如何获取当前配置的IPV6地址的前缀长度（掩码）

#include <sys/socket.h>
#include <netdb.h>
#include <ifaddrs.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <net/if.h>
#include <netinet/in.h>
#include <sys/types.h>

int
main(int argc, char *argv[])
{
struct ifaddrs *ifaddr, *ifa;
int family, s;
char host[NI_MAXHOST];
char host_netmask[NI_MAXHOST];

struct sockaddr_in6  *pip6 = NULL;
int i;

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

/* Walk through linked list, maintaining head pointer so we
can free list later */

for (ifa = ifaddr; ifa != NULL; ifa = ifa->ifa_next) {
if (ifa->ifa_addr == NULL)
continue;

family = ifa->ifa_addr->sa_family;

/* Display interface name and family (including symbolic
form of the latter for the common families) */

printf("%s  address family: %d%s\n",
ifa->ifa_name, family,
(family == AF_PACKET) ? " (AF_PACKET)" :
(family == AF_INET) ?   " (AF_INET)" :
(family == AF_INET6) ?  " (AF_INET6)" : "");

/* For an AF_INET* interface address, display the address */

if (family == AF_INET || family == AF_INET6) {
s = getnameinfo(ifa->ifa_addr,
(family == AF_INET) ? sizeof(struct sockaddr_in) :
sizeof(struct sockaddr_in6),
host, NI_MAXHOST, NULL, 0, NI_NUMERICHOST);
if (s != 0) {
printf("getnameinfo() failed: %s\n", gai_strerror(s));
exit(EXIT_FAILURE);
}
getnameinfo(ifa->ifa_netmask,
(family == AF_INET)? sizeof(struct sockaddr_in): sizeof(struct sockaddr_in6),
host_netmask, NI_MAXHOST, NULL, 0, NI_NUMERICHOST);
if(AF_INET6==family)
{
pip6 = (struct sockaddr_in6 *)(ifa->ifa_netmask);
char *p = (char *)&(pip6->sin6_addr);
for(i=0;i<16;i++)
{
printf("%d ",*p);
p++;
}
}

printf("\taddress: <%s>\n", host);
printf("\tnetmaks: <%s>\n", host_netmask);
}
}

freeifaddrs(ifaddr);
exit(EXIT_SUCCESS);
}