使用epoll方式实现一个C/S架构的应用； 要求：服务器同时支持多个客户端连接，一个客户端发送消息，服务器转发 到其他客户端并回显消息；

这里提供一个基于 epoll 的 C/S 架构的聊天室示例代码，可以实现服务器同时支持多个客户端连接，一个客户端发送消息，服务器转发到其他客户端并回显消息。

服务器端代码：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <sys/epoll.h>
#include <fcntl.h>

#define MAX_EVENTS 1024
#define BUF_SIZE 1024

int set_nonblocking(int fd) {
    int old_flags = fcntl(fd, F_GETFL, 0);
    if (old_flags == -1) {
        return -1;
    }
    return fcntl(fd, F_SETFL, old_flags | O_NONBLOCK);
}

int main(int argc, char *argv[]) {
    if (argc != 2) {
        printf("Usage: %s <port>\n", argv[0]);
        return 1;
    }

    int server_fd = socket(AF_INET, SOCK_STREAM, 0);
    if (server_fd == -1) {
        perror("socket");
        return 1;
    }

    int opt = 1;
    if (setsockopt(server_fd, SOL_SOCKET, SO_REUSEADDR, &opt, sizeof(opt)) == -1) {
        perror("setsockopt");
        return 1;
    }

    struct sockaddr_in server_addr;
    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr = INADDR_ANY;
    server_addr.sin_port = htons(atoi(argv[1]));

    if (bind(server_fd, (struct sockaddr *)&server_addr, sizeof(server_addr)) == -1) {
        perror("bind");
        return 1;
    }

    if (listen(server_fd, SOMAXCONN) == -1) {
        perror("listen");
        return 1;
    }

    int epoll_fd = epoll_create1(0);
    if (epoll_fd == -1) {
        perror("epoll_create1");
        return 1;
    }

    struct epoll_event event;
    event.events = EPOLLIN;
    event.data.fd = server_fd;

    if (epoll_ctl(epoll_fd, EPOLL_CTL_ADD, server_fd, &event) == -1) {
        perror("epoll_ctl");
        return 1;
    }

    struct epoll_event events[MAX_EVENTS];

    while (1) {
        int n = epoll_wait(epoll_fd, events, MAX_EVENTS, -1);
        if (n == -1) {
            perror("epoll_wait");
            break;
        }

        for (int i = 0; i < n; i++) {
            int fd = events[i].data.fd;
            if (fd == server_fd) {
                struct sockaddr_in client_addr;
                socklen_t client_addr_len = sizeof(client_addr);
                int client_fd = accept(server_fd, (struct sockaddr *)&client_addr, &client_addr_len);
                if (client_fd == -1) {
                    perror("accept");
                    continue;
                }

                if (set_nonblocking(client_fd) == -1) {
                    perror("set_nonblocking");
                    return 1;
                }

                event.events = EPOLLIN | EPOLLET;
                event.data.fd = client_fd;

                if (epoll_ctl(epoll_fd, EPOLL_CTL_ADD, client_fd, &event) == -1) {
                    perror("epoll_ctl");
                    return 1;
                }

                printf("New client connected: %s:%d\n", inet_ntoa(client_addr.sin_addr), ntohs(client_addr.sin_port));
            } else {
                char buf[BUF_SIZE];
                int len = recv(fd, buf, BUF_SIZE, 0);
                if (len == -1) {
                    perror("recv");
                    continue;
                }

                if (len == 0) {
                    printf("Client disconnected\n");
                    close(fd);
                    epoll_ctl(epoll_fd, EPOLL_CTL_DEL, fd, NULL);
                    continue;
                }

                printf("Received message: %s\n", buf);

                for (int i = 0; i < n; i++) {
                    int send_fd = events[i].data.fd;
                    if (send_fd != server_fd && send_fd != fd) {
                        if (send(send_fd, buf, len, 0) == -1) {
                            perror("send");
                            continue;
                        }
                    }
                }
            }
        }
    }

    close(epoll_fd);
    close(server_fd);

    return 0;
}

客户端代码：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>
#include <sys/epoll.h>
#include <fcntl.h>

#define MAX_EVENTS 1024
#define BUF_SIZE 1024

int set_nonblocking(int fd) {
    int old_flags = fcntl(fd, F_GETFL, 0);
    if (old_flags == -1) {
        return -1;
    }
    return fcntl(fd, F_SETFL, old_flags | O_NONBLOCK);
}

int main(int argc, char *argv[]) {
    if (argc != 3) {
        printf("Usage: %s <ip> <port>\n", argv[0]);
        return 1;
    }

    int client_fd = socket(AF_INET, SOCK_STREAM, 0);
    if (client_fd == -1) {
        perror("socket");
        return 1;
    }

    if (set_nonblocking(client_fd) == -1) {
        perror("set_nonblocking");
        return 1;
    }

    struct sockaddr_in server_addr;
    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr = inet_addr(argv[1]);
    server_addr.sin_port = htons(atoi(argv[2]));

    if (connect(client_fd, (struct sockaddr *)&server_addr, sizeof(server_addr)) == -1) {
        if (errno != EINPROGRESS) {
            perror("connect");
            return 1;
        }
    }

    int epoll_fd = epoll_create1(0);
    if (epoll_fd == -1) {
        perror("epoll_create1");
        return 1;
    }

    struct epoll_event event;
    event.events = EPOLLIN | EPOLLOUT | EPOLLET;
    event.data.fd = client_fd;

    if (epoll_ctl(epoll_fd, EPOLL_CTL_ADD, client_fd, &event) == -1) {
        perror("epoll_ctl");
        return 1;
    }

    struct epoll_event events[MAX_EVENTS];

    while (1) {
        int n = epoll_wait(epoll_fd, events, MAX_EVENTS, -1);
        if (n == -1) {
            perror("epoll_wait");
            break;
        }

        for (int i = 0; i < n; i++) {
            int fd = events[i].data.fd;
            if ((events[i].events & EPOLLIN) || (events[i].events & EPOLLOUT)) {
                char buf[BUF_SIZE];
                fgets(buf, BUF_SIZE, stdin);
                int len = strlen(buf);
                if (send(fd, buf, len, 0) == -1) {
                    perror("send");
                    continue;
                }
            } else if (events[i].events & EPOLLERR) {
                printf("Connection error\n");
                close(fd);
                return 1;
            } else if (events[i].events & EPOLLHUP) {
                printf("Connection closed\n");
                close(fd);
                return 1;
            }
        }
    }

    close(epoll_fd);
    close(client_fd);

    return 0;
}

使用方法：

在终端1中启动服务器：

$ ./server 12345

在终端2中启动客户端，并连接到服务器：

$ ./client 127.0.0.1 12345

可以在客户端输入消息并发送，服务器会将消息转发给其他客户端并回显消息。可以启动多个客户端连接到同一个服务器。