Question

CancelIo() is supposed to cancel all pending I/O operations associated with the calling thread. In my experience, CancelIo() sometimes cancels future I/O operations as well. Given:

ReadFile(port, buffer, length, &bytesTransferred, overlapped);

1. If I invoke CancelIo(port) immediately before the read, GetQueuedCompletionStatus() will block forever, never receiving the read operation.
2. If I invoke CancelIo(port) immediately after the read, GetQueuedCompletionStatus() will return 0 with GetLastError()==ERROR_OPERATION_ABORTED
3. If I invoke CancelIo(port) and there are no pending or subsequent reads, GetQueuedCompletionStatus() will block forever.

The key point here is that there is no way to detect when CancelIo() has finished executing. How can I ensure that CancelIo() is done executing and it is safe to issue further read requests?

PS: Looking at http://osdir.com/ml/lib.boost.asio.user/2008-02/msg00074.html and http://www.boost.org/doc/libs/1_44_0/doc/html/boost_asio/using.html it sounds like CancelIo() is not really usable. Must customer requires Windows XP support. What are my options?

NOTE: I am reading from a serial port.

Answer 1

CancelIo() works fine. I misunderstood my code.

Upon further investigation it turns out that the code was invoking CancelIo() followed by ReadFile() with a timeout INFINITE. The completion port was never getting notified of the read because the remote end was never sending anything. In other words, CancelIo() did not cancel subsequent operations.

I found some eye-opening documentation here:

Be careful when coding for asynchronous I/O because the system reserves the right to make an operation synchronous if it needs to. Therefore, it is best if you write the program to correctly handle an I/O operation that may be completed either synchronously or asynchronously. The sample code demonstrates this consideration.

It turns out that device drivers may choose to treat an asynchronous operation in a synchronous manner if the data being read is already cached by the device driver. Upon further investigation, I discovered that when CancelIo() was being invoked before ReadFile() it would sometimes cause the latter to return synchronously. I have no idea why the completion port was never getting notified of ReadFile() after a CancelIo() but I can no longer reproduce this problem.

The completion port is signaled regardless of whether ReadFile() is synchronous or asynchronous.