A macro processor for Unix and GCOS which is more
flexible than cpp. m4 copies its input to the output,
expanding macros which can be either built-in or user-defined.
m4 has built-in functions for including files, running Unix
commands, doing integer arithmetic, manipulating text in
various ways and recursing. m4 can be used either as a
front-end to a compiler or as a stand-alone tool.
sendmail's configuration file (/etc/sendmail.cf) is writen
in m4 macros.
There is a {GNU m4 v1.1
(ftp://prep.ai.mit.edu/pub/gnu/m4-1.0.tar.Z)} by Francois
Pinard
and a public domain version
by Ozan Yigit and Richard A. O'Keefe
(FTP from any 386BSD, NetBSD or
FreeBSD archive). A Macintosh version is {here
(ftp://nic.switch.ch/pub/software/mac/src/mpw-c/)}.
See also m3, m5.
["The M4 Macro Processor",
Kernighan & Ritchie, Jul 1977].
