GetQueuedCompletionStatus function

Attempts to dequeue an I/O completion packet from the specified I/O completion port. If there is no completion packet queued, the function waits for a pending I/O operation associated with the completion port to complete.

To dequeue multiple I/O completion packets at once, use the GetQueuedCompletionStatusEx function.


BOOL WINAPI GetQueuedCompletionStatus(
  _In_   HANDLE CompletionPort,
  _Out_  LPDWORD lpNumberOfBytes,
  _Out_  PULONG_PTR lpCompletionKey,
  _Out_  LPOVERLAPPED *lpOverlapped,
  _In_   DWORD dwMilliseconds


CompletionPort [in]

A handle to the completion port. To create a completion port, use the CreateIoCompletionPort function.

lpNumberOfBytes [out]

A pointer to a variable that receives the number of bytes transferred during an I/O operation that has completed.

lpCompletionKey [out]

A pointer to a variable that receives the completion key value associated with the file handle whose I/O operation has completed. A completion key is a per-file key that is specified in a call to CreateIoCompletionPort.

lpOverlapped [out]

A pointer to a variable that receives the address of the OVERLAPPED structure that was specified when the completed I/O operation was started.

Even if you have passed the function a file handle associated with a completion port and a valid OVERLAPPED structure, an application can prevent completion port notification. This is done by specifying a valid event handle for the hEvent member of the OVERLAPPED structure, and setting its low-order bit. A valid event handle whose low-order bit is set keeps I/O completion from being queued to the completion port.

dwMilliseconds [in]

The number of milliseconds that the caller is willing to wait for a completion packet to appear at the completion port. If a completion packet does not appear within the specified time, the function times out, returns FALSE, and sets *lpOverlapped to NULL.

If dwMilliseconds is INFINITE, the function will never time out. If dwMilliseconds is zero and there is no I/O operation to dequeue, the function will time out immediately.

Return value

Returns nonzero (TRUE) if successful or zero (FALSE) otherwise.

To get extended error information, call GetLastError.

For more information, see the Remarks section.


This function associates a thread with the specified completion port. A thread can be associated with at most one completion port.

If a call to GetQueuedCompletionStatus fails because the completion port handle associated with it is closed while the call is outstanding, the function returns FALSE, *lpOverlapped will be NULL, and GetLastError will return ERROR_ABANDONED_WAIT_0.

Windows Server 2003 and Windows XP:  Closing the completion port handle while a call is outstanding will not result in the previously stated behavior. The function will continue to wait until an entry is removed from the port or until a time-out occurs, if specified as a value other than INFINITE.

If theGetQueuedCompletionStatus function succeeds, it dequeued a completion packet for a successful I/O operation from the completion port and has stored information in the variables pointed to by the following parameters: lpNumberOfBytes, lpCompletionKey, and lpOverlapped. Upon failure (the return value is FALSE), those same parameters can contain particular value combinations as follows:

  • If *lpOverlapped is NULL, the function did not dequeue a completion packet from the completion port. In this case, the function does not store information in the variables pointed to by the lpNumberOfBytes and lpCompletionKey parameters, and their values are indeterminate.
  • If *lpOverlapped is not NULL and the function dequeues a completion packet for a failed I/O operation from the completion port, the function stores information about the failed operation in the variables pointed to by lpNumberOfBytes, lpCompletionKey, and lpOverlapped. To get extended error information, call GetLastError.

For more information on I/O completion port theory, usage, and associated functions, see I/O Completion Ports.



Server Message Block (SMB) 3.0 protocol


SMB 3.0 Transparent Failover (TFO)


SMB 3.0 with Scale-out File Shares (SO)


Cluster Shared Volume File System (CsvFS)


Resilient File System (ReFS)




Minimum supported client

Windows XP [desktop apps only]

Minimum supported server

Windows Server 2003 [desktop apps only]


WinBase.h (include Windows.h)





See also

Overview Topics
File Management Functions
I/O Completion Ports
Using the Windows Headers




© 2014 Microsoft