请从struct rtentry 中获取网关信息，请写出C语言程序

可以使用系统库函数`ioctl`来获取路由表信息，然后从中筛选出默认路由的网关地址。以下是一个示例程序：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <arpa/inet.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <net/route.h>
#include <net/if_arp.h>

int main()
{
    struct rtentry route;
    struct sockaddr_in *sa_in;

    /* Clear the route struct */
    memset(&route, 0, sizeof(route));
    route.rt_dst.sa_family = AF_INET;
    ((struct sockaddr_in*)&route.rt_dst)->sin_addr.s_addr = 0;
    route.rt_flags = RTF_UP;

    /* Get the default route */
    if (ioctl(socket(AF_INET, SOCK_DGRAM, IPPROTO_IP), SIOCGETRT, &route) == -1) {
        perror("ioctl(SIOCGETRT)");
        exit(EXIT_FAILURE);
    }

    /* Check if the route is a default route */
    if (route.rt_dst.sa_family == AF_INET && ((struct sockaddr_in*)&route.rt_dst)->sin_addr.s_addr == 0) {
        sa_in = (struct sockaddr_in*)&route.rt_gateway;
        uint32_t gateway = ntohl(sa_in->sin_addr.s_addr);
        printf("Gateway address: %d.%d.%d.%d\n", (gateway >> 24) & 0xff, (gateway >> 16) & 0xff, (gateway >> 8) & 0xff, gateway & 0xff);
    } else {
        fprintf(stderr, "Default route not found\n");
        exit(EXIT_FAILURE);
    }

    return 0;
}

该程序会获取默认路由的网关地址。注意，该程序需要以root权限运行才能成功获取路由表信息。