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NNTP-Posting-Date: Wed, 05 Oct 2005 18:35:08 -0500
From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
References: <1128525722.605730.281980@g43g2000cwa.googlegroups.com>
 <87mzlnomca.fsf@ludovic-brenta.org>
Subject: Re: volatile vs aliased
Date: Wed, 5 Oct 2005 18:38:52 -0500
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Date: 2005-10-05T18:38:52-05:00
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"Ludovic Brenta" <ludovic@ludovic-brenta.org> wrote in message
news:87mzlnomca.fsf@ludovic-brenta.org...
...
>  In your situation, it seems that you want something like this
> (which, incidentally, I use very often):
>
> type T is ...;
>
> procedure Read_Variable (At_Address : in  System.Address;
>                          Into       : out T) is
>    V : aliased T;
>    -- suppress default initialisation, I know what I'm doing.
>    pragma Import (Ada, V);
>    for V'Address use At_Address;
> begin
>    -- perform my own explicit validation of the variable, perhaps
>    -- using 'Valid; then, copy into Into:
>    Into := V;
> end Read_Variable;

This seems like a good time to mention that I think the explicit use of
System.Address in Ada 95 and Ada 200Y code is usually a mistake. Since
pragma Convention can be used to ensure that general access types have the
appropriate representation, its rare that Address needs to be used for
interfacing. (There is only a handful of uses of Address in Claw, for
example.)

Moreover, when you *do* need to use it, Address_to_Access_Conversions is the
best way to convert it, not an overlay (which at best blocks optimizations
and at worst won't even work right).

type T is ...;

procedure Read_Variable (At_Address : in  System.Address;
                         Into       : out T) is
   package AAC is new System.Address_to_Access_Conversions (T);
   V : AAC.Object_Pointer := AAC.To_Pointer (At_Address);
begin
    -- perform my own explicit validation of the pointer's contents, perhaps
    -- using 'Valid; then, copy into Into:
    Into := V.all;
end Read_Variable;

But it is better still to declare an appropriate type and never use Address
in the first place:

type Pointer_T is access all T;
pragma Convention (C, Pointer_T);

Address clauses should be restricted to mapping to hardware, IMHO.

                          Randy.