Removing a handle from a I/O completion port and other questions about IOCP

Question

The CreateIoCompletionPort function allows the creation of a new I/O completion port and the registration of file handles to an existing I/O completion port.

Then, I can use any function, like a recv on a socket or a ReadFile on a file with a OVERLAPPED structure to start an asynchronous operation.

I have to check whether the function call returned synchronously although it was called with an OVERLAPPED structure and in this case handle it directly. In the other case, when ERROR_IO_PENDING is returned, I can use the GetQueuedCompletionStatus function to be notified when the operation completes.

The question which arise are:

• How can I remove a handle from the I/O completion port? For example, when I add sockets to the IOCP, how can I remove closed ones? Should I just re-register another socket with the same completion key?

• Also, is there a way to make the calls ALWAYS go over the I/O completion port and don't return synchronously?

• And finally, is it possible for example to recv asynchronously but to send synchronously? For example when a simple echo service is implemented: Can I wait with an asynchronous recv for new data but send the response in a synchronous way so that code complexity is reduced? In my case, I wouldn't recv a second time anyways before the first request was processed.

• What happens if an asynchronous ReadFile has been requested, but before it completes, a WriteFile to the same file should be processed. Will the ReadFile be cancelled with an error message and I have to restart the read process as soon as the write is complete? Or do I have to cancel the ReadFile manually before writing? This question arises in combination with a communication device; so, the write and read should not do problems if happening concurrently.

Answer 1

How can I remove a handle from the I/O completion port?

In my experience you can't disassociate a handle from a completion port. However, you may disable completion port notification by setting the low-order bit of your OVERLAPPED structure's hEvent field: See the documentation for GetQueuedCompletionStatus.

For example, when I add sockets to the IOCP, how can I remove closed ones? Should I just re-register another socket with the same completion key?

It is not necessary to explicitly disassociate a handle from an I/O completion port; closing the handle is sufficient. You may associate multiple handles with the same completion key; the best way to figure out which request is associated with the I/O completion is by using the OVERLAPPED structure. In fact, you may even extend OVERLAPPED to store additional data.

Also, is there a way to make the calls ALWAYS go over the I/O completion port and don't return synchronously?

That is the default behavior, even when ReadFile/WriteFile returns TRUE. You must explicitly call SetFileCompletionNotificationModes to tell Windows to not enqueue a completion packet when TRUE and ERROR_SUCCESS are returned.

is it possible for example to recv asynchronously but to send synchronously?

Not by using recv and send; you need to use functions that accept OVERLAPPED structures, such as WSARecv, WSASend, or alternatively ReadFile and WriteFile. It might be more handy to use the latter if your code is meant to work multiple types of I/O handles, such as both sockets and named pipes. Those functions provide a synchronous mode, so if you use those them you can mix asynchronous and synchronous calls.

What happens if an asynchronous ReadFile has been requested, but before it completes, a WriteFile to the same file should be processed?

There is no implicit cancellation. As long as you're using separate OVERLAPPED structures for each read/write to a full-duplex device, I see no reason why you can't do concurrent I/O operations.