Power Messages
The system sends power messages to all applications and installable drivers
whenever a power management event occurs or whenever an application calls the
SetSystemPowerState function to suspend operation. The system sends these messages through the
WM_POWERBROADCAST message, setting the
wParam parameter to the message type. For example, the message type,
PBT_APMPOWERSTATUSCHANGE, indicates a system power status change message.
The system sends a
PBT_APMQUERYSUSPEND message to request permission to suspend system operation. The system expects
each application and driver to determine whether the requested event should
occur and to return TRUE or BROADCAST_QUERY_DENY indicating this decision. Any
application or driver can deny the request and prevent it from occurring.
The system sends a
PBT_APMSUSPEND message immediately before suspending operation. This gives applications and
drivers one last chance to prepare for the event before it occurs. In many
cases, the system sends these messages without requesting permission to do so. This
happens, for example, if an application forces suspension with the
SetSystemPowerState function.
The system sends the
PBT_APMQUERYSUSPENDFAILED message whenever a requested event is denied. These messages are intended to
notify applications and drivers to continue operation as usual.
The system sends the
PBT_APMRESUMESUSPEND or
PBT_APMRESUMECRITICAL message when system operation has been restored.
Note The
WM_POWER message, previously available to applications and drivers for use with power
management, is maintained for backward compatibility. All current applications
and installable drivers should use and process the
WM_POWERBROADCAST message instead.
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