Timer Objects and DPCs

Any driver can use a timer object within a nonarbitrary thread context to time out operations within the driver's other routines, or to perform a periodic operation. A driver calls KeInitializeTimer or KeInitializeTimerEx to set up a timer object.

A timer can be set to expire just once, or to expire repeatedly after a given interval. KeSetTimer always sets a timer that will expire just once. KeSetTimerEx accepts an optional Period parameter, which specifies a recurring timer interval.

An optional CustomTimerDpc routine (a type of deferred procedure call) can be associated with either a notification timer or a synchronization timer. This routine executes when the specified time interval expires. For more information, see Using Timer Objects.

A timer can be a notification timer or a synchronization timer.

• When a notification timer is signaled, all waiting threads have their wait satisfied. The state of the timer remains signaled until it is explicitly reset.
• When a synchronization timer expires, its state is set to Signaled until a single waiting thread is released. Then the timer is reset to the Not-Signaled state.

KeInitializeTimer always creates notification timers. KeInitializeTimerEx accepts a Type parameter, which can be NotificationTimer or SynchronizationTimer.

This section contains the following topics, which provide more information about timer objects and DPCs:

Using Timer Objects

Registering and Queuing a CustomTimerDpc Routine

Providing CustomTimerDpc Context Information

Using a CustomTimerDpc Routine