linux中select函数

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

#define PORT 8080
#define MAX_CLIENTS 10

int main() {
    int server_fd, new_socket, max_sd, activity, i, valread, sd;
    int client_sockets[MAX_CLIENTS];
    fd_set readfds;

    struct sockaddr_in address;
    int addrlen = sizeof(address);
    char buffer[1024] = {0};

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

    // 绑定
    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);
    }

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

    // 初始化客户端 socket 数组
    for (i = 0; i < MAX_CLIENTS; i++) {
        client_sockets[i] = 0;
    }

    while (1) {
        // 清空文件描述符集合
        FD_ZERO(&readfds);

        // 添加服务器 socket 到集合
        FD_SET(server_fd, &readfds);
        max_sd = server_fd;

        // 添加客户端 socket 到集合
        for (i = 0; i < MAX_CLIENTS; i++) {
            sd = client_sockets[i];
            if (sd > 0) {
                FD_SET(sd, &readfds);
            }
            if (sd > max_sd) {
                max_sd = sd;
            }
        }

        // 等待 I/O 事件
        activity = select(max_sd + 1, &readfds, NULL, NULL, NULL);

        if ((activity < 0) && (errno != EINTR)) {
            printf("select error");
        }

        // 如果是服务器 socket 就绪
        if (FD_ISSET(server_fd, &readfds)) {
            if ((new_socket = accept(server_fd, (struct sockaddr *)&address, (socklen_t*)&addrlen)) < 0) {
                perror("accept");
                exit(EXIT_FAILURE);
            }

            // 添加新连接的客户端 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++) {
            sd = client_sockets[i];
            if (FD_ISSET(sd, &readfds)) {
                if ((valread = read(sd, buffer, 1024)) == 0) {
                    // 客户端断开连接
                    close(sd);
                    client_sockets[i] = 0;
                } else {
                    // 处理接收到的数据
                    printf("Received: %s\n", buffer);
                    send(sd, "ACK", 3, 0);
                }
            }
        }
    }

    return 0;
}

if (FD_ISSET(sd, &readfds)) {
    // sd 就绪，可以进行读操作
}

struct timeval timeout;
timeout.tv_sec = 5;  // 5 秒
timeout.tv_usec = 0;

activity = select(max_sd + 1, &readfds, NULL, NULL, &timeout);