The 2 specifies the number of threads that synchronize (nthread in sync. c). each thread sits in a tight loop. in each loop iteration, a thread calls synchronizer() and then sleeps for some random number of microseconds. the assert triggers, because one thread leaves the synchronizer before the other thread has reached the synchronizer. the desired behavior is that all threads should block until nthread threads have called synchronizer. your goal is to achieve the desired behavior.