On Tue, 31 May 2011, [email protected] 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
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