M4 (computer language)
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- The title of this article is incorrect because of technical limitations. The correct title is "m4 macro language".
m4 is a macro processing language designed by Brian Kernighan and Dennis Ritchie in 1977. It was the original macro engine used to implement Rational Fortran, and is shipped with most Unix variants. One of the most widespread present-day uses is as part of the GNU autoconf.
A macro processor is a text-replacement tool. Its chief use is to re-use text templates, typically in programming applications, but also in text editing, text processing applications.
Macro processors were prevalent when assembly language programming was the common tool of programmers. In these early days of programming, the programmers noted that much of their program was repeated text. Simple means to re-use this text were invented. Programmers soon discovered it easy to not only reuse entire blocks of text, but, on occasion, substitute different values for similar parameters. This defined the usage range of macro processors.
m4 offers these facilities:
- text replacement
- parameter substitution
- file inclusion
- string manipulation
- conditional evaluation
- arithmetic expressions
- system interface
- programmer diagnostics
Unlike most earlier macro processors, it is not targeted at any particular computer or human language; historically, however, it was developed for supporting the Ratfor dialect of Fortran. Unlike some other macro processors, m4 is Turing-complete as well as a practical programming language.
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Example
The following fragment is a simple example that could be part of an HTML generation library. It defines a macro to automatically number sections:
define(`H2_COUNT', 0) define(`H2', `define(`H2_COUNT', incr(H2_COUNT))' dnl `<h2>H2_COUNT. $1</h2>') H2(First Section) H2(Second Section) H2(Conclusion)
After being processed with m4, the following text will be generated:
<h2>1. First Section</h2> <h2>2. Second Section</h2> <h2>3. Conclusion</h2>
Free software implementations
There is a GNU version of m4. FreeBSD, NetBSD, and OpenBSD also provide independent implementionations of the m4 language.
External links
References
Brian W. Kernighan and Dennis M. Ritchie. The M4 macro processor. Technical report, Bell Laboratories, Murray Hill, New Jersey, USA, 1977. [1] (http://wolfram.schneider.org/bsd/7thEdManVol2/m4/m4.pdf)
Kenneth J. Turner. Exploiting the m4 macro language. Technical Report CSM-126, Department of Computing Science and Mathematics, University of Stirling, Scotland, September 1994. [2] (http://www.cs.stir.ac.uk/~kjt/research/pdf/expl-m4.pdf)
René Seindal. GNU M4 Manual. GNU Press. 2004. [3] (http://www.gnu.org/software/m4/manual/)pl:M4