epoll_wait总是设置EPOLLOUT位

，并添加案例代码。文章分为以下几个段落：简介、EPOLLOUT的作用、EPOLLOUT的使用方式、案例代码与解析。

c++
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#define MAX_EVENTS 10
#define BUFFER_SIZE 1024
int main() {
    int sockfd, epollfd, n;
    struct epoll_event event, events[MAX_EVENTS];
    char buffer[BUFFER_SIZE];
    // 创建socket
    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) {
        perror("socket error");
        return 1;
    }
    // 设置非阻塞
    int flags = fcntl(sockfd, F_GETFL, 0);
    fcntl(sockfd, F_SETFL, flags | O_NONBLOCK);
    // 绑定地址和端口
    struct sockaddr_in server_addr;
    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(8888);
    server_addr.sin_addr.s_addr = htonl(INADDR_ANY);
    bind(sockfd, (struct sockaddr*)&server_addr, sizeof(server_addr));
    // 监听
    listen(sockfd, 5);
    // 创建epoll实例
    epollfd = epoll_create1(0);
    if (epollfd == -1) {
        perror("epoll_create1 error");
        return 1;
    }
    // 添加监听的套接字到epoll事件集合中
    event.events = EPOLLIN | EPOLLOUT;
    event.data.fd = sockfd;
    if (epoll_ctl(epollfd, EPOLL_CTL_ADD, sockfd, &event) == -1) {
        perror("epoll_ctl error");
        return 1;
    }
    while (1) {
        n = epoll_wait(epollfd, events, MAX_EVENTS, -1);
        if (n == -1) {
            perror("epoll_wait error");
            return 1;
        }
        for (int i = 0; i < n; ++i) {
            if (events[i].data.fd == sockfd) {
                // 有新的连接
                struct sockaddr_in client_addr;
                socklen_t client_len = sizeof(client_addr);
                int clientfd = accept(sockfd, (struct sockaddr*)&client_addr, &client_len);
                if (clientfd == -1) {
                    perror("accept error");
                    return 1;
                }
                // 设置非阻塞
                int flags = fcntl(clientfd, F_GETFL, 0);
                fcntl(clientfd, F_SETFL, flags | O_NONBLOCK);
                // 添加新的套接字到epoll事件集合中
                event.events = EPOLLOUT;
                event.data.fd = clientfd;
                if (epoll_ctl(epollfd, EPOLL_CTL_ADD, clientfd, &event) == -1) {
                    perror("epoll_ctl error");
                    return 1;
                }
            } else if (events[i].events & EPOLLOUT) {
                // 可写事件
                int clientfd = events[i].data.fd;
                const char* message = "Hello, client!";
                write(clientfd, message, strlen(message));
                // 关闭连接
                epoll_ctl(epollfd, EPOLL_CTL_DEL, clientfd, NULL);
                close(clientfd);
            }
        }
    }
    close(sockfd);
    return 0;
}