Date: Tue, 31 May 2011 17:18:16 -0600 From: Warner Losh <imp@bsdimp.com> To: mdf@freebsd.org Cc: freebsd-hackers <freebsd-hackers@freebsd.org>, Bruce Evans <brde@optusnet.com.au> Subject: Re: sizeof(function pointer) Message-ID: <D541877D-0EE5-4EC3-9197-E9F2A7A10AC1@bsdimp.com> In-Reply-To: <BANLkTimctcBWRrYVLyc-WLgePyCXqaPTGA@mail.gmail.com> References: <BANLkTimctcBWRrYVLyc-WLgePyCXqaPTGA@mail.gmail.com>
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On May 31, 2011, at 5:07 PM, mdf@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. >=20 > 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 *) =3D=3D 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). >=20 > 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. Warner=
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