This routine attempts to acquire a read-write lock for read
    access, but does not wait for the lock if it not immediately
    available.

    If no thread already holds the lock for write access and there
    are no writers waiting to acquire the lock, the lock for read
    access is granted to the calling thread and this routine returns.
    If a thread already holds the lock for read access, the lock is
    granted and this routine returns.

    If some thread already holds the lock for write access, the
    calling thread will not acquire the read lock. Results are
    undefined if the calling thread has already acquired a lock for
    write access on rwlock when this routine is called.

    A thread can hold multiple, concurrent locks for read access on
    the same read-write lock. In a given thread, for each call to
    this routine that successfully acquires the same read-write lock
    for read access, a corresponding call to pthread_rwlock_unlock()
    must be issued.

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