SEM_WAIT(2)

NAME
sem_wait, sem_trywait - lock a semaphore

SYNOPSIS
#include <semaphore.h>
int
sem_wait(sem_t *sem);
int
sem_trywait(sem_t *sem);

DESCRIPTION
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 aquired or until the call is interrupted by a signal. Alternatively, the sem_trywait() function will fail if the semaphore is already locked, rather than blocking on the semaphore.

If successful (the lock was aquired), sem_wait() and sem_trywait() will return 0. Otherwise, -1 is returned and errno is set, and the state of the semaphore is unchanged.

ERRORS
sem_wait() and sem_trywait() succeed unless:

[EAGAIN]
    The semaphore is already locked.

[EINVAL]
    sem is not a valid semaphore descriptor.

[EDEADLK]
    A deadlock was detected.

[EINTR]
    The call was interrupted by a signal.

NOTES
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.

SEE ALSO
semctl(2), semget(2), semop(2), sem_open(2), sem_post(2)

HISTORY
sem_wait() and sem_trywait() are specified in the POSIX Realtime Extension (1003.1b-1993/1003.1i-1995).