C++ appcrash USER32.dll

Question

  Problem Event Name: APPCRASH
  Application Name: program.exe
  Application Version: 0.0.0.0
  Application Timestamp: 537374b6
  Fault Module Name: USER32.dll
  Fault Module Version: 6.3.9600.16384
  Fault Module Timestamp: 52157ca5
  Exception Code: c0000005
  Exception Offset: 0000949d
  OS Version: 6.3.9600.2.0.0.256.48
  Locale ID: 1049
  Additional Information 1: 5861
  Additional Information 2: 5861822e1919d7c014bbb064c64908b2
  Additional Information 3: 3a20
  Additional Information 4: 3a20a93c34687143a5bf7d33f1cf3ccc

Im trying to make C++ winapi program, the its only function is to draw a window with a button that toggles cursor move and click, but right after window is shown, the app crashes before able to do something. In window procedure i have only WM_COMMAND, WM_DESTROY and default return DefWindowProc() cases. The WinMain function code:

    WNDCLASSEX wcx;
    int X, Y;
    MSG msg;
    RECT srect;

    GetWindowRect(GetDesktopWindow(), &srect);
    X = srect.right;
    Y = srect.bottom;

    wcx.cbSize = sizeof(WNDCLASSEX);
    wcx.style = CS_HREDRAW | CS_VREDRAW | CS_DROPSHADOW;
    wcx.lpfnWndProc = MainProc;
    wcx.cbClsExtra = 0;
    wcx.cbWndExtra = 0;
    wcx.hInstance = hinst;
    wcx.hIcon = 0;
    wcx.hCursor = 0;
    wcx.hbrBackground = (HBRUSH)(COLOR_WINDOW + 1);
    wcx.lpszMenuName = 0;
    wcx.lpszClassName = L"wcname";
    wcx.hIconSm = 0;

    if (!RegisterClassEx(&wcx)){
        MessageBox(0, L".", L"RegisterClass failed", 0);
        return 1;
    }

    hwnd = CreateWindowEx(
        0,
        L"wcname",
        L"off",
        WS_OVERLAPPEDWINDOW,
        X / 2 - 112,
        Y / 2 - 40,
        224,
        80,
        0,
        0,
        hinst,
        0
    );

    if (!hwnd){
        MessageBox(0, L".", L"CreateWindow failed", 0);
        return 1;
    }

    GetClientRect(hwnd, &srect);
    X = srect.right;
    Y = srect.bottom;

    btn = CreateWindowEx(
        0,
        L"button",
        L"Turn on",
        WS_VISIBLE | WS_CHILD | BS_PUSHBUTTON,
        0, 0,
        X, Y,
        hwnd,
        (HMENU)BTN,
        hinst,
        0
    );

    ShowWindow(hwnd, nCmdShow);
    UpdateWindow(hwnd);

    while (GetMessage(0, hwnd, 0, 0)){
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }

    return 0;

The HWNDs are global variables, BTN and MAIN are macros.

Answer 1

The lpMsg argument to GetMessage is not allowed to be NULL (0).

This call:

while (GetMessage(0, hwnd, 0, 0))

Needs to be:

while (GetMessage(&msg, hwnd, 0, 0))