3.linux系统编程之网络

linux系统编程之网络

网络编程是Linux系统编程中最重要的部分之一，下面我将详细介绍Linux下的网络编程接口、核心概念和实际应用。

一、网络编程基础

1. 核心概念

• Socket：网络通信的基本端点，是操作系统提供的抽象接口
• IP地址：标识网络中的主机（IPv4 32位，IPv6 128位）
• 端口号：标识主机上的特定进程（16位整数，0-65535）
• 协议：TCP（可靠连接）和UDP（无连接不可靠）

2. 网络编程基本流程

TCP服务端流程：

1. 创建socket
2. 绑定(bind)地址和端口
3. 监听(listen)连接
4. 接受(accept)连接
5. 读写数据
6. 关闭连接

TCP客户端流程：

1. 创建socket
2. 连接(connect)服务器
3. 读写数据
4. 关闭连接

二、核心API详解

1. socket创建

#include <sys/types.h>
#include <sys/socket.h>

int socket(int domain, int type, int protocol);

• domain：协议族
  • AF_INET：IPv4
  • AF_INET6：IPv6
  • AF_UNIX：本地通信
• type：通信类型
  • SOCK_STREAM：TCP
  • SOCK_DGRAM：UDP
• protocol：通常为0

2. 地址绑定

#include <sys/socket.h>

int bind(int sockfd, const struct sockaddr *addr, socklen_t addrlen);

• sockfd：socket文件描述符
• addr：地址结构体指针
• addrlen：地址结构体长度

3. 监听连接

#include <sys/socket.h>

int listen(int sockfd, int backlog);

• backlog：等待连接队列的最大长度

4. 接受连接

#include <sys/socket.h>

int accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen);

• 返回新的socket描述符用于通信

5. 建立连接

#include <sys/socket.h>

int connect(int sockfd, const struct sockaddr *addr, socklen_t addrlen);

三、完整代码示例

1. TCP服务端示例

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

#define PORT 8080
#define BUFFER_SIZE 1024

int main() {
    int server_fd, new_socket;
    struct sockaddr_in address;
    int opt = 1;
    int addrlen = sizeof(address);
    char buffer[BUFFER_SIZE] = {0};
    char *response = "Hello from server";

    // 创建socket文件描述符
    if ((server_fd = socket(AF_INET, SOCK_STREAM, 0)) == 0) {
        perror("socket failed");
        exit(EXIT_FAILURE);
    }

    // 设置socket选项
    if (setsockopt(server_fd, SOL_SOCKET, SO_REUSEADDR | SO_REUSEPORT, &opt, sizeof(opt))) {
        perror("setsockopt");
        exit(EXIT_FAILURE);
    }

    address.sin_family = AF_INET;
    address.sin_addr.s_addr = INADDR_ANY;
    address.sin_port = htons(PORT);

    // 绑定socket到端口
    if (bind(server_fd, (struct sockaddr *)&address, sizeof(address)) < 0) {
        perror("bind failed");
        exit(EXIT_FAILURE);
    }

    // 监听
    if (listen(server_fd, 3) < 0) {
        perror("listen");
        exit(EXIT_FAILURE);
    }

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

    // 接受连接
    if ((new_socket = accept(server_fd, (struct sockaddr *)&address, (socklen_t*)&addrlen)) < 0) {
        perror("accept");
        exit(EXIT_FAILURE);
    }

    printf("Connection accepted from %s:%d\n", 
           inet_ntoa(address.sin_addr), ntohs(address.sin_port));

    // 读取客户端数据
    read(new_socket, buffer, BUFFER_SIZE);
    printf("Client says: %s\n", buffer);

    // 发送响应
    send(new_socket, response, strlen(response), 0);
    printf("Response sent\n");

    // 关闭连接
    close(new_socket);
    close(server_fd);

    return 0;
}

2. TCP客户端示例

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

#define PORT 8080
#define BUFFER_SIZE 1024

int main() {
    int sock = 0;
    struct sockaddr_in serv_addr;
    char *hello = "Hello from client";
    char buffer[BUFFER_SIZE] = {0};

    // 创建socket
    if ((sock = socket(AF_INET, SOCK_STREAM, 0)) < 0) {
        perror("socket creation error");
        return -1;
    }

    serv_addr.sin_family = AF_INET;
    serv_addr.sin_port = htons(PORT);

    // 转换IP地址
    if (inet_pton(AF_INET, "127.0.0.1", &serv_addr.sin_addr) <= 0) {
        perror("invalid address");
        return -1;
    }

    // 连接服务器
    if (connect(sock, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0) {
        perror("connection failed");
        return -1;
    }

    // 发送数据
    send(sock, hello, strlen(hello), 0);
    printf("Hello message sent\n");

    // 读取响应
    read(sock, buffer, BUFFER_SIZE);
    printf("Server says: %s\n", buffer);

    // 关闭socket
    close(sock);

    return 0;
}

3. UDP服务端示例

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

#define PORT 8080
#define BUFFER_SIZE 1024

int main() {
    int sockfd;
    char buffer[BUFFER_SIZE];
    struct sockaddr_in servaddr, cliaddr;

    // 创建socket
    if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) {
        perror("socket creation failed");
        exit(EXIT_FAILURE);
    }

    memset(&servaddr, 0, sizeof(servaddr));
    memset(&cliaddr, 0, sizeof(cliaddr));

    servaddr.sin_family = AF_INET;
    servaddr.sin_addr.s_addr = INADDR_ANY;
    servaddr.sin_port = htons(PORT);

    // 绑定socket
    if (bind(sockfd, (const struct sockaddr *)&servaddr, sizeof(servaddr)) < 0) {
        perror("bind failed");
        exit(EXIT_FAILURE);
    }

    printf("UDP server listening on port %d...\n", PORT);

    socklen_t len;
    int n;

    len = sizeof(cliaddr);

    // 接收数据
    n = recvfrom(sockfd, (char *)buffer, BUFFER_SIZE, 
                 MSG_WAITALL, (struct sockaddr *)&cliaddr, &len);
    buffer[n] = '\0';
    printf("Client : %s\n", buffer);

    // 发送响应
    sendto(sockfd, "Hello from UDP server", strlen("Hello from UDP server"), 
           MSG_CONFIRM, (const struct sockaddr *)&cliaddr, len);
    printf("Response sent.\n");

    close(sockfd);
    return 0;
}

四、高级网络编程

1. 多客户端处理（fork方式）

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

#define PORT 8080
#define BUFFER_SIZE 1024

void handle_client(int client_socket) {
    char buffer[BUFFER_SIZE] = {0};
    char *response = "Hello from server";
    
    read(client_socket, buffer, BUFFER_SIZE);
    printf("Received: %s\n", buffer);
    
    send(client_socket, response, strlen(response), 0);
    printf("Response sent\n");
    
    close(client_socket);
    exit(0);
}

int main() {
    int server_fd, new_socket;
    struct sockaddr_in address;
    int opt = 1;
    int addrlen = sizeof(address);
    
    if ((server_fd = socket(AF_INET, SOCK_STREAM, 0)) == 0) {
        perror("socket failed");
        exit(EXIT_FAILURE);
    }
    
    if (setsockopt(server_fd, SOL_SOCKET, SO_REUSEADDR | SO_REUSEPORT, &opt, sizeof(opt))) {
        perror("setsockopt");
        exit(EXIT_FAILURE);
    }
    
    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) {
        perror("bind failed");
        exit(EXIT_FAILURE);
    }
    
    if (listen(server_fd, 3) < 0) {
        perror("listen");
        exit(EXIT_FAILURE);
    }
    
    printf("Server listening on port %d...\n", PORT);
    
    while(1) {
        if ((new_socket = accept(server_fd, (struct sockaddr *)&address, (socklen_t*)&addrlen)) < 0) {
            perror("accept");
            continue;
        }
        
        printf("Connection accepted from %s:%d\n", 
               inet_ntoa(address.sin_addr), ntohs(address.sin_port));
        
        pid_t pid = fork();
        if (pid == 0) {
            // 子进程处理客户端
            close(server_fd);
            handle_client(new_socket);
        } else if (pid > 0) {
            // 父进程继续监听
            close(new_socket);
            // 回收僵尸进程
            while(waitpid(-1, NULL, WNOHANG) > 0);
        } else {
            perror("fork");
            close(new_socket);
        }
    }
    
    return 0;
}

2. I/O多路复用（select）

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

#define PORT 8080
#define BUFFER_SIZE 1024
#define MAX_CLIENTS 10

int main() {
    int server_fd, new_socket, client_sockets[MAX_CLIENTS];
    struct sockaddr_in address;
    int opt = 1;
    int addrlen = sizeof(address);
    char buffer[BUFFER_SIZE] = {0};
    fd_set readfds;
    int max_sd, activity, i, valread;
    
    // 初始化客户端socket数组
    for (i = 0; i < MAX_CLIENTS; i++) {
        client_sockets[i] = 0;
    }
    
    // 创建socket
    if ((server_fd = socket(AF_INET, SOCK_STREAM, 0)) == 0) {
        perror("socket failed");
        exit(EXIT_FAILURE);
    }
    
    // 设置socket选项
    if (setsockopt(server_fd, SOL_SOCKET, SO_REUSEADDR | SO_REUSEPORT, &opt, sizeof(opt))) {
        perror("setsockopt");
        exit(EXIT_FAILURE);
    }
    
    address.sin_family = AF_INET;
    address.sin_addr.s_addr = INADDR_ANY;
    address.sin_port = htons(PORT);
    
    // 绑定socket
    if (bind(server_fd, (struct sockaddr *)&address, sizeof(address)) < 0) {
        perror("bind failed");
        exit(EXIT_FAILURE);
    }
    
    // 监听
    if (listen(server_fd, 3) < 0) {
        perror("listen");
        exit(EXIT_FAILURE);
    }
    
    printf("Server listening on port %d...\n", PORT);
    
    while(1) {
        // 清空socket集合
        FD_ZERO(&readfds);
        
        // 添加主socket到集合
        FD_SET(server_fd, &readfds);
        max_sd = server_fd;
        
        // 添加子socket到集合
        for (i = 0; i < MAX_CLIENTS; i++) {
            int sd = client_sockets[i];
            if (sd > 0) {
                FD_SET(sd, &readfds);
            }
            if (sd > max_sd) {
                max_sd = sd;
            }
        }
        
        // 等待活动
        activity = select(max_sd + 1, &readfds, NULL, NULL, NULL);
        
        if ((activity < 0) && (errno != EINTR)) {
            perror("select error");
        }
        
        // 如果是主socket活动，表示有新连接
        if (FD_ISSET(server_fd, &readfds)) {
            if ((new_socket = accept(server_fd, (struct sockaddr *)&address, (socklen_t*)&addrlen)) < 0) {
                perror("accept");
                exit(EXIT_FAILURE);
            }
            
            printf("New connection, socket fd: %d, IP: %s, Port: %d\n",
                   new_socket, inet_ntoa(address.sin_addr), ntohs(address.sin_port));
            
            // 添加新socket到数组
            for (i = 0; i < MAX_CLIENTS; i++) {
                if (client_sockets[i] == 0) {
                    client_sockets[i] = new_socket;
                    break;
                }
            }
        }
        
        // 处理客户端数据
        for (i = 0; i < MAX_CLIENTS; i++) {
            int sd = client_sockets[i];
            
            if (FD_ISSET(sd, &readfds)) {
                if ((valread = read(sd, buffer, BUFFER_SIZE)) == 0) {
                    // 客户端断开连接
                    getpeername(sd, (struct sockaddr*)&address, (socklen_t*)&addrlen);
                    printf("Host disconnected, IP %s, Port %d\n",
                           inet_ntoa(address.sin_addr), ntohs(address.sin_port));
                    
                    close(sd);
                    client_sockets[i] = 0;
                } else {
                    // 处理客户端消息
                    buffer[valread] = '\0';
                    printf("Received: %s\n", buffer);
                    send(sd, "Hello from server", strlen("Hello from server"), 0);
                }
            }
        }
    }
    
    return 0;
}

五、网络编程注意事项

1. 错误处理：所有网络调用都应检查返回值并处理错误
2. 字节序：使用htons、htonl、ntohs、ntohl转换网络和主机字节序
3. 地址重用：设置SO_REUSEADDR选项避免地址占用问题
4. 缓冲区管理：注意缓冲区溢出和边界检查
5. 资源释放：确保所有socket都被正确关闭
6. 并发控制：根据需求选择多进程、多线程或I/O多路复用
7. 超时处理：设置适当的超时避免无限等待