C 语言 select 多路复用示例：高效处理多个客户端连接

下面是一个使用select多路复用的示例代码：

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

#define MAX_CLIENTS 5

int main() {
    int master_socket, addrlen, new_socket, client_socket[MAX_CLIENTS], max_clients = MAX_CLIENTS, activity, i, valread, sd;
    int max_sd;
    struct sockaddr_in address;
    char buffer[1025];

    fd_set readfds;

    for (i = 0; i < max_clients; i++) {
        client_socket[i] = 0;
    }

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

    address.sin_family = AF_INET;
    address.sin_addr.s_addr = INADDR_ANY;
    address.sin_port = htons(8888);

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

    if (listen(master_socket, 3) < 0) {
        perror("listen");
        exit(EXIT_FAILURE);
    }

    addrlen = sizeof(address);
    puts("Waiting for connections ...");

    while (1) {
        FD_ZERO(&readfds);
        FD_SET(master_socket, &readfds);
        max_sd = master_socket;

        for (i = 0; i < max_clients; i++) {
            sd = client_socket[i];

            if (sd > 0) {
                FD_SET(sd, &readfds);
            }

            if (sd > max_sd) {
                max_sd = sd;
            }
        }

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

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

        if (FD_ISSET(master_socket, &readfds)) {
            if ((new_socket = accept(master_socket, (struct sockaddr *)&address, (socklen_t*)&addrlen)) < 0) {
                perror("accept");
                exit(EXIT_FAILURE);
            }

            printf("New connection, socket fd is %d, ip is : %s, port : %d\n", new_socket, inet_ntoa(address.sin_addr), ntohs(address.sin_port));

            for (i = 0; i < max_clients; i++) {
                if (client_socket[i] == 0) {
                    client_socket[i] = new_socket;
                    printf("Adding to list of sockets as %d\n", i);
                    break;
                }
            }
        }

        for (i = 0; i < max_clients; i++) {
            sd = client_socket[i];

            if (FD_ISSET(sd, &readfds)) {
                if ((valread = read(sd, buffer, 1024)) == 0) {
                    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_socket[i] = 0;
                } else {
                    buffer[valread] = '\0';
                    send(sd, buffer, strlen(buffer), 0);
                }
            }
        }
    }

    return 0;
}

这段代码创建了一个TCP服务器，它可以同时处理多个客户端连接。使用select函数来实现多路复用，以便在有事件发生时进行处理。首先，创建一个master_socket作为服务器的监听套接字，并使用bind和listen函数进行绑定和监听。然后，使用select来监听master_socket和所有客户端套接字的读事件。当有新连接到达时，使用accept函数接受连接，并将新的客户端套接字添加到client_socket数组中。当有数据到达或客户端断开连接时，通过FD_ISSET来判断是哪个套接字发生了事件，并进行相应的处理操作。