File: autoconf.info,  Node: One-Shot Macros,  Prev: Suggested Ordering,  Up: Dependencies Between Macros

10.3.3 One-Shot Macros
----------------------

Some macros should be called only once, either because calling them
multiple time is unsafe, or because it is bad style.  For instance
Autoconf ensures that ‘AC_CANONICAL_BUILD’ and cousins (*note
Canonicalizing::) are evaluated only once, because it makes no sense to
run these expensive checks more than once.  Such one-shot macros can be
defined using ‘AC_DEFUN_ONCE’.

 -- Macro: AC_DEFUN_ONCE (MACRO-NAME, MACRO-BODY)
     Declare macro MACRO-NAME like ‘AC_DEFUN’ would (*note Macro
     Definitions::), but add additional logic that guarantees that only
     the first use of the macro (whether by direct expansion or
     ‘AC_REQUIRE’) causes an expansion of MACRO-BODY; the expansion will
     occur before the start of any enclosing macro defined by
     ‘AC_DEFUN’.  Subsequent expansions are silently ignored.
     Generally, it does not make sense for MACRO-BODY to use parameters
     such as ‘$1’.

   Prior to Autoconf 2.64, a macro defined by ‘AC_DEFUN_ONCE’ would emit
a warning if it was directly expanded a second time, so for portability,
it is better to use ‘AC_REQUIRE’ than direct invocation of MACRO-NAME
inside a macro defined by ‘AC_DEFUN’ (*note Prerequisite Macros::).