Question

How can I freeze a task?

I mean, if I have a task

task body My_Task is
begin 
  accept Start;
  loop
    Put ("1");
    Put ("2");
    Put ("3");
    ...
    Put ("n");
  end loop;
end My_Task;

is there a way that I can “freeze” the task in its current state? If, for instance, the execution finished executing Put ("2");, how can I freeze it and later I can turn it to continue? I want to provoque a freeze from outside the task, and also from outside, order it to continue.

Update

I could sure implement, if I had the spec like

type State_Type is
  (RUN,
   FROZEN);

task type My_Task (State : State_Type) is
   entry Start;
end My_Task;

the body

task body My_Task is
begin 
  accept Start;
  loop
    Put ("1");
    Put ("2");
    Put ("3");
    ...
    Put ("n");

    loop 
     if State = RUN then exit; end if;
    end loop;
  end loop;
end My_Task;

but it would not be the case because I had to wait for the nth Put instruction line (i.e., the task would not be actually frozen, because the inside loop would be running).

Answer 1

You can use a “selective accept” to allow your task to be interrupted by an external caller (another task or the main program). You can’t (easily) interrupt a task at an arbitrary point in its execution; instead, the task itself needs to determine when it will accept entry calls.

So you’ll probably want to replace your sequence

Put("1");
Put("2");
Put("3");
...
Put("n");

by a loop that calls Put once on each iteration, with a select statement that’s executed each time.

Here’s a demo program that I just threw together. The counter increments once per second (approximately). The counter value is printed if Running is true; otherwise the loop proceeds silently. The main program alternatively calls Pause and Resume every 3 seconds.

The else clause in the select statement cause it to continue executing if neither entry has been called.

with Ada.Text_IO; use Ada.Text_IO;
procedure T is

    task Looper is
        entry Pause;
        entry Resume;
    end Looper;

    task body Looper is
        Running: Boolean := True;
        Count: Integer := 0;
    begin
        loop
            select
                accept Pause do
                    Running := False;
                end;
            or
                accept Resume do
                    Running := True;
                end;
            else
                null;
            end select;
            delay 1.0;
            if Running then
                Put_Line("Running, Count = " & Integer'Image(Count));
            end if;
            Count := Count + 1;
        end loop;
    end Looper;

begin -- T
    loop
        delay 3.0;
        Looper.Pause;
        delay 3.0;
        Looper.Resume;
    end loop;
end T;

There may be a more elegant approach than this. It’s been a long time since I’ve really used Ada.