通过struct rtable获取eth0的网关，请写出C程序

好的，以下是获取eth0网关的C程序示例：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <net/if.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <linux/rtnetlink.h>
#include <sys/socket.h>

#define BUF_SIZE 4096

int main() {
    int sock_fd;
    char buf[BUF_SIZE];
    struct nlmsghdr *nl_hdr;
    struct rtmsg *rt_msg;
    struct rtattr *rt_attr;
    int len, rt_len;
    struct in_addr gw;

    // 创建socket
    sock_fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);
    if (sock_fd < 0) {
        perror("create socket");
        exit(1);
    }

    // 构造请求消息
    memset(buf, 0, BUF_SIZE);
    nl_hdr = (struct nlmsghdr *)buf;
    nl_hdr->nlmsg_len = NLMSG_LENGTH(sizeof(struct rtmsg));
    nl_hdr->nlmsg_type = RTM_GETROUTE;
    nl_hdr->nlmsg_flags = NLM_F_REQUEST | NLM_F_DUMP;
    nl_hdr->nlmsg_seq = 0;
    nl_hdr->nlmsg_pid = getpid();

    rt_msg = (struct rtmsg *)NLMSG_DATA(nl_hdr);
    rt_msg->rtm_family = AF_INET;
    rt_msg->rtm_table = RT_TABLE_MAIN;
    rt_msg->rtm_protocol = RTPROT_UNSPEC;
    rt_msg->rtm_scope = RT_SCOPE_UNIVERSE;

    // 发送请求消息
    if (send(sock_fd, buf, nl_hdr->nlmsg_len, 0) < 0) {
        perror("send");
        exit(1);
    }

    // 接收回复消息
    len = recv(sock_fd, buf, BUF_SIZE, 0);
    if (len < 0) {
        perror("recv");
        exit(1);
    }

    // 解析回复消息
    nl_hdr = (struct nlmsghdr *)buf;
    while (NLMSG_OK(nl_hdr, len)) {
        if (nl_hdr->nlmsg_type == RTM_NEWROUTE) {
            rt_msg = (struct rtmsg *)NLMSG_DATA(nl_hdr);
            if (rt_msg->rtm_family == AF_INET) {
                rt_attr = (struct rtattr *)RTM_RTA(rt_msg);
                rt_len = RTM_PAYLOAD(nl_hdr);
                while (RTA_OK(rt_attr, rt_len)) {
                    if (rt_attr->rta_type == RTA_GATEWAY) {
                        memcpy(&gw.s_addr, RTA_DATA(rt_attr), sizeof(gw.s_addr));
                        printf("eth0 gateway: %s\n", inet_ntoa(gw));
                        close(sock_fd);
                        return 0;
                    }
                    rt_attr = RTA_NEXT(rt_attr, rt_len);
                }
            }
        }
        nl_hdr = NLMSG_NEXT(nl_hdr, len);
    }

    printf("eth0 gateway not found\n");
    close(sock_fd);
    return 0;
}

这个程序通过 Netlink 和 RTNETLINK 协议获取内核路由表信息，从中找到eth0的网关。注意该程序需要在root权限下运行。