I want to constantly read from and write to a port (

is it specified how you have to constantly read from and write to a port or is that implementation detail up to you?

because while- loops and threads I the least optimal way to do it

Now I know that I have to declare a class inheritted from qThread class and inside its run function,

Why? Is this a required exercise to use a thread, or are you just doing it because you think you need to??

Oh, now I see

@jsulm I was asked to do this. I was asked to send a text in while(1) loop.

EDIT OK, I have seen now you have answered that you must do it this way. Seems terrible to me! But OK :)

        QByteArray data = serial->read(1);
        for(int i=0; i<data.size(); i++)
        {
                qDebug() << data[i];
        }

Is this part reading the datas byte by byte and store them inside an array and print the datas?

I asked already why threads. Seems to be an exercise.

possibly, but the OP only states the while loop as a task. Not threads specifically.

@nanor can you word exactly, what you want to do ?

if (serial.waitForReadyRead(currentWaitTimeout)) {
    QByteArray responseData = serial.readAll();

I don't know if any serial data is available to read at the instant you open the port. And you only try to read one byte once.

#include "mythread.h"

MyThread::MyThread(QObject *parent) : QThread(parent)
{

}

void MyThread::run()
{
    serial = new QSerialPort();
    serial->setPortName("COM1");

    for(const auto &serialPortInfo : QSerialPortInfo::availablePorts())
    {
       qDebug() << "find serial port: " << serialPortInfo.portName() ;
    }

    serial->open(QIODevice::ReadWrite);


    if(serial->isOpen())
    {
        serial->setBaudRate(QSerialPort::Baud9600);
        serial->setDataBits(QSerialPort::Data8);
        serial->setParity(QSerialPort::NoParity);
        serial->setStopBits(QSerialPort::OneStop);
        serial->setFlowControl(QSerialPort::NoFlowControl);



        QByteArray data = serial->read(1);
        for(int i=0; i<data.size(); i++)
         {
                qDebug() << data[i];
         }




    }

    else
    {
      qDebug() << "can't open the port";
    }

}

But still I can't get the messages. I send data from hercules.

You have now changed your code...

but nothing is printed

So either it never enters the loop, or it never has anything to read.

while(1)
        {
            QByteArray data = serial->read(1);
            for(int i=0; i<data.size(); i++)
            {
                qDebug() << data[i];
            }


        }

serial = new QSerialPort(this);

Remove "this".
If it is still crashing then please use the debugger to see where exactly and post the stack trace after crash.

mainwindow.h:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QtSerialPort/QSerialPort>
#include "mythread.h"

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

    QSerialPort *serial;

    QThread *myThread;


private slots:
    void on_pushButton_clicked();

private:
    Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H

mythread.h:

#ifndef MYTHREAD_H
#define MYTHREAD_H

#include <QObject>
#include <QThread>
#include <QtSerialPort/QSerialPort>
#include <QtSerialPort/QSerialPortInfo>
#include <QDebug>

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

    void run();

    QSerialPort *serial;

signals:

public slots:
};

#endif // MYTHREAD_H

mainwindow.cpp:

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QtSerialPort/QSerialPort>
#include <QtSerialPort/QSerialPortInfo>
#include <QDebug>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    myThread->start();

}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::on_pushButton_clicked()
{
    qDebug() << "salam";
}

mythread.cpp:

#include "mythread.h"

MyThread::MyThread(QObject *parent) : QThread(parent)
{

}

void MyThread::run()
{
    serial = new QSerialPort(this);
    serial->setPortName("COM1");

    for(const auto &serialPortInfo : QSerialPortInfo::availablePorts())
    {
       qDebug() << "find serial port: " << serialPortInfo.portName() ;
    }

    serial->open(QIODevice::ReadWrite);


    if(serial->isOpen())
    {
        serial->setBaudRate(QSerialPort::Baud9600);
        serial->setDataBits(QSerialPort::Data8);
        serial->setParity(QSerialPort::NoParity);
        serial->setStopBits(QSerialPort::OneStop);
        serial->setFlowControl(QSerialPort::NoFlowControl);

         while(1)
        {
            serial->read(1);

       }

    }

    else
    {
      qDebug() << "can't open the port";
    }

}

the error:

10:33:50: Starting C:\Users\nanor\OneDrive\Desktop\qt codes\build-serialportreading3-Desktop_Qt_5_12_2_MinGW_32_bit-Debug\debug\serialportreading3.exe...
ASSERT: "timeout >= 0" in file thread\qmutex.cpp, line 582
10:33:52: The program has unexpectedly finished.
10:33:52: The process was ended forcefully.
10:33:52: C:/Users/nanor/OneDrive/Desktop/qt codes/build-serialportreading3-Desktop_Qt_5_12_2_MinGW_32_bit-Debug/debug/serialportreading3.exe crashed.