epoll non blocking 상태에서 sendfile 사용하기

void set_nonblocking(int fd)
{
    int opts;
    opts=fcntl(fd, F_GETFL);
    if (opts < 0)
    {
        perror("fcntl failed\n");
        return;
    }
    opts = opts | O_NONBLOCK;
    if(fcntl(fd, F_SETFL, opts) < 0)
    {
        perror("fcntl failed\n");
        return;
    }
    return;
}
 
... 중략 ...
    ev.events = EPOLLIN;
    ev.data.fd = server_fd;
    epoll_ctl(epoll_fd, EPOLL_CTL_ADD, server_fd, &ev);
    while (1)
    {
        num_fd = epoll_wait(epoll_fd, events, MAXEVENTS, 500);
        for (i=0; i<num_fd; i++)
        {
            if(events[i].data.fd == server_fd)
            {
                client_fd = accept(server_fd, (struct sockaddr *) &client_addr, &len);
                // set_nonblocking(client_fd);
                ev.data.fd = client_fd;
                ev.events = EPOLLIN;
                epoll_ctl(epoll_fd, EPOLL_CTL_ADD, client_fd, &ev);
                printf("%d user connect ...\n", ev.data.fd);
            }
            else if(events[i].events & EPOLLIN)
            {
                client_fd = events[i].data.fd;
                ev.data.fd = client_fd;
                ev.events = EPOLLOUT;
                epoll_ctl(epoll_fd, EPOLL_CTL_MOD, client_fd, &ev);
                printf("recv ...\n");
                recv(client_fd, buf, 512, 0);
                printf("%s", buf);
            }
            else if(events[i].events & EPOLLOUT)
            {
                char sendmsg[1024];
                ssize_t offset = 0;
                struct stat stat_buf;
 
                int len;
                char buf[1024];
                int fd = open("a.tar.gz", O_RDONLY);
                if (fd == -1) {
                    fprintf(stderr, "unable to open  %s\n", strerror(errno));
                    exit(1);
                }
                fstat(fd, &stat_buf);
                client_fd = events[i].data.fd;
                sprintf(sendmsg, "HTTP/1.1 200 OK\r\ncontent-type: application/x-octetstream\r\ncontent-disposition: attachment; filename=\"a.tar.gz\"\r\ncontent-length: %zd\r\n\r\n", stat_buf.st_size);
                send(client_fd, sendmsg, strlen(sendmsg), 0);
                /*int opts;
                opts=fcntl(fd, F_GETFL);
                opts = opts | O_NONBLOCK;
                fcntl(fd, F_SETFL, opts);
                fcntl(fd, F_SETFD, FD_CLOEXEC);*/
                ret = sendfile (client_fd, fd, &offset, stat_buf.st_size);
                ev.data.fd = client_fd;
                epoll_ctl(epoll_fd, EPOLL_CTL_DEL, client_fd, &ev);
                close(fd);
                close(client_fd);
                printf("send ... size = [%d]\n", ret);
            }
        }
    }