The sem_close function makes a semaphore available for reuse
    by deallocating any system resources allocated for use by the
    current process for the named semaphore indicated by sem.

    If the semaphore has not been removed with a call to sem_unlink,
    sem_close does not change the current state of the semaphore.

    If the semaphore has been removed with a call to sem_unlink after
    the most recent call to sem_open with O_CREAT, the semaphore is
    no longer available after all processes that opened the semaphore
    close it.