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.

See also m3, m5.

["The M4 Macro Processor", Kernighan & Ritchie, Jul 1977].