@dircategory Text Processing Tools * m4: (m4). A powerful macro processor.
Copyright (C) 1989, 90, 91, 92, 93, 94, 98, 99, 2000 Free Software Foundation, Inc.
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This first chapter explains what is GNU
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 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 UNIX commands, doing integer
arithmetic, manipulating text in various ways, recursion, etc...
m4 can be used either as a front-end to a compiler, or as a
macro processor in its own right.
m4 macro processor is widely available on all UNIXes.
Usually, only a small percentage of users are aware of its existence.
However, those who do often become commited users. The growing
popularity of GNU Autoconf, which prerequires GNU
generating the `configure' scripts, is an incentive
for many to install it, while these people will not themselves
m4 is mostly compatible with the
System V, Release 3 version, except for some minor differences.
See section Compatibility with other versions of
m4, for more details.
Some people found
m4 to be fairly addictive. They first use
m4 for simple problems, then take bigger and bigger challenges,
learning how to write complex
m4 sets of macros along the way.
Once really addicted, users pursue writing of sophisticated
applications even to solve simple problems, devoting more time
m4 scripts than doing real work. Beware that
m4 may be dangerous for the health of compulsive programmers.
The historical notes included here are fairly incomplete, and not authoritative at all. Please knowledgeable users help us to more properly write this section.
GPM has been an important ancestor of
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".
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.
m4 was the engine for Rational FORTRAN preprocessor,
that is, the
ratfor equivalent of
The format of the
m4 command is:
m4[option...] [macro-definitions...] [input-file...]
All options begin with `-', or if long option names are used, with
a `--'. A long option name need not be written completely, and
unambigous prefix is sufficient.
m4 understands the following
m4without reading any input-files.
m4without reading any input-files.
m4, for a list of these.
m4once the first warning has been issued, considering all of them to be fatal.
m4search dir for included files that are not found in the current working directory. See section Searching for include files, for more details.
m4interactive. This means that all output will be unbuffered, and interrupts will be ignored.
m4is used as a front end to a compiler. Source file name and line number information is conveyed by directives of the form `#line linenum "filename"', which are inserted as needed into the middle of the input. Such directives mean that the following line originated or was expanded from the contents of input file filename at line linenum. The `"filename"' part is often omitted when the file name did not change from the previous directive. Synchronisation directives are always given on complete lines per themselves. When a synchronisation discrepancy occurs in the middle of an output line, the associated synchronisation directive is delayed until the beginning of the next generated line. See section Turning sync lines on and off within
m4, for runtime control.
m4implementations. (see section Changing the lexical structure of words and section Experimental features in GNU m4).
m4input 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, while these do not necessarily consume much memory or stack space. Through clever usage of rescanning loops, one can request complex, time-consuming computations to
m4with useful results. Putting limitations in this area would break
m4power. There are many pathological cases: `define(`a', `a')a' is only the simplest example (but see section Compatibility with other versions of
m4). Expecting GNU
m4to 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!
m4, but do nothing in this implementation.
m4, and were controlling the number of possible diversions which could be used at the same time. They do nothing, because there is no fixed limit anymore.
Macro definitions and deletions can be made on the command line, by using the `-D' and `-U' options. They have the following format:
-U, so these can change the macros.
The 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.
The input files are read in the sequence given. The standard input can only be read once, so the file name `-' should only appear once on the command line.
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)
`firstname.lastname@example.org'. Please include the version number of
you are using. You can get this information with the command `m4
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
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
Example of input line =>Output line from m4 error-->and an error message
As each of the predefined macros in
m4 is described, a prototype
call of the macro will be shown, giving descriptive names to the
regexp(string, regexp, opt replacement)
All macro arguments in
m4 are strings, but some are given special
interpretation, e.g., as numbers, file names, regular expressions, etc.
The `opt' before the third argument shows that this argument is optional--if it is left out, it is taken to be the empty string. An ellipsis (`...') last in the argument list indicates that any number of arguments may follow.
This document consistently writes and uses builtin, without an
hyphen, as if it were an English word. This is how the
primitive is spelled within
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