Condition Variables

• There are many cases where a thread wishes to check whether a condition is true before continuing its execution
• A naive approach could just constantly check a global (spin)
  • hugely inefficient as this wastes CPU time

Definition and Routines

• To wait for a condition to become true, a thread can make use of what is known as a condition variable.
• A condition variable is an explicit queue that threads can put themselves on when some state of execution (i.e., some condition) is not as desired (by waiting on the condition)

Simple Example

1 int done = 0;
2 pthread_mutex_t m = PTHREAD_MUTEX_INITIALIZER;
3 pthread_cond_t c = PTHREAD_COND_INITIALIZER;
4
5 void thr_exit() {
6   Pthread_mutex_lock(&m);
7   done = 1;
8   Pthread_cond_signal(&c);
9   Pthread_mutex_unlock(&m);
10 }
11
12 void *child(void *arg) {
13  printf("child\n");
14  thr_exit();
15  return NULL;
16 }
17
18 void thr_join() {
19  Pthread_mutex_lock(&m);
20  while (done == 0)
21  Pthread_cond_wait(&c, &m);
22  Pthread_mutex_unlock(&m);
23 }

Semantics

• When checking your condition use a while loop (Mesa Semantics)
• Hoare semantics, is harder to build but provides a stronger guarantee that the woken thread will run immediately upon being woken
• Virtually every system ever built employs Mesa semantics.

Spurious Wakeups

In some thread packages, due to details of the implementation, it is possible that two threads get woken up though just a single signal has taken place. Spurious wakeups are further reason to re-check the condition a thread is waiting on.