void silc_rwlock_wrlock(SilcRwLock rwlock);

    Acquires write lock of the read/write lock `rwlock'.  If the `rwlock'
    is locked by a writer or a reader the current thread will block until
    the other thread(s) have issued silc_rwlock_unlock for the `rwlock'.
    A thread may acquire the write lock only once.  A deadlock may occur
    if thread attempts to acquire the write lock when it has already done
    so.  If `rwlock' is NULL this function has no effect.