为什么WriteFile()总是返回异常代码6

void CCallLeeKuDlg::OnBtn1() 
{
    // TODO: Add your control notification handler code here
    HANDLE m_hComm;
    COMMTIMEOUTS m_CommTimeouts;

    char *szPort = new char[10];
    char *szBaud = new char[50];

    UINT uPort = 1;
    UINT uBaud = 19200;
    char parity = 'N';
    UINT databits = 8;
    UINT stopbits = 1;
    DWORD dwCommEvents = EV_RXCHAR | EV_CTS;
    UINT nBufferSize = 512;
    sprintf(szPort, "COM%d", uPort);
    sprintf(szBaud, "baud=%d parity=%c data=%d stop=%d", uBaud, parity, databits, stopbits);

    // initialize critical section
//    InitializeCriticalSection(&m_csCommunicationSync);

    //利用一般开启档案的 CreatFile()即可开启 serial port device用 CreateFile() API.
    m_hComm = CreateFile(szPort,                        // communication port string (COMX)
                         GENERIC_READ | GENERIC_WRITE,    // read/write types
                         0,                                // comm devices must be opened with exclusive access
                         NULL,                            // no security attributes
                         OPEN_EXISTING,                    // comm devices must use OPEN_EXISTING
                         FILE_FLAG_OVERLAPPED,            // Async I/O
                         0);

    
    //设置超时时间 set the timeout values
    m_CommTimeouts.ReadIntervalTimeout = 1000;
    m_CommTimeouts.ReadTotalTimeoutMultiplier = 1000;
    m_CommTimeouts.ReadTotalTimeoutConstant = 1000;
    m_CommTimeouts.WriteTotalTimeoutMultiplier = 1000;
    m_CommTimeouts.WriteTotalTimeoutConstant = 1000;
    
    if(SetCommTimeouts(m_hComm, &m_CommTimeouts))
    {    
        //设置串口事件               
        if (SetCommMask(m_hComm, dwCommEvents))
        {
            //设定及取得通讯状态
            SetCommPortState(m_hComm, uBaud, parity, databits, stopbits);
        }
        else
            AfxMessageBox("SetCommMask Failed!");
    }
    else
        AfxMessageBox("SetCommTimeouts Failed!");

    // flush the port ???
    PurgeComm(m_hComm, PURGE_RXCLEAR | PURGE_TXCLEAR | PURGE_RXABORT | PURGE_TXABORT);

//    HANDLE hFile = m_hComm;
    CString strSend = "isabc";

    OVERLAPPED m_ov;//???
    BOOL bWrite = TRUE;
    BOOL bResult = TRUE;

    DWORD BytesSent = 0;

    // Initailize variables
    m_ov.Offset = 0;
    m_ov.OffsetHigh = 0;

    // Clear buffer
    PurgeComm(m_hComm, PURGE_RXCLEAR | PURGE_TXCLEAR | PURGE_RXABORT | PURGE_TXABORT);

    bResult = WriteFile(m_hComm,                    // Handle to COMM Port
                        strSend.GetBuffer(strSend.GetLength()),                    // Pointer to message buffer in calling finction
                        strSend.GetLength(),    // Length of message to send by isabc
                        &BytesSent,                // Where to store the number of bytes sent
                        &m_ov);                    // Overlapped structure
    if(!bResult)
    {
        DWORD dwError = GetLastError();
        CString strErr;
        strErr.Format("Error Code :%d",dwError);
        AfxMessageBox(strErr);
        switch (dwError)
        {
        case ERROR_IO_PENDING:
            {
                // continue to GetOverlappedResults()
                BytesSent = 0;
                bWrite = FALSE;
                break;
            }
        default:
            {
                // all other error codes
                AfxMessageBox("WriteFile() happen other error!");
            }
        }
    }
    else


    if (!bWrite)
    {
        bWrite = TRUE;
    
        bResult = GetOverlappedResult(m_hComm,    // Handle to COMM port 
                                      &m_ov,        // Overlapped structure
                                      &BytesSent,        // Stores number of bytes sent
                                      TRUE);             // Wait flag

        LeaveCriticalSection(&m_csCommunicationSync);

        // deal with the error code 
        if (!bResult)  
        {
            AfxMessageBox("GetOverlappedResults() in WriteFile()");
        }    
    } // end if (!bWrite)

    if (BytesSent != strSend.GetLength())
    {
        TRACE("WARNING: WriteFile() error.. Bytes Sent: %d; Message Length: %d\n", BytesSent, strSend.GetLength());
    }
    
}