NAME

sem_wait, sem_timedwait, sem_trywait — decrement (lock) a semaphore

SYNOPSIS

POSIX Threads Library (libpthread, -lpthread)
#include <semaphore.h>

int
sem_wait(sem_t *sem);

int
sem_timedwait(sem_t *sem, const struct timespec *abstime);

int
sem_trywait(sem_t *sem);

DESCRIPTION

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_timedwait() function decrements (locks) the semaphore pointed to by sem, but blocks if the value of sem is zero, until either the value is non-zero and can be decremented or the system time specified by abstime is reached.

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.

RETURN VALUES

The sem_wait(), sem_timedwait(), and sem_trywait() functions return the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

ERRORS

sem_wait(), sem_timedwait(), and sem_trywait() will fail if:

[EINVAL]

sem points to an invalid semaphore.

Additionally, sem_wait() and sem_timedwait() will fail if:

[EINTR]

The call was interrupted by a signal.

Additionally, sem_timedwait() will fail if:

[EINVAL]

abstime was NULL or specified a nanosecond value less than zero or greater than 1000 million.

[ETIMEDOUT]

The semaphore value was zero and could not be decremented before abstime was reached.

Additionally, sem_trywait() will fail if:

[EAGAIN]

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

SEE ALSO

sem_destroy(3), sem_getvalue(3), sem_init(3), sem_open(3), sem_post(3)

STANDARDS

sem_wait(), sem_timedwait(), and sem_trywait() conform to IEEE Std 1003.1-2008 (“POSIX.1”).