sem_post(2)sem_post(2)NAMEsem_post - unlock a POSIX semaphore
SYNOPSISDESCRIPTION
is used to post the semaphore referenced by sem. The calling thread
will not return from its call to unless it can either: increment the
semaphore value, if there are no blocked threads on this semaphore;
give the semaphore to a blocked thread, if there are any blocked
threads on this semaphore; or have an error condition.
If the semaphore value is < 0, the semaphore has blocked threads, wait‐
ing for it to become available (the absolute value of the semaphore's
value indicates the number of waiters at that moment). If the sema‐
phore value is >= 0, the semaphore has no waiters.
If the semaphore has no waiters at the time its value is checked, the
semaphore's value will be atomically incremented, with respect to the
checking of its value, up to its maximum value as specified by If the
semaphore has waiters at the time its value is checked, the semaphore
value is not changed. Instead, the calling thread will attempt to wake
up a waiter. If the semaphore has waiters having realtime priorities,
the thread must wake up the highest priority waiter. Otherwise the
thread at the head of the channel queue is woken up.
When a waiter is successfully woken, the semaphore being posted will be
given to the woken waiter. In other words, the state of the semaphore
remains unchanged. Instead, the semaphore being posted will be inher‐
ited by the waiter being woken from this call to
If the specified semaphore referred to by sem is a named semaphore,
then this semaphore must have been opened by the calling process with
The calling process must have both read and write permissions on the
semaphore to perform this operation. The routine may be called asyn‐
chronously, i.e. from a signal handler.
To use this function, link in the realtime library by specifying on the
compiler or linker command line.
EXAMPLES
The following call to will post the semaphore sem.
RETURN VALUE
A successful call to will return 0 and the calling thread would have
posted the semaphore. Otherwise, the call to will return -1 with errno
set to the appropriate value of the error condition.
ERRORS
fails and does not perform the requested operation if any of the fol‐
lowing conditions are encountered:
[EPERM] The calling process does not have the privileges
necessary to post the semaphore.
[EINVAL] The argument sem does not refer to a valid sema‐
phore.
SEE ALSO
<semaphore.h>.
STANDARDS CONFORMANCEsem_post(2)
