Qt-- realize TCP communication

Tips ： This is the learning record , If there is an error , Please contact the author , Modest and educated .

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Preface

Life always comes and goes , Don't wait for a long time .

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One 、TCP agreement

Transmission control protocol （TCP,Transmission Control Protocol） It's a connection oriented 、 reliable 、 Transport layer communication protocol based on byte stream .

advantage ：
（1） Flow based approach ;
（2） Connection oriented ;
（3） Reliable means of communication ;
（4） When the network condition is not good, try to reduce the bandwidth cost caused by retransmission ;
（5） Communication connection maintenance is oriented to the two endpoints of communication , Without considering the intermediate network segment and node .

See in detail ：《TCP website 》

To satisfy TCP These characteristics of the agreement ,TCP The agreement makes the following provisions ：
① Data fragmentation ： Segment the user data at the sending end , Regroup at the receiving end , from TCP Determine the size of the segments and control the fragmentation and recombination ;
② Arrival confirmation ： When the receiving end receives the fragmentation data , Send a confirmation to the sender according to the slice data serial number ;
③ Timeout retransmission ： The sender starts the timeout timer when sending the fragment , If you don't receive the corresponding confirmation after the timer timeout , Resend slice ;
④ The sliding window ：TCP Each side of the connection has a fixed receive buffer size , The receiving end only allows the other end to send the data that the receiving end buffer can accept ,TCP Provide flow control on the basis of sliding windows , Prevent faster hosts from overflowing buffers on slower hosts ;
⑤ Out of order processing ： As IP Datagram to transmit TCP Pieces may arrive out of order ,TCP The received data will be reordered , Deliver the received data to the application layer in the correct order ;
⑥ Reprocessing ： As IP Datagram to transmit TCP The segments will repeat ,TCP The receiver of must discard the duplicate data ;
⑦ data verification ：TCP Will keep its head and data checked and , This is an end-to-end inspection and , The purpose is to detect any changes in data during transmission . If you receive a patch check and there is an error ,TCP This fragment will be discarded , Failure to acknowledge receipt of this message segment causes the peer to time out and resend .

《 The legendary TCP Three handshakes of 》

The host computer acts as the client , Actively connect to the server , send data , receive data .
Simple understanding ：
A send out ： I want to send you a message , are you ready ？
B send out ： I'm ready , You send it ！
A send out ： well , I'll send it .
then A Start sending .
This is the three handshakes on the understanding level .
The following will be relevant QT Simple example of .

Two 、TCP Communication steps

1.TCP The implementation process of the server

a key ：
A. call listen Listening port

B. Connecting signals newConnection, Call in slot function nextPendingConnection Get connected socket.

（1）.h Import header file

#include <QTcpServer>
#include <QTcpSocket>

（2） establish QTcpServer Object used as a listening socket ;

QTcpServer *m_s;
QTcpSocket *m_tcp;

In the constructor new

// Put in constructor 
m_s = new QTcpServer(this);

（3） Use listen() Method to monitor the network card ip And port

void MainWindow::on_bt_listen_clicked()
{
    
    unsigned short port = ui->lineEdit->text().toUShort();
    bool ret =m_s->listen(QHostAddress::Any, port);
    if(ret)
    {
    
        ui->textEdit->append(" Start the server successfully ");
    }
    else {
    
        ui->textEdit->append("error");
    }
    qDebug()<<ret<<m_s->errorString();
}

（4） Send a message

void MainWindow::on_bt_sendmsg_clicked()
{
    
    QString msg = ui->text_send->toPlainText();
    m_tcp->write(msg.toUtf8());
    ui->textEdit->append(" The server said :"+msg);
}

（5） Close the connection

void MainWindow::on_bt_close_clicked()
{
    
    // Actively disconnect from the client 
    if(NULL == m_tcp)
    {
    
        return;
    }
    m_tcp->disconnectFromHost();
    m_tcp->close();
    m_tcp = NULL;
}

（6） Trigger the slot function above , Use... In the constructor connect()
When there is data transmission , client QTcpSocket Will trigger readyRead() The signal , then readAll（） Read the data and display it .

connect(m_s,&QTcpServer::newConnection,this,[=]()
    {
    
        m_tcp = m_s->nextPendingConnection();
        ui->textEdit->append(" Successfully established a connection with the client !");
        connect(m_tcp,&QTcpSocket::readyRead,this,[=]()
        {
    
            QByteArray data = m_tcp->readAll();
            ui->textEdit->append(" Client said :"+data);
        });
        connect(m_tcp,&QTcpSocket::disconnected,this,[=]()
        {
    
            ui->textEdit->append(" Client disconnected !");
            m_tcp->close();
            m_tcp->deleteLater();
        });

    });

2.TCP The implementation process of the client

a key ：
A. call connectToHost Connect to server

B. call waitForConnected Determine if the connection is successful

C. Connecting signals readyRead Slot function , Reading data asynchronously

D. call waitForReadyRead, Blocking read data

E. call disconnected disconnect

F. call state() == QAbstractSocket::UnconnectedState and waitForDisconnected Determine whether to disconnect

（1）.h Import header file

#include <QTcpSocket>

（2） establish QTcpSocket Socket

QTcpSocket *tcpsocket;// Communication socket 

In the constructor new

tcpsocket = new QTcpSocket(this);

（3） Use this object to connect to the server

void MainWindow::on_bt_lianjie_clicked()
{
      
    qint16 port = ui->tcp_com->text().toInt();
    //unsigned short ip = ui->ip->text().toUShort();
    QString ip = ui->tcp_add->text();
    tcpsocket->connectToHost(QHostAddress(ip),port);
    if(tcpsocket->waitForConnected(3000))
    {
    
        ui->textdisplay->append(" Successfully connected to the server ！");
    }
    else
    {
    
        ui->textdisplay->append(" The connection fails , Please check IP Address and port ！");
    }
}

（4） Send a message write（）

void MainWindow::on_pushButton_clicked()
{
    
    QString msg = ui->text_send->toPlainText();
    ui->textdisplay->append(" Client said :"+msg);
    // Get the contents of the edit box 
    QString str = ui->text_send->toPlainText();
    // send data 
    tcpsocket->write(str.toUtf8().data());
    ui->textdisplay->append(" The client sends ："+str);
}

（5） disconnect

void MainWindow::on_bt_duankai_clicked()
{
    
    // Take the initiative to disconnect from the other party 
    tcpsocket->disconnectFromHost();
    tcpsocket->close();
    // Disconnection succeeded 
    if (tcpsocket->state() == QAbstractSocket::UnconnectedState || tcpsocket->waitForDisconnected(1000))
    {
    
        ui->textdisplay->append(" connection dropped ！");
    }
    else
    {
    
        ui->textdisplay->append(" Unable to disconnect from server ！");
    }
}

（6） Trigger the slot function above , Use... In the constructor connect()
When the client's nested word socket When new data arrives in the receive buffer , Will be issued readRead() The signal

connect(tcpsocket,&QTcpSocket::connected,this,[=]()
    {
    
        ui->textdisplay->append(" Successfully established a connection with the server !");
    });
    connect(tcpsocket,&QTcpSocket::readyRead,this,[=]()
    {
    
        QByteArray data = tcpsocket->readAll();
        ui->textdisplay->append(" The server said :"+data);
    });
    connect(tcpsocket,&QTcpSocket::disconnected,this,[=]()
    {
    
        //m_tcp->close();
        //m_tcp->deleteLater();
        ui->textdisplay->append(" Disconnected from server !");
    });

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summary

Tips ： Good at summarizing , More further .