Win32: WaitCommEvent & GetQueuedCompletionStatus funny...

tendofan

Hi all,

I'm experiencing something odd using WaitCommEvent with IO Completion
ports - I've looked everywhere for some clues as to what's going on,
but to no avail, so I'm hoping that some of you may have come across
something like it. I've got a number of IO Completion port threads,
and for each IO operation, I've got a class derived from the
OVERLAPPED structure.

class Context : public OVERLAPPED
{
...
public:
void OnIOComplete(DWORD dwXFerred, DWORD dwErr)
{
m_pHandler->OnIOComplete(dwXFerred, dwErr);
}

private:
CIOHandler* m_pHandler;
}

For ReadFile, WriteFile and WaitCommEvent operations, I've got a
separate CIOHandler object, and I set up the Context class with this
as necessary.

In the IO Completion port thread, the code looks a little like this:

DWORD dwXFerred = 0L;
Context* pContext = NULL;
ULONG* pCompletion = NULL;


...

if (TRUE == ::GetCompletionStatus(hIOCP, &dwXFerred, &pCompletion,
(OVERLAPPED**)&pContext, 1000))
{
pContext->OnIOComplete(dwXFerred, 0);
}

Now... *deep breath* - initially, prior to client connection, I issue
a WaitCommEvent on the port for the EV_RING event, when this
happens, a connection script runs, i.e. a series of reads operations,
making sure that the "RING" and "CONNECT" strings are returned from
the modem. All is good so far, but then, after the connect is
received, I issue another WaitCommEvent, waiting for EV_ERR or
EV_BREAK, and at the same time, issue a ReadFile against the port.
What seems to be happening, is that in the IO Completion Port thread,
GetQueuedCompletionStatus is returning TRUE, with the dwXFerred
parameter set to 4, but there's no data in the port, and the Context
object returned is the one related to the WaitCommEvent. This
handler re-issues the WaitCommEvent, as one's supposed to do, and I
end up in a very busy loop.

Both SysInternal's PortMon and a line monitor show that no data is in
the port at all, so I'm wondering what's going on. Any ideas?

Tendofan