PostMessage not working with posting custom message - c++

Question

I used
PostMessage(NULL,WM_DUCKWND,0,0);
where
#define WM_DUCKWND (WM_USER +4)
to send user-defined msg to all windows in current thread.
DETAILS
this is straight in the main function
(DUCKPROC_CLASS_MSG_NAME and DUCKPROC_WINDOW_MSG_NAME are all user-defined macros)
//create message-only window
WNDCLASS wndc={};
wndc.lpfnWndProc = MsgWindowProc;
wndc.hInstance = hInstance;
wndc.lpszClassName = DUCKPROC_CLASS_MSG_NAME;
RegisterClass(&wndc);
auto hw=CreateWindowEx(NULL, DUCKPROC_CLASS_MSG_NAME, DUCKPROC_WINDOW_MSG_NAME, NULL, 0, 0, 0, 0, HWND_MESSAGE, NULL, hInstance, NULL);
//post
PostMessage(NULL,WM_DUCKWND,0,0);
//message loop
MSG msg = {};
while (GetMessage(&msg, NULL, 0, 0))
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
and used WindProc like this
LRESULT CALLBACK MsgWindowProc(HWND hwnd,UINT uMsg,WPARAM wParam,LPARAM lParam)
{
switch (uMsg)
{
case WM_DUCKWND:
[BREAKPOINT][BREAKPOINT][BREAKPOINT][BREAKPOINT]
return 0;
}
return DefWindowProc(hwnd, uMsg, wParam, lParam);
}
However, the breakpoint isn't triggered as supposed.
How's that wrong?

Answer 1

"all windows in current thread."
No, that's not correct. HWND==NULL sends the message to the thread message queue. That's the message queue you process with your GetMessage(HWND==NULL) loop.
DispatchMessage is the function which looks at HWND in msg, and chooses the correct window proc. It does so by looking up the window class of that HWND.
Since HWND==NULL does not have a window class, it does not have a window proc either, and the message is not dispatched to any window.
If you want to send WM_DUCKWND(HWND==NULL) to all your windows, you'll have to dispatch it yourself. In this simple example, that's as simple as setting msg.hWnd=hw for msg.message==WM_DUCKWND && msg.hWnd==NULL.
Side note: it really should be WM_APP+4; the WM_USER range is for messages internal to a window class. The thread message queue is shared by windows, so you shouldn't post WM_USER messages to it.