请写出netlink 非阻塞的实现程序，用C语言实现

以下是使用 epoll 实现 netlink 非阻塞通信的示例代码：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/socket.h>
#include <linux/netlink.h>
#include <sys/epoll.h>

#define MAX_EVENTS 10
#define NETLINK_USER 31

int main() {
    int sockfd, epfd, ret;
    struct sockaddr_nl src_addr, dest_addr;
    struct nlmsghdr *nlh = NULL;
    struct iovec iov;
    struct msghdr msg;
    struct epoll_event ev, events[MAX_EVENTS];

    // 创建 netlink socket
    sockfd = socket(PF_NETLINK, SOCK_RAW, NETLINK_USER);
    if (sockfd < 0) {
        perror("socket");
        exit(EXIT_FAILURE);
    }

    // 绑定本地地址
    memset(&src_addr, 0, sizeof(src_addr));
    src_addr.nl_family = AF_NETLINK;
    src_addr.nl_pid = getpid();
    src_addr.nl_groups = 0;
    if (bind(sockfd, (struct sockaddr*)&src_addr, sizeof(src_addr)) < 0) {
        perror("bind");
        exit(EXIT_FAILURE);
    }

    // 设置目标地址
    memset(&dest_addr, 0, sizeof(dest_addr));
    dest_addr.nl_family = AF_NETLINK;
    dest_addr.nl_pid = 0; // 内核
    dest_addr.nl_groups = 0;

    // 初始化 netlink 消息头
    nlh = (struct nlmsghdr *)malloc(NLMSG_SPACE(1024));
    memset(nlh, 0, NLMSG_SPACE(1024));
    nlh->nlmsg_len = NLMSG_SPACE(1024);
    nlh->nlmsg_pid = getpid();
    nlh->nlmsg_flags = 0;

    // 设置 netlink 消息体
    strcpy(NLMSG_DATA(nlh), "Hello from user space!");

    // 初始化 iov 和 msg 结构体
    memset(&iov, 0, sizeof(iov));
    iov.iov_base = (void *)nlh;
    iov.iov_len = nlh->nlmsg_len;
    memset(&msg, 0, sizeof(msg));
    msg.msg_name = (void *)&dest_addr;
    msg.msg_namelen = sizeof(dest_addr);
    msg.msg_iov = &iov;
    msg.msg_iovlen = 1;

    // 发送 netlink 消息
    ret = sendmsg(sockfd, &msg, 0);
    if (ret < 0) {
        perror("sendmsg");
        exit(EXIT_FAILURE);
    }

    // 创建 epoll 实例
    epfd = epoll_create1(0);
    if (epfd < 0) {
        perror("epoll_create1");
        exit(EXIT_FAILURE);
    }

    // 将 netlink socket 注册到 epoll 实例中
    ev.events = EPOLLIN | EPOLLET;
    ev.data.fd = sockfd;
    if (epoll_ctl(epfd, EPOLL_CTL_ADD, sockfd, &ev) < 0) {
        perror("epoll_ctl");
        exit(EXIT_FAILURE);
    }

    // 等待事件发生
    while (1) {
        int nfds = epoll_wait(epfd, events, MAX_EVENTS, -1);
        if (nfds < 0) {
            perror("epoll_wait");
            exit(EXIT_FAILURE);
        }

        // 处理每个事件
        for (int i = 0; i < nfds; i++) {
            if (events[i].data.fd == sockfd) {
                // 读取 netlink 消息
                while (1) {
                    char buffer[1024];
                    struct nlmsghdr *nlh_recv;
                    struct iovec iov_recv;
                    struct msghdr msg_recv;
                    int len;

                    memset(&iov_recv, 0, sizeof(iov_recv));
                    iov_recv.iov_base = buffer;
                    iov_recv.iov_len = sizeof(buffer);
                    memset(&msg_recv, 0, sizeof(msg_recv));
                    msg_recv.msg_name = &src_addr;
                    msg_recv.msg_namelen = sizeof(src_addr);
                    msg_recv.msg_iov = &iov_recv;
                    msg_recv.msg_iovlen = 1;

                    len = recvmsg(sockfd, &msg_recv, MSG_DONTWAIT);
                    if (len < 0) {
                        if (errno == EAGAIN || errno == EWOULDBLOCK) {
                            break; // 没有更多数据了，跳出循环
                        }
                        perror("recvmsg");
                        exit(EXIT_FAILURE);
                    }

                    nlh_recv = (struct nlmsghdr *)buffer;
                    while (NLMSG_OK(nlh_recv, len)) {
                        if (nlh_recv->nlmsg_type == NLMSG_DONE) {
                            break;
                        }
                        printf("Received message: %s\n", (char*)NLMSG_DATA(nlh_recv));
                        nlh_recv = NLMSG_NEXT(nlh_recv, len);
                    }
                }
            }
        }
    }

    // 关闭 socket
    close(sockfd);

    return 0;
}

该程序发送一个 netlink 消息到内核，并使用 epoll 实现非阻塞地读取内核发送的回复消息。当没有消息可读时，程序会阻塞在 epoll_wait 函数上。需要注意的是，在读取 netlink 消息时，需要通过循环来读取所有可用的消息。