sem_post(2) System Calls Manual sem_post(2)
sem_post - unlock a POSIX semaphore
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, waiting for it to become available (the absolute value of the semaphore's
value indicates the number of waiters at that moment). If the semaphore 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 priori-
ties, 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 sema-
phore remains unchanged. Instead, the semaphore being posted will be inherited 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.
The following call to will post the semaphore sem.
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.
fails and does not perform the requested operation if any of the following 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 semaphore.