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Category:M4: Difference between revisions
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correction, changequote() for the quote chars, not changecom()
(m4 ; should a GNU m4 page be created for the implementation?) |
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{{language
|exec=interpreted
|safety=safe
|untyped=yes
|tag=m4
}}
'''m4''' reads text input, expands macros in the text, and prints the result. It can be used as a front-end to a compiler or assembler, or for general purpose expanding etc.
Various builtin macros can do integer arithmetic, run shell commands, access files, divert output to temporary buffers for re-ordering, etc. <code>define</code> creates new macros.
<lang m4>define(`foo', `blah blah: $1')
foo(123)
=>
blah blah: 123</lang>
Control flow is limited to <code>ifelse</code>, but it's easy to construct loops by recursion. GNU m4 includes examples of macros implementing various general-purpose loops.
Quoting data values against premature or unwanted expansion can be a little tricky. The default quote characters are <code>`</code> and <code>'</code>. If they would occur in text too often then <code>changequote()</code> can set something else. Autoconf changes to <code>[</code> and <code>]</code> since <code>`</code> and <code>'</code> occur often in its [[Bourne Shell]] output.
When a macro expands, its value (with <code>$1</code> etc parameters substituted) is re-read as input. This is how macro definitions can contain further macros to expand.
<lang m4>define(`foo', `bar(`$1',x,`$2')')</lang>
Various m4 implementations, including BSD, have a fixed limit on the amount "push-back" text to be re-read. GNU m4 has no limit except available memory. A limit restricts the size of macro values and the data they might operate on. Cutting data into pieces can keep expansions to a reasonable size.
The simple text re-reading means that macro calls are "properly tail recursive". If an expansion ends with another macro call then that call can re-expand recursively or by co-routining endlessly. But a tail call must be the very last thing, no newline or other fixed text after. See [[Factorial#M4|Factorial]] for an example of such recursion.
One implementation of this Unix macro processor m4 is the GNU m4
* [[wp:m4 (computer language)|m4 on wikipedia]]
* [http://www.gnu.org/software/m4/ GNU m4]
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