1. Locks

mutex = mutual exclusion, only one thread can hold the lock at any time

  pthread_mutex_t lock = PTHREAD_MUTEX_INITIALIZER;
  ...
  Pthread_mutex_lock(&lock); // wrapper; exits on failure
  balance = balance + 1;
  Pthread_mutex_unlock(&lock);

How to implement?

Issues: Correctness (even on multiprocessors), Performance (time overhead), Fairness (no starve)