Re: [Boost-users] [thread] Condition::notify_one, wait

From: Anthony Williams (anthony_w.geo_at_[hidden])
Date: 2008-04-08 10:54:35

Anteru <newsgroups_at_[hidden]> writes:

> The documentation of Condition::notify_one says it unblocks one of the 
> threads waiting currently on the condition -- what happens if I have 
> several threads waiting and I call notify_one twice?

If there are multiple threads waiting, the first call to notify_one() should
wake one, and the second call should wake another.

> It seems that on Windows/x64/VC9, only on thread wakes up if notify_one 
> is called twice in a row. Is this right?

That would be incorrect behaviour if there really is two threads waiting.

> Second, wait says: "The thread will unblock when notified by a call to 
> this->notify_one()  or this->notify_all(), or spuriously". Is it likely 
> that a condition unblocks spuriously? After all, I'd expect to unblock 
> only after calling notify_* and not "by accident". Is there any way to 
> get diagnostic output when a wait has been called spuriously?

It is not possible to detect a spurious wake: if it was, then the
implementation would resume waiting. However, spurious wakes should be rare
occurrences.

> The problem I'm having at the moment is: I have PIMPLed Boost.Thread 
> behind some simple wrappers, and I'm using them like this here:
>
> Lock lock (mutex_);
>
> if (queue.empty ())
> {
> 	waitForItem.wait (lock);
> }
>
> with Lock being a wrapper for scoped_lock, mutex being a wrapper for 
> mutex and waitForItem being a condition, and funnily enough it goes 
> waiting even though there is an item in queue (i.e., queue.empty () == 
> false). I'm not sure where this is coming from (all access to queue is 
> protected using mutexes), only candidate at the moment is that when 
> inserting, I call notify_one, and if two items get inserted one right 
> after the other, I assume that only one thread is woken up and hence the 
> other remains waiting although there is an item in the queue.

Notifies are not sticky, so if there is no thread waiting when notify_one() is
called, the notify is lost.

> It works when I use notify_all (), but this does not smell right to me :/

Can you show me some code?

Anthony
-- 
Anthony Williams            | Just Software Solutions Ltd
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