Description

   This routine acquires a read-write lock for read access. 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.

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

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

   If a thread is interrupted (via a Tru64 UNIX signal or an OpenVMS
   AST) while waiting for a read-write lock for read access, upon
   return from the interrupt routine the thread resumes waiting for
   the lock as if it had not been interrupted.