#include <windows.h>
#include <stdio.h>
#include <string.h>
 
// Global variables
HINSTANCE hInst;
UINT uMyMsgWndMoving;
HINSTANCE mhInst;
HWND mhWnd; // handle to own window (initialize in WinMain)
HDC mhDC; // device context (initialize in each Paint function calling)
 
// Function prototypes.
int WINAPI WinMain(HINSTANCE, HINSTANCE, LPSTR, int);
LRESULT CALLBACK MainWndProc(HWND, UINT, WPARAM, LPARAM);
void onPaint(HWND hWnd);
BOOL CALLBACK PrintEnumWindowsProc(HWND hWnd,LPARAM lParam);
 
// Application entry point.
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow) {
    MSG msg;
    BOOL bRet;
    WNDCLASS wcx; // register class
 
    hInst = hInstance; // Save the application-instance handle.
    uMyMsgWndMoving = RegisterWindowMessage("MyMessage");
 
    // Fill in the window class structure with parameters that describe the main window.
    wcx.style = CS_HREDRAW | CS_VREDRAW; // redraw if size changes
    wcx.lpfnWndProc = (WNDPROC) MainWndProc; // points to window procedure
    wcx.cbClsExtra = 0; // no extra class memory
    wcx.cbWndExtra = 0; // no extra window memory
    wcx.hInstance = hInstance; // handle to instance
    wcx.hIcon = LoadIcon(NULL, IDI_APPLICATION); // predefined app. icon
    wcx.hCursor = LoadCursor(NULL, IDC_ARROW); // predefined arrow
    wcx.hbrBackground = GetStockObject(WHITE_BRUSH); // white background brush
    wcx.lpszMenuName = (LPCSTR) "MainMenu"; // name of menu resource
    wcx.lpszClassName = (LPCSTR) "ITUClass"; // name of window class
 
    // Register the window class.
    if (!RegisterClass(&wcx)) {
	return FALSE;
    }
 
    // create window of registered class
    mhWnd = CreateWindow("ITUClass", "ITU", WS_OVERLAPPEDWINDOW, 0, 0, 200, 200, (HWND) NULL, (HMENU) NULL, hInstance, (LPVOID) NULL);
    if (!mhWnd) {
	return FALSE;
    }
 
    // Show the window and send a WM_PAINT message to the window procedure.
    // Record the current cursor position.
    ShowWindow(mhWnd, nCmdShow);
    UpdateWindow(mhWnd);
 
    while ((bRet = GetMessage(&msg, NULL, 0, 0 )) != 0) {
        if (bRet == -1) {
            // handle the error and possibly exit
        } else {
            TranslateMessage(&msg);
            DispatchMessage(&msg);
        }
    }
 
    PostMessage(HWND_BROADCAST, uMyMsgWndMoving, 0, 0);
    return (int) msg.wParam;
}
 
LRESULT CALLBACK MainWndProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam) {
    InvalidateRect(hWnd, NULL, 0);
 
    // Some of the application instance is moving
    if (uMsg == uMyMsgWndMoving) {
        InvalidateRect(hWnd, NULL, 1);
        return 0;
    }
 
    switch (uMsg) {
	case WM_CREATE:
	    // Initialize the window.
	    return 0;
 
	case WM_SIZE:
            // Set the size and position of the window.
	    PostMessage(HWND_BROADCAST, uMyMsgWndMoving, 0, 0);
	    return 0;
 
	case WM_MOVE:
	    // Set the new window position. (Send it to all application instances.)
	    PostMessage(HWND_BROADCAST, uMyMsgWndMoving, 0, 0);
	    return 0;
 
	case WM_PAINT:
            // Paint the window's client area.
            onPaint(hWnd);
            return 0;
 
	case WM_DESTROY:
            // Clean up window-specific data objects.
            PostQuitMessage(0);
            return 0;
 
        default:
            return DefWindowProc(hWnd, uMsg, wParam, lParam);
    }
}
 
void onPaint(HWND hWnd) {
    PAINTSTRUCT ps; // information can be used to paint the client area of a window owned by that application  
    mhDC = BeginPaint(hWnd, &ps); // prepares the specified window for painting (use global variable)
    EnumWindows(&PrintEnumWindowsProc, 0); // process PrintEnumWindowsProc function to all existed windows
    DeleteDC(mhDC); // deletes the specified device context
    EndPaint(hWnd, &ps); // marks the end of painting in the specified window
}
 
BOOL CALLBACK PrintEnumWindowsProc(HWND hWnd, LPARAM lParam) {
    char WClassName[256];
    char mWClassName[256];
    RECT myRect, otherRect;
 
    GetClassName(hWnd, WClassName,  256); // get class name of some system window
    GetClassName(mhWnd, mWClassName, 256); // get class name of your own window
 
    if ((strcmp(WClassName, mWClassName) == 0) && (hWnd != mhWnd)) {
	GetWindowRect(mhWnd, &myRect);
	GetWindowRect(hWnd, &otherRect);
 
	int myX = (myRect.right - myRect.left) / 2;
	int myY = (myRect.bottom - myRect.top) / 2;
	int otherX = otherRect.left - myRect.left + (otherRect.right - otherRect.left) / 2;
	int otherY = otherRect.top - myRect.top + (otherRect.bottom - otherRect.top) / 2;
	MoveToEx(mhDC, myX, myY, NULL);
	LineTo(mhDC, otherX, otherY);
    }
 
    return TRUE;
}