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本帖最后由 kutong 于 2010-11-11 17:53 编辑
下面这段代码是UNP第一卷,第17章中的一个程序,route/getrt.c
我有一个疑问,想请教各位,在第19行和第35行有同样一条语句
rtm = (struct rt_msghdr *) buf;
我不明白这条语句为什么需要出现两次?
麻烦大家指点,先谢过了
#include "unproute.h"
2 #define BUFLEN (sizeof(struct rt_msghdr) + 512)
3 /* sizeof(struct sockaddr_in6) * 8 = 192 */
4 #define SEQ 9999
5 int
6 main(int argc, char **argv)
7 {
8 int sockfd;
9 char *buf;
10 pid_t pid;
11 ssize_t n;
12 struct rt_msghdr *rtm;
13 struct sockaddr *sa, *rti_info[RTAX_MAX];
14 struct sockaddr_in *sin;
15 if (argc != 2)
16 err_quit("usage: getrt <IPaddress>");
17 sockfd = Socket(AF_ROUTE, SOCK_RAW, 0); /* need superuser privileges */
18 buf = Calloc(1, BUFLEN); /* and initialized to 0 */
19 rtm = (struct rt_msghdr *) buf;
20 rtm->rtm_msglen = sizeof(struct rt_msghdr) + sizeof(struct sockaddr_in);
21 rtm->rtm_version = RTM_VERSION;
22 rtm->rtm_type = RTM_GET;
23 rtm->rtm_addrs = RTA_DST;
24 rtm->rtm_pid = pid = getpid();
25 rtm->rtm_seq = SEQ;
26 sin = (struct sockaddr_in *) (rtm + 1);
27 sin->sin_len = sizeof(struct sockaddr_in);
28 sin->sin_family = AF_INET;
29 Inet_pton(AF_INET, argv[1], &sin->sin_addr);
30 Write(sockfd, rtm, rtm->rtm_msglen);
31 do {
32 n = Read(sockfd, rtm, BUFLEN);
33 } while (rtm->rtm_type != RTM_GET || rtm->rtm_seq != SEQ ||
34 rtm->rtm_pid != pid);
35 rtm = (struct rt_msghdr *) buf;
36 sa = (struct sockaddr *) (rtm + 1);
37 get_rtaddrs(rtm->rtm_addrs, sa, rti_info);
38 if ( (sa = rti_info[RTAX_DST]) != NULL)
39 printf("dest: %s\n", Sock_ntop_host(sa, sa->sa_len));
40 if ( (sa = rti_info[RTAX_GATEWAY]) != NULL)
41 printf("gateway: %s\n", Sock_ntop_host(sa, sa->sa_len));
42 if ( (sa = rti_info[RTAX_NETMASK]) != NULL)
43 printf("netmask: %s\n", Sock_masktop(sa, sa->sa_len));
44 if ( (sa = rti_info[RTAX_GENMASK]) != NULL)
45 printf("genmask: %s\n", Sock_masktop(sa, sa->sa_len));
46 exit(0);
47 } |
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发表于 2010-11-11 17:51
