(autoconf.info)Generic Functions

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4.3.2 Generic Function Checks
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These macros are used to find functions not covered by the particular
test macros.  If the functions might be in libraries other than the
default C library, first call 'AC_CHECK_LIB' for those libraries.  If
you need to check the behavior of a function as well as find out whether
it is present, you have to write your own test for it (Note: Writing
Tests).

 -- Macro: AC_CHECK_FUNC (FUNCTION, [ACTION-IF-FOUND [,
          ACTION-IF-NOT-FOUND]])
     If C function FUNCTION is available, run shell commands
     ACTION-IF-FOUND, otherwise ACTION-IF-NOT-FOUND.  If you just want
     to define a symbol if the function is available, consider using
     'AC_CHECK_FUNCS' instead.  This macro checks for functions with C
     linkage even when 'AC_LANG_CPLUSPLUS' has been called, since C++ is
     more standardized than C is.  (Note: Language Choice, for more
     information about selecting the language for checks.)

 -- Macro: AC_CHECK_FUNCS (FUNCTION... [, ACTION-IF-FOUND [,
          ACTION-IF-NOT-FOUND]])
     For each given FUNCTION in the whitespace-separated argument list
     that is available, define 'HAVE_FUNCTION' (in all capitals).  If
     ACTION-IF-FOUND is given, it is additional shell code to execute
     when one of the functions is found.  You can give it a value of
     'break' to break out of the loop on the first match.  If
     ACTION-IF-NOT-FOUND is given, it is executed when one of the
     functions is not found.

 -- Macro: AC_REPLACE_FUNCS (FUNCTION...)
     Like calling 'AC_CHECK_FUNCS' using an ACTION-IF-NOT-FOUND that
     adds 'FUNCTION.o' to the value of the output variable 'LIBOBJS'.
     You can declare a function for which your replacement version is
     used by enclosing the prototype in '#ifndef HAVE_FUNCTION'.  If the
     system has the function, it probably declares it in a header file
     you should be including, so you shouldn't redeclare it, lest your
     declaration conflict.