mktime: add libc-config dependency

[gnulib.git] / doc / standards.texi

\input texinfo @c -*-texinfo-*-
@c %**start of header
@setfilename standards.info
@settitle GNU Coding Standards
@c This date is automagically updated when you save this file:
@set lastupdate August 3, 2018
@c %**end of header

@dircategory GNU organization
@direntry
* Standards: (standards).       GNU coding standards.
@end direntry

@c @setchapternewpage odd
@setchapternewpage off

@c Put everything in one index (arbitrarily chosen to be the concept index).
@syncodeindex fn cp
@syncodeindex ky cp
@syncodeindex pg cp
@syncodeindex vr cp

@c This is used by a cross ref in make-stds.texi
@set CODESTD  1

@copying
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, 2010,
2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018 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
any later version published by the Free Software Foundation; with no
Invariant Sections, no Front-Cover Texts, and no Back-Cover
Texts.  A copy of the license is included in the section entitled
``GNU Free Documentation License''.
@end copying

@titlepage
@title GNU Coding Standards
@author Richard Stallman, et al.
@author last updated @value{lastupdate}
@page
@vskip 0pt plus 1filll
@insertcopying
@end titlepage

@contents

@ifnottex
@node Top
@top GNU Coding Standards

@insertcopying
@end ifnottex

@menu
* Preface::                     About the GNU Coding Standards.
* Legal Issues::                Keeping free software free.
* Design Advice::               General program design.
* Program Behavior::            Program behavior for all programs
* Writing C::                   Making the best use of C.
* Documentation::               Documenting programs.
* Managing Releases::           The release process.
* References::                  Mentioning non-free software or documentation.
* GNU Free Documentation License::  Copying and sharing this manual.
* Index::

@end menu

@node Preface
@chapter About the GNU Coding Standards

The GNU Coding Standards were written by Richard Stallman and other GNU
Project volunteers.  Their purpose is to make the GNU system clean,
consistent, and easy to install.  This document can also be read as a
guide to writing portable, robust and reliable programs.  It focuses on
programs written in C, but many of the rules and principles are useful
even if you write in another programming language.  The rules often
state reasons for writing in a certain way.

@cindex where to obtain @code{standards.texi}
@cindex downloading this manual
If you did not obtain this file directly from the GNU project and
recently, please check for a newer version.  You can get the GNU
Coding Standards from the GNU web server in many
different formats, including the Texinfo source, PDF, HTML, DVI, plain
text, and more, at: @uref{https://www.gnu.org/prep/standards/}.

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{https://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{https://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.
Sometimes, you might suggest that such standards be added to this
document.  If you think your standards would be generally useful, please
do suggest them.

You should also set standards for your package on many questions not
addressed or not firmly specified here.  The most important point is to
be self-consistent---try to stick to the conventions you pick, and try
to document them as much as possible.  That way, your program will be
more maintainable by others.

The GNU Hello program serves as an example of how to follow the GNU
coding standards for a trivial program.
@uref{https://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

This chapter discusses how you can make sure that GNU software
avoids legal difficulties, and other related issues.

@menu
* Reading Non-Free Code::       Referring to proprietary programs.
* Contributions::               Accepting contributions.
* Trademarks::                  How we deal with trademark issues.
@end menu

@node Reading Non-Free Code
@section Referring to Proprietary Programs
@cindex proprietary programs
@cindex avoiding proprietary code

Don't in any circumstances refer to Unix source code for or during
your work on GNU!  (Or to any other proprietary programs.)

If you have a vague recollection of the internals of a Unix program,
this does not absolutely mean you can't write an imitation of it, but
do try to organize the imitation internally along different lines,
because this is likely to make the details of the Unix version
irrelevant and dissimilar to your results.

For example, Unix utilities were generally optimized to minimize
memory use; if you go for speed instead, your program will be very
different.  You could keep the entire input file in memory and scan it
there instead of using stdio.  Use a smarter algorithm discovered more
recently than the Unix program.  Eliminate use of temporary files.  Do
it in one pass instead of two (we did this in the assembler).

Or, on the contrary, emphasize simplicity instead of speed.  For some
applications, the speed of today's computers makes simpler algorithms
adequate.

Or go for generality.  For example, Unix programs often have static
tables or fixed-size strings, which make for arbitrary limits; use
dynamic allocation instead.  Make sure your program handles NULs and
other funny characters in the input files.  Add a programming language
for extensibility and write part of the program in that language.

Or turn some parts of the program into independently usable libraries.
Or use a simple garbage collector instead of tracking precisely when
to free memory, or use a new GNU facility such as obstacks.


@node Contributions
@section Accepting Contributions
@cindex legal papers
@cindex accepting contributions

If the program you are working on is copyrighted by the Free Software
Foundation, then when someone else sends you a piece of code to add to
the program, we need legal papers to use it---just as we asked you to
sign papers initially.  @emph{Each} person who makes a nontrivial
contribution to a program must sign some sort of legal papers in order
for us to have clear title to the program; the main author alone is not
enough.

So, before adding in any contributions from other people, please tell
us, so we can arrange to get the papers.  Then wait until we tell you
that we have received the signed papers, before you actually use the
contribution.

This applies both before you release the program and afterward.  If
you receive diffs to fix a bug, and they make significant changes, we
need legal papers for that change.

This also applies to comments and documentation files.  For copyright
law, comments and code are just text.  Copyright applies to all kinds of
text, so we need legal papers for all kinds.

We know it is frustrating to ask for legal papers; it's frustrating for
us as well.  But if you don't wait, you are going out on a limb---for
example, what if the contributor's employer won't sign a disclaimer?
You might have to take that code out again!

You don't need papers for changes of a few lines here or there, since
they are not significant for copyright purposes.  Also, you don't need
papers if all you get from the suggestion is some ideas, not actual code
which you use.  For example, if someone sent you one implementation, but
you write a different implementation of the same idea, you don't need to
get papers.

The very worst thing is if you forget to tell us about the other
contributor.  We could be very embarrassed in court some day as a
result.

We have more detailed advice for maintainers of GNU packages.  If you
have reached the stage of maintaining a GNU program (whether released
or not), please take a look: @pxref{Legal Matters,,, maintain,
Information for GNU Maintainers}.


@node Trademarks
@section Trademarks
@cindex trademarks

Please do not include any trademark acknowledgements in GNU software
packages or documentation.

Trademark acknowledgements are the statements that such-and-such is a
trademark of so-and-so.  The GNU Project has no objection to the basic
idea of trademarks, but these acknowledgements feel like kowtowing,
and there is no legal requirement for them, so we don't use them.

What is legally required, as regards other people's trademarks, is to
avoid using them in ways which a reader might reasonably understand as
naming or labeling our own programs or activities.  For example, since
``Objective C'' is (or at least was) a trademark, we made sure to say
that we provide a ``compiler for the Objective C language'' rather
than an ``Objective C compiler''.  The latter would have been meant as
a shorter way of saying the former, but it does not explicitly state
the relationship, so it could be misinterpreted as using ``Objective
C'' as a label for the compiler rather than for the language.

Please don't use ``win'' as an abbreviation for Microsoft Windows in
GNU software or documentation.  In hacker terminology, calling
something a ``win'' is a form of praise.  You're free to praise
Microsoft Windows on your own if you want, but please don't do so in
GNU packages.  Please write ``Windows'' in full, or abbreviate it to
``w.''  @xref{System Portability}.

@node Design Advice
@chapter General Program Design
@cindex program design

This chapter discusses some of the issues you should take into
account when designing your program.

@c                         Standard or ANSI C
@c
@c In 1989 the American National Standards Institute (ANSI) standardized
@c C   as  standard  X3.159-1989.    In  December   of  that   year  the
@c International Standards Organization ISO  adopted the ANSI C standard
@c making  minor changes.   In 1990  ANSI then  re-adopted  ISO standard
@c C. This version of C is known as either ANSI C or Standard C.

@c A major revision of the C Standard appeared in 1999.

@menu
* Source Language::             Which languages to use.
* Compatibility::               Compatibility with other implementations.
* Using Extensions::            Using non-standard features.
* Standard C::                  Using standard C features.
* Conditional Compilation::     Compiling code only if a conditional is true.
@end menu

@node Source Language
@section Which Languages to Use
@cindex programming languages

When you want to use a language that gets compiled and runs at high
speed, the best language to use is C.  C++ is ok too, but please don't
make heavy use of templates.  So is Java, if you compile it.

When highest efficiency is not required, other languages commonly used
in the free software community, such as Lisp, Scheme, Python, Ruby, and
Java, are OK too.  Scheme, as implemented by GNU@tie{}Guile, plays a
particular role in the GNU System: it is the preferred language to
extend programs written in C/C++, and also a fine language for a wide
range of applications.  The more GNU components use Guile and Scheme,
the more users are able to extend and combine them (@pxref{The Emacs
Thesis,,, guile, GNU Guile Reference Manual}).

Many programs are designed to be extensible: they include an interpreter
for a language that is higher level than C.  Often much of the program
is written in that language, too.  The Emacs editor pioneered this
technique.

@cindex Guile
@cindex GNOME and Guile
The standard extensibility interpreter for GNU software is Guile
(@uref{https://www.gnu.org/@/software/@/guile/}), which implements the
language Scheme (an especially clean and simple dialect of Lisp).
Guile also includes bindings for GTK+/GNOME, making it practical to
write modern GUI functionality within Guile.  We don't reject programs
written in other ``scripting languages'' such as Perl and Python, but
using Guile is the path that will lead to overall consistency of the
GNU system.


@node Compatibility
@section Compatibility with Other Implementations
@cindex compatibility with C and POSIX standards
@cindex C compatibility
@cindex POSIX compatibility

With occasional exceptions, utility programs and libraries for GNU
should be upward compatible with those in Berkeley Unix, and upward
compatible with Standard C if Standard C specifies their
behavior, and upward compatible with POSIX if POSIX specifies
their behavior.

When these standards conflict, it is useful to offer compatibility
modes for each of them.

@cindex options for compatibility
Standard C and POSIX prohibit many kinds of extensions.  Feel
free to make the extensions anyway, and include a @samp{--ansi},
@samp{--posix}, or @samp{--compatible} option to turn them off.
However, if the extension has a significant chance of breaking any real
programs or scripts, then it is not really upward compatible.  So you
should try to redesign its interface to make it upward compatible.

@cindex @code{POSIXLY_CORRECT}, environment variable
Many GNU programs suppress extensions that conflict with POSIX if the
environment variable @code{POSIXLY_CORRECT} is defined (even if it is
defined with a null value).  Please make your program recognize this
variable if appropriate.

When a feature is used only by users (not by programs or command
files), and it is done poorly in Unix, feel free to replace it
completely with something totally different and better.  (For example,
@code{vi} is replaced with Emacs.)  But it is nice to offer a compatible
feature as well.  (There is a free @code{vi} clone, so we offer it.)

Additional useful features are welcome regardless of whether
there is any precedent for them.

@node Using Extensions
@section Using Non-standard Features
@cindex non-standard extensions

Many GNU facilities that already exist support a number of convenient
extensions over the comparable Unix facilities.  Whether to use these
extensions in implementing your program is a difficult question.

On the one hand, using the extensions can make a cleaner program.
On the other hand, people will not be able to build the program
unless the other GNU tools are available.  This might cause the
program to work on fewer kinds of machines.

With some extensions, it might be easy to provide both alternatives.
For example, you can define functions with a ``keyword'' @code{INLINE}
and define that as a macro to expand into either @code{inline} or
nothing, depending on the compiler.

In general, perhaps it is best not to use the extensions if you can
straightforwardly do without them, but to use the extensions if they
are a big improvement.

An exception to this rule are the large, established programs (such as
Emacs) which run on a great variety of systems.  Using GNU extensions in
such programs would make many users unhappy, so we don't do that.

Another exception is for programs that are used as part of compilation:
anything that must be compiled with other compilers in order to
bootstrap the GNU compilation facilities.  If these require the GNU
compiler, then no one can compile them without having them installed
already.  That would be extremely troublesome in certain cases.

@node Standard C
@section Standard C and Pre-Standard C
@cindex ANSI C standard

1989 Standard C is widespread enough now that it is ok to use its
features in programs.  There is one exception: do not ever use the
``trigraph'' feature of Standard C.

The 1999 and 2011 editions of Standard C are not fully supported
on all platforms.  If you aim to support compilation by
compilers other than GCC, you should not require these C
features in your programs.  It is ok to use these features
conditionally when the compiler supports them.

If your program is only meant to compile with GCC, then you can
use these features if GCC supports them, when they give substantial
benefit.

However, it is easy to support pre-standard compilers in most programs,
so if you know how to do that, feel free.

@cindex function prototypes
To support pre-standard C, instead of writing function definitions in
standard prototype form,

@example
int
foo (int x, int y)
@dots{}
@end example

@noindent
write the definition in pre-standard style like this,

@example
int
foo (x, y)
     int x, y;
@dots{}
@end example

@noindent
and use a separate declaration to specify the argument prototype:

@example
int foo (int, int);
@end example

You need such a declaration anyway, in a header file, to get the benefit
of prototypes in all the files where the function is called.  And once
you have the declaration, you normally lose nothing by writing the
function definition in the pre-standard style.

This technique does not work for integer types narrower than @code{int}.
If you think of an argument as being of a type narrower than @code{int},
declare it as @code{int} instead.

There are a few special cases where this technique is hard to use.  For
example, if a function argument needs to hold the system type
@code{dev_t}, you run into trouble, because @code{dev_t} is shorter than
@code{int} on some machines; but you cannot use @code{int} instead,
because @code{dev_t} is wider than @code{int} on some machines.  There
is no type you can safely use on all machines in a non-standard
definition.  The only way to support non-standard C and pass such an
argument is to check the width of @code{dev_t} using Autoconf and choose
the argument type accordingly.  This may not be worth the trouble.

In order to support pre-standard compilers that do not recognize
prototypes, you may want to use a preprocessor macro like this:

@example
/* Declare the prototype for a general external function.  */
#if defined (__STDC__) || defined (WINDOWSNT)
#define P_(proto) proto
#else
#define P_(proto) ()
#endif
@end example

@node Conditional Compilation
@section Conditional Compilation

When supporting configuration options already known when building your
program we prefer using @code{if (... )} over conditional compilation,
as in the former case the compiler is able to perform more extensive
checking of all possible code paths.

For example, please write

@smallexample
  if (HAS_FOO)
    ...
  else
    ...
@end smallexample

@noindent
instead of:

@smallexample
  #ifdef HAS_FOO
    ...
  #else
    ...
  #endif
@end smallexample

A modern compiler such as GCC will generate exactly the same code in
both cases, and we have been using similar techniques with good success
in several projects.  Of course, the former method assumes that
@code{HAS_FOO} is defined as either 0 or 1.

While this is not a silver bullet solving all portability problems,
and is not always appropriate, following this policy would have saved
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
an easy workaround.  Simply introduce another macro
@code{HAS_REVERSIBLE_CC_MODE} as in the following example:

@smallexample
  #ifdef REVERSIBLE_CC_MODE
  #define HAS_REVERSIBLE_CC_MODE 1
  #else
  #define HAS_REVERSIBLE_CC_MODE 0
  #endif
@end smallexample

@node Program Behavior
@chapter Program Behavior for All Programs

This chapter describes conventions for writing robust
software.  It also describes general standards for error messages, the
command line interface, and how libraries should behave.

@menu
* Non-GNU Standards::           We consider standards such as POSIX;
                                  we don't "obey" them.
* Semantics::                   Writing robust programs.
* Libraries::                   Library behavior.
* Errors::                      Formatting error messages.
* 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.
* File Usage::                  Which files to use, and where.
@end menu

@node Non-GNU Standards
@section Non-GNU Standards

The GNU Project regards standards published by other organizations as
suggestions, not orders.  We consider those standards, but we do not
``obey'' them.  In developing a GNU program, you should implement
an outside standard's specifications when that makes the GNU system
better overall in an objective sense.  When it doesn't, you shouldn't.

In most cases, following published standards is convenient for
users---it means that their programs or scripts will work more
portably.  For instance, GCC implements nearly all the features of
Standard C as specified by that standard.  C program developers would
be unhappy if it did not.  And GNU utilities mostly follow
specifications of POSIX.2; shell script writers and users would be
unhappy if our programs were incompatible.

But we do not follow either of these specifications rigidly, and there
are specific points on which we decided not to follow them, so as to
make the GNU system better for users.

For instance, Standard C says that nearly all extensions to C are
prohibited.  How silly!  GCC implements many extensions, some of which
were later adopted as part of the standard.  If you want these
constructs to give an error message as ``required'' by the standard,
you must specify @samp{--pedantic}, which was implemented only so that
we can say ``GCC is a 100% implementation of the standard'', not
because there is any reason to actually use it.

POSIX.2 specifies that @samp{df} and @samp{du} must output sizes by
default in units of 512 bytes.  What users want is units of 1k, so
that is what we do by default.  If you want the ridiculous behavior
``required'' by POSIX, you must set the environment variable
@samp{POSIXLY_CORRECT} (which was originally going to be named
@samp{POSIX_ME_HARDER}).

GNU utilities also depart from the letter of the POSIX.2 specification
when they support long-named command-line options, and intermixing
options with ordinary arguments.  This minor incompatibility with
POSIX is never a problem in practice, and it is very useful.

In particular, don't reject a new feature, or remove an old one,
merely because a standard says it is ``forbidden'' or ``deprecated''.


@node Semantics
@section Writing Robust Programs

@cindex arbitrary limits on data
Avoid arbitrary limits on the length or number of @emph{any} data
structure, including file names, lines, files, and symbols, by allocating
all data structures dynamically.  In most Unix utilities, ``long lines
are silently truncated''.  This is not acceptable in a GNU utility.

@cindex @code{NUL} characters
@findex libiconv
Utilities reading files should not drop NUL characters, or any other
nonprinting characters.  Programs should work properly with multibyte
character encodings, such as UTF-8.  You can use libiconv to deal with
a range of encodings.

@cindex error messages
Check every system call for an error return, unless you know you wish
to ignore errors.  Include the system error text (from
@code{strerror}, or equivalent) in @emph{every} error message
resulting from a failing system call, as well as the name of the file
if any and the name of the utility.  Just ``cannot open foo.c'' or
``stat failed'' is not sufficient.

@cindex @code{malloc} return value
@cindex memory allocation failure
Check every call to @code{malloc} or @code{realloc} to see if it
returned @code{NULL}.  Check @code{realloc} even if you are making the
block smaller; in a system that rounds block sizes to a power of 2,
@code{realloc} may get a different block if you ask for less space.

You must expect @code{free} to alter the contents of the block that was
freed.  Anything you want to fetch from the block, you must fetch before
calling @code{free}.

If @code{malloc} fails in a noninteractive program, make that a fatal
error.  In an interactive program (one that reads commands from the
user), it is better to abort the command and return to the command
reader loop.  This allows the user to kill other processes to free up
virtual memory, and then try the command again.

@cindex command-line arguments, decoding
Use @code{getopt_long} to decode arguments, unless the argument syntax
makes this unreasonable.

When static storage is to be written in during program execution, use
explicit C code to initialize it.  This way, restarting the program
(without reloading it), or part of it, will reinitialize those
variables.  Reserve C initialized declarations for data that will not
be changed.
@c ADR: why?

Try to avoid low-level interfaces to obscure Unix data structures (such
as file directories, utmp, or the layout of kernel memory), since these
are less likely to work compatibly.  If you need to find all the files
in a directory, use @code{readdir} or some other high-level interface.
These are supported compatibly by GNU.

@cindex signal handling
The preferred signal handling facilities are the BSD variant of
@code{signal}, and the POSIX @code{sigaction} function; the
alternative USG @code{signal} interface is an inferior design.

Nowadays, using the POSIX signal functions may be the easiest way
to make a program portable.  If you use @code{signal}, then on GNU/Linux
systems running GNU libc version 1, you should include
@file{bsd/signal.h} instead of @file{signal.h}, so as to get BSD
behavior.  It is up to you whether to support systems where
@code{signal} has only the USG behavior, or give up on them.

@cindex impossible conditions
In error checks that detect ``impossible'' conditions, just abort.
There is usually no point in printing any message.  These checks
indicate the existence of bugs.  Whoever wants to fix the bugs will have
to read the source code and run a debugger.  So explain the problem with
comments in the source.  The relevant data will be in variables, which
are easy to examine with the debugger, so there is no point moving them
elsewhere.

Do not use a count of errors as the exit status for a program.
@emph{That does not work}, because exit status values are limited to 8
bits (0 through 255).  A single run of the program might have 256
errors; if you try to return 256 as the exit status, the parent process
will see 0 as the status, and it will appear that the program succeeded.

@cindex temporary files
@cindex @code{TMPDIR} environment variable
If you make temporary files, check the @code{TMPDIR} environment
variable; if that variable is defined, use the specified directory
instead of @file{/tmp}.

In addition, be aware that there is a possible security problem when
creating temporary files in world-writable directories.  In C, you can
avoid this problem by creating temporary files in this manner:

@example
fd = open (filename, O_WRONLY | O_CREAT | O_EXCL, 0600);
@end example

@noindent
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}).


@node Libraries
@section Library Behavior
@cindex libraries

Try to make library functions reentrant.  If they need to do dynamic
storage allocation, at least try to avoid any nonreentrancy aside from
that of @code{malloc} itself.

Here are certain name conventions for libraries, to avoid name
conflicts.

Choose a name prefix for the library, more than two characters long.
All external function and variable names should start with this
prefix.  In addition, there should only be one of these in any given
library member.  This usually means putting each one in a separate
source file.

An exception can be made when two external symbols are always used
together, so that no reasonable program could use one without the
other; then they can both go in the same file.

External symbols that are not documented entry points for the user
should have names beginning with @samp{_}.  The @samp{_} should be
followed by the chosen name prefix for the library, to prevent
collisions with other libraries.  These can go in the same files with
user entry points if you like.

Static functions and variables can be used as you like and need not
fit any naming convention.

@node Errors
@section Formatting Error Messages
@cindex formatting error messages
@cindex error messages, formatting

Error messages from compilers should look like this:

@example
@var{sourcefile}:@var{lineno}: @var{message}
@end example

@noindent
If you want to mention the column number, use one of these formats:

@example
@var{sourcefile}:@var{lineno}:@var{column}: @var{message}
@var{sourcefile}:@var{lineno}.@var{column}: @var{message}

@end example

@noindent
Line numbers should start from 1 at the beginning of the file, and
column numbers should start from 1 at the beginning of the line.
(Both of these conventions are chosen for compatibility.)  Calculate
column numbers assuming that space and all ASCII printing characters
have equal width, and assuming tab stops every 8 columns.  For
non-ASCII characters, Unicode character widths should be used when in
a UTF-8 locale; GNU libc and GNU gnulib provide suitable
@code{wcwidth} functions.

The error message can also give both the starting and ending positions
of the erroneous text.  There are several formats so that you can
avoid redundant information such as a duplicate line number.
Here are the possible formats:

@example
@var{sourcefile}:@var{line1}.@var{column1}-@var{line2}.@var{column2}: @var{message}
@var{sourcefile}:@var{line1}.@var{column1}-@var{column2}: @var{message}
@var{sourcefile}:@var{line1}-@var{line2}: @var{message}
@end example

@noindent
When an error is spread over several files, you can use this format:

@example
@var{file1}:@var{line1}.@var{column1}-@var{file2}:@var{line2}.@var{column2}: @var{message}
@end example

Error messages from other noninteractive programs should look like this:

@example
@var{program}:@var{sourcefile}:@var{lineno}: @var{message}
@end example

@noindent
when there is an appropriate source file, or like this:

@example
@var{program}: @var{message}
@end example

@noindent
when there is no relevant source file.

If you want to mention the column number, use this format:

@example
@var{program}:@var{sourcefile}:@var{lineno}:@var{column}: @var{message}
@end example

In an interactive program (one that is reading commands from a
terminal), it is better not to include the program name in an error
message.  The place to indicate which program is running is in the
prompt or with the screen layout.  (When the same program runs with
input from a source other than a terminal, it is not interactive and
would do best to print error messages using the noninteractive style.)

The string @var{message} should not begin with a capital letter when
it follows a program name and/or file name, because that isn't the
beginning of a sentence.  (The sentence conceptually starts at the
beginning of the line.)  Also, it should not end with a period.

Error messages from interactive programs, and other messages such as
usage messages, should start with a capital letter.  But they should not
end with a period.

@node User Interfaces
@section Standards for Interfaces Generally

@cindex program name and its behavior
@cindex behavior, dependent on program's name
Please don't make the behavior of a utility depend on the name used
to invoke it.  It is useful sometimes to make a link to a utility
with a different name, and that should not change what it does.

Instead, use a run time option or a compilation switch or both to
select among the alternate behaviors.  You can also build two versions
of the program, with different names and different default behaviors.

@cindex output device and program's behavior
Likewise, please don't make the behavior of a command-line program
depend on the type of output device it gets as standard output or
standard input.  Device independence is an important principle of the
system's design; do not compromise it merely to save someone from
typing an option now and then.  (Variation in error message syntax
when using a terminal is ok, because that is a side issue that people
do not depend on.)

If you think one behavior is most useful when the output is to a
terminal, and another is most useful when the output is a file or a
pipe, then it is usually best to make the default behavior the one
that is useful with output to a terminal, and have an option for the
other behavior.  You can also build two different versions of the
program with different names.

There is an exception for programs whose output in certain cases is
binary data.  Sending such output to a terminal is useless and can
cause trouble.  If such a program normally sends its output to stdout,
it should detect, in these cases, when the output is a terminal and
give an error message instead.  The @code{-f} option should override
this exception, thus permitting the output to go to the terminal.

Compatibility requires certain programs to depend on the type of output
device.  It would be disastrous if @code{ls} or @code{sh} did not do so
in the way all users expect.  In some of these cases, we supplement the
program with a preferred alternate version that does not depend on the
output device type.  For example, we provide a @code{dir} program much
like @code{ls} except that its default output format is always
multi-column format.

@node Graphical Interfaces
@section Standards for Graphical Interfaces
@cindex graphical user interface
@cindex interface styles
@cindex user interface styles

@cindex GTK+
@cindex GNUstep
When you write a program that provides a graphical user interface,
please make it work with the X Window System, using the GTK+ toolkit
or the GNUstep 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
@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.

Please make your program interoperate with access technology such as
screen readers (see
@url{https://www.gnu.org/accessibility/accessibility.html}).  This should
be automatic if you use GTK+.

@node Command-Line Interfaces
@section Standards for Command Line Interfaces
@cindex command-line interface

@findex getopt
It is a good idea to follow the POSIX guidelines for the
command-line options of a program.  The easiest way to do this is to use
@code{getopt} to parse them.  Note that the GNU version of @code{getopt}
will normally permit options anywhere among the arguments unless the
special argument @samp{--} is used.  This is not what POSIX
specifies; it is a GNU extension.

@cindex long-named options
Please define long-named options that are equivalent to the
single-letter Unix-style options.  We hope to make GNU more user
friendly this way.  This is easy to do with the GNU function
@code{getopt_long}.

One of the advantages of long-named options is that they can be
consistent from program to program.  For example, users should be able
to expect the ``verbose'' option of any GNU program which has one, to be
spelled precisely @samp{--verbose}.  To achieve this uniformity, look at
the table of common long-option names when you choose the option names
for your program (@pxref{Option Table}).

It is usually a good idea for file names given as ordinary arguments to
be input files only; any output files would be specified using options
(preferably @samp{-o} or @samp{--output}).  Even if you allow an output
file name as an ordinary argument for compatibility, try to provide an
option as another way to specify it.  This will lead to more consistency
among GNU utilities, and fewer idiosyncrasies for users to remember.

@cindex standard command-line options
@cindex options, standard command-line
@cindex CGI programs, standard options for
@cindex PATH_INFO, specifying standard options as
All programs should support two standard options: @samp{--version}
and @samp{--help}.  CGI programs should accept these as command-line
options, and also if given as the @env{PATH_INFO}; for instance,
visiting @indicateurl{http://example.org/p.cgi/--help} in a browser should
output the same information as invoking @samp{p.cgi --help} from the
command line.

@menu
* --version::       The standard output for --version.
* --help::          The standard output for --help.
@end menu

@node --version
@subsection @option{--version}

@cindex @samp{--version} output

The standard @code{--version} option should direct the program to
print information about its name, version, origin and legal status,
all on standard output, and then exit successfully.  Other options and
arguments should be ignored once this is seen, and the program should
not perform its normal function.

@cindex canonical name of a program
@cindex program's canonical name
The first line is meant to be easy for a program to parse; the version
number proper starts after the last space.  In addition, it contains
the canonical name for this program, in this format:

@example
GNU Emacs 19.30
@end example

@noindent
The program's name should be a constant string; @emph{don't} compute it
from @code{argv[0]}.  The idea is to state the standard or canonical
name for the program, not its file name.  There are other ways to find
out the precise file name where a command is found in @code{PATH}.

If the program is a subsidiary part of a larger package, mention the
package name in parentheses, like this:

@example
emacsserver (GNU Emacs) 19.30
@end example

@noindent
If the package has a version number which is different from this
program's version number, you can mention the package version number
just before the close-parenthesis.

If you @emph{need} to mention the version numbers of libraries which
are distributed separately from the package which contains this program,
you can do so by printing an additional line of version info for each
library you want to mention.  Use the same format for these lines as for
the first line.

Please do not mention all of the libraries that the program uses ``just
for completeness''---that would produce a lot of unhelpful clutter.
Please mention library version numbers only if you find in practice that
they are very important to you in debugging.

The following line, after the version number line or lines, should be a
copyright notice.  If more than one copyright notice is called for, put
each on a separate line.

Next should follow a line stating the license, preferably using one of
abbreviations below, and a brief statement that the program is free
software, and that users are free to copy and change it.  Also mention
that there is no warranty, to the extent permitted by law.  See
recommended wording below.

It is ok to finish the output with a list of the major authors of the
program, as a way of giving credit.

Here's an example of output that follows these rules:

@smallexample
GNU hello 2.3
Copyright (C) 2007 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <https://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
@end smallexample

You should adapt this to your program, of course, filling in the proper
year, copyright holder, name of program, and the references to
distribution terms, and changing the rest of the wording as necessary.

This copyright notice only needs to mention the most recent year in
which changes were made---there's no need to list the years for previous
versions' changes.  You don't have to mention the name of the program in
these notices, if that is inconvenient, since it appeared in the first
line.  (The rules are different for copyright notices in source files;
@pxref{Copyright Notices,,,maintain,Information for GNU Maintainers}.)

Translations of the above lines must preserve the validity of the
copyright notices (@pxref{Internationalization}).  If the translation's
character set supports it, the @samp{(C)} should be replaced with the
copyright symbol, as follows:

@ifinfo
(the official copyright symbol, which is the letter C in a circle);
@end ifinfo
@ifnotinfo
@copyright{}
@end ifnotinfo

Write the word ``Copyright'' exactly like that, in English.  Do not
translate it into another language.  International treaties recognize
the English word ``Copyright''; translations into other languages do not
have legal significance.

Finally, here is the table of our suggested license abbreviations.
Any abbreviation can be followed by @samp{v@var{version}[+]}, meaning
that particular version, or later versions with the @samp{+}, as shown
above.  In the case of a GNU license, @emph{always} indicate the permitted
versions in this way.

In the case of exceptions for extra permissions with the GPL, we use
@samp{/} for a separator; the version number can follow the license
abbreviation as usual, as in the examples below.

@table @asis
@item GPL
GNU General Public License, @url{https://www.gnu.org/@/licenses/@/gpl.html}.

@item LGPL
GNU Lesser General Public License, @url{https://www.gnu.org/@/licenses/@/lgpl.html}.

@item GPL/Ada
GNU GPL with the exception for Ada.

@item Apache
The Apache Software Foundation license,
@url{https://www.apache.org/@/licenses}.

@item Artistic
The Artistic license used for Perl, @url{https://dev.perl.org/licenses/artistic.html}.

@item Expat
The Expat license, @url{http://www.jclark.com/@/xml/@/copying.txt}.

@item MPL
The Mozilla Public License, @url{https://www.mozilla.org/@/MPL/}.

@item OBSD
The original (4-clause) BSD license, incompatible with the GNU GPL
@url{https://www.xfree86.org/@/3.3.6/@/COPYRIGHT2.html#6}.

@item PHP
The license used for PHP, @url{http://www.php.net/@/license/}.

@item public domain
The non-license that is being in the public domain,
@url{https://www.gnu.org/@/licenses/@/license-list.html#PublicDomain}.

@item Python
The license for Python,
@url{https://directory.fsf.org/wiki?title=License:Python2.0.1}.

@item RBSD
The revised (3-clause) BSD, compatible with the GNU GPL,@*
@url{https://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{https://www.xfree86.org/@/3.3.6/@/COPYRIGHT2.html#3}.

@item Zlib
The license for Zlib, @url{http://www.gzip.org/@/zlib/@/zlib_license.html}.

@end table

More information about these licenses and many more are on the GNU
licensing web pages,
@url{https://www.gnu.org/@/licenses/@/license-list.html}.


@node --help
@subsection @option{--help}

@cindex @samp{--help} output

The standard @code{--help} option should output brief documentation
for how to invoke the program, on standard output, then exit
successfully.  Other options and arguments should be ignored once this
is seen, and the program should not perform its normal function.

@cindex address for bug reports
@cindex bug reports
Near the end of the @samp{--help} option's output, please place lines
giving the email address for bug reports, the package's home page
(normally @indicateurl{https://www.gnu.org/software/@var{pkg}}, and the
general page for help using GNU programs.  The format should be like this:

@example
Report bugs to: @var{mailing-address}
@var{pkg} home page: <https://www.gnu.org/software/@var{pkg}/>
General help using GNU software: <https://www.gnu.org/gethelp/>
@end example

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
@cindex table of long options

Here is a table of long options used by GNU programs.  It is surely
incomplete, but we aim to list all the options that a new program might
want to be compatible with.  If you use names not already in the table,
please send @email{bug-standards@@gnu.org} a list of them, with their
meanings, so we can update the table.

@c Please leave newlines between items in this table; it's much easier
@c to update when it isn't completely squashed together and unreadable.
@c When there is more than one short option for a long option name, put
@c a semicolon between the lists of the programs that use them, not a
@c period.   --friedman

@table @samp
@item after-date
@samp{-N} in @code{tar}.

@item all
@samp{-a} in @code{du}, @code{ls}, @code{nm}, @code{stty}, @code{uname},
and @code{unexpand}.

@item all-text
@samp{-a} in @code{diff}.

@item almost-all
@samp{-A} in @code{ls}.

@item append
@samp{-a} in @code{etags}, @code{tee}, @code{time};
@samp{-r} in @code{tar}.

@item archive
@samp{-a} in @code{cp}.

@item archive-name
@samp{-n} in @code{shar}.

@item arglength
@samp{-l} in @code{m4}.

@item ascii
@samp{-a} in @code{diff}.

@item assign
@samp{-v} in @code{gawk}.

@item assume-new
@samp{-W} in @code{make}.

@item assume-old
@samp{-o} in @code{make}.

@item auto-check
@samp{-a} in @code{recode}.

@item auto-pager
@samp{-a} in @code{wdiff}.

@item auto-reference
@samp{-A} in @code{ptx}.

@item avoid-wraps
@samp{-n} in @code{wdiff}.

@item background
For server programs, run in the background.

@item backward-search
@samp{-B} in @code{ctags}.

@item basename
@samp{-f} in @code{shar}.

@item batch
Used in GDB.

@item baud
Used in GDB.

@item before
@samp{-b} in @code{tac}.

@item binary
@samp{-b} in @code{cpio} and @code{diff}.

@item bits-per-code
@samp{-b} in @code{shar}.

@item block-size
Used in @code{cpio} and @code{tar}.

@item blocks
@samp{-b} in @code{head} and @code{tail}.

@item break-file
@samp{-b} in @code{ptx}.

@item brief
Used in various programs to make output shorter.

@item bytes
@samp{-c} in @code{head}, @code{split}, and @code{tail}.

@item c@t{++}
@samp{-C} in @code{etags}.

@item catenate
@samp{-A} in @code{tar}.

@item cd
Used in various programs to specify the directory to use.

@item changes
@samp{-c} in @code{chgrp} and @code{chown}.

@item classify
@samp{-F} in @code{ls}.

@item colons
@samp{-c} in @code{recode}.

@item command
@samp{-c} in @code{su};
@samp{-x} in GDB.

@item compare
@samp{-d} in @code{tar}.

@item compat
Used in @code{gawk}.

@item compress
@samp{-Z} in @code{tar} and @code{shar}.

@item concatenate
@samp{-A} in @code{tar}.

@item confirmation
@samp{-w} in @code{tar}.

@item context
Used in @code{diff}.

@item copyleft
@samp{-W copyleft} in @code{gawk}.

@item copyright
@samp{-C} in @code{ptx}, @code{recode}, and @code{wdiff};
@samp{-W copyright} in @code{gawk}.

@item core
Used in GDB.

@item count
@samp{-q} in @code{who}.

@item count-links
@samp{-l} in @code{du}.

@item create
Used in @code{tar} and @code{cpio}.

@item cut-mark
@samp{-c} in @code{shar}.

@item cxref
@samp{-x} in @code{ctags}.

@item date
@samp{-d} in @code{touch}.

@item debug
@samp{-d} in @code{make} and @code{m4};
@samp{-t} in Bison.

@item define
@samp{-D} in @code{m4}.

@item defines
@samp{-d} in Bison and @code{ctags}.

@item delete
@samp{-D} in @code{tar}.

@item dereference
@samp{-L} in @code{chgrp}, @code{chown}, @code{cpio}, @code{du},
@code{ls}, and @code{tar}.

@item dereference-args
@samp{-D} in @code{du}.

@item device
Specify an I/O device (special file name).

@item diacritics
@samp{-d} in @code{recode}.

@item dictionary-order
@samp{-d} in @code{look}.

@item diff
@samp{-d} in @code{tar}.

@item digits
@samp{-n} in @code{csplit}.

@item directory
Specify the directory to use, in various programs.  In @code{ls}, it
means to show directories themselves rather than their contents.  In
@code{rm} and @code{ln}, it means to not treat links to directories
specially.

@item discard-all
@samp{-x} in @code{strip}.

@item discard-locals
@samp{-X} in @code{strip}.

@item dry-run
@samp{-n} in @code{make}.

@item ed
@samp{-e} in @code{diff}.

@item elide-empty-files
@samp{-z} in @code{csplit}.

@item end-delete
@samp{-x} in @code{wdiff}.

@item end-insert
@samp{-z} in @code{wdiff}.

@item entire-new-file
@samp{-N} in @code{diff}.

@item environment-overrides
@samp{-e} in @code{make}.

@item eof
@samp{-e} in @code{xargs}.

@item epoch
Used in GDB.

@item error-limit
Used in @code{makeinfo}.

@item error-output
@samp{-o} in @code{m4}.

@item escape
@samp{-b} in @code{ls}.

@item exclude-from
@samp{-X} in @code{tar}.

@item exec
Used in GDB.

@item exit
@samp{-x} in @code{xargs}.

@item exit-0
@samp{-e} in @code{unshar}.

@item expand-tabs
@samp{-t} in @code{diff}.

@item expression
@samp{-e} in @code{sed}.

@item extern-only
@samp{-g} in @code{nm}.

@item extract
@samp{-i} in @code{cpio};
@samp{-x} in @code{tar}.

@item faces
@samp{-f} in @code{finger}.

@item fast
@samp{-f} in @code{su}.

@item fatal-warnings
@samp{-E} in @code{m4}.

@item file
@samp{-f} in @code{gawk}, @code{info}, @code{make}, @code{mt},
@code{sed}, and @code{tar}.

@item field-separator
@samp{-F} in @code{gawk}.

@item file-prefix
@samp{-b} in Bison.

@item file-type
@samp{-F} in @code{ls}.

@item files-from
@samp{-T} in @code{tar}.

@item fill-column
Used in @code{makeinfo}.

@item flag-truncation
@samp{-F} in @code{ptx}.

@item fixed-output-files
@samp{-y} in Bison.

@item follow
@samp{-f} in @code{tail}.

@item footnote-style
Used in @code{makeinfo}.

@item force
@samp{-f} in @code{cp}, @code{ln}, @code{mv}, and @code{rm}.

@item force-prefix
@samp{-F} in @code{shar}.

@item foreground
For server programs, run in the foreground;
in other words, don't do anything special to run the server
in the background.

@item format
Used in @code{ls}, @code{time}, and @code{ptx}.

@item freeze-state
@samp{-F} in @code{m4}.

@item fullname
Used in GDB.

@item gap-size
@samp{-g} in @code{ptx}.

@item get
@samp{-x} in @code{tar}.

@item graphic
@samp{-i} in @code{ul}.

@item graphics
@samp{-g} in @code{recode}.

@item group
@samp{-g} in @code{install}.

@item gzip
@samp{-z} in @code{tar} and @code{shar}.

@item hashsize
@samp{-H} in @code{m4}.

@item header
@samp{-h} in @code{objdump} and @code{recode}

@item heading
@samp{-H} in @code{who}.

@item help
Used to ask for brief usage information.

@item here-delimiter
@samp{-d} in @code{shar}.

@item hide-control-chars
@samp{-q} in @code{ls}.

@item html
In @code{makeinfo}, output HTML.

@item idle
@samp{-u} in @code{who}.

@item ifdef
@samp{-D} in @code{diff}.

@item ignore
@samp{-I} in @code{ls};
@samp{-x} in @code{recode}.

@item ignore-all-space
@samp{-w} in @code{diff}.

@item ignore-backups
@samp{-B} in @code{ls}.

@item ignore-blank-lines
@samp{-B} in @code{diff}.

@item ignore-case
@samp{-f} in @code{look} and @code{ptx};
@samp{-i} in @code{diff} and @code{wdiff}.

@item ignore-errors
@samp{-i} in @code{make}.

@item ignore-file
@samp{-i} in @code{ptx}.

@item ignore-indentation
@samp{-I} in @code{etags}.

@item ignore-init-file
@samp{-f} in Oleo.

@item ignore-interrupts
@samp{-i} in @code{tee}.

@item ignore-matching-lines
@samp{-I} in @code{diff}.

@item ignore-space-change
@samp{-b} in @code{diff}.

@item ignore-zeros
@samp{-i} in @code{tar}.

@item include
@samp{-i} in @code{etags};
@samp{-I} in @code{m4}.

@item include-dir
@samp{-I} in @code{make}.

@item incremental
@samp{-G} in @code{tar}.

@item info
@samp{-i}, @samp{-l}, and @samp{-m} in Finger.

@item init-file
In some programs, specify the name of the file to read as the user's
init file.

@item initial
@samp{-i} in @code{expand}.

@item initial-tab
@samp{-T} in @code{diff}.

@item inode
@samp{-i} in @code{ls}.

@item interactive
@samp{-i} in @code{cp}, @code{ln}, @code{mv}, @code{rm};
@samp{-e} in @code{m4};
@samp{-p} in @code{xargs};
@samp{-w} in @code{tar}.

@item intermix-type
@samp{-p} in @code{shar}.

@item iso-8601
Used in @code{date}

@item jobs
@samp{-j} in @code{make}.

@item just-print
@samp{-n} in @code{make}.

@item keep-going
@samp{-k} in @code{make}.

@item keep-files
@samp{-k} in @code{csplit}.

@item kilobytes
@samp{-k} in @code{du} and @code{ls}.

@item language
@samp{-l} in @code{etags}.

@item less-mode
@samp{-l} in @code{wdiff}.

@item level-for-gzip
@samp{-g} in @code{shar}.

@item line-bytes
@samp{-C} in @code{split}.

@item lines
Used in @code{split}, @code{head}, and @code{tail}.

@item link
@samp{-l} in @code{cpio}.

@item lint
@itemx lint-old
Used in @code{gawk}.

@item list
@samp{-t} in @code{cpio};
@samp{-l} in @code{recode}.

@item list
@samp{-t} in @code{tar}.

@item literal
@samp{-N} in @code{ls}.

@item load-average
@samp{-l} in @code{make}.

@item login
Used in @code{su}.

@item machine
Used in @code{uname}.

@item macro-name
@samp{-M} in @code{ptx}.

@item mail
@samp{-m} in @code{hello} and @code{uname}.

@item make-directories
@samp{-d} in @code{cpio}.

@item makefile
@samp{-f} in @code{make}.

@item mapped
Used in GDB.

@item max-args
@samp{-n} in @code{xargs}.

@item max-chars
@samp{-n} in @code{xargs}.

@item max-lines
@samp{-l} in @code{xargs}.

@item max-load
@samp{-l} in @code{make}.

@item max-procs
@samp{-P} in @code{xargs}.

@item mesg
@samp{-T} in @code{who}.

@item message
@samp{-T} in @code{who}.

@item minimal
@samp{-d} in @code{diff}.

@item mixed-uuencode
@samp{-M} in @code{shar}.

@item mode
@samp{-m} in @code{install}, @code{mkdir}, and @code{mkfifo}.

@item modification-time
@samp{-m} in @code{tar}.

@item multi-volume
@samp{-M} in @code{tar}.

@item name-prefix
@samp{-a} in Bison.

@item nesting-limit
@samp{-L} in @code{m4}.

@item net-headers
@samp{-a} in @code{shar}.

@item new-file
@samp{-W} in @code{make}.

@item no-builtin-rules
@samp{-r} in @code{make}.

@item no-character-count
@samp{-w} in @code{shar}.

@item no-check-existing
@samp{-x} in @code{shar}.

@item no-common
@samp{-3} in @code{wdiff}.

@item no-create
@samp{-c} in @code{touch}.

@item no-defines
@samp{-D} in @code{etags}.

@item no-deleted
@samp{-1} in @code{wdiff}.

@item no-dereference
@samp{-d} in @code{cp}.

@item no-inserted
@samp{-2} in @code{wdiff}.

@item no-keep-going
@samp{-S} in @code{make}.

@item no-lines
@samp{-l} in Bison.

@item no-piping
@samp{-P} in @code{shar}.

@item no-prof
@samp{-e} in @code{gprof}.

@item no-regex
@samp{-R} in @code{etags}.

@item no-sort
@samp{-p} in @code{nm}.

@item no-splash
Don't print a startup splash screen.

@item no-split
Used in @code{makeinfo}.

@item no-static
@samp{-a} in @code{gprof}.

@item no-time
@samp{-E} in @code{gprof}.

@item no-timestamp
@samp{-m} in @code{shar}.

@item no-validate
Used in @code{makeinfo}.

@item no-wait
Used in @code{emacsclient}.

@item no-warn
Used in various programs to inhibit warnings.

@item node
@samp{-n} in @code{info}.

@item nodename
@samp{-n} in @code{uname}.

@item nonmatching
@samp{-f} in @code{cpio}.

@item nstuff
@samp{-n} in @code{objdump}.

@item null
@samp{-0} in @code{xargs}.

@item number
@samp{-n} in @code{cat}.

@item number-nonblank
@samp{-b} in @code{cat}.

@item numeric-sort
@samp{-n} in @code{nm}.

@item numeric-uid-gid
@samp{-n} in @code{cpio} and @code{ls}.

@item nx
Used in GDB.

@item old-archive
@samp{-o} in @code{tar}.

@item old-file
@samp{-o} in @code{make}.

@item one-file-system
@samp{-l} in @code{tar}, @code{cp}, and @code{du}.

@item only-file
@samp{-o} in @code{ptx}.

@item only-prof
@samp{-f} in @code{gprof}.

@item only-time
@samp{-F} in @code{gprof}.

@item options
@samp{-o} in @code{getopt}, @code{fdlist}, @code{fdmount},
@code{fdmountd}, and @code{fdumount}.

@item output
In various programs, specify the output file name.

@item output-prefix
@samp{-o} in @code{shar}.

@item override
@samp{-o} in @code{rm}.

@item overwrite
@samp{-c} in @code{unshar}.

@item owner
@samp{-o} in @code{install}.

@item paginate
@samp{-l} in @code{diff}.

@item paragraph-indent
Used in @code{makeinfo}.

@item parents
@samp{-p} in @code{mkdir} and @code{rmdir}.

@item pass-all
@samp{-p} in @code{ul}.

@item pass-through
@samp{-p} in @code{cpio}.

@item port
@samp{-P} in @code{finger}.

@item portability
@samp{-c} in @code{cpio} and @code{tar}.

@item posix
Used in @code{gawk}.

@item prefix-builtins
@samp{-P} in @code{m4}.

@item prefix
@samp{-f} in @code{csplit}.

@item preserve
Used in @code{tar} and @code{cp}.

@item preserve-environment
@samp{-p} in @code{su}.

@item preserve-modification-time
@samp{-m} in @code{cpio}.

@item preserve-order
@samp{-s} in @code{tar}.

@item preserve-permissions
@samp{-p} in @code{tar}.

@item print
@samp{-l} in @code{diff}.

@item print-chars
@samp{-L} in @code{cmp}.

@item print-data-base
@samp{-p} in @code{make}.

@item print-directory
@samp{-w} in @code{make}.

@item print-file-name
@samp{-o} in @code{nm}.

@item print-symdefs
@samp{-s} in @code{nm}.

@item printer
@samp{-p} in @code{wdiff}.

@item prompt
@samp{-p} in @code{ed}.

@item proxy
Specify an HTTP proxy.

@item query-user
@samp{-X} in @code{shar}.

@item question
@samp{-q} in @code{make}.

@item quiet
Used in many programs to inhibit the usual output.  Every
program accepting @samp{--quiet} should accept @samp{--silent} as a
synonym.

@item quiet-unshar
@samp{-Q} in @code{shar}

@item quote-name
@samp{-Q} in @code{ls}.

@item rcs
@samp{-n} in @code{diff}.

@item re-interval
Used in @code{gawk}.

@item read-full-blocks
@samp{-B} in @code{tar}.

@item readnow
Used in GDB.

@item recon
@samp{-n} in @code{make}.

@item record-number
@samp{-R} in @code{tar}.

@item recursive
Used in @code{chgrp}, @code{chown}, @code{cp}, @code{ls}, @code{diff},
and @code{rm}.

@item reference
@samp{-r} in @code{touch}.

@item references
@samp{-r} in @code{ptx}.

@item regex
@samp{-r} in @code{tac} and @code{etags}.

@item release
@samp{-r} in @code{uname}.

@item reload-state
@samp{-R} in @code{m4}.

@item relocation
@samp{-r} in @code{objdump}.

@item rename
@samp{-r} in @code{cpio}.

@item replace
@samp{-i} in @code{xargs}.

@item report-identical-files
@samp{-s} in @code{diff}.

@item reset-access-time
@samp{-a} in @code{cpio}.

@item reverse
@samp{-r} in @code{ls} and @code{nm}.

@item reversed-ed
@samp{-f} in @code{diff}.

@item right-side-defs
@samp{-R} in @code{ptx}.

@item same-order
@samp{-s} in @code{tar}.

@item same-permissions
@samp{-p} in @code{tar}.

@item save
@samp{-g} in @code{stty}.

@item se
Used in GDB.

@item sentence-regexp
@samp{-S} in @code{ptx}.

@item separate-dirs
@samp{-S} in @code{du}.

@item separator
@samp{-s} in @code{tac}.

@item sequence
Used by @code{recode} to chose files or pipes for sequencing passes.

@item shell
@samp{-s} in @code{su}.

@item show-all
@samp{-A} in @code{cat}.

@item show-c-function
@samp{-p} in @code{diff}.

@item show-ends
@samp{-E} in @code{cat}.

@item show-function-line
@samp{-F} in @code{diff}.

@item show-tabs
@samp{-T} in @code{cat}.

@item silent
Used in many programs to inhibit the usual output.
Every program accepting
@samp{--silent} should accept @samp{--quiet} as a synonym.

@item size
@samp{-s} in @code{ls}.

@item socket
Specify a file descriptor for a network server to use for its socket,
instead of opening and binding a new socket.  This provides a way to
run, in a non-privileged process, a server that normally needs a
reserved port number.

@item sort
Used in @code{ls}.

@item source
@samp{-W source} in @code{gawk}.

@item sparse
@samp{-S} in @code{tar}.

@item speed-large-files
@samp{-H} in @code{diff}.

@item split-at
@samp{-E} in @code{unshar}.

@item split-size-limit
@samp{-L} in @code{shar}.

@item squeeze-blank
@samp{-s} in @code{cat}.

@item start-delete
@samp{-w} in @code{wdiff}.

@item start-insert
@samp{-y} in @code{wdiff}.

@item starting-file
Used in @code{tar} and @code{diff} to specify which file within
a directory to start processing with.

@item statistics
@samp{-s} in @code{wdiff}.

@item stdin-file-list
@samp{-S} in @code{shar}.

@item stop
@samp{-S} in @code{make}.

@item strict
@samp{-s} in @code{recode}.

@item strip
@samp{-s} in @code{install}.

@item strip-all
@samp{-s} in @code{strip}.

@item strip-debug
@samp{-S} in @code{strip}.

@item submitter
@samp{-s} in @code{shar}.

@item suffix
@samp{-S} in @code{cp}, @code{ln}, @code{mv}.

@item suffix-format
@samp{-b} in @code{csplit}.

@item sum
@samp{-s} in @code{gprof}.

@item summarize
@samp{-s} in @code{du}.

@item symbolic
@samp{-s} in @code{ln}.

@item symbols
Used in GDB and @code{objdump}.

@item synclines
@samp{-s} in @code{m4}.

@item sysname
@samp{-s} in @code{uname}.

@item tabs
@samp{-t} in @code{expand} and @code{unexpand}.

@item tabsize
@samp{-T} in @code{ls}.

@item terminal
@samp{-T} in @code{tput} and @code{ul}.
@samp{-t} in @code{wdiff}.

@item text
@samp{-a} in @code{diff}.

@item text-files
@samp{-T} in @code{shar}.

@item time
Used in @code{ls} and @code{touch}.

@item timeout
Specify how long to wait before giving up on some operation.

@item to-stdout
@samp{-O} in @code{tar}.

@item total
@samp{-c} in @code{du}.

@item touch
@samp{-t} in @code{make}, @code{ranlib}, and @code{recode}.

@item trace
@samp{-t} in @code{m4}.

@item traditional
@samp{-t} in @code{hello};
@samp{-W traditional} in @code{gawk};
@samp{-G} in @code{ed}, @code{m4}, and @code{ptx}.

@item tty
Used in GDB.

@item typedefs
@samp{-t} in @code{ctags}.

@item typedefs-and-c++
@samp{-T} in @code{ctags}.

@item typeset-mode
@samp{-t} in @code{ptx}.

@item uncompress
@samp{-z} in @code{tar}.

@item unconditional
@samp{-u} in @code{cpio}.

@item undefine
@samp{-U} in @code{m4}.

@item undefined-only
@samp{-u} in @code{nm}.

@item update
@samp{-u} in @code{cp}, @code{ctags}, @code{mv}, @code{tar}.

@item usage
Used in @code{gawk}; same as @samp{--help}.

@item uuencode
@samp{-B} in @code{shar}.

@item vanilla-operation
@samp{-V} in @code{shar}.

@item verbose
Print more information about progress.  Many programs support this.

@item verify
@samp{-W} in @code{tar}.

@item version
Print the version number.

@item version-control
@samp{-V} in @code{cp}, @code{ln}, @code{mv}.

@item vgrind
@samp{-v} in @code{ctags}.

@item volume
@samp{-V} in @code{tar}.

@item what-if
@samp{-W} in @code{make}.

@item whole-size-limit
@samp{-l} in @code{shar}.

@item width
@samp{-w} in @code{ls} and @code{ptx}.

@item word-regexp
@samp{-W} in @code{ptx}.

@item writable
@samp{-T} in @code{who}.

@item zeros
@samp{-z} in @code{gprof}.
@end table

@node OID Allocations
@section OID Allocations
@cindex OID allocations for GNU
@cindex SNMP
@cindex LDAP
@cindex X.509

The OID (object identifier) 1.3.6.1.4.1.11591 has been assigned to the
GNU Project (thanks to Sergey Poznyakoff).  These are used for SNMP,
LDAP, X.509 certificates, and so on.  The web site
@url{https://www.alvestrand.no/objectid} has a (voluntary) listing of
many OID assignments.

If you need a new slot for your GNU package, write
@email{maintainers@@gnu.org}.  Here is a list of arcs currently
assigned:

@example
@include gnu-oids.texi
@end example


@node Memory Usage
@section Memory Usage
@cindex memory usage

If a program typically uses just a few meg of memory, don't bother making any
effort to reduce memory usage.  For example, if it is impractical for
other reasons to operate on files more than a few meg long, it is
reasonable to read entire input files into memory to operate on them.

However, for programs such as @code{cat} or @code{tail}, that can
usefully operate on very large files, it is important to avoid using a
technique that would artificially limit the size of files it can handle.
If a program works by lines and could be applied to arbitrary
user-supplied input files, it should keep only a line in memory, because
this is not very hard and users will want to be able to operate on input
files that are bigger than will fit in memory all at once.

If your program creates complicated data structures, just make them in
memory and give a fatal error if @code{malloc} returns @code{NULL}.

@pindex valgrind
@cindex memory leak
Memory analysis tools 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 such a tool.


@node File Usage
@section File Usage
@cindex file usage

Programs should be prepared to operate when @file{/usr} and @file{/etc}
are read-only file systems.  Thus, if the program manages log files,
lock files, backup files, score files, or any other files which are
modified for internal purposes, these files should not be stored in
@file{/usr} or @file{/etc}.

There are two exceptions.  @file{/etc} is used to store system
configuration information; it is reasonable for a program to modify
files in @file{/etc} when its job is to update the system configuration.
Also, if the user explicitly asks to modify one file in a directory, it
is reasonable for the program to store other files in the same
directory.

@node Writing C
@chapter Making The Best Use of C

This chapter provides advice on how best to use the C language
when writing GNU software.

@menu
* Formatting::                  Formatting your source code.
* Comments::                    Commenting your work.
* Syntactic Conventions::       Clean use of C constructs.
* Names::                       Naming variables, functions, and files.
* System Portability::          Portability among different operating systems.
* CPU Portability::             Supporting the range of CPU types.
* System Functions::            Portability and ``standard'' library functions.
* Internationalization::        Techniques for internationalization.
* Character Set::               Use ASCII by default.
* Quote Characters::            Use "..." or '...' in the C locale.
* Mmap::                        How you can safely use @code{mmap}.
@end menu

@node Formatting
@section Formatting Your Source Code
@cindex formatting source code

@cindex line length
@cindex length of source lines
Please keep the length of source lines to 79 characters or less, for
maximum readability in the widest range of environments.

@cindex open brace
@cindex braces, in C source
@cindex function definitions, formatting
It is important to put the open-brace that starts the body of a C
function in column one, so that they will start a defun.  Several
tools look for open-braces in column one to find the beginnings of C
functions.  These tools will not work on code not formatted that way.

Avoid putting open-brace, open-parenthesis or open-bracket in column
one when they are inside a function, so that they won't start a defun.
The open-brace that starts a @code{struct} body can go in column one
if you find it useful to treat that definition as a defun.

It is also important for function definitions to start the name of the
function in column one.  This helps people to search for function
definitions, and may also help certain tools recognize them.  Thus,
using Standard C syntax, the format is this:

@example
static char *
concat (char *s1, char *s2)
@{
  @dots{}
@}
@end example

@noindent
or, if you want to use traditional C syntax, format the definition like
this:

@example
static char *
concat (s1, s2)        /* Name starts in column one here */
     char *s1, *s2;
@{                     /* Open brace in column one here */
  @dots{}
@}
@end example

In Standard C, if the arguments don't fit nicely on one line,
split it like this:

@example
int
lots_of_args (int an_integer, long a_long, short a_short,
              double a_double, float a_float)
@dots{}
@end example

@cindex @code{struct} types, formatting
@cindex @code{enum} types, formatting
For @code{struct} and @code{enum} types, likewise put the braces in
column one, unless the whole contents fits on one line:

@example
struct foo
@{
  int a, b;
@}
@exdent @r{or}
struct foo @{ int a, b; @}
@end example

The rest of this section gives our recommendations for other aspects of
C formatting style, which is also the default style of the @code{indent}
program in version 1.2 and newer.  It corresponds to the options

@smallexample
-nbad -bap -nbc -bbo -bl -bli2 -bls -ncdb -nce -cp1 -cs -di2
-ndj -nfc1 -nfca -hnl -i2 -ip5 -lp -pcs -psl -nsc -nsob
@end smallexample

We don't think of these recommendations as requirements, because it
causes no problems for users if two different programs have different
formatting styles.

But whatever style you use, please use it consistently, since a mixture
of styles within one program tends to look ugly.  If you are
contributing changes to an existing program, please follow the style of
that program.

For the body of the function, our recommended style looks like this:

@example
if (x < foo (y, z))
  haha = bar[4] + 5;
else
  @{
    while (z)
      @{
        haha += foo (z, z);
        z--;
      @}
    return ++x + bar ();
  @}
@end example

@cindex spaces before open-paren
We find it easier to read a program when it has spaces before the
open-parentheses and after the commas.  Especially after the commas.

When you split an expression into multiple lines, split it
before an operator, not after one.  Here is the right way:

@cindex expressions, splitting
@example
if (foo_this_is_long && bar > win (x, y, z)
    && remaining_condition)
@end example

Try to avoid having two operators of different precedence at the same
level of indentation.  For example, don't write this:

@example
mode = (inmode[j] == VOIDmode
        || GET_MODE_SIZE (outmode[j]) > GET_MODE_SIZE (inmode[j])
        ? outmode[j] : inmode[j]);
@end example

Instead, use extra parentheses so that the indentation shows the nesting:

@example
mode = ((inmode[j] == VOIDmode
         || (GET_MODE_SIZE (outmode[j]) > GET_MODE_SIZE (inmode[j])))
        ? outmode[j] : inmode[j]);
@end example

Insert extra parentheses so that Emacs will indent the code properly.
For example, the following indentation looks nice if you do it by hand,

@example
v = rup->ru_utime.tv_sec*1000 + rup->ru_utime.tv_usec/1000
    + rup->ru_stime.tv_sec*1000 + rup->ru_stime.tv_usec/1000;
@end example

@noindent
but Emacs would alter it.  Adding a set of parentheses produces
something that looks equally nice, and which Emacs will preserve:

@example
v = (rup->ru_utime.tv_sec*1000 + rup->ru_utime.tv_usec/1000
     + rup->ru_stime.tv_sec*1000 + rup->ru_stime.tv_usec/1000);
@end example

Format do-while statements like this:

@example
do
  @{
    a = foo (a);
  @}
while (a > 0);
@end example

@cindex formfeed
@cindex control-L
Please use formfeed characters (control-L) to divide the program into
pages at logical places (but not within a function).  It does not matter
just how long the pages are, since they do not have to fit on a printed
page.  The formfeeds should appear alone on lines by themselves.

@node Comments
@section Commenting Your Work
@cindex commenting

Every program should start with a comment saying briefly what it is for.
Example: @samp{fmt - filter for simple filling of text}.  This comment
should be at the top of the source file containing the @samp{main}
function of the program.

Also, please write a brief comment at the start of each source file,
with the file name and a line or two about the overall purpose of the
file.

Please write the comments in a GNU program in English, because English
is the one language that nearly all programmers in all countries can
read.  If you do not write English well, please write comments in
English as well as you can, then ask other people to help rewrite them.
If you can't write comments in English, please find someone to work with
you and translate your comments into English.

Please put a comment on each function saying what the function does,
what sorts of arguments it gets, and what the possible values of
arguments mean and are used for.  It is not necessary to duplicate in
words the meaning of the C argument declarations, if a C type is being
used in its customary fashion.  If there is anything nonstandard about
its use (such as an argument of type @code{char *} which is really the
address of the second character of a string, not the first), or any
possible values that would not work the way one would expect (such as,
that strings containing newlines are not guaranteed to work), be sure
to say so.

Also explain the significance of the return value, if there is one.

Please put two spaces after the end of a sentence in your comments, so
that the Emacs sentence commands will work.  Also, please write
complete sentences and capitalize the first word.  If a lower-case
identifier comes at the beginning of a sentence, don't capitalize it!
Changing the spelling makes it a different identifier.  If you don't
like starting a sentence with a lower case letter, write the sentence
differently (e.g., ``The identifier lower-case is @dots{}'').

The comment on a function is much clearer if you use the argument
names to speak about the argument values.  The variable name itself
should be lower case, but write it in upper case when you are speaking
about the value rather than the variable itself.  Thus, ``the inode
number NODE_NUM'' rather than ``an inode''.

There is usually no purpose in restating the name of the function in
the comment before it, because readers can see that for themselves.
There might be an exception when the comment is so long that the function
itself would be off the bottom of the screen.

There should be a comment on each static variable as well, like this:

@example
/* Nonzero means truncate lines in the display;
   zero means continue them.  */
int truncate_lines;
@end example

@cindex conditionals, comments for
@cindex @code{#endif}, commenting
Every @samp{#endif} should have a comment, except in the case of short
conditionals (just a few lines) that are not nested.  The comment should
state the condition of the conditional that is ending, @emph{including
its sense}.  @samp{#else} should have a comment describing the condition
@emph{and sense} of the code that follows.  For example:

@example
@group
#ifdef foo
  @dots{}
#else /* not foo */
  @dots{}
#endif /* not foo */
@end group
@group
#ifdef foo
  @dots{}
#endif /* foo */
@end group
@end example

@noindent
but, by contrast, write the comments this way for a @samp{#ifndef}:

@example
@group
#ifndef foo
  @dots{}
#else /* foo */
  @dots{}
#endif /* foo */
@end group
@group
#ifndef foo
  @dots{}
#endif /* not foo */
@end group
@end example

@node Syntactic Conventions
@section Clean Use of C Constructs
@cindex syntactic conventions

@cindex implicit @code{int}
@cindex function argument, declaring
Please explicitly declare the types of all objects.  For example, you
should explicitly declare all arguments to functions, and you should
declare functions to return @code{int} rather than omitting the
@code{int}.

@cindex compiler warnings
@cindex @samp{-Wall} compiler option
Some programmers like to use the GCC @samp{-Wall} option, and change the
code whenever it issues a warning.  If you want to do this, then do.
Other programmers prefer not to use @samp{-Wall}, because it gives
warnings for valid and legitimate code which they do not want to change.
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 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
should go in a header file.  Don't put @code{extern} declarations inside
functions.

@cindex temporary variables
It used to be common practice to use the same local variables (with
names like @code{tem}) over and over for different values within one
function.  Instead of doing this, it is better to declare a separate local
variable for each distinct purpose, and give it a name which is
meaningful.  This not only makes programs easier to understand, it also
facilitates optimization by good compilers.  You can also move the
declaration of each local variable into the smallest scope that includes
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.
Start a new declaration on each line, instead.  For example, instead
of this:

@example
@group
int    foo,
       bar;
@end group
@end example

@noindent
write either this:

@example
int foo, bar;
@end example

@noindent
or this:

@example
int foo;
int bar;
@end example

@noindent
(If they are global variables, each should have a comment preceding it
anyway.)

When you have an @code{if}-@code{else} statement nested in another
@code{if} statement, always put braces around the @code{if}-@code{else}.
Thus, never write like this:

@example
if (foo)
  if (bar)
    win ();
  else
    lose ();
@end example

@noindent
always like this:

@example
if (foo)
  @{
    if (bar)
      win ();
    else
      lose ();
  @}
@end example

If you have an @code{if} statement nested inside of an @code{else}
statement, either write @code{else if} on one line, like this,

@example
if (foo)
  @dots{}
else if (bar)
  @dots{}
@end example

@noindent
with its @code{then}-part indented like the preceding @code{then}-part,
or write the nested @code{if} within braces like this:

@example
if (foo)
  @dots{}
else
  @{
    if (bar)
      @dots{}
  @}
@end example

Don't declare both a structure tag and variables or typedefs in the
same declaration.  Instead, declare the structure tag separately
and then use it to declare the variables or typedefs.

Try to avoid assignments inside @code{if}-conditions (assignments
inside @code{while}-conditions are ok).  For example, don't write
this:

@example
if ((foo = (char *) malloc (sizeof *foo)) == NULL)
  fatal ("virtual memory exhausted");
@end example

@noindent
instead, write this:

@example
foo = (char *) malloc (sizeof *foo);
if (foo == NULL)
  fatal ("virtual memory exhausted");
@end example

@node Names
@section Naming Variables, Functions, and Files

@cindex names of variables, functions, and files
The names of global variables and functions in a program serve as
comments of a sort.  So don't choose terse names---instead, look for
names that give useful information about the meaning of the variable or
function.  In a GNU program, names should be English, like other
comments.

Local variable names can be shorter, because they are used only within
one context, where (presumably) comments explain their purpose.

Try to limit your use of abbreviations in symbol names.  It is ok to
make a few abbreviations, explain what they mean, and then use them
frequently, but don't use lots of obscure abbreviations.

Please use underscores to separate words in a name, so that the Emacs
word commands can be useful within them.  Stick to lower case; reserve
upper case for macros and @code{enum} constants, and for name-prefixes
that follow a uniform convention.

For example, you should use names like @code{ignore_space_change_flag};
don't use names like @code{iCantReadThis}.

Variables that indicate whether command-line options have been
specified should be named after the meaning of the option, not after
the option-letter.  A comment should state both the exact meaning of
the option and its letter.  For example,

@example
@group
/* Ignore changes in horizontal whitespace (-b).  */
int ignore_space_change_flag;
@end group
@end example

When you want to define names with constant integer values, use
@code{enum} rather than @samp{#define}.  GDB knows about enumeration
constants.

@cindex file-name limitations
@pindex doschk
You might want to make sure that none of the file names would conflict
if the files were loaded onto an MS-DOS file system which shortens the
names.  You can use the program @code{doschk} to test for this.

Some GNU programs were designed to limit themselves to file names of 14
characters or less, to avoid file name conflicts if they are read into
older System V systems.  Please preserve this feature in the existing
GNU programs that have it, but there is no need to do this in new GNU
programs.  @code{doschk} also reports file names longer than 14
characters.


@node System Portability
@section Portability between System Types
@cindex portability, between system types

In the Unix world, ``portability'' refers to porting to different Unix
versions.  For a GNU program, this kind of portability is desirable, but
not paramount.

The primary purpose of GNU software is to run on top of the GNU kernel,
compiled with the GNU C compiler, on various types of CPU.  So the
kinds of portability that are absolutely necessary are quite limited.
But it is important to support Linux-based GNU systems, since they
are the form of GNU that is popular.

Beyond that, it is good to support the other free operating systems
(*BSD), and it is nice to support other Unix-like systems if you want
to.  Supporting a variety of Unix-like systems is desirable, although
not paramount.  It is usually not too hard, so you may as well do it.
But you don't have to consider it an obligation, if it does turn out to
be hard.

@pindex autoconf
The easiest way to achieve portability to most Unix-like systems is to
use Autoconf.  It's unlikely that your program needs to know more
information about the host platform than Autoconf can provide, simply
because most of the programs that need such knowledge have already been
written.

Avoid using the format of semi-internal data bases (e.g., directories)
when there is a higher-level alternative (@code{readdir}).

@cindex non-POSIX systems, and portability
As for systems that are not like Unix, such as MSDOS, Windows, VMS, MVS,
and older Macintosh systems, supporting them is often a lot of work.
When that is the case, it is better to spend your time adding features
that will be useful on GNU and GNU/Linux, rather than on supporting
other incompatible systems.

If you do support Windows, please do not abbreviate it as ``win''.
@xref{Trademarks}.

Usually we write the name ``Windows'' in full, but when brevity is
very important (as in file names and some symbol names), we abbreviate
it to ``w''.  In GNU Emacs, for instance, we use @samp{w32} in file
names of Windows-specific files, but the macro for Windows
conditionals is called @code{WINDOWSNT}.  In principle there could
also be @samp{w64}.

It is a good idea to define the ``feature test macro''
@code{_GNU_SOURCE} when compiling your C files.  When you compile on GNU
or GNU/Linux, this will enable the declarations of GNU library extension
functions, and that will usually give you a compiler error message if
you define the same function names in some other way in your program.
(You don't have to actually @emph{use} these functions, if you prefer
to make the program more portable to other systems.)

But whether or not you use these GNU extensions, you should avoid
using their names for any other meanings.  Doing so would make it hard
to move your code into other GNU programs.

@node CPU Portability
@section Portability between CPUs

@cindex data types, and portability
@cindex portability, and data types
Even GNU systems will differ because of differences among CPU
types---for example, difference in byte ordering and alignment
requirements.  It is absolutely essential to handle these differences.
However, don't make any effort to cater to the possibility that an
@code{int} will be less than 32 bits.  We don't support 16-bit machines
in GNU.

You need not cater to the possibility that @code{long} will be smaller
than pointers and @code{size_t}.  We know of one such platform: 64-bit
programs on Microsoft Windows.  If you care about making your package
run on Windows using Mingw64, you would need to deal with 8-byte
pointers and 4-byte @code{long}, which would break this code:

@example
printf ("size = %lu\n", (unsigned long) sizeof array);
printf ("diff = %ld\n", (long) (pointer2 - pointer1));
@end example

Whether to support Mingw64, and Windows in general, in your package is
your choice.  The GNU Project doesn't say you have any responsibility to
do so.  Our goal is to replace proprietary systems, including Windows,
not to enhance them.  If people pressure you to make your program run
on Windows, and you are not interested, you can respond with, ``Switch
to GNU/Linux --- your freedom depends on 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
work with them.  One way to print an @code{off_t} value portably is to
print its digits yourself, one by one.

Don't assume that the address of an @code{int} object is also the
address of its least-significant byte.  This is false on big-endian
machines.  Thus, don't make the following mistake:

@example
int c;
@dots{}
while ((c = getchar ()) != EOF)
  write (file_descriptor, &c, 1);
@end example

@noindent Instead, use @code{unsigned char} as follows.  (The @code{unsigned}
is for portability to unusual systems where @code{char} is signed and
where there is integer overflow checking.)

@example
int c;
while ((c = getchar ()) != EOF)
  @{
    unsigned char u = c;
    write (file_descriptor, &u, 1);
  @}
@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
cases where casting pointers to integers is essential---such as, a Lisp
interpreter which stores type information as well as an address in one
word---you'll have to make explicit provisions to handle different word
sizes.  You will also need to make provision for systems in which 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

Historically, C implementations differed substantially, and many
systems lacked a full implementation of ANSI/ISO C89.  Nowadays,
however, all practical systems have a C89 compiler and GNU C supports
almost all of C99 and some of C11.  Similarly, most systems implement
POSIX.1-2001 libraries and tools, and many have POSIX.1-2008.

Hence, there is little reason to support old C or non-POSIX systems,
and you may want to take advantage of standard C and POSIX 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

@cindex Gnulib
@uref{https://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.


@node Internationalization
@section Internationalization
@cindex internationalization

@pindex gettext
GNU has a library called GNU gettext that makes it easy to translate the
messages in a program into various languages.  You should use this
library in every program.  Use English for the messages as they appear
in the program, and let gettext provide the way to translate them into
other languages.

Using GNU gettext involves putting a call to the @code{gettext} macro
around each string that might need translation---like this:

@example
printf (gettext ("Processing file '%s'..."), file);
@end example

@noindent
This permits GNU gettext to replace the string @code{"Processing file
'%s'..."} with a translated version.

Once a program uses gettext, please make a point of writing calls to
@code{gettext} when you add new strings that call for translation.

Using GNU gettext in a package involves specifying a @dfn{text domain
name} for the package.  The text domain name is used to separate the
translations for this package from the translations for other packages.
Normally, the text domain name should be the same as the name of the
package---for example, @samp{coreutils} for the GNU core utilities.

@cindex message text, and internationalization
To enable gettext to work well, avoid writing code that makes
assumptions about the structure of words or sentences.  When you want
the precise text of a sentence to vary depending on the data, use two or
more alternative string constants each containing a complete sentences,
rather than inserting conditionalized words or phrases into a single
sentence framework.

Here is an example of what not to do:

@smallexample
printf ("%s is full", capacity > 5000000 ? "disk" : "floppy disk");
@end smallexample

If you apply gettext to all strings, like this,

@smallexample
printf (gettext ("%s is full"),
        capacity > 5000000 ? gettext ("disk") : gettext ("floppy disk"));
@end smallexample

@noindent
the translator will hardly know that "disk" and "floppy disk" are meant to
be substituted in the other string.  Worse, in some languages (like French)
the construction will not work: the translation of the word "full" depends
on the gender of the first part of the sentence; it happens to be not the
same for "disk" as for "floppy disk".

Complete sentences can be translated without problems:

@example
printf (capacity > 5000000 ? gettext ("disk is full")
        : gettext ("floppy disk is full"));
@end example

A similar problem appears at the level of sentence structure with this
code:

@example
printf ("#  Implicit rule search has%s been done.\n",
        f->tried_implicit ? "" : " not");
@end example

@noindent
Adding @code{gettext} calls to this code cannot give correct results for
all languages, because negation in some languages requires adding words
at more than one place in the sentence.  By contrast, adding
@code{gettext} calls does the job straightforwardly if the code starts
out like this:

@example
printf (f->tried_implicit
        ? "#  Implicit rule search has been done.\n",
        : "#  Implicit rule search has not been done.\n");
@end example

Another example is this one:

@example
printf ("%d file%s processed", nfiles,
        nfiles != 1 ? "s" : "");
@end example

@noindent
The problem with this example is that it assumes that plurals are made
by adding `s'.  If you apply gettext to the format string, like this,

@example
printf (gettext ("%d file%s processed"), nfiles,
        nfiles != 1 ? "s" : "");
@end example

@noindent
the message can use different words, but it will still be forced to use
`s' for the plural.  Here is a better way, with gettext being applied to
the two strings independently:

@example
printf ((nfiles != 1 ? gettext ("%d files processed")
         : gettext ("%d file processed")),
        nfiles);
@end example

@noindent
But this still doesn't work for languages like Polish, which has three
plural forms: one for nfiles == 1, one for nfiles == 2, 3, 4, 22, 23, 24, ...
and one for the rest.  The GNU @code{ngettext} function solves this problem:

@example
printf (ngettext ("%d files processed", "%d file processed", nfiles),
        nfiles);
@end example


@node Character Set
@section Character Set
@cindex character set
@cindex encodings
@cindex ASCII characters
@cindex non-ASCII characters

Sticking to the ASCII character set (plain text, 7-bit characters) is
preferred in GNU source code comments, text documents, and other
contexts, unless there is good reason to do something else because of
the application domain.  For example, if source code deals with the
French Revolutionary calendar, it is OK if its literal strings contain
accented characters in month names like ``Flor@'eal''.  Also, it is OK
(but not required) to use non-ASCII characters to represent proper
names of contributors in change logs (@pxref{Change Logs}).

If you need to use non-ASCII characters, you should normally stick
with one encoding, certainly within a single file.  UTF-8 is likely to
be the best choice.


@node Quote Characters
@section Quote Characters
@cindex quote characters
@cindex locale-specific quote characters
@cindex left quote
@cindex right quote
@cindex opening quote
@cindex single quote
@cindex double quote
@cindex grave accent
@set txicodequoteundirected
@set txicodequotebacktick

In the C locale, the output of GNU programs should stick to plain
ASCII for quotation characters in messages to users: preferably 0x22
(@samp{"}) or 0x27 (@samp{'}) for both opening and closing quotes.
Although GNU programs traditionally used 0x60 (@samp{`}) for opening
and 0x27 (@samp{'}) for closing quotes, nowadays quotes @samp{`like
this'} are typically rendered asymmetrically, so quoting @samp{"like
this"} or @samp{'like this'} typically looks better.

It is ok, but not required, for GNU programs to generate
locale-specific quotes in non-C locales.  For example:

@example
printf (gettext ("Processing file '%s'..."), file);
@end example

@noindent
Here, a French translation might cause @code{gettext} to return the
string @code{"Traitement de fichier
@guilsinglleft{}@tie{}%s@tie{}@guilsinglright{}..."}, yielding quotes
more appropriate for a French locale.

Sometimes a program may need to use opening and closing quotes
directly.  By convention, @code{gettext} translates the string
@samp{"`"} to the opening quote and the string @samp{"'"} to the
closing quote, and a program can use these translations.  Generally,
though, it is better to translate quote characters in the context of
longer strings.

If the output of your program is ever likely to be parsed by another
program, it is good to provide an option that makes this parsing
reliable.  For example, you could escape special characters using
conventions from the C language or the Bourne shell.  See for example
the option @option{--quoting-style} of GNU @code{ls}.

@clear txicodequoteundirected
@clear txicodequotebacktick


@node Mmap
@section Mmap
@findex mmap

If you use @code{mmap} to read or write files, don't assume it either
works on all files or fails for all files.  It may work on some files
and fail on others.

The proper way to use @code{mmap} is to try it on the specific file for
which you want to use it---and if @code{mmap} doesn't work, fall back on
doing the job in another way using @code{read} and @code{write}.

The reason this precaution is needed is that the GNU kernel (the HURD)
provides a user-extensible file system, in which there can be many
different kinds of ``ordinary files''.  Many of them support
@code{mmap}, but some do not.  It is important to make programs handle
all these kinds of files.


@node Documentation
@chapter Documenting Programs
@cindex documentation

A GNU program should ideally come with full free documentation, adequate
for both reference and tutorial purposes.  If the package can be
programmed or extended, the documentation should cover programming or
extending it, as well as just using it.

@menu
* GNU Manuals::                 Writing proper manuals.
* Doc Strings and Manuals::     Compiling doc strings doesn't make a manual.
* Manual Structure Details::    Specific structure conventions.
* License for Manuals::         Writing the distribution terms for a manual.
* Manual Credits::              Giving credit to documentation contributors.
* Printed Manuals::             Mentioning the printed manual.
* NEWS File::                   NEWS files supplement manuals.
* Change Logs::                 Recording changes.
* Man Pages::                   Man pages are secondary.
* Reading other Manuals::       How far you can go in learning
                                from other manuals.
@end menu

@node GNU Manuals
@section GNU Manuals

The preferred document format for the GNU system is the Texinfo
formatting language.  Every GNU package should (ideally) have
documentation in Texinfo both for reference and for learners.  Texinfo
makes it possible to produce a good quality formatted book, using
@TeX{}, and to generate an Info file.  It is also possible to generate
HTML output from Texinfo source.  See the Texinfo manual, either the
hardcopy, or the on-line version available through @code{info} or the
Emacs Info subsystem (@kbd{C-h i}).

Nowadays some other formats such as Docbook and Sgmltexi can be
converted automatically into Texinfo.  It is ok to produce the Texinfo
documentation by conversion this way, as long as it gives good results.

Make sure your manual is clear to a reader who knows nothing about the
topic and reads it straight through.  This means covering basic topics
at the beginning, and advanced topics only later.  This also means
defining every specialized term when it is first used.

Programmers tend to carry over the structure of the program as the
structure for its documentation.  But this structure is not
necessarily good for explaining how to use the program; it may be
irrelevant and confusing for a user.

Instead, the right way to structure documentation is according to the
concepts and questions that a user will have in mind when reading it.
This principle applies at every level, from the lowest (ordering
sentences in a paragraph) to the highest (ordering of chapter topics
within the manual).  Sometimes this structure of ideas matches the
structure of the implementation of the software being documented---but
often they are different.  An important part of learning to write good
documentation is to learn to notice when you have unthinkingly
structured the documentation like the implementation, stop yourself,
and look for better alternatives.

For example, each program in the GNU system probably ought to be
documented in one manual; but this does not mean each program should
have its own manual.  That would be following the structure of the
implementation, rather than the structure that helps the user
understand.

Instead, each manual should cover a coherent @emph{topic}.  For example,
instead of a manual for @code{diff} and a manual for @code{diff3}, we
have one manual for ``comparison of files'' which covers both of those
programs, as well as @code{cmp}.  By documenting these programs
together, we can make the whole subject clearer.

The manual which discusses a program should certainly document all of
the program's command-line options and all of its commands.  It should
give examples of their use.  But don't organize the manual as a list
of features.  Instead, organize it logically, by subtopics.  Address
the questions that a user will ask when thinking about the job that
the program does.  Don't just tell the reader what each feature can
do---say what jobs it is good for, and show how to use it for those
jobs.  Explain what is recommended usage, and what kinds of usage
users should avoid.

In general, a GNU manual should serve both as tutorial and reference.
It should be set up for convenient access to each topic through Info,
and for reading straight through (appendixes aside).  A GNU manual
should give a good introduction to a beginner reading through from the
start, and should also provide all the details that hackers want.
The Bison manual is a good example of this---please take a look at it
to see what we mean.

That is not as hard as it first sounds.  Arrange each chapter as a
logical breakdown of its topic, but order the sections, and write their
text, so that reading the chapter straight through makes sense.  Do
likewise when structuring the book into chapters, and when structuring a
section into paragraphs.  The watchword is, @emph{at each point, address
the most fundamental and important issue raised by the preceding text.}

If necessary, add extra chapters at the beginning of the manual which
are purely tutorial and cover the basics of the subject.  These provide
the framework for a beginner to understand the rest of the manual.  The
Bison manual provides a good example of how to do this.

To serve as a reference, a manual should have an Index that lists all
the functions, variables, options, and important concepts that are
part of the program.  One combined Index should do for a short manual,
but sometimes for a complex package it is better to use multiple
indices.  The Texinfo manual includes advice on preparing good index
entries, see @ref{Index Entries, , Making Index Entries, texinfo, GNU
Texinfo}, and see @ref{Indexing Commands, , Defining the Entries of an
Index, texinfo, GNU Texinfo}.

Don't use Unix man pages as a model for how to write GNU documentation;
most of them are terse, badly structured, and give inadequate
explanation of the underlying concepts.  (There are, of course, some
exceptions.)  Also, Unix man pages use a particular format which is
different from what we use in GNU manuals.

Please include an email address in the manual for where to report
bugs @emph{in the text of the manual}.

Please do not use the term ``pathname'' that is used in Unix
documentation; use ``file name'' (two words) instead.  We use the term
``path'' only for search paths, which are lists of directory names.

Please do not use the term ``illegal'' to refer to erroneous input to
a computer program.  Please use ``invalid'' for this, and reserve the
term ``illegal'' for activities prohibited by law.

Please do not write @samp{()} after a function name just to indicate
it is a function.  @code{foo ()} is not a function, it is a function
call with no arguments.

Whenever possible, please stick to the active voice, avoiding the
passive, and use the present tense, not the future tense.  For
instance, write ``The function @code{foo} returns a list containing
@var{a} and @var{b}'' rather than ``A list containing @var{a} and
@var{b} will be returned.''  One advantage of the active voice is it
requires you to state the subject of the sentence; with the passive
voice, you might omit the subject, which leads to vagueness.

It is proper to use the future tense when grammar demands it, as in,
``If you type @kbd{x}, the computer will self-destruct in 10
seconds.''

@node Doc Strings and Manuals
@section Doc Strings and Manuals

Some programming systems, such as Emacs, provide a documentation string
for each function, command or variable.  You may be tempted to write a
reference manual by compiling the documentation strings and writing a
little additional text to go around them---but you must not do it.  That
approach is a fundamental mistake.  The text of well-written
documentation strings will be entirely wrong for a manual.

A documentation string needs to stand alone---when it appears on the
screen, there will be no other text to introduce or explain it.
Meanwhile, it can be rather informal in style.

The text describing a function or variable in a manual must not stand
alone; it appears in the context of a section or subsection.  Other text
at the beginning of the section should explain some of the concepts, and
should often make some general points that apply to several functions or
variables.  The previous descriptions of functions and variables in the
section will also have given information about the topic.  A description
written to stand alone would repeat some of that information; this
redundancy looks bad.  Meanwhile, the informality that is acceptable in
a documentation string is totally unacceptable in a manual.

The only good way to use documentation strings in writing a good manual
is to use them as a source of information for writing good text.

@node Manual Structure Details
@section Manual Structure Details
@cindex manual structure

The title page of the manual should state the version of the programs or
packages documented in the manual.  The Top node of the manual should
also contain this information.  If the manual is changing more
frequently than or independent of the program, also state a version
number for the manual in both of these places.

Each program documented in the manual should have a node named
@samp{@var{program} Invocation} or @samp{Invoking @var{program}}.  This
node (together with its subnodes, if any) should describe the program's
command line arguments and how to run it (the sort of information people
would look for in a man page).  Start with an @samp{@@example}
containing a template for all the options and arguments that the program
uses.

Alternatively, put a menu item in some menu whose item name fits one of
the above patterns.  This identifies the node which that item points to
as the node for this purpose, regardless of the node's actual name.

The @samp{--usage} feature of the Info reader looks for such a node
or menu item in order to find the relevant text, so it is essential
for every Texinfo file to have one.

If one manual describes several programs, it should have such a node for
each program described in the manual.

@node License for Manuals
@section License for Manuals
@cindex license for manuals

Please use the GNU Free Documentation License for all GNU manuals that
are more than a few pages long.  Likewise for a collection of short
documents---you only need one copy of the GNU FDL for the whole
collection.  For a single short document, you can use a very permissive
non-copyleft license, to avoid taking up space with a long license.

See @uref{https://www.gnu.org/copyleft/fdl-howto.html} for more explanation
of how to employ the GFDL.

Note that it is not obligatory to include a copy of the GNU GPL or GNU
LGPL in a manual whose license is neither the GPL nor the LGPL.  It can
be a good idea to include the program's license in a large manual; in a
short manual, whose size would be increased considerably by including
the program's license, it is probably better not to include it.

@node Manual Credits
@section Manual Credits
@cindex credits for manuals

Please credit the principal human writers of the manual as the authors,
on the title page of the manual.  If a company sponsored the work, thank
the company in a suitable place in the manual, but do not cite the
company as an author.

@node Printed Manuals
@section Printed Manuals

The FSF publishes some GNU manuals in printed form.  To encourage sales
of these manuals, the on-line versions of the manual should mention at
the very start that the printed manual is available and should point at
information for getting it---for instance, with a link to the page
@url{https://www.gnu.org/order/order.html}.  This should not be included
in the printed manual, though, because there it is redundant.

It is also useful to explain in the on-line forms of the manual how the
user can print out the manual from the sources.

@node NEWS File
@section The NEWS File
@cindex @file{NEWS} file

In addition to its manual, the package should have a file named
@file{NEWS} which contains a list of user-visible changes worth
mentioning.  In each new release, add items to the front of the file and
identify the version they pertain to.  Don't discard old items; leave
them in the file after the newer items.  This way, a user upgrading from
any previous version can see what is new.

If the @file{NEWS} file gets very long, move some of the older items
into a file named @file{ONEWS} and put a note at the end referring the
user to that file.

@node Change Logs
@section Change Logs
@cindex change logs

Keep a change log to describe all the changes made to program source
files.  The purpose of this is so that people investigating bugs in the
future will know about the changes that might have introduced the bug.
Often a new bug can be found by looking at what was recently changed.
More importantly, change logs can help you eliminate conceptual
inconsistencies between different parts of a program, by giving you a
history of how the conflicting concepts arose and who they came from.

@menu
* Change Log Concepts::
* Style of Change Logs::
* Simple Changes::
* Conditional Changes::
* Indicating the Part Changed::
@end menu

@node Change Log Concepts
@subsection Change Log Concepts

@cindex change set
@cindex batch of changes
You can think of the change log as a conceptual ``undo list'' which
states how earlier versions were different from the current version.
People can see the current version; they don't need the change log to
tell them what is in it.  What they want from a change log is a clear
explanation of how the earlier version differed.  Each @dfn{entry} in
a change log describes either an individual change or the smallest
batch of changes that belong together, also known as a @dfn{change
set}.

@cindex title, change log entry
@cindex description, change log entry
It is a good idea to start the change log entry with a description
of the overall change.  This should be as long as needed to give
a clear description.

Then give a list of names of the entities or definitions that you
changed, according to the files they are in, and what was changed
in each one.  @xref{Style of Change Logs}.

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
you.

Instead of using a file named @file{ChangeLog}, you can record the
change log information as log entries in a version control system such
as RCS or CVS.  This can be converted automatically 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.

The best place to explain how parts of the new code work with other code
is in comments in the code, not in the change log.

If you think that a change calls for explanation of @emph{why} the
change was needed---that is, what problem the old code had such that
it required this change---you're probably right.  Please put the
explanation in comments in the code, where people will see it whenever
they see the code.  An example of such an explanation is, ``This
function used to be iterative, but that failed when MUMBLE was a
tree.''  (Though such a simple reason would not need this kind of
explanation.)

The best place for other kinds of explanation of the change is in the
change log entry.

The easiest way to add an entry to @file{ChangeLog} is with the Emacs
command @kbd{M-x add-change-log-entry}.  An individual change should
have an asterisk, the name of the changed file, and then in
parentheses the name of the changed functions, variables or whatever,
followed by a colon.  Then describe the changes you made to that
function or variable.

@node Style of Change Logs
@subsection Style of Change Logs
@cindex change logs, style

Here are some simple examples of change log entries, starting with the
header line that says who made the change and when it was installed,
followed by descriptions of specific changes.  (These examples are
drawn from Emacs and GCC.)

@example
1998-08-17  Richard Stallman  <rms@@gnu.org>

* register.el (insert-register): Return nil.
(jump-to-register): Likewise.

* sort.el (sort-subr): Return nil.

* tex-mode.el (tex-bibtex-file, tex-file, tex-region):
Restart the tex shell if process is gone or stopped.
(tex-shell-running): New function.

* expr.c (store_one_arg): Round size up for move_block_to_reg.
(expand_call): Round up when emitting USE insns.
* stmt.c (assign_parms): Round size up for move_block_from_reg.
@end example

It's important to name the changed function or variable in full.  Don't
abbreviate function or variable names, and don't combine them.
Subsequent maintainers will often search for a function name to find all
the change log entries that pertain to it; if you abbreviate the name,
they won't find it when they search.

For example, some people are tempted to abbreviate groups of function
names by writing @samp{* register.el (@{insert,jump-to@}-register)};
this is not a good idea, since searching for @code{jump-to-register} or
@code{insert-register} would not find that entry.

Separate unrelated change log entries with blank lines.  Don't put
blank lines between individual changes of an entry.  You can omit the
file name and the asterisk when successive individual changes are in
the same file.

Break long lists of function names by closing continued lines with
@samp{)}, rather than @samp{,}, and opening the continuation with
@samp{(} as in this example:

@example
* keyboard.c (menu_bar_items, tool_bar_items)
(Fexecute_extended_command): Deal with 'keymap' property.
@end example

When you install someone else's changes, put the contributor's name in
the change log entry rather than in the text of the entry.  In other
words, write this:

@example
2002-07-14  John Doe  <jdoe@@gnu.org>

        * sewing.c: Make it sew.
@end example

@noindent
rather than this:

@example
2002-07-14  Usual Maintainer  <usual@@gnu.org>

        * sewing.c: Make it sew.  Patch by jdoe@@gnu.org.
@end example

As for the date, that should be the date you applied the change.

@node Simple Changes
@subsection Simple Changes

Certain simple kinds of changes don't need much detail in the change
log.

When you change the calling sequence of a function in a simple fashion,
and you change all the callers of the function to use the new calling
sequence, there is no need to make individual entries for all the
callers that you changed.  Just write in the entry for the function
being called, ``All callers changed''---like this:

@example
* keyboard.c (Fcommand_execute): New arg SPECIAL.
All callers changed.
@end example

When you change just comments or doc strings, it is enough to write an
entry for the file, without mentioning the functions.  Just ``Doc
fixes'' is enough for the change log.

There's no technical need to make change log entries for non-software
files (manuals, help files, media files, etc.).  This is because they
are not susceptible to bugs that are hard to understand.  To correct
an error, you need not know the history of the erroneous passage; it
is enough to compare what the file says with the actual facts.

However, you should keep change logs for non-software files when the
project gets copyright assignments from its contributors, so as to
make the records of authorship more accurate.

@node Conditional Changes
@subsection Conditional Changes
@cindex conditional changes, and change logs
@cindex change logs, conditional changes

Source files can often contain code that is conditional to build-time
or static conditions.  For example, C programs can contain
compile-time @code{#if} conditionals; programs implemented in
interpreted languages can contain module imports of function
definitions that are only performed for certain versions of the
interpreter; and Automake @file{Makefile.am} files can contain
variable definitions or target declarations that are only to be
considered if a configure-time Automake conditional is true.

Many changes are conditional as well: sometimes you add a new variable,
or function, or even a new program or library, which is entirely
dependent on a build-time condition.  It is useful to indicate
in the change log the conditions for which a change applies.

Our convention for indicating conditional changes is to use
@emph{square brackets around the name of the condition}.

Conditional changes can happen in numerous scenarios and with many
variations, so here are some examples to help clarify.  This first
example describes changes in C, Perl, and Python files which are
conditional but do not have an associated function or entity name:

@example
* xterm.c [SOLARIS2]: Include <string.h>.
* FilePath.pm [$^O eq 'VMS']: Import the VMS::Feature module.
* framework.py [sys.version_info < (2, 6)]: Make "with" statement
  available by importing it from __future__,
  to support also python 2.5.
@end example

Our other examples will for simplicity be limited to C, as the minor
changes necessary to adapt them to other languages should be
self-evident.

Next, here is an entry describing a new definition which is entirely
conditional: the C macro @code{FRAME_WINDOW_P} is defined (and used)
only when the macro @code{HAVE_X_WINDOWS} is defined:

@example
* frame.h [HAVE_X_WINDOWS] (FRAME_WINDOW_P): Macro defined.
@end example

Next, an entry for a change within the function @code{init_display},
whose definition as a whole is unconditional, but the changes
themselves are contained in a @samp{#ifdef HAVE_LIBNCURSES}
conditional:

@example
* dispnew.c (init_display) [HAVE_LIBNCURSES]: If X, call tgetent.
@end example

Finally, here is an entry for a change that takes effect only when
a certain macro is @emph{not} defined:

@example
* host.c (gethostname) [!HAVE_SOCKETS]: Replace with winsock version.
@end example


@node Indicating the Part Changed
@subsection Indicating the Part Changed

Indicate the part of a function which changed by using angle brackets
enclosing an indication of what the changed part does.  Here is an entry
for a change in the part of the function @code{sh-while-getopts} that
deals with @code{sh} commands:

@example
* progmodes/sh-script.el (sh-while-getopts) <sh>: Handle case that
user-specified option string is empty.
@end example


@node Man Pages
@section Man Pages
@cindex man pages

In the GNU project, man pages are secondary.  It is not necessary or
expected for every GNU program to have a man page, but some of them do.
It's your choice whether to include a man page in your program.

When you make this decision, consider that supporting a man page
requires continual effort each time the program is changed.  The time
you spend on the man page is time taken away from more useful work.

For a simple program which changes little, updating the man page may be
a small job.  Then there is little reason not to include a man page, if
you have one.

For a large program that changes a great deal, updating a man page may
be a substantial burden.  If a user offers to donate a man page, you may
find this gift costly to accept.  It may be better to refuse the man
page unless the same person agrees to take full responsibility for
maintaining it---so that you can wash your hands of it entirely.  If
this volunteer later ceases to do the job, then don't feel obliged to
pick it up yourself; it may be better to withdraw the man page from the
distribution until someone else agrees to update it.

When a program changes only a little, you may feel that the
discrepancies are small enough that the man page remains useful without
updating.  If so, put a prominent note near the beginning of the man
page stating that you don't maintain it and that the Texinfo manual
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
(@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
Manuals}).

Finally, the GNU help2man program
(@uref{https://www.gnu.org/software/help2man/}) is one way to automate
generation of a man page, in this case from @option{--help} output.
This is sufficient in many cases.

@node Reading other Manuals
@section Reading other Manuals

There may be non-free books or documentation files that describe the
program you are documenting.

It is ok to use these documents for reference, just as the author of a
new algebra textbook can read other books on algebra.  A large portion
of any non-fiction book consists of facts, in this case facts about how
a certain program works, and these facts are necessarily the same for
everyone who writes about the subject.  But be careful not to copy your
outline structure, wording, tables or examples from preexisting non-free
documentation.  Copying from free documentation may be ok; please check
with the FSF about the individual case.

@node Managing Releases
@chapter The Release Process
@cindex releasing

Making a release is more than just bundling up your source files in a
tar file and putting it up for FTP.  You should set up your software so
that it can be configured to run on a variety of systems.  Your Makefile
should conform to the GNU standards described below, and your directory
layout should also conform to the standards discussed below.  Doing so
makes it easy to include your package into the larger framework of
all GNU software.

@menu
* Configuration::               How configuration of GNU packages should work.
* Makefile Conventions::        Makefile conventions.
* Releases::                    Making releases
@end menu

@node Configuration
@section How Configuration Should Work
@cindex program configuration

@pindex configure
Each GNU distribution should come with a shell script named
@code{configure}.  This script is given arguments which describe the
kind of machine and system you want to compile the program for.
The @code{configure} script must record the configuration options so
that they affect compilation.

The description here is the specification of the interface for the
@code{configure} script in GNU packages.  Many packages implement it
using GNU Autoconf (@pxref{Top,, Introduction, autoconf, Autoconf})
and/or GNU Automake (@pxref{Top,, Introduction, automake, Automake}),
but you do not have to use these tools.  You can implement it any way
you like; for instance, by making @code{configure} be a wrapper around
a completely different configuration system.

Another way for the @code{configure} script to operate is to make a
link from a standard name such as @file{config.h} to the proper
configuration file for the chosen system.  If you use this technique,
the distribution should @emph{not} contain a file named
@file{config.h}.  This is so that people won't be able to build the
program without configuring it first.

Another thing that @code{configure} can do is to edit the Makefile.  If
you do this, the distribution should @emph{not} contain a file named
@file{Makefile}.  Instead, it should include a file @file{Makefile.in} which
contains the input used for editing.  Once again, this is so that people
won't be able to build the program without configuring it first.

If @code{configure} does write the @file{Makefile}, then @file{Makefile}
should have a target named @file{Makefile} which causes @code{configure}
to be rerun, setting up the same configuration that was set up last
time.  The files that @code{configure} reads should be listed as
dependencies of @file{Makefile}.

All the files which are output from the @code{configure} script should
have comments at the beginning stating that they were generated
automatically using @code{configure}.  This is so that users won't think
of trying to edit them by hand.

The @code{configure} script should write a file named @file{config.status}
which describes which configuration options were specified when the
program was last configured.  This file should be a shell script which,
if run, will recreate the same configuration.

The @code{configure} script should accept an option of the form
@samp{--srcdir=@var{dirname}} to specify the directory where sources are found
(if it is not the current directory).  This makes it possible to build
the program in a separate directory, so that the actual source directory
is not modified.

If the user does not specify @samp{--srcdir}, then @code{configure} should
check both @file{.} and @file{..} to see if it can find the sources.  If
it finds the sources in one of these places, it should use them from
there.  Otherwise, it should report that it cannot find the sources, and
should exit with nonzero status.

Usually the easy way to support @samp{--srcdir} is by editing a
definition of @code{VPATH} into the Makefile.  Some rules may need to
refer explicitly to the specified source directory.  To make this
possible, @code{configure} can add to the Makefile a variable named
@code{srcdir} whose value is precisely the specified directory.

In addition, the @samp{configure} script should take options
corresponding to most of the standard directory variables
(@pxref{Directory Variables}).  Here is the list:

@example
--prefix --exec-prefix --bindir --sbindir --libexecdir --sysconfdir
--sharedstatedir --localstatedir --runstatedir
--libdir --includedir --oldincludedir
--datarootdir --datadir --infodir --localedir --mandir --docdir
--htmldir --dvidir --pdfdir --psdir
@end example

The @code{configure} script should also take an argument which specifies the
type of system to build the program for.  This argument should look like
this:

@example
@var{cpu}-@var{company}-@var{system}
@end example

For example, an Athlon-based GNU/Linux system might be
@samp{i686-pc-linux-gnu}.

The @code{configure} script needs to be able to decode all plausible
alternatives for how to describe a machine.  Thus,
@samp{athlon-pc-gnu/linux} would be a valid alias.  There is a shell
script called
@uref{https://git.savannah.gnu.org/@/gitweb/@/?p=config.git;a=blob_plain;f=config.sub;hb=HEAD,
@file{config.sub}} that you can use as a subroutine to validate system
types and canonicalize aliases.

The @code{configure} script should also take the option
@option{--build=@var{buildtype}}, which should be equivalent to a
plain @var{buildtype} argument.  For example, @samp{configure
--build=i686-pc-linux-gnu} is equivalent to @samp{configure
i686-pc-linux-gnu}.  When the build type is not specified by an option
or argument, the @code{configure} script should normally guess it using
the shell script
@uref{https://git.savannah.gnu.org/@/gitweb/@/?p=config.git;a=blob_plain;f=config.guess;hb=HEAD,
@file{config.guess}}.

@cindex optional features, configure-time
Other options are permitted to specify in more detail the software
or hardware present on the machine, to include or exclude optional parts
of the package, or to adjust the name of some tools or arguments to them:

@table @samp
@item --enable-@var{feature}@r{[}=@var{parameter}@r{]}
Configure the package to build and install an optional user-level
facility called @var{feature}.  This allows users to choose which
optional features to include.  Giving an optional @var{parameter} of
@samp{no} should omit @var{feature}, if it is built by default.

No @samp{--enable} option should @strong{ever} cause one feature to
replace another.  No @samp{--enable} option should ever substitute one
useful behavior for another useful behavior.  The only proper use for
@samp{--enable} is for questions of whether to build part of the program
or exclude it.

@item --with-@var{package}
@c @r{[}=@var{parameter}@r{]}
The package @var{package} will be installed, so configure this package
to work with @var{package}.

@c  Giving an optional @var{parameter} of
@c @samp{no} should omit @var{package}, if it is used by default.

Possible values of @var{package} include
@samp{gnu-as} (or @samp{gas}), @samp{gnu-ld}, @samp{gnu-libc},
@samp{gdb},
@samp{x},
and
@samp{x-toolkit}.

Do not use a @samp{--with} option to specify the file name to use to
find certain files.  That is outside the scope of what @samp{--with}
options are for.

@item @var{variable}=@var{value}
Set the value of the variable @var{variable} to @var{value}.  This is
used to override the default values of commands or arguments in the
build process.  For example, the user could issue @samp{configure
CFLAGS=-g CXXFLAGS=-g} to build with debugging information and without
the default optimization.

Specifying variables as arguments to @code{configure}, like this:
@example
./configure CC=gcc
@end example
is preferable to setting them in environment variables:
@example
CC=gcc ./configure
@end example
as it helps to recreate the same configuration later with
@file{config.status}.  However, both methods should be supported.
@end table

All @code{configure} scripts should accept all of the ``detail''
options and the variable settings, whether or not they make any
difference to the particular package at hand.  In particular, they
should accept any option that starts with @samp{--with-} or
@samp{--enable-}.  This is so users will be able to configure an
entire GNU source tree at once with a single set of options.

You will note that the categories @samp{--with-} and @samp{--enable-}
are narrow: they @strong{do not} provide a place for any sort of option
you might think of.  That is deliberate.  We want to limit the possible
configuration options in GNU software.  We do not want GNU programs to
have idiosyncratic configuration options.

Packages that perform part of the compilation process may support
cross-compilation.  In such a case, the host and target machines for the
program may be different.

The @code{configure} script should normally treat the specified type of
system as both the host and the target, thus producing a program which
works for the same type of machine that it runs on.

To compile a program to run on a host type that differs from the build
type, use the configure option @option{--host=@var{hosttype}}, where
@var{hosttype} uses the same syntax as @var{buildtype}.  The host type
normally defaults to the build type.

To configure a cross-compiler, cross-assembler, or what have you, you
should specify a target different from the host, using the configure
option @samp{--target=@var{targettype}}.  The syntax for
@var{targettype} is the same as for the host type.  So the command would
look like this:

@example
./configure --host=@var{hosttype} --target=@var{targettype}
@end example

The target type normally defaults to the host type.
Programs for which cross-operation is not meaningful need not accept the
@samp{--target} option, because configuring an entire operating system for
cross-operation is not a meaningful operation.

Some programs have ways of configuring themselves automatically.  If
your program is set up to do this, your @code{configure} script can simply
ignore most of its arguments.

@comment The makefile standards are in a separate file that is also
@comment included by make.texinfo.  Done by roland@gnu.ai.mit.edu on 1/6/93.
@comment For this document, turn chapters into sections, etc.
@lowersections
@include make-stds.texi
@raisesections

@node Releases
@section Making Releases
@cindex packaging

@cindex version numbers, for releases
You should identify each release with a pair of version numbers, a
major version and a minor.  We have no objection to using more than
two numbers, but it is very unlikely that you really need them.

Package the distribution of @code{Foo version 69.96} up in a gzipped tar
file with the name @file{foo-69.96.tar.gz}.  It should unpack into a
subdirectory named @file{foo-69.96}.

Building and installing the program should never modify any of the files
contained in the distribution.  This means that all the files that form
part of the program in any way must be classified into @dfn{source
files} and @dfn{non-source files}.  Source files are written by humans
and never changed automatically; non-source files are produced from
source files by programs under the control of the Makefile.

@cindex @file{README} file
The distribution should contain a file named @file{README} with a
general overview of the package:

@itemize
@item the name of the package;

@item the version number of the package, or refer to where in the
package the version can be found;

@item a general description of what the package does;

@item a reference to the file @file{INSTALL}, which
should in turn contain an explanation of the installation procedure;

@item a brief explanation of any unusual top-level directories or
files, or other hints for readers to find their way around the source;

@item a reference 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.LESSER}.
@end itemize

Naturally, all the source files must be in the distribution.  It is
okay to include non-source files in the distribution along with the
source files they are generated from, provided they are up-to-date
with the source they are made from, and machine-independent, so that
normal building of the distribution will never modify them.  We
commonly include non-source files produced by Autoconf, Automake,
Bison, @code{flex}, @TeX{}, and @code{makeinfo}; this helps avoid
unnecessary dependencies between our distributions, so that users can
install whichever versions of whichever packages they like.  Do not
induce new dependencies on other software lightly.

Non-source files that might actually be modified by building and
installing the program should @strong{never} be included in the
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 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
systems that don't support symbolic links.  Also, don't use multiple
names for one file in different directories, because certain file
systems cannot handle this and that prevents unpacking the
distribution.

Try to make sure that all the file names will be unique on MS-DOS.  A
name on MS-DOS consists of up to 8 characters, optionally followed by a
period and up to three characters.  MS-DOS will truncate extra
characters both before and after the period.  Thus,
@file{foobarhacker.c} and @file{foobarhacker.o} are not ambiguous; they
are truncated to @file{foobarha.c} and @file{foobarha.o}, which are
distinct.

@cindex @file{texinfo.tex}, in a distribution
Include in your distribution a copy of the @file{texinfo.tex} you used
to test print any @file{*.texinfo} or @file{*.texi} files.

Likewise, if your program uses small GNU software packages like regex,
getopt, obstack, or termcap, include them in the distribution file.
Leaving them out would make the distribution file a little smaller at
the expense of possible inconvenience to a user who doesn't know what
other files to get.

@node References
@chapter References to Non-Free Software and Documentation
@cindex references to non-free material

A GNU program should not recommend, promote, or grant legitimacy to
the use of any non-free program.  Proprietary software is a social and
ethical problem, and our aim is to put an end to that problem.  We
can't stop some people from writing proprietary programs, or stop
other people from using them, but we can and should refuse to
advertise them to new potential customers, or to give the public the
idea that their existence is ethical.

The GNU definition of free software is found on the GNU web site at
@url{https://www.gnu.org/@/philosophy/@/free-sw.html}, and the definition
of free documentation is found at
@url{https://www.gnu.org/@/philosophy/@/free-doc.html}.  The terms ``free''
and ``non-free'', used in this document, refer to those definitions.

A list of important licenses and whether they qualify as free is in
@url{https://www.gnu.org/@/licenses/@/license-list.html}.  If it is not
clear whether a license qualifies as free, please ask the GNU Project
by writing to @email{licensing@@gnu.org}.  We will answer, and if the
license is an important one, we will add it to the list.

When a non-free program or system is well known, you can mention it in
passing---that is harmless, since users who might want to use it
probably already know about it.  For instance, it is fine to explain
how to build your package on top of some widely used non-free
operating system, or how to use it together with some widely used
non-free program.

However, you should give only the necessary information to help those
who already use the non-free program to use your program with
it---don't give, or refer to, any further information about the
proprietary program, and don't imply that the proprietary program
enhances your program, or that its existence is in any way a good
thing.  The goal should be that people already using the proprietary
program will get the advice they need about how to use your free
program with it, while people who don't already use the proprietary
program will not see anything likely to lead them to take an interest
in it.

If a non-free program or system is obscure in your program's domain,
your program should not mention or support it at all, since doing so
would tend to popularize the non-free program more than it popularizes
your program.  (You cannot hope to find many additional users for your
program among the users of Foobar, if the existence of Foobar is not
generally known among people who might want to use your program.)

Sometimes a program is free software in itself but depends on a
non-free platform in order to run.  For instance, many Java programs
depend on some non-free Java libraries.  To recommend or promote such
a program is to promote the other programs it needs.  This is why we
are careful about listing Java programs in the Free Software
Directory: we don't want to promote the non-free Java libraries.

We hope this particular problem with Java will be gone by and by, as
we replace the remaining non-free standard Java libraries with free
software, but the general principle will remain the same: don't
recommend, promote or legitimize programs that depend on non-free
software to run.

Some free programs strongly encourage the use of non-free software.  A
typical example is @command{mplayer}.  It is free software in itself,
and the free code can handle some kinds of files.  However,
@command{mplayer} recommends use of non-free codecs for other kinds of
files, and users that install @command{mplayer} are very likely to
install those codecs along with it.  To recommend @command{mplayer}
is, in effect, to promote use of the non-free codecs.

Thus, you should not recommend programs that strongly encourage the
use of non-free software.  This is why we do not list
@command{mplayer} in the Free Software Directory.

A GNU package should not refer the user to any non-free documentation
for free software.  Free documentation that can be included in free
operating systems is essential for completing the GNU system, or any
free operating system, so encouraging it is a priority; to recommend
use of documentation that we are not allowed to include undermines the
impetus for the community to produce documentation that we can
include.  So GNU packages should never recommend non-free
documentation.

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 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
program is promoting that program, so please do not make links to (or
mention by name) web sites that contain such material.  This policy is
relevant particularly for the web pages for a GNU package.

Following links from nearly any web site can lead eventually to
non-free software; this is inherent in the nature of the web.  So it
makes no sense to criticize a site for having such links.  As long as
the site does not itself recommend a non-free program, there is no
need to consider the question of the sites that it links to for other
reasons.

Thus, for example, you should not refer to AT&T's web site if that
recommends AT&T's non-free software packages; you should not refer to
a page @var{p} that links to AT&T's site presenting it as a place to get
some non-free program, because that part of the page @var{p} itself
recommends and legitimizes the non-free program.  However, that @var{p}
contains a link to AT&T's web site for some other purpose (such as
long-distance telephone service) is not an objection against it.

A web page recommends a program in a particularly strong way if it
requires users to run that program in order to use the page.  Many
pages contain Javascript code which they recommend in this way.  This
Javascript code may be free or nonfree, but nonfree is the usual case.

If the purpose for which you would refer to the page cannot be carried
out without running nonfree Javascript code, then you should not refer
to it.  Thus, if the purpose of referring to the page is for people to
view a video, or subscribing to a mailing list, and the viewing or
subscribing fail to work if the user's browser blocks the nonfree
Javascript code, then don't refer to that page.

The extreme case is that of web sites which depend on nonfree
Javascript code even to @emph{see} the contents of the pages.  Any
site hosted on @indicateurl{wix.com} has this problem, and so do some
other sites.  Referring people to such pages to read their contents
is, in effect, urging them to run those nonfree programs---so please
don't refer to those pages.  (Such pages also break the Web, so they
deserve condemnation for two reasons.)

Instead, please quote excerpts from the page to make your point,
or find another place to refer to that information.

@node GNU Free Documentation License
@appendix GNU Free Documentation License

@cindex FDL, GNU Free Documentation License
@include fdl.texi

@node Index
@unnumbered Index
@printindex cp

@bye

Local variables:
eval: (add-hook 'before-save-hook 'time-stamp)
time-stamp-start: "@set lastupdate "
time-stamp-end: "$"
time-stamp-format: "%:b %:d, %:y"
compile-command: "cd work.s && make"
End: