Re: Ravenscar vs selective wait

[Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org>]

On 03/23/2015 11:48 PM, Simon Wright wrote:
> Simon Wright <simon@pushface.org> writes:
> 
>> Is it possible to construct an analog of the selective wait using the
>> Ravenscar profile?
>>
>>    Turn_On_The_Lamp;
>>    Next := Clock + Milliseconds (500);
>>    select
>>       accept Reset do
>>          Next := Clock + Milliseconds (500);
>>       end Reset;
>>    or
>>       delay until Next;
>>       Turn_Off_The_Lamp;
>>    end select;
> 
> This is rather simplified from what I want to achieve, and I can think
> of an ugly solution for it.
> 
>    Turn_On_The_Lamp;
>    Next := Clock + Milliseconds (500);  -- Next is a shared variable
>    loop
>       delay until Next;
>       if Next <= Clock then
>          exit;
>       else
>          -- something else in the program has updated Next
>       end if;
>    end loop;
>    Turn_Off_The_Lamp;
> 
> But what if I need the next wake-up to be *earlier*?

Seems to me you could do what you originally asked about with 3 tasks and a PO:

Lamp task:

   Turn_On;
   Next := Clock + Interval;
   PO.Set_Expiration (Expiration_Time => Next);
   loop
      PO.Wait (Result => Wait_Result);
      case Wait_Result is
      when Reset =>
         Next := Clock + Interval;
         PO.Set_Expiration (Expiration_Time => Next);
      when Timeout =>
         Turn_Off;
         exit;
      end case;
   end loop;

PO:

   procedure Set_Expiration (Expiration_Time : in Time);
   procedure Delay_Expired;
   procedure Reset_Expiration;
   function Expiration return Time;
   entry Wait (Result : out Wait_Result_ID);

The delay task would poll function Expiration and compare it to Clock, calling
Delay_Expired when Expiration >= Clock.

Some 3rd task would call Reset_Expiration when the expiration should be extended.

With some more complication this could also allow resetting to an earlier time.

-- 
Jeff Carter
"What's the amount of the insult?"
Never Give a Sucker an Even Break
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