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References: <LIednWDbPNgXGbPZRVnysQ@pipex.net>
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Subject: Re: Any way of persuading GNAT/GCC to implement a true overlay and
 not a pointer?
Date: Sat, 1 Apr 2006 22:33:39 +0100
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Date: 2006-04-01T22:33:39+01:00
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>>>>>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.
>>>>
>>>>Could you show some more code?  Probably initialization is causing
>>>>your problems, which can be fixed with a pragma Import.
>>>
>>>The following is an example of the problem:
>>>
>>>package TestPackage
>>>is
>>>
>>>  type MyRecordType is
>>>  record
>>>     element1 : Positive;
>>>     element2 : Positive;
>>>  end record;
>>>
>>>  type MyArrayType is array(1..2) of Positive;
>>>
>>>  myX : MyRecordType;
>>>  pragma volatile(myX);
>>>  myY : MyArrayType;
>>>  pragma volatile(myY);
>>>  for myY'Address use myX'Address;
>>>
>>>
>>>end TestPackage;
>>>
>>>The following fragments of the resulting MAP file (GCC 3.4.2 mingw32) 
>>>show the problem...
>>>
>>>.data          0x00472b00       0x10 ./testpackage.o
>>>               0x00472b00                testpackage__myy
>>>.data          0x00472b10       0x10 ./nextpackage.o
>>>
>>><..... later in the file...>
>>>
>>>COMMON         0x004a2770       0x20 ./testpackage.o
>>>               0x004a2770                testpackage__myx
>>>               0x004a2780                testpackage_E
>>>
>>>
>>>Clearly myX and myY do NOT share the same address.  I am also puzzled as 
>>>to why the variables have been put in differen memory sections...
>>>
>>
>> Note that including a pragma import did NOT solve the problem - the code 
>> looked like this
>>
>>    myX : MyRecordType;
>>    pragma volatile(myX);
>>    myY : MyArrayType;
>>    pragma volatile(myY);
>>    for myY'Address use myX'Address;
>>    pragma Import (Ada, myY);
>>
>>
>> But the map file exhibited the same problem as before.
>
> Try this.
>
> package TestPackage
> is
>
>    type MyRecordType is
>    record
>       element1 : Positive;
>       element2 : Positive;
>    end record;
>
>    type MyArrayType is array(1..2) of Positive;
>
>    myX : MyRecordType;
>    pragma Volatile(Myx);
>    Pragma Export(Ada, Myx, "MyX");
>
>    myY : MyArrayType;
>    pragma Volatile(Myy);
>    pragma Import(Ada, Myy, "MyX");
>
> end TestPackage;
>
> Not sure exactly what your end desired result is but assuming you really 
> need to do this then perhaps this works.

Interestingly with this approach any reference to myY disappears from the 
Map file and debug symbol table.
myY then ceases to be a useful symbol for debugging purposes external to the 
program  (which was the original reason to have the overlay!).....