This routine releases a lock acquisition held on the read-write
    lock object referenced by rwlock. Results are undefined if rwlock
    is not held by the calling thread.

    If this routine is called to release a lock for read access on
    rwlock and the calling thread also currently holds other locks
    for read access on rwlock, the read-write lock object remains
    in the read locked state. If this routine releases the calling
    thread's last lock for read access on rwlock, the calling thread
    is no longer one of the owners of the lock object.

    If this routine is called to release a lock for write access
    on rwlock, the lock object is put in the unlocked state with no
    owners.

    If a call to this routine results in the read-write lock object
    becoming unlocked and there are multiple threads waiting to
    acquire that lock for write access, the Threads Library uses
    the scheduling policy of those waiting threads to determine
    which thread next acquires the lock object for write access. If
    there are multiple threads waiting to acquire the read-write lock
    object for read access, the Threads Library uses the scheduling
    policy of those waiting threads to determine the order in which
    those threads acquire the lock for read access. If there are
    multiple threads waiting to acquire the read-write lock object
    for both read and write access, it is unspecified whether a
    thread waiting for read access or for write access next acquires
    the lock object.

    If the read-write lock object referenced by rwlock is not
    initialized, the results of calling this routine are undefined.