Re: Basic question about select

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Wed, 28 Apr 2010 18:16:44 +0000 (UTC), John Wilkinson wrote:

> select
>     	PID.Call;  --an entry call to task PID
>     	Status := P;
> then abort
>     	delay 10.0;
>     	Status := Q;
> end select;
> 
> Is the above valid Ada code? It's been handed to me by someone, I 
> don't really know much about Ada, and I'm supposed to understand 
> what it does. I can't find any information on "abort" or even 
> "then".

See ARM 9.7.4 Asynchronous Transfer of Control

> If it's valid, what does it do?

It waits 10s and then assigns Q to Status. But if within these 10s + the
time needed to assign Status, the task PID accepts Call, then the sequence
introduced by "then abort" is aborted, Call is performed, and finally, P is
assigned to Status.

Note that the recommended way to do this is the timed entry call ARM 9.7.2:

select
   PID.Call;  -- call to task PID
   Status := P;
or delay 10.0; -- wait to longer than 10s
   Status := Q;
end select;

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de