Implementing an N process barrier using semaphores

Question

I'm currently training for an OS exam with previous iterations and I came across this:

Implement a "N Process Barrier", that is, making sure that each process out of a group of them waits, at some point in its respective execution, for the other processes to reach their given point.

You have the following ops available:

init(sem,value), wait(sem) and signal(sem)

N is an arbitrary number. I can make it so that it works for a given number of processes, but not for any number.

Any ideas? It's OK to reply with the pseudo-code, this is not an assignment, just personal study.

Answer 1

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