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From: "Jeffrey R. Carter" <spam.jrcarter.not@acm.nospam.org>
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Subject: Re: Tasking problem (delay)
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Date: 2007-08-15T18:32:33+00:00
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Gerd wrote:
> 
> WD_Delay: Duaration := 1000.0; -- so do not trigger on start

Assuming this is "Duration" ...

> task body startup is
> begin
>   ...
>  WD_Delay := 1.0;
>  ...
> end startup;
> 
> task body worker is
> begin
>   loop
>     select
>        accept Data (...)
>    or
>      delay WD_Timeout;

... and this is "WD_Delay"

>      trigger_WD;
>     end select;
>   end loop;
> end worker;

The expression in the delay statement (in this case "WD_Delay") is 
evaluated once upon reaching the select statement. So this will wait for 
a call to Data, or for the value of WD_Delay at the time the select 
statement was reached, whichever comes 1st.

Using unprotected shared variables is not a good idea anyway. I'd 
suggest something more like:

task Worker is
    entry Startup_Complete;
    entry Data (...);
end Worker;

task body Worker is
    WD_Delay : constant := 1.0;
begin -- Worker
    select
       accept Startup_Complete;
    or
       terminate;
    end select;

    Forever : loop
       select
          accept Data ...
       or
          delay WD_Delay;

          Trigger_WD;
       end select;
    end loop Forever;
end Worker;

If Worker needs to be able to accept data before startup is complete, 
this would become

task body Worker is
    WD_Delay : Duration := 1_000.0;
begin -- Worker
    Forever : loop
       select
          accept Data ...
       or
          accept Startup_Complete;

          WD_Delay := 1.0;
       or
          delay WD_Delay;

          Trigger_WD;
       end select;
    end loop Forever;
end Worker;

-- 
Jeff Carter
"Clear? Why, a 4-yr-old child could understand this
report. Run out and find me a 4-yr-old child. I can't
make head or tail out of it."
Duck Soup
94