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From: "REH" <spamjunk@stny.rr.com>
Newsgroups: comp.lang.ada
Subject: Re: volatile vs aliased
Date: 5 Oct 2005 11:39:26 -0700
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Date: 2005-10-05T11:39:26-07:00
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Ludovic Brenta wrote:
> I like this warning given by Apex; I think it would be nice if GNAT
> would also warn in this situation.
>
> pragma Volatile (Variable) says the compiler must not optimise away
> any reads or writes to that variable, and that it may not add extra
> reads or writes beyond those you explicitly request in your program
> text.  You want to use that for hardware registers, where a "read"
> operation may have a side effect such as changing the device's state.
>
> Whenever you access a variable through an address value, the variable
> is aliased, so you should say that explicitly.  Otherwise, the
> compiler may be tempted to place the variable in a processor register,
> which has no address.
>
> In addition, pragma Import (Ada, Variable) says that the compiler may
> not insert default initialisations for that variable, or assume
> anything about the value of the variable, before you write to it.  But
> the compiler may do optimisations, because it is allowed to assume
> that no side effects occur when you read from or write to the variable
> (that is, side effects not explicitly requested from the program
> text).  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;
>
> --
> Ludovic Brenta.

VERY helpful!  Thank you so much.

I'm dealing with a situation where I have to do symbol table lookups
from the OS which return an address to procedure or function.  This is
done in several places in the code.  The reason is I have to support
different hardware dynamically.  For example, if I am on board A, I
call Foo.  The comparible subprogram on B is Bar.  So, I cannot just
make pragma exports for them both as one will be undefined.  Anyways,
to make it easier, I wanted to make a generic which just returned an
access type without having to do the pragma Import and for X'address in
every case.  Then calling the subprogram is simple.  My first pass is:

  generic
    type Data_Type is private;
    type Data_Ptr is access all Data_Type;
  function Address_To_Access(Addr : System.Address) return Data_Ptr;

  function Address_To_Access(Addr : System.Address) return Data_Ptr is
  begin
    if Addr = System.Null_Address then
      return null;
    else
      declare
        Data: aliased Data_Type;
        for Data'Address use Addr;
      begin
        return Data'Unchecked_Access;
      end;
    end if;
  end Address_To_Access;


Two questions:
1) Is this even a smart thing to do?
2) How would I define a generic like the above for an subprogram with
an unknown signature?

Thanks again for the help.

REH