Is it possible to track a PostMessage between processes?

Question

We have a system where there are typically two processes running on the same system. One process handles the GUI and the other runs like a service (although for historical reasons, it's not a service, just an exe with no visible window).

The two processes undertake IPC mainly via registered messages asynchronously - i.e. we use RegisterWindowMessage() in both processes to define a large'ish set of messages that effectively form the API to the server process.

I have written a "hands-free" monitoring application that uses SetWindowsHookEx() to monitor and display the message queues of both processes and provide some level of decoding of the way the API is being utilised and how notifications are being propagated to the GUI process (each individual window can subscribe to notifications from the server directly).

So, there are a large number of messages in both directions so I have filtering and summary counts etc. so I can focus on particular activity. All this can be done without affecting the live code, which is good.

This all works well, but it now would be very useful to be able to "tag" a message originating in the GUI so I can trace the same message when it's processed by the server. This would be enormously useful for debugging and diagnosing system issues, but I can't find a clean way (actually I can't find any way!) of doing this without adding such support to our registered message API, which would be a lot of work and involves more risk than I'm comfortable with at the moment. It gets further complicated by the fact that the server pre-processes some messages and then does a PostMessage() back to itself to perform the action, so the originating message can get "lost".

Has anyone here tackled this type of problem? If so, can you give me some pointers? If not, then are there any documented or undocumented ways of adding a small block of data to a Windows message and retrieving it later? I've looked at SetMessageExtraInfo() but that seems to be per-queue rather than per-message.

Answer 1

FindWindow or FindWindowEx will give you the details of the GUI Window. Compare the details with message intercepted