Spurios wake up and condition variables

Question

I was reading this nice blog from A Williams:

http://www.justsoftwaresolutions.co.uk/threading/condition-variable-spurious-wakes.html

and one thing bugs me: when thread wakes up because of the spurious wake does it have mutex locked?

boost::mutex::scoped_lock lock(the_mutex);
while(the_queue.empty())
{
    the_condition_variable.wait(lock);
}

I guess so because otherwise call to .empty would be unsafe but Im not sure.

Answer 1

Yes, it does have the mutex locked. Basically, the mutex gets released only while the thread is blocked in the_condition_variable.wait(). Spurious wakeup or not, the mutex is locked everywhere else in the code that you show.

From the documentation for boost::condition_variable::wait():

Postcondition:

lock is locked by the current thread.