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Defining I/O Control Codes

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When defining new IOCTLs, it is important to remember the following rules:

  • If a new IOCTL will be available to user-mode software components, the IOCTL must be used with IRP_MJ_DEVICE_CONTROL requests. User-mode components send IRP_MJ_DEVICE_CONTROL requests by calling the DeviceIoControl, which is a Win32 function.
  • If a new IOCTL will be available only to kernel-mode driver components, the IOCTL must be used with IRP_MJ_INTERNAL_DEVICE_CONTROL requests. Kernel-mode components create IRP_MJ_INTERNAL_DEVICE_CONTROL requests by calling IoBuildDeviceIoControlRequest. For more information, see Creating IOCTL Requests in Drivers.

An I/O control code is a 32-bit value that consists of several fields. The following figure illustrates the layout of I/O control codes.

diagram illustrating the i/o control code layout.

Use the system-supplied CTL_CODE macro, which is defined in Wdm.h and Ntddk.h, to define new I/O control codes. The definition of a new IOCTL code, whether intended for use with IRP_MJ_DEVICE_CONTROL or IRP_MJ_INTERNAL_DEVICE_CONTROL requests, uses the following format:

#define IOCTL_Device_Function CTL_CODE(DeviceType, Function, Method, Access)

Choose a descriptive constant name for the IOCTL, of the form IOCTL_Device_Function, where Device indicates the type of device and Function indicates the operation. An example constant name is IOCTL_VIDEO_ENABLE_CURSOR.

Supply the following parameters to the CTL_CODE macro:

DeviceType
Identifies the device type. This value must match the value that is set in the DeviceType member of the driver's DEVICE_OBJECT structure. (See Specifying Device Types). Values of less than 0x8000 are reserved for Microsoft. Values of 0x8000 and higher can be used by vendors. Note that the vendor-assigned values set the Common bit.

FunctionCode
Identifies the function to be performed by the driver. Values of less than 0x800 are reserved for Microsoft. Values of 0x800 and higher can be used by vendors. Note that the vendor-assigned values set the Custom bit.

TransferType
Indicates how the system will pass data between the caller of DeviceIoControl (or IoBuildDeviceIoControlRequest) and the driver that handles the IRP.

Use one of the following system-defined constants:

METHOD_BUFFERED
Specifies the buffered I/O method, which is typically used for transferring small amounts of data per request. Most I/O control codes for device and intermediate drivers use this TransferType value.

For information about how the system specifies data buffers for METHOD_BUFFERED I/O control codes, see Buffer Descriptions for I/O Control Codes.

For more information about buffered I/O, see Using Buffered I/O.

METHOD_IN_DIRECT or METHOD_OUT_DIRECT
Specifies the direct I/O method, which is typically used for reading or writing large amounts of data, using DMA or PIO, that must be transferred quickly.

Specify METHOD_IN_DIRECT if the caller of DeviceIoControl or IoBuildDeviceIoControlRequest will pass data to the driver.

Specify METHOD_OUT_DIRECT if the caller of DeviceIoControl or IoBuildDeviceIoControlRequest will receive data from the driver.

For information about how the system specifies data buffers for METHOD_IN_DIRECT and METHOD_OUT_DIRECT I/O control codes, see Buffer Descriptions for I/O Control Codes.

For more information about direct I/O, see Using Direct I/O.

METHOD_NEITHER
Specifies neither buffered nor direct I/O. The I/O manager does not provide any system buffers or MDLs. The IRP supplies the user-mode virtual addresses of the input and output buffers that were specified to DeviceIoControl or IoBuildDeviceIoControlRequest, without validating or mapping them.

For information about how the system specifies data buffers for METHOD_NEITHER I/O control codes, see Buffer Descriptions for I/O Control Codes.

This method can be used only if the driver can be guaranteed to be running in the context of the thread that originated the I/O control request. Only a highest-level kernel-mode driver is guaranteed to meet this condition, so METHOD_NEITHER is seldom used for the I/O control codes that are passed to low-level device drivers.

With this method, the highest-level driver must determine whether to set up buffered or direct access to user data on receipt of the request, possibly must lock down the user buffer, and must wrap its access to the user buffer in a structured exception handler (see Handling Exceptions). Otherwise, the originating user-mode caller might change the buffered data before the driver can use it, or the caller could be swapped out just as the driver is accessing the user buffer.

For more information, see Using Neither Buffered Nor Direct I/O.

RequiredAccess
Indicates the type of access that a caller must request when opening the file object that represents the device (see IRP_MJ_CREATE). The I/O manager will create IRPs and call the driver with a particular I/O control code only if the caller has requested the specified access rights. RequiredAccess is specified by using the following system-defined constants:

FILE_ANY_ACCESS
The I/O manager sends the IRP for any caller that has a handle to the file object that represents the target device object.

FILE_READ_DATA
The I/O manager sends the IRP only for a caller with read access rights, allowing the underlying device driver to transfer data from the device to system memory.

FILE_WRITE_DATA
The I/O manager sends the IRP only for a caller with write access rights, allowing the underlying device driver to transfer data from system memory to its device.

FILE_READ_DATA and FILE_WRITE_DATA can be ORed together if the caller must have both read and write access rights.

Some system-defined I/O control codes have a RequiredAccess value of FILE_ANY_ACCESS, which allows the caller to send the particular IOCTL regardless of the access granted to the device. Examples include I/O control codes that are sent to drivers of exclusive devices.

Other system-defined I/O control codes require the caller to have read access rights, write access rights, or both. For example, the following definition of the public I/O control code IOCTL_DISK_SET_PARTITION_INFO shows that this I/O request can be sent to a driver only if the caller has both read and write access rights:

#define IOCTL_DISK_SET_PARTITION_INFO\
        CTL_CODE(IOCTL_DISK_BASE, 0x008, METHOD_BUFFERED,\
        FILE_READ_DATA | FILE_WRITE_DATA)

Note

Before specifying FILE_ANY_ACCESS for a new IOCTL code, you must be absolutely certain that allowing unrestricted access to your device does not create a possible path for malicious users to compromise the system.

Drivers can use IoValidateDeviceIoControlAccess to perform stricter access checking than that provided by an IOCTL's RequiredAccess bits.

Other useful macros

The following macros are useful for extracting the 16-bit DeviceType and 2-bit TransferType fields from an IOCTL code:

#define DEVICE_TYPE_FROM_CTL_CODE(ctrlCode)   (((ULONG)(ctrlCode & 0xffff0000)) >> 16)
#define METHOD_FROM_CTL_CODE(ctrlCode)        ((ULONG)(ctrlCode & 3))

These macros are defined in Wdm.h and Ntddk.h.