Editing M4 (computer language)

{{lowercase|m4}}
{{Short description|General-purpose macro processor}}
{{Infobox programming language
| name                   = m4
| logo                   = 
| paradigm               = [[macro (computer science)|macro]]
| year                   = {{Start date and age|1977}}
| designer               = [[Brian Kernighan]], [[Dennis Ritchie]]
| developer              = 
| latest_release_version = 
| latest_release_date    = 
| implementations        = [[GNU m4]]
| typing                 = 
| influenced_by          = 
| operating_system       = 
| license                = 
| website                = 
}}
'''m4''' is a [[general-purpose macro processor]] included in most [[Unix-like]] operating systems, and is a component of the [[POSIX]] standard.

The language was designed by [[Brian Kernighan]] and [[Dennis Ritchie]] for the original versions of [[UNIX]]. It is an extension of an earlier macro processor, m3, written by Ritchie for an unknown AP-3 minicomputer.<ref name='KR1977'>Brian W. Kernighan and Dennis M. Ritchie. The m4 macro processor. Technical report, Bell Laboratories, Murray Hill, New Jersey, USA, 1977. [http://wolfram.schneider.org/bsd/7thEdManVol2/m4/m4.pdf pdf]</ref>

The macro [[preprocessor]] operates as a text-replacement tool. It is employed to re-use text templates, typically in [[computer programming]] applications, but also in text editing and text-processing applications. Most users require m4 as a dependency of GNU [[autoconf]] and [[GNU Bison]].

==History==

Macro processors became popular when [[programmer]]s commonly used [[assembly language]]. In those early days of programming, programmers noted that much of their programs consisted of repeated text, and they invented simple means for reusing this text. Programmers soon discovered the advantages not only of reusing entire blocks of text, but also of substituting different values for similar parameters. This defined the usage range of macro processors at the time.<ref>{{citation |url=https://www.gnu.org/software/m4/manual/html_node/History.html |title=History of GNU m4}}</ref>

In the 1960s, an early general-purpose macro processor, M6, was in use at [[Bell Labs|AT&T Bell Laboratories]], which was developed by [[Douglas McIlroy]], [[Robert Morris (cryptographer)|Robert Morris]] and Andrew Hall.<ref>{{cite report |last=Hall |first=Andrew D. |title=The M6 Macro Processor. Computing Science Technical Report #2 |year=1972 |publisher=Bell Labs |url=https://plan9.io/cm/cs/cstr/2.pdf}}</ref>

Kernighan and Ritchie developed m4 in 1977, basing it on the ideas of [[Christopher Strachey]]. The distinguishing features of this style of macro preprocessing included:
* free-form syntax (not line-based like a typical macro preprocessor designed for assembly-language processing)
* the high degree of re-expansion (a macro's arguments get expanded twice: once during scanning and once at interpretation time)

The implementation of [[Ratfor|Rational Fortran]] used m4 as its macro engine from the beginning, and most [[Unix]] variants ship with it.

{{As of |2024}} many applications continue to use m4 as part of the [[GNU]] Project's [[autoconf]]. It also appears in the configuration process of [[sendmail]] (a widespread{{fact|date=December 2024}} [[mail transfer agent]]) and for generating footprints in the [[gEDA]] toolsuite. The [[SELinux]] Reference Policy relies heavily on the m4 macro processor.

m4 has many uses in [[Automatic programming|code generation]], but (as with any macro processor) problems can be hard to debug.<ref name='Turner1994'>Kenneth J. Turner. Exploiting the m4 macro language. Technical Report CSM-126, Department of Computing Science and Mathematics, University of Stirling, Scotland, September 1994. [http://www.cs.stir.ac.uk/~kjt/research/pdf/expl-m4.pdf pdf]</ref>

==Features==
m4 offers these facilities:

* a [[Free-form language|free-form syntax]], rather than [[line-based syntax]]
* a high degree of macro expansion (arguments get expanded during scan and again during interpretation)
* text replacement
* parameter substitution
* file inclusion
* string manipulation
* conditional evaluation
* arithmetic expressions
* system interface
* programmer diagnostics
* programming language independent
* human language independent
* provides programming language capabilities

Unlike most earlier macro processors, m4 does not target any particular computer or human language; historically, however, its development originated for supporting the [[Ratfor]] dialect of [[Fortran]]. Unlike some other macro processors, m4 is [[Turing-complete]] as well as a practical programming language.

Unquoted identifiers which match defined macros are replaced with their definitions. Placing identifiers in quotes suppresses expansion until possibly later, such as when a quoted string is expanded as part of macro replacement. Unlike most languages, strings in m4 are quoted using the [[backtick]] (`) as the starting [[delimiter]], and [[apostrophe]] (') as the ending delimiter. Separate starting and ending delimiters allows the arbitrary nesting of quotation marks in strings to be used, allowing a fine degree of control of how and when macro expansion takes place in different parts of a string.

==Example==
The following fragment gives a simple example that could form part of a library for generating [[HTML]] code. It defines a commented macro to number sections automatically:

<syntaxhighlight lang="html">
divert(-1)

m4 has multiple output queues that can be manipulated with the
`divert' macro. Valid queues range from 0 to 10, inclusive, with
the default queue being 0. As an extension, GNU m4 supports more
diversions, limited only by integer type size.

Calling the `divert' macro with an invalid queue causes text to be
discarded until another call.  Note that even while output is being
discarded, quotes around `divert' and other macros are needed to
prevent expansion.

# Macros aren't expanded within comments, meaning that keywords such
# as divert and other built-ins may be used without consequence.

# HTML utility macro:

define(`H2_COUNT', 0)

# The H2_COUNT macro is redefined every time the H2 macro is used:

define(`H2',
	`define(`H2_COUNT', incr(H2_COUNT))<h2>H2_COUNT. $1</h2>')

divert(1)dnl
dnl
dnl The dnl macro causes m4 to discard the rest of the line, thus
dnl preventing unwanted blank lines from appearing in the output.
dnl
H2(First Section)
H2(Second Section)
H2(Conclusion)
dnl
divert(0)dnl
dnl
<HTML>
undivert(1)dnl One of the queues is being pushed to output.
</HTML>
</syntaxhighlight>

Processing this code with m4 generates the following text:

<syntaxhighlight lang="html">
<HTML>
<h2>1. First Section</h2>
<h2>2. Second Section</h2>
<h2>3. Conclusion</h2>
</HTML>
</syntaxhighlight>

==Implementations==
[[FreeBSD]], [[NetBSD]], and [[OpenBSD]] provide independent implementations of the m4 language. Furthermore, the [[Heirloom Project]] Development Tools includes a free version of the m4 language, derived from [[OpenSolaris]].

M4 has been included in the [[Inferno (operating system)|Inferno]] operating system. This implementation is more closely related to the original m4 developed by Kernighan and Ritchie in [[Version 7 Unix]] than its more sophisticated relatives in [[UNIX System V]] and [[POSIX]].<ref>{{man|1|m4|Inferno}}</ref>

''GNU m4'' is an implementation of m4 for the [[GNU Project]].<ref name='GNUm4web'>GNU m4 web site [https://www.gnu.org/software/m4/ "GNU M4"], accessed January 25, 2020.</ref><ref name='GNUm4manual'>GNU m4 manual, online and for download in HTML, PDF, and other forms. [https://www.gnu.org/software/m4/manual/index.html "GNU M4 &mdash; GNU macro processor"], accessed January 25, 2020.</ref>  It is designed to avoid many kinds of arbitrary limits found in traditional m4 implementations, such as maximum line lengths, maximum size of a macro and number of macros. Removing such arbitrary limits is one of the stated goals of the GNU Project.<ref>[https://www.gnu.org/prep/standards/html_node/Semantics.html "GNU Coding Standards: Writing Robust Programs"].
quote: "Avoid arbitrary limits on the length or number of any data structure".
</ref>

The GNU [[Autoconf]] package makes extensive use of the features of GNU m4.

GNU m4 is currently maintained by Gary V. Vaughan and Eric Blake.<ref name='GNUm4web'/> GNU m4 is [[free software]], released under the terms of the [[GNU General Public License]].

==See also==
* [[C preprocessor]]
* [[Macro (computer science)]]
* [[make (software)|Make]]
* [[Template processor]]
* [[Web template system]]
* {{Annotated link|AWK}}

==References==
{{Reflist}}

==External links==
{{Wikibooks|Guide to Unix|Commands}}
* [https://www.gnu.org/software/m4/m4.html GNU m4 website]
* [https://www.gnu.org/software/m4/manual/m4.html GNU m4 manual]
* [http://mbreen.com/m4.html m4 tutorial]
* [http://www.jpeek.com/articles/linuxmag/2005-02/ Macro Magic: m4, Part One] and [http://www.jpeek.com/articles/linuxmag/2005-03/ Part Two]

{{Unix commands}}

{{Use mdy dates|date=October 2018}}

{{DEFAULTSORT:M4 (Computer Language)}}
[[Category:Macro programming languages]]
[[Category:Unix programming tools]]
[[Category:Unix SUS2008 utilities]]
[[Category:Inferno (operating system) commands]]