The PBT_APMSUSPEND is sent immediately before the computer is suspended. This message is typically sent when all applications and installable drivers have returned TRUE to a previous PBT_APMQUERYSUSPEND message. A window receives this message through the WM_POWERBROADCAST message.
dwData = (DWORD) lParam;
No return value.
An application should process this message by completing all tasks necessary to save data. This message may also be sent, without a prior PBT_APMQUERYSUSPEND message, if an application or device driver uses the SetSystemPowerState function to force suspension.