m4
This manual is for GNU M4 (version 1.4.7, 23 September 2006), a package containing an implementation of the m4 macro language.
Copyright © 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 René Seindal, with
subsequent changes by François Pinard and other volunteers
on the Internet. All names and email addresses can be found in the
files AUTHORS and THANKS from the GNU M4 distribution.
This is release 1.4.7. 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.
--- The Detailed Node Listing ---
Introduction and preliminaries
Lexical and syntactic conventions
How to invoke macros
How to define new macros
Conditionals, loops, and recursion
How to debug macros and input
Input control
File inclusion
Diverting and undiverting output
Macros for text handling
Macros for doing arithmetic
Macros for running shell commands
Miscellaneous builtin macros
Fast loading of frozen state
Compatibility with other versions of m4
How to make copies of this manual
Indices of concepts and macros
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.
m4m4 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.
See 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.
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.
René 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).
François 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, and 1.4.7.
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.
m4The 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. Unless POSIXLY_CORRECT is set in the environment, options may be intermixed with files. 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, without POSIXLY_CORRECT, 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 POSIXLY_CORRECT, it is equivalent to m4 -Q -P -D foo -d -- a ./-d+f).
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.
Several options control the overall operation of m4:
--helpm4 without reading any input files.
--versionm4 without reading any input files.
-E--fatal-warningsm4 once the first warning has been
issued, considering all of them to be fatal.
-i--interactive-em4 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-Q--quiet--silent-W REGEXP--word-regexp=REGEXPm4
implementations (see Changeword).
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]-I DIRECTORY--include=DIRECTORYm4 search DIRECTORY for included files that are not
found in the current working directory. See Search Path, for more
details. This option may be given more than once.
-s--synclinesm4 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=NAMEThere 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 constraints) in GNU m4.
-G--traditional-H NUM--hashsize=NUM-L NUM--nesting-limit=NUMThe 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 see 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 NUMm4, but
do nothing in this implementation. They may disappear in future
releases, and issue a warning to that effect.
-N NUM--diversions=NUMm4, 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.
GNU m4 comes with a feature of freezing internal state
(see Frozen files). This can be used to speed up m4
execution when reusing a common initialization script.
-F FILE--freeze-state=FILE-R FILE--reload-state=FILEFinally, there are several options for aiding in debugging m4
scripts.
-d[FLAGS]--debug[=FLAGS]--debugfile=FILE-o FILE--error-output=FILEdumpdef 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. See 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-t NAME--trace=NAMEThe remaining arguments on the command line are taken to be input file names. If no names are present, the standard input is read. A file name of - is taken to mean the 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.
If none of the input files invoked m4exit (see 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 (see 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.
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.
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.,
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 inm4, 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.
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 (see Changequote),
comments (see Changecom), and macro names (see Indir), with the
exception of the nul character (the zero byte `'\0'').
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
(see Macros). Names are case-sensitive.
Examples of legal names are: `foo', `_tmp', and `name01'.
m4A 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. See Changequote, for more information.
m4 inputComments 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. See Changecom, for more
information.
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.
m4 copies input to outputAs 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.
This process continues until there are no more macro calls to expand and all the input has been consumed.
This chapter covers macro invocation, macro arguments and how macro expansion is treated.
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.
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, see Invoking m4) that renames all builtin macro 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, See 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. See 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.
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.
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, see Invoking m4). 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 leading unquoted 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.
define(
^D
error-->m4:stdin:1: ERROR: end of file in argument list
Each argument has leading unquoted 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, See 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 (see Changeword).
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'.
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.
The normal way to define or redefine macros is to use the builtin
define:
Defines name to expand to expansion. If expansion is not given, it is taken to be empty.
The expansion of
defineis void. The macrodefineis 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.
See 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
(see Pushdef). Some other implementations of m4 replace all
definitions of a macro with define. See 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
and 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.
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, $1')
=>
define(exch(``expansion text'', ``macro''))
=>
macro
=>expansion text
See Quoting Arguments, for an explanation of the double quotes.
(You should try and improve this example so that clients of exch
do not have to double quote. see Answers)
GNU m4 allows the number following the `$' 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.
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
See 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
A macro definition can be removed with undefine:
For each argument, remove the macro name. The macro names must necessarily be quoted, since they will be expanded otherwise.
The expansion of
undefineis void. The macroundefineis 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.
It is possible to rename an already defined macro. To do this, you need
the builtin defn:
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(andpushdef), and is silently converted to an empty string in most other contexts.The macro
defnis 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
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:
Analogous to
defineandundefine.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,pushdefbehaves exactly likedefine.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,popdefbehaves likeundefine.The expansion of both
pushdefandpopdefis void. The macrospushdefandpopdefare 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.
Any macro can be called indirectly with indir:
Results in a call to the macro name, which is passed the rest of the arguments. If name is not defined, an error message is printed, and the expansion is void.
The macro
indiris 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'
=>
Builtin macros can be called indirectly with builtin:
Results in a call to the builtin name, which is passed the rest of the arguments. If name does not name a builtin, an error message is printed, and the expansion is void.
The macro
builtinis 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', `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,
see Invoking m4) 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 `'
=>
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.
There are two different builtin conditionals in m4. The first is
ifdef:
If name is defined as a macro,
ifdefexpands 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
ifdefis 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
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:
Used with only one argument, the
ifelsesimply discards it and produces no output.If called with three or four arguments,
ifelseexpands 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,
ifelseexpands into equal-1, otherwise the first three arguments are discarded and the processing starts again.The macro
ifelseis 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.
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:
Takes any number of arguments, and expands to all its arguments except arg1, separated by commas, with each argument quoted.
The macro
shiftis recognized only with parameters.
shift
=>shift
shift(`bar')
=>
shift(`foo', `bar', `baz')
=>bar,baz
An example of the use of shift is this macro:
It is implemented as:
define(`reverse', `ifelse(`$#', `0', , `$#', `1', ``$1'',
`reverse(shift($@)), `$1'')')
=>
reverse
=>
reverse(`foo')
=>foo
reverse(`foo', `bar', `gnats', `and gnus')
=>and gnus, gnats, bar, foo
While not a very interesting macro, it does show how simple loops can be
made with shift, ifelse and recursion. It also shows
that shift is usually used with `$@'.
Here is an example of a loop macro that implements a simple for loop.
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 it is finished. If it has not finished, it increments
the iteration variable (using the predefined macro incr,
see Incr), and recurses.
Here is the actual implementation of forloop, distributed as
examples/forloop.m4 in this package:
undivert(`forloop.m4')
=>divert(`-1')
=># forloop(var, from, to, stmt)
=>define(`forloop',
=> `pushdef(`$1', `$2')_forloop(`$1', `$2', `$3', `$4')popdef(`$1')')
=>define(`_forloop',
=> `$4`'ifelse($1, `$3', ,
=> `define(`$1', incr($1))_forloop(`$1', `$2', `$3', `$4')')')
=>divert`'dnl
=>
Notice the careful use of quotes. Only three macro arguments are unquoted, each for its own reason. Try to find out why these three arguments are left unquoted, and see what happens if they are quoted.
Now, even though these two macros are useful, they are still not robust enough for general use. They lack even basic error handling of cases like start value less than final value, and the first argument not being a name. Correcting these errors are left as an exercise to the reader.
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.
If you want to see what a name expands into, you can use the builtin
dumpdef:
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
dumpdefis 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: ``$0'1'
=>f2
f(popdef(`f')dumpdef(`f'))
error-->m4:stdin:3: undefined macro `f'
=>f1
See Debug Levels, for information on controlling the details of the display.
It is possible to trace macro calls and expansions through the builtins
traceon and traceoff:
When called without any arguments,
traceonandtraceoffwill turn tracing on and off, respectively, for all defined macros. When called with arguments, only the named macros are affected, whether or not they are currently defined.The expansion of
traceonandtraceoffis void.
Whenever a traced macro is called and the arguments have been collected, the call is displayed. If the expansion of the macro call is not void, the expansion can be displayed after the call. The output is printed to the current debug file (usually standard error).
define(`foo', `Hello World.')
=>
define(`echo', `$@')
=>
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 (see Invoking m4).
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 (see 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
See Debug Levels, for information on controlling the details of the display.
The -d option to m4 (see Invoking m4) 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:
atraceon.
cetraceon.
filpqtm4.
xVIf 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:
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
debugmodeis 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
Debug and tracing output can be redirected to files using either the
--debugfile option to m4 (see Invoking m4), or with the
builtin macro debugfile:
Sends all further debug and trace output to file. If file is empty, debug and trace output are discarded. If
debugfileis called without any arguments, debug and trace output are sent to standard error. This does not affect warnings, error messages, orerrprintoutput, which are always sent to standard error. If file cannot be opened, the current debug file is unchanged.The expansion of
debugfileis 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
This chapter describes various builtin macros for controlling the input
to m4.
The builtin dnl stands for “Discard to Next Line”:
All characters, up to and including the next newline, are discarded without performing any macro expansion.
The expansion of
dnlis 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 (see 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:2: 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.
define(`hi', `HI')
=>
m4wrap(`m4wrap(`2 hi
')0 hi dnl 1 hi')
=>
^D
error-->m4: Warning: end of file treated as newline
=>0 HI 2 HI
The default quote delimiters can be changed with the builtin
changequote:
This sets start as the new begin-quote delimiter and end as the new end-quote delimiter. If any of the arguments are missing, the default quotes (
`and') are used instead of the void arguments.The expansion of
changequoteis 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.
changequote(`[[[', `]]]')
=>
define([[[foo]]], [[[Macro [[[[[foo]]]]].]]])
=>
foo
=>Macro [[foo]].
Changing the quotes to the empty strings will effectively disable the quoting mechanism, leaving no way to quote text.
define(`foo', `Macro `FOO'.')
=>
changequote(, )
=>
foo
=>Macro `FOO'.
`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 a potential macro name, the
quoting mechanism is effectively disabled. Unless you use
changeword (see Changeword), this means that start
should not begin with a letter or `_' (underscore).
define(`hi', `HI')
=>
changequote(`q', `Q')
=>
q hi Q hi
=>q HI Q HI
changequote
=>
changequote(`-', `EOF')
=>
- hi EOF hi
=> 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.
`dangling quote
^D
error-->m4:stdin:1: ERROR: end of file in string
The default comment delimiters can be changed with the builtin
macro changecom:
This sets start as the new begin-comment delimiter and end as the new end-comment delimiter. If only one argument is provided, newline becomes the new end-comment delimiter. The comment delimiters can be of any length. Omitting the first argument, or using the empty string as the first argument, disables comments.
The expansion of
changecomis 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 an empty string
for the first argument, disables the commenting mechanism completely.
To restore the original comment start of `#', you must explicitly
ask for it.
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.
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 or `_' (underscore), and that neither the start-quote nor the start-comment string should be a prefix of the other.
define(`hi', `HI')
=>
changecom(`q', `Q')
=>
q hi Q hi
=>q hi Q HI
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:1: ERROR: end of file in comment
The macrochangewordand all associated functionality is experimental. It is only available if the --enable-changeword option was given toconfigure, at GNUm4installation 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:
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
changewordis void. The macrochangewordis 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.
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:
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
m4wrapis void. The macrom4wrapis 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: ERROR: end of file in argument list
m4 allows you to include named files at any point in the input.
There are two builtin macros in m4 for including files:
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
includeandsincludeis therefore the contents of file.If file does not exist (or cannot be read), the expansion is void, and
includewill fail with an error whilesincludeis silent. The empty string counts as a file that does not exist.The macros
includeandsincludeare 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 (see Invoking m4) pointing to the examples
directory shipped as part of the GNU m4 package. The
file 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.
GNU m4 allows included files to be found in other directories
than the current working directory.
If a file is not found in the current working directory, and the file name is not absolute, the file will be looked for in a specified search path. First, the directories specified with the -I option will be searched, in the order found on the command line (see Invoking m4). Second, 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 (see Debug Levels) can help isolate the problem.
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.
So, it is theoretically possible that the number of diversions be
limited by the number of available file descriptors.
Output is diverted using divert:
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
divertis 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 non-existent 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 how to avoid them.
divert(`-1')
define(`foo', `Macro `foo'.')
define(`bar', `Macro `bar'.')
divert
=>
This is a common programming idiom in m4.
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 (see Invoking m4). 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.
Diverted text can be undiverted explicitly using the builtin
undivert:
Undiverts the 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, if number is not numeric, treat it as a file name instead.
The expansion of
undivertis 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
(see Include). To illustrate the difference, the file
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.
The current diversion is tracked by the builtin divnum:
Initial divnum
=>Initial 0
divert(`1')
Diversion one: divnum
divert(`2')
Diversion two: divnum
^D
=>
=>Diversion one: 1
=>
=>Diversion two: 2
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:
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. see Answers)
There are a number of builtins in m4 for manipulating text in
various ways, extracting substrings, searching, substituting, and so on.
The length of a string can be calculated by len:
Expands to the length of string, as a decimal number.
The macro
lenis recognized only with parameters.
len()
=>0
len(`abcdef')
=>6
Searching for substrings is done with index:
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,
indexexpands to `-1'.The macro
indexis 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
Searching for regular expressions is done with the builtin
regexp:
Searches for regexp in string. The syntax for regular expressions is the same as in GNU Emacs. See Syntax of Regular Expressions in the GNU Emacs Manual.
If replacement is omitted,
regexpexpands 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,
regexpchanges 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,regexpexpands to the empty string.The macro
regexpis 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
Substrings are extracted with substr:
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
substris 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
Character translation is done with translit:
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 are deleted from the expansion. If replacement is omitted, all characters in string that are present in chars are deleted from the expansion.
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.
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 macro
translitis 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
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.
Omitting chars evokes a warning, but still produces output.
translit(`abc')
error-->m4:stdin:1: Warning: too few arguments to builtin `translit'
=>abc
Global substitution in a string is done by patsubst:
Searches string for matches of regexp, and substitutes replacement for each match. The syntax for regular expressions is the same as in GNU Emacs (see 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
patsubstis 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.
Expand to text, but with capitalization changed:
upcasechanges all letters to upper case,downcasechanges all letters to lower case, andcapitalizechanges 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
Formatted output can be made with format:
Works much like the C function
printf. The first argument format-string can contain `%' specifications which are satisfied by additional arguments, and the expansion offormatis the formatted string.The macro
formatis 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 in See Loops, 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.
Integer arithmetic is included in m4, with a C-like syntax. As
convenient shorthands, there are builtins for simple increment and
decrement operations.
Increment and decrement of integers are supported using the builtins
incr and decr:
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
incranddecrare 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
Integer expressions are evaluated with eval:
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
evalis recognized only with parameters.
Expressions can contain the following operators, listed in order of decreasing precedence.
+ -*** / %+ -<< >>== != > >= < <=!~&^|&&||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
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.
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.
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, see Invoking m4), 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
Any shell command can be executed, using syscmd:
Executes shell-command as a shell command.
The expansion of
syscmdis void, not the output from shell-command! Output or error messages from shell-command are not read bym4. See Esyscmd, if you need to process the command output.Prior to executing the command,
m4flushes its buffers. The default standard input, output and error of shell-command are the same as those ofm4.The macro
syscmdis 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, see Invoking m4), 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.
If you want m4 to read the output of a shell command, use
esyscmd:
Expands to the standard output of the shell command shell-command.
Prior to executing the command,
m4flushes its buffers. The default standard input and error output of shell-command are the same as those ofm4. The error output of shell-command is not a part of the expansion: it will appear along with the error output ofm4.The macro
esyscmdis 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.
To see whether a shell command succeeded, use sysval:
Expands to the exit status of the last shell command run with
syscmdoresyscmd. 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
Commands specified to syscmd or esyscmd might need a
temporary file, for output or for some other purpose.
There is a builtin macro, maketemp, for making temporary file
names:
Expands to a name of a new, empty file, made from the string template, which should end with the string `XXXXXX'. The six
Xcharacters are then replaced with random data, in order to make the file name unique.The macro
maketempis recognized only with parameters.
maketemp(`/tmp/fooXXXXXX')
=>/tmp/fooa07346
Traditional implementations of m4 replaced the trailing `X'
sequence with the process id, without creating the file; meaning you
only get one result no matter how many times you use maketemp on the
same string. As of this release, POSIX is considering the
addition of a new macro mkstemp that behaves like GNU
maketemp, so a future version of GNU M4 may have
changes in this area.
This chapter describes various builtins, that do not really belong in any of the previous chapters.
You can print error messages using errprint:
Prints message and the rest of the arguments on the standard error output, separated by spaces.
The expansion of
errprintis void. The macroerrprintis recognized only with parameters.
errprint(`Invalid arguments to forloop
')
error-->Invalid arguments to forloop
=>
A trailing newline is not printed automatically, so it must be
supplied as part of the argument, as in the example. BSD
implementations of m4 do append a trailing newline on each
errprint call, while some other implementations only print the
first argument.
To make it possible to specify the location of an error, three utility builtins exist:
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,
see Invoking m4), and the
`f' and `l' flags of debugmode (see 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
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
=>
Currently, all text wrapped with m4wrap (see M4wrap) behaves
as though it came from line 0 of the file “”. It is hoped that a
future release of m4 can overcome this limitation and remember
which file invoked the call to m4wrap.
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 (see Syscmd) to pick the same version of
m4 that is currently running, rather than whatever version of
m4 happens to be first in PATH.
m4If you need to exit from m4 before the entire input has been
read, you can use m4exit:
Causes
m4to 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.
A common use of this is to abort processing:
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
(see M4wrap) is not reread. (This macro has a subtle bug, when
invoked from wrapped text. You should try to see if you can find it and
correct it. see Answers)
m4wrap(`This text is lost to `m4exit'.')
=>
divert(`1') And so is this.
divert
=>
m4exit
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 out, 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.
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.
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 (see 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.
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>D number, len <NL> str <NL>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>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>T len1 , len2 <NL> str1 str2 <NL>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>m4 1.4.7 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.
m4This 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.
m4This 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 (see Invoking m4), unless
overridden by other command line options.
$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 (see 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
divert (see Divert) macro can manage more than 9
diversions. GNU m4 treats all positive numbers as valid
diversions, rather than discarding diversions greater than 9.
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 (see Search Path).
undivert can be non-numeric, in which case the named
file will be included uninterpreted in the output (see Undivert).
format builtin, which
is modeled after the C library function printf (see Format).
regexp (see Regexp) and patsubst
(see Patsubst) builtins.
m4 with
esyscmd (see Esyscmd).
builtin
(see Builtin).
indir (see Indir).
__program__,
__file__, and __line__ (see Location).
dumpdef and macro tracing can be
controlled with debugmode (see Debug Levels).
debugfile (see Debug Output).
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. See 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.
m4 not in GNU m4The 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.
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.
include (see Include) when a file is unreadable,
eval (see Eval) when an argument cannot be parsed, or using
m4exit (see M4exit) with a non-numeric argument).
m4 correctly handles multiple instances
of `-' on the command line.
m4wrap (see 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 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
a`'define`'b would expand to ab.
But GNU m4 ignores certain builtins if they have missing
arguments, giving adefineb for the above example.
define(`f',`1') (see 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').
syscmd (see 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.
maketemp (see Maketemp) to replace
the trailing `X' characters with the m4 process id, giving
the same result on identical input, without creating any files, which
leaves the door open for a data race in which other processes can create
a file by the same name. GNU m4 actually creates a temporary
file for each invocation of maketemp, which means that the output
of the macro is different even if the input is identical.
changequote(arg)
(see Changequote) to use newline as the close quote, but GNU
m4 uses `'' as the close quote. Meanwhile, some
traditional implementations use arg as the close quote, making it
impossible to nest quotes. For predictable results, never call
changequote with just one argument.
m4 give macros a higher precedence than
comments when parsing, meaning that if the start delimiter given to
changecom (see 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.
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 (see 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.
m4 treats traceon (see 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.
eval (see 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 `**'.
translit (see Translit) to treat
each character of the second and third arguments literally, but GNU
m4 treats `-' as a range operator.
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.
There are a few other incompatibilities between this implementation of
m4, and the System V version.
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.
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.
Some of the examples in this manuals are buggy, for demonstration purposes. Correctly working macros are presented here.
The exch macro (see 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', `$1'')
=>
define(exch(`expansion text', `macro'))
=>
macro
=>expansion text
The cleardivert macro (see Cleardiv) cannot, as it stands, be
called without arguments to clear all pending diversions. That is
because using undivert with an empty string for an argument is different
than using it with no arguments at all. Compare the earlier definition
with one that takes the number of arguments into account:
define(`cleardivert',
`pushdef(`_n', divnum)divert(`-1')undivert($@)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
=>
The fatal_error macro (see M4exit) does not quite match the
format of internal error messages when invoked inside wrapped text, due
to the current limitations of __file__ (see Location) when
invoked inside m4wrap. Since m4 omits the file and line
number from its warning messages when there is no current file (or
equivalently, when the current line is 0, since all files start at line
1), a better implementation would be:
define(`fatal_error',
`errprint(__program__:ifelse(__line__, `0', `',
`__file__:__line__:')` fatal error: $*
')m4exit(`1')')
=>
m4wrap(`divnum(`demo of internal message')
fatal_error(`inside wrapped text')')
=>
^D
error-->m4: Warning: excess arguments to builtin `divnum' ignored
=>0
error-->m4: fatal error: inside wrapped text
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m4: M4exitm4 exit: M4exitm4 macrosReferences are exclusively to the places where a builtin is introduced the first time.
__file__: Location__gnu__: Platform macros__line__: Location__os2__: Platform macros__program__: Location__unix__: Platform macros__windows__: Platform macrosbuiltin: Builtincapitalize: Patsubstchangecom: Changecomchangequote: Changequotechangeword: Changewordcleardivert: Cleardivdebugfile: Debug Outputdebugmode: Debug Levelsdecr: Incrdefine: Definedefn: Defndivert: Divertdivnum: Divnumdnl: Dnldowncase: Patsubstdumpdef: Dumpdeferrprint: Errprintesyscmd: Esyscmdeval: Evalexample: Manualfatal_error: M4exitforloop: Loopsformat: Formatifdef: Ifdefifelse: Ifelseinclude: Includeincr: Incrindex: Index macroindir: Indirlen: Lenm4exit: M4exitm4wrap: M4wrapmaketemp: Maketempos2: Platform macrospatsubst: Patsubstpopdef: Pushdefpushdef: Pushdefregexp: Regexpreverse: Loopsshift: Loopssinclude: Includesubstr: Substrsyscmd: Syscmdsysval: Sysvaltraceoff: Tracetraceon: Tracetranslit: Translitundefine: Undefineundivert: Undivertunix: Platform macrosupcase: Patsubstwindows: Platform macros