GNU Coding Standards

Table of Contents

GNU Coding Standards

The GNU coding standards, last updated June 12, 2020.

Copyright © 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2020 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License”.

1 About the GNU Coding Standards

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

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

If you are maintaining an official GNU package, in addition to this document, please read and follow the GNU maintainer information (see Contents in 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 https://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 https://savannah.gnu.org/projects/gnustandards.

These standards cover the minimum of what is important when writing a GNU package. Likely, the need for additional standards will come up. Sometimes, you might suggest that such standards be added to this document. If you think your standards would be generally useful, please do suggest them.

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

The GNU Hello program serves as an example of how to follow the GNU coding standards for a trivial program. https://www.gnu.org/software/hello/hello.html.

This release of the GNU Coding Standards was last updated June 12, 2020.

2 Keeping Free Software Free

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

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

2.1 Referring to Proprietary Programs

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

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

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

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

2.2 Accepting Contributions

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

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

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

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

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

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

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

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

2.3 Trademarks

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

Trademark acknowledgments 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 acknowledgments feel like kowtowing, and there is no legal requirement for them, so we don’t use them.

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

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

3 General Program Design

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

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

3.1 Which Languages to Use

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

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

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

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

3.2 Compatibility with Other Implementations

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

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

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

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

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

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

3.3 Using Non-standard Features

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

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

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

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

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

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

3.4 Standard C and Pre-Standard C

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

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

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

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

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

3.5 Conditional Compilation

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

For example, please write

if (HAS_FOO) ... 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

4 Program Behavior for All Programs

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

4.1 Non-GNU Standards

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

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

4.2 Writing Robust Programs

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

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

Check every system call for an error return, unless you know you wish to ignore errors. Include the system error text (from strerror , 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 NULL . 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.

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. This way, restarting the program (without reloading it), or part of it, will reinitialize those variables. Reserve C initialized declarations for data that will not be changed.

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 Gnulib (see mkstemps in Gnulib ).

In bash, use set -C (long name noclobber ) to avoid this problem. In addition, the mktemp utility is a more general solution for creating temporary files from shell scripts (see mktemp invocation in GNU Coreutils ).

4.3 Library Behavior

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

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

4.4 Formatting Error Messages

Error messages from compilers should look like this:

sourcefile : lineno : message

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

sourcefile : lineno : column : message sourcefile : 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. For non-ASCII characters, Unicode character widths should be used when in a UTF-8 locale; GNU libc and GNU gnulib provide suitable wcwidth functions.

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

sourcefile : line1 . column1 - line2 . column2 : message sourcefile : line1 . column1 - column2 : message sourcefile : line1 - line2 : message

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

file1 : line1 . column1 - file2 : line2 . column2 : message

Error messages from other noninteractive programs should look like this:

program : sourcefile : 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 : sourcefile : 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.

4.5 Standards for Interfaces Generally

Please don’t make the behavior of a utility depend on the name used to invoke it. It is useful sometimes to make a link to a utility with a different name, and that should not change what it does. Thus, if you make foo a link to ls , the program should behave the same regardless of which of those names is used to invoke it.

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

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

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

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

Compatibility requires certain programs to depend on the type of output device. It would be disastrous if 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.

4.6 Finding the Program’s Executable and Associated Files

A program may need to find the executable file it was started with, so as to relaunch the same program. It may need to find associated files, either source files or files constructed by building, that it uses at run time.

The way to find them starts with looking at argv[0] .

If that string contains a slash, it is by convention the file name of the executable and its directory part is the directory that contained the executable. This is the case when the program was not found through PATH , which normally means it was built but not installed, and run from the build directory. The program can use the argv[0] file name to relaunch itself, and can look in its directory part for associated files. If that file name is not absolute, then it is relative to the working directory in which the program started.

If argv[0] does not contain a slash, it is a command name whose executable was found via PATH . The program should search for that name in the directories in PATH , interpreting . as the working directory that was current when the program started.

If this procedure finds the executable, we call the directory it was found in the invocation directory. The program should check for the presence in that directory of the associated files it needs.

If the program’s executable is normally built in a subdirectory of the main build directory, and the main build directory contains associated files (perhaps including subdirectories), the program should look at the parent of the invocation directory, checking for the associated files and subdirectories the main build directory should contain.

If the invocation directory doesn’t contain what’s needed, but the executable file name is a symbolic link, the program should try using the link target’s containing directory as the invocation directory.

If this procedure doesn’t come up with an invocation directory that is valid—normally the case for an installed program that was found via PATH —the program should look for the associated files in the directories where the program’s makefile installs them. See Directory Variables.

Providing valid information in argv[0] is a convention, not guaranteed. Well-behaved programs that launch other programs, such as shells, follow the convention; your code should follow it too, when launching other programs. But it is always possible to launch the program and give a nonsensical value in argv[0] .

Therefore, any program that needs to know the location of its executable, or that of of other associated files, should offer the user environment variables to specify those locations explicitly.

Don’t give special privilege, such as with the setuid bit, to programs that will search heuristically for associated files or for their own executables when invoked that way. Limit that privilege to programs that find associated files in hard-coded installed locations such as under /usr and /etc .

See Bourne Shell Variables in Bash Reference Manual , for more information about PATH .

4.7 Standards for Graphical Interfaces

When you write a program that provides a graphical user interface, please make it work with the X Window System, using the GTK+ toolkit or the GNUstep toolkit, unless the functionality specifically requires some alternative (for example, “displaying jpeg images while in console mode”).

In addition, please provide a command-line interface to control the functionality. (In many cases, the graphical user interface can be a separate program which invokes the command-line program.) This is so that the same jobs can be done from scripts.

Please also consider providing a D-bus interface for use from other running programs, such as within GNOME. (GNOME used to use CORBA for this, but that is being phased out.) In addition, consider providing a library interface (for use from C), and perhaps a keyboard-driven console interface (for use by users from console mode). Once you are doing the work to provide the functionality and the graphical interface, these won’t be much extra work.

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

4.8 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 output for –version. • --help: The standard output for –help.

4.8.1 --version

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

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

GNU Emacs 19.30

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

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

emacsserver (GNU Emacs) 19.30

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

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

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

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

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

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

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

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

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 in 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:

©

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 ‘ v version [+] ’, meaning that particular version, or later versions with the ‘ + ’, as shown above. In the case of a GNU license, always indicate the permitted versions in this way.

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, https://www.gnu.org/licenses/gpl.html. LGPL GNU Lesser General Public License, https://www.gnu.org/licenses/lgpl.html. GPL/Ada GNU GPL with the exception for Ada. Apache The Apache Software Foundation license, https://directory.fsf.org/wiki/License:Apache2.0. Artistic The Artistic license used for Perl, https://directory.fsf.org/wiki/License:ArtisticLicense2.0. Expat The Expat license, https://directory.fsf.org/wiki/License:Expat. MPL The Mozilla Public License, https://directory.fsf.org/wiki/License:MPLv2.0. OBSD The original (4-clause) BSD license, incompatible with the GNU GPL,

https://directory.fsf.org/wiki/License:BSD_4Clause. PHP The license used for PHP, https://directory.fsf.org/wiki/License:PHPv3.01. public domain The non-license that is being in the public domain,

https://www.gnu.org/licenses/license-list.html#PublicDomain. Python The license for Python, https://directory.fsf.org/wiki/License:Python2.0.1. RBSD The revised (3-clause) BSD, compatible with the GNU GPL,

https://directory.fsf.org/wiki/License:BSD_3Clause. X11 The simple non-copyleft license used for most versions of the X Window System, https://directory.fsf.org/wiki/License:X11. Zlib The license for Zlib, https://directory.fsf.org/wiki/License:Zlib.

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

4.8.2 --help

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

Near the end of the ‘ --help ’ option’s output, please place lines giving the email address for bug reports, the package’s home page (normally ‘ https://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: <https://www.gnu.org/software/ pkg /> General help using GNU software: <https://www.gnu.org/gethelp/>

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

4.9 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 in 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.

4.10 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 .

4.11 OID Allocations

The OID (object identifier) 1.3.6.1.4.1.11591 has been assigned to the GNU Project (thanks to Sergey Poznyakoff). These are used for SNMP, LDAP, X.509 certificates, and so on. The web site https://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:

1.3.6.1.4.1.11591 GNU 1.3.6.1.4.1.11591.1 GNU Radius 1.3.6.1.4.1.11591.2 GnuPG 1.3.6.1.4.1.11591.2.1 notation 1.3.6.1.4.1.11591.2.1.1 pkaAddress 1.3.6.1.4.1.11591.3 GNU Radar 1.3.6.1.4.1.11591.4 GNU GSS 1.3.6.1.4.1.11591.5 GNU Mailutils 1.3.6.1.4.1.11591.6 GNU Shishi 1.3.6.1.4.1.11591.7 GNU Radio 1.3.6.1.4.1.11591.8 GNU Dico 1.3.6.1.4.1.11591.9 GNU Rush 1.3.6.1.4.1.11591.12 digestAlgorithm 1.3.6.1.4.1.11591.12.2 TIGER/192 1.3.6.1.4.1.11591.13 encryptionAlgorithm 1.3.6.1.4.1.11591.13.2 Serpent 1.3.6.1.4.1.11591.13.2.1 Serpent-128-ECB 1.3.6.1.4.1.11591.13.2.2 Serpent-128-CBC 1.3.6.1.4.1.11591.13.2.3 Serpent-128-OFB 1.3.6.1.4.1.11591.13.2.4 Serpent-128-CFB 1.3.6.1.4.1.11591.13.2.21 Serpent-192-ECB 1.3.6.1.4.1.11591.13.2.22 Serpent-192-CBC 1.3.6.1.4.1.11591.13.2.23 Serpent-192-OFB 1.3.6.1.4.1.11591.13.2.24 Serpent-192-CFB 1.3.6.1.4.1.11591.13.2.41 Serpent-256-ECB 1.3.6.1.4.1.11591.13.2.42 Serpent-256-CBC 1.3.6.1.4.1.11591.13.2.43 Serpent-256-OFB 1.3.6.1.4.1.11591.13.2.44 Serpent-256-CFB 1.3.6.1.4.1.11591.14 CRC algorithms 1.3.6.1.4.1.11591.14.1 CRC 32 1.3.6.1.4.1.11591.15 ellipticCurve 1.3.6.1.4.1.11591.15.1 Ed25519

4.12 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 NULL .

Memory analysis tools 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 such a tool.

4.13 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.

5 Making The Best Use of C

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

5.1 Formatting Your Source Code

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

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) …

For struct and enum types, likewise put the braces in column one, unless the whole contents fits on one line:

struct foo { int a, b; } or struct foo { int a, b; }

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.

5.2 Commenting Your Work

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

Also, please write a brief comment at the start of each source file, with the file name and a line or two about the overall purpose of the 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 readers can see that for themselves. There might be an exception when the comment is so long that the function itself would be off the bottom of the screen.

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

/* 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 */

5.3 Clean Use of C Constructs

Please explicitly declare the types of all objects. For example, you should explicitly declare all arguments to functions, and you should declare functions to return int rather than omitting the int .

Some programmers like to use the GCC ‘ -Wall ’ option, and change the code whenever it issues a warning. If you want to do this, then do. Other programmers prefer not to use ‘ -Wall ’, because it gives warnings for valid and legitimate code which they do not want to change. If you want to do this, then do. The compiler should be your servant, not your master.

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 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)) == NULL) fatal ("virtual memory exhausted");

instead, write this:

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

5.4 Naming Variables, Functions, and Files

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

5.5 Portability between System Types

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

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

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

The easiest way to achieve portability to most Unix-like systems is to use Autoconf. It’s unlikely that your program needs to know more information about the host platform than Autoconf can provide, simply because most of the programs that need such knowledge have already been 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 MS-DOS, 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”. See Trademarks.

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

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.

5.6 Portability between CPUs

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

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

printf ("size = %lu

", (unsigned long) sizeof array); printf ("diff = %ld

", (long) (pointer2 - pointer1));

Whether to support Mingw64, and Windows in general, in your package is your choice. The GNU Project doesn’t say you have any responsibility to do so. Our goal is to replace proprietary systems, including Windows, not to enhance them. If people pressure you to make your program run on Windows, and you are not interested, you can respond with, “Switch to GNU/Linux — your freedom depends on it.”

Predefined file-size types like 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); }

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.

5.7 Calling System Functions

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

Hence, there is little reason to support old C or non-POSIX systems, and you may want to take advantage of standard C and POSIX to write clearer, more portable, or faster code. You should use standard interfaces where possible; but if GNU extensions make your program more maintainable, powerful, or otherwise better, don’t hesitate to use them. In any case, don’t make your own declaration of system functions; that’s a recipe for conflict.

Despite the standards, nearly every library function has some sort of portability issue on some system or another. Here are some examples:

open Names with trailing / ’s are mishandled on many platforms. printf long double may be unimplemented; floating values Infinity and NaN are often mishandled; output for large precisions may be incorrect. readlink May return int instead of ssize_t . scanf On Windows, errno is not set on failure.

Gnulib is a big help in this regard. Gnulib provides implementations of standard interfaces on many of the systems that lack them, including portable implementations of enhanced GNU interfaces, thereby making their use portable, and of POSIX-1.2008 interfaces, some of which are missing even on up-to-date GNU systems.

Gnulib also provides many useful non-standard interfaces; for example, C implementations of standard data structures (hash tables, binary trees), error-checking type-safe wrappers for memory allocation functions ( xmalloc , xrealloc ), and output of error messages.

Gnulib integrates with GNU Autoconf and Automake to remove much of the burden of writing portable code from the programmer: Gnulib makes your configure script automatically determine what features are missing and use the Gnulib code to supply the missing pieces.

The Gnulib and Autoconf manuals have extensive sections on portability: Introduction in Gnulib and see Portable C and C++ in Autoconf . Please consult them for many more details.

5.8 Internationalization

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

Using GNU gettext involves putting a call to the gettext macro around each string that might need translation—like this:

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

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.

", 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.

", : "# Implicit rule search has not been done.

");

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);

5.9 Character Set

Sticking to the ASCII character set (plain text, 7-bit characters) is preferred in GNU source code comments, text documents, and other contexts, unless there is good reason to do something else because of the application domain. For example, if source code deals with the French Revolutionary calendar, it is OK if its literal strings contain accented characters in month names like “Floréal”. Also, it is OK (but not required) 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, certainly within a single file. UTF-8 is likely to be the best choice.

5.10 Quote Characters

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

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

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

Here, a French translation might cause gettext to return the string "Traitement de fichier ‹ %s ›..." , yielding quotes more appropriate for a French locale.

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

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

5.11 Mmap

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

The proper way to use 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.

6 Documenting Programs

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

6.1 GNU Manuals

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

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

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

Remember that the audience for a GNU manual (and other GNU documentation) is global, and that it will be used for years, maybe decades. This means that the reader could have very different cultural reference points. Decades from now, all but old folks will have very different cultural reference points; many things that "everyone knows about" today may be mostly forgotten.

For this reason, try to avoid writing in a way that depends on cultural reference points for proper understanding, or that refers to them in ways that would impede reading for someone that doesn’t recognize them.

Likewise, be conservative in your choice of words (aside from technical terms), linguistic constructs, and spelling: aim to make them intelligible to readers from ten years ago. In any contest for trendiness, GNU writing should not even qualify to enter.

It is ok to refer once in a rare while to spatially or temporally localized reference points or facts, if it is directly pertinent or as an aside. Changing these few things (which in any case stand out) when they no longer make sense will not be a lot of work.

By contrast, it is always proper to refer to concepts of GNU and the free software movement, when they are pertinent. These are a central part of our message, so we should take advantage of opportunities to mention them. They are fundamental moral positions, so they will rarely if ever change.

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 lists all the functions, variables, options, and important concepts that are part of the program. One combined Index should do for a short manual, but sometimes for a complex package it is better to use multiple indices. The Texinfo manual includes advice on preparing good index entries, see Making Index Entries in GNU Texinfo , and see Defining the Entries of an Index in 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.

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

It is proper to use the future tense when grammar demands it, as in, “If you type x , the computer will self-destruct in 10 seconds.”

6.2 Doc Strings and Manuals

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

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

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

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

6.3 Manual Structure Details

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

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

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

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

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

6.4 License for Manuals

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

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

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

6.5 Manual Credits

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

6.6 Printed Manuals

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

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

6.7 The NEWS File

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

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

6.8 Change Logs

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

Therefore, change logs should be detailed enough and accurate enough to provide the information commonly required for such software forensics. Specifically, change logs should make finding answers to the following questions easy:

What changes affected a particular source file?

Was a particular source file renamed or moved, and if so, as part of what change?

What changes affected a given function or macro or definition of a data structure?

Was a function (or a macro or the definition of a data structure) renamed or moved from anot