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Date: Fri, 5 Sep 2014 13:40:29 -0700 (PDT)
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Subject: Re: Strange behavior
From: Laurent <daemon2@internet.lu>
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Xref: number.nntp.dca.giganews.com comp.lang.ada:188884
Date: 2014-09-05T13:40:29-07:00
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Am Freitag, 5. September 2014 22:34:17 UTC+2 schrieb Robert A Duff:
> Laurent writes:
>=20
>=20
>=20
> > with Ada.Text_IO;
>=20
> > procedure Two_Cooperating_Tasks is
>=20
> >   =20
>=20
> >    task type Simple_Task (Message : Character; How_Many : Positive);
>=20
> >   =20
>=20
> >    task body Simple_Task is
>=20
> >      =20
>=20
> >    begin -- Simple_Task
>=20
> >      =20
>=20
> >       for Count in 1 .. How_Many loop
>=20
> >          Ada.Text_IO.Put ("Hello from Task " & Message);
>=20
> >          Ada.Text_IO.New_Line;
>=20
> >          delay 0.1;
>=20
> >       end loop;
>=20
> >      =20
>=20
> >    end Simple_Task;
>=20
> >   =20
>=20
> >    Task_A : Simple_Task (Message =3D> 'A', How_Many =3D> 5);
>=20
> >    Task_B : Simple_Task (Message =3D> 'B', How_Many =3D> 5);
>=20
> >   =20
>=20
> > begin -- Two_Cooperating_Tasks
>=20
> >   =20
>=20
> >    null;
>=20
> >   =20
>=20
> > end Two_Cooperating_Tasks;
>=20
>=20
>=20
> The above causes erroneous execution, which means that anything can
>=20
> happen.  Two tasks are writing to standard output without proper
>=20
> synchronization.  (I would prefer the language defined each output
>=20
> call to be atomic, but it doesn't.  And that wouldn't help much in your
>=20
> example, because you have two calls, Put and New_Line.  You could
>=20
> combine those into a single Put_Line, by the way.)
>=20
>=20
>=20
> If you have only one processor, then the output shown in the text is
>=20
> likely to happen.  But you can't depend on that.  And anyway, nowadays
>=20
> you almost certainly have more than one processor, so the tasks are
>=20
> executing in parallel.
>=20
>=20
>=20
> Synchronization via delay statements is a wrong way to do.
>=20
>=20
>=20
> - Bob

Yes I understand that. The following examples in the book actually are abou=
t this problem and that's the introduction for the protected types. I was j=
ust 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 d=
ifferent.

Thanks to all for the enlightenment

Laurent