serial communication with IOCP

hello :
i use IOCP in serial communication, RS232.
 
   BOOL bOK = ::GetQueuedCompletionStatus(m_hCompletion, 
    &dwTrans, (LPDWORD)&dwKey, (LPOVERLAPPED*)&lpol, INFINITE);
 
the function above won't block when the serial port no data comming.
the return value is TRUE.
GetQueuedCompletionStatus should blocked, when no data received,
but it always returns, and the dwTrans is Zero, implying no data.
when there's some data received, the dwTrans is the number of the data,
and i can fetch data from buffer, that is correct.
Each time i get a read-finished notification, i read the data, and post another
read request, eg. calling "ReadFile".
 If i comment out "ReadFile", GetQueuedCompletionStatus did blocked, but 
i can never receive any data anymore.
 
so, how to let the function(GetQueuedCompletionStatus ) block, when there's no data ???
by the way, the similar codes works well with Winsock.
thanks.


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