Re: Ada equivalent for C-pointer?

[Nick Roberts <nick.roberts@acm.org>]

Niklas Holsti <nobody@nowhere.fi> wrote:

> But according to LRM B.3(68), for an "in out" parameter of an *elementary*
> type (as here) the pointer that is passed to the C function is a pointer
> to a *temporary copy* of the parameter (here Number), not a pointer to the
> parameter itself. This may not be what the C function expects. For
> example, it would not make sense for "wibble" to save the pointer for
> later use.
> 
> (Perhaps this detail is explained in the Wiki page that was referenced.
> For some reason, I couldn't reach that page to read it.)

I think the idea is as follows. Supposing a call is made to Wibble with an 
actual parameter corresponding to Number called Actual. The call first makes
a copy of the value of Actual, then it passes a pointer to that copy as the
parameter Number, and makes the call. Upon return, the value in that
temporary copy is then copied back into Actual. (Hence, it 'works' :-)

The idea is that Actual may, in fact, be a register, or something other than
a straightforward memory location. The temporary copy will be in a
straightforward memory location (suitable to be pointed at by a C pointer).
However, I suspect a possible optimisation, if it so happens that Actual is
in fact stored in a straightforward memory location, is to simply pass a
pointer to Actual; theoretically, the semantics should be the same (in the
absence of aliasing effects).

However, this is all based upon an Implementation Advice clause.
Implementations are necessarily permitted to do something else.

-- 
Nick Roberts