Re: Allocators design flaw

[David Thompson <dave.thompson2@verizon.net>]

On Sat, 14 Oct 2017 16:56:04 +0100, Simon Wright <simon@pushface.org>
wrote:

> Victor Porton <porton@narod.ru> writes:
> 
> > Simon Wright wrote:
<snip>
> >>   struct chrs {
> >>     char c[10];
> >>   }
> >> 
> >> only *needs* alignment of 1
> >> 
> >>   struct chrs {
> >>     int last;
> >>     char c[256];
> >>   }
> >
> > No, all C structs share the same alignment.
> 
That's quite wrong. No C standard has ever required it, and I've used
several conforming C implementations where different struct types have
different alignments. 

There are restrictions on _pointers to structs_ (and unions) as I've
just described in another post, but not the structs/unions themselves.

> I agree that malloc() will always allocate aligned on some boundary
> (16-byte? 32-byte?) but that's not the same as the alignment of the
> struct itself.
> 
malloc (and calloc and realloc) are required to provide the 'maximal'
alignment of _any_ type supported by the compiler (which I've never
seen more than 8 bytes) and allowed to provide more if they want, for
example because it is sometimes beneficial (like SSE data on x86) or
because the allocator strategy works better (maybe buddy).

> See e.g.
> https://en.wikipedia.org/wiki/Data_structure_alignment#Typical_alignment_of_C_structs_on_x86
> 
Don't need typical, this is in the standard.