Use sem_wait(3RT) to block the calling thread until the semaphore count pointed to by sem becomes greater than zero, then atomically decrement the count.
int sem_wait(sem_t *sem);
#include <semaphore.h> sem_t sem; int ret; ret = sem_wait(&sem); /* wait for semaphore */
sem_wait() returns zero after completing successfully. Any other return value indicates that an error occurred. When any of the following conditions occurs, the function fails and returns the corresponding value.
sem points to an illegal address.
A signal interrupted this function.