read from USB serial port of raspberry pi with serialGetchar()

[isan]

I use wiringSerial.h library to read from USB port of raspi in thread

void MyThread::run()
{
 
    qDebug("Thread id inside run %d",(int)QThread::currentThreadId());
 
    int fd ,value;
 
    if ((fd = serialOpen ("/dev/ttyACM0",230400)) < 0)
    {
        fprintf (stderr, "Unable to open serial device: %s\n", strerror (errno)) ;
    }
 
   while (serialDataAvail(fd)>-2)
    {
 
        value=serialGetchar (fd) ; 
 
       msleep(1); 
       emit signalValueUpdated(value);
    }
    serialClose(fd);
 
}

void MyThread::run()
{

    qDebug("Thread id inside run %d",(int)QThread::currentThreadId());

    int fd ,value;

    if ((fd = serialOpen ("/dev/ttyACM0",230400)) < 0)
    {
        fprintf (stderr, "Unable to open serial device: %s\n", strerror (errno)) ;
    }

   while (serialDataAvail(fd)>-2)
    {

        value=serialGetchar (fd) ; 

       msleep(1); 
       emit signalValueUpdated(value);
    }
    serialClose(fd);

}

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the data that received is a number between -1 to 255
when use value in emit signal Value Updated(value); or print it
the value is not the same value that send to raspberry pi, what should I do?