Cannot write a data to a virtual serial port ?

elypheldia

I am new in QT developing. I opened a virtual serial port, but I cannot write a data to this. How can I write ?

main.cpp

#include <QCoreApplication>
#include "myserialport.h"
int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    MySerialPort s;
    s.openSerialPort();


    return a.exec();
}

myserialport.h

#ifndef MYSERIALPORT_H
#define MYSERIALPORT_H

#include <QObject>
#include <QSerialPort>

class MySerialPort : public QObject
{
    Q_OBJECT
public:
    explicit MySerialPort(QObject *parent = nullptr);
    void openSerialPort();
    void closeSerialPort();
    void writeData(const QByteArray &);


signals:

public slots:
    void readData();
private:
    QSerialPort *serial;
};

#endif // MYSERIALPORT_H

myserialport.cpp

#include "myserialport.h"
#include <QSerialPort>
#include <QDebug>

MySerialPort::MySerialPort(QObject *parent) : QObject(parent)
{
    serial=new QSerialPort(this);
    connect(serial,SIGNAL(readyRead()),this,SLOT(readData()));

}
void MySerialPort::openSerialPort()
{
    serial->setPortName("/dev/tnt0");
    serial->setBaudRate(QSerialPort::Baud9600);
    serial->setDataBits(QSerialPort::Data8);
    serial->setFlowControl(QSerialPort::NoFlowControl);
    serial->setStopBits(QSerialPort::OneStop);
    serial->setParity(QSerialPort::NoParity);
    if(serial->open(QIODevice::ReadWrite))
    {
        qDebug()<<"opened";


    }
    else
    {
        qDebug()<<QSerialPort::PermissionError;
    }

}
void MySerialPort::closeSerialPort()
{
    if(serial->isOpen())
    {
        serial->close();
    }
}
void MySerialPort::writeData(const QByteArray &data)
{
    serial->write(data);
}
void MySerialPort::readData()
{
    QByteArray data="hello";
    writeData(data);
    serial->readAll();
    qDebug()<<data;
}

J.Hilk

@elypheldia said in Cannot write a data to a virtual serial port ?:

@J-Hilk I already did this. But I get no output. I am very new, sorry. Thx to all for replies.

thats why I said:

make sure you actually write something before you expect readData to be called

change your openSerialPort to this:

void MySerialPort::openSerialPort()
{
    serial->setPortName("/dev/tnt0");
    serial->setBaudRate(QSerialPort::Baud9600);
    serial->setDataBits(QSerialPort::Data8);
    serial->setFlowControl(QSerialPort::NoFlowControl);
    serial->setStopBits(QSerialPort::OneStop);
    serial->setParity(QSerialPort::NoParity);
    if(serial->open(QIODevice::ReadWrite))
    {
        qDebug()<<"opened";
       QByteArray data="hello";
           writeData(data);

    }
    else
    {
        qDebug()<<QSerialPort::PermissionError;
    }

}

it will, at the very, least attempt to write something, and should return a bytes written value and/or an error message

jsulm

@elypheldia said in Cannot write a data to a virtual serial port ?:

void MySerialPort::readData()
{
QByteArray data="hello";
writeData(data);
serial->readAll();
qDebug()<<data;
}

Why do you write data in a port connected to readyRead signal?
You should at least first read all the data and then write.

elypheldia

@jsulm Hey, thx for reply. As I said that I am very new, and I know, I have terrible mistakes. Could you give me some example about write and read data to understand?

jsulm

@elypheldia Well, simply change order:

void MySerialPort::readData()
{
    serial->readAll();
    QByteArray data="hello";
    writeData(data);
    qDebug()<<data;
}

elypheldia

@jsulm I tried your code, but I still can't see the data in the terminal. What is the reason of this ?

jsulm

@elypheldia What does serial->write(data); return?
Also, connect a slot to https://doc.qt.io/qt-5/qserialport.html#errorOccurred and see whether there es any error.

elypheldia

@jsulm I added this slot to handle error.

connect(serial, SIGNAL(error(QSerialPort::SerialPortError)), this,
    SLOT(handleError(QSerialPort::SerialPortError)));

void MySerialPort::handleError(QSerialPort::SerialPortError error)
{
    if (error == QSerialPort::NoError) {
        qDebug()<<"no error";
    }
}

And, I got "no error " output.
I added to writeData function

qDebug()<<"written";

But, I did not get output. Port is opened, but couldn't write.

JonB

@elypheldia
@jsulm asked you to

What does serial->write(data); return?

Can you just confirm/tell us what that write() returns?

elypheldia

@JonB number of written bytes

JonB

@elypheldia
Then we would suggest to you (I believe!) that the bytes are being sent. So what is your "proof" that they are not? For example, for all I know if you expect a "response" your write might not be correct, it might require a \n or something at the end.

Further, you call serial->readAll(); immediately after executing the write(). I don't know about serial ports, but in general this is not right in Qt, the data sent back may arrive a while later. You are supposed to use readyRead() signal for asynchronous operation. [OK, I see you do that. Your protocol first waits for bytes to arrive initially, then writes something back; is your device then supposed to send something back to you? If so, it will then ping-pong endlessly?] Unless you/ @jsulm know better?

I know Wireshark can be used on PC to view TCP/IP/socket traffic. Is there a tool you can use to verify whether your bytes are being sent to the Linux serial device?

jsulm

@elypheldia said in Cannot write a data to a virtual serial port ?:

number of written bytes

Could you please tell us the number?

elypheldia

@jsulm I cannot. Because, I dont know how to learn number of written bytes.

J.Hilk

@elypheldia said in Cannot write a data to a virtual serial port ?:

@jsulm I cannot. Because, I dont know how to learn number of written bytes.

are you joking ?

you already use qDebug() in your code example, use it some more
qDebug() << "Bytes Written:" << serial->write(data);

that said, make sure you actually write something before you expect readData to be called.

elypheldia

@J-Hilk I already did this. But I get no output. I am very new, sorry. Thx to all for replies.

J.Hilk

@elypheldia said in Cannot write a data to a virtual serial port ?:

@J-Hilk I already did this. But I get no output. I am very new, sorry. Thx to all for replies.

thats why I said:

make sure you actually write something before you expect readData to be called

change your openSerialPort to this:

void MySerialPort::openSerialPort()
{
    serial->setPortName("/dev/tnt0");
    serial->setBaudRate(QSerialPort::Baud9600);
    serial->setDataBits(QSerialPort::Data8);
    serial->setFlowControl(QSerialPort::NoFlowControl);
    serial->setStopBits(QSerialPort::OneStop);
    serial->setParity(QSerialPort::NoParity);
    if(serial->open(QIODevice::ReadWrite))
    {
        qDebug()<<"opened";
       QByteArray data="hello";
           writeData(data);

    }
    else
    {
        qDebug()<<QSerialPort::PermissionError;
    }

}

it will, at the very, least attempt to write something, and should return a bytes written value and/or an error message

elypheldia

@J-Hilk Thank you all guys for your patient and sorry for my basic mistakes :). I fixed my issue. As you said, I expect the read without writing a data to serial port.