Re: Size of a pointer to a tagged record

["David Thompson" <david.thompson1@worldnet.att.net>]

Steven Murdoch <news01+Steven.Murdoch@cl.cam.ac.uk> wrote :
> In article <wccvg29d7q8.fsf@shell01.TheWorld.com>,
>  Robert A Duff <bobduff@shell01.TheWorld.com> writes:
...
> >One thing you might think about is taking 'Size on the Ada side,
> >converting that to bytes (using System.Storage_Unit), and passing
> >that to the C code at some point.  The C code might be able to
> >use that size, or else assert that it is correct.
>
> I will try that too, the size is set during run time so there
> should not be any problem setting it. IIRC Malloc takes a number of
> octets so I could use a variant of Jeff Carters suggestion such as
> (Parameter_Access'Size + 7) / 8
> This should be the right thing to pass in, even if Storage_Unit is not 8
>
C malloc et al measure size in bytes as defined by that
C implementation, which must be at least 8 bits but may
be more, for exactly the same reasons SSU may vary;
the value is provided by macro CHAR_BIT in <limits.h>.

Neither standard requires C CHAR_BIT to equal Ada SSU,
it would be out of scope;  but logically they should be the same,
and if your implementation doesn't do so I think you're out of luck.

--
- David.Thompson 1 now at worldnet.att.net