Rescuing thread messages from modal loops via message filters

As we have seen recently, thread messages are eaten by modal loops because they have nowhere to go when dispatched. However, there is a way to see them before they vanish, provided the modal loop is cooperative.

The WH_MSGFILTER message hook allows you to receive messages passed to the CallMsgFilter function. Fortunately, all the modal loops in the window manager use CallMsgFilter to allow the thread to capture thread messages before they are lost. Therefore, this gives you a way to snoop on messages as they travel through modal loops.

Let's add a message filter to the program we wrote last time to see how messages pass through a message filter. Note that this is the wrong way to solve the problem. The correct solution was illustrated last time. I'm doing it the wrong way to illustrate message filters since they are not well-understood. (For example, a valid reason for a message filter would to prevent the menu loop from seeing certain input.)

Start with the program from last the before we changed the PostThreadMessage to a PostMessage, then make the following changes:


LRESULT CALLBACK MsgFilterProc(int code, WPARAM wParam, LPARAM lParam)
 MSG* pmsg = (MSG*)lParam;
 if (code >= 0 && IsThreadMessage(pmsg)) return TRUE;
 return CallNextHookEx(g_hhkMSGF, code, wParam, lParam);

OnCreate(HWND hwnd, LPCREATESTRUCT lpcs)
 g_hhkMSGF = SetWindowsHookEx(WH_MSGFILTER, MsgFilterProc,
    NULL, GetCurrentThreadId());
 if (!g_hhkMSGF) return FALSE;
 DWORD dwThread;
 HANDLE hThread = CreateThread(NULL, 0, ThreadProc,
       UintToPtr(GetCurrentThreadId()), 0, &dwThread);

Here, we installed a message filter hook on our thread so that we can seem messages as they pass through modal loops. The code parameter tells us what type of modal loop retrieved the message; we ignore it here since we want to do our filtering for all modal loops.

Run this program and observe that the beeps are no longer lost because our message filter is getting a chance to see them and react to them.

The message filter trick relies on all modal loops sending the messages they retrieve through a message filter before dispatching them. If you are writing code that is going into a library, and you have a modal loop, then you too should call the message filter before dispatching messages you've retrieved, in case the program using your library wants to do something with the message.

MSG msg;
while (GetMessage(&msg, NULL, 0, 0)) {
 if (!CallMsgFilter(&msg, MSGF_MYLIBRARY)) {

The value MSGF_MYLIBRARY is an arbitrary positive value you can choose and document in your library's header file. You can see examples of this in the commctrl.h header file:

#define MSGF_COMMCTRL_BEGINDRAG     0x4200

These are the message filters called by the modal loops in the shell common controls library.

One question you might ask is, "Why use a message filter hook instead of a GetMessage hook?"

Message filter hooks are less expensive than GetMessage hooks because they are called only upon request, as opposed to a GetMessage hook which is called for every retrieved message. Message filter hooks also tell you which modal loop is doing the filtering, in case you want to adjust your behavior accordingly.

The downside of message filter hooks is that all modal loops need to remember to call CallMsgFilter as part of their dispatch loop.

Comments (11)
  1. Universalis says:

    Some modal loops call CallMsgFilter, some don’t.

    Do all modal loops within Windows itself call CallMsgFilter? If not, is there a list of the ones that do and the ones that don’t?

  2. waleri says:

    You mentioned before, that the thread message is discarded by DispatchMessage() – does this mean that GetMessage() still retrieves it from the message queu correctly? If so, isn’t it easier to process those messages in the message pump loop and therefore avoid using message hooks, etc?

  3. waleri says:

    In other words, why do you use a message hook, rather than simply pass the message to some message handler function?

  4. Ben Cooke says:

    waleri, I think the idea here is that you may not have control over the modal loop as it may be handled by someone else’s code or by Windows itself. Obviously there’s no way to "get inside" someone else’s message loop without a hook.

  5. All the modal loops I’ve seen, and yet this is the first time I’ve heard of CallMsgFilter. Of COURSE people aren’t going to remember to call it, assuming they care at all!

    I still stand by what I wrote three articles ago. There should be a callback(s) for thread messages, which gets called if DispatchMessage() sees that it has been set. Imagine that, it’d even be backwards-compatible.

    On the other hand, I use MFC with its goofy GetMessage hook, so why should I care…

  6. Clearly a simple callback isn’t good enough – what if two modules both want to register a callback? Who wins? It would have to be a new hook type – but since you only care about thread messages, the existing WH_GETMESSAGE hook works just fine.

  7. "what if two modules both want to register a callback? Who wins?"

    Ask whoever wrote SetWindowSubclass(…). They seemed to figure it out. :-)

  8. Ivo says:

    I’m with Joshua on this one. I’ve never heard of CallMsgFilter before. :) When should I call it? Every time I do a modal message loop? In my main application message loop?

    The MSDN is not clear at all on this. It says "The system calls CallMsgFilter to enable applications…" Sounds like it is to be called by the system, and not by application code.

  9. Joshua: Precisely, SetWindowSubclass uses a hook model rather than a single callback. And hey check it out there’s already a hook WH_GETMESSAGE you can use to capture thread messages.

  10. asdf says:

    So why is this the wrong way?

  11. Putting together pieces you already know.

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