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 <firstname.lastname@example.org> and a public domain version by Ozan Yigit <email@example.com> and Richard A. O'Keefe <firstname.lastname@example.org.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].
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Nearby terms: m2 « M2toM3 « M3 « m4 » M5 » ma » MAC