Re: Question on interface Ada to C

[Simon Wright <simon@pushface.org>]

James Alan Farrell <anonymous@anonymous.com> writes:

>  type stuff is integer; -- just for example
> 
>  type List_Type is array(Integer range <>) of stuff;
>  
>  package MyPointers is 
>               new System.Address_To_Access_Conversions(List_Type);
>  subtype List_Pointer is MyPointers.Object_Pointer;
> 
>  procedure MyProc
>      (Items     : in out List_Pointer;
>       Nitems    : in out Integer) is
> 
>       List : List_Type := function_that_returns_a_list;

List is on the stack

> 
>    begin
>       Nitems := List'Length;
>       Items  := MyPointers.To_Pointer(List'Address);
>    end;

List has gone away!

I think you're going to need to allocate the particular array and get
the C code to free it (well, that's dangerous, perhaps you should
supply an Ada subprogram to actually do the freeing so as to be sure
the right storage pool is used; or perhaps you could import malloc?)

And I don't think C will understand a List_Pointer, because List_Type
is unconstrained so any concrete List may have bounds with it. There's
no reason to suppose that List'Address is the same as List
(List'First)'Address. The C is expecting effectively a System.Address,
so make your Items parameter a System.Address (why is it in out, BTW?)

One "trick" is to declare a local subtype which is constrained by the
actual bounds.

  List : List_Type := function_that_returns_a_list;
  type actual_list_type is list_type (list'range); -- or suchlike
  -- instantiate System.Address_To_Access_Conversions for actual_list_type
  function malloc (n : natural) return system.address;
  pragma import (c, malloc, "malloc");
begin
  Items := malloc (actual_list_type'max_size_in_storage_elements);
  -- copy the contents of List using the System.Address_To_Access_Conversions
  -- instantiation above

Good grief, that looks complicated, I dare say I've missed something
simple ..

-- 
Simon Wright                               100% Ada, no bugs.