23.2 Reusing Existing Macros
It goes without saying that it makes sense to reuse macros where
possible--indeed, a search of the Autoconf macro archive might turn up a
macro which does exactly what you want, alleviating the need to write a
macro at all (see section 23.5.1 Autoconf macro archive).
It's more likely, though, that there will be generic, parameterized
tests available that you can use to help you get your job done.
Autoconf"s `generic' tests provide one such collection of macros. A
macro that wants to test for support of a new language keyword, for
example, should rely on the
AC_TRY_COMPILE macro. This macro can
be used to attempt to compile a small program and detect a failure due
to, say, a syntax error.
In any case, it is good practice when reusing macros to adhere to their
publicized interface--do not rely on implementation details such as
shell variables used to record the test's result unless this is
explicitly mentioned as part of the macro's behavior. Macros in the
Autoconf core can, and do, change their implementation from time to
Reusing a macro does not imply that the macro is necessarily invoked
from within the definition of your macro. Sometimes you might just want
to rely on some action performed by a macro earlier in the configuration
run--this is still a form of reuse. In these cases, it is necessary to
ensure that this macro has indeed run at least once before your macro is
invoked. It is possible to state such a dependency by invoking the
AC_REQUIRE macro at the beginning of your macro's definition.
Should you need to write a macro from scratch, the following sections
will provide guidelines for writing better macros.