Re: Access and alias

[Jeff Carter <jrcarter@acm.org>]

Alfred Hilscher wrote:
> 
> Jeff Carter wrote:
> > If you have to pass null, access types (with convention C) work fine. If
> > you don't have to pass null, in out works for procedures, and access
> > parameters work for functions. All keep the type checking that addresses
> > lose:
> >
> > -- int func (int* I);
> >
> > function F1 (I : System.Address) return Int;
> > pragma Import (C, F1, "func");
> >
> > function F2 (I : access Int) return Int;
> > pragma Import (C, F2, "func");
> >
> > I : aliased Int;
> > J : Int;
> > S : String := Get;

I see I forgot to put "aliased" on the declaration of S. Sorry.

> >
> > ...
> >
> > J := F1 (I'Address); -- works as intended
> > J := F1 (S'Address); -- works as not intended
> > J := F2 (I'access); -- works as intended
> > J := F2 (S'access); -- compilation error
> 
> OK. But I would do a little change:
> 
> Instead of:
> function F2 (I : access Int) return Int;
> pragma Import (C, F2, "func");
> 
> Would write
> type Int_Access is access all INT;
> function F2 (I : Int_Access) return Int;
> pragma Import (C, F2, "func");
> 
> This avoids some problems that I had with anonymous access types.

Yes, if you have to pass null, this is the way to do it. BE sure to put
a

pragma Convention (C, Int_Access)

in there. Then you declare

P : Int_Access;

...

J := F2 (P);

and presumably the C function knows what to do with the null.


-- 
Jeff Carter
"We burst our pimples at you."
Monty Python & the Holy Grail