WinApi message loop, Postmessage works like SendMessage - c++

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.