4

I'm trying to use wait_for function of std::condition_variable compiled with Visual C++ 2017 however it returns random unexpected results with the following simple code (which never signals the variable, so it's supposed to always return timeout):

condition_variable m_cv;
mutex m_mutex;

void ThreadProc()
{
   while (1)
   {
      unique_lock<mutex> lk(m_mutex);
      cv_status r = m_cv.wait_for(lk, chrono::milliseconds(200));

      printf("Result %s\n", r == cv_status::timeout ? "timeout" : "no timeout");

      Sleep(100);
   }
}

int _tmain(int argc, _TCHAR* argv[])
{
   thread th(ThreadProc);
   th.join();

   return 0;
}

The results are random like:

Result no timeout
Result timeout
Result no timeout
Result timeout
Result timeout
Result timeout
Result timeout
Result no timeout

Am I doing something wrong or this is a compiler bug?

Improve this question
7
  • 3
    "...It may also be unblocked spuriously...." see note to (1), try using (2) from: en.cppreference.com/w/cpp/thread/condition_variable/wait_for Commented May 25, 2018 at 4:59
  • Hmm, that seems ridiculous, then what's the point of this function? :) Commented May 25, 2018 at 5:02
  • 1
    The point of the function is to wait for a condition to become true. You use it inside a loop that repeats the wait until the condition is true. That guards against spurious wake-ups. Commented May 25, 2018 at 6:52
  • The point is to make sure it never waits longer than 200 milliseconds. Use wait_until if you want accurate regular wake-up times (still checking for spurious events). Commented May 25, 2018 at 7:27
  • I see thanks, but why isn't that check done within wait function? Why would anyone need spurious return?? Commented May 25, 2018 at 12:10
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