VMS Help POSIX Threads, PTHREAD routines, pthread_rwlock_trywrlock, Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    This routine attempts to acquire the read-write lock referenced
    by rwlock for write access. If any thread already holds that lock
    for write access or read access, this routine fails and returns
    [EBUSY] and the calling thread does not wait for the lock to
    become available.

    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.

    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.