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Microsoft KB Archive/175551

From BetaArchive Wiki

INFO: Serial Communications on Windows CE HPC devices


The information in this article applies to:

  • Microsoft Win32 Application Programming Interface (API), included with:
    • Microsoft Windows CE


Unlike other Win32 platforms, the Windows CE serial driver is implemented as overlapped. This means it can handle multiple concurrent I/O requests even though file handles must be opened as non-overlapped.


On most Win32 platforms, the I/O manager serializes non-overlapped serial communications. This implies that if there is an I/O request that is blocked, any subsequent I/O request will also be blocked until the pending I/O request completes. This is true even if each request was made from a separate thread.

While the Windows CE API does not support overlapped I/O, the serial driver is implemented as overlapped, that is, it can handle multiple concurrent I/O requests even though file handles are opened as non-overlapped. Therefore, serial communications programs on Windows CE devices have the following implementation option, which will not work on other Win32 platforms. A reading thread may call WaitCommEvent and when a RX_CHAR event arrives then call ReadFile to read characters from the port. The exact amount of data to read can be obtained by calling ClearCommError. A separate thread may simultaneously write to the port without hanging. The following sample code demonstrates this read technique.

Sample Code

   ReadThread (LPVOID pvarg)

     DWORD   BytesTransferred;
     DWORD   fdwCommMask;
     DWORD   dwErrors;
     COMSTAT Stat;
     COMMTIMEOUTS CommTimeouts;
     DWORD   t;
     BYTE    ReadBuffer[1024];

     GetCommTimeouts (hPort, &CommTimeouts);
     CommTimeouts.ReadIntervalTimeout = MAXDWORD;
     CommTimeouts.ReadTotalTimeoutMultiplier = 0;
     CommTimeouts.ReadTotalTimeoutConstant = 0;  // Read timeout at 50msec
     SetCommTimeouts (hPort, &CommTimeouts);

     SetCommMask (hPort, EV_RXCHAR | EV_CTS | EV_DSR | EV_RLSD | EV_RING);

     while (!v_bStopThread) {
        WaitCommEvent (hPort, &fdwCommMask, 0);

        // Reset the comm Mask
        SetCommMask (hPort, EV_RXCHAR | EV_CTS | EV_DSR | EV_RING);

        if (fdwCommMask &= EV_RXCHAR) {
            // Loop getting data.

         // Need to loop because our buffer is only 1024 bytes
                while (TRUE)
                    ClearCommError( hPort,&dwErrors,&Stat);
                    if (!Stat.cbInQue)
                        ReadFile (  hPort,
                            // do something with data
                   // Loop around and check for more data
                   // In case additional byte has arrived while reading.


     return 0;


Additional query words:

Keywords : kbAPI kbKernBase kbOSWinCEsearch kbComPort
Issue type : kbinfo
Technology : kbAudDeveloper kbWin32sSearch kbWin32API

Last Reviewed: January 15, 2000
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