The sem_getvalue function updates a location referenced by the
    sval argument with the value of semaphore sem. The updated value
    represents an actual semaphore value that occurred during the
    call, but may not be the actual value of the semaphore at the
    time that the value is returned to the calling process.

    If the semaphore is locked, the value returned will either be
    zero or a negative number indicating the number of processes
    waiting for the semaphore at some time during the call.