Manual Pages  — SEM_WAIT

sem_wait, sem_trywait – decrement (lock) a semaphore

LIBRARY SYNOPSIS DESCRIPTION RETURN VALUES ERRORS SEE ALSO STANDARDS

Standard C Library (libc, -lc)

#include <semaphore.h>

int
sem_wait(sem_t *sem);

int
sem_trywait(sem_t *sem);

The sem_wait() function decrements (locks) the semaphore pointed to by sem, but blocks if the value of sem is zero, until the value is non-zero and the value can be decremented.

The sem_trywait() function decrements (locks) the semaphore pointed to by sem only if the value is non-zero. Otherwise, the semaphore is not decremented and an error is returned.

Upon successful completion, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

The sem_wait() and sem_trywait() functions will fail if:

[EINVAL]
	The sem argument points to an invalid semaphore.

Additionally, sem_wait() will fail if:

[EINTR]
	A signal interrupted this function.

Additionally, sem_trywait() will fail if:

[EAGAIN]
	The semaphore value was zero, and thus could not be decremented.

The sem_wait() and sem_trywait() functions conform to IEEE Std 1003.1-96 ("POSIX.1").