Socket详解及C++应用示例

一、Socket基本概念

1.1 定义

Socket（套接字）是计算机网络中进程间通信的一种机制，它提供了不同主机或同一主机上不同进程之间进行数据交换的端点。Socket本质上是一个编程接口（API），封装了TCP/IP协议栈的复杂实现，使开发者能便捷地实现网络通信。

1.2 核心作用

• 实现跨进程/跨主机的数据传输
• 提供可靠的字节流传输（TCP）或无连接的数据报传输（UDP）
• 支持多种网络协议（TCP、UDP、ICMP等）
• 允许应用程序通过端口号区分不同服务

1.3 主要类型

1.4 通信模型

Socket通信基于客户端-服务器（C/S）模型：

• 服务器：创建Socket → 绑定端口 → 监听连接 → 接受请求 → 数据交互
• 客户端：创建Socket → 连接服务器 → 数据交互

二、TCP Socket编程示例（C++）

2.1 TCP服务器端实现

#include <iostream>
#include <cstring>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>

const int PORT = 8080;
const int BUFFER_SIZE = 1024;

int main() {
// 1. 创建TCP套接字
int server_fd = socket(AF_INET, SOCK_STREAM, 0);
if (server_fd == –1) {
std::cerr << "Socket creation failed: " << strerror(errno) << std::endl;
return –1;
}

// 2. 设置地址重用，避免端口占用问题
int opt = 1;
if (setsockopt(server_fd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt))) {
std::cerr << "Setsockopt failed: " << strerror(errno) << std::endl;
close(server_fd);
return –1;
}

// 3. 绑定地址和端口
sockaddr_in address;
int addrlen = sizeof(address);
address.sin_family = AF_INET;
address.sin_addr.s_addr = INADDR_ANY; // 监听所有网络接口
address.sin_port = htons(PORT); // 转换为网络字节序

if (bind(server_fd, (struct sockaddr *)&address, sizeof(address)) < 0) {
std::cerr << "Bind failed: " << strerror(errno) << std::endl;
close(server_fd);
return –1;
}

// 4. 监听连接请求（最大等待队列长度为3）
if (listen(server_fd, 3) < 0) {
std::cerr << "Listen failed: " << strerror(errno) << std::endl;
close(server_fd);
return –1;
}
std::cout << "Server listening on port " << PORT << std::endl;

// 5. 接受客户端连接
int new_socket;
sockaddr_in client_addr;
int client_addr_len = sizeof(client_addr);
if ((new_socket = accept(server_fd, (struct sockaddr *)&client_addr,
(socklen_t*)&client_addr_len)) < 0) {
std::cerr << "Accept failed: " << strerror(errno) << std::endl;
close(server_fd);
return –1;
}

char client_ip[INET_ADDRSTRLEN];
inet_ntop(AF_INET, &client_addr.sin_addr, client_ip, INET_ADDRSTRLEN);
std::cout << "Accepted connection from " << client_ip << ":"
<< ntohs(client_addr.sin_port) << std::endl;

// 6. 数据交互
char buffer[BUFFER_SIZE] = {0};
ssize_t valread = read(new_socket, buffer, BUFFER_SIZE);
std::cout << "Received: " << buffer << std::endl;

const char *response = "Hello from TCP server";
send(new_socket, response, strlen(response), 0);
std::cout << "Response sent" << std::endl;

// 7. 关闭连接
close(new_socket);
close(server_fd);
return 0;
}

2.2 TCP客户端实现

#include <iostream>
#include <cstring>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>

const char* SERVER_IP = "127.0.0.1";
const int PORT = 8080;
const int BUFFER_SIZE = 1024;

int main() {
// 1. 创建TCP套接字
int sock = socket(AF_INET, SOCK_STREAM, 0);
if (sock == –1) {
std::cerr << "Socket creation failed: " << strerror(errno) << std::endl;
return –1;
}

// 2. 设置服务器地址
sockaddr_in serv_addr;
serv_addr.sin_family = AF_INET;
serv_addr.sin_port = htons(PORT);

// 转换IP地址为二进制格式
if (inet_pton(AF_INET, SERVER_IP, &serv_addr.sin_addr) <= 0) {
std::cerr << "Invalid address or address not supported" << std::endl;
close(sock);
return –1;
}

// 3. 连接服务器
if (connect(sock, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0) {
std::cerr << "Connection failed: " << strerror(errno) << std::endl;
close(sock);
return –1;
}
std::cout << "Connected to server " << SERVER_IP << ":" << PORT << std::endl;

// 4. 发送数据
const char *message = "Hello from TCP client";
send(sock, message, strlen(message), 0);
std::cout << "Message sent" << std::endl;

// 5. 接收响应
char buffer[BUFFER_SIZE] = {0};
ssize_t valread = read(sock, buffer, BUFFER_SIZE);
std::cout << "Received: " << buffer << std::endl;

// 6. 关闭连接
close(sock);
return 0;
}

三、UDP Socket编程示例（C++）

3.1 UDP服务器端实现

#include <iostream>
#include <cstring>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>

const int PORT = 8080;
const int BUFFER_SIZE = 1024;

int main() {
// 1. 创建UDP套接字
int server_fd = socket(AF_INET, SOCK_DGRAM, 0);
if (server_fd == –1) {
std::cerr << "Socket creation failed: " << strerror(errno) << std::endl;
return –1;
}

// 2. 设置地址重用
int opt = 1;
if (setsockopt(server_fd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt))) {
std::cerr << "Setsockopt failed: " << strerror(errno) << std::endl;
close(server_fd);
return –1;
}

// 3. 绑定地址和端口
sockaddr_in address;
int addrlen = sizeof(address);
address.sin_family = AF_INET;
address.sin_addr.s_addr = INADDR_ANY;
address.sin_port = htons(PORT);

if (bind(server_fd, (struct sockaddr *)&address, sizeof(address)) < 0) {
std::cerr << "Bind failed: " << strerror(errno) << std::endl;
close(server_fd);
return –1;
}
std::cout << "UDP Server listening on port " << PORT << std::endl;

// 4. 接收和发送数据
char buffer[BUFFER_SIZE] = {0};
sockaddr_in client_addr;
int client_addr_len = sizeof(client_addr);

while (true) {
// 接收客户端消息
ssize_t len = recvfrom(server_fd, buffer, BUFFER_SIZE, 0,
(struct sockaddr *)&client_addr, (socklen_t*)&client_addr_len);
if (len < 0) {
std::cerr << "Recvfrom failed: " << strerror(errno) << std::endl;
continue;
}
buffer[len] = '\\0'; // 添加字符串结束符

char client_ip[INET_ADDRSTRLEN];
inet_ntop(AF_INET, &client_addr.sin_addr, client_ip, INET_ADDRSTRLEN);
std::cout << "Received from " << client_ip << ":" << ntohs(client_addr.sin_port)
<< ": " << buffer << std::endl;

// 发送响应
const char *response = "Hello from UDP server";
sendto(server_fd, response, strlen(response), 0,
(struct sockaddr *)&client_addr, client_addr_len);
}

// 注意：UDP服务器通常不会主动关闭，此处为示例完整性添加
close(server_fd);
return 0;
}

3.2 UDP客户端实现

#include <iostream>
#include <cstring>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>

const char* SERVER_IP = "127.0.0.1";
const int PORT = 8080;
const int BUFFER_SIZE = 1024;

int main() {
// 1. 创建UDP套接字
int sock = socket(AF_INET, SOCK_DGRAM, 0);
if (sock == –1) {
std::cerr << "Socket creation failed: " << strerror(errno) << std::endl;
return –1;
}

// 2. 设置服务器地址
sockaddr_in serv_addr;
serv_addr.sin_family = AF_INET;
serv_addr.sin_port = htons(PORT);

if (inet_pton(AF_INET, SERVER_IP, &serv_addr.sin_addr) <= 0) {
std::cerr << "Invalid address or address not supported" << std::endl;
close(sock);
return –1;
}

// 3. 发送数据
const char *message = "Hello from UDP client";
sendto(sock, message, strlen(message), 0,
(struct sockaddr *)&serv_addr, sizeof(serv_addr));
std::cout << "Message sent to " << SERVER_IP << ":" << PORT << std::endl;

// 4. 接收响应
char buffer[BUFFER_SIZE] = {0};
sockaddr_in server_response_addr;
int server_addr_len = sizeof(server_response_addr);

ssize_t len = recvfrom(sock, buffer, BUFFER_SIZE, 0,
(struct sockaddr *)&server_response_addr, (socklen_t*)&server_addr_len);
if (len < 0) {
std::cerr << "Recvfrom failed: " << strerror(errno) << std::endl;
close(sock);
return –1;
}
buffer[len] = '\\0';
std::cout << "Received: " << buffer << std::endl;

// 5. 关闭套接字
close(sock);
return 0;
}

四、跨平台注意事项

4.1 Windows与Linux的主要差异

4.2 跨平台示例代码（TCP服务器）

#include <iostream>
#include <cstring>
#ifdef _WIN32
#include <winsock2.h>
#pragma comment(lib, "ws2_32.lib")
#else
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <errno.h>
#endif

const int PORT = 8080;
const int BUFFER_SIZE = 1024;

int main() {
#ifdef _WIN32
// Windows初始化Winsock
WSADATA wsaData;
if (WSAStartup(MAKEWORD(2, 2), &wsaData) != 0) {
std::cerr << "WSAStartup failed: " << WSAGetLastError() << std::endl;
return –1;
}
#endif

// 创建套接字代码（与前面相同）
int server_fd = socket(AF_INET, SOCK_STREAM, 0);
if (server_fd == –1) {
std::cerr << "Socket creation failed: "
#ifdef _WIN32
<< WSAGetLastError()
#else
<< strerror(errno)
#endif
<< std::endl;
#ifdef _WIN32
WSACleanup();
#endif
return –1;
}

// … 其余代码（绑定、监听、接受连接等）…

// 关闭资源
#ifdef _WIN32
closesocket(server_fd);
WSACleanup();
#else
close(server_fd);
#endif
return 0;
}

五、现代C++网络编程选项

5.1 C++标准网络库（C++20 Networking TS）

C++20引入了标准网络库，提供了现代、类型安全的网络编程接口：

#include <iostream>
#include <string>
#include <asio.hpp>

using asio::ip::tcp;

int main() {
try {
asio::io_context io_context;

tcp::resolver resolver(io_context);
auto endpoints = resolver.resolve("example.com", "daytime");

tcp::socket socket(io_context);
asio::connect(socket, endpoints);

std::string data;
asio::error_code error;
asio::read(socket, asio::dynamic_buffer(data), error);

if (error == asio::error::eof) {
std::cout << "Received: " << data << std::endl;
} else if (error) {
throw std::system_error(error);
}
} catch (std::exception& e) {
std::cerr << "Exception: " << e.what() << std::endl;
}

return 0;
}

5.2 Boost.Asio库

Boost.Asio是广泛使用的跨平台网络库，也是C++标准网络库的基础：

#include <iostream>
#include <boost/asio.hpp>

using boost::asio::ip::tcp;

int main() {
try {
boost::asio::io_context io_context;
tcp::socket socket(io_context);
tcp::resolver resolver(io_context);
boost::asio::connect(socket, resolver.resolve("localhost", "8080"));

std::string request = "Hello from Boost.Asio client";
boost::asio::write(socket, boost::asio::buffer(request));

char reply[1024];
size_t len = socket.read_some(boost::asio::buffer(reply));
std::cout << "Received: " << std::string(reply, len) << std::endl;
} catch (std::exception& e) {
std::cerr << "Exception: " << e.what() << std::endl;
}

return 0;
}

六、Socket应用场景与常见问题

6.1 典型应用场景

• 客户端-服务器通信：Web浏览器与服务器、移动应用后端交互
• 实时数据传输：在线游戏、视频会议、实时监控
• 分布式系统：微服务间通信、进程间通信（IPC）
• 物联网设备：传感器数据上报、设备控制指令

6.2 常见问题及解决方案

6.3 安全考虑

• 使用TLS/SSL加密传输（如HTTPS）
• 验证客户端身份（IP过滤、令牌认证）
• 限制连接频率，防止DoS攻击
• 输入验证，防止缓冲区溢出

七、总结

Socket是网络编程的基础，提供了进程间通信的通用接口。本文介绍了Socket的基本概念、TCP/UDP协议的C++实现示例、跨平台注意事项以及现代C++网络编程选项。通过掌握Socket编程，开发者可以构建各种网络应用，从简单的客户端-服务器程序到复杂的分布式系统。

实际开发中，除了基础的BSD Socket API，还可以考虑使用更高级的库如Boost.Asio或C++20标准网络库，以提高开发效率和代码质量。同时，需要注意网络编程中的错误处理、性能优化和安全问题，确保应用的可靠性和稳定性。