The GNU coding standards, last updated July 22, 2007.
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Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License”.
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.
This release of the GNU Coding Standards was last updated July 22, 2007.
If you did not obtain this file directly from the GNU project and recently, please check for a newer version. You can get the GNU Coding Standards from the GNU web server in many different formats, including the Texinfo source, PDF, HTML, DVI, plain text, and more, at: http://www.gnu.org/prep/standards/.
Corrections or suggestions for this document should be sent to bug-standards@gnu.org. If you make a suggestion, please include a suggested new wording for it; our time is limited. We prefer a context diff to the standards.texi or make-stds.texi files, but if you don't have those files, please mail your suggestion anyway.
These standards cover the minimum of what is important when writing a GNU package. Likely, the need for additional standards will come up. Sometimes, you might suggest that such standards be added to this document. If you think your standards would be generally useful, please do suggest them.
You should also set standards for your package on many questions not addressed or not firmly specified here. The most important point is to be self-consistent—try to stick to the conventions you pick, and try to document them as much as possible. That way, your program will be more maintainable by others.
The GNU Hello program serves as an example of how to follow the GNU coding standards for a trivial program. http://www.gnu.org/software/hello/hello.html.
This chapter discusses how you can make sure that GNU software avoids legal difficulties, and other related issues.
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.
If the program you are working on is copyrighted by the Free Software Foundation, then when someone else sends you a piece of code to add to the program, we need legal papers to use it—just as we asked you to sign papers initially. Each person who makes a nontrivial contribution to a program must sign some sort of legal papers in order for us to have clear title to the program; the main author alone is not enough.
So, before adding in any contributions from other people, please tell us, so we can arrange to get the papers. Then wait until we tell you that we have received the signed papers, before you actually use the contribution.
This applies both before you release the program and afterward. If you receive diffs to fix a bug, and they make significant changes, we need legal papers for that change.
This also applies to comments and documentation files. For copyright law, comments and code are just text. Copyright applies to all kinds of text, so we need legal papers for all kinds.
We know it is frustrating to ask for legal papers; it's frustrating for us as well. But if you don't wait, you are going out on a limb—for example, what if the contributor's employer won't sign a disclaimer? You might have to take that code out again!
You don't need papers for changes of a few lines here or there, since they are not significant for copyright purposes. Also, you don't need papers if all you get from the suggestion is some ideas, not actual code which you use. For example, if someone sent you one implementation, but you write a different implementation of the same idea, you don't need to get papers.
The very worst thing is if you forget to tell us about the other contributor. We could be very embarrassed in court some day as a result.
We have more detailed advice for maintainers of programs; if you have reached the stage of actually maintaining a program for GNU (whether released or not), please ask us for a copy. It is also available online for your perusal: http://www.gnu.org/prep/maintain/.
Please do not include any trademark acknowledgements in GNU software packages or documentation.
Trademark acknowledgements are the statements that such-and-such is a trademark of so-and-so. The GNU Project has no objection to the basic idea of trademarks, but these acknowledgements feel like kowtowing, and there is no legal requirement for them, so we don't use them.
What is legally required, as regards other people's trademarks, is to avoid using them in ways which a reader might reasonably understand as naming or labeling our own programs or activities. For example, since “Objective C” is (or at least was) a trademark, we made sure to say that we provide a “compiler for the Objective C language” rather than an “Objective C compiler”. The latter would have been meant as a shorter way of saying the former, but it does not explicitly state the relationship, so it could be misinterpreted as using “Objective C” as a label for the compiler rather than for the language.
Please don't use “win” as an abbreviation for Microsoft Windows in GNU software or documentation. In hacker terminology, calling something a “win” is a form of praise. If you wish to praise Microsoft Windows when speaking on your own, by all means do so, but not in GNU software. Usually we write the name “Windows” in full, but when brevity is very important (as in file names and sometimes symbol names), we abbreviate it to “w”. For instance, the files and functions in Emacs that deal with Windows start with ‘w32’.
This chapter discusses some of the issues you should take into account when designing your program.
When you want to use a language that gets compiled and runs at high speed, the best language to use is C. Using another language is like using a non-standard feature: it will cause trouble for users. Even if GCC supports the other language, users may find it inconvenient to have to install the compiler for that other language in order to build your program. For example, if you write your program in C++, people will have to install the GNU C++ compiler in order to compile your program.
C has one other advantage over C++ and other compiled languages: more people know C, so more people will find it easy to read and modify the program if it is written in C.
So in general it is much better to use C, rather than the comparable alternatives.
But there are two exceptions to that conclusion:
Many programs are designed to be extensible: they include an interpreter for a language that is higher level than C. Often much of the program is written in that language, too. The Emacs editor pioneered this technique.
The standard extensibility interpreter for GNU software is GUILE (http://www.gnu.org/software/guile/), which implements the language Scheme (an especially clean and simple dialect of Lisp). We don't reject programs written in other “scripting languages” such as Perl and Python, but using GUILE is very important for the overall consistency of the GNU system.
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.
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.
1989 Standard C is widespread enough now that it is ok to use its features in new programs. There is one exception: do not ever use the “trigraph” feature of Standard C.
1999 Standard C is not widespread yet, so please do not require its features in programs. It is ok to use its features if they are present.
However, it is easy to support pre-standard compilers in most programs, so if you know how to do that, feel free. If a program you are maintaining has such support, you should try to keep it working.
To support pre-standard C, instead of writing function definitions in standard prototype form,
int
foo (int x, int y)
...
write the definition in pre-standard style like this,
int
foo (x, y)
int x, y;
...
and use a separate declaration to specify the argument prototype:
int foo (int, int);
You need such a declaration anyway, in a header file, to get the benefit of prototypes in all the files where the function is called. And once you have the declaration, you normally lose nothing by writing the function definition in the pre-standard style.
This technique does not work for integer types narrower than int.
If you think of an argument as being of a type narrower than int,
declare it as int instead.
There are a few special cases where this technique is hard to use. For
example, if a function argument needs to hold the system type
dev_t, you run into trouble, because dev_t is shorter than
int on some machines; but you cannot use int instead,
because dev_t is wider than int on some machines. There
is no type you can safely use on all machines in a non-standard
definition. The only way to support non-standard C and pass such an
argument is to check the width of dev_t using Autoconf and choose
the argument type accordingly. This may not be worth the trouble.
In order to support pre-standard compilers that do not recognize prototypes, you may want to use a preprocessor macro like this:
/* Declare the prototype for a general external function. */
#if defined (__STDC__) || defined (WINDOWSNT)
#define P_(proto) proto
#else
#define P_(proto) ()
#endif
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
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.
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.”
Avoid arbitrary limits on the length or number of any data structure, including file names, lines, files, and symbols, by allocating all data structures dynamically. In most Unix utilities, “long lines are silently truncated”. This is not acceptable in a GNU utility.
Utilities reading files should not drop NUL characters, or any other nonprinting characters including those with codes above 0177. The only sensible exceptions would be utilities specifically intended for interface to certain types of terminals or printers that can't handle those characters. Whenever possible, try to make programs work properly with sequences of bytes that represent multibyte characters, using encodings such as UTF-8 and others.
Check every system call for an error return, unless you know you wish to
ignore errors. Include the system error text (from perror or
equivalent) in every error message resulting from a failing
system call, as well as the name of the file if any and the name of the
utility. Just “cannot open foo.c” or “stat failed” is not
sufficient.
Check every call to malloc or realloc to see if it
returned zero. Check realloc even if you are making the block
smaller; in a system that rounds block sizes to a power of 2,
realloc may get a different block if you ask for less space.
In Unix, realloc can destroy the storage block if it returns
zero. GNU realloc does not have this bug: if it fails, the
original block is unchanged. Feel free to assume the bug is fixed. If
you wish to run your program on Unix, and wish to avoid lossage in this
case, you can use the GNU malloc.
You must expect free to alter the contents of the block that was
freed. Anything you want to fetch from the block, you must fetch before
calling free.
If malloc fails in a noninteractive program, make that a fatal
error. In an interactive program (one that reads commands from the
user), it is better to abort the command and return to the command
reader loop. This allows the user to kill other processes to free up
virtual memory, and then try the command again.
Use getopt_long to decode arguments, unless the argument syntax
makes this unreasonable.
When static storage is to be written in during program execution, use explicit C code to initialize it. Reserve C initialized declarations for data that will not be changed.
Try to avoid low-level interfaces to obscure Unix data structures (such
as file directories, utmp, or the layout of kernel memory), since these
are less likely to work compatibly. If you need to find all the files
in a directory, use readdir or some other high-level interface.
These are supported compatibly by GNU.
The preferred signal handling facilities are the BSD variant of
signal, and the posix sigaction function; the
alternative USG signal interface is an inferior design.
Nowadays, using the posix signal functions may be the easiest way
to make a program portable. If you use signal, then on GNU/Linux
systems running GNU libc version 1, you should include
bsd/signal.h instead of signal.h, so as to get BSD
behavior. It is up to you whether to support systems where
signal has only the USG behavior, or give up on them.
In error checks that detect “impossible” conditions, just abort. There is usually no point in printing any message. These checks indicate the existence of bugs. Whoever wants to fix the bugs will have to read the source code and run a debugger. So explain the problem with comments in the source. The relevant data will be in variables, which are easy to examine with the debugger, so there is no point moving them elsewhere.
Do not use a count of errors as the exit status for a program. That does not work, because exit status values are limited to 8 bits (0 through 255). A single run of the program might have 256 errors; if you try to return 256 as the exit status, the parent process will see 0 as the status, and it will appear that the program succeeded.
If you make temporary files, check the TMPDIR environment
variable; if that variable is defined, use the specified directory
instead of /tmp.
In addition, be aware that there is a possible security problem when creating temporary files in world-writable directories. In C, you can avoid this problem by creating temporary files in this manner:
fd = open(filename, O_WRONLY | O_CREAT | O_EXCL, 0600);
or by using the mkstemps function from libiberty.
In bash, use set -C to avoid this problem.
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.
Error messages from compilers should look like this:
source-file-name:lineno: message
If you want to mention the column number, use one of these formats:
source-file-name:lineno:column: message
source-file-name:lineno.column: message
Line numbers should start from 1 at the beginning of the file, and column numbers should start from 1 at the beginning of the line. (Both of these conventions are chosen for compatibility.) Calculate column numbers assuming that space and all ASCII printing characters have equal width, and assuming tab stops every 8 columns.
The error message can also give both the starting and ending positions of the erroneous text. There are several formats so that you can avoid redundant information such as a duplicate line number. Here are the possible formats:
source-file-name:lineno-1.column-1-lineno-2.column-2: message
source-file-name:lineno-1.column-1-column-2: message
source-file-name:lineno-1-lineno-2: message
When an error is spread over several files, you can use this format:
file-1:lineno-1.column-1-file-2:lineno-2.column-2: message
Error messages from other noninteractive programs should look like this:
program:source-file-name:lineno: message
when there is an appropriate source file, or like this:
program: message
when there is no relevant source file.
If you want to mention the column number, use this format:
program:source-file-name:lineno:column: message
In an interactive program (one that is reading commands from a terminal), it is better not to include the program name in an error message. The place to indicate which program is running is in the prompt or with the screen layout. (When the same program runs with input from a source other than a terminal, it is not interactive and would do best to print error messages using the noninteractive style.)
The string message should not begin with a capital letter when it follows a program name and/or file name, because that isn't the beginning of a sentence. (The sentence conceptually starts at the beginning of the line.) Also, it should not end with a period.
Error messages from interactive programs, and other messages such as usage messages, should start with a capital letter. But they should not end with a period.
Please don't make the behavior of a utility depend on the name used to invoke it. It is useful sometimes to make a link to a utility with a different name, and that should not change what it does.
Instead, use a run time option or a compilation switch or both to select among the alternate behaviors.
Likewise, please don't make the behavior of the program depend on the type of output device it is used with. Device independence is an important principle of the system's design; do not compromise it merely to save someone from typing an option now and then. (Variation in error message syntax when using a terminal is ok, because that is a side issue that people do not depend on.)
If you think one behavior is most useful when the output is to a terminal, and another is most useful when the output is a file or a pipe, then it is usually best to make the default behavior the one that is useful with output to a terminal, and have an option for the other behavior.
Compatibility requires certain programs to depend on the type of output
device. It would be disastrous if ls or sh did not do so
in the way all users expect. In some of these cases, we supplement the
program with a preferred alternate version that does not depend on the
output device type. For example, we provide a dir program much
like ls except that its default output format is always
multi-column format.
When you write a program that provides a graphical user interface, please make it work with X Windows and the GTK+ toolkit unless the functionality specifically requires some alternative (for example, “displaying jpeg images while in console mode”).
In addition, please provide a command-line interface to control the functionality. (In many cases, the graphical user interface can be a separate program which invokes the command-line program.) This is so that the same jobs can be done from scripts.
Please also consider providing a CORBA interface (for use from GNOME), 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.
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.
The standard --version option should direct the program to
print information about its name, version, origin and legal status,
all on standard output, and then exit successfully. Other options and
arguments should be ignored once this is seen, and the program should
not perform its normal function.
The first line is meant to be easy for a program to parse; the version number proper starts after the last space. In addition, it contains the canonical name for this program, in this format:
GNU Emacs 19.30
The program's name should be a constant string; don't compute it
from argv[0]. The idea is to state the standard or canonical
name for the program, not its file name. There are other ways to find
out the precise file name where a command is found in PATH.
If the program is a subsidiary part of a larger package, mention the package name in parentheses, like this:
emacsserver (GNU Emacs) 19.30
If the package has a version number which is different from this program's version number, you can mention the package version number just before the close-parenthesis.
If you need to mention the version numbers of libraries which are distributed separately from the package which contains this program, you can do so by printing an additional line of version info for each library you want to mention. Use the same format for these lines as for the first line.
Please do not mention all of the libraries that the program uses “just for completeness”—that would produce a lot of unhelpful clutter. Please mention library version numbers only if you find in practice that they are very important to you in debugging.
The following line, after the version number line or lines, should be a copyright notice. If more than one copyright notice is called for, put each on a separate line.
Next should follow a line stating the license, preferably using one of abbrevations below, and a brief statement that the program is free software, and that users are free to copy and change it. Also mention that there is no warranty, to the extent permitted by law. See recommended wording below.
It is ok to finish the output with a list of the major authors of the program, as a way of giving credit.
Here's an example of output that follows these rules:
GNU hello 2.3
Copyright (C) 2007 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
You should adapt this to your program, of course, filling in the proper year, copyright holder, name of program, and the references to distribution terms, and changing the rest of the wording as necessary.
This copyright notice only needs to mention the most recent year in which changes were made—there's no need to list the years for previous versions' changes. You don't have to mention the name of the program in these notices, if that is inconvenient, since it appeared in the first line. (The rules are different for copyright notices in source files; see Copyright Notices.)
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 ‘vversion[+]’, meaning that particular version, or later versions with the ‘+’, as shown above.
In the case of exceptions for extra permissions with the GPL, we use ‘/’ for a separator; the version number can follow the license abbreviation as usual, as in the examples below.
GNU GPL with the exception for Ada.
More information about these licenses and many more are on the GNU licensing web pages, http://www.gnu.org/licenses/license-list.html.
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 there should be a line that says where to mail bug reports. It should have this format:
Report bugs to mailing-address.
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.
tar.
du, ls, nm, stty, uname,
and unexpand.
diff.
ls.
etags, tee, time;
‘-r’ in tar.
cp.
shar.
m4.
diff.
gawk.
recode.
wdiff.
ptx.
wdiff.
ctags.
shar.
tac.
cpio and diff.
shar.
cpio and tar.
head and tail.
ptx.
head, split, and tail.
etags.
tar.
chgrp and chown.
ls.
recode.
su;
‘-x’ in GDB.
tar.
gawk.
tar and shar.
tar.
tar.
diff.
gawk.
ptx, recode, and wdiff;
‘-W copyright’ in gawk.
who.
du.
tar and cpio.
shar.
ctags.
touch.
m4;
‘-t’ in Bison.
m4.
ctags.
tar.
chgrp, chown, cpio, du,
ls, and tar.
du.
recode.
look.
tar.
csplit.
ls, it
means to show directories themselves rather than their contents. In
rm and ln, it means to not treat links to directories
specially.
strip.
strip.
diff.
csplit.
wdiff.
wdiff.
diff.
xargs.
makeinfo.
m4.
ls.
tar.
xargs.
unshar.
diff.
sed.
nm.
cpio;
‘-x’ in tar.
finger.
su.
m4.
info, gawk, Make, mt, and tar;
‘-n’ in sed;
‘-r’ in touch.
gawk.
ls.
tar.
makeinfo.
ptx.
tail.
makeinfo.
cp, ln, mv, and rm.
shar.
ls, time, and ptx.
m4.
ptx.
tar.
ul.
recode.
install.
tar and shar.
m4.
objdump and recode
who.
shar.
ls.
makeinfo, output HTML.
who.
diff.
ls;
‘-x’ in recode.
diff.
ls.
diff.
look and ptx;
‘-i’ in diff and wdiff.
ptx.
etags.
tee.
diff.
diff.
tar.
etags;
‘-I’ in m4.
tar.
expand.
diff.
ls.
cp, ln, mv, rm;
‘-e’ in m4;
‘-p’ in xargs;
‘-w’ in tar.
shar.
date
csplit.
du and ls.
etags.
wdiff.
shar.
split.
split, head, and tail.
cpio.
gawk.
cpio;
‘-l’ in recode.
tar.
ls.
su.
uname.
ptx.
hello and uname.
cpio.
xargs.
xargs.
xargs.
xargs.
who.
who.
diff.
shar.
install, mkdir, and mkfifo.
tar.
tar.
m4.
shar.
shar.
shar.
wdiff.
touch.
etags.
wdiff.
cp.
wdiff.
shar.
gprof.
etags.
nm.
makeinfo.
gprof.
gprof.
shar.
makeinfo.
emacsclient.
info.
uname.
cpio.
objdump.
xargs.
cat.
cat.
nm.
cpio and ls.
tar.
tar, cp, and du.
ptx.
gprof.
gprof.
getopt, fdlist, fdmount,
fdmountd, and fdumount.
shar.
rm.
unshar.
install.
diff.
makeinfo.
mkdir and rmdir.
ul.
cpio.
finger.
cpio and tar.
gawk.
m4.
csplit.
tar and cp.
su.
cpio.
tar.
tar.
diff.
cmp.
nm.
nm.
wdiff.
ed.
shar.
shar
ls.
diff.
gawk.
tar.
tar.
chgrp, chown, cp, ls, diff,
and rm.
makeinfo.
ptx.
tac and etags.
uname.
m4.
objdump.
cpio.
xargs.
diff.
cpio.
ls and nm.
diff.
ptx.
tar.
tar.
stty.
ptx.
du.
tac.
recode to chose files or pipes for sequencing passes.
su.
cat.
diff.
cat.
diff.
cat.
ls.
ls.
gawk.
tar.
diff.
unshar.
shar.
cat.
wdiff.
wdiff.
tar and diff to specify which file within
a directory to start processing with.
wdiff.
shar.
recode.
install.
strip.
strip.
shar.
cp, ln, mv.
csplit.
gprof.
du.
ln.
objdump.
m4.
uname.
expand and unexpand.
ls.
tput and ul.
‘-t’ in wdiff.
diff.
shar.
ls and touch.
tar.
du.
ranlib, and recode.
m4.
hello;
‘-W traditional’ in gawk;
‘-G’ in ed, m4, and ptx.
ctags.
ctags.
ptx.
tar.
cpio.
m4.
nm.
cp, ctags, mv, tar.
gawk; same as ‘--help’.
shar.
shar.
tar.
cp, ln, mv.
ctags.
tar.
shar.
ls and ptx.
ptx.
who.
gprof.
If a program typically uses just a few meg of memory, don't bother making any effort to reduce memory usage. For example, if it is impractical for other reasons to operate on files more than a few meg long, it is reasonable to read entire input files into memory to operate on them.
However, for programs such as cat or tail, that can
usefully operate on very large files, it is important to avoid using a
technique that would artificially limit the size of files it can handle.
If a program works by lines and could be applied to arbitrary
user-supplied input files, it should keep only a line in memory, because
this is not very hard and users will want to be able to operate on input
files that are bigger than will fit in memory all at once.
If your program creates complicated data structures, just make them in
memory and give a fatal error if malloc returns zero.
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.
This chapter provides advice on how best to use the C language when writing GNU software.
It is important to put the open-brace that starts the body of a C function in column one, so that they will start a defun. Several tools look for open-braces in column one to find the beginnings of C functions. These tools will not work on code not formatted that way.
Avoid putting open-brace, open-parenthesis or open-bracket in column
one when they are inside a function, so that they won't start a defun.
The open-brace that starts a struct body can go in column one
if you find it useful to treat that definition as a defun.
It is also important for function definitions to start the name of the function in column one. This helps people to search for function definitions, and may also help certain tools recognize them. Thus, using Standard C syntax, the format is this:
static char *
concat (char *s1, char *s2)
{
...
}
or, if you want to use traditional C syntax, format the definition like this:
static char *
concat (s1, s2) /* Name starts in column one here */
char *s1, *s2;
{ /* Open brace in column one here */
...
}
In Standard C, if the arguments don't fit nicely on one line, split it like this:
int
lots_of_args (int an_integer, long a_long, short a_short,
double a_double, float a_float)
...
The rest of this section gives our recommendations for other aspects of
C formatting style, which is also the default style of the indent
program in version 1.2 and newer. It corresponds to the options
-nbad -bap -nbc -bbo -bl -bli2 -bls -ncdb -nce -cp1 -cs -di2
-ndj -nfc1 -nfca -hnl -i2 -ip5 -lp -pcs -psl -nsc -nsob
We don't think of these recommendations as requirements, because it causes no problems for users if two different programs have different formatting styles.
But whatever style you use, please use it consistently, since a mixture of styles within one program tends to look ugly. If you are contributing changes to an existing program, please follow the style of that program.
For the body of the function, our recommended style looks like this:
if (x < foo (y, z))
haha = bar[4] + 5;
else
{
while (z)
{
haha += foo (z, z);
z--;
}
return ++x + bar ();
}
We find it easier to read a program when it has spaces before the open-parentheses and after the commas. Especially after the commas.
When you split an expression into multiple lines, split it before an operator, not after one. Here is the right way:
if (foo_this_is_long && bar > win (x, y, z)
&& remaining_condition)
Try to avoid having two operators of different precedence at the same level of indentation. For example, don't write this:
mode = (inmode[j] == VOIDmode
|| GET_MODE_SIZE (outmode[j]) > GET_MODE_SIZE (inmode[j])
? outmode[j] : inmode[j]);
Instead, use extra parentheses so that the indentation shows the nesting:
mode = ((inmode[j] == VOIDmode
|| (GET_MODE_SIZE (outmode[j]) > GET_MODE_SIZE (inmode[j])))
? outmode[j] : inmode[j]);
Insert extra parentheses so that Emacs will indent the code properly. For example, the following indentation looks nice if you do it by hand,
v = rup->ru_utime.tv_sec*1000 + rup->ru_utime.tv_usec/1000
+ rup->ru_stime.tv_sec*1000 + rup->ru_stime.tv_usec/1000;
but Emacs would alter it. Adding a set of parentheses produces something that looks equally nice, and which Emacs will preserve:
v = (rup->ru_utime.tv_sec*1000 + rup->ru_utime.tv_usec/1000
+ rup->ru_stime.tv_sec*1000 + rup->ru_stime.tv_usec/1000);
Format do-while statements like this:
do
{
a = foo (a);
}
while (a > 0);
Please use formfeed characters (control-L) to divide the program into pages at logical places (but not within a function). It does not matter just how long the pages are, since they do not have to fit on a printed page. The formfeeds should appear alone on lines by themselves.
Every program should start with a comment saying briefly what it is for. Example: ‘fmt - filter for simple filling of text’. This comment should be at the top of the source file containing the ‘main’ function of the program.
Also, please write a brief comment at the start of each source file, with the file name and a line or two about the overall purpose of the file.
Please write the comments in a GNU program in English, because English is the one language that nearly all programmers in all countries can read. If you do not write English well, please write comments in English as well as you can, then ask other people to help rewrite them. If you can't write comments in English, please find someone to work with you and translate your comments into English.
Please put a comment on each function saying what the function does,
what sorts of arguments it gets, and what the possible values of
arguments mean and are used for. It is not necessary to duplicate in
words the meaning of the C argument declarations, if a C type is being
used in its customary fashion. If there is anything nonstandard about
its use (such as an argument of type char * which is really the
address of the second character of a string, not the first), or any
possible values that would not work the way one would expect (such as,
that strings containing newlines are not guaranteed to work), be sure
to say so.
Also explain the significance of the return value, if there is one.
Please put two spaces after the end of a sentence in your comments, so that the Emacs sentence commands will work. Also, please write complete sentences and capitalize the first word. If a lower-case identifier comes at the beginning of a sentence, don't capitalize it! Changing the spelling makes it a different identifier. If you don't like starting a sentence with a lower case letter, write the sentence differently (e.g., “The identifier lower-case is ...”).
The comment on a function is much clearer if you use the argument names to speak about the argument values. The variable name itself should be lower case, but write it in upper case when you are speaking about the value rather than the variable itself. Thus, “the inode number NODE_NUM” rather than “an inode”.
There is usually no purpose in restating the name of the function in the comment before it, because the reader can see that for himself. There might be an exception when the comment is so long that the function itself would be off the bottom of the screen.
There should be a comment on each static variable as well, like this:
/* Nonzero means truncate lines in the display;
zero means continue them. */
int truncate_lines;
Every ‘#endif’ should have a comment, except in the case of short conditionals (just a few lines) that are not nested. The comment should state the condition of the conditional that is ending, including its sense. ‘#else’ should have a comment describing the condition and sense of the code that follows. For example:
#ifdef foo
...
#else /* not foo */
...
#endif /* not foo */
#ifdef foo
...
#endif /* foo */
but, by contrast, write the comments this way for a ‘#ifndef’:
#ifndef foo
...
#else /* foo */
...
#endif /* foo */
#ifndef foo
...
#endif /* not foo */
Please explicitly declare the types of all objects. For example, you
should explicitly declare all arguments to functions, and you should
declare functions to return int rather than omitting the
int.
Some programmers like to use the GCC ‘-Wall’ option, and change the code whenever it issues a warning. If you want to do this, then do. Other programmers prefer not to use ‘-Wall’, because it gives warnings for valid and legitimate code which they do not want to change. If you want to do this, then do. The compiler should be your servant, not your master.
Declarations of external functions and functions to appear later in the
source file should all go in one place near the beginning of the file
(somewhere before the first function definition in the file), or else
should go in a header file. Don't put extern declarations inside
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.
Don't declare multiple variables in one declaration that spans lines. Start a new declaration on each line, instead. For example, instead of this:
int foo,
bar;
write either this:
int foo, bar;
or this:
int foo;
int bar;
(If they are global variables, each should have a comment preceding it anyway.)
When you have an if-else statement nested in another
if statement, always put braces around the if-else.
Thus, never write like this:
if (foo)
if (bar)
win ();
else
lose ();
always like this:
if (foo)
{
if (bar)
win ();
else
lose ();
}
If you have an if statement nested inside of an else
statement, either write else if on one line, like this,
if (foo)
...
else if (bar)
...
with its then-part indented like the preceding then-part,
or write the nested if within braces like this:
if (foo)
...
else
{
if (bar)
...
}
Don't declare both a structure tag and variables or typedefs in the same declaration. Instead, declare the structure tag separately and then use it to declare the variables or typedefs.
Try to avoid assignments inside if-conditions (assignments
inside while-conditions are ok). For example, don't write
this:
if ((foo = (char *) malloc (sizeof *foo)) == 0)
fatal ("virtual memory exhausted");
instead, write this:
foo = (char *) malloc (sizeof *foo);
if (foo == 0)
fatal ("virtual memory exhausted");
Don't make the program ugly to placate lint. Please don't insert any
casts to void. Zero without a cast is perfectly fine as a null
pointer constant, except when calling a varargs function.
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.
In the Unix world, “portability” refers to porting to different Unix versions. For a GNU program, this kind of portability is desirable, but not paramount.
The primary purpose of GNU software is to run on top of the GNU kernel, compiled with the GNU C compiler, on various types of cpu. So the kinds of portability that are absolutely necessary are quite limited. But it is important to support Linux-based GNU systems, since they are the form of GNU that is popular.
Beyond that, it is good to support the other free operating systems (*BSD), and it is nice to support other Unix-like systems if you want to. Supporting a variety of Unix-like systems is desirable, although not paramount. It is usually not too hard, so you may as well do it. But you don't have to consider it an obligation, if it does turn out to be hard.
The easiest way to achieve portability to most Unix-like systems is to use Autoconf. It's unlikely that your program needs to know more information about the host platform than Autoconf can provide, simply because most of the programs that need such knowledge have already been written.
Avoid using the format of semi-internal data bases (e.g., directories)
when there is a higher-level alternative (readdir).
As for systems that are not like Unix, such as MSDOS, Windows, VMS, MVS, and older Macintosh systems, supporting them is often a lot of work. When that is the case, it is better to spend your time adding features that will be useful on GNU and GNU/Linux, rather than on supporting other incompatible systems.
If you do support Windows, please do not abbreviate it as “win”. In
hacker terminology, calling something a “win” is a form of praise.
You're free to praise Microsoft Windows on your own if you want, but
please don't do this in GNU packages. Instead of abbreviating
“Windows” to “un”, you can write it in full or abbreviate it to
“woe” or “w”. In GNU Emacs, for instance, we use ‘w32’ in
file names of Windows-specific files, but the macro for Windows
conditionals is called WINDOWSNT.
It is a good idea to define the “feature test macro”
_GNU_SOURCE when compiling your C files. When you compile on GNU
or GNU/Linux, this will enable the declarations of GNU library extension
functions, and that will usually give you a compiler error message if
you define the same function names in some other way in your program.
(You don't have to actually use these functions, if you prefer
to make the program more portable to other systems.)
But whether or not you use these GNU extensions, you should avoid using their names for any other meanings. Doing so would make it hard to move your code into other GNU programs.
Even GNU systems will differ because of differences among cpu
types—for example, difference in byte ordering and alignment
requirements. It is absolutely essential to handle these differences.
However, don't make any effort to cater to the possibility that an
int will be less than 32 bits. We don't support 16-bit machines
in GNU.
Similarly, don't make any effort to cater to the possibility that
long will be smaller than predefined types like size_t.
For example, the following code is ok:
printf ("size = %lu\n", (unsigned long) sizeof array);
printf ("diff = %ld\n", (long) (pointer2 - pointer1));
1989 Standard C requires this to work, and we know of only one counterexample: 64-bit programs on Microsoft Windows. We will leave it to those who want to port GNU programs to that environment to figure out how to do it.
Predefined file-size types like off_t are an exception: they are
longer than long on many platforms, so code like the above won't
work with them. One way to print an off_t value portably is to
print its digits yourself, one by one.
Don't assume that the address of an int object is also the
address of its least-significant byte. This is false on big-endian
machines. Thus, don't make the following mistake:
int c;
...
while ((c = getchar ()) != EOF)
write (file_descriptor, &c, 1);
Instead, use unsigned char as follows. (The unsigned
is for portability to unusual systems where char is signed and
where there is integer overflow checking.)
int c;
while ((c = getchar ()) != EOF)
{
unsigned char u = c;
write (file_descriptor, &u, 1);
}
It used to be ok to not worry about the difference between pointers
and integers when passing arguments to functions. However, on most
modern 64-bit machines pointers are wider than int.
Conversely, integer types like long long int and off_t
are wider than pointers on most modern 32-bit machines. Hence it's
often better nowadays to use prototypes to define functions whose
argument types are not trivial.
In particular, if functions accept varying argument counts or types they should be declared using prototypes containing ‘...’ and defined using stdarg.h. For an example of this, please see the Gnulib error module, which declares and defines the following function:
/* Print a message with `fprintf (stderr, FORMAT, ...)';
if ERRNUM is nonzero, follow it with ": " and strerror (ERRNUM).
If STATUS is nonzero, terminate the program with `exit (STATUS)'. */
void error (int status, int errnum, const char *format, ...);
A simple way to use the Gnulib error module is to obtain the two source files error.c and error.h from the Gnulib library source code repository at http://savannah.gnu.org/cgi-bin/viewcvs/gnulib/gnulib/lib/. Here's a sample use:
#include "error.h"
#include <errno.h>
#include <stdio.h>
char *program_name = "myprogram";
FILE *
xfopen (char const *name)
{
FILE *fp = fopen (name, "r");
if (! fp)
error (1, errno, "cannot read %s", name);
return fp;
}
Avoid casting pointers to integers if you can. Such casts greatly
reduce portability, and in most programs they are easy to avoid. In the
cases where casting pointers to integers is essential—such as, a Lisp
interpreter which stores type information as well as an address in one
word—you'll have to make explicit provisions to handle different word
sizes. You will also need to make provision for systems in which the
normal range of addresses you can get from malloc starts far away
from zero.
C implementations differ substantially. Standard C reduces but does not eliminate the incompatibilities; meanwhile, many GNU packages still support pre-standard compilers because this is not hard to do. This chapter gives recommendations for how to use the more-or-less standard C library functions to avoid unnecessary loss of portability.
sprintf. It returns the number of
characters written on some systems, but not on all systems.
vfprintf is not always available.
main should be declared to return type int. It should
terminate either by calling exit or by returning the integer
status code; make sure it cannot ever return an undefined value.
Almost any declaration for a system function is wrong on some system. To minimize conflicts, leave it to the system header files to declare system functions. If the headers don't declare a function, let it remain undeclared.
While it may seem unclean to use a function without declaring it, in practice this works fine for most system library functions on the systems where this really happens; thus, the disadvantage is only theoretical. By contrast, actual declarations have frequently caused actual conflicts.
malloc or
realloc.
Most GNU programs use those functions just once, in functions
conventionally named xmalloc and xrealloc. These
functions call malloc and realloc, respectively, and
check the results.
Because xmalloc and xrealloc are defined in your program,
you can declare them in other files without any risk of type conflict.
On most systems, int is the same length as a pointer; thus, the
calls to malloc and realloc work fine. For the few
exceptional systems (mostly 64-bit machines), you can use
conditionalized declarations of malloc and
realloc—or put these declarations in configuration files
specific to those systems.
That causes less of a problem than you might think. The newer standard string functions should be avoided anyway because many systems still don't support them. The string functions you can use are these:
strcpy strncpy strcat strncat
strlen strcmp strncmp
strchr strrchr
The copy and concatenate functions work fine without a declaration as
long as you don't use their values. Using their values without a
declaration fails on systems where the width of a pointer differs from
the width of int, and perhaps in other cases. It is trivial to
avoid using their values, so do that.
The compare functions and strlen work fine without a declaration
on most systems, possibly all the ones that GNU software runs on.
You may find it necessary to declare them conditionally on a
few systems.
The search functions must be declared to return char *. Luckily,
there is no variation in the data type they return. But there is
variation in their names. Some systems give these functions the names
index and rindex; other systems use the names
strchr and strrchr. Some systems support both pairs of
names, but neither pair works on all systems.
You should pick a single pair of names and use it throughout your
program. (Nowadays, it is better to choose strchr and
strrchr for new programs, since those are the standard
names.) Declare both of those names as functions returning char
*. On systems which don't support those names, define them as macros
in terms of the other pair. For example, here is what to put at the
beginning of your file (or in a header) if you want to use the names
strchr and strrchr throughout:
#ifndef HAVE_STRCHR
#define strchr index
#endif
#ifndef HAVE_STRRCHR
#define strrchr rindex
#endif
char *strchr ();
char *strrchr ();
Here we assume that HAVE_STRCHR and HAVE_STRRCHR are
macros defined in systems where the corresponding functions exist.
One way to get them properly defined is to use Autoconf.
GNU has a library called GNU gettext that makes it easy to translate the messages in a program into various languages. You should use this library in every program. Use English for the messages as they appear in the program, and let gettext provide the way to translate them into other languages.
Using GNU gettext involves putting a call to the gettext macro
around each string that might need translation—like this:
printf (gettext ("Processing file `%s'..."));
This permits GNU gettext to replace the string "Processing file
`%s'..." with a translated version.
Once a program uses gettext, please make a point of writing calls to
gettext when you add new strings that call for translation.
Using GNU gettext in a package involves specifying a text domain name for the package. The text domain name is used to separate the translations for this package from the translations for other packages. Normally, the text domain name should be the same as the name of the package—for example, ‘coreutils’ for the GNU core utilities.
To enable gettext to work well, avoid writing code that makes assumptions about the structure of words or sentences. When you want the precise text of a sentence to vary depending on the data, use two or more alternative string constants each containing a complete sentences, rather than inserting conditionalized words or phrases into a single sentence framework.
Here is an example of what not to do:
printf ("%s is full", capacity > 5000000 ? "disk" : "floppy disk");
If you apply gettext to all strings, like this,
printf (gettext ("%s is full"),
capacity > 5000000 ? gettext ("disk") : gettext ("floppy disk"));
the translator will hardly know that "disk" and "floppy disk" are meant to be substituted in the other string. Worse, in some languages (like French) the construction will not work: the translation of the word "full" depends on the gender of the first part of the sentence; it happens to be not the same for "disk" as for "floppy disk".
Complete sentences can be translated without problems:
printf (capacity > 5000000 ? gettext ("disk is full")
: gettext ("floppy disk is full"));
A similar problem appears at the level of sentence structure with this code:
printf ("# Implicit rule search has%s been done.\n",
f->tried_implicit ? "" : " not");
Adding gettext calls to this code cannot give correct results for
all languages, because negation in some languages requires adding words
at more than one place in the sentence. By contrast, adding
gettext calls does the job straightforwardly if the code starts
out like this:
printf (f->tried_implicit
? "# Implicit rule search has been done.\n",
: "# Implicit rule search has not been done.\n");
Another example is this one:
printf ("%d file%s processed", nfiles,
nfiles != 1 ? "s" : "");
The problem with this example is that it assumes that plurals are made by adding `s'. If you apply gettext to the format string, like this,
printf (gettext ("%d file%s processed"), nfiles,
nfiles != 1 ? "s" : "");
the message can use different words, but it will still be forced to use `s' for the plural. Here is a better way, with gettext being applied to the two strings independently:
printf ((nfiles != 1 ? gettext ("%d files processed")
: gettext ("%d file processed")),
nfiles);
But this still doesn't work for languages like Polish, which has three
plural forms: one for nfiles == 1, one for nfiles == 2, 3, 4, 22, 23, 24, ...
and one for the rest. The GNU ngettext function solves this problem:
printf (ngettext ("%d files processed", "%d file processed", nfiles),
nfiles);
Sticking to the ASCII character set (plain text, 7-bit characters) is preferred in GNU source code comments, text documents, and other contexts, unless there is good reason to do something else because of the application domain. For example, if source code deals with the French Revolutionary calendar, it is OK if its literal strings contain accented characters in month names like “Floréal”. Also, it is OK to use non-ASCII characters to represent proper names of contributors in change logs (see Change Logs).
If you need to use non-ASCII characters, you should normally stick with one encoding, as one cannot in general mix encodings reliably.
In the C locale, GNU programs should stick to plain ASCII for quotation characters in messages to users: preferably 0x60 (‘`’) for left quotes and 0x27 (‘'’) for right quotes. It is ok, but not required, to use locale-specific quotes in other locales.
The Gnulib quote and
quotearg modules provide a reasonably straightforward way to
support locale-specific quote characters, as well as taking care of
other issues, such as quoting a filename that itself contains a quote
character. See the Gnulib documentation for usage details.
In any case, the documentation for your program should clearly specify how it does quoting, if different than the preferred method of ‘`’ and ‘'’. This is especially important if the output of your program is ever likely to be parsed by another program.
Quotation characters are a difficult area in the computing world at this time: there are no true left or right quote characters in Latin1; the ‘`’ character we use was standardized there as a grave accent. Moreover, Latin1 is still not universally usable.
Unicode contains the unambiguous quote characters required, and its common encoding UTF-8 is upward compatible with Latin1. However, Unicode and UTF-8 are not universally well-supported, either.
This may change over the next few years, and then we will revisit this.
Don't assume that mmap either works on all files or fails
for all files. It may work on some files and fail on others.
The proper way to use mmap is to try it on the specific file for
which you want to use it—and if mmap doesn't work, fall back on
doing the job in another way using read and write.
The reason this precaution is needed is that the GNU kernel (the HURD)
provides a user-extensible file system, in which there can be many
different kinds of “ordinary files.” Many of them support
mmap, but some do not. It is important to make programs handle
all these kinds of files.
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.
The preferred document format for the GNU system is the Texinfo
formatting language. Every GNU package should (ideally) have
documentation in Texinfo both for reference and for learners. Texinfo
makes it possible to produce a good quality formatted book, using
TeX, and to generate an Info file. It is also possible to generate
HTML output from Texinfo source. See the Texinfo manual, either the
hardcopy, or the on-line version available through info or the
Emacs Info subsystem (C-h i).
Nowadays some other formats such as Docbook and Sgmltexi can be converted automatically into Texinfo. It is ok to produce the Texinfo documentation by conversion this way, as long as it gives good results.
Make sure your manual is clear to a reader who knows nothing about the topic and reads it straight through. This means covering basic topics at the beginning, and advanced topics only later. This also means defining every specialized term when it is first used.
Programmers tend to carry over the structure of the program as the structure for its documentation. But this structure is not necessarily good for explaining how to use the program; it may be irrelevant and confusing for a user.
Instead, the right way to structure documentation is according to the concepts and questions that a user will have in mind when reading it. This principle applies at every level, from the lowest (ordering sentences in a paragraph) to the highest (ordering of chapter topics within the manual). Sometimes this structure of ideas matches the structure of the implementation of the software being documented—but often they are different. An important part of learning to write good documentation is to learn to notice when you have unthinkingly structured the documentation like the implementation, stop yourself, and look for better alternatives.
For example, each program in the GNU system probably ought to be documented in one manual; but this does not mean each program should have its own manual. That would be following the structure of the implementation, rather than the structure that helps the user understand.
Instead, each manual should cover a coherent topic. For example,
instead of a manual for diff and a manual for diff3, we
have one manual for “comparison of files” which covers both of those
programs, as well as cmp. By documenting these programs
together, we can make the whole subject clearer.
The manual which discusses a program should certainly document all of the program's command-line options and all of its commands. It should give examples of their use. But don't organize the manual as a list of features. Instead, organize it logically, by subtopics. Address the questions that a user will ask when thinking about the job that the program does. Don't just tell the reader what each feature can do—say what jobs it is good for, and show how to use it for those jobs. Explain what is recommended usage, and what kinds of usage users should avoid.
In general, a GNU manual should serve both as tutorial and reference. It should be set up for convenient access to each topic through Info, and for reading straight through (appendixes aside). A GNU manual should give a good introduction to a beginner reading through from the start, and should also provide all the details that hackers want. The Bison manual is a good example of this—please take a look at it to see what we mean.
That is not as hard as it first sounds. Arrange each chapter as a logical breakdown of its topic, but order the sections, and write their text, so that reading the chapter straight through makes sense. Do likewise when structuring the book into chapters, and when structuring a section into paragraphs. The watchword is, at each point, address the most fundamental and important issue raised by the preceding text.
If necessary, add extra chapters at the beginning of the manual which are purely tutorial and cover the basics of the subject. These provide the framework for a beginner to understand the rest of the manual. The Bison manual provides a good example of how to do this.
To serve as a reference, a manual should have an Index that list all the functions, variables, options, and important concepts that are part of the program. One combined Index should do for a short manual, but sometimes for a complex package it is better to use multiple indices. The Texinfo manual includes advice on preparing good index entries, see Making Index Entries, and see Defining the Entries of an Index.
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.
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.
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.
Please use the GNU Free Documentation License for all GNU manuals that are more than a few pages long. Likewise for a collection of short documents—you only need one copy of the GNU FDL for the whole collection. For a single short document, you can use a very permissive non-copyleft license, to avoid taking up space with a long license.
See http://www.gnu.org/copyleft/fdl-howto.html for more explanation of how to employ the GFDL.
Note that it is not obligatory to include a copy of the GNU GPL or GNU LGPL in a manual whose license is neither the GPL nor the LGPL. It can be a good idea to include the program's license in a large manual; in a short manual, whose size would be increased considerably by including the program's license, it is probably better not to include it.
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.
The FSF publishes some GNU manuals in printed form. To encourage sales of these manuals, the on-line versions of the manual should mention at the very start that the printed manual is available and should point at information for getting it—for instance, with a link to the page http://www.gnu.org/order/order.html. This should not be included in the printed manual, though, because there it is redundant.
It is also useful to explain in the on-line forms of the manual how the user can print out the manual from the sources.
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.
Keep a change log to describe all the changes made to program source files. The purpose of this is so that people investigating bugs in the future will know about the changes that might have introduced the bug. Often a new bug can be found by looking at what was recently changed. More importantly, change logs can help you eliminate conceptual inconsistencies between different parts of a program, by giving you a history of how the conflicting concepts arose and who they came from.
You can think of the change log as a conceptual “undo list” which explains how earlier versions were different from the current version. People can see the current version; they don't need the change log to tell them what is in it. What they want from a change log is a clear explanation of how the earlier version differed.
The change log file is normally called ChangeLog and covers an entire directory. Each directory can have its own change log, or a directory can use the change log of its parent directory–it's up to you.
Another alternative is to record change log information with a version
control system such as RCS or CVS. This can be converted automatically
to a ChangeLog file using rcs2log; in Emacs, the command
C-x v a (vc-update-change-log) does the job.
There's no need to describe the full purpose of the changes or how they work together. If you think that a change calls for explanation, you're probably right. Please do explain it—but please put the explanation in comments in the code, where people will see it whenever they see the code. For example, “New function” is enough for the change log when you add a function, because there should be a comment before the function definition to explain what it does.
In the past, we recommended not mentioning changes in non-software files (manuals, help files, etc.) in change logs. However, we've been advised that it is a good idea to include them, for the sake of copyright records.
However, sometimes it is useful to write one line to describe the overall purpose of a batch of changes.
The easiest way to add an entry to ChangeLog is with the Emacs command M-x add-change-log-entry. An entry should have an asterisk, the name of the changed file, and then in parentheses the name of the changed functions, variables or whatever, followed by a colon. Then describe the changes you made to that function or variable.
Here are some simple examples of change log entries, starting with the header line that says who made the change and when it was installed, followed by descriptions of specific changes. (These examples are drawn from Emacs and GCC.)
1998-08-17 Richard Stallman <rms@gnu.org>
* register.el (insert-register): Return nil.
(jump-to-register): Likewise.
* sort.el (sort-subr): Return nil.
* tex-mode.el (tex-bibtex-file, tex-file, tex-region):
Restart the tex shell if process is gone or stopped.
(tex-shell-running): New function.
* expr.c (store_one_arg): Round size up for move_block_to_reg.
(expand_call): Round up when emitting USE insns.
* stmt.c (assign_parms): Round size up for move_block_from_reg.
It's important to name the changed function or variable in full. Don't abbreviate function or variable names, and don't combine them. Subsequent maintainers will often search for a function name to find all the change log entries that pertain to it; if you abbreviate the name, they won't find it when they search.
For example, some people are tempted to abbreviate groups of function
names by writing ‘* register.el ({insert,jump-to}-register)’;
this is not a good idea, since searching for jump-to-register or
insert-register would not find that entry.
Separate unrelated change log entries with blank lines. When two entries represent parts of the same change, so that they work together, then don't put blank lines between them. Then you can omit the file name and the asterisk when successive entries are in the same file.
Break long lists of function names by closing continued lines with ‘)’, rather than ‘,’, and opening the continuation with ‘(’ as in this example:
* keyboard.c (menu_bar_items, tool_bar_items)
(Fexecute_extended_command): Deal with `keymap' property.
When you install someone else's changes, put the contributor's name in the change log entry rather than in the text of the entry. In other words, write this:
2002-07-14 John Doe <jdoe@gnu.org>
* sewing.c: Make it sew.
rather than this:
2002-07-14 Usual Maintainer <usual@gnu.org>
* sewing.c: Make it sew. Patch by jdoe@gnu.org.
As for the date, that should be the date you applied the change.
Certain simple kinds of changes don't need much detail in the change log.
When you change the calling sequence of a function in a simple fashion, and you change all the callers of the function to use the new calling sequence, there is no need to make individual entries for all the callers that you changed. Just write in the entry for the function being called, “All callers changed”—like this:
* keyboard.c (Fcommand_execute): New arg SPECIAL.
All callers changed.
When you change just comments or doc strings, it is enough to write an entry for the file, without mentioning the functions. Just “Doc fixes” is enough for the change log.
There's no technical need to make change log entries for documentation files. This is because documentation is not susceptible to bugs that are hard to fix. Documentation does not consist of parts that must interact in a precisely engineered fashion. To correct an error, you need not know the history of the erroneous passage; it is enough to compare what the documentation says with the way the program actually works.
However, you should keep change logs for documentation files when the project gets copyright assignments from its contributors, so as to make the records of authorship more accurate.
C programs often contain compile-time #if conditionals. Many
changes are conditional; sometimes you add a new definition which is
entirely contained in a conditional. It is very useful to indicate in
the change log the conditions for which the change applies.
Our convention for indicating conditional changes is to use square brackets around the name of the condition.
Here is a simple example, describing a change which is conditional but does not have a function or entity name associated with it:
* xterm.c [SOLARIS2]: Include string.h.
Here is an entry describing a new definition which is entirely
conditional. This new definition for the macro FRAME_WINDOW_P is
used only when HAVE_X_WINDOWS is defined:
* frame.h [HAVE_X_WINDOWS] (FRAME_WINDOW_P): Macro defined.
Here is an entry for a change within the function init_display,
whose definition as a whole is unconditional, but the changes themselves
are contained in a ‘#ifdef HAVE_LIBNCURSES’ conditional:
* dispnew.c (init_display) [HAVE_LIBNCURSES]: If X, call tgetent.
Here is an entry for a change that takes affect only when a certain macro is not defined:
(gethostname) [!HAVE_SOCKETS]: Replace with winsock version.
Indicate the part of a function which changed by using angle brackets
enclosing an indication of what the changed part does. Here is an entry
for a change in the part of the function sh-while-getopts that
deals with sh commands:
* progmodes/sh-script.el (sh-while-getopts) <sh>: Handle case that
user-specified option string is empty.
In the GNU project, man pages are secondary. It is not necessary or expected for every GNU program to have a man page, but some of them do. It's your choice whether to include a man page in your program.
When you make this decision, consider that supporting a man page requires continual effort each time the program is changed. The time you spend on the man page is time taken away from more useful work.
For a simple program which changes little, updating the man page may be a small job. Then there is little reason not to include a man page, if you have one.
For a large program that changes a great deal, updating a man page may be a substantial burden. If a user offers to donate a man page, you may find this gift costly to accept. It may be better to refuse the man page unless the same person agrees to take full responsibility for maintaining it—so that you can wash your hands of it entirely. If this volunteer later ceases to do the job, then don't feel obliged to pick it up yourself; it may be better to withdraw the man page from the distribution until someone else agrees to update it.
When a program changes only a little, you may feel that the discrepancies are small enough that the man page remains useful without updating. If so, put a prominent note near the beginning of the man page explaining that you don't maintain it and that the Texinfo manual is more authoritative. The note should say how to access the Texinfo documentation.
Be sure that man pages include a copyright statement and free license. The simple all-permissive license is appropriate for simple man pages:
Copying and distribution of this file, with or without modification,
are permitted in any medium without royalty provided the copyright
notice and this notice are preserved.
For long man pages, with enough explanation and documentation that they can be considered true manuals, use the GFDL (see License for Manuals).
Finally, the GNU help2man program (http://www.gnu.org/software/help2man/) is one way to automate generation of a man page, in this case from --help output. This is sufficient in many cases.
There may be non-free books or documentation files that describe the program you are documenting.
It is ok to use these documents for reference, just as the author of a new algebra textbook can read other books on algebra. A large portion of any non-fiction book consists of facts, in this case facts about how a certain program works, and these facts are necessarily the same for everyone who writes about the subject. But be careful not to copy your outline structure, wording, tables or examples from preexisting non-free documentation. Copying from free documentation may be ok; please check with the FSF about the individual case.
Making a release is more than just bundling up your source files in a tar file and putting it up for FTP. You should set up your software so that it can be configured to run on a variety of systems. Your Makefile should conform to the GNU standards described below, and your directory layout should also conform to the standards discussed below. Doing so makes it easy to include your package into the larger framework of all GNU software.
Each GNU distribution should come with a shell script named
configure. This script is given arguments which describe the
kind of machine and system you want to compile the program for.
The configure script must record the configuration options so
that they affect compilation.
One way to do this is to make a link from a standard name such as config.h to the proper configuration file for the chosen system. If you use this technique, the distribution should not contain a file named config.h. This is so that people won't be able to build the program without configuring it first.
Another thing that configure can do is to edit the Makefile. If
you do this, the distribution should not contain a file named
Makefile. Instead, it should include a file Makefile.in which
contains the input used for editing. Once again, this is so that people
won't be able to build the program without configuring it first.
If configure does write the Makefile, then Makefile
should have a target named Makefile which causes configure
to be rerun, setting up the same configuration that was set up last
time. The files that configure reads should be listed as
dependencies of Makefile.
All the files which are output from the configure script should
have comments at the beginning explaining that they were generated
automatically using configure. This is so that users won't think
of trying to edit them by hand.
The configure script should write a file named config.status
which describes which configuration options were specified when the
program was last configured. This file should be a shell script which,
if run, will recreate the same configuration.
The configure script should accept an option of the form
‘--srcdir=dirname’ to specify the directory where sources are found
(if it is not the current directory). This makes it possible to build
the program in a separate directory, so that the actual source directory
is not modified.
If the user does not specify ‘--srcdir’, then configure should
check both . and .. to see if it can find the sources. If
it finds the sources in one of these places, it should use them from
there. Otherwise, it should report that it cannot find the sources, and
should exit with nonzero status.
Usually the easy way to support ‘--srcdir’ is by editing a
definition of VPATH into the Makefile. Some rules may need to
refer explicitly to the specified source directory. To make this
possible, configure can add to the Makefile a variable named
srcdir whose value is precisely the specified directory.
The configure script should also take an argument which specifies the
type of system to build the program for. This argument should look like
this:
cpu-company-system
For example, an Athlon-based GNU/Linux system might be ‘i686-pc-linux-gnu’.
The configure script needs to be able to decode all plausible
alternatives for how to describe a machine. Thus,
‘athlon-pc-gnu/linux’ would be a valid alias. There is a shell
script called
config.sub that you can use as a subroutine to validate system
types and canonicalize aliases.
The configure script should also take the option
--build=buildtype, which should be equivalent to a
plain buildtype argument. For example, ‘configure
--build=i686-pc-linux-gnu’ is equivalent to ‘configure
i686-pc-linux-gnu’. When the build type is not specified by an option
or argument, the configure script should normally guess it using
the shell script
config.guess.
Other options are permitted to specify in more detail the software or hardware present on the machine, to include or exclude optional parts of the package, or to adjust the name of some tools or arguments to them:
No ‘--enable’ option should ever cause one feature to
replace another. No ‘--enable’ option should ever substitute one
useful behavior for another useful behavior. The only proper use for
‘--enable’ is for questions of whether to build part of the program
or exclude it.
Possible values of package include ‘gnu-as’ (or ‘gas’), ‘gnu-ld’, ‘gnu-libc’, ‘gdb’, ‘x’, and ‘x-toolkit’.
Do not use a ‘--with’ option to specify the file name to use to
find certain files. That is outside the scope of what ‘--with’
options are for.
Specifying variables as arguments to configure, like this:
./configure CC=gcc
is preferable to setting them in environment variables:
CC=gcc ./configure
as it helps to recreate the same configuration later with config.status.
All configure scripts should accept all of the “detail”
options and the variable settings, whether or not they make any
difference to the particular package at hand. In particular, they
should accept any option that starts with ‘--with-’ or
‘--enable-’. This is so users will be able to configure an
entire GNU source tree at once with a single set of options.
You will note that the categories ‘--with-’ and ‘--enable-’ are narrow: they do not provide a place for any sort of option you might think of. That is deliberate. We want to limit the possible configuration options in GNU software. We do not want GNU programs to have idiosyncratic configuration options.
Packages that perform part of the compilation process may support cross-compilation. In such a case, the host and target machines for the program may be different.
The configure script should normally treat the specified type of
system as both the host and the target, thus producing a program which
works for the same type of machine that it runs on.
To compile a program to run on a host type that differs from the build type, use the configure option --host=hosttype, where hosttype uses the same syntax as buildtype. The host type normally defaults to the build type.
To configure a cross-compiler, cross-assembler, or what have you, you should specify a target different from the host, using the configure option ‘--target=targettype’. The syntax for targettype is the same as for the host type. So the command would look like this:
./configure --host=hosttype --target=targettype
The target type normally defaults to the host type. Programs for which cross-operation is not meaningful need not accept the ‘--target’ option, because configuring an entire operating system for cross-operation is not a meaningful operation.
Some programs have ways of configuring themselves automatically. If
your program is set up to do this, your configure script can simply
ignore most of its arguments.
This describes conventions for writing the Makefiles for GNU programs. Using Automake will help you write a Makefile that follows these conventions.
Every Makefile should contain this line:
SHELL = /bin/sh
to avoid trouble on systems where the SHELL variable might be
inherited from the environment. (This is never a problem with GNU
make.)
Different make programs have incompatible suffix lists and
implicit rules, and this sometimes creates confusion or misbehavior. So
it is a good idea to set the suffix list explicitly using only the
suffixes you need in the particular Makefile, like this:
.SUFFIXES:
.SUFFIXES: .c .o
The first line clears out the suffix list, the second introduces all suffixes which may be subject to implicit rules in this Makefile.
Don't assume that . is in the path for command execution. When you need to run programs that are a part of your package during the make, please make sure that it uses ./ if the program is built as part of the make or $(srcdir)/ if the file is an unchanging part of the source code. Without one of these prefixes, the current search path is used.
The distinction between ./ (the build directory) and $(srcdir)/ (the source directory) is important because users can build in a separate directory using the ‘--srcdir’ option to configure. A rule of the form:
foo.1 : foo.man sedscript
sed -e sedscript foo.man > foo.1
will fail when the build directory is not the source directory, because foo.man and sedscript are in the source directory.
When using GNU make, relying on ‘VPATH’ to find the source
file will work in the case where there is a single dependency file,
since the make automatic variable ‘$<’ will represent the
source file wherever it is. (Many versions of make set ‘$<’
only in implicit rules.) A Makefile target like
foo.o : bar.c
$(CC) -I. -I$(srcdir) $(CFLAGS) -c bar.c -o foo.o
should instead be written as
foo.o : bar.c
$(CC) -I. -I$(srcdir) $(CFLAGS) -c $< -o $@
in order to allow ‘VPATH’ to work correctly. When the target has multiple dependencies, using an explicit ‘$(srcdir)’ is the easiest way to make the rule work well. For example, the target above for foo.1 is best written as:
foo.1 : foo.man sedscript
sed -e $(srcdir)/sedscript $(srcdir)/foo.man > $@
GNU distributions usually contain some files which are not source files—for example, Info files, and the output from Autoconf, Automake, Bison or Flex. Since these files normally appear in the source directory, they should always appear in the source directory, not in the build directory. So Makefile rules to update them should put the updated files in the source directory.
However, if a file does not appear in the distribution, then the Makefile should not put it in the source directory, because building a program in ordinary circumstances should not modify the source directory in any way.
Try to make the build and installation targets, at least (and all their
subtargets) work correctly with a parallel make.
Write the Makefile commands (and any shell scripts, such as
configure) to run in sh, not in csh. Don't use any
special features of ksh or bash.
The configure script and the Makefile rules for building and
installation should not use any utilities directly except these:
cat cmp cp diff echo egrep expr false grep install-info
ln ls mkdir mv pwd rm rmdir sed sleep sort tar test touch true
The compression program gzip can be used in the dist rule.
Stick to the generally supported options for these programs. For example, don't use ‘mkdir -p’, convenient as it may be, because most systems don't support it.
It is a good idea to avoid creating symbolic links in makefiles, since a few systems don't support them.
The Makefile rules for building and installation can also use compilers
and related programs, but should do so via make variables so that the
user can substitute alternatives. Here are some of the programs we
mean:
ar bison cc flex install ld ldconfig lex
make makeinfo ranlib texi2dvi yacc
Use the following make variables to run those programs:
$(AR) $(BISON) $(CC) $(FLEX) $(INSTALL) $(LD) $(LDCONFIG) $(LEX)
$(MAKE) $(MAKEINFO) $(RANLIB) $(TEXI2DVI) $(YACC)
When you use ranlib or ldconfig, you should make sure
nothing bad happens if the system does not have the program in question.
Arrange to ignore an error from that command, and print a message before
the command to tell the user that failure of this command does not mean
a problem. (The Autoconf ‘AC_PROG_RANLIB’ macro can help with
this.)
If you use symbolic links, you should implement a fallback for systems that don't have symbolic links.
Additional utilities that can be used via Make variables are:
chgrp chmod chown mknod
It is ok to use other utilities in Makefile portions (or scripts) intended only for particular systems where you know those utilities exist.
Makefiles should provide variables for overriding certain commands, options, and so on.
In particular, you should run most utility programs via variables.
Thus, if you use Bison, have a variable named BISON whose default
value is set with ‘BISON = bison’, and refer to it with
$(BISON) whenever you need to use Bison.
File management utilities such as ln, rm, mv, and
so on, need not be referred to through variables in this way, since users
don't need to replace them with other programs.
Each program-name variable should come with an options variable that is
used to supply options to the program. Append ‘FLAGS’ to the
program-name variable name to get the options variable name—for
example, BISONFLAGS. (The names CFLAGS for the C
compiler, YFLAGS for yacc, and LFLAGS for lex, are
exceptions to this rule, but we keep them because they are standard.)
Use CPPFLAGS in any compilation command that runs the
preprocessor, and use LDFLAGS in any compilation command that
does linking as well as in any direct use of ld.
If there are C compiler options that must be used for proper
compilation of certain files, do not include them in CFLAGS.
Users expect to be able to specify CFLAGS freely themselves.
Instead, arrange to pass the necessary options to the C compiler
independently of CFLAGS, by writing them explicitly in the
compilation commands or by defining an implicit rule, like this:
CFLAGS = -g
ALL_CFLAGS = -I. $(CFLAGS)
.c.o:
$(CC) -c $(CPPFLAGS) $(ALL_CFLAGS) $<
Do include the ‘-g’ option in CFLAGS, because that is not
required for proper compilation. You can consider it a default
that is only recommended. If the package is set up so that it is
compiled with GCC by default, then you might as well include ‘-O’
in the default value of CFLAGS as well.
Put CFLAGS last in the compilation command, after other variables
containing compiler options, so the user can use CFLAGS to
override the others.
CFLAGS should be used in every invocation of the C compiler,
both those which do compilation and those which do linking.
Every Makefile should define the variable INSTALL, which is the
basic command for installing a file into the system.
Every Makefile should also define the variables INSTALL_PROGRAM
and INSTALL_DATA. (The default for INSTALL_PROGRAM should
be $(INSTALL); the default for INSTALL_DATA should be
${INSTALL} -m 644.) Then it should use those variables as the
commands for actual installation, for executables and non-executables
respectively. Minimal use of these variables is as follows:
$(INSTALL_PROGRAM) foo $(bindir)/foo
$(INSTALL_DATA) libfoo.a $(libdir)/libfoo.a
However, it is preferable to support a DESTDIR prefix on the
target files, as explained in the next section.
Always use a file name, not a directory name, as the second argument of the installation commands. Use a separate command for each file to be installed.
DESTDIR: support for staged installs
DESTDIR is a variable prepended to each installed target file,
like this:
$(INSTALL_PROGRAM) foo $(DESTDIR)$(bindir)/foo
$(INSTALL_DATA) libfoo.a $(DESTDIR)$(libdir)/libfoo.a
The DESTDIR variable is specified by the user on the make
command line. For example:
make DESTDIR=/tmp/stage install
DESTDIR should be supported only in the install* and
uninstall* targets, as those are the only targets where it is
useful.
If your installation step would normally install /usr/local/bin/foo and /usr/local/lib/libfoo.a, then an installation invoked as in the example above would install /tmp/stage/usr/local/bin/foo and /tmp/stage/usr/local/lib/libfoo.a instead.
Prepending the variable DESTDIR to each target in this way
provides for staged installs, where the installed files are not
placed directly into their expected location but are instead copied
into a temporary location (DESTDIR). However, installed files
maintain their relative directory structure and any embedded file names
will not be modified.
You should not set the value of DESTDIR in your Makefile
at all; then the files are installed into their expected locations by
default. Also, specifying DESTDIR should not change the
operation of the software in any way, so its value should not be
included in any file contents.
DESTDIR support is commonly used in package creation. It is
also helpful to users who want to understand what a given package will
install where, and to allow users who don't normally have permissions
to install into protected areas to build and install before gaining
those permissions. Finally, it can be useful with tools such as
stow, where code is installed in one place but made to appear
to be installed somewhere else using symbolic links or special mount
operations. So, we strongly recommend GNU packages support
DESTDIR, though it is not an absolute requirement.
Installation directories should always be named by variables, so it is easy to install in a nonstandard place. The standard names for these variables and the values they should have in GNU packages are described below. They are based on a standard file system layout; variants of it are used in GNU/Linux and other modern operating systems.
Installers are expected to override these values when calling make (e.g., make prefix=/usr install or configure (e.g., configure --prefix=/usr). GNU packages should not try to guess which value should be appropriate for these variables on the system they are being installed onto: use the default settings specified here so that all GNU packages behave identically, allowing the installer to achieve any desired layout.
These first two variables set the root for the installation. All the other installation directories should be subdirectories of one of these two, and nothing should be directly installed into these two directories.
prefixprefix should be /usr/local.
When building the complete GNU system, the prefix will be empty and
/usr will be a symbolic link to /.
(If you are using Autoconf, write it as ‘@prefix@’.)
Running ‘make install’ with a different value of prefix from
the one used to build the program should not recompile the
program.
exec_prefixexec_prefix should
be $(prefix).
(If you are using Autoconf, write it as ‘@exec_prefix@’.)
Generally, $(exec_prefix) is used for directories that contain
machine-specific files (such as executables and subroutine libraries),
while $(prefix) is used directly for other directories.
Running ‘make install’ with a different value of exec_prefix
from the one used to build the program should not recompile the
program.
Executable programs are installed in one of the following directories.
bindirsbindirlibexecdirThe definition of ‘libexecdir’ is the same for all packages, so you should install your data in a subdirectory thereof. Most packages install their data under $(libexecdir)/package-name/, possibly within additional subdirectories thereof, such as $(libexecdir)/package-name/machine/version.
Data files used by the program during its execution are divided into categories in two ways.
This makes for six different possibilities. However, we want to discourage the use of architecture-dependent files, aside from object files and libraries. It is much cleaner to make other data files architecture-independent, and it is generally not hard.
Here are the variables Makefiles should use to specify directories to put these various kinds of files in:
This should normally be /usr/local/share, but write it as $(datarootdir). (If you are using Autoconf, write it as ‘@datadir@’.)
The definition of ‘datadir’ is the same for all packages, so you
should install your data in a subdirectory thereof. Most packages
install their data under $(datadir)/package-name/.
Do not install executables here in this directory (they probably belong
in $(libexecdir) or $(sbindir)). Also do not install
files that are modified in the normal course of their use (programs
whose purpose is to change the configuration of the system excluded).
Those probably belong in $(localstatedir).
These variables specify the directory for installing certain specific types of files, if your program has them. Every GNU package should have Info files, so every program needs ‘infodir’, but not all need ‘libdir’ or ‘lispdir’.
Most compilers other than GCC do not look for header files in directory
/usr/local/include. So installing the header files this way is
only useful with GCC. Sometimes this is not a problem because some
libraries are only really intended to work with GCC. But some libraries
are intended to work with other compilers. They should install their
header files in two places, one specified by includedir and one
specified by oldincludedir.
The Makefile commands should check whether the value of
oldincludedir is empty. If it is, they should not try to use
it; they should cancel the second installation of the header files.
A package should not replace an existing header in this directory unless
the header came from the same package. Thus, if your Foo package
provides a header file foo.h, then it should install the header
file in the oldincludedir directory if either (1) there is no
foo.h there or (2) the foo.h that exists came from the Foo
package.
To tell whether foo.h came from the Foo package, put a magic
string in the file—part of a comment—and grep for that string.
infodir is separate from
docdir for compatibility with existing practice.
$(docdir) by default. (If
you are using Autoconf, write them as ‘@htmldir@’,
‘@dvidir@’, etc.) Packages which supply several translations
of their documentation should install them in
‘$(htmldir)/’ll, ‘$(pdfdir)/’ll, etc. where
ll is a locale abbreviation such as ‘en’ or ‘pt_BR’.
libdir should normally be
/usr/local/lib, but write it as $(exec_prefix)/lib.
(If you are using Autoconf, write it as ‘@libdir@’.)
If you are using Autoconf, write the default as ‘@lispdir@’. In order to make ‘@lispdir@’ work, you need the following lines in your configure.in file:
lispdir='${datarootdir}/emacs/site-lisp'
AC_SUBST(lispdir)
Unix-style man pages are installed in one of the following:
And finally, you should set the following variable:
configure shell script.
(If you are using Autoconf, use ‘srcdir = @srcdir@’.)
For example:
# Common prefix for installation directories.
# NOTE: This directory must exist when you start the install.
prefix = /usr/local
datarootdir = $(prefix)/share
datadir = $(datarootdir)
exec_prefix = $(prefix)
# Where to put the executable for the command `gcc'.
bindir = $(exec_prefix)/bin
# Where to put the directories used by the compiler.
libexecdir = $(exec_prefix)/libexec
# Where to put the Info files.
infodir = $(datarootdir)/info
If your program installs a large number of files into one of the
standard user-specified directories, it might be useful to group them
into a subdirectory particular to that program. If you do this, you
should write the install rule to create these subdirectories.
Do not expect the user to include the subdirectory name in the value of any of the variables listed above. The idea of having a uniform set of variable names for installation directories is to enable the user to specify the exact same values for several different GNU packages. In order for this to be useful, all the packages must be designed so that they will work sensibly when the user does so.
At times, not all of these variables may be implemented in the current release of Autoconf and/or Automake; but as of Autoconf 2.60, we believe all of them are. When any are missing, the descriptions here serve as specifications for what Autoconf will implement. As a programmer, you can either use a development version of Autoconf or avoid using these variables until a stable release is made which supports them.
All GNU programs should have the following targets in their Makefiles:
By default, the Make rules should compile and link with ‘-g’, so
that executable programs have debugging symbols. Users who don't mind
being helpless can strip the executables later if they wish.
Do not strip executables when installing them. Devil-may-care users can
use the install-strip target to do that.
If possible, write the install target rule so that it does not
modify anything in the directory where the program was built, provided
‘make all’ has just been done. This is convenient for building the
program under one user name and installing it under another.
The commands should create all the directories in which files are to be
installed, if they don't already exist. This includes the directories
specified as the values of the variables prefix and
exec_prefix, as well as all subdirectories that are needed.
One way to do this is by means of an installdirs target
as described below.
Use ‘-’ before any command for installing a man page, so that
make will ignore any errors. This is in case there are systems
that don't have the Unix man page documentation system installed.
The way to install Info files is to copy them into $(infodir)
with $(INSTALL_DATA) (see Command Variables), and then run
the install-info program if it is present. install-info
is a program that edits the Info dir file to add or update the
menu entry for the given Info file; it is part of the Texinfo package.
Here is a sample rule to install an Info file:
$(DESTDIR)$(infodir)/foo.info: foo.info
$(POST_INSTALL)
# There may be a newer info file in . than in srcdir.
-if test -f foo.info; then d=.; \
else d=$(srcdir); fi; \
$(INSTALL_DATA) $$d/foo.info $(DESTDIR)$@; \
# Run install-info only if it exists.
# Use `if' instead of just prepending `-' to the
# line so we notice real errors from install-info.
# We use `$(SHELL) -c' because some shells do not
# fail gracefully when there is an unknown command.
if $(SHELL) -c 'install-info --version' \
>/dev/null 2>&1; then \
install-info --dir-file=$(DESTDIR)$(infodir)/dir \
$(DESTDIR)$(infodir)/foo.info; \
else true; fi
When writing the install target, you must classify all the
commands into three categories: normal ones, pre-installation
commands and post-installation commands. See Install Command Categories.
install target.
When you have many documentation files to install, we recommend that
you avoid collisions and clutter by arranging for these targets to
install in subdirectories of the appropriate installation directory,
such as htmldir. As one example, if your package has multiple
manuals, and you wish to install HTML documentation with many files
(such as the “split” mode output by makeinfo --html), you'll
certainly want to use subdirectories, or two nodes with the same name
in different manuals will overwrite each other.
Please make these install-format targets invoke the
commands for the format target, for example, by making
format a dependency.
This rule should not modify the directories where compilation is done, only the directories where files are installed.
The uninstallation commands are divided into three categories, just like
the installation commands. See Install Command Categories.
install, but strip the executable files while installing
them. In simple cases, this target can use the install target in
a simple way:
install-strip:
$(MAKE) INSTALL_PROGRAM='$(INSTALL_PROGRAM) -s' \
install
But if the package installs scripts as well as real executables, the
install-strip target can't just refer to the install
target; it has to strip the executables but not the scripts.
install-strip should not strip the executables in the build
directory which are being copied for installation. It should only strip
the copies that are installed.
Normally we do not recommend stripping an executable unless you are sure
the program has no bugs. However, it can be reasonable to install a
stripped executable for actual execution while saving the unstripped
executable elsewhere in case there is a bug.
Delete .dvi files here if they are not part of the distribution.
distclean, plus
more: C source files produced by Bison, tags tables, Info files, and
so on.
The reason we say “almost everything” is that running the command
‘make maintainer-clean’ should not delete configure even
if configure can be remade using a rule in the Makefile. More
generally, ‘make maintainer-clean’ should not delete anything
that needs to exist in order to run configure and then begin to
build the program. Also, there is no need to delete parent
directories that were created with ‘mkdir -p’, since they could
have existed anyway. These are the only exceptions;
maintainer-clean should delete everything else that can be
rebuilt.
The ‘maintainer-clean’ target is intended to be used by a maintainer of the package, not by ordinary users. You may need special tools to reconstruct some of the files that ‘make maintainer-clean’ deletes. Since these files are normally included in the distribution, we don't take care to make them easy to reconstruct. If you find you need to unpack the full distribution again, don't blame us.
To help make users aware of this, the commands for the special
maintainer-clean target should start with these two:
@echo 'This command is intended for maintainers to use; it'
@echo 'deletes files that may need special tools to rebuild.'
info: foo.info
foo.info: foo.texi chap1.texi chap2.texi
$(MAKEINFO) $(srcdir)/foo.texi
You must define the variable MAKEINFO in the Makefile. It should
run the makeinfo program, which is part of the Texinfo
distribution.
Normally a GNU distribution comes with Info files, and that means the
Info files are present in the source directory. Therefore, the Make
rule for an info file should update it in the source directory. When
users build the package, ordinarily Make will not update the Info files
because they will already be up to date.
all target; the user must manually invoke them.
Here's an example rule for generating DVI files from Texinfo:
dvi: foo.dvi
foo.dvi: foo.texi chap1.texi chap2.texi
$(TEXI2DVI) $(srcdir)/foo.texi
You must define the variable TEXI2DVI in the Makefile. It should
run the program texi2dvi, which is part of the Texinfo
distribution.1 Alternatively,
write just the dependencies, and allow GNU make to provide the command.
Here's another example, this one for generating HTML from Texinfo:
html: foo.html
foo.html: foo.texi chap1.texi chap2.texi
$(TEXI2HTML) $(srcdir)/foo.texi
Again, you would define the variable TEXI2HTML in the Makefile;
for example, it might run makeinfo --no-split --html
(makeinfo is part of the Texinfo distribution).
For example, the distribution tar file of GCC version 1.40 unpacks into a subdirectory named gcc-1.40.
The easiest way to do this is to create a subdirectory appropriately
named, use ln or cp to install the proper files in it, and
then tar that subdirectory.
Compress the tar file with gzip. For example, the actual
distribution file for GCC version 1.40 is called gcc-1.40.tar.gz.
The dist target should explicitly depend on all non-source files
that are in the distribution, to make sure they are up to date in the
distribution.
See Making Releases.
The following targets are suggested as conventional names, for programs in which they are useful.
installcheckinstalldirs # Make sure all installation directories (e.g. $(bindir))
# actually exist by making them if necessary.
installdirs: mkinstalldirs
$(srcdir)/mkinstalldirs $(bindir) $(datadir) \
$(libdir) $(infodir) \
$(mandir)
or, if you wish to support DESTDIR,
# Make sure all installation directories (e.g. $(bindir))
# actually exist by making them if necessary.
installdirs: mkinstalldirs
$(srcdir)/mkinstalldirs \
$(DESTDIR)$(bindir) $(DESTDIR)$(datadir) \
$(DESTDIR)$(libdir) $(DESTDIR)$(infodir) \
$(DESTDIR)$(mandir)
This rule should not modify the directories where compilation is done. It should do nothing but create installation directories.
When writing the install target, you must classify all the
commands into three categories: normal ones, pre-installation
commands and post-installation commands.
Normal commands move files into their proper places, and set their modes. They may not alter any files except the ones that come entirely from the package they belong to.
Pre-installation and post-installation commands may alter other files; in particular, they can edit global configuration files or data bases.
Pre-installation commands are typically executed before the normal commands, and post-installation commands are typically run after the normal commands.
The most common use for a post-installation command is to run
install-info. This cannot be done with a normal command, since
it alters a file (the Info directory) which does not come entirely and
solely from the package being installed. It is a post-installation
command because it needs to be done after the normal command which
installs the package's Info files.
Most programs don't need any pre-installation commands, but we have the feature just in case it is needed.
To classify the commands in the install rule into these three
categories, insert category lines among them. A category line
specifies the category for the commands that follow.
A category line consists of a tab and a reference to a special Make variable, plus an optional comment at the end. There are three variables you can use, one for each category; the variable name specifies the category. Category lines are no-ops in ordinary execution because these three Make variables are normally undefined (and you should not define them in the makefile).
Here are the three possible category lines, each with a comment that explains what it means:
$(PRE_INSTALL) # Pre-install commands follow. $(POST_INSTALL) # Post-install commands follow. $(NORMAL_INSTALL) # Normal commands follow.
If you don't use a category line at the beginning of the install
rule, all the commands are classified as normal until the first category
line. If you don't use any category lines, all the commands are
classified as normal.
These are the category lines for uninstall:
$(PRE_UNINSTALL) # Pre-uninstall commands follow. $(POST_UNINSTALL) # Post-uninstall commands follow. $(NORMAL_UNINSTALL) # Normal commands follow.
Typically, a pre-uninstall command would be used for deleting entries from the Info directory.
If the install or uninstall target has any dependencies
which act as subroutines of installation, then you should start
each dependency's commands with a category line, and start the
main target's commands with a category line also. This way, you can
ensure that each command is placed in the right category regardless of
which of the dependencies actually run.
Pre-installation and post-installation commands should not run any programs except for these:
[ basename bash cat chgrp chmod chown cmp cp dd diff echo
egrep expand expr false fgrep find getopt grep gunzip gzip
hostname install install-info kill ldconfig ln ls md5sum
mkdir mkfifo mknod mv printenv pwd rm rmdir sed sort tee
test touch true uname xargs yes
The reason for distinguishing the commands in this way is for the sake of making binary packages. Typically a binary package contains all the executables and other files that need to be installed, and has its own method of installing them—so it does not need to run the normal installation commands. But installing the binary package does need to execute the pre-installation and post-installation commands.
Programs to build binary packages work by extracting the pre-installation and post-installation commands. Here is one way of extracting the pre-installation commands (the -s option to make is needed to silence messages about entering subdirectories):
make -s -n install -o all \
PRE_INSTALL=pre-install \
POST_INSTALL=post-install \
NORMAL_INSTALL=normal-install \
| gawk -f pre-install.awk
where the file pre-install.awk could contain this:
$0 ~ /^(normal-install|post-install)[ \t]*$/ {on = 0}
on {print $0}
$0 ~ /^pre-install[ \t]*$/ {on = 1}
You should identify each release with a pair of version numbers, a major version and a minor. We have no objection to using more than two numbers, but it is very unlikely that you really need them.
Package the distribution of Foo version 69.96 up in a gzipped tar
file with the name foo-69.96.tar.gz. It should unpack into a
subdirectory named foo-69.96.
Building and installing the program should never modify any of the files contained in the distribution. This means that all the files that form part of the program in any way must be classified into source files and non-source files. Source files are written by humans and never changed automatically; non-source files are produced from source files by programs under the control of the Makefile.
The distribution should contain a file named README which gives the name of the package, and a general description of what it does. It is also good to explain the purpose of each of the first-level subdirectories in the package, if there are any. The README file should either state the version number of the package, or refer to where in the package it can be found.
The README file should refer to the file INSTALL, which should contain an explanation of the installation procedure.
The README file should also refer to the file which contains the copying conditions. The GNU GPL, if used, should be in a file called COPYING. If the GNU LGPL is used, it should be in a file called COPYING.LIB.
Naturally, all the source files must be in the distribution. It is okay
to include non-source files in the distribution, provided they are
up-to-date and machine-independent, so that building the distribution
normally will never modify them. We commonly include non-source files
produced by Bison, lex, TeX, and makeinfo; this helps avoid
unnecessary dependencies between our distributions, so that users can
install whichever packages they want to install.
Non-source files that might actually be modified by building and installing the program should never be included in the distribution. So if you do distribute non-source files, always make sure they are up to date when you make a new distribution.
Make sure that the directory into which the distribution unpacks (as
well as any subdirectories) are all world-writable (octal mode 777).
This is so that old versions of tar which preserve the
ownership and permissions of the files from the tar archive will be
able to extract all the files even if the user is unprivileged.
Make sure that all the files in the distribution are world-readable.
Don't include any symbolic links in the distribution itself. If the tar file contains symbolic links, then people cannot even unpack it on systems that don't support symbolic links. Also, don't use multiple names for one file in different directories, because certain file systems cannot handle this and that prevents unpacking the distribution.
Try to make sure that all the file names will be unique on MS-DOS. A name on MS-DOS consists of up to 8 characters, optionally followed by a period and up to three characters. MS-DOS will truncate extra characters both before and after the period. Thus, foobarhacker.c and foobarhacker.o are not ambiguous; they are truncated to foobarha.c and foobarha.o, which are distinct.
Include in your distribution a copy of the texinfo.tex you used to test print any *.texinfo or *.texi files.
Likewise, if your program uses small GNU software packages like regex, getopt, obstack, or termcap, include them in the distribution file. Leaving them out would make the distribution file a little smaller at the expense of possible inconvenience to a user who doesn't know what other files to get.
A GNU program should not recommend use of any non-free program. We can't stop some people from writing proprietary programs, or stop other people from using them, but we can and should refuse to advertise them to new potential customers. Proprietary software is a social and ethical problem, and the point of GNU is to solve that problem.
The GNU definition of free software is found on the GNU web site at http://www.gnu.org/philosophy/free-sw.html, and the definition of free documentation is found at http://www.gnu.org/philosophy/free-doc.html. A list of important licenses and whether they qualify as free is in http://www.gnu.org/licenses/license-list.html. The terms “free” and “non-free”, used in this document, refer to that definition. If it is not clear whether a license qualifies as free under this definition, please ask the GNU Project by writing to licensing@gnu.org. We will answer, and if the license is an important one, we will add it to the list.
When a non-free program or system is well known, you can mention it in passing—that is harmless, since users who might want to use it probably already know about it. For instance, it is fine to explain how to build your package on top of some widely used non-free operating system, or how to use it together with some widely used non-free program.
However, you should give only the necessary information to help those who already use the non-free program to use your program with it—don't give, or refer to, any further information about the proprietary program, and don't imply that the proprietary program enhances your program, or that its existence is in any way a good thing. The goal should be that people already using the proprietary program will get the advice they need about how to use your free program with it, while people who don't already use the proprietary program will not see anything to lead them to take an interest in it.
If a non-free program or system is obscure in your program's domain, your program should not mention or support it at all, since doing so would tend to popularize the non-free program more than it popularizes your program. (You cannot hope to find many additional users among the users of Foobar if the users of Foobar are few.)
Sometimes a program is free software in itself but depends on a non-free platform in order to run. For instance, many Java programs depend on the parts of Sun's Java implementation which are not yet free software, and won't run on the GNU Java Compiler (which does not yet have all the features) or won't run with the GNU Java libraries. We hope this particular problem will be gone in a few months, when Sun makes the standard Java libraries free software, but of course the general principle remains: you should not recommend programs that depend on non-free software to run.
Some free programs encourage the use of non-free software. A typical example is mplayer. It is free software in itself, and the free code can handle some kinds of files. However, mplayer recommends use of non-free codecs for other kinds of files, and users that install mplayer are very likely to install those codecs along with it. To recommend mplayer is, in effect, to recommend the non-free codecs. We must not do that, so we cannot recommend mplayer either.
In general, you should also not recommend programs that themselves strongly recommend the use of non-free software.
A GNU package should not refer the user to any non-free documentation for free software. Free documentation that can be included in free operating systems is essential for completing the GNU system, or any free operating system, so it is a major focus of the GNU Project; to recommend use of documentation that we are not allowed to use in GNU would weaken the impetus for the community to produce documentation that we can include. So GNU packages should never recommend non-free documentation.
By contrast, it is ok to refer to journal articles and textbooks in the comments of a program for explanation of how it functions, even though they be non-free. This is because we don't include such things in the GNU system even if we are allowed to—they are outside the scope of an operating system project.
Referring to a web site that describes or recommends a non-free program is in effect promoting that software, so please do not make links (or mention by name) web sites that contain such material. This policy is relevant particularly for the web pages for a GNU package.
Following links from nearly any web site can lead to non-free software; this is an inescapable aspect of the nature of the web, and in itself is no objection to linking to a site. As long as the site does not itself recommend a non-free program, there is no need be concerned about the sites it links to for other reasons.
Thus, for example, you should not make a link to AT&T's web site, because that recommends AT&T's non-free software packages; you should not make a link to a site that links to AT&T's site saying it is a place to get a non-free program; but if a site you want to link to refers to AT&T's web site in some other context (such as long-distance telephone service), that is not a problem.
Copyright © 2000,2001,2002 Free Software Foundation, Inc.
51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
The purpose of this License is to make a manual, textbook, or other functional and useful document free in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.
This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.
We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.
This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free license, unlimited in duration, to use that work under the conditions stated herein. The “Document”, below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as “you”. You accept the license if you copy, modify or distribute the work in a way requiring permission under copyright law.
A “Modified Version” of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language.
A “Secondary Section” is a named appendix or a front-matter section of the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Document's overall subject (or to related matters) and contains nothing that could fall directly within that overall subject. (Thus, if the Document is in part a textbook of mathematics, a Secondary Section may not explain any mathematics.) The relationship could be a matter of historical connection with the subject or with related matters, or of legal, commercial, philosophical, ethical or political position regarding them.
The “Invariant Sections” are certain Secondary Sections whose titles are designated, as being those of Invariant Sections, in the notice that says that the Document is released under this License. If a section does not fit the above definition of Secondary then it is not allowed to be designated as Invariant. The Document may contain zero Invariant Sections. If the Document does not identify any Invariant Sections then there are none.
The “Cover Texts” are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, in the notice that says that the Document is released under this License. A Front-Cover Text may be at most 5 words, and a Back-Cover Text may be at most 25 words.
A “Transparent” copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, that is suitable for revising the document straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters. A copy made in an otherwise Transparent file format whose markup, or absence of markup, has been arranged to thwart or discourage subsequent modification by readers is not Transparent. An image format is not Transparent if used for any substantial amount of text. A copy that is not “Transparent” is called “Opaque”.
Examples of suitable formats for Transparent copies include plain ascii without markup, Texinfo input format, LaTeX input format, SGML or XML using a publicly available DTD, and standard-conforming simple HTML, PostScript or PDF designed for human modification. Examples of transparent image formats include PNG, XCF and JPG. Opaque formats include proprietary formats that can be read and edited only by proprietary word processors, SGML or XML for which the DTD and/or processing tools are not generally available, and the machine-generated HTML, PostScript or PDF produced by some word processors for output purposes only.
The “Title Page” means, for a printed book, the title page itself, plus such following pages as are needed to hold, legibly, the material this License requires to appear in the title page. For works in formats which do not have any title page as such, “Title Page” means the text near the most prominent appearance of the work's title, preceding the beginning of the body of the text.
A section “Entitled XYZ” means a named subunit of the Document whose title either is precisely XYZ or contains XYZ in parentheses following text that translates XYZ in another language. (Here XYZ stands for a specific section name mentioned below, such as “Acknowledgements”, “Dedications”, “Endorsements”, or “History”.) To “Preserve the Title” of such a section when you modify the Document means that it remains a section “Entitled XYZ” according to this definition.
The Document may include Warranty Disclaimers next to the notice which states that this License applies to the Document. These Warranty Disclaimers are considered to be included by reference in this License, but only as regards disclaiming warranties: any other implication that these Warranty Disclaimers may have is void and has no effect on the meaning of this License.
You may copy and distribute the Document in any medium, either commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute. However, you may accept compensation in exchange for copies. If you distribute a large enough number of copies you must also follow the conditions in section 3.
You may also lend copies, under the same conditions stated above, and you may publicly display copies.
If you publish printed copies (or copies in media that commonly have printed covers) of the Document, numbering more than 100, and the Document's license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects.
If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages.
If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a computer-network location from which the general network-using public has access to download using public-standard network protocols a complete Transparent copy of the Document, free of added material. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public.
It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document.
You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:
If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version's license notice. These titles must be distinct from any other section titles.
You may add a section Entitled “Endorsements”, provided it contains nothing but endorsements of your Modified Version by various parties—for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.
You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one.
The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version.
You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that you preserve all their Warranty Disclaimers.
The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.
In the combination, you must combine any sections Entitled “History” in the various original documents, forming one section Entitled “History”; likewise combine any sections Entitled “Acknowledgements”, and any sections Entitled “Dedications”. You must delete all sections Entitled “Endorsements.”
You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.
You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.
A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an “aggregate” if the copyright resulting from the compilation is not used to limit the legal rights of the compilation's users beyond what the individual works permit. When the Document is included in an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document.
If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document's Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.
Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warranty Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail.
If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.
You may not copy, modify, sublicense, or distribute the Document except as expressly provided for under this License. Any other attempt to copy, modify, sublicense or distribute the Document is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.
The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.
Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation.
To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:
Copyright (C) year your name.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2
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''.
If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with...Texts.” line with this:
with the Invariant Sections being list their titles, with
the Front-Cover Texts being list, and with the Back-Cover Texts
being list.
If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.
If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.
#endif, commenting: Commentsautoconf: System Portabilitybindir: Directory Variablesconfigure: ConfigurationDESTDIR: DESTDIRdoschk: Namesexec_prefix: Directory Variablesgetopt: Command-Line Interfacesgettext: Internationalizationint: Syntactic Conventionslibexecdir: Directory Variableslint: Syntactic Conventionsmalloc return value: Semanticsmmap: MmapNUL characters: SemanticsPOSIXLY_CORRECT, environment variable: Compatibilityprefix: Directory Variablessbindir: Directory VariablesTMPDIR environment variable: Semanticsstandards.texi: Preface[1] texi2dvi uses TeX to do the real work
of formatting. TeX is not distributed with Texinfo.