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From: Tarek Ghaleb <invalid@invalid.org>
Newsgroups: comp.lang.ada
Subject: Re: System.Address to Access to function/procedure conversion
Date: Mon, 29 Jul 2013 07:06:06 +0000 (UTC)
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Date: 2013-07-29T07:06:06+00:00
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On 2013-07-29, Shark8 <onewingedshark@gmail.com> wrote:
> On Sunday, July 28, 2013 4:54:27 PM UTC-6, Tarek Ghaleb wrote:
>> On 2013-07-28, Shark8 wrote:
>> 
>> >> I agree completely. Using C convention is the way to do it. But is it
>> >> *possible* to call a function using its address? (even if not a such a
>> >> great idea)
>> 
>> > Yes. It's actually absurdly easy. Here's a sample of how to do it in
>> > Ada 2012, the same will work in earliet versions of Ada using
>> > Pargmas... all the way back to 83, IIUC.
>
> Oops; a little overzealous there.
> You have to use attribute definition clauses, which IIRC came in Ada 95.
> Below compiles and runs in GNAT using Ada83-mode:
>
> --  Pragma Ada_2012;
> Pragma Ada_83;
> Pragma Assertion_Policy( Check );
>
> With
> Text_IO,
> System;
>
> Procedure Test is
>
> Begin
>     Text_IO.Put_Line("Starting Test:");
>
>
>     ACTUAL_TEST:
>     declare
>
> 	-- Note: Pragma CONVENTION is non-standard.
> 	Procedure K;
> 	Pragma Convention( C, K );
> 	
> 	Procedure K is
> 	begin
> 	    Text_IO.Put_Line( "K was called" );
> 	end K;
> 	
> 	Procedure Execute( Addr : System.Address ) is
> 	    -- Note: Pragma IMPORT is non-standard.
> 	    Procedure Stub;
> 	    Pragma Import( C, Stub );
>
> 	    -- Note: The Ada 83 LRM, 13.5 (Address Clauses) gives the following:
> 	    --         for CONTROL use at 16#0020#; -- assuming that
> 	    --         SYSTEM.ADDRESS is an integer
> 	    --   with the following warning:
> 	    --    Address clauses should not be used to achieve overlays of
> 	    --    objects or overlays of program units. Nor should a given
> 	    --    interrupt be linked to more than one entry. Any program using
> 	    --    address clauses to achieve such effects is erroneous.      
>
> 	    Use Text_IO;
> 	Begin
> 	    Put_Line( "Executing:" );
> 	    Put( ASCII.HT );
> 	    Stub;
> 	End Execute;
> 	
>     begin
> 	Execute( K'Address );
>     end ACTUAL_TEST;
>     
>     
>     Text_IO.Put_Line("Testing complete.");
> End Test;

That's an _exhaustively_ thorough answer :-))

Actually, I've used 

  For A'Address Use B'Address;

before, but never thought of using it for a subprogram: the way you
showed in the example above.

Tarek.

-- 
Rotten wood cannot be carved. -- Confucius, "Analects", Book 5, Ch. 9

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