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Date: Sat, 01 Apr 2006 09:33:33 -0500
From: Jeffrey Creem <jeff@thecreems.com>
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Subject: Re: Any way of persuading GNAT/GCC to implement a true overlay and
 not a pointer?
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Date: 2006-04-01T09:33:33-05:00
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Doobs wrote:
> I was under the impression that code of the following form :
> 
> X : <Some Type>;
> Y : <Some Type>;
> for Y'Address use X'Address;
> 
> would result in an overlay in the resulting code.   However on a project 
> where external test equipment showed that the contents of  X were NOT the 
> same as Y I investigated further.  I checked on both a Windows host version 
> of GCC (3.4.2) and a target cross compiler (GCC 3.4.4) and on both the 
> construct above is actually implemented using indirection (a pointer).  The 
> map file and symbol table show that irrespective of the data type and size 
> of Y it is always implemented as a pointer to X.  Although this appears 
> semantially identical to an overlay as far as the progam is concerned it is 
> NOT identical as far as an external observer is concerned.    I have a bit 
> packed record which I wanted to return as an array of longwords to pretty 
> dumb test equipment. The easiest way *would* be to overlay a longword 
> array - however this would not appear to be possible.....
>  
> 
> 

Try applying pragma volatile to both declarations. Also for completeness 
you should add a pragma import to Y to avoid any default initialization.

There is an AI that talks (to some exent) to this issue.

I am not entirely sure that the volatile will fix your problem because I 
  don't quite get what the resulting behaviour is that you are 
seeing/not seeing.