Intel XENIX 286 Programmers Guide (86) - Tenox.tc

CHAPTER 11
m4: MACRO PROCESSOR
The m4 macro processor defines and processes specially defined strings of characters
called macros. By defining a set of macros to be processed by m4, a programming
language can be enhanced to make it more structured, more readable, and more
appropriate for a particular application.
The #define statement in C and the analogous define in RATFOR are examples of the
basic facility provided by any macro processor--replacement of text by other text.
Besides the straightforward replacement of one string of text by another, m4 provides
• Macros with arguments
• Conditional macro expansions
• Arithmetic expressions
• File manipulation facilities
• String processing functions
The basic operation of m4 is copying its input to its output. As the input is read, each
alphanumeric token (that is, string of letters and digits) is checked. If the token is the
name of a macro, then the name of the macro is replaced by its defining text. The
resulting string is reread by m4. Macros may also be called with arguments, in which
case the arguments are collected and substituted in the right places in the defining text
before m4 rescans the text.
m4 provides a collection of about 20 built-in macros. In addition, the user can define
new macros. Built-in and user-defined macros work in exactly the same way, except
that some of the built-in macros have side effects on the state of the process.
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