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://gnu.org/pub/gnu/m4-1.0.tar.Z)} by Francois Pinard and a public domain version by Ozan YigIT and Richard A. O' Keefe IT.OZ.AU> (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].