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From: Jean-Pierre Rosen <rosen@adalog.fr>
Newsgroups: comp.lang.ada
Subject: Re: Simulating OS semaphore behavior
Date: Fri, 25 Aug 2006 19:31:53 +0200
Organization: Adalog
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Date: 2006-08-25T19:31:53+02:00
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Dmitry A. Kazakov a �crit :
> 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.
> 

However, this will cause the signaling task to wait until some task 
calls wait. Generally, if there is no waiting task, Signal should do 
nothing. The classical barrier is as follows:

protected type Barrier is
	entry Wait;
	procedure Signal;
	function Count return Natural;
private
	Arrived : Boolean := False;
end Barrier;

protected body Barrieris
	entry Wait when Arrived is
	begin
		if Wait'COUNT = 0 then
			Arrived := False;
		end if;
	end Wait;

	procedure Signal is
	begin
		if Wait'COUNT > 0 then
			Arrived := True;
		end if;
	end Signal;

	function Count return Natural is
	begin
		return Wait'COUNT;
	end Count;
end Barrier;
-- 
---------------------------------------------------------
            J-P. Rosen (rosen@adalog.fr)
Visit Adalog's web site at http://www.adalog.fr