Re: Interfacing to C non-void procedure with "out" parameters

[Jeff Carter <jrcarter001@my-deja.com>]

In article <85nurq$cr0$1@nnrp1.deja.com>,
  Ted Dennison <dennison@telepath.com> wrote:
> In article <85nqaf$95o$1@nnrp1.deja.com>,
>   Jeff Carter <jrcarter001@my-deja.com> wrote:
> > In article <85ngl3$10s$1@nnrp1.deja.com>,
> >   Ted Dennison <dennison@telepath.com> wrote:
> > > First you interface to the routine to create your thin binding:
> > >
> > >    function esGetValue (Key   : in Interfaces.C.Int;
> > >                         Value : in System.Address
> > >                        ) return Interfaces.C.Int;
> > >    pragma Import (C, esGetValue, "esGetValue");
> > There's no reason to use System.Address here. Use an access
parameter:
>
> If you mean doing a "Value : access Interfaces.C.Int", that is
sometimes
> doable. The problem with that is than many C interfaces, particularly
> system calls, have optional pointer parameters where "null" is an
> acceptable value. An access parameter does not allow "null" to be
passed
> in. So in general they aren't always useful

This is sometimes the case, though it doesn't appear to be so here.
When it is the case, and you want to pass null, use an access type

type Int_Ptr is access all Interfaces.C.Int;

...

Value : in Int_Ptr; -- in function specification

...

Value => null -- in function call

System.Address loses all type checking. You can pass the address of
_anything_. An access parameter or access type limits you to the
address of something of the correct type. System.Address is best used
when the type is unknown, such as "void*".

--
Jeff Carter
"Now go away or I shall taunt you a second time."
-- Monty Python and the Holy Grail


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