Last checked against version &m4-contversion;. m4 is a macro processor. It copies input to output, expanding macros as it goes. Macros are either built-in or user-defined and can take any number of arguments. Besides just doing macro expansion, m4 has built-in functions for including named files, running Unix commands, doing integer arithmetic, manipulating text in various ways, recursion, etc. The m4 program can be used either as a front-end to a compiler or as a macro processor in its own right.