Книга: Writing Windows WDM Device Drivers

Device Objects and Device Extensions

Device Objects and Device Extensions

The kernel stores information about devices in device objects, DEVICE_OBJECT structures. The relevant DEVICE_OBJECT is passed to your driver callback routines for each interaction with a device.

For example, a Win32 call to ReadFile results in a Read IRP (i.e., with major function code IRP_MJ_READ) being sent to your device. The DriverEntry routine must set up a handler for the relevant callback. In the Wdm1 driver, this routine, Wdm1Read, is passed a pointer to the relevant device object and a pointer to the IRP.

The device object is "owned" by the kernel, but has a few fields that should be used by a device driver, as described later. However, a driver can define a block of memory called a device extension, which it can use for whatever it wants. The device extension structure for each Wdm1 device is defined in Wdm1.h as shown in Listing 5.1. Your device extension structures will probably start with the same fields as Wdm1.

Listing 5.1 Wdm1 Device Extension definition

typedef struct _WDM1_DEVICE_EXTENSION {
 PDEVICE_OBJECT fdo;
 PDEVICE_OBJECT NextStackDevice;
 UNICODE_STRING ifSymlinkName;
} WDM1_DEVICE_EXTENSION, *PWDM1_DEVICE_EXTENSION;

A device is created using the IoCreateDevice kernel call. The DeviceExtensionSize parameter gives the size of the device extension required. IoCreateDevice allocates the memory for you from the nonpaged pool. Access the device extension through the device object DeviceExtension field. For example, Wdm1 accesses the its device extension using the following code.

PWDM1_DEVICE_EXTENSION dx = (PWDM1_DEVICE_EXTENSION)fdo->DeviceExtension;

A device object is deleted using IoDeleteDevice. This routine deallocates the device extension memory. Make sure that you clean up all your device-related objects before calling IoDeleteDevice.

Оглавление книги