使用C语言写一个select函数的监听socket 接受demo

下面是一个简单的使用select函数监听socket并接受数据的示例代码，其中使用了TCP协议：

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

#define MAX_CLIENTS 10
#define BUFFER_SIZE 1024

int main(int argc, char *argv[]) {
    int server_fd, max_fd, activity, i, valread, new_socket, sd;
    struct sockaddr_in server_addr, client_addr;
    char buffer[BUFFER_SIZE];
    fd_set readfds;

    // Create socket
    if ((server_fd = socket(AF_INET, SOCK_STREAM, 0)) == 0) {
        perror("socket failed");
        exit(EXIT_FAILURE);
    }

    // Set socket options
    int opt = 1;
    if (setsockopt(server_fd, SOL_SOCKET, SO_REUSEADDR | SO_REUSEPORT, &opt, sizeof(opt))) {
        perror("setsockopt failed");
        exit(EXIT_FAILURE);
    }

    // Bind socket to address
    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr = INADDR_ANY;
    server_addr.sin_port = htons(8080);
    if (bind(server_fd, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0) {
        perror("bind failed");
        exit(EXIT_FAILURE);
    }

    // Listen for incoming connections
    if (listen(server_fd, MAX_CLIENTS) < 0) {
        perror("listen failed");
        exit(EXIT_FAILURE);
    }

    // Accept incoming connections and add them to the set of file descriptors to monitor
    int addrlen = sizeof(client_addr);
    int client_sockets[MAX_CLIENTS] = {0};
    for (i = 0; i < MAX_CLIENTS; ++i) {
        new_socket = accept(server_fd, (struct sockaddr *)&client_addr, (socklen_t*)&addrlen);
        if (new_socket < 0) {
            perror("accept failed");
            exit(EXIT_FAILURE);
        }
        printf("New connection, socket fd is %d, IP is : %s, port : %d\n", new_socket, inet_ntoa(client_addr.sin_addr), ntohs(client_addr.sin_port));

        // Add new socket to set of file descriptors to monitor
        client_sockets[i] = new_socket;
    }

    // Main loop to monitor file descriptors for activity
    while (1) {
        // Clear the set of file descriptors to monitor
        FD_ZERO(&readfds);

        // Add server socket and client sockets to the set of file descriptors to monitor
        FD_SET(server_fd, &readfds);
        max_fd = server_fd;
        for (i = 0; i < MAX_CLIENTS; ++i) {
            sd = client_sockets[i];
            if (sd > 0) {
                FD_SET(sd, &readfds);
            }
            if (sd > max_fd) {
                max_fd = sd;
            }
        }

        // Wait for activity on any of the monitored file descriptors
        activity = select(max_fd + 1, &readfds, NULL, NULL, NULL);
        if (activity < 0) {
            perror("select error");
            exit(EXIT_FAILURE);
        }

        // If there is activity on the server socket, it means there is a new incoming connection
        if (FD_ISSET(server_fd, &readfds)) {
            new_socket = accept(server_fd, (struct sockaddr *)&client_addr, (socklen_t*)&addrlen);
            if (new_socket < 0) {
                perror("accept error");
                exit(EXIT_FAILURE);
            }
            printf("New connection, socket fd is %d, IP is : %s, port : %d\n", new_socket, inet_ntoa(client_addr.sin_addr), ntohs(client_addr.sin_port));

            // Add new socket to set of file descriptors to monitor
            for (i = 0; i < MAX_CLIENTS; ++i) {
                if (client_sockets[i] == 0) {
                    client_sockets[i] = new_socket;
                    break;
                }
            }
        }

        // Check each client socket for activity
        for (i = 0; i < MAX_CLIENTS; ++i) {
            sd = client_sockets[i];
            if (FD_ISSET(sd, &readfds)) {
                // Receive data from client
                valread = read(sd, buffer, BUFFER_SIZE);
                if (valread == 0) {
                    // Client disconnected
                    printf("Client disconnected, socket fd is %d\n", sd);
                    close(sd);
                    client_sockets[i] = 0;
                } else {
                    // Display received data and send response back to client
                    printf("Received data from client %d: %s\n", sd, buffer);
                    send(sd, buffer, strlen(buffer), 0);
                }
            }
        }
    }

    return 0;
}

在上述代码中，我们首先创建了一个服务器套接字，然后将它绑定到指定的端口并开始监听。接着，在一个循环中，我们使用select函数来等待任何一个文件描述符上的活动，当有活动时，我们就处理该活动。如果是服务器套接字上的活动，说明有新的客户端连接，我们接受该连接并将其添加到要监听的文件描述符集合中。如果是客户端套接字上的活动，说明客户端已经发送数据，我们读取该数据并向客户端发送回复。如果客户端已经断开连接，我们将其从要监听的文件描述符集合中移除，并关闭该套接字。