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Using a System-Defined Callback Object

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The system defines three callback objects for driver use:

\Callback\SetSystemTime

\Callback\PowerState

\Callback\ProcessorAdd

Drivers that use the system time (for example, file system drivers) might register for the \Callback\SetSystemTime callback object. This callback provides for notification when the system time changes.

The \Callback\PowerState callback object provides for notification when one of the following occurs:

  • The system switches from AC to DC power or vice versa.

  • The system power policy changes as the result of a user or application request.

  • A transition to a system sleep or shutdown state is imminent. A driver can request notification so that it can lock code into memory in anticipation of shutdown. Callback routines will be notified before the power manager sends the system set-power IRP.

The \Callback\ProcessorAdd callback provides notification when a new processor is added to the system.

To use a system-defined callback, a driver initializes an attribute block by calling InitializeObjectAttributes with the callback's name, then opens the callback object (ExCreateCallback), just as for a driver-defined callback. The driver should not request that the callback object be created.

With the handle returned by ExCreateCallback, the driver calls ExRegisterCallback to register a notification routine, passing a pointer to an arbitrary context and a pointer to its routine. A driver can register its callback routine any time. When the specified condition occurs, the system calls the registered callback routine at IRQL<=DISPATCH_LEVEL.

When the driver no longer requires notification, it should call ExUnregisterCallback to delete its callback routine from the list of registered callbacks and to remove its reference to the callback object.