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Creating a Device Object

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A monolithic driver must create a device object for each physical, logical, or virtual device for which it handles I/O requests. A driver that does not create a device object for a device does not receive any IRPs for the device.

In some technology areas, a minidriver that is associated with a class driver or port driver does not have to create its own device objects. Instead, the class or port driver creates the device object, and receives all IRPs for the device. The class or port driver then uses a driver-specific method to pass the I/O request to the minidriver. See the documentation for your particular technology area to determine if Microsoft supplies a class or port driver that creates device objects on behalf of your driver.

Drivers call either IoCreateDevice or IoCreateDeviceSecure to create their device objects. For more information about which routine to use, see the following sections.

Creating Device Objects for WDM Function and Filter Drivers

Creating Device Objects for WDM Bus Drivers

Creating Device Objects for Non-WDM Drivers

When the driver creates a device object, it supplies the following information to IoCreateDevice or IoCreateDeviceSecure:

  • The size of the device's device extension. The device extension is a system-allocated storage area that the driver can use for device-specific storage. For more information, see Device Extensions.

  • A system-defined constant, indicating the DeviceType represented by the device object. For more information, see Specifying Device Types.

  • One or more ORed, system-defined constants that indicate the device characteristics for the device. For more information, see Specifying Device Characteristics.

  • A Boolean value, named Exclusive, that specifies whether a bit in the device object's Flags should be set with DO_EXCLUSIVE, indicating the driver services an exclusive device, such as a video, serial, parallel, or sound device. WDM drivers must set Exclusive to FALSE. For more information, see Specifying Exclusive Access to Device Objects.

  • A pointer to the driver object for the driver. A WDM function or filter driver receives a pointer to its driver object as a parameter to its AddDevice routine. All drivers receive a pointer to their driver object in their DriverEntry routine. The system uses this pointer to associate the driver with its device object.

  • An optional pointer to a null-terminated Unicode string (DeviceName) naming the device. WDM drivers, other than bus drivers, do not supply a device name; doing so bypasses the PnP manager's security features. For more information, see Named Device Objects.

If the call to IoCreateDevice or IoCreateDeviceSecure succeeds, the I/O manager provides storage for the device object itself and for all other data structures associated with the device object, including the device extension, which it initializes with zeros.

Creating Device Objects for WDM Function and Filter Drivers

WDM drivers, other than bus drivers, call IoCreateDevice to create their device objects. Most WDM drivers create their device objects from within their AddDevice routines. Some drivers, such as disk drivers that must respond to drive layout IOCTLs, call IoCreateDevice from a dispatch routine.

Unless device type-specific sections of the Windows Driver Kit (WDK) documentation state otherwise, your driver should create its device objects in its AddDevice routine. For more information, see Writing an AddDevice Routine.

Creating Device Objects for WDM Bus Drivers

A WDM bus driver creates a PDO when it is enumerating a new device in response to an IRP_MN_QUERY_DEVICE_RELATIONS request, if the relation type is BusRelations.

The following rules determine if a bus driver calls IoCreateDevice or IoCreateDeviceSecure to create a device object:

  • If a device can be used in raw mode, then it must call IoCreateDeviceSecure.

  • If the device is not raw-mode capable, then the bus driver can use either IoCreateDevice or IoCreateDeviceSecure. IoCreateDevice can be used when the default system security for devices on the bus is adequate; IoCreateDeviceSecure can be used to specify a more stringent security descriptor. For more information, see Controlling Device Access.

Creating Device Objects for Non-WDM Drivers

A non-WDM driver uses IoCreateDevice to create unnamed device objects, and IoCreateDeviceSecure to create named device objects. Note the unnamed device objects of a non-WDM driver are not accessible from user mode, so the driver usually must create at least one named object.