sprintf() и WriteFile(), влияющие на строковый буфер

    sprintf(buf,"%s!\n", tagBuffer);
    WriteFile(hSerial,buf,strlen(buf), &dwBytesWritten,&ovWrite);

//keep polling as long as stop character '-' is not read
while(szRxChar != '-')
{
    // Check if a read is outstanding
    if (HasOverlappedIoCompleted(&ovRead))
    {
        // Issue a serial port read
        if (!ReadFile(hSerial,&szRxChar,1,
                &dwBytesRead,&ovRead))
        {
            DWORD dwErr = GetLastError();
            if (dwErr!=ERROR_IO_PENDING)
                return dwErr;
        }
    }

    // resets tagBuffer in case tagBuffer is out of sync
    time_t t_time = time(0);
    char buf[50];
    if (HasOverlappedIoCompleted(&ovWrite))
    {
        i=0;                                       
    }

    // Wait 5 seconds for serial input
    if (!(HasOverlappedIoCompleted(&ovRead)))
    {
        WaitForSingleObject(hReadEvent,RESET_TIME);
    }

    // Check if serial input has arrived
    if (GetOverlappedResult(hSerial,&ovRead,
            &dwBytesRead,FALSE))
    {
        // Wait for the write
        GetOverlappedResult(hSerial,&ovWrite,
            &dwBytesWritten,TRUE);

        if( strlen(tagBuffer) >= PACKET_LENGTH )
        {
            i = 0;
        }

        //load tagBuffer with byte stream
        tagBuffer[i] = szRxChar;
        i++;
        tagBuffer[i] = 0; //char arrays are \0 terminated

        //run query with tagBuffer  
        sprintf(query,"select type from rfid where rfidnum=\"");
        strcat(query, tagBuffer);
        strcat(query, "\"");
        mysql_real_query(&mysql,query,(unsigned int)strlen(query));

        //process result and send back to handheld
        res = mysql_use_result(&mysql);
        while(row = mysql_fetch_row(res))
        {
           printf("result of query is %s\n",row[0]);

           string str = "";
           str = string(row[0]);

           if( str == "book" )
           {
               WriteFile(hSerial,BOOK_INDICATOR,strlen(BOOK_INDICATOR),
                &dwBytesWritten,&ovWrite);
           }
           else if ( str == "shelf" )
           {
               WriteFile(hSerial,SHELF_INDICATOR,strlen(SHELF_INDICATOR),
                &dwBytesWritten,&ovWrite);
           }
           else //this else doesn't work
           {
               WriteFile(hSerial,NOK,strlen(NOK),
                &dwBytesWritten,&ovWrite);
           }
        }


        mysql_free_result(res);

        // Display a response to input
        //printf("query is %s!\n", query);
        //printf("strlen(tagBuffer) is %d!\n", strlen(tagBuffer));

        //without these, tagBuffer only holds the last character
        sprintf(buf,"%s!\n", tagBuffer);
        WriteFile(hSerial,buf,strlen(buf), &dwBytesWritten,&ovWrite);

    }
}