How can my app find the sender of a windows message?

Question

I have an app which uses a keyboard hook procedure in a library. The wParam in the hook for one message is 255 which we think is "(reserved / OEMClear)". I'd like to work out the source of this message as it causes my application to crash in the library, and given it shouldn't be happening it would be good to identify it. The message comes in repeatedly on only one PC we have - other computers don't see the message at all.

So, is there a way to trace the source of a message sent to a window please, or all those on the system?

Answer 1

There is no built-in way to find out who sent the window message, not even win32k keeps track of this; you might be able to find it out with a kernel debugger and a conditional breakpoint.

However, I would argue that you don't really need this information; you need to make your app properly handle any message sent to it.

Answer 2

(I originally suggested using Spy++ or winspector, but they do not hook into the sending of messages. That doesn't even make sense! A window receives messages but they don't send them, a thread does that. I'll leave my suggestion about using a debugger.)

Sometimes debugging can help. Try downloading the windows PDB files and setting a breakpoint that hits only when one of these messages occur. Looking at the call stack at that point can often shed some light on why things are happening. Posted messages and messages send from other processes will foil this approach.