(autoconf.info)Macro Definitions

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9.1 Macro Definitions

Autoconf macros are defined using the `AC_DEFUN' macro, which is
similar to the M4 builtin `m4_define' macro.  In addition to defining a
macro, `AC_DEFUN' adds to it some code that is used to constrain the
order in which macros are called (Note: Prerequisite Macros).

   An Autoconf macro definition looks like this:


   You can refer to any arguments passed to the macro as `$1', `$2',
etc.  Note: How to define new macros, for more
complete information on writing M4 macros.

   Be sure to properly quote both the MACRO-BODY _and_ the MACRO-NAME
to avoid any problems if the macro happens to have been previously

   Each macro should have a header comment that gives its prototype,
and a brief description.  When arguments have default values, display
them in the prototype.  For example:

     # --------------------------------------
       [{ AS_MESSAGE([error: $1], [2])
          exit m4_default([$2], [1]); }])

   Comments about the macro should be left in the header comment.  Most
other comments make their way into `configure', so just keep using `#'
to introduce comments.

   If you have some special comments about pure M4 code, comments that
make no sense in `configure' and in the header comment, then use the
builtin `dnl': it causes M4 to discard the text through the next

   Keep in mind that `dnl' is rarely needed to introduce comments;
`dnl' is more useful to get rid of the newlines following macros that
produce no output, such as `AC_REQUIRE'.

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