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From: mheaney@on2.com (Matthew Heaney)
Newsgroups: comp.lang.ada
Subject: Re: Q re pack & aliased
Date: 22 Apr 2003 15:51:51 -0700
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tmoran@acm.org wrote in message news:<Cjepa.568837$L1.166648@sccrnsc02>...
> >The first True-Color-VGA had 24-bit-sized pixels. How would you have
> >addressed them?
>   I have pre-Gnat 3.15p programs that ran with 24 bit pixels, so this
> is apparently a new problem.  Would someone with a newer Gnat please
> try the program below?  On 3.15p it gives a warning on aliased, and ignores
> the packing.  Removing the word "aliased" makes it pack correctly.
> 
> with ada.text_io;
> procedure testpack is
>   type triples is range 0 .. 2**24-1;
>   for triples'size use 24;
>   type bunches is array(integer range <>) of aliased triples;
>   pragma pack(bunches);
>   x : bunches(1 .. 10);
> begin
>   ada.text_io.put_line(integer'image(x'size/80)
>                        & " should = 3, not 4");
> end testpack;


Pragma Pack and aliased components don't really go together.  Think
about it: the pragma is there to tell the compiler not to allocate
array components on "natural" boundaries, which would otherwise be
addressable.

Get rid of the pragma Pack, and use a component size clause:

   type bunches is array (integer range <>) of aliased triples;
   for bunches'Component_Size use 24;

Pragma Pack is really intended to give a hint to the compiler that you
wish to make a space optimization -- do not use it to specify
representation.  That's what the T'Component_Size is for.

Don't use pragma Pack unless you know what you're doing.

Your program is broken.