sizeof(function pointer)

[Robert Watson]

On Tue, 31 May 2011, 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).

I think you're OK for MFC purposes, but that in general, we shouldn't assume 
that they are the same size.  I.e., we should use a function pointer type 
where we mean a function pointer type, and never write code that casts a 
function pointer to a regular pointer.  (Which the change is fine with respect 
to, I believe).

I'm doing some research on an experimental architecture where certain types of 
function pointers are 256-bit.  This has some interesting consequences; we 
haven't yet gotten to investigating C language extensions/compatibility, but 
that will follow in the next year or so.  (We also have 256-bit data 
references, similar to pointers, for use in some environments, which will also 
prove interesting.  I'm not yet convinced we'll try to use a general pointer 
type for them, but perhaps instead extend the language to have a qualified 
type of some sort).

Robert