目录

• 1. UDP通信概述
• 2. UDP消息传送的三种模式
• 3. QUdpSocket类的接口函数
• 4. UDP单播和广播代码示例
• • 4.1 测试说明
  • 4.2 MainWindow.h
  • 4.3 MainWindow.cpp
  • 4.4 界面展示
• 5. UDP组播代码示例
• • 5.1 组播的特性
  • 5.2 MainWindow.h
  • 5.3 MainWindow.cpp
  • 5.4 界面展示

1. UDP通信概述

UDP是无连接、不可靠、面向数据报（datagram）的协议，可以应用于对可靠性要求不高的场合。与TCP通信不同，UDP通信无需预先建立持久的socket连接，UDP每次发送数据报都需要指定目标地址和端口。

QUdpSocket以数据报传输数据，而不是以连续的数据流。发送数据报使用函数QUdpSocket::writeDatagram()，数据报的长度一般少于512字节，每个数据报包含发送者和接收者的IP地址和端口等信息。
UDP数据接收，首先要使用QUdpSocket::bind()绑定一个端口，绑定端口后，socket的状态会变为已绑定状态“BoundState”。当有数据报传入时，QudpSocket会自动发射readyRead()信号，在其槽函数中使用QUdpSocket::readDatagram()进行数据读取。abort()为解除绑定，解除后socket状态变为未连接状态“UnconnectedState”。

2. UDP消息传送的三种模式

单播模式（unicast）：一个UDP客户端发送数据报到指定地址和端口的另一UDP客户端，是一对一的数据传输。

广播模式（broadcast）：一个UDP客户端发出的数据报，在同一网络范围内其他所有的UDP客户端都可以收到。QUdpSocket支持IPv4广播。需要在数据报中指定接收端地址为QHostAddress::Broadcast，一般的广播地址是255.255.255.255。

组播模式（multicast）：UDP客户端加入到另一个组播IP地址的多播组，成员向组播地址发送的数据报，其加入组播的所有成员都可以接收到，类似于QQ群功能。QUdpSocket::joinMulticastGroup()函数实现加入多播组的功能。

在单播、广播和多播模式下，UDP程序都是对等的，不像TCP通信分为客户端和服务端。
TCP通信只有单播模式。UDP通信虽然不能保证数据传输的准确性，但是具有灵活性，一般的即时通信软件都是基于UDP通信的。

3. QUdpSocket类的接口函数

bool bind(quint16 port = 0) 为UDP通信绑定一个端口

qint64 writeDatagram(QByteArray& datagram, QHostAddress& host, quint16 port) 向目标地址和端口的UDP客户端发送数据报，返回成功发送的字节数，数据报的长度一般不超过512字节。

bool hasPendingDatagrams() 当至少有一个数据报需要读取时，返回true

qint64 pendingDatagramSize() 返回第一个待读取的数据报的大小

qint64 readDatagram(char* data, qint64 maxSize) 读取一个数据报，返回成功读取的字节数

qint64 readDatagram(char* data, qint64 maxSize, QHostAddress* address, quint16* port) 读取一个数据报，返回成功读取的字节数。发送方的主机地址和端口存储在*address和*port中(除非指针为0)

bool joinMulticastGroup(QHostAddress& groupAddress) 加入一个多播组

bool leaveMulticastGroup(QHostAddress& groupAddress) 离开一个多播组

void abort() 终止当前连接并重置套接字。通常在析构函数中写入。与disconnectFromHost()不同，该函数立即关闭套接字，丢弃写入缓冲区中的任何挂起数据。

4. UDP单播和广播代码示例

4.1 测试说明

本实例实现UDP通信的单播和广播。两个实例可以运行在同一台计算机上，也可以运行在不同的计算机上。
这里的两个实例是运行在同一台计算机上，需要注意，在同一台计算机上运行时，两个实例需要绑定不同的端口。例如实例A绑定端口1600，实例B绑定端口3200，实例A向实例B发送数据报时，需要指定实例B的端口，这样实例B才能收到数据。
如果两个实例在不同的计算机上运行，则端口可以一样，因为IP地址不同了，不会导致绑定时发生冲突。一般的UDP通信程序都是在不同的计算机上运行的，约定一个固定的端口作为通信端口。

4.2 MainWindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include namespace Ui {class MainWindow;
}class MainWindow : public QMainWindow {Q_OBJECTpublic:explicit MainWindow(QWidget* parent = 0);~MainWindow();private slots:void slotActBindPort();void slotActUnbindPort();void slotActClearText();void slotActQuit();void slotSocketStateChanged(QAbstractSocket::SocketState socketState);void slotBtnSend();void slotBtnBroad();void slotSocketReadyRead();  //读取socket传入的数据private:Ui::MainWindow* ui;QAction* m_pActBindPort;QAction* m_pActUnbindPort;QAction* m_pActClearText;QAction* m_pActQuit;QWidget* m_pCentralWidget;QLabel* m_pLabBindPort;QLabel* m_PLabTargetAddr;QLabel* m_pLabTargetPort;QSpinBox* m_pSpinBindPort;QComboBox* m_pComboTargetAddr;QSpinBox* m_pSpinTargetPort;QLineEdit* m_pLineEdit;QPushButton* m_pBtnSend;QPushButton* m_pBtnBroad;QPlainTextEdit* m_pPlainText;QLabel* m_pLabState;QUdpSocket* m_pUdpSocket;QString getLocalIP();
};#endif  // MAINWINDOW_H

4.3 MainWindow.cpp

#include "mainwindow.h"#include "ui_mainwindow.h"MainWindow::MainWindow(QWidget* parent) : QMainWindow(parent), ui(new Ui::MainWindow) {ui->setupUi(this);this->setWindowIcon(QIcon(":/new/prefix1/res/TitleIcon.png"));this->setWindowTitle(QStringLiteral("UDP Send/Receiver"));//工具栏ui->mainToolBar->setToolButtonStyle(Qt::ToolButtonTextBesideIcon);m_pActBindPort = new QAction(QIcon(":/new/prefix1/res/绑定端口.png"), QStringLiteral("绑定端口"), this);m_pActUnbindPort = new QAction(QIcon(":/new/prefix1/res/解除绑定.png"), QStringLiteral("结束绑定"), this);m_pActClearText = new QAction(QIcon(":/new/prefix1/res/清空文本.png"), QStringLiteral("清空文本"), this);m_pActQuit = new QAction(QIcon(":/new/prefix1/res/退出.png"), QStringLiteral("退出"), this);ui->mainToolBar->addAction(m_pActBindPort);ui->mainToolBar->addAction(m_pActUnbindPort);ui->mainToolBar->addSeparator();ui->mainToolBar->addAction(m_pActClearText);ui->mainToolBar->addSeparator();ui->mainToolBar->addAction(m_pActQuit);//界面布局m_pCentralWidget = new QWidget(this);QHBoxLayout* HLay1 = new QHBoxLayout;m_pLabBindPort = new QLabel(QStringLiteral("绑定端口"), m_pCentralWidget);m_pSpinBindPort = new QSpinBox(m_pCentralWidget);m_pSpinBindPort->setMinimum(1);m_pSpinBindPort->setMaximum(65535);m_pSpinBindPort->setValue(1600);m_PLabTargetAddr = new QLabel(QStringLiteral("目标地址"), m_pCentralWidget);m_pComboTargetAddr = new QComboBox(m_pCentralWidget);m_pLabTargetPort = new QLabel(QStringLiteral("目标端口"), m_pCentralWidget);m_pSpinTargetPort = new QSpinBox(m_pCentralWidget);m_pSpinTargetPort->setMinimum(1);m_pSpinTargetPort->setMaximum(65535);m_pSpinTargetPort->setValue(3200);HLay1->addWidget(m_pLabBindPort, 1, Qt::AlignRight);HLay1->addWidget(m_pSpinBindPort, 2);HLay1->addWidget(m_PLabTargetAddr, 1, Qt::AlignRight);HLay1->addWidget(m_pComboTargetAddr, 4);HLay1->addWidget(m_pLabTargetPort, 1, Qt::AlignRight);HLay1->addWidget(m_pSpinTargetPort, 2);QHBoxLayout* HLay2 = new QHBoxLayout;m_pLineEdit = new QLineEdit(m_pCentralWidget);m_pBtnSend = new QPushButton(QStringLiteral("发送消息"), m_pCentralWidget);m_pBtnBroad = new QPushButton(QStringLiteral("广播消息"), m_pCentralWidget);HLay2->addWidget(m_pLineEdit);HLay2->addWidget(m_pBtnSend);HLay2->addWidget(m_pBtnBroad);QVBoxLayout* VLay = new QVBoxLayout(m_pCentralWidget);  //主布局必须设置parent，否则不会显示布局// QVBoxLayout* VLay = new QVBoxLayout();VLay->addLayout(HLay1);VLay->addLayout(HLay2);m_pPlainText = new QPlainTextEdit(m_pCentralWidget);VLay->addWidget(m_pPlainText);this->setCentralWidget(m_pCentralWidget);this->setLayout(VLay);  //设置为窗体的主布。在指定了主布局的parent之后，这句话可有可无QString localIP = getLocalIP();this->setWindowTitle(this->windowTitle() + "---IP:" + localIP);m_pComboTargetAddr->addItem(localIP);m_pUdpSocket = new QUdpSocket(this);//状态栏m_pLabState = new QLabel(QStringLiteral("socket状态："), this);m_pLabState->setMinimumWidth(150);ui->statusBar->addWidget(m_pLabState);// connectconnect(m_pActBindPort, &QAction::triggered, this, &MainWindow::slotActBindPort);connect(m_pActUnbindPort, &QAction::triggered, this, &MainWindow::slotActUnbindPort);connect(m_pActClearText, &QAction::triggered, this, &MainWindow::slotActClearText);connect(m_pActQuit, &QAction::triggered, this, &MainWindow::slotActQuit);connect(m_pBtnSend, &QPushButton::clicked, this, &MainWindow::slotBtnSend);connect(m_pBtnBroad, &QPushButton::clicked, this, &MainWindow::slotBtnBroad);connect(m_pUdpSocket, &QUdpSocket::stateChanged, this, &MainWindow::slotSocketStateChanged);connect(m_pUdpSocket, &QUdpSocket::readyRead, this, &MainWindow::slotSocketReadyRead);
}MainWindow::~MainWindow() {m_pUdpSocket->abort();delete m_pUdpSocket;m_pUdpSocket = nullptr;delete ui;
}void MainWindow::slotActBindPort() {quint16 port = m_pSpinBindPort->value();  //本机UDP端口if (m_pUdpSocket->bind(port)) {m_pPlainText->appendPlainText("**已成功绑定");m_pPlainText->appendPlainText("绑定端口：" + QString::number(m_pUdpSocket->localPort()));//使能m_pActBindPort->setEnabled(false);m_pActUnbindPort->setEnabled(true);} else {m_pPlainText->appendPlainText("绑定失败");}
}void MainWindow::slotActUnbindPort() {m_pUdpSocket->abort();  //解除绑定m_pPlainText->appendPlainText("**已解除绑定");m_pActBindPort->setEnabled(true);m_pActUnbindPort->setEnabled(false);
}void MainWindow::slotActClearText() { m_pPlainText->clear(); }void MainWindow::slotActQuit() {QMessageBox::StandardButton button = QMessageBox::question(this, "", "是否要退出？");if (button == QMessageBox::StandardButton::Yes)this->close();
}void MainWindow::slotSocketStateChanged(QAbstractSocket::SocketState socketState) {switch (socketState) {case QAbstractSocket::UnconnectedState: m_pLabState->setText("socket状态：UnconnectedState"); break;case QAbstractSocket::HostLookupState: m_pLabState->setText("socket状态：HostLookupState"); break;case QAbstractSocket::ConnectingState: m_pLabState->setText("socket状态：ConnectingState"); break;case QAbstractSocket::ConnectedState: m_pLabState->setText("socket状态：ConnectedState"); break;case QAbstractSocket::BoundState: m_pLabState->setText("socket状态：BoundState"); break;case QAbstractSocket::ClosingState: m_pLabState->setText("socket状态：ClosingState"); break;default: break;}
}void MainWindow::slotBtnSend() {QString msg = m_pLineEdit->text();QByteArray str = msg.toUtf8();QString targetIp = m_pComboTargetAddr->currentText();  //目标IPQHostAddress targetAddr(targetIp);quint16 targetPort = m_pSpinTargetPort->value();  //目标端口m_pUdpSocket->writeDatagram(str, targetAddr, targetPort);m_pPlainText->appendPlainText("[out] " + msg);m_pLineEdit->clear();m_pLineEdit->setFocus();
}void MainWindow::slotBtnBroad() {QString msg = m_pLineEdit->text();QByteArray str = msg.toUtf8();quint16 targetPort = m_pSpinTargetPort->value();m_pUdpSocket->writeDatagram(str, QHostAddress::Broadcast, targetPort);m_pPlainText->appendPlainText("[out] " + msg);m_pLineEdit->clear();m_pLineEdit->setFocus();
}void MainWindow::slotSocketReadyRead() {while (m_pUdpSocket->hasPendingDatagrams()) {QByteArray dataGram;dataGram.resize(m_pUdpSocket->pendingDatagramSize());QHostAddress peerAddress;quint16 peerPort;m_pUdpSocket->readDatagram(dataGram.data(), dataGram.size(), &peerAddress, &peerPort);QString str = dataGram.data();QString peer = "[From " + peerAddress.toString() + ":" + QString::number(peerPort) + "]";m_pPlainText->appendPlainText(peer + str);}
}QString MainWindow::getLocalIP() {QString hostName = QHostInfo::localHostName();QHostInfo hostInfo = QHostInfo::fromName(hostName);QString localIP = "";QList addrList = hostInfo.addresses();if (!addrList.isEmpty()) {for (int i = 0; i < addrList.size(); i++) {QHostAddress addr = addrList.at(i);if (QAbstractSocket::IPv4Protocol == addr.protocol()) {localIP = addr.toString();break;}}}return localIP;
}

4.4 界面展示

5. UDP组播代码示例

5.1 组播的特性

组播报文的目的地址使用D类IP地址，关于组播IP地址，有以下约定：

1. 224.0.0.0 ~ 224.0.0.255为预留的组播地址（永久组地址），地址224.0.0.0保留不做分配，其他地址供路由协议使用。
2. 224.0.1.0 ~ 224.0.1.255是公用组播地址，可以用于Internet。
3. 224.0.2.0 ~ 238.255.255.255为用户可用的组播地址（临时组地址），全网范围内有效。
4. 239.0.0.0 ~ 239.255.255.255为本地管理组播地址，仅在特定的本地范围内有效。所以，若是在家庭或办公室局域网内测试UDP组播功能，可以使用这些IP。

5.2 MainWindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include namespace Ui {class MainWindow;
}class MainWindow : public QMainWindow {Q_OBJECTpublic:explicit MainWindow(QWidget* parent = 0);~MainWindow();private slots:void slotActJoinMulti();void slotActLeaveMulti();void slotActClearText();void slotActQuit();void slotSocketStateChanged(QAbstractSocket::SocketState socketState);void slotBtnMultiMsg();void slotReadyRead();private:Ui::MainWindow* ui;QAction* m_pActJoinMulti;QAction* m_pActLeaveMulti;QAction* m_pActClearText;QAction* m_pActQuit;QWidget* m_pCentralWidget;QLabel* m_pLabPort;QLabel* m_pLabAddr;QSpinBox* m_pSpinPort;QComboBox* m_pComboAddr;QLineEdit* m_pLineEdit;QPushButton* m_pBtnSendMulti;QPlainTextEdit* m_pPlainText;QLabel* m_pLabState;QUdpSocket* m_pUdpSocket;QHostAddress m_multicastAddr;QString getLocalIP();
};#endif  // MAINWINDOW_H

5.3 MainWindow.cpp

#include "mainwindow.h"#include "ui_mainwindow.h"MainWindow::MainWindow(QWidget* parent) : QMainWindow(parent), ui(new Ui::MainWindow) {ui->setupUi(this);this->setWindowTitle(QStringLiteral("UDP Multicast"));//工具栏ui->mainToolBar->setToolButtonStyle(Qt::ToolButtonTextBesideIcon);m_pActJoinMulti = new QAction(QIcon(":/new/prefix1/res/添加群组.png"), QStringLiteral("加入组播"), this);m_pActLeaveMulti = new QAction(QIcon(":/new/prefix1/res/退出群组.png"), QStringLiteral("退出组播"), this);m_pActClearText = new QAction(QIcon(":/new/prefix1/res/清空.png"), QStringLiteral("清空文本"), this);m_pActQuit = new QAction(QIcon(":/new/prefix1/res/退出.png"), QStringLiteral("退出"), this);ui->mainToolBar->addAction(m_pActJoinMulti);ui->mainToolBar->addAction(m_pActLeaveMulti);ui->mainToolBar->addSeparator();ui->mainToolBar->addAction(m_pActClearText);ui->mainToolBar->addSeparator();ui->mainToolBar->addAction(m_pActQuit);//界面布局m_pCentralWidget = new QWidget(this);m_pLabPort = new QLabel(QStringLiteral("组播端口"), m_pCentralWidget);m_pSpinPort = new QSpinBox(m_pCentralWidget);m_pSpinPort->setMinimum(1);m_pSpinPort->setMaximum(65535);m_pSpinPort->setValue(3200);m_pLabAddr = new QLabel(QStringLiteral("组播地址"), m_pCentralWidget);m_pComboAddr = new QComboBox(m_pCentralWidget);m_pComboAddr->setEditable(true);  //下拉框可编辑输入m_pComboAddr->addItem("239.0.0.1");// 正则匹配 D类IP：224.0.0.0~239.255.255.255// .必须使用转义字符\，否则.会匹配任意字符// C++中"\"在字符串中表示要用"\\"// 是 - 不是 ~ ； 是[0-9]不是[0~9]QRegExp regExp("^(22[4-9]|23[0-9])(\\.((\\d)|([1-9]\\d)|(1\\d{2})|(2[0-4]\\d)|(25[0-5]))){3}$");QValidator* pValidator = new QRegExpValidator(regExp, this);m_pComboAddr->setValidator(pValidator);QHBoxLayout* HLay1 = new QHBoxLayout();HLay1->addWidget(m_pLabPort, 1, Qt::AlignRight);HLay1->addWidget(m_pSpinPort, 1);HLay1->addWidget(m_pLabAddr, 1, Qt::AlignRight);HLay1->addWidget(m_pComboAddr, 2);m_pLineEdit = new QLineEdit(m_pCentralWidget);m_pBtnSendMulti = new QPushButton(QStringLiteral("组播消息"), m_pCentralWidget);QHBoxLayout* HLay2 = new QHBoxLayout();HLay2->addWidget(m_pLineEdit, 4);HLay2->addWidget(m_pBtnSendMulti, 1);m_pPlainText = new QPlainTextEdit(m_pCentralWidget);QVBoxLayout* VLay = new QVBoxLayout(m_pCentralWidget);VLay->addLayout(HLay1);VLay->addLayout(HLay2);VLay->addWidget(m_pPlainText);this->setCentralWidget(m_pCentralWidget);this->setLayout(VLay);//状态栏m_pLabState = new QLabel(QStringLiteral("socket状态："), this);m_pLabState->setMinimumWidth(150);ui->statusBar->addWidget(m_pLabState);QString str = getLocalIP();this->setWindowTitle(this->windowTitle() + "---IP:" + str);m_pUdpSocket = new QUdpSocket(this);m_pUdpSocket->setSocketOption(QAbstractSocket::MulticastTtlOption, 1);// connectconnect(m_pActJoinMulti, &QAction::triggered, this, &MainWindow::slotActJoinMulti);connect(m_pActLeaveMulti, &QAction::triggered, this, &MainWindow::slotActLeaveMulti);connect(m_pActClearText, &QAction::triggered, this, &MainWindow::slotActClearText);connect(m_pActQuit, &QAction::triggered, this, &MainWindow::slotActQuit);connect(m_pUdpSocket, &QUdpSocket::stateChanged, this, &MainWindow::slotSocketStateChanged);connect(m_pBtnSendMulti, &QPushButton::clicked, this, &MainWindow::slotBtnMultiMsg);connect(m_pUdpSocket, &QUdpSocket::readyRead, this, &MainWindow::slotReadyRead);
}MainWindow::~MainWindow() { delete ui; }void MainWindow::slotActJoinMulti() {QString ip = m_pComboAddr->currentText();m_multicastAddr = QHostAddress(ip);quint16 multicastPort = m_pSpinPort->value();if (m_pUdpSocket->bind(QHostAddress::AnyIPv4, multicastPort, QUdpSocket::ShareAddress)) {m_pUdpSocket->joinMulticastGroup(m_multicastAddr);  //加入多播组m_pPlainText->appendPlainText("**加入组播成功");m_pPlainText->appendPlainText("**组播地址IP：" + ip);m_pPlainText->appendPlainText("**绑定端口：" + QString::number(multicastPort));m_pActJoinMulti->setEnabled(false);m_pActLeaveMulti->setEnabled(true);m_pComboAddr->setEditable(false);} else {m_pPlainText->appendPlainText("**绑定端口失败");}
}void MainWindow::slotActLeaveMulti() {m_pUdpSocket->leaveMulticastGroup(m_multicastAddr);  //退出组播m_pUdpSocket->abort();                               //解除绑定m_pActJoinMulti->setEnabled(true);m_pActLeaveMulti->setEnabled(false);m_pComboAddr->setEnabled(true);m_pComboAddr->setEditable(true);m_pPlainText->appendPlainText("**已退出组播，解除端口绑定");
}void MainWindow::slotActClearText() { m_pPlainText->clear(); }void MainWindow::slotActQuit() {QMessageBox::StandardButton button = QMessageBox::question(this, "", "是否退出？");if (QMessageBox::StandardButton::Yes == button) {this->close();}
}void MainWindow::slotSocketStateChanged(QAbstractSocket::SocketState socketState) {// case并不包含所有的情况，因为没有写listening的情况，所以就需要写defaultswitch (socketState) {case QAbstractSocket::UnconnectedState: m_pLabState->setText("socket状态：UnconnectedState"); break;case QAbstractSocket::HostLookupState: m_pLabState->setText("socket状态：HostLookupState"); break;case QAbstractSocket::ConnectingState: m_pLabState->setText("socket状态：ConnectingState"); break;case QAbstractSocket::ConnectedState: m_pLabState->setText("socket状态：ConnectedState"); break;case QAbstractSocket::BoundState: m_pLabState->setText("socket状态：BoundState"); break;case QAbstractSocket::ClosingState: m_pLabState->setText("socket状态：ClosingState"); break;default: break;}
}void MainWindow::slotBtnMultiMsg() {QString msg = m_pLineEdit->text();QByteArray str = msg.toUtf8();quint16 multiPort = m_pSpinPort->value();m_pUdpSocket->writeDatagram(str, m_multicastAddr, multiPort);m_pPlainText->appendPlainText("[multicast] " + msg);m_pLineEdit->clear();m_pLineEdit->setFocus();
}void MainWindow::slotReadyRead() {while (m_pUdpSocket->hasPendingDatagrams()) {QByteArray dataGram;dataGram.resize(m_pUdpSocket->pendingDatagramSize());QHostAddress peerAddr;quint16 peerPort;m_pUdpSocket->readDatagram(dataGram.data(), dataGram.size(), &peerAddr, &peerPort);QString str = dataGram.data();QString peer = "[From " + peerAddr.toString() + ":" + QString::number(peerPort) + "] ";m_pPlainText->appendPlainText(peer + str);qDebug() << m_pUdpSocket->peerAddress();qDebug() << m_pUdpSocket->localAddress().toString();qDebug() << m_pUdpSocket->localPort();}
}QString MainWindow::getLocalIP() {QString localName = QHostInfo::localHostName();QHostInfo hostInfo = QHostInfo::fromName(localName);QList addrList = hostInfo.addresses();QString localIP = "";if (!addrList.isEmpty()) {for (int i = 0; i < addrList.size(); i++) {QHostAddress addr = addrList.at(i);if (QAbstractSocket::IPv4Protocol == addr.protocol()) {localIP = addr.toString();break;}}}return localIP;
}

5.4 界面展示