4.4.1 Naming Device Objects

NT drivers’ device objects have names for two basic reasons:

• So that higher-level NT drivers can layer themselves over lower-level NT drivers

  The DriverEntry routine of each higher-level NT driver must pass a pointer to the name of the next-lower-level driver’s device object when it calls IoGetDeviceObjectPointer or IoAttachDevice. NT drivers at every level should name their device objects to support the addition of value-added, higher-level NT drivers.

• So that user-mode applications can carry out I/O operations

  Any driver of a physical, logical, or virtual device to which user-mode code can direct I/O requests must name the device object it creates. For example, when an application or end user calls a Win32® function to connect to a printer port, such as LPT1, the Win32 subsystem translates the application I/O request into a call to one of the NT I/O Manager’s system services with the name of a file object that represents the printer port.

  Such a named file object has an associated symbolic link object in the Win32 namespace, \??, that aliases the Win32 name to the name of the appropriate device object in the NT namespace for device objects. The NT Object Manager and I/O Manager parse the input file object name and chase the symbolic link to identify the target device for the I/O operation. Then, the I/O Manager sets up an IRP for the system parallel class driver.

The system uses a simple naming scheme to identify the NT devices in every machine. In general, each device object that can be the target of an I/O request or can be connected to by a higher-level NT driver has a name of the form:

\Device\GenericDeviceTypeDigit

where

GenericDeviceType indicates the kind of device, such as Harddisk, Serial, Tape, etc.

and

The Digit suffix indicates the zero-based number of this device, depending on how many other devices of the same type have already been named.

Every NT driver must follow these naming conventions for device objects.

Note that each GenericDeviceType guarantees that all device objects representing a particular type of device have the same name, so NT higher-level drivers can readily find device objects representing the set of underlying physical devices over which such a higher-level driver can layer itself. Note also that the Digit suffix guarantees that each device object of that type has a unique name in every Windows NT machine. For more information about how to assign an appropriate Digit suffix to a named device object, see also the section on using the registry in Chapter 16.

The DriverEntry routine is responsible for setting up a unique-to-the-machine Unicode string to name each device object that an NT driver creates, except for any device object that cannot be a target for an I/O request or that cannot be connected to by a higher-level driver.