System Wake-up Events

Your application can restore an On-Now capable computer that is in a sleeping state to the working state by using a scheduled time or a device event. This is known as a wake-up event. Use a waitable timer object to specify the time at which the system should wake. To create the object, use the CreateWaitableTimer function. To set the timer, use the SetWaitableTimer function. The pDueTime parameter specifies when the timer will be signaled. To specify that the system should wake when the timer is signaled, set the fResume parameter to TRUE.

When the system wakes automatically because of an event (other than power switch or user activity), the system automatically sets its idle timer to at least 1 minute. This timer allows applications to call the SetThreadExecutionState function to indicate that they are busy. This enables the system to shut down without affecting the user. The following criteria determine whether a system shutdown is needed:

• If the system wakes automatically, it shuts down as soon as the last job is completed, as indicated by the process calling SetThreadExecutionState.
• If the system wakes automatically, but the user provides new input while the event is handled, the system does not shut down when the last job finishes.
• If the system is on and the user is active, the system does not enter the sleeping state.
• If the system is on, no applications are busy, and the user is inactive for a long time, the system enters the sleeping state when the timer expires.

When the system wakes automatically, the system broadcasts the PBT_APMRESUMEAUTOMATIC event to all applications. Because the user is not present, most applications should do nothing. Event-handling applications, such as fax servers, should handle their events. If you wish to determine whether the system is in this state, call the IsSystemResumeAutomatic function.

If the system wakes due to user activity after broadcasting PBT_APMRESUMEAUTOMATIC, the system broadcasts the PBT_APMRESUMESUSPEND event and turns on the display. Your application should reopen files that it closed when the system entered the sleeping state, and it should prepare for user input.

If an application called SetSystemPowerState with fForce set to TRUE, no applications will receive the PBT_APMQUERYSUSPEND event when the system enters the sleeping state. If the system is carrying out a critical suspension, no applications will receive the PBT_APMQUERYSUSPEND or PBT_APMSUSPEND event; they will receive only the PBT_APMRESUMECRITICAL event.