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From: ncohen@watson.ibm.com (Norman H. Cohen)
Subject: Re: ADA task
Date: 1996/09/13
Message-ID: <51cjp0$q7k@watnews1.watson.ibm.com>#1/1
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distribution: world
references: <5174qu$o1p@nr1.ottawa.istar.net>
organization: IBM T.J. Watson Research Center
reply-to: ncohen@watson.ibm.com
newsgroups: comp.lang.ada
Date: 1996-09-13T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <5174qu$o1p@nr1.ottawa.istar.net>, "Roumen Roupski"
<Roumen.Roupski@Pika.Ca> writes: 

|> 1. How can I implement a task running at regular intervals, for example
|> every 50ms +/- 5ms. The task activation must be guaranteed too.

By a "task", you seem to mean a piece of code to do certain work one
time. At regular intervals, the task is "run" to accomplish this work.

In Ada, a "task" is a thread of control, and the typical approach to
periodic processing is for this thread to remain in existence throughout
the program, executing a loop in which it spends most of its time
sleeping (thus not consuming CPU) and wakes up at the right times to do
its work: 

   task Cyclic_Processor;

   task body Cyclic_Processor is
      Next_Wakeup : Calendar.Time := Calendar.Clock;
      Interval    : constant Duration := 0.050;
   begin
      loop
         delay until Next_Wakeup;
         Do_Some_Work;
         Next_Wakeup := Next_Wakeup + Interval;
      end loop;
   end Cyclic_Processor;

The "delay until" statement is a new feature in Ada 95.  In Ada 83 you
would write

   delay Next_Wakeup - Calendar.Clock;

which delays for the specified amount of time (in this case, the time
from now until Next_Wakeup).

--
Norman H. Cohen    ncohen@watson.ibm.com