WinApi message loop, Postmessage works like SendMessage

Question

Hello can Somebody answer me why when I run this program the order of MessageBoxes is 1,2,4,3 instead of 1,2,3,4. In my opinion program should end executing WM_PAINT procedure before start WM_USER+11, why it isn't?

// Win32Project6.cpp : Defines the entry point for the application.
//

#include "stdafx.h"
#include "Win32Project6.h"

#define MAX_LOADSTRING 100

// Global Variables:
HINSTANCE hInst;                                // current instance
TCHAR szTitle[MAX_LOADSTRING];                  // The title bar text
TCHAR szWindowClass[MAX_LOADSTRING];            // the main window class name

// Forward declarations of functions included in this code module:
ATOM                MyRegisterClass(HINSTANCE hInstance);
BOOL                InitInstance(HINSTANCE, int);
LRESULT CALLBACK    WndProc(HWND, UINT, WPARAM, LPARAM);
INT_PTR CALLBACK    About(HWND, UINT, WPARAM, LPARAM);


DWORD thread(LPVOID lpdwThreadParam);

int APIENTRY _tWinMain(_In_ HINSTANCE hInstance,
                     _In_opt_ HINSTANCE hPrevInstance,
                     _In_ LPTSTR    lpCmdLine,
                     _In_ int       nCmdShow)
{
    UNREFERENCED_PARAMETER(hPrevInstance);
    UNREFERENCED_PARAMETER(lpCmdLine);

    // TODO: Place code here.
    MSG msg;
    HACCEL hAccelTable;

    // Initialize global strings
    LoadString(hInstance, IDS_APP_TITLE, szTitle, MAX_LOADSTRING);
    LoadString(hInstance, IDC_WIN32PROJECT6, szWindowClass, MAX_LOADSTRING);
    MyRegisterClass(hInstance);

    // Perform application initialization:
    if (!InitInstance (hInstance, nCmdShow))
    {
        return FALSE;
    }

    hAccelTable = LoadAccelerators(hInstance, MAKEINTRESOURCE(IDC_WIN32PROJECT6));

    // Main message loop:
    while (GetMessage(&msg, NULL, 0, 0))
    {
        if (!TranslateAccelerator(msg.hwnd, hAccelTable, &msg))
        {
            TranslateMessage(&msg);
            DispatchMessage(&msg);
        }
    }

    return (int) msg.wParam;
}



//
//  FUNCTION: MyRegisterClass()
//
//  PURPOSE: Registers the window class.
//
ATOM MyRegisterClass(HINSTANCE hInstance)
{
    WNDCLASSEX wcex;

    wcex.cbSize = sizeof(WNDCLASSEX);

    wcex.style          = CS_HREDRAW | CS_VREDRAW;
    wcex.lpfnWndProc    = WndProc;
    wcex.cbClsExtra     = 0;
    wcex.cbWndExtra     = 0;
    wcex.hInstance      = hInstance;
    wcex.hIcon          = LoadIcon(hInstance, MAKEINTRESOURCE(IDI_WIN32PROJECT6));
    wcex.hCursor        = LoadCursor(NULL, IDC_ARROW);
    wcex.hbrBackground  = (HBRUSH)(COLOR_WINDOW+1);
    wcex.lpszMenuName   = MAKEINTRESOURCE(IDC_WIN32PROJECT6);
    wcex.lpszClassName  = szWindowClass;
    wcex.hIconSm        = LoadIcon(wcex.hInstance, MAKEINTRESOURCE(IDI_SMALL));

    return RegisterClassEx(&wcex);
}

//
//   FUNCTION: InitInstance(HINSTANCE, int)
//
//   PURPOSE: Saves instance handle and creates main window
//
//   COMMENTS:
//
//        In this function, we save the instance handle in a global variable and
//        create and display the main program window.
//
BOOL InitInstance(HINSTANCE hInstance, int nCmdShow)
{
   HWND hWnd;

   hInst = hInstance; // Store instance handle in our global variable

   hWnd = CreateWindow(szWindowClass, szTitle, WS_OVERLAPPEDWINDOW,
      CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, NULL, NULL, hInstance, NULL);

   if (!hWnd)
   {
      return FALSE;
   }
//   PostMessage(hWnd, WM_USER + 11, 0, 0);
   MessageBox(0,"1","Message",0);
   MessageBox(0, "2", "Message", 0);
   ShowWindow(hWnd, nCmdShow);
   UpdateWindow(hWnd);

   return TRUE;
}

//
//  FUNCTION: WndProc(HWND, UINT, WPARAM, LPARAM)
//
//  PURPOSE:  Processes messages for the main window.
//
//  WM_COMMAND  - process the application menu
//  WM_PAINT    - Paint the main window
//  WM_DESTROY  - post a quit message and return
//
//
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    int wmId, wmEvent;
    PAINTSTRUCT ps;
    HDC hdc;

    switch (message)
    {
    case WM_COMMAND:
        wmId    = LOWORD(wParam);
        wmEvent = HIWORD(wParam);
        // Parse the menu selections:
        switch (wmId)
        {
        case IDM_ABOUT:
            DialogBox(hInst, MAKEINTRESOURCE(IDD_ABOUTBOX), hWnd, About);
            break;
        case IDM_EXIT:
            DestroyWindow(hWnd);
            break;
        default:
            return DefWindowProc(hWnd, message, wParam, lParam);
        }
        break;
    case WM_USER+11:
        MessageBox(hWnd,"4","Message",0);
        break;
    case WM_PAINT:
        hdc = BeginPaint(hWnd, &ps);
        CreateThread(0, 0, (LPTHREAD_START_ROUTINE)&thread, &hWnd, 0, 0);
        MessageBox(hWnd, "3", "Message", 0);
        // TODO: Add any drawing code here...
        EndPaint(hWnd, &ps);
        break;
    case WM_DESTROY:
        PostQuitMessage(0);
        break;
    default:
        return DefWindowProc(hWnd, message, wParam, lParam);
    }
    return 0;
}

// Message handler for about box.
INT_PTR CALLBACK About(HWND hDlg, UINT message, WPARAM wParam, LPARAM lParam)
{
    UNREFERENCED_PARAMETER(lParam);
    switch (message)
    {
    case WM_INITDIALOG:
        return (INT_PTR)TRUE;

    case WM_COMMAND:
        if (LOWORD(wParam) == IDOK || LOWORD(wParam) == IDCANCEL)
        {
            EndDialog(hDlg, LOWORD(wParam));
            return (INT_PTR)TRUE;
        }
        break;
    }
    return (INT_PTR)FALSE;
}


DWORD thread(LPVOID lpdwThreadParam)
{

    PostMessage(*(HWND*)(lpdwThreadParam), WM_USER + 11, 0, 0);

    return 0;
}

Answer 1

Some notes about this code:

• You should NOT create threads in WM_PAINT. Nor should you call MessageBox in WM_PAINT. WM_PAINT is only for painting your window; no other logic should be executed here. The system optimizes WM_PAINT calls and behavior can get really tricky when you start overstaying your welcome in this handler.
• You should also never cast a function call like you do with (LPTHREAD_START_ROUTINE)&thread. If your function is not the right type, change the function prototype; don't just try and hide the compiler's warnings.
• You should probably use _beginthread, because the CRT has some initialization to perform.
• You should pass hWnd as the thread parameter, not &hWnd. The pointer can go out of scope and become invalid. This is a critical bug.

To really see what's going on, you should fix these errors first.

To answer your question, the different threads have no guarantee of speed. After you split off a thread, there is now no telling what will happen next. They are asynchronous. Maybe the thread code runs faster than your main thread, maybe vice-versa.

Keep in mind that MessageBox has its own internal message loop. So the callstack at WM_USER+11 (MessageBox4) is going to look something like this:

MessageBox (4)
WndProc (WM_USER+11)
DispatchMessage
MessageBox (3), before it's actually shown
WndProc (WM_PAINT)
DispatchMessage
WinMain

So you can see that if the thread posts the message quickly enough, it will be processed before MessageBox(3) will be shown.

My guess is if you use a lighter debugging technique (OutputDebugString for example), you'll observe more predictable behavior.

Answer 2

WM_PAINT is one of the special low-priority messages.

The WM_PAINT message, the WM_TIMER message, and the WM_QUIT message, [...] are kept in the queue and are forwarded to the window procedure only when the queue contains no other messages.

MSDN - Queued Messages.

See also The Old New Thing - Paint messages will come in as fast as you let them.