What can cause Windows to unhook a low level (global) keyboard hook?

Question

We have some global keyboard hooks installed via SetWindowsHookEx with WH_KEYBOARD_LL that appear to randomly get unhooked by Windows.

We

Answer 1

There are two things I have thought of that might help you figure out where the problem is.

1. To help isolate the problem's location, run another WH_KEYBOARD_LL hook simultaneously with your current hook and have it do nothing other than pass the data on down the hook chain. When you find out that your original hook is unhooked, check and see if this "dummy" hook was also unhooked. If the "dummy" hook was also unhooked, you can be fairly certain the problem is outside of your hook (i.e. in Windows or something related to your process as a whole?) If the "dummy" hooks was not unhooked, then the problem is probably somewhere within your hook.

2. Log the information that comes to your hook via the callback and run it until the hook gets unhooked. Repeat this a number of times and examine the logged data to see if you can discern a pattern leading up to the unhooking.

I would try these one at a time, just in case either would affect the others' outcome. If after that you have no leads on what the problem could be, you might try running them together.