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From: dbrown@vigra.com (David Brown)
Subject: Generic address to access conversion.
Date: 1996/12/26
Message-ID: <oszpz09aor.fsf@ted.vigra.com>#1/1
X-Deja-AN: 206101805
sender: dbrown@ted.vigra.com
organization: VisiCom Laboratories, Inc.
newsgroups: comp.lang.ada
Date: 1996-12-26T00:00:00+00:00
List-Id: <comp.lang.ada>


I am implementing a specialized memory manager that manages a small
region of non-virtual wired memory.  This manager would almost use
ordinary pool semantics except that the memory manager needs to be
able to move regions of memory around.  To use this, I expect
something like:

   Fixed_Memory.Lock (...);
   Accessor := ....Accessor (Handle);
   ...
   Fixed_Memory.Unlock (...);

where Accessor is an access variable that will be valid until the
memory is unlocked.

I have implemented the manager but am having difficulty coming up with
a way of implementing (or even specifying) the Accessor.  The memory
manager currently keeps track of an offset into

   type Byte is mod 2 ** System.Storage_Unit;
   for Byte'Size use System.Storage_Unit;
   type Byte_Array is array (Offset range <>) of Byte;
   pragma Pack (Byte_Array);

for the current location of the region.  The problem I am having is
converting this to a given user's access type.  I can make the code
work by using an Unchecked_Conversion as well as
System.Address_To_Access_Conversions, however it only works for
discriminated types, and this I only suspect is because my compiler
(GNAT 3.07) uses simple pointers for access variables.

Thanks,
David Brown
dbrown@vigra.com