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From: dewar@merv.cs.nyu.edu (Robert Dewar)
Subject: Re: aliased & packed?
Date: 1997/05/10
Message-ID: <dewar.863322720@merv>#1/1
X-Deja-AN: 240812787
References: <33734C0C.6904@bix.com>
Organization: New York University
Newsgroups: comp.lang.ada
Date: 1997-05-10T00:00:00+00:00
List-Id: <comp.lang.ada>


Tom Moran said

<<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?>>

I don't understand, what is Image? The whole array? Of course you can
take the access of a packed array, but then the *array* needs to be
aliased, not its components. The components only need to be aliased
if you need the access of individual array elements, but clearly that
makes no sense if the elements are on arbitrary bit boundaries.

If what you want to do makes sense here, it can be done, please explain
more clearly what you are trying to do precisely!