Sending messages from a client to server using network on the same machine

[qcoderpro]

Hi all,

This is my first attempt to get my feet wet with network programming. After reading some references I decided to learn the stuff in action by creating the following two QtQuick projects called server and client using Qt 6.2 on my Windows machine.
The purpose is to be able to connect to a server (using an IP and port) and then send messages to it. I guess it's a simple task for networking but since I'm completely new to this area there may be silly mistakes in the code of the two projects.

First client.qml
Client.h:

#include <QDataStream>
#include <QTcpSocket>

class QTcpSocket;

class Client : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QVector<QString> addresses READ readAddress CONSTANT);

public:
    explicit Client(QObject *parent = nullptr);
    QVector<QString> readAddress() const;

public slots:
    void sendMessage(QString, QString); // Called from the front-end
    QString readMessage(); // Called from the front-end

private:
    QTcpSocket* tcpSocket { nullptr };
    QDataStream in;
    QVector<QString> addresses;
};

Client.cpp:

#include "client.h"
#include <QtNetwork>
#include <QtCore>

Client::Client(QObject *parent)
    : QObject{parent}
    , tcpSocket(new QTcpSocket(this))
{
    /* find out name and IP addresses of this machine
        non-localhost addresses add localhost addresses
        and add all to the vector (addresses)
   */ 
  // Addresses all are shown properly on the front-end part

    in.setDevice(tcpSocket);
    in.setVersion(QDataStream::Qt_4_0);
    connect(tcpSocket, &QIODevice::readyRead, this, &Client::readMessage);
}

void Client::sendMessage(QString ip, QString port)
{
    tcpSocket->abort();
    tcpSocket->connectToHost(ip, port.toInt());
}

QVector<QString> Client::readAddress() const
{
    return addresses;
}

QString Client::readMessage() {
    in.startTransaction();

    QString message;
    in >> message;

    if (!in.commitTransaction())
        return "Error";
    return message;
}

And this is server.qml
Server.h:

#include <QObject>

class QTcpServer;
class QTcpSocket;

class Server : public QObject
{
    Q_OBJECT

public:
    explicit Server(QObject *parent = nullptr);

public slots:
    QString initServer();
    QString setMessage();

private:
    QTcpServer* tcpServer { nullptr };
    QTcpSocket* tcpSocket { nullptr };
    QDataStream in;
};

Server.cpp:

#include "server.h"
#include <QtNetwork>
#include <QtCore>

Server::Server(QObject *parent)
    : QObject{parent}
    , tcpServer(new QTcpServer(this))
    , tcpSocket(new QTcpSocket(this))
{
    initServer();
}

QString Server::initServer() {

    tcpServer = new QTcpServer(this);
    if(!tcpServer->listen())
        return "Server Unable to start the server: " +
                tcpServer->errorString();

    QString ipAddress;
    QList<QHostAddress> ipAddressesList = QNetworkInterface::allAddresses();

   /* use the first non-local IPv4 address if we did not find one, use IPv4 
        localhost, then return IP and port number. 
   */
    in.setDevice(tcpSocket);
    in.setVersion(QDataStream::Qt_4_0);
}

QString Server::setMessage()
{
    in.startTransaction();

    QString message;
    in >> message;

    if (!in.commitTransaction())
        return "commitTransaction error" ;

    return message;
}

When both projects run at the same time, firstly this message appears in Application Output: QDataStream: No transaction in progress

Then after selecting the IP and port number on the Client (based on what Server has provided) and clicking on the Send button to send a simple Hello message, errors bellow will come up on the apps':

[SGaist]

Hi,

You are not handling new connections in your server part.

[qcoderpro]

@SGaist

I explored server.cpp well but am not sure how to do that. :|

[SGaist]

Did you check the fortune client and server examples ?

[qcoderpro]

@SGaist
Yes. There a button on the client causes a message to be sent from the server to it which is different from mine. On my projects, the client itself sends messages to the sever.

[SGaist]

You are not handling new connection. See newConnection.

[qcoderpro]

@SGaist

I want to define: connect(tcpServer, &QTcpServer::newConnection, this, &Server:: ...); but don't know what slot to be called for that. Please remember my server app is the receiver of the messages and there's only a lost, setMessage(), defined in it apart from initServer().

[SGaist]

It means that you need to store the socket that is created to handle the connection with your client.
Then connect that socket readyRead signal to the slot that will process the message.

[qcoderpro]

@SGaist
I'm really confused. Let's get it straight, please.
Just to emphasize: The goal of the two projects is that: First the Server app provides IP and port number. Then the Client app uses those IP and port to connect to the Server, then using a textfiled and a send button it (the client) sends messages to the server.

Now I've got two questions:

1. Are the projects Server and Client defined in the first post of this thread right for that goal?
2. If so, I assume I need to add connect(tcpServer, &QTcpServer::newConnection, this, &Server::setMessage); to the Server app and connect(tcpSocket, &QIODevice::readyRead, this, &Client::sendMessage); to the Client app. Right up to here?

[SGaist]

Client and server separation are correct however, what you are missing is the handling of the socket server side. When a new connection is attempted, it will create a socket on the server to allow communication with the client, and that's why you need to store that socket to be able to do communication with it.

[qcoderpro]

You mentioned "storing a socket" a couple of times, but I don't know what you mean by storing a socket!

What I know from rereading your previous messages up to here is that I need to set the connection below in the server side to store the coming socket (how?):
connect(tcpServer, &QTcpServer::newConnection, this, &Server:: ...);
Then it's required to use that stored socket in: connect(tcpSocket, &QIODevice::readyRead, this, &Client::setMessage); (the setMessage slot will set the message to the label)

Is it not possible to write a few lines of code for the projects to get them to work, please? I'm sure I will definitely learn the whole stuff used for the both projects by reading your messages all (again) and looking at the codes.

[SGaist]

This is enough to receive data from your client and automatically delete the socket on disconnection however it will require to use the sender method to retrieve the data:

void Server::onNewConnection()
{
    QTcpSocket *clientConnection = tcpServer->nextPendingConnection();
    connect(clientConnection, &QAbstractSocket::disconnected,
            clientConnection, &QObject::deleteLater);
    connect(clientConnection, &QAbstractSocket::readyRead, this, &Server::processMessage);
}

The other solution would be to use a lambda rather than a slot.
Lastly, depending on what else you want to do with the socket object, you should add a member variable to your Server class to store it, whether it's a QTcpSocket pointer or a vector of them will depend on how much connections you are going to allow to your server.

[qcoderpro]

The example is much harder than I thought of! :|
I had to simplify the examples as much as possible to get the topic and can get them to work as expected. I'm sorry that I send the examples part of which may be repetitive.
(The purpose is the same: connecting client to the server and then sending messages to it)
client.h

class QTcpSocket;

class Client : public QObject
{
    Q_OBJECT

public:
    explicit Client(QObject *parent = nullptr);

public slots:
    void sendMessage(QString, QString);

private:
    QTcpSocket* tcpSocket { nullptr };
    QDataStream in;
};

client.cpp

#include "client.h"
#include <QtNetwork>

Client::Client(QObject *parent)
    : QObject{parent}
    , tcpSocket(new QTcpSocket(this))
{
    in.setDevice(tcpSocket);
    in.setVersion(QDataStream::Qt_4_0);
}

void Client::sendMessage(QString ip, QString port)
{
    tcpSocket->abort();
    tcpSocket->connectToHost(ip, port.toInt());
}

main.qml

Window {
    width: 300
    height: 200
    visible: true
    title: qsTr("Client")
    color: "lightblue"

    ColumnLayout {
        anchors.fill: parent

        TextField {
            id: ipAddrs
        }
        TextField {
                id: portNum
            }

        RowLayout {
            Layout.alignment: Qt.AlignBottom

            TextField {
                id: txtField
                Layout.fillWidth: true
            }
            Button {
                text: qsTr("Send")
                onClicked:
                myObject.sendMessage(ipAddrs.text.toString(), portNum.text.toString())
            }
        }
    }

    MyObject{
        id: myObject
    }
}

server.h

class QTcpServer;
class QTcpSocket;

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

public slots:
    QString initServer();
    QString setMessage();
    void onNewConnection();

private:
    QTcpServer* tcpServer { nullptr };
    QTcpSocket* tcpSocket { nullptr };
    QDataStream in;
};

server.cpp

#include "server.h"
#include <QtNetwork>
#include <QtCore>

Server::Server(QObject *parent)
    : QObject{parent}
    , tcpServer(new QTcpServer(this))
    , tcpSocket(new QTcpSocket(this))
{
    initServer();
    in.setDevice(tcpSocket);
    in.setVersion(QDataStream::Qt_4_0);
    connect(tcpServer, &QTcpServer::newConnection, this, &Server::onNewConnection);
}

QString Server::initServer() {

    tcpServer = new QTcpServer(this);
    if(!tcpServer->listen())
        return "Server Unable to start the server: " +
                tcpServer->errorString();

    QString ipAddress;
    QList<QHostAddress> ipAddressesList = QNetworkInterface::allAddresses();

    // use the first non-local IPv4 address
    for(int i=0; i<ipAddressesList.size(); ++i)
        if(ipAddressesList.at(i) != QHostAddress::LocalHost &&
                ipAddressesList.at(i).toIPv4Address()) {
            ipAddress = ipAddressesList.at(i).toString();
            break;
        }

    // if we did not find one, use IPv4 localhost
    if(ipAddress.isEmpty())
        ipAddress = QHostAddress(QHostAddress::LocalHost).toString();

    return " The server is running on\n\n IP: " +
            ipAddress + "\n port: " + QString::number(tcpServer->serverPort())
            + "\n\n Run the Client example now.";
}

void Server::onNewConnection()
{
    QTcpSocket *clientConnection = tcpServer->nextPendingConnection();
    connect(clientConnection, &QAbstractSocket::disconnected,
            clientConnection, &QObject::deleteLater);
    connect(clientConnection, &QAbstractSocket::readyRead, this, &Server::setMessage);
}

QString Server::setMessage()
{
    in.startTransaction();

    QString message;
    in >> message;

    if (!in.commitTransaction())
       return "commitTransaction error" ;

    return message;
}

main.qml

Window {
    width: 300
    height: 200
    visible: true
    title: qsTr("Server")
    color: "lightblue"

    ColumnLayout {
        anchors.fill: parent

        Label {
            text: myObject.initServer()
        }

        Label {
            id: msgLabel
            text: myObject.setMessage()
        }
    }

    MyObject{
        id: myObject
    }
}

For now I have only a question. Are the projects to this point fine? I mean I know that there may be many things to be added to them but isn't anything redundant up to here?

[qcoderpro]

When I run both projects, I get this issue for the client project:
QIODevice::read (QTcpSocket): device not open and the error message: commitTransaction error
on the server's project's user interface. Let's for now ignore these.

I type the IP and port numbers provided by the server UI into the text fields of the client project's UI and write "Hi" in the bottom text field and click on the button Send there for which the sendMessage slot is called and it connectes to the server successfully.

void Client::sendMessage(QString ip, QString port)
{
    tcpSocket->abort();
    tcpSocket->connectToHost(ip, port.toInt());
}

Afterwards, we've this connection in the Server project's constructor:
connect(tcpServer, &QTcpServer::newConnection, this, &Server::onNewConnection);
As well as:

in.setDevice(tcpSocket);
in.setVersion(QDataStream::Qt_4_0);

The connection above calls onNewConnection slot since there's a new connection signal.

void Server::onNewConnection()
{
    QTcpSocket *clientConnection = tcpServer->nextPendingConnection();
    connect(clientConnection, &QAbstractSocket::disconnected,
            clientConnection, &QObject::deleteLater);
    connect(clientConnection, &QAbstractSocket::readyRead, this, &Server::setMessage);
}

In that function the setMessage slot is called:

QString Server::setMessage()
{
    in.startTransaction();

    QString message;
    in >> message;

    if (!in.commitTransaction())
       return "commitTransaction error" ;

    return message;
}

in is already set to the socket in the constructor, so it's expected that it's able to get the message sent ('Hi') and return that message to the server's label front-end part where that slot is called (too)!
But in reality no message is shown on the server's UI!

Could you please tell me where the first mistake is in the code so that I can firstly fix it and then we go for the others?

[SGaist]

First thing: you initialize your QDataStream on the wrong socket, it shall use the socket matching the connection that was established.

[qcoderpro]

@SGaist

Do you mean to move

in.setDevice(tcpSocket);
in.setVersion(QDataStream::Qt_4_0);

from the server's constructor into the onNewConnection slot this way, please?

void Server::onNewConnection()
{
    QTcpSocket *clientConnection = tcpServer->nextPendingConnection();
    connect(clientConnection, &QAbstractSocket::disconnected,
            clientConnection, &QObject::deleteLater);
    in.setDevice(clientConnection);
    in.setVersion(QDataStream::Qt_4_0);
    connect(clientConnection, &QAbstractSocket::readyRead, this, &Server::setMessage);
}

The error message when running the projects both at the same time: QDataStream: No transaction in progress

[qcoderpro]

No further help? :|

[SGaist]

Do you also use QDataStream to prepare the data to send ?

[qcoderpro]

This post is deleted!

[qcoderpro]

@SGaist

Yes, you're right. I partly changed all six files to match the requirements as follows:
The server part:
server.h:

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

public slots:
    QString initServer();
    void setMessage();
    QString getMessage() const;
    void onNewConnection();

private:
    QTcpServer* tcpServer { nullptr };
    QDataStream in;
    QString message;
};

server.cpp:

Server::Server(QObject *parent)
    : QObject{parent}
    , tcpServer(new QTcpServer(this))
{
    initServer();
    connect(tcpServer, &QTcpServer::newConnection, this, &Server::onNewConnection);
}

QString Server::initServer() {
//.. Provide the IP address and port number for the client
}

void Server::onNewConnection()
{
    QTcpSocket *clientConnection = tcpServer->nextPendingConnection();
    connect(clientConnection, &QAbstractSocket::disconnected,
            clientConnection, &QObject::deleteLater);
    in.setDevice(clientConnection);
    in.setVersion(QDataStream::Qt_4_0);
    connect(clientConnection, &QAbstractSocket::readyRead, this, &Server::setMessage);
}

void Server::setMessage()
{
    in.startTransaction();
    QString msg;
    in >> msg;

    if (!in.commitTransaction())
        message = "commitTransaction error" ;
    else
        message = msg;
}

QString Server::getMessage() const
{
    return message;
}

server's qml file:

 ColumnLayout {
        anchors.fill: parent

        Label {
            text: myObj.initServer()
        }

        Label {
            id: msgLabel
            text: myObj.getMessage()
        }
    }

    ServerClass{
        id: myObj
    }

The client part:

client.h:

class Client : public QObject
{
    Q_OBJECT

public:
    explicit Client(QObject *parent = nullptr);

public slots:
    void sendAddress(QString, QString);
    void sendMessage(const QString&);

private:
    QTcpSocket* tcpSocket { nullptr };
    QDataStream out;
};

client.cpp:

Client::Client(QObject *parent)
    : QObject{parent}
    , tcpSocket(new QTcpSocket(this))
{
    out.setDevice(tcpSocket);
    out.setVersion(QDataStream::Qt_4_0);
}

void Client::sendAddress(QString ip, QString port)
{
    tcpSocket->abort();
    tcpSocket->connectToHost(ip, port.toInt());
}

void Client::sendMessage(const QString& message)
{
    out.startTransaction();
    out << message;

    if (!out.commitTransaction())
        return ;
}

client's qml file:

 ColumnLayout {
        anchors.fill: parent

        TextField {
            id: ipAddrs
        }
        TextField {
                id: portNum
            }
        Button {
            text: "Send Address"
            onClicked: myObj.sendAddress(ipAddrs.text.toString(), portNum.text.toString())
        }

        RowLayout {
            Layout.alignment: Qt.AlignBottom

            TextField {
                id: txtField
                Layout.fillWidth: true
            }
            Button {
                text: qsTr("Send")
                onClicked: myObj.sendMessage(txtField.text)
            }
        }
    }

    ClientClass{
        id: myObj
    }
}

After running both projects this way, and typing the IP address and port number (given by the server UI) on the client UI and writing a text message there I click on the Send button, but nothing is shown on the server UI.

I'm almost sure now the goal is closer but there're still a number of mistakes that need to be worked out.