FreeBSD Device Drivers

Condition Variables

Condition variables synchronize the execution of two or more threads based upon the value of an object. In contrast, locks synchronize threads by controlling their access to objects.

Condition variables are used in conjunction with locks to “block” threads until a condition is true. It works like this: A thread first acquires the foo lock. Then it examines the condition. If the condition is false, it sleeps on the bar condition variable. While asleep on bar, threads relinquish foo. A thread that causes the condition to be true wakes up the threads sleeping on bar. Threads woken up in this manner reacquire foo before proceeding.