Date: Thu, 15 Feb 2007 17:14:47 +0100 From: Pascal Hofstee <caelian@gmail.com> To: Robert Watson <rwatson@FreeBSD.org> Cc: Dag-Erling =?ISO-8859-1?Q?Sm=F8rgrav?= <des@des.no>, hackers@freebsd.org Subject: Re: a question regarding <sys/shm.h> Message-ID: <1171556087.9285.1.camel@chekov> In-Reply-To: <20070215133647.V79543@fledge.watson.org> References: <45C04593.2090704@gmail.com> <20070131085206.GW892@turion.vk2pj.dyndns.org> <45C06167.60401@gmail.com> <86odofjyua.fsf@dwp.des.no> <20070131105024.L91177@fledge.watson.org> <d8a0b7620702150446g5638b86aw3d9124d5b1a7ecd7@mail.gmail.com> <20070215133647.V79543@fledge.watson.org>
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On Thu, 2007-02-15 at 13:41 +0000, Robert Watson wrote: > Unfortunately, things are a bit more tricky. The problem is not so much the > API, where converting size_t/int is a relative non-event, rather, the ABI. By > changing the size of a field in a data structure, you may change the layout of > the structure, and hence the offset of other fields. This offset information > is compiled into binaries that access the structure -- hence being part of the > ABI. On i386, the change from int to size_t doesn't modify the ABI, as both > int and size_t are 32-bit. However, on 64-bit platforms, int is 32-bit and > size_t is 64-bit: > > sledge:/tmp> uname -a > FreeBSD sledge.freebsd.org 7.0-CURRENT FreeBSD 7.0-CURRENT #898: Wed Feb 14 > 14:20:16 UTC 2007 root@sledge.freebsd.org:/h/src/sys/amd64/compile/SLEDGE > amd64 > sledge:/tmp> ./size_t > sizeof int: 4 > sizeof size_t: 8 > > In practice, this means that all of the later fields in the data structure > will be offset by 4 bytes. This will affect any application that accesses > later fields in the structure but isn't recompiled. This is why DES and I > have been discussing this change as requiring kernel compatibility code, which > would provide new system calls working with the new layout, and retain old > system calls working with the old layout. So we'd need to provide a new > shmctl() with the new structure, and an oshmctl() with the old layout. While > doing that, it makes sense to do all the other ABI-related things that we'd > like to get out of the way, such as fixing the types in shm_perm. I understand ... i'll leave this up to you guys .. you have obviously a lot more hands on experience in these kinds of matters :) -- Pascal Hofstee
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