Re: Simulating OS semaphore behavior

["REH" <me@you.com>]

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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:1gx5jdfxrewj6.19ufu2gqaakge.dlg@40tude.net...
> On Fri, 25 Aug 2006 19:31:53 +0200, Jean-Pierre Rosen wrote:
>
>> 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.
>
> Why? The Signal's barrier is open when the Wait's queue is empty.
>
> Surely there are subtleties with the above:
>
> 1. The same task might get the same event twice if it anew queues itself 
> to
> Wait before emptying the queue.

I want to avoid this.

How about:

protected type Event is
    entry Wait;
    procedure Signal;
private:
    Arrived : Boolean := False;
    entry Wait_More;
end Event;

protected body Event is
    entry Wait when not Arrived is
        requeue Wait_More;
    end Wait;

    procedure Signal is
    begin
        Arrived := True;
    end Signal;

    entry Wait_More when Arrived is
        Arrived := Wait_More'Count > 0;
    end Wait_More;
end Event;