Re: System.Address to Access to function/procedure conversion

[Shark8 <onewingedshark@gmail.com>]

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.      
	    --
	    --  In Ada 95 we get propper Attribute Definition Clauses and so
	    --  we would use the following:
	    --    For Stub'Address Use Addr;
	    For Stub use at Addr;

	    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;