IWDFIoQueue::RetrieveNextRequest method (wudfddi.h)
[Warning: UMDF 2 is the latest version of UMDF and supersedes UMDF 1. All new UMDF drivers should be written using UMDF 2. No new features are being added to UMDF 1 and there is limited support for UMDF 1 on newer versions of Windows 10. Universal Windows drivers must use UMDF 2. For more info, see Getting Started with UMDF.]
The RetrieveNextRequest method retrieves the next I/O request from an I/O queue.
Syntax
HRESULT RetrieveNextRequest(
[out] IWDFIoRequest **ppRequest
);
Parameters
[out] ppRequest
A pointer to a buffer that receives a pointer to the IWDFIoRequest interface for the next request object, or receives NULL if the queue is empty or if the next request is not found.
Return value
RetrieveNextRequest returns one of the following values:
| Return code | Description |
|---|---|
|
The next I/O request was successfully retrieved from the I/O queue. |
|
The queue is not dispatching requests. This situation occurs if the device undergoes a power state transition and all of the queues are stopped from dispatching requests or if the driver explicitly called IWDFIoQueue::Stop to stop dispatching requests. This situation can also occur if the driver attempts to remove a request from a manual queue that is power managed and that is powered down or if the queue is paused. |
|
No requests were in the queue. |
|
The call was made to retrieve the request from a parallel queue. |
RetrieveNextRequest might also return other HRESULT values.
Remarks
If a driver configures an I/O queue for manual dispatching of I/O requests, the driver can call the RetrieveNextRequest method to obtain the next request from the queue. For more information about manually dispatching I/O requests, see Configuring Dispatch Mode for an I/O Queue.
If a driver configures an I/O queue for sequential dispatching of I/O requests, the driver can still call the RetrieveNextRequest method to obtain the next request from the queue without receiving an error. Although the framework permits the driver to call RetrieveNextRequest to retrieve a request from a sequential queue, the driver should only call RetrieveNextRequest before the driver completes the current request. Otherwise, if the driver attempts to call RetrieveNextRequest after the driver completes the current request, a race condition might occur. This race condition occurs between the framework's automatic dispatching of the next request from the sequential queue and the driver's call to RetrieveNextRequest to retrieve the next request.
Examples
The following code example, which is from the umdf_fx2 sample driver, polls the queue for requests for as long as requests can be retrieved. The code first verifies if requests are associated with a specific file object.
VOID
CMyDevice::ServiceSwitchChangeQueue(
__in SWITCH_STATE NewState,
__in HRESULT CompletionStatus,
__in_opt IWDFFile *SpecificFile
)
{
IWDFIoRequest *fxRequest;
HRESULT enumHr = S_OK;
do {
HRESULT hr;
//
// Get the next request.
//
if (NULL != SpecificFile) {
enumHr = m_SwitchChangeQueue->RetrieveNextRequestByFileObject(
SpecificFile,
&fxRequest
);
}
else {
enumHr = m_SwitchChangeQueue->RetrieveNextRequest(&fxRequest);
}
//
// If a request was retrieved, complete it.
//
if (S_OK == enumHr) {
if (S_OK == CompletionStatus) {
IWDFMemory *fxMemory;
//
// Copy the result to the request buffer.
//
fxRequest->GetOutputMemory(&fxMemory);
hr = fxMemory->CopyFromBuffer(0,
&NewState,
sizeof(SWITCH_STATE));
fxMemory->Release();
}
else {
hr = CompletionStatus;
}
//
// Complete the request with the copy or
// completion status.
//
if (S_OK == hr) {
fxRequest->CompleteWithInformation(hr,
sizeof(SWITCH_STATE));
}
else {
fxRequest->Complete(hr);
}
fxRequest->Release();
}
}
while (S_OK == enumHr);
}
Requirements
| Requirement | Value |
|---|---|
| End of support | Unavailable in UMDF 2.0 and later. |
| Target Platform | Desktop |
| Minimum UMDF version | 1.5 |
| Header | wudfddi.h (include Wudfddi.h) |
| DLL | WUDFx.dll |