Re: [boost] Probably bug in condition::timed_wait()

From: Peter Dimov (pdimov_at_[hidden])
Date: 2006-10-08 08:48:33

Roland Schwarz wrote:
> Peter Dimov wrote:
>> Does this mean that timed_wait( lock, xt, pred ) has a bug? Instead
>> of
>>
>> while (!pred()) { if (!timed_wait(lock, xt)) return false; } return
>> true;
>>
>> it needs to be
>>
>> while (!pred()) { if (!timed_wait(lock, xt)) return pred(); } return
>> true;
>>
>> otherwise a 'false' return can consume a signal meant for someone
>> else without notifying the caller to handle the pred() == true case?
>
> The documentation states:
>
> Returns: false if xt is reached, otherwise true.
>
> So if it returned pred instead you wouldn't get the correct result.
> I.e. when using the timed_wait you are deliberately missing the
> signal when it arrives out of time. It simply does not matter if
> the signal arrived after timeout.

It does matter. Consider the case where two threads are awaiting an element 
to be pushed into a queue, one uses timed_wait, the other uses wait. The 
"element pushed" signal arrives concurrently with the timeout, the 
timed_wait thread is awakened, sees timeout, returns false to its caller, 
who consequently doesn't pop the element. The other thread waits forever, 
and the element remains in the queue.