请从struct rtentry 中获取eth0的网关信息，请写出C语言程序

可以使用系统库函数`getaddrinfo`和`getifaddrs`来获取指定接口的IP地址和子网掩码，然后计算出网关地址。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 <net/if.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <net/route.h>
#include <net/if_arp.h>
#include <ifaddrs.h>

#define ETH_NAME "eth0"

int main()
{
    struct ifaddrs *ifaddr, *ifa;
    struct rtentry route;
    struct sockaddr_in *sa_in;
    char buf[1024];

    if (getifaddrs(&ifaddr) == -1) {
        perror("getifaddrs");
        exit(EXIT_FAILURE);
    }

    /* Find the interface address */
    for (ifa = ifaddr; ifa != NULL; ifa = ifa->ifa_next) {
        if (!strcmp(ifa->ifa_name, ETH_NAME) && ifa->ifa_addr->sa_family == AF_INET) {
            sa_in = (struct sockaddr_in*)ifa->ifa_addr;
            break;
        }
    }

    if (!ifa) {
        fprintf(stderr, "Interface %s not found\n", ETH_NAME);
        exit(EXIT_FAILURE);
    }

    /* Calculate the network address and netmask */
    uint32_t netaddr = ntohl(sa_in->sin_addr.s_addr);
    uint32_t netmask = 0xffffffff;
    if (ioctl(socket(AF_INET, SOCK_DGRAM, IPPROTO_IP), SIOCGIFNETMASK, &buf) != -1) {
        struct ifreq *ifreq = (struct ifreq*)buf;
        sa_in = (struct sockaddr_in*)&ifreq->ifr_addr;
        netmask = ntohl(sa_in->sin_addr.s_addr);
    }

    /* Calculate the broadcast address */
    uint32_t broadcast = (netaddr & netmask) | ~netmask;

    /* Find the default route */
    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;
    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);
    }

    freeifaddrs(ifaddr);
    return 0;
}

该程序会获取指定名称的网络接口的IP地址和子网掩码，然后计算出网关地址。注意，该程序需要以root权限运行才能成功获取路由表信息。