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This is Info file ../info/standards.info, produced by Makeinfo version
1.68 from the input file standards.texi.

START-INFO-DIR-ENTRY
* Standards: (standards).        GNU coding standards.
END-INFO-DIR-ENTRY

   GNU Coding Standards Copyright (C) 1992, 1993, 1994, 1995, 1996 Free
Software Foundation, Inc.

   Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

   Permission is granted to copy and distribute modified versions of
this manual under the conditions for verbatim copying, provided that
the entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

   Permission is granted to copy and distribute translations of this
manual into another language, under the above conditions for modified
versions, except that this permission notice may be stated in a
translation approved by the Free Software Foundation.

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File: standards.info,  Node: Top,  Next: Preface,  Prev: (dir),  Up: (dir)

Version
*******

   Last updated 17 May 1996.

* Menu:

* Preface::                     About the GNU Coding Standards
* Intellectual Property::       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

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File: standards.info,  Node: Preface,  Next: Intellectual Property,  Prev: Top,  Up: Top

About the GNU Coding Standards
******************************

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

   Corrections or suggestions regarding this document should be sent to
`gnu@prep.ai.mit.edu'.  If you make a suggestion, please include a
suggested new wording for it; our time is limited.  We prefer a context
diff to the `standards.texi' or `make-stds.texi' files, but if you
don't have those files, please mail your suggestion anyway.

   This release of the GNU Coding Standards was last updated 17 May
1996.

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File: standards.info,  Node: Intellectual Property,  Next: Design Advice,  Prev: Preface,  Up: Top

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

   This node discusses how you can make sure that GNU software remains
unencumbered.

* Menu:

* Reading Non-Free Code::       Referring to Proprietary Programs
* Contributions::               Accepting Contributions

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File: standards.info,  Node: Reading Non-Free Code,  Next: Contributions,  Up: Intellectual Property

Referring to Proprietary Programs
=================================

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

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

   For example, Unix utilities were generally optimized to minimize
memory use; if you go for speed instead, your program will be very
different.  You could keep the entire input file in core 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.

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File: standards.info,  Node: Contributions,  Prev: Reading Non-Free Code,  Up: Intellectual Property

Accepting Contributions
=======================

   If someone else sends you a piece of code to add to the program you
are working on, we need legal papers to use it--the same sort of legal
papers we will need to get from you.  *Each* significant contributor 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, 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 it.

   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 you write a different solution to the
problem, you don't need to get papers.

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

   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.

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File: standards.info,  Node: Design Advice,  Next: Program Behavior,  Prev: Intellectual Property,  Up: Top

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

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

* Menu:

* Compatibility::               Compatibility with other implementations
* Using Extensions::            Using non-standard features
* ANSI C::                      Using ANSI C features
* Source Language::             Using languages other than C

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File: standards.info,  Node: Compatibility,  Next: Using Extensions,  Up: Design Advice

Compatibility with Other Implementations
========================================

   With occasional exceptions, utility programs and libraries for GNU
should be upward compatible with those in Berkeley Unix, and upward
compatible with ANSI C if ANSI 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.

   ANSI 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.  Try to redesign its interface.

   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 not in Berkeley Unix are welcome.
Additional programs with no counterpart in Unix may be useful, but our
first priority is usually to duplicate what Unix already has.

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File: standards.info,  Node: Using Extensions,  Next: ANSI C,  Prev: Compatibility,  Up: Design Advice

Using Non-standard Features
===========================

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

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

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

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

   An exception to this rule are the large, established programs (such
as Emacs) which run on a great variety of systems.  Such programs would
be broken by use of GNU extensions.

   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 no good.

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File: standards.info,  Node: ANSI C,  Next: Source Language,  Prev: Using Extensions,  Up: Design Advice

ANSI C and pre-ANSI C
=====================

   Do not ever use the "trigraph" feature of ANSI C.

   ANSI C is widespread enough now that it is ok to write new programs
that use ANSI C features (and therefore will not work in non-ANSI
compilers).  And if a program is already written in ANSI C, there's no
need to convert it to support non-ANSI compilers.

   However, it is easy to support non-ANSI compilers in most programs,
so you might still consider doing so when you write a program.  Instead
of writing function definitions in ANSI prototype form,

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

write the definition in pre-ANSI 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 ANSI C prototypes in all the files where the function is
called.  And once you have it, you lose nothing by writing the function
definition in the pre-ANSI style.

   If you don't know non-ANSI C, there's no need to learn it; just
write in ANSI C.

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File: standards.info,  Node: Source Language,  Prev: ANSI C,  Up: Design Advice

Using Languages Other Than C
============================

   Using a language other than C is like using a non-standard feature:
it will cause trouble for users.  Even if GCC supports the other
language, users may find it inconvenient to have to install the
compiler for that other language in order to build your program.  So
please write in C.

   There are three exceptions for this rule:

   * It is okay to use a special language if the same program contains
     an interpreter for that language.

     For example, if your program links with GUILE, it is ok to write
     part of the program in Scheme or another language supported by
     GUILE.

   * It is okay to use another language in a tool specifically intended
     for use with that language.

     This is okay because the only people who want to build the tool
     will be those who have installed the other language anyway.

   * If an application is not of extremely widespread interest, then
     perhaps it's not important if the application is inconvenient to
     install.

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File: standards.info,  Node: Program Behavior,  Next: Writing C,  Prev: Design Advice,  Up: Top

Program Behavior for All Programs
*********************************

   This node describes how to write robust software. It also describes
general standards for error messages, the command line interface, and
how libraries should behave.

* Menu:

* Semantics::                   Writing robust programs
* Libraries::                   Library behavior
* Errors::                      Formatting error messages
* User Interfaces::             Standards for command line interfaces
* Memory Usage::                When and how to care about memory needs

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File: standards.info,  Node: Semantics,  Next: Libraries,  Up: Program Behavior

Writing Robust Programs
=======================

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

   Utilities reading files should not drop NUL characters, or any other
nonprinting characters *including those with codes above 0177*.  The
only sensible exceptions would be utilities specifically intended for
interface to certain types of printers that can't handle those
characters.

   Check every system call for an error return, unless you know you
wish to ignore errors.  Include the system error text (from `perror' or
equivalent) in *every* error message resulting from a failing system
call, as well as the name of the file if any and the name of the
utility.  Just "cannot open foo.c" or "stat failed" is not sufficient.

   Check every call to `malloc' or `realloc' to see if it returned
zero.  Check `realloc' even if you are making the block smaller; in a
system that rounds block sizes to a power of 2, `realloc' may get a
different block if you ask for less space.

   In Unix, `realloc' can destroy the storage block if it returns zero.
GNU `realloc' does not have this bug: if it fails, the original block
is unchanged.  Feel free to assume the bug is fixed.  If you wish to
run your program on Unix, and wish to avoid lossage in this case, you
can use the GNU `malloc'.

   You must expect `free' to alter the contents of the block that was
freed.  Anything you want to fetch from the block, you must fetch before
calling `free'.

   If `malloc' fails in a noninteractive program, make that a fatal
error.  In an interactive program (one that reads commands from the
user), it is better to abort the command and return to the command
reader loop.  This allows the user to kill other processes to free up
virtual memory, and then try the command again.

   Use `getopt_long' to decode arguments, unless the argument syntax
makes this unreasonable.

   When static storage is to be written in during program execution, use
explicit C code to initialize it.  Reserve C initialized declarations
for data that will not be changed.

   Try to avoid low-level interfaces to obscure Unix data structures
(such as file directories, utmp, or the layout of kernel memory), since
these are less likely to work compatibly.  If you need to find all the
files in a directory, use `readdir' or some other high-level interface.
These will be supported compatibly by GNU.

   By default, the GNU system will provide the signal handling
functions of BSD and of POSIX.  So GNU software should be written to use
these.

   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'.

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File: standards.info,  Node: Libraries,  Next: Errors,  Prev: Semantics,  Up: Program Behavior

Library Behavior
================

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

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

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

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

   External symbols that are not documented entry points for the user
should have names beginning with `_'.  They should also contain 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.

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File: standards.info,  Node: Errors,  Next: User Interfaces,  Prev: Libraries,  Up: Program Behavior

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

   Error messages from compilers should look like this:

     SOURCE-FILE-NAME:LINENO: MESSAGE

   Error messages from other noninteractive programs should look like
this:

     PROGRAM:SOURCE-FILE-NAME:LINENO: MESSAGE

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

     PROGRAM: MESSAGE

when there is no relevant source file.

   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.  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.

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File: standards.info,  Node: User Interfaces,  Next: Memory Usage,  Prev: Errors,  Up: Program Behavior

Standards for Command Line Interfaces
=====================================

   Please don't make the behavior of a utility depend on the name used
to invoke it.  It is useful sometimes to make a link to a utility with
a different name, and that should not change what it does.

   Instead, use a run time option or a compilation switch or both to
select among the alternate behaviors.

   Likewise, please don't make the behavior of the program depend on the
type of output device it is used with.  Device independence is an
important principle of the system's design; do not compromise it merely
to save someone from typing an option now and then.

   If you think one behavior is most useful when the output is to a
terminal, and another is most useful when the output is a file or a
pipe, then it is usually best to make the default behavior the one that
is useful with output to a terminal, and have an option for the other
behavior.

   Compatibility requires certain programs to depend on the type of
output device.  It would be disastrous if `ls' or `sh' did not do so in
the way all users expect.  In some of these cases, we supplement the
program with a preferred alternate version that does not depend on the
output device type.  For example, we provide a `dir' program much like
`ls' except that its default output format is always multi-column
format.

   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.  The table appears below.

   If you use names not already in the table, please send
`gnu@prep.ai.mit.edu' a list of them, with their meanings, so we can
update the 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').  Even if you allow an output file name as an
ordinary argument for compatibility, try to provide a suitable option
as well.  This will lead to more consistency among GNU utilities, so
that there are fewer idiosyncracies for users to remember.

   Programs should support an option `--version' which prints the
program's version number on standard output and exits successfully, and
an option `--help' which prints option usage information on standard
output and exits successfully.  These options should inhibit the normal
function of the command; they should do nothing except print the
requested information.

   Here is the table of long options used by GNU programs.

`after-date'
     `-N' in `tar'.

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

`all-text'
     `-a' in `diff'.

`almost-all'
     `-A' in `ls'.

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

`archive'
     `-a' in `cp'.

`archive-name'
     `-n' in `shar'.

`arglength'
     `-l' in `m4'.

`ascii'
     `-a' in `diff'.

`assign'
     `-v' in `gawk'.

`assume-new'
     `-W' in Make.

`assume-old'
     `-o' in Make.

`auto-check'
     `-a' in `recode'.

`auto-pager'
     `-a' in `wdiff'.

`auto-reference'
     `-A' in `ptx'.

`avoid-wraps'
     `-n' in `wdiff'.

`backward-search'
     `-B' in `ctags'.

`basename'
     `-f' in `shar'.

`batch'
     Used in GDB.

`baud'
     Used in GDB.

`before'
     `-b' in `tac'.

`binary'
     `-b' in `cpio' and `diff'.

`bits-per-code'
     `-b' in `shar'.

`block-size'
     Used in `cpio' and `tar'.

`blocks'
     `-b' in `head' and `tail'.

`break-file'
     `-b' in `ptx'.

`brief'
     Used in various programs to make output shorter.

`bytes'
     `-c' in `head', `split', and `tail'.

`c++'
     `-C' in `etags'.

`catenate'
     `-A' in `tar'.

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

`changes'
     `-c' in `chgrp' and `chown'.

`classify'
     `-F' in `ls'.

`colons'
     `-c' in `recode'.

`command'
     `-c' in `su'; `-x' in GDB.

`compare'
     `-d' in `tar'.

`compat'
     Used in `gawk'.

`compress'
     `-Z' in `tar' and `shar'.

`concatenate'
     `-A' in `tar'.

`confirmation'
     `-w' in `tar'.

`context'
     Used in `diff'.

`copyleft'
     `-W copyleft' in `gawk'.

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

`core'
     Used in GDB.

`count'
     `-q' in `who'.

`count-links'
     `-l' in `du'.

`create'
     Used in `tar' and `cpio'.

`cut-mark'
     `-c' in `shar'.

`cxref'
     `-x' in `ctags'.

`date'
     `-d' in `touch'.

`debug'
     `-d' in Make and `m4'; `-t' in Bison.

`define'
     `-D' in `m4'.

`defines'
     `-d' in Bison and `ctags'.

`delete'
     `-D' in `tar'.

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

`dereference-args'
     `-D' in `du'.

`diacritics'
     `-d' in `recode'.

`dictionary-order'
     `-d' in `look'.

`diff'
     `-d' in `tar'.

`digits'
     `-n' in `csplit'.

`directory'
     Specify the directory to use, in various programs.  In `ls', it
     means to show directories themselves rather than their contents.
     In `rm' and `ln', it means to not treat links to directories
     specially.

`discard-all'
     `-x' in `strip'.

`discard-locals'
     `-X' in `strip'.

`dry-run'
     `-n' in Make.

`ed'
     `-e' in `diff'.

`elide-empty-files'
     `-z' in `csplit'.

`end-delete'
     `-x' in `wdiff'.

`end-insert'
     `-z' in `wdiff'.

`entire-new-file'
     `-N' in `diff'.

`environment-overrides'
     `-e' in Make.

`eof'
     `-e' in `xargs'.

`epoch'
     Used in GDB.

`error-limit'
     Used in `makeinfo'.

`error-output'
     `-o' in `m4'.

`escape'
     `-b' in `ls'.

`exclude-from'
     `-X' in `tar'.

`exec'
     Used in GDB.

`exit'
     `-x' in `xargs'.

`exit-0'
     `-e' in `unshar'.

`expand-tabs'
     `-t' in `diff'.

`expression'
     `-e' in `sed'.

`extern-only'
     `-g' in `nm'.

`extract'
     `-i' in `cpio'; `-x' in `tar'.

`faces'
     `-f' in `finger'.

`fast'
     `-f' in `su'.

`fatal-warnings'
     `-E' in `m4'.

`file'
     `-f' in `info', `gawk', Make, `mt', and `tar'; `-n' in `sed'; `-r'
     in `touch'.

`field-separator'
     `-F' in `gawk'.

`file-prefix'
     `-b' in Bison.

`file-type'
     `-F' in `ls'.

`files-from'
     `-T' in `tar'.

`fill-column'
     Used in `makeinfo'.

`flag-truncation'
     `-F' in `ptx'.

`fixed-output-files'
     `-y' in Bison.

`follow'
     `-f' in `tail'.

`footnote-style'
     Used in `makeinfo'.

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

`force-prefix'
     `-F' in `shar'.

`format'
     Used in `ls', `time', and `ptx'.

`freeze-state'
     `-F' in `m4'.

`fullname'
     Used in GDB.

`gap-size'
     `-g' in `ptx'.

`get'
     `-x' in `tar'.

`graphic'
     `-i' in `ul'.

`graphics'
     `-g' in `recode'.

`group'
     `-g' in `install'.

`gzip'
     `-z' in `tar' and `shar'.

`hashsize'
     `-H' in `m4'.

`header'
     `-h' in `objdump' and `recode'

`heading'
     `-H' in `who'.

`help'
     Used to ask for brief usage information.

`here-delimiter'
     `-d' in `shar'.

`hide-control-chars'
     `-q' in `ls'.

`idle'
     `-u' in `who'.

`ifdef'
     `-D' in `diff'.

`ignore'
     `-I' in `ls'; `-x' in `recode'.

`ignore-all-space'
     `-w' in `diff'.

`ignore-backups'
     `-B' in `ls'.

`ignore-blank-lines'
     `-B' in `diff'.

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

`ignore-errors'
     `-i' in Make.

`ignore-file'
     `-i' in `ptx'.

`ignore-indentation'
     `-I' in `etags'.

`ignore-init-file'
     `-f' in Oleo.

`ignore-interrupts'
     `-i' in `tee'.

`ignore-matching-lines'
     `-I' in `diff'.

`ignore-space-change'
     `-b' in `diff'.

`ignore-zeros'
     `-i' in `tar'.

`include'
     `-i' in `etags'; `-I' in `m4'.

`include-dir'
     `-I' in Make.

`incremental'
     `-G' in `tar'.

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

`initial'
     `-i' in `expand'.

`initial-tab'
     `-T' in `diff'.

`inode'
     `-i' in `ls'.

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

`intermix-type'
     `-p' in `shar'.

`jobs'
     `-j' in Make.

`just-print'
     `-n' in Make.

`keep-going'
     `-k' in Make.

`keep-files'
     `-k' in `csplit'.

`kilobytes'
     `-k' in `du' and `ls'.

`language'
     `-l' in `etags'.

`less-mode'
     `-l' in `wdiff'.

`level-for-gzip'
     `-g' in `shar'.

`line-bytes'
     `-C' in `split'.

`lines'
     Used in `split', `head', and `tail'.

`link'
     `-l' in `cpio'.

`lint'
`lint-old'
     Used in `gawk'.

`list'
     `-t' in `cpio'; `-l' in `recode'.

`list'
     `-t' in `tar'.

`literal'
     `-N' in `ls'.

`load-average'
     `-l' in Make.

`login'
     Used in `su'.

`machine'
     No listing of which programs already use this; someone should
     check to see if any actually do and tell `gnu@prep.ai.mit.edu'.

`macro-name'
     `-M' in `ptx'.

`mail'
     `-m' in `hello' and `uname'.

`make-directories'
     `-d' in `cpio'.

`makefile'
     `-f' in Make.

`mapped'
     Used in GDB.

`max-args'
     `-n' in `xargs'.

`max-chars'
     `-n' in `xargs'.

`max-lines'
     `-l' in `xargs'.

`max-load'
     `-l' in Make.

`max-procs'
     `-P' in `xargs'.

`mesg'
     `-T' in `who'.

`message'
     `-T' in `who'.

`minimal'
     `-d' in `diff'.

`mixed-uuencode'
     `-M' in `shar'.

`mode'
     `-m' in `install', `mkdir', and `mkfifo'.

`modification-time'
     `-m' in `tar'.

`multi-volume'
     `-M' in `tar'.

`name-prefix'
     `-a' in Bison.

`nesting-limit'
     `-L' in `m4'.

`net-headers'
     `-a' in `shar'.

`new-file'
     `-W' in Make.

`no-builtin-rules'
     `-r' in Make.

`no-character-count'
     `-w' in `shar'.

`no-check-existing'
     `-x' in `shar'.

`no-common'
     `-3' in `wdiff'.

`no-create'
     `-c' in `touch'.

`no-defines'
     `-D' in `etags'.

`no-deleted'
     `-1' in `wdiff'.

`no-dereference'
     `-d' in `cp'.

`no-inserted'
     `-2' in `wdiff'.

`no-keep-going'
     `-S' in Make.

`no-lines'
     `-l' in Bison.

`no-piping'
     `-P' in `shar'.

`no-prof'
     `-e' in `gprof'.

`no-regex'
     `-R' in `etags'.

`no-sort'
     `-p' in `nm'.

`no-split'
     Used in `makeinfo'.

`no-static'
     `-a' in `gprof'.

`no-time'
     `-E' in `gprof'.

`no-timestamp'
     `-m' in `shar'.

`no-validate'
     Used in `makeinfo'.

`no-warn'
     Used in various programs to inhibit warnings.

`node'
     `-n' in `info'.

`nodename'
     `-n' in `uname'.

`nonmatching'
     `-f' in `cpio'.

`nstuff'
     `-n' in `objdump'.

`null'
     `-0' in `xargs'.

`number'
     `-n' in `cat'.

`number-nonblank'
     `-b' in `cat'.

`numeric-sort'
     `-n' in `nm'.

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

`nx'
     Used in GDB.

`old-archive'
     `-o' in `tar'.

`old-file'
     `-o' in Make.

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

`only-file'
     `-o' in `ptx'.

`only-prof'
     `-f' in `gprof'.

`only-time'
     `-F' in `gprof'.

`output'
     In various programs, specify the output file name.

`output-prefix'
     `-o' in `shar'.

`override'
     `-o' in `rm'.

`overwrite'
     `-c' in `unshar'.

`owner'
     `-o' in `install'.

`paginate'
     `-l' in `diff'.

`paragraph-indent'
     Used in `makeinfo'.

`parents'
     `-p' in `mkdir' and `rmdir'.

`pass-all'
     `-p' in `ul'.

`pass-through'
     `-p' in `cpio'.

`port'
     `-P' in `finger'.

`portability'
     `-c' in `cpio' and `tar'.

`posix'
     Used in `gawk'.

`prefix-builtins'
     `-P' in `m4'.

`prefix'
     `-f' in `csplit'.

`preserve'
     Used in `tar' and `cp'.

`preserve-environment'
     `-p' in `su'.

`preserve-modification-time'
     `-m' in `cpio'.

`preserve-order'
     `-s' in `tar'.

`preserve-permissions'
     `-p' in `tar'.

`print'
     `-l' in `diff'.

`print-chars'
     `-L' in `cmp'.

`print-data-base'
     `-p' in Make.

`print-directory'
     `-w' in Make.

`print-file-name'
     `-o' in `nm'.

`print-symdefs'
     `-s' in `nm'.

`printer'
     `-p' in `wdiff'.

`prompt'
     `-p' in `ed'.

`query-user'
     `-X' in `shar'.

`question'
     `-q' in Make.

`quiet'
     Used in many programs to inhibit the usual output.  *Please note:*
     every program accepting `--quiet' should accept `--silent' as a
     synonym.

`quiet-unshar'
     `-Q' in `shar'

`quote-name'
     `-Q' in `ls'.

`rcs'
     `-n' in `diff'.

`re-interval'
     Used in `gawk'.

`read-full-blocks'
     `-B' in `tar'.

`readnow'
     Used in GDB.

`recon'
     `-n' in Make.

`record-number'
     `-R' in `tar'.

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

`reference-limit'
     Used in `makeinfo'.

`references'
     `-r' in `ptx'.

`regex'
     `-r' in `tac' and `etags'.

`release'
     `-r' in `uname'.

`reload-state'
     `-R' in `m4'.

`relocation'
     `-r' in `objdump'.

`rename'
     `-r' in `cpio'.

`replace'
     `-i' in `xargs'.

`report-identical-files'
     `-s' in `diff'.

`reset-access-time'
     `-a' in `cpio'.

`reverse'
     `-r' in `ls' and `nm'.

`reversed-ed'
     `-f' in `diff'.

`right-side-defs'
     `-R' in `ptx'.

`same-order'
     `-s' in `tar'.

`same-permissions'
     `-p' in `tar'.

`save'
     `-g' in `stty'.

`se'
     Used in GDB.

`sentence-regexp'
     `-S' in `ptx'.

`separate-dirs'
     `-S' in `du'.

`separator'
     `-s' in `tac'.

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

`shell'
     `-s' in `su'.

`show-all'
     `-A' in `cat'.

`show-c-function'
     `-p' in `diff'.

`show-ends'
     `-E' in `cat'.

`show-function-line'
     `-F' in `diff'.

`show-tabs'
     `-T' in `cat'.

`silent'
     Used in many programs to inhibit the usual output.  *Please note:*
     every program accepting `--silent' should accept `--quiet' as a
     synonym.

`size'
     `-s' in `ls'.

`sort'
     Used in `ls'.

`source'
     `-W source' in `gawk'.

`sparse'
     `-S' in `tar'.

`speed-large-files'
     `-H' in `diff'.

`split-at'
     `-E' in `unshar'.

`split-size-limit'
     `-L' in `shar'.

`squeeze-blank'
     `-s' in `cat'.

`start-delete'
     `-w' in `wdiff'.

`start-insert'
     `-y' in `wdiff'.

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

`statistics'
     `-s' in `wdiff'.

`stdin-file-list'
     `-S' in `shar'.

`stop'
     `-S' in Make.

`strict'
     `-s' in `recode'.

`strip'
     `-s' in `install'.

`strip-all'
     `-s' in `strip'.

`strip-debug'
     `-S' in `strip'.

`submitter'
     `-s' in `shar'.

`suffix'
     `-S' in `cp', `ln', `mv'.

`suffix-format'
     `-b' in `csplit'.

`sum'
     `-s' in `gprof'.

`summarize'
     `-s' in `du'.

`symbolic'
     `-s' in `ln'.

`symbols'
     Used in GDB and `objdump'.

`synclines'
     `-s' in `m4'.

`sysname'
     `-s' in `uname'.

`tabs'
     `-t' in `expand' and `unexpand'.

`tabsize'
     `-T' in `ls'.

`terminal'
     `-T' in `tput' and `ul'.  `-t' in `wdiff'.

`text'
     `-a' in `diff'.

`text-files'
     `-T' in `shar'.

`time'
     Used in `ls' and `touch'.

`to-stdout'
     `-O' in `tar'.

`total'
     `-c' in `du'.

`touch'
     `-t' in Make, `ranlib', and `recode'.

`trace'
     `-t' in `m4'.

`traditional'
     `-t' in `hello'; `-W traditional' in `gawk'; `-G' in `ed', `m4',
     and `ptx'.

`tty'
     Used in GDB.

`typedefs'
     `-t' in `ctags'.

`typedefs-and-c++'
     `-T' in `ctags'.

`typeset-mode'
     `-t' in `ptx'.

`uncompress'
     `-z' in `tar'.

`unconditional'
     `-u' in `cpio'.

`undefine'
     `-U' in `m4'.

`undefined-only'
     `-u' in `nm'.

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

`usage'
     Used in `gawk'; same as `--help'.

`uuencode'
     `-B' in `shar'.

`vanilla-operation'
     `-V' in `shar'.

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

`verify'
     `-W' in `tar'.

`version'
     Print the version number.

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

`vgrind'
     `-v' in `ctags'.

`volume'
     `-V' in `tar'.

`what-if'
     `-W' in Make.

`whole-size-limit'
     `-l' in `shar'.

`width'
     `-w' in `ls' and `ptx'.

`word-regexp'
     `-W' in `ptx'.

`writable'
     `-T' in `who'.

`zeros'
     `-z' in `gprof'.

\1f
File: standards.info,  Node: Memory Usage,  Prev: User Interfaces,  Up: Program Behavior

Memory Usage
============

   If it 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 core 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 core all at once.

   If your program creates complicated data structures, just make them
in core and give a fatal error if `malloc' returns zero.

\1f
File: standards.info,  Node: Writing C,  Next: Documentation,  Prev: Program Behavior,  Up: Top

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

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

* Menu:

* Formatting::                  Formatting Your Source Code
* Comments::                    Commenting Your Work
* Syntactic Conventions::       Clean Use of C Constructs
* Names::                       Naming Variables and Functions
* System Portability::          Portability between different operating systems
* CPU Portability::             Supporting the range of CPU types
* System Functions::            Portability and "standard" library functions
* Internationalization::        Techniques for internationalization

\1f
File: standards.info,  Node: Formatting,  Next: Comments,  Up: Writing C

Formatting Your Source Code
===========================

   It is important to put the open-brace that starts the body of a C
function in column zero, and avoid putting any other open-brace or
open-parenthesis or open-bracket in column zero.  Several tools look
for open-braces in column zero to find the beginnings of C functions.
These tools will not work on code not formatted that way.

   It is also important for function definitions to start the name of
the function in column zero.  This helps people to search for function
definitions, and may also help certain tools recognize them.  Thus, the
proper format is this:

     static char *
     concat (s1, s2)        /* Name starts in column zero here */
          char *s1, *s2;
     {                     /* Open brace in column zero here */
       ...
     }

or, if you want to use ANSI C, format the definition like this:

     static char *
     concat (char *s1, char *s2)
     {
       ...
     }

   In ANSI 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 the body of the function, we prefer code formatted 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,
but Emacs would mess it up:

     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 adding a set of parentheses solves the problem:

     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.

\1f
File: standards.info,  Node: Comments,  Next: Syntactic Conventions,  Prev: Formatting,  Up: Writing C

Commenting Your Work
====================

   Every program should start with a comment saying briefly what it is
for.  Example: `fmt - filter for simple filling of text'.

   Please put a comment on each function saying what the function does,
what sorts of arguments it gets, and what the possible values of
arguments mean and are used for.  It is not necessary to duplicate in
words the meaning of the C argument declarations, if a C type is being
used in its customary fashion.  If there is anything nonstandard about
its use (such as an argument of type `char *' which is really the
address of the second character of a string, not the first), or any
possible values that would not work the way one would expect (such as,
that strings containing newlines are not guaranteed to work), be sure
to say so.

   Also explain the significance of the return value, if there is one.

   Please put two spaces after the end of a sentence in your comments,
so that the Emacs sentence commands will work.  Also, please write
complete sentences and capitalize the first word.  If a lower-case
identifier comes at the beginning of a sentence, don't capitalize it!
Changing the spelling makes it a different identifier.  If you don't
like starting a sentence with a lower case letter, write the sentence
differently (e.g., "The identifier lower-case is ...").

   The comment on a function is much clearer if you use the argument
names to speak about the argument values.  The variable name itself
should be lower case, but write it in upper case when you are speaking
about the value rather than the variable itself.  Thus, "the inode
number NODE_NUM" rather than "an inode".

   There is usually no purpose in restating the name of the function in
the comment before it, because the reader can see that for himself.
There might be an exception when the comment is so long that the
function itself would be off the bottom of the screen.

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

     /* Nonzero means truncate lines in the display;
        zero means continue them.  */
     int truncate_lines;

   Every `#endif' should have a comment, except in the case of short
conditionals (just a few lines) that are not nested.  The comment should
state the condition of the conditional that is ending, *including its
sense*.  `#else' should have a comment describing the condition *and
sense* of the code that follows.  For example:

     #ifdef foo
       ...
     #else /* not foo */
       ...
     #endif /* not foo */

but, by contrast, write the comments this way for a `#ifndef':

     #ifndef foo
       ...
     #else /* foo */
       ...
     #endif /* foo */

\1f
File: standards.info,  Node: Syntactic Conventions,  Next: Names,  Prev: Comments,  Up: Writing C

Clean Use of C Constructs
=========================

   Please explicitly declare all arguments to functions.  Don't omit
them just because they are `int's.

   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 declare a separate local
variable for each distinct purpose, and give it a name which is
meaningful.  This not only makes programs easier to understand, it also
facilitates optimization by good compilers.  You can also move the
declaration of each local variable into the smallest scope that includes
all its uses.  This makes the program even cleaner.

   Don't use local variables or parameters that shadow global
identifiers.

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

     int    foo,
            bar;

write either this:

     int foo, bar;

or this:

     int foo;
     int bar;

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

   When you have an `if'-`else' statement nested in another `if'
statement, always put braces around the `if'-`else'.  Thus, never write
like this:

     if (foo)
       if (bar)
         win ();
       else
         lose ();

always like this:

     if (foo)
       {
         if (bar)
           win ();
         else
           lose ();
       }

   If you have an `if' statement nested inside of an `else' statement,
either write `else if' on one line, like this,

     if (foo)
       ...
     else if (bar)
       ...

with its `then'-part indented like the preceding `then'-part, or write
the nested `if' within braces like this:

     if (foo)
       ...
     else
       {
         if (bar)
           ...
       }

   Don't declare both a structure tag and variables or typedefs in the
same declaration.  Instead, declare the structure tag separately and
then use it to declare the variables or typedefs.

   Try to avoid assignments inside `if'-conditions.  For example, don't
write this:

     if ((foo = (char *) malloc (sizeof *foo)) == 0)
       fatal ("virtual memory exhausted");

instead, write this:

     foo = (char *) malloc (sizeof *foo);
     if (foo == 0)
       fatal ("virtual memory exhausted");

   Don't make the program ugly to placate `lint'.  Please don't insert
any casts to `void'.  Zero without a cast is perfectly fine as a null
pointer constant, except when calling a varargs function.

\1f
File: standards.info,  Node: Names,  Next: System Portability,  Prev: Syntactic Conventions,  Up: Writing C

Naming Variables and Functions
==============================

   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.

   Use file names of 14 characters or less, to avoid creating gratuitous
problems on older System V systems.  You can use the program `doschk'
to test for this.  `doschk' also tests for potential name conflicts if
the files were loaded onto an MS-DOS file system--something you may or
may not care about.

\1f
File: standards.info,  Node: System Portability,  Next: CPU Portability,  Prev: Names,  Up: Writing C

Portability between System Types
================================

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

   The primary purpose of GNU software is to run on top of the GNU
kernel, compiled with the GNU C compiler, on various types of CPU.  The
amount and kinds of variation among GNU systems on different CPUs will
be comparable to the variation among Linux-based GNU systems or among
BSD systems today.  So the kinds of portability that are absolutely
necessary are quite limited.

   But many users do run GNU software on non-GNU Unix or Unix-like
systems.  So supporting a variety of Unix-like systems is desirable,
although not paramount.

   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, the
Macintosh, VMS, and MVS, supporting them is usually so much work that it
is better if you don't.

   The planned GNU kernel is not finished yet, but you can tell which
facilities it will provide by looking at the GNU C Library Manual.  The
GNU kernel is based on Mach, so the features of Mach will also be
available.  However, if you use Mach features, you'll probably have
trouble debugging your program today.