Advanced Mac OS X Programming: The Big Nerd Ranch Guide

Semaphores

GCD provides semaphores – objects that threads can wait on and signal with (where signal is not a Unix signal). There are two kinds of semaphores: binary semaphores and counting semaphores.

A binary semaphore is a single flag. The flag starts out “signaled.” Waiting on it claims the flag. Any further attempts to wait on it require blocking until it is signaled again.

A counting semaphore is often initialized with the count of some resource and then used to allocate that resource. Waiting on the semaphore claims a unit, and signaling releases that claim. Only when the units have run out (the semaphore’s count being zero) does waiting on the semaphore actually block, requiring waiting for a unit to become available. Waiting can block, ...