Editing Semaphore (programming) (section)

===Login queue===
Consider a system that can only support ten users (S=10). Whenever a user logs in, P is called, decrementing the semaphore ''S'' by 1. Whenever a user logs out, V is called, incrementing ''S'' by 1 representing a login slot that has become available. When ''S'' is 0, any users wishing to log in must wait until ''S'' increases. The login request is enqueued onto a FIFO queue until a slot is freed. [[Mutual exclusion]] is used to ensure that requests are enqueued in order. Whenever ''S'' increases (login slots available), a login request is dequeued, and the user owning the request is allowed to log in. If ''S'' is already greater than 0, then login requests are immediately dequeued.