Implementing an N process barrier using semaphores

This is well presented in The Little Book of Semaphores.

n = the number of threads
count = 0
mutex = Semaphore(1)
barrier = Semaphore(0)


mutex.wait()
count = count + 1
mutex.signal()

if count == n: barrier.signal() # unblock ONE thread

barrier.wait()
barrier.signal() # once we are unblocked, it's our duty to unblock the next thread