why do I need std::condition_variable?

Question

I found that std::condition_variable is very difficult to use due to spurious wakeups. So sometimes I need to set a flags such as:

atomic

        

Answer 1

The purpose of std::condition_variable is to wait for some condition to become true. It is not designed to be just a receiver of a notify. You might use it, for example, when a consumer thread needs to wait for a queue to become non-empty.

T get_from_queue() {
   std::unique_lock l(the_mutex);
   while (the_queue.empty()) {
     the_condition_variable.wait(l);
   }
   // the above loop is _exactly_ equivalent to the_condition_variable.wait(l, [&the_queue](){ return !the_queue.empty(); }
   // now we have the mutex and the invariant (that the_queue be non-empty) is true
   T retval = the_queue.top();
   the_queue.pop();
   return retval;
}

put_in_queue(T& v) {
  std::unique_lock l(the_mutex);
  the_queue.push(v);
  the_condition_variable.notify_one();  // the queue is non-empty now, so wake up one of the blocked consumers (if there is one) so they can retest.
}

The consumer (get_from_queue) is not waiting for the condition variable, they are waiting for the condition the_queue.empty(). The condition variable gives you the way to put them to sleep while they are waiting, simultaneously releasing the mutex and doing so in a way that avoids race conditions where you miss wake ups.

The condition you are waiting on should be protected by a mutex (the one you release when you wait on the condition variable.) This means that the condition rarely (if ever) needs to be an atomic. You are always accessing it from within a mutex.