Re: Interfacing to C, output arrays

[Simon Wright <simon@pogner.demon.co.uk>]

dewar@merv.cs.nyu.edu (Robert Dewar) writes:

> Simon asks about:
> 
> with Interfaces.C;
> package Inter is
>   type Arr is array (Integer range <>) of Interfaces.C.Int;
>   procedure Accessor (Result : out Arr);
> end Inter;
> 
> package body Inter is
>   procedure Accessor (Result : out Arr) is
>     subtype This_Arr is Arr (Result'Range);
>     function Accessor (N : Interfaces.C.Int;
>                        Result : This_Arr) return Interfaces.C.Int;
>     pragma Import (C, Accessor, "accessor");
>     Count : Interfaces.C.Int;
>   begin
>     Count := Accessor (N => Result'Length, Result => Result);
>   end Accessor;
> end Inter;
> 
> And he surmises that RM B.3(70) makes this safe.
> 
> Well, I certainly think that GNAT is right to output a warning here, at
> the least it is very ugly to have a C routine modifying an in parameter,
> and indeed I think a compiler would be within its rights (though perhaps
> not well advised) to assume that the function call could not modify the
> array.

Yes, I quite understand that; the problem is that the C subprogram is
a function and therefore can't be declared with an out parameter. I
guess I could use the DEC pragma (valued_subprogram??), but that would
be a tad implementation-dependent!

> It would be much cleaner to pass an access to the array or to use a
> procedure call and make the parameter out or in out.

The revised version below uses the access idea, at the cost of a copy.
Is this the only proper solution?

> Also note that RM B.3(70) is only implementation advice, not a requirement.

Point taken.

-- 
with Interfaces.C;
package Inter is
  type Arr is array (Integer range <>) of Interfaces.C.Int;
  procedure Accessor (Result : out Arr);
end Inter;

package body Inter is
  procedure Accessor (Result : out Arr) is
    subtype This_Arr is Arr (Result'Range);
    function Accessor (N : Interfaces.C.Int;
                       Result : access This_Arr) return Interfaces.C.Int;
    pragma Import (C, Accessor, "accessor");
    Count : Interfaces.C.Int;
    Res : aliased This_Arr;
  begin
    Count := Accessor (N => Result'Length, Result => Res'Access);
    Result := Res;
  end Accessor;
end Inter;