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.

> 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.

> has to remain interposable).  If there is no relationship to malloc, GCC
> essentially supports unbounded alignment on many targets.  How is it possible
> to have a meaningful definition of max_align_t under such circumstances?

max_align_t only covers fundamental alignments, not necessarily all 
alignments that are supported.

-- 
Joseph S. Myers
joseph@codesourcery.com