Your screen saver module must include a ScreenSaverProc window procedure to receive and process messages for the screen saver window. The ScreenSaverProc window procedure must pass unprocessed messages to the DefScreenSaverProc function rather than to the DefWindowProc function.
Your ScreenSaverProc window procedure can substitute its own actions for the message handling performed by DefScreenSaverProc. For information about how the DefScreenSaverProc function responds to key window messages, see Section 14.5, “Functions.”
The ScreenSaverProc window procedure must be exported by including it in the EXPORTS section of your module-definition (.DEF) file.