int sem_post (
The sem_post function unlocks the specified semaphore by performing the semaphore unlock operation on that semaphore. The appropriate function (sem_open for named semaphores or sem_init for unnamed semaphores) must be called for a semaphore before you can call the locking and unlocking functions, sem_wait, sem_trywait, and sem_post.
If the semaphore value after a sem_post function is positive, no processes were blocked waiting for the semaphore to be unlocked; the semaphore value is incremented. If the semaphore value after a sem_post function is zero, one of the processes blocked waiting for the semaphore is allowed to return successfully from its call to sem_wait.
If more than one process is blocked while waiting for the semaphore, only one process is unblocked and the state of the semaphore remains unchanged when the sem_post function returns. The process to be unblocked is selected according to the scheduling policies and priorities of all blocked processes. If the scheduling policy is SCHED_FIFO or SCHED_RR, the highest priority waiting process is unblocked. If more than one process of that priority is blocked, then the process that has waited the longest amount of time is unblocked.
The sem_post function can be called from a signal-catching function.
On successful completion, the sem_post function returns the value 0 and performs a semaphore unlock operation, unblocking a process.
Otherwise, the function returns the value -1 and sets errno to indicate the error. The state of the semaphore remains unchanged.
The sem_post function fails under the following condition:
Functions: sem_trywait(3), sem_wait(3) delim off