{-B} in @code{ctags}.
1234 @samp{-f} in @code{shar}.
1243 @samp{-b} in @code{tac}.
1246 @samp{-b} in @code{cpio} and @code{diff}.
1249 @samp{-b} in @code{shar}.
1252 Used in @code{cpio} and @code{tar}.
1255 @samp{-b} in @code{head} and @code{tail}.
1258 @samp{-b} in @code{ptx}.
1261 Used in various programs to make output shorter.
1264 @samp{-c} in @code{head}, @code{split}, and @code{tail}.
1267 @samp{-C} in @code{etags}.
1270 @samp{-A} in @code{tar}.
1273 Used in various programs to specify the directory to use.
1276 @samp{-c} in @code{chgrp} and @code{chown}.
1279 @samp{-F} in @code{ls}.
1282 @samp{-c} in @code{recode}.
1285 @samp{-c} in @code{su};
1289 @samp{-d} in @code{tar}.
1292 Used in @code{gawk}.
1295 @samp{-Z} in @code{tar} and @code{shar}.
1298 @samp{-A} in @code{tar}.
1301 @samp{-w} in @code{tar}.
1304 Used in @code{diff}.
1307 @samp{-W copyleft} in @code{gawk}.
1310 @samp{-C} in @code{ptx}, @code{recode}, and @code{wdiff};
1311 @samp{-W copyright} in @code{gawk}.
1317 @samp{-q} in @code{who}.
1320 @samp{-l} in @code{du}.
1323 Used in @code{tar} and @code{cpio}.
1326 @samp{-c} in @code{shar}.
1329 @samp{-x} in @code{ctags}.
1332 @samp{-d} in @code{touch}.
1335 @samp{-d} in @code{make} and @code{m4};
1339 @samp{-D} in @code{m4}.
1342 @samp{-d} in Bison and @code{ctags}.
1345 @samp{-D} in @code{tar}.
1348 @samp{-L} in @code{chgrp}, @code{chown}, @code{cpio}, @code{du},
1349 @code{ls}, and @code{tar}.
1351 @item dereference-args
1352 @samp{-D} in @code{du}.
1355 Specify an I/O device (special file name).
1358 @samp{-d} in @code{recode}.
1360 @item dictionary-order
1361 @samp{-d} in @code{look}.
1364 @samp{-d} in @code{tar}.
1367 @samp{-n} in @code{csplit}.
1370 Specify the directory to use, in various programs. In @code{ls}, it
1371 means to show directories themselves rather than their contents. In
1372 @code{rm} and @code{ln}, it means to not treat links to directories
1376 @samp{-x} in @code{strip}.
1378 @item discard-locals
1379 @samp{-X} in @code{strip}.
1382 @samp{-n} in @code{make}.
1385 @samp{-e} in @code{diff}.
1387 @item elide-empty-files
1388 @samp{-z} in @code{csplit}.
1391 @samp{-x} in @code{wdiff}.
1394 @samp{-z} in @code{wdiff}.
1396 @item entire-new-file
1397 @samp{-N} in @code{diff}.
1399 @item environment-overrides
1400 @samp{-e} in @code{make}.
1403 @samp{-e} in @code{xargs}.
1409 Used in @code{makeinfo}.
1412 @samp{-o} in @code{m4}.
1415 @samp{-b} in @code{ls}.
1418 @samp{-X} in @code{tar}.
1424 @samp{-x} in @code{xargs}.
1427 @samp{-e} in @code{unshar}.
1430 @samp{-t} in @code{diff}.
1433 @samp{-e} in @code{sed}.
1436 @samp{-g} in @code{nm}.
1439 @samp{-i} in @code{cpio};
1440 @samp{-x} in @code{tar}.
1443 @samp{-f} in @code{finger}.
1446 @samp{-f} in @code{su}.
1448 @item fatal-warnings
1449 @samp{-E} in @code{m4}.
1452 @samp{-f} in @code{gawk}, @code{info}, @code{make}, @code{mt},
1453 @code{sed}, and @code{tar}.
1455 @item field-separator
1456 @samp{-F} in @code{gawk}.
1462 @samp{-F} in @code{ls}.
1465 @samp{-T} in @code{tar}.
1468 Used in @code{makeinfo}.
1470 @item flag-truncation
1471 @samp{-F} in @code{ptx}.
1473 @item fixed-output-files
1477 @samp{-f} in @code{tail}.
1479 @item footnote-style
1480 Used in @code{makeinfo}.
1483 @samp{-f} in @code{cp}, @code{ln}, @code{mv}, and @code{rm}.
1486 @samp{-F} in @code{shar}.
1489 For server programs, run in the foreground;
1490 in other words, don't do anything special to run the server
1494 Used in @code{ls}, @code{time}, and @code{ptx}.
1497 @samp{-F} in @code{m4}.
1503 @samp{-g} in @code{ptx}.
1506 @samp{-x} in @code{tar}.
1509 @samp{-i} in @code{ul}.
1512 @samp{-g} in @code{recode}.
1515 @samp{-g} in @code{install}.
1518 @samp{-z} in @code{tar} and @code{shar}.
1521 @samp{-H} in @code{m4}.
1524 @samp{-h} in @code{objdump} and @code{recode}
1527 @samp{-H} in @code{who}.
1530 Used to ask for brief usage information.
1532 @item here-delimiter
1533 @samp{-d} in @code{shar}.
1535 @item hide-control-chars
1536 @samp{-q} in @code{ls}.
1539 In @code{makeinfo}, output HTML.
1542 @samp{-u} in @code{who}.
1545 @samp{-D} in @code{diff}.
1548 @samp{-I} in @code{ls};
1549 @samp{-x} in @code{recode}.
1551 @item ignore-all-space
1552 @samp{-w} in @code{diff}.
1554 @item ignore-backups
1555 @samp{-B} in @code{ls}.
1557 @item ignore-blank-lines
1558 @samp{-B} in @code{diff}.
1561 @samp{-f} in @code{look} and @code{ptx};
1562 @samp{-i} in @code{diff} and @code{wdiff}.
1565 @samp{-i} in @code{make}.
1568 @samp{-i} in @code{ptx}.
1570 @item ignore-indentation
1571 @samp{-I} in @code{etags}.
1573 @item ignore-init-file
1576 @item ignore-interrupts
1577 @samp{-i} in @code{tee}.
1579 @item ignore-matching-lines
1580 @samp{-I} in @code{diff}.
1582 @item ignore-space-change
1583 @samp{-b} in @code{diff}.
1586 @samp{-i} in @code{tar}.
1589 @samp{-i} in @code{etags};
1590 @samp{-I} in @code{m4}.
1593 @samp{-I} in @code{make}.
1596 @samp{-G} in @code{tar}.
1599 @samp{-i}, @samp{-l}, and @samp{-m} in Finger.
1602 In some programs, specify the name of the file to read as the user's
1606 @samp{-i} in @code{expand}.
1609 @samp{-T} in @code{diff}.
1612 @samp{-i} in @code{ls}.
1615 @samp{-i} in @code{cp}, @code{ln}, @code{mv}, @code{rm};
1616 @samp{-e} in @code{m4};
1617 @samp{-p} in @code{xargs};
1618 @samp{-w} in @code{tar}.
1621 @samp{-p} in @code{shar}.
1627 @samp{-j} in @code{make}.
1630 @samp{-n} in @code{make}.
1633 @samp{-k} in @code{make}.
1636 @samp{-k} in @code{csplit}.
1639 @samp{-k} in @code{du} and @code{ls}.
1642 @samp{-l} in @code{etags}.
1645 @samp{-l} in @code{wdiff}.
1647 @item level-for-gzip
1648 @samp{-g} in @code{shar}.
1651 @samp{-C} in @code{split}.
1654 Used in @code{split}, @code{head}, and @code{tail}.
1657 @samp{-l} in @code{cpio}.
1661 Used in @code{gawk}.
1664 @samp{-t} in @code{cpio};
1665 @samp{-l} in @code{recode}.
1668 @samp{-t} in @code{tar}.
1671 @samp{-N} in @code{ls}.
1674 @samp{-l} in @code{make}.
1680 Used in @code{uname}.
1683 @samp{-M} in @code{ptx}.
1686 @samp{-m} in @code{hello} and @code{uname}.
1688 @item make-directories
1689 @samp{-d} in @code{cpio}.
1692 @samp{-f} in @code{make}.
1698 @samp{-n} in @code{xargs}.
1701 @samp{-n} in @code{xargs}.
1704 @samp{-l} in @code{xargs}.
1707 @samp{-l} in @code{make}.
1710 @samp{-P} in @code{xargs}.
1713 @samp{-T} in @code{who}.
1716 @samp{-T} in @code{who}.
1719 @samp{-d} in @code{diff}.
1721 @item mixed-uuencode
1722 @samp{-M} in @code{shar}.
1725 @samp{-m} in @code{install}, @code{mkdir}, and @code{mkfifo}.
1727 @item modification-time
1728 @samp{-m} in @code{tar}.
1731 @samp{-M} in @code{tar}.
1737 @samp{-L} in @code{m4}.
1740 @samp{-a} in @code{shar}.
1743 @samp{-W} in @code{make}.
1745 @item no-builtin-rules
1746 @samp{-r} in @code{make}.
1748 @item no-character-count
1749 @samp{-w} in @code{shar}.
1751 @item no-check-existing
1752 @samp{-x} in @code{shar}.
1755 @samp{-3} in @code{wdiff}.
1758 @samp{-c} in @code{touch}.
1761 @samp{-D} in @code{etags}.
1764 @samp{-1} in @code{wdiff}.
1766 @item no-dereference
1767 @samp{-d} in @code{cp}.
1770 @samp{-2} in @code{wdiff}.
1773 @samp{-S} in @code{make}.
1779 @samp{-P} in @code{shar}.
1782 @samp{-e} in @code{gprof}.
1785 @samp{-R} in @code{etags}.
1788 @samp{-p} in @code{nm}.
1791 Don't print a startup splash screen.
1794 Used in @code{makeinfo}.
1797 @samp{-a} in @code{gprof}.
1800 @samp{-E} in @code{gprof}.
1803 @samp{-m} in @code{shar}.
1806 Used in @code{makeinfo}.
1809 Used in @code{emacsclient}.
1812 Used in various programs to inhibit warnings.
1815 @samp{-n} in @code{info}.
1818 @samp{-n} in @code{uname}.
1821 @samp{-f} in @code{cpio}.
1824 @samp{-n} in @code{objdump}.
1827 @samp{-0} in @code{xargs}.
1830 @samp{-n} in @code{cat}.
1832 @item number-nonblank
1833 @samp{-b} in @code{cat}.
1836 @samp{-n} in @code{nm}.
1838 @item numeric-uid-gid
1839 @samp{-n} in @code{cpio} and @code{ls}.
1845 @samp{-o} in @code{tar}.
1848 @samp{-o} in @code{make}.
1850 @item one-file-system
1851 @samp{-l} in @code{tar}, @code{cp}, and @code{du}.
1854 @samp{-o} in @code{ptx}.
1857 @samp{-f} in @code{gprof}.
1860 @samp{-F} in @code{gprof}.
1863 @samp{-o} in @code{getopt}, @code{fdlist}, @code{fdmount},
1864 @code{fdmountd}, and @code{fdumount}.
1867 In various programs, specify the output file name.
1870 @samp{-o} in @code{shar}.
1873 @samp{-o} in @code{rm}.
1876 @samp{-c} in @code{unshar}.
1879 @samp{-o} in @code{install}.
1882 @samp{-l} in @code{diff}.
1884 @item paragraph-indent
1885 Used in @code{makeinfo}.
1888 @samp{-p} in @code{mkdir} and @code{rmdir}.
1891 @samp{-p} in @code{ul}.
1894 @samp{-p} in @code{cpio}.
1897 @samp{-P} in @code{finger}.
1900 @samp{-c} in @code{cpio} and @code{tar}.
1903 Used in @code{gawk}.
1905 @item prefix-builtins
1906 @samp{-P} in @code{m4}.
1909 @samp{-f} in @code{csplit}.
1912 Used in @code{tar} and @code{cp}.
1914 @item preserve-environment
1915 @samp{-p} in @code{su}.
1917 @item preserve-modification-time
1918 @samp{-m} in @code{cpio}.
1920 @item preserve-order
1921 @samp{-s} in @code{tar}.
1923 @item preserve-permissions
1924 @samp{-p} in @code{tar}.
1927 @samp{-l} in @code{diff}.
1930 @samp{-L} in @code{cmp}.
1932 @item print-data-base
1933 @samp{-p} in @code{make}.
1935 @item print-directory
1936 @samp{-w} in @code{make}.
1938 @item print-file-name
1939 @samp{-o} in @code{nm}.
1942 @samp{-s} in @code{nm}.
1945 @samp{-p} in @code{wdiff}.
1948 @samp{-p} in @code{ed}.
1951 Specify an HTTP proxy.
1954 @samp{-X} in @code{shar}.
1957 @samp{-q} in @code{make}.
1960 Used in many programs to inhibit the usual output. Every
1961 program accepting @samp{--quiet} should accept @samp{--silent} as a
1965 @samp{-Q} in @code{shar}
1968 @samp{-Q} in @code{ls}.
1971 @samp{-n} in @code{diff}.
1974 Used in @code{gawk}.
1976 @item read-full-blocks
1977 @samp{-B} in @code{tar}.
1983 @samp{-n} in @code{make}.
1986 @samp{-R} in @code{tar}.
1989 Used in @code{chgrp}, @code{chown}, @code{cp}, @code{ls}, @code{diff},
1993 @samp{-r} in @code{touch}.
1996 @samp{-r} in @code{ptx}.
1999 @samp{-r} in @code{tac} and @code{etags}.
2002 @samp{-r} in @code{uname}.
2005 @samp{-R} in @code{m4}.
2008 @samp{-r} in @code{objdump}.
2011 @samp{-r} in @code{cpio}.
2014 @samp{-i} in @code{xargs}.
2016 @item report-identical-files
2017 @samp{-s} in @code{diff}.
2019 @item reset-access-time
2020 @samp{-a} in @code{cpio}.
2023 @samp{-r} in @code{ls} and @code{nm}.
2026 @samp{-f} in @code{diff}.
2028 @item right-side-defs
2029 @samp{-R} in @code{ptx}.
2032 @samp{-s} in @code{tar}.
2034 @item same-permissions
2035 @samp{-p} in @code{tar}.
2038 @samp{-g} in @code{stty}.
2043 @item sentence-regexp
2044 @samp{-S} in @code{ptx}.
2047 @samp{-S} in @code{du}.
2050 @samp{-s} in @code{tac}.
2053 Used by @code{recode} to chose files or pipes for sequencing passes.
2056 @samp{-s} in @code{su}.
2059 @samp{-A} in @code{cat}.
2061 @item show-c-function
2062 @samp{-p} in @code{diff}.
2065 @samp{-E} in @code{cat}.
2067 @item show-function-line
2068 @samp{-F} in @code{diff}.
2071 @samp{-T} in @code{cat}.
2074 Used in many programs to inhibit the usual output.
2075 Every program accepting
2076 @samp{--silent} should accept @samp{--quiet} as a synonym.
2079 @samp{-s} in @code{ls}.
2082 Specify a file descriptor for a network server to use for its socket,
2083 instead of opening and binding a new socket. This provides a way to
2084 run, in a non-privileged process, a server that normally needs a
2085 reserved port number.
2091 @samp{-W source} in @code{gawk}.
2094 @samp{-S} in @code{tar}.
2096 @item speed-large-files
2097 @samp{-H} in @code{diff}.
2100 @samp{-E} in @code{unshar}.
2102 @item split-size-limit
2103 @samp{-L} in @code{shar}.
2106 @samp{-s} in @code{cat}.
2109 @samp{-w} in @code{wdiff}.
2112 @samp{-y} in @code{wdiff}.
2115 Used in @code{tar} and @code{diff} to specify which file within
2116 a directory to start processing with.
2119 @samp{-s} in @code{wdiff}.
2121 @item stdin-file-list
2122 @samp{-S} in @code{shar}.
2125 @samp{-S} in @code{make}.
2128 @samp{-s} in @code{recode}.
2131 @samp{-s} in @code{install}.
2134 @samp{-s} in @code{strip}.
2137 @samp{-S} in @code{strip}.
2140 @samp{-s} in @code{shar}.
2143 @samp{-S} in @code{cp}, @code{ln}, @code{mv}.
2146 @samp{-b} in @code{csplit}.
2149 @samp{-s} in @code{gprof}.
2152 @samp{-s} in @code{du}.
2155 @samp{-s} in @code{ln}.
2158 Used in GDB and @code{objdump}.
2161 @samp{-s} in @code{m4}.
2164 @samp{-s} in @code{uname}.
2167 @samp{-t} in @code{expand} and @code{unexpand}.
2170 @samp{-T} in @code{ls}.
2173 @samp{-T} in @code{tput} and @code{ul}.
2174 @samp{-t} in @code{wdiff}.
2177 @samp{-a} in @code{diff}.
2180 @samp{-T} in @code{shar}.
2183 Used in @code{ls} and @code{touch}.
2186 Specify how long to wait before giving up on some operation.
2189 @samp{-O} in @code{tar}.
2192 @samp{-c} in @code{du}.
2195 @samp{-t} in @code{make}, @code{ranlib}, and @code{recode}.
2198 @samp{-t} in @code{m4}.
2201 @samp{-t} in @code{hello};
2202 @samp{-W traditional} in @code{gawk};
2203 @samp{-G} in @code{ed}, @code{m4}, and @code{ptx}.
2209 @samp{-t} in @code{ctags}.
2211 @item typedefs-and-c++
2212 @samp{-T} in @code{ctags}.
2215 @samp{-t} in @code{ptx}.
2218 @samp{-z} in @code{tar}.
2221 @samp{-u} in @code{cpio}.
2224 @samp{-U} in @code{m4}.
2226 @item undefined-only
2227 @samp{-u} in @code{nm}.
2230 @samp{-u} in @code{cp}, @code{ctags}, @code{mv}, @code{tar}.
2233 Used in @code{gawk}; same as @samp{--help}.
2236 @samp{-B} in @code{shar}.
2238 @item vanilla-operation
2239 @samp{-V} in @code{shar}.
2242 Print more information about progress. Many programs support this.
2245 @samp{-W} in @code{tar}.
2248 Print the version number.
2250 @item version-control
2251 @samp{-V} in @code{cp}, @code{ln}, @code{mv}.
2254 @samp{-v} in @code{ctags}.
2257 @samp{-V} in @code{tar}.
2260 @samp{-W} in @code{make}.
2262 @item whole-size-limit
2263 @samp{-l} in @code{shar}.
2266 @samp{-w} in @code{ls} and @code{ptx}.
2269 @samp{-W} in @code{ptx}.
2272 @samp{-T} in @code{who}.
2275 @samp{-z} in @code{gprof}.
2278 @node OID Allocations
2279 @section OID Allocations
2280 @cindex OID allocations for GNU
2285 The OID (object identifier) 1.3.6.1.4.1.11591 has been assigned to the
2286 GNU Project (thanks to Werner Koch). These are used for SNMP, LDAP,
2287 X.509 certificates, and so on. The web site
2288 @url{http://www.alvestrand.no/objectid} has a (voluntary) listing of
2289 many OID assignments.
2291 If you need a new slot for your GNU package, write
2292 @email{maintainers@@gnu.org}. Here is a list of arcs currently
2296 @include gnu-oids.texi
2301 @section Memory Usage
2302 @cindex memory usage
2304 If a program typically uses just a few meg of memory, don't bother making any
2305 effort to reduce memory usage. For example, if it is impractical for
2306 other reasons to operate on files more than a few meg long, it is
2307 reasonable to read entire input files into memory to operate on them.
2309 However, for programs such as @code{cat} or @code{tail}, that can
2310 usefully operate on very large files, it is important to avoid using a
2311 technique that would artificially limit the size of files it can handle.
2312 If a program works by lines and could be applied to arbitrary
2313 user-supplied input files, it should keep only a line in memory, because
2314 this is not very hard and users will want to be able to operate on input
2315 files that are bigger than will fit in memory all at once.
2317 If your program creates complicated data structures, just make them in
2318 memory and give a fatal error if @code{malloc} returns zero.
2322 Memory analysis tools such as @command{valgrind} can be useful, but
2323 don't complicate a program merely to avoid their false alarms. For
2324 example, if memory is used until just before a process exits, don't
2325 free it simply to silence such a tool.
2332 Programs should be prepared to operate when @file{/usr} and @file{/etc}
2333 are read-only file systems. Thus, if the program manages log files,
2334 lock files, backup files, score files, or any other files which are
2335 modified for internal purposes, these files should not be stored in
2336 @file{/usr} or @file{/etc}.
2338 There are two exceptions. @file{/etc} is used to store system
2339 configuration information; it is reasonable for a program to modify
2340 files in @file{/etc} when its job is to update the system configuration.
2341 Also, if the user explicitly asks to modify one file in a directory, it
2342 is reasonable for the program to store other files in the same
2346 @chapter Making The Best Use of C
2348 This chapter provides advice on how best to use the C language
2349 when writing GNU software.
2352 * Formatting:: Formatting your source code.
2353 * Comments:: Commenting your work.
2354 * Syntactic Conventions:: Clean use of C constructs.
2355 * Names:: Naming variables, functions, and files.
2356 * System Portability:: Portability among different operating systems.
2357 * CPU Portability:: Supporting the range of CPU types.
2358 * System Functions:: Portability and ``standard'' library functions.
2359 * Internationalization:: Techniques for internationalization.
2360 * Character Set:: Use ASCII by default.
2361 * Quote Characters:: Use "..." or '...' in the C locale.
2362 * Mmap:: How you can safely use @code{mmap}.
2366 @section Formatting Your Source Code
2367 @cindex formatting source code
2370 @cindex braces, in C source
2371 @cindex function definitions, formatting
2372 It is important to put the open-brace that starts the body of a C
2373 function in column one, so that they will start a defun. Several
2374 tools look for open-braces in column one to find the beginnings of C
2375 functions. These tools will not work on code not formatted that way.
2377 Avoid putting open-brace, open-parenthesis or open-bracket in column
2378 one when they are inside a function, so that they won't start a defun.
2379 The open-brace that starts a @code{struct} body can go in column one
2380 if you find it useful to treat that definition as a defun.
2382 It is also important for function definitions to start the name of the
2383 function in column one. This helps people to search for function
2384 definitions, and may also help certain tools recognize them. Thus,
2385 using Standard C syntax, the format is this:
2389 concat (char *s1, char *s2)
2396 or, if you want to use traditional C syntax, format the definition like
2401 concat (s1, s2) /* Name starts in column one here */
2403 @{ /* Open brace in column one here */
2408 In Standard C, if the arguments don't fit nicely on one line,
2413 lots_of_args (int an_integer, long a_long, short a_short,
2414 double a_double, float a_float)
2418 @cindex @code{struct} types, formatting
2419 @cindex @code{enum} types, formatting
2420 For @code{struct} and @code{enum} types, likewise put the braces in
2421 column one, unless the whole contents fits on one line:
2429 struct foo @{ int a, b; @}
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 readers can see that for themselves.
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 it hurts readability to silence them with
2658 unnecessary casts, wrappers, and other complications. For example,
2659 please don't insert casts to @code{void} or calls to do-nothing
2660 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 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-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 CPUs
2904 @cindex data types, and portability
2905 @cindex portability, and data types
2906 Even GNU systems will differ because of differences among 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, all practical systems have a C89 compiler and GNU C supports
2978 almost all of C99 and some of C11. Similarly, most systems implement
2979 POSIX.1-2001 libraries and tools, and many have POSIX.1-2008.
2981 Hence, there is little reason to support old C or non-POSIX systems,
2982 and you may want to take advantage of standard C and POSIX 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'..."), file);
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 (but not required) to use non-ASCII characters to represent proper
3175 names of contributors in change logs (@pxref{Change Logs}).
3177 If you need to use non-ASCII characters, you should normally stick
3178 with one encoding, certainly within a single file. UTF-8 is likely to
3182 @node Quote Characters
3183 @section Quote Characters
3184 @cindex quote characters
3185 @cindex locale-specific quote characters
3188 @cindex opening quote
3189 @cindex single quote
3190 @cindex double quote
3191 @cindex grave accent
3192 @set txicodequoteundirected
3193 @set txicodequotebacktick
3195 In the C locale, the output of GNU programs should stick to plain
3196 ASCII for quotation characters in messages to users: preferably 0x22
3197 (@samp{"}) or 0x27 (@samp{'}) for both opening and closing quotes.
3198 Although GNU programs traditionally used 0x60 (@samp{`}) for opening
3199 and 0x27 (@samp{'}) for closing quotes, nowadays quotes @samp{`like
3200 this'} are typically rendered asymmetrically, so quoting @samp{"like
3201 this"} or @samp{'like this'} typically looks better.
3203 It is ok, but not required, for GNU programs to generate
3204 locale-specific quotes in non-C locales. For example:
3207 printf (gettext ("Processing file '%s'..."), file);
3211 Here, a French translation might cause @code{gettext} to return the
3212 string @code{"Traitement de fichier
3213 @guilsinglleft{}@tie{}%s@tie{}@guilsinglright{}..."}, yielding quotes
3214 more appropriate for a French locale.
3216 Sometimes a program may need to use opening and closing quotes
3217 directly. By convention, @code{gettext} translates the string
3218 @samp{"`"} to the opening quote and the string @samp{"'"} to the
3219 closing quote, and a program can use these translations. Generally,
3220 though, it is better to translate quote characters in the context of
3223 If the output of your program is ever likely to be parsed by another
3224 program, it is good to provide an option that makes this parsing
3225 reliable. For example, you could escape special characters using
3226 conventions from the C language or the Bourne shell. See for example
3227 the option @option{--quoting-style} of GNU @code{ls}.
3229 @clear txicodequoteundirected
3230 @clear txicodequotebacktick
3237 Don't assume that @code{mmap} either works on all files or fails
3238 for all files. It may work on some files and fail on others.
3240 The proper way to use @code{mmap} is to try it on the specific file for
3241 which you want to use it---and if @code{mmap} doesn't work, fall back on
3242 doing the job in another way using @code{read} and @code{write}.
3244 The reason this precaution is needed is that the GNU kernel (the HURD)
3245 provides a user-extensible file system, in which there can be many
3246 different kinds of ``ordinary files''. Many of them support
3247 @code{mmap}, but some do not. It is important to make programs handle
3248 all these kinds of files.
3252 @chapter Documenting Programs
3253 @cindex documentation
3255 A GNU program should ideally come with full free documentation, adequate
3256 for both reference and tutorial purposes. If the package can be
3257 programmed or extended, the documentation should cover programming or
3258 extending it, as well as just using it.
3261 * GNU Manuals:: Writing proper manuals.
3262 * Doc Strings and Manuals:: Compiling doc strings doesn't make a manual.
3263 * Manual Structure Details:: Specific structure conventions.
3264 * License for Manuals:: Writing the distribution terms for a manual.
3265 * Manual Credits:: Giving credit to documentation contributors.
3266 * Printed Manuals:: Mentioning the printed manual.
3267 * NEWS File:: NEWS files supplement manuals.
3268 * Change Logs:: Recording changes.
3269 * Man Pages:: Man pages are secondary.
3270 * Reading other Manuals:: How far you can go in learning
3275 @section GNU Manuals
3277 The preferred document format for the GNU system is the Texinfo
3278 formatting language. Every GNU package should (ideally) have
3279 documentation in Texinfo both for reference and for learners. Texinfo
3280 makes it possible to produce a good quality formatted book, using
3281 @TeX{}, and to generate an Info file. It is also possible to generate
3282 HTML output from Texinfo source. See the Texinfo manual, either the
3283 hardcopy, or the on-line version available through @code{info} or the
3284 Emacs Info subsystem (@kbd{C-h i}).
3286 Nowadays some other formats such as Docbook and Sgmltexi can be
3287 converted automatically into Texinfo. It is ok to produce the Texinfo
3288 documentation by conversion this way, as long as it gives good results.
3290 Make sure your manual is clear to a reader who knows nothing about the
3291 topic and reads it straight through. This means covering basic topics
3292 at the beginning, and advanced topics only later. This also means
3293 defining every specialized term when it is first used.
3295 Programmers tend to carry over the structure of the program as the
3296 structure for its documentation. But this structure is not
3297 necessarily good for explaining how to use the program; it may be
3298 irrelevant and confusing for a user.
3300 Instead, the right way to structure documentation is according to the
3301 concepts and questions that a user will have in mind when reading it.
3302 This principle applies at every level, from the lowest (ordering
3303 sentences in a paragraph) to the highest (ordering of chapter topics
3304 within the manual). Sometimes this structure of ideas matches the
3305 structure of the implementation of the software being documented---but
3306 often they are different. An important part of learning to write good
3307 documentation is to learn to notice when you have unthinkingly
3308 structured the documentation like the implementation, stop yourself,
3309 and look for better alternatives.
3311 For example, each program in the GNU system probably ought to be
3312 documented in one manual; but this does not mean each program should
3313 have its own manual. That would be following the structure of the
3314 implementation, rather than the structure that helps the user
3317 Instead, each manual should cover a coherent @emph{topic}. For example,
3318 instead of a manual for @code{diff} and a manual for @code{diff3}, we
3319 have one manual for ``comparison of files'' which covers both of those
3320 programs, as well as @code{cmp}. By documenting these programs
3321 together, we can make the whole subject clearer.
3323 The manual which discusses a program should certainly document all of
3324 the program's command-line options and all of its commands. It should
3325 give examples of their use. But don't organize the manual as a list
3326 of features. Instead, organize it logically, by subtopics. Address
3327 the questions that a user will ask when thinking about the job that
3328 the program does. Don't just tell the reader what each feature can
3329 do---say what jobs it is good for, and show how to use it for those
3330 jobs. Explain what is recommended usage, and what kinds of usage
3333 In general, a GNU manual should serve both as tutorial and reference.
3334 It should be set up for convenient access to each topic through Info,
3335 and for reading straight through (appendixes aside). A GNU manual
3336 should give a good introduction to a beginner reading through from the
3337 start, and should also provide all the details that hackers want.
3338 The Bison manual is a good example of this---please take a look at it
3339 to see what we mean.
3341 That is not as hard as it first sounds. Arrange each chapter as a
3342 logical breakdown of its topic, but order the sections, and write their
3343 text, so that reading the chapter straight through makes sense. Do
3344 likewise when structuring the book into chapters, and when structuring a
3345 section into paragraphs. The watchword is, @emph{at each point, address
3346 the most fundamental and important issue raised by the preceding text.}
3348 If necessary, add extra chapters at the beginning of the manual which
3349 are purely tutorial and cover the basics of the subject. These provide
3350 the framework for a beginner to understand the rest of the manual. The
3351 Bison manual provides a good example of how to do this.
3353 To serve as a reference, a manual should have an Index that lists all
3354 the functions, variables, options, and important concepts that are
3355 part of the program. One combined Index should do for a short manual,
3356 but sometimes for a complex package it is better to use multiple
3357 indices. The Texinfo manual includes advice on preparing good index
3358 entries, see @ref{Index Entries, , Making Index Entries, texinfo, GNU
3359 Texinfo}, and see @ref{Indexing Commands, , Defining the Entries of an
3360 Index, texinfo, GNU Texinfo}.
3362 Don't use Unix man pages as a model for how to write GNU documentation;
3363 most of them are terse, badly structured, and give inadequate
3364 explanation of the underlying concepts. (There are, of course, some
3365 exceptions.) Also, Unix man pages use a particular format which is
3366 different from what we use in GNU manuals.
3368 Please include an email address in the manual for where to report
3369 bugs @emph{in the text of the manual}.
3371 Please do not use the term ``pathname'' that is used in Unix
3372 documentation; use ``file name'' (two words) instead. We use the term
3373 ``path'' only for search paths, which are lists of directory names.
3375 Please do not use the term ``illegal'' to refer to erroneous input to
3376 a computer program. Please use ``invalid'' for this, and reserve the
3377 term ``illegal'' for activities prohibited by law.
3379 Please do not write @samp{()} after a function name just to indicate
3380 it is a function. @code{foo ()} is not a function, it is a function
3381 call with no arguments.
3383 @node Doc Strings and Manuals
3384 @section Doc Strings and Manuals
3386 Some programming systems, such as Emacs, provide a documentation string
3387 for each function, command or variable. You may be tempted to write a
3388 reference manual by compiling the documentation strings and writing a
3389 little additional text to go around them---but you must not do it. That
3390 approach is a fundamental mistake. The text of well-written
3391 documentation strings will be entirely wrong for a manual.
3393 A documentation string needs to stand alone---when it appears on the
3394 screen, there will be no other text to introduce or explain it.
3395 Meanwhile, it can be rather informal in style.
3397 The text describing a function or variable in a manual must not stand
3398 alone; it appears in the context of a section or subsection. Other text
3399 at the beginning of the section should explain some of the concepts, and
3400 should often make some general points that apply to several functions or
3401 variables. The previous descriptions of functions and variables in the
3402 section will also have given information about the topic. A description
3403 written to stand alone would repeat some of that information; this
3404 redundancy looks bad. Meanwhile, the informality that is acceptable in
3405 a documentation string is totally unacceptable in a manual.
3407 The only good way to use documentation strings in writing a good manual
3408 is to use them as a source of information for writing good text.
3410 @node Manual Structure Details
3411 @section Manual Structure Details
3412 @cindex manual structure
3414 The title page of the manual should state the version of the programs or
3415 packages documented in the manual. The Top node of the manual should
3416 also contain this information. If the manual is changing more
3417 frequently than or independent of the program, also state a version
3418 number for the manual in both of these places.
3420 Each program documented in the manual should have a node named
3421 @samp{@var{program} Invocation} or @samp{Invoking @var{program}}. This
3422 node (together with its subnodes, if any) should describe the program's
3423 command line arguments and how to run it (the sort of information people
3424 would look for in a man page). Start with an @samp{@@example}
3425 containing a template for all the options and arguments that the program
3428 Alternatively, put a menu item in some menu whose item name fits one of
3429 the above patterns. This identifies the node which that item points to
3430 as the node for this purpose, regardless of the node's actual name.
3432 The @samp{--usage} feature of the Info reader looks for such a node
3433 or menu item in order to find the relevant text, so it is essential
3434 for every Texinfo file to have one.
3436 If one manual describes several programs, it should have such a node for
3437 each program described in the manual.
3439 @node License for Manuals
3440 @section License for Manuals
3441 @cindex license for manuals
3443 Please use the GNU Free Documentation License for all GNU manuals that
3444 are more than a few pages long. Likewise for a collection of short
3445 documents---you only need one copy of the GNU FDL for the whole
3446 collection. For a single short document, you can use a very permissive
3447 non-copyleft license, to avoid taking up space with a long license.
3449 See @uref{http://www.gnu.org/copyleft/fdl-howto.html} for more explanation
3450 of how to employ the GFDL.
3452 Note that it is not obligatory to include a copy of the GNU GPL or GNU
3453 LGPL in a manual whose license is neither the GPL nor the LGPL. It can
3454 be a good idea to include the program's license in a large manual; in a
3455 short manual, whose size would be increased considerably by including
3456 the program's license, it is probably better not to include it.
3458 @node Manual Credits
3459 @section Manual Credits
3460 @cindex credits for manuals
3462 Please credit the principal human writers of the manual as the authors,
3463 on the title page of the manual. If a company sponsored the work, thank
3464 the company in a suitable place in the manual, but do not cite the
3465 company as an author.
3467 @node Printed Manuals
3468 @section Printed Manuals
3470 The FSF publishes some GNU manuals in printed form. To encourage sales
3471 of these manuals, the on-line versions of the manual should mention at
3472 the very start that the printed manual is available and should point at
3473 information for getting it---for instance, with a link to the page
3474 @url{http://www.gnu.org/order/order.html}. This should not be included
3475 in the printed manual, though, because there it is redundant.
3477 It is also useful to explain in the on-line forms of the manual how the
3478 user can print out the manual from the sources.
3481 @section The NEWS File
3482 @cindex @file{NEWS} file
3484 In addition to its manual, the package should have a file named
3485 @file{NEWS} which contains a list of user-visible changes worth
3486 mentioning. In each new release, add items to the front of the file and
3487 identify the version they pertain to. Don't discard old items; leave
3488 them in the file after the newer items. This way, a user upgrading from
3489 any previous version can see what is new.
3491 If the @file{NEWS} file gets very long, move some of the older items
3492 into a file named @file{ONEWS} and put a note at the end referring the
3496 @section Change Logs
3499 Keep a change log to describe all the changes made to program source
3500 files. The purpose of this is so that people investigating bugs in the
3501 future will know about the changes that might have introduced the bug.
3502 Often a new bug can be found by looking at what was recently changed.
3503 More importantly, change logs can help you eliminate conceptual
3504 inconsistencies between different parts of a program, by giving you a
3505 history of how the conflicting concepts arose and who they came from.
3508 * Change Log Concepts::
3509 * Style of Change Logs::
3511 * Conditional Changes::
3512 * Indicating the Part Changed::
3515 @node Change Log Concepts
3516 @subsection Change Log Concepts
3519 @cindex batch of changes
3520 You can think of the change log as a conceptual ``undo list'' which
3521 explains how earlier versions were different from the current version.
3522 People can see the current version; they don't need the change log to
3523 tell them what is in it. What they want from a change log is a clear
3524 explanation of how the earlier version differed. Each @dfn{entry} in
3525 a change log describes either an individual change or the smallest
3526 batch of changes that belong together, also known as a @dfn{change
3528 @cindex title, change log entry
3529 @cindex description, change log entry
3530 For later reference or for summarizing, sometimes it is useful to
3531 start the entry with a one-line description (sometimes called a
3532 @dfn{title}) to describe its overall purpose.
3534 In the past, we recommended not mentioning changes in non-software
3535 files (manuals, help files, media files, etc.)@: in change logs.
3536 However, we've been advised that it is a good idea to include them,
3537 for the sake of copyright records.
3539 The change log file is normally called @file{ChangeLog} and covers an
3540 entire directory. Each directory can have its own change log, or a
3541 directory can use the change log of its parent directory---it's up to
3544 Another alternative is to record change log information with a version
3545 control system such as RCS or CVS. This can be converted automatically
3546 to a @file{ChangeLog} file using @code{rcs2log}; in Emacs, the command
3547 @kbd{C-x v a} (@code{vc-update-change-log}) does the job.
3549 For changes to code, there's no need to describe the full purpose of
3550 the changes or how they work together. If you think that a change
3551 calls for explanation, you're probably right. Please do explain
3552 it---but please put the full explanation in comments in the code,
3553 where people will see it whenever they see the code. For example,
3554 ``New function'' is enough for the change log when you add a function,
3555 because there should be a comment before the function definition to
3556 explain what it does.
3558 For changes to files that do not support a comment syntax (e.g., media
3559 files), it is ok to include the full explanation in the change log file,
3560 after the title and before the list of individual changes.
3562 The easiest way to add an entry to @file{ChangeLog} is with the Emacs
3563 command @kbd{M-x add-change-log-entry}. An individual change should
3564 have an asterisk, the name of the changed file, and then in
3565 parentheses the name of the changed functions, variables or whatever,
3566 followed by a colon. Then describe the changes you made to that
3567 function or variable.
3570 @node Style of Change Logs
3571 @subsection Style of Change Logs
3572 @cindex change logs, style
3574 Here are some simple examples of change log entries, starting with the
3575 header line that says who made the change and when it was installed,
3576 followed by descriptions of specific changes. (These examples are
3577 drawn from Emacs and GCC.)
3580 1998-08-17 Richard Stallman <rms@@gnu.org>
3582 * register.el (insert-register): Return nil.
3583 (jump-to-register): Likewise.
3585 * sort.el (sort-subr): Return nil.
3587 * tex-mode.el (tex-bibtex-file, tex-file, tex-region):
3588 Restart the tex shell if process is gone or stopped.
3589 (tex-shell-running): New function.
3591 * expr.c (store_one_arg): Round size up for move_block_to_reg.
3592 (expand_call): Round up when emitting USE insns.
3593 * stmt.c (assign_parms): Round size up for move_block_from_reg.
3596 It's important to name the changed function or variable in full. Don't
3597 abbreviate function or variable names, and don't combine them.
3598 Subsequent maintainers will often search for a function name to find all
3599 the change log entries that pertain to it; if you abbreviate the name,
3600 they won't find it when they search.
3602 For example, some people are tempted to abbreviate groups of function
3603 names by writing @samp{* register.el (@{insert,jump-to@}-register)};
3604 this is not a good idea, since searching for @code{jump-to-register} or
3605 @code{insert-register} would not find that entry.
3607 Separate unrelated change log entries with blank lines. Don't put
3608 blank lines between individual changes of an entry. You can omit the
3609 file name and the asterisk when successive individual changes are in
3612 Break long lists of function names by closing continued lines with
3613 @samp{)}, rather than @samp{,}, and opening the continuation with
3614 @samp{(} as in this example:
3617 * keyboard.c (menu_bar_items, tool_bar_items)
3618 (Fexecute_extended_command): Deal with 'keymap' property.
3621 When you install someone else's changes, put the contributor's name in
3622 the change log entry rather than in the text of the entry. In other
3626 2002-07-14 John Doe <jdoe@@gnu.org>
3628 * sewing.c: Make it sew.
3635 2002-07-14 Usual Maintainer <usual@@gnu.org>
3637 * sewing.c: Make it sew. Patch by jdoe@@gnu.org.
3640 As for the date, that should be the date you applied the change.
3642 @node Simple Changes
3643 @subsection Simple Changes
3645 Certain simple kinds of changes don't need much detail in the change
3648 When you change the calling sequence of a function in a simple fashion,
3649 and you change all the callers of the function to use the new calling
3650 sequence, there is no need to make individual entries for all the
3651 callers that you changed. Just write in the entry for the function
3652 being called, ``All callers changed''---like this:
3655 * keyboard.c (Fcommand_execute): New arg SPECIAL.
3656 All callers changed.
3659 When you change just comments or doc strings, it is enough to write an
3660 entry for the file, without mentioning the functions. Just ``Doc
3661 fixes'' is enough for the change log.
3663 There's no technical need to make change log entries for documentation
3664 files. This is because documentation is not susceptible to bugs that
3665 are hard to fix. Documentation does not consist of parts that must
3666 interact in a precisely engineered fashion. To correct an error, you
3667 need not know the history of the erroneous passage; it is enough to
3668 compare what the documentation says with the way the program actually
3671 However, you should keep change logs for documentation files when the
3672 project gets copyright assignments from its contributors, so as to
3673 make the records of authorship more accurate.
3675 @node Conditional Changes
3676 @subsection Conditional Changes
3677 @cindex conditional changes, and change logs
3678 @cindex change logs, conditional changes
3680 Source files can often contain code that is conditional to build-time
3681 or static conditions. For example, C programs can contain
3682 compile-time @code{#if} conditionals; programs implemented in
3683 interpreted languages can contain module imports of function
3684 definitions that are only performed for certain versions of the
3685 interpreter; and Automake @file{Makefile.am} files can contain
3686 variable definitions or target declarations that are only to be
3687 considered if a configure-time Automake conditional is true.
3689 Many changes are conditional as well: sometimes you add a new variable,
3690 or function, or even a new program or library, which is entirely
3691 dependent on a build-time condition. It is useful to indicate
3692 in the change log the conditions for which a change applies.
3694 Our convention for indicating conditional changes is to use
3695 @emph{square brackets around the name of the condition}.
3697 Conditional changes can happen in numerous scenarios and with many
3698 variations, so here are some examples to help clarify. This first
3699 example describes changes in C, Perl, and Python files which are
3700 conditional but do not have an associated function or entity name:
3703 * xterm.c [SOLARIS2]: Include <string.h>.
3704 * FilePath.pm [$^O eq 'VMS']: Import the VMS::Feature module.
3705 * framework.py [sys.version_info < (2, 6)]: Make "with" statement
3706 available by importing it from __future__,
3707 to support also python 2.5.
3710 Our other examples will for simplicity be limited to C, as the minor
3711 changes necessary to adapt them to other languages should be
3714 Next, here is an entry describing a new definition which is entirely
3715 conditional: the C macro @code{FRAME_WINDOW_P} is defined (and used)
3716 only when the macro @code{HAVE_X_WINDOWS} is defined:
3719 * frame.h [HAVE_X_WINDOWS] (FRAME_WINDOW_P): Macro defined.
3722 Next, an entry for a change within the function @code{init_display},
3723 whose definition as a whole is unconditional, but the changes
3724 themselves are contained in a @samp{#ifdef HAVE_LIBNCURSES}
3728 * dispnew.c (init_display) [HAVE_LIBNCURSES]: If X, call tgetent.
3731 Finally, here is an entry for a change that takes effect only when
3732 a certain macro is @emph{not} defined:
3735 * host.c (gethostname) [!HAVE_SOCKETS]: Replace with winsock version.
3739 @node Indicating the Part Changed
3740 @subsection Indicating the Part Changed
3742 Indicate the part of a function which changed by using angle brackets
3743 enclosing an indication of what the changed part does. Here is an entry
3744 for a change in the part of the function @code{sh-while-getopts} that
3745 deals with @code{sh} commands:
3748 * progmodes/sh-script.el (sh-while-getopts) <sh>: Handle case that
3749 user-specified option string is empty.
3757 In the GNU project, man pages are secondary. It is not necessary or
3758 expected for every GNU program to have a man page, but some of them do.
3759 It's your choice whether to include a man page in your program.
3761 When you make this decision, consider that supporting a man page
3762 requires continual effort each time the program is changed. The time
3763 you spend on the man page is time taken away from more useful work.
3765 For a simple program which changes little, updating the man page may be
3766 a small job. Then there is little reason not to include a man page, if
3769 For a large program that changes a great deal, updating a man page may
3770 be a substantial burden. If a user offers to donate a man page, you may
3771 find this gift costly to accept. It may be better to refuse the man
3772 page unless the same person agrees to take full responsibility for
3773 maintaining it---so that you can wash your hands of it entirely. If
3774 this volunteer later ceases to do the job, then don't feel obliged to
3775 pick it up yourself; it may be better to withdraw the man page from the
3776 distribution until someone else agrees to update it.
3778 When a program changes only a little, you may feel that the
3779 discrepancies are small enough that the man page remains useful without
3780 updating. If so, put a prominent note near the beginning of the man
3781 page explaining that you don't maintain it and that the Texinfo manual
3782 is more authoritative. The note should say how to access the Texinfo
3785 Be sure that man pages include a copyright statement and free license.
3786 The simple all-permissive license is appropriate for simple man pages
3787 (@pxref{License Notices for Other Files,,,maintain,Information for GNU
3790 For long man pages, with enough explanation and documentation that
3791 they can be considered true manuals, use the GFDL (@pxref{License for
3794 Finally, the GNU help2man program
3795 (@uref{http://www.gnu.org/software/help2man/}) is one way to automate
3796 generation of a man page, in this case from @option{--help} output.
3797 This is sufficient in many cases.
3799 @node Reading other Manuals
3800 @section Reading other Manuals
3802 There may be non-free books or documentation files that describe the
3803 program you are documenting.
3805 It is ok to use these documents for reference, just as the author of a
3806 new algebra textbook can read other books on algebra. A large portion
3807 of any non-fiction book consists of facts, in this case facts about how
3808 a certain program works, and these facts are necessarily the same for
3809 everyone who writes about the subject. But be careful not to copy your
3810 outline structure, wording, tables or examples from preexisting non-free
3811 documentation. Copying from free documentation may be ok; please check
3812 with the FSF about the individual case.
3814 @node Managing Releases
3815 @chapter The Release Process
3818 Making a release is more than just bundling up your source files in a
3819 tar file and putting it up for FTP. You should set up your software so
3820 that it can be configured to run on a variety of systems. Your Makefile
3821 should conform to the GNU standards described below, and your directory
3822 layout should also conform to the standards discussed below. Doing so
3823 makes it easy to include your package into the larger framework of
3827 * Configuration:: How configuration of GNU packages should work.
3828 * Makefile Conventions:: Makefile conventions.
3829 * Releases:: Making releases
3833 @section How Configuration Should Work
3834 @cindex program configuration
3837 Each GNU distribution should come with a shell script named
3838 @code{configure}. This script is given arguments which describe the
3839 kind of machine and system you want to compile the program for.
3840 The @code{configure} script must record the configuration options so
3841 that they affect compilation.
3843 The description here is the specification of the interface for the
3844 @code{configure} script in GNU packages. Many packages implement it
3845 using GNU Autoconf (@pxref{Top,, Introduction, autoconf, Autoconf})
3846 and/or GNU Automake (@pxref{Top,, Introduction, automake, Automake}),
3847 but you do not have to use these tools. You can implement it any way
3848 you like; for instance, by making @code{configure} be a wrapper around
3849 a completely different configuration system.
3851 Another way for the @code{configure} script to operate is to make a
3852 link from a standard name such as @file{config.h} to the proper
3853 configuration file for the chosen system. If you use this technique,
3854 the distribution should @emph{not} contain a file named
3855 @file{config.h}. This is so that people won't be able to build the
3856 program without configuring it first.
3858 Another thing that @code{configure} can do is to edit the Makefile. If
3859 you do this, the distribution should @emph{not} contain a file named
3860 @file{Makefile}. Instead, it should include a file @file{Makefile.in} which
3861 contains the input used for editing. Once again, this is so that people
3862 won't be able to build the program without configuring it first.
3864 If @code{configure} does write the @file{Makefile}, then @file{Makefile}
3865 should have a target named @file{Makefile} which causes @code{configure}
3866 to be rerun, setting up the same configuration that was set up last
3867 time. The files that @code{configure} reads should be listed as
3868 dependencies of @file{Makefile}.
3870 All the files which are output from the @code{configure} script should
3871 have comments at the beginning explaining that they were generated
3872 automatically using @code{configure}. This is so that users won't think
3873 of trying to edit them by hand.
3875 The @code{configure} script should write a file named @file{config.status}
3876 which describes which configuration options were specified when the
3877 program was last configured. This file should be a shell script which,
3878 if run, will recreate the same configuration.
3880 The @code{configure} script should accept an option of the form
3881 @samp{--srcdir=@var{dirname}} to specify the directory where sources are found
3882 (if it is not the current directory). This makes it possible to build
3883 the program in a separate directory, so that the actual source directory
3886 If the user does not specify @samp{--srcdir}, then @code{configure} should
3887 check both @file{.} and @file{..} to see if it can find the sources. If
3888 it finds the sources in one of these places, it should use them from
3889 there. Otherwise, it should report that it cannot find the sources, and
3890 should exit with nonzero status.
3892 Usually the easy way to support @samp{--srcdir} is by editing a
3893 definition of @code{VPATH} into the Makefile. Some rules may need to
3894 refer explicitly to the specified source directory. To make this
3895 possible, @code{configure} can add to the Makefile a variable named
3896 @code{srcdir} whose value is precisely the specified directory.
3898 In addition, the @samp{configure} script should take options
3899 corresponding to most of the standard directory variables
3900 (@pxref{Directory Variables}). Here is the list:
3903 --prefix --exec-prefix --bindir --sbindir --libexecdir --sysconfdir
3904 --sharedstatedir --localstatedir --libdir --includedir --oldincludedir
3905 --datarootdir --datadir --infodir --localedir --mandir --docdir
3906 --htmldir --dvidir --pdfdir --psdir
3909 The @code{configure} script should also take an argument which specifies the
3910 type of system to build the program for. This argument should look like
3914 @var{cpu}-@var{company}-@var{system}
3917 For example, an Athlon-based GNU/Linux system might be
3918 @samp{i686-pc-linux-gnu}.
3920 The @code{configure} script needs to be able to decode all plausible
3921 alternatives for how to describe a machine. Thus,
3922 @samp{athlon-pc-gnu/linux} would be a valid alias. There is a shell
3924 @uref{http://git.savannah.gnu.org/@/gitweb/@/?p=config.git;a=blob_plain;f=config.sub;hb=HEAD,
3925 @file{config.sub}} that you can use as a subroutine to validate system
3926 types and canonicalize aliases.
3928 The @code{configure} script should also take the option
3929 @option{--build=@var{buildtype}}, which should be equivalent to a
3930 plain @var{buildtype} argument. For example, @samp{configure
3931 --build=i686-pc-linux-gnu} is equivalent to @samp{configure
3932 i686-pc-linux-gnu}. When the build type is not specified by an option
3933 or argument, the @code{configure} script should normally guess it using
3935 @uref{http://git.savannah.gnu.org/@/gitweb/@/?p=config.git;a=blob_plain;f=config.guess;hb=HEAD,
3936 @file{config.guess}}.
3938 @cindex optional features, configure-time
3939 Other options are permitted to specify in more detail the software
3940 or hardware present on the machine, to include or exclude optional parts
3941 of the package, or to adjust the name of some tools or arguments to them:
3944 @item --enable-@var{feature}@r{[}=@var{parameter}@r{]}
3945 Configure the package to build and install an optional user-level
3946 facility called @var{feature}. This allows users to choose which
3947 optional features to include. Giving an optional @var{parameter} of
3948 @samp{no} should omit @var{feature}, if it is built by default.
3950 No @samp{--enable} option should @strong{ever} cause one feature to
3951 replace another. No @samp{--enable} option should ever substitute one
3952 useful behavior for another useful behavior. The only proper use for
3953 @samp{--enable} is for questions of whether to build part of the program
3956 @item --with-@var{package}
3957 @c @r{[}=@var{parameter}@r{]}
3958 The package @var{package} will be installed, so configure this package
3959 to work with @var{package}.
3961 @c Giving an optional @var{parameter} of
3962 @c @samp{no} should omit @var{package}, if it is used by default.
3964 Possible values of @var{package} include
3965 @samp{gnu-as} (or @samp{gas}), @samp{gnu-ld}, @samp{gnu-libc},
3971 Do not use a @samp{--with} option to specify the file name to use to
3972 find certain files. That is outside the scope of what @samp{--with}
3975 @item @var{variable}=@var{value}
3976 Set the value of the variable @var{variable} to @var{value}. This is
3977 used to override the default values of commands or arguments in the
3978 build process. For example, the user could issue @samp{configure
3979 CFLAGS=-g CXXFLAGS=-g} to build with debugging information and without
3980 the default optimization.
3982 Specifying variables as arguments to @code{configure}, like this:
3986 is preferable to setting them in environment variables:
3990 as it helps to recreate the same configuration later with
3991 @file{config.status}. However, both methods should be supported.
3994 All @code{configure} scripts should accept all of the ``detail''
3995 options and the variable settings, whether or not they make any
3996 difference to the particular package at hand. In particular, they
3997 should accept any option that starts with @samp{--with-} or
3998 @samp{--enable-}. This is so users will be able to configure an
3999 entire GNU source tree at once with a single set of options.
4001 You will note that the categories @samp{--with-} and @samp{--enable-}
4002 are narrow: they @strong{do not} provide a place for any sort of option
4003 you might think of. That is deliberate. We want to limit the possible
4004 configuration options in GNU software. We do not want GNU programs to
4005 have idiosyncratic configuration options.
4007 Packages that perform part of the compilation process may support
4008 cross-compilation. In such a case, the host and target machines for the
4009 program may be different.
4011 The @code{configure} script should normally treat the specified type of
4012 system as both the host and the target, thus producing a program which
4013 works for the same type of machine that it runs on.
4015 To compile a program to run on a host type that differs from the build
4016 type, use the configure option @option{--host=@var{hosttype}}, where
4017 @var{hosttype} uses the same syntax as @var{buildtype}. The host type
4018 normally defaults to the build type.
4020 To configure a cross-compiler, cross-assembler, or what have you, you
4021 should specify a target different from the host, using the configure
4022 option @samp{--target=@var{targettype}}. The syntax for
4023 @var{targettype} is the same as for the host type. So the command would
4027 ./configure --host=@var{hosttype} --target=@var{targettype}
4030 The target type normally defaults to the host type.
4031 Programs for which cross-operation is not meaningful need not accept the
4032 @samp{--target} option, because configuring an entire operating system for
4033 cross-operation is not a meaningful operation.
4035 Some programs have ways of configuring themselves automatically. If
4036 your program is set up to do this, your @code{configure} script can simply
4037 ignore most of its arguments.
4039 @comment The makefile standards are in a separate file that is also
4040 @comment included by make.texinfo. Done by roland@gnu.ai.mit.edu on 1/6/93.
4041 @comment For this document, turn chapters into sections, etc.
4043 @include make-stds.texi
4047 @section Making Releases
4050 @cindex version numbers, for releases
4051 You should identify each release with a pair of version numbers, a
4052 major version and a minor. We have no objection to using more than
4053 two numbers, but it is very unlikely that you really need them.
4055 Package the distribution of @code{Foo version 69.96} up in a gzipped tar
4056 file with the name @file{foo-69.96.tar.gz}. It should unpack into a
4057 subdirectory named @file{foo-69.96}.
4059 Building and installing the program should never modify any of the files
4060 contained in the distribution. This means that all the files that form
4061 part of the program in any way must be classified into @dfn{source
4062 files} and @dfn{non-source files}. Source files are written by humans
4063 and never changed automatically; non-source files are produced from
4064 source files by programs under the control of the Makefile.
4066 @cindex @file{README} file
4067 The distribution should contain a file named @file{README} with a
4068 general overview of the package:
4071 @item the name of the package;
4073 @item the version number of the package, or refer to where in the
4074 package the version can be found;
4076 @item a general description of what the package does;
4078 @item a reference to the file @file{INSTALL}, which
4079 should in turn contain an explanation of the installation procedure;
4081 @item a brief explanation of any unusual top-level directories or
4082 files, or other hints for readers to find their way around the source;
4084 @item a reference to the file which contains the copying conditions.
4085 The GNU GPL, if used, should be in a file called @file{COPYING}. If
4086 the GNU LGPL is used, it should be in a file called
4087 @file{COPYING.LESSER}.
4090 Naturally, all the source files must be in the distribution. It is
4091 okay to include non-source files in the distribution along with the
4092 source files they are generated from, provided they are up-to-date
4093 with the source they are made from, and machine-independent, so that
4094 normal building of the distribution will never modify them. We
4095 commonly include non-source files produced by Autoconf, Automake,
4096 Bison, @code{flex}, @TeX{}, and @code{makeinfo}; this helps avoid
4097 unnecessary dependencies between our distributions, so that users can
4098 install whichever versions of whichever packages they like. Do not
4099 induce new dependencies on other software lightly.
4101 Non-source files that might actually be modified by building and
4102 installing the program should @strong{never} be included in the
4103 distribution. So if you do distribute non-source files, always make
4104 sure they are up to date when you make a new distribution.
4106 Make sure that all the files in the distribution are world-readable, and
4107 that directories are world-readable and world-searchable (octal mode 755).
4108 We used to recommend that all directories in the distribution also be
4109 world-writable (octal mode 777), because ancient versions of @code{tar}
4110 would otherwise not cope when extracting the archive as an unprivileged
4111 user. That can easily lead to security issues when creating the archive,
4112 however, so now we recommend against that.
4114 Don't include any symbolic links in the distribution itself. If the tar
4115 file contains symbolic links, then people cannot even unpack it on
4116 systems that don't support symbolic links. Also, don't use multiple
4117 names for one file in different directories, because certain file
4118 systems cannot handle this and that prevents unpacking the
4121 Try to make sure that all the file names will be unique on MS-DOS. A
4122 name on MS-DOS consists of up to 8 characters, optionally followed by a
4123 period and up to three characters. MS-DOS will truncate extra
4124 characters both before and after the period. Thus,
4125 @file{foobarhacker.c} and @file{foobarhacker.o} are not ambiguous; they
4126 are truncated to @file{foobarha.c} and @file{foobarha.o}, which are
4129 @cindex @file{texinfo.tex}, in a distribution
4130 Include in your distribution a copy of the @file{texinfo.tex} you used
4131 to test print any @file{*.texinfo} or @file{*.texi} files.
4133 Likewise, if your program uses small GNU software packages like regex,
4134 getopt, obstack, or termcap, include them in the distribution file.
4135 Leaving them out would make the distribution file a little smaller at
4136 the expense of possible inconvenience to a user who doesn't know what
4140 @chapter References to Non-Free Software and Documentation
4141 @cindex references to non-free material
4143 A GNU program should not recommend, promote, or grant legitimacy to
4144 the use of any non-free program. Proprietary software is a social and
4145 ethical problem, and our aim is to put an end to that problem. We
4146 can't stop some people from writing proprietary programs, or stop
4147 other people from using them, but we can and should refuse to
4148 advertise them to new potential customers, or to give the public the
4149 idea that their existence is ethical.
4151 The GNU definition of free software is found on the GNU web site at
4152 @url{http://www.gnu.org/@/philosophy/@/free-sw.html}, and the definition
4153 of free documentation is found at
4154 @url{http://www.gnu.org/@/philosophy/@/free-doc.html}. The terms ``free''
4155 and ``non-free'', used in this document, refer to those definitions.
4157 A list of important licenses and whether they qualify as free is in
4158 @url{http://www.gnu.org/@/licenses/@/license-list.html}. If it is not
4159 clear whether a license qualifies as free, please ask the GNU Project
4160 by writing to @email{licensing@@gnu.org}. We will answer, and if the
4161 license is an important one, we will add it to the list.
4163 When a non-free program or system is well known, you can mention it in
4164 passing---that is harmless, since users who might want to use it
4165 probably already know about it. For instance, it is fine to explain
4166 how to build your package on top of some widely used non-free
4167 operating system, or how to use it together with some widely used
4170 However, you should give only the necessary information to help those
4171 who already use the non-free program to use your program with
4172 it---don't give, or refer to, any further information about the
4173 proprietary program, and don't imply that the proprietary program
4174 enhances your program, or that its existence is in any way a good
4175 thing. The goal should be that people already using the proprietary
4176 program will get the advice they need about how to use your free
4177 program with it, while people who don't already use the proprietary
4178 program will not see anything likely to lead them to take an interest
4181 If a non-free program or system is obscure in your program's domain,
4182 your program should not mention or support it at all, since doing so
4183 would tend to popularize the non-free program more than it popularizes
4184 your program. (You cannot hope to find many additional users for your
4185 program among the users of Foobar, if the existence of Foobar is not
4186 generally known among people who might want to use your program.)
4188 Sometimes a program is free software in itself but depends on a
4189 non-free platform in order to run. For instance, many Java programs
4190 depend on some non-free Java libraries. To recommend or promote such
4191 a program is to promote the other programs it needs. This is why we
4192 are careful about listing Java programs in the Free Software
4193 Directory: we don't want to promote the non-free Java libraries.
4195 We hope this particular problem with Java will be gone by and by, as
4196 we replace the remaining non-free standard Java libraries with free
4197 software, but the general principle will remain the same: don't
4198 recommend, promote or legitimize programs that depend on non-free
4201 Some free programs strongly encourage the use of non-free software. A
4202 typical example is @command{mplayer}. It is free software in itself,
4203 and the free code can handle some kinds of files. However,
4204 @command{mplayer} recommends use of non-free codecs for other kinds of
4205 files, and users that install @command{mplayer} are very likely to
4206 install those codecs along with it. To recommend @command{mplayer}
4207 is, in effect, to promote use of the non-free codecs.
4209 Thus, you should not recommend programs that strongly encourage the
4210 use of non-free software. This is why we do not list
4211 @command{mplayer} in the Free Software Directory.
4213 A GNU package should not refer the user to any non-free documentation
4214 for free software. Free documentation that can be included in free
4215 operating systems is essential for completing the GNU system, or any
4216 free operating system, so encouraging it is a priority; to recommend
4217 use of documentation that we are not allowed to include undermines the
4218 impetus for the community to produce documentation that we can
4219 include. So GNU packages should never recommend non-free
4222 By contrast, it is ok to refer to journal articles and textbooks in
4223 the comments of a program for explanation of how it functions, even
4224 though they are non-free. This is because we don't include such
4225 things in the GNU system even if they are free---they are outside the
4226 scope of what a software distribution needs to include.
4228 Referring to a web site that describes or recommends a non-free
4229 program is promoting that program, so please do not make links to (or
4230 mention by name) web sites that contain such material. This policy is
4231 relevant particularly for the web pages for a GNU package.
4233 Following links from nearly any web site can lead eventually to
4234 non-free software; this is inherent in the nature of the web. So it
4235 makes no sense to criticize a site for having such links. As long as
4236 the site does not itself recommend a non-free program, there is no
4237 need to consider the question of the sites that it links to for other
4240 Thus, for example, you should not refer to AT&T's web site if that
4241 recommends AT&T's non-free software packages; you should not refer to
4242 a site that links to AT&T's site presenting it as a place to get some
4243 non-free program, because that link recommends and legitimizes the
4244 non-free program. However, that a site contains a link to AT&T's web
4245 site for some other purpose (such as long-distance telephone service)
4246 is not an objection against it.
4248 @node GNU Free Documentation License
4249 @appendix GNU Free Documentation License
4251 @cindex FDL, GNU Free Documentation License
4261 eval: (add-hook 'write-file-hooks 'time-stamp)
4262 time-stamp-start: "@set lastupdate "
4264 time-stamp-format: "%:b %:d, %:y"
4265 compile-command: "cd work.s && make"