Is it possible to detect when a low-level keyboard hook has been automatically disconnected by Windows?

Question

I am working on a program that uses keyboard hooks. However, when the PC that the program is running on is just slightly overloaded, it causes Windows to disconnect the hook

Answer 1

You can't "detect" this, and you absolutely shouldn't need to. What you're describing is a feature, specifically one introduced in Windows 7 to protect your system from rogue applications.

The applicable documentation describes it thusly (pay particular attention to the bolded section):

The hook procedure should process a message in less time than the data entry specified in the LowLevelHooksTimeout value in the following registry key:

HKEY_CURRENT_USER\Control Panel\Desktop

The value is in milliseconds. If the hook procedure times out, the system passes the message to the next hook. However, on Windows 7 and later, the hook is silently removed without being called. There is no way for the application to know whether the hook is removed.

The solution here is most certainly not to figure out a way to "detect" when the hook is uninstalled and reinstall it. You should have figured out that you're doing something wrong when the operating system uninstalled the hook the first time.

The actual solution is to redesign your application to return from the hook procedure more quickly. Ideally, you should return almost immediately. If you need to run some type of intensive calculation in response to the low level messages (and I can't really imagine why you would), then you should store the information you receive, return from the hook procedure, and do your processing at a later time (probably on a separate thread).

In fact, that's almost exactly what the documentation continues on to suggest:

Note: Debug hooks cannot track this type of low level keyboard hooks. If the application must use low level hooks, it should run the hooks on a dedicated thread that passes the work off to a worker thread and then immediately returns. In most cases where the application needs to use low level hooks, it should monitor raw input instead. This is because raw input can asynchronously monitor mouse and keyboard messages that are targeted for other threads more effectively than low level hooks can. For more information on raw input, see Raw Input.