Semaphores

Mutex locks, as mentioned earlier, are generally considered the simplest of synchronization tools. But sometimes we need a more robust tool that can also provide more sophisticated ways for processes to synchronize their activities.

A semaphore S is an integer variable that, apart from initialization, is accessed only through two standard atomic operations: wait() and signal().

tip

A mutex is particular case of semaphor with S = 1.

POSIX API

sem_init()

int sem_init(sem_t *sem, int pshared, unisigned int value);

This sets the semaphore sem with S = value. Use 0 for pshared.

sem_wait() and sem_post()

sem_wait() decrements S with one unit, but if S=0 then the function waits for the value to rise before further decrementing, thus blocking the thread!

sem_post() increments S with one unit and if there are blocked threads, then it frees the one who waited the most in the queue.

To destroy a semaphore, use sem_destroy(sem_t *sem).