C语言Socket网络编程完全指南

Socket 是 Unix/Linux 下网络编程的核心接口。本文将详细介绍 TCP 与 UDP 编程模型，从基础 socket 创建到并发服务器实现，帮助你掌握网络通信技能。

Socket 编程基础

Socket（套接字）是进程间通信的一种方式，支持本地和跨主机通信。常见的 socket 类型有：

• TCP (SOCK_STREAM) - 面向连接、可靠传输
• UDP (SOCK_DGRAM) - 无连接、不可靠但高效
• 原始 Socket (SOCK_RAW) - 访问底层协议

TCP 客户端与服务器

TCP 编程遵循「创建 socket → 绑定地址 → 监听 → 接受连接 → 通信」流程：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>

#define PORT 8080

int main(void) {
    /* 1. 创建 socket */
    int server_fd = socket(AF_INET, SOCK_STREAM, 0);
    if (server_fd < 0) {
        perror("socket failed");
        exit(EXIT_FAILURE);
    }

    /* 2. 绑定地址 */
    struct sockaddr_in addr;
    memset(&addr, 0, sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_addr.s_addr = INADDR_ANY;  /* 监听所有网卡 */
    addr.sin_port = htons(PORT);

    if (bind(server_fd, (struct sockaddr *)&addr, sizeof(addr)) < 0) {
        perror("bind failed");
        close(server_fd);
        exit(EXIT_FAILURE);
    }

    /* 3. 监听连接 */
    if (listen(server_fd, 10) < 0) {
        perror("listen failed");
        close(server_fd);
        exit(EXIT_FAILURE);
    }

    printf("Server listening on port %d\n", PORT);

    /* 4. 接受客户端连接 */
    struct sockaddr_in client_addr;
    socklen_t client_len = sizeof(client_addr);
    int client_fd = accept(server_fd, (struct sockaddr *)&client_addr, &client_len);

    char buffer[1024];
    read(client_fd, buffer, sizeof(buffer));
    printf("Received: %s\n", buffer);

    char *msg = "Hello from server!";
    write(client_fd, msg, strlen(msg));

    close(client_fd);
    close(server_fd);
    return 0;
}

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>

#define SERVER_IP "127.0.0.1"
#define PORT 8080

int main(void) {
    int sock = socket(AF_INET, SOCK_STREAM, 0);

    struct sockaddr_in server_addr;
    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(PORT);
    inet_pton(AF_INET, SERVER_IP, &server_addr.sin_addr);

    connect(sock, (struct sockaddr *)&server_addr, sizeof(server_addr));

    char *msg = "Hello from client!";
    write(sock, msg, strlen(msg));

    char buffer[1024];
    read(sock, buffer, sizeof(buffer));
    printf("Server: %s\n", buffer);

    close(sock);
    return 0;
}

UDP 编程

UDP 是无连接协议，通信更简单但不可靠：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>

int main(void) {
    int sock = socket(AF_INET, SOCK_DGRAM, 0);

    struct sockaddr_in addr;
    memset(&addr, 0, sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_addr.s_addr = INADDR_ANY;
    addr.sin_port = htons(9000);

    bind(sock, (struct sockaddr *)&addr, sizeof(addr));

    char buffer[1024];
    struct sockaddr_in client_addr;
    socklen_t len = sizeof(client_addr);

    recvfrom(sock, buffer, sizeof(buffer), 0,
                 (struct sockaddr *)&client_addr, &len);

    printf("Received: %s\n", buffer);
    sendto(sock, "ACK", 3, 0,
              (struct sockaddr *)&client_addr, len);

    close(sock);
    return 0;
}

并发服务器

处理多客户端时，常用 fork 或线程池实现并发：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <signal.h>

void handle_client(int client_fd) {
    char buffer[1024];
    read(client_fd, buffer, sizeof(buffer));
    printf("Client: %s\n", buffer);
    write(client_fd, "OK", 2);
    close(client_fd);
}

int main(void) {
    signal(SIGCHLD, SIG_IGN);  /* 避免僵尸进程 */

    int server_fd = socket(AF_INET, SOCK_STREAM, 0);
    struct sockaddr_in addr = {0};
    addr.sin_port = htons(8080);
    bind(server_fd, (struct sockaddr *)&addr, sizeof(addr));
    listen(server_fd, 10);

    while (1) {
        int client_fd = accept(server_fd, NULL, NULL);
        if (fork() == 0) {  /* 子进程处理客户端 */
            close(server_fd);
            handle_client(client_fd);
            exit(0);
        }
        close(client_fd);  /* 父进程关闭客户端套接字 */
    }
    return 0;
}

网络编程注意事项

1. 字节序转换 - 使用 htons/ntohs、htonl/ntohl 处理网络字节序
2. 地址复用 - 设置 SO_REUSEADDR 避免端口绑定失败
3. 错误处理 - 注意处理 EINTR 等中断错误
4. 缓冲区大小 - 合理设置 socket 缓冲区避免丢包
5. 资源释放 - 确保 close() 在适当时机调用

总结

Socket 编程是 C 语言网络开发的基础。TCP 提供可靠连接，UDP 更适合实时性要求高的场景。掌握基本的服务器和客户端编写，理解并发处理方式，才能完成实际的网络应用开发。