On 03/17/2010 05:00 PM, Grégoire Sutre wrote: > Hi Eric, > >> AC_CHECK_FUNCS leaves HAVE_FUNC undefined if it is missing, but defines >> HAVE_FUNC to 1 if it is present. It is much easier to write: >> >> #if HAVE_FUNC > > In that case you only need to write: > > #ifdef HAVE_FUNC > > which is just as simple, and is compliant with -Wundef.
Then you contrast it with AC_CHECK_DECL, which defines HAVE_FUNC_DECL to either 0 or 1, but always defines it. If you use #ifdef in those situations, you lose (you typed 3 more bytes, and you get the wrong result). It is a maintenance nightmare to remember which variables are defined as 0 and which are left undefined, and much simpler to blindly use #if than it is to decide when #ifdef is necessary. > Moreover, the documentation [1] of AC_CHECK_FUNCS actually does *not* > guarantee that the value will be 1, it could as well be 0 (or -1234): Actually, it DOES guarantee that it will be 1, but the guarantee is (admittedly indirectly) specified elsewhere in the manual (that is, AC_DEFINE defaults to 1 in the absence of any other value, and AC_CHECK_FUNCS does not provide an interface to inject any other value. Autoconf will NOT be changing the use of AC_CHECK_FUNCS; but I am open to doc patches to make this point more clear. -- Eric Blake ebl...@redhat.com +1-801-349-2682 Libvirt virtualization library http://libvirt.org
signature.asc
Description: OpenPGP digital signature
