EV_RXCHAR event received, but ReadFile sometimes failed to read a byte from serial port

Hi, All

I have a machine connected to PC using serial port. PC sends request
to the machine and the machine responds. My code is copied below (most
code are from MSDN "serial communication in win32")

When debug the code, I found that EV_RXCHAR is received. but when I
issue the ReadFile, it sometimes (NOT ALWAYS) failed to read a byte,
the read operation time out. Don't know why, please help me.

BOOL ProcessEvent(HANDLE hComm,DWORD dwCommEvent, RESPONSE *response)
{
    BOOL fRING, fRLSD, fRXCHAR, fRXFLAG, fTXEMPTY;
    BOOL fBREAK, fCTS, fDSR, fERR;
    OVERLAPPED osReader = {0};
    unsigned char chRead;
    BOOL fRes=TRUE;
    DWORD dwRead;
    int i=0;
    BOOL fContinue=TRUE;

    fCTS = EV_CTS & dwCommEvent;
    fDSR = EV_DSR & dwCommEvent;
    fERR = EV_ERR & dwCommEvent;
    fRING = EV_RING & dwCommEvent;
    fRLSD = EV_RLSD & dwCommEvent;
    fBREAK = EV_BREAK & dwCommEvent;
    fRXCHAR = EV_RXCHAR & dwCommEvent;
    fRXFLAG = EV_RXFLAG & dwCommEvent;
    fTXEMPTY = EV_TXEMPTY & dwCommEvent;

    if(fRXCHAR) //issue read
    {
        printf("Char Received\n");

        while(fContinue)
        {
            dwRead=ReadChar(hComm,&chRead);
            if(dwRead)
            {
                printf("one byte: read %02X, response->len=%d\n",chRead, response-
                
response->response[response->len]=chRead;
                response->len++;
                printf("total len=%d\n", response->len);
            }
            else
            {
                printf("Can't read more characters, total=%d\n",response->len);
                for(i=1;i<=response->len;i++)
                {
                    printf("%02X ",response->response[i-1]);
                    if((i%16)==0)
                    {
                        printf("\n");
                    }
                }
                printf("\ndwRead=0\n");
                fContinue=FALSE;
            }

        }

        return FALSE; //return FALSE to stop WaitCommEvent
    }

    return TRUE; //return TRUE to issue another WaitCommEvent
}

int ReadChar(HANDLE hComm, unsigned char *chRead)
{
    DWORD dwRead=0;
    BOOL fWaitingOnRead = FALSE;
    OVERLAPPED osReader = {0};
    DWORD dwRes;
    DWORD dwRetry=0;
    DWORD dwError;
    COMSTAT commstat;

    printf("calling ReadChar\n");

    osReader.hEvent=CreateEvent(NULL, TRUE, FALSE, NULL);
    if(osReader.hEvent==NULL)
    {
        return ERROR_SYSTEM;
    }

    ClearCommError(hComm,&dwError,&commstat);
    printf("there are %d bytes in queue\n",commstat.cbInQue);
    dwRes=ReadFile(hComm, chRead,1, &dwRead, &osReader); //read one byte
    //printf("ReadFile return %d\n",dwRes);

    if (!dwRes) //ReadFile Error
    {
        if (GetLastError() != ERROR_IO_PENDING)
        {
            printf("ReadBuffer System Error\n");
        }
        else
        {
            printf("ReadChar, IO pending\n");
            fWaitingOnRead=TRUE;
        }
    }
    else
    {
        // read completed immediately
        printf("ReadChar returns immediately, %d bytes read\n",dwRead);
    }

    while(fWaitingOnRead && (dwRetry<3)) //wait only once
    {
        dwRes = WaitForSingleObject(osReader.hEvent, READ_TIMEOUT);
        switch(dwRes)
        {
            // Read completed.
        case WAIT_OBJECT_0:
            if (!GetOverlappedResult(hComm, &osReader, &dwRead, FALSE))
            {
                printf("ReadChar, Error in GetOverlappedResult\n");
            }
            else
            {
                printf("ReadChar successfully, %d bytes read\n",dwRead);
            }
            fWaitingOnRead=FALSE;
            break;

        case WAIT_TIMEOUT: //continue wait
            printf("ReadChar Timeout\n");
            dwRetry++;
            break; //get out of the loop
        default:
            printf("ReadChar Unknown Error\n");
            fWaitingOnRead=FALSE;
            break;
        }
    }

    CloseHandle(osReader.hEvent);

    return dwRead;
}