多路 I/O 复用监听 3 个套接字的数据

多路 I/O 复用可以通过 select 或者 epoll 等技术实现同时监听多个套接字的数据。以下是使用 select 监听 3 个套接字的数据的示例代码：

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

#define PORT1 8000
#define PORT2 8001
#define PORT3 8002

int main()
{
    int sockfd1, sockfd2, sockfd3, maxfd;
    fd_set readfds;
    struct sockaddr_in addr1, addr2, addr3;
    char buffer[1024];

    // create socket 1
    sockfd1 = socket(AF_INET, SOCK_STREAM, 0);
    memset(&addr1, 0, sizeof(addr1));
    addr1.sin_family = AF_INET;
    addr1.sin_port = htons(PORT1);
    addr1.sin_addr.s_addr = htonl(INADDR_ANY);
    bind(sockfd1, (struct sockaddr *)&addr1, sizeof(addr1));
    listen(sockfd1, 5);

    // create socket 2
    sockfd2 = socket(AF_INET, SOCK_STREAM, 0);
    memset(&addr2, 0, sizeof(addr2));
    addr2.sin_family = AF_INET;
    addr2.sin_port = htons(PORT2);
    addr2.sin_addr.s_addr = htonl(INADDR_ANY);
    bind(sockfd2, (struct sockaddr *)&addr2, sizeof(addr2));
    listen(sockfd2, 5);

    // create socket 3
    sockfd3 = socket(AF_INET, SOCK_STREAM, 0);
    memset(&addr3, 0, sizeof(addr3));
    addr3.sin_family = AF_INET;
    addr3.sin_port = htons(PORT3);
    addr3.sin_addr.s_addr = htonl(INADDR_ANY);
    bind(sockfd3, (struct sockaddr *)&addr3, sizeof(addr3));
    listen(sockfd3, 5);

    // set up file descriptor set
    FD_ZERO(&readfds);
    FD_SET(sockfd1, &readfds);
    FD_SET(sockfd2, &readfds);
    FD_SET(sockfd3, &readfds);
    maxfd = sockfd3 + 1;

    while (1) {
        fd_set tmpfds = readfds;
        int ret = select(maxfd, &tmpfds, NULL, NULL, NULL);
        if (ret < 0) {
            perror("select");
            exit(EXIT_FAILURE);
        } else if (ret == 0) {
            continue;
        }

        if (FD_ISSET(sockfd1, &tmpfds)) {
            struct sockaddr_in client_addr;
            socklen_t client_len = sizeof(client_addr);
            int clientfd = accept(sockfd1, (struct sockaddr *)&client_addr, &client_len);
            printf("new connection from %s:%d\n", inet_ntoa(client_addr.sin_addr), ntohs(client_addr.sin_port));
            FD_SET(clientfd, &readfds);
            if (clientfd >= maxfd) {
                maxfd = clientfd + 1;
            }
        }

        if (FD_ISSET(sockfd2, &tmpfds)) {
            struct sockaddr_in client_addr;
            socklen_t client_len = sizeof(client_addr);
            int clientfd = accept(sockfd2, (struct sockaddr *)&client_addr, &client_len);
            printf("new connection from %s:%d\n", inet_ntoa(client_addr.sin_addr), ntohs(client_addr.sin_port));
            FD_SET(clientfd, &readfds);
            if (clientfd >= maxfd) {
                maxfd = clientfd + 1;
            }
        }

        if (FD_ISSET(sockfd3, &tmpfds)) {
            struct sockaddr_in client_addr;
            socklen_t client_len = sizeof(client_addr);
            int clientfd = accept(sockfd3, (struct sockaddr *)&client_addr, &client_len);
            printf("new connection from %s:%d\n", inet_ntoa(client_addr.sin_addr), ntohs(client_addr.sin_port));
            FD_SET(clientfd, &readfds);
            if (clientfd >= maxfd) {
                maxfd = clientfd + 1;
            }
        }

        for (int i = sockfd3 + 1; i < maxfd; i++) {
            if (FD_ISSET(i, &tmpfds)) {
                int n = recv(i, buffer, sizeof(buffer), 0);
                if (n <= 0) {
                    close(i);
                    FD_CLR(i, &readfds);
                    printf("connection closed\n");
                } else {
                    buffer[n] = '\0';
                    printf("received data from %d: %s\n", i, buffer);
                }
            }
        }
    }

    close(sockfd1);
    close(sockfd2);
    close(sockfd3);

    return 0;
}

这段代码同时监听了 3 个套接字，当有客户端连接时，会打印出客户端的 IP 地址和端口号。当有数据到达时，会打印出数据和对应的套接字文件描述符。当连接关闭时，会打印出连接关闭的信息。可以根据自己的需要，修改代码以满足实际需求。