How to block a message using WH_GETMESSAGE hook?

Question

In the CBTProc hook, there's: return 0 to allow the msg, 1 to deny .

What about the WH_GETMESSAGE hook? How I could 'block' a message from being executed?

In this example :

LRESULT CALLBACK GetMsgProc(_In_ int nCode, _In_ WPARAM wParam, _In_ LPARAM lParam
)
{
    if (nCode < 0)
        return CallNextHookEx(nullptr, nCode, wParam, lParam);

    MSG* pMsg = (MSG*)lParam;

    if ((pMsg->message == WM_KEYDOWN) && (wParam == PM_REMOVE))
    {

    }

    return CallNextHookEx(g->getmsgproc, nCode, wParam, lParam);

}

When the code gets inside of the block if ((pMsg->message == WM_KEYDOWN) , how i could deny it from executing the WM_KEYDOWN msg?

Do I need to replace pMsg->message with something else or what?

Inside of both return CallNextHookEx(... the first parameter must be nullptr ?

Answer 1

What about the WH_GETMESSAGE hook? How I could 'block' a message from being executed?

You can't block the message. Once all hook procedures in the chain have exited, the message is then passed to the original caller of (Get|Peek)Message() . There is no option to avoid that.

Do I need to replace pMsg->message with something else or what?

Yes, you can modify the message. For instance, you could set its message field to WM_NULL so the caller will effectively ignore it.

Inside of both return CallNextHookEx(... the first parameter must be nullptr ?

It doesn't really matter what you set it to. In very old versions of Windows (Win9x/ME), it had to be set to the actual HHOOK , but that hasn't been true for a very long. Since NT4/Win2K, the parameter is completely ignored. So yes, setting it to nullptr is fine.