what if notify() is called before wait()?

Question

I have a situation where a notify() 'can' be called before a wait().

I am trying to make a simulator to schedule its next event when I 'notify' him by sending him messages. So I have devised a wait->notify->scedule chain

void Broker::pause()
{
    boost::unique_lock<boost::mutex> lock(m_pause_mutex);
    {
        std::cout << "pausing the simulation" << std::endl;
        m_cond_cnn.wait(lock);
        std::cout << "Simulation UNpaused" << std::endl;
        // the following line causes the current function to be called at 
        // a later time, and a notify() can happen before the current function
        // is called again
        Simulator::Schedule(MilliSeconds(xxx), &Broker::pause, this);
    }
}

void Broker::messageReceiveCallback(std::string message) {
    boost::unique_lock<boost::mutex> lock(m_pause_mutex);
    {
        m_cond_cnn.notify_one();
    }
}

the problem here is that: there can be situations that a notify() is called before its wait() is called.

Is there a solution for such situation? thank you

Answer 1

Condition variables can hardly be used alone, if only because, as you noticed, they only wake the currently waiting threads. There's also the matter of spurious wake-ups (ie. the condition variable can sometimes wake up a thread without any corresponding notify having been called). To work properly, condition variables usually need another variable to maintain a more reliable state.

To solve both those problems, in your case you just need to add a boolean flag:

boost::unique_lock<boost::mutex> lock(m_pause_mutex);
while (!someFlag)
    m_cond_cnn.wait(lock);
someFlag = false;

//...

boost::unique_lock<boost::mutex> lock(m_pause_mutex);
someFlag = true;
m_cond_cnn.notify_one();