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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Strange behavior
Date: Fri, 05 Sep 2014 16:34:41 -0400
Organization: The World Public Access UNIX, Brookline, MA
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Date: 2014-09-05T16:34:41-04:00
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Laurent <daemon2@internet.lu> 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