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From: "John B. Matthews" <nospam@nospam.invalid>
Newsgroups: comp.lang.ada
Subject: Re: Barrier re-evaluation issue with GNAT 4.3.2
Date: Wed, 30 Sep 2009 11:59:09 -0400
Organization: The Wasteland
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Date: 2009-09-30T11:59:09-04:00
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In article <h9vp0g$1kn$1@news.eternal-september.org>,
 Reto Buerki <reet@codelabs.ch> wrote:

> John B. Matthews wrote:
> > Using Pragma Detect_Blocking, as suggested by Bob Duff, will tell 
> > you if the compiler's warning is actualized at run-time. As this is 
> > a work-around and your original code seems correct, you might 
> > arrange things so that the original can be restored easily upon 
> > upgrade or port.
> 
> The code runs correctly even though Program_Error is raised when
> compiled with the Detect_Blocking pragma.
> 
> I think it's good enough as a workaround, we will upgrade to GNAT 
> 4.4.x as soon as possible.

Excellent, although upgrading has it's own perils. :-)

Another alternative, suggested by Dmitry A. Kazakov, is to replace entry 
Wait with procedure Wait, which obviates the need for Signal: 

with Ada.Text_IO;
with Alarm_Clock;

procedure Main is
   My_Alarm : Alarm_Clock.Alarm_Type;
begin
   Ada.Text_IO.Put_Line ("Setting alarm ...");
   My_Alarm.Set;

   Ada.Text_IO.Put_Line ("Waiting for alarm ...");
   My_Alarm.Wait;

   Ada.Text_IO.Put_Line ("ALARM!!");
end Main;

with Ada.Real_Time.Timing_Events;

package Alarm_Clock is

   protected type Alarm_Type is
      procedure Set;
      procedure Wait;
   end Alarm_Type;

end Alarm_Clock;

package body Alarm_Clock is

   use Ada.Real_Time;
   Timer : Timing_Events.Timing_Event;
   Fired : Boolean := False;

   protected body Alarm_Type is

      procedure Wakeup(Event : in out Timing_Events.Timing_Event) is
      begin
         Fired := True;
      end Wakeup;

      procedure Set is
      begin
         Timer.Set_Handler(In_Time => Seconds (2),
                           Handler => Wakeup'Access);
      end Set;

      procedure Wait is
      begin
         loop
            exit when Fired;
         end loop;
      end Wait;

   end Alarm_Type;

end Alarm_Clock;

> Thanks for all your help!

I enjoyed the opportunity to examine this interesting question.

-- 
John B. Matthews
trashgod at gmail dot com
<http://sites.google.com/site/drjohnbmatthews>