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.

The main use of semaphores is to enforce an order of operations among threads.

Systems programming

In POSIX systems

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