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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: How to pass two dimensional arrays to C
Newsgroups: comp.lang.ada
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Date: Fri, 28 Jul 2006 14:27:53 +0200
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Date: 2006-07-28T14:27:57+02:00
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On 28 Jul 2006 04:06:59 -0700, Jerry wrote:

> In my project to write a binding for a popular plotter written in C,
> I've hit a brick wall and the problem might be associated with passing
> two-dimensionaal arrays to C.

Well, C does not have arrays (of either dimensionality), so you cannot pass
an Ada (true) array to C, as-is. You can pass a pointer to its first
element. 2D arrays in C are often emulated by using pointers to pointers.
This structure is even more distant from Ada's 2D array.

> I'm using GNAT on OS X and Xcode. I can
> pass one-dimensional arrays OK. Here are the pertinent parts from an
> example where the problem happens:
> 
>       The C
> 
> typedef double PLFLT;
> typedef int PLINT;
> 
> void c_plmesh(PLFLT *x, PLFLT *y, PLFLT **z, PLINT nx, PLINT ny, PLINT
> opt);

[...] 
> This appears during the call to plmesh. Any ideas? 

I'd suggest you to start with the Annex B.3. What you need is probably

with Interfaces.C;
with Interfaces.C.Pointers;

type PLFLT is new Interfaces.C.Double;
type PLINT is new Interfaces.C.Int;

type PLFLT_Array is array (PLINT range <>) of aliased PLFLT;
package PLFLT_Pointers is
   new Interfaces.C.Pointers
       (  Index              => PLINT,
          Element            => PLFLT,
          Element_Array      => PLFLT_Array,
          Default_Terminator => 0.0
       );
subtype PLFLT_Ptr is PLFLT_Pointers.Pointer;

type PLFLT_Ptr_Array is array (PLINT range <>) of aliased PLFLT_Ptr;
package PLFLT_Pointers_To_Pointers is
   new Interfaces.C.Pointers
       (  Index              => PLINT,
          Element            => PLFLT_Ptr,
          Element_Array      => PLFLT_Ptr_Array,
          Default_Terminator => null
       );
subtype PLFLT_Ptr_Ptr is PLFLT_Pointers_To_Pointers.Pointer;

procedure plmesh
          (  x   : PLFLT_Ptr;
             y   : PLFLT_Ptr;
             z   : PLFLT_Ptr_Ptr;
             nx  : PLINT;
             ny  : PLINT;
             opt : PLINT
          );
pragma Import(C, plmesh, "c_plmesh"); 

You might wish to write a wrapper to plmesh to be able to pass PLFLT_Array
and PLFLT_Ptr_Array down to it.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de