Re: Strange behavior

[Laurent <daemon2@internet.lu>]

Am Freitag, 5. September 2014 22:34:17 UTC+2 schrieb Robert A Duff:
> Laurent writes:
> 
> 
> 
> > with Ada.Text_IO;
> 
> > procedure Two_Cooperating_Tasks is
> 
> >    
> 
> >    task type Simple_Task (Message : Character; How_Many : Positive);
> 
> >    
> 
> >    task body Simple_Task is
> 
> >       
> 
> >    begin -- Simple_Task
> 
> >       
> 
> >       for Count in 1 .. How_Many loop
> 
> >          Ada.Text_IO.Put ("Hello from Task " & Message);
> 
> >          Ada.Text_IO.New_Line;
> 
> >          delay 0.1;
> 
> >       end loop;
> 
> >       
> 
> >    end Simple_Task;
> 
> >    
> 
> >    Task_A : Simple_Task (Message => 'A', How_Many => 5);
> 
> >    Task_B : Simple_Task (Message => 'B', How_Many => 5);
> 
> >    
> 
> > begin -- Two_Cooperating_Tasks
> 
> >    
> 
> >    null;
> 
> >    
> 
> > end Two_Cooperating_Tasks;
> 
> 
> 
> The above causes erroneous execution, which means that anything can
> 
> happen.  Two tasks are writing to standard output without proper
> 
> synchronization.  (I would prefer the language defined each output
> 
> call to be atomic, but it doesn't.  And that wouldn't help much in your
> 
> example, because you have two calls, Put and New_Line.  You could
> 
> combine those into a single Put_Line, by the way.)
> 
> 
> 
> If you have only one processor, then the output shown in the text is
> 
> likely to happen.  But you can't depend on that.  And anyway, nowadays
> 
> you almost certainly have more than one processor, so the tasks are
> 
> executing in parallel.
> 
> 
> 
> Synchronization via delay statements is a wrong way to do.
> 
> 
> 
> - Bob

Yes I understand that. The following examples in the book actually are about this problem and that's the introduction for the protected types. I was just astonished that a so simple program can already behave so unpredictable. Ok the book was written in 1998 so at this time the result was probably different.

Thanks to all for the enlightenment

Laurent