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From: "David C. Hoos, Sr." <david.c.hoos.sr@ada95.com>
Subject: Re: System.Address_to_Access_Conversions
Date: 1998/07/14
Message-ID: <6ofqvs$alm@hacgate2.hac.com>#1/1
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References: <6odddl$k94$1@nnrp1.dejanews.com> <35AB9C59.74E529E0@magic.fr>
 <6ofn8e$5ff$1@nnrp1.dejanews.com>
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Organization: Hughes Aircraft Company
Newsgroups: comp.lang.ada
Date: 1998-07-14T00:00:00+00:00
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jsanchor@my-dejanews.com wrote in message
<6ofn8e$5ff$1@nnrp1.dejanews.com>...
>> No, the package is correct. I want to convert a system address to an
access
>type.--


With all due respect, sir you are wrong.  Take a look at the package
specification at the end of this message.
Despite the package name, the package exports functions which convert both
ways -- i.e., To_Address, and To_Pointer.
Now, look at the function which returns an access value. The parameter of
that function is of type System.Address, and the return type is
Object_Pointer which is in turn declared as being an access to the generic
formal type Object.  Therefore, the type with which the package must be
instantiated is the type of the object which you wish to locate at your
specified address location -- not an access type or an address type, but the
actual object type.

However, there is another aspect of your original example which is a
problem.  Typically you will want to use a record representation clause to
make the memory layout correspond to the harware requirements.  Using a
tagged type in such a sitiuation is not a good idea, because the tag is a
"hidden" data field within the record which certainly will have no
correspondence with the hardware-required memory layout.

David C. Hoos, Sr.

-- begin package System.Address_To_Access_Conversions --
generic
  type Object(<>) is limited private;
package System.Address_To_Access_Conversions is
  pragma Preelaborate(Address_To_Access_Conversions);
  type Object_Pointer is access all Object;
  function To_Pointer(Value : Address) return Object_Pointer;
  function To_Address(Value : Object_Pointer) return Address;
  pragma Convention(Intrinsic, To_Pointer);
  pragma Convention(Intrinsic, To_Address);
end System.Address_To_Access_Conversions;
-- end package System.Address_To_Access_Conversions --