NAME
sem_wait,
sem_timedwait, sem_trywait
— decrement (lock) a
semaphore
SYNOPSIS
#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
NULLor 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”).