[Applies to KMDF only]

A driver's EvtDmaTransactionDmaTransferComplete event callback function is called when the system-mode controller has completed the current DMA transfer.



void EvtWdfDmaTransactionDmaTransferComplete(



A handle to a DMA transaction object representing the DMA transfer that has just completed.


A handle to the framework device object that the driver specified when it called WdfDmaTransactionCreate.


The context pointer that the driver specified in a previous call to WdfDmaTransactionSetTransferCompleteCallback.


A WDF_DMA_DIRECTION-typed value that specifies the direction of the completing DMA transfer operation.


A DMA_COMPLETION_STATUS-typed value that specifies the status of the transfer.

Return value



The hardware for a bus-master DMA device typically issues an interrupt when a DMA transfer is complete. The driver then completes the DMA transfer in its EvtInterruptDpc callback function.

However, the hardware for a system-mode DMA device does not always signal DMA transfer completion by issuing an interrupt. To receive notification of DMA transfer completion, a driver for a system-mode DMA device can instead register an EvtDmaTransactionDmaTransferComplete event callback function by calling WdfDmaTransactionSetTransferCompleteCallback.

The framework calls EvtDmaTransactionDmaTransferComplete after the system DMA controller has completed the transfer, once for each transfer in a transaction.

From within its EvtDmaTransactionDmaTransferComplete callback, the driver can call the following methods to notify the framework that the transfer has completed:

WdfDmaTransactionDmaCompleted WdfDmaTransactionDmaCompletedFinal WdfDmaTransactionDmaCompletedWithLength The driver might not call one of the previous methods from EvtDmaTransactionDmaTransferComplete, opting instead to create a timer object or schedule a DPC to complete the transfer later, as needed. After WdfDmaTransactionDmaCompletedXxx returns TRUE, indicating that no more transfers are needed to complete the DMA transaction, the driver can optionally call WdfDmaTransactionExecute to initiate a subsequent transaction.

If the driver calls WdfDmaTransactionStopSystemTransfer, the framework calls EvtDmaTransactionDmaTransferComplete with a Status value of DmaCancelled. In this case, the driver should call WdfDmaTransactionDmaCompletedFinal from within EvtDmaTransactionDmaTransferComplete, and then can continue with request processing.

The driver must not manipulate the data buffers associated with the transaction until after WdfDmaTransactionDmaCompletedXxx has returned TRUE.

The driver can call WdfDmaTransactionRelease from within EvtDmaTransactionDmaTransferComplete if it needs to terminate the DMA transaction.

For more information about system-mode DMA, see Supporting System-Mode DMA.


To define an EvtDmaTransactionDmaTransferComplete callback function, you must first provide a function declaration that identifies the type of callback function you’re defining. Windows provides a set of callback function types for drivers. Declaring a function using the callback function types helps Code Analysis for Drivers, Static Driver Verifier (SDV), and other verification tools find errors, and it’s a requirement for writing drivers for the Windows operating system.

For example, to define an EvtDmaTransactionDmaTransferComplete callback function that is named MyDmaTransactionDmaTransferComplete, use the EVT_WDF_DMA_TRANSACTION_DMA_TRANSFER_COMPLETE type as shown in this code example:

Then, implement your callback function as follows.
    WDFDEVICE /* Device */,
    WDFCONTEXT Context,
    WDF_DMA_DIRECTION /* Direction */,
    PREQUEST_CONTEXT requestContext = (PREQUEST_CONTEXT) Context;
    NTSTATUS requestStatus;
    bool overrideStatus = true;
    size_t bytesTransferred;
if (DmaStatus == DmaComplete) {
    // Normal transfer completion.  Indicate this to the framework and see 
    // if there's more work to do.
    if (WdfDmaTransactionDmaCompleted(Transaction, &requestStatus) == FALSE) {
        // There are more DMA transfers to come.  The transaction 
        // may already have been completed on another processor.  
        // Return without touching it again.
        goto exit;

    requestStatus = STATUS_SUCCESS;
else {

    // Complete the entire transaction.  But throw out the status and 
    // use one derived from the DmaStatus.
    WdfDmaTransactionDmaCompletedFinal(Transaction, 0, &requestStatus);        
    // Error or cancellation.  Indicate that this was the final transfer to 
    // the framework.
    if (DmaStatus == DmaError) {
        requestStatus = STATUS_DEVICE_DATA_ERROR;
    else {

        // Cancel status should only be triggered by timeout or cancel.  Rely on 
        // someone having already set the status, which means we should lose
        // the race for BeginCompletion below.
        requestStatus = STATUS_PENDING;
        overrideStatus = false;

// Begin completion.  There's nothing special to do here if cancel or
// timeout got there first.
BeginCompletion(requestContext, requestStatus, overrideStatus);

// Record the number of bytes we transferred.
bytesTransferred = WdfDmaTransactionGetBytesTransferred(

WdfRequestSetInformation(requestContext->Request, bytesTransferred);

// Success, error or cancel, this was the last transfer in the 
// transaction.  Attempt to complete the request.
AttemptRequestCompletion(requestContext, true);

exit: return; }

bool BeginCompletion( __in PREQUEST_CONTEXT RequestContext, __in NTSTATUS CompletionStatus, __in bool ForceStatusUpdate ) { bool completionStarted;

// Grab the object lock and mark the beginning of 
// completion.

completionStarted = RequestContext->CompletionStarted;
RequestContext->CompletionStarted = true;

if ((completionStarted == false) || 
    (ForceStatusUpdate == true)) {
    RequestContext->CompletionStatus = CompletionStatus;


return !completionStarted;


VOID AttemptRequestCompletion( __in PREQUEST_CONTEXT RequestContext, __in bool TransferComplete ) { LONG refCount;

NT_ASSERTMSG("No thread has begun completion", 
             RequestContext->CompletionStarted == true);

if (TransferComplete) {
    // Unmark the request cancelable.  If that succeeds then drop the cancel reference
    if (WdfRequestUnmarkCancelable(RequestContext->Request) == STATUS_SUCCESS) {
        refCount = InterlockedDecrement(&(RequestContext->CompletionRefCount));
        NT_ASSERTMSGW(L"Reference count should not have gone to zero yet",
                      refCount != 0);
    // Stop the timer if it's been started.
    if (RequestContext->TimerStarted == true) {
        if (WdfTimerStop(RequestContext->Timer, FALSE) == TRUE) {
            // The timer was queued but will never run.  Drop its 
            // reference count.
            refCount = InterlockedDecrement(&RequestContext->CompletionRefCount);
            NT_ASSERTMSG("Completion reference count should not reach zero until "
                         L"this routine calls AttemptRequestCompletion",
                         refCount > 0);

// Drop this caller's reference.  If that was the last one then 
// complete the request.
refCount = InterlockedDecrement(&(RequestContext->CompletionRefCount));

if (refCount == 0) {
    NT_ASSERTMSGW(L"Execution reference was released, but execution "
                  L"path did not set a completion status for the "
                  RequestContext->CompletionStatus != STATUS_PENDING);
    // Timers are disposed of at passive level.  If we leave it attached to 
    // the request then we can hit a verifier issue, since the request 
    // needs to be immediately disposable at dispatch-level.
    // Delete the timer now so that we can complete the request safely.
    // At this point the timer has either expired or been succesfully 
    // cancelled so there's no race with the timer routine.
    if (RequestContext->Timer != NULL) {
        RequestContext->Timer = NULL;



The EVT_WDF_DMA_TRANSACTION_DMA_TRANSFER_COMPLETE function type is defined in the WdfDmaTransaction.h header file. To more accurately identify errors when you run the code analysis tools, be sure to add the _Use_decl_annotations_ annotation to your function definition. The _Use_decl_annotations_ annotation ensures that the annotations that are applied to the EVT_WDF_DMA_TRANSACTION_DMA_TRANSFER_COMPLETE function type in the header file are used. For more information about the requirements for function declarations, see Declaring Functions by Using Function Role Types for KMDF Drivers. For information about _Use_decl_annotations_, see Annotating Function Behavior.


Minimum supported client Windows 8
Target Platform Universal
Minimum KMDF version 1.11
Header wdfdmatransaction.h (include Wdf.h)

See also