Interfacing to C, output arrays

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

The code below shows an interface to a C function
  int accessor(int n, int* a);
which fills the n-element array a with data.

The code works just fine with Gnat 3.07 (OSF, Linux) but I think I may
be just 'lucky'.

The purpose of the subtype This_Arr is to make sure that the parameter
passed to the C code doesn't have any 'extraneous' constraint info at
the front (in fact, Gnat seems to pass the address of the first
element even to the Ada procedure anyway). Gnat warns that Result is
never assigned a value (that is, the Result out parameter to the Ada
procedure), so I'm concerned that another compiler might handle things
differently.

The Rationale doesn't cover the case of array parameters
(Interfaces.C.Pointers doesn't seem relevant here), but LRM B.3(70)
seems to indicate that I'm OK. Any thoughts?

--
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;