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Re: [PATCH] elf: dl-minimal malloc needs to respect fundamental alignment
- From: "H.J. Lu" <hjl dot tools at gmail dot com>
- To: Florian Weimer <fweimer at redhat dot com>
- Cc: GNU C Library <libc-alpha at sourceware dot org>
- Date: Tue, 21 Jun 2016 06:20:27 -0700
- Subject: Re: [PATCH] elf: dl-minimal malloc needs to respect fundamental alignment
- Authentication-results: sourceware.org; auth=none
- References: <20160621111702 dot 39A5B402F6E95 at oldenburg dot str dot redhat dot com> <CAMe9rOpx1Vbr6_n8+SvGCe92TO5e+KdC3hNcNAN3LvfXhd79Ow at mail dot gmail dot com> <6ca009d9-433e-31f5-c3c0-c84ecc587059 at redhat dot com> <CAMe9rOpLkftyZ+4y=1E3pbrKRU-i0kE4bdQLGaCac_Q9v1cV6g at mail dot gmail dot com> <0ef86d34-0642-8e47-9fe4-dcd7638c6153 at redhat dot com>
On Tue, Jun 21, 2016 at 6:06 AM, Florian Weimer <fweimer@redhat.com> wrote:
> On 06/21/2016 03:00 PM, H.J. Lu wrote:
>
>>> MALLOC_ALIGNMENT is potentially larger. malloc/tst-malloc-thread-fail
>>> tests
>>> for alignment. To my knowledge, it passes on all regularly tested
>>> architectures after commit dea39b13e2958a7f0e75b5594a06d97d61cc439f.
>>
>>
>> MALLOC_ALIGNMENT is kind of mapped to the malloc alignment of
>> a psABI. Shouldn't ld.so malloc have the same alignment of libc malloc?
>
>
> I don't see why. MALLOC_ALIGNMENT has to match both the ABI constraint and
> the malloc/malloc.c implementation constraint (which requires a minimum
> alignment of 2 * sizeof (size_t)).
My understanding is since the minimum constraint of malloc alignment
<= ABI alignment, MALLOC_ALIGNMENT == ABI alignment. Do you
have a glibc platform where it isn't true?
> Other mallocs do not have matching implementation constraints, and it is
> standard practice (in non-glibc mallocs) to lower the alignment for
> allocations which are smaller in size than _Alignof (max_align_t), although
> this is not compliant with C11.
>
> Florian
--
H.J.