Date: Fri, 16 Feb 2007 12:56:02 +0000 (GMT) From: Robert Watson <rwatson@FreeBSD.org> To: Pascal Hofstee <caelian@gmail.com> 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: <20070216125514.O38234@fledge.watson.org> In-Reply-To: <1171556087.9285.1.camel@chekov> 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> <1171556087.9285.1.camel@chekov>
next in thread | previous in thread | raw e-mail | index | archive | help
On Thu, 15 Feb 2007, Pascal Hofstee wrote: > 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 :) Well -- don't let this discourage you from working on it, I'm just pointing out that there are some more details to work on before it will be done :-). I'm happy to advise further as the work moves along, but unfortunately don't have time to do it myself at this point. It's something I would very much like to see happen, though! Robert N M Watson Computer Laboratory University of Cambridge
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20070216125514.O38234>