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 by Francois
Pinard <pinard@iro.umontreal.ca> and a public domain version
by Ozan Yigit <oz@sis.yorku.ca> and Richard A. O'Keefe
<ok@goanna.cs.rmit.OZ.AU> (FTP from any 386BSD, NetBSD or
FreeBSD archive). A Macintosh version is here.