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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Freezing a task
Date: Fri, 18 Nov 2011 09:47:58 +0100
Organization: cbb software GmbH
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Date: 2011-11-18T09:47:58+01:00
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On Thu, 17 Nov 2011 17:23:55 -0800 (PST), Rego, P. wrote:

>>    Paused : Boolean := False;
>> begin
>>    loop
>>       if Paused then
>>          accept Release;
>>          Paused := False;
>>       else
>>          select
>>             accept Pause;
>>             Paused := True;
>>          else
>>             ... -- Do stuff
>>          end select;
>>       end if;
>>    end loop;
> 
> So I can use
> 
> Paused : Boolean := False;
> begin
>     accept Start;
>     loop
>        if Paused then
>           accept Release;
>           Paused := False;
>        else
>           select
>              accept Pause;
>              Paused := True;
>           else
>              ... -- Do stuff
>           end select;
>        end if;
>     end loop;
> 
> and if I would want to add an abort entry? What should I do?

Note that the above has a deadlock when you call to Pause or Release twice.
It is easy to correct, but a protected object solution would be more
robust. See the Jeffrey's response, however, I would use an exception
rather than output parameter:

Pending_Abort : exception;
protected type Control is
   procedure Process;
   procedure Pause;
   procedure Stop;
   entry Get;
private
   Paused : Boolean := False;
   Exiting : Boolean := False;
end Control;

protected body Control is
   procedure Process is
   begin
       Paused := False;
   end Process;
   procedure Pause is
   begin
       Paused := True;
   end Pause;
   procedure Stop is
       Exiting := True;
   end Stop;
   entry Get when not Paused or Exiting is
   begin
       if Exiting then
         raise Pending_Abort;
      end if;
   end Get;
private
   Paused : Boolean := False;
   Exiting : Boolean := False;
end Control;

task body T is
begin
    loop
       Control_Instance.Get;
       ... -- Do stuff
       Control_Instance.Get;
       ... -- Do other stuff
       Control_Instance.Get;
       ... -- Do yet another stuff
    end loop;
exception
   when Pending_Abort =>
        null;
end T;

You don't need Start entry of the task. Just make Paused initially true.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de