source: trunk/essentials/sys-devel/m4/doc/m4.info

This is m4.info, produced by makeinfo version 4.8 from m4.texinfo.

   This manual is for GNU M4 (version 1.4.8, 20 November 2006), a
package containing an implementation of the m4 macro language.

   Copyright (C) 1989, 1990, 1991, 1992, 1993, 1994, 2004, 2005, 2006
Free Software Foundation, Inc.

     Permission is granted to copy, distribute and/or modify this
     document under the terms of the GNU Free Documentation License,
     Version 1.2 or any later version published by the Free Software
     Foundation; with no Invariant Sections, no Front-Cover Texts, and
     no Back-Cover Texts.  A copy of the license is included in the
     section entitled "GNU Free Documentation License."

INFO-DIR-SECTION GNU programming tools
START-INFO-DIR-ENTRY
* M4: (m4).                     A powerful macro processor.
END-INFO-DIR-ENTRY


File: m4.info,  Node: Top,  Next: Preliminaries,  Up: (dir)

GNU M4
******

This manual is for GNU M4 (version 1.4.8, 20 November 2006), a package
containing an implementation of the m4 macro language.

   Copyright (C) 1989, 1990, 1991, 1992, 1993, 1994, 2004, 2005, 2006
Free Software Foundation, Inc.

     Permission is granted to copy, distribute and/or modify this
     document under the terms of the GNU Free Documentation License,
     Version 1.2 or any later version published by the Free Software
     Foundation; with no Invariant Sections, no Front-Cover Texts, and
     no Back-Cover Texts.  A copy of the license is included in the
     section entitled "GNU Free Documentation License."

   GNU `m4' is an implementation of the traditional UNIX macro
processor.  It is mostly SVR4 compatible, although it has some
extensions (for example, handling more than 9 positional parameters to
macros).  `m4' also has builtin functions for including files, running
shell commands, doing arithmetic, etc.  Autoconf needs GNU `m4' for
generating `configure' scripts, but not for running them.

   GNU `m4' was originally written by Rene' Seindal, with subsequent
changes by Franc,ois Pinard and other volunteers on the Internet.  All
names and email addresses can be found in the files `m4-1.4.8/AUTHORS'
and `m4-1.4.8/THANKS' from the GNU M4 distribution.

   This is release 1.4.8.  It is now considered stable:  future
releases in the 1.4.x series are only meant to fix bugs, increase speed,
or improve documentation.  However...

   An experimental feature, which would improve `m4' usefulness, allows
for changing the syntax for what is a "word" in `m4'.  You should use:
     ./configure --enable-changeword
   if you want this feature compiled in.  The current implementation
slows down `m4' considerably and is hardly acceptable.  In the future,
`m4' 2.0 will come with a different set of new features that provide
similar capabilities, but without the inefficiencies, so changeword
will go away and _you should not count on it_.

* Menu:

* Preliminaries::               Introduction and preliminaries
* Invoking m4::                 Invoking `m4'
* Syntax::                      Lexical and syntactic conventions

* Macros::                      How to invoke macros
* Definitions::                 How to define new macros
* Conditionals::                Conditionals, loops, and recursion

* Debugging::                   How to debug macros and input

* Input Control::               Input control
* File Inclusion::              File inclusion
* Diversions::                  Diverting and undiverting output

* Text handling::               Macros for text handling
* Arithmetic::                  Macros for doing arithmetic
* Shell commands::              Macros for running shell commands
* Miscellaneous::               Miscellaneous builtin macros
* Frozen files::                Fast loading of frozen state

* Compatibility::               Compatibility with other versions of `m4'
* Answers::                     Correct version of some examples
* Copying This Manual::         How to make copies of this manual
* Indices::                     Indices of concepts and macros

 --- The Detailed Node Listing ---

Introduction and preliminaries

* Intro::                       Introduction to `m4'
* History::                     Historical references
* Bugs::                        Problems and bugs
* Manual::                      Using this manual

Invoking `m4'

* Operation modes::             Command line options for operation modes
* Preprocessor features::       Command line options for preprocessor features
* Limits control::              Command line options for limits control
* Frozen state::                Command line options for frozen state
* Debugging options::           Command line options for debugging
* Command line files::          Specifying input files on the command line

Lexical and syntactic conventions

* Names::                       Macro names
* Quoted strings::              Quoting input to `m4'
* Comments::                    Comments in `m4' input
* Other tokens::                Other kinds of input tokens
* Input processing::            How `m4' copies input to output

How to invoke macros

* Invocation::                  Macro invocation
* Inhibiting Invocation::       Preventing macro invocation
* Macro Arguments::             Macro arguments
* Quoting Arguments::           On Quoting Arguments to macros
* Macro expansion::             Expanding macros

How to define new macros

* Define::                      Defining a new macro
* Arguments::                   Arguments to macros
* Pseudo Arguments::            Special arguments to macros
* Undefine::                    Deleting a macro
* Defn::                        Renaming macros
* Pushdef::                     Temporarily redefining macros

* Indir::                       Indirect call of macros
* Builtin::                     Indirect call of builtins

Conditionals, loops, and recursion

* Ifdef::                       Testing if a macro is defined
* Ifelse::                      If-else construct, or multibranch
* Shift::                       Recursion in `m4'
* Forloop::                     Iteration by counting
* Foreach::                     Iteration by list contents

How to debug macros and input

* Dumpdef::                     Displaying macro definitions
* Trace::                       Tracing macro calls
* Debug Levels::                Controlling debugging output
* Debug Output::                Saving debugging output

Input control

* Dnl::                         Deleting whitespace in input
* Changequote::                 Changing the quote characters
* Changecom::                   Changing the comment delimiters
* Changeword::                  Changing the lexical structure of words
* M4wrap::                      Saving text until end of input

File inclusion

* Include::                     Including named files
* Search Path::                 Searching for include files

Diverting and undiverting output

* Divert::                      Diverting output
* Undivert::                    Undiverting output
* Divnum::                      Diversion numbers
* Cleardivert::                 Discarding diverted text

Macros for text handling

* Len::                         Calculating length of strings
* Index macro::                 Searching for substrings
* Regexp::                      Searching for regular expressions
* Substr::                      Extracting substrings
* Translit::                    Translating characters
* Patsubst::                    Substituting text by regular expression
* Format::                      Formatting strings (printf-like)

Macros for doing arithmetic

* Incr::                        Decrement and increment operators
* Eval::                        Evaluating integer expressions

Macros for running shell commands

* Platform macros::             Determining the platform
* Syscmd::                      Executing simple commands
* Esyscmd::                     Reading the output of commands
* Sysval::                      Exit status
* Mkstemp::                     Making temporary files

Miscellaneous builtin macros

* Errprint::                    Printing error messages
* Location::                    Printing current location
* M4exit::                      Exiting from `m4'

Fast loading of frozen state

* Using frozen files::          Using frozen files
* Frozen file format::          Frozen file format

Compatibility with other versions of `m4'

* Extensions::                  Extensions in GNU M4
* Incompatibilities::           Facilities in System V m4 not in GNU M4
* Other Incompatibilities::     Other incompatibilities

Correct version of some examples

* Improved exch::               Solution for `exch'
* Improved forloop::            Solution for `forloop'
* Improved foreach::            Solution for `foreach'
* Improved cleardivert::        Solution for `cleardivert'
* Improved fatal_error::        Solution for `fatal_error'

How to make copies of this manual

* GNU Free Documentation License::  License for copying this manual

Indices of concepts and macros

* Concept index::               Index for many concepts
* Macro index::                 Index for all `m4' macros


File: m4.info,  Node: Preliminaries,  Next: Invoking m4,  Prev: Top,  Up: Top

1 Introduction and preliminaries
********************************

This first chapter explains what GNU `m4' is, where `m4' comes from,
how to read and use this documentation, how to call the `m4' program,
and how to report bugs about it.  It concludes by giving tips for
reading the remainder of the manual.

   The following chapters then detail all the features of the `m4'
language.

* Menu:

* Intro::                       Introduction to `m4'
* History::                     Historical references
* Bugs::                        Problems and bugs
* Manual::                      Using this manual


File: m4.info,  Node: Intro,  Next: History,  Up: Preliminaries

1.1 Introduction to `m4'
========================

`m4' is a macro processor, in the sense that it copies its input to the
output, expanding macros as it goes.  Macros are either builtin or
user-defined, and can take any number of arguments.  Besides just doing
macro expansion, `m4' has builtin functions for including named files,
running shell commands, doing integer arithmetic, manipulating text in
various ways, performing recursion, etc....  `m4' can be used either as
a front-end to a compiler, or as a macro processor in its own right.

   The `m4' macro processor is widely available on all UNIXes, and has
been standardized by POSIX.  Usually, only a small percentage of users
are aware of its existence.  However, those who find it often become
committed users.  The popularity of GNU Autoconf, which requires GNU
`m4' for _generating_ `configure' scripts, is an incentive for many to
install it, while these people will not themselves program in `m4'.
GNU `m4' is mostly compatible with the System V, Release 3 version,
except for some minor differences.  *Note Compatibility::, for more
details.

   Some people find `m4' to be fairly addictive.  They first use `m4'
for simple problems, then take bigger and bigger challenges, learning
how to write complex sets of `m4' macros along the way.  Once really
addicted, users pursue writing of sophisticated `m4' applications even
to solve simple problems, devoting more time debugging their `m4'
scripts than doing real work.  Beware that `m4' may be dangerous for
the health of compulsive programmers.


File: m4.info,  Node: History,  Next: Bugs,  Prev: Intro,  Up: Preliminaries

1.2 Historical references
=========================

`GPM' was an important ancestor of `m4'.  See C. Stratchey: "A General
Purpose Macro generator", Computer Journal 8,3 (1965), pp. 225 ff.
`GPM' is also succinctly described into David Gries classic "Compiler
Construction for Digital Computers".

   The classic B. Kernighan and P.J. Plauger: "Software Tools",
Addison-Wesley, Inc. (1976) describes and implements a Unix
macro-processor language, which inspired Dennis Ritchie to write `m3',
a macro processor for the AP-3 minicomputer.

   Kernighan and Ritchie then joined forces to develop the original
`m4', as described in "The M4 Macro Processor", Bell Laboratories
(1977).  It had only 21 builtin macros.

   While `GPM' was more _pure_, `m4' is meant to deal with the true
intricacies of real life: macros can be recognized without being
pre-announced, skipping whitespace or end-of-lines is easier, more
constructs are builtin instead of derived, etc.

   Originally, the Kernighan and Plauger macro-processor, and then
`m3', formed the engine for the Rational FORTRAN preprocessor, that is,
the `Ratfor' equivalent of `cpp'.  Later, `m4' was used as a frontend
for `Ratfor', `C' and `Cobol'.

   Rene' Seindal released his implementation of `m4', GNU `m4', in
1990, with the aim of removing the artificial limitations in many of
the traditional `m4' implementations, such as maximum line length,
macro size, or number of macros.

   The late Professor A. Dain Samples described and implemented a
further evolution in the form of `M5': "User's Guide to the M5 Macro
Language: 2nd edition", Electronic Announcement on comp.compilers
newsgroup (1992).

   Franc,ois Pinard took over maintenance of GNU `m4' in 1992, until
1994 when he released GNU `m4' 1.4, which was the stable release for 10
years.  It was at this time that GNU Autoconf decided to require GNU
`m4' as its underlying engine, since all other implementations of `m4'
had too many limitations.

   More recently, in 2004, Paul Eggert released 1.4.1 and 1.4.2 which
addressed some long standing bugs in the venerable 1.4 release.  Then
in 2005 Gary V. Vaughan collected together the many patches to GNU `m4'
1.4 that were floating around the net and released 1.4.3 and 1.4.4.
And in 2006, Eric Blake joined the team and prepared patches for the
release of 1.4.5, 1.4.6, 1.4.7, and 1.4.8.

   Meanwhile, development has continued on new features for `m4', such
as dynamic module loading and additional builtins.  When complete, GNU
`m4' 2.0 will start a new series of releases.


File: m4.info,  Node: Bugs,  Next: Manual,  Prev: History,  Up: Preliminaries

1.3 Problems and bugs
=====================

If you have problems with GNU M4 or think you've found a bug, please
report it.  Before reporting a bug, make sure you've actually found a
real bug.  Carefully reread the documentation and see if it really says
you can do what you're trying to do.  If it's not clear whether you
should be able to do something or not, report that too; it's a bug in
the documentation!

   Before reporting a bug or trying to fix it yourself, try to isolate
it to the smallest possible input file that reproduces the problem.
Then send us the input file and the exact results `m4' gave you.  Also
say what you expected to occur; this will help us decide whether the
problem was really in the documentation.

   Once you've got a precise problem, send e-mail to (Internet)
<bug-m4@gnu.org>.  Please include the version number of `m4' you are
using.  You can get this information with the command `m4 --version'.
Also provide details about the platform you are executing on.

   Non-bug suggestions are always welcome as well.  If you have
questions about things that are unclear in the documentation or are
just obscure features, please report them too.


File: m4.info,  Node: Manual,  Prev: Bugs,  Up: Preliminaries

1.4 Using this manual
=====================

This manual contains a number of examples of `m4' input and output, and
a simple notation is used to distinguish input, output and error
messages from `m4'.  Examples are set out from the normal text, and
shown in a fixed width font, like this

     This is an example of an example!

   To distinguish input from output, all output from `m4' is prefixed
by the string `=>', and all error messages by the string `error-->'.
Thus

     Example of input line
     =>Output line from m4
     error-->and an error message

   The sequence `^D' in an example indicates the end of the input file.
The majority of these examples are self-contained, and you can run them
with similar results by invoking `m4 -d'.  In fact, the testsuite that
is bundled in the GNU M4 package consists of the examples in this
document!

   As each of the predefined macros in `m4' is described, a prototype
call of the macro will be shown, giving descriptive names to the
arguments, e.g.,

 -- Composite: example (STRING, [COUNT = `1'], [ARGUMENT]...)
     This is a sample prototype.  There is not really a macro named
     `example', but this documents that if there were, it would be a
     Composite macro, rather than a Builtin.  It requires at least one
     argument, STRING.  Remember that in `m4', there must not be a
     space between the macro name and the opening parenthesis, unless
     it was intended to call the macro without any arguments.  The
     brackets around COUNT and ARGUMENT show that these arguments are
     optional.  If COUNT is omitted, the macro behaves as if count were
     `1', whereas if ARGUMENT is omitted, the macro behaves as if it
     were the empty string.  A blank argument is not the same as an
     omitted argument.  For example, `example(`a')', `example(`a',`1')',
     and `example(`a',`1',)' would behave identically with COUNT set to
     `1'; while `example(`a',)' and `example(`a',`')' would explicitly
     pass the empty string for COUNT.  The ellipses (`...') show that
     the macro processes additional arguments after ARGUMENT, rather
     than ignoring them.

   All macro arguments in `m4' are strings, but some are given special
interpretation, e.g., as numbers, file names, regular expressions, etc.
The documentation for each macro will state how the parameters are
interpreted, and what happens if the argument cannot be parsed
according to the desired interpretation.  Unless specified otherwise, a
parameter specified to be a number is parsed as a decimal, even if the
argument has leading zeros; and parsing the empty string as a number
results in 0 rather than an error, although a warning will be issued.

   This document consistently writes and uses "builtin", without a
hyphen, as if it were an English word.  This is how the `builtin'
primitive is spelled within `m4'.


File: m4.info,  Node: Invoking m4,  Next: Syntax,  Prev: Preliminaries,  Up: Top

2 Invoking `m4'
***************

The format of the `m4' command is:

     `m4' [OPTION...] [FILE...]

   All options begin with `-', or if long option names are used, with
`--'.  A long option name need not be written completely, any
unambiguous prefix is sufficient.  POSIX requires `m4' to recognize
arguments intermixed with files, even when `POSIXLY_CORRECT' is set in
the environment.  Most options take effect at startup regardless of
their position, but some are documented below as taking effect after
any files that occurred earlier in the command line.  The argument `--'
is a marker to denote the end of options.

   With short options, options that do not take arguments may be
combined into a single command line argument with subsequent options,
options with mandatory arguments may be provided either as a single
command line argument or as two arguments, and options with optional
arguments must be provided as a single argument.  In other words, `m4
-QPDfoo -d a -d+f' is equivalent to `m4 -Q -P -D foo -d -d+f -- ./a',
although the latter form is considered canonical.

   With long options, options with mandatory arguments may be provided
with an equal sign (`=') in a single argument, or as two arguments, and
options with optional arguments must be provided as a single argument.
In other words, `m4 --def foo --debug a' is equivalent to `m4
--define=foo --debug= -- ./a', although the latter form is considered
canonical (not to mention more robust, in case a future version of `m4'
introduces an option named `--default').

   `m4' understands the following options, grouped by functionality.

* Menu:

* Operation modes::             Command line options for operation modes
* Preprocessor features::       Command line options for preprocessor features
* Limits control::              Command line options for limits control
* Frozen state::                Command line options for frozen state
* Debugging options::           Command line options for debugging
* Command line files::          Specifying input files on the command line


File: m4.info,  Node: Operation modes,  Next: Preprocessor features,  Up: Invoking m4

2.1 Command line options for operation modes
============================================

Several options control the overall operation of `m4':

`--help'
     Print a help summary on standard output, then immediately exit
     `m4' without reading any input files or performing any other
     actions.

`--version'
     Print the version number of the program on standard output, then
     immediately exit `m4' without reading any input files or
     performing any other actions.

`-E'
`--fatal-warnings'
     Stop execution and exit `m4' once the first warning has been
     issued, considering all of them to be fatal.

`-i'
`--interactive'
`-e'
     Makes this invocation of `m4' interactive.  This means that all
     output will be unbuffered, and interrupts will be ignored.  The
     spelling `-e' exists for compatibility with other `m4'
     implementations, and issues a warning because it may be withdrawn
     in a future version of GNU M4.

`-P'
`--prefix-builtins'
     Internally modify _all_ builtin macro names so they all start with
     the prefix `m4_'.  For example, using this option, one should write
     `m4_define' instead of `define', and `m4___file__' instead of
     `__file__'.  This option has no effect if `-R' is also specified.

`-Q'
`--quiet'
`--silent'
     Suppress warnings, such as missing or superfluous arguments in
     macro calls, or treating the empty string as zero.

`-W REGEXP'
`--word-regexp=REGEXP'
     Use REGEXP as an alternative syntax for macro names.  This
     experimental option will not be present in all GNU `m4'
     implementations (*note Changeword::).


File: m4.info,  Node: Preprocessor features,  Next: Limits control,  Prev: Operation modes,  Up: Invoking m4

2.2 Command line options for preprocessor features
==================================================

Several options allow `m4' to behave more like a preprocessor.  Macro
definitions and deletions can be made on the command line, the search
path can be altered, and the output file can track where the input came
from.  These features occur with the following options:

`-D NAME[=VALUE]'
`--define=NAME[=VALUE]'
     This enters NAME into the symbol table, before any input files are
     read.  If `=VALUE' is missing, the value is taken to be the empty
     string.  The VALUE can be any string, and the macro can be defined
     to take arguments, just as if it was defined from within the
     input.  This option may be given more than once; order with
     respect to file names is significant, and redefining the same NAME
     loses the previous value.

`-I DIRECTORY'
`--include=DIRECTORY'
     Make `m4' search DIRECTORY for included files that are not found
     in the current working directory.  *Note Search Path::, for more
     details.  This option may be given more than once.

`-s'
`--synclines'
     Generate synchronization lines, for use by the C preprocessor or
     other similar tools.  Order is significant with respect to file
     names.  This option is useful, for example, when `m4' is used as a
     front end to a compiler.  Source file name and line number
     information is conveyed by directives of the form `#line LINENUM
     "FILE"', which are inserted as needed into the middle of the
     output.  Such directives mean that the following line originated
     or was expanded from the contents of input file FILE at line
     LINENUM.  The `"FILE"' part is often omitted when the file name
     did not change from the previous directive.

     Synchronization directives are always given on complete lines by
     themselves.  When a synchronization discrepancy occurs in the
     middle of an output line, the associated synchronization directive
     is delayed until the beginning of the next generated line.

`-U NAME'
`--undefine=NAME'
     This deletes any predefined meaning NAME might have.  Obviously,
     only predefined macros can be deleted in this way.  This option
     may be given more than once; undefining a NAME that does not have a
     definition is silently ignored.  Order is significant with respect
     to file names.


File: m4.info,  Node: Limits control,  Next: Frozen state,  Prev: Preprocessor features,  Up: Invoking m4

2.3 Command line options for limits control
===========================================

There are some limits within `m4' that can be tuned.  For
compatibility, `m4' also accepts some options that control limits in
other implementations, but which are automatically unbounded (limited
only by your hardware and operating system constraints) in GNU `m4'.

`-G'
`--traditional'
     Suppress all the extensions made in this implementation, compared
     to the System V version.  *Note Compatibility::, for a list of
     these.

`-H NUM'
`--hashsize=NUM'
     Make the internal hash table for symbol lookup be NUM entries big.
     For better performance, the number should be prime, but this is not
     checked.  The default is 509 entries.  It should not be necessary
     to increase this value, unless you define an excessive number of
     macros.

`-L NUM'
`--nesting-limit=NUM'
     Artificially limit the nesting of macro calls to NUM levels,
     stopping program execution if this limit is ever exceeded.  When
     not specified, nesting is limited to 1024 levels.  A value of zero
     means unlimited; but then heavily nested code could potentially
     cause a stack overflow.

     The precise effect of this option might be more correctly
     associated with textual nesting than dynamic recursion.  It has
     been useful when some complex `m4' input was generated by
     mechanical means.  Most users would never need this option.  If
     shown to be obtrusive, this option (which is still experimental)
     might well disappear.

     This option does _not_ have the ability to break endless
     rescanning loops, since these do not necessarily consume much
     memory or stack space.  Through clever usage of rescanning loops,
     one can request complex, time-consuming computations from `m4'
     with useful results.  Putting limitations in this area would break
     `m4' power.  There are many pathological cases:
     `define(`a', `a')a' is only the simplest example (but *note
     Compatibility::).  Expecting GNU `m4' to detect these would be a
     little like expecting a compiler system to detect and diagnose
     endless loops: it is a quite _hard_ problem in general, if not
     undecidable!

`-B NUM'
`-S NUM'
`-T NUM'
     These options are present for compatibility with System V `m4', but
     do nothing in this implementation.  They may disappear in future
     releases, and issue a warning to that effect.

`-N NUM'
`--diversions=NUM'
     These options are present only for compatibility with previous
     versions of GNU `m4', and were controlling the number of possible
     diversions which could be used at the same time.  They do nothing,
     because there is no fixed limit anymore.  They may disappear in
     future releases, and issue a warning to that effect.


File: m4.info,  Node: Frozen state,  Next: Debugging options,  Prev: Limits control,  Up: Invoking m4

2.4 Command line options for frozen state
=========================================

GNU `m4' comes with a feature of freezing internal state (*note Frozen
files::).  This can be used to speed up `m4' execution when reusing a
common initialization script.

`-F FILE'
`--freeze-state=FILE'
     Once execution is finished, write out the frozen state on the
     specified FILE.  It is conventional, but not required, for FILE to
     end in `.m4f'.

`-R FILE'
`--reload-state=FILE'
     Before execution starts, recover the internal state from the
     specified frozen FILE.  The options `-D', `-U', and `-t' take
     effect after state is reloaded, but before the input files are
     read.


File: m4.info,  Node: Debugging options,  Next: Command line files,  Prev: Frozen state,  Up: Invoking m4

2.5 Command line options for debugging
======================================

Finally, there are several options for aiding in debugging `m4' scripts.

`-d[FLAGS]'
`--debug[=FLAGS]'
     Set the debug-level according to the flags FLAGS.  The debug-level
     controls the format and amount of information presented by the
     debugging functions.  *Note Debug Levels::, for more details on
     the format and meaning of FLAGS.  If omitted, FLAGS defaults to
     `aeq'.

`--debugfile=FILE'
`-o FILE'
`--error-output=FILE'
     Redirect `dumpdef' output, debug messages, and trace output to the
     named FILE.  Warnings, error messages, and `errprint' output are
     still printed to standard error.  If unspecified, debug output goes
     to standard error; if empty, debug output is discarded.  *Note
     Debug Output::, for more details.  The spellings `-o' and
     `--error-output' are misleading and inconsistent with other GNU
     tools; for now they are silently accepted as synonyms of
     `--debugfile', but in a future version of M4, using them will
     cause a warning to be issued.

`-l NUM'
`--arglength=NUM'
     Restrict the size of the output generated by macro tracing to NUM
     characters per trace line.  If unspecified or zero, output is
     unlimited.  *Note Debug Levels::, for more details.

`-t NAME'
`--trace=NAME'
     This enables tracing for the macro NAME, at any point where it is
     defined.  NAME need not be defined when this option is given.
     This option may be given more than once, and order is significant
     with respect to file names.  *Note Trace::, for more details.


File: m4.info,  Node: Command line files,  Prev: Debugging options,  Up: Invoking m4

2.6 Specifying input files on the command line
==============================================

The remaining arguments on the command line are taken to be input file
names.  If no names are present, standard input is read.  A file name
of `-' is taken to mean standard input.  It is conventional, but not
required, for input files to end in `.m4'.

   The input files are read in the sequence given.  Standard input can
be read more than once, so the file name `-' may appear multiple times
on the command line; this makes a difference when input is from a
terminal or other special file type.  It is an error if an input file
ends in the middle of argument collection, a comment, or a quoted
string.

   The options `--define' (`-D'), `--undefine' (`-U'), `--synclines'
(`-s'), and `--trace' (`-t') only take effect after processing input
from any file names that occur earlier on the command line.

   If none of the input files invoked `m4exit' (*note M4exit::), the
exit status of `m4' will be 0 for success, 1 for general failure (such
as problems with reading an input file), and 63 for version mismatch
(*note Using frozen files::).

   If you need to read a file whose name starts with a `-', you can
specify it as `./-file', or use `--' to mark the end of options.


File: m4.info,  Node: Syntax,  Next: Macros,  Prev: Invoking m4,  Up: Top

3 Lexical and syntactic conventions
***********************************

As `m4' reads its input, it separates it into "tokens".  A token is
either a name, a quoted string, or any single character, that is not a
part of either a name or a string.  Input to `m4' can also contain
comments.  GNU `m4' does not yet understand locales; all operations are
byte-oriented rather than character-oriented.  However, `m4' is
eight-bit clean, so you can use non-ASCII characters in quoted strings
(*note Changequote::), comments (*note Changecom::), and macro names
(*note Indir::), with the exception of the NUL character (the zero byte
`'\0'').

* Menu:

* Names::                       Macro names
* Quoted strings::              Quoting input to `m4'
* Comments::                    Comments in `m4' input
* Other tokens::                Other kinds of input tokens
* Input processing::            How `m4' copies input to output


File: m4.info,  Node: Names,  Next: Quoted strings,  Up: Syntax

3.1 Macro names
===============

A name is any sequence of letters, digits, and the character `_'
(underscore), where the first character is not a digit.  `m4' will use
the longest such sequence found in the input.  If a name has a macro
definition, it will be subject to macro expansion (*note Macros::).
Names are case-sensitive.

   Examples of legal names are: `foo', `_tmp', and `name01'.


File: m4.info,  Node: Quoted strings,  Next: Comments,  Prev: Names,  Up: Syntax

3.2 Quoting input to `m4'
=========================

A quoted string is a sequence of characters surrounded by quote
strings, defaulting to ``' and `'', where the nested begin and end
quotes within the string are balanced.  The value of a string token is
the text, with one level of quotes stripped off.  Thus

     `'
     =>

is the empty string, and double-quoting turns into single-quoting.

     ``quoted''
     =>`quoted'

   The quote characters can be changed at any time, using the builtin
macro `changequote'.  *Note Changequote::, for more information.


File: m4.info,  Node: Comments,  Next: Other tokens,  Prev: Quoted strings,  Up: Syntax

3.3 Comments in `m4' input
==========================

Comments in `m4' are normally delimited by the characters `#' and
newline.  All characters between the comment delimiters are ignored,
but the entire comment (including the delimiters) is passed through to
the output--comments are _not_ discarded by `m4'.

   Comments cannot be nested, so the first newline after a `#' ends the
comment.  The commenting effect of the begin-comment string can be
inhibited by quoting it.

     `quoted text' # `commented text'
     =>quoted text # `commented text'
     `quoting inhibits' `#' `comments'
     =>quoting inhibits # comments

   The comment delimiters can be changed to any string at any time,
using the builtin macro `changecom'.  *Note Changecom::, for more
information.


File: m4.info,  Node: Other tokens,  Next: Input processing,  Prev: Comments,  Up: Syntax

3.4 Other kinds of input tokens
===============================

Any character, that is neither a part of a name, nor of a quoted string,
nor a comment, is a token by itself.  When not in the context of macro
expansion, all of these tokens are just copied to output.  However,
during macro expansion, whitespace characters (space, tab, newline,
formfeed, carriage return, vertical tab), parentheses (`(' and `)'),
comma (`,'), and dollar (`$') have additional roles, explained later.


File: m4.info,  Node: Input processing,  Prev: Other tokens,  Up: Syntax

3.5 How `m4' copies input to output
===================================

As `m4' reads the input token by token, it will copy each token
directly to the output immediately.

   The exception is when it finds a word with a macro definition.  In
that case `m4' will calculate the macro's expansion, possibly reading
more input to get the arguments.  It then inserts the expansion in front
of the remaining input.  In other words, the resulting text from a macro
call will be read and parsed into tokens again.

   `m4' expands a macro as soon as possible.  If it finds a macro call
when collecting the arguments to another, it will expand the second
call first.  For a running example, examine how `m4' handles this input:

     format(`Result is %d', eval(`2**15'))

First, `m4' sees that the token `format' is a macro name, so it
collects the tokens `(', ``Result is %d'', `,', and ` ', before
encountering another potential macro.  Sure enough, `eval' is a macro
name, so the nested argument collection picks up `(', ``2**15'', and
`)', invoking the eval macro with the lone argument of `2**15'.  The
expansion of `eval(2**15)' is `32768', which is then rescanned as the
five tokens `3', `2', `7', `6', and `8'; and combined with the next
`)', the format macro now has all its arguments, as if the user had
typed:

     format(`Result is %d', 32768)

The format macro expands to `Result is 32768', and we have another
round of scanning for the tokens `Result', ` ', `is', ` ', `3', `2',
`7', `6', and `8'.  None of these are macros, so the final output is

     =>Result is 32768

   The order in which `m4' expands the macros can be explored using the
trace facilities of GNU `m4' (*note Trace::).

   This process continues until there are no more macro calls to expand
and all the input has been consumed.


File: m4.info,  Node: Macros,  Next: Definitions,  Prev: Syntax,  Up: Top

4 How to invoke macros
**********************

This chapter covers macro invocation, macro arguments and how macro
expansion is treated.

* Menu:

* Invocation::                  Macro invocation
* Inhibiting Invocation::       Preventing macro invocation
* Macro Arguments::             Macro arguments
* Quoting Arguments::           On Quoting Arguments to macros
* Macro expansion::             Expanding macros


File: m4.info,  Node: Invocation,  Next: Inhibiting Invocation,  Up: Macros

4.1 Macro invocation
====================

Macro invocations has one of the forms

     name

which is a macro invocation without any arguments, or

     name(arg1, arg2, ..., argN)

which is a macro invocation with N arguments.  Macros can have any
number of arguments.  All arguments are strings, but different macros
might interpret the arguments in different ways.

   The opening parenthesis _must_ follow the NAME directly, with no
spaces in between.  If it does not, the macro is called with no
arguments at all.

   For a macro call to have no arguments, the parentheses _must_ be
left out.  The macro call

     name()

is a macro call with one argument, which is the empty string, not a call
with no arguments.


File: m4.info,  Node: Inhibiting Invocation,  Next: Macro Arguments,  Prev: Invocation,  Up: Macros

4.2 Preventing macro invocation
===============================

An innovation of the `m4' language, compared to some of its
predecessors (like Stratchey's `GPM', for example), is the ability to
recognize macro calls without resorting to any special, prefixed
invocation character.  While generally useful, this feature might
sometimes be the source of spurious, unwanted macro calls.  So, GNU
`m4' offers several mechanisms or techniques for inhibiting the
recognition of names as macro calls.

   First of all, many builtin macros cannot meaningfully be called
without arguments.  For any of these macros, whenever an opening
parenthesis does not immediately follow their name, the builtin macro
call is not triggered.  This solves the most usual cases, like for
`include' or `eval'.  Later in this document, the sentence "This macro
is recognized only with parameters" refers to this specific provision.

   There is also a command line option (`--prefix-builtins', or `-P',
*note Invoking m4: Operation modes.) that renames all builtin macros
with a prefix of `m4_' at startup.  The option has no effect whatsoever
on user defined macros.  For example, with this option, one has to
write `m4_dnl' and even `m4_m4exit'.  It also has no effect on whether
a macro requires parameters.

   Another alternative is to redefine problematic macros to a name less
likely to cause conflicts, *Note Definitions::.

   If your version of GNU `m4' has the `changeword' feature compiled
in, it offers far more flexibility in specifying the syntax of macro
names, both builtin or user-defined.  *Note Changeword::, for more
information on this experimental feature.

   Of course, the simplest way to prevent a name from being interpreted
as a call to an existing macro is to quote it.  The remainder of this
section studies a little more deeply how quoting affects macro
invocation, and how quoting can be used to inhibit macro invocation.

   Even if quoting is usually done over the whole macro name, it can
also be done over only a few characters of this name (provided, of
course, that the unquoted portions are not also a macro).  It is also
possible to quote the empty string, but this works only _inside_ the
name.  For example:

     `divert'
     =>divert
     `d'ivert
     =>divert
     di`ver't
     =>divert
     div`'ert
     =>divert

all yield the string `divert'.  While in both:

     `'divert
     =>
     divert`'
     =>

the `divert' builtin macro will be called, which expands to the empty
string.

   The output of macro evaluations is always rescanned.  The following
example would yield the string `de', exactly as if `m4' has been given
`substr(`abcde', `3', `2')' as input:

     define(`x', `substr(ab')
     =>
     define(`y', `cde, `3', `2')')
     =>
     x`'y
     =>de

   Unquoted strings on either side of a quoted string are subject to
being recognized as macro names.  In the following example, quoting the
empty string allows for the second `macro' to be recognized as such:

     define(`macro', `m')
     =>
     macro(`m')macro
     =>mmacro
     macro(`m')`'macro
     =>mm

   Quoting may prevent recognizing as a macro name the concatenation of
a macro expansion with the surrounding characters.  In this example:

     define(`macro', `di$1')
     =>
     macro(`v')`ert'
     =>divert
     macro(`v')ert
     =>

the input will produce the string `divert'.  When the quotes were
removed, the `divert' builtin was called instead.


File: m4.info,  Node: Macro Arguments,  Next: Quoting Arguments,  Prev: Inhibiting Invocation,  Up: Macros

4.3 Macro arguments
===================

When a name is seen, and it has a macro definition, it will be expanded
as a macro.

   If the name is followed by an opening parenthesis, the arguments
will be collected before the macro is called.  If too few arguments are
supplied, the missing arguments are taken to be the empty string.
However, some builtins are documented to behave differently for a
missing optional argument than for an explicit empty string.  If there
are too many arguments, the excess arguments are ignored.  Unquoted
leading whitespace is stripped off all arguments, but whitespace
generated by a macro expansion or occuring after a macro that expanded
to an empty string remains intact.  Whitespace includes space, tab,
newline, carriage return, vertical tab, and formfeed.

     define(`macro', `$1')
     =>
     macro( unquoted leading space lost)
     =>unquoted leading space lost
     macro(` quoted leading space kept')
     => quoted leading space kept
     macro(
      divert `unquoted space kept after expansion')
     => unquoted space kept after expansion
     macro(macro(`
     ')`whitespace from expansion kept')
     =>
     =>whitespace from expansion kept
     macro(`unquoted trailing whitespace kept'
     )
     =>unquoted trailing whitespace kept
     =>

   Normally `m4' will issue warnings if a builtin macro is called with
an inappropriate number of arguments, but it can be suppressed with the
`--quiet' command line option (or `--silent', or `-Q', *note Invoking
m4: Operation modes.).  For user defined macros, there is no check of
the number of arguments given.

   Macros are expanded normally during argument collection, and whatever
commas, quotes and parentheses that might show up in the resulting
expanded text will serve to define the arguments as well.  Thus, if FOO
expands to `, b, c', the macro call

     bar(a foo, d)

is a macro call with four arguments, which are `a ', `b', `c' and `d'.
To understand why the first argument contains whitespace, remember that
unquoted leading whitespace is never part of an argument, but trailing
whitespace always is.

   It is possible for a macro's definition to change during argument
collection, in which case the expansion uses the definition that was in
effect at the time the opening `(' was seen.

     define(`f', `1')
     =>
     f(define(`f', `2'))
     =>1
     f
     =>2

   It is an error if the end of file occurs while collecting arguments.

     hello world
     =>hello world
     define(
     ^D
     error-->m4:stdin:2: ERROR: end of file in argument list


File: m4.info,  Node: Quoting Arguments,  Next: Macro expansion,  Prev: Macro Arguments,  Up: Macros

4.4 On Quoting Arguments to macros
==================================

Each argument has unquoted leading whitespace removed.  Within each
argument, all unquoted parentheses must match.  For example, if FOO is
a macro,

     foo(() (`(') `(')

is a macro call, with one argument, whose value is `() (() ('.  Commas
separate arguments, except when they occur inside quotes, comments, or
unquoted parentheses.  *Note Pseudo Arguments::, for examples.

   It is common practice to quote all arguments to macros, unless you
are sure you want the arguments expanded.  Thus, in the above example
with the parentheses, the `right' way to do it is like this:

     foo(`() (() (')

   It is, however, in certain cases necessary or convenient to leave out
quotes for some arguments, and there is nothing wrong in doing it.  It
just makes life a bit harder, if you are not careful.  For consistency,
this manual follows the rule of thumb that each layer of parentheses
introduces another layer of single quoting, except when showing the
consequences of quoting rules.  This is done even when the quoted string
cannot be a macro, such as with integers when you have not changed the
syntax via `changeword' (*note Changeword::).


File: m4.info,  Node: Macro expansion,  Prev: Quoting Arguments,  Up: Macros

4.5 Macro expansion
===================

When the arguments, if any, to a macro call have been collected, the
macro is expanded, and the expansion text is pushed back onto the input
(unquoted), and reread.  The expansion text from one macro call might
therefore result in more macros being called, if the calls are included,
completely or partially, in the first macro calls' expansion.

   Taking a very simple example, if FOO expands to `bar', and BAR
expands to `Hello world', the input

     foo

will expand first to `bar', and when this is reread and expanded, into
`Hello world'.


File: m4.info,  Node: Definitions,  Next: Conditionals,  Prev: Macros,  Up: Top

5 How to define new macros
**************************

Macros can be defined, redefined and deleted in several different ways.
Also, it is possible to redefine a macro without losing a previous
value, and bring back the original value at a later time.

* Menu:

* Define::                      Defining a new macro
* Arguments::                   Arguments to macros
* Pseudo Arguments::            Special arguments to macros
* Undefine::                    Deleting a macro
* Defn::                        Renaming macros
* Pushdef::                     Temporarily redefining macros

* Indir::                       Indirect call of macros
* Builtin::                     Indirect call of builtins


File: m4.info,  Node: Define,  Next: Arguments,  Up: Definitions

5.1 Defining a macro
====================

The normal way to define or redefine macros is to use the builtin
`define':

 -- Builtin: define (NAME, [EXPANSION])
     Defines NAME to expand to EXPANSION.  If EXPANSION is not given,
     it is taken to be empty.

     The expansion of `define' is void.  The macro `define' is
     recognized only with parameters.

   The following example defines the macro FOO to expand to the text
`Hello World.'.

     define(`foo', `Hello world.')
     =>
     foo
     =>Hello world.

   The empty line in the output is there because the newline is not a
part of the macro definition, and it is consequently copied to the
output.  This can be avoided by use of the macro `dnl'.  *Note Dnl::,
for details.

   The first argument to `define' should be quoted; otherwise, if the
macro is already defined, you will be defining a different macro.  This
example shows the problems with underquoting, since we did not want to
redefine `one':

     define(foo, one)
     =>
     define(foo, two)
     =>
     one
     =>two

   GNU `m4' normally replaces only the _topmost_ definition of a macro
if it has several definitions from `pushdef' (*note Pushdef::).  Some
other implementations of `m4' replace all definitions of a macro with
`define'.  *Note Incompatibilities::, for more details.

   As a GNU extension, the first argument to `define' does not have to
be a simple word.  It can be any text string, even the empty string.  A
macro with a non-standard name cannot be invoked in the normal way, as
the name is not recognized.  It can only be referenced by the builtins
`Indir' (*note Indir::) and `Defn' (*note Defn::).

   Arrays and associative arrays can be simulated by using this trick.

     define(`array', `defn(format(``array[%d]'', `$1'))')
     =>
     define(`array_set', `define(format(``array[%d]'', `$1'), `$2')')
     =>
     array_set(`4', `array element no. 4')
     =>
     array_set(`17', `array element no. 17')
     =>
     array(`4')
     =>array element no. 4
     array(eval(`10 + 7'))
     =>array element no. 17

   Change the `%d' to `%s' and it is an associative array.


File: m4.info,  Node: Arguments,  Next: Pseudo Arguments,  Prev: Define,  Up: Definitions

5.2 Arguments to macros
=======================

Macros can have arguments.  The Nth argument is denoted by `$n' in the
expansion text, and is replaced by the Nth actual argument, when the
macro is expanded.  Replacement of arguments happens before rescanning,
regardless of how many nesting levels of quoting appear in the
expansion.  Here is an example of a macro with two arguments.  It
simply exchanges the order of the two arguments.

     define(`exch', `$2, $1')
     =>
     exch(`arg1', `arg2')
     =>arg2, arg1

   This can be used, for example, if you like the arguments to `define'
to be reversed.

     define(`exch', `$2, $1')
     =>
     define(exch(``expansion text'', ``macro''))
     =>
     macro
     =>expansion text

   *Note Quoting Arguments::, for an explanation of the double quotes.
(You should try and improve this example so that clients of `exch' do
not have to double quote; or *note Answers: Improved exch.).

   GNU `m4' allows the number following the `$' to consist of one or
more digits, allowing macros to have any number of arguments.  This is
not so in UNIX implementations of `m4', which only recognize one digit.

   As a special case, the zeroth argument, `$0', is always the name of
the macro being expanded.

     define(`test', ``Macro name: $0'')
     =>
     test
     =>Macro name: test

   If you want quoted text to appear as part of the expansion text,
remember that quotes can be nested in quoted strings.  Thus, in

     define(`foo', `This is macro `foo'.')
     =>
     foo
     =>This is macro foo.

The `foo' in the expansion text is _not_ expanded, since it is a quoted
string, and not a name.


File: m4.info,  Node: Pseudo Arguments,  Next: Undefine,  Prev: Arguments,  Up: Definitions

5.3 Special arguments to macros
===============================

There is a special notation for the number of actual arguments supplied,
and for all the actual arguments.

   The number of actual arguments in a macro call is denoted by `$#' in
the expansion text.  Thus, a macro to display the number of arguments
given can be

     define(`nargs', `$#')
     =>
     nargs
     =>0
     nargs()
     =>1
     nargs(`arg1', `arg2', `arg3')
     =>3
     nargs(`commas can be quoted, like this')
     =>1
     nargs(arg1#inside comments, commas do not separate arguments
     still arg1)
     =>1
     nargs((unquoted parentheses, like this, group arguments))
     =>1

   The notation `$*' can be used in the expansion text to denote all
the actual arguments, unquoted, with commas in between.  For example

     define(`echo', `$*')
     =>
     echo(arg1,    arg2, arg3 , arg4)
     =>arg1,arg2,arg3 ,arg4

   Often each argument should be quoted, and the notation `$@' handles
that.  It is just like `$*', except that it quotes each argument.  A
simple example of that is:

     define(`echo', `$@')
     =>
     echo(arg1,    arg2, arg3 , arg4)
     =>arg1,arg2,arg3 ,arg4

   Where did the quotes go?  Of course, they were eaten, when the
expanded text were reread by `m4'.  To show the difference, try

     define(`echo1', `$*')
     =>
     define(`echo2', `$@')
     =>
     define(`foo', `This is macro `foo'.')
     =>
     echo1(foo)
     =>This is macro This is macro foo..
     echo1(`foo')
     =>This is macro foo.
     echo2(foo)
     =>This is macro foo.
     echo2(`foo')
     =>foo

*Note Trace::, if you do not understand this.  As another example of the
difference, remember that comments encountered in arguments are passed
untouched to the macro, and that quoting disables comments.

     define(`echo1', `$*')
     =>
     define(`echo2', `$@')
     =>
     define(`foo', `bar')
     =>
     echo1(#foo'foo
     foo)
     =>#foo'foo
     =>bar
     echo2(#foo'foo
     foo)
     =>#foobar
     =>bar'

   A `$' sign in the expansion text, that is not followed by anything
`m4' understands, is simply copied to the macro expansion, as any other
text is.

     define(`foo', `$$$ hello $$$')
     =>
     foo
     =>$$$ hello $$$

   If you want a macro to expand to something like `$12', the judicious
use of nested quoting can put a safe character between the `$' and the
next character, relying on the rescanning to remove the nested quote.
This will prevent `m4' from interpreting the `$' sign as a reference to
an argument.

     define(`foo', `no nested quote: $1')
     =>
     foo(`arg')
     =>no nested quote: arg
     define(`foo', `nested quote around $: `$'1')
     =>
     foo(`arg')
     =>nested quote around $: $1
     define(`foo', `nested empty quote after $: $`'1')
     =>
     foo(`arg')
     =>nested empty quote after $: $1
     define(`foo', `nested quote around next character: $`1'')
     =>
     foo(`arg')
     =>nested quote around next character: $1
     define(`foo', `nested quote around both: `$1'')
     =>
     foo(`arg')
     =>nested quote around both: arg


File: m4.info,  Node: Undefine,  Next: Defn,  Prev: Pseudo Arguments,  Up: Definitions

5.4 Deleting a macro
====================

A macro definition can be removed with `undefine':

 -- Builtin: undefine (NAME...)
     For each argument, remove the macro NAME.  The macro names must
     necessarily be quoted, since they will be expanded otherwise.

     The expansion of `undefine' is void.  The macro `undefine' is
     recognized only with parameters.

     foo bar blah
     =>foo bar blah
     define(`foo', `some')define(`bar', `other')define(`blah', `text')
     =>
     foo bar blah
     =>some other text
     undefine(`foo')
     =>
     foo bar blah
     =>foo other text
     undefine(`bar', `blah')
     =>
     foo bar blah
     =>foo bar blah

   Undefining a macro inside that macro's expansion is safe; the macro
still expands to the definition that was in effect at the `('.

     define(`f', ``$0':$1')
     =>
     f(f(f(undefine(`f')`hello world')))
     =>f:f:f:hello world
     f(`bye')
     =>f(bye)

   It is not an error for NAME to have no macro definition.  In that
case, `undefine' does nothing.


File: m4.info,  Node: Defn,  Next: Pushdef,  Prev: Undefine,  Up: Definitions

5.5 Renaming macros
===================

It is possible to rename an already defined macro.  To do this, you need
the builtin `defn':

 -- Builtin: defn (NAME)
     Expands to the _quoted definition_ of NAME.  If the argument is
     not a defined macro, the expansion is void.

     If NAME is a user-defined macro, the quoted definition is simply
     the quoted expansion text.  If, instead, NAME is a builtin, the
     expansion is a special token, which points to the builtin's
     internal definition.  This token is only meaningful as the second
     argument to `define' (and `pushdef'), and is silently converted to
     an empty string in most other contexts.

     The macro `defn' is recognized only with parameters.

   Its normal use is best understood through an example, which shows
how to rename `undefine' to `zap':

     define(`zap', defn(`undefine'))
     =>
     zap(`undefine')
     =>
     undefine(`zap')
     =>undefine(zap)

   In this way, `defn' can be used to copy macro definitions, and also
definitions of builtin macros.  Even if the original macro is removed,
the other name can still be used to access the definition.

   The fact that macro definitions can be transferred also explains why
you should use `$0', rather than retyping a macro's name in its
definition:

     define(`foo', `This is `$0'')
     =>
     define(`bar', defn(`foo'))
     =>
     bar
     =>This is bar

   Macros used as string variables should be referred through `defn',
to avoid unwanted expansion of the text:

     define(`string', `The macro dnl is very useful
     ')
     =>
     string
     =>The macro 
     defn(`string')
     =>The macro dnl is very useful
     =>

   However, it is important to remember that `m4' rescanning is purely
textual.  If an unbalanced end-quote string occurs in a macro
definition, the rescan will see that embedded quote as the termination
of the quoted string, and the remainder of the macro's definition will
be rescanned unquoted.  Thus it is a good idea to avoid unbalanced
end-quotes in macro definitions or arguments to macros.

     define(`foo', a'a)
     =>
     define(`a', `A')
     =>
     define(`echo', `$@')
     =>
     foo
     =>A'A
     defn(`foo')
     =>aA'
     echo(foo)
     =>AA'

   Using `defn' to generate special tokens for builtin macros outside
of expected contexts can sometimes trigger warnings.  But most of the
time, such tokens are silently converted to the empty string.

     defn(`defn')
     =>
     define(defn(`divnum'), `cannot redefine a builtin token')
     error-->m4:stdin:2: Warning: define: invalid macro name ignored
     =>
     divnum
     =>0


File: m4.info,  Node: Pushdef,  Next: Indir,  Prev: Defn,  Up: Definitions

5.6 Temporarily redefining macros
=================================

It is possible to redefine a macro temporarily, reverting to the
previous definition at a later time.  This is done with the builtins
`pushdef' and `popdef':

 -- Builtin: pushdef (NAME, [EXPANSION])
 -- Builtin: popdef (NAME...)
     Analogous to `define' and `undefine'.

     These macros work in a stack-like fashion.  A macro is temporarily
     redefined with `pushdef', which replaces an existing definition of
     NAME, while saving the previous definition, before the new one is
     installed.  If there is no previous definition, `pushdef' behaves
     exactly like `define'.

     If a macro has several definitions (of which only one is
     accessible), the topmost definition can be removed with `popdef'.
     If there is no previous definition, `popdef' behaves like
     `undefine'.

     The expansion of both `pushdef' and `popdef' is void.  The macros
     `pushdef' and `popdef' are recognized only with parameters.

     define(`foo', `Expansion one.')
     =>
     foo
     =>Expansion one.
     pushdef(`foo', `Expansion two.')
     =>
     foo
     =>Expansion two.
     pushdef(`foo', `Expansion three.')
     =>
     pushdef(`foo', `Expansion four.')
     =>
     popdef(`foo')
     =>
     foo
     =>Expansion three.
     popdef(`foo', `foo')
     =>
     foo
     =>Expansion one.
     popdef(`foo')
     =>
     foo
     =>foo

   If a macro with several definitions is redefined with `define', the
topmost definition is _replaced_ with the new definition.  If it is
removed with `undefine', _all_ the definitions are removed, and not
only the topmost one.

     define(`foo', `Expansion one.')
     =>
     foo
     =>Expansion one.
     pushdef(`foo', `Expansion two.')
     =>
     foo
     =>Expansion two.
     define(`foo', `Second expansion two.')
     =>
     foo
     =>Second expansion two.
     undefine(`foo')
     =>
     foo
     =>foo

   Local variables within macros are made with `pushdef' and `popdef'.
At the start of the macro a new definition is pushed, within the macro
it is manipulated and at the end it is popped, revealing the former
definition.

   It is possible to temporarily redefine a builtin with `pushdef' and
`defn'.


File: m4.info,  Node: Indir,  Next: Builtin,  Prev: Pushdef,  Up: Definitions

5.7 Indirect call of macros
===========================

Any macro can be called indirectly with `indir':

 -- Builtin: indir (NAME, [ARGS...])
     Results in a call to the macro NAME, which is passed the rest of
     the arguments ARGS.  If NAME is not defined, an error message is
     printed, and the expansion is void.

     The macro `indir' is recognized only with parameters.

   This can be used to call macros with computed or "invalid" names
(`define' allows such names to be defined):

     define(`$$internal$macro', `Internal macro (name `$0')')
     =>
     $$internal$macro
     =>$$internal$macro
     indir(`$$internal$macro')
     =>Internal macro (name $$internal$macro)

   The point is, here, that larger macro packages can have private
macros defined, that will not be called by accident.  They can _only_ be
called through the builtin `indir'.

   One other point to observe is that argument collection occurs before
`indir' invokes NAME, so if argument collection changes the value of
NAME, that will be reflected in the final expansion.  This is different
than the behavior when invoking macros directly, where the definition
that was in effect before argument collection is used.

     define(`f', `1')
     =>
     f(define(`f', `2'))
     =>1
     indir(`f', define(`f', `3'))
     =>3
     indir(`f', undefine(`f'))
     error-->m4:stdin:4: undefined macro `f'
     =>

   When handed the result of `defn' (*note Defn::) as one of its
arguments, `indir' defers to the invoked NAME for whether a token
representing a builtin is recognized or flattened to the empty string.

     indir(defn(`defn'), `divnum')
     error-->m4:stdin:1: Warning: indir: invalid macro name ignored
     =>
     indir(`define', defn(`defn'), `divnum')
     error-->m4:stdin:2: Warning: define: invalid macro name ignored
     =>
     indir(`define', `foo', defn(`divnum'))
     =>
     foo
     =>0
     indir(`divert', defn(`foo'))
     error-->m4:stdin:5: empty string treated as 0 in builtin `divert'
     =>


File: m4.info,  Node: Builtin,  Prev: Indir,  Up: Definitions

5.8 Indirect call of builtins
=============================

Builtin macros can be called indirectly with `builtin':

 -- Builtin: builtin (NAME, [ARGS...])
     Results in a call to the builtin NAME, which is passed the rest of
     the arguments ARGS.  If NAME does not name a builtin, an error
     message is printed, and the expansion is void.

     The macro `builtin' is recognized only with parameters.

   This can be used even if NAME has been given another definition that
has covered the original, or been undefined so that no macro maps to
the builtin.

     pushdef(`define', `hidden')
     =>
     undefine(`undefine')
     =>
     define(`foo', `bar')
     =>hidden
     foo
     =>foo
     builtin(`define', `foo', defn(`divnum'))
     =>
     foo
     =>0
     builtin(`define', `foo', `BAR')
     =>
     foo
     =>BAR
     undefine(`foo')
     =>undefine(foo)
     foo
     =>BAR
     builtin(`undefine', `foo')
     =>
     foo
     =>foo

   The NAME argument only matches the original name of the builtin,
even when the `--prefix-builtins' option (or `-P', *note Invoking m4:
Operation modes.) is in effect.  This is different from `indir', which
only tracks current macro names.

   Note that `indir' and `builtin' can be used to invoke builtins
without arguments, even when they normally require parameters to be
recognized; but it will provoke a warning, and result in a void
expansion.

     builtin
     =>builtin
     builtin()
     error-->m4:stdin:2: undefined builtin `'
     =>
     builtin(`builtin')
     error-->m4:stdin:3: Warning: too few arguments to builtin `builtin'
     =>
     builtin(`builtin',)
     error-->m4:stdin:4: undefined builtin `'
     =>


File: m4.info,  Node: Conditionals,  Next: Debugging,  Prev: Definitions,  Up: Top

6 Conditionals, loops, and recursion
************************************

Macros, expanding to plain text, perhaps with arguments, are not quite
enough.  We would like to have macros expand to different things, based
on decisions taken at run-time.  For that, we need some kind of
conditionals.  Also, we would like to have some kind of loop construct,
so we could do something a number of times, or while some condition is
true.

* Menu:

* Ifdef::                       Testing if a macro is defined
* Ifelse::                      If-else construct, or multibranch
* Shift::                       Recursion in `m4'
* Forloop::                     Iteration by counting
* Foreach::                     Iteration by list contents


File: m4.info,  Node: Ifdef,  Next: Ifelse,  Up: Conditionals

6.1 Testing if a macro is defined
=================================

There are two different builtin conditionals in `m4'.  The first is
`ifdef':

 -- Builtin: ifdef (NAME, STRING-1, [STRING-2])
     If NAME is defined as a macro, `ifdef' expands to STRING-1,
     otherwise to STRING-2.  If STRING-2 is omitted, it is taken to be
     the empty string (according to the normal rules).

     The macro `ifdef' is recognized only with parameters.

     ifdef(`foo', ``foo' is defined', ``foo' is not defined')
     =>foo is not defined
     define(`foo', `')
     =>
     ifdef(`foo', ``foo' is defined', ``foo' is not defined')
     =>foo is defined
     ifdef(`no_such_macro', `yes', `no', `extra argument')
     error-->m4:stdin:4: Warning: excess arguments to builtin `ifdef' ignored
     =>no


File: m4.info,  Node: Ifelse,  Next: Shift,  Prev: Ifdef,  Up: Conditionals

6.2 If-else construct, or multibranch
=====================================

The other conditional, `ifelse', is much more powerful.  It can be used
as a way to introduce a long comment, as an if-else construct, or as a
multibranch, depending on the number of arguments supplied:

 -- Builtin: ifelse (COMMENT)
 -- Builtin: ifelse (STRING-1, STRING-2, EQUAL, [NOT-EQUAL])
 -- Builtin: ifelse (STRING-1, STRING-2, EQUAL-1, STRING-3, STRING-4,
          EQUAL-2, ...)
     Used with only one argument, the `ifelse' simply discards it and
     produces no output.

     If called with three or four arguments, `ifelse' expands into
     EQUAL, if STRING-1 and STRING-2 are equal (character for
     character), otherwise it expands to NOT-EQUAL.  A final fifth
     argument is ignored, after triggering a warning.

     If called with six or more arguments, and STRING-1 and STRING-2
     are equal, `ifelse' expands into EQUAL-1, otherwise the first
     three arguments are discarded and the processing starts again.

     The macro `ifelse' is recognized only with parameters.

   Using only one argument is a common `m4' idiom for introducing a
block comment, as an alternative to repeatedly using `dnl'.  This
special usage is recognized by GNU `m4', so that in this case, the
warning about missing arguments is never triggered.

     ifelse(`some comments')
     =>
     ifelse(`foo', `bar')
     error-->m4:stdin:2: Warning: too few arguments to builtin `ifelse'
     =>

   Using three or four arguments provides decision points.

     ifelse(`foo', `bar', `true')
     =>
     ifelse(`foo', `foo', `true')
     =>true
     define(`foo', `bar')
     =>
     ifelse(foo, `bar', `true', `false')
     =>true
     ifelse(foo, `foo', `true', `false')
     =>false

   Notice how the first argument was used unquoted; it is common to
compare the expansion of a macro with a string.  With this macro, you
can now reproduce the behavior of many of the builtins, where the macro
is recognized only with arguments.

     define(`foo', `ifelse(`$#', `0', ``$0'', `arguments:$#')')
     =>
     foo
     =>foo
     foo()
     =>arguments:1
     foo(`a', `b', `c')
     =>arguments:3

   However, `ifelse' can take more than four arguments.  If given more
than four arguments, `ifelse' works like a `case' or `switch' statement
in traditional programming languages.  If STRING-1 and STRING-2 are
equal, `ifelse' expands into EQUAL-1, otherwise the procedure is
repeated with the first three arguments discarded.  This calls for an
example:

     ifelse(`foo', `bar', `third', `gnu', `gnats')
     error-->m4:stdin:1: Warning: excess arguments to builtin `ifelse' ignored
     =>gnu
     ifelse(`foo', `bar', `third', `gnu', `gnats', `sixth')
     =>
     ifelse(`foo', `bar', `third', `gnu', `gnats', `sixth', `seventh')
     =>seventh
     ifelse(`foo', `bar', `3', `gnu', `gnats', `6', `7', `8')
     error-->m4:stdin:4: Warning: excess arguments to builtin `ifelse' ignored
     =>7

   Naturally, the normal case will be slightly more advanced than these
examples.  A common use of `ifelse' is in macros implementing loops of
various kinds.


File: m4.info,  Node: Shift,  Next: Forloop,  Prev: Ifelse,  Up: Conditionals

6.3 Recursion in `m4'
=====================

There is no direct support for loops in `m4', but macros can be
recursive.  There is no limit on the number of recursion levels, other
than those enforced by your hardware and operating system.

   Loops can be programmed using recursion and the conditionals
described previously.

   There is a builtin macro, `shift', which can, among other things, be
used for iterating through the actual arguments to a macro:

 -- Builtin: shift (ARG1, ...)
     Takes any number of arguments, and expands to all its arguments
     except ARG1, separated by commas, with each argument quoted.

     The macro `shift' is recognized only with parameters.

     shift
     =>shift
     shift(`bar')
     =>
     shift(`foo', `bar', `baz')
     =>bar,baz

   An example of the use of `shift' is this macro:

 -- Composite: reverse (...)
     Takes any number of arguments, and reverses their order.

   It is implemented as:

     define(`reverse', `ifelse(`$#', `0', , `$#', `1', ``$1'',
                               `reverse(shift($@)), `$1'')')
     =>
     reverse
     =>
     reverse(`foo')
     =>foo
     reverse(`foo', `bar', `gnats', `and gnus')
     =>and gnus, gnats, bar, foo

   While not a very interesting macro, it does show how simple loops
can be made with `shift', `ifelse' and recursion.  It also shows that
`shift' is usually used with `$@'.  Sometimes, a recursive algorithm
requires adding quotes to each element:

 -- Composite: quote (...)
 -- Composite: dquote (...)
 -- Composite: dquote_elt (...)
     Takes any number of arguments, and adds quoting.  With `quote',
     only one level of quoting is added, effectively removing whitespace
     after commas and turning multiple arguments into a single string.
     With `dquote', two levels of quoting are added, one around each
     element, and one around the list.  And with `dquote_elt', two
     levels of quoting are added around each element.

   An actual implementation of these three macros is distributed as
`m4-1.4.8/examples/quote.m4' in this package.  First, let's examine
their usage:

     include(`quote.m4')
     =>
     -quote-dquote-dquote_elt-
     =>----
     -quote()-dquote()-dquote_elt()-
     =>--`'-`'-
     -quote(`1')-dquote(`1')-dquote_elt(`1')-
     =>-1-`1'-`1'-
     -quote(`1', `2')-dquote(`1', `2')-dquote_elt(`1', `2')-
     =>-1,2-`1',`2'-`1',`2'-
     define(`n', `$#')dnl
     -n(quote(`1', `2'))-n(dquote(`1', `2'))-n(dquote_elt(`1', `2'))-
     =>-1-1-2-
     dquote(dquote_elt(`1', `2'))
     =>``1'',``2''
     dquote_elt(dquote(`1', `2'))
     =>``1',`2''

   The last two lines show that when given two arguments, `dquote'
results in one string, while `dquote_elt' results in two.  Now, examine
the implementation.  Note that `quote' and `dquote_elt' make decisions
based on their number of arguments, so that when called without
arguments, they result in nothing instead of a quoted empty string;
this is so that it is possible to distinquish between no arguments and
an empty first argument.  `dquote', on the other hand, results in a
string no matter what, since it is still possible to tell whether it
was invoked without arguments based on the resulting string.

     undivert(`quote.m4')dnl
     =>divert(`-1')
     =># quote(args) - convert args to single-quoted string
     =>define(`quote', `ifelse(`$#', `0', `', ``$*'')')
     =># dquote(args) - convert args to quoted list of quoted strings
     =>define(`dquote', ``$@'')
     =># dquote_elt(args) - convert args to list of double-quoted strings
     =>define(`dquote_elt', `ifelse(`$#', `0', `', `$#', `1', ```$1''',
     =>                             ```$1'',$0(shift($@))')')
     =>divert`'dnl


File: m4.info,  Node: Forloop,  Next: Foreach,  Prev: Shift,  Up: Conditionals

6.4 Iteration by counting
=========================

Here is an example of a loop macro that implements a simple for loop.

 -- Composite: forloop (ITERATOR, START, END, TEXT)
     Takes the name in ITERATOR, which must be a valid macro name, and
     successively assign it each integer value from START to END,
     inclusive.  For each assignment to ITERATOR, append TEXT to the
     expansion of the `forloop'.  TEXT may refer to ITERATOR.  Any
     definition of ITERATOR prior to this invocation is restored.

   It can, for example, be used for simple counting:

     include(`forloop.m4')
     =>
     forloop(`i', `1', `8', `i ')
     =>1 2 3 4 5 6 7 8 

   For-loops can be nested, like:

     include(`forloop.m4')
     =>
     forloop(`i', `1', `4', `forloop(`j', `1', `8', ` (i, j)')
     ')
     => (1, 1) (1, 2) (1, 3) (1, 4) (1, 5) (1, 6) (1, 7) (1, 8)
     => (2, 1) (2, 2) (2, 3) (2, 4) (2, 5) (2, 6) (2, 7) (2, 8)
     => (3, 1) (3, 2) (3, 3) (3, 4) (3, 5) (3, 6) (3, 7) (3, 8)
     => (4, 1) (4, 2) (4, 3) (4, 4) (4, 5) (4, 6) (4, 7) (4, 8)
     =>

   The implementation of the `forloop' macro is fairly straightforward.
The `forloop' macro itself is simply a wrapper, which saves the
previous definition of the first argument, calls the internal macro
`_forloop', and re-establishes the saved definition of the first
argument.

   The macro `_forloop' expands the fourth argument once, and tests to
see if the iterator has reached the final value.  If it has not
finished, it increments the iterator (using the predefined macro
`incr', *note Incr::), and recurses.

   Here is an actual implementation of `forloop', distributed as
`m4-1.4.8/examples/forloop.m4' in this package:

     undivert(`forloop.m4')dnl
     =>divert(`-1')
     =># forloop(var, from, to, stmt) - simple version
     =>define(`forloop', `pushdef(`$1', `$2')_forloop($@)popdef(`$1')')
     =>define(`_forloop',
     =>       `$4`'ifelse($1, `$3', `', `define(`$1', incr($1))$0($@)')')
     =>divert`'dnl

   Notice the careful use of quotes.  Certain macro arguments are left
unquoted, each for its own reason.  Try to find out _why_ these
arguments are left unquoted, and see what happens if they are quoted.
(As presented, these two macros are useful but not very robust for
general use.  They lack even basic error handling for cases like START
less than END, END not numeric, or ITERATOR not being a macro name.
See if you can improve these macros; or *note Answers: Improved
forloop.).


File: m4.info,  Node: Foreach,  Prev: Forloop,  Up: Conditionals

6.5 Iteration by list contents
==============================

Here is an example of a loop macro that implements list iteration.

 -- Composite: foreach (ITERATOR, PAREN-LIST, TEXT)
 -- Composite: foreachq (ITERATOR, QUOTE-LIST, TEXT)
     Takes the name in ITERATOR, which must be a valid macro name, and
     successively assign it each value from PAREN-LIST or QUOTE-LIST.
     In `foreach', PAREN-LIST is a comma-separated list of elements
     contained in parentheses.  In `foreachq', QUOTE-LIST is a
     comma-separated list of elements contained in a quoted string.
     For each assignment to ITERATOR, append TEXT to the overall
     expansion.  TEXT may refer to ITERATOR.  Any definition of
     ITERATOR prior to this invocation is restored.

   As an example, this displays each word in a list inside of a
sentence, using an implementation of `foreach' distributed as
`m4-1.4.8/examples/foreach.m4', and `foreachq' in
`m4-1.4.8/examples/foreachq.m4'.

     include(`foreach.m4')
     =>
     foreach(`x', (foo, bar, foobar), `Word was: x
     ')dnl
     =>Word was: foo
     =>Word was: bar
     =>Word was: foobar
     include(`foreachq.m4')
     =>
     foreachq(`x', `foo, bar, foobar', `Word was: x
     ')dnl
     =>Word was: foo
     =>Word was: bar
     =>Word was: foobar

   It is possible to be more complex; each element of the PAREN-LIST or
QUOTE-LIST can itself be a list, to pass as further arguments to a
helper macro.  This example generates a shell case statement:

     include(`foreach.m4')
     =>
     define(`_case', `  $1)
         $2=" $1";;
     ')dnl
     define(`_cat', `$1$2')dnl
     case $`'1 in
     =>case $1 in
     foreach(`x', `(`(`a', `vara')', `(`b', `varb')', `(`c', `varc')')',
             `_cat(`_case', x)')dnl
     =>  a)
     =>    vara=" a";;
     =>  b)
     =>    varb=" b";;
     =>  c)
     =>    varc=" c";;
     esac
     =>esac

   The implementation of the `foreach' macro is a bit more involved; it
is a wrapper around two helper macros.  First, `_arg1' is needed to
grab the first element of a list.  Second, `_foreach' implements the
recursion, successively walking through the original list.  Here is a
simple implementation of `foreach':

     undivert(`foreach.m4')dnl
     =>divert(`-1')
     =># foreach(x, (item_1, item_2, ..., item_n), stmt)
     =>#   parenthesized list, simple version
     =>define(`foreach', `pushdef(`$1')_foreach($@)popdef(`$1')')
     =>define(`_arg1', `$1')
     =>define(`_foreach', `ifelse(`$2', `()', `',
     =>  `define(`$1', _arg1$2)$3`'$0(`$1', (shift$2), `$3')')')
     =>divert`'dnl

   Unfortunately, that implementation is not robust to macro names as
list elements.  Each iteration of `_foreach' is stripping another layer
of quotes, leading to erratic results if list elements are not already
fully expanded.  The first cut at implementing `foreachq' takes this
into account.  Also, when using quoted elements in a PAREN-LIST, the
overall list must be quoted.  A QUOTE-LIST has the nice property of
requiring fewer characters to create a list containing the same quoted
elements.  To see the difference between the two macros, we attempt to
pass double-quoted macro names in a list, expecting the macro name on
output after one layer of quotes is removed during list iteration and
the final layer removed during the final rescan:

     define(`a', `1')define(`b', `2')define(`c', `3')
     =>
     include(`foreach.m4')
     =>
     include(`foreachq.m4')
     =>
     foreach(`x', `(``a'', ``(b'', ``c)'')', `x
     ')
     =>1
     =>(2)1
     =>
     =>, x
     =>)
     foreachq(`x', ```a'', ``(b'', ``c)''', `x
     ')dnl
     =>a
     =>(b
     =>c)

   Obviously, `foreachq' did a better job; here is its implementation:

     undivert(`foreachq.m4')dnl
     =>include(`quote.m4')dnl
     =>divert(`-1')
     =># foreachq(x, `item_1, item_2, ..., item_n', stmt)
     =>#   quoted list, simple version
     =>define(`foreachq', `pushdef(`$1')_foreachq($@)popdef(`$1')')
     =>define(`_arg1', `$1')
     =>define(`_foreachq', `ifelse(quote($2), `', `',
     =>  `define(`$1', `_arg1($2)')$3`'$0(`$1', `shift($2)', `$3')')')
     =>divert`'dnl

   Notice that `_foreachq' had to use the helper macro `quote' defined
earlier (*note Shift::), to ensure that the embedded `ifelse' call does
not go haywire if a list element contains a comma.  Unfortunately, this
implementation of `foreachq' has its own severe flaw.  Whereas the
`foreach' implementation was linear, this macro is quadratic in the
number of list elements, and is much more likely to trip up the limit
set by the command line option `--nesting-limit' (or `-L', *note
Invoking m4: Limits control.).  (It is possible to have robust
iteration with linear behavior for either list style.  See if you can
learn from the best elements of both of these implementations to create
robust macros; or *note Answers: Improved foreach.).


File: m4.info,  Node: Debugging,  Next: Input Control,  Prev: Conditionals,  Up: Top

7 How to debug macros and input
*******************************

When writing macros for `m4', they often do not work as intended on the
first try (as is the case with most programming languages).
Fortunately, there is support for macro debugging in `m4'.

* Menu:

* Dumpdef::                     Displaying macro definitions
* Trace::                       Tracing macro calls
* Debug Levels::                Controlling debugging output
* Debug Output::                Saving debugging output


File: m4.info,  Node: Dumpdef,  Next: Trace,  Up: Debugging

7.1 Displaying macro definitions
================================

If you want to see what a name expands into, you can use the builtin
`dumpdef':

 -- Builtin: dumpdef ([NAMES...])
     Accepts any number of arguments.  If called without any arguments,
     it displays the definitions of all known names, otherwise it
     displays the definitions of the NAMES given.  The output is
     printed to the current debug file (usually standard error), and is
     sorted by name.  If an unknown name is encountered, a warning is
     printed.

     The expansion of `dumpdef' is void.

     define(`foo', `Hello world.')
     =>
     dumpdef(`foo')
     error-->foo: `Hello world.'
     =>
     dumpdef(`define')
     error-->define: <define>
     =>

   The last example shows how builtin macros definitions are displayed.
The definition that is dumped corresponds to what would occur if the
macro were to be called at that point, even if other definitions are
still live due to redefining a macro during argument collection.

     pushdef(`f', ``$0'1')pushdef(`f', ``$0'2')
     =>
     f(popdef(`f')dumpdef(`f'))
     error-->f: ``$0'1'
     =>f2
     f(popdef(`f')dumpdef(`f'))
     error-->m4:stdin:3: undefined macro `f'
     =>f1

   *Note Debug Levels::, for information on controlling the details of
the display.


File: m4.info,  Node: Trace,  Next: Debug Levels,  Prev: Dumpdef,  Up: Debugging

7.2 Tracing macro calls
=======================

It is possible to trace macro calls and expansions through the builtins
`traceon' and `traceoff':

 -- Builtin: traceon ([NAMES...])
 -- Builtin: traceoff ([NAMES...])
     When called without any arguments, `traceon' and `traceoff' will
     turn tracing on and off, respectively, for all defined macros.

     When called with arguments, only the macros listed in NAMES are
     affected, whether or not they are currently defined.

     The expansion of `traceon' and `traceoff' is void.

   Whenever a traced macro is called and the arguments have been
collected, the call is displayed.  If the expansion of the macro call
is not void, the expansion can be displayed after the call.  The output
is printed to the current debug file (defaulting to standard error,
*note Debug Output::).

     define(`foo', `Hello World.')
     =>
     define(`echo', `$@')
     =>
     traceon(`foo', `echo')
     =>
     foo
     error-->m4trace: -1- foo -> `Hello World.'
     =>Hello World.
     echo(`gnus', `and gnats')
     error-->m4trace: -1- echo(`gnus', `and gnats') -> ``gnus',`and gnats''
     =>gnus,and gnats

   The number between dashes is the depth of the expansion.  It is one
most of the time, signifying an expansion at the outermost level, but it
increases when macro arguments contain unquoted macro calls.  The
maximum number that will appear between dashes is controlled by the
option `--nesting-limit' (*note Invoking m4: Limits control.).

   Tracing by name is an attribute that is preserved whether the macro
is defined or not.  This allows the `-t' option to select macros to
trace before those macros are defined.

     traceoff(`foo')
     =>
     traceon(`foo')
     =>
     foo
     =>foo
     define(`foo', `bar')
     =>
     foo
     error-->m4trace: -1- foo -> `bar'
     =>bar
     undefine(`foo')
     =>
     ifdef(`foo', `yes', `no')
     =>no
     indir(`foo')
     error-->m4:stdin:8: undefined macro `foo'
     =>
     define(`foo', `blah')
     =>
     foo
     error-->m4trace: -1- foo -> `blah'
     =>blah
     traceoff
     =>
     foo
     =>blah

   Tracing even works on builtins.  However, `defn' (*note Defn::) does
not transfer tracing status.

     traceon(`eval', `m4_divnum')
     =>
     define(`m4_eval', defn(`eval'))
     =>
     define(`m4_divnum', defn(`divnum'))
     =>
     eval(divnum)
     error-->m4trace: -1- eval(`0') -> `0'
     =>0
     m4_eval(m4_divnum)
     error-->m4trace: -2- m4_divnum -> `0'
     =>0

   *Note Debug Levels::, for information on controlling the details of
the display.


File: m4.info,  Node: Debug Levels,  Next: Debug Output,  Prev: Trace,  Up: Debugging

7.3 Controlling debugging output
================================

The `-d' option to `m4' (*note Invoking m4: Debugging options.)
controls the amount of details presented, when using the macros
described in the preceding sections.

   The FLAGS following the option can be one or more of the following:

`a'
     Show the actual arguments in each macro call.  This applies to all
     macro calls if the `t' flag is used, otherwise only the macros
     covered by calls of `traceon'.

`c'
     Show several trace lines for each macro call.  A line is shown
     when the macro is seen, but before the arguments are collected; a
     second line when the arguments have been collected and a third
     line after the call has completed.

`e'
     Show the expansion of each macro call, if it is not void.  This
     applies to all macro calls if the `t' flag is used, otherwise only
     the macros covered by calls of `traceon'.

`f'
     Show the name of the current input file in each trace output line.

`i'
     Print a message each time the current input file is changed,
     giving file name and input line number.

`l'
     Show the current input line number in each trace output line.

`p'
     Print a message when a named file is found through the path search
     mechanism (*note Search Path::), giving the actual file name used.

`q'
     Quote actual arguments and macro expansions in the display with the
     current quotes.

`t'
     Trace all macro calls made in this invocation of `m4'.

`x'
     Add a unique `macro call id' to each line of the trace output.
     This is useful in connection with the `c' flag above.

`V'
     A shorthand for all of the above flags.

   If no flags are specified with the `-d' option, the default is
`aeq'.  The examples throughout this manual assume the default flags.

   There is a builtin macro `debugmode', which allows on-the-fly
control of the debugging output format:

 -- Builtin: debugmode ([FLAGS])
     The argument FLAGS should be a subset of the letters listed above.
     As special cases, if the argument starts with a `+', the flags are
     added to the current debug flags, and if it starts with a `-', they
     are removed.  If no argument is present, all debugging flags are
     cleared (as if no `-d' was given), and with an empty argument the
     flags are reset to the default of `aeq'.

     The expansion of `debugmode' is void.

     define(`foo', `FOO')
     =>
     traceon(`foo')
     =>
     debugmode()
     =>
     foo
     error-->m4trace: -1- foo -> `FOO'
     =>FOO
     debugmode
     =>
     foo
     error-->m4trace: -1- foo
     =>FOO
     debugmode(`+l')
     =>
     foo
     error-->m4trace:8: -1- foo
     =>FOO


File: m4.info,  Node: Debug Output,  Prev: Debug Levels,  Up: Debugging

7.4 Saving debugging output
===========================

Debug and tracing output can be redirected to files using either the
`--debugfile' option to `m4' (*note Invoking m4: Debugging options.),
or with the builtin macro `debugfile':

 -- Builtin: debugfile ([FILE])
     Sends all further debug and trace output to FILE, opened in append
     mode.  If FILE is the empty string, debug and trace output are
     discarded.  If `debugfile' is called without any arguments, debug
     and trace output are sent to standard error.  This does not affect
     warnings, error messages, or `errprint' output, which are always
     sent to standard error.  If FILE cannot be opened, the current
     debug file is unchanged, and an error is issued.

     The expansion of `debugfile' is void.

     traceon(`divnum')
     =>
     divnum(`extra')
     error-->m4:stdin:2: Warning: excess arguments to builtin `divnum' ignored
     error-->m4trace: -1- divnum(`extra') -> `0'
     =>0
     debugfile()
     =>
     divnum(`extra')
     error-->m4:stdin:4: Warning: excess arguments to builtin `divnum' ignored
     =>0
     debugfile
     =>
     divnum
     error-->m4trace: -1- divnum -> `0'
     =>0


File: m4.info,  Node: Input Control,  Next: File Inclusion,  Prev: Debugging,  Up: Top

8 Input control
***************

This chapter describes various builtin macros for controlling the input
to `m4'.

* Menu:

* Dnl::                         Deleting whitespace in input
* Changequote::                 Changing the quote characters
* Changecom::                   Changing the comment delimiters
* Changeword::                  Changing the lexical structure of words
* M4wrap::                      Saving text until end of input


File: m4.info,  Node: Dnl,  Next: Changequote,  Up: Input Control

8.1 Deleting whitespace in input
================================

The builtin `dnl' stands for "Discard to Next Line":

 -- Builtin: dnl
     All characters, up to and including the next newline, are discarded
     without performing any macro expansion.  A warning is issued if
     the end of the file is encountered without a newline.

     The expansion of `dnl' is void.

   It is often used in connection with `define', to remove the newline
that follows the call to `define'.  Thus

     define(`foo', `Macro `foo'.')dnl A very simple macro, indeed.
     foo
     =>Macro foo.

   The input up to and including the next newline is discarded, as
opposed to the way comments are treated (*note Comments::).

   Usually, `dnl' is immediately followed by an end of line or some
other whitespace.  GNU `m4' will produce a warning diagnostic if `dnl'
is followed by an open parenthesis.  In this case, `dnl' will collect
and process all arguments, looking for a matching close parenthesis.
All predictable side effects resulting from this collection will take
place.  `dnl' will return no output.  The input following the matching
close parenthesis up to and including the next newline, on whatever
line containing it, will still be discarded.

     dnl(`args are ignored, but side effects occur',
     define(`foo', `like this')) while this text is ignored: undefine(`foo')
     error-->m4:stdin:1: Warning: excess arguments to builtin `dnl' ignored
     See how `foo' was defined, foo?
     =>See how foo was defined, like this?

   If the end of file is encountered without a newline character, a
warning is issued and dnl stops consuming input.

     m4wrap(`m4wrap(`2 hi
     ')0 hi dnl 1 hi')
     =>
     define(`hi', `HI')
     =>
     ^D
     error-->m4:stdin:1: Warning: end of file treated as newline
     =>0 HI 2 HI


File: m4.info,  Node: Changequote,  Next: Changecom,  Prev: Dnl,  Up: Input Control

8.2 Changing the quote characters
=================================

The default quote delimiters can be changed with the builtin
`changequote':

 -- Builtin: changequote ([START = ``'], [END = `''])
     This sets START as the new begin-quote delimiter and END as the
     new end-quote delimiter.  If both arguments are missing, the
     default quotes (``' and `'') are used.  If START is void, then
     quoting is disabled.  Otherwise, if END is missing or void, the
     default end-quote delimiter (`'') is used.  The quote delimiters
     can be of any length.

     The expansion of `changequote' is void.

     changequote(`[', `]')
     =>
     define([foo], [Macro [foo].])
     =>
     foo
     =>Macro foo.

   The quotation strings can safely contain eight-bit characters.  If
no single character is appropriate, START and END can be of any length.
Other implementations cap the delimiter length to five characters, but
GNU has no inherent limit.

     changequote(`[[[', `]]]')
     =>
     define([[[foo]]], [[[Macro [[[[[foo]]]]].]]])
     =>
     foo
     =>Macro [[foo]].

   Calling `changequote' with START as the empty string will
effectively disable the quoting mechanism, leaving no way to quote text.
However, using an empty string is not portable, as some other
implementations of `m4' revert to the default quoting, while others
preserve the prior non-empty delimiter.  If START is not empty, then an
empty END will use the default end-quote delimiter of `'', as
otherwise, it would be impossible to end a quoted string.  Again, this
is not portable, as some other `m4' implementations reuse START as the
end-quote delimiter, while others preserve the previous non-empty
value.  Omitting both arguments restores the default begin-quote and
end-quote delimiters; fortunately this behavior is portable to all
implementations of `m4'.

     define(`foo', `Macro `FOO'.')
     =>
     changequote(`', `')
     =>
     foo
     =>Macro `FOO'.
     `foo'
     =>`Macro `FOO'.'
     changequote(`,)
     =>
     foo
     =>Macro FOO.

   There is no way in `m4' to quote a string containing an unmatched
begin-quote, except using `changequote' to change the current quotes.

   If the quotes should be changed from, say, `[' to `[[', temporary
quote characters have to be defined.  To achieve this, two calls of
`changequote' must be made, one for the temporary quotes and one for
the new quotes.

   Macros are recognized in preference to the begin-quote string, so if
a prefix of START can be recognized as part of a potential macro name,
the quoting mechanism is effectively disabled.  Unless you use
`changeword' (*note Changeword::), this means that START should not
begin with a letter, digit, or `_' (underscore).  However, even though
quoted strings are not recognized, the quote characters can still be
discerned in macro expansion and in trace output.

     define(`echo', `$@')
     =>
     define(`hi', `HI')
     =>
     changequote(`q', `Q')
     =>
     q hi Q hi
     =>q HI Q HI
     echo(hi)
     =>qHIQ
     changequote
     =>
     changequote(`-', `EOF')
     =>
     - hi EOF hi
     => hi  HI
     changequote
     =>
     changequote(`1', `2')
     =>
     hi1hi2
     =>hi1hi2
     hi 1hi2
     =>HI hi

   Quotes are recognized in preference to argument collection.  In
particular, if START is a single `(', then argument collection is
effectively disabled.  For portability with other implementations, it
is a good idea to avoid `(', `,', and `)' as the first character in
START.

     define(`echo', `$#:$@:')
     =>
     define(`hi', `HI')
     =>
     changequote(`(',`)')
     =>
     echo(hi)
     =>0::hi
     changequote
     =>
     changequote(`((', `))')
     =>
     echo(hi)
     =>1:HI:
     echo((hi))
     =>0::hi
     changequote
     =>
     changequote(`,', `)')
     =>
     echo(hi,hi)bye)
     =>1:HIhibye:

   If END is a prefix of START, the end-quote will be recognized in
preference to a nested begin-quote.  In particular, changing the quotes
to have the same string for START and END disables nesting of quotes.
When quote nesting is disabled, it is impossible to double-quote
strings across macro expansions, so using the same string is not done
very often.

     define(`hi', `HI')
     =>
     changequote(`""', `"')
     =>
     ""hi"""hi"
     =>hihi
     ""hi" ""hi"
     =>hi hi
     ""hi"" "hi"
     =>hi" "HI"
     changequote
     =>
     `hi`hi'hi'
     =>hi`hi'hi
     changequote(`"', `"')
     =>
     "hi"hi"hi"
     =>hiHIhi

   It is an error if the end of file occurs within a quoted string.

     `hello world'
     =>hello world
     `dangling quote
     ^D
     error-->m4:stdin:2: ERROR: end of file in string


File: m4.info,  Node: Changecom,  Next: Changeword,  Prev: Changequote,  Up: Input Control

8.3 Changing the comment delimiters
===================================

The default comment delimiters can be changed with the builtin macro
`changecom':

 -- Builtin: changecom ([START], [END = `<NL>'])
     This sets START as the new begin-comment delimiter and END as the
     new end-comment delimiter.  If both arguments are missing, or
     START is void, then comments are disabled.  Otherwise, if END is
     missing or void, the default end-comment delimiter of newline is
     used.  The comment delimiters can be of any length.

     The expansion of `changecom' is void.

     define(`comment', `COMMENT')
     =>
     # A normal comment
     =># A normal comment
     changecom(`/*', `*/')
     =>
     # Not a comment anymore
     =># Not a COMMENT anymore
     But: /* this is a comment now */ while this is not a comment
     =>But: /* this is a comment now */ while this is not a COMMENT

   Note how comments are copied to the output, much as if they were
quoted strings.  If you want the text inside a comment expanded, quote
the begin-comment delimiter.

   Calling `changecom' without any arguments, or with START as the
empty string, will effectively disable the commenting mechanism.  To
restore the original comment start of `#', you must explicitly ask for
it.  If START is not empty, then an empty END will use the default
end-comment delimiter of newline, as otherwise, it would be impossible
to end a comment.  However, this is not portable, as some other `m4'
implementations preserve the previous non-empty delimiters instead.

     define(`comment', `COMMENT')
     =>
     changecom
     =>
     # Not a comment anymore
     =># Not a COMMENT anymore
     changecom(`#', `')
     =>
     # comment again
     =># comment again

   The comment strings can safely contain eight-bit characters.  If no
single character is appropriate, START and END can be of any length.
Other implementations cap the delimiter length to five characters, but
GNU has no inherent limit.

   Comments are recognized in preference to macros.  However, this is
not compatible with other implementations, where macros and even quoting
takes precedence over comments, so it may change in a future release.
For portability, this means that START should not begin with a letter,
digit, or `_' (underscore), and that neither the start-quote nor the
start-comment string should be a prefix of the other.

     define(`hi', `HI')
     =>
     define(`hi1hi2', `hello')
     =>
     changecom(`q', `Q')
     =>
     q hi Q hi
     =>q hi Q HI
     changecom(`1', `2')
     =>
     hi1hi2
     =>hello
     hi 1hi2
     =>HI 1hi2

   Comments are recognized in preference to argument collection.  In
particular, if START is a single `(', then argument collection is
effectively disabled.  For portability with other implementations, it
is a good idea to avoid `(', `,', and `)' as the first character in
START.

     define(`echo', `$#:$@:')
     =>
     define(`hi', `HI')
     =>
     changecom(`(',`)')
     =>
     echo(hi)
     =>0::(hi)
     changecom
     =>
     changecom(`((', `))')
     =>
     echo(hi)
     =>1:HI:
     echo((hi))
     =>0::((hi))
     changecom(`,', `)')
     =>
     echo(hi,hi)bye)
     =>1:HI,hi)bye:

   It is an error if the end of file occurs within a comment.

     changecom(`/*', `*/')
     =>
     /*dangling comment
     ^D
     error-->m4:stdin:2: ERROR: end of file in comment


File: m4.info,  Node: Changeword,  Next: M4wrap,  Prev: Changecom,  Up: Input Control

8.4 Changing the lexical structure of words
===========================================

     The macro `changeword' and all associated functionality is
     experimental.  It is only available if the `--enable-changeword'
     option was given to `configure', at GNU `m4' installation time.
     The functionality will go away in the future, to be replaced by
     other new features that are more efficient at providing the same
     capabilities.  _Do not rely on it_.  Please direct your comments
     about it the same way you would do for bugs.

   A file being processed by `m4' is split into quoted strings, words
(potential macro names) and simple tokens (any other single character).
Initially a word is defined by the following regular expression:

     [_a-zA-Z][_a-zA-Z0-9]*

   Using `changeword', you can change this regular expression:

 -- Optional builtin: changeword (REGEX)
     Changes the regular expression for recognizing macro names to be
     REGEX.  If REGEX is empty, use `[_a-zA-Z][_a-zA-Z0-9]*'.  REGEX
     must obey the constraint that every prefix of the desired final
     pattern is also accepted by the regular expression.  If REGEX
     contains grouping parentheses, the macro invoked is the portion
     that matched the first group, rather than the entire matching
     string.

     The expansion of `changeword' is void.  The macro `changeword' is
     recognized only with parameters.

   Relaxing the lexical rules of `m4' might be useful (for example) if
you wanted to apply translations to a file of numbers:

     ifdef(`changeword', `', `errprint(` skipping: no changeword support
     ')m4exit(`77')')dnl
     changeword(`[_a-zA-Z0-9]+')
     =>
     define(`1', `0')1
     =>0

   Tightening the lexical rules is less useful, because it will
generally make some of the builtins unavailable.  You could use it to
prevent accidental call of builtins, for example:

     ifdef(`changeword', `', `errprint(` skipping: no changeword support
     ')m4exit(`77')')dnl
     define(`_indir', defn(`indir'))
     =>
     changeword(`_[_a-zA-Z0-9]*')
     =>
     esyscmd(`foo')
     =>esyscmd(foo)
     _indir(`esyscmd', `echo hi')
     =>hi
     =>

   Because `m4' constructs its words a character at a time, there is a
restriction on the regular expressions that may be passed to
`changeword'.  This is that if your regular expression accepts `foo',
it must also accept `f' and `fo'.

     ifdef(`changeword', `', `errprint(` skipping: no changeword support
     ')m4exit(`77')')dnl
     define(`foo
     ', `bar
     ')
     =>
     dnl This example wants to recognize changeword, dnl, and `foo\n'.
     dnl First, we check that our regexp will match.
     regexp(`changeword', `[cd][a-z]*\|foo[
     ]')
     =>0
     regexp(`foo
     ', `[cd][a-z]*\|foo[
     ]')
     =>0
     regexp(`f', `[cd][a-z]*\|foo[
     ]')
     =>-1
     foo
     =>foo
     changeword(`[cd][a-z]*\|foo[
     ]')
     =>
     dnl Even though `foo\n' matches, we forgot to allow `f'.
     foo
     =>foo
     changeword(`[cd][a-z]*\|fo*[
     ]?')
     =>
     dnl Now we can call `foo\n'.
     foo
     =>bar

   `changeword' has another function.  If the regular expression
supplied contains any grouped subexpressions, then text outside the
first of these is discarded before symbol lookup.  So:

     ifdef(`changeword', `', `errprint(` skipping: no changeword support
     ')m4exit(`77')')dnl
     ifdef(`__unix__', ,
           `errprint(` skipping: syscmd does not have unix semantics
     ')m4exit(`77')')dnl
     changecom(`/*', `*/')dnl
     define(`foo', `bar')dnl
     changeword(`#\([_a-zA-Z0-9]*\)')
     =>
     #esyscmd(`echo foo \#foo')
     =>foo bar
     =>

   `m4' now requires a `#' mark at the beginning of every macro
invocation, so one can use `m4' to preprocess plain text without losing
various words like `divert'.

   In `m4', macro substitution is based on text, while in TeX, it is
based on tokens.  `changeword' can throw this difference into relief.
For example, here is the same idea represented in TeX and `m4'.  First,
the TeX version:

     \def\a{\message{Hello}}
     \catcode`\@=0
     \catcode`\\=12
     @a
     @bye
     =>Hello

Then, the `m4' version:

     ifdef(`changeword', `', `errprint(` skipping: no changeword support
     ')m4exit(`77')')dnl
     define(`a', `errprint(`Hello')')dnl
     changeword(`@\([_a-zA-Z0-9]*\)')
     =>
     @a
     =>errprint(Hello)

   In the TeX example, the first line defines a macro `a' to print the
message `Hello'.  The second line defines <@> to be usable instead of
<\> as an escape character.  The third line defines <\> to be a normal
printing character, not an escape.  The fourth line invokes the macro
`a'.  So, when TeX is run on this file, it displays the message `Hello'.

   When the `m4' example is passed through `m4', it outputs
`errprint(Hello)'.  The reason for this is that TeX does lexical
analysis of macro definition when the macro is _defined_.  `m4' just
stores the text, postponing the lexical analysis until the macro is
_used_.

   You should note that using `changeword' will slow `m4' down by a
factor of about seven, once it is changed to something other than the
default regular expression.  You can invoke `changeword' with the empty
string to restore the default word definition, and regain the parsing
speed.


File: m4.info,  Node: M4wrap,  Prev: Changeword,  Up: Input Control

8.5 Saving text until end of input
==================================

It is possible to `save' some text until the end of the normal input has
been seen.  Text can be saved, to be read again by `m4' when the normal
input has been exhausted.  This feature is normally used to initiate
cleanup actions before normal exit, e.g., deleting temporary files.

   To save input text, use the builtin `m4wrap':

 -- Builtin: m4wrap (STRING, ...)
     Stores STRING in a safe place, to be reread when end of input is
     reached.  As a GNU extension, additional arguments are
     concatenated with a space to the STRING.

     The expansion of `m4wrap' is void.  The macro `m4wrap' is
     recognized only with parameters.

     define(`cleanup', `This is the `cleanup' action.
     ')
     =>
     m4wrap(`cleanup')
     =>
     This is the first and last normal input line.
     =>This is the first and last normal input line.
     ^D
     =>This is the cleanup action.

   The saved input is only reread when the end of normal input is seen,
and not if `m4exit' is used to exit `m4'.

   It is safe to call `m4wrap' from saved text, but then the order in
which the saved text is reread is undefined.  If `m4wrap' is not used
recursively, the saved pieces of text are reread in the opposite order
in which they were saved (LIFO--last in, first out).  However, this
behavior is likely to change in a future release, to match POSIX, so
you should not depend on this order.

   Here is an example of implementing a factorial function using
`m4wrap':

     define(`f', `ifelse(`$1', `0', `Answer: 0!=1
     ', eval(`$1>1'), `0', `Answer: $2$1=eval(`$2$1')
     ', `m4wrap(`f(decr(`$1'), `$2$1*')')')')
     =>
     f(`10')
     =>
     ^D
     =>Answer: 10*9*8*7*6*5*4*3*2*1=3628800

   Invocations of `m4wrap' at the same recursion level are concatenated
and rescanned as usual:

     define(`aa', `AA
     ')
     =>
     m4wrap(`a')m4wrap(`a')
     =>
     ^D
     =>AA

however, the transition between recursion levels behaves like an end of
file condition between two input files.

     m4wrap(`m4wrap(`)')len(abc')
     =>
     ^D
     error-->m4:stdin:1: ERROR: end of file in argument list


File: m4.info,  Node: File Inclusion,  Next: Diversions,  Prev: Input Control,  Up: Top

9 File inclusion
****************

`m4' allows you to include named files at any point in the input.

* Menu:

* Include::                     Including named files
* Search Path::                 Searching for include files


File: m4.info,  Node: Include,  Next: Search Path,  Up: File Inclusion

9.1 Including named files
=========================

There are two builtin macros in `m4' for including files:

 -- Builtin: include (FILE)
 -- Builtin: sinclude (FILE)
     Both macros cause the file named FILE to be read by `m4'.  When
     the end of the file is reached, input is resumed from the previous
     input file.

     The expansion of `include' and `sinclude' is therefore the
     contents of FILE.

     If FILE does not exist (or cannot be read), the expansion is void,
     and `include' will fail with an error while `sinclude' is silent.
     The empty string counts as a file that does not exist.

     The macros `include' and `sinclude' are recognized only with
     parameters.

     include(`none')
     error-->m4:stdin:1: cannot open `none': No such file or directory
     =>
     include()
     error-->m4:stdin:2: cannot open `': No such file or directory
     =>
     sinclude(`none')
     =>
     sinclude()
     =>

   The rest of this section assumes that `m4' is invoked with the `-I'
option (*note Invoking m4: Preprocessor features.)  pointing to the
`m4-1.4.8/examples' directory shipped as part of the GNU `m4' package.
The file `m4-1.4.8/examples/incl.m4' in the distribution contains the
lines:

     Include file start
     foo
     Include file end

   Normally file inclusion is used to insert the contents of a file
into the input stream.  The contents of the file will be read by `m4'
and macro calls in the file will be expanded:

     define(`foo', `FOO')
     =>
     include(`incl.m4')
     =>Include file start
     =>FOO
     =>Include file end
     =>

   The fact that `include' and `sinclude' expand to the contents of the
file can be used to define macros that operate on entire files.  Here
is an example, which defines `bar' to expand to the contents of
`incl.m4':

     define(`bar', include(`incl.m4'))
     =>
     This is `bar':  >>bar<<
     =>This is bar:  >>Include file start
     =>foo
     =>Include file end
     =><<

   This use of `include' is not trivial, though, as files can contain
quotes, commas, and parentheses, which can interfere with the way the
`m4' parser works.  GNU `m4' seamlessly concatenates the file contents
with the next character, even if the included file ended in the middle
of a comment, string, or macro call.  These conditions are only treated
as end of file errors if specified as input files on the command line.

   In GNU `m4', an alternative method of reading files is using
`undivert' (*note Undivert::) on a named file.


File: m4.info,  Node: Search Path,  Prev: Include,  Up: File Inclusion

9.2 Searching for include files
===============================

GNU `m4' allows included files to be found in other directories than
the current working directory.

   If the `--prepend-include' or `-B' command-line option was provided
(*note Invoking m4: Preprocessor features.), those directories are
searched first, in reverse order that those options were listed on the
command line.  Then `m4' looks in the current working directory.  Next
comes the directories specified with the `--include' or `-I' option, in
the order found on the command line.  Finally, if the `M4PATH'
environment variable is set, it is expected to contain a
colon-separated list of directories, which will be searched in order.

   If the automatic search for include-files causes trouble, the `p'
debug flag (*note Debug Levels::) can help isolate the problem.


File: m4.info,  Node: Diversions,  Next: Text handling,  Prev: File Inclusion,  Up: Top

10 Diverting and undiverting output
***********************************

Diversions are a way of temporarily saving output.  The output of `m4'
can at any time be diverted to a temporary file, and be reinserted into
the output stream, "undiverted", again at a later time.

   Numbered diversions are counted from 0 upwards, diversion number 0
being the normal output stream.  The number of simultaneous diversions
is limited mainly by the memory used to describe them, because GNU `m4'
tries to keep diversions in memory.  However, there is a limit to the
overall memory usable by all diversions taken altogether (512K,
currently).  When this maximum is about to be exceeded, a temporary
file is opened to receive the contents of the biggest diversion still
in memory, freeing this memory for other diversions.  When creating the
temporary file, `m4' honors the value of the environment variable
`TMPDIR', and falls back to `/tmp'.  So, it is theoretically possible
that the number and aggregate size of diversions is limited only by
available disk space.

   Diversions make it possible to generate output in a different order
than the input was read.  It is possible to implement topological
sorting dependencies.  For example, GNU Autoconf makes use of
diversions under the hood to ensure that the expansion of a prerequisite
macro appears in the output prior to the expansion of a dependent macro,
regardless of which order the two macros were invoked in the user's
input file.

* Menu:

* Divert::                      Diverting output
* Undivert::                    Undiverting output
* Divnum::                      Diversion numbers
* Cleardivert::                 Discarding diverted text


File: m4.info,  Node: Divert,  Next: Undivert,  Up: Diversions

10.1 Diverting output
=====================

Output is diverted using `divert':

 -- Builtin: divert ([NUMBER = `0'])
     The current diversion is changed to NUMBER.  If NUMBER is left out
     or empty, it is assumed to be zero.  If NUMBER cannot be parsed,
     the diversion is unchanged.

     The expansion of `divert' is void.

   When all the `m4' input will have been processed, all existing
diversions are automatically undiverted, in numerical order.

     divert(`1')
     This text is diverted.
     divert
     =>
     This text is not diverted.
     =>This text is not diverted.
     ^D
     =>
     =>This text is diverted.

   Several calls of `divert' with the same argument do not overwrite
the previous diverted text, but append to it.  Diversions are printed
after any wrapped text is expanded.

     define(`text', `TEXT')
     =>
     divert(`1')`diverted text.'
     divert
     =>
     m4wrap(`Wrapped text preceeds ')
     =>
     ^D
     =>Wrapped TEXT preceeds diverted text.

   If output is diverted to a negative diversion, it is simply
discarded.  This can be used to suppress unwanted output.  A common
example of unwanted output is the trailing newlines after macro
definitions.  Here is a common programming idiom in `m4' for avoiding
them.

     divert(`-1')
     define(`foo', `Macro `foo'.')
     define(`bar', `Macro `bar'.')
     divert
     =>

   Traditional implementations only supported ten diversions.  But as a
GNU extension, diversion numbers can be as large as positive integers
will allow, rather than treating a multi-digit diversion number as a
request to discard text.

     divert(eval(`1<<28'))world
     divert(`2')hello
     ^D
     =>hello
     =>world

   Note that `divert' is an English word, but also an active macro
without arguments.  When processing plain text, the word might appear in
normal text and be unintentionally swallowed as a macro invocation.  One
way to avoid this is to use the `-P' option to rename all builtins
(*note Invoking m4: Operation modes.).  Another is to write a wrapper
that requires a parameter to be recognized.

     We decided to divert the stream for irrigation.
     =>We decided to  the stream for irrigation.
     define(`divert', `ifelse(`$#', `0', ``$0'', `builtin(`$0', $@)')')
     =>
     divert(`-1')
     Ignored text.
     divert(`0')
     =>
     We decided to divert the stream for irrigation.
     =>We decided to divert the stream for irrigation.


File: m4.info,  Node: Undivert,  Next: Divnum,  Prev: Divert,  Up: Diversions

10.2 Undiverting output
=======================

Diverted text can be undiverted explicitly using the builtin `undivert':

 -- Builtin: undivert ([DIVERSIONS...])
     Undiverts the numeric DIVERSIONS given by the arguments, in the
     order given.  If no arguments are supplied, all diversions are
     undiverted, in numerical order.

     As a GNU extension, DIVERSIONS may contain non-numeric strings,
     which are treated as the names of files to copy into the output
     without expansion.  A warning is issued if a file could not be
     opened.

     The expansion of `undivert' is void.

     divert(`1')
     This text is diverted.
     divert
     =>
     This text is not diverted.
     =>This text is not diverted.
     undivert(`1')
     =>
     =>This text is diverted.
     =>

   Notice the last two blank lines.  One of them comes from the newline
following `undivert', the other from the newline that followed the
`divert'!  A diversion often starts with a blank line like this.

   When diverted text is undiverted, it is _not_ reread by `m4', but
rather copied directly to the current output, and it is therefore not
an error to undivert into a diversion.  Undiverting the empty string is
the same as specifying diversion 0; in either case nothing happens
since the output has already been flushed.

     divert(`1')diverted text
     divert
     =>
     undivert()
     =>
     undivert(`0')
     =>
     undivert
     =>diverted text
     =>

   When a diversion has been undiverted, the diverted text is discarded,
and it is not possible to bring back diverted text more than once.

     divert(`1')
     This text is diverted first.
     divert(`0')undivert(`1')dnl
     =>
     =>This text is diverted first.
     undivert(`1')
     =>
     divert(`1')
     This text is also diverted but not appended.
     divert(`0')undivert(`1')dnl
     =>
     =>This text is also diverted but not appended.

   Attempts to undivert the current diversion are silently ignored.
Thus, when the current diversion is not 0, the current diversion does
not get rearranged among the other diversions.

     divert(`1')one
     divert(`2')two
     divert(`3')three
     divert(`2')undivert`'dnl
     divert`'undivert`'dnl
     =>two
     =>one
     =>three

   GNU `m4' allows named files to be undiverted.  Given a non-numeric
argument, the contents of the file named will be copied, uninterpreted,
to the current output.  This complements the builtin `include' (*note
Include::).  To illustrate the difference, the file
`m4-1.4.8/examples/foo' contains the word `bar':

     define(`bar', `BAR')
     =>
     undivert(`foo')
     =>bar
     =>
     include(`foo')
     =>BAR
     =>

   If the file is not found (or cannot be read), an error message is
issued, and the expansion is void.


File: m4.info,  Node: Divnum,  Next: Cleardivert,  Prev: Undivert,  Up: Diversions

10.3 Diversion numbers
======================

The current diversion is tracked by the builtin `divnum':

 -- Builtin: divnum
     Expands to the number of the current diversion.

     Initial divnum
     =>Initial 0
     divert(`1')
     Diversion one: divnum
     divert(`2')
     Diversion two: divnum
     ^D
     =>
     =>Diversion one: 1
     =>
     =>Diversion two: 2


File: m4.info,  Node: Cleardivert,  Prev: Divnum,  Up: Diversions

10.4 Discarding diverted text
=============================

Often it is not known, when output is diverted, whether the diverted
text is actually needed.  Since all non-empty diversion are brought back
on the main output stream when the end of input is seen, a method of
discarding a diversion is needed.  If all diversions should be
discarded, the easiest is to end the input to `m4' with `divert(`-1')'
followed by an explicit `undivert':

     divert(`1')
     Diversion one: divnum
     divert(`2')
     Diversion two: divnum
     divert(`-1')
     undivert
     ^D

No output is produced at all.

   Clearing selected diversions can be done with the following macro:

 -- Composite: cleardivert ([DIVERSIONS...])
     Discard the contents of each of the listed numeric DIVERSIONS.

     define(`cleardivert',
     `pushdef(`_n', divnum)divert(`-1')undivert($@)divert(_n)popdef(`_n')')
     =>

   It is called just like `undivert', but the effect is to clear the
diversions, given by the arguments.  (This macro has a nasty bug!  You
should try to see if you can find it and correct it; or *note Answers:
Improved cleardivert.).


File: m4.info,  Node: Text handling,  Next: Arithmetic,  Prev: Diversions,  Up: Top

11 Macros for text handling
***************************

There are a number of builtins in `m4' for manipulating text in various
ways, extracting substrings, searching, substituting, and so on.

* Menu:

* Len::                         Calculating length of strings
* Index macro::                 Searching for substrings
* Regexp::                      Searching for regular expressions
* Substr::                      Extracting substrings
* Translit::                    Translating characters
* Patsubst::                    Substituting text by regular expression
* Format::                      Formatting strings (printf-like)


File: m4.info,  Node: Len,  Next: Index macro,  Up: Text handling

11.1 Calculating length of strings
==================================

The length of a string can be calculated by `len':

 -- Builtin: len (STRING)
     Expands to the length of STRING, as a decimal number.

     The macro `len' is recognized only with parameters.

     len()
     =>0
     len(`abcdef')
     =>6


File: m4.info,  Node: Index macro,  Next: Regexp,  Prev: Len,  Up: Text handling

11.2 Searching for substrings
=============================

Searching for substrings is done with `index':

 -- Builtin: index (STRING, SUBSTRING)
     Expands to the index of the first occurrence of SUBSTRING in
     STRING.  The first character in STRING has index 0.  If SUBSTRING
     does not occur in STRING, `index' expands to `-1'.

     The macro `index' is recognized only with parameters.

     index(`gnus, gnats, and armadillos', `nat')
     =>7
     index(`gnus, gnats, and armadillos', `dag')
     =>-1

   Omitting SUBSTRING evokes a warning, but still produces output.

     index(`abc')
     error-->m4:stdin:1: Warning: too few arguments to builtin `index'
     =>0


File: m4.info,  Node: Regexp,  Next: Substr,  Prev: Index macro,  Up: Text handling

11.3 Searching for regular expressions
======================================

Searching for regular expressions is done with the builtin `regexp':

 -- Builtin: regexp (STRING, REGEXP, [REPLACEMENT])
     Searches for REGEXP in STRING.  The syntax for regular expressions
     is the same as in GNU Emacs.  *Note Syntax of Regular Expressions:
     (emacs)Regexps.

     If REPLACEMENT is omitted, `regexp' expands to the index of the
     first match of REGEXP in STRING.  If REGEXP does not match
     anywhere in STRING, it expands to -1.

     If REPLACEMENT is supplied, and there was a match, `regexp'
     changes the expansion to this argument, with `\N' substituted by
     the text matched by the Nth parenthesized sub-expression of
     REGEXP, up to nine sub-expressions.  The escape `\&' is replaced
     by the text of the entire regular expression matched.  For all
     other characters, `\' treats the next character literally.  A
     warning is issued if there were fewer sub-expressions than the
     `\N' requested, or if there is a trailing `\'.  If there was no
     match, `regexp' expands to the empty string.

     The macro `regexp' is recognized only with parameters.

     regexp(`GNUs not Unix', `\<[a-z]\w+')
     =>5
     regexp(`GNUs not Unix', `\<Q\w*')
     =>-1
     regexp(`GNUs not Unix', `\w\(\w+\)$', `*** \& *** \1 ***')
     =>*** Unix *** nix ***
     regexp(`GNUs not Unix', `\<Q\w*', `*** \& *** \1 ***')
     =>

   Here are some more examples on the handling of backslash:

     regexp(`abc', `\(b\)', `\\\10\a')
     =>\b0a
     regexp(`abc', `b', `\1\')
     error-->m4:stdin:2: Warning: sub-expression 1 not present
     error-->m4:stdin:2: Warning: trailing \ ignored in replacement
     =>
     regexp(`abc', `\(\(d\)?\)\(c\)', `\1\2\3\4\5\6')
     error-->m4:stdin:3: Warning: sub-expression 4 not present
     error-->m4:stdin:3: Warning: sub-expression 5 not present
     error-->m4:stdin:3: Warning: sub-expression 6 not present
     =>c

   Omitting REGEXP evokes a warning, but still produces output.

     regexp(`abc')
     error-->m4:stdin:1: Warning: too few arguments to builtin `regexp'
     =>0


File: m4.info,  Node: Substr,  Next: Translit,  Prev: Regexp,  Up: Text handling

11.4 Extracting substrings
==========================

Substrings are extracted with `substr':

 -- Builtin: substr (STRING, FROM, [LENGTH])
     Expands to the substring of STRING, which starts at index FROM,
     and extends for LENGTH characters, or to the end of STRING, if
     LENGTH is omitted.  The starting index of a string is always 0.
     The expansion is empty if there is an error parsing FROM or
     LENGTH, if FROM is beyond the end of STRING, or if LENGTH is
     negative.

     The macro `substr' is recognized only with parameters.

     substr(`gnus, gnats, and armadillos', `6')
     =>gnats, and armadillos
     substr(`gnus, gnats, and armadillos', `6', `5')
     =>gnats

   Omitting FROM evokes a warning, but still produces output.

     substr(`abc')
     error-->m4:stdin:1: Warning: too few arguments to builtin `substr'
     =>abc
     substr(`abc',)
     error-->m4:stdin:2: empty string treated as 0 in builtin `substr'
     =>abc


File: m4.info,  Node: Translit,  Next: Patsubst,  Prev: Substr,  Up: Text handling

11.5 Translating characters
===========================

Character translation is done with `translit':

 -- Builtin: translit (STRING, CHARS, [REPLACEMENT])
     Expands to STRING, with each character that occurs in CHARS
     translated into the character from REPLACEMENT with the same index.

     If REPLACEMENT is shorter than CHARS, the excess characters of
     CHARS are deleted from the expansion; if CHARS is shorter, the
     excess characters in REPLACEMENT are silently ignored.  If
     REPLACEMENT is omitted, all characters in STRING that are present
     in CHARS are deleted from the expansion.  If a character appears
     more than once in CHARS, only the first instance is used in making
     the translation.  Only a single translation pass is made, even if
     characters in REPLACEMENT also appear in CHARS.

     As a GNU extension, both CHARS and REPLACEMENT can contain
     character-ranges, e.g., `a-z' (meaning all lowercase letters) or
     `0-9' (meaning all digits).  To include a dash `-' in CHARS or
     REPLACEMENT, place it first or last in the entire string, or as
     the last character of a range.  Back-to-back ranges can share a
     common endpoint.  It is not an error for the last character in the
     range to be `larger' than the first.  In that case, the range runs
     backwards, i.e., `9-0' means the string `9876543210'.  The
     expansion of a range is dependent on the underlying encoding of
     characters, so using ranges is not always portable between
     machines.

     The macro `translit' is recognized only with parameters.

     translit(`GNUs not Unix', `A-Z')
     =>s not nix
     translit(`GNUs not Unix', `a-z', `A-Z')
     =>GNUS NOT UNIX
     translit(`GNUs not Unix', `A-Z', `z-a')
     =>tmfs not fnix
     translit(`+,-12345', `+--1-5', `<;>a-c-a')
     =><;>abcba
     translit(`abcdef', `aabdef', `bcged')
     =>bgced

   In the ASCII encoding, the first example deletes all uppercase
letters, the second converts lowercase to uppercase, and the third
`mirrors' all uppercase letters, while converting them to lowercase.
The two first cases are by far the most common, even though they are not
portable to EBCDIC or other encodings.  The fourth example shows a
range ending in `-', as well as back-to-back ranges.  The final example
shows that `a' is mapped to `b', not `c'; the resulting `b' is not
further remapped to `g'; the `d' and `e' are swapped, and the `f' is
discarded.

   Omitting CHARS evokes a warning, but still produces output.

     translit(`abc')
     error-->m4:stdin:1: Warning: too few arguments to builtin `translit'
     =>abc


File: m4.info,  Node: Patsubst,  Next: Format,  Prev: Translit,  Up: Text handling

11.6 Substituting text by regular expression
============================================

Global substitution in a string is done by `patsubst':

 -- Builtin: patsubst (STRING, REGEXP, [REPLACEMENT])
     Searches STRING for matches of REGEXP, and substitutes REPLACEMENT
     for each match.  The syntax for regular expressions is the same as
     in GNU Emacs (*note Regexp::).

     The parts of STRING that are not covered by any match of REGEXP
     are copied to the expansion.  Whenever a match is found, the
     search proceeds from the end of the match, so a character from
     STRING will never be substituted twice.  If REGEXP matches a
     string of zero length, the start position for the search is
     incremented, to avoid infinite loops.

     When a replacement is to be made, REPLACEMENT is inserted into the
     expansion, with `\N' substituted by the text matched by the Nth
     parenthesized sub-expression of PATSUBST, for up to nine
     sub-expressions.  The escape `\&' is replaced by the text of the
     entire regular expression matched.  For all other characters, `\'
     treats the next character literally.  A warning is issued if there
     were fewer sub-expressions than the `\N' requested, or if there is
     a trailing `\'.

     The REPLACEMENT argument can be omitted, in which case the text
     matched by REGEXP is deleted.

     The macro `patsubst' is recognized only with parameters.

     patsubst(`GNUs not Unix', `^', `OBS: ')
     =>OBS: GNUs not Unix
     patsubst(`GNUs not Unix', `\<', `OBS: ')
     =>OBS: GNUs OBS: not OBS: Unix
     patsubst(`GNUs not Unix', `\w*', `(\&)')
     =>(GNUs)() (not)() (Unix)()
     patsubst(`GNUs not Unix', `\w+', `(\&)')
     =>(GNUs) (not) (Unix)
     patsubst(`GNUs not Unix', `[A-Z][a-z]+')
     =>GN not 
     patsubst(`GNUs not Unix', `not', `NOT\')
     error-->m4:stdin:6: Warning: trailing \ ignored in replacement
     =>GNUs NOT Unix

   Here is a slightly more realistic example, which capitalizes
individual word or whole sentences, by substituting calls of the macros
`upcase' and `downcase' into the strings.

 -- Composite: upcase (TEXT)
 -- Composite: downcase (TEXT)
 -- Composite: capitalize (TEXT)
     Expand to TEXT, but with capitalization changed: `upcase' changes
     all letters to upper case, `downcase' changes all letters to lower
     case, and `capitalize' changes the first character of each word to
     upper case and the remaining characters to lower case.

     define(`upcase', `translit(`$*', `a-z', `A-Z')')dnl
     define(`downcase', `translit(`$*', `A-Z', `a-z')')dnl
     define(`capitalize1',
            `regexp(`$1', `^\(\w\)\(\w*\)',
                    `upcase(`\1')`'downcase(`\2')')')dnl
     define(`capitalize',
            `patsubst(`$1', `\w+', `capitalize1(`\&')')')dnl
     capitalize(`GNUs not Unix')
     =>Gnus Not Unix

   While `regexp' replaces the whole input with the replacement as soon
as there is a match, `patsubst' replaces each _occurrence_ of a match
and preserves non-matching pieces:

     define(`patreg',
     `patsubst($@)
     regexp($@)')dnl
     patreg(`bar foo baz Foo', `foo\|Foo', `FOO')
     =>bar FOO baz FOO
     =>FOO
     patreg(`aba abb 121', `\(.\)\(.\)\1', `\2\1\2')
     =>bab abb 212
     =>bab

   Omitting REGEXP evokes a warning, but still produces output.

     patsubst(`abc')
     error-->m4:stdin:1: Warning: too few arguments to builtin `patsubst'
     =>abc


File: m4.info,  Node: Format,  Prev: Patsubst,  Up: Text handling

11.7 Formatted output
=====================

Formatted output can be made with `format':

 -- Builtin: format (FORMAT-STRING, ...)
     Works much like the C function `printf'.  The first argument
     FORMAT-STRING can contain `%' specifications which are satisfied
     by additional arguments, and the expansion of `format' is the
     formatted string.

     The macro `format' is recognized only with parameters.

   Its use is best described by a few examples:

     define(`foo', `The brown fox jumped over the lazy dog')
     =>
     format(`The string "%s" uses %d characters', foo, len(foo))
     =>The string "The brown fox jumped over the lazy dog" uses 38 characters
     format(`%.0f', `56789.9876')
     =>56790
     len(format(`%-*X', `300', `1'))
     =>300

   Using the `forloop' macro defined earlier (*note Forloop::), this
example shows how `format' can be used to produce tabular output.

     include(`forloop.m4')
     =>
     forloop(`i', `1', `10', `format(`%6d squared is %10d
     ', i, eval(i**2))')
     =>     1 squared is          1
     =>     2 squared is          4
     =>     3 squared is          9
     =>     4 squared is         16
     =>     5 squared is         25
     =>     6 squared is         36
     =>     7 squared is         49
     =>     8 squared is         64
     =>     9 squared is         81
     =>    10 squared is        100
     =>

   The builtin `format' is modeled after the ANSI C `printf' function,
and supports these `%' specifiers: `c', `s', `d', `o', `x', `X', `u',
`e', `E', `f', `F', `g', `G', and `%'; it supports field widths and
precisions, and the modifiers `+', `-', ` ', `0', `#', `h' and `l'.
For more details on the functioning of `printf', see the C Library
Manual.

   For now, unrecognized specifiers are silently ignored, but it is
anticipated that a future release of GNU `m4' will support more
specifiers, and give warnings when problems are encountered.  Likewise,
escape sequences are not yet recognized.


File: m4.info,  Node: Arithmetic,  Next: Shell commands,  Prev: Text handling,  Up: Top

12 Macros for doing arithmetic
******************************

Integer arithmetic is included in `m4', with a C-like syntax.  As
convenient shorthands, there are builtins for simple increment and
decrement operations.

* Menu:

* Incr::                        Decrement and increment operators
* Eval::                        Evaluating integer expressions


File: m4.info,  Node: Incr,  Next: Eval,  Up: Arithmetic

12.1 Decrement and increment operators
======================================

Increment and decrement of integers are supported using the builtins
`incr' and `decr':

 -- Builtin: incr (NUMBER)
 -- Builtin: decr (NUMBER)
     Expand to the numerical value of NUMBER, incremented or
     decremented, respectively, by one.  Except for the empty string,
     the expansion is empty if NUMBER could not be parsed.

     The macros `incr' and `decr' are recognized only with parameters.

     incr(`4')
     =>5
     decr(`7')
     =>6
     incr()
     error-->m4:stdin:3: empty string treated as 0 in builtin `incr'
     =>1
     decr()
     error-->m4:stdin:4: empty string treated as 0 in builtin `decr'
     =>-1


File: m4.info,  Node: Eval,  Prev: Incr,  Up: Arithmetic

12.2 Evaluating integer expressions
===================================

Integer expressions are evaluated with `eval':

 -- Builtin: eval (EXPRESSION, [RADIX = `10'], [WIDTH])
     Expands to the value of EXPRESSION.  The expansion is empty if an
     error is encountered while parsing the arguments.  If specified,
     RADIX and WIDTH control the format of the output.

     The macro `eval' is recognized only with parameters.

   Expressions can contain the following operators, listed in order of
decreasing precedence.

`+ -'
     Unary plus and minus

`**'
     Exponentiation

`*  /  %'
     Multiplication, division and modulo

`+  -'
     Addition and subtraction

`<<  >>'
     Shift left or right

`==  !=  >  >=  <  <='
     Relational operators

`!'
     Logical negation

`~'
     Bitwise negation

`&'
     Bitwise and

`^'
     Bitwise exclusive-or

`|'
     Bitwise or

`&&'
     Logical and

`||'
     Logical or

   All operators, except exponentiation, are left associative.

   Note that some older `m4' implementations use `^' as an alternate
operator for exponentiation, although POSIX requires the C behavior of
bitwise exclusive-or.  On the other hand, the precedence of `~' and `!'
are different in GNU `m4' than they are in C, matching the precedence
in traditional `m4' implementations.  This behavior is likely to change
in a future version to match POSIX, so use parentheses to force the
desired precedence.

   Within EXPRESSION, (but not RADIX or WIDTH), numbers without a
special prefix are decimal.  A simple `0' prefix introduces an octal
number.  `0x' introduces a hexadecimal number.  `0b' introduces a
binary number.  `0r' introduces a number expressed in any radix between
1 and 36: the prefix should be immediately followed by the decimal
expression of the radix, a colon, then the digits making the number.
For radix 1, leading zeros are ignored and all remaining digits must be
`1'; for all other radices, the digits are `0', `1', `2', ....  Beyond
`9', the digits are `a', `b' ... up to `z'.  Lower and upper case
letters can be used interchangeably in numbers prefixes and as number
digits.

   Parentheses may be used to group subexpressions whenever needed.
For the relational operators, a true relation returns `1', and a false
relation return `0'.

   Here are a few examples of use of `eval'.

     eval(`-3 * 5')
     =>-15
     eval(index(`Hello world', `llo') >= 0)
     =>1
     eval(`0r1:0111 + 0b100 + 0r3:12')
     =>12
     define(`square', `eval(`('$1`)**2')')
     =>
     square(`9')
     =>81
     square(square(`5')`+1')
     =>676
     define(`foo', `666')
     =>
     eval(`foo/6')
     error-->m4:stdin:8: bad expression in eval: foo/6
     =>
     eval(foo/6)
     =>111

   As the last two lines show, `eval' does not handle macro names, even
if they expand to a valid expression (or part of a valid expression).
Therefore all macros must be expanded before they are passed to `eval'.

   All evaluation is done with 32-bit signed integers, assuming
2's-complement with wrap-around.  The shift operators are defined in GNU
`m4' by doing an implicit bit-wise and of the right-hand operand with
0x1f, and sign-extension with right shift.

     eval(0x80000000 / -1)
     =>-2147483648
     eval(0x80000000 % -1)
     =>0
     eval(0x7fffffff)
     =>2147483647
     incr(eval(0x7fffffff))
     =>-2147483648
     eval(-4 >> 33)
     =>-2

   If RADIX is specified, it specifies the radix to be used in the
expansion.  The default radix is 10; this is also the case if RADIX is
the empty string.  It is an error if the radix is outside the range of
1 through 36, inclusive.  The result of `eval' is always taken to be
signed.  No radix prefix is output, and for radices greater than 10,
the digits are lower case.  The WIDTH argument specifies the minimum
output width, excluding any negative sign.  The result is zero-padded
to extend the expansion to the requested width.  It is an error if the
width is negative.  On error, the expansion of `eval' is empty.

     eval(`666', `10')
     =>666
     eval(`666', `11')
     =>556
     eval(`666', `6')
     =>3030
     eval(`666', `6', `10')
     =>0000003030
     eval(`-666', `6', `10')
     =>-0000003030
     eval(`10', `', `0')
     =>10
     `0r1:'eval(`10', `1', `11')
     =>0r1:01111111111
     eval(`10', `16')
     =>a


File: m4.info,  Node: Shell commands,  Next: Miscellaneous,  Prev: Arithmetic,  Up: Top

13 Macros for running shell commands
************************************

There are a few builtin macros in `m4' that allow you to run shell
commands from within `m4'.

   Note that the definition of a valid shell command is system
dependent.  On UNIX systems, this is the typical `/bin/sh'.  But on
other systems, such as native Windows, the shell has a different syntax
of commands that it understands.  Some examples in this chapter assume
`/bin/sh', and also demonstrate how to quit early with a known exit
value if this is not the case.

* Menu:

* Platform macros::             Determining the platform
* Syscmd::                      Executing simple commands
* Esyscmd::                     Reading the output of commands
* Sysval::                      Exit status
* Mkstemp::                     Making temporary files


File: m4.info,  Node: Platform macros,  Next: Syscmd,  Up: Shell commands

13.1 Determining the platform
=============================

Sometimes it is desirable for an input file to know which platform `m4'
is running on.  GNU `m4' provides several macros that are predefined to
expand to the empty string; checking for their existence will confirm
platform details.

 -- Optional builtin: __gnu__
 -- Optional builtin: __os2__
 -- Optional builtin: os2
 -- Optional builtin: __unix__
 -- Optional builtin: unix
 -- Optional builtin: __windows__
 -- Optional builtin: windows
     Each of these macros is conditionally defined as needed to
     describe the environment of `m4'.  If defined, each macro expands
     to the empty string.

   When GNU extensions are in effect (that is, when you did not use the
`-G' option, *note Invoking m4: Limits control.), GNU `m4' will define
the macro `__gnu__' to expand to the empty string.

     __gnu__
     =>
     ifdef(`__gnu__', `Extensions are active')
     =>Extensions are active

   On UNIX systems, GNU `m4' will define `__unix__' by default, or
`unix' when the `-G' option is specified.

   On native Windows systems, GNU `m4' will define `__windows__' by
default, or `windows' when the `-G' option is specified.

   On OS/2 systems, GNU `m4' will define `__os2__' by default, or `os2'
when the `-G' option is specified.

   If GNU `m4' does not provide a platform macro for your system,
please report that as a bug.

     define(`provided', `0')
     =>
     ifdef(`__unix__', `define(`provided', incr(provided))')
     =>
     ifdef(`__windows__', `define(`provided', incr(provided))')
     =>
     ifdef(`__os2__', `define(`provided', incr(provided))')
     =>
     provided
     =>1


File: m4.info,  Node: Syscmd,  Next: Esyscmd,  Prev: Platform macros,  Up: Shell commands

13.2 Executing simple commands
==============================

Any shell command can be executed, using `syscmd':

 -- Builtin: syscmd (SHELL-COMMAND)
     Executes SHELL-COMMAND as a shell command.

     The expansion of `syscmd' is void, _not_ the output from
     SHELL-COMMAND!  Output or error messages from SHELL-COMMAND are
     not read by `m4'.  *Note Esyscmd::, if you need to process the
     command output.

     Prior to executing the command, `m4' flushes its buffers.  The
     default standard input, output and error of SHELL-COMMAND are the
     same as those of `m4'.

     The macro `syscmd' is recognized only with parameters.

     define(`foo', `FOO')
     =>
     syscmd(`echo foo')
     =>foo
     =>

   Note how the expansion of `syscmd' keeps the trailing newline of the
command, as well as using the newline that appeared after the macro.

   As an example of SHELL-COMMAND using the same standard input as
`m4', the command line `echo "m4wrap(\`syscmd(\`cat')')" | m4' will
tell `m4' to read all of its input before executing the wrapped text,
then hand a valid (albeit emptied) pipe as standard input for the `cat'
subcommand.  Therefore, you should be careful when using standard input
(either by specifying no files, or by passing `-' as a file name on the
command line, *note Invoking m4: Command line files.), and also
invoking subcommands via `syscmd' or `esyscmd' that consume data from
standard input.  When standard input is a seekable file, the subprocess
will pick up with the next character not yet processed by `m4'; when it
is a pipe or other non-seekable file, there is no guarantee how much
data will already be buffered by `m4' and thus unavailable to the child.


File: m4.info,  Node: Esyscmd,  Next: Sysval,  Prev: Syscmd,  Up: Shell commands

13.3 Reading the output of commands
===================================

If you want `m4' to read the output of a shell command, use `esyscmd':

 -- Builtin: esyscmd (SHELL-COMMAND)
     Expands to the standard output of the shell command SHELL-COMMAND.

     Prior to executing the command, `m4' flushes its buffers.  The
     default standard input and standard error of SHELL-COMMAND are the
     same as those of `m4'.  The error output of SHELL-COMMAND is not a
     part of the expansion: it will appear along with the error output
     of `m4'.

     The macro `esyscmd' is recognized only with parameters.

     define(`foo', `FOO')
     =>
     esyscmd(`echo foo')
     =>FOO
     =>

   Note how the expansion of `esyscmd' keeps the trailing newline of
the command, as well as using the newline that appeared after the macro.

   Just as with `syscmd', care must be exercised when sharing standard
input between `m4' and the child process of `esyscmd'.


File: m4.info,  Node: Sysval,  Next: Mkstemp,  Prev: Esyscmd,  Up: Shell commands

13.4 Exit status
================

To see whether a shell command succeeded, use `sysval':

 -- Builtin: sysval
     Expands to the exit status of the last shell command run with
     `syscmd' or `esyscmd'.  Expands to 0 if no command has been run
     yet.

     syscmd(`false')
     =>
     ifelse(sysval, `0', `zero', `non-zero')
     =>non-zero
     syscmd(`exit 2')
     =>
     sysval
     =>2
     syscmd(`true')
     =>
     sysval
     =>0
     esyscmd(`false')
     =>
     ifelse(sysval, `0', `zero', `non-zero')
     =>non-zero
     esyscmd(`exit 2')
     =>
     sysval
     =>2
     esyscmd(`true')
     =>
     sysval
     =>0

   `sysval' results in 127 if there was a problem executing the
command, for example, if the system-imposed argument length is exceeded,
or if there were not enough resources to fork.  It is not possible to
distinguish between failed execution and successful execution that had
an exit status of 127.

   On UNIX platforms, where it is possible to detect when command
execution is terminated by a signal, rather than a normal exit, the
result is the signal number shifted left by eight bits.

     dnl This test assumes kill is a shell builtin, and that signals are
     dnl recognizable.
     ifdef(`__unix__', ,
           `errprint(` skipping: syscmd does not have unix semantics
     ')m4exit(`77')')dnl
     syscmd(`kill -13 $$')
     =>
     sysval
     =>3328
     esyscmd(`kill -9 $$')
     =>
     sysval
     =>2304


File: m4.info,  Node: Mkstemp,  Prev: Sysval,  Up: Shell commands

13.5 Making temporary files
===========================

Commands specified to `syscmd' or `esyscmd' might need a temporary
file, for output or for some other purpose.  There is a builtin macro,
`mkstemp', for making a temporary file:

 -- Builtin: mkstemp (TEMPLATE)
 -- Builtin: maketemp (TEMPLATE)
     Expands to a name of a new, empty file, made from the string
     TEMPLATE, which should end with the string `XXXXXX'.  The six `X'
     characters are then replaced with random characters matching the
     regular expression `[a-zA-Z0-9._-]', in order to make the file
     name unique.  If fewer than six `X' characters are found at the end
     of `template', the result will be longer than the template.  The
     created file will have access permissions as if by `chmod =rw,go=',
     meaning that the current umask of the `m4' process is taken into
     account, and at most only the current user can read and write the
     file.

     The traditional behavior, standardized by POSIX, is that
     `maketemp' merely replaces the trailing `X' with the process id,
     without creating a file, and without ensuring that the resulting
     string is a unique file name.  In part, this means that using the
     same TEMPLATE twice in the same input file will result in the same
     expansion.  This behavior is a security hole, as it is very easy
     for another process to guess the name that will be generated, and
     thus interfere with a subsequent use of `syscmd' trying to
     manipulate that file name.  Hence, POSIX has recommended that all
     new implementations of `m4' provide the secure `mkstemp' builtin,
     and that users of `m4' check for its existence.

     The macros `mkstemp' and `maketemp' are recognized only with
     parameters.

   If you try this next example, you will most likely get different
output for the two file names, since the replacement characters are
randomly chosen:

     maketemp(`/tmp/fooXXXXXX')
     =>/tmp/fooa07346
     ifdef(`mkstemp', `define(`maketemp', defn(`mkstemp'))',
           `define(`mkstemp', defn(`maketemp'))dnl
     errprint(`warning: potentially insecure maketemp implementation
     ')')
     =>
     mkstemp(`doc')
     =>docQv83Uw

   Unless you use the `--traditional' command line option (or `-G',
*note Invoking m4: Limits control.), the GNU version of `maketemp' is
secure.  This means that using the same template to multiple calls will
generate multiple files.  However, we recommend that you use the new
`mkstemp' macro, introduced in GNU M4 1.4.8, which is secure even in
traditional mode.

     syscmd(`echo foo??????')dnl
     =>foo??????
     define(`file1', maketemp(`fooXXXXXX'))dnl
     ifelse(esyscmd(`echo foo??????'), `foo??????', `no file', `created')
     =>created
     define(`file2', maketemp(`fooXX'))dnl
     define(`file3', mkstemp(`fooXXXXXX'))dnl
     ifelse(len(file1), len(file2), `same length', `different')
     =>same length
     ifelse(file1, file2, `same', `different file')
     =>different file
     ifelse(file2, file3, `same', `different file')
     =>different file
     ifelse(file1, file3, `same', `different file')
     =>different file
     syscmd(`rm 'file1 file2 file3)
     =>
     sysval
     =>0


File: m4.info,  Node: Miscellaneous,  Next: Frozen files,  Prev: Shell commands,  Up: Top

14 Miscellaneous builtin macros
*******************************

This chapter describes various builtins, that do not really belong in
any of the previous chapters.

* Menu:

* Errprint::                    Printing error messages
* Location::                    Printing current location
* M4exit::                      Exiting from `m4'


File: m4.info,  Node: Errprint,  Next: Location,  Up: Miscellaneous

14.1 Printing error messages
============================

You can print error messages using `errprint':

 -- Builtin: errprint (MESSAGE, ...)
     Prints MESSAGE and the rest of the arguments to standard error,
     separated by spaces.  Standard error is used, regardless of the
     `--debugfile' option (*note Invoking m4: Debugging options.).

     The expansion of `errprint' is void.  The macro `errprint' is
     recognized only with parameters.

     errprint(`Invalid arguments to forloop
     ')
     error-->Invalid arguments to forloop
     =>
     errprint(`1')errprint(`2',`3
     ')
     error-->12 3
     =>

   A trailing newline is _not_ printed automatically, so it should be
supplied as part of the argument, as in the example.  Unfortunately, the
exact output of `errprint' is not very portable to other `m4'
implementations: POSIX requires that all arguments be printed, but some
implementations of `m4' only print the first.  Furthermore, some BSD
implementations always append a newline for each `errprint' call,
regardless of whether the last argument already had one, and POSIX is
silent on whether this is acceptable.


File: m4.info,  Node: Location,  Next: M4exit,  Prev: Errprint,  Up: Miscellaneous

14.2 Printing current location
==============================

To make it possible to specify the location of an error, three utility
builtins exist:

 -- Builtin: __file__
 -- Builtin: __line__
 -- Builtin: __program__
     Expand to the quoted name of the current input file, the current
     input line number in that file, and the quoted name of the current
     invocation of `m4'.

     errprint(__program__:__file__:__line__: `input error
     ')
     error-->m4:stdin:1: input error
     =>

   Line numbers start at 1 for each file.  If the file was found due to
the `-I' option or `M4PATH' environment variable, that is reflected in
the file name.  The syncline option (`-s', *note Invoking m4:
Preprocessor features.), and the `f' and `l' flags of `debugmode'
(*note Debug Levels::), also use this notion of current file and line.
Redefining the three location macros has no effect on syncline, debug,
or warning message output.  Assume this example is run in the
`m4-1.4.8/checks' directory of the GNU M4 package, using
`--include=../examples' in the command line to find the file `incl.m4'
mentioned earlier:

     define(`foo', ``$0' called at __file__:__line__')
     =>
     foo
     =>foo called at stdin:2
     include(`incl.m4')
     =>Include file start
     =>foo called at ../examples/incl.m4:2
     =>Include file end
     =>

   The location of macros invoked during the rescanning of macro
expansion text corresponds to the location in the file where the
expansion was triggered, regardless of how many newline characters the
expansion text contains.  As of GNU M4 1.4.8, the location of text
wrapped with `m4wrap' (*note M4wrap::) is the point at which the
`m4wrap' was invoked.  Previous versions, however, behaved as though
wrapped text came from line 0 of the file "".

     define(`echo', `$@')
     =>
     define(`foo', `echo(__line__
     __line__)')
     =>
     echo(__line__
     __line__)
     =>4
     =>5
     m4wrap(`foo
     ')
     =>
     foo(errprint(__line__
     __line__
     ))
     error-->8
     error-->9
     =>8
     =>8
     __line__
     =>11
     ^D
     =>6
     =>6

   The `__program__' macro behaves like `$0' in shell terminology.  If
you invoke `m4' through an absolute path or a link with a different
spelling, rather than by relying on a `PATH' search for plain `m4', it
will affect how `__program__' expands.  The intent is that you can use
it to produce error messages with the same formatting that `m4'
produces internally.  It can also be used within `syscmd' (*note
Syscmd::) to pick the same version of `m4' that is currently running,
rather than whatever version of `m4' happens to be first in `PATH'.  It
was first introduced in GNU M4 1.4.6.


File: m4.info,  Node: M4exit,  Prev: Location,  Up: Miscellaneous

14.3 Exiting from `m4'
======================

If you need to exit from `m4' before the entire input has been read,
you can use `m4exit':

 -- Builtin: m4exit ([CODE = `0'])
     Causes `m4' to exit, with exit status CODE.  If CODE is left out,
     the exit status is zero.  If CODE cannot be parsed, or is outside
     the range of 0 to 255, the exit status is one.  No further input
     is read, and all wrapped and diverted text is discarded.

     m4wrap(`This text is lost due to `m4exit'.')
     =>
     divert(`1') So is this.
     divert
     =>
     m4exit And this is never read.

   A common use of this is to abort processing:

 -- Composite: fatal_error (MESSAGE)
     Abort processing with an error message and non-zero status.  Prefix
     MESSAGE with details about where the error occurred, and print the
     resulting string to standard error.

     define(`fatal_error',
            `errprint(__program__:__file__:__line__`: fatal error: $*
     ')m4exit(`1')')
     =>
     fatal_error(`this is a BAD one, buster')
     error-->m4:stdin:4: fatal error: this is a BAD one, buster

   After this macro call, `m4' will exit with exit status 1.  This macro
is only intended for error exits, since the normal exit procedures are
not followed, e.g., diverted text is not undiverted, and saved text
(*note M4wrap::) is not reread.  (This macro could be made more robust
to earlier versions of `m4'.  You should try to see if you can find
weaknesses and correct them; or *note Answers: Improved fatal_error.).

   Note that it is still possible for the exit status to be different
than what was requested by `m4exit'.  If `m4' detects some other error,
such as a write error on standard output, the exit status will be
non-zero even if `m4exit' requested zero.

   If standard input is seekable, then the file will be positioned at
the next unread character.  If it is a pipe or other non-seekable file,
then there are no guarantees how much data `m4' might have read into
buffers, and thus discarded.


File: m4.info,  Node: Frozen files,  Next: Compatibility,  Prev: Miscellaneous,  Up: Top

15 Fast loading of frozen state
*******************************

Some bigger `m4' applications may be built over a common base
containing hundreds of definitions and other costly initializations.
Usually, the common base is kept in one or more declarative files,
which files are listed on each `m4' invocation prior to the user's
input file, or else each input file uses `include'.

   Reading the common base of a big application, over and over again,
may be time consuming.  GNU `m4' offers some machinery to speed up the
start of an application using lengthy common bases.

* Menu:

* Using frozen files::          Using frozen files
* Frozen file format::          Frozen file format


File: m4.info,  Node: Using frozen files,  Next: Frozen file format,  Up: Frozen files

15.1 Using frozen files
=======================

Suppose a user has a library of `m4' initializations in `base.m4',
which is then used with multiple input files:

     m4 base.m4 input1.m4
     m4 base.m4 input2.m4
     m4 base.m4 input3.m4

   Rather than spending time parsing the fixed contents of `base.m4'
every time, the user might rather execute:

     m4 -F base.m4f base.m4

once, and further execute, as often as needed:

     m4 -R base.m4f input1.m4
     m4 -R base.m4f input2.m4
     m4 -R base.m4f input3.m4

with the varying input.  The first call, containing the `-F' option,
only reads and executes file `base.m4', defining various application
macros and computing other initializations.  Once the input file
`base.m4' has been completely processed, GNU `m4' produces on
`base.m4f' a "frozen" file, that is, a file which contains a kind of
snapshot of the `m4' internal state.

   Later calls, containing the `-R' option, are able to reload the
internal state of `m4', from `base.m4f', _prior_ to reading any other
input files.  This means instead of starting with a virgin copy of
`m4', input will be read after having effectively recovered the effect
of a prior run.  In our example, the effect is the same as if file
`base.m4' has been read anew.  However, this effect is achieved a lot
faster.

   Only one frozen file may be created or read in any one `m4'
invocation.  It is not possible to recover two frozen files at once.
However, frozen files may be updated incrementally, through using `-R'
and `-F' options simultaneously.  For example, if some care is taken,
the command:

     m4 file1.m4 file2.m4 file3.m4 file4.m4

could be broken down in the following sequence, accumulating the same
output:

     m4 -F file1.m4f file1.m4
     m4 -R file1.m4f -F file2.m4f file2.m4
     m4 -R file2.m4f -F file3.m4f file3.m4
     m4 -R file3.m4f file4.m4

   Some care is necessary because not every effort has been made for
this to work in all cases.  In particular, the trace attribute of
macros is not handled, nor the current setting of `changeword'.
Currently, `m4wrap' and `sysval' also have problems.  Also,
interactions for some options of `m4', being used in one call and not
in the next, have not been fully analyzed yet.  On the other end, you
may be confident that stacks of `pushdef' definitions are handled
correctly, as well as undefined or renamed builtins, and changed
strings for quotes or comments.  And future releases of GNU M4 will
improve on the utility of frozen files.

   When an `m4' run is to be frozen, the automatic undiversion which
takes place at end of execution is inhibited.  Instead, all positively
numbered diversions are saved into the frozen file.  The active
diversion number is also transmitted.

   A frozen file to be reloaded need not reside in the current
directory.  It is looked up the same way as an `include' file (*note
Search Path::).

   If the frozen file was generated with a newer version of `m4', and
contains directives that an older `m4' cannot parse, attempting to load
the frozen file with option `-R' will cause `m4' to exit with status 63
to indicate version mismatch.


File: m4.info,  Node: Frozen file format,  Prev: Using frozen files,  Up: Frozen files

15.2 Frozen file format
=======================

Frozen files are sharable across architectures.  It is safe to write a
frozen file on one machine and read it on another, given that the
second machine uses the same or newer version of GNU `m4'.  It is
conventional, but not required, to give a frozen file the suffix of
`.m4f'.

   These are simple (editable) text files, made up of directives, each
starting with a capital letter and ending with a newline (<NL>).
Wherever a directive is expected, the character `#' introduces a
comment line; empty lines are also ignored if they are not part of an
embedded string.  In the following descriptions, each LEN refers to the
length of the corresponding strings STR in the next line of input.
Numbers are always expressed in decimal.  There are no escape
characters.  The directives are:

`C LEN1 , LEN2 <NL> STR1 STR2 <NL>'
     Uses STR1 and STR2 as the begin-comment and end-comment strings.
     If omitted, then `#' and <NL> are the comment delimiters.

`D NUMBER, LEN <NL> STR <NL>'
     Selects diversion NUMBER, making it current, then copy STR in the
     current diversion.  NUMBER may be a negative number for a
     non-existing diversion.  To merely specify an active selection,
     use this command with an empty STR.  With 0 as the diversion
     NUMBER, STR will be issued on standard output at reload time.  GNU
     `m4' will not produce the `D' directive with non-zero length for
     diversion 0, but this can be done with manual edits.  This
     directive may appear more than once for the same diversion, in
     which case the diversion is the concatenation of the various uses.
     If omitted, then diversion 0 is current.

`F LEN1 , LEN2 <NL> STR1 STR2 <NL>'
     Defines, through `pushdef', a definition for STR1 expanding to the
     function whose builtin name is STR2.  If the builtin does not
     exist (for example, if the frozen file was produced by a copy of
     `m4' compiled with changeword support, but the version of `m4'
     reloading was compiled without it), the reload is silent, but any
     subsequent use of the definition of STR1 will result in a warning.
     This directive may appear more than once for the same name, and
     its order, along with `T', is important.  If omitted, you will
     have no access to any builtins.

`Q LEN1 , LEN2 <NL> STR1 STR2 <NL>'
     Uses STR1 and STR2 as the begin-quote and end-quote strings.  If
     omitted, then ``' and `'' are the quote delimiters.

`T LEN1 , LEN2 <NL> STR1 STR2 <NL>'
     Defines, though `pushdef', a definition for STR1 expanding to the
     text given by STR2.  This directive may appear more than once for
     the same name, and its order, along with `F', is important.

`V NUMBER <NL>'
     Confirms the format of the file.  `m4' 1.4.8 only creates and
     understands frozen files where NUMBER is 1.  This directive must
     be the first non-comment in the file, and may not appear more than
     once.


File: m4.info,  Node: Compatibility,  Next: Answers,  Prev: Frozen files,  Up: Top

16 Compatibility with other versions of `m4'
********************************************

This chapter describes the differences between this implementation of
`m4', and the implementation found under UNIX, notably System V,
Release 3.

   There are also differences in BSD flavors of `m4'.  No attempt is
made to summarize these here.

* Menu:

* Extensions::                  Extensions in GNU M4
* Incompatibilities::           Facilities in System V m4 not in GNU M4
* Other Incompatibilities::     Other incompatibilities


File: m4.info,  Node: Extensions,  Next: Incompatibilities,  Up: Compatibility

16.1 Extensions in GNU `m4'
===========================

This version of `m4' contains a few facilities that do not exist in
System V `m4'.  These extra facilities are all suppressed by using the
`-G' command line option (*note Invoking m4: Limits control.), unless
overridden by other command line options.

   * In the `$'N notation for macro arguments, N can contain several
     digits, while the System V `m4' only accepts one digit.  This
     allows macros in GNU `m4' to take any number of arguments, and not
     only nine (*note Arguments::).

     This means that `define(`foo', `$11')' is ambiguous between
     implementations.  To portably choose between grabbing the first
     parameter and appending 1 to the expansion, or grabbing the
     eleventh parameter, you can do the following:

          define(`a1', `A1')
          =>
          dnl First argument, concatenated with 1
          define(`_1', `$1')define(`first1', `_1($@)1')
          =>
          dnl Eleventh argument, portable
          define(`_9', `$9')define(`eleventh', `_9(shift(shift($@)))')
          =>
          dnl Eleventh argument, GNU style
          define(`Eleventh', `$11')
          =>
          first1(`a', `b', `c', `d', `e', `f', `g', `h', `i', `j', `k')
          =>A1
          eleventh(`a', `b', `c', `d', `e', `f', `g', `h', `i', `j', `k')
          =>k
          Eleventh(`a', `b', `c', `d', `e', `f', `g', `h', `i', `j', `k')
          =>k

   * The `divert' (*note Divert::) macro can manage more than 9
     diversions.  GNU `m4' treats all positive numbers as valid
     diversions, rather than discarding diversions greater than 9.

   * Files included with `include' and `sinclude' are sought in a user
     specified search path, if they are not found in the working
     directory.  The search path is specified by the `-I' option and the
     `M4PATH' environment variable (*note Search Path::).

   * Arguments to `undivert' can be non-numeric, in which case the named
     file will be included uninterpreted in the output (*note
     Undivert::).

   * Formatted output is supported through the `format' builtin, which
     is modeled after the C library function `printf' (*note Format::).

   * Searches and text substitution through regular expressions are
     supported by the `regexp' (*note Regexp::) and `patsubst' (*note
     Patsubst::) builtins.

   * The output of shell commands can be read into `m4' with `esyscmd'
     (*note Esyscmd::).

   * There is indirect access to any builtin macro with `builtin'
     (*note Builtin::).

   * Macros can be called indirectly through `indir' (*note Indir::).

   * The name of the program, the current input file, and the current
     input line number are accessible through the builtins
     `__program__', `__file__', and `__line__' (*note Location::).

   * The format of the output from `dumpdef' and macro tracing can be
     controlled with `debugmode' (*note Debug Levels::).

   * The destination of trace and debug output can be controlled with
     `debugfile' (*note Debug Output::).

   * The `maketemp' (*note Mkstemp::) macro behaves like `mkstemp',
     creating a new file with a unique name on every invocation, rather
     than following the insecure behavior of replacing the trailing `X'
     characters with the `m4' process id.

   In addition to the above extensions, GNU `m4' implements the
following command line options: `-F', `-G', `-I', `-L', `-R', `-V',
`-W', `-d', `-i', `-l', `--debugfile' and `-t'.  *Note Invoking m4::,
for a description of these options.

   Also, the debugging and tracing facilities in GNU `m4' are much more
extensive than in most other versions of `m4'.


File: m4.info,  Node: Incompatibilities,  Next: Other Incompatibilities,  Prev: Extensions,  Up: Compatibility

16.2 Facilities in System V `m4' not in GNU `m4'
================================================

The version of `m4' from System V contains a few facilities that have
not been implemented in GNU `m4' yet.  Additionally, POSIX requires
some behaviors that GNU `m4' has not implemented yet.  Relying on these
behaviors is non-portable, as a future release of GNU `m4' may change.

   * System V `m4' supports multiple arguments to `defn', and POSIX
     requires it.  This is not yet implemented in GNU `m4'.
     Unfortunately, this means it is not possible to mix builtins and
     other text into a single macro; a helper macro is required.

   * POSIX requires an application to exit with non-zero status if it
     wrote an error message to stderr.  This has not yet been
     consistently implemented for the various builtins that are
     required to issue an error (such as `include' (*note Include::)
     when a file is unreadable, `eval' (*note Eval::) when an argument
     cannot be parsed, or using `m4exit' (*note M4exit::) with a
     non-numeric argument).

   * Some traditional implementations only allow reading standard input
     once, but GNU `m4' correctly handles multiple instances of `-' on
     the command line.

   * POSIX requires `m4wrap' (*note M4wrap::) to act in FIFO (first-in,
     first-out) order, but GNU `m4' currently uses LIFO order.
     Furthermore, POSIX states that only the first argument to `m4wrap'
     is saved for later evaluation, bug GNU `m4' saves and processes
     all arguments, with output separated by spaces.

     However, it is possible to emulate POSIX behavior by including the
     file `m4-1.4.8/examples/wrapfifo.m4' from the distribution:

          undivert(`wrapfifo.m4')dnl
          =>dnl Redefine m4wrap to have FIFO semantics.
          =>define(`_m4wrap_level', `0')dnl
          =>define(`m4wrap',
          =>`ifdef(`m4wrap'_m4wrap_level,
          =>       `define(`m4wrap'_m4wrap_level,
          =>               defn(`m4wrap'_m4wrap_level)`$1')',
          =>       `builtin(`m4wrap', `define(`_m4wrap_level',
          =>                                  incr(_m4wrap_level))dnl
          =>m4wrap'_m4wrap_level)dnl
          =>define(`m4wrap'_m4wrap_level, `$1')')')dnl
          include(`wrapfifo.m4')
          =>
          m4wrap(`a`'m4wrap(`c
          ', `d')')m4wrap(`b')
          =>
          ^D
          =>abc

   * POSIX requires that all builtins that require arguments, but are
     called without arguments, behave as though empty strings had been
     passed.  For example, `a`'define`'b' would expand to `ab'.  But
     GNU `m4' ignores certain builtins if they have missing arguments,
     giving `adefineb' for the above example.

   * Traditional implementations handle `define(`f',`1')' (*note
     Define::) by undefining the entire stack of previous definitions,
     and if doing `undefine(`f')' first.  GNU `m4' replaces just the top
     definition on the stack, as if doing `popdef(`f')' followed by
     `pushdef(`f',`1')'.

   * POSIX requires `syscmd' (*note Syscmd::) to evaluate command
     output for macro expansion, but this appears to be a mistake in
     POSIX since traditional implementations did not do this.  GNU `m4'
     follows traditional behavior in `syscmd', and provides the
     extension `esyscmd' that provides the POSIX semantics.

   * At one point, POSIX required `changequote(ARG)' (*note
     Changequote::) to use newline as the close quote, but this was a
     bug, and the next version of POSIX is anticipated to state that
     using empty strings or just one argument is unspecified.
     Meanwhile, the GNU `m4' behavior of treating an empty end-quote
     delimiter as `'' is not portable, as Solaris treats it as
     repeating the start-quote delimiter, and BSD treats it as leaving
     the previous end-quote delimiter unchanged.  For predictable
     results, never call changequote with just one argument, or with
     empty strings for arguments.

   * At one point, POSIX required `changecom(ARG,)' (*note Changecom::)
     to make it impossible to end a comment, but this is a bug, and the
     next version of POSIX is anticipated to state that using empty
     strings is unspecified.  Meanwhile, the GNU `m4' behavior of
     treating an empty end-comment delimiter as newline is not
     portable, as BSD treats it as leaving the previous end-comment
     delimiter unchanged.  It is also impossible in BSD implementations
     to disable comments, even though that is required by POSIX.  For
     predictable results, never call changecom with empty strings for
     arguments.

   * Most implementations of `m4' give macros a higher precedence than
     comments when parsing, meaning that if the start delimiter given to
     `changecom' (*note Changecom::) starts with a macro name, comments
     are effectively disabled.  POSIX does not specify what the
     precedence is, so the GNU `m4' parser recognizes comments, then
     macros, then quoted strings.

   * Traditional implementations allow argument collection, but not
     string and comment processing, to span file boundaries.  Thus, if
     `a.m4' contains `len(', and `b.m4' contains `abc)', `m4 a.m4 b.m4'
     outputs `3' with traditional `m4', but gives an error message that
     the end of file was encountered inside a macro with GNU `m4'.  On
     the other hand, traditional implementations do end of file
     processing for files included with `include' or `sinclude' (*note
     Include::), while GNU `m4' seamlessly integrates the content of
     those files.  Thus `include(`a.m4')include(`b.m4')' will output
     `3' instead of giving an error.

   * Traditional `m4' treats `traceon' (*note Trace::) without
     arguments as a global variable, independent of named macro tracing.
     Also, once a macro is undefined, named tracing of that macro is
     lost.  On the other hand, when GNU `m4' encounters `traceon'
     without arguments, it turns tracing on for all existing
     definitions at the time, but does not trace future definitions;
     `traceoff' without arguments turns tracing off for all definitions
     regardless of whether they were also traced by name; and tracing
     by name, such as with `-tfoo' at the command line or
     `traceon(`foo')' in the input, is an attribute that is preserved
     even if the macro is currently undefined.

   * POSIX requires `eval' (*note Eval::) to treat all operators with
     the same precedence as C.  However, GNU `m4' currently follows the
     traditional precedence of other `m4' implementations, where
     bitwise and logical negation (`~' and `!') have lower precedence
     than equality operators, rather than equal precedence with other
     unary operators.  Use explicit parentheses to ensure proper
     precedence.  As extensions to POSIX, GNU `m4' treats the shift
     operators `<<' and `>>' as well-defined on signed integers (even
     though they are not in C), and adds the exponentiation operator
     `**'.

   * POSIX requires `translit' (*note Translit::) to treat each
     character of the second and third arguments literally, but GNU
     `m4' treats `-' as a range operator.

   * POSIX requires `m4' to honor the locale environment variables of
     `LANG', `LC_ALL', `LC_CTYPE', `LC_MESSAGES', and `NLSPATH', but
     this has not yet been implemented in GNU `m4'.

   * POSIX states that only unquoted leading newlines and blanks (that
     is, space and tab) are ignored when collecting macro arguments.
     However, this appears to be a bug in POSIX, since most traditional
     implementations also ignore all whitespace (formfeed, carriage
     return, and vertical tab).  GNU `m4' follows tradition and ignores
     all leading unquoted whitespace.


File: m4.info,  Node: Other Incompatibilities,  Prev: Incompatibilities,  Up: Compatibility

16.3 Other incompatibilities
============================

There are a few other incompatibilities between this implementation of
`m4', and the System V version.

   * GNU `m4' implements sync lines differently from System V `m4',
     when text is being diverted.  GNU `m4' outputs the sync lines when
     the text is being diverted, and System V `m4' when the diverted
     text is being brought back.

     The problem is which lines and file names should be attached to
     text that is being, or has been, diverted.  System V `m4' regards
     all the diverted text as being generated by the source line
     containing the `undivert' call, whereas GNU `m4' regards the
     diverted text as being generated at the time it is diverted.

     The sync line option is used mostly when using `m4' as a front end
     to a compiler.  If a diverted line causes a compiler error, the
     error messages should most probably refer to the place where the
     diversion were made, and not where it was inserted again.

   * GNU `m4' makes no attempt at prohibiting self-referential
     definitions like:

          define(`x', `x')
          =>
          define(`x', `x ')
          =>

     There is nothing inherently wrong with defining `x' to return `x'.
     The wrong thing is to expand `x' unquoted.  In `m4', one might
     use macros to hold strings, as we do for variables in other
     programming languages, further checking them with:

          ifelse(defn(`HOLDER'), `VALUE', ...)

     In cases like this one, an interdiction for a macro to hold its own
     name would be a useless limitation.  Of course, this leaves more
     rope for the GNU `m4' user to hang himself!  Rescanning hangs may
     be avoided through careful programming, a little like for endless
     loops in traditional programming languages.


File: m4.info,  Node: Answers,  Next: Copying This Manual,  Prev: Compatibility,  Up: Top

17 Correct version of some examples
***********************************

Some of the examples in this manuals are buggy or not very robust, for
demonstration purposes.  Improved versions of these composite macros are
presented here.

* Menu:

* Improved exch::               Solution for `exch'
* Improved forloop::            Solution for `forloop'
* Improved foreach::            Solution for `foreach'
* Improved cleardivert::        Solution for `cleardivert'
* Improved fatal_error::        Solution for `fatal_error'


File: m4.info,  Node: Improved exch,  Next: Improved forloop,  Up: Answers

17.1 Solution for `exch'
========================

The `exch' macro (*note Arguments::) as presented requires clients to
double quote their arguments.  A nicer definition, which lets clients
follow the rule of thumb of one level of quoting per level of
parentheses, involves adding quotes in the definition of `exch', as
follows:

     define(`exch', ``$2', `$1'')
     =>
     define(exch(`expansion text', `macro'))
     =>
     macro
     =>expansion text


File: m4.info,  Node: Improved forloop,  Next: Improved foreach,  Prev: Improved exch,  Up: Answers

17.2 Solution for `forloop'
===========================

The `forloop' macro (*note Forloop::) as presented earlier can go into
an infinite loop if given an iterator that is not parsed as a macro
name.  It does not do any sanity checking on its numeric bounds, and
only permits decimal numbers for bounds.  Here is an improved version,
shipped as `m4-1.4.8/examples/forloop2.m4'; this version also optimizes
based on the fact that the starting bound does not need to be passed to
the helper `_forloop'.

     undivert(`forloop2.m4')dnl
     =>divert(`-1')
     =># forloop(var, from, to, stmt) - improved version:
     =>#   works even if VAR is not a strict macro name
     =>#   performs sanity check that FROM is larger than TO
     =>#   allows complex numerical expressions in TO and FROM
     =>define(`forloop', `ifelse(eval(`($3) >= ($2)'), `1',
     =>  `pushdef(`$1', eval(`$2'))_forloop(`$1',
     =>    eval(`$3'), `$4')popdef(`$1')')')
     =>define(`_forloop',
     =>  `$3`'ifelse(indir(`$1'), `$2', `',
     =>    `define(`$1', incr(indir(`$1')))$0($@)')')
     =>divert`'dnl
     include(`forloop2.m4')
     =>
     forloop(`i', `2', `1', `no iteration occurs')
     =>
     forloop(`', `1', `2', ` odd iterator name')
     => odd iterator name odd iterator name
     forloop(`i', `5 + 5', `0xc', ` 0x`'eval(i, `16')')
     => 0xa 0xb 0xc
     forloop(`i', `a', `b', `non-numeric bounds')
     error-->m4:stdin:6: bad expression in eval (bad input): (b) >= (a)
     =>

   Of course, it is possible to make even more improvements, such as
adding an optional step argument, or allowing iteration through
descending sequences.  GNU Autoconf provides some of these additional
bells and whistles in its `m4_for' macro.


File: m4.info,  Node: Improved foreach,  Next: Improved cleardivert,  Prev: Improved forloop,  Up: Answers

17.3 Solution for `foreach'
===========================

The `foreach' and `foreachq' macros (*note Foreach::) as presented
earlier each have flaws.  First, we will examine and fix the quadratic
behavior of `foreachq':

     include(`foreachq.m4')
     =>
     traceon(`shift')debugmode(`aq')
     =>
     foreachq(`x', ``1', `2', `3', `4'', `x
     ')dnl
     =>1
     error-->m4trace: -3- shift(`1', `2', `3', `4')
     error-->m4trace: -2- shift(`1', `2', `3', `4')
     =>2
     error-->m4trace: -4- shift(`1', `2', `3', `4')
     error-->m4trace: -3- shift(`2', `3', `4')
     error-->m4trace: -3- shift(`1', `2', `3', `4')
     error-->m4trace: -2- shift(`2', `3', `4')
     =>3
     error-->m4trace: -5- shift(`1', `2', `3', `4')
     error-->m4trace: -4- shift(`2', `3', `4')
     error-->m4trace: -3- shift(`3', `4')
     error-->m4trace: -4- shift(`1', `2', `3', `4')
     error-->m4trace: -3- shift(`2', `3', `4')
     error-->m4trace: -2- shift(`3', `4')
     =>4
     error-->m4trace: -6- shift(`1', `2', `3', `4')
     error-->m4trace: -5- shift(`2', `3', `4')
     error-->m4trace: -4- shift(`3', `4')
     error-->m4trace: -3- shift(`4')

   Each successive iteration was adding more quoted `shift'
invocations, and the entire list contents were passing through every
iteration.  In general, when recursing, it is a good idea to make the
recursion use fewer arguments, rather than adding additional quoted
uses of `shift'.  By doing so, `m4' uses less memory, invokes fewer
macros, is less likely to run into machine limits, and most
importantly, performs faster.  The fixed version of `foreachq' can be
found in `m4-1.4.8/examples/foreachq2.m4':

     include(`foreachq2.m4')
     =>
     undivert(`foreachq2.m4')dnl
     =>include(`quote.m4')dnl
     =>divert(`-1')
     =># foreachq(x, `item_1, item_2, ..., item_n', stmt)
     =>#   quoted list, improved version
     =>define(`foreachq', `pushdef(`$1')_foreachq($@)popdef(`$1')')
     =>define(`_arg1q', ``$1'')
     =>define(`_rest', `ifelse(`$#', `1', `', `dquote(shift($@))')')
     =>define(`_foreachq', `ifelse(`$2', `', `',
     =>  `define(`$1', _arg1q($2))$3`'$0(`$1', _rest($2), `$3')')')
     =>divert`'dnl
     traceon(`shift')debugmode(`aq')
     =>
     foreachq(`x', ``1', `2', `3', `4'', `x
     ')dnl
     =>1
     error-->m4trace: -3- shift(`1', `2', `3', `4')
     =>2
     error-->m4trace: -3- shift(`2', `3', `4')
     =>3
     error-->m4trace: -3- shift(`3', `4')
     =>4

   Note that the fixed version calls unquoted helper macros in
`_foreachq' to trim elements immediately; those helper macros in turn
must re-supply the layer of quotes lost in the macro invocation.
Contrast the use of `_arg1q', which quotes the first list element, with
`_arg1' of the earlier implementation that returned the first list
element directly.

   For a different approach, the improved version of `foreach',
available in `m4-1.4.8/examples/foreach2.m4', simply overquotes the
arguments to `_foreach' to begin with, using `dquote_elt'.  Then
`_foreach' can just use `_arg1' to remove the extra layer of quoting
that was added up front:

     include(`foreach2.m4')
     =>
     undivert(`foreach2.m4')dnl
     =>include(`quote.m4')dnl
     =>divert(`-1')
     =># foreach(x, (item_1, item_2, ..., item_n), stmt)
     =>#   parenthesized list, improved version
     =>define(`foreach', `pushdef(`$1')_foreach(`$1',
     =>  (dquote(dquote_elt$2)), `$3')popdef(`$1')')
     =>define(`_arg1', `$1')
     =>define(`_foreach', `ifelse(`$2', `(`')', `',
     =>  `define(`$1', _arg1$2)$3`'$0(`$1', (dquote(shift$2)), `$3')')')
     =>divert`'dnl
     traceon(`shift')debugmode(`aq')
     =>
     foreach(`x', `(`1', `2', `3', `4')', `x
     ')dnl
     error-->m4trace: -4- shift(`1', `2', `3', `4')
     error-->m4trace: -4- shift(`2', `3', `4')
     error-->m4trace: -4- shift(`3', `4')
     =>1
     error-->m4trace: -3- shift(``1'', ``2'', ``3'', ``4'')
     =>2
     error-->m4trace: -3- shift(``2'', ``3'', ``4'')
     =>3
     error-->m4trace: -3- shift(``3'', ``4'')
     =>4
     error-->m4trace: -3- shift(``4'')

   In summary, recursion over list elements is trickier than it
appeared at first glance, but provides a powerful idiom within `m4'
processing.  As a final demonstration, both list styles are now able to
handle several scenarios that would wreak havoc on the original
implementations.  This points out one other difference between the two
list styles.  `foreach' evaluates unquoted list elements only once, in
preparation for calling `_foreach'.  But `foreachq' evaluates unquoted
list elements twice while visiting the first list element, once in
`_arg1q' and once in `_rest'.  When deciding which list style to use,
one must take into account whether repeating the side effects of
unquoted list elements will have any detrimental effects.

     include(`foreach2.m4')
     =>
     include(`foreachq2.m4')
     =>
     dnl 0-element list:
     foreach(`x', `', `<x>') / foreachq(`x', `', `<x>')
     => / 
     dnl 1-element list of empty element
     foreach(`x', `()', `<x>') / foreachq(`x', ``'', `<x>')
     =><> / <>
     dnl 2-element list of empty elements
     foreach(`x', `(`',`')', `<x>') / foreachq(`x', ``',`'', `<x>')
     =><><> / <><>
     dnl 1-element list of a comma
     foreach(`x', `(`,')', `<x>') / foreachq(`x', ``,'', `<x>')
     =><,> / <,>
     dnl 2-element list of unbalanced parentheses
     foreach(`x', `(`(', `)')', `<x>') / foreachq(`x', ``(', `)'', `<x>')
     =><(><)> / <(><)>
     define(`active', `ACT, IVE')
     =>
     traceon(`active')
     =>
     dnl list of unquoted macros; expansion occurs before recursion
     foreach(`x', `(active, active)', `<x>
     ')dnl
     error-->m4trace: -4- active -> `ACT, IVE'
     error-->m4trace: -4- active -> `ACT, IVE'
     =><ACT>
     =><IVE>
     =><ACT>
     =><IVE>
     foreachq(`x', `active, active', `<x>
     ')dnl
     error-->m4trace: -3- active -> `ACT, IVE'
     error-->m4trace: -3- active -> `ACT, IVE'
     =><ACT>
     error-->m4trace: -3- active -> `ACT, IVE'
     error-->m4trace: -3- active -> `ACT, IVE'
     =><IVE>
     =><ACT>
     =><IVE>
     dnl list of quoted macros; expansion occurs during recursion
     foreach(`x', `(`active', `active')', `<x>
     ')dnl
     error-->m4trace: -1- active -> `ACT, IVE'
     =><ACT, IVE>
     error-->m4trace: -1- active -> `ACT, IVE'
     =><ACT, IVE>
     foreachq(`x', ``active', `active'', `<x>
     ')dnl
     error-->m4trace: -1- active -> `ACT, IVE'
     =><ACT, IVE>
     error-->m4trace: -1- active -> `ACT, IVE'
     =><ACT, IVE>
     dnl list of double-quoted macro names; no expansion
     foreach(`x', `(``active'', ``active'')', `<x>
     ')dnl
     =><active>
     =><active>
     foreachq(`x', ```active'', ``active''', `<x>
     ')dnl
     =><active>
     =><active>


File: m4.info,  Node: Improved cleardivert,  Next: Improved fatal_error,  Prev: Improved foreach,  Up: Answers

17.4 Solution for `cleardivert'
===============================

The `cleardivert' macro (*note Cleardivert::) cannot, as it stands, be
called without arguments to clear all pending diversions.  That is
because using undivert with an empty string for an argument is different
than using it with no arguments at all.  Compare the earlier definition
with one that takes the number of arguments into account:

     define(`cleardivert',
       `pushdef(`_n', divnum)divert(`-1')undivert($@)divert(_n)popdef(`_n')')
     =>
     divert(`1')one
     divert
     =>
     cleardivert
     =>
     undivert
     =>one
     =>
     define(`cleardivert',
       `pushdef(`_num', divnum)divert(`-1')ifelse(`$#', `0',
         `undivert`'', `undivert($@)')divert(_num)popdef(`_num')')
     =>
     divert(`2')two
     divert
     =>
     cleardivert
     =>
     undivert
     =>


File: m4.info,  Node: Improved fatal_error,  Prev: Improved cleardivert,  Up: Answers

17.5 Solution for `fatal_error'
===============================

The `fatal_error' macro (*note M4exit::) is not robust to versions of
GNU M4 earlier than 1.4.8, where invoking `__file__' (*note Location::)
inside `m4wrap' would result in an empty string, and `__line__'
resulted in `0' even though all files start at line 1.  Furthermore,
versions earlier than 1.4.6 did not support the `__program__' macro.
If you want `fatal_error' to work across the entire 1.4.x release
series, a better implementation would be:

     define(`fatal_error',
       `errprint(ifdef(`__program__', `__program__', ``m4'')'dnl
     `:ifelse(__line__, `0', `',
         `__file__:__line__:')` fatal error: $*
     ')m4exit(`1')')
     =>
     m4wrap(`divnum(`demo of internal message')
     fatal_error(`inside wrapped text')')
     =>
     ^D
     error-->m4:stdin:6: Warning: excess arguments to builtin `divnum' ignored
     =>0
     error-->m4:stdin:6: fatal error: inside wrapped text


File: m4.info,  Node: Copying This Manual,  Next: Indices,  Prev: Answers,  Up: Top

Appendix A How to make copies of this manual
********************************************

* Menu:

* GNU Free Documentation License::  License for copying this manual


File: m4.info,  Node: GNU Free Documentation License,  Up: Copying This Manual

A.1 GNU Free Documentation License
==================================

                      Version 1.2, November 2002

     Copyright (C) 2000,2001,2002 Free Software Foundation, Inc.
     51 Franklin St, Fifth Floor, Boston, MA  02110-1301, USA

     Everyone is permitted to copy and distribute verbatim copies
     of this license document, but changing it is not allowed.

  0. PREAMBLE

     The purpose of this License is to make a manual, textbook, or other
     functional and useful document "free" in the sense of freedom: to
     assure everyone the effective freedom to copy and redistribute it,
     with or without modifying it, either commercially or
     noncommercially.  Secondarily, this License preserves for the
     author and publisher a way to get credit for their work, while not
     being considered responsible for modifications made by others.

     This License is a kind of "copyleft", which means that derivative
     works of the document must themselves be free in the same sense.
     It complements the GNU General Public License, which is a copyleft
     license designed for free software.

     We have designed this License in order to use it for manuals for
     free software, because free software needs free documentation: a
     free program should come with manuals providing the same freedoms
     that the software does.  But this License is not limited to
     software manuals; it can be used for any textual work, regardless
     of subject matter or whether it is published as a printed book.
     We recommend this License principally for works whose purpose is
     instruction or reference.

  1. APPLICABILITY AND DEFINITIONS

     This License applies to any manual or other work, in any medium,
     that contains a notice placed by the copyright holder saying it
     can be distributed under the terms of this License.  Such a notice
     grants a world-wide, royalty-free license, unlimited in duration,
     to use that work under the conditions stated herein.  The
     "Document", below, refers to any such manual or work.  Any member
     of the public is a licensee, and is addressed as "you".  You
     accept the license if you copy, modify or distribute the work in a
     way requiring permission under copyright law.

     A "Modified Version" of the Document means any work containing the
     Document or a portion of it, either copied verbatim, or with
     modifications and/or translated into another language.

     A "Secondary Section" is a named appendix or a front-matter section
     of the Document that deals exclusively with the relationship of the
     publishers or authors of the Document to the Document's overall
     subject (or to related matters) and contains nothing that could
     fall directly within that overall subject.  (Thus, if the Document
     is in part a textbook of mathematics, a Secondary Section may not
     explain any mathematics.)  The relationship could be a matter of
     historical connection with the subject or with related matters, or
     of legal, commercial, philosophical, ethical or political position
     regarding them.

     The "Invariant Sections" are certain Secondary Sections whose
     titles are designated, as being those of Invariant Sections, in
     the notice that says that the Document is released under this
     License.  If a section does not fit the above definition of
     Secondary then it is not allowed to be designated as Invariant.
     The Document may contain zero Invariant Sections.  If the Document
     does not identify any Invariant Sections then there are none.

     The "Cover Texts" are certain short passages of text that are
     listed, as Front-Cover Texts or Back-Cover Texts, in the notice
     that says that the Document is released under this License.  A
     Front-Cover Text may be at most 5 words, and a Back-Cover Text may
     be at most 25 words.

     A "Transparent" copy of the Document means a machine-readable copy,
     represented in a format whose specification is available to the
     general public, that is suitable for revising the document
     straightforwardly with generic text editors or (for images
     composed of pixels) generic paint programs or (for drawings) some
     widely available drawing editor, and that is suitable for input to
     text formatters or for automatic translation to a variety of
     formats suitable for input to text formatters.  A copy made in an
     otherwise Transparent file format whose markup, or absence of
     markup, has been arranged to thwart or discourage subsequent
     modification by readers is not Transparent.  An image format is
     not Transparent if used for any substantial amount of text.  A
     copy that is not "Transparent" is called "Opaque".

     Examples of suitable formats for Transparent copies include plain
     ASCII without markup, Texinfo input format, LaTeX input format,
     SGML or XML using a publicly available DTD, and
     standard-conforming simple HTML, PostScript or PDF designed for
     human modification.  Examples of transparent image formats include
     PNG, XCF and JPG.  Opaque formats include proprietary formats that
     can be read and edited only by proprietary word processors, SGML or
     XML for which the DTD and/or processing tools are not generally
     available, and the machine-generated HTML, PostScript or PDF
     produced by some word processors for output purposes only.

     The "Title Page" means, for a printed book, the title page itself,
     plus such following pages as are needed to hold, legibly, the
     material this License requires to appear in the title page.  For
     works in formats which do not have any title page as such, "Title
     Page" means the text near the most prominent appearance of the
     work's title, preceding the beginning of the body of the text.

     A section "Entitled XYZ" means a named subunit of the Document
     whose title either is precisely XYZ or contains XYZ in parentheses
     following text that translates XYZ in another language.  (Here XYZ
     stands for a specific section name mentioned below, such as
     "Acknowledgements", "Dedications", "Endorsements", or "History".)
     To "Preserve the Title" of such a section when you modify the
     Document means that it remains a section "Entitled XYZ" according
     to this definition.

     The Document may include Warranty Disclaimers next to the notice
     which states that this License applies to the Document.  These
     Warranty Disclaimers are considered to be included by reference in
     this License, but only as regards disclaiming warranties: any other
     implication that these Warranty Disclaimers may have is void and
     has no effect on the meaning of this License.

  2. VERBATIM COPYING

     You may copy and distribute the Document in any medium, either
     commercially or noncommercially, provided that this License, the
     copyright notices, and the license notice saying this License
     applies to the Document are reproduced in all copies, and that you
     add no other conditions whatsoever to those of this License.  You
     may not use technical measures to obstruct or control the reading
     or further copying of the copies you make or distribute.  However,
     you may accept compensation in exchange for copies.  If you
     distribute a large enough number of copies you must also follow
     the conditions in section 3.

     You may also lend copies, under the same conditions stated above,
     and you may publicly display copies.

  3. COPYING IN QUANTITY

     If you publish printed copies (or copies in media that commonly
     have printed covers) of the Document, numbering more than 100, and
     the Document's license notice requires Cover Texts, you must
     enclose the copies in covers that carry, clearly and legibly, all
     these Cover Texts: Front-Cover Texts on the front cover, and
     Back-Cover Texts on the back cover.  Both covers must also clearly
     and legibly identify you as the publisher of these copies.  The
     front cover must present the full title with all words of the
     title equally prominent and visible.  You may add other material
     on the covers in addition.  Copying with changes limited to the
     covers, as long as they preserve the title of the Document and
     satisfy these conditions, can be treated as verbatim copying in
     other respects.

     If the required texts for either cover are too voluminous to fit
     legibly, you should put the first ones listed (as many as fit
     reasonably) on the actual cover, and continue the rest onto
     adjacent pages.

     If you publish or distribute Opaque copies of the Document
     numbering more than 100, you must either include a
     machine-readable Transparent copy along with each Opaque copy, or
     state in or with each Opaque copy a computer-network location from
     which the general network-using public has access to download
     using public-standard network protocols a complete Transparent
     copy of the Document, free of added material.  If you use the
     latter option, you must take reasonably prudent steps, when you
     begin distribution of Opaque copies in quantity, to ensure that
     this Transparent copy will remain thus accessible at the stated
     location until at least one year after the last time you
     distribute an Opaque copy (directly or through your agents or
     retailers) of that edition to the public.

     It is requested, but not required, that you contact the authors of
     the Document well before redistributing any large number of
     copies, to give them a chance to provide you with an updated
     version of the Document.

  4. MODIFICATIONS

     You may copy and distribute a Modified Version of the Document
     under the conditions of sections 2 and 3 above, provided that you
     release the Modified Version under precisely this License, with
     the Modified Version filling the role of the Document, thus
     licensing distribution and modification of the Modified Version to
     whoever possesses a copy of it.  In addition, you must do these
     things in the Modified Version:

       A. Use in the Title Page (and on the covers, if any) a title
          distinct from that of the Document, and from those of
          previous versions (which should, if there were any, be listed
          in the History section of the Document).  You may use the
          same title as a previous version if the original publisher of
          that version gives permission.

       B. List on the Title Page, as authors, one or more persons or
          entities responsible for authorship of the modifications in
          the Modified Version, together with at least five of the
          principal authors of the Document (all of its principal
          authors, if it has fewer than five), unless they release you
          from this requirement.

       C. State on the Title page the name of the publisher of the
          Modified Version, as the publisher.

       D. Preserve all the copyright notices of the Document.

       E. Add an appropriate copyright notice for your modifications
          adjacent to the other copyright notices.

       F. Include, immediately after the copyright notices, a license
          notice giving the public permission to use the Modified
          Version under the terms of this License, in the form shown in
          the Addendum below.

       G. Preserve in that license notice the full lists of Invariant
          Sections and required Cover Texts given in the Document's
          license notice.

       H. Include an unaltered copy of this License.

       I. Preserve the section Entitled "History", Preserve its Title,
          and add to it an item stating at least the title, year, new
          authors, and publisher of the Modified Version as given on
          the Title Page.  If there is no section Entitled "History" in
          the Document, create one stating the title, year, authors,
          and publisher of the Document as given on its Title Page,
          then add an item describing the Modified Version as stated in
          the previous sentence.

       J. Preserve the network location, if any, given in the Document
          for public access to a Transparent copy of the Document, and
          likewise the network locations given in the Document for
          previous versions it was based on.  These may be placed in
          the "History" section.  You may omit a network location for a
          work that was published at least four years before the
          Document itself, or if the original publisher of the version
          it refers to gives permission.

       K. For any section Entitled "Acknowledgements" or "Dedications",
          Preserve the Title of the section, and preserve in the
          section all the substance and tone of each of the contributor
          acknowledgements and/or dedications given therein.

       L. Preserve all the Invariant Sections of the Document,
          unaltered in their text and in their titles.  Section numbers
          or the equivalent are not considered part of the section
          titles.

       M. Delete any section Entitled "Endorsements".  Such a section
          may not be included in the Modified Version.

       N. Do not retitle any existing section to be Entitled
          "Endorsements" or to conflict in title with any Invariant
          Section.

       O. Preserve any Warranty Disclaimers.

     If the Modified Version includes new front-matter sections or
     appendices that qualify as Secondary Sections and contain no
     material copied from the Document, you may at your option
     designate some or all of these sections as invariant.  To do this,
     add their titles to the list of Invariant Sections in the Modified
     Version's license notice.  These titles must be distinct from any
     other section titles.

     You may add a section Entitled "Endorsements", provided it contains
     nothing but endorsements of your Modified Version by various
     parties--for example, statements of peer review or that the text
     has been approved by an organization as the authoritative
     definition of a standard.

     You may add a passage of up to five words as a Front-Cover Text,
     and a passage of up to 25 words as a Back-Cover Text, to the end
     of the list of Cover Texts in the Modified Version.  Only one
     passage of Front-Cover Text and one of Back-Cover Text may be
     added by (or through arrangements made by) any one entity.  If the
     Document already includes a cover text for the same cover,
     previously added by you or by arrangement made by the same entity
     you are acting on behalf of, you may not add another; but you may
     replace the old one, on explicit permission from the previous
     publisher that added the old one.

     The author(s) and publisher(s) of the Document do not by this
     License give permission to use their names for publicity for or to
     assert or imply endorsement of any Modified Version.

  5. COMBINING DOCUMENTS

     You may combine the Document with other documents released under
     this License, under the terms defined in section 4 above for
     modified versions, provided that you include in the combination
     all of the Invariant Sections of all of the original documents,
     unmodified, and list them all as Invariant Sections of your
     combined work in its license notice, and that you preserve all
     their Warranty Disclaimers.

     The combined work need only contain one copy of this License, and
     multiple identical Invariant Sections may be replaced with a single
     copy.  If there are multiple Invariant Sections with the same name
     but different contents, make the title of each such section unique
     by adding at the end of it, in parentheses, the name of the
     original author or publisher of that section if known, or else a
     unique number.  Make the same adjustment to the section titles in
     the list of Invariant Sections in the license notice of the
     combined work.

     In the combination, you must combine any sections Entitled
     "History" in the various original documents, forming one section
     Entitled "History"; likewise combine any sections Entitled
     "Acknowledgements", and any sections Entitled "Dedications".  You
     must delete all sections Entitled "Endorsements."

  6. COLLECTIONS OF DOCUMENTS

     You may make a collection consisting of the Document and other
     documents released under this License, and replace the individual
     copies of this License in the various documents with a single copy
     that is included in the collection, provided that you follow the
     rules of this License for verbatim copying of each of the
     documents in all other respects.

     You may extract a single document from such a collection, and
     distribute it individually under this License, provided you insert
     a copy of this License into the extracted document, and follow
     this License in all other respects regarding verbatim copying of
     that document.

  7. AGGREGATION WITH INDEPENDENT WORKS

     A compilation of the Document or its derivatives with other
     separate and independent documents or works, in or on a volume of
     a storage or distribution medium, is called an "aggregate" if the
     copyright resulting from the compilation is not used to limit the
     legal rights of the compilation's users beyond what the individual
     works permit.  When the Document is included in an aggregate, this
     License does not apply to the other works in the aggregate which
     are not themselves derivative works of the Document.

     If the Cover Text requirement of section 3 is applicable to these
     copies of the Document, then if the Document is less than one half
     of the entire aggregate, the Document's Cover Texts may be placed
     on covers that bracket the Document within the aggregate, or the
     electronic equivalent of covers if the Document is in electronic
     form.  Otherwise they must appear on printed covers that bracket
     the whole aggregate.

  8. TRANSLATION

     Translation is considered a kind of modification, so you may
     distribute translations of the Document under the terms of section
     4.  Replacing Invariant Sections with translations requires special
     permission from their copyright holders, but you may include
     translations of some or all Invariant Sections in addition to the
     original versions of these Invariant Sections.  You may include a
     translation of this License, and all the license notices in the
     Document, and any Warranty Disclaimers, provided that you also
     include the original English version of this License and the
     original versions of those notices and disclaimers.  In case of a
     disagreement between the translation and the original version of
     this License or a notice or disclaimer, the original version will
     prevail.

     If a section in the Document is Entitled "Acknowledgements",
     "Dedications", or "History", the requirement (section 4) to
     Preserve its Title (section 1) will typically require changing the
     actual title.

  9. TERMINATION

     You may not copy, modify, sublicense, or distribute the Document
     except as expressly provided for under this License.  Any other
     attempt to copy, modify, sublicense or distribute the Document is
     void, and will automatically terminate your rights under this
     License.  However, parties who have received copies, or rights,
     from you under this License will not have their licenses
     terminated so long as such parties remain in full compliance.

 10. FUTURE REVISIONS OF THIS LICENSE

     The Free Software Foundation may publish new, revised versions of
     the GNU Free Documentation License from time to time.  Such new
     versions will be similar in spirit to the present version, but may
     differ in detail to address new problems or concerns.  See
     `http://www.gnu.org/copyleft/'.

     Each version of the License is given a distinguishing version
     number.  If the Document specifies that a particular numbered
     version of this License "or any later version" applies to it, you
     have the option of following the terms and conditions either of
     that specified version or of any later version that has been
     published (not as a draft) by the Free Software Foundation.  If
     the Document does not specify a version number of this License,
     you may choose any version ever published (not as a draft) by the
     Free Software Foundation.

ADDENDUM: How to use this License for your documents
====================================================

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and license
notices just after the title page:

       Copyright (C)  YEAR  YOUR NAME.
       Permission is granted to copy, distribute and/or modify this document
       under the terms of the GNU Free Documentation License, Version 1.2
       or any later version published by the Free Software Foundation;
       with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
       Texts.  A copy of the license is included in the section entitled ``GNU
       Free Documentation License''.

   If you have Invariant Sections, Front-Cover Texts and Back-Cover
Texts, replace the "with...Texts." line with this:

         with the Invariant Sections being LIST THEIR TITLES, with
         the Front-Cover Texts being LIST, and with the Back-Cover Texts
         being LIST.

   If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the
situation.

   If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of
free software license, such as the GNU General Public License, to
permit their use in free software.


File: m4.info,  Node: Indices,  Prev: Copying This Manual,  Up: Top

Appendix B Indices of concepts and macros
*****************************************

* Menu:

* Concept index::               Index for many concepts
* Macro index::                 Index for all `m4' macros


File: m4.info,  Node: Concept index,  Next: Macro index,  Up: Indices

B.1 Index for many concepts
===========================

[index]
* Menu:

* arguments to macros:                   Macro Arguments.      (line  6)
* Arguments to macros:                   Arguments.            (line  6)
* arguments to macros, special:          Pseudo Arguments.     (line  6)
* arguments, quoted macro:               Quoting Arguments.    (line  6)
* arithmetic:                            Arithmetic.           (line  6)
* arrays:                                Define.               (line 52)
* builtins, indirect call of:            Builtin.              (line  6)
* call of builtins, indirect:            Builtin.              (line  6)
* call of macros, indirect:              Indir.                (line  6)
* changing comment delimiters:           Changecom.            (line  6)
* changing the quote delimiters:         Changequote.          (line  6)
* characters, translating:               Translit.             (line  6)
* command line, file names on the:       Command line files.   (line  6)
* command line, macro definitions on the: Preprocessor features.
                                                               (line  6)
* command line, options:                 Invoking m4.          (line 10)
* commands, exit status from shell:      Sysval.               (line  6)
* commands, running shell:               Shell commands.       (line  6)
* commands, running UNIX:                Shell commands.       (line  6)
* comment delimiters, changing:          Changecom.            (line  6)
* comments:                              Comments.             (line  6)
* comments, copied to output:            Changecom.            (line 29)
* comparing strings:                     Ifelse.               (line  6)
* compatibility:                         Compatibility.        (line  6)
* conditionals:                          Ifdef.                (line  6)
* controlling debugging output:          Debug Levels.         (line  6)
* counting loops:                        Forloop.              (line  6)
* debugging output, controlling:         Debug Levels.         (line  6)
* debugging output, saving:              Debug Output.         (line  6)
* decrement operator:                    Incr.                 (line  6)
* defining new macros:                   Definitions.          (line  6)
* definitions, displaying macro:         Dumpdef.              (line  6)
* deleting macros:                       Undefine.             (line  6)
* deleting whitespace in input:          Dnl.                  (line  6)
* discarding diverted text:              Cleardivert.          (line  6)
* displaying macro definitions:          Dumpdef.              (line  6)
* diversion numbers:                     Divnum.               (line  6)
* diverted text, discarding:             Cleardivert.          (line  6)
* diverting output to files:             Divert.               (line  6)
* dumping into frozen file:              Using frozen files.   (line  6)
* error messages, printing:              Errprint.             (line  6)
* evaluation, of integer expressions:    Eval.                 (line  6)
* executing shell commands:              Shell commands.       (line  6)
* executing UNIX commands:               Shell commands.       (line  6)
* exit status from shell commands:       Sysval.               (line  6)
* exiting from m4:                       M4exit.               (line  6)
* expansion of macros:                   Macro expansion.      (line  6)
* expansion, tracing macro:              Trace.                (line  6)
* expressions, evaluation of integer:    Eval.                 (line  6)
* extracting substrings:                 Substr.               (line  6)
* fast loading of frozen files:          Using frozen files.   (line  6)
* FDL, GNU Free Documentation License:   GNU Free Documentation License.
                                                               (line  6)
* file format, frozen file:              Frozen file format.   (line  6)
* file inclusion <1>:                    Undivert.             (line 81)
* file inclusion:                        File Inclusion.       (line  6)
* file names, on the command line:       Command line files.   (line  6)
* files, diverting output to:            Divert.               (line  6)
* files, names of temporary:             Mkstemp.              (line  6)
* for each loops:                        Foreach.              (line  6)
* for loops:                             Forloop.              (line  6)
* formatted output:                      Format.               (line  6)
* frozen file format:                    Frozen file format.   (line  6)
* frozen files for fast loading:         Using frozen files.   (line  6)
* GNU extensions <1>:                    Indir.                (line  6)
* GNU extensions <2>:                    Patsubst.             (line  6)
* GNU extensions <3>:                    Extensions.           (line  6)
* GNU extensions <4>:                    Regexp.               (line  6)
* GNU extensions <5>:                    Using frozen files.   (line  6)
* GNU extensions <6>:                    Format.               (line  6)
* GNU extensions <7>:                    Undivert.             (line 81)
* GNU extensions <8>:                    Divert.               (line 54)
* GNU extensions <9>:                    Search Path.          (line  6)
* GNU extensions <10>:                   Define.               (line 41)
* GNU extensions <11>:                   Arguments.            (line 32)
* GNU extensions <12>:                   Esyscmd.              (line  6)
* GNU extensions <13>:                   Debug Output.         (line  6)
* GNU extensions <14>:                   Mkstemp.              (line 52)
* GNU extensions <15>:                   Builtin.              (line  6)
* GNU extensions:                        Debug Levels.         (line 59)
* included files, search path for:       Search Path.          (line  6)
* inclusion, of files <1>:               Undivert.             (line 81)
* inclusion, of files:                   File Inclusion.       (line  6)
* increment operator:                    Incr.                 (line  6)
* indirect call of builtins:             Builtin.              (line  6)
* indirect call of macros:               Indir.                (line  6)
* initialization, frozen states:         Using frozen files.   (line  6)
* input tokens:                          Syntax.               (line  6)
* input, saving:                         M4wrap.               (line  6)
* integer arithmetic:                    Arithmetic.           (line  6)
* integer expression evaluation:         Eval.                 (line  6)
* iterating over lists:                  Foreach.              (line  6)
* length of strings:                     Len.                  (line  6)
* lexical structure of words:            Changeword.           (line  6)
* License:                               Copying This Manual.  (line  6)
* local variables:                       Pushdef.              (line 75)
* loops:                                 Shift.                (line 10)
* loops, counting:                       Forloop.              (line  6)
* loops, list iteration:                 Foreach.              (line  6)
* macro definitions, on the command line: Preprocessor features.
                                                               (line  6)
* macro expansion, tracing:              Trace.                (line  6)
* macro invocation:                      Invocation.           (line  6)
* macros, arguments to <1>:              Macro Arguments.      (line  6)
* macros, arguments to:                  Arguments.            (line  6)
* macros, displaying definitions:        Dumpdef.              (line  6)
* macros, expansion of:                  Macro expansion.      (line  6)
* macros, how to define new:             Definitions.          (line  6)
* macros, how to delete:                 Undefine.             (line  6)
* macros, how to rename:                 Defn.                 (line  6)
* macros, indirect call of:              Indir.                (line  6)
* macros, quoted arguments to:           Quoting Arguments.    (line  6)
* macros, recursive:                     Shift.                (line  6)
* macros, special arguments to:          Pseudo Arguments.     (line  6)
* macros, temporary redefinition of:     Pushdef.              (line  6)
* messages, printing error:              Errprint.             (line  6)
* multibranches:                         Ifelse.               (line 66)
* names:                                 Names.                (line  6)
* options, command line:                 Invoking m4.          (line 10)
* output, diverting to files:            Divert.               (line  6)
* output, formatted:                     Format.               (line  6)
* output, saving debugging:              Debug Output.         (line  6)
* pattern substitution:                  Patsubst.             (line  6)
* platform macro:                        Platform macros.      (line 31)
* platform macros:                       Platform macros.      (line  6)
* POSIXLY_CORRECT:                       Invoking m4.          (line 10)
* printing error messages:               Errprint.             (line  6)
* quote delimiters, changing the:        Changequote.          (line  6)
* quoted macro arguments:                Quoting Arguments.    (line  6)
* quoted string:                         Quoted strings.       (line  6)
* recursive macros:                      Shift.                (line  6)
* redefinition of macros, temporary:     Pushdef.              (line  6)
* regular expressions <1>:               Patsubst.             (line  6)
* regular expressions:                   Regexp.               (line  6)
* reloading a frozen file:               Using frozen files.   (line  6)
* renaming macros:                       Defn.                 (line  6)
* running shell commands:                Shell commands.       (line  6)
* running UNIX commands:                 Shell commands.       (line  6)
* saving debugging output:               Debug Output.         (line  6)
* saving input:                          M4wrap.               (line  6)
* search path for included files:        Search Path.          (line  6)
* shell commands, exit status from:      Sysval.               (line  6)
* shell commands, running:               Shell commands.       (line  6)
* special arguments to macros:           Pseudo Arguments.     (line  6)
* status of shell commands:              Sysval.               (line  6)
* status, setting m4 exit:               M4exit.               (line  6)
* strings, length of:                    Len.                  (line  6)
* substitution by regular expression:    Patsubst.             (line  6)
* substrings, extracting:                Substr.               (line  6)
* temporary file names:                  Mkstemp.              (line  6)
* temporary redefinition of macros:      Pushdef.              (line  6)
* tokens:                                Syntax.               (line  6)
* tracing macro expansion:               Trace.                (line  6)
* translating characters:                Translit.             (line  6)
* undefining macros:                     Undefine.             (line  6)
* UNIX commands, exit status from:       Sysval.               (line  6)
* UNIX commands, running:                Shell commands.       (line  6)
* variables, local:                      Pushdef.              (line 75)
* words, lexical structure of:           Changeword.           (line  6)


File: m4.info,  Node: Macro index,  Prev: Concept index,  Up: Indices

B.2 Index for all `m4' macros
=============================

References are exclusively to the places where a builtin is introduced
the first time.

[index]
* Menu:

* __file__:                              Location.             (line 10)
* __gnu__:                               Platform macros.      (line 12)
* __line__:                              Location.             (line 11)
* __os2__:                               Platform macros.      (line 13)
* __program__:                           Location.             (line 12)
* __unix__:                              Platform macros.      (line 15)
* __windows__:                           Platform macros.      (line 17)
* builtin:                               Builtin.              (line  9)
* capitalize:                            Patsubst.             (line 55)
* changecom:                             Changecom.            (line 10)
* changequote:                           Changequote.          (line 10)
* changeword:                            Changeword.           (line 23)
* cleardivert:                           Cleardivert.          (line 26)
* debugfile:                             Debug Output.         (line 11)
* debugmode:                             Debug Levels.         (line 63)
* decr:                                  Incr.                 (line 11)
* define:                                Define.               (line 10)
* defn:                                  Defn.                 (line 10)
* divert:                                Divert.               (line  9)
* divnum:                                Divnum.               (line  9)
* dnl:                                   Dnl.                  (line  9)
* downcase:                              Patsubst.             (line 54)
* dquote:                                Shift.                (line 53)
* dquote_elt:                            Shift.                (line 54)
* dumpdef:                               Dumpdef.              (line 10)
* errprint:                              Errprint.             (line  9)
* esyscmd:                               Esyscmd.              (line  9)
* eval:                                  Eval.                 (line  9)
* example:                               Manual.               (line 32)
* fatal_error:                           M4exit.               (line 25)
* foreach:                               Foreach.              (line  9)
* foreachq:                              Foreach.              (line 10)
* forloop:                               Forloop.              (line  9)
* format:                                Format.               (line  9)
* ifdef:                                 Ifdef.                (line 10)
* ifelse:                                Ifelse.               (line 12)
* include:                               Include.              (line  9)
* incr:                                  Incr.                 (line 10)
* index:                                 Index macro.          (line  9)
* indir:                                 Indir.                (line  9)
* len:                                   Len.                  (line  9)
* m4exit:                                M4exit.               (line 10)
* m4wrap:                                M4wrap.               (line 14)
* maketemp:                              Mkstemp.              (line 12)
* mkstemp:                               Mkstemp.              (line 11)
* os2:                                   Platform macros.      (line 14)
* patsubst:                              Patsubst.             (line  9)
* popdef:                                Pushdef.              (line 12)
* pushdef:                               Pushdef.              (line 11)
* quote:                                 Shift.                (line 52)
* regexp:                                Regexp.               (line  9)
* reverse:                               Shift.                (line 32)
* shift:                                 Shift.                (line 17)
* sinclude:                              Include.              (line 10)
* substr:                                Substr.               (line  9)
* syscmd:                                Syscmd.               (line  9)
* sysval:                                Sysval.               (line  9)
* traceoff:                              Trace.                (line 11)
* traceon:                               Trace.                (line 10)
* translit:                              Translit.             (line  9)
* undefine:                              Undefine.             (line  9)
* undivert:                              Undivert.             (line  9)
* unix:                                  Platform macros.      (line 16)
* upcase:                                Patsubst.             (line 53)
* windows:                               Platform macros.      (line 18)



Tag Table:
Node: Top840
Node: Preliminaries9196
Node: Intro9882
Node: History11514
Node: Bugs14146
Node: Manual15408
Node: Invoking m418340
Node: Operation modes20488
Node: Preprocessor features22199
Node: Limits control24701
Node: Frozen state27637
Node: Debugging options28436
Node: Command line files30175
Node: Syntax31538
Node: Names32539
Node: Quoted strings33001
Node: Comments33650
Node: Other tokens34517
Node: Input processing35095
Node: Macros36982
Node: Invocation37476
Node: Inhibiting Invocation38277
Node: Macro Arguments41848
Node: Quoting Arguments44539
Node: Macro expansion45861
Node: Definitions46529
Node: Define47314
Node: Arguments49520
Node: Pseudo Arguments51268
Node: Undefine54483
Node: Defn55613
Node: Pushdef58346
Node: Indir60680
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Node: Debug Output88255
Node: Input Control89526
Node: Dnl90063
Node: Changequote91964
Node: Changecom96760
Node: Changeword100276
Node: M4wrap105717
Node: File Inclusion107978
Node: Include108295
Node: Search Path110895
Node: Diversions111812
Node: Divert113603
Node: Undivert116129
Node: Divnum119011
Node: Cleardivert119475
Node: Text handling120680
Node: Len121403
Node: Index macro121788
Node: Regexp122559
Node: Substr124808
Node: Translit125859
Node: Patsubst128581
Node: Format132121
Node: Arithmetic134188
Node: Incr134637
Node: Eval135412
Node: Shell commands139821
Node: Platform macros140743
Node: Syscmd142487
Node: Esyscmd144292
Node: Sysval145340
Node: Mkstemp146895
Node: Miscellaneous150197
Node: Errprint150630
Node: Location151849
Node: M4exit154651
Node: Frozen files156738
Node: Using frozen files157519
Node: Frozen file format160759
Node: Compatibility163822
Node: Extensions164437
Node: Incompatibilities168204
Node: Other Incompatibilities176116
Node: Answers178044
Node: Improved exch178661
Node: Improved forloop179199
Node: Improved foreach181035
Node: Improved cleardivert187974
Node: Improved fatal_error188957
Node: Copying This Manual190019
Node: GNU Free Documentation License190275
Node: Indices212669
Node: Concept index212949
Node: Macro index224692

End Tag Table