Re: Ada array vs C pointer (call by reference)

[Adam Beneschan <adam@irvine.com>]

On Jun 27, 3:14 pm, Keith Thompson <ks...@mib.org> wrote:
> Maciej Sobczak <see.my.homep...@gmail.com> writes:
> > On 27 Cze, 18:11, Keith Thompson <ks...@mib.org> wrote:
> > > > Ada *cannot* guarantee that Interfaces.C.C_Float is the *right* float,
> > > > because it does not verify how the C code was compiled and there is a
> > > > lot of freedom given to C implementers in this area.
> > > > Same for all other types.
>
> > > No, Ada can't guarantee it directly.  Instead, Ada requires the
> > > implementation to guarantee it.
>
> > And how the implementation can guarantee it without mandating the
> > representation on the C compiler?
>
> It doesn't *mandate* the representation used by the C compiler; the
> author of the Interfaces.C implementation has to *know* the
> representation used by the C compiler.

Which is, of course, impossible without a crystal ball.  Anyway, I
don't see anything in the RM that makes any such requirement.  The
language involved uses a lot of "corresponds to" phrasing, which I
think is deliberately fuzzy enough to avoid *requiring* an Ada
implementation to do something that cannot realistically be done in a
guaranteed manner.  I think the idea is that the implementor is just
expected to define the types in a way that is probably going to make
things work right.  That's about as strong a statement as one can
make.  But, of course, it's still different from the standard Ada
types like Integer and Float; there are no such expectations put on
Ada implementations that the type representations correspond at all to
C types.

                                  -- Adam