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From: Ludovic Brenta <ludovic@ludovic-brenta.org>
Newsgroups: comp.lang.ada
Subject: Re: volatile vs aliased
References: <1128525722.605730.281980@g43g2000cwa.googlegroups.com>
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Date: Wed, 05 Oct 2005 21:46:14 +0200
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Date: 2005-10-05T21:46:14+02:00
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"REH" <spamjunk@stny.rr.com> writes:
> 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?

Why do you have to use a generic?  Why not just:

procedure Call_Procedure (At_Address : in System.Address) is
   procedure Proc;
   for Proc'Address use At_Address;
begin
   Proc;
end Call_Procedure;

The above effectively converts from an address to an
access-to-procedure, and calls the procedure.  If what you want is
convert from an address to an access-to-data, you should probably use
Ada.Address_To_Access_Conversions.

> 2) How would I define a generic like the above for an subprogram with
> an unknown signature?

I don't know how you could call a procedure with unknown parameters.
Or perhaps you mean with one known parameter which is the address of
a record containing arbitrary parameters?

generic
   type Parameter_Type is limited private; -- [1]
procedure Call_Procedure (At_Address : in System.Address;
                          Params     : in Parameter_Type);

procedure Call_Procedure (At_Address : in System.Address;
                          Params     : in Parameter_Type) is
   procedure Proc (Params : in System.Address);
   for Proc'Address use At_Address;
begin
   Proc (Params'Address);
end Call_Procedure;

Is this close to what you want?

[1] I think the type has to be declared limited, so that it is always
passed by reference to Call_Procedure.  This guarantees that the
parameter has an address, and is not in a register.  I have not
compiled or tried this code, so please take it with due care.

-- 
Ludovic Brenta.