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From: Stephen Leake <Stephen.Leake@gsfc.nasa.gov>
Subject: Re: aliased & packed?
Date: 1997/05/09
Message-ID: <33739A50.7B99@gsfc.nasa.gov>#1/1
X-Deja-AN: 240657386
References: <33734C0C.6904@bix.com>
Organization: NASA Goddard Space Flight Center -- Greenbelt, Maryland USA
Reply-To: Stephen.Leake@gsfc.nasa.gov
Newsgroups: comp.lang.ada
Date: 1997-05-09T00:00:00+00:00
List-Id: <comp.lang.ada>


Tom Moran wrote:
> 
> Clearly you can't do it.  I'd like to use pragma Pack for bits in a
> black/white bitmap array.  That's easy to manipulate.  But to actually
> display it I need to do a system call and pass the address.  If it
> wasn't packed, Image'unchecked_access would be appropriate.  But
> that won't work because Image would have to be aliased, and it can't
> be because it's packed. Must I drop 'Pack' and make it an array of
> bytes and do my own masking, shifting, etc?  What's the cleanest/most
> portable/best way to accomplish the task?

Since the system call wants the address, use 'address (converted to
Long_Integer or whatever is required). You have to do some conversion to
pass to a system call; converting from 'unchecked_access is no "cleaner"
than converting from 'address.

As for portability, there will be a different system call to display
bitmaps, so you'll have to change a lot more than the attribute!
-- 
- Stephe