WdfIoTargetFormatRequestForRead function (wdfiotarget.h)

Article
04/18/2022

[Applies to KMDF and UMDF]

The WdfIoTargetFormatRequestForRead method builds a read request for an I/O target but does not send the request.

Syntax

NTSTATUS WdfIoTargetFormatRequestForRead(
  [in]           WDFIOTARGET       IoTarget,
  [in]           WDFREQUEST        Request,
  [in, optional] WDFMEMORY         OutputBuffer,
  [in, optional] PWDFMEMORY_OFFSET OutputBufferOffset,
  [in, optional] PLONGLONG         DeviceOffset
);

Parameters

[in] IoTarget

A handle to a local or remote I/O target object that was obtained from a previous call to WdfDeviceGetIoTarget or WdfIoTargetCreate, or from a method that a specialized I/O target supplies.

[in] Request

A handle to a framework request object. For more information, see the following Remarks section.

[in, optional] OutputBuffer

A handle to a framework memory object. This object represents a buffer that will receive data from the I/O target. This parameter is optional and can be NULL. For more information about this parameter, see the following Remarks section.

[in, optional] OutputBufferOffset

A pointer to a caller-allocated WDFMEMORY_OFFSET structure that supplies optional byte offset and length values. The framework uses these values to determine the beginning address and length, within the output buffer, for the data transfer. If this pointer is NULL, the data transfer begins at the beginning of the output buffer, and the transfer size is the buffer size.

[in, optional] DeviceOffset

A pointer to a variable that specifies a starting offset for the transfer. The I/O target (that is, the next-lower driver) defines how to use this value. For example, the drivers in a disk's driver stack might specify an offset from the beginning of the disk. The I/O target obtains this information in the Parameters.Read.DeviceOffset member of the request's WDF_REQUEST_PARAMETERS structure. This pointer is optional. Most drivers set this pointer to NULL.

Return value

WdfIoTargetFormatRequestForRead returns STATUS_SUCCESS if the operation succeeds. Otherwise, this method might return one of the following values:

Return code	Description
STATUS_INVALID_PARAMETER	An invalid parameter was detected.
STATUS_INVALID_DEVICE_REQUEST	The transfer length was larger than the buffer length, or the I/O request was already queued to an I/O target.
STATUS_INSUFFICIENT_RESOURCES	The framework could not allocate system resources (typically memory).
STATUS_REQUEST_NOT_ACCEPTED	The I/O request packet (IRP) that the Request parameter represents does not provide enough IO_STACK_LOCATION structures to allow the driver to forward the request.

This method also might return other NTSTATUS values.

A bug check occurs if the driver supplies an invalid object handle.

Remarks

Use the WdfIoTargetFormatRequestForRead method, followed by the WdfRequestSend method, to send read requests either synchronously or asynchronously. Alternatively, use the WdfIoTargetSendReadSynchronously method to send read requests synchronously.

You can forward an I/O request that your driver received in an I/O queue, or you can create and send a new request. In either case, the framework requires a request object and some buffer space.

To forward an I/O request that your driver received in an I/O queue:

Specify the received request's handle for the WdfIoTargetFormatRequestForRead method's Request parameter.
Use the received request's output buffer for the WdfIoTargetFormatRequestForRead method's OutputBuffer parameter.
The driver must call WdfRequestRetrieveOutputMemory to obtain a handle to a framework memory object that represents the request's output buffer and must use that handle as the value for OutputBuffer.

For more information about forwarding an I/O request, see Forwarding I/O Requests.

Drivers often divide received I/O requests into smaller requests that they send to an I/O target, so your driver might create new requests.

To create a new I/O request:

Create a new request object and supply its handle for the WdfIoTargetFormatRequestForRead method's Request parameter.
Call WdfRequestCreate to preallocate one or more request objects. You can reuse these request objects by calling WdfRequestReuse. Your driver's EvtDriverDeviceAdd callback function can preallocate request objects for a device.
Provide buffer space, and supply the buffer's handle for the WdfIoTargetFormatRequestForRead method's OutputBuffer parameter.
Your driver must specify this buffer space as a WDFMEMORY handle to framework-managed memory. Your driver can do either of the following:
- Call WdfMemoryCreate or WdfMemoryCreatePreallocated to create a new memory buffer, if you want the driver to pass a new buffer to the I/O target.
- Call WdfRequestRetrieveOutputMemory to obtain a handle to the memory object that represents a received I/O request's buffer, if you want the driver to pass that buffer's contents to the I/O target.
Note that if your driver calls WdfRequestRetrieveOutputMemory and passes the memory handle to WdfIoTargetFormatRequestForRead, the driver must not complete the received I/O request until after the driver deletes, reuses, or reformats the new, driver-created request object. (WdfIoTargetFormatRequestForRead increments the memory object's reference count. Deleting, reusing, or reformatting a request object decrements the memory object's reference count.)

Some I/O targets accept read requests that have a zero-length buffer. For such I/O targets, your driver can specify NULL for the OutputBuffer parameter.

After a driver calls WdfIoTargetFormatRequestForRead to format an I/O request, it must call WdfRequestSend to send the request (either synchronously or asynchronously) to an I/O target.

Multiple calls to WdfIoTargetFormatRequestForRead that use the same request do not cause additional resource allocations. Therefore, to reduce the chance that WdfRequestCreate will return STATUS_INSUFFICIENT_RESOURCES, your driver's EvtDriverDeviceAdd callback function can call WdfRequestCreate to preallocate one or more request objects for a device. The driver can subsequently reuse (call WdfRequestReuse), reformat (call WdfIoTargetFormatRequestForRead), and resend (call WdfRequestSend) each request object without risking a STATUS_INSUFFICIENT_RESOURCES return value from a later call to WdfRequestCreate. All subsequent calls to WdfIoTargetFormatRequestForRead for the reused request object will return STATUS_SUCCESS, if parameter values do not change. (If the driver does not call the same request-formatting method each time, additional resources might be allocated.)

For information about obtaining status information after an I/O request completes, see Obtaining Completion Information.

For more information about WdfIoTargetFormatRequestForRead, see Sending I/O Requests to General I/O Targets.

For more information about I/O targets, see Using I/O Targets.

Examples

The following code example creates a framework memory object for a read request's output buffer, formats the read request, registers a CompletionRoutine callback function, and sends the read request to an I/O target.

WDFREQUEST  request;
NTSTATUS  status;
WDFMEMORY  memory;
WDF_OBJECT_ATTRIBUTES  attributes;

WDF_OBJECT_ATTRIBUTES_INIT(&attributes);
status = WdfMemoryCreate(
                         &attributes,
                         NonPagedPool,
                         DRIVER_TAG,
                         READ_BUF_SIZE,
                         &memory,
                         NULL
                         );

if (!NT_SUCCESS(status)) {
    return status;
}

status = WdfIoTargetFormatRequestForRead(
                                         IoTarget,
                                         request,
                                         memory,
                                         NULL,
                                         NULL
                                         );
if (!NT_SUCCESS(status)) {
        return status;
}
WdfRequestSetCompletionRoutine(
                               request,
                               MyReadRequestCompletionRoutine,
                               targetInfo
                               );
if (WdfRequestSend(
                   request,
                   IoTarget,
                   WDF_NO_SEND_OPTIONS
                   ) == FALSE) {
        status = WdfRequestGetStatus(request);
}

Requirements

Requirement	Value
Target Platform	Universal
Minimum KMDF version	1.0
Minimum UMDF version	2.0
Header	wdfiotarget.h (include Wdf.h)
Library	Wdf01000.sys (KMDF); WUDFx02000.dll (UMDF)
IRQL	<=DISPATCH_LEVEL
DDI compliance rules	DriverCreate(kmdf), KmdfIrql(kmdf), KmdfIrql2(kmdf), KmdfIrqlExplicit(kmdf), RequestFormattedValid(kmdf), RequestSendAndForgetNoFormatting(kmdf), RequestSendAndForgetNoFormatting2(kmdf)