Version
*******

The GNU coding standards, last updated September 9, 2010.

Copyright © 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''.

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: http://www.gnu.org/prep/standards/.

If you are maintaining an official GNU package, in addition to this document, please read and follow the GNU maintainer information (see Contents (Information for Maintainers of GNU Software)).

If you want to receive diffs for every change to these GNU documents, join the mailing list gnustandards-commit@gnu.org, via the web interface at http://lists.gnu.org/mailman/listinfo/gnustandards-commit. Archives are also available there.

Please send corrections or suggestions for this document to bug-standards@gnu.org.  If you make a suggestion, please include a suggested new wording for it, to help us consider the suggestion efficiently.  We prefer a context diff to the Texinfo source, but if that's difficult for you, you can make a context diff for some other version of this document, or propose it in any way that makes it clear.  The source repository for this document can be found at http://savannah.gnu.org/projects/gnustandards.

These standards cover the minimum of what is important when writing a GNU package.  Likely, the need for additional standards will come up. 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. http://www.gnu.org/software/hello/hello.html.

This release of the GNU Coding Standards was last updated September 9, 2010.

Keeping Free Software Free
**************************

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

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 adequate.

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

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

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 enough.

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

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

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

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

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

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

We have more detailed advice for maintainers of 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: http://www.gnu.org/prep/maintain/.

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.

General Program Design
**********************

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

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 alternatives.

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 yourself.

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 (http://www.gnu.org/software/guile/), which implements the language Scheme (an especially clean and simple dialect of Lisp). Guile also includes bindings for GTK+/GNOME, making it practical to write modern GUI functionality within Guile.  We don't reject programs written in other ``scripting languages'' such as Perl and Python, but using Guile is very important for the overall consistency of the GNU system.

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.

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 cases.

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,

     int
     foo (int x, int y)
     ...

write the definition in pre-standard style like this,

     int
     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
     #else
     #define P_(proto) ()
     #endif

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)
         ...
       else
         ...

instead of:

       #ifdef HAS_FOO
         ...
       #else
         ...
       #endif

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:

       #ifdef REVERSIBLE_CC_MODE
       #define HAS_REVERSIBLE_CC_MODE 1
       #else
       #define HAS_REVERSIBLE_CC_MODE 0
       #endif

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.

Non-GNU Standards
=================

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

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

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

For instance, Standard C says that nearly all extensions to C are prohibited.  How silly!  GCC implements many extensions, some of which were later adopted as part of the standard.  If you want these constructs to give an error message as ``required'' by the standard, you must specify --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.''

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 elsewhere.

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.

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 conflicts.

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

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

External symbols that are not documented entry points for the user should have names beginning with _.  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.

Formatting Error Messages
=========================

Error messages from compilers should look like this:

     SOURCE-FILE-NAME:LINENO: MESSAGE

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

     SOURCE-FILE-NAME:LINENO:COLUMN: MESSAGE
     SOURCE-FILE-NAME:LINENO.COLUMN: MESSAGE

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:

     SOURCE-FILE-NAME:LINENO-1.COLUMN-1-LINENO-2.COLUMN-2: MESSAGE
     SOURCE-FILE-NAME:LINENO-1.COLUMN-1-COLUMN-2: MESSAGE
     SOURCE-FILE-NAME:LINENO-1-LINENO-2: MESSAGE

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

     FILE-1:LINENO-1.COLUMN-1-FILE-2:LINENO-2.COLUMN-2: MESSAGE

Error messages from other noninteractive programs should look like this:

     PROGRAM:SOURCE-FILE-NAME:LINENO: MESSAGE

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

     PROGRAM: MESSAGE

when there is no relevant source file.

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

     PROGRAM:SOURCE-FILE-NAME:LINENO:COLUMN: MESSAGE

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.

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 behavior.

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 format.

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.

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 getopt_long.

One of the advantages of long-named options is that they can be consistent from program to program.  For example, users should be able to expect the ``verbose'' option of any GNU program which has one, to be spelled precisely --verbose.  To achieve this uniformity, look at the table of common long-option names when you choose the option names for your program (see 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 http://example.org/p.cgi/--help in a browser should output the same information as invoking p.cgi --help from the command line.

--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 <http://gnu.org/licenses/gpl.html>
     This is free software: you are free to change and redistribute it.
     There is NO WARRANTY, to the extent permitted by law.

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; see Copyright Notices (Information for GNU Maintainers).)

Translations of the above lines must preserve the validity of the copyright notices (see 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.

GPL
     GNU General Public License, http://www.gnu.org/licenses/gpl.html.

LGPL
     GNU Lesser General Public License, http://www.gnu.org/licenses/lgpl.html.

GPL/Ada
     GNU GPL with the exception for Ada.

Apache
     The Apache Software Foundation license, http://www.apache.org/licenses.

Artistic
     The Artistic license used for Perl, http://www.perlfoundation.org/legal.

Expat
     The Expat license, http://www.jclark.com/xml/copying.txt.

MPL
     The Mozilla Public License, http://www.mozilla.org/MPL/.

OBSD
     The original (4-clause) BSD license, incompatible with the GNU GPL http://www.xfree86.org/3.3.6/COPYRIGHT2.html#6.

PHP
     The license used for PHP, http://www.php.net/license/.

public domain
     The non-license that is being in the public domain, http://www.gnu.org/licenses/license-list.html#PublicDomain.

Python
     The license for Python, http://www.python.org/2.0.1/license.html.

RBSD
     The revised (3-clause) BSD, compatible with the GNU GPL,

     http://www.xfree86.org/3.3.6/COPYRIGHT2.html#5.

X11
     The simple non-copyleft license used for most versions of the X Window System, http://www.xfree86.org/3.3.6/COPYRIGHT2.html#3.

Zlib
     The license for Zlib, http://www.gzip.org/zlib/zlib_license.html.

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

--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 http://www.gnu.org/software/PKG, and the general page for help using GNU programs.  The format should be like this:

     Report bugs to: MAILING-ADDRESS
     PKG home page: <http://www.gnu.org/software/PKG/>
     General help using GNU software: <http://www.gnu.org/gethelp/>

It is ok to mention other appropriate mailing lists and web pages.

Standards for Dynamic Plug-in Interfaces
========================================

Another aspect of keeping free programs free is encouraging development of free plug-ins, and discouraging development of proprietary plug-ins.  Many GNU programs will not have anything like plug-ins at all, but those that do should follow these practices.

First, the general plug-in architecture design should closely tie the plug-in to the original code, such that the plug-in and the base program are parts of one extended program.  For GCC, for example, plug-ins receive and modify GCC's internal data structures, and so clearly form an extended program with the base GCC.

Second, you should require plug-in developers to affirm that their plug-ins are released under an appropriate license.  This should be enforced with a simple programmatic check.  For GCC, again for example, a plug-in must define the global symbol plugin_is_GPL_compatible, thus asserting that the plug-in is released under a GPL-compatible license (see Plugins (GCC Internals)).

By adding this check to your program you are not creating a new legal requirement.  The GPL itself requires plug-ins to be free software, licensed compatibly.  As long as you have followed the first rule above to keep plug-ins closely tied to your original program, the GPL and AGPL already require those plug-ins to be released under a compatible license.  The symbol definition in the plug-in---or whatever equivalent works best in your program---makes it harder for anyone who might distribute proprietary plug-ins to legally defend themselves.  If a case about this got to court, we can point to that symbol as evidence that the plug-in developer understood that the license had this requirement.

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 bug-standards@gnu.org a list of them, with their meanings, so we can update the table.

after-date
     -N in tar.

all
     -a in du, ls, nm, stty, uname, and unexpand.

all-text
     -a in diff.

almost-all
     -A in ls.

append
     -a in etags, tee, time; -r in tar.

archive
     -a in cp.

archive-name
     -n in shar.

arglength
     -l in m4.

ascii
     -a in diff.

assign
     -v in gawk.

assume-new
     -W in make.

assume-old
     -o in make.

auto-check
     -a in recode.

auto-pager
     -a in wdiff.

auto-reference
     -A in ptx.

avoid-wraps
     -n in wdiff.

background
     For server programs, run in the background.

backward-search
     -B in ctags.

basename
     -f in shar.

batch
     Used in GDB.

baud
     Used in GDB.

before
     -b in tac.

binary
     -b in cpio and diff.

bits-per-code
     -b in shar.

block-size
     Used in cpio and tar.

blocks
     -b in head and tail.

break-file
     -b in ptx.

brief
     Used in various programs to make output shorter.

bytes
     -c in head, split, and tail.

c++
     -C in etags.

catenate
     -A in tar.

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

changes
     -c in chgrp and chown.

classify
     -F in ls.

colons
     -c in recode.

command
     -c in su; -x in GDB.

compare
     -d in tar.

compat
     Used in gawk.

compress
     -Z in tar and shar.

concatenate
     -A in tar.

confirmation
     -w in tar.

context
     Used in diff.

copyleft
     -W copyleft in gawk.

copyright
     -C in ptx, recode, and wdiff; -W copyright in gawk.

core
     Used in GDB.

count
     -q in who.

count-links
     -l in du.

create
     Used in tar and cpio.

cut-mark
     -c in shar.

cxref
     -x in ctags.

date
     -d in touch.

debug
     -d in make and m4; -t in Bison.

define
     -D in m4.

defines
     -d in Bison and ctags.

delete
     -D in tar.

dereference
     -L in chgrp, chown, cpio, du, ls, and tar.

dereference-args
     -D in du.

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

diacritics
     -d in recode.

dictionary-order
     -d in look.

diff
     -d in tar.

digits
     -n in csplit.

directory
     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 specially.

discard-all
     -x in strip.

discard-locals
     -X in strip.

dry-run
     -n in make.

ed
     -e in diff.

elide-empty-files
     -z in csplit.

end-delete
     -x in wdiff.

end-insert
     -z in wdiff.

entire-new-file
     -N in diff.

environment-overrides
     -e in make.

eof
     -e in xargs.

epoch
     Used in GDB.

error-limit
     Used in makeinfo.

error-output
     -o in m4.

escape
     -b in ls.

exclude-from
     -X in tar.

exec
     Used in GDB.

exit
     -x in xargs.

exit-0
     -e in unshar.

expand-tabs
     -t in diff.

expression
     -e in sed.

extern-only
     -g in nm.

extract
     -i in cpio; -x in tar.

faces
     -f in finger.

fast
     -f in su.

fatal-warnings
     -E in m4.

file
     -f in gawk, info, make, mt, sed, and tar.

field-separator
     -F in gawk.

file-prefix
     -b in Bison.

file-type
     -F in ls.

files-from
     -T in tar.

fill-column
     Used in makeinfo.

flag-truncation
     -F in ptx.

fixed-output-files
     -y in Bison.

follow
     -f in tail.

footnote-style
     Used in makeinfo.

force
     -f in cp, ln, mv, and rm.

force-prefix
     -F in shar.

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

format
     Used in ls, time, and ptx.

freeze-state
     -F in m4.

fullname
     Used in GDB.

gap-size
     -g in ptx.

get
     -x in tar.

graphic
     -i in ul.

graphics
     -g in recode.

group
     -g in install.

gzip
     -z in tar and shar.

hashsize
     -H in m4.

header
     -h in objdump and recode

heading
     -H in who.

help
     Used to ask for brief usage information.

here-delimiter
     -d in shar.

hide-control-chars
     -q in ls.

html
     In makeinfo, output HTML.

idle
     -u in who.

ifdef
     -D in diff.

ignore
     -I in ls; -x in recode.

ignore-all-space
     -w in diff.

ignore-backups
     -B in ls.

ignore-blank-lines
     -B in diff.

ignore-case
     -f in look and ptx; -i in diff and wdiff.

ignore-errors
     -i in make.

ignore-file
     -i in ptx.

ignore-indentation
     -I in etags.

ignore-init-file
     -f in Oleo.

ignore-interrupts
     -i in tee.

ignore-matching-lines
     -I in diff.

ignore-space-change
     -b in diff.

ignore-zeros
     -i in tar.

include
     -i in etags; -I in m4.

include-dir
     -I in make.

incremental
     -G in tar.

info
     -i, -l, and -m in Finger.

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

initial
     -i in expand.

initial-tab
     -T in diff.

inode
     -i in ls.

interactive
     -i in cp, ln, mv, rm; -e in m4; -p in xargs; -w in tar.

intermix-type
     -p in shar.

iso-8601
     Used in date

jobs
     -j in make.

just-print
     -n in make.

keep-going
     -k in make.

keep-files
     -k in csplit.

kilobytes
     -k in du and ls.

language
     -l in etags.

less-mode
     -l in wdiff.

level-for-gzip
     -g in shar.

line-bytes
     -C in split.

lines
     Used in split, head, and tail.

link
     -l in cpio.

lint
lint-old
     Used in gawk.

list
     -t in cpio; -l in recode.

list
     -t in tar.

literal
     -N in ls.

load-average
     -l in make.

login
     Used in su.

machine
     Used in uname.

macro-name
     -M in ptx.

mail
     -m in hello and uname.

make-directories
     -d in cpio.

makefile
     -f in make.

mapped
     Used in GDB.

max-args
     -n in xargs.

max-chars
     -n in xargs.

max-lines
     -l in xargs.

max-load
     -l in make.

max-procs
     -P in xargs.

mesg
     -T in who.

message
     -T in who.

minimal
     -d in diff.

mixed-uuencode
     -M in shar.

mode
     -m in install, mkdir, and mkfifo.

modification-time
     -m in tar.

multi-volume
     -M in tar.

name-prefix
     -a in Bison.

nesting-limit
     -L in m4.

net-headers
     -a in shar.

new-file
     -W in make.

no-builtin-rules
     -r in make.

no-character-count
     -w in shar.

no-check-existing
     -x in shar.

no-common
     -3 in wdiff.

no-create
     -c in touch.

no-defines
     -D in etags.

no-deleted
     -1 in wdiff.

no-dereference
     -d in cp.

no-inserted
     -2 in wdiff.

no-keep-going
     -S in make.

no-lines
     -l in Bison.

no-piping
     -P in shar.

no-prof
     -e in gprof.

no-regex
     -R in etags.

no-sort
     -p in nm.

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

no-split
     Used in makeinfo.

no-static
     -a in gprof.

no-time
     -E in gprof.

no-timestamp
     -m in shar.

no-validate
     Used in makeinfo.

no-wait
     Used in emacsclient.

no-warn
     Used in various programs to inhibit warnings.

node
     -n in info.

nodename
     -n in uname.

nonmatching
     -f in cpio.

nstuff
     -n in objdump.

null
     -0 in xargs.

number
     -n in cat.

number-nonblank
     -b in cat.

numeric-sort
     -n in nm.

numeric-uid-gid
     -n in cpio and ls.

nx
     Used in GDB.

old-archive
     -o in tar.

old-file
     -o in make.

one-file-system
     -l in tar, cp, and du.

only-file
     -o in ptx.

only-prof
     -f in gprof.

only-time
     -F in gprof.

options
     -o in getopt, fdlist, fdmount, fdmountd, and fdumount.

output
     In various programs, specify the output file name.

output-prefix
     -o in shar.

override
     -o in rm.

overwrite
     -c in unshar.

owner
     -o in install.

paginate
     -l in diff.

paragraph-indent
     Used in makeinfo.

parents
     -p in mkdir and rmdir.

pass-all
     -p in ul.

pass-through
     -p in cpio.

port
     -P in finger.

portability
     -c in cpio and tar.

posix
     Used in gawk.

prefix-builtins
     -P in m4.

prefix
     -f in csplit.

preserve
     Used in tar and cp.

preserve-environment
     -p in su.

preserve-modification-time
     -m in cpio.

preserve-order
     -s in tar.

preserve-permissions
     -p in tar.

print
     -l in diff.

print-chars
     -L in cmp.

print-data-base
     -p in make.

print-directory
     -w in make.

print-file-name
     -o in nm.

print-symdefs
     -s in nm.

printer
     -p in wdiff.

prompt
     -p in ed.

proxy
     Specify an HTTP proxy.

query-user
     -X in shar.

question
     -q in make.

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

quiet-unshar
     -Q in shar

quote-name
     -Q in ls.

rcs
     -n in diff.

re-interval
     Used in gawk.

read-full-blocks
     -B in tar.

readnow
     Used in GDB.

recon
     -n in make.

record-number
     -R in tar.

recursive
     Used in chgrp, chown, cp, ls, diff, and rm.

reference
     -r in touch.

references
     -r in ptx.

regex
     -r in tac and etags.

release
     -r in uname.

reload-state
     -R in m4.

relocation
     -r in objdump.

rename
     -r in cpio.

replace
     -i in xargs.

report-identical-files
     -s in diff.

reset-access-time
     -a in cpio.

reverse
     -r in ls and nm.

reversed-ed
     -f in diff.

right-side-defs
     -R in ptx.

same-order
     -s in tar.

same-permissions
     -p in tar.

save
     -g in stty.

se
     Used in GDB.

sentence-regexp
     -S in ptx.

separate-dirs
     -S in du.

separator
     -s in tac.

sequence
     Used by recode to chose files or pipes for sequencing passes.

shell
     -s in su.

show-all
     -A in cat.

show-c-function
     -p in diff.

show-ends
     -E in cat.

show-function-line
     -F in diff.

show-tabs
     -T in cat.

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

size
     -s in ls.

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

sort
     Used in ls.

source
     -W source in gawk.

sparse
     -S in tar.

speed-large-files
     -H in diff.

split-at
     -E in unshar.

split-size-limit
     -L in shar.

squeeze-blank
     -s in cat.

start-delete
     -w in wdiff.

start-insert
     -y in wdiff.

starting-file
     Used in tar and diff to specify which file within a directory to start processing with.

statistics
     -s in wdiff.

stdin-file-list
     -S in shar.

stop
     -S in make.

strict
     -s in recode.

strip
     -s in install.

strip-all
     -s in strip.

strip-debug
     -S in strip.

submitter
     -s in shar.

suffix
     -S in cp, ln, mv.

suffix-format
     -b in csplit.

sum
     -s in gprof.

summarize
     -s in du.

symbolic
     -s in ln.

symbols
     Used in GDB and objdump.

synclines
     -s in m4.

sysname
     -s in uname.

tabs
     -t in expand and unexpand.

tabsize
     -T in ls.

terminal
     -T in tput and ul. -t in wdiff.

text
     -a in diff.

text-files
     -T in shar.

time
     Used in ls and touch.

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

to-stdout
     -O in tar.

total
     -c in du.

touch
     -t in make, ranlib, and recode.

trace
     -t in m4.

traditional
     -t in hello; -W traditional in gawk; -G in ed, m4, and ptx.

tty
     Used in GDB.

typedefs
     -t in ctags.

typedefs-and-c++
     -T in ctags.

typeset-mode
     -t in ptx.

uncompress
     -z in tar.

unconditional
     -u in cpio.

undefine
     -U in m4.

undefined-only
     -u in nm.

update
     -u in cp, ctags, mv, tar.

usage
     Used in gawk; same as --help.

uuencode
     -B in shar.

vanilla-operation
     -V in shar.

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

verify
     -W in tar.

version
     Print the version number.

version-control
     -V in cp, ln, mv.

vgrind
     -v in ctags.

volume
     -V in tar.

what-if
     -W in make.

whole-size-limit
     -l in shar.

width
     -w in ls and ptx.

word-regexp
     -W in ptx.

writable
     -T in who.

zeros
     -z in gprof.

OID Allocations
===============

The OID (object identifier) 1.3.6.1.4.1.11591 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 http://www.alvestrand.no/objectid has a (voluntary) listing of many OID assignments.

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

          [file gnu-oids.texi not included]

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.

Memory leak detectors such as valgrind can be useful, but don't complicate a program merely to avoid their false alarms.  For example, if memory is used until just before a process exits, don't free it simply to silence a leak detector.

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 /etc.

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 directory.

Making The Best Use of C
************************

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

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 this:

     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:

     int
     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;
     else
       {
         while (z)
           {
             haha += foo (z, 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 nesting:

     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:

     do
       {
         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.

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 file.

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

Please put a comment on each function saying what the function does, what sorts of arguments it gets, and what the possible values of arguments mean and are used for.  It is not necessary to duplicate in words the meaning of the C argument declarations, if a C type is being used in its customary fashion.  If there is anything nonstandard about its use (such as an argument of type 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 */

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.

Don't make the program ugly just to placate static analysis tools such as lint, clang, and GCC with extra warnings options such as -Wconversion and -Wundef.  These tools can help find bugs and unclear code, but they can also generate so many false alarms that that it hurts readability to silence them with unnecessary casts, wrappers, and other complications.  For example, please don't insert casts to void or calls to do-nothing functions merely to pacify a lint checker.

Declarations of external functions and functions to appear later in the source file should all go in one place near the beginning of the file (somewhere before the first function definition in the file), or else should go in a header file.  Don't put extern declarations inside functions.

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 identifiers. GCC's -Wshadow option can detect this problem.

Don't declare multiple variables in one declaration that spans lines. Start a new declaration on each line, instead.  For example, instead of this:

     int    foo,
            bar;

write either this:

     int foo, bar;

or this:

     int foo;
     int bar;

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

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 ();
       else
         lose ();

always like this:

     if (foo)
       {
         if (bar)
           win ();
         else
           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)
       ...
     else
       {
         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");

This example uses zero without a cast as a null pointer constant. This is perfectly fine, except that a cast is needed when calling a varargs function or when using sizeof.

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 comments.

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

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

Please use underscores to separate words in a name, so that the Emacs word commands can be useful within them.  Stick to lower case; reserve upper case for macros and 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 characters.

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 written.

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.

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 GNU.

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 (http://www.gnu.org/software/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 http://git.savannah.gnu.org/gitweb/?p=gnulib.git. Here's a sample use:

     #include "error.h"
     #include <errno.h>
     #include <stdio.h>

     char *program_name = "myprogram";

     FILE *
     xfopen (char const *name)
     {
       FILE *fp = fopen (name, "r");
       if (! fp)
         error (1, errno, "cannot read %s", name);
       return fp;
     }

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.

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 conflict.

  * 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 systems.

  * 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 systems.

     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:

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

          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.

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")),
             nfiles);

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 problem:

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

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 ``Floréal''.  Also, it is OK to use non-ASCII characters to represent proper names of contributors in change logs (see 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.

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 (http://www.gnu.org/software/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 this.

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.

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.

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 understand.

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 avoid.

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

That is not as hard as it first sounds.  Arrange each chapter as a logical breakdown of its topic, but order the sections, and write their text, so that reading the chapter straight through makes sense.  Do likewise when structuring the book into chapters, and when structuring a section into paragraphs.  The watchword is, 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 Making Index Entries (GNU Texinfo), and see Defining the Entries of an Index (GNU Texinfo).

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

Please include an email address in the manual for where to report bugs 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.

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.

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.

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 http://www.gnu.org/copyleft/fdl-howto.html for more explanation of how to employ the GFDL.

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

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.

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 http://www.gnu.org/order/order.html.  This should not be included in the printed manual, though, because there it is redundant.

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

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.

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.

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 you.

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

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  <rms@gnu.org>

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

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

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

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

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 example:

     * 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  <jdoe@gnu.org>

             * sewing.c: Make it sew.

rather than this:

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

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

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

Simple Changes
--------------

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

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

     * 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.

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.

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.

Man Pages
=========

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

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

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

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

When a program changes only a little, you may feel that the discrepancies are small enough that the man page remains useful without updating.  If so, put a prominent note near the beginning of the man page explaining that you don't maintain it and that the Texinfo manual is more authoritative.  The note should say how to access the Texinfo documentation.

Be sure that man pages include a copyright statement and free license. The simple all-permissive license is appropriate for simple man pages (see License Notices for Other Files (Information for GNU Maintainers)).

For long man pages, with enough explanation and documentation that they can be considered true manuals, use the GFDL (see License for Manuals).

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

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.

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.

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 (see Introduction (Autoconf)) and/or GNU Automake (see Introduction (Automake)), 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 system.

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 first.

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 Makefile.in which contains the input used for editing.  Once again, this is so that people won't be able to build the program without configuring it first.

If 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 (see 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:

     CPU-COMPANY-SYSTEM

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

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 (http://git.savannah.gnu.org/gitweb/?p=config.git;a=blob_plain;f=config.sub;hb=HEAD) 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 (http://git.savannah.gnu.org/gitweb/?p=config.git;a=blob_plain;f=config.guess;hb=HEAD).

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

--enable-FEATURE[=PARAMETER]
     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.

--with-PACKAGE
     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.

VARIABLE=VALUE
     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 make-stds.texi not included]

Making Releases
===============

You should identify each release with a pair of version numbers, a major version and a minor.  We have no objection to using more than two numbers, but it is very unlikely that you really need them.

Package the distribution of Foo version 69.96 up in a gzipped tar file with the name foo-69.96.tar.gz.  It should unpack into a subdirectory named foo-69.96.

Building and installing the program should never modify any of the files contained in the distribution.  This means that all the files that form part of the program in any way must be classified into source files and non-source files.  Source files are written by humans and never changed automatically; non-source files are produced from source files by programs under the control of the Makefile.

The distribution should contain a file named README which gives the name of the package, and a general description of what it does.  It is also good to explain the purpose of each of the first-level subdirectories in the package, if there are any.  The README file should either state the version number of the package, or refer to where in the package it can be found.

The README file should refer to the file INSTALL, which should contain an explanation of the installation procedure.

The README file should also refer to the file which contains the copying conditions.  The GNU GPL, if used, should be in a file called COPYING.  If the GNU LGPL is used, it should be in a file called COPYING.LESSER.

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

Non-source files that might actually be modified by building and installing the program should never be included in the distribution.  So if you do distribute non-source files, always make sure they are up to date when you make a new distribution.

Make sure that all the files in the distribution are world-readable, and that directories are world-readable and world-searchable (octal mode 755). We used to recommend that all directories in the distribution also be world-writable (octal mode 777), because ancient versions of tar would otherwise not cope when extracting the archive as an unprivileged user.  That can easily lead to security issues when creating the archive, however, so now we recommend against that.

Don't include any symbolic links in the distribution itself.  If the tar file contains symbolic links, then people cannot even unpack it on systems that don't support symbolic links.  Also, don't use multiple names for one file in different directories, because certain file systems cannot handle this and that prevents unpacking the distribution.

Try to make sure that all the file names will be unique on MS-DOS.  A name on MS-DOS consists of up to 8 characters, optionally followed by a period and up to three characters.  MS-DOS will truncate extra characters both before and after the period.  Thus, foobarhacker.c and foobarhacker.o are not ambiguous; they are truncated to foobarha.c and foobarha.o, which are distinct.

Include in your distribution a copy of the texinfo.tex you used to test print any *.texinfo or *.texi files.

Likewise, if your program uses small GNU software packages like regex, getopt, obstack, or termcap, include them in the distribution file. Leaving them out would make the distribution file a little smaller at the expense of possible inconvenience to a user who doesn't know what other files to get.

References to Non-Free Software and Documentation
*************************************************

A GNU program should not recommend, promote, or grant legitimacy to the use of any non-free program.  Proprietary software is a social and ethical problem, and our aim is to put an end to that problem.  We can't stop some people from writing proprietary programs, or stop other people from using them, but we can and should refuse to advertise them to new potential customers, or to give the public the idea that their existence is ethical.

The GNU definition of free software is found on the GNU web site at http://www.gnu.org/philosophy/free-sw.html, and the definition of free documentation is found at http://www.gnu.org/philosophy/free-doc.html.  The terms ``free'' and ``non-free'', used in this document, refer to those definitions.

A list of important licenses and whether they qualify as free is in http://www.gnu.org/licenses/license-list.html.  If it is not clear whether a license qualifies as free, please ask the GNU Project by writing to licensing@gnu.org.  We will answer, and if the license is an important one, we will add it to the list.

When a non-free program or system is well known, you can mention it in passing---that is harmless, since users who might want to use it probably already know about it.  For instance, it is fine to explain how to build your package on top of some widely used non-free operating system, or how to use it together with some widely used non-free program.

However, you should give only the necessary information to help those who already use the non-free program to use your program with it---don't give, or refer to, any further information about the proprietary program, and don't imply that the proprietary program enhances your program, or that its existence is in any way a good thing.  The goal should be that people already using the proprietary program will get the advice they need about how to use your free program with it, while people who don't already use the proprietary program will not see anything likely to lead them to take an interest in it.

If a non-free program or system is obscure in your program's domain, your program should not mention or support it at all, since doing so would tend to popularize the non-free program more than it popularizes your program.  (You cannot hope to find many additional users for your program among the users of Foobar, if the existence of Foobar is not generally known among people who might want to use your program.)

Sometimes a program is free software in itself but depends on a non-free platform in order to run.  For instance, many Java programs depend on some non-free Java libraries.  To recommend or promote such a program is to promote the other programs it needs.  This is why we are careful about listing Java programs in the Free Software Directory: we don't want to promote the non-free Java libraries.

We hope this particular problem with Java will be gone by and by, as we replace the remaining non-free standard Java libraries with free software, but the general principle will remain the same: don't recommend, promote or legitimize programs that depend on non-free software to run.

Some free programs strongly encourage the use of non-free software.  A typical example is mplayer.  It is free software in itself, and the free code can handle some kinds of files.  However, mplayer recommends use of non-free codecs for other kinds of files, and users that install mplayer are very likely to install those codecs along with it.  To recommend mplayer is, in effect, to promote use of the non-free codecs.

Thus, you should not recommend programs that strongly encourage the use of non-free software.  This is why we do not list mplayer in the Free Software Directory.

A GNU package should not refer the user to any non-free documentation for free software.  Free documentation that can be included in free operating systems is essential for completing the GNU system, or any free operating system, so encouraging it is a priority; to recommend use of documentation that we are not allowed to include undermines the impetus for the community to produce documentation that we can include.  So GNU packages should never recommend non-free documentation.

By contrast, it is ok to refer to journal articles and textbooks in the comments of a program for explanation of how it functions, even though they are non-free.  This is because we don't include such things in the GNU system even if they are free---they are outside the scope of what a software distribution needs to include.

Referring to a web site that describes or recommends a non-free program is promoting that program, so please do not make links (or mention by name) web sites that contain such material.  This policy is relevant particularly for the web pages for a GNU package.

Following links from nearly any web site can lead eventually to non-free software; this is inherent in the nature of the web.  So it makes no sense to criticize a site for having such links.  As long as the site does not itself recommend a non-free program, there is no need to consider the question of the sites that it links to for other reasons.

Thus, for example, you should not refer to AT&T's web site if that recommends AT&T's non-free software packages; you should not refer to a site that links to AT&T's site presenting it as a place to get some non-free program, because that link recommends and legitimizes the non-free program.  However, that a site contains a link to AT&T's web site for some other purpose (such as long-distance telephone service) is not an objection against it.

GNU Free Documentation License
******************************

     [file fdl.texi not included]