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RIOCreateCompletionQueue function

The RIOCreateCompletionQueue function creates an I/O completion queue of a specific size for use with the Winsock registered I/O extensions.

Syntax


RIO_CQ  RIOCreateCompletionQueue(
  _In_      DWORD QueueSize,
  _In_opt_  RIO_NOTIFICATION_COMPLETION NotificationCompletion
);

Parameters

QueueSize [in]

The size, in number of entries, of the completion queue to create.

NotificationCompletion [in, optional]

The type of notification completion to use based on the Type member of the RIO_NOTIFICATION_COMPLETION structure (I/O completion or event notification).

If the Type member is set to RIO_EVENT_COMPLETION, then the Event member of the RIO_NOTIFICATION_COMPLETION structure must be set.

If the Type member is set to RIO_IOCP_COMPLETION, then the Iocp member of the RIO_NOTIFICATION_COMPLETION structure must be set and the Iocp.Overlapped member of the RIO_NOTIFICATION_COMPLETION structure must not be NULL.

If the NotificationCompletion parameter is NULL, this specifies no notification completion is used and that polling must be used to determine completion.

Return value

If no error occurs, the RIOCreateCompletionQueue function returns a descriptor referencing a new completion queue. Otherwise, a value of RIO_INVALID_CQ is returned, and a specific error code can be retrieved by calling the WSAGetLastError function.

Return codeDescription
WSAEFAULT

The system detected an invalid pointer address in attempting to use a pointer argument in a call.

WSAEINVAL

An invalid parameter was passed to the function.

This error is returned if the QueueSize parameter is less than 1 or greater than RIO_MAX_CQ_SIZE defined in the Mswsockdef.h header file.

WSAENOBUFS

Sufficient memory could not be allocated. This error is returned if there was insufficient memory to allocate the completion queue requested based on the QueueSize parameter.

 

Remarks

The RIOCreateCompletionQueue function creates an I/O completion queue of a specific size. The size of the completion queue restricts the set of registered I/O sockets that can be associated with the completion queue. For more information, see the RIOCreateRequestQueue function.

When creating a RIO_CQ, the RIO_NOTIFICATION_COMPLETION structure pointed to by the NotificationCompletion parameter determines how the application will receive completion queue notifications. If a RIO_NOTIFICATION_COMPLETION structure is provided when creating the completion queue, the application may call the RIONotify function to request a completion queue notification. Normally this notification occurs when the completion queue is not empty. This may happen immediately or when the next completion entry is inserted into the completion queue. However, send and receive requests may be flagged as RIO_MSG_DONT_NOTIFY. Completion queue notification and will never be triggered as a result of such requests. If the completion queue contains only entries with the RIO_MSG_DONT_NOTIFY flag set, the completion queue notification will not be triggered. Also, when a new entry enters the completion queue, the completion queue notification is only triggered if the RIO_MSG_DONT_NOTIFY flag was not set on the associated request. Any completed requests can still be retrieved by polling using the RIODequeueCompletion function. Once a completion queue notification is issued, the application must call the RIONotify function in order to receive another completion queue notification. When a completion queue notification occurs, the application typically calls the RIODequeueCompletion function to dequeue the completed send or receive requests.

Two options are available for completion queue notification.

  • Event handles.
  • I/O completion ports

If the Type member of the RIO_NOTIFICATION_COMPLETION structure is set to RIO_EVENT_COMPLETION, an event handle is used to signal completion queue notifications. An event handle is provided as the EventNotify.EventHandle member in the RIO_NOTIFICATION_COMPLETION structure passed to the RIOCreateCompletionQueue function. The Event.EventHandle member should contain the handle for an event created by the WSACreateEvent or CreateEvent function. To receive the RIONotify completion, the application should wait on the specified event handle using WSAWaitForMultipleEvents or a similar wait routine. The completion of the RIONotify function for this RIO_CQ will signal the event. The Event.NotifyReset member in the RIO_NOTIFICATION_COMPLETION structure passed to the RIOCreateCompletionQueue function indicates whether or not the event should be reset as part of a call to the RIONotify function. If the application plans to reset and reuse the event, the application can reduce overhead by setting the Event.NotifyReset member to a non-zero value. This causes the event to be automatically reset by the RIONotify function when the notification occurs. This mitigates the need to call the WSAResetEvent function to reset the event between calls to the RIONotify function.

If the Type member of the RIO_NOTIFICATION_COMPLETION structure is set to RIO_IOCP_COMPLETION, an I/O completion port is used to signal completion queue notifications. An I/O completion port handle is provided as the Iocp.IocpHandle member in the RIO_NOTIFICATION_COMPLETION structure passed to the RIOCreateCompletionQueue function. The completion of the RIONotify function for this RIO_CQ will queue an entry to the I/O completion port which can be retrieved using the GetQueuedCompletionStatus or GetQueuedCompletionStatusEx function. A queued entry will have the returned lpCompletionKey parameter value set to the value specified in Iocp.CompletionKey member of the RIO_NOTIFICATION_COMPLETION structure and the Iocp.Overlapped member in the RIO_NOTIFICATION_COMPLETION structure will be a non-NULL value.

In terms of its usage, completion queue notification is designed to wake up a waiting application thread so that the thread can examine the completion queue. Waking and scheduling a thread comes at a cost, so if this happens too frequently it will have a negative impact on the application performance. The RIO_MSG_DONT_NOTIFY flag is provided so that the application can control the frequency of these events and limit their over impact on performance.

Note  For purposes of efficiency, access to the completion queues (RIO_CQ structs) and request queues (RIO_RQ structs) are not protected by synchronization primitives. If you need to access a completion or request queue from multiple threads, access should be coordinated by a critical section, slim reader write lock or similar mechanism. This locking is not needed for access by a single thread. Different threads can access separate requests/completion queues without locks. The need for synchronization occurs only when multiple threads try to access the same queue. Synchronization is also required if multiple threads issue sends and receives on the same socket because the send and receive operations use the socket’s request queue.

Note  The function pointer to the RIOCreateCompletionQueue function must be obtained at run time by making a call to the WSAIoctl function with the SIO_GET_MULTIPLE_EXTENSION_FUNCTION_POINTER opcode specified. The input buffer passed to the WSAIoctl function must contain WSAID_MULTIPLE_RIO, a globally unique identifier (GUID) whose value identifies the Winsock registered I/O extension functions. On success, the output returned by the WSAIoctl function contains a pointer to the RIO_EXTENSION_FUNCTION_TABLE structure that contains pointers to the Winsock registered I/O extension functions. The SIO_GET_MULTIPLE_EXTENSION_FUNCTION_POINTER IOCTL is defined in the Ws2def.h header file. The WSAID_MULTIPLE_RIO GUID is defined in the Mswsock.h header file.

Windows Phone 8: This function is supported for Windows Phone Store apps on Windows Phone 8 and later.

Windows 8.1 and Windows Server 2012 R2: This function is supported for Windows Store apps on Windows 8.1, Windows Server 2012 R2, and later.

Requirements

Minimum supported client

Windows 8.1, Windows 8 [desktop apps only]

Minimum supported server

Windows Server 2012 [desktop apps only]

Minimum supported phone

Windows Phone 8

Header

Mswsock.h

Library

Mswsock.lib

DLL

Mswsock.dll

See also

CreateEvent
GetQueuedCompletionStatus
GetQueuedCompletionStatusEx
RIO_CQ
RIO_EXTENSION_FUNCTION_TABLE
RIOCloseCompletionQueue
RIOCreateRequestQueue
RIONotify
WSACreateEvent
WSAResetEvent
WSAWaitForMultipleEvents
WSAIoctl

 

 

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