Qt5 Tutorial QTcpSocket - 2020

In this tutorial, we will learn how to download a file using QTcpSocket. We're taking blocking approach (synchronous). In other words, we use waitFor...() functions (e.g., QTcpSocket::waitForConnected()) until the operation has completed, instead of connecting to signals.

Note: Qt5 document

The QTcpSocket class provides a TCP socket.

TCP (Transmission Control Protocol) is a reliable, stream-oriented, connection-oriented transport protocol. It is especially well suited for continuous transmission of data.

QTcpSocket is a convenience subclass of QAbstractSocket that allows you to establish a TCP connection and transfer streams of data. See the QAbstractSocket documentation for details.

For TCP Socket in general, please visit my C++ Tutorials: Socket - Server and Client.

We'll start with Qt Console Application.

First, we need to add network module to our project file, QTcpSocket.pro:

QT       += core
QT       += network
QT       -= gui

TARGET = QTcpSocket
CONFIG   += console
CONFIG   -= app_bundle

TEMPLATE = app


SOURCES += main.cpp

Then, we want to create a new class called MyTcpSocket.

Let's do work on main.cpp.

We need to create an instance of MyTcpSocket, and then call a our key driver function call, doConnect():

// main.cpp

#include 
#include "mytcpsocket.h"

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    
    MyTcpSocket s;
    s.doConnect();
            
    return a.exec();
}

Now, let's implement the doConnect() method. But before doing that we should write some lines of code in the mytcpsocket.h:

1. #include <QTcpSocket>
2. the prototype of the method, doConnect()
3. Socket object (pointer)

#ifndef MYTCPSOCKET_H
#define MYTCPSOCKET_H

#include <QObject>
#include <QTcpSocket>
#include <QDebug>

class MyTcpSocket : public QObject
{
    Q_OBJECT
public:
    explicit MyTcpSocket(QObject *parent = 0);
    
    void doConnect();

signals:
    
public slots:

private:
    QTcpSocket *socket;
    
};

#endif // MYTCPSOCKET_H

Once header file is done, let's move on to implementation file, mytcpsocket.cpp:

#include "mytcpsocket.h"

MyTcpSocket::MyTcpSocket(QObject *parent) :
    QObject(parent)
{
}

void MyTcpSocket::doConnect()
{
    socket = new QTcpSocket(this);

    socket->connectToHost("google.com", 80);

    if(socket->waitForConnected(5000))
    {
        qDebug() << "Connected!";

        // send
        socket->write("Hello server\r\n\r\n");
        socket->waitForBytesWritten(1000);
        socket->waitForReadyRead(3000);
        
        qDebug() << "Reading: " << socket->bytesAvailable();

        // get the data
        qDebug() << socket->readAll();
        
        // close the connection
        socket->close();
    }
    else
    {
        qDebug() << "Not connected!";
    }
}

The void QAbstractSocket::connectToHost(const QString & hostName, quint16 port, OpenMode openMode = ReadWrite, NetworkLayerProtocol protocol = AnyIPProtocol) function attempts to make a connection to hostName on the given port. The protocol parameter can be used to specify which network protocol to use (eg. IPv4 or IPv6).

The socket is opened in the given openMode and first enters HostLookupState, then performs a host name lookup of hostName. If the lookup succeeds, hostFound() is emitted and QAbstractSocket enters ConnectingState. It then attempts to connect to the address or addresses returned by the lookup. Finally, if a connection is established, QAbstractSocket enters ConnectedState and emits connected().

At any point, the socket can emit error() to signal that an error occurred.

The hostName may be an IP address in string form (e.g., "43.195.83.32"), or it may be a host name (e.g., "example.com"). QAbstractSocket will do a lookup only if required. port is in native byte order.

The bool QAbstractSocket::waitForConnected(int msecs = 30000) waits until the socket is connected, up to msecs milliseconds. If the connection has been established, this function returns true; otherwise it returns false. In the case where it returns false, we can call error() to determine the cause of the error.

For buffered devices, the bool QIODevice::waitForBytesWritten(int msecs) function waits until a payload of buffered written data has been written to the device and the bytesWritten() signal has been emitted, or until msecs milliseconds have passed. If msecs is -1, this function will not time out. For unbuffered devices, it returns immediately.

It returns true if a payload of data was written to the device; otherwise returns false (i.e. if the operation timed out, or if an error occurred).

This function can operate without an event loop. It is useful when writing non-GUI applications and when performing I/O operations in a non-GUI thread.

The bool QAbstractSocket::waitForReadyRead(int msecs = 30000) is reimplemented from QIODevice::waitForReadyRead().

This function blocks until new data is available for reading and the readyRead() signal has been emitted. The function will timeout after msecs milliseconds; the default timeout is 30000 milliseconds.

The function returns true if the readyRead() signal is emitted and there is new data available for reading; otherwise it returns false (if an error occurred or the operation timed out).

The qint64 QAbstractSocket::bytesAvailable() const is reimplemented from QIODevice::bytesAvailable().

This function returns the number of incoming bytes that are waiting to be read.

The QByteArray QIODevice::readAll() reads all available data from the device, and returns it as a QByteArray.

Run the code, then we get:

Connected!
Reading:  1068
"HTTP/1.0 400 Bad Request
Content-Type: text/html; charset=UTF-8
Content-Length: 925
Date: Wed, 18 Sep 2013 05:09:31 GMT
Server: GFE/2.0

<!DOCTYPE html>
<html lang=en>
  <meta charset=utf-8>
  <meta name=viewport content="initial-scale=1, minimum-scale=1, width=device-wi
dth">
  <title>Error 400 (Bad Request)!!1</title>
  <style>
    *{margin:0;padding:0}html,code{font:15px/22px arial,sans-serif}html{backgrou
nd:#fff;color:#222;padding:15px}body{margin:7% auto 0;max-width:390px;min-height
:180px;padding:30px 0 15px}* > body{background:url(//www.google.com/images/error
s/robot.png) 100% 5px no-repeat;padding-right:205px}p{margin:11px 0 22px;overflo
w:hidden}ins{color:#777;text-decoration:none}a img{border:0}@media screen and (m
ax-width:772px){body{background:none;margin-top:0;max-width:none;padding-right:0
}}
  </style>
  <a href=//www.google.com/><img src=//www.google.com/images/errors/logo_sm.gif
alt=Google></a>
  <p><b>400.</b> <ins>ThatÆs an error.</ins>
  <p>Your client has issued a malformed or illegal request.  <ins>ThatÆs all we
know.</ins>
"

Here are the files used in this tutorial.

main.cpp:

#include <QCoreApplication>
#include "mytcpsocket.h"

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    
    MyTcpSocket s;
    s.doConnect();

    return a.exec();
}

mytcpsocket.h:

#ifndef MYTCPSOCKET_H
#define MYTCPSOCKET_H

#include <QObject>
#include <QTcpSocket>
#include <QDebug>

class MyTcpSocket : public QObject
{
    Q_OBJECT
public:
    explicit MyTcpSocket(QObject *parent = 0);
    
    void doConnect();

signals:
    
public slots:

private:
    QTcpSocket *socket;
    
};

#endif // MYTCPSOCKET_H

mytcpsocket.cpp:

#include "mytcpsocket.h"

MyTcpSocket::MyTcpSocket(QObject *parent) :
    QObject(parent)
{
}

void MyTcpSocket::doConnect()
{
    socket = new QTcpSocket(this);

    socket->connectToHost("google.com", 80);

    if(socket->waitForConnected(5000))
    {
        qDebug() << "Connected!";

        // send
        socket->write("Hello server\r\n\r\n");
        socket->waitForBytesWritten(1000);
        socket->waitForReadyRead(3000);
        
        qDebug() << "Reading: " << socket->bytesAvailable();

        // get the data
        qDebug() << socket->readAll();
        
        // close the connection
        socket->close();
    }
    else
    {
        qDebug() << "Not connected!";
    }
}

Qt 5 Tutorial

1. Hello World
2. Signals and Slots
3. Q_OBJECT Macro
4. MainWindow and Action
5. MainWindow and ImageViewer using Designer A
6. MainWindow and ImageViewer using Designer B
7. Layouts
8. Layouts without Designer
9. Grid Layouts
10. Splitter
11. QDir
12. QFile (Basic)
13. Resource Files (.qrc)
14. QComboBox
15. QListWidget
16. QTreeWidget
17. QAction and Icon Resources
18. QStatusBar
19. QMessageBox
20. QTimer
21. QList
22. QListIterator
23. QMutableListIterator
24. QLinkedList
25. QMap
26. QHash
27. QStringList
28. QTextStream
29. QMimeType and QMimeDatabase
30. QFile (Serialization I)
31. QFile (Serialization II - Class)
32. Tool Tips in HTML Style and with Resource Images
33. QPainter
34. QBrush and QRect
35. QPainterPath and QPolygon
36. QPen and Cap Style
37. QBrush and QGradient
38. QPainter and Transformations
39. QGraphicsView and QGraphicsScene
40. Customizing Items by inheriting QGraphicsItem
41. QGraphicsView Animation
42. FFmpeg Converter using QProcess
43. QProgress Dialog - Modal and Modeless
44. QVariant and QMetaType
45. QtXML - Writing to a file
46. QtXML - QtXML DOM Reading
47. QThreads - Introduction
48. QThreads - Creating Threads
49. Creating QThreads using QtConcurrent
50. QThreads - Priority
51. QThreads - QMutex
52. QThreads - GuiThread
53. QtConcurrent QProgressDialog with QFutureWatcher
54. QSemaphores - Producer and Consumer
55. QThreads - wait()
56. MVC - ModelView with QListView and QStringListModel
57. MVC - ModelView with QTreeView and QDirModel
58. MVC - ModelView with QTreeView and QFileSystemModel
59. MVC - ModelView with QTableView and QItemDelegate
60. QHttp - Downloading Files
61. QNetworkAccessManager and QNetworkRequest - Downloading Files
62. Qt's Network Download Example - Reconstructed
63. QNetworkAccessManager - Downloading Files with UI and QProgressDialog
64. QUdpSocket
65. QTcpSocket
66. QTcpSocket with Signals and Slots
67. QTcpServer - Client and Server
68. QTcpServer - Loopback Dialog
69. QTcpServer - Client and Server using MultiThreading
70. QTcpServer - Client and Server using QThreadPool
71. Asynchronous QTcpServer - Client and Server using QThreadPool
72. Qt Quick2 QML Animation - A
73. Qt Quick2 QML Animation - B
74. Short note on Ubuntu Install
75. OpenGL with QT5
76. Qt5 Webkit : Web Browser with QtCreator using QWebView Part A
77. Qt5 Webkit : Web Browser with QtCreator using QWebView Part B
78. Video Player with HTML5 QWebView and FFmpeg Converter
79. Qt5 Add-in and Visual Studio 2012
80. Qt5.3 Installation on Ubuntu 14.04
81. Qt5.5 Installation on Ubuntu 14.04
82. Short note on deploying to Windows