Copyright (C) 1992 Free Software Foundation 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.

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

Version
*******

Last updated 9 May 1992.

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.

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 change, 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.

I 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.

Change Logs
***********

Keep a change log for each directory, describing the changes made to source files in that directory.  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 eliminate conceptual inconsistencies between different parts of a program; they can give you a history of how the conflicting concepts arose.

Use the Emacs command M-x add-change to start a new entry in the change log.  An entry should have an asterisk, the name of the changed file, and then in parentheses the name of the changed functions, variables or whatever, followed by a colon.  Then describe the changes you made to that function or variable.

Separate unrelated entries with blank lines.  When two entries represent parts of the same change, so that they work together, then don't put blank lines between them.  Then you can omit the file name and the asterisk when successive entries are in the same file.

Here are some examples:

     * 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.

There's no need to describe here the full purpose of the changes or how they work together.  It is better to put this explanation in comments in the code.  That's why just ``New function'' is enough; there is a comment with the function in the source to explain what it does.

However, sometimes it is useful to write one line to describe the overall purpose of a large batch of changes.

When you change the calling sequence of a function in a simple fashion, and you change all the callers of the function, there is no need to make individual entries for all the callers.  Just write in the entry for the function being called, ``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.  Write just, ``Doc fix.''  There's no need to keep a change log 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.

Compatibility with Other Implementations
****************************************

With certain 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 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.

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.

Makefile Conventions
********************

This chapter describes conventions for writing Makefiles.

General Conventions for Makefiles
=================================

Every Makefile should contain this line:

     SHELL = /bin/sh

to avoid trouble on systems where the SHELL variable might be inherited from the environment.

Don't assume that . is in the path for command execution.  When you need to run programs that are files in the current directory, always use ./ to make sure the proper file is run regardless of the current path.

Standard Targets for Users
==========================

All GNU programs should have the following targets in their Makefiles:

all
     Compile the entire program.

install
     Compile the program and copy the executables, libraries, and so on to the file names where they should reside for actual use.  If there is a simple test to verify that a program is properly installed then run that test.

clean
     Delete all files from the current directory that are normally created by building the program.  Don't delete the files that record the configuration.  Also preserve files that could be made by building, but normally aren't because the distribution comes with them.

distclean
     Delete all files from the current directory that are created by configuring or building the program.  If you have unpacked the source and built the program without creating any other files, make distclean should leave only the files that were in the distribution.

mostlyclean
     Like clean, but may refrain from deleting a few files that people normally don't want to recompile.  For example, the mostlyclean target for GCC does not delete libgcc.a, because recompiling it is rarely necessary and takes a lot of time.

realclean
     Delete everything from the current directory that can be reconstructed with this Makefile.  This typically includes everything deleted by distclean, plus more: C source files produced by Bison, tags tables, info files, and so on.

TAGS
     Update a tags table for this program.

dist
     Create a distribution tar file for this program.  The tar file should be set up so that the file names in the tar file start with a subdirectory name which is the name of the package it is a distribution for.  This name can include the version number.

     For example, the distribution tar file of GCC version 1.40 unpacks into a subdirectory named gcc-1.40.

     The easiest way to do this is to create a subdirectory appropriately named, use ln or cp to install the proper files in it, and then tar that subdirectory.

     The dist target should explicitly depend on all non-source files that are in the distribution, to make sure they are up to date in the distribution.  See Releases.

check
     Perform self-tests (if any).  The user must build the program before running the tests, but need not install the program; you should write the self-tests so that they work when the program is built but not installed.

Variables for Specifying Commands
=================================

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.

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 name CFLAGS is an exception to this rule, but we keep it because it is standard.)

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.

Every Makefile should define the variable INSTALL, which is the basic command for installing a file into the system.

Every Makefile should also define variables INSTALL_PROGRAM and INSTALL_DATA.  (The default for each of these should be $(INSTALL).)  Then it should use those variables as the commands for actual installation, for executables and nonexecutables respectively.  Use these variables as follows:

     $(INSTALL_PROGRAM) foo ${bindir}/foo
     $(INSTALL_DATA) libfoo.a ${libdir}/libfoo.a

(Always use a file name, not a directory name, as the second argument. Use a separate command for each file to be installed.)

Variables for Installation Directories
======================================

Installation directories should always be named by variables, so it is easy to install in a nonstandard place.  The standard names for these variables are:

prefix
     A prefix used in constructing the default values of the variables listed below.  The default value of prefix should be /usr/local (at least for now).

exec_prefix
     A prefix used in constructing the default values of the some of the variables listed below.  The default value of exec_prefix should be $(prefix).

     Generally, $(exec_prefix) is used for directories that contain machine-specific files (such as executables and subroutine libraries), while $(prefix) is used directly for other directories.

bindir
     The directory for installing executable programs that users can run. This should normally be /usr/local/bin, but it should be written as $(exec_prefix)/bin.

libdir
     The directory for installing executable files to be run by the program rather than by users.  Object files and libraries of object code should also go in this directory.  The idea is that this directory is used for files that pertain to a specific machine architecture, but need not be in the path for commands.  The value of libdir should normally be /usr/local/lib, but it should be written as $(exec_prefix)/lib.

datadir
     The directory for installing read-only data files which the programs refer to while they run.  This directory is used for files which are independent of the type of machine being used.  This should normally be /usr/local/lib, but it should be written as $(prefix)/lib.

statedir
     The directory for installing data files which the programs modify while they run.  These files should be independent of the type of machine being used, and it should be possible to share them among machines at a network installation.  This should normally be /usr/local/lib, but it should be written as $(prefix)/lib.

includedir
     The directory for installing #include header files to be included by user programs.  This should normally be /usr/local/include, but it should be written as $(prefix)/include.

     Most compilers other than GCC do not look for header files in /usr/local/include.  So installing the header files this way is only useful with GCC.  Sometimes this is not a problem because some libraries are only really intended to work with GCC.  But some libraries are intended to work with other compilers.  They should install their header files in two places, one specified by includedir and one specified by oldincludedir

oldincludedir
     The directory for installing #include header files for use with compilers other than GCC.  This should normally be /usr/include.

     The Makefile commands should check whether the value of oldincludedir is empty.  If it is, they should not try to use it; they should cancel the second installation of the header files.

mandir
     The directory for installing the man pages (if any) for this package. It should include the suffix for the proper section of the manual---usually 1 for a utility.

man1dir
     The directory for installing section 1 man pages.

man2dir
     The directory for installing section 2 man pages.

...
     Use these names instead of mandir if the package needs to install man pages in more than one section of the manual.

     Don't make the primary documentation for any GNU software be a man page.  Write a manual in Texinfo instead.  Man pages are just for the sake of people running GNU software on Unix, which is a secondary application only.

manext
     The file name extension for the installed man page.  This should contain a period followed by the appropriate digit.

infodir
     The directory for installing the info files for this package.  By default, it should be /usr/local/info, but it should be written as $(prefix)/info.

srcdir
     The directory for the sources being compiled.  The value of this variable is normally inserted by the configure shell script.

For example:

     # Common prefix for installation directories.
     # NOTE: This directory must exist when you start installation.
     prefix = /usr/local
     exec_prefix = $(prefix)
     # Directory in which to put the executable for the command `gcc'
     bindir = $(exec_prefix)/bin
     # Directory in which to put the directories used by the compiler.
     libdir = $(exec_prefix)/lib
     # Directory in which to put the Info files.
     infodir = $(prefix)/info

How Configuration Should Work
*****************************

Each GNU distribution should come with a shell script named configure.  This script is given arguments which describe the kind of machine and system you want to compile the program for.

The configure script must record the configuration options so that they affect compilation.

One way to do this is to make a link from a standard name such as config.h to the proper configuration file for the chosen system. If you use this technique, the distribution should not contain a file named config.h.  This is so that people won't be able to build the program without configuring it first.

Another thing that configure can do is to edit the Makefile.  If you do this, the distribution should not contain a file named Makefile.  Instead, include a file Makefile.in which contains the input used for editing.  Once again, this is so that people won't be able to build the program without configuring it first.

If configure does write the Makefile, then Makefile should have a target named Makefile which causes configure to be rerun, setting up the same configuration that was set up last time.  The files that configure reads should be listed as dependencies of Makefile.

All the files which are output from the configure script should have comments at the beginning explaining that they were generated automatically using configure.  This is so that users won't think of trying to edit them by hand.

The configure script should write a file named config.status which describes which configuration options were specified when the program was last configured.  This file should be a shell script which, if run, will recreate the same configuration.

The configure script should accept an option of the form --srcdir=DIRNAME to specify the directory where sources are found (if it is not the current directory).  This makes it possible to build the program in a separate directory, so that the actual source directory is not modified.

If the user does not specify --srcdir, then configure should check both . and .. to see if it can find the sources.  If it finds the sources in one of these places, it should use them from there.  Otherwise, it should report that it cannot find the sources, and should exit with nonzero status.

Usually the easy way to support --srcdir is by editing a definition of VPATH into the Makefile.  Some rules may need to refer explicitly to the specified source directory.  To make this possible, configure can add to the Makefile a variable named srcdir whose value is precisely the specified directory.

The configure script should also take an argument which specifies the type of system to build the program for.  This argument should look like this:

     CPU-COMPANY-SYSTEM

For example, a Sun 3 might be m68k-sun-sunos4.1.

The configure script needs to be able to decode all plausible alternatives for how to describe a machine.  Thus, sun3-sunos4.1 would be a valid alias.  So would sun3-bsd4.2, since SunOS is basically BSD and no other BSD system is used on a Sun.  For many programs, vax-dec-ultrix would be an alias for vax-dec-bsd, simply because the differences between Ultrix and BSD are rarely noticeable, but a few programs might need to distinguish them.

There is a shell script called config.sub that you can use as a subroutine to validate system types and canonicalize aliases.

Other options are permitted to specify in more detail the software or hardware are present on the machine:

--with-PACKAGE
     The package PACKAGE will be installed, so configure this package to work with PACKAGE.

     Possible values of PACKAGE include x, gnu-as (or gas), gnu-ld, gnu-libc, and gdb.

--nfp
     The target machine has no floating point processor.

--gas
     The target machine assembler is GAS, the GNU assembler. This is obsolete; use --with-gnu-as instead.

--x
     The target machine has the X Window system installed. This is obsolete; use --with-x instead.

All configure scripts should accept all of these ``detail'' options, whether or not they make any difference to the particular package at hand.  In particular, they should accept any option that starts with --with-.  This is so users will be able to configure an entire GNU source tree at once with a single set of options.

Packages that perform part of compilation 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.

The way to build a cross-compiler, cross-assembler, or what have you, is to specify the option --host=HOSTTYPE when running configure.  This specifies the host system without changing the type of target system.  The syntax for HOSTTYPE is the same as described above.

Programs for which cross-operation is not meaningful need not accept the --host option, because configuring an entire operating system for cross-operation is not a meaningful thing.

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.

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.

     Thus, it is not a problem that GNU Emacs contains code written in Emacs Lisp, because it comes with a Lisp interpreter.

  * 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.

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.

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 identifer 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 */

Clean Use of C Constructs
*************************

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

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.

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.

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 System V.

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.

Since most computer systems do not yet implement ANSI C, using the ANSI C features is effectively using a GNU extension, so the same considerations apply.  (Except for ANSI features that we discourage, such as trigraphs---don't ever use them.)

Program Behaviour for All Programs
**********************************

Avoid arbitrary limits on the length or number of any data structure, including filenames, 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.

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.

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 filename.  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.

Library Behaviour
*****************

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.

Portability As It Applies to GNU
********************************

Much of what is called ``portability'' in the Unix world refers to porting to different Unix versions.  This is not relevant to GNU software, because its purpose is to run on top of one and only one kernel, the GNU kernel, compiled with one and only one C compiler, the GNU C compiler.  The amount and kinds of variation among GNU systems on different cpu's will be like the variation among Berkeley 4.3 systems on different cpu's.

It is difficult to be sure exactly what facilities the GNU kernel will provide, since it isn't finished yet.  Therefore, assume you can use anything in 4.3; just avoid using the format of semi-internal data bases (e.g., directories) when there is a higher-level alternative (readdir).

You can freely assume any reasonably standard facilities in the C language, libraries or kernel, because we will find it necessary to support these facilities in the full GNU system, whether or not we have already done so.  The fact that there may exist kernels or C compilers that lack these facilities is irrelevant as long as the GNU kernel and C compiler support them.

It remains necessary to worry about differences among cpu types, such as the difference in byte ordering and alignment restrictions.  It's unlikely that 16-bit machines will ever be supported by GNU, so there is no point in spending any time to consider the possibility that an int will be less than 32 bits.

You can assume that all pointers have the same format, regardless of the type they point to, and that this is really an integer. There are some weird machines where this isn't true, but they aren't important; don't waste time catering to them.  Besides, eventually we will put function prototypes into all GNU programs, and that will probably make your program work even on weird machines.

Since some important machines (including the 68000) are big-endian, it is important not to assume that the address of an int object is also the address of its least-significant byte.  Thus, don't make the following mistake:

     int c;
     ...
     while ((c = getchar()) != EOF)
             write(file_descriptor, &c, 1);

You can assume that it is reasonable to use a meg of memory.  Don't strain to reduce memory usage unless it can get to that level.  If your program creates complicated data structures, just make them in core and give a fatal error if malloc returns zero.

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.

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.

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.

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, and an option --help which prints option usage information.

Documenting Programs
********************

Please use Texinfo for documenting GNU programs.  See the Texinfo manual, either the hardcopy or the version in the GNU Emacs Info sub-system (C-h i).

See existing GNU texinfo files (e.g. those under the man/ directory in the GNU Emacs Distribution) for examples.

The title page of the manual should state the version of the program which the manual applies to.  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.

The manual should document all command-line arguments and all commands.  It should give examples of their use.  But don't organize the manual as a list of features.  Instead, organize it by the concepts a user will have before reaching that point in the manual. Address the goals that a user will have in mind, and explain how to accomplish them.

Making Releases
***************

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

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

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

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

Make sure that no file name in the distribution is no more than 14 characters long.  Nowadays, there are systems that adhere to a foolish interpretation of the POSIX standard which holds that they should refuse to open a longer name, rather than truncating as they did in the past.

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

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

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