sizeof(function pointer)

[Alexander Kabaev]

On Tue, 31 May 2011 17:18:16 -0600
Warner Losh <imp at bsdimp.com> wrote:

> 
> On May 31, 2011, at 5:07 PM, mdf at freebsd.org wrote:
> 
> > 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.
> 
> It is the same on MIPS too for all three ABIs that we support (and
> all ABIs that I know about).  It is true on ARM as well.
> 
> Usually it is different only on segmented architectures like 16-bit
> x86.
> 

Not so on ia64, where they have special function descriptor type.

-- 
Alexander Kabaev
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