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From: Stephen Leake <stephen_leake@stephe-leake.org>
Newsgroups: comp.lang.ada
Subject: Re: What shoudl GNAT do wrt variant record and representation
 clauses?
References: <49494faf-1e2d-43a1-a2b0-fd25af3850cc@googlegroups.com>
Date: Wed, 04 Feb 2015 03:59:28 -0600
Message-ID: <85oapa3slb.fsf@stephe-leake.org>
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Date: 2015-02-04T03:59:28-06:00
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Lucretia <laguest9000@googlemail.com> writes:

> Hi,
>
> I've been back on SDLAda, one bit I've been dreading, setting up a
> unit test to check to make sure my representations of the pixel
> formats are the same as what the C macros produce.

Use -fdump-ada-spec, then you don't need to write records or tests.

>    type Pixel_Orders (Pixel_Type : Pixel_Types := Unknown) is
>       record
>          case Pixel_Type is
>             when Index_1 | Index_4 | Index_8 =>
>                Indexed_Order : Bitmap_Pixel_Order;
>
>             when Packed_8 | Packed_16 | Packed_32 =>
>                Packed_Order  : Packed_Component_Order;
>
>             when Array_U8 | Array_U16 | Array_U32 | Array_F16 | Array_F32 =>
>                Array_Order   : Array_Component_Order;
>
>             when others =>
>                null;
>          end case;
>       end record with
>         Unchecked_Union => True,
>         Convention      => C,
>         Size            => 4;
>
>    for Pixel_Orders use
>       record
>          Indexed_Order at 0 range 0 .. 2;
>          Packed_Order  at 0 range 0 .. 3;
>          Array_Order   at 0 range 0 .. 3;
>       end record;

> So, is this correct behaviour?

Your original version says there is one bit that is unspecified for
Pixel_Type in Index_1 | Index_4 | Index_8, so yes, this is correct
behavior.

Another way to handle the extra bit is:

          case Pixel_Type is
             when Index_1 | Index_4 | Index_8 =>
                Indexed_Order : Bitmap_Pixel_Order;
                Spare_1 : Boolean;

>If so, is there a compiler flag to catch this?

Consulting the secret documentation:

(info "(gnat_ugn) Warning Message Control")
C-s record

turns up "-gnatw.h (holes in record layouts)"

--
-- Stephe