Description

   This routine attempts to acquire a read-write lock for write
   access. If any thread already has acquired the lock for write
   access or read access, the lock is not granted and the calling
   thread blocks until it can acquire the lock. A thread can hold
   only one lock for write access on a read-write lock.

   Results are undefined if the calling thread holds the read-write
   lock (whether for read or write access) at the time 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 write access, upon
   return from the interrupt routine the thread resumes waiting for
   the lock as if it had not been interrupted.

   Realtime applications can encounter priority inversion when using
   read-write locks. The problem occurs when a high-priority thread
   acquires a read-write lock that is about to be unlocked (that
   is, posted) by a low-priority thread, but the low-priority thread
   is preempted by a medium-priority thread. This scenario leads to
   priority inversion in that a high-priority thread is blocked by
   lower-priority threads for an unlimited period of time. During
   system design, realtime programmers must take into account the
   possibility of priority inversion and can deal with it in a
   number of ways, such as by having critical sections that are
   guarded by read-write locks execute at a high priority, so that
   a thread cannot be preempted while executing in its critical
   section.