Multithreaded Programming Guide

Comparing Primitives

The most basic synchronization primitive in threads is the mutual exclusion lock. So, it is the most efficient mechanism in both memory use and execution time. The basic use of a mutual exclusion lock is to serialize access to a resource.

The next most efficient primitive in threads is the condition variable. The basic use of a condition variable is to block on a change of state; that is it provides a thread wait facility. Remember that a mutex lock must be acquired before blocking on a condition variable and must be unlocked after returning from pthread_cond_wait(). The mutex lock must also be held across the change of state that occurs before the corresponding call to pthread_cond_signal().

The semaphore uses more memory than the condition variable. It is easier to use in some circumstances because a semaphore variable functions on state rather than on control. Unlike a lock, a semaphore does not have an owner. Any thread can increment a semaphore that has blocked.

The read-write lock permits concurrent reads and exclusive writes to a protected resource. The read-write lock is a single entity that can be locked in read or write mode. To modify a resource, a thread must first acquire the exclusive write lock. An exclusive write lock is not permitted until all read locks have been released.