File Handle IO Targets in UMDF Drivers


File Handle I/O Targets in UMDF Drivers

A UMDF driver communicates with some devices by using a Windows file handle, as in the following examples:

  • A device that is exposed through the Windows socket API.

  • A device that is emulated using Windows named pipes.

  • A device that is emulated using the Windows file system.

The driver obtains the file handle by calling a Windows function such as socket, CreateFile, or CreateNamedPipe that returns a file handle.

A driver can bind this handle to a framework I/O target by using the FileHandle I/O target. The driver can then use the framework's I/O target interface to send I/O requests to the file handle, thereby getting all the benefits of I/O targets.

The IWDFFileHandleTargetFactory interface creates an I/O target that is associated with a file handle. A driver gets a pointer to this interface by querying the framework device object. The driver can then call the CreateFileHandleTarget method and pass the file handle to create the I/O target object in the framework.

The framework device object is the parent of the I/O target object, so the lifetime of the I/O target object is by default the same as that of the device object. If the driver finishes using an I/O target object while the device object remains active, the driver can delete the I/O target object by calling IWDFObject::DeleteWdfObject.

Listing 9-16 shows how a UMDF driver creates a FileHandle I/O target that represents a named pipe.

Listing 9-16: Creating a file handle I/O target in a UMDF driver

image from book
 HANDLE m_WriteHandle;          // Device Handle IWDFIoTarget * m_WriteTarget;  // I/O target HRESULT hr = S_OK; IWDFFileHandleTargetFactory * pFileHandleTargetFactory = NULL; // Create a pipe and get the handle. m_WriteHandle = CreateNamedPipe(NP_NAME,                                 . . . //Additional parameters omitted for brevity); if (SUCCEEDED(hr)) {     hr = m_FxDevice->QueryInterface(IID_PPV_ARGS(&pFileHandleTargetFactory));     if (SUCCEEDED(hr)) {         hr = pFileHandleTargetFactory->CreateFileHandleTarget (m_WriteHandle,                                                                &m_WriteTarget);     } } . . . //Additional code omitted SAFE_RELEASE(pFileHandleTargetFactory); 
image from book

In the listing, the driver calls the Windows CreateNamedPipe function to open a handle to a named pipe. If this function succeeds, the driver queries the framework's device object for a pointer to the IWDFFileHandleTargetFactory interface. It then calls the CreateFileHandleTarget method, which creates a framework I/O target object that corresponds to the file handle and returns a pointer to the I/O target object's IWDFIoTarget interface. When the driver has finished using the IWDFFileHandleTargetFactory interface, it releases its reference on the interface.

After creating the I/O target, the driver can call methods on the IWDFIoTarget and IWDFIoTargetStateManagement interfaces to format I/O requests for the target device, get information about the target, and manage the state of the target.

image from book
Why Should I Use a File Handle I/O Target Instead of the Windows API?

You should use a file handle I/O target if your driver can create a Windows file handle for the target and your UMDF driver is not loaded as part of the device stack for the target devnode. For example, if your driver uses a socket, you should use a file handle I/O target. However, if your driver loads as part of the device stack for the target devnode, you should just use the default I/O target.

Although you can use the Windows API directly, you should consider using a file handle I/O target for the following reasons:

  • I/O flows through all the drivers in the UMDF device stack.

  • You get all the advantages of a WDF I/O target: the ability to manage the state of the target, I/O processing based on the target's state, and the ability to send I/O with various configurations, such as with or without a time-out or in a synchronous or asynchronous manner.

  • The I/O target coordinates I/O completion and cancellation, which can be tricky to get right especially if your driver handles multiple outstanding I/O requests.

For the same reasons, you should use an I/O target to communicate with the kernel-mode portion of your device stack.
-Praveen Rao, Windows Driver Foundation Team, Microsoft

image from book




Developing Drivers with the Microsoft Windows Driver Foundation
Developing Drivers with the Windows Driver Foundation (Pro Developer)
ISBN: 0735623740
EAN: 2147483647
Year: 2007
Pages: 224

Similar book on Amazon
Windows Internals, Part 1: Covering Windows Server 2008 R2 and Windows 7
Windows Internals, Part 1: Covering Windows Server 2008 R2 and Windows 7
Windowsu00ae Internals: Including Windows Server 2008 and Windows Vista, Fifth Edition (Pro Developer)
Windowsu00ae Internals: Including Windows Server 2008 and Windows Vista, Fifth Edition (Pro Developer)
Advanced Windows Debugging
Advanced Windows Debugging
The Windows 2000 Device Driver Book: A Guide for Programmers (2nd Edition)
The Windows 2000 Device Driver Book: A Guide for Programmers (2nd Edition)

flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net