@setfilename standards.info
@settitle GNU Coding Standards
@c This date is automagically updated when you save this file:
-@set lastupdate April 8, 2009
+@set lastupdate March 28, 2011
@c %**end of header
@dircategory GNU organization
The GNU coding standards, last updated @value{lastupdate}.
Copyright @copyright{} 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999,
-2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009 Free Software
-Foundation, Inc.
+2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011
+Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
@contents
@ifnottex
-@node Top, Preface, (dir), (dir)
-@top Version
+@node Top
+@top GNU Coding Standards
@insertcopying
@end ifnottex
even if you write in another programming language. The rules often
state reasons for writing in a certain way.
-This release of the GNU Coding Standards was last updated
-@value{lastupdate}.
-
@cindex where to obtain @code{standards.texi}
@cindex downloading this manual
If you did not obtain this file directly from the GNU project and
different formats, including the Texinfo source, PDF, HTML, DVI, plain
text, and more, at: @uref{http://www.gnu.org/prep/standards/}.
-Corrections or suggestions for this document should be sent to
-@email{bug-standards@@gnu.org}. If you make a suggestion, please include a
-suggested new wording for it; our time is limited. We prefer a context
-diff to the @file{standards.texi} or @file{make-stds.texi} files, but if
-you don't have those files, please mail your suggestion anyway.
+If you are maintaining an official GNU package, in addition to this
+document, please read and follow the GNU maintainer information
+(@pxref{Top, , Contents, maintain, Information for Maintainers of GNU
+Software}).
+
+@cindex @code{gnustandards-commit@@gnu.org} mailing list
+If you want to receive diffs for every change to these GNU documents,
+join the mailing list @code{gnustandards-commit@@gnu.org}, via the web
+interface at
+@url{http://lists.gnu.org/mailman/listinfo/gnustandards-commit}.
+Archives are also available there.
+
+@cindex @code{bug-standards@@gnu.org} email address
+@cindex Savannah repository for gnustandards
+@cindex gnustandards project repository
+Please send corrections or suggestions for this document to
+@email{bug-standards@@gnu.org}. If you make a suggestion, please
+include a suggested new wording for it, to help us consider the
+suggestion efficiently. We prefer a context diff to the Texinfo
+source, but if that's difficult for you, you can make a context diff
+for some other version of this document, or propose it in any way that
+makes it clear. The source repository for this document can be found
+at @url{http://savannah.gnu.org/projects/gnustandards}.
These standards cover the minimum of what is important when writing a
GNU package. Likely, the need for additional standards will come up.
coding standards for a trivial program.
@uref{http://www.gnu.org/software/hello/hello.html}.
+This release of the GNU Coding Standards was last updated
+@value{lastupdate}.
+
+
@node Legal Issues
@chapter Keeping Free Software Free
@cindex legal aspects
GCC developers many hours, or even days, per year.
In the case of function-like macros like @code{REVERSIBLE_CC_MODE} in
-GCC which cannot be simply used in @code{if( ...)} statements, there is
+GCC which cannot be simply used in @code{if (...)} statements, there is
an easy workaround. Simply introduce another macro
@code{HAS_REVERSIBLE_CC_MODE} as in the following example:
* User Interfaces:: Standards about interfaces generally.
* Graphical Interfaces:: Standards for graphical interfaces.
* Command-Line Interfaces:: Standards for command line interfaces.
+* Dynamic Plug-In Interfaces:: Standards for dynamic plug-in interfaces.
* Option Table:: Table of long options.
* OID Allocations:: Table of OID slots for GNU.
* Memory Usage:: When and how to care about memory needs.
avoid this problem by creating temporary files in this manner:
@example
-fd = open(filename, O_WRONLY | O_CREAT | O_EXCL, 0600);
+fd = open (filename, O_WRONLY | O_CREAT | O_EXCL, 0600);
@end example
@noindent
-or by using the @code{mkstemps} function from libiberty.
+or by using the @code{mkstemps} function from Gnulib
+(@pxref{mkstemps,,, gnulib, Gnulib}).
+
+In bash, use @code{set -C} (long name @code{noclobber}) to avoid this
+problem. In addition, the @code{mktemp} utility is a more general
+solution for creating temporary files from shell scripts
+(@pxref{mktemp invocation,,, coreutils, GNU Coreutils}).
-In bash, use @code{set -C} to avoid this problem.
@node Libraries
@section Library Behavior
@node Graphical Interfaces
@section Standards for Graphical Interfaces
@cindex graphical user interface
+@cindex interface styles
+@cindex user interface styles
-@cindex gtk+
+@cindex GTK+
When you write a program that provides a graphical user interface,
-please make it work with X Windows and the GTK+ toolkit unless the
-functionality specifically requires some alternative (for example,
-``displaying jpeg images while in console mode'').
+please make it work with the X Window System and the GTK+ toolkit
+unless the functionality specifically requires some alternative (for
+example, ``displaying jpeg images while in console mode'').
In addition, please provide a command-line interface to control the
functionality. (In many cases, the graphical user interface can be a
separate program which invokes the command-line program.) This is
so that the same jobs can be done from scripts.
-@cindex corba
-@cindex gnome
-Please also consider providing a CORBA interface (for use from GNOME), a
-library interface (for use from C), and perhaps a keyboard-driven
-console interface (for use by users from console mode). Once you are
-doing the work to provide the functionality and the graphical interface,
-these won't be much extra work.
+@cindex CORBA
+@cindex GNOME
+@cindex D-bus
+@cindex keyboard interface
+@cindex library interface
+Please also consider providing a D-bus interface for use from other
+running programs, such as within GNOME. (GNOME used to use CORBA
+for this, but that is being phased out.) In addition, consider
+providing a library interface (for use from C), and perhaps a
+keyboard-driven console interface (for use by users from console
+mode). Once you are doing the work to provide the functionality and
+the graphical interface, these won't be much extra work.
@node Command-Line Interfaces
@item LGPL
GNU Lesser General Public License, @url{http://www.gnu.org/@/licenses/@/lgpl.html}.
-@item GPL/Guile
-GNU GPL with the exception for Guile; for example, GPLv3+/Guile means
-the GNU GPL version 3 or later, with the extra exception for Guile.
-
@item GPL/Ada
GNU GPL with the exception for Ada.
The license for Python, @url{http://www.python.org/@/2.0.1/@/license.html}.
@item RBSD
-The revised (3-clause) BSD, compatible with the GNU GPL,
+The revised (3-clause) BSD, compatible with the GNU GPL,@*
@url{http://www.xfree86.org/@/3.3.6/@/COPYRIGHT2.html#5}.
@item X11
The simple non-copyleft license used for most versions of the X Window
-system, @url{http://www.xfree86.org/@/3.3.6/@/COPYRIGHT2.html#3}.
+System, @url{http://www.xfree86.org/@/3.3.6/@/COPYRIGHT2.html#3}.
@item Zlib
The license for Zlib, @url{http://www.gzip.org/@/zlib/@/zlib_license.html}.
It is ok to mention other appropriate mailing lists and web pages.
+@node Dynamic Plug-In Interfaces
+@section Standards for Dynamic Plug-in Interfaces
+@cindex plug-ins
+@cindex dynamic plug-ins
+
+Another aspect of keeping free programs free is encouraging
+development of free plug-ins, and discouraging development of
+proprietary plug-ins. Many GNU programs will not have anything like
+plug-ins at all, but those that do should follow these
+practices.
+
+First, the general plug-in architecture design should closely tie the
+plug-in to the original code, such that the plug-in and the base
+program are parts of one extended program. For GCC, for example,
+plug-ins receive and modify GCC's internal data structures, and so
+clearly form an extended program with the base GCC.
+
+@vindex plugin_is_GPL_compatible
+Second, you should require plug-in developers to affirm that their
+plug-ins are released under an appropriate license. This should be
+enforced with a simple programmatic check. For GCC, again for
+example, a plug-in must define the global symbol
+@code{plugin_is_GPL_compatible}, thus asserting that the plug-in is
+released under a GPL-compatible license (@pxref{Plugins,, Plugins,
+gccint, GCC Internals}).
+
+By adding this check to your program you are not creating a new legal
+requirement. The GPL itself requires plug-ins to be free software,
+licensed compatibly. As long as you have followed the first rule above
+to keep plug-ins closely tied to your original program, the GPL and AGPL
+already require those plug-ins to be released under a compatible
+license. The symbol definition in the plug-in---or whatever equivalent
+works best in your program---makes it harder for anyone who might
+distribute proprietary plug-ins to legally defend themselves. If a case
+about this got to court, we can point to that symbol as evidence that
+the plug-in developer understood that the license had this requirement.
+
+
@node Option Table
@section Table of Long Options
@cindex long option names
If your program creates complicated data structures, just make them in
memory and give a fatal error if @code{malloc} returns zero.
+@pindex valgrind
+@cindex memory leak
+Memory leak detectors such as @command{valgrind} can be useful, but
+don't complicate a program merely to avoid their false alarms. For
+example, if memory is used until just before a process exits, don't
+free it simply to silence a leak detector.
+
@node File Usage
@section File Usage
@cindex file usage
If you want to do this, then do. The compiler should be your servant,
not your master.
+@pindex clang
+@pindex lint
+Don't make the program ugly just to placate static analysis tools such
+as @command{lint}, @command{clang}, and GCC with extra warnings
+options such as @option{-Wconversion} and @option{-Wundef}. These
+tools can help find bugs and unclear code, but they can also generate
+so many false alarms that that it hurts readability to silence them
+with unnecessary casts, wrappers, and other complications. For
+example, please don't insert casts to @code{void} or calls to
+do-nothing functions merely to pacify a lint checker.
+
Declarations of external functions and functions to appear later in the
source file should all go in one place near the beginning of the file
(somewhere before the first function definition in the file), or else
all its uses. This makes the program even cleaner.
Don't use local variables or parameters that shadow global identifiers.
+GCC's @samp{-Wshadow} option can detect this problem.
@cindex multiple variables in a line
Don't declare multiple variables in one declaration that spans lines.
fatal ("virtual memory exhausted");
@end example
-@pindex lint
-Don't make the program ugly to placate @code{lint}. Please don't insert any
-casts to @code{void}. Zero without a cast is perfectly fine as a null
-pointer constant, except when calling a varargs function.
+This example uses zero without a cast as a null pointer constant.
+This is perfectly fine, except that a cast is needed when calling a
+varargs function or when using @code{sizeof}.
@node Names
@section Naming Variables, Functions, and Files
programs. @code{doschk} also reports file names longer than 14
characters.
+
@node System Portability
@section Portability between System Types
@cindex portability, between system types
@end example
1989 Standard C requires this to work, and we know of only one
-counterexample: 64-bit programs on Microsoft Windows. We will
-leave it to those who want to port GNU programs to that environment
-to figure out how to do it.
+counterexample: 64-bit programs on Microsoft Windows. We will leave
+it to those who want to port GNU programs to that environment to
+figure out how to do it.
Predefined file-size types like @code{off_t} are an exception: they are
longer than @code{long} on many platforms, so code like the above won't
@}
@end example
-It used to be ok to not worry about the difference between pointers
-and integers when passing arguments to functions. However, on most
-modern 64-bit machines pointers are wider than @code{int}.
-Conversely, integer types like @code{long long int} and @code{off_t}
-are wider than pointers on most modern 32-bit machines. Hence it's
-often better nowadays to use prototypes to define functions whose
-argument types are not trivial.
-
-In particular, if functions accept varying argument counts or types
-they should be declared using prototypes containing @samp{...} and
-defined using @file{stdarg.h}. For an example of this, please see the
-@uref{http://www.gnu.org/software/gnulib/, Gnulib} error module, which
-declares and defines the following function:
-
-@example
-/* Print a message with `fprintf (stderr, FORMAT, ...)';
- if ERRNUM is nonzero, follow it with ": " and strerror (ERRNUM).
- If STATUS is nonzero, terminate the program with `exit (STATUS)'. */
-
-void error (int status, int errnum, const char *format, ...);
-@end example
-
-A simple way to use the Gnulib error module is to obtain the two
-source files @file{error.c} and @file{error.h} from the Gnulib library
-source code repository at
-@uref{http://git.savannah.gnu.org/@/gitweb/@/?p=gnulib.git}.
-Here's a sample use:
-
-@example
-#include "error.h"
-#include <errno.h>
-#include <stdio.h>
-
-char *program_name = "myprogram";
-
-FILE *
-xfopen (char const *name)
-@{
- FILE *fp = fopen (name, "r");
- if (! fp)
- error (1, errno, "cannot read %s", name);
- return fp;
-@}
-@end example
-
@cindex casting pointers to integers
Avoid casting pointers to integers if you can. Such casts greatly
reduce portability, and in most programs they are easy to avoid. In the
normal range of addresses you can get from @code{malloc} starts far away
from zero.
+
@node System Functions
@section Calling System Functions
+
+@cindex C library functions, and portability
+@cindex POSIX functions, and portability
@cindex library functions, and portability
@cindex portability, and library functions
-C implementations differ substantially. Standard C reduces but does
-not eliminate the incompatibilities; meanwhile, many GNU packages still
-support pre-standard compilers because this is not hard to do. This
-chapter gives recommendations for how to use the more-or-less standard C
-library functions to avoid unnecessary loss of portability.
-
-@itemize @bullet
-@item
-Don't use the return value of @code{sprintf}. It returns the number of
-characters written on some systems, but not on all systems.
-
-@item
-Be aware that @code{vfprintf} is not always available.
-
-@item
-@code{main} should be declared to return type @code{int}. It should
-terminate either by calling @code{exit} or by returning the integer
-status code; make sure it cannot ever return an undefined value.
-
-@cindex declaration for system functions
-@item
-Don't declare system functions explicitly.
-
-Almost any declaration for a system function is wrong on some system.
-To minimize conflicts, leave it to the system header files to declare
-system functions. If the headers don't declare a function, let it
-remain undeclared.
-
-While it may seem unclean to use a function without declaring it, in
-practice this works fine for most system library functions on the
-systems where this really happens; thus, the disadvantage is only
-theoretical. By contrast, actual declarations have frequently caused
-actual conflicts.
-
-@item
-If you must declare a system function, don't specify the argument types.
-Use an old-style declaration, not a Standard C prototype. The more you
-specify about the function, the more likely a conflict.
-
-@item
-In particular, don't unconditionally declare @code{malloc} or
-@code{realloc}.
-
-Most GNU programs use those functions just once, in functions
-conventionally named @code{xmalloc} and @code{xrealloc}. These
-functions call @code{malloc} and @code{realloc}, respectively, and
-check the results.
-
-Because @code{xmalloc} and @code{xrealloc} are defined in your program,
-you can declare them in other files without any risk of type conflict.
-
-On most systems, @code{int} is the same length as a pointer; thus, the
-calls to @code{malloc} and @code{realloc} work fine. For the few
-exceptional systems (mostly 64-bit machines), you can use
-@strong{conditionalized} declarations of @code{malloc} and
-@code{realloc}---or put these declarations in configuration files
-specific to those systems.
-
-@cindex string library functions
-@item
-The string functions require special treatment. Some Unix systems have
-a header file @file{string.h}; others have @file{strings.h}. Neither
-file name is portable. There are two things you can do: use Autoconf to
-figure out which file to include, or don't include either file.
-
-@item
-If you don't include either strings file, you can't get declarations for
-the string functions from the header file in the usual way.
-
-That causes less of a problem than you might think. The newer standard
-string functions should be avoided anyway because many systems still
-don't support them. The string functions you can use are these:
-
-@example
-strcpy strncpy strcat strncat
-strlen strcmp strncmp
-strchr strrchr
-@end example
-
-The copy and concatenate functions work fine without a declaration as
-long as you don't use their values. Using their values without a
-declaration fails on systems where the width of a pointer differs from
-the width of @code{int}, and perhaps in other cases. It is trivial to
-avoid using their values, so do that.
-
-The compare functions and @code{strlen} work fine without a declaration
-on most systems, possibly all the ones that GNU software runs on.
-You may find it necessary to declare them @strong{conditionally} on a
-few systems.
-
-The search functions must be declared to return @code{char *}. Luckily,
-there is no variation in the data type they return. But there is
-variation in their names. Some systems give these functions the names
-@code{index} and @code{rindex}; other systems use the names
-@code{strchr} and @code{strrchr}. Some systems support both pairs of
-names, but neither pair works on all systems.
-
-You should pick a single pair of names and use it throughout your
-program. (Nowadays, it is better to choose @code{strchr} and
-@code{strrchr} for new programs, since those are the standard
-names.) Declare both of those names as functions returning @code{char
-*}. On systems which don't support those names, define them as macros
-in terms of the other pair. For example, here is what to put at the
-beginning of your file (or in a header) if you want to use the names
-@code{strchr} and @code{strrchr} throughout:
-
-@example
-#ifndef HAVE_STRCHR
-#define strchr index
-#endif
-#ifndef HAVE_STRRCHR
-#define strrchr rindex
-#endif
+Historically, C implementations differed substantially, and many
+systems lacked a full implementation of ANSI/ISO C89. Nowadays,
+however, very few systems lack a C89 compiler and GNU C supports
+almost all of C99. Similarly, most systems implement POSIX.1-1993
+libraries and tools, and many have POSIX.1-2001.
+
+Hence, there is little reason to support old C or non-POSIX systems,
+and you may want to take advantage of C99 and POSIX-1.2001 to write
+clearer, more portable, or faster code. You should use standard
+interfaces where possible; but if GNU extensions make your program
+more maintainable, powerful, or otherwise better, don't hesitate to
+use them. In any case, don't make your own declaration of system
+functions; that's a recipe for conflict.
+
+Despite the standards, nearly every library function has some sort of
+portability issue on some system or another. Here are some examples:
+
+@table @code
+@item open
+Names with trailing @code{/}'s are mishandled on many platforms.
+
+@item printf
+@code{long double} may be unimplemented; floating values Infinity and
+NaN are often mishandled; output for large precisions may be
+incorrect.
+
+@item readlink
+May return @code{int} instead of @code{ssize_t}.
+
+@item scanf
+On Windows, @code{errno} is not set on failure.
+@end table
-char *strchr ();
-char *strrchr ();
-@end example
-@end itemize
+@cindex Gnulib
+@uref{http://www.gnu.org/software/gnulib/, Gnulib} is a big help in
+this regard. Gnulib provides implementations of standard interfaces
+on many of the systems that lack them, including portable
+implementations of enhanced GNU interfaces, thereby making their use
+portable, and of POSIX-1.2008 interfaces, some of which are missing
+even on up-to-date GNU systems.
+
+@findex xmalloc, in Gnulib
+@findex error messages, in Gnulib
+@findex data structures, in Gnulib
+Gnulib also provides many useful non-standard interfaces; for example,
+C implementations of standard data structures (hash tables, binary
+trees), error-checking type-safe wrappers for memory allocation
+functions (@code{xmalloc}, @code{xrealloc}), and output of error
+messages.
+
+Gnulib integrates with GNU Autoconf and Automake to remove much of the
+burden of writing portable code from the programmer: Gnulib makes your
+configure script automatically determine what features are missing and
+use the Gnulib code to supply the missing pieces.
+
+The Gnulib and Autoconf manuals have extensive sections on
+portability: @ref{Top,, Introduction, gnulib, Gnulib} and
+@pxref{Portable C and C++,,, autoconf, Autoconf}. Please consult them
+for many more details.
-Here we assume that @code{HAVE_STRCHR} and @code{HAVE_STRRCHR} are
-macros defined in systems where the corresponding functions exist.
-One way to get them properly defined is to use Autoconf.
@node Internationalization
@section Internationalization
The change log file is normally called @file{ChangeLog} and covers an
entire directory. Each directory can have its own change log, or a
-directory can use the change log of its parent directory--it's up to
+directory can use the change log of its parent directory---it's up to
you.
Another alternative is to record change log information with a version
to a @file{ChangeLog} file using @code{rcs2log}; in Emacs, the command
@kbd{C-x v a} (@code{vc-update-change-log}) does the job.
-There's no need to describe the full purpose of the changes or how they
-work together. If you think that a change calls for explanation, you're
-probably right. Please do explain it---but please put the explanation
-in comments in the code, where people will see it whenever they see the
-code. For example, ``New function'' is enough for the change log when
-you add a function, because there should be a comment before the
-function definition to explain what it does.
+There's no need to describe the full purpose of the changes or how
+they work together. However, sometimes it is useful to write one line
+to describe the overall purpose of a change or a batch of changes. If
+you think that a change calls for explanation, you're probably right.
+Please do explain it---but please put the full explanation in comments
+in the code, where people will see it whenever they see the code. For
+example, ``New function'' is enough for the change log when you add a
+function, because there should be a comment before the function
+definition to explain what it does.
In the past, we recommended not mentioning changes in non-software
files (manuals, help files, etc.) in change logs. However, we've been
advised that it is a good idea to include them, for the sake of
copyright records.
-However, sometimes it is useful to write one line to describe the
-overall purpose of a batch of changes.
-
The easiest way to add an entry to @file{ChangeLog} is with the Emacs
command @kbd{M-x add-change-log-entry}. An entry should have an
asterisk, the name of the changed file, and then in parentheses the name
is more authoritative. The note should say how to access the Texinfo
documentation.
-Be sure that man pages include a copyright statement and free
-license. The simple all-permissive license is appropriate for simple
-man pages:
-
-@example
-Copying and distribution of this file, with or without modification,
-are permitted in any medium without royalty provided the copyright
-notice and this notice are preserved.
-@end example
+Be sure that man pages include a copyright statement and free license.
+The simple all-permissive license is appropriate for simple man pages
+(@pxref{License Notices for Other Files,,,maintain,Information for GNU
+Maintainers}).
For long man pages, with enough explanation and documentation that
they can be considered true manuals, use the GFDL (@pxref{License for
CC=gcc ./configure
@end example
as it helps to recreate the same configuration later with
-@file{config.status}.
+@file{config.status}. However, both methods should be supported.
@end table
All @code{configure} scripts should accept all of the ``detail''
The @file{README} file should also refer to the file which contains the
copying conditions. The GNU GPL, if used, should be in a file called
@file{COPYING}. If the GNU LGPL is used, it should be in a file called
-@file{COPYING.LIB}.
+@file{COPYING.LESSER}.
Naturally, all the source files must be in the distribution. It is okay
to include non-source files in the distribution, provided they are
distribution. So if you do distribute non-source files, always make
sure they are up to date when you make a new distribution.
-Make sure that the directory into which the distribution unpacks (as
-well as any subdirectories) are all world-writable (octal mode 777).
-This is so that old versions of @code{tar} which preserve the
-ownership and permissions of the files from the tar archive will be
-able to extract all the files even if the user is unprivileged.
-
-Make sure that all the files in the distribution are world-readable.
+Make sure that all the files in the distribution are world-readable, and
+that directories are world-readable and world-searchable (octal mode 755).
+We used to recommend that all directories in the distribution also be
+world-writable (octal mode 777), because ancient versions of @code{tar}
+would otherwise not cope when extracting the archive as an unprivileged
+user. That can easily lead to security issues when creating the archive,
+however, so now we recommend against that.
Don't include any symbolic links in the distribution itself. If the tar
file contains symbolic links, then people cannot even unpack it on
By contrast, it is ok to refer to journal articles and textbooks in
the comments of a program for explanation of how it functions, even
though they are non-free. This is because we don't include such
-things in the GNU system even they are free---they are outside the
+things in the GNU system even if they are free---they are outside the
scope of what a software distribution needs to include.
Referring to a web site that describes or recommends a non-free
time-stamp-start: "@set lastupdate "
time-stamp-end: "$"
time-stamp-format: "%:b %:d, %:y"
-compile-command: "make just-standards"
+compile-command: "cd work.s && make"
End:
projects / gnulib.git / blobdiff