Implement C23 memalignment

[Florian Weimer]

* Joseph Myers:

> On Thu, 2 Oct 2025, Florian Weimer wrote:
>
>> * Joseph Myers:
>> 
>> > diff --git a/manual/memory.texi b/manual/memory.texi
>> > index 6a70168e61..1e30631125 100644
>> > --- a/manual/memory.texi
>> > +++ b/manual/memory.texi
>> > @@ -1135,6 +1135,17 @@ The @code{valloc} function is obsolete and @code{aligned_alloc} or
>> >  @code{posix_memalign} should be used instead.
>> >  @end deftypefun
>> >  
>> > +You can determine the alignment of a pointer with the
>> > +@code{memalignment} function.
>> > +
>> > +@deftypefun size_t memalignment (void *@var{p})
>> > +@standards{C23, stdlib.h}
>> > +@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
>> > +
>> > +This function, defined in C23, returns the alignment of @var{p}, as a
>> > +power of two.  If @var{p} is a null pointer, it returns zero.
>> > +@end deftypefun
>> 
>> Does the pointer have to be a valid object pointer?  I think we should
>> support arbitrary bit patterns as a GNU extension.
>
> C23 suggests it has to be a valid pointer to an object (or a null 
> pointer).  This implementation supports arbitrary bit patterns.

Please document this (both aspects) in the manual update.

Thanks,
Florian