The GNU coding standards, last updated July 25, 2016.
Copyright © 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License”.
| • Preface: | About the GNU Coding Standards. | |
| • Legal Issues: | Keeping free software free. | |
| • Design Advice: | General program design. | |
| • Program Behavior: | Program behavior for all programs | |
| • Writing C: | Making the best use of C. | |
| • Documentation: | Documenting programs. | |
| • Managing Releases: | The release process. | |
| • References: | Mentioning non-free software or documentation. | |
| • GNU Free Documentation License: | Copying and sharing this manual. | |
| • Index: | ||
Next: Legal Issues, Previous: Top, Up: Top [Contents][Index]
The GNU Coding Standards were written by Richard Stallman and other GNU Project volunteers. Their purpose is to make the GNU system clean, consistent, and easy to install. This document can also be read as a guide to writing portable, robust and reliable programs. It focuses on programs written in C, but many of the rules and principles are useful even if you write in another programming language. The rules often state reasons for writing in a certain way.
If you did not obtain this file directly from the GNU project and recently, please check for a newer version. You can get the GNU Coding Standards from the GNU web server in many different formats, including the Texinfo source, PDF, HTML, DVI, plain text, and more, at: http://www.gnu.org/prep/standards/.
If you are maintaining an official GNU package, in addition to this document, please read and follow the GNU maintainer information (see Contents in Information for Maintainers of GNU Software).
If you want to receive diffs for every change to these GNU documents,
join the mailing list<EMAIL_ADDRESS>, via the web
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http://lists.gnu.org/mailman/listinfo/gnustandards-commit.
Archives are also available there.
Please send corrections or suggestions for this document to<EMAIL_ADDRESS>If you make a suggestion, please include a suggested new wording for it, to help us consider the suggestion efficiently. We prefer a context diff to the Texinfo source, but if that’s difficult for you, you can make a context diff for some other version of this document, or propose it in any way that makes it clear. The source repository for this document can be found at http://savannah.gnu.org/projects/gnustandards.
These standards cover the minimum of what is important when writing a GNU package. Likely, the need for additional standards will come up. Sometimes, you might suggest that such standards be added to this document. If you think your standards would be generally useful, please do suggest them.
You should also set standards for your package on many questions not addressed or not firmly specified here. The most important point is to be self-consistent—try to stick to the conventions you pick, and try to document them as much as possible. That way, your program will be more maintainable by others.
The GNU Hello program serves as an example of how to follow the GNU coding standards for a trivial program. http://www.gnu.org/software/hello/hello.html.
This release of the GNU Coding Standards was last updated July 25, 2016.
Next: Design Advice, Previous: Preface, Up: Top [Contents][Index]
This chapter discusses how you can make sure that GNU software avoids legal difficulties, and other related issues.
| • Reading Non-Free Code: | Referring to proprietary programs. | |
| • Contributions: | Accepting contributions. | |
| • Trademarks: | How we deal with trademark issues. |
Next: Contributions, Up: Legal Issues [Contents][Index]
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.
Next: Trademarks, Previous: Reading Non-Free Code, Up: Legal Issues [Contents][Index]
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.  You might have to take that code out again!
You don’t need papers for changes of a few lines here or there, since they are not significant for copyright purposes. Also, you don’t need papers if all you get from the suggestion is some ideas, not actual code which you use. For example, if someone sent you one implementation, but you write a different implementation of the same idea, you don’t need to get papers.
The very worst thing is if you forget to tell us about the other contributor. We could be very embarrassed in court some day as a result.
We have more detailed advice for maintainers of GNU packages. If you have reached the stage of maintaining a GNU program (whether released or not), please take a look: see Legal Matters in Information for GNU Maintainers.
Previous: Contributions, Up: Legal Issues [Contents][Index]
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’.
Next: Program Behavior, Previous: Legal Issues, Up: Top [Contents][Index]
This chapter discusses some of the issues you should take into account when designing your program.
| • Source Language: | Which languages to use. | |
| • Compatibility: | Compatibility with other implementations. | |
| • Using Extensions: | Using non-standard features. | |
| • Standard C: | Using standard C features. | |
| • Conditional Compilation: | Compiling code only if a conditional is true. |
Next: Compatibility, Up: Design Advice [Contents][Index]
When you want to use a language that gets compiled and runs at high speed, the best language to use is C. C++ is ok too, but please don’t make heavy use of templates. So is Java, if you compile it.
When highest efficiency is not required, other languages commonly used in the free software community, such as Lisp, Scheme, Python, Ruby, and Java, are OK too. Scheme, as implemented by GNU Guile, plays a particular role in the GNU System: it is the preferred language to extend programs written in C/C++, and also a fine language for a wide range of applications. The more GNU components use Guile and Scheme, the more users are able to extend and combine them (see The Emacs Thesis in GNU Guile Reference Manual).
Many programs are designed to be extensible: they include an interpreter for a language that is higher level than C. Often much of the program is written in that language, too. The Emacs editor pioneered this technique.
The standard extensibility interpreter for GNU software is Guile (http://www.gnu.org/software/guile/), which implements the language Scheme (an especially clean and simple dialect of Lisp). Guile also includes bindings for GTK+/GNOME, making it practical to write modern GUI functionality within Guile. We don’t reject programs written in other “scripting languages” such as Perl and Python, but using Guile is the path that will lead to overall consistency of the GNU system.
Next: Using Extensions, Previous: Source Language, Up: Design Advice [Contents][Index]
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.
Next: Standard C, Previous: Compatibility, Up: Design Advice [Contents][Index]
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.
Next: Conditional Compilation, Previous: Using Extensions, Up: Design Advice [Contents][Index]
1989 Standard C is widespread enough now that it is ok to use its features in programs. There is one exception: do not ever use the “trigraph” feature of Standard C.
The 1999 and 2011 editions of Standard C are not fully supported on all platforms. If you aim to support compilation by compilers other than GCC, you should not require these C features in your programs. It is ok to use these features conditionally when the compiler supports them.
If your program is only meant to compile with GCC, then you can use these features if GCC supports them, when they give substantial benefit.
However, it is easy to support pre-standard compilers in most programs, so if you know how to do that, feel free.
To support pre-standard C, instead of writing function definitions in standard prototype form,
int foo (int x, int y) …
write the definition in pre-standard style like this,
int foo (x, y) int x, y; …
and use a separate declaration to specify the argument prototype:
int foo (int, int);
You need such a declaration anyway, in a header file, to get the benefit of prototypes in all the files where the function is called. And once you have the declaration, you normally lose nothing by writing the function definition in the pre-standard style.
This technique does not work for integer types narrower than int
.
If you think of an argument as being of a type narrower than int
,
declare it as int
instead.
There are a few special cases where this technique is hard to use. For
example, if a function argument needs to hold the system type
dev_t
, you run into trouble, because dev_t
is shorter than
int
on some machines; but you cannot use int
instead,
because dev_t
is wider than int
on some machines. There
is no type you can safely use on all machines in a non-standard
definition. The only way to support non-standard C and pass such an
argument is to check the width of dev_t
using Autoconf and choose
the argument type accordingly. This may not be worth the trouble.
In order to support pre-standard compilers that do not recognize prototypes, you may want to use a preprocessor macro like this:
/* Declare the prototype for a general external function. */ #if defined (__STDC__) || defined (WINDOWSNT) #define P_(proto) proto #else #define P_(proto) () #endif
Previous: Standard C, Up: Design Advice [Contents][Index]
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
Next: Writing C, Previous: Design Advice, Up: Top [Contents][Index]
This chapter describes conventions for writing robust software. It also describes general standards for error messages, the command line interface, and how libraries should behave.
| • Non-GNU Standards: | We consider standards such as POSIX; we don’t "obey" them. | |
| • Semantics: | Writing robust programs. | |
| • Libraries: | Library behavior. | |
| • Errors: | Formatting error messages. | |
| • User Interfaces: | Standards about interfaces generally. | |
| • Graphical Interfaces: | Standards for graphical interfaces. | |
| • Command-Line Interfaces: | Standards for command line interfaces. | |
| • Dynamic Plug-In Interfaces: | Standards for dynamic plug-in interfaces. | |
| • Option Table: | Table of long options. | |
| • OID Allocations: | Table of OID slots for GNU. | |
| • Memory Usage: | When and how to care about memory needs. | |
| • File Usage: | Which files to use, and where. |
Next: Semantics, Up: Program Behavior [Contents][Index]
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”.
Next: Libraries, Previous: Non-GNU Standards, Up: Program Behavior [Contents][Index]
Avoid arbitrary limits on the length or number of any data structure, including file names, lines, files, and symbols, by allocating all data structures dynamically. In most Unix utilities, “long lines are silently truncated”. This is not acceptable in a GNU utility.
Utilities reading files should not drop NUL characters, or any other nonprinting characters. Programs should work properly with multibyte character encodings, such as UTF-8. You can use libiconv to deal with a range of encodings.
Check every system call for an error return, unless you know you wish
to ignore errors. Include the system error text (from
strerror
, or equivalent) in every error message
resulting from a failing system call, as well as the name of the file
if any and the name of the utility. Just “cannot open foo.c” or
“stat failed” is not sufficient.
Check every call to malloc
or realloc
to see if it
returned NULL
. Check realloc
even if you are making the
block smaller; in a system that rounds block sizes to a power of 2,
realloc
may get a different block if you ask for less space.
You must expect free
to alter the contents of the block that was
freed. Anything you want to fetch from the block, you must fetch before
calling free
.
If malloc
fails in a noninteractive program, make that a fatal
error. In an interactive program (one that reads commands from the
user), it is better to abort the command and return to the command
reader loop. This allows the user to kill other processes to free up
virtual memory, and then try the command again.
Use getopt_long
to decode arguments, unless the argument syntax
makes this unreasonable.
When static storage is to be written in during program execution, use explicit C code to initialize it. This way, restarting the program (without reloading it), or part of it, will reinitialize those variables. Reserve C initialized declarations for data that will not be changed.
Try to avoid low-level interfaces to obscure Unix data structures (such
as file directories, utmp, or the layout of kernel memory), since these
are less likely to work compatibly. If you need to find all the files
in a directory, use readdir
or some other high-level interface.
These are supported compatibly by GNU.
The preferred signal handling facilities are the BSD variant of
signal
, and the POSIX sigaction
function; the
alternative USG signal
interface is an inferior design.
Nowadays, using the POSIX signal functions may be the easiest way
to make a program portable. If you use signal
, then on GNU/Linux
systems running GNU libc version 1, you should include
bsd/signal.h instead of signal.h, so as to get BSD
behavior. It is up to you whether to support systems where
signal
has only the USG behavior, or give up on them.
In error checks that detect “impossible” conditions, just abort. There is usually no point in printing any message. These checks indicate the existence of bugs. Whoever wants to fix the bugs will have to read the source code and run a debugger. So explain the problem with comments in the source. The relevant data will be in variables, which are easy to examine with the debugger, so there is no point moving them elsewhere.
Do not use a count of errors as the exit status for a program. That does not work, because exit status values are limited to 8 bits (0 through 255). A single run of the program might have 256 errors; if you try to return 256 as the exit status, the parent process will see 0 as the status, and it will appear that the program succeeded.
If you make temporary files, check the TMPDIR
environment
variable; if that variable is defined, use the specified directory
instead of /tmp.
In addition, be aware that there is a possible security problem when creating temporary files in world-writable directories. In C, you can avoid this problem by creating temporary files in this manner:
fd = open (filename, O_WRONLY | O_CREAT | O_EXCL, 0600);
or by using the mkstemps
function from Gnulib
(see mkstemps in Gnulib).
In bash, use set -C
(long name noclobber
) to avoid this
problem. In addition, the mktemp
utility is a more general
solution for creating temporary files from shell scripts
(see mktemp invocation in GNU Coreutils).
Next: Errors, Previous: Semantics, Up: Program Behavior [Contents][Index]
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.
Next: User Interfaces, Previous: Libraries, Up: Program Behavior [Contents][Index]
Error messages from compilers should look like this:
sourcefile:lineno: message
If you want to mention the column number, use one of these formats:
sourcefile:lineno:column: message sourcefile:lineno.column: message
Line numbers should start from 1 at the beginning of the file, and
column numbers should start from 1 at the beginning of the line.
(Both of these conventions are chosen for compatibility.) Calculate
column numbers assuming that space and all ASCII printing characters
have equal width, and assuming tab stops every 8 columns. For
non-ASCII characters, Unicode character widths should be used when in
a UTF-8 locale; GNU libc and GNU gnulib provide suitable
wcwidth
functions.
The error message can also give both the starting and ending positions of the erroneous text. There are several formats so that you can avoid redundant information such as a duplicate line number. Here are the possible formats:
sourcefile:line1.column1-line2.column2: message sourcefile:line1.column1-column2: message sourcefile:line1-line2: message
When an error is spread over several files, you can use this format:
file1:line1.column1-file2:line2.column2: message
Error messages from other noninteractive programs should look like this:
program:sourcefile:lineno: message
when there is an appropriate source file, or like this:
program: message
when there is no relevant source file.
If you want to mention the column number, use this format:
program:sourcefile:lineno:column: message
In an interactive program (one that is reading commands from a terminal), it is better not to include the program name in an error message. The place to indicate which program is running is in the prompt or with the screen layout. (When the same program runs with input from a source other than a terminal, it is not interactive and would do best to print error messages using the noninteractive style.)
The string message should not begin with a capital letter when it follows a program name and/or file name, because that isn’t the beginning of a sentence. (The sentence conceptually starts at the beginning of the line.) Also, it should not end with a period.
Error messages from interactive programs, and other messages such as usage messages, should start with a capital letter. But they should not end with a period.
Next: Graphical Interfaces, Previous: Errors, Up: Program Behavior [Contents][Index]
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. You can also build two versions of the program, with different names and different default behaviors.
Likewise, please don’t make the behavior of a command-line program depend on the type of output device it gets as standard output or standard input. Device independence is an important principle of the system’s design; do not compromise it merely to save someone from typing an option now and then. (Variation in error message syntax when using a terminal is ok, because that is a side issue that people do not depend on.)
If you think one behavior is most useful when the output is to a terminal, and another is most useful when the output is a file or a pipe, then it is usually best to make the default behavior the one that is useful with output to a terminal, and have an option for the other behavior. You can also build two different versions of the program with different names.
There is an exception for programs whose output in certain cases is
binary data. Sending such output to a terminal is useless and can
cause trouble. If such a program normally sends its output to stdout,
it should detect, in these cases, when the output is a terminal and
give an error message instead. The -f
option should override
this exception, thus permitting the output to go to the terminal.
Compatibility requires certain programs to depend on the type of output
device. It would be disastrous if ls
or sh
did not do so
in the way all users expect. In some of these cases, we supplement the
program with a preferred alternate version that does not depend on the
output device type. For example, we provide a dir
program much
like ls
except that its default output format is always
multi-column format.
Next: Command-Line Interfaces, Previous: User Interfaces, Up: Program Behavior [Contents][Index]
When you write a program that provides a graphical user interface, please make it work with the X Window System, using the GTK+ toolkit or the GNUstep toolkit, unless the functionality specifically requires some alternative (for example, “displaying jpeg images while in console mode”).
In addition, please provide a command-line interface to control the functionality. (In many cases, the graphical user interface can be a separate program which invokes the command-line program.) This is so that the same jobs can be done from scripts.
Please also consider providing a D-bus interface for use from other running programs, such as within GNOME. (GNOME used to use CORBA for this, but that is being phased out.) In addition, consider providing a library interface (for use from C), and perhaps a keyboard-driven console interface (for use by users from console mode). Once you are doing the work to provide the functionality and the graphical interface, these won’t be much extra work.
Please make your program interoperate with access technology such as screen readers (see http://www.gnu.org/accessibility/accessibility.html). This should be automatic if you use GTK+.
Next: Dynamic Plug-In Interfaces, Previous: Graphical Interfaces, Up: Program Behavior [Contents][Index]
It is a good idea to follow the POSIX guidelines for the
command-line options of a program. The easiest way to do this is to use
getopt
to parse them. Note that the GNU version of getopt
will normally permit options anywhere among the arguments unless the
special argument ‘--’ is used. This is not what POSIX
specifies; it is a GNU extension.
Please define long-named options that are equivalent to the
single-letter Unix-style options. We hope to make GNU more user
friendly this way. This is easy to do with the GNU function
getopt_long
.
One of the advantages of long-named options is that they can be consistent from program to program. For example, users should be able to expect the “verbose” option of any GNU program which has one, to be spelled precisely ‘--verbose’. To achieve this uniformity, look at the table of common long-option names when you choose the option names for your program (see Option Table).
It is usually a good idea for file names given as ordinary arguments to be input files only; any output files would be specified using options (preferably ‘-o’ or ‘--output’). Even if you allow an output file name as an ordinary argument for compatibility, try to provide an option as another way to specify it. This will lead to more consistency among GNU utilities, and fewer idiosyncrasies for users to remember.
All programs should support two standard options: ‘--version’
and ‘--help’. CGI programs should accept these as command-line
options, and also if given as the PATH_INFO
; for instance,
visiting ‘http://example.org/p.cgi/--help
’ in a browser should
output the same information as invoking ‘p.cgi --help’ from the
command line.
| • --version: | The standard output for –version. | |
| • --help: | The standard output for –help. |
Next: --help, Up: Command-Line Interfaces [Contents][Index]
The standard --version
option should direct the program to
print information about its name, version, origin and legal status,
all on standard output, and then exit successfully. Other options and
arguments should be ignored once this is seen, and the program should
not perform its normal function.
The first line is meant to be easy for a program to parse; the version number proper starts after the last space. In addition, it contains the canonical name for this program, in this format:
GNU Emacs 19.30
The program’s name should be a constant string; don’t compute it
from argv[0]
. The idea is to state the standard or canonical
name for the program, not its file name. There are other ways to find
out the precise file name where a command is found in PATH
.
If the program is a subsidiary part of a larger package, mention the package name in parentheses, like this:
emacsserver (GNU Emacs) 19.30
If the package has a version number which is different from this program’s version number, you can mention the package version number just before the close-parenthesis.
If you need to mention the version numbers of libraries which are distributed separately from the package which contains this program, you can do so by printing an additional line of version info for each library you want to mention. Use the same format for these lines as for the first line.
Please do not mention all of the libraries that the program uses “just for completeness”—that would produce a lot of unhelpful clutter. Please mention library version numbers only if you find in practice that they are very important to you in debugging.
The following line, after the version number line or lines, should be a copyright notice. If more than one copyright notice is called for, put each on a separate line.
Next should follow a line stating the license, preferably using one of abbreviations below, and a brief statement that the program is free software, and that users are free to copy and change it. Also mention that there is no warranty, to the extent permitted by law. See recommended wording below.
It is ok to finish the output with a list of the major authors of the program, as a way of giving credit.
Here’s an example of output that follows these rules:
GNU hello 2.3 Copyright (C) 2007 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <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 in Information for GNU Maintainers.)
Translations of the above lines must preserve the validity of the copyright notices (see Internationalization). If the translation’s character set supports it, the ‘(C)’ should be replaced with the copyright symbol, as follows:
©
Write the word “Copyright” exactly like that, in English. Do not translate it into another language. International treaties recognize the English word “Copyright”; translations into other languages do not have legal significance.
Finally, here is the table of our suggested license abbreviations. Any abbreviation can be followed by ‘vversion[+]’, meaning that particular version, or later versions with the ‘+’, as shown above. In the case of a GNU license, always indicate the permitted versions in this way.
In the case of exceptions for extra permissions with the GPL, we use ‘/’ for a separator; the version number can follow the license abbreviation as usual, as in the examples below.
GNU General Public License, http://www.gnu.org/licenses/gpl.html.
GNU Lesser General Public License, http://www.gnu.org/licenses/lgpl.html.
GNU GPL with the exception for Ada.
The Apache Software Foundation license, http://www.apache.org/licenses.
The Artistic license used for Perl, http://dev.perl.org/licenses/artistic.html.
The Expat license, http://www.jclark.com/xml/copying.txt.
The Mozilla Public License, http://www.mozilla.org/MPL/.
The original (4-clause) BSD license, incompatible with the GNU GPL http://www.xfree86.org/3.3.6/COPYRIGHT2.html#6.
The license used for PHP, http://www.php.net/license/.
The non-license that is being in the public domain, http://www.gnu.org/licenses/license-list.html#PublicDomain.
The license for Python, http://directory.fsf.org/wiki?title=License:Python2.0.1.
The revised (3-clause) BSD, compatible with the GNU GPL,
http://www.xfree86.org/3.3.6/COPYRIGHT2.html#5.
The simple non-copyleft license used for most versions of the X Window System, http://www.xfree86.org/3.3.6/COPYRIGHT2.html#3.
The license for Zlib, http://www.gzip.org/zlib/zlib_license.html.
More information about these licenses and many more are on the GNU licensing web pages, http://www.gnu.org/licenses/license-list.html.
Previous: --version, Up: Command-Line Interfaces [Contents][Index]
The standard --help
option should output brief documentation
for how to invoke the program, on standard output, then exit
successfully. Other options and arguments should be ignored once this
is seen, and the program should not perform its normal function.
Near the end of the ‘--help’ option’s output, please place lines
giving the email address for bug reports, the package’s home page
(normally ‘http://www.gnu.org/software/pkg
’, and the
general page for help using GNU programs. The format should be like this:
Report bugs to: mailing-address pkg home page: <http://www.gnu.org/software/pkg/> General help using GNU software: <http://www.gnu.org/gethelp/>
It is ok to mention other appropriate mailing lists and web pages.
Next: Option Table, Previous: Command-Line Interfaces, Up: Program Behavior [Contents][Index]
Another aspect of keeping free programs free is encouraging development of free plug-ins, and discouraging development of proprietary plug-ins. Many GNU programs will not have anything like plug-ins at all, but those that do should follow these practices.
First, the general plug-in architecture design should closely tie the plug-in to the original code, such that the plug-in and the base program are parts of one extended program. For GCC, for example, plug-ins receive and modify GCC’s internal data structures, and so clearly form an extended program with the base GCC.
Second, you should require plug-in developers to affirm that their
plug-ins are released under an appropriate license. This should be
enforced with a simple programmatic check. For GCC, again for
example, a plug-in must define the global symbol
plugin_is_GPL_compatible
, thus asserting that the plug-in is
released under a GPL-compatible license (see Plugins in GCC Internals).
By adding this check to your program you are not creating a new legal requirement. The GPL itself requires plug-ins to be free software, licensed compatibly. As long as you have followed the first rule above to keep plug-ins closely tied to your original program, the GPL and AGPL already require those plug-ins to be released under a compatible license. The symbol definition in the plug-in—or whatever equivalent works best in your program—makes it harder for anyone who might distribute proprietary plug-ins to legally defend themselves. If a case about this got to court, we can point to that symbol as evidence that the plug-in developer understood that the license had this requirement.
Next: OID Allocations, Previous: Dynamic Plug-In Interfaces, Up: Program Behavior [Contents][Index]
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<EMAIL_ADDRESS>a list of them, with their meanings, so we can update the table.
‘-N’ in tar
.
‘-a’ in du
, ls
, nm
, stty
, uname
,
and unexpand
.
‘-a’ in diff
.
‘-A’ in ls
.
‘-a’ in etags
, tee
, time
;
‘-r’ in tar
.
‘-a’ in cp
.
‘-n’ in shar
.
‘-l’ in m4
.
‘-a’ in diff
.
‘-v’ in gawk
.
‘-W’ in make
.
‘-o’ in make
.
‘-a’ in recode
.
‘-a’ in wdiff
.
‘-A’ in ptx
.
‘-n’ in wdiff
.
For server programs, run in the background.
‘-B’ in ctags
.
‘-f’ in shar
.
Used in GDB.
Used in GDB.
‘-b’ in tac
.
‘-b’ in cpio
and diff
.
‘-b’ in shar
.
Used in cpio
and tar
.
‘-b’ in head
and tail
.
‘-b’ in ptx
.
Used in various programs to make output shorter.
‘-c’ in head
, split
, and tail
.
‘-C’ in etags
.
‘-A’ in tar
.
Used in various programs to specify the directory to use.
‘-c’ in chgrp
and chown
.
‘-F’ in ls
.
‘-c’ in recode
.
‘-c’ in su
;
‘-x’ in GDB.
‘-d’ in tar
.
Used in gawk
.
‘-Z’ in tar
and shar
.
‘-A’ in tar
.
‘-w’ in tar
.
Used in diff
.
‘-W copyleft’ in gawk
.
‘-C’ in ptx
, recode
, and wdiff
;
‘-W copyright’ in gawk
.
Used in GDB.
‘-q’ in who
.
‘-l’ in du
.
Used in tar
and cpio
.
‘-c’ in shar
.
‘-x’ in ctags
.
‘-d’ in touch
.
‘-d’ in make
and m4
;
‘-t’ in Bison.
‘-D’ in m4
.
‘-d’ in Bison and ctags
.
‘-D’ in tar
.
‘-L’ in chgrp
, chown
, cpio
, du
,
ls
, and tar
.
‘-D’ in du
.
Specify an I/O device (special file name).
‘-d’ in recode
.
‘-d’ in look
.
‘-d’ in tar
.
‘-n’ in csplit
.
Specify the directory to use, in various programs. In ls
, it
means to show directories themselves rather than their contents. In
rm
and ln
, it means to not treat links to directories
specially.
‘-x’ in strip
.
‘-X’ in strip
.
‘-n’ in make
.
‘-e’ in diff
.
‘-z’ in csplit
.
‘-x’ in wdiff
.
‘-z’ in wdiff
.
‘-N’ in diff
.
‘-e’ in make
.
‘-e’ in xargs
.
Used in GDB.
Used in makeinfo
.
‘-o’ in m4
.
‘-b’ in ls
.
‘-X’ in tar
.
Used in GDB.
‘-x’ in xargs
.
‘-e’ in unshar
.
‘-t’ in diff
.
‘-e’ in sed
.
‘-g’ in nm
.
‘-i’ in cpio
;
‘-x’ in tar
.
‘-f’ in finger
.
‘-f’ in su
.
‘-E’ in m4
.
‘-f’ in gawk
, info
, make
, mt
,
sed
, and tar
.
‘-F’ in gawk
.
‘-b’ in Bison.
‘-F’ in ls
.
‘-T’ in tar
.
Used in makeinfo
.
‘-F’ in ptx
.
‘-y’ in Bison.
‘-f’ in tail
.
Used in makeinfo
.
‘-f’ in cp
, ln
, mv
, and rm
.
‘-F’ in shar
.
For server programs, run in the foreground; in other words, don’t do anything special to run the server in the background.
Used in ls
, time
, and ptx
.
‘-F’ in m4
.
Used in GDB.
‘-g’ in ptx
.
‘-x’ in tar
.
‘-i’ in ul
.
‘-g’ in recode
.
‘-g’ in install
.
‘-z’ in tar
and shar
.
‘-H’ in m4
.
‘-h’ in objdump
and recode
‘-H’ in who
.
Used to ask for brief usage information.
‘-d’ in shar
.
‘-q’ in ls
.
In makeinfo
, output HTML.
‘-u’ in who
.
‘-D’ in diff
.
‘-I’ in ls
;
‘-x’ in recode
.
‘-w’ in diff
.
‘-B’ in ls
.
‘-B’ in diff
.
‘-f’ in look
and ptx
;
‘-i’ in diff
and wdiff
.
‘-i’ in make
.
‘-i’ in ptx
.
‘-I’ in etags
.
‘-f’ in Oleo.
‘-i’ in tee
.
‘-I’ in diff
.
‘-b’ in diff
.
‘-i’ in tar
.
‘-i’ in etags
;
‘-I’ in m4
.
‘-I’ in make
.
‘-G’ in tar
.
‘-i’, ‘-l’, and ‘-m’ in Finger.
In some programs, specify the name of the file to read as the user’s init file.
‘-i’ in expand
.
‘-T’ in diff
.
‘-i’ in ls
.
‘-i’ in cp
, ln
, mv
, rm
;
‘-e’ in m4
;
‘-p’ in xargs
;
‘-w’ in tar
.
‘-p’ in shar
.
Used in date
‘-j’ in make
.
‘-n’ in make
.
‘-k’ in make
.
‘-k’ in csplit
.
‘-k’ in du
and ls
.
‘-l’ in etags
.
‘-l’ in wdiff
.
‘-g’ in shar
.
‘-C’ in split
.
Used in split
, head
, and tail
.
‘-l’ in cpio
.
Used in gawk
.
‘-t’ in cpio
;
‘-l’ in recode
.
‘-t’ in tar
.
‘-N’ in ls
.
‘-l’ in make
.
Used in su
.
Used in uname
.
‘-M’ in ptx
.
‘-m’ in hello
and uname
.
‘-d’ in cpio
.
‘-f’ in make
.
Used in GDB.
‘-n’ in xargs
.
‘-n’ in xargs
.
‘-l’ in xargs
.
‘-l’ in make
.
‘-P’ in xargs
.
‘-T’ in who
.
‘-T’ in who
.
‘-d’ in diff
.
‘-M’ in shar
.
‘-m’ in install
, mkdir
, and mkfifo
.
‘-m’ in tar
.
‘-M’ in tar
.
‘-a’ in Bison.
‘-L’ in m4
.
‘-a’ in shar
.
‘-W’ in make
.
‘-r’ in make
.
‘-w’ in shar
.
‘-x’ in shar
.
‘-3’ in wdiff
.
‘-c’ in touch
.
‘-D’ in etags
.
‘-1’ in wdiff
.
‘-d’ in cp
.
‘-2’ in wdiff
.
‘-S’ in make
.
‘-l’ in Bison.
‘-P’ in shar
.
‘-e’ in gprof
.
‘-R’ in etags
.
‘-p’ in nm
.
Don’t print a startup splash screen.
Used in makeinfo
.
‘-a’ in gprof
.
‘-E’ in gprof
.
‘-m’ in shar
.
Used in makeinfo
.
Used in emacsclient
.
Used in various programs to inhibit warnings.
‘-n’ in info
.
‘-n’ in uname
.
‘-f’ in cpio
.
‘-n’ in objdump
.
‘-0’ in xargs
.
‘-n’ in cat
.
‘-b’ in cat
.
‘-n’ in nm
.
‘-n’ in cpio
and ls
.
Used in GDB.
‘-o’ in tar
.
‘-o’ in make
.
‘-l’ in tar
, cp
, and du
.
‘-o’ in ptx
.
‘-f’ in gprof
.
‘-F’ in gprof
.
‘-o’ in getopt
, fdlist
, fdmount
,
fdmountd
, and fdumount
.
In various programs, specify the output file name.
‘-o’ in shar
.
‘-o’ in rm
.
‘-c’ in unshar
.
‘-o’ in install
.
‘-l’ in diff
.
Used in makeinfo
.
‘-p’ in mkdir
and rmdir
.
‘-p’ in ul
.
‘-p’ in cpio
.
‘-P’ in finger
.
‘-c’ in cpio
and tar
.
Used in gawk
.
‘-P’ in m4
.
‘-f’ in csplit
.
Used in tar
and cp
.
‘-p’ in su
.
‘-m’ in cpio
.
‘-s’ in tar
.
‘-p’ in tar
.
‘-l’ in diff
.
‘-L’ in cmp
.
‘-p’ in make
.
‘-w’ in make
.
‘-o’ in nm
.
‘-s’ in nm
.
‘-p’ in wdiff
.
‘-p’ in ed
.
Specify an HTTP proxy.
‘-X’ in shar
.
‘-q’ in make
.
Used in many programs to inhibit the usual output. Every program accepting ‘--quiet’ should accept ‘--silent’ as a synonym.
‘-Q’ in shar
‘-Q’ in ls
.
‘-n’ in diff
.
Used in gawk
.
‘-B’ in tar
.
Used in GDB.
‘-n’ in make
.
‘-R’ in tar
.
Used in chgrp
, chown
, cp
, ls
, diff
,
and rm
.
‘-r’ in touch
.
‘-r’ in ptx
.
‘-r’ in tac
and etags
.
‘-r’ in uname
.
‘-R’ in m4
.
‘-r’ in objdump
.
‘-r’ in cpio
.
‘-i’ in xargs
.
‘-s’ in diff
.
‘-a’ in cpio
.
‘-r’ in ls
and nm
.
‘-f’ in diff
.
‘-R’ in ptx
.
‘-s’ in tar
.
‘-p’ in tar
.
‘-g’ in stty
.
Used in GDB.
‘-S’ in ptx
.
‘-S’ in du
.
‘-s’ in tac
.
Used by recode
to chose files or pipes for sequencing passes.
‘-s’ in su
.
‘-A’ in cat
.
‘-p’ in diff
.
‘-E’ in cat
.
‘-F’ in diff
.
‘-T’ in cat
.
Used in many programs to inhibit the usual output. Every program accepting ‘--silent’ should accept ‘--quiet’ as a synonym.
‘-s’ in ls
.
Specify a file descriptor for a network server to use for its socket, instead of opening and binding a new socket. This provides a way to run, in a non-privileged process, a server that normally needs a reserved port number.
Used in ls
.
‘-W source’ in gawk
.
‘-S’ in tar
.
‘-H’ in diff
.
‘-E’ in unshar
.
‘-L’ in shar
.
‘-s’ in cat
.
‘-w’ in wdiff
.
‘-y’ in wdiff
.
Used in tar
and diff
to specify which file within
a directory to start processing with.
‘-s’ in wdiff
.
‘-S’ in shar
.
‘-S’ in make
.
‘-s’ in recode
.
‘-s’ in install
.
‘-s’ in strip
.
‘-S’ in strip
.
‘-s’ in shar
.
‘-S’ in cp
, ln
, mv
.
‘-b’ in csplit
.
‘-s’ in gprof
.
‘-s’ in du
.
‘-s’ in ln
.
Used in GDB and objdump
.
‘-s’ in m4
.
‘-s’ in uname
.
‘-t’ in expand
and unexpand
.
‘-T’ in ls
.
‘-T’ in tput
and ul
.
‘-t’ in wdiff
.
‘-a’ in diff
.
‘-T’ in shar
.
Used in ls
and touch
.
Specify how long to wait before giving up on some operation.
‘-O’ in tar
.
‘-c’ in du
.
‘-t’ in make
, ranlib
, and recode
.
‘-t’ in m4
.
‘-t’ in hello
;
‘-W traditional’ in gawk
;
‘-G’ in ed
, m4
, and ptx
.
Used in GDB.
‘-t’ in ctags
.
‘-T’ in ctags
.
‘-t’ in ptx
.
‘-z’ in tar
.
‘-u’ in cpio
.
‘-U’ in m4
.
‘-u’ in nm
.
‘-u’ in cp
, ctags
, mv
, tar
.
Used in gawk
; same as ‘--help’.
‘-B’ in shar
.
‘-V’ in shar
.
Print more information about progress. Many programs support this.
‘-W’ in tar
.
Print the version number.
‘-V’ in cp
, ln
, mv
.
‘-v’ in ctags
.
‘-V’ in tar
.
‘-W’ in make
.
‘-l’ in shar
.
‘-w’ in ls
and ptx
.
‘-W’ in ptx
.
‘-T’ in who
.
‘-z’ in gprof
.
Next: Memory Usage, Previous: Option Table, Up: Program Behavior [Contents][Index]
The OID (object identifier) <IP_ADDRESS>.4.1.11591 has been assigned to the GNU Project (thanks to Sergey Poznyakoff). These are used for SNMP, LDAP, X.509 certificates, and so on. The web site http://www.alvestrand.no/objectid has a (voluntary) listing of many OID assignments.
If you need a new slot for your GNU package, write<EMAIL_ADDRESS>Here is a list of arcs currently assigned:
<IP_ADDRESS>.4.1.11591 GNU <IP_ADDRESS>.4.1.11591.1 GNU Radius <IP_ADDRESS>.4.1.11591.2 GnuPG <IP_ADDRESS>.4.1.11591.2.1 notation <IP_ADDRESS>.4.1.115<IP_ADDRESS> pkaAddress <IP_ADDRESS>.4.1.11591.3 GNU Radar <IP_ADDRESS>.4.1.11591.4 GNU GSS <IP_ADDRESS>.4.1.11591.5 GNU Mailutils <IP_ADDRESS>.4.1.11591.6 GNU Shishi <IP_ADDRESS>.4.1.11591.7 GNU Radio <IP_ADDRESS>.4.1.11591.8 GNU Dico <IP_ADDRESS>.4.1.11591.9 GNU Rush <IP_ADDRESS>.4.1.11591.12 digestAlgorithm <IP_ADDRESS>.4.1.11591.12.2 TIGER/192 <IP_ADDRESS>.4.1.11591.13 encryptionAlgorithm <IP_ADDRESS>.4.1.11591.13.2 Serpent <IP_ADDRESS>.4.1.115<IP_ADDRESS> Serpent-128-ECB <IP_ADDRESS>.4.1.115<IP_ADDRESS> Serpent-128-CBC <IP_ADDRESS>.4.1.115<IP_ADDRESS> Serpent-128-OFB <IP_ADDRESS>.4.1.115<IP_ADDRESS> Serpent-128-CFB <IP_ADDRESS>.4.1.115<IP_ADDRESS> Serpent-192-ECB <IP_ADDRESS>.4.1.115<IP_ADDRESS> Serpent-192-CBC <IP_ADDRESS>.4.1.115<IP_ADDRESS> Serpent-192-OFB <IP_ADDRESS>.4.1.115<IP_ADDRESS> Serpent-192-CFB <IP_ADDRESS>.4.1.115<IP_ADDRESS> Serpent-256-ECB <IP_ADDRESS>.4.1.115<IP_ADDRESS> Serpent-256-CBC <IP_ADDRESS>.4.1.115<IP_ADDRESS> Serpent-256-OFB <IP_ADDRESS>.4.1.115<IP_ADDRESS> Serpent-256-CFB <IP_ADDRESS>.4.1.11591.14 CRC algorithms <IP_ADDRESS>.4.1.11591.14.1 CRC 32 <IP_ADDRESS>.4.1.11591.15 ellipticCurve <IP_ADDRESS>.4.1.11591.15.1 Ed25519
Next: File Usage, Previous: OID Allocations, Up: Program Behavior [Contents][Index]
If a program typically uses just a few meg of memory, don’t bother making any effort to reduce memory usage. For example, if it is impractical for other reasons to operate on files more than a few meg long, it is reasonable to read entire input files into memory to operate on them.
However, for programs such as cat
or tail
, that can
usefully operate on very large files, it is important to avoid using a
technique that would artificially limit the size of files it can handle.
If a program works by lines and could be applied to arbitrary
user-supplied input files, it should keep only a line in memory, because
this is not very hard and users will want to be able to operate on input
files that are bigger than will fit in memory all at once.
If your program creates complicated data structures, just make them in
memory and give a fatal error if malloc
returns NULL
.
Memory analysis tools such as valgrind
can be useful, but
don’t complicate a program merely to avoid their false alarms. For
example, if memory is used until just before a process exits, don’t
free it simply to silence such a tool.
Previous: Memory Usage, Up: Program Behavior [Contents][Index]
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.
Next: Documentation, Previous: Program Behavior, Up: Top [Contents][Index]
This chapter provides advice on how best to use the C language when writing GNU software.
| • Formatting: | Formatting your source code. | |
| • Comments: | Commenting your work. | |
| • Syntactic Conventions: | Clean use of C constructs. | |
| • Names: | Naming variables, functions, and files. | |
| • System Portability: | Portability among different operating systems. | |
| • CPU Portability: | Supporting the range of CPU types. | |
| • System Functions: | Portability and “standard” library functions. | |
| • Internationalization: | Techniques for internationalization. | |
| • Character Set: | Use ASCII by default. | |
| • Quote Characters: | Use "..." or ’...’ in the C locale. | |
| • Mmap: | How you can safely use mmap .
|
Please keep the length of source lines to 79 characters or less, for maximum readability in the widest range of environments.
It is important to put the open-brace that starts the body of a C function in column one, so that they will start a defun. Several tools look for open-braces in column one to find the beginnings of C functions. These tools will not work on code not formatted that way.
Avoid putting open-brace, open-parenthesis or open-bracket in column
one when they are inside a function, so that they won’t start a defun.
The open-brace that starts a struct
body can go in column one
if you find it useful to treat that definition as a defun.
It is also important for function definitions to start the name of the function in column one. This helps people to search for function definitions, and may also help certain tools recognize them. Thus, using Standard C syntax, the format is this:
static char * concat (char *s1, char *s2) { … }
or, if you want to use traditional C syntax, format the definition like this:
static char * concat (s1, s2) /* Name starts in column one here */ char *s1, *s2; { /* Open brace in column one here */ … }
In Standard C, if the arguments don’t fit nicely on one line, split it like this:
int lots_of_args (int an_integer, long a_long, short a_short, double a_double, float a_float) …
For struct
and enum
types, likewise put the braces in
column one, unless the whole contents fits on one line:
struct foo { int a, b; }
or
struct foo { int a, b; }
The rest of this section gives our recommendations for other aspects of
C formatting style, which is also the default style of the indent
program in version 1.2 and newer. It corresponds to the options
-nbad -bap -nbc -bbo -bl -bli2 -bls -ncdb -nce -cp1 -cs -di2 -ndj -nfc1 -nfca -hnl -i2 -ip5 -lp -pcs -psl -nsc -nsob
We don’t think of these recommendations as requirements, because it causes no problems for users if two different programs have different formatting styles.
But whatever style you use, please use it consistently, since a mixture of styles within one program tends to look ugly. If you are contributing changes to an existing program, please follow the style of that program.
For the body of the function, our recommended style looks like this:
if (x < foo (y, z)) haha = bar[4] + 5; else { while (z) { haha += foo (z, z); z--; } return ++x + bar (); }
We find it easier to read a program when it has spaces before the open-parentheses and after the commas. Especially after the commas.
When you split an expression into multiple lines, split it before an operator, not after one. Here is the right way:
if (foo_this_is_long && bar > win (x, y, z) && remaining_condition)
Try to avoid having two operators of different precedence at the same level of indentation. For example, don’t write this:
mode = (inmode[j] == VOIDmode || GET_MODE_SIZE (outmode[j]) > GET_MODE_SIZE (inmode[j]) ? outmode[j] : inmode[j]);
Instead, use extra parentheses so that the indentation shows the nesting:
mode = ((inmode[j] == VOIDmode || (GET_MODE_SIZE (outmode[j]) > GET_MODE_SIZE (inmode[j]))) ? outmode[j] : inmode[j]);
Insert extra parentheses so that Emacs will indent the code properly. For example, the following indentation looks nice if you do it by hand,
v = rup->ru_utime.tv_sec*1000 + rup->ru_utime.tv_usec/1000 + rup->ru_stime.tv_sec*1000 + rup->ru_stime.tv_usec/1000;
but Emacs would alter it. Adding a set of parentheses produces something that looks equally nice, and which Emacs will preserve:
v = (rup->ru_utime.tv_sec*1000 + rup->ru_utime.tv_usec/1000 + rup->ru_stime.tv_sec*1000 + rup->ru_stime.tv_usec/1000);
Format do-while statements like this:
do { a = foo (a); } while (a > 0);
Please use formfeed characters (control-L) to divide the program into pages at logical places (but not within a function). It does not matter just how long the pages are, since they do not have to fit on a printed page. The formfeeds should appear alone on lines by themselves.
Next: Syntactic Conventions, Previous: Formatting, Up: Writing C [Contents][Index]
Every program should start with a comment saying briefly what it is for. Example: ‘fmt - filter for simple filling of text’. This comment should be at the top of the source file containing the ‘main’ function of the program.
Also, please write a brief comment at the start of each source file, with the file name and a line or two about the overall purpose of the file.
Please write the comments in a GNU program in English, because English is the one language that nearly all programmers in all countries can read. If you do not write English well, please write comments in English as well as you can, then ask other people to help rewrite them. If you can’t write comments in English, please find someone to work with you and translate your comments into English.
Please put a comment on each function saying what the function does,
what sorts of arguments it gets, and what the possible values of
arguments mean and are used for. It is not necessary to duplicate in
words the meaning of the C argument declarations, if a C type is being
used in its customary fashion. If there is anything nonstandard about
its use (such as an argument of type char *
which is really the
address of the second character of a string, not the first), or any
possible values that would not work the way one would expect (such as,
that strings containing newlines are not guaranteed to work), be sure
to say so.
Also explain the significance of the return value, if there is one.
Please put two spaces after the end of a sentence in your comments, so that the Emacs sentence commands will work. Also, please write complete sentences and capitalize the first word. If a lower-case identifier comes at the beginning of a sentence, don’t capitalize it! Changing the spelling makes it a different identifier. If you don’t like starting a sentence with a lower case letter, write the sentence differently (e.g., “The identifier lower-case is …”).
The comment on a function is much clearer if you use the argument names to speak about the argument values. The variable name itself should be lower case, but write it in upper case when you are speaking about the value rather than the variable itself. Thus, “the inode number NODE_NUM” rather than “an inode”.
There is usually no purpose in restating the name of the function in the comment before it, because readers can see that for themselves. There might be an exception when the comment is so long that the function itself would be off the bottom of the screen.
There should be a comment on each static variable as well, like this:
/* Nonzero means truncate lines in the display; zero means continue them. */ int truncate_lines;
Every ‘#endif’ should have a comment, except in the case of short conditionals (just a few lines) that are not nested. The comment should state the condition of the conditional that is ending, including its sense. ‘#else’ should have a comment describing the condition and sense of the code that follows. For example:
#ifdef foo … #else /* not foo */ … #endif /* not foo */
#ifdef foo … #endif /* foo */
but, by contrast, write the comments this way for a ‘#ifndef’:
#ifndef foo … #else /* foo */ … #endif /* foo */
#ifndef foo … #endif /* not foo */
Please explicitly declare the types of all objects. For example, you
should explicitly declare all arguments to functions, and you should
declare functions to return int
rather than omitting the
int
.
Some programmers like to use the GCC ‘-Wall’ option, and change the code whenever it issues a warning. If you want to do this, then do. Other programmers prefer not to use ‘-Wall’, because it gives warnings for valid and legitimate code which they do not want to change. If you want to do this, then do. The compiler should be your servant, not your master.
Don’t make the program ugly just to placate static analysis tools such
as lint
, clang
, and GCC with extra warnings
options such as -Wconversion and -Wundef. These
tools can help find bugs and unclear code, but they can also generate
so many false alarms that it hurts readability to silence them with
unnecessary casts, wrappers, and other complications. For example,
please don’t insert casts to void
or calls to do-nothing
functions merely to pacify a lint checker.
Declarations of external functions and functions to appear later in the
source file should all go in one place near the beginning of the file
(somewhere before the first function definition in the file), or else
should go in a header file. Don’t put extern
declarations inside
functions.
It used to be common practice to use the same local variables (with
names like tem
) over and over for different values within one
function. Instead of doing this, it is better to declare a separate local
variable for each distinct purpose, and give it a name which is
meaningful. This not only makes programs easier to understand, it also
facilitates optimization by good compilers. You can also move the
declaration of each local variable into the smallest scope that includes
all its uses. This makes the program even cleaner.
Don’t use local variables or parameters that shadow global identifiers. GCC’s ‘-Wshadow’ option can detect this problem.
Don’t declare multiple variables in one declaration that spans lines. Start a new declaration on each line, instead. For example, instead of this:
int foo, bar;
write either this:
int foo, bar;
or this:
int foo; int bar;
(If they are global variables, each should have a comment preceding it anyway.)
When you have an if
-else
statement nested in another
if
statement, always put braces around the if
-else
.
Thus, never write like this:
if (foo) if (bar) win (); else lose ();
always like this:
if (foo) { if (bar) win (); else lose (); }
If you have an if
statement nested inside of an else
statement, either write else if
on one line, like this,
if (foo) … else if (bar) …
with its then
-part indented like the preceding then
-part,
or write the nested if
within braces like this:
if (foo) … else { if (bar) … }
Don’t declare both a structure tag and variables or typedefs in the same declaration. Instead, declare the structure tag separately and then use it to declare the variables or typedefs.
Try to avoid assignments inside if
-conditions (assignments
inside while
-conditions are ok). For example, don’t write
this:
if ((foo = (char *) malloc (sizeof *foo)) == NULL) fatal ("virtual memory exhausted");
instead, write this:
foo = (char *) malloc (sizeof *foo); if (foo == NULL) fatal ("virtual memory exhausted");
Next: System Portability, Previous: Syntactic Conventions, Up: Writing C [Contents][Index]
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.
Next: CPU Portability, Previous: Names, Up: Writing C [Contents][Index]
In the Unix world, “portability” refers to porting to different Unix versions. For a GNU program, this kind of portability is desirable, but not paramount.
The primary purpose of GNU software is to run on top of the GNU kernel, compiled with the GNU C compiler, on various types of CPU. So the kinds of portability that are absolutely necessary are quite limited. But it is important to support Linux-based GNU systems, since they are the form of GNU that is popular.
Beyond that, it is good to support the other free operating systems (*BSD), and it is nice to support other Unix-like systems if you want to. Supporting a variety of Unix-like systems is desirable, although not paramount. It is usually not too hard, so you may as well do it. But you don’t have to consider it an obligation, if it does turn out to be hard.
The easiest way to achieve portability to most Unix-like systems is to use Autoconf. It’s unlikely that your program needs to know more information about the host platform than Autoconf can provide, simply because most of the programs that need such knowledge have already been written.
Avoid using the format of semi-internal data bases (e.g., directories)
when there is a higher-level alternative (readdir
).
As for systems that are not like Unix, such as MSDOS, Windows, VMS, MVS, and older Macintosh systems, supporting them is often a lot of work. When that is the case, it is better to spend your time adding features that will be useful on GNU and GNU/Linux, rather than on supporting other incompatible systems.
If you do support Windows, please do not abbreviate it as “win”. In
hacker terminology, calling something a “win” is a form of praise.
You’re free to praise Microsoft Windows on your own if you want, but
please don’t do this in GNU packages. Instead of abbreviating
“Windows” to “win”, you can write it in full or abbreviate it to
“woe” or “w”. In GNU Emacs, for instance, we use ‘w32’ in
file names of Windows-specific files, but the macro for Windows
conditionals is called WINDOWSNT
.
It is a good idea to define the “feature test macro”
_GNU_SOURCE
when compiling your C files. When you compile on GNU
or GNU/Linux, this will enable the declarations of GNU library extension
functions, and that will usually give you a compiler error message if
you define the same function names in some other way in your program.
(You don’t have to actually use these functions, if you prefer
to make the program more portable to other systems.)
But whether or not you use these GNU extensions, you should avoid using their names for any other meanings. Doing so would make it hard to move your code into other GNU programs.
Next: System Functions, Previous: System Portability, Up: Writing C [Contents][Index]
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); }
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.
Next: Internationalization, Previous: CPU Portability, Up: Writing C [Contents][Index]
Historically, C implementations differed substantially, and many systems lacked a full implementation of ANSI/ISO C89. Nowadays, however, all practical systems have a C89 compiler and GNU C supports almost all of C99 and some of C11. Similarly, most systems implement POSIX.1-2001 libraries and tools, and many have POSIX.1-2008.
Hence, there is little reason to support old C or non-POSIX systems, and you may want to take advantage of standard C and POSIX to write clearer, more portable, or faster code. You should use standard interfaces where possible; but if GNU extensions make your program more maintainable, powerful, or otherwise better, don’t hesitate to use them. In any case, don’t make your own declaration of system functions; that’s a recipe for conflict.
Despite the standards, nearly every library function has some sort of portability issue on some system or another. Here are some examples:
open
Names with trailing /
’s are mishandled on many platforms.
printf
long double
may be unimplemented; floating values Infinity and
NaN are often mishandled; output for large precisions may be
incorrect.
readlink
May return int
instead of ssize_t
.
scanf
On Windows, errno
is not set on failure.
Gnulib is a big help in this regard. Gnulib provides implementations of standard interfaces on many of the systems that lack them, including portable implementations of enhanced GNU interfaces, thereby making their use portable, and of POSIX-1.2008 interfaces, some of which are missing even on up-to-date GNU systems.
Gnulib also provides many useful non-standard interfaces; for example,
C implementations of standard data structures (hash tables, binary
trees), error-checking type-safe wrappers for memory allocation
functions (xmalloc
, xrealloc
), and output of error
messages.
Gnulib integrates with GNU Autoconf and Automake to remove much of the burden of writing portable code from the programmer: Gnulib makes your configure script automatically determine what features are missing and use the Gnulib code to supply the missing pieces.
The Gnulib and Autoconf manuals have extensive sections on portability: Introduction in Gnulib and see Portable C and C++ in Autoconf. Please consult them for many more details.
Next: Character Set, Previous: System Functions, Up: Writing C [Contents][Index]
GNU has a library called GNU gettext that makes it easy to translate the messages in a program into various languages. You should use this library in every program. Use English for the messages as they appear in the program, and let gettext provide the way to translate them into other languages.
Using GNU gettext involves putting a call to the gettext
macro
around each string that might need translation—like this:
printf (gettext ("Processing file '%s'..."), file);
This permits GNU gettext to replace the string "Processing file
'%s'..."
with a translated version.
Once a program uses gettext, please make a point of writing calls to
gettext
when you add new strings that call for translation.
Using GNU gettext in a package involves specifying a text domain name for the package. The text domain name is used to separate the translations for this package from the translations for other packages. Normally, the text domain name should be the same as the name of the package—for example, ‘coreutils’ for the GNU core utilities.
To enable gettext to work well, avoid writing code that makes assumptions about the structure of words or sentences. When you want the precise text of a sentence to vary depending on the data, use two or more alternative string constants each containing a complete sentences, rather than inserting conditionalized words or phrases into a single sentence framework.
Here is an example of what not to do:
printf ("%s is full", capacity > 5000000 ? "disk" : "floppy disk");
If you apply gettext to all strings, like this,
printf (gettext ("%s is full"), capacity > 5000000 ? gettext ("disk") : gettext ("floppy disk"));
the translator will hardly know that "disk" and "floppy disk" are meant to be substituted in the other string. Worse, in some languages (like French) the construction will not work: the translation of the word "full" depends on the gender of the first part of the sentence; it happens to be not the same for "disk" as for "floppy disk".
Complete sentences can be translated without problems:
printf (capacity > 5000000 ? gettext ("disk is full") : gettext ("floppy disk is full"));
A similar problem appears at the level of sentence structure with this code:
printf ("# Implicit rule search has%s been done.\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);
Next: Quote Characters, Previous: Internationalization, Up: Writing C [Contents][Index]
Sticking to the ASCII character set (plain text, 7-bit characters) is preferred in GNU source code comments, text documents, and other contexts, unless there is good reason to do something else because of the application domain. For example, if source code deals with the French Revolutionary calendar, it is OK if its literal strings contain accented characters in month names like “Floréal”. Also, it is OK (but not required) to use non-ASCII characters to represent proper names of contributors in change logs (see Change Logs).
If you need to use non-ASCII characters, you should normally stick with one encoding, certainly within a single file. UTF-8 is likely to be the best choice.
Next: Mmap, Previous: Character Set, Up: Writing C [Contents][Index]
In the C locale, the output of GNU programs should stick to plain ASCII for quotation characters in messages to users: preferably 0x22 (‘"’) or 0x27 (‘'’) for both opening and closing quotes. Although GNU programs traditionally used 0x60 (‘`’) for opening and 0x27 (‘'’) for closing quotes, nowadays quotes ‘`like this'’ are typically rendered asymmetrically, so quoting ‘"like this"’ or ‘'like this'’ typically looks better.
It is ok, but not required, for GNU programs to generate locale-specific quotes in non-C locales. For example:
printf (gettext ("Processing file '%s'..."), file);
Here, a French translation might cause gettext
to return the
string "Traitement de fichier
‹ %s ›..."
, yielding quotes
more appropriate for a French locale.
Sometimes a program may need to use opening and closing quotes
directly. By convention, gettext
translates the string
‘"`"’ to the opening quote and the string ‘"'"’ to the
closing quote, and a program can use these translations. Generally,
though, it is better to translate quote characters in the context of
longer strings.
If the output of your program is ever likely to be parsed by another
program, it is good to provide an option that makes this parsing
reliable. For example, you could escape special characters using
conventions from the C language or the Bourne shell. See for example
the option --quoting-style of GNU ls
.
Previous: Quote Characters, Up: Writing C [Contents][Index]
If you use mmap
to read or write files, don’t assume it either
works on all files or fails for all files. It may work on some files
and fail on others.
The proper way to use mmap
is to try it on the specific file for
which you want to use it—and if mmap
doesn’t work, fall back on
doing the job in another way using read
and write
.
The reason this precaution is needed is that the GNU kernel (the HURD)
provides a user-extensible file system, in which there can be many
different kinds of “ordinary files”. Many of them support
mmap
, but some do not. It is important to make programs handle
all these kinds of files.
Next: Managing Releases, Previous: Writing C, Up: Top [Contents][Index]
A GNU program should ideally come with full free documentation, adequate for both reference and tutorial purposes. If the package can be programmed or extended, the documentation should cover programming or extending it, as well as just using it.
| • GNU Manuals: | Writing proper manuals. | |
| • Doc Strings and Manuals: | Compiling doc strings doesn’t make a manual. | |
| • Manual Structure Details: | Specific structure conventions. | |
| • License for Manuals: | Writing the distribution terms for a manual. | |
| • Manual Credits: | Giving credit to documentation contributors. | |
| • Printed Manuals: | Mentioning the printed manual. | |
| • NEWS File: | NEWS files supplement manuals. | |
| • Change Logs: | Recording changes. | |
| • Man Pages: | Man pages are secondary. | |
| • Reading other Manuals: | How far you can go in learning from other manuals. |
Next: Doc Strings and Manuals, Up: Documentation [Contents][Index]
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 lists all the functions, variables, options, and important concepts that are part of the program. One combined Index should do for a short manual, but sometimes for a complex package it is better to use multiple indices. The Texinfo manual includes advice on preparing good index entries, see Making Index Entries in GNU Texinfo, and see Defining the Entries of an Index in GNU Texinfo.
Don’t use Unix man pages as a model for how to write GNU documentation; most of them are terse, badly structured, and give inadequate explanation of the underlying concepts. (There are, of course, some exceptions.) Also, Unix man pages use a particular format which is different from what we use in GNU manuals.
Please include an email address in the manual for where to report bugs in the text of the manual.
Please do not use the term “pathname” that is used in Unix documentation; use “file name” (two words) instead. We use the term “path” only for search paths, which are lists of directory names.
Please do not use the term “illegal” to refer to erroneous input to a computer program. Please use “invalid” for this, and reserve the term “illegal” for activities prohibited by law.
Please do not write ‘()’ after a function name just to indicate
it is a function. foo ()
is not a function, it is a function
call with no arguments.
Whenever possible, please stick to the active voice, avoiding the
passive, and use the present tense, not the future teste. For
instance, write “The function foo
returns a list containing
a and b” rather than “A list containing a and
b will be returned.” One advantage of the active voice is it
requires you to state the subject of the sentence; with the passive
voice, you might omit the subject, which leads to vagueness.
It is proper to use the future tense when grammar demands it, as in, “If you type x, the computer will self-destruct in 10 seconds.”
Next: Manual Structure Details, Previous: GNU Manuals, Up: Documentation [Contents][Index]
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.
Next: License for Manuals, Previous: Doc Strings and Manuals, Up: Documentation [Contents][Index]
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.
Next: Manual Credits, Previous: Manual Structure Details, Up: Documentation [Contents][Index]
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.
Next: Printed Manuals, Previous: License for Manuals, Up: Documentation [Contents][Index]
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.
Next: NEWS File, Previous: Manual Credits, Up: Documentation [Contents][Index]
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.
Next: Change Logs, Previous: Printed Manuals, Up: Documentation [Contents][Index]
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.
Next: Man Pages, Previous: NEWS File, Up: Documentation [Contents][Index]
Keep a change log to describe all the changes made to program source files. The purpose of this is so that people investigating bugs in the future will know about the changes that might have introduced the bug. Often a new bug can be found by looking at what was recently changed. More importantly, change logs can help you eliminate conceptual inconsistencies between different parts of a program, by giving you a history of how the conflicting concepts arose and who they came from.
| • Change Log Concepts: | ||
| • Style of Change Logs: | ||
| • Simple Changes: | ||
| • Conditional Changes: | ||
| • Indicating the Part Changed: |
Next: Style of Change Logs, Up: Change Logs [Contents][Index]
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. Each entry in a change log describes either an individual change or the smallest batch of changes that belong together, also known as a change set. For later reference or for summarizing, sometimes it is useful to start the entry with a one-line description (sometimes called a title) to describe its overall purpose.
In the past, we recommended not mentioning changes in non-software files (manuals, help files, media files, etc.) in change logs. However, we’ve been advised that it is a good idea to include them, for the sake of copyright records.
The 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.
For changes to code, 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 full explanation in comments in the code, where people will see it whenever they see the code. For example, “New function” is enough for the change log when you add a function, because there should be a comment before the function definition to explain what it does, how to call it, and so on.
For changes to files that do not support a comment syntax (e.g., media files), it is ok to include the full explanation in the change log file, after the title and before the list of individual changes.
The easiest way to add an entry to ChangeLog is with the Emacs command M-x add-change-log-entry. An individual change 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.
Next: Simple Changes, Previous: Change Log Concepts, Up: Change Logs [Contents][Index]
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<EMAIL_ADDRESS>* 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. Don’t put blank lines between individual changes of an entry. You can omit the file name and the asterisk when successive individual changes 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<EMAIL_ADDRESS>* sewing.c: Make it sew.
rather than this:
2002-07-14 Usual Maintainer<EMAIL_ADDRESS>* sewing.c: Make it sew. Patch by<EMAIL_ADDRESS>As for the date, that should be the date you applied the change.
Next: Conditional Changes, Previous: Style of Change Logs, Up: Change Logs [Contents][Index]
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.
Next: Indicating the Part Changed, Previous: Simple Changes, Up: Change Logs [Contents][Index]
Source files can often contain code that is conditional to build-time
or static conditions. For example, C programs can contain
compile-time #if
conditionals; programs implemented in
interpreted languages can contain module imports of function
definitions that are only performed for certain versions of the
interpreter; and Automake Makefile.am files can contain
variable definitions or target declarations that are only to be
considered if a configure-time Automake conditional is true.
Many changes are conditional as well: sometimes you add a new variable, or function, or even a new program or library, which is entirely dependent on a build-time condition. It is useful to indicate in the change log the conditions for which a change applies.
Our convention for indicating conditional changes is to use square brackets around the name of the condition.
Conditional changes can happen in numerous scenarios and with many variations, so here are some examples to help clarify. This first example describes changes in C, Perl, and Python files which are conditional but do not have an associated function or entity name:
* xterm.c [SOLARIS2]: Include <string.h>. * FilePath.pm [$^O eq 'VMS']: Import the VMS::Feature module. * framework.py [sys.version_info < (2, 6)]: Make "with" statement available by importing it from __future__, to support also python 2.5.
Our other examples will for simplicity be limited to C, as the minor changes necessary to adapt them to other languages should be self-evident.
Next, here is an entry describing a new definition which is entirely
conditional: the C macro FRAME_WINDOW_P
is defined (and used)
only when the macro HAVE_X_WINDOWS
is defined:
* frame.h [HAVE_X_WINDOWS] (FRAME_WINDOW_P): Macro defined.
Next, 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.
Finally, here is an entry for a change that takes effect only when a certain macro is not defined:
* host.c (gethostname) [!HAVE_SOCKETS]: Replace with winsock version.
Previous: Conditional Changes, Up: Change Logs [Contents][Index]
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.
Next: Reading other Manuals, Previous: Change Logs, Up: Documentation [Contents][Index]
In the GNU project, man pages are secondary. It is not necessary or expected for every GNU program to have a man page, but some of them do. It’s your choice whether to include a man page in your program.
When you make this decision, consider that supporting a man page requires continual effort each time the program is changed. The time you spend on the man page is time taken away from more useful work.
For a simple program which changes little, updating the man page may be a small job. Then there is little reason not to include a man page, if you have one.
For a large program that changes a great deal, updating a man page may be a substantial burden. If a user offers to donate a man page, you may find this gift costly to accept. It may be better to refuse the man page unless the same person agrees to take full responsibility for maintaining it—so that you can wash your hands of it entirely. If this volunteer later ceases to do the job, then don’t feel obliged to pick it up yourself; it may be better to withdraw the man page from the distribution until someone else agrees to update it.
When a program changes only a little, you may feel that the discrepancies are small enough that the man page remains useful without updating. If so, put a prominent note near the beginning of the man page explaining that you don’t maintain it and that the Texinfo manual is more authoritative. The note should say how to access the Texinfo documentation.
Be sure that man pages include a copyright statement and free license. The simple all-permissive license is appropriate for simple man pages (see License Notices for Other Files in Information for GNU Maintainers).
For long man pages, with enough explanation and documentation that they can be considered true manuals, use the GFDL (see License for Manuals).
Finally, the GNU help2man program (http://www.gnu.org/software/help2man/) is one way to automate generation of a man page, in this case from --help output. This is sufficient in many cases.
Previous: Man Pages, Up: Documentation [Contents][Index]
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.
Next: References, Previous: Documentation, Up: Top [Contents][Index]
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.
| • Configuration: | How configuration of GNU packages should work. | |
| • Makefile Conventions: | Makefile conventions. | |
| • Releases: | Making releases |
Next: Makefile Conventions, Up: Managing Releases [Contents][Index]
Each GNU distribution should come with a shell script named
configure
. This script is given arguments which describe the
kind of machine and system you want to compile the program for.
The configure
script must record the configuration options so
that they affect compilation.
The description here is the specification of the interface for the
configure
script in GNU packages. Many packages implement it
using GNU Autoconf (see Introduction in Autoconf)
and/or GNU Automake (see Introduction in Automake),
but you do not have to use these tools. You can implement it any way
you like; for instance, by making configure
be a wrapper around
a completely different configuration system.
Another way for the configure
script to operate is to make a
link from a standard name such as config.h to the proper
configuration file for the chosen system. If you use this technique,
the distribution should not contain a file named
config.h. This is so that people won’t be able to build the
program without configuring it first.
Another thing that configure
can do is to edit the Makefile. If
you do this, the distribution should not contain a file named
Makefile. Instead, it should include a file Makefile.in which
contains the input used for editing. Once again, this is so that people
won’t be able to build the program without configuring it first.
If configure
does write the Makefile, then Makefile
should have a target named Makefile which causes configure
to be rerun, setting up the same configuration that was set up last
time. The files that configure
reads should be listed as
dependencies of Makefile.
All the files which are output from the configure
script should
have comments at the beginning explaining that they were generated
automatically using configure
. This is so that users won’t think
of trying to edit them by hand.
The configure
script should write a file named config.status
which describes which configuration options were specified when the
program was last configured. This file should be a shell script which,
if run, will recreate the same configuration.
The configure
script should accept an option of the form
‘--srcdir=dirname’ to specify the directory where sources are found
(if it is not the current directory). This makes it possible to build
the program in a separate directory, so that the actual source directory
is not modified.
If the user does not specify ‘--srcdir’, then configure
should
check both . and .. to see if it can find the sources. If
it finds the sources in one of these places, it should use them from
there. Otherwise, it should report that it cannot find the sources, and
should exit with nonzero status.
Usually the easy way to support ‘--srcdir’ is by editing a
definition of VPATH
into the Makefile. Some rules may need to
refer explicitly to the specified source directory. To make this
possible, configure
can add to the Makefile a variable named
srcdir
whose value is precisely the specified directory.
In addition, the ‘configure’ script should take options corresponding to most of the standard directory variables (see Directory Variables). Here is the list:
--prefix --exec-prefix --bindir --sbindir --libexecdir --sysconfdir --sharedstatedir --localstatedir --runstatedir --libdir --includedir --oldincludedir --datarootdir --datadir --infodir --localedir --mandir --docdir --htmldir --dvidir --pdfdir --psdir
The configure
script should also take an argument which specifies the
type of system to build the program for. This argument should look like
this:
cpu-company-system
For example, an Athlon-based GNU/Linux system might be ‘i686-pc-linux-gnu’.
The configure
script needs to be able to decode all plausible
alternatives for how to describe a machine. Thus,
‘athlon-pc-gnu/linux’ would be a valid alias. There is a shell
script called
config.sub 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:
Configure the package to build and install an optional user-level facility called feature. This allows users to choose which optional features to include. Giving an optional parameter of ‘no’ should omit feature, if it is built by default.
No ‘--enable’ option should ever cause one feature to replace another. No ‘--enable’ option should ever substitute one useful behavior for another useful behavior. The only proper use for ‘--enable’ is for questions of whether to build part of the program or exclude it.
The package package will be installed, so configure this package to work with package.
Possible values of package include ‘gnu-as’ (or ‘gas’), ‘gnu-ld’, ‘gnu-libc’, ‘gdb’, ‘x’, and ‘x-toolkit’.
Do not use a ‘--with’ option to specify the file name to use to find certain files. That is outside the scope of what ‘--with’ options are for.
Set the value of the variable variable to value. This is used to override the default values of commands or arguments in the build process. For example, the user could issue ‘configure CFLAGS=-g CXXFLAGS=-g’ to build with debugging information and without the default optimization.
Specifying variables as arguments to configure
, like this:
./configure CC=gcc
is preferable to setting them in environment variables:
CC=gcc ./configure
as it helps to recreate the same configuration later with config.status. However, both methods should be supported.
All configure
scripts should accept all of the “detail”
options and the variable settings, whether or not they make any
difference to the particular package at hand. In particular, they
should accept any option that starts with ‘--with-’ or
‘--enable-’. This is so users will be able to configure an
entire GNU source tree at once with a single set of options.
You will note that the categories ‘--with-’ and ‘--enable-’ are narrow: they do not provide a place for any sort of option you might think of. That is deliberate. We want to limit the possible configuration options in GNU software. We do not want GNU programs to have idiosyncratic configuration options.
Packages that perform part of the compilation process may support cross-compilation. In such a case, the host and target machines for the program may be different.
The configure
script should normally treat the specified type of
system as both the host and the target, thus producing a program which
works for the same type of machine that it runs on.
To compile a program to run on a host type that differs from the build type, use the configure option --host=hosttype, where hosttype uses the same syntax as buildtype. The host type normally defaults to the build type.
To configure a cross-compiler, cross-assembler, or what have you, you should specify a target different from the host, using the configure option ‘--target=targettype’. The syntax for targettype is the same as for the host type. So the command would look like this:
./configure --host=hosttype --target=targettype
The target type normally defaults to the host type. Programs for which cross-operation is not meaningful need not accept the ‘--target’ option, because configuring an entire operating system for cross-operation is not a meaningful operation.
Some programs have ways of configuring themselves automatically. If
your program is set up to do this, your configure
script can simply
ignore most of its arguments.
Next: Releases, Previous: Configuration, Up: Managing Releases [Contents][Index]
This describes conventions for writing the Makefiles for GNU programs. Using Automake will help you write a Makefile that follows these conventions. For more information on portable Makefiles, see POSIX and Portable Make Programming in Autoconf.
| • Makefile Basics: | General conventions for Makefiles. | |
| • Utilities in Makefiles: | Utilities to be used in Makefiles. | |
| • Command Variables: | Variables for specifying commands. | |
| • DESTDIR: | Supporting staged installs. | |
| • Directory Variables: | Variables for installation directories. | |
| • Standard Targets: | Standard targets for users. | |
| • Install Command Categories: | Three categories of commands in the ‘install’ rule: normal, pre-install and post-install. |
Next: Utilities in Makefiles, Up: Makefile Conventions [Contents][Index]
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 $(srcdir)/ (the source directory) is important because users can build in a separate directory using the ‘--srcdir’ option to configure. A rule of the form:
foo.1 : foo.man sedscript sed -f sedscript foo.man > foo.1
will fail when the build directory is not the source directory, because foo.man and sedscript are in the source directory.
When using GNU make
, relying on ‘VPATH’ to find the source
file will work in the case where there is a single dependency file,
since the make
automatic variable ‘$<’ 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 -f $(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
.
Next: Command Variables, Previous: Makefile Basics, Up: Makefile Conventions [Contents][Index]
Write the Makefile commands (and any shell scripts, such as
configure
) to run under sh
(both the traditional Bourne
shell and the POSIX shell), not csh
. Don’t use any
special features of ksh
or bash
, or POSIX features
not widely supported in traditional Bourne sh
.
The configure
script and the Makefile rules for building and
installation should not use any utilities directly except these:
awk cat cmp cp diff echo egrep expr false grep install-info ln ls mkdir mv printf pwd rm rmdir sed sleep sort tar test touch tr true
Compression programs such as gzip
can be used in the
dist
rule.
Generally, stick to the widely-supported (usually POSIX-specified) options and features of these programs. For example, don’t use ‘mkdir -p’, convenient as it may be, because a few systems don’t support it at all and with others, it is not safe for parallel execution. For a list of known incompatibilities, see Portable Shell Programming in Autoconf.
It is a good idea to avoid creating symbolic links in makefiles, since a few file 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.
Next: DESTDIR, Previous: Utilities in Makefiles, Up: Makefile Conventions [Contents][Index]
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.
It is acceptable, but not required, to install multiple files in one command, with the final argument being a directory, as in:
$(INSTALL_PROGRAM) foo bar baz $(bindir)
Next: Directory Variables, Previous: Command Variables, Up: Makefile Conventions [Contents][Index]
DESTDIR
: Support for Staged InstallsDESTDIR
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 as an absolute file name. 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.
Next: Standard Targets, Previous: DESTDIR, Up: Makefile Conventions [Contents][Index]
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.
All installation directories, and their parent directories, should be created (if necessary) before they are installed into.
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.
prefix
A prefix used in constructing the default values of the variables listed
below. The default value of prefix
should be /usr/local.
When building the complete GNU system, the prefix will be empty and
/usr will be a symbolic link to /.
(If you are using Autoconf, write it as ‘<EMAIL_ADDRESS>Running ‘make install’ with a different value of prefix
from
the one used to build the program should not recompile the
program.
exec_prefix
A prefix used in constructing the default values of some of the
variables listed below. The default value of exec_prefix
should
be $(prefix)
.
(If you are using Autoconf, write it as ‘<EMAIL_ADDRESS>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.
bindir
The directory for installing executable programs that users can run. This should normally be /usr/local/bin, but write it as $(exec_prefix)/bin. (If you are using Autoconf, write it as ‘<EMAIL_ADDRESS>sbindir
The directory for installing executable programs that can be run from the shell, but are only generally useful to system administrators. This should normally be /usr/local/sbin, but write it as $(exec_prefix)/sbin. (If you are using Autoconf, write it as ‘<EMAIL_ADDRESS>libexecdir
The directory for installing executable programs to be run by other programs rather than by users. This directory should normally be /usr/local/libexec, but write it as $(exec_prefix)/libexec. (If you are using Autoconf, write it as ‘<EMAIL_ADDRESS>The definition of ‘libexecdir’ is the same for all packages, so you should install your data in a subdirectory thereof. Most packages install their data under $(libexecdir)/package-name/, possibly within additional subdirectories thereof, such as $(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: