Question

In an OpenGL win32 program developed in Visual Studio C++ 2010, the debug version works correctly no matter what, the release version works only if I insert a MessageBox() diagnostic message within a certain range of my code. Of course this is a problem because now I cannot debug my release version using MessageBox() which is what I usually do. Moreover, if I put the MessageBox() in another part of code (inside the offending Display loop), instead of making the program work correctly it will make the cursor spaz into hourglass/non-hourglass and it will not display a MessageBox(), almost as if the MessageBox() code is being called every display frame but is not doing anything right.

Insight as to what might be causing the bug, if MessageBox() fixes it magically in some places.

Answer 1

MessageBox() has a windows message pump cycle built into it to service window messages.

Somewhere in your application at a very low level within your design, you need a windows message loop to service messages for the window hosting your openGL content. This should run from within the thread that created the window. The ill effects you are seeing in other places could easily be caused by contention side-effects when MessageBox() is invoked from a different thread.

Here is an intro to the operation of window messages at MSDN.

Using Messages and Message Queues @ MSDN

Very simply put (via a code sample from wikipedia), you need a standing loop akin to the following:

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow)
{
    MSG msg;
    while(GetMessage(&msg, NULL, 0, 0) > 0)
    {
        TranslateMessage(&msg);
        DispatchMessage(&msg);
    }
    return msg.wParam;
}

Depending on how you have handled updating the window, if you are using paint messages (WM_PAINT), you need to ensure these are dispatched as quickly as possible, or you may need to suppress them entirely if you are drawing direct to the the device DC. I’m mentioning this because you may find additional bugs once a message loop is properly setup.

WM_PAINT message @ MSDN