NetBSD kqueue : inconsistent type for user data (kevent.udata)

I just fired up a VM and did a fresh install of NetBSD 10.1. Its /usr/include/sys/event.h defines the struct kevent's udata field as a void *, and it has definitions for EVFILT_USER filters.

Looking at the change logs, the type of the field and support for user filters was added between NetBSD 9 and 10.0. That your 10.1 install doesn't have an updated header that reflects this (The first line should say the version of the file is 1.54) suggests that you upgraded from a 9.X and something went wrong so that the updated headers weren't installed over the ones from the old release, or that something else is screwy with your installation. A normal 10.1 installation does follow the documentation.