SEM_POST(3C)SEM_POST(3C)NAMEsem_post - increment the count of a semaphore
SYNOPSIS
#include <semaphore.h>
int sem_post(sem_t *sem);
DESCRIPTION
The sem_post() function unlocks the semaphore referenced by sem by per‐
forming a semaphore unlock operation on that semaphore.
If the semaphore value resulting from this operation is positive, then
no threads were blocked waiting for the semaphore to become unlocked;
the semaphore value is simply incremented.
If the value of the semaphore resulting from this operation is 0, then
one of the threads blocked waiting for the semaphore will be allowed
to return successfully from its call to sem_wait(3C). If the symbol
_POSIX_PRIORITY_SCHEDULING is defined, the thread to be unblocked will
be chosen in a manner appropriate to the scheduling policies and param‐
eters in effect for the blocked threads. In the case of the schedulers
SCHED_FIFO and SCHED_RR, the highest priority waiting thread will be
unblocked, and if there is more than one highest priority thread
blocked waiting for the semaphore, then the highest priority thread
that has been waiting the longest will be unblocked. If the symbol
_POSIX_PRIORITY_SCHEDULING is not defined, the choice of a thread to
unblock is unspecified.
RETURN VALUES
If successful, sem_post() returns 0; otherwise it returns −1 and sets
errno to indicate the error.
ERRORS
The sem_post() function will fail if:
EINVAL
The sem argument does not refer to a valid semaphore.
ENOSYS
The sem_post() function is not supported by the system.
EOVERFLOW
The semaphore value exceeds SEM_VALUE_MAX.
USAGE
The sem_post() function is reentrant with respect to signals and may be
invoked from a signal-catching function. The semaphore functionality
described on this manual page is for the POSIX (see standards(5))
threads implementation. For the documentation of the Solaris threads
interface, see semaphore(3C)).
EXAMPLES
See sem_wait(3C).
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌────────────────────┬───────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├────────────────────┼───────────────────┤
│Interface Stability │ Committed │
├────────────────────┼───────────────────┤
│MT-Level │ Async-Signal-Safe │
├────────────────────┼───────────────────┤
│Standard │ See standards(5). │
└────────────────────┴───────────────────┘
SEE ALSOsched_setscheduler(3C), sem_wait(3C), semaphore(3C), attributes(5),
standards(5)
Feb 5, 2008 SEM_POST(3C)
