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5.5.3 Generic Function Checks

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 (see section 6. 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(C++) has been called, since C is more standardized than C++. (see section 6.7 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 function in the whitespace-separated argument list, define HAVE_function (in all capitals) if it is available. 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.

Autoconf follows a philosophy that was formed over the years by those who have struggled for portability: isolate the portability issues in specific files, and then program as if you were in a POSIX environment. Some functions may be missing or unfixable, and your package must be ready to replace them.

Use the first three of the following macros to specify a function to be replaced, and the last one (AC_REPLACE_FUNCS) to check for and replace the function if needed.

Macro: AC_LIBOBJ (function)
Specify that `function.c' must be included in the executables to replace a missing or broken implementation of function.

Technically, it adds `function.$ac_objext' to the output variable LIBOBJS and calls AC_LIBSOURCE for `function.c'. You should not directly change LIBOBJS, since this is not traceable.

Macro: AC_LIBSOURCE (file)
Specify that file might be needed to compile the project. If you need to know what files might be needed by a `configure.ac', you should trace AC_LIBSOURCE. file must be a literal.

This macro is called automatically from AC_LIBOBJ, but you must call it explicitly if you pass a shell variable to AC_LIBOBJ. In that case, since shell variables cannot be traced statically, you must pass to AC_LIBSOURCE any possible files that the shell variable might cause AC_LIBOBJ to need. For example, if you want to pass a variable $foo_or_bar to AC_LIBOBJ that holds either "foo" or "bar", you should do:


There is usually a way to avoid this, however, and you are encouraged to simply call AC_LIBOBJ with literal arguments.

Note that this macro replaces the obsolete AC_LIBOBJ_DECL, with slightly different semantics: the old macro took the function name, e.g. foo, as its argument rather than the file name.

Macro: AC_LIBSOURCES (files)
Like AC_LIBSOURCE, but accepts one or more files in a comma-separated M4 list. Thus, the above example might be rewritten:

AC_LIBSOURCES([foo.c, bar.c])

Macro: AC_REPLACE_FUNCS (function...)
Like AC_CHECK_FUNCS, but uses `AC_LIBOBJ(function)' as action-if-not-found. You can declare your replacement function by enclosing the prototype in `#if !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.

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