Initializing a Device Object

After IoCreateDevice returns, giving the caller a pointer to a DeviceObject that contains a pointer to the device extension, drivers must set up certain fields in the device objects for their respective physical, logical, and/or virtual devices.

IoCreateDevice sets the StackSize field of a newly created device object to one. A lowest-level driver can ignore this field. When a higher-level driver calls IoAttachDeviceToDeviceStack to attach itself to the next-lower driver, that routine automatically sets the StackSize field in the device object to that of the next-lower driver's device object plus one. For some device types, however, the higher-level driver might need to set the StackSize field to a greater value, as noted in the device-specific documentation. Setting the stack size ensures that IRPs sent to the higher-level driver will contain a driver-specific I/O stack location, plus the correct number of I/O stack locations for all lower-level drivers in the chain.

IoCreateDevice sets the AlignmentRequirement field of a newly created device object to the processor's data cache line size minus one, to ensure that buffers used in direct I/O are aligned correctly. After IoCreateDevice returns, lowest-level physical device drivers must do the following:

  1. Subtract one from the alignment requirement of the device.

  2. Compare the result of stepĀ 1 with the current value of the device object's AlignmentRequirement.

  3. If the device's alignment requirement is greater, set AlignmentRequirement to the result of stepĀ 1. Otherwise, leave the AlignmentRequirement value as set by IoCreateDevice.

After any higher-level driver chains itself over another driver by calling IoGetDeviceObjectPointer, the higher-level driver must set the AlignmentRequirement field of its newly created device object to that of the next-lower-level driver's device object. As a general rule, a higher-level driver should not change this value. If a higher-level driver calls IoAttachDevice or IoAttachDeviceToDeviceStack, those routines automatically set the AlignmentRequirement field in the device object to that of the lower-level driver's device object.

IoGetDeviceObjectPointer returns pointers both to the lower-level driver's device object and to the associated file object. Only an FSD (or, possibly, another highest-level driver) can use the returned file object pointer. An intermediate driver that calls IoGetDeviceObjectPointer should save this file object pointer so it can be dereferenced by calling ObDereferenceObject when the driver is unloaded.

After an FSD mounts the volume containing the file object that represents a lower driver's device object, an intermediate driver cannot chain itself between the file system and the lower driver by calling IoAttachDevice or IoAttachDeviceToDeviceStack. Additionally, an FSD can set the SectorSize member of the device object based on the geometry of the underlying volume hardware when a mount occurs. For more information, see DEVICE_OBJECT.

An intermediate or lowest-level driver also sets a bit in the device object's Flags by ORing it either with DO_DIRECT_IO or with DO_BUFFERED_IO in every device object it creates. Highest-level drivers of logical or virtual devices can avoid setting Flags for either buffered or direct I/O if the driver writer decides the additional work involved will pay off in better driver performance. An intermediate driver must set up the Flags field of its device object to match that of the next-lower driver's device object.

Setting up a device object Flags field with DO_DIRECT_IO or DO_BUFFERED_IO determines how the I/O manager passes access to user buffers in all data transfer requests subsequently sent to the driver.

The driver can then set any other device-dependent values in the device object. For example, non-WDM drivers for removable-media devices must OR the device object's Flags member with DO_VERIFY_VOLUME if they detect (or suspect) a change in media during I/O operations. (See Supporting Removable Media for more information.) Drivers of devices that require inrush power must OR the Flags member with DO_POWER_INRUSH, and drivers of devices that are not on the system paging path must OR the Flags member with DO_POWER_PAGABLE. Function and filter drivers must clear the DO_DEVICE_INITIALIZING flag.

After initializing the device object, a driver can also initialize any Kernel-defined objects and other system-defined data structures for which it has provided storage in the device extension. Precisely when a driver performs these tasks depends on its device, the type of the object, and/or the nature of the data. In general, any objects or data structures that can persist through PnP start and stop requests can be initialized in the AddDevice routine. Those that require resource information provided with a PnP IRP_MN_START_DEVICE request, or that might require changes when the device is stopped and/or restarted, should be initialized when the driver handles the IRP_MN_START_DEVICE request. For more information about AddDevice routines, see Writing an AddDevice Routine.