QSerialPort outside main function not working

[julienchz]

Hello, I'm trying to get data from an USB connection with QSerialPort. When I am running the following code from the main function, everything works properly. However, when I try to run this code somewhere else, I manage to open the connection but I can't read any data. What could be the reason of this beahvior ?

    QList<QSerialPortInfo> serialPortInfoList = QSerialPortInfo::availablePorts();
    QSerialPortInfo currentPortInfo;;
    for (QSerialPortInfo& portInfo: serialPortInfoList)
    {
        if (portInfo.description() == "USB-SERIAL CH340")
        {
            qDebug() << "Valid port found";
            currentPortInfo = portInfo;
        }
    }
    
    QSerialPort serialPort(currentPortInfo);
    serialPort.setBaudRate(57600);
    
    if (!serialPort.setDataBits(QSerialPort::Data8))
        qDebug() << "Data bits not set";
    if (!serialPort.setFlowControl(QSerialPort::HardwareControl))
        qDebug() << "Flow control not set";
    if (!serialPort.setParity(QSerialPort::NoParity))
        qDebug() << "Parity not set";
    if (!serialPort.setStopBits(QSerialPort::OneStop))
        qDebug() << "Stop bits not set";
    if (!serialPort.open(QIODevice::ReadOnly))
    {
        qDebug() << "Failed to open port";
    }
    
    QObject::connect(&serialPort, &QSerialPort::readyRead, [&]
    {
        //this is called when readyRead() is emitted
        qDebug() << "New data available: " << serialPort.bytesAvailable();
        QByteArray datas = serialPort.readAll();
        qDebug() << datas;
    });

Thank you for your help

[JonB]

@julienchz

QSerialPort serialPort(currentPortInfo);
QObject::connect(&serialPort, &QSerialPort::readyRead, [&] { ... }

serialPort is local variable. If in main() its lifetime is as long as your program. If you put this elsewhere think about its scope/lifetime.

[julienchz]

@JonB I tried to use it as an attribute of a class inheriting QObject, but without success

[Christian Ehrlicher]

@julienchz said in QSerialPort outside main function not working:

I tried to use it as an attribute of a class inheriting QObject, but without success

please show the code

[JonB]

@julienchz That in itself says nothing about the instance's lifetime.

[julienchz]

@Christian-Ehrlicher @JonB Here is my class

class SerialPortReader : public QObject
{
    Q_OBJECT
public:
    explicit SerialPortReader(QObject *parent = nullptr);

protected:
    QSerialPortInfo FindSerialPortInfo();

private slots:
    void HandleReadyRead();
    void HandleTimeout();
    void HandleError(QSerialPort::SerialPortError error);

private:
    const QString PORT_DESCRIPTION = "USB-SERIAL CH340";
    const qint32 BAUD_RATE = QSerialPort::Baud57600;

    QSerialPort *m_serialPort;
    QByteArray m_readData;
    QTextStream m_standardOutput;
    QTimer m_timer;
};

SerialPortReader::SerialPortReader(QObject *parent) :
    QObject(parent),
    m_serialPort(new QSerialPort(this))
{
    QSerialPortInfo portInfo = FindSerialPortInfo();
    if (portInfo.isNull())
        return;


    m_serialPort->setPort(portInfo);
    // m_serialPort = new QSerialPort(portInfo, this);
    if (!m_serialPort->setBaudRate(BAUD_RATE))
        qDebug() << "Baud rate not set";
    if (!m_serialPort->setDataBits(QSerialPort::Data8))
        qDebug() << "Data bits not set";
    if (!m_serialPort->setFlowControl(QSerialPort::HardwareControl))
        qDebug() << "Flow control not set";
    if (!m_serialPort->setParity(QSerialPort::NoParity))
        qDebug() << "Parity not set";
    if (!m_serialPort->setStopBits(QSerialPort::OneStop))
        qDebug() << "Stop bits not set";

    if (!m_serialPort->open(QIODevice::ReadOnly))
    {
        qDebug() << "Failed to open port";
        return;
    }

    qDebug() << "Port correctly opened";

    connect(m_serialPort, &QSerialPort::readyRead, this, &SerialPortReader::HandleReadyRead);
    connect(m_serialPort, &QSerialPort::errorOccurred, this, &SerialPortReader::HandleError);
    connect(&m_timer, &QTimer::timeout, this, &SerialPortReader::HandleTimeout);

    m_timer.start(5000);
}

QSerialPortInfo SerialPortReader::FindSerialPortInfo()
{
    QList<QSerialPortInfo> serialPortInfoList = QSerialPortInfo::availablePorts();
    for (QSerialPortInfo& portInfo: serialPortInfoList)
    {
        if (portInfo.description() == PORT_DESCRIPTION)
        {
            qDebug() << "Valid port found";
            return portInfo;
        }
    }

    qDebug() << "No valid port found";
    return QSerialPortInfo();
}


void SerialPortReader::HandleReadyRead()
{
    m_readData.append(m_serialPort->readAll());

    if (!m_timer.isActive())
        m_timer.start(5000);
}

void SerialPortReader::HandleTimeout()
{
    if (m_readData.isEmpty()) {
        qDebug() << QObject::tr("No data was currently available "
                                        "for reading from port");
    } else {
        qDebug() << QObject::tr("Data successfully received from port");
        qDebug() << m_readData;
    }
}

void SerialPortReader::HandleError(QSerialPort::SerialPortError serialPortError)
{
    if (serialPortError == QSerialPort::ReadError) {
        qDebug() << QObject::tr("An I/O error occurred while reading "
                                        "the data from port, error:");
    }
}

[Christian Ehrlicher]

And what 'does not work' with this code?
The timeout handling looks strange... why wait 5000ms before doing something with the received data?

[julienchz]

@Christian-Ehrlicher It was from an example I commented it to try. The opening of the port is successful but the signal readyRead is never triggered

[Christian Ehrlicher]

@julienchz Are you sure that there is data coming in? And do you have a running eventloop (QCoreApplication::exec() in your main.cpp?)

[JonB]

@julienchz
Does your SerialPortReader instance (wherever you create it) live as long as the readyRead()s you are expecting.

When I am running the following code from the main function, everything works properly. However, when I try to run this code somewhere else, I manage to open the connection but I can't read any data

If that is the case lifetime seems the obvious candidate.

[julienchz]

@Christian-Ehrlicher Yes i'am sure there is data coming