WdfRequestRetrieveOutputBuffer function (wdfrequest.h)

[Applies to KMDF and UMDF]

The WdfRequestRetrieveOutputBuffer method retrieves an I/O request's output buffer.

Syntax

NTSTATUS WdfRequestRetrieveOutputBuffer(
 [in] WDFREQUEST Request,
 [in] size_t MinimumRequiredSize,
 [out] PVOID *Buffer,
 [out, optional] size_t *Length
);

Parameters

[in] Request

A handle to a framework request object.

[in] MinimumRequiredSize

The minimum buffer size, in bytes, that the driver needs to process the I/O request.

[out] Buffer

A pointer to a location that receives the buffer's address.

[out, optional] Length

A pointer to a location that receives the buffer's size, in bytes. This parameter is optional and can be NULL.

Return value

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

This method might also return other NTSTATUS values.

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

Remarks

A request's output buffer receives information, such as data from a disk, that the driver provides to the originator of the request. Your driver can call WdfRequestRetrieveOutputBuffer to obtain the output buffer for a read request or a device I/O control request, but not for a write request (because write requests do not provide output data).

The WdfRequestRetrieveOutputBuffer method retrieves the output buffer for I/O requests that use the buffered I/O method or the direct I/O method for accessing data buffers. If the request's I/O control code is IRP_MJ_INTERNAL_DEVICE_CONTROL, or if the request came from another kernel-mode driver, WdfRequestRetrieveOutputBuffer also supports I/O requests that use neither buffered nor direct I/O.

If WdfRequestRetrieveOutputBuffer returns STATUS_SUCCESS, the driver receives the address and, optionally, the size of the output buffer.

The driver can access the retrieved buffer until it completes the I/O request that the Request parameter represents.

Instead of calling WdfRequestRetrieveOutputBuffer, the driver can call WdfRequestRetrieveOutputMemory, which creates a framework memory object that represents the buffer.

For more information about WdfRequestRetrieveOutputBuffer, see Accessing Data Buffers in Framework-Based Drivers.

Examples

The following code example is part of an EvtIoDeviceControl callback function. This example obtains a USB device's configuration descriptor and places the descriptor in the I/O request's output buffer.

VOID
MyEvtIoDeviceControl(
 IN WDFQUEUE Queue,
 IN WDFREQUEST Request,
 IN size_t OutputBufferLength,
 IN size_t InputBufferLength,
 IN ULONG IoControlCode 
 )
{
 WDFDEVICE device;
 PDEVICE_CONTEXT pDevContext;
 size_t bytesReturned = 0;
 NTSTATUS status;

 device = WdfIoQueueGetDevice(Queue);
 //
 // GetDeviceContext is a driver-defined function 
 // to retrieve device object context space.
 //
 pDevContext = GetDeviceContext(device);

 switch(IoControlCode) {

 case IOCTL_OSRUSBFX2_GET_CONFIG_DESCRIPTOR: {
 
 PUSB_CONFIGURATION_DESCRIPTOR configurationDescriptor = NULL;
 USHORT requiredSize;

 //
 // First, get the size of the USB configuration descriptor.
 //
 status = WdfUsbTargetDeviceRetrieveConfigDescriptor(
 pDevContext->UsbDevice,
 NULL,
 &requiredSize
 );
 if (status == STATUS_BUFFER_TOO_SMALL) {
 break;
 }

 //
 // Get the buffer. Make sure the buffer is big
 // enough to hold the configuration descriptor.
 //
 status = WdfRequestRetrieveOutputBuffer(
 Request, 
 (size_t)requiredSize,
 &configurationDescriptor,
 NULL
 );
 if(!NT_SUCCESS(status)){
 break;
 }
 //
 // Now get the config descriptor.
 //
 status = WdfUsbTargetDeviceRetrieveConfigDescriptor(
 pDevContext->UsbDevice,
 configurationDescriptor,
 &requiredSize
 );
 if (!NT_SUCCESS(status)) {
 break;
 }

 bytesReturned = requiredSize;
 }
 break;
...
 (Other case statements removed.)
...
 default:
 status = STATUS_INVALID_DEVICE_REQUEST;
 break;
 }
 //
 // Complete the request.
 //
 WdfRequestCompleteWithInformation(
 Request,
 status,
 bytesReturned
 );
 return;
}

Requirements

See also

WdfRequestRetrieveInputBuffer

WdfRequestRetrieveOutputMemory