sizeof(function pointer)

[mdf at FreeBSD.org]

I am looking into potentially MFC'ing r212367 and related, that adds
drains to sbufs.  The reason for MFC is that several pieces of new
code in CURRENT are using the drain functionality and it would make
MFCing those changes much easier.

The problem is that r212367 added a pointer to a drain function in the
sbuf (it replaced a pointer to void).  The C standard doesn't
guarantee that a void * and a function pointer have the same size,
though its true on amd64, i386 and I believe PPC.  What I'm wondering
is, though not guaranteed by the standard, is it *practically* true
that sizeof(void *) == sizeof(int(*)(void)), such that an MFC won't
break binary compatibility for any supported architecture?  (The
standard does guarantee, though not in words, that all function
pointers have the same size, since it guarantees that pointers to
functions can be cast to other pointers to functions and back without
changing the value).

Another possibility is to malloc a blob that is sizeof(int(*)(void))
and store that in a renamed s_unused; this is a bit messier but
guaranteed to work.  I'd just rather the code be an MCF instead of a
partial re-write.

Thanks,
matthew