Qt not receiving serial data from arduino

Question

I am writing a C++ program using Qt that reads data from the serial line. The "producer" of the data is an Arduino UNO board.

The Arduino code is very simple. Currently, it just generates a random number and sends it over the serial line. Here is the code:

long randNumber;

void setup() {
  Serial.begin(9600);
  randomSeed(analogRead(0));
}

void loop() {
  randNumber = random(300);
  Serial.write(randNumber);
  delay(10);
}

I have used the Arduino's "serial monitor" to verify that data is coming across the serial line.

On the Qt side of things, I have a worker thread that is supposed to read the serial data and update a plot on the UI accordingly. The worker thread is definitely running, and this code is getting executed (I've checked). I only have one device in my "port list" that shows up, and it is the Arduino, so currently I connect right to it. Here is the code:

void doWork ()
{
    QList<QSerialPortInfo> port_list = QSerialPortInfo::availablePorts();
    QSerialPort serialPort;
    serialPort.setPort(port_list[0]);
    serialPort.setBaudRate(QSerialPort::Baud9600);

    if (!serialPort.open(QIODevice::ReadWrite))
    {
        cout << QObject::tr("Failed to open port COM3, error: %1").arg(serialPort.errorString()).toStdString() << endl;
    }

    while(!abort_thread)
    {
        int bytes_available = serialPort.bytesAvailable();
        if (bytes_available >= 4)
        {
            QByteArray byte_array = serialPort.read(4);
            int data = byte_array.toInt();
            emit signalDataReady(data);
        }
    }

    serialPort.close();
}

Unfortunately, there are never any bytes available. It opens the serial port successfully, but no bytes come through. What am I doing wrong here? Any ideas? Thanks!

Answer 1

You are forgot to add waitForReadyRead() before bytesAvailable().