The sem_timedwait function locks the semaphore referenced by
    sem as in the sem_wait function. But if the semaphore cannot be
    locked without waiting for another process or thread to unlock
    the semaphore by performing a sem_post function, this wait
    terminates when the specified timeout expires.

    The timeout expires when the absolute time specified by abs_
    timeout passes, as measured by the clock on which timeouts are
    based (that is, when the value of that clock equals or exceeds
    abs_timeout, or if the absolute time specified by abs_timeout has
    already been passed at the time of the call.

    The function will not fail with a timeout if the semaphore can be
    locked immediately. The validity of abs_timeout does not need to
    be checked if the semaphore can be locked immediately.