Re: [PATCH v2] rust: alloc: satisfy `aligned_alloc` requirements

[DJ Delorie <dj@redhat.com>]

Alejandro Colomar <alx@kernel.org> writes:
> -  aligned_alloc()
>	It makes sure that the input is a power of two, or it fails.
>
> -  posix_memalign()
>
> 	. . . It requires that the input is power of
> 	two,
>
> I wonder why glibc silently overaligns aligned_alloc() without reporting
> an error for an alignment of 2, while it reports an error for an
> alignment of 3.  It doesn't make much sense at first glance.  No
> standard seems to require that, so it looks like an arbitrary choice.

Because 2 is a power of two, but 3 isn't.  No power of two is a multiple
of 3.

GNU malloc only supports alignments that are powers of two.  The
resulting addresses might *happen* to be multiples of other numbers, but
you cannot request that.

As for why, ask Posix:

"If the value of alignment is not a valid alignment supported by the
 implementation, a null pointer shall be returned."
 [EINVAL]
    The value of alignment is not a valid alignment supported by the
    implementation."