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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Interfacing C type, unconstrained array with record
Date: Sat, 16 Oct 2010 17:41:40 -0400
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Date: 2010-10-16T17:41:40-04:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:

> You need a flat array, e.g.
>
>    with Interfaces;  use Interfaces;
>    with Interfaces.C;  use Interfaces.C;

I'd probably write:

    with Interfaces.C;  use Interfaces;

and then refer to C.int instead of Int.
No big deal.

>    type SDL_Color is record
>       R, G, B, Unused : Unsigned_8;
>    end record;
>    pragma Convention (C, SDL_Color);
>    type SDL_Color_Array is array (Positive) of aliased SDL_Color;

I think you want this indexed by the same type as N_Colors.
And why does it need to have aliased components?
So perhaps:

    type SDL_Color_Array is array (Int range 1..1_000_000) of SDL_Color;

>    pragma Convention (C, SDL_Color_Array);
>    type SDL_Palette is record
>       N_Colors : Int;
>       Colors   : SDL_Color_Array;
>    end record;
>    pragma Convention (C, SDL_Palette);
>
> Now, you cannot directly declare a SDL_Palette variable, because it is too
> large. You should do it as you would do in C, i.e. allocate some amount of
> memory and then set structure pointer to it. You do not need 'new' or
> access types in Ada. It can be on the stack like this:
>
>    with System.Storage_Elements;  use System.Storage_Elements;
>       ...
>    Raw_Memory : Storage_Array (1..1_024); -- Memory
>    Palette    : SDL_Palette;
>    for Palette'Address use Raw_Memory'Address;

I think you need to worry about alignment here.
Perhaps:

    pragma Assert(SDL_Palette'Alignment = C.int'Alignment);
    for Raw_Memory'Alignment use C.int'Alignment;

> begin
>    Palette.N_Colors := 20;
>
> Of course computing memory size from N_Colors is up to you (as in C).

You might want to say:

    Colors: SDL_Color_Array renames Palette.Colors(1..Palette.N_Colors);

and then refer to components of Colors, so you get array bounds checking.

- Bob