sem_wait(2) BSD System Calls Manual sem_wait(2)
sem_trywait, sem_wait -- lock a semaphore
#include <semaphore.h> int sem_trywait(sem_t *sem); int sem_wait(sem_t *sem);
The semaphore referenced by sem is locked. When calling sem_wait(), if the semaphore's value is zero, the calling thread will block until the lock is acquired or until the call is interrupted by a signal. Alterna- tively, the sem_trywait() function will fail if the semaphore is already locked, rather than blocking on the semaphore. If successful (the lock was acquired), sem_wait() and sem_trywait() will return 0. Otherwise, -1 is returned and errno is set, and the state of the semaphore is unchanged.
sem_wait() and sem_trywait() succeed unless: [EAGAIN] The semaphore is already locked. [EDEADLK] A deadlock was detected. [EINTR] The call was interrupted by a signal. [EINVAL] sem is not a valid semaphore descriptor.
Applications may encounter a priority inversion while using semaphores. When a thread is waiting on a semaphore which is about to be posted by a lower-priority thread and the lower-priority thread is preempted by another thread (of medium priority), a priority inversion has occured, and the higher-priority thread will be blocked for an unlimited time period. Programmers using the realtime functionality of the system should take care to avoid priority inversions.
sem_wait() and sem_trywait() are specified in the POSIX Realtime Exten- sion (1003.1b-1993/1003.1i-1995). Darwin June 8, 2000 Darwin
Mac OS X 10.9.1 - Generated Mon Jan 6 13:32:42 CST 2014