Description

   This routine attempts to acquire a read-write lock for read
   access. If the lock cannot be granted, the routine returns
   without waiting.

   When a thread calls this routine, an attempt is made to
   immediately acquire the lock for read access. If the lock is
   acquired, zero (0) is returned. If a holder of the lock for write
   access exists, [EBUSY] is returned.

   If the lock cannot be acquired for read access immediately, the
   calling program does not wait for the lock to be released.