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Date: Sun, 7 Jul 2019 19:17:48 +0100
From: Chris Hall <musl@...h.uk>
To: musl@...ts.openwall.com
Subject: malloc() alignment and max_align_t
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
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