From: Chris Hall <musl@gmch.uk>
To: musl@lists.openwall.com
Subject: malloc() alignment and max_align_t
Date: Sun, 7 Jul 2019 19:17:48 +0100 [thread overview]
Message-ID: <6b44b4a2-0047-6304-8c86-058236dc1c74@gmch.uk> (raw)
In-Reply-To: <7dfceb33-b903-de2a-7abe-825f44512f0c@gmch.uk>
The C11 Standard says that malloc() will return a pointer which "is
suitably aligned so that it may be assigned to a pointer to any type of
object with a fundamental alignment requirement", and it says that
_Alignof(max_align_t) is equal to the "fundamental alignment".
I note that:
* musl malloc() works to an alignment SIZE_ALIGN, which is
#defined'd to 4*sizeof(size_t) == 32 for 64-bit processors.
* musl defines max_align_t:
typedef struct { long long __ll;
long double __ld; } max_align_t;
(same like other libc, FWIW).
_Alignof(max_align_t) == 16 for x86_64 (at least),
because that's the ABI alignment for long double.
Clearly, C11 does not require malloc() to align exactly as max_align_t,
and bigger is fine.
But I'm curious as to why SIZE_ALIGN is twice as big as it needs to be ?
Thanks,
Chris
next prev parent reply other threads:[~2019-07-07 18:17 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <1561802993.8028.ezmlm@lists.openwall.com>
2019-06-29 11:48 ` Detecting musl at compile and/or configure time Chris Hall
2019-06-29 13:27 ` A. Wilcox
2019-06-29 13:58 ` Szabolcs Nagy
2019-06-30 11:48 ` Chris Hall
2019-07-07 18:17 ` Chris Hall [this message]
2019-07-07 19:22 ` malloc() alignment and max_align_t Markus Wichmann
2019-07-08 0:16 ` Rich Felker
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