获取网络节点的网关地址，请写出C程序

以下是获取网络节点的网关地址的 C 语言程序：

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

#define BUFSIZE 8192

int main() {
    int sockfd = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE);
    if (sockfd < 0) {
        perror("socket");
        return -1;
    }

    struct sockaddr_nl sa;
    memset(&sa, 0, sizeof(sa));
    sa.nl_family = AF_NETLINK;
    sa.nl_groups = RTMGRP_IPV4_ROUTE;
    if (bind(sockfd, (struct sockaddr *)&sa, sizeof(sa)) < 0) {
        perror("bind");
        return -1;
    }

    char buf[BUFSIZE];
    memset(buf, 0, sizeof(buf));
    struct iovec iov = {buf, sizeof(buf)};
    struct msghdr msg = {NULL, 0, &iov, 1, NULL, 0, 0};

    int len = recvmsg(sockfd, &msg, 0);
    if (len < 0) {
        perror("recvmsg");
        return -1;
    }

    struct nlmsghdr *nh;
    for (nh = (struct nlmsghdr *)buf; NLMSG_OK(nh, len); nh = NLMSG_NEXT(nh, len)) {
        if (nh->nlmsg_type == NLMSG_DONE) break;
        if (nh->nlmsg_type == NLMSG_ERROR) {
            perror("netlink error");
            return -1;
        }

        struct rtmsg *rtm = (struct rtmsg *)NLMSG_DATA(nh);
        if (rtm->rtm_family != AF_INET) continue;
        if (rtm->rtm_table != RT_TABLE_MAIN) continue;
        if (!(rtm->rtm_flags & RTM_F_GATEWAY)) continue;

        struct rtattr *rta;
        int rtl;
        for (rta = (struct rtattr *)RTM_RTA(rtm), rtl = RTM_PAYLOAD(nh); rta && rtl; rta = RTA_NEXT(rta, rtl)) {
            if (rta->rta_type != RTA_GATEWAY) continue;
            struct in_addr *gateway = (struct in_addr *)RTA_DATA(rta);
            printf("Gateway Address: %s\n", inet_ntoa(*gateway));
            return 0;
        }
    }

    close(sockfd);

    return 0;
}

该程序通过使用 netlink 协议的接口，获取系统的路由表信息，并从中提取主路由表（RT_TABLE_MAIN）中的网关地址。程序将获取到的网关地址打印到控制台。需要注意的是，该程序需要在 root 权限下运行，才能正确地获取路由表信息。