From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.5-pre1 (2020-06-20) on
	ip-172-31-74-118.ec2.internal
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=3.0 tests=BAYES_00 autolearn=ham
	autolearn_force=no version=3.4.5-pre1
Date: 31 Aug 93 20:36:04 GMT
From: dog.ee.lbl.gov!agate!howland.reston.ans.net!math.ohio-state.edu!magnus.ac
s.ohio-state.edu!csn!news.den.mmc.com!iplmail!alcyone!rgilbert@ucbvax.Berkeley.
EDU  (Bob Gilbert)
Subject: Re: Data Overlays
Message-ID: <1993Aug31.203604.29444@iplmail.orl.mmc.com>
List-Id: <comp.lang.ada>

In article 29091@software.org, smithd@software.org (Doug Smith) writes:
>In article <1993Aug18.125540.1393@iplmail.orl.mmc.com> rgilbert@orl.mmc.com wr
ites:
>>
>>As far as the technique of:
>>
>>       X : TYPE_1;
>>       Y : TYPE_2;
>>       for Y use at X'address;
>>
>>The LRM 13.5 states that address clauses should not be used to achieve overla
ys
>>of objects...  Any program using address clauses to achieve such effects is
>>erroneous.  What is the rationale for this?
>>
>> [stuff deleted]
>>
>>                             Bob
>
>
>And...if you use this technique, don't forget what happens when the
>variable is elaborated:
>
>   procedure P(X : Some_Type) is
>      Y : Some_Pointer_Type; -- or record with initialization
>      for Y use at X'Address;
>   begin
>      -- Y has likely been initialized to null, and X has probably
>      -- been changed.  But, since this program is erroneous,
>      -- it is fine for the compiler to do whatever it wants ;^)
>   end;
>   
>Doug
>smithd@software.org

Absolutely, and somewhat unrelated to data overlays...

   I have experienced this and it can be a problem when attempting to
   map to read-only (VME bus system) addresses.  Since most compilers
   will attempt to initialize objects using representation clauses,
   during program elaboration the resulting write to the address
   results in a VME bus error.  The solution is to use access types
   and assign the address to the access object using Unchecked_Conversion.
   This BTW, is similar to the other method I have used for implementing
   data overlays.

                     Bob