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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Simulating OS semaphore behavior
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Date: Fri, 25 Aug 2006 17:09:47 +0200
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Date: 2006-08-25T17:09:47+02:00
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On 25 Aug 2006 08:00:48 -0700, REH wrote:

> Abstractly, I want to allow an arbitrary number tasks to wait for a
> particular event, and when the event occurs, wake all those tasks.
> Thereafter, tasks can again block waiting for the next occurrence.
> Something like:
> 
> protected type Event is
>   entry Wait;
>   procedure Signal;
> end Event;
> 
> Implementing the above for one task seems simple enough, but how would
> it been done for an arbitrary number such that the behavior is:
> 1. Wait's guard is initially false.
> 2. some tasks call Wait.
> 3. Signal is called.
> 4. Wait's guard becomes true.
> 5. all tasks currently queued on Wait are allowed to continue.
> 6. Wait's guard becomes false.

That looks like a classic automatic event for multiple tasks. Make Signal
an entry:

   protected body Event is
      entry Wait when Signal'Count > 0 is
      begin
         null;
      end Wait;
      entry Signal when Wait'Count = 0 is
      begin
         null;
      end Signal;
   end Event;

Signal is blocked until all waiting tasks get released. There is no race
condition because fresh attempts to Wait are blocked if a signal is
pending.

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