WdfDeviceInitSetIoTypeEx function (wdfdevice.h)

[Applies to KMDF and UMDF]

The WdfDeviceInitSetIoTypeEx method sets the method or preference for how a driver will access the data buffers that are included in read and write requests, as well as device I/O control requests, for a specified device.


void WdfDeviceInitSetIoTypeEx(
  PWDFDEVICE_INIT     DeviceInit,



[in] A pointer to a WDFDEVICE_INIT structure.


[in] Pointer to WDF_IO_TYPE_CONFIG structure initialized using WDF_IO_TYPE_CONFIG_INIT macro.

Return value



If you are writing a driver using KMDF version 1.11 or earlier, you must instead use WdfDeviceInitSetIoType.

KMDF A KMDF driver calls WdfDeviceInitSetIoTypeEx to set a buffer-access method for read and write requests. For device I/O control requests, the framework uses the buffer type that is encoded in the I/O control code (IOCTL).

UMDF A UMDF driver calls WdfDeviceInitSetIoTypeEx to register preferences for read and write requests, as well as device I/O control requests. The values that a UMDF driver provides to WdfDeviceInitSetIoTypeEx are only preferences, and are not guaranteed to be used by the framework. Your driver can call WdfDeviceGetDeviceStackIoType to determine the buffer access methods that UMDF has assigned to a device's read/write requests and I/O control requests. For I/O control requests, the access method that the framework uses might differ from the access method specified in the IOCTL and the access method requested by the driver.

If a driver calls WdfDeviceInitSetIoTypeEx, it must do so before it calls WdfDeviceCreate.

If the driver does not call WdfDeviceInitSetIoTypeEx, the framework sets the driver's buffer-access method to WdfDeviceIoBuffered, for the specified device.

Calling WdfDeviceInitSetIoTypeEx from a KMDF filter driver has no effect. For KMDF filter drivers, the framework uses the I/O type that the next-lower driver in the driver stack specifies.

A UMDF filter driver can, however, register preferences for buffer-access method by calling WdfDeviceInitSetIoTypeEx.

All UMDF drivers in a driver stack must use the same method for accessing a device's buffers. If UMDF determines that some drivers prefer either buffered I/O or direct I/O for a device while other drivers prefer only buffered I/O for the device, UMDF uses buffered I/O for all drivers. If one or more of a stack's drivers prefer only buffered I/O while others prefer only direct I/O, UMDF logs an event to the system event log and does not start the driver stack.

For more information about buffer-access methods, see Accessing Data Buffers.

This method is the UMDF 2.0 equivalent of IWDFDeviceInitialize2::SetIoTypePreference.


The following code example initializes a WDF_IO_TYPE_CONFIG structure, sets the driver's buffer-access preferences to direct I/O, specifies that transfers smaller than 32 KB should use buffered I/O, and calls WdfDeviceInitSetIoTypeEx.

ioConfig.ReadWriteIoType = WdfDeviceIoDirect;
ioConfig.DeviceControlIoType = WdfDeviceIoDirect;
ioConfig.DirectTransferThreshold = 32;

WdfDeviceInitSetIoTypeEx(DeviceInit, &ioConfig);


Target Platform Universal
Minimum KMDF version 1.13
Minimum UMDF version 2.0
Header wdfdevice.h (include Wdf.h)
Library Wdf01000.sys (KMDF); WUDFx02000.dll (UMDF)

See also