C++ 网络编程基础：Socket API 与 TCP 通信

三次握手(建立连接):
  Client ──SYN────▶ Server
  Client ◀─SYN+Ack─ Server
  Client ──Ack────▶ Server

四次挥手(关闭连接):
  Client ──FIN────▶ Server
  Client ◀─Ack───── Server
  Client ◀─FIN───── Server
  Client ──Ack────▶ Server

服务器: socket() → bind() → listen() → accept() → recv/send → close()
客户端: socket() → connect() → send/recv → close()

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

int main() {
    // 1. 创建 socket
    int server_fd = socket(AF_INET, SOCK_STREAM, 0);

    // 2. 绑定地址
    sockaddr_in address{};
    address.sin_family = AF_INET;
    address.sin_addr.s_addr = INADDR_ANY;   // 监听所有网卡
    address.sin_port = htons(8080);
    bind(server_fd, (sockaddr*)&address, sizeof(address));

    // 3. 监听
    listen(server_fd, 5);

    // 4. 接受连接(阻塞,直到有客户端连入)
    sockaddr_in client_addr{};
    socklen_t addr_len = sizeof(client_addr);
    int client_fd = accept(server_fd, (sockaddr*)&client_addr, &addr_len);

    // 5. 收发数据
    char buffer[1024] = {0};
    recv(client_fd, buffer, sizeof(buffer), 0);
    std::cout << "收到: " << buffer << std::endl;

    send(client_fd, "Hello!", 6, 0);

    // 6. 关闭
    close(client_fd);
    close(server_fd);
    return 0;
}