Recursive message loop - winapi

Question

I am having trouble correctly running a message loop from within a message handler. In effect replicating how DialogBox() processes messages, minus all of the windowing.
Simply invoking GetMessage() from within a message handler nearly works except when the WM_SYSKEYDOWN event opening the system menu also triggers the entry into a sub-loop. After this weird things happen, with keys presses being swallowed and WM_MOUSEMOVE messages relative to the system menu getting sent to the main window.
For the record this happens both in Windows 8 and XP.
To give some context I am attempting a threading model where a (windowless) worker thread communicates through blocking SendMessage calls back to the main window acting as a server. These actions may require further input or depend on other I/O and thus regular messages need to be processed until the reply is ready.
I'm fairly certain that this is a basic mistake or misunderstanding on my part, just like last time I posted here, but I can't quite seem to work out what I'm doing wrong on my own.
Here is my repro case. Try navigating after pressing ALT+SPACE to open the system menu,
#include <windows.h>
BOOL update;
LRESULT WINAPI WindowProc(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam) {
MSG msg;
char text[256];
switch(uMsg) {
case WM_DESTROY:
ExitProcess(0);
// Trigger an update on input
case WM_SYSKEYDOWN:
update = TRUE;
break;
// Display the update from the worker thread, returning once it is time to
// ask for the next one
case WM_USER:
wsprintf(text, TEXT("%u"), (unsigned int) lParam);
SetWindowText(hwnd, text);
while(!update && GetMessage(&msg, NULL, 0, 0) > 0) {
TranslateMessage(&msg);
DispatchMessage(&msg);
}
update = FALSE;
return 0;
}
return DefWindowProc(hwnd, uMsg, wParam, lParam);
}
DWORD WINAPI ThreadProc(void *hwnd) {
// Submit updates as quickly as possible
LONG sequence = 1;
for(;;)
SendMessage(hwnd, WM_USER, 0, sequence++);
}
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
LPSTR lpCommandLine, int nCmdShow) {
HWND hwnd;
MSG msg;
// Create our window
WNDCLASS windowClass = { 0 };
windowClass.lpfnWndProc = WindowProc;
windowClass.hIcon = LoadIcon(NULL, IDI_APPLICATION);
windowClass.hCursor = NULL;
windowClass.lpszClassName = TEXT("Repro");
RegisterClass(&windowClass);
hwnd = CreateWindow(TEXT("Repro"), TEXT("Repro"),
WS_VISIBLE | WS_OVERLAPPED | WS_CAPTION | WS_SYSMENU | WS_MINIMIZEBOX,
CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, NULL, NULL,
hInstance, 0);
// Launch the worker thread
CreateThread(NULL, 0, ThreadProc, hwnd, 0, NULL);
// And run the primary message loop
while(GetMessage(&msg, NULL, 0, 0) > 0) {
TranslateMessage(&msg);
DispatchMessage(&msg);
}
return 0;
}

Answer 1

Modal message loops are perfectly fine. Raymond Chen has a series of articles on writing modal message loops properly.
One thing I notice: Your thread should post the message, not send it; SendMessage calls directly into the window proc. Don't use PostThreadMessage, either; that's designed for threads without visible UI (and the nested DispatchMessage won't know how to dispatch the thread message, resulting in dropped messages).