Semaphore

A semaphore is a mechanism used to prevent race conditions between threads. Semaphores have a value that is shared between threads (and optionally processes). It has two fundamental operations wait and post:

• wait — decrements the value atomically. Doesn’t return until the value is greater than 0. The integer value cannot go less than 0.
• post — increments the value atomically.

In POSIX systems

The semaphore API is given in semaphore.h. It has the following functions, all of which return 0 upon success. It’s a fairly similar API to using mutexes.

• sem_init
• sem_destroy
• sem_wait
• sem_trywait
• sem_post