Re: Thick bindings to a C library and gnattest: suggestions?

[Maurizio Tomasi <ziotom78@gmail.com>]

On Monday, July 1, 2013 11:45:04 AM UTC+2, Dmitry A. Kazakov wrote:
> Why don't you simply pass the array down to the C subprogram? You can do
> something like:
> 
>    type Double_Array is array (Positive range <>)
>       of aliased Interfaces.C.double;
>    pragma Convention (C, Double_Array);
>    procedure Foo (A : Double_Array);
> 
> Implementation:
> 
>    type Double_Ptr is access all Interfaces.C.double;
>    pragma Convention (C, Double_Ptr);
> 
>    procedure Foo (A : Double_Array) is
>    --
>    -- Assuming foo's signature in C:
>    --
>    --    foo (double * a, unsigned n);
>    --
>       procedure Internal (A : Double_Ptr; N : Interfaces.C.unsigned);
>       pragma Import (C, Internal, "foo");
>    begin
>       Internal (A (A'First)'Access, A'Length);
>    end Foo;

Hi Dmitry,

  I like a lot this solution, but apparently I am not able even to implement this correctly. I wrote a dummy package to make my problem clearer:

with Interfaces.C;

procedure Test_Package is
   
   type Vector is array (Positive range <>) of aliased Natural;
   
   type Pointer is access all Natural;
   pragma Convention (C, Pointer); 
   
   type Const_Pointer is access constant Natural;
   pragma Convention (C, Const_Pointer); 

   procedure Read_Vector_From_File (A : out Vector) is 
      
      procedure Internal (A : Pointer; N : Interfaces.C.unsigned); 
      pragma Import (C, Internal, "read_vector"); 
      
   begin 
      Internal (A (A'First)'Access, A'Length); 
   end Read_Vector_From_File; 
   
   procedure Write_Vector_To_File (A : Vector) is 
      
      procedure Internal (A : Const_Pointer; N : Interfaces.C.unsigned); 
      pragma Import (C, Internal, "write_vector"); 
      
   begin 
      Internal (A (A'First)'Access, A'Length); 
   end Write_Vector_To_File; 
   
   A : Vector (1 .. 3);
   
begin
   Read_Vector_From_File (A);
   Write_Vector_To_File (A);
end Test_Package;

When I compile this program, both occurrences of "A (A'First)'Access" in Read_Vector and Write_Vector make GNAT complain:

test_package.adb:19:17: non-local pointer cannot point to local object
test_package.adb:28:17: non-local pointer cannot point to local object

I do not understand what causes this problem: in both cases the address of A's first element is not stored in a global variable but used as the argument to a procedure call. So how can this be a "non-local pointer"?

Maurizio.