path: root/etc/standards.texi

\input texinfo @c -*-texinfo-*-
@c %**start of header
@setfilename standards.info
@settitle GNU Coding Standards
@c UPDATE THIS DATE WHENEVER YOU MAKE CHANGES!
@set lastupdate 16 January 1997
@c %**end of header

@ifinfo
@format
START-INFO-DIR-ENTRY
* Standards: (standards).        GNU coding standards.
END-INFO-DIR-ENTRY
@end format
@end ifinfo

@c @setchapternewpage odd
@setchapternewpage off

@c This is used by a cross ref in make-stds.texi
@set CODESTD  1
@iftex
@set CHAPTER chapter
@end iftex
@ifinfo
@set CHAPTER node
@end ifinfo

@ifinfo
GNU Coding Standards
Copyright (C) 1992, 1993, 1994, 1995, 1996 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

@ignore
Permission is granted to process this file through TeX and print the
results, provided the printed document carries copying permission
notice identical to this one except for the removal of this paragraph
(this paragraph not being relevant to the printed manual).
@end ignore

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the entire
resulting derived work is distributed under the terms of a permission
notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions,
except that this permission notice may be stated in a translation approved
by the Free Software Foundation.
@end ifinfo

@titlepage
@title GNU Coding Standards
@author Richard Stallman
@author last updated @value{lastupdate}
@page

@vskip 0pt plus 1filll
Copyright @copyright{} 1992, 1993, 1994, 1995, 1996 Free Software Foundation, Inc.

Permission is granted to make and distribute verbatim copies of
this manual provided the copyright notice and this permission notice
are preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the entire
resulting derived work is distributed under the terms of a permission
notice identical to this one.

Permission is granted to copy and distribute translations of this manual
into another language, under the above conditions for modified versions,
except that this permission notice may be stated in a translation approved
by the Free Software Foundation.
@end titlepage

@ifinfo
@node Top, Preface, (dir), (dir)
@top Version

Last updated @value{lastupdate}.
@end ifinfo

@menu
* Preface::                     About the GNU Coding Standards
* Intellectual Property::       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
@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.

Corrections or suggestions regarding this document should be sent to
@code{gnu@@prep.ai.mit.edu}.  If you make a suggestion, please include a
suggested new wording for it; our time is limited.  We prefer a context
diff to the @file{standards.texi} or @file{make-stds.texi} files, but if
you don't have those files, please mail your suggestion anyway.

This release of the GNU Coding Standards was last updated
@value{lastupdate}.

@node Intellectual Property
@chapter Keeping Free Software Free

This @value{CHAPTER} discusses how you can make sure that GNU software
remains unencumbered.

@menu
* Reading Non-Free Code::       Referring to Proprietary Programs
* Contributions::               Accepting Contributions
@end menu

@node Reading Non-Free Code
@section Referring to Proprietary Programs

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 core 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

If someone else sends you a piece of code to add to the program you are
working on, we need legal papers to use it---the same sort of legal
papers we will need to get from you.  @emph{Each} significant
contributor 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, 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 it.

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 you write a different solution to the
problem, you don't need to get papers.

We know this is frustrating; 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!

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.

@node Design Advice
@chapter General Program Design

This @value{CHAPTER} discusses some of the issues you should take into
account when designing your program.

@menu
* Compatibility::               Compatibility with other implementations
* Using Extensions::            Using non-standard features
* ANSI C::                      Using ANSI C features
* Source Language::             Using languages other than C
@end menu

@node Compatibility
@section Compatibility with Other Implementations

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

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

@sc{ansi} C and @sc{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.  Try to
redesign its interface.

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 not in Berkeley Unix are welcome.
Additional programs with no counterpart in Unix may be useful,
but our first priority is usually to duplicate what Unix already
has.

@node Using Extensions
@section Using Non-standard Features

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.  Such programs would
be broken by use of GNU extensions.

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 no good.

@node ANSI C
@section @sc{ansi} C and pre-@sc{ansi} C

Do not ever use the ``trigraph'' feature of @sc{ansi} C.

@sc{ansi} C is widespread enough now that it is ok to write new programs
that use @sc{ansi} C features (and therefore will not work in
non-@sc{ansi} compilers).  And if a program is already written in
@sc{ansi} C, there's no need to convert it to support non-@sc{ansi}
compilers.

However, it is easy to support non-@sc{ansi} compilers in most programs,
so you might still consider doing so when you write a program.  Instead
of writing function definitions in @sc{ansi} prototype form,

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

@noindent
write the definition in pre-@sc{ansi} 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 @sc{ansi} C prototypes in all the files where the function is called.
And once you have it, you lose nothing by writing the function
definition in the pre-@sc{ansi} style.

If you don't know non-@sc{ansi} C, there's no need to learn it; just
write in @sc{ansi} C.

@node Source Language
@section Using Languages Other Than C

Using a language other than C is like using a non-standard feature: it
will cause trouble for users.  Even if GCC supports the other language,
users may find it inconvenient to have to install the compiler for that
other language in order to build your program.  So please write in C.

There are three exceptions for this rule:

@itemize @bullet
@item
It is okay to use a special language if the same program contains an
interpreter for that language.

For example, if your program links with GUILE, it is ok to write part of
the program in Scheme or another language supported by GUILE.

@item
It is okay to use another language in a tool specifically intended for
use with that language.

This is okay because the only people who want to build the tool will be
those who have installed the other language anyway.

@item
If an application is not of extremely widespread interest, then perhaps
it's not important if the application is inconvenient to install.
@end itemize

@node Program Behavior
@chapter Program Behavior for All Programs

This @value{CHAPTER} describes how to write robust software. It also
describes general standards for error messages, the command line interface,
and how libraries should behave.

@menu
* Semantics::                   Writing robust programs
* Libraries::                   Library behavior
* Errors::                      Formatting error messages
* User Interfaces::             Standards for command line interfaces
* Option Table::                Table of long options.
* Memory Usage::                When and how to care about memory needs
@end menu

@node Semantics
@section Writing Robust Programs

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.

Utilities reading files should not drop NUL characters, or any other
nonprinting characters @emph{including those with codes above 0177}.  The
only sensible exceptions would be utilities specifically intended for
interface to certain types of printers that can't handle those characters.

Check every system call for an error return, unless you know you wish to
ignore errors.  Include the system error text (from @code{perror} 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.

Check every call to @code{malloc} or @code{realloc} to see if it
returned zero.  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.

In Unix, @code{realloc} can destroy the storage block if it returns
zero.  GNU @code{realloc} does not have this bug: if it fails, the
original block is unchanged.  Feel free to assume the bug is fixed.  If
you wish to run your program on Unix, and wish to avoid lossage in this
case, you can use the GNU @code{malloc}.

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.

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.  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 will be supported compatibly by GNU.

By default, the GNU system will provide the signal handling functions of
@sc{BSD} and of @sc{POSIX}.  So GNU software should be written to use
these.

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.

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

@node Libraries
@section Library Behavior

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{_}.  They should also contain
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

Error messages from compilers should look like this:

@example
@var{source-file-name}:@var{lineno}: @var{message}
@end example

Error messages from other noninteractive programs should look like this:

@example
@var{program}:@var{source-file-name}:@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.

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.  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 Command Line Interfaces

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.

Likewise, please don't make the behavior of the program depend on the
type of output device it is used with.  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.

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.

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.

It is a good idea to follow the @sc{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 @sc{POSIX}
specifies; it is a GNU extension.

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 idiosyncracies for users to remember.

All programs should support two standard options: @samp{--version}
and @samp{--help}.

@table @code
@item --version
This option should direct the program to 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.

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 @strong{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 don't mention all 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 brief statement that the program is free software,
and that users are free to copy and change it on certain conditions.  If
the program is covered by the GNU GPL, say so here.  Also mention that
there is no warranty, to the extent permitted by law.

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 Emacs 19.34.5
Copyright (C) 1996 Free Software Foundation, Inc.
GNU Emacs comes with NO WARRANTY, to the extent permitted by law.
You may redistribute copies of GNU Emacs
under the terms of the GNU General Public License.
For more information about these matters, see the files named COPYING.
@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.

@item --help
This 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.

Near the end of the @samp{--help} option's output there should be a line
that says where to mail bug reports.  It should have this format:

@example
Report bugs to @var{mailing-address}.
@end example
@end table

@node Option Table
@section 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 @samp{gnu@@prep.ai.mit.edu} 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 Make.

@item assume-old
@samp{-o} in 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 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 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 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 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 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{info}, @code{gawk}, Make, @code{mt}, and @code{tar};
@samp{-n} in @code{sed};
@samp{-r} in @code{touch}.

@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 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 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 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 Make.

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

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

@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 jobs
@samp{-j} in Make.

@item just-print
@samp{-n} in Make.

@item keep-going
@samp{-k} in 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 Make.

@item login
Used in @code{su}.

@item machine
No listing of which programs already use this;
someone should check to
see if any actually do and tell @code{gnu@@prep.ai.mit.edu}.

@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 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 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 Make.

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

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

@item question
@samp{-q} in Make.

@item quiet
Used in many programs to inhibit the usual output.  @strong{Note:} 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 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-limit
Used in @code{makeinfo}.

@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.
@strong{Note:} every program accepting
@samp{--silent} should accept @samp{--quiet} as a synonym.

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

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

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

@item touch
@samp{-t} in 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 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 Memory Usage
@section Memory Usage

If it 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 core 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 core all at once.

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

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

This @value{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 and Functions
* System Portability::          Portability between different operating systems
* CPU Portability::             Supporting the range of CPU types
* System Functions::            Portability and ``standard'' library functions
* Internationalization::        Techniques for internationalization
* Mmap::                        How you can safely use @code{mmap}.
@end menu

@node Formatting
@section Formatting Your Source Code

It is important to put the open-brace that starts the body of a C
function in column zero, and avoid putting any other open-brace or
open-parenthesis or open-bracket in column zero.  Several tools look
for open-braces in column zero to find the beginnings of C functions.
These tools will not work on code not formatted that way.

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

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

@noindent
or, if you want to use @sc{ansi} C, format the definition like this:

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

In @sc{ansi} 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

For the body of the function, we prefer code formatted 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

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:

@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,
but Emacs would mess it up:

@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

But adding a set of parentheses solves the problem:

@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

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

Every program should start with a comment saying briefly what it is for.
Example: @samp{fmt - filter for simple filling of text}.

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 the reader can see that for himself.
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

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
@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
@end example


@node Syntactic Conventions
@section Clean Use of C Constructs

Please explicitly declare all arguments to functions.
Don't omit them just because they are @code{int}s.

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.

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 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.

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.  For example,
don't write this:

@example
if ((foo = (char *) malloc (sizeof *foo)) == 0)
  fatal ("virtual memory exhausted");
@end example

@noindent
instead, write this:

@example
foo = (char *) malloc (sizeof *foo);
if (foo == 0)
  fatal ("virtual memory exhausted");
@end example

Don't make the program ugly to placate @code{lint}.  Please don't insert any
casts to @code{void}.  Zero without a cast is perfectly fine as a null
pointer constant, except when calling a varargs function.

@node  Names
@section Naming Variables and Functions

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.

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.

Use file names of 14 characters or less, to avoid creating gratuitous
problems on older System V systems.  You can use the program
@code{doschk} to test for this.  @code{doschk} also tests for potential
name conflicts if the files were loaded onto an MS-DOS file
system---something you may or may not care about.

@node System Portability
@section 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 @sc{cpu}.  The
amount and kinds of variation among GNU systems on different @sc{cpu}s
will be comparable to the variation among Linux-based GNU systems or
among BSD systems today.  So the kinds of portability that are absolutely
necessary are quite limited.

But many users do run GNU software on non-GNU Unix or Unix-like systems.
So supporting a variety of Unix-like systems is desirable, although not
paramount.

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}).

As for systems that are not like Unix, such as MSDOS, Windows, the
Macintosh, VMS, and MVS, supporting them is usually so much work that it
is better if you don't.

The planned GNU kernel is not finished yet, but you can tell which
facilities it will provide by looking at the GNU C Library Manual.  The
GNU kernel is based on Mach, so the features of Mach will also be
available.  However, if you use Mach features, you'll probably have
trouble debugging your program today.

@node CPU Portability
@section Portability between @sc{cpu}s

Even GNU systems will differ because of differences among @sc{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.

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

When calling functions, you need not worry about the difference between
pointers of various types, or between pointers and integers.  On most
machines, there's no difference anyway.  As for the few machines where
there is a difference, all of them support @sc{ansi} C, so you can use
prototypes (conditionalized to be active only in @sc{ansi} C) to make
the code work on those systems.

In certain cases, it is ok to pass integer and pointer arguments
indiscriminately to the same function, and use no prototype on any
system.  For example, many GNU programs have error-reporting functions
that pass their arguments along to @code{printf} and friends:

@example
error (s, a1, a2, a3)
     char *s;
     int a1, a2, a3;
@{
  fprintf (stderr, "error: ");
  fprintf (stderr, s, a1, a2, a3);
@}
@end example

@noindent
In practice, this works on all machines, and it is much simpler than any
``correct'' alternative.  Be sure @emph{not} to use a prototype
for such functions.

However, avoid casting pointers to integers unless you really need to.
These assumptions really 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---it is ok to do so, but you'll have to
make explicit provisions to handle different word sizes.

@node System Functions
@section Calling System Functions

C implementations differ substantially.  @sc{ansi} C reduces but does not
eliminate the incompatibilities; meanwhile, many users wish to compile
GNU software with pre-@sc{ansi} compilers.  This chapter gives
recommendations for how to use the more or less standard C library
functions to avoid unnecessary loss of portability.

@itemize @bullet
@item
Don't use the value of @code{sprintf}.  It returns the number of
characters written on some systems, but not on all systems.

@item
@code{main} should be declared to return type @code{int}.  It should
terminate either by calling @code{exit} or by returning the integer
status code; make sure it cannot ever return an undefined value.

@item
Don't declare system functions explicitly.

Almost any declaration for a system function is wrong on some system.
To minimize conflicts, leave it to the system header files to declare
system functions.  If the headers don't declare a function, let it
remain undeclared.

While it may seem unclean to use a function without declaring it, in
practice this works fine for most system library functions on the
systems where this really happens; thus, the disadvantage is only
theoretical.  By contrast, actual declarations have frequently caused
actual conflicts.

@item
If you must declare a system function, don't specify the argument types.
Use an old-style declaration, not an @sc{ansi} prototype.  The more you
specify about the function, the more likely a conflict.

@item
In particular, don't unconditionally declare @code{malloc} or
@code{realloc}.

Most GNU programs use those functions just once, in functions
conventionally named @code{xmalloc} and @code{xrealloc}.  These
functions call @code{malloc} and @code{realloc}, respectively, and
check the results.

Because @code{xmalloc} and @code{xrealloc} are defined in your program,
you can declare them in other files without any risk of type conflict.

On most systems, @code{int} is the same length as a pointer; thus, the
calls to @code{malloc} and @code{realloc} work fine.  For the few
exceptional systems (mostly 64-bit machines), you can use
@strong{conditionalized} declarations of @code{malloc} and
@code{realloc}---or put these declarations in configuration files
specific to those systems.

@item
The string functions require special treatment.  Some Unix systems have
a header file @file{string.h}; others have @file{strings.h}.  Neither
file name is portable.  There are two things you can do: use Autoconf to
figure out which file to include, or don't include either file.

@item
If you don't include either strings file, you can't get declarations for
the string functions from the header file in the usual way.

That causes less of a problem than you might think.  The newer @sc{ansi}
string functions should be avoided anyway because many systems still
don't support them.  The string functions you can use are these:

@example
strcpy   strncpy   strcat   strncat
strlen   strcmp    strncmp
strchr   strrchr
@end example

The copy and concatenate functions work fine without a declaration as
long as you don't use their values.  Using their values without a
declaration fails on systems where the width of a pointer differs from
the width of @code{int}, and perhaps in other cases.  It is trivial to
avoid using their values, so do that.

The compare functions and @code{strlen} work fine without a declaration
on most systems, possibly all the ones that GNU software runs on.
You may find it necessary to declare them @strong{conditionally} on a
few systems.

The search functions must be declared to return @code{char *}.  Luckily,
there is no variation in the data type they return.  But there is
variation in their names.  Some systems give these functions the names
@code{index} and @code{rindex}; other systems use the names
@code{strchr} and @code{strrchr}.  Some systems support both pairs of
names, but neither pair works on all systems.

You should pick a single pair of names and use it throughout your
program.  (Nowadays, it is better to choose @code{strchr} and
@code{strrchr} for new programs, since those are the standard @sc{ansi}
names.)  Declare both of those names as functions returning @code{char
*}.  On systems which don't support those names, define them as macros
in terms of the other pair.  For example, here is what to put at the
beginning of your file (or in a header) if you want to use the names
@code{strchr} and @code{strrchr} throughout:

@example
#ifndef HAVE_STRCHR
#define strchr index
#endif
#ifndef HAVE_STRRCHR
#define strrchr rindex
#endif

char *strchr ();
char *strrchr ();
@end example
@end itemize

Here we assume that @code{HAVE_STRCHR} and @code{HAVE_STRRCHR} are
macros defined in systems where the corresponding functions exist.
One way to get them properly defined is to use Autoconf.

@node Internationalization
@section Internationalization

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'..."));
@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{fileutils} for the GNU file utilities.

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:

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

@noindent
The problem with that 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:

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

@noindent
This way, you can apply gettext to each of the two strings
independently:

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

@noindent
This can any method of forming the plural of the word for ``file'', and
also handles languages that require agreement in the word for
``processed''.

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 straightfowardly 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

@node Mmap
@section Mmap

Don't assume that @code{mmap} 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

@menu
* GNU Manuals::                 Writing proper manuals.
* Manual Structure Details::    Specific structure conventions.
* 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 way to document part of the GNU system is to write a
manual in the Texinfo formatting language.  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}).

Programmers often find it most natural to structure the documentation
following the structure of the implementation, which they know.  But
this structure is not necessarily good for explaining how to use the
program; it may be irrelevant and confusing for a user.

At every level, from the sentences in a paragraph to the grouping of
topics into separate manuals, the right way to structure documentation
is according to the concepts and questions that a user will have in mind
when reading it.  Sometimes this structure of ideas matches the
structure of the implementation of the software being documented---but
often they are different.  Often the most important part of learning to
write good documentation is learning to notice when you are structuring
the documentation like the implementation, and think about 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 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.

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.

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.

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
exceptions.)  Also Unix man pages use a particular format which is
different from what we use in GNU manuals.

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 file 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 violations of law.

@node Manual Structure Details
@section Manual Structure Details

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 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 in a man page for).  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.

There will be automatic features for specifying a program name and
quickly reading just this part of its manual.

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

@node NEWS File
@section The 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

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::         
@end menu

@node Change Log Concepts
@subsection Change Log Concepts

You can think of the change log as a conceptual ``undo list'' which
explains 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.

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.

Another alternative is to record change log information with a version
control system such as RCS or CVS.  This can be converted automatically
to a @file{ChangeLog} file.

There's no need to describe the full purpose of the changes or how they
work together.  If you think that a change calls for explanation, you're
probably right.  Please do explain it---but please put the explanation
in comments in the code, where people will see it whenever they see the
code.  For example, ``New function'' is enough for the change log when
you add a function, because there should be a comment before the
function definition to explain what it does.

However, sometimes it is useful to write one line to describe the
overall purpose of a batch of changes.

The easiest way to add an entry to @file{ChangeLog} is with the Emacs
command @kbd{M-x add-change-log-entry}.  An entry should have an
asterisk, the name of the changed file, and then in parentheses the name
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

Here are some examples of change log entries:

@example
* 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.  When two
entries represent parts of the same change, so that they work together,
then don't put blank lines between them.  Then you can omit the file
name and the asterisk when successive entries are in the same file.

@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, 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.''

@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 need to make change log entries for documentation files.
This is because documentation is not susceptible to bugs that are hard
to fix.  Documentation does not consist of parts that must interact in a
precisely engineered fashion.  To correct an error, you need not know
the history of the erroneous passage; it is enough to compare what the
documentation says with the way the program actually works.

@node Conditional Changes
@subsection Conditional Changes

C programs often contain compile-time @code{#if} conditionals.  Many
changes are conditional; sometimes you add a new definition which is
entirely contained in a conditional.  It is very useful to indicate in
the change log the conditions for which the change applies.

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

Here is a simple example, describing a change which is conditional but
does not have a function or entity name associated with it:

@example
* xterm.c [SOLARIS2]: Include string.h.
@end example

Here is an entry describing a new definition which is entirely
conditional.  This new definition for the macro @code{FRAME_WINDOW_P} is
used only when @code{HAVE_X_WINDOWS} is defined:

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

Here is 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

Here is an entry for a change that takes affect only when
a certain macro is @emph{not} defined:

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

@node Man Pages
@section 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 explaining that you don't maintain it and that the Texinfo manual
is more authoritative.  The note should say how to access the Texinfo
documentation.

@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

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 Should Work
* Makefile Conventions::	Makefile Conventions
* Releases::                    Making Releases
@end menu

@node Configuration
@section How Configuration Should Work

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.

One way to do this 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 explaining 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.

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, a Sun 3 might be @samp{m68k-sun-sunos4.1}.

The @code{configure} script needs to be able to decode all plausible
alternatives for how to describe a machine.  Thus, @samp{sun3-sunos4.1}
would be a valid alias.  For many programs, @samp{vax-dec-ultrix} would
be an alias for @samp{vax-dec-bsd}, simply because the differences
between Ultrix and @sc{BSD} are rarely noticeable, but a few programs
might need to distinguish them.
@c Real 4.4BSD now runs on some Suns.

There is a shell script called @file{config.sub} that you can use
as a subroutine to validate system types and canonicalize aliases.

Other options are permitted to specify in more detail the software
or hardware present on the machine, and include or exclude optional
parts of the package:

@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{x}, @samp{x-toolkit},
@samp{gnu-as} (or @samp{gas}), @samp{gnu-ld}, @samp{gnu-libc}, and
@samp{gdb}.

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 --nfp
The target machine has no floating point processor.

@item --gas
The target machine assembler is GAS, the GNU assembler.
This is obsolete; users should use @samp{--with-gnu-as} instead.

@item --x
The target machine has the X Window System installed.
This is obsolete; users should use @samp{--with-x} instead.
@end table

All @code{configure} scripts should accept all of these ``detail''
options, 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.

The way to build a cross-compiler, cross-assembler, or what have you, is
to specify the option @samp{--host=@var{hosttype}} when running
@code{configure}.  This specifies the host system without changing the
type of target system.  The syntax for @var{hosttype} is the same as
described above.

Bootstrapping a cross-compiler requires compiling it on a machine other
than the host it will run on.  Compilation packages accept a
configuration option @samp{--build=@var{hosttype}} for specifying the
configuration on which you will compile them, in case that is different
from the host.

Programs for which cross-operation is not meaningful need not accept the
@samp{--host} option, because configuring an entire operating system for
cross-operation is not a meaningful thing.

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

Package the distribution of Foo version 69.96 in a gzipped tar file
named @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.

Naturally, all the source files must be in the distribution.  It is okay
to include non-source files in the distribution, provided they are
up-to-date and machine-independent, so that building the distribution
normally will never modify them.  We commonly include non-source files
produced by Bison, @code{lex}, @TeX{}, and @code{makeinfo}; this helps avoid
unnecessary dependencies between our distributions, so that users can
install whichever packages they want to install.

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 the directory into which the distribution unpacks (as
well as any subdirectories) are all world-writable (octal mode 777).
This is so that old versions of @code{tar} which preserve the
ownership and permissions of the files from the tar archive will be
able to extract all the files even if the user is unprivileged.

Make sure that all the files in the distribution are world-readable.

Make sure that no file name in the distribution is more than 14
characters long.  Likewise, no file created by building the program
should have a name longer than 14 characters.  The reason for this is
that some systems adhere to a foolish interpretation of the POSIX
standard, and refuse to open a longer name, rather than truncating as
they did in the past.

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.

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.

@contents

@bye