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From: "Jerry" <lanceboyle@qwest.net>
Newsgroups: comp.lang.ada
Subject: Re: How to use Annex G.3 Vectors and Matrices in bindings to C arrays
Date: 13 Nov 2006 03:20:02 -0800
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Date: 2006-11-13T03:20:02-08:00
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Jeffrey R. Carter wrote:
> Jerry wrote:
> >
> > I wonder if I am incorrect in believing that the binding to C-arrays
> > need aliased components. Like I said, the reason for that is a little
> > fuzzy to me right now. But I do know that if I remove the aliased
> > component requirement from Long_Float_Array_1D the binding will not
> > compile.
>
> What is the error message that you get? Are you taking 'access of
> components of such arrays?
>
I'm afraid I'll have to apologize for not being accurate in my initial
post. Indeed, I am not having any problems (as I erroneously claimed)
with 1D arrays. The problem arises with 2D arrays, about which I
pestered the list earlier. With further apologies for posting a bit
more code that I would like, the following is a typical binding that I
made in which it was necessary to gain access to 2D C arrays.



procedure Mesh_3D_Base_Contour
   (x, y           : Long_Float_Array_1D; -- surface definition points
    z              : in out Long_Float_Array_2D; -- height of surface
at definition points
    Options        : Integer;
    Contour_Levels : Long_Float_Array_1D) is -- levels at which to draw
contours

    package PL_Float_Pointers_Local is new Interfaces.C.Pointers
       (Index              => Integer,
        Element            => Long_Float,
        Element_Array      => Long_Float_Array_1D,
        Default_Terminator => 0.0);
    use type PL_Float_Pointers_Local.Pointer; -- as in RM B.3.2
    type PL_Float_Pointer_Array_Local is array (Integer range <>) of
PL_Float_Pointers_Local.Pointer; -- array of pointers to Long_Floats
which represent the first element of each row of z in C-land

    Index_Of_First_Column : Integer := z'First(2);
    z_As_Pointers : PL_Float_Pointer_Array_Local (z'range(1));

    procedure
    plmeshc_local(x : PL_Float_Array; y : PL_Float_Array; z :
PL_Float_Pointer_Array_Local; nx : Integer; ny : Integer; opt :
Integer; clevel : PL_Float_Array; nlevel : PLINT);
    pragma Import(C, plmeshc_local, "c_plmeshc");

begin
    for Index in z'range(1) loop
        z_As_Pointers(Index) := z(Index, Index_Of_First_Column)'access;
    end loop;
    plmeshc_local(x, y, z_As_Pointers, x'Length, y'Length, Options,
Contour_Levels, Contour_Levels'Length); -- pass z_As_Pointers here
rather than z
end Mesh_3D_Base_Contour;



The related array definitions are:
    type Long_Float_Array_1D is array (Integer range <>) of aliased
Long_Float;
    type Long_Float_Array_2D is array (Integer range <>, Integer range
<>) of aliased Long_Float;

The "thin" binding Ada procedure plmeshc is defined:
    procedure plmeshc(x : PL_Float_Array; y : PL_Float_Array; z :
PL_Float_Array_2D; nx : PLINT; ny : PLINT; opt : PLINT;
          clevel : PL_Float_Array; nlevel : PLINT);
    pragma Import(C, plmeshc, "c_plmeshc");


And the original C procedure is defined as here:

http://plplot.sourceforge.net/docbook-manual/plplot-html-5.6.1/plmeshc.html

So it appears that I need to use access variables to mimic the 2D C
arrays, but the Annex G.3 Matrix definitions do not have aliased
elements. So, if you re-read my original post and substitute "2D" for
"1D" my original questions still stand.

Jerry