Re: Question on interface Ada to C

[Dale Stanbrough <MrNoSpam@bigpoop.net.au>]

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


> Thanks,
> James Alan Farrell
> GrammaTech.
> 
>  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;
> 
>    begin
>       Nitems := List'Length;
>       Items  := MyPointers.To_Pointer(List'Address);
>    end;

You are right in thinking it is not good code.

The variable List is allocated on the stack/secondary head, and should
go away when the procedure ends. The pointer to it will no longer
be valid once you return from this procedure.


Better to have function_that_returns_a_list be
   function_that_returns_a_pointer_to_a_list

and return that....


      type List_Type_Access is access all List_Type;
      -- declare this type where it is visible to your
      -- other function.


      List_Ptr :+ list_type_access := function_that_returns_a_pointer...
   begin
      Nitems := List.all'Length;
      Items  := MyPointers.To_Pointer(List.all(1)'Address);
   end;


there's probably other stuff you should look at in the Interfaces.C.*
package family - there will be a better way than this.


Dale

-- 
dstanbro@spam.o.matic.bigpond.net.au