NAME
sem_wait,
sem_timedwait,
sem_trywait —
decrement (lock) a
semaphore
LIBRARY
POSIX Real-time Library (librt, -lrt)
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
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 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
sem_post(3)
STANDARDS
sem_wait() and
sem_trywait() conform to
ISO/IEC 9945-1:1996 (“POSIX.1”).