Synchronization can be achieved by means other than locks and condition variables. One alternative is semaphores. A semaphore is used to regulate traf?c in a critical section. A semaphore is implemented as a non-negative integer counter with atomic increment and decrement operations, up() and down(), respectively. Whenever the decrement operation would make the counter negative, the semaphore blocks instead.  down(sem): tries to decrease the counter by one; if the counter is zero, blocks until the counter is greater than zero. down() is analogous to wait().

 up(sem): increments the counter; wakes up one waiting process. up() is analogous to sig- nal().

Semaphores in computer science were introduced by the Dutch computer scientist Edsger Dijkstra, and so down() is sometimes called P(), from the Dutch for proberen, roughly "try [to decrement]", and up() is sometimes called V(), from the Dutch for verhogen, roughly "increment". Notice that we can use semaphores to impose both mutual exclusion and ordering constraints. In some sense, semaphores, with two operations, are more elegant than monitors, with ?ve operations (namely, lock(), unlock(), wait(), signal(), and broadcast()).

Some people, however, ?nd semaphores less intuitive thanmonitors. There is also an important difference between semaphores and signals: signals have no memory, whereas semaphores do have memory.