show ip route命令的代码实现

show ip route 命令是用于显示路由表的命令，它的代码实现需要借助于网络编程中的路由表操作函数，例如在 Linux 系统中，可以使用如下的代码实现：

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

#define BUFSIZE 8192

int main(int argc, char **argv) {
    int fd, len, rta_len;
    char buf[BUFSIZE];
    struct nlmsghdr *nlh;
    struct rtmsg *rtm;
    struct rtattr *rta;
    char dest[32], gw[32], netmask[32];

    memset(buf, 0, sizeof(buf));
    if ((fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE)) < 0) {
        perror("socket");
        exit(1);
    }

    nlh = (struct nlmsghdr *)buf;
    rtm = (struct rtmsg *)NLMSG_DATA(nlh);

    nlh->nlmsg_len = NLMSG_LENGTH(sizeof(struct rtmsg));
    nlh->nlmsg_type = RTM_GETROUTE;
    nlh->nlmsg_flags = NLM_F_DUMP | NLM_F_REQUEST;
    nlh->nlmsg_seq = 1;
    rtm->rtm_family = AF_INET;
    rtm->rtm_table = RT_TABLE_MAIN;

    if (send(fd, nlh, nlh->nlmsg_len, 0) < 0) {
        perror("send");
        exit(1);
    }

    while ((len = recv(fd, buf, BUFSIZE, 0)) > 0) {
        for (nlh = (struct nlmsghdr *)buf; NLMSG_OK(nlh, len); nlh = NLMSG_NEXT(nlh, len)) {
            if (nlh->nlmsg_type == NLMSG_DONE) {
                goto done;
            }
            if (nlh->nlmsg_type != RTM_NEWROUTE) {
                continue;
            }
            rtm = (struct rtmsg *)NLMSG_DATA(nlh);
            rta = (struct rtattr *)RTM_RTA(rtm);
            rta_len = RTM_PAYLOAD(nlh);

            memset(dest, 0, sizeof(dest));
            memset(gw, 0, sizeof(gw));
            memset(netmask, 0, sizeof(netmask));

            while (RTA_OK(rta, rta_len)) {
                switch (rta->rta_type) {
                    case RTA_DST:
                        inet_ntop(rtm->rtm_family, RTA_DATA(rta), dest, sizeof(dest));
                        break;
                    case RTA_GATEWAY:
                        inet_ntop(rtm->rtm_family, RTA_DATA(rta), gw, sizeof(gw));
                        break;
                    case RTA_NETMASK:
                        inet_ntop(rtm->rtm_family, RTA_DATA(rta), netmask, sizeof(netmask));
                        break;
                    default:
                        break;
                }
                rta = RTA_NEXT(rta, rta_len);
            }
            printf("%s/%s via %s\n", dest, netmask, gw);
        }
    }

done:
    close(fd);
    return 0;
}

以上代码是使用 Netlink 接口实现的，通过发送 RTM_GETROUTE 消息向内核请求路由表信息，然后解析返回的消息并输出。