From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00
	autolearn=unavailable autolearn_force=no version=3.4.4
Path: 
 eternal-september.org!reader01.eternal-september.org!reader02.eternal-september.org!news.eternal-september.org!mx02.eternal-september.org!.POSTED!not-for-mail
From: Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org>
Newsgroups: comp.lang.ada
Subject: Re: Ravenscar vs selective wait
Date: Tue, 24 Mar 2015 00:25:55 -0700
Organization: Also freenews.netfront.net; news.tornevall.net;
 news.eternal-september.org
Message-ID: <mer3gh$lph$1@dont-email.me>
References: <lyoanjv0g6.fsf@pushface.org> <lyk2y6vqkk.fsf@pushface.org>
Mime-Version: 1.0
Content-Type: text/plain; charset=windows-1252
Content-Transfer-Encoding: 7bit
Injection-Date: Tue, 24 Mar 2015 07:25:07 +0000 (UTC)
Injection-Info: mx02.eternal-september.org;
 posting-host="3f77efd256cd7097938236a53a6861ee";
	logging-data="22321"; mail-complaints-to="abuse@eternal-september.org";
	posting-account="U2FsdGVkX18Z7FVM35FE6s46ZsicNuXuQDdYRyEYV/w="
User-Agent: Mozilla/5.0 (X11; Linux x86_64;
 rv:31.0) Gecko/20100101 Thunderbird/31.5.0
In-Reply-To: <lyk2y6vqkk.fsf@pushface.org>
Cancel-Lock: sha1:NLvaSufsyJbt8QSQdSs453JkKro=
Xref: news.eternal-september.org comp.lang.ada:25230
Date: 2015-03-24T00:25:55-07:00
List-Id: <comp.lang.ada>

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
104