The execution sequence after conditional variable notify

Question

There are two threads (Call them T1 and T2) that sync with each other by boost condition variable and mutex like:

boost::condition_variable global_cond;
boost::mutex global_mutex;
boost::unique_lock<boost::mutex> lock( global_mutex);

thread1() {
  global_cond.notify_one();
  code_block_a();
}

tread2() {
  global_cond.wait(lock)
  code_block_b();
}

Let's say I can gugarntee that thread2 come to wait first and then thread1 will do the notify.

My question is, is that determinastic that code_block_a() or code_block_b() will execute first?

Generally? No. In your code? Who knows. Maybe if you posted it. — Michael Foukarakis, Nov 10 '16 at 14:23

score 1 · Answer 1 · answered Nov 10 '16 at 14:31

Not guaranteed. The system may perform context switching right after thread1 called notify_one() and allow thread2() to run. And it may not.

Please note that your code is generally buggy because global_cond.wait(lock) can be spuriously woken up and tread2 can run code_block_b() even before thread1() has run.

The execution sequence after conditional variable notify

1 Answers1