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This first chapter explains what is GNU m4, 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
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.
The 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 m4 for
generating the `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 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 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.
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 m4. See
C. Stratchey: "A General Purpose Macro generator", Computer Journal
8,3 (1965), pp. 225 ff. GPM is also succintly described into
David Gries classic "Compiler Construction for Digital Computers".
While GPM was pure, m4 was meant to deal more
with the true intricacies of real life: macros could be recognized
with being pre-announced, skipping whitespace or end-of-lines was
made easier, more constructs were builtin instead of derived, etc.
Originally, m4 was the engine for Rational FORTRAN preprocessor,
that is, the ratfor equivalent of cpp.
Stop execution and exit m4 once the first warning has been
issued, considering all of them to be fatal.
Set the debug-level according to the flags flags. The debug-level
controls the format and amount of information presented by the debugging
functions. See section Controlling debugging output for more details on the format and
meaning of flags.
Redirect debug and trace output to the named file. Error messages are
still printed on the standard error output. See section Saving debugging output for
Make m4 search dir for included files that are not found in
the current working directory. See section Searching for include files for more details.
Makes this invocation of m4 interactive. This means that all
output will be unbuffered, and interrupts will be ignored.
Generate synchronisation lines, for use by the C preprocessor or other
similar tools. This is useful, for example, when m4 is used as a
front end to a compiler. Source file name and line number information
is conveyed by directives of the form `#line linenum
"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.
Internally modify all builtin macro names so they all start with
the prefix `m4_'. For example, using this option, one should write
`m4_define' instead of `define', and `m4___file__'
instead of `__file__'.
Make the internal hash table for symbol lookup be n entries big.
The number should be prime. The default is 509 entries. It should not
be necessary to increase this value, unless you define an excessive
number of macros.
Artificially limit the nesting of macro calls to n levels,
stopping program execution if this limit is ever exceeded. When not
specified, nesting is limited to 250 levels.
The precise effect of this option might be more correctly associated
with textual nesting than dynamic recursion. It has been useful
when some complex m4 input was generated by mechanical means.
Most users would never need this option. If shown to be obtrusive,
this option (which is still experimental) might well disappear.
This option does not have the ability to break endless
rescanning loops, 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 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 section Compatibility with other versions of m4). 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!
Suppress warnings about missing or superflous arguments in macro calls.
These options are present for compatibility with System V m4, but
do nothing in this implementation.
These options are present only for compatibility with previous
versions of GNU m4, and were controlling the number of possible
diversions which could be used at the same time. They do nothing,
because there is no fixed limit anymore.
Macro definitions and deletions can be made on the command line, by
using the `-D' and `-U' options. They have the following
This enters name into the symbol table, before any input files are
read. If `=value' is missing, the value is taken to be the
empty string. The value can be any string, and the macro can be
defined to take arguments, just as if it was defined from within the
This deletes any predefined meaning name might have. Obviously,
only predefined macros can be deleted in this way.
This enters name into the symbol table, as undefined but traced.
The macro will consequently be traced from the point it is defined.
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)
`email@example.com' or (UUCP)
`mit-eddie!prep.ai.mit.edu!bug-gnu-utils'. Please include the
version number of m4 you are using. You can get this information
with the command `m4 --version'.
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
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, filenames, 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 builtin
primitive is spelled within m4.