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From: Gautier write-only <gautier_niouzes@hotmail.com>
Newsgroups: comp.lang.ada
Subject: Q: Stopping a task running a long simulation
Date: Thu, 11 Mar 2010 05:08:24 -0800 (PST)
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Date: 2010-03-11T05:08:24-08:00
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Hello,
Being still a bit new to tasking, I have a question of how to
terminate correctly a certain kind of task.
I have a task type which is waiting for running a potentially long
simulation; it is kind of a "daemon".
(the rationale of using a task type is that I have several simulations
that can be run in parallel, hence an array of these tasks).
Now this simulation occurs within a GUI, where the user might want to
stop it.
Here is the task type I've defined:

  task type Simulation_type is
    entry Start;
    entry Run( [some parameters] );
    entry Stop;
  end;

  Simulation: array(PF_version) of Simulation_type;

  task body Simulation_type is
  begin
    accept Start;
    loop
      select
        accept Stop;
        exit;
      or
        accept Run( [some parameters] ) do
          -- some quick parameter passing
        end Run;
        Compute( [some parameters] );
      or
        delay 0.2; -- relax
      end select;
    end loop;
  end Simulation_type;

For aborting the simulation, I have tried this (happens when the main
GUI window is destroyed):
    for v in PF_version loop
      abort Daemons.Simulation(v);
    end loop;
but (as expected if I've understood Barnes' chapter correctly), the
task waits for 'Compute' to complete.
At least, it is what happens with GNAT under Windows: despite the
above, the GUI closes but the program somehow is still alive and CPU-
busy.

I have an alternative to that.
'Compute' has a generic 'Feedback' procedure for showing progress.
I could with that way give a Boolean, user_abort, to 'Feedback', and
'Compute' would stop when an ad-hoc exception is raised, and return
normally on its own.
Meanwhile, I could call the 'Stop' entry which could be soon be
activated, since Compute would stop quickly, and the daemon task would
be terminated, and I could close the shop properly.

Now my question: is it the right way of doing it (I'm sure it will
work), or is there something more straightforward ?

TIA

Gautier