Windows CE enables you to synchronously perform the WriteFile, ReadFile, and WaitCommEvent functions.
Windows CE does not support the overlapped I/O features of Windows NT. The lpOverlapped parameter to ReadFile or WriteFile must be NULL. Therefore, Windows CE cannot signal the event that is passed in when the I/O operation is completed. However, Windows CE does support simultaneous synchronous or asynchronous calls to ReadFile or WriteFile made by separate threads that are overlapped in time; this is not supported in Windows NT.