sem_post - release a posix named or unnamed semaphore
int sem_post(sem_t *sem<b>);
sem_post atomically increments the value of the semaphore located at
address sem. If any processes are blocked on the semaphore, the highest
priority process waiting the longest is unblocked and made runnable.
The address sem may reside anywhere in the calling processes address
space, but the process must have read and write access to sem.
sem_post will fail if the following is true:
[EINVAL] The semaphore located at address sem is not a valid posix
named or unnamed semaphore.
sem_init(3C), sem_destroy(3C), sem_getvalue(3C), sem_post(3C),
sem_wait(3C), sem_trywait(3C), sem_open(3C), sem_close(3C),
Upon successful completion, a value of 0 is returned to the calling
process. Otherwise, a value of -1 is returned and errno is set to
indicate the error.
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