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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.

Syntax

BOOL WINAPI GetQueuedCompletionStatus(
  __in   HANDLE CompletionPort,
  __out  LPDWORD lpNumberOfBytes,
  __out  PULONG_PTR lpCompletionKey,
  __out  LPOVERLAPPED *lpOverlapped,
  __in   DWORD dwMilliseconds
);

Parameters

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

If the function dequeues a completion packet for a successful I/O operation from the completion port, the return value is nonzero. The function stores information in the variables pointed to by the lpNumberOfBytes, lpCompletionKey, and lpOverlapped parameters.

If *lpOverlapped is NULL and the function does not dequeue a completion packet from the completion port, the return value is zero. The function does not store information in the variables pointed to by the lpNumberOfBytes and lpCompletionKey parameters. To get extended error information, call GetLastError. If the function did not dequeue a completion packet because the wait timed out, GetLastError returns WAIT_TIMEOUT.

If *lpOverlapped is not NULL and the function dequeues a completion packet for a failed I/O operation from the completion port, the return value is zero. The function stores information in the variables pointed to by lpNumberOfBytes, lpCompletionKey, and lpOverlapped. To get extended error information, call GetLastError.

Remarks

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

This function returns TRUE when at least one pending I/O is completed.

This function returns FALSE when no I/O operation was dequeued. This typically means that an error occurred while processing the parameters to this call, or that the CompletionPort handle was closed or is otherwise invalid. The GetLastError function provides extended error information.

Starting with Windows Vista, if a call to GetQueuedCompletionStatus fails because the handle associated with it is closed, the function returns FALSE and GetLastError will return ERROR_ABANDONED_WAIT_0.

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

Requirements