NAME
sem_wait,
sem_timedwait, sem_trywait
— decrement (lock) a
semaphore
LIBRARY
library “librt”
SYNOPSIS
#include
<semaphore.h>
int
sem_wait(sem_t
*sem);
int
sem_timedwait(sem_t
*sem, const struct
timespec * restrict abstime);
int
sem_trywait(sem_t
*sem);
DESCRIPTION
Thesem_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 performs the same action, but will not wait beyond
abstime for the value of sem to
be non-zero.
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() function returns 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_trywait() will fail
if:
- [
EAGAIN] - The semaphore value was zero, and thus could not be decremented.
sem_timedwait() will also fail if:
- [
ETIMEDOUT] - The semaphore value was zero and abstime was reached.
SEE ALSO
STANDARDS
sem_wait() and
sem_trywait() conform to ISO/IEC
9945-1:1996 (“POSIX.1”).