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From: "Jean-Pierre Rosen" <rosen@adalog.fr>
Newsgroups: comp.lang.ada
Subject: Re: Multiple entry tasks
Date: Thu, 19 Apr 2001 10:17:58 +0200
Organization: Adalog
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Date: 2001-04-19T10:17:58+02:00
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"Marin David Condic" <marin.condic.auntie.spam@pacemicro.com> a �crit dans
le message news: 9bkevj$61k$1@nh.pace.co.uk...
> Sounds like you want some version of guarded entries in a select
statement.
> I have not attempted to do this before (kids: don't try this at home - we
> *are* trained professionals!), nor have I run this past a compiler (spank
me
> for this later if it doesn't work) but I *think* you want some version of
> the following:
>
> select
>     when (Entry2'Count <= 0) and (Entry1'Count <= 0) =>
>         accept Entry3 ;
>     when (Entry1'Count <= 0) =>
>         accept Entry2 ;
> or
>     accept Entry1;
> else
>     terminate;
> end select ;
>
This is wrong. Guards are evaluated only when the select is entered (unlike
POs), therefore if a task cancels its call (through timed entry call) while
the guards are being evaluated, you may end up in a situation of infinite
blocking.
The 'Count attribute should be used only with great care outside a PO. The
proper way of checking if a queue is empty is to use the else part of the
select statement. Therefore:

select
   accept Entry1;
else
   select
      accept Entry2;
   else
      select
         accept Entry1;
      or
         accept Entry2;
      or
         accept Entry3;
      or
         terminate;
      end select;
   end select;
end select;

Now, an exercise to see who is following. Those who attended an Adalog
training session are excluded from the competition :-)
1) Show that the sequence above exhibits a race condition
2) Show that the race condition is perfectly acceptable.

--
---------------------------------------------------------
           J-P. Rosen (rosen@adalog.fr)
Visit Adalog's web site at http://www.adalog.fr