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Re: glibc 2.21 - Machine maintainers, please test your machines.
- From: Torvald Riegel <triegel at redhat dot com>
- To: Andreas Schwab <schwab at suse dot de>
- Cc: Chris Metcalf <cmetcalf at ezchip dot com>, "Carlos O'Donell" <carlos at redhat dot com>, GNU C Library <libc-alpha at sourceware dot org>, "H.J. Lu" <hjl dot tools at gmail dot com>, David Miller <davem at davemloft dot net>, Richard Henderson <rth at redhat dot com>, Mike Frysinger <vapier at gentoo dot org>, "Joseph S. Myers" <joseph at codesourcery dot com>, Kaz Kojima <kkojima at rr dot iij4u dot or dot jp>, Thomas Schwinge <thomas at codesourcery dot com>, Marcus Shawcroft <marcus dot shawcroft at linaro dot org>, Chung-Lin Tang <chunglin_tang at mentor dot com>, Adhemerval Zanella <azanella at linux dot vnet dot ibm dot com>, Andreas Krebbel <krebbel at linux dot vnet dot ibm dot com>
- Date: Tue, 27 Jan 2015 11:52:18 +0100
- Subject: Re: glibc 2.21 - Machine maintainers, please test your machines.
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On Mon, 2015-01-26 at 23:56 +0100, Andreas Schwab wrote:
> Torvald Riegel <triegel@redhat.com> writes:
>
> > I don't think so, because in a badly 4B-aligned (ie, no 8B-aligned) use,
> > struct new_sem is at offset 4 of the semaphore. If you copy that to a
> > 8B-aligned semaphore just bit-by-bit, then the actual data will still
> > start at offset 4, but to_new_sem will assume offset 0.
>
> It is undefined to use a copy of a sem_t in any of the semaphore calls.
I think we all agree on that. Nonetheless, as I mentioned previously,
I'm not actually aware of where in POSIX (or C?) this would be
explicitly forbidden. So if you know that, I'd be interested in getting
a reference.