/sys$common/syshlp/helplib.hlb POSIX Threads, TIS routines, tis_write_trylock, Description  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    This routine attempts to acquire a read-write lock for write
    access. The routine attempts to immediately acquire the lock.
    If the lock is acquired, zero (0) is returned. If the lock is
    held by another thread (for either read or write access), [EBUSY]
    is returned and the calling thread does not wait for the write-
    access lock to be acquired.

    Note that it is a coding error to attempt to acquire the lock
    for write access if the lock is already held by the calling
    thread. (However, this routine returns [EBUSY] anyway, because
    no ownership error-checking takes place.)