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* Standards: (standards). GNU coding standards.
The GNU coding standards, last updated April 12, 2010.
Copyright (C) 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000,
2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010 Free Software
Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with no
Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
Texts. A copy of the license is included in the section entitled "GNU
Free Documentation License".

File:, Node: Top, Next: Preface, Prev: (dir), Up: (dir)
The GNU coding standards, last updated April 12, 2010.
Copyright (C) 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000,
2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010 Free Software
Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with no
Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
Texts. A copy of the license is included in the section entitled "GNU
Free Documentation License".
* Menu:
* Preface:: About the GNU Coding Standards.
* Legal Issues:: Keeping free software free.
* Design Advice:: General program design.
* Program Behavior:: Program behavior for all programs
* Writing C:: Making the best use of C.
* Documentation:: Documenting programs.
* Managing Releases:: The release process.
* References:: Mentioning non-free software or documentation.
* GNU Free Documentation License:: Copying and sharing this manual.
* Index::

File:, Node: Preface, Next: Legal Issues, Prev: Top, Up: Top
1 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.
If you did not obtain this file directly from the GNU project and
recently, please check for a newer version. You can get the GNU Coding
Standards from the GNU web server in many different formats, including
the Texinfo source, PDF, HTML, DVI, plain text, and more, at:
If you are maintaining an official GNU package, in addition to this
document, please read and follow the GNU maintainer information (*note
Contents: (maintain)Top.).
If you want to receive diffs for every change to these GNU documents,
join the mailing list `', via the web
interface at
`'. Archives
are also available there.
Please send corrections or suggestions for this document to
<>. If you make a suggestion, please include a
suggested new wording for it, to help us consider the suggestion
efficiently. We prefer a context diff to the Texinfo source, but if
that's difficult for you, you can make a context diff for some other
version of this document, or propose it in any way that makes it clear.
The source repository for this document can be found at
These standards cover the minimum of what is important when writing a
GNU package. Likely, the need for additional standards will come up.
Sometimes, you might suggest that such standards be added to this
document. If you think your standards would be generally useful, please
do suggest them.
You should also set standards for your package on many questions not
addressed or not firmly specified here. The most important point is to
be self-consistent--try to stick to the conventions you pick, and try
to document them as much as possible. That way, your program will be
more maintainable by others.
The GNU Hello program serves as an example of how to follow the GNU
coding standards for a trivial program.
This release of the GNU Coding Standards was last updated April 12,

File:, Node: Legal Issues, Next: Design Advice, Prev: Preface, Up: Top
2 Keeping Free Software Free
This chapter discusses how you can make sure that GNU software avoids
legal difficulties, and other related issues.
* Menu:
* Reading Non-Free Code:: Referring to proprietary programs.
* Contributions:: Accepting contributions.
* Trademarks:: How we deal with trademark issues.

File:, Node: Reading Non-Free Code, Next: Contributions, Up: Legal Issues
2.1 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 memory and scan it
there instead of using stdio. Use a smarter algorithm discovered more
recently than the Unix program. Eliminate use of temporary files. Do
it in one pass instead of two (we did this in the assembler).
Or, on the contrary, emphasize simplicity instead of speed. For some
applications, the speed of today's computers makes simpler algorithms
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

File:, Node: Contributions, Next: Trademarks, Prev: Reading Non-Free Code, Up: Legal Issues
2.2 Accepting Contributions
If the program you are working on is copyrighted by the Free Software
Foundation, then when someone else sends you a piece of code to add to
the program, we need legal papers to use it--just as we asked you to
sign papers initially. _Each_ person who makes a nontrivial
contribution to a program must sign some sort of legal papers in order
for us to have clear title to the program; the main author alone is not
So, before adding in any contributions from other people, please tell
us, so we can arrange to get the papers. Then wait until we tell you
that we have received the signed papers, before you actually use the
This applies both before you release the program and afterward. If
you receive diffs to fix a bug, and they make significant changes, we
need legal papers for that change.
This also applies to comments and documentation files. For copyright
law, comments and code are just text. Copyright applies to all kinds of
text, so we need legal papers for all kinds.
We know it is frustrating to ask for legal papers; it's frustrating
for us as well. But if you don't wait, you are going out on a limb--for
example, what if the contributor's employer won't sign a disclaimer?
You might have to take that code out again!
You don't need papers for changes of a few lines here or there, since
they are not significant for copyright purposes. Also, you don't need
papers if all you get from the suggestion is some ideas, not actual code
which you use. For example, if someone sent you one implementation, but
you write a different implementation of the same idea, you don't need to
get papers.
The very worst thing is if you forget to tell us about the other
contributor. We could be very embarrassed in court some day as a
We have more detailed advice for maintainers of programs; if you have
reached the stage of actually maintaining a program for GNU (whether
released or not), please ask us for a copy. It is also available
online for your perusal: `'.

File:, Node: Trademarks, Prev: Contributions, Up: Legal Issues
2.3 Trademarks
Please do not include any trademark acknowledgements in GNU software
packages or documentation.
Trademark acknowledgements are the statements that such-and-such is a
trademark of so-and-so. The GNU Project has no objection to the basic
idea of trademarks, but these acknowledgements feel like kowtowing, and
there is no legal requirement for them, so we don't use them.
What is legally required, as regards other people's trademarks, is to
avoid using them in ways which a reader might reasonably understand as
naming or labeling our own programs or activities. For example, since
"Objective C" is (or at least was) a trademark, we made sure to say
that we provide a "compiler for the Objective C language" rather than
an "Objective C compiler". The latter would have been meant as a
shorter way of saying the former, but it does not explicitly state the
relationship, so it could be misinterpreted as using "Objective C" as a
label for the compiler rather than for the language.
Please don't use "win" as an abbreviation for Microsoft Windows in
GNU software or documentation. In hacker terminology, calling
something a "win" is a form of praise. If you wish to praise Microsoft
Windows when speaking on your own, by all means do so, but not in GNU
software. Usually we write the name "Windows" in full, but when
brevity is very important (as in file names and sometimes symbol
names), we abbreviate it to "w". For instance, the files and functions
in Emacs that deal with Windows start with `w32'.

File:, Node: Design Advice, Next: Program Behavior, Prev: Legal Issues, Up: Top
3 General Program Design
This chapter discusses some of the issues you should take into account
when designing your program.
* Menu:
* Source Language:: Which languages to use.
* Compatibility:: Compatibility with other implementations.
* Using Extensions:: Using non-standard features.
* Standard C:: Using standard C features.
* Conditional Compilation:: Compiling code only if a conditional is true.

File:, Node: Source Language, Next: Compatibility, Up: Design Advice
3.1 Which Languages to Use
When you want to use a language that gets compiled and runs at high
speed, the best language to use is C. Using another language 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. For example, if you write your program in C++, people will
have to install the GNU C++ compiler in order to compile your program.
C has one other advantage over C++ and other compiled languages: more
people know C, so more people will find it easy to read and modify the
program if it is written in C.
So in general it is much better to use C, rather than the comparable
But there are two exceptions to that conclusion:
* It is no problem to use another language to write a tool
specifically intended for use with that language. That is because
the only people who want to build the tool will be those who have
installed the other language anyway.
* If an application is of interest only to a narrow part of the
community, then the question of which language it is written in
has less effect on other people, so you may as well please
Many programs are designed to be extensible: they include an
interpreter for a language that is higher level than C. Often much of
the program is written in that language, too. The Emacs editor
pioneered this technique.
The standard extensibility interpreter for GNU software is Guile
(`'), which implements the language
Scheme (an especially clean and simple dialect of Lisp). Guile also
includes bindings for GTK+/GNOME, making it practical to write modern
GUI functionality within Guile. We don't reject programs written in
other "scripting languages" such as Perl and Python, but using Guile is
very important for the overall consistency of the GNU system.

File:, Node: Compatibility, Next: Using Extensions, Prev: Source Language, Up: Design Advice
3.2 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 Standard C if Standard C specifies their behavior, and
upward compatible with POSIX if POSIX specifies their behavior.
When these standards conflict, it is useful to offer compatibility
modes for each of them.
Standard C and POSIX prohibit many kinds of extensions. Feel free
to make the extensions anyway, and include a `--ansi', `--posix', or
`--compatible' option to turn them off. However, if the extension has
a significant chance of breaking any real programs or scripts, then it
is not really upward compatible. So you should try to redesign its
interface to make it upward compatible.
Many GNU programs suppress extensions that conflict with POSIX if the
environment variable `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,
`vi' is replaced with Emacs.) But it is nice to offer a compatible
feature as well. (There is a free `vi' clone, so we offer it.)
Additional useful features are welcome regardless of whether there
is any precedent for them.

File:, Node: Using Extensions, Next: Standard C, Prev: Compatibility, Up: Design Advice
3.3 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" `INLINE' and
define that as a macro to expand into either `inline' or nothing,
depending on the compiler.
In general, perhaps it is best not to use the extensions if you can
straightforwardly do without them, but to use the extensions if they
are a big improvement.
An exception to this rule are the large, established programs (such
as Emacs) which run on a great variety of systems. Using GNU
extensions in such programs would make many users unhappy, so we don't
do that.
Another exception is for programs that are used as part of
compilation: anything that must be compiled with other compilers in
order to bootstrap the GNU compilation facilities. If these require
the GNU compiler, then no one can compile them without having them
installed already. That would be extremely troublesome in certain

File:, Node: Standard C, Next: Conditional Compilation, Prev: Using Extensions, Up: Design Advice
3.4 Standard C and Pre-Standard C
1989 Standard C is widespread enough now that it is ok to use its
features in new programs. There is one exception: do not ever use the
"trigraph" feature of Standard C.
1999 Standard C is not widespread yet, so please do not require its
features in programs. It is ok to use its features if they are present.
However, it is easy to support pre-standard compilers in most
programs, so if you know how to do that, feel free. If a program you
are maintaining has such support, you should try to keep it working.
To support pre-standard C, instead of writing function definitions in
standard prototype form,
foo (int x, int y)
write the definition in pre-standard style like this,
foo (x, y)
int x, y;
and use a separate declaration to specify the argument prototype:
int foo (int, int);
You need such a declaration anyway, in a header file, to get the
benefit of prototypes in all the files where the function is called.
And once you have the declaration, you normally lose nothing by writing
the function definition in the pre-standard style.
This technique does not work for integer types narrower than `int'.
If you think of an argument as being of a type narrower than `int',
declare it as `int' instead.
There are a few special cases where this technique is hard to use.
For example, if a function argument needs to hold the system type
`dev_t', you run into trouble, because `dev_t' is shorter than `int' on
some machines; but you cannot use `int' instead, because `dev_t' is
wider than `int' on some machines. There is no type you can safely use
on all machines in a non-standard definition. The only way to support
non-standard C and pass such an argument is to check the width of
`dev_t' using Autoconf and choose the argument type accordingly. This
may not be worth the trouble.
In order to support pre-standard compilers that do not recognize
prototypes, you may want to use a preprocessor macro like this:
/* Declare the prototype for a general external function. */
#if defined (__STDC__) || defined (WINDOWSNT)
#define P_(proto) proto
#define P_(proto) ()

File:, Node: Conditional Compilation, Prev: Standard C, Up: Design Advice
3.5 Conditional Compilation
When supporting configuration options already known when building your
program we prefer using `if (... )' over conditional compilation, as in
the former case the compiler is able to perform more extensive checking
of all possible code paths.
For example, please write
if (HAS_FOO)
instead of:
#ifdef HAS_FOO
A modern compiler such as GCC will generate exactly the same code in
both cases, and we have been using similar techniques with good success
in several projects. Of course, the former method assumes that
`HAS_FOO' is defined as either 0 or 1.
While this is not a silver bullet solving all portability problems,
and is not always appropriate, following this policy would have saved
GCC developers many hours, or even days, per year.
In the case of function-like macros like `REVERSIBLE_CC_MODE' in GCC
which cannot be simply used in `if (...)' statements, there is an easy
workaround. Simply introduce another macro `HAS_REVERSIBLE_CC_MODE' as
in the following example:

File:, Node: Program Behavior, Next: Writing C, Prev: Design Advice, Up: Top
4 Program Behavior for All Programs
This chapter describes conventions for writing robust software. It
also describes general standards for error messages, the command line
interface, and how libraries should behave.
* Menu:
* Non-GNU Standards:: We consider standards such as POSIX;
we don't "obey" them.
* Semantics:: Writing robust programs.
* Libraries:: Library behavior.
* Errors:: Formatting error messages.
* User Interfaces:: Standards about interfaces generally.
* Graphical Interfaces:: Standards for graphical interfaces.
* Command-Line Interfaces:: Standards for command line interfaces.
* Option Table:: Table of long options.
* OID Allocations:: Table of OID slots for GNU.
* Memory Usage:: When and how to care about memory needs.
* File Usage:: Which files to use, and where.

File:, Node: Non-GNU Standards, Next: Semantics, Up: Program Behavior
4.1 Non-GNU Standards
The GNU Project regards standards published by other organizations as
suggestions, not orders. We consider those standards, but we do not
"obey" them. In developing a GNU program, you should implement an
outside standard's specifications when that makes the GNU system better
overall in an objective sense. When it doesn't, you shouldn't.
In most cases, following published standards is convenient for
users--it means that their programs or scripts will work more portably.
For instance, GCC implements nearly all the features of Standard C as
specified by that standard. C program developers would be unhappy if
it did not. And GNU utilities mostly follow specifications of POSIX.2;
shell script writers and users would be unhappy if our programs were
But we do not follow either of these specifications rigidly, and
there are specific points on which we decided not to follow them, so as
to make the GNU system better for users.
For instance, Standard C says that nearly all extensions to C are
prohibited. How silly! GCC implements many extensions, some of which
were later adopted as part of the standard. If you want these
constructs to give an error message as "required" by the standard, you
must specify `--pedantic', which was implemented only so that we can
say "GCC is a 100% implementation of the standard," not because there
is any reason to actually use it.
POSIX.2 specifies that `df' and `du' must output sizes by default in
units of 512 bytes. What users want is units of 1k, so that is what we
do by default. If you want the ridiculous behavior "required" by
POSIX, you must set the environment variable `POSIXLY_CORRECT' (which
was originally going to be named `POSIX_ME_HARDER').
GNU utilities also depart from the letter of the POSIX.2
specification when they support long-named command-line options, and
intermixing options with ordinary arguments. This minor
incompatibility with POSIX is never a problem in practice, and it is
very useful.
In particular, don't reject a new feature, or remove an old one,
merely because a standard says it is "forbidden" or "deprecated."

File:, Node: Semantics, Next: Libraries, Prev: Non-GNU Standards, Up: Program Behavior
4.2 Writing Robust Programs
Avoid arbitrary limits on the length or number of _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 _including those with codes above 0177_. The
only sensible exceptions would be utilities specifically intended for
interface to certain types of terminals or printers that can't handle
those characters. Whenever possible, try to make programs work
properly with sequences of bytes that represent multibyte characters,
using encodings such as UTF-8 and others.
Check every system call for an error return, unless you know you
wish to ignore errors. Include the system error text (from `perror' or
equivalent) in _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 `malloc' or `realloc' to see if it returned
zero. Check `realloc' even if you are making the block smaller; in a
system that rounds block sizes to a power of 2, `realloc' may get a
different block if you ask for less space.
In Unix, `realloc' can destroy the storage block if it returns zero.
GNU `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 `malloc'.
You must expect `free' to alter the contents of the block that was
freed. Anything you want to fetch from the block, you must fetch before
calling `free'.
If `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 `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.
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 `readdir' or some other high-level interface.
These are supported compatibly by GNU.
The preferred signal handling facilities are the BSD variant of
`signal', and the POSIX `sigaction' function; the alternative USG
`signal' interface is an inferior design.
Nowadays, using the POSIX signal functions may be the easiest way to
make a program portable. If you use `signal', then on GNU/Linux
systems running GNU libc version 1, you should include `bsd/signal.h'
instead of `signal.h', so as to get BSD behavior. It is up to you
whether to support systems where `signal' has only the USG behavior, or
give up on them.
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
Do not use a count of errors as the exit status for a program.
_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 `TMPDIR' environment
variable; if that variable is defined, use the specified directory
instead of `/tmp'.
In addition, be aware that there is a possible security problem when
creating temporary files in world-writable directories. In C, you can
avoid this problem by creating temporary files in this manner:
fd = open (filename, O_WRONLY | O_CREAT | O_EXCL, 0600);
or by using the `mkstemps' function from libiberty.
In bash, use `set -C' to avoid this problem.

File:, Node: Libraries, Next: Errors, Prev: Semantics, Up: Program Behavior
4.3 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 `malloc' itself.
Here are certain name conventions for libraries, to avoid name
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 `_'. The `_' should be followed by
the chosen name prefix for the library, to prevent collisions with
other libraries. These can go in the same files with user entry points
if you like.
Static functions and variables can be used as you like and need not
fit any naming convention.

File:, Node: Errors, Next: User Interfaces, Prev: Libraries, Up: Program Behavior
4.4 Formatting Error Messages
Error messages from compilers should look like this:
If you want to mention the column number, use one of these formats:
Line numbers should start from 1 at the beginning of the file, and
column numbers should start from 1 at the beginning of the line. (Both
of these conventions are chosen for compatibility.) Calculate column
numbers assuming that space and all ASCII printing characters have
equal width, and assuming tab stops every 8 columns.
The error message can also give both the starting and ending
positions of the erroneous text. There are several formats so that you
can avoid redundant information such as a duplicate line number. Here
are the possible formats:
When an error is spread over several files, you can use this format:
Error messages from other noninteractive programs should look like
when there is an appropriate source file, or like this:
when there is no relevant source file.
If you want to mention the column number, use this format:
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 MESSAGE should not begin with a capital letter when it
follows a program name and/or file name, because that isn't the
beginning of a sentence. (The sentence conceptually starts at the
beginning of the line.) Also, it should not end with a period.
Error messages from interactive programs, and other messages such as
usage messages, should start with a capital letter. But they should not
end with a period.

File:, Node: User Interfaces, Next: Graphical Interfaces, Prev: Errors, Up: Program Behavior
4.5 Standards for Interfaces Generally
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. (Variation in error
message syntax when using a terminal is ok, because that is a side issue
that people do not depend on.)
If you think one behavior is most useful when the output is to a
terminal, and another is most useful when the output is a file or a
pipe, then it is usually best to make the default behavior the one that
is useful with output to a terminal, and have an option for the other
Compatibility requires certain programs to depend on the type of
output device. It would be disastrous if `ls' or `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 `dir' program much like
`ls' except that its default output format is always multi-column

File:, Node: Graphical Interfaces, Next: Command-Line Interfaces, Prev: User Interfaces, Up: Program Behavior
4.6 Standards for Graphical Interfaces
When you write a program that provides a graphical user interface,
please make it work with the X Window System and the GTK+ toolkit
unless the functionality specifically requires some alternative (for
example, "displaying jpeg images while in console mode").
In addition, please provide a command-line interface to control the
functionality. (In many cases, the graphical user interface can be a
separate program which invokes the command-line program.) This is so
that the same jobs can be done from scripts.
Please also consider providing a D-bus interface for use from other
running programs, such as within GNOME. (GNOME used to use CORBA for
this, but that is being phased out.) In addition, consider providing a
library interface (for use from C), and perhaps a keyboard-driven
console interface (for use by users from console mode). Once you are
doing the work to provide the functionality and the graphical
interface, these won't be much extra work.

File:, Node: Command-Line Interfaces, Next: Option Table, Prev: Graphical Interfaces, Up: Program Behavior
4.7 Standards for Command Line Interfaces
It is a good idea to follow the POSIX guidelines for the command-line
options of a program. The easiest way to do this is to use `getopt' to
parse them. Note that the GNU version of `getopt' will normally permit
options anywhere among the arguments unless the special argument `--'
is used. This is not what 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
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 `--verbose'. To achieve this uniformity, look at the
table of common long-option names when you choose the option names for
your program (*note 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 `-o' or `--output'). Even if you allow an output
file name as an ordinary argument for compatibility, try to provide an
option as another way to specify it. This will lead to more consistency
among GNU utilities, and fewer idiosyncrasies for users to remember.
All programs should support two standard options: `--version' and
`--help'. CGI programs should accept these as command-line options,
and also if given as the `PATH_INFO'; for instance, visiting
`' in a browser should output the same
information as invoking `p.cgi --help' from the command line.
* Menu:
* --version:: The standard output for --version.
* --help:: The standard output for --help.

File:, Node: --version, Next: --help, Up: Command-Line Interfaces
4.7.1 `--version'
The standard `--version' option should direct the program to print
information about its name, version, origin and legal status, all on
standard output, and then exit successfully. Other options and
arguments should be ignored once this is seen, and the program should
not perform its normal function.
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:
GNU Emacs 19.30
The program's name should be a constant string; _don't_ compute it from
`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 `PATH'.
If the program is a subsidiary part of a larger package, mention the
package name in parentheses, like this:
emacsserver (GNU Emacs) 19.30
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 _need_ to mention the version numbers of libraries which are
distributed separately from the package which contains this program,
you can do so by printing an additional line of version info for each
library you want to mention. Use the same format for these lines as for
the first line.
Please do not mention all of the libraries that the program uses
"just for completeness"--that would produce a lot of unhelpful clutter.
Please mention library version numbers only if you find in practice that
they are very important to you in debugging.
The following line, after the version number line or lines, should
be a copyright notice. If more than one copyright notice is called
for, put each on a separate line.
Next should follow a line stating the license, preferably using one
of abbrevations below, and a brief statement that the program is free
software, and that users are free to copy and change it. Also mention
that there is no warranty, to the extent permitted by law. See
recommended wording below.
It is ok to finish the output with a list of the major authors of the
program, as a way of giving credit.
Here's an example of output that follows these rules:
GNU hello 2.3
Copyright (C) 2007 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
You should adapt this to your program, of course, filling in the
proper year, copyright holder, name of program, and the references to
distribution terms, and changing the rest of the wording as necessary.
This copyright notice only needs to mention the most recent year in
which changes were made--there's no need to list the years for previous
versions' changes. You don't have to mention the name of the program in
these notices, if that is inconvenient, since it appeared in the first
line. (The rules are different for copyright notices in source files;
*note Copyright Notices: (maintain)Copyright Notices.)
Translations of the above lines must preserve the validity of the
copyright notices (*note Internationalization::). If the translation's
character set supports it, the `(C)' should be replaced with the
copyright symbol, as follows:
(the official copyright symbol, which is the letter C in a circle);
Write the word "Copyright" exactly like that, in English. Do not
translate it into another language. International treaties recognize
the English word "Copyright"; translations into other languages do not
have legal significance.
Finally, here is the table of our suggested license abbreviations.
Any abbreviation can be followed by `vVERSION[+]', meaning that
particular version, or later versions with the `+', as shown above.
In the case of exceptions for extra permissions with the GPL, we use
`/' for a separator; the version number can follow the license
abbreviation as usual, as in the examples below.
GNU General Public License, `'.
GNU Lesser General Public License,
GNU GPL with the exception for Ada.
The Apache Software Foundation license,
The Artistic license used for Perl,
The Expat license, `'.
The Mozilla Public License, `'.
The original (4-clause) BSD license, incompatible with the GNU GPL
The license used for PHP, `'.
public domain
The non-license that is being in the public domain,
The license for Python, `'.
The revised (3-clause) BSD, compatible with the GNU GPL,
The simple non-copyleft license used for most versions of the X
Window System, `'.
The license for Zlib, `'.
More information about these licenses and many more are on the GNU
licensing web pages, `'.

File:, Node: --help, Prev: --version, Up: Command-Line Interfaces
4.7.2 `--help'
The standard `--help' option should output brief documentation for how
to invoke the program, on standard output, then exit successfully.
Other options and arguments should be ignored once this is seen, and
the program should not perform its normal function.
Near the end of the `--help' option's output, please place lines
giving the email address for bug reports, the package's home page
(normally <>, and the general page for
help using GNU programs. The format should be like this:
Report bugs to: MAILING-ADDRESS
PKG home page: <>
General help using GNU software: <>
It is ok to mention other appropriate mailing lists and web pages.

File:, Node: Option Table, Next: OID Allocations, Prev: Command-Line Interfaces, Up: Program Behavior
4.8 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 <> a list of them, with their
meanings, so we can update the table.
`-N' in `tar'.
`-a' in `du', `ls', `nm', `stty', `uname', and `unexpand'.
`-a' in `diff'.
`-A' in `ls'.
`-a' in `etags', `tee', `time'; `-r' in `tar'.
`-a' in `cp'.
`-n' in `shar'.
`-l' in `m4'.
`-a' in `diff'.
`-v' in `gawk'.
`-W' in `make'.
`-o' in `make'.
`-a' in `recode'.
`-a' in `wdiff'.
`-A' in `ptx'.
`-n' in `wdiff'.
For server programs, run in the background.
`-B' in `ctags'.
`-f' in `shar'.
Used in GDB.
Used in GDB.
`-b' in `tac'.
`-b' in `cpio' and `diff'.
`-b' in `shar'.
Used in `cpio' and `tar'.
`-b' in `head' and `tail'.
`-b' in `ptx'.
Used in various programs to make output shorter.
`-c' in `head', `split', and `tail'.
`-C' in `etags'.
`-A' in `tar'.
Used in various programs to specify the directory to use.
`-c' in `chgrp' and `chown'.
`-F' in `ls'.
`-c' in `recode'.
`-c' in `su'; `-x' in GDB.
`-d' in `tar'.
Used in `gawk'.
`-Z' in `tar' and `shar'.
`-A' in `tar'.
`-w' in `tar'.
Used in `diff'.
`-W copyleft' in `gawk'.
`-C' in `ptx', `recode', and `wdiff'; `-W copyright' in `gawk'.
Used in GDB.
`-q' in `who'.
`-l' in `du'.
Used in `tar' and `cpio'.
`-c' in `shar'.
`-x' in `ctags'.
`-d' in `touch'.
`-d' in `make' and `m4'; `-t' in Bison.
`-D' in `m4'.
`-d' in Bison and `ctags'.
`-D' in `tar'.
`-L' in `chgrp', `chown', `cpio', `du', `ls', and `tar'.
`-D' in `du'.
Specify an I/O device (special file name).
`-d' in `recode'.
`-d' in `look'.
`-d' in `tar'.
`-n' in `csplit'.
Specify the directory to use, in various programs. In `ls', it
means to show directories themselves rather than their contents.
In `rm' and `ln', it means to not treat links to directories
`-x' in `strip'.
`-X' in `strip'.
`-n' in `make'.
`-e' in `diff'.
`-z' in `csplit'.
`-x' in `wdiff'.
`-z' in `wdiff'.
`-N' in `diff'.
`-e' in `make'.
`-e' in `xargs'.
Used in GDB.
Used in `makeinfo'.
`-o' in `m4'.
`-b' in `ls'.
`-X' in `tar'.
Used in GDB.
`-x' in `xargs'.
`-e' in `unshar'.
`-t' in `diff'.
`-e' in `sed'.
`-g' in `nm'.
`-i' in `cpio'; `-x' in `tar'.
`-f' in `finger'.
`-f' in `su'.
`-E' in `m4'.
`-f' in `gawk', `info', `make', `mt', `sed', and `tar'.
`-F' in `gawk'.
`-b' in Bison.
`-F' in `ls'.
`-T' in `tar'.
Used in `makeinfo'.
`-F' in `ptx'.
`-y' in Bison.
`-f' in `tail'.
Used in `makeinfo'.
`-f' in `cp', `ln', `mv', and `rm'.
`-F' in `shar'.
For server programs, run in the foreground; in other words, don't
do anything special to run the server in the background.
Used in `ls', `time', and `ptx'.
`-F' in `m4'.
Used in GDB.
`-g' in `ptx'.
`-x' in `tar'.
`-i' in `ul'.
`-g' in `recode'.
`-g' in `install'.
`-z' in `tar' and `shar'.
`-H' in `m4'.
`-h' in `objdump' and `recode'
`-H' in `who'.
Used to ask for brief usage information.
`-d' in `shar'.
`-q' in `ls'.
In `makeinfo', output HTML.
`-u' in `who'.
`-D' in `diff'.
`-I' in `ls'; `-x' in `recode'.
`-w' in `diff'.
`-B' in `ls'.
`-B' in `diff'.
`-f' in `look' and `ptx'; `-i' in `diff' and `wdiff'.
`-i' in `make'.
`-i' in `ptx'.
`-I' in `etags'.
`-f' in Oleo.
`-i' in `tee'.
`-I' in `diff'.
`-b' in `diff'.
`-i' in `tar'.
`-i' in `etags'; `-I' in `m4'.
`-I' in `make'.
`-G' in `tar'.
`-i', `-l', and `-m' in Finger.
In some programs, specify the name of the file to read as the
user's init file.
`-i' in `expand'.
`-T' in `diff'.
`-i' in `ls'.
`-i' in `cp', `ln', `mv', `rm'; `-e' in `m4'; `-p' in `xargs';
`-w' in `tar'.
`-p' in `shar'.
Used in `date'
`-j' in `make'.
`-n' in `make'.
`-k' in `make'.
`-k' in `csplit'.
`-k' in `du' and `ls'.
`-l' in `etags'.
`-l' in `wdiff'.
`-g' in `shar'.
`-C' in `split'.
Used in `split', `head', and `tail'.
`-l' in `cpio'.
Used in `gawk'.
`-t' in `cpio'; `-l' in `recode'.
`-t' in `tar'.
`-N' in `ls'.
`-l' in `make'.
Used in `su'.
Used in `uname'.
`-M' in `ptx'.
`-m' in `hello' and `uname'.
`-d' in `cpio'.
`-f' in `make'.
Used in GDB.
`-n' in `xargs'.
`-n' in `xargs'.
`-l' in `xargs'.
`-l' in `make'.
`-P' in `xargs'.
`-T' in `who'.
`-T' in `who'.
`-d' in `diff'.
`-M' in `shar'.
`-m' in `install', `mkdir', and `mkfifo'.
`-m' in `tar'.
`-M' in `tar'.
`-a' in Bison.
`-L' in `m4'.
`-a' in `shar'.
`-W' in `make'.
`-r' in `make'.
`-w' in `shar'.
`-x' in `shar'.
`-3' in `wdiff'.
`-c' in `touch'.
`-D' in `etags'.
`-1' in `wdiff'.
`-d' in `cp'.
`-2' in `wdiff'.
`-S' in `make'.
`-l' in Bison.
`-P' in `shar'.
`-e' in `gprof'.
`-R' in `etags'.
`-p' in `nm'.
Don't print a startup splash screen.
Used in `makeinfo'.
`-a' in `gprof'.
`-E' in `gprof'.
`-m' in `shar'.
Used in `makeinfo'.
Used in `emacsclient'.
Used in various programs to inhibit warnings.
`-n' in `info'.
`-n' in `uname'.
`-f' in `cpio'.
`-n' in `objdump'.
`-0' in `xargs'.
`-n' in `cat'.
`-b' in `cat'.
`-n' in `nm'.
`-n' in `cpio' and `ls'.
Used in GDB.
`-o' in `tar'.
`-o' in `make'.
`-l' in `tar', `cp', and `du'.
`-o' in `ptx'.
`-f' in `gprof'.
`-F' in `gprof'.
`-o' in `getopt', `fdlist', `fdmount', `fdmountd', and `fdumount'.
In various programs, specify the output file name.
`-o' in `shar'.
`-o' in `rm'.
`-c' in `unshar'.
`-o' in `install'.
`-l' in `diff'.
Used in `makeinfo'.
`-p' in `mkdir' and `rmdir'.
`-p' in `ul'.
`-p' in `cpio'.
`-P' in `finger'.
`-c' in `cpio' and `tar'.
Used in `gawk'.
`-P' in `m4'.
`-f' in `csplit'.
Used in `tar' and `cp'.
`-p' in `su'.
`-m' in `cpio'.
`-s' in `tar'.
`-p' in `tar'.
`-l' in `diff'.
`-L' in `cmp'.
`-p' in `make'.
`-w' in `make'.
`-o' in `nm'.
`-s' in `nm'.
`-p' in `wdiff'.
`-p' in `ed'.
Specify an HTTP proxy.
`-X' in `shar'.
`-q' in `make'.
Used in many programs to inhibit the usual output. Every program
accepting `--quiet' should accept `--silent' as a synonym.
`-Q' in `shar'
`-Q' in `ls'.
`-n' in `diff'.
Used in `gawk'.
`-B' in `tar'.
Used in GDB.
`-n' in `make'.
`-R' in `tar'.
Used in `chgrp', `chown', `cp', `ls', `diff', and `rm'.
`-r' in `touch'.
`-r' in `ptx'.
`-r' in `tac' and `etags'.
`-r' in `uname'.
`-R' in `m4'.
`-r' in `objdump'.
`-r' in `cpio'.
`-i' in `xargs'.
`-s' in `diff'.
`-a' in `cpio'.
`-r' in `ls' and `nm'.
`-f' in `diff'.
`-R' in `ptx'.
`-s' in `tar'.
`-p' in `tar'.
`-g' in `stty'.
Used in GDB.
`-S' in `ptx'.
`-S' in `du'.
`-s' in `tac'.
Used by `recode' to chose files or pipes for sequencing passes.
`-s' in `su'.
`-A' in `cat'.
`-p' in `diff'.
`-E' in `cat'.
`-F' in `diff'.
`-T' in `cat'.
Used in many programs to inhibit the usual output. Every program
accepting `--silent' should accept `--quiet' as a synonym.
`-s' in `ls'.
Specify a file descriptor for a network server to use for its
socket, instead of opening and binding a new socket. This
provides a way to run, in a non-privileged process, a server that
normally needs a reserved port number.
Used in `ls'.
`-W source' in `gawk'.
`-S' in `tar'.
`-H' in `diff'.
`-E' in `unshar'.
`-L' in `shar'.
`-s' in `cat'.
`-w' in `wdiff'.
`-y' in `wdiff'.
Used in `tar' and `diff' to specify which file within a directory
to start processing with.
`-s' in `wdiff'.
`-S' in `shar'.
`-S' in `make'.
`-s' in `recode'.
`-s' in `install'.
`-s' in `strip'.
`-S' in `strip'.
`-s' in `shar'.
`-S' in `cp', `ln', `mv'.
`-b' in `csplit'.
`-s' in `gprof'.
`-s' in `du'.
`-s' in `ln'.
Used in GDB and `objdump'.
`-s' in `m4'.
`-s' in `uname'.
`-t' in `expand' and `unexpand'.
`-T' in `ls'.
`-T' in `tput' and `ul'. `-t' in `wdiff'.
`-a' in `diff'.
`-T' in `shar'.
Used in `ls' and `touch'.
Specify how long to wait before giving up on some operation.
`-O' in `tar'.
`-c' in `du'.
`-t' in `make', `ranlib', and `recode'.
`-t' in `m4'.
`-t' in `hello'; `-W traditional' in `gawk'; `-G' in `ed', `m4',
and `ptx'.
Used in GDB.
`-t' in `ctags'.
`-T' in `ctags'.
`-t' in `ptx'.
`-z' in `tar'.
`-u' in `cpio'.
`-U' in `m4'.
`-u' in `nm'.
`-u' in `cp', `ctags', `mv', `tar'.
Used in `gawk'; same as `--help'.
`-B' in `shar'.
`-V' in `shar'.
Print more information about progress. Many programs support this.
`-W' in `tar'.
Print the version number.
`-V' in `cp', `ln', `mv'.
`-v' in `ctags'.
`-V' in `tar'.
`-W' in `make'.
`-l' in `shar'.
`-w' in `ls' and `ptx'.
`-W' in `ptx'.
`-T' in `who'.
`-z' in `gprof'.

File:, Node: OID Allocations, Next: Memory Usage, Prev: Option Table, Up: Program Behavior
4.9 OID Allocations
The OID (object identifier) has been assigned to the
GNU Project (thanks to Werner Koch). These are used for SNMP, LDAP,
X.509 certificates, and so on. The web site
`' has a (voluntary) listing of many
OID assignments.
If you need a new slot for your GNU package, write
<>. Here is a list of arcs currently assigned: GNU GNU Radius GnuPG notation pkaAddress GNU Radar GNU GSS GNU Mailutils GNU Shishi GNU Radio digestAlgorithm TIGER/192 encryptionAlgorithm Serpent Serpent-128-ECB Serpent-128-CBC Serpent-128-OFB Serpent-128-CFB Serpent-192-ECB Serpent-192-CBC Serpent-192-OFB Serpent-192-CFB Serpent-256-ECB Serpent-256-CBC Serpent-256-OFB Serpent-256-CFB CRC algorithms CRC 32

File:, Node: Memory Usage, Next: File Usage, Prev: OID Allocations, Up: Program Behavior
4.10 Memory Usage
If a program typically uses just a few meg of memory, don't bother
making any effort to reduce memory usage. For example, if it is
impractical for other reasons to operate on files more than a few meg
long, it is reasonable to read entire input files into memory to
operate on them.
However, for programs such as `cat' or `tail', that can usefully
operate on very large files, it is important to avoid using a technique
that would artificially limit the size of files it can handle. If a
program works by lines and could be applied to arbitrary user-supplied
input files, it should keep only a line in memory, because this is not
very hard and users will want to be able to operate on input files that
are bigger than will fit in memory all at once.
If your program creates complicated data structures, just make them
in memory and give a fatal error if `malloc' returns zero.

File:, Node: File Usage, Prev: Memory Usage, Up: Program Behavior
4.11 File Usage
Programs should be prepared to operate when `/usr' and `/etc' are
read-only file systems. Thus, if the program manages log files, lock
files, backup files, score files, or any other files which are modified
for internal purposes, these files should not be stored in `/usr' or
There are two exceptions. `/etc' is used to store system
configuration information; it is reasonable for a program to modify
files in `/etc' when its job is to update the system configuration.
Also, if the user explicitly asks to modify one file in a directory, it
is reasonable for the program to store other files in the same

File:, Node: Writing C, Next: Documentation, Prev: Program Behavior, Up: Top
5 Making The Best Use of C
This chapter provides advice on how best to use the C language when
writing GNU software.
* Menu:
* Formatting:: Formatting your source code.
* Comments:: Commenting your work.
* Syntactic Conventions:: Clean use of C constructs.
* Names:: Naming variables, functions, and files.
* System Portability:: Portability among different operating systems.
* CPU Portability:: Supporting the range of CPU types.
* System Functions:: Portability and ``standard'' library functions.
* Internationalization:: Techniques for internationalization.
* Character Set:: Use ASCII by default.
* Quote Characters:: Use `...' in the C locale.
* Mmap:: How you can safely use `mmap'.

File:, Node: Formatting, Next: Comments, Up: Writing C
5.1 Formatting Your Source Code
It is important to put the open-brace that starts the body of a C
function in column one, so that they will start a defun. Several tools
look for open-braces in column one to find the beginnings of C
functions. These tools will not work on code not formatted that way.
Avoid putting open-brace, open-parenthesis or open-bracket in column
one when they are inside a function, so that they won't start a defun.
The open-brace that starts a `struct' body can go in column one if you
find it useful to treat that definition as a defun.
It is also important for function definitions to start the name of
the function in column one. This helps people to search for function
definitions, and may also help certain tools recognize them. Thus,
using Standard C syntax, the format is this:
static char *
concat (char *s1, char *s2)
or, if you want to use traditional C syntax, format the definition like
static char *
concat (s1, s2) /* Name starts in column one here */
char *s1, *s2;
{ /* Open brace in column one here */
In Standard C, if the arguments don't fit nicely on one line, split
it like this:
lots_of_args (int an_integer, long a_long, short a_short,
double a_double, float a_float)
The rest of this section gives our recommendations for other aspects
of C formatting style, which is also the default style of the `indent'
program in version 1.2 and newer. It corresponds to the options
-nbad -bap -nbc -bbo -bl -bli2 -bls -ncdb -nce -cp1 -cs -di2
-ndj -nfc1 -nfca -hnl -i2 -ip5 -lp -pcs -psl -nsc -nsob
We don't think of these recommendations as requirements, because it
causes no problems for users if two different programs have different
formatting styles.
But whatever style you use, please use it consistently, since a
mixture of styles within one program tends to look ugly. If you are
contributing changes to an existing program, please follow the style of
that program.
For the body of the function, our recommended style looks like this:
if (x < foo (y, z))
haha = bar[4] + 5;
while (z)
haha += foo (z, z);
return ++x + bar ();
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:
if (foo_this_is_long && bar > win (x, y, z)
&& remaining_condition)
Try to avoid having two operators of different precedence at the same
level of indentation. For example, don't write this:
mode = (inmode[j] == VOIDmode
|| GET_MODE_SIZE (outmode[j]) > GET_MODE_SIZE (inmode[j])
? outmode[j] : inmode[j]);
Instead, use extra parentheses so that the indentation shows the
mode = ((inmode[j] == VOIDmode
|| (GET_MODE_SIZE (outmode[j]) > GET_MODE_SIZE (inmode[j])))
? outmode[j] : inmode[j]);
Insert extra parentheses so that Emacs will indent the code properly.
For example, the following indentation looks nice if you do it by hand,
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;
but Emacs would alter it. Adding a set of parentheses produces
something that looks equally nice, and which Emacs will preserve:
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);
Format do-while statements like this:
a = foo (a);
while (a > 0);
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.

File:, Node: Comments, Next: Syntactic Conventions, Prev: Formatting, Up: Writing C
5.2 Commenting Your Work
Every program should start with a comment saying briefly what it is for.
Example: `fmt - filter for simple filling of text'. This comment
should be at the top of the source file containing the `main' function
of the program.
Also, please write a brief comment at the start of each source file,
with the file name and a line or two about the overall purpose of the
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 `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 ...").
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:
/* Nonzero means truncate lines in the display;
zero means continue them. */
int truncate_lines;
Every `#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, _including its
sense_. `#else' should have a comment describing the condition _and
sense_ of the code that follows. For example:
#ifdef foo
#else /* not foo */
#endif /* not foo */
#ifdef foo
#endif /* foo */
but, by contrast, write the comments this way for a `#ifndef':
#ifndef foo
#else /* foo */
#endif /* foo */
#ifndef foo
#endif /* not foo */

File:, Node: Syntactic Conventions, Next: Names, Prev: Comments, Up: Writing C
5.3 Clean Use of C Constructs
Please explicitly declare the types of all objects. For example, you
should explicitly declare all arguments to functions, and you should
declare functions to return `int' rather than omitting the `int'.
Some programmers like to use the GCC `-Wall' option, and change the
code whenever it issues a warning. If you want to do this, then do.
Other programmers prefer not to use `-Wall', because it gives warnings
for valid and legitimate code which they do not want to change. If you
want to do this, then do. The compiler should be your servant, not
your master.
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 `extern' declarations inside
It used to be common practice to use the same local variables (with
names like `tem') over and over for different values within one
function. Instead of doing this, it is better to declare a separate
local variable for each distinct purpose, and give it a name which is
meaningful. This not only makes programs easier to understand, it also
facilitates optimization by good compilers. You can also move the
declaration of each local variable into the smallest scope that includes
all its uses. This makes the program even cleaner.
Don't use local variables or parameters that shadow global
Don't declare multiple variables in one declaration that spans lines.
Start a new declaration on each line, instead. For example, instead of
int foo,
write either this:
int foo, bar;
or this:
int foo;
int bar;
(If they are global variables, each should have a comment preceding it
When you have an `if'-`else' statement nested in another `if'
statement, always put braces around the `if'-`else'. Thus, never write
like this:
if (foo)
if (bar)
win ();
lose ();
always like this:
if (foo)
if (bar)
win ();
lose ();
If you have an `if' statement nested inside of an `else' statement,
either write `else if' on one line, like this,
if (foo)
else if (bar)
with its `then'-part indented like the preceding `then'-part, or write
the nested `if' within braces like this:
if (foo)
if (bar)
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 `if'-conditions (assignments inside
`while'-conditions are ok). For example, don't write this:
if ((foo = (char *) malloc (sizeof *foo)) == 0)
fatal ("virtual memory exhausted");
instead, write this:
foo = (char *) malloc (sizeof *foo);
if (foo == 0)
fatal ("virtual memory exhausted");
Don't make the program ugly to placate `lint'. Please don't insert
any casts to `void'. Zero without a cast is perfectly fine as a null
pointer constant, except when calling a varargs function.

File:, Node: Names, Next: System Portability, Prev: Syntactic Conventions, Up: Writing C
5.4 Naming Variables, Functions, and Files
The names of global variables and functions in a program serve as
comments of a sort. So don't choose terse names--instead, look for
names that give useful information about the meaning of the variable or
function. In a GNU program, names should be English, like other
Local variable names can be shorter, because they are used only
within one context, where (presumably) comments explain their purpose.
Try to limit your use of abbreviations in symbol names. It is ok to
make a few abbreviations, explain what they mean, and then use them
frequently, but don't use lots of obscure abbreviations.
Please use underscores to separate words in a name, so that the Emacs
word commands can be useful within them. Stick to lower case; reserve
upper case for macros and `enum' constants, and for name-prefixes that
follow a uniform convention.
For example, you should use names like `ignore_space_change_flag';
don't use names like `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,
/* Ignore changes in horizontal whitespace (-b). */
int ignore_space_change_flag;
When you want to define names with constant integer values, use
`enum' rather than `#define'. GDB knows about enumeration constants.
You might want to make sure that none of the file names would
conflict if the files were loaded onto an MS-DOS file system which
shortens the names. You can use the program `doschk' to test for this.
Some GNU programs were designed to limit themselves to file names of
14 characters or less, to avoid file name conflicts if they are read
into older System V systems. Please preserve this feature in the
existing GNU programs that have it, but there is no need to do this in
new GNU programs. `doschk' also reports file names longer than 14

File:, Node: System Portability, Next: CPU Portability, Prev: Names, Up: Writing C
5.5 Portability between System Types
In the Unix world, "portability" refers to porting to different Unix
versions. For a GNU program, this kind of portability is desirable, but
not paramount.
The primary purpose of GNU software is to run on top of the GNU
kernel, compiled with the GNU C compiler, on various types of CPU. So
the kinds of portability that are absolutely necessary are quite
limited. But it is important to support Linux-based GNU systems, since
they are the form of GNU that is popular.
Beyond that, it is good to support the other free operating systems
(*BSD), and it is nice to support other Unix-like systems if you want
to. Supporting a variety of Unix-like systems is desirable, although
not paramount. It is usually not too hard, so you may as well do it.
But you don't have to consider it an obligation, if it does turn out to
be hard.
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
Avoid using the format of semi-internal data bases (e.g.,
directories) when there is a higher-level alternative (`readdir').
As for systems that are not like Unix, such as MSDOS, Windows, VMS,
MVS, and older Macintosh systems, supporting them is often a lot of
work. When that is the case, it is better to spend your time adding
features that will be useful on GNU and GNU/Linux, rather than on
supporting other incompatible systems.
If you do support Windows, please do not abbreviate it as "win". In
hacker terminology, calling something a "win" is a form of praise.
You're free to praise Microsoft Windows on your own if you want, but
please don't do this in GNU packages. Instead of abbreviating
"Windows" to "win", you can write it in full or abbreviate it to "woe"
or "w". In GNU Emacs, for instance, we use `w32' in file names of
Windows-specific files, but the macro for Windows conditionals is
called `WINDOWSNT'.
It is a good idea to define the "feature test macro" `_GNU_SOURCE'
when compiling your C files. When you compile on GNU or GNU/Linux,
this will enable the declarations of GNU library extension functions,
and that will usually give you a compiler error message if you define
the same function names in some other way in your program. (You don't
have to actually _use_ these functions, if you prefer to make the
program more portable to other systems.)
But whether or not you use these GNU extensions, you should avoid
using their names for any other meanings. Doing so would make it hard
to move your code into other GNU programs.

File:, Node: CPU Portability, Next: System Functions, Prev: System Portability, Up: Writing C
5.6 Portability between CPUs
Even GNU systems will differ because of differences among CPU
types--for example, difference in byte ordering and alignment
requirements. It is absolutely essential to handle these differences.
However, don't make any effort to cater to the possibility that an
`int' will be less than 32 bits. We don't support 16-bit machines in
Similarly, don't make any effort to cater to the possibility that
`long' will be smaller than predefined types like `size_t'. For
example, the following code is ok:
printf ("size = %lu\n", (unsigned long) sizeof array);
printf ("diff = %ld\n", (long) (pointer2 - pointer1));
1989 Standard C requires this to work, and we know of only one
counterexample: 64-bit programs on Microsoft Windows. We will leave it
to those who want to port GNU programs to that environment to figure
out how to do it.
Predefined file-size types like `off_t' are an exception: they are
longer than `long' on many platforms, so code like the above won't work
with them. One way to print an `off_t' value portably is to print its
digits yourself, one by one.
Don't assume that the address of an `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:
int c;
while ((c = getchar ()) != EOF)
write (file_descriptor, &c, 1);
Instead, use `unsigned char' as follows. (The `unsigned' is for
portability to unusual systems where `char' is signed and where there
is integer overflow checking.)
int c;
while ((c = getchar ()) != EOF)
unsigned char u = c;
write (file_descriptor, &u, 1);
It used to be ok to not worry about the difference between pointers
and integers when passing arguments to functions. However, on most
modern 64-bit machines pointers are wider than `int'. Conversely,
integer types like `long long int' and `off_t' are wider than pointers
on most modern 32-bit machines. Hence it's often better nowadays to
use prototypes to define functions whose argument types are not trivial.
In particular, if functions accept varying argument counts or types
they should be declared using prototypes containing `...' and defined
using `stdarg.h'. For an example of this, please see the Gnulib
( error module, which declares and
defines the following function:
/* Print a message with `fprintf (stderr, FORMAT, ...)';
if ERRNUM is nonzero, follow it with ": " and strerror (ERRNUM).
If STATUS is nonzero, terminate the program with `exit (STATUS)'. */
void error (int status, int errnum, const char *format, ...);
A simple way to use the Gnulib error module is to obtain the two
source files `error.c' and `error.h' from the Gnulib library source
code repository at `'.
Here's a sample use:
#include "error.h"
#include <errno.h>
#include <stdio.h>
char *program_name = "myprogram";
xfopen (char const *name)
FILE *fp = fopen (name, "r");
if (! fp)
error (1, errno, "cannot read %s", name);
return fp;
Avoid casting pointers to integers if you can. Such casts greatly
reduce portability, and in most programs they are easy to avoid. In the
cases where casting pointers to integers is essential--such as, a Lisp
interpreter which stores type information as well as an address in one
word--you'll have to make explicit provisions to handle different word
sizes. You will also need to make provision for systems in which the
normal range of addresses you can get from `malloc' starts far away
from zero.

File:, Node: System Functions, Next: Internationalization, Prev: CPU Portability, Up: Writing C
5.7 Calling System Functions
C implementations differ substantially. Standard C reduces but does
not eliminate the incompatibilities; meanwhile, many GNU packages still
support pre-standard compilers because this is not hard to do. This
chapter gives recommendations for how to use the more-or-less standard C
library functions to avoid unnecessary loss of portability.
* Don't use the return value of `sprintf'. It returns the number of
characters written on some systems, but not on all systems.
* Be aware that `vfprintf' is not always available.
* `main' should be declared to return type `int'. It should
terminate either by calling `exit' or by returning the integer
status code; make sure it cannot ever return an undefined value.
* 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.
* If you must declare a system function, don't specify the argument
types. Use an old-style declaration, not a Standard C prototype.
The more you specify about the function, the more likely a
* In particular, don't unconditionally declare `malloc' or `realloc'.
Most GNU programs use those functions just once, in functions
conventionally named `xmalloc' and `xrealloc'. These functions
call `malloc' and `realloc', respectively, and check the results.
Because `xmalloc' and `xrealloc' are defined in your program, you
can declare them in other files without any risk of type conflict.
On most systems, `int' is the same length as a pointer; thus, the
calls to `malloc' and `realloc' work fine. For the few
exceptional systems (mostly 64-bit machines), you can use
*conditionalized* declarations of `malloc' and `realloc'--or put
these declarations in configuration files specific to those
* The string functions require special treatment. Some Unix systems
have a header file `string.h'; others have `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.
* If you don't include either strings file, you can't get
declarations for the string functions from the header file in the
usual way.
That causes less of a problem than you might think. The newer
standard string functions should be avoided anyway because many
systems still don't support them. The string functions you can
use are these:
strcpy strncpy strcat strncat
strlen strcmp strncmp
strchr strrchr
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 `int', and perhaps in other cases. It
is trivial to avoid using their values, so do that.
The compare functions and `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 *conditionally* on a few
The search functions must be declared to return `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 `index' and `rindex'; other systems use the names `strchr'
and `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 `strchr' and `strrchr'
for new programs, since those are the standard names.) Declare
both of those names as functions returning `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
`strchr' and `strrchr' throughout:
#define strchr index
#define strrchr rindex
char *strchr ();
char *strrchr ();
Here we assume that `HAVE_STRCHR' and `HAVE_STRRCHR' are macros
defined in systems where the corresponding functions exist. One way to
get them properly defined is to use Autoconf.

File:, Node: Internationalization, Next: Character Set, Prev: System Functions, Up: Writing C
5.8 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 `gettext' macro
around each string that might need translation--like this:
printf (gettext ("Processing file `%s'..."));
This permits GNU gettext to replace the string `"Processing file
`%s'..."' with a translated version.
Once a program uses gettext, please make a point of writing calls to
`gettext' when you add new strings that call for translation.
Using GNU gettext in a package involves specifying a "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, `coreutils' for the GNU core 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:
printf ("%s is full", capacity > 5000000 ? "disk" : "floppy disk");
If you apply gettext to all strings, like this,
printf (gettext ("%s is full"),
capacity > 5000000 ? gettext ("disk") : gettext ("floppy disk"));
the translator will hardly know that "disk" and "floppy disk" are meant
to be substituted in the other string. Worse, in some languages (like
French) the construction will not work: the translation of the word
"full" depends on the gender of the first part of the sentence; it
happens to be not the same for "disk" as for "floppy disk".
Complete sentences can be translated without problems:
printf (capacity > 5000000 ? gettext ("disk is full")
: gettext ("floppy disk is full"));
A similar problem appears at the level of sentence structure with
this code:
printf ("# Implicit rule search has%s been done.\n",
f->tried_implicit ? "" : " not");
Adding `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 `gettext'
calls does the job straightforwardly if the code starts out like this:
printf (f->tried_implicit
? "# Implicit rule search has been done.\n",
: "# Implicit rule search has not been done.\n");
Another example is this one:
printf ("%d file%s processed", nfiles,
nfiles != 1 ? "s" : "");
The problem with this example is that it assumes that plurals are made
by adding `s'. If you apply gettext to the format string, like this,
printf (gettext ("%d file%s processed"), nfiles,
nfiles != 1 ? "s" : "");
the message can use different words, but it will still be forced to use
`s' for the plural. Here is a better way, with gettext being applied to
the two strings independently:
printf ((nfiles != 1 ? gettext ("%d files processed")
: gettext ("%d file processed")),
But this still doesn't work for languages like Polish, which has three
plural forms: one for nfiles == 1, one for nfiles == 2, 3, 4, 22, 23,
24, ... and one for the rest. The GNU `ngettext' function solves this
printf (ngettext ("%d files processed", "%d file processed", nfiles),

File:, Node: Character Set, Next: Quote Characters, Prev: Internationalization, Up: Writing C
5.9 Character Set
Sticking to the ASCII character set (plain text, 7-bit characters) is
preferred in GNU source code comments, text documents, and other
contexts, unless there is good reason to do something else because of
the application domain. For example, if source code deals with the
French Revolutionary calendar, it is OK if its literal strings contain
accented characters in month names like "Flore'al". Also, it is OK to
use non-ASCII characters to represent proper names of contributors in
change logs (*note Change Logs::).
If you need to use non-ASCII characters, you should normally stick
with one encoding, as one cannot in general mix encodings reliably.

File:, Node: Quote Characters, Next: Mmap, Prev: Character Set, Up: Writing C
5.10 Quote Characters
In the C locale, GNU programs should stick to plain ASCII for quotation
characters in messages to users: preferably 0x60 (``') for left quotes
and 0x27 (`'') for right quotes. It is ok, but not required, to use
locale-specific quotes in other locales.
The Gnulib ( `quote' and
`quotearg' modules provide a reasonably straightforward way to support
locale-specific quote characters, as well as taking care of other
issues, such as quoting a filename that itself contains a quote
character. See the Gnulib documentation for usage details.
In any case, the documentation for your program should clearly
specify how it does quoting, if different than the preferred method of
``' and `''. This is especially important if the output of your
program is ever likely to be parsed by another program.
Quotation characters are a difficult area in the computing world at
this time: there are no true left or right quote characters in Latin1;
the ``' character we use was standardized there as a grave accent.
Moreover, Latin1 is still not universally usable.
Unicode contains the unambiguous quote characters required, and its
common encoding UTF-8 is upward compatible with Latin1. However,
Unicode and UTF-8 are not universally well-supported, either.
This may change over the next few years, and then we will revisit

File:, Node: Mmap, Prev: Quote Characters, Up: Writing C
5.11 Mmap
Don't assume that `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 `mmap' is to try it on the specific file for
which you want to use it--and if `mmap' doesn't work, fall back on
doing the job in another way using `read' and `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 `mmap', but
some do not. It is important to make programs handle all these kinds
of files.

File:, Node: Documentation, Next: Managing Releases, Prev: Writing C, Up: Top
6 Documenting Programs
A GNU program should ideally come with full free documentation, adequate
for both reference and tutorial purposes. If the package can be
programmed or extended, the documentation should cover programming or
extending it, as well as just using it.
* Menu:
* GNU Manuals:: Writing proper manuals.
* Doc Strings and Manuals:: Compiling doc strings doesn't make a manual.
* Manual Structure Details:: Specific structure conventions.
* License for Manuals:: Writing the distribution terms for a manual.
* Manual Credits:: Giving credit to documentation contributors.
* Printed Manuals:: Mentioning the printed manual.
* NEWS File:: NEWS files supplement manuals.
* Change Logs:: Recording changes.
* Man Pages:: Man pages are secondary.
* Reading other Manuals:: How far you can go in learning
from other manuals.

File:, Node: GNU Manuals, Next: Doc Strings and Manuals, Up: Documentation
6.1 GNU Manuals
The preferred document format for the GNU system is the Texinfo
formatting language. Every GNU package should (ideally) have
documentation in Texinfo both for reference and for learners. Texinfo
makes it possible to produce a good quality formatted book, using TeX,
and to generate an Info file. It is also possible to generate HTML
output from Texinfo source. See the Texinfo manual, either the
hardcopy, or the on-line version available through `info' or the Emacs
Info subsystem (`C-h i').
Nowadays some other formats such as Docbook and Sgmltexi can be
converted automatically into Texinfo. It is ok to produce the Texinfo
documentation by conversion this way, as long as it gives good results.
Make sure your manual is clear to a reader who knows nothing about
the topic and reads it straight through. This means covering basic
topics at the beginning, and advanced topics only later. This also
means defining every specialized term when it is first used.
Programmers tend to carry over the structure of the program as the
structure for its documentation. But this structure is not necessarily
good for explaining how to use the program; it may be irrelevant and
confusing for a user.
Instead, the right way to structure documentation is according to the
concepts and questions that a user will have in mind when reading it.
This principle applies at every level, from the lowest (ordering
sentences in a paragraph) to the highest (ordering of chapter topics
within the manual). Sometimes this structure of ideas matches the
structure of the implementation of the software being documented--but
often they are different. An important part of learning to write good
documentation is to learn to notice when you have unthinkingly
structured the documentation like the implementation, stop yourself,
and look for better alternatives.
For example, each program in the GNU system probably ought to be
documented in one manual; but this does not mean each program should
have its own manual. That would be following the structure of the
implementation, rather than the structure that helps the user
Instead, each manual should cover a coherent _topic_. For example,
instead of a manual for `diff' and a manual for `diff3', we have one
manual for "comparison of files" which covers both of those programs,
as well as `cmp'. By documenting these programs together, we can make
the whole subject clearer.
The manual which discusses a program should certainly document all of
the program's command-line options and all of its commands. It should
give examples of their use. But don't organize the manual as a list of
features. Instead, organize it logically, by subtopics. Address the
questions that a user will ask when thinking about the job that the
program does. Don't just tell the reader what each feature can do--say
what jobs it is good for, and show how to use it for those jobs.
Explain what is recommended usage, and what kinds of usage users should
In general, a GNU manual should serve both as tutorial and reference.
It should be set up for convenient access to each topic through Info,
and for reading straight through (appendixes aside). A GNU manual
should give a good introduction to a beginner reading through from the
start, and should also provide all the details that hackers want. The
Bison manual is a good example of this--please take a look at it to see
what we mean.
That is not as hard as it first sounds. Arrange each chapter as a
logical breakdown of its topic, but order the sections, and write their
text, so that reading the chapter straight through makes sense. Do
likewise when structuring the book into chapters, and when structuring a
section into paragraphs. The watchword is, _at each point, address the
most fundamental and important issue raised by the preceding text._
If necessary, add extra chapters at the beginning of the manual which
are purely tutorial and cover the basics of the subject. These provide
the framework for a beginner to understand the rest of the manual. The
Bison manual provides a good example of how to do this.
To serve as a reference, a manual should have an Index that list all
the functions, variables, options, and important concepts that are part
of the program. One combined Index should do for a short manual, but
sometimes for a complex package it is better to use multiple indices.
The Texinfo manual includes advice on preparing good index entries, see
*Note Making Index Entries: (texinfo)Index Entries, and see *Note
Defining the Entries of an Index: (texinfo)Indexing Commands.
Don't use Unix man pages as a model for how to write GNU
documentation; most of them are terse, badly structured, and give
inadequate explanation of the underlying concepts. (There are, of
course, some exceptions.) Also, Unix man pages use a particular format
which is different from what we use in GNU manuals.
Please include an email address in the manual for where to report
bugs _in the text of the manual_.
Please do not use the term "pathname" that is used in Unix
documentation; use "file name" (two words) instead. We use the term
"path" only for search paths, which are lists of directory names.
Please do not use the term "illegal" to refer to erroneous input to
a computer program. Please use "invalid" for this, and reserve the
term "illegal" for activities prohibited by law.
Please do not write `()' after a function name just to indicate it
is a function. `foo ()' is not a function, it is a function call with
no arguments.

File:, Node: Doc Strings and Manuals, Next: Manual Structure Details, Prev: GNU Manuals, Up: Documentation
6.2 Doc Strings and Manuals
Some programming systems, such as Emacs, provide a documentation string
for each function, command or variable. You may be tempted to write a
reference manual by compiling the documentation strings and writing a
little additional text to go around them--but you must not do it. That
approach is a fundamental mistake. The text of well-written
documentation strings will be entirely wrong for a manual.
A documentation string needs to stand alone--when it appears on the
screen, there will be no other text to introduce or explain it.
Meanwhile, it can be rather informal in style.
The text describing a function or variable in a manual must not stand
alone; it appears in the context of a section or subsection. Other text
at the beginning of the section should explain some of the concepts, and
should often make some general points that apply to several functions or
variables. The previous descriptions of functions and variables in the
section will also have given information about the topic. A description
written to stand alone would repeat some of that information; this
redundancy looks bad. Meanwhile, the informality that is acceptable in
a documentation string is totally unacceptable in a manual.
The only good way to use documentation strings in writing a good
manual is to use them as a source of information for writing good text.

File:, Node: Manual Structure Details, Next: License for Manuals, Prev: Doc Strings and Manuals, Up: Documentation
6.3 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 have a node named
`PROGRAM Invocation' or `Invoking PROGRAM'. This node (together with
its subnodes, if any) should describe the program's command line
arguments and how to run it (the sort of information people would look
for in a man page). Start with an `@example' containing a template for
all the options and arguments that the program uses.
Alternatively, put a menu item in some menu whose item name fits one
of the above patterns. This identifies the node which that item points
to as the node for this purpose, regardless of the node's actual name.
The `--usage' feature of the Info reader looks for such a node or
menu item in order to find the relevant text, so it is essential for
every Texinfo file to have one.
If one manual describes several programs, it should have such a node
for each program described in the manual.

File:, Node: License for Manuals, Next: Manual Credits, Prev: Manual Structure Details, Up: Documentation
6.4 License for Manuals
Please use the GNU Free Documentation License for all GNU manuals that
are more than a few pages long. Likewise for a collection of short
documents--you only need one copy of the GNU FDL for the whole
collection. For a single short document, you can use a very permissive
non-copyleft license, to avoid taking up space with a long license.
See `' for more explanation
of how to employ the GFDL.
Note that it is not obligatory to include a copy of the GNU GPL or
GNU LGPL in a manual whose license is neither the GPL nor the LGPL. It
can be a good idea to include the program's license in a large manual;
in a short manual, whose size would be increased considerably by
including the program's license, it is probably better not to include

File:, Node: Manual Credits, Next: Printed Manuals, Prev: License for Manuals, Up: Documentation
6.5 Manual Credits
Please credit the principal human writers of the manual as the authors,
on the title page of the manual. If a company sponsored the work, thank
the company in a suitable place in the manual, but do not cite the
company as an author.

File:, Node: Printed Manuals, Next: NEWS File, Prev: Manual Credits, Up: Documentation
6.6 Printed Manuals
The FSF publishes some GNU manuals in printed form. To encourage sales
of these manuals, the on-line versions of the manual should mention at
the very start that the printed manual is available and should point at
information for getting it--for instance, with a link to the page
`'. This should not be included in
the printed manual, though, because there it is redundant.
It is also useful to explain in the on-line forms of the manual how
the user can print out the manual from the sources.

File:, Node: NEWS File, Next: Change Logs, Prev: Printed Manuals, Up: Documentation
6.7 The NEWS File
In addition to its manual, the package should have a file named `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 `NEWS' file gets very long, move some of the older items into
a file named `ONEWS' and put a note at the end referring the user to
that file.

File:, Node: Change Logs, Next: Man Pages, Prev: NEWS File, Up: Documentation
6.8 Change Logs
Keep a change log to describe all the changes made to program source
files. The purpose of this is so that people investigating bugs in the
future will know about the changes that might have introduced the bug.
Often a new bug can be found by looking at what was recently changed.
More importantly, change logs can help you eliminate conceptual
inconsistencies between different parts of a program, by giving you a
history of how the conflicting concepts arose and who they came from.
* Menu:
* Change Log Concepts::
* Style of Change Logs::
* Simple Changes::
* Conditional Changes::
* Indicating the Part Changed::

File:, Node: Change Log Concepts, Next: Style of Change Logs, Up: Change Logs
6.8.1 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 `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
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 `ChangeLog' file using `rcs2log'; in Emacs, the
command `C-x v a' (`vc-update-change-log') does the job.
There's no need to describe the full purpose of the changes or how
they work together. However, sometimes it is useful to write one line
to describe the overall purpose of a change or a batch of changes. If
you think that a change calls for explanation, you're probably right.
Please do explain it--but please put the full explanation in comments
in the code, where people will see it whenever they see the code. For
example, "New function" is enough for the change log when you add a
function, because there should be a comment before the function
definition to explain what it does.
In the past, we recommended not mentioning changes in non-software
files (manuals, help files, etc.) in change logs. However, we've been
advised that it is a good idea to include them, for the sake of
copyright records.
The easiest way to add an entry to `ChangeLog' is with the Emacs
command `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.

File:, Node: Style of Change Logs, Next: Simple Changes, Prev: Change Log Concepts, Up: Change Logs
6.8.2 Style of Change Logs
Here are some simple examples of change log entries, starting with the
header line that says who made the change and when it was installed,
followed by descriptions of specific changes. (These examples are
drawn from Emacs and GCC.)
1998-08-17 Richard Stallman <>
* 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.
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 `* register.el ({insert,jump-to}-register)'; this is
not a good idea, since searching for `jump-to-register' or
`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.
Break long lists of function names by closing continued lines with
`)', rather than `,', and opening the continuation with `(' as in this
* keyboard.c (menu_bar_items, tool_bar_items)
(Fexecute_extended_command): Deal with `keymap' property.
When you install someone else's changes, put the contributor's name
in the change log entry rather than in the text of the entry. In other
words, write this:
2002-07-14 John Doe <>
* sewing.c: Make it sew.
rather than this:
2002-07-14 Usual Maintainer <>
* sewing.c: Make it sew. Patch by
As for the date, that should be the date you applied the change.

File:, Node: Simple Changes, Next: Conditional Changes, Prev: Style of Change Logs, Up: Change Logs
6.8.3 Simple Changes
Certain simple kinds of changes don't need much detail in the change
When you change the calling sequence of a function in a simple
fashion, and you change all the callers of the function to use the new
calling sequence, there is no need to make individual entries for all
the callers that you changed. Just write in the entry for the function
being called, "All callers changed"--like this:
* keyboard.c (Fcommand_execute): New arg SPECIAL.
All callers changed.
When you change just comments or doc strings, it is enough to write
an entry for the file, without mentioning the functions. Just "Doc
fixes" is enough for the change log.
There's no technical need to make change log entries for
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.
However, you should keep change logs for documentation files when the
project gets copyright assignments from its contributors, so as to make
the records of authorship more accurate.

File:, Node: Conditional Changes, Next: Indicating the Part Changed, Prev: Simple Changes, Up: Change Logs
6.8.4 Conditional Changes
C programs often contain compile-time `#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:
* xterm.c [SOLARIS2]: Include string.h.
Here is an entry describing a new definition which is entirely
conditional. This new definition for the macro `FRAME_WINDOW_P' is
used only when `HAVE_X_WINDOWS' is defined:
* frame.h [HAVE_X_WINDOWS] (FRAME_WINDOW_P): Macro defined.
Here is an entry for a change within the function `init_display',
whose definition as a whole is unconditional, but the changes themselves
are contained in a `#ifdef HAVE_LIBNCURSES' conditional:
* dispnew.c (init_display) [HAVE_LIBNCURSES]: If X, call tgetent.
Here is an entry for a change that takes affect only when a certain
macro is _not_ defined:
(gethostname) [!HAVE_SOCKETS]: Replace with winsock version.

File:, Node: Indicating the Part Changed, Prev: Conditional Changes, Up: Change Logs
6.8.5 Indicating the Part Changed
Indicate the part of a function which changed by using angle brackets
enclosing an indication of what the changed part does. Here is an entry
for a change in the part of the function `sh-while-getopts' that deals
with `sh' commands:
* progmodes/sh-script.el (sh-while-getopts) <sh>: Handle case that
user-specified option string is empty.

File:, Node: Man Pages, Next: Reading other Manuals, Prev: Change Logs, Up: Documentation
6.9 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
Be sure that man pages include a copyright statement and free
license. The simple all-permissive license is appropriate for simple
man pages (*note License Notices for Other Files: (maintain)License
Notices for Other Files.).
For long man pages, with enough explanation and documentation that
they can be considered true manuals, use the GFDL (*note License for
Finally, the GNU help2man program
(`') is one way to automate
generation of a man page, in this case from `--help' output. This is
sufficient in many cases.

File:, Node: Reading other Manuals, Prev: Man Pages, Up: Documentation
6.10 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.

File:, Node: Managing Releases, Next: References, Prev: Documentation, Up: Top
7 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 of GNU packages should work.
* Makefile Conventions:: Makefile conventions.
* Releases:: Making releases

File:, Node: Configuration, Next: Makefile Conventions, Up: Managing Releases
7.1 How Configuration Should Work
Each GNU distribution should come with a shell script named
`configure'. This script is given arguments which describe the kind of
machine and system you want to compile the program for. The
`configure' script must record the configuration options so that they
affect compilation.
The description here is the specification of the interface for the
`configure' script in GNU packages. Many packages implement it using
GNU Autoconf (*note Introduction: (autoconf)Top.) and/or GNU Automake
(*note Introduction: (automake)Top.), but you do not have to use these
tools. You can implement it any way you like; for instance, by making
`configure' be a wrapper around a completely different configuration
Another way for the `configure' script to operate is to make a link
from a standard name such as `config.h' to the proper configuration
file for the chosen system. If you use this technique, the
distribution should _not_ contain a file named `config.h'. This is so
that people won't be able to build the program without configuring it
Another thing that `configure' can do is to edit the Makefile. If
you do this, the distribution should _not_ contain a file named
`Makefile'. Instead, it should include a file `' 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 `configure' does write the `Makefile', then `Makefile' should
have a target named `Makefile' which causes `configure' to be rerun,
setting up the same configuration that was set up last time. The files
that `configure' reads should be listed as dependencies of `Makefile'.
All the files which are output from the `configure' script should
have comments at the beginning explaining that they were generated
automatically using `configure'. This is so that users won't think of
trying to edit them by hand.
The `configure' script should write a file named `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 `configure' script should accept an option of the form
`--srcdir=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 `--srcdir', then `configure' should
check both `.' and `..' 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 `--srcdir' is by editing a
definition of `VPATH' into the Makefile. Some rules may need to refer
explicitly to the specified source directory. To make this possible,
`configure' can add to the Makefile a variable named `srcdir' whose
value is precisely the specified directory.
In addition, the `configure' script should take options
corresponding to most of the standard directory variables (*note
Directory Variables::). Here is the list:
--prefix --exec-prefix --bindir --sbindir --libexecdir --sysconfdir
--sharedstatedir --localstatedir --libdir --includedir --oldincludedir
--datarootdir --datadir --infodir --localedir --mandir --docdir
--htmldir --dvidir --pdfdir --psdir
The `configure' script should also take an argument which specifies
the type of system to build the program for. This argument should look
like this:
For example, an Athlon-based GNU/Linux system might be
The `configure' script needs to be able to decode all plausible
alternatives for how to describe a machine. Thus,
`athlon-pc-gnu/linux' would be a valid alias. There is a shell script
called `config.sub'
that you can use as a subroutine to validate system types and
canonicalize aliases.
The `configure' script should also take the option
`--build=BUILDTYPE', which should be equivalent to a plain BUILDTYPE
argument. For example, `configure --build=i686-pc-linux-gnu' is
equivalent to `configure i686-pc-linux-gnu'. When the build type is
not specified by an option or argument, the `configure' script should
normally guess it using the shell script `config.guess'
Other options are permitted to specify in more detail the software
or hardware present on the machine, to include or exclude optional parts
of the package, or to adjust the name of some tools or arguments to
Configure the package to build and install an optional user-level
facility called FEATURE. This allows users to choose which
optional features to include. Giving an optional PARAMETER of
`no' should omit FEATURE, if it is built by default.
No `--enable' option should *ever* cause one feature to replace
another. No `--enable' option should ever substitute one useful
behavior for another useful behavior. The only proper use for
`--enable' is for questions of whether to build part of the program
or exclude it.
The package PACKAGE will be installed, so configure this package
to work with PACKAGE.
Possible values of PACKAGE include `gnu-as' (or `gas'), `gnu-ld',
`gnu-libc', `gdb', `x', and `x-toolkit'.
Do not use a `--with' option to specify the file name to use to
find certain files. That is outside the scope of what `--with'
options are for.
Set the value of the variable VARIABLE to VALUE. This is used to
override the default values of commands or arguments in the build
process. For example, the user could issue `configure CFLAGS=-g
CXXFLAGS=-g' to build with debugging information and without the
default optimization.
Specifying variables as arguments to `configure', like this:
./configure CC=gcc
is preferable to setting them in environment variables:
CC=gcc ./configure
as it helps to recreate the same configuration later with
`config.status'. However, both methods should be supported.
All `configure' scripts should accept all of the "detail" options
and the variable settings, whether or not they make any difference to
the particular package at hand. In particular, they should accept any
option that starts with `--with-' or `--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 `--with-' and `--enable-' are
narrow: they *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 `configure' script should normally treat the specified type of
system as both the host and the target, thus producing a program which
works for the same type of machine that it runs on.
To compile a program to run on a host type that differs from the
build type, use the configure option `--host=HOSTTYPE', where HOSTTYPE
uses the same syntax as BUILDTYPE. The host type normally defaults to
the build type.
To configure a cross-compiler, cross-assembler, or what have you, you
should specify a target different from the host, using the configure
option `--target=TARGETTYPE'. The syntax for TARGETTYPE is the same as
for the host type. So the command would look like this:
./configure --host=HOSTTYPE --target=TARGETTYPE
The target type normally defaults to the host type. Programs for
which cross-operation is not meaningful need not accept the `--target'
option, because configuring an entire operating system for
cross-operation is not a meaningful operation.
Some programs have ways of configuring themselves automatically. If
your program is set up to do this, your `configure' script can simply
ignore most of its arguments.

File:, Node: Makefile Conventions, Next: Releases, Prev: Configuration, Up: Managing Releases
7.2 Makefile Conventions
This node describes conventions for writing the Makefiles for GNU
programs. Using Automake will help you write a Makefile that follows
these conventions.
* Menu:
* Makefile Basics:: General conventions for Makefiles.
* Utilities in Makefiles:: Utilities to be used in Makefiles.
* Command Variables:: Variables for specifying commands.
* DESTDIR:: Supporting staged installs.
* Directory Variables:: Variables for installation directories.
* Standard Targets:: Standard targets for users.
* Install Command Categories:: Three categories of commands in the `install'
rule: normal, pre-install and post-install.

File:, Node: Makefile Basics, Next: Utilities in Makefiles, Up: Makefile Conventions
7.2.1 General Conventions for Makefiles
Every Makefile should contain this line:
SHELL = /bin/sh
to avoid trouble on systems where the `SHELL' variable might be
inherited from the environment. (This is never a problem with GNU
Different `make' programs have incompatible suffix lists and
implicit rules, and this sometimes creates confusion or misbehavior. So
it is a good idea to set the suffix list explicitly using only the
suffixes you need in the particular Makefile, like this:
.SUFFIXES: .c .o
The first line clears out the suffix list, the second introduces all
suffixes which may be subject to implicit rules in this Makefile.
Don't assume that `.' is in the path for command execution. When
you need to run programs that are a part of your package during the
make, please make sure that it uses `./' if the program is built as
part of the make or `$(srcdir)/' if the file is an unchanging part of
the source code. Without one of these prefixes, the current search
path is used.
The distinction between `./' (the "build directory") and
`$(srcdir)/' (the "source directory") is important because users can
build in a separate directory using the `--srcdir' option to
`configure'. A rule of the form:
foo.1 : sedscript
sed -e sedscript > foo.1
will fail when the build directory is not the source directory, because
`' and `sedscript' are in the source directory.
When using GNU `make', relying on `VPATH' to find the source file
will work in the case where there is a single dependency file, since
the `make' automatic variable `$<' will represent the source file
wherever it is. (Many versions of `make' set `$<' only in implicit
rules.) A Makefile target like
foo.o : bar.c
$(CC) -I. -I$(srcdir) $(CFLAGS) -c bar.c -o foo.o
should instead be written as
foo.o : bar.c
$(CC) -I. -I$(srcdir) $(CFLAGS) -c $< -o $@
in order to allow `VPATH' to work correctly. When the target has
multiple dependencies, using an explicit `$(srcdir)' is the easiest way
to make the rule work well. For example, the target above for `foo.1'
is best written as:
foo.1 : sedscript
sed -e $(srcdir)/sedscript $(srcdir)/ > $@
GNU distributions usually contain some files which are not source
files--for example, Info files, and the output from Autoconf, Automake,
Bison or Flex. Since these files normally appear in the source
directory, they should always appear in the source directory, not in the
build directory. So Makefile rules to update them should put the
updated files in the source directory.
However, if a file does not appear in the distribution, then the
Makefile should not put it in the source directory, because building a
program in ordinary circumstances should not modify the source directory
in any way.
Try to make the build and installation targets, at least (and all
their subtargets) work correctly with a parallel `make'.

File:, Node: Utilities in Makefiles, Next: Command Variables, Prev: Makefile Basics, Up: Makefile Conventions
7.2.2 Utilities in Makefiles
Write the Makefile commands (and any shell scripts, such as
`configure') to run in `sh', not in `csh'. Don't use any special
features of `ksh' or `bash'.
The `configure' script and the Makefile rules for building and
installation should not use any utilities directly except these:
cat cmp cp diff echo egrep expr false grep install-info
ln ls mkdir mv pwd rm rmdir sed sleep sort tar test touch true
The compression program `gzip' can be used in the `dist' rule.
Stick to the generally supported options for these programs. For
example, don't use `mkdir -p', convenient as it may be, because most
systems don't support it.
It is a good idea to avoid creating symbolic links in makefiles,
since a few systems don't support them.
The Makefile rules for building and installation can also use
compilers and related programs, but should do so via `make' variables
so that the user can substitute alternatives. Here are some of the
programs we mean:
ar bison cc flex install ld ldconfig lex
make makeinfo ranlib texi2dvi yacc
Use the following `make' variables to run those programs:
When you use `ranlib' or `ldconfig', you should make sure nothing
bad happens if the system does not have the program in question.
Arrange to ignore an error from that command, and print a message before
the command to tell the user that failure of this command does not mean
a problem. (The Autoconf `AC_PROG_RANLIB' macro can help with this.)
If you use symbolic links, you should implement a fallback for
systems that don't have symbolic links.
Additional utilities that can be used via Make variables are:
chgrp chmod chown mknod
It is ok to use other utilities in Makefile portions (or scripts)
intended only for particular systems where you know those utilities

File:, Node: Command Variables, Next: DESTDIR, Prev: Utilities in Makefiles, Up: Makefile Conventions
7.2.3 Variables for Specifying Commands
Makefiles should provide variables for overriding certain commands,
options, and so on.
In particular, you should run most utility programs via variables.
Thus, if you use Bison, have a variable named `BISON' whose default
value is set with `BISON = bison', and refer to it with `$(BISON)'
whenever you need to use Bison.
File management utilities such as `ln', `rm', `mv', and so on, need
not be referred to through variables in this way, since users don't
need to replace them with other programs.
Each program-name variable should come with an options variable that
is used to supply options to the program. Append `FLAGS' to the
program-name variable name to get the options variable name--for
example, `BISONFLAGS'. (The names `CFLAGS' for the C compiler,
`YFLAGS' for yacc, and `LFLAGS' for lex, are exceptions to this rule,
but we keep them because they are standard.) Use `CPPFLAGS' in any
compilation command that runs the preprocessor, and use `LDFLAGS' in
any compilation command that does linking as well as in any direct use
of `ld'.
If there are C compiler options that _must_ be used for proper
compilation of certain files, do not include them in `CFLAGS'. Users
expect to be able to specify `CFLAGS' freely themselves. Instead,
arrange to pass the necessary options to the C compiler independently
of `CFLAGS', by writing them explicitly in the compilation commands or
by defining an implicit rule, like this:
Do include the `-g' option in `CFLAGS', because that is not
_required_ for proper compilation. You can consider it a default that
is only recommended. If the package is set up so that it is compiled
with GCC by default, then you might as well include `-O' in the default
value of `CFLAGS' as well.
Put `CFLAGS' last in the compilation command, after other variables
containing compiler options, so the user can use `CFLAGS' to override
the others.
`CFLAGS' should be used in every invocation of the C compiler, both
those which do compilation and those which do linking.
Every Makefile should define the variable `INSTALL', which is the
basic command for installing a file into the system.
Every Makefile should also define the variables `INSTALL_PROGRAM'
and `INSTALL_DATA'. (The default for `INSTALL_PROGRAM' should be
`$(INSTALL)'; the default for `INSTALL_DATA' should be `${INSTALL} -m
644'.) Then it should use those variables as the commands for actual
installation, for executables and non-executables respectively.
Minimal use of these variables is as follows:
$(INSTALL_PROGRAM) foo $(bindir)/foo
$(INSTALL_DATA) libfoo.a $(libdir)/libfoo.a
However, it is preferable to support a `DESTDIR' prefix on the
target files, as explained in the next section.
Always use a file name, not a directory name, as the second argument of
the installation commands. Use a separate command for each file to be

File:, Node: DESTDIR, Next: Directory Variables, Prev: Command Variables, Up: Makefile Conventions
7.2.4 `DESTDIR': support for staged installs
`DESTDIR' is a variable prepended to each installed target file, like
$(INSTALL_PROGRAM) foo $(DESTDIR)$(bindir)/foo
$(INSTALL_DATA) libfoo.a $(DESTDIR)$(libdir)/libfoo.a
The `DESTDIR' variable is specified by the user on the `make'
command line. For example:
make DESTDIR=/tmp/stage install
`DESTDIR' should be supported only in the `install*' and `uninstall*'
targets, as those are the only targets where it is useful.
If your installation step would normally install
`/usr/local/bin/foo' and `/usr/local/lib/libfoo.a', then an
installation invoked as in the example above would install
`/tmp/stage/usr/local/bin/foo' and `/tmp/stage/usr/local/lib/libfoo.a'
Prepending the variable `DESTDIR' to each target in this way
provides for "staged installs", where the installed files are not
placed directly into their expected location but are instead copied
into a temporary location (`DESTDIR'). However, installed files
maintain their relative directory structure and any embedded file names
will not be modified.
You should not set the value of `DESTDIR' in your `Makefile' at all;
then the files are installed into their expected locations by default.
Also, specifying `DESTDIR' should not change the operation of the
software in any way, so its value should not be included in any file
`DESTDIR' support is commonly used in package creation. It is also
helpful to users who want to understand what a given package will
install where, and to allow users who don't normally have permissions
to install into protected areas to build and install before gaining
those permissions. Finally, it can be useful with tools such as
`stow', where code is installed in one place but made to appear to be
installed somewhere else using symbolic links or special mount
operations. So, we strongly recommend GNU packages support `DESTDIR',
though it is not an absolute requirement.

File:, Node: Directory Variables, Next: Standard Targets, Prev: DESTDIR, Up: Makefile Conventions
7.2.5 Variables for Installation Directories
Installation directories should always be named by variables, so it is
easy to install in a nonstandard place. The standard names for these
variables and the values they should have in GNU packages are described
below. They are based on a standard file system layout; variants of it
are used in GNU/Linux and other modern operating systems.
Installers are expected to override these values when calling `make'
(e.g., `make prefix=/usr install' or `configure' (e.g., `configure
--prefix=/usr'). GNU packages should not try to guess which value
should be appropriate for these variables on the system they are being
installed onto: use the default settings specified here so that all GNU
packages behave identically, allowing the installer to achieve any
desired layout.
These first two variables set the root for the installation. All the
other installation directories should be subdirectories of one of these
two, and nothing should be directly installed into these two
A prefix used in constructing the default values of the variables
listed below. The default value of `prefix' should be
`/usr/local'. When building the complete GNU system, the prefix
will be empty and `/usr' will be a symbolic link to `/'. (If you
are using Autoconf, write it as `@prefix@'.)
Running `make install' with a different value of `prefix' from the
one used to build the program should _not_ recompile the program.
A prefix used in constructing the default values of some of the
variables listed below. The default value of `exec_prefix' should
be `$(prefix)'. (If you are using Autoconf, write it as
Generally, `$(exec_prefix)' is used for directories that contain
machine-specific files (such as executables and subroutine
libraries), while `$(prefix)' is used directly for other
Running `make install' with a different value of `exec_prefix'
from the one used to build the program should _not_ recompile the
Executable programs are installed in one of the following
The directory for installing executable programs that users can
run. This should normally be `/usr/local/bin', but write it as
`$(exec_prefix)/bin'. (If you are using Autoconf, write it as
The directory for installing executable programs that can be run
from the shell, but are only generally useful to system
administrators. This should normally be `/usr/local/sbin', but
write it as `$(exec_prefix)/sbin'. (If you are using Autoconf,
write it as `@sbindir@'.)
The directory for installing executable programs to be run by other
programs rather than by users. This directory should normally be
`/usr/local/libexec', but write it as `$(exec_prefix)/libexec'.
(If you are using Autoconf, write it as `@libexecdir@'.)
The definition of `libexecdir' is the same for all packages, so
you should install your data in a subdirectory thereof. Most
packages install their data under `$(libexecdir)/PACKAGE-NAME/',
possibly within additional subdirectories thereof, such as
Data files used by the program during its execution are divided into
categories in two ways.
* Some files are normally modified by programs; others are never
normally modified (though users may edit some of these).
* Some files are architecture-independent and can be shared by all
machines at a site; some are architecture-dependent and can be
shared only by machines of the same kind and operating system;
others may never be shared between two machines.
This makes for six different possibilities. However, we want to
discourage the use of architecture-dependent files, aside from object
files and libraries. It is much cleaner to make other data files
architecture-independent, and it is generally not hard.
Here are the variables Makefiles should use to specify directories
to put these various kinds of files in:
The root of the directory tree for read-only
architecture-independent data files. This should normally be
`/usr/local/share', but write it as `$(prefix)/share'. (If you
are using Autoconf, write it as `@datarootdir@'.) `datadir''s
default value is based on this variable; so are `infodir',
`mandir', and others.
The directory for installing idiosyncratic read-only
architecture-independent data files for this program. This is
usually the same place as `datarootdir', but we use the two
separate variables so that you can move these program-specific
files without altering the location for Info files, man pages, etc.
This should normally be `/usr/local/share', but write it as
`$(datarootdir)'. (If you are using Autoconf, write it as
The definition of `datadir' is the same for all packages, so you
should install your data in a subdirectory thereof. Most packages
install their data under `$(datadir)/PACKAGE-NAME/'.
The directory for installing read-only data files that pertain to a
single machine-that is to say, files for configuring a host.
Mailer and network configuration files, `/etc/passwd', and so
forth belong here. All the files in this directory should be
ordinary ASCII text files. This directory should normally be
`/usr/local/etc', but write it as `$(prefix)/etc'. (If you are
using Autoconf, write it as `@sysconfdir@'.)
Do not install executables here in this directory (they probably
belong in `$(libexecdir)' or `$(sbindir)'). Also do not install
files that are modified in the normal course of their use (programs
whose purpose is to change the configuration of the system
excluded). Those probably belong in `$(localstatedir)'.
The directory for installing architecture-independent data files
which the programs modify while they run. This should normally be
`/usr/local/com', but write it as `$(prefix)/com'. (If you are
using Autoconf, write it as `@sharedstatedir@'.)
The directory for installing data files which the programs modify
while they run, and that pertain to one specific machine. Users
should never need to modify files in this directory to configure
the package's operation; put such configuration information in
separate files that go in `$(datadir)' or `$(sysconfdir)'.
`$(localstatedir)' should normally be `/usr/local/var', but write
it as `$(prefix)/var'. (If you are using Autoconf, write it as
These variables specify the directory for installing certain specific
types of files, if your program has them. Every GNU package should
have Info files, so every program needs `infodir', but not all need
`libdir' or `lispdir'.
The directory for installing header files to be included by user
programs with the C `#include' preprocessor directive. This
should normally be `/usr/local/include', but write it as
`$(prefix)/include'. (If you are using Autoconf, write it as
Most compilers other than GCC do not look for header files in
directory `/usr/local/include'. So installing the header files
this way is only useful with GCC. Sometimes this is not a problem
because some libraries are only really intended to work with GCC.
But some libraries are intended to work with other compilers.
They should install their header files in two places, one
specified by `includedir' and one specified by `oldincludedir'.
The directory for installing `#include' header files for use with
compilers other than GCC. This should normally be `/usr/include'.
(If you are using Autoconf, you can write it as `@oldincludedir@'.)
The Makefile commands should check whether the value of
`oldincludedir' is empty. If it is, they should not try to use
it; they should cancel the second installation of the header files.
A package should not replace an existing header in this directory
unless the header came from the same package. Thus, if your Foo
package provides a header file `foo.h', then it should install the
header file in the `oldincludedir' directory if either (1) there
is no `foo.h' there or (2) the `foo.h' that exists came from the
Foo package.
To tell whether `foo.h' came from the Foo package, put a magic
string in the file--part of a comment--and `grep' for that string.
The directory for installing documentation files (other than Info)
for this package. By default, it should be
`/usr/local/share/doc/YOURPKG', but it should be written as
`$(datarootdir)/doc/YOURPKG'. (If you are using Autoconf, write
it as `@docdir@'.) The YOURPKG subdirectory, which may include a
version number, prevents collisions among files with common names,
such as `README'.
The directory for installing the Info files for this package. By
default, it should be `/usr/local/share/info', but it should be
written as `$(datarootdir)/info'. (If you are using Autoconf,
write it as `@infodir@'.) `infodir' is separate from `docdir' for
compatibility with existing practice.
Directories for installing documentation files in the particular
format. They should all be set to `$(docdir)' by default. (If
you are using Autoconf, write them as `@htmldir@', `@dvidir@',
etc.) Packages which supply several translations of their
documentation should install them in `$(htmldir)/'LL,
`$(pdfdir)/'LL, etc. where LL is a locale abbreviation such as
`en' or `pt_BR'.
The directory for object files and libraries of object code. Do
not install executables here, they probably ought to go in
`$(libexecdir)' instead. The value of `libdir' should normally be
`/usr/local/lib', but write it as `$(exec_prefix)/lib'. (If you
are using Autoconf, write it as `@libdir@'.)
The directory for installing any Emacs Lisp files in this package.
By default, it should be `/usr/lo