On Thu, 9 Apr 2015, Florian Weimer wrote:
> If I declare a type with an _Alignas specification, requiring an
> alignment which is less than _Alignof (max_align_t), that is a,
> fundamental alignment, then I can allocate such an object with malloc.
> That is, this code is valid:
>
> struct S {
> _Alignas (max_align_t) char a;
> };
Subject to DR#444 (the C11 syntax currently doesn't actually allow
_Alignas specifiers inside struct and unions at all).
> struct S *p = malloc (sizeof (struct S));
For arrays of this type to work, sizeof (struct S) must actually be a
multiple of its alignment (so 16, not 1).
> But I see nothing in the x86-64 ABI document that implies that malloc
> (1) must be 16-byte-aligned. The fact that there are types of larger
It's C90 DR#075 that requires malloc (1) to return a pointer suitably
aligned for all types (including long double). (That is, all types that
can be defined using C90 standard syntax.)
> Before C11, this was perfectly conforming. I doubt it was the intention
No, it was never conforming for malloc (1) to return memory not aligned
for long double.
All C11 (+ DRs) does is restrict the requirement from *all* types to all
types that aren't over-aligned, along with introducing the possibility of
over-aligned types.
--
Joseph S. Myers
[email protected]
