On 09/06/2016 01:25 PM, Joseph Myers wrote:
On Tue, 6 Sep 2016, Florian Weimer wrote:
Why aren't there any users? The standard isn't very clear what the value of
_Alignof (max_align_t) actually means. Does the phrase “all contexts” include
pointers returned malloc? Even if the requested size is smaller than the
fundamental alignment? Did those who wrote the standard expect there to be
*any* relationship between malloc and max_align_t?
See my cleanup of the wording in DR#445
<http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2059.htm#dr_445>, which
is intended to reflect the intent and stay compatible with C99. malloc
should be usable to allocate memory for any type from the standard library
headers, including max_align_t.
And the old text for malloc says:
“
The pointer returned if the allocation succeeds is suitably aligned so
that it may be assigned to a pointer to any type of object with a
fundamental alignment requirement and then used to access such an object
or an array of such objects in the space allocated (until the space is
explicitly deallocated).
”
So that's what ties the two things together. I still don't like what's
implied in PR66661, that all object sizes have to be multiples of the
fundamental alignment.
The existing situation is that most mallocs to do not provide _Alignof
(max_align_t) alignment unconditionally. But they can provide arbitrarily
large alignment with aligned_alloc/memalign-style interfaces.
Well, that's a conformance bug in the implementation as a whole. The
nonconforming modes in question are still useful and it's useful for GCC
to support such mallocs.
PR66661 shows that GCC does not want to support such mallocs (or even
glibc's malloc).
Florian
