Question

I have two threads in C++. One thread called alarm thread runs the function raiseAlarm() and the other thread called print thread runs the function called printMetrics. At a fixed interval, raiseAlarm sets an atomic variable to true. When the variable is true, printMetrics thread, which is spinning on the value of this atomic variable, prints some data. When I run this application, nothing happens. But if I put a cout anywhere in raiseAlarm, everything works fine. Why?

void Client::raiseAlarm()
{
    bool no = false;
    while(!stop.load(std::memory_order_acquire))
    {
        //cout << "about to sleep\n";
        this_thread::sleep_for(std::chrono::seconds(captureInterval));
        while(!alarm.compare_exchange_weak(no, true, std::memory_order_acq_rel))
        {
            no = false;
        }
    }
}

void Client::printMetrics()
{
    bool yes = true;
    while(!stop.load(std::memory_order_acquire))
    {
        while(!alarm.compare_exchange_weak(yes, false, std::memory_order_acq_rel) )
        {
            yes = true;
        }

        cout << "Msgs Rcvd: " << metrics.rcv_total.load(std::memory_order_acquire);
        cout << "Msgs Sent: " << metrics.snd_total.load(std::memory_order_acquire);
        cout << "Min latency: " << metrics.min_latency.load(std::memory_order_acquire);
        cout << "Max latency: " << metrics.max_latency.load(std::memory_order_acquire);
        metrics.reset();
    }

}

Answer 1

Just a suggestion because I’m not so savvy with concurrency in C++, but make sure you don’t forget to flush your output stream. Either stick a cout << flush; after all of your cout lines or add an << endl to each one (which will automatically flush your stream).