根据传入接口名获取相应的ip地址

//传入的参数是接口名，如eth0,eth1等，返回值是ip地址

char * get_iface_ip(const char *ifname)
{
#if defined(__linux__)
	struct ifreq if_data;
	struct in_addr in;
	char *ip_str;
	int sockd;
	u_int32_t ip;

	/* Create a socket */
	if ((sockd = socket (AF_INET, SOCK_PACKET, htons(0x8086))) < 0) {
		debug(LOG_ERR, "socket(): %s", strerror(errno));
		return NULL;
	}

	/* Get IP of internal interface */
	strcpy (if_data.ifr_name, ifname);

	/* Get the IP address */
	if (ioctl (sockd, SIOCGIFADDR, &if_data) < 0) {
		debug(LOG_ERR, "ioctl(): SIOCGIFADDR %s", strerror(errno));
		return NULL;
	}
	memcpy ((void *) &ip, (void *) &if_data.ifr_addr.sa_data + 2, 4);
	in.s_addr = ip;

	ip_str = inet_ntoa(in);
	close(sockd);
	return safe_strdup(ip_str);
#elif defined(__NetBSD__)
	struct ifaddrs *ifa, *ifap;
	char *str = NULL;

	if (getifaddrs(&ifap) == -1) {
		debug(LOG_ERR, "getifaddrs(): %s", strerror(errno));
		return NULL;
	}
	/* XXX arbitrarily pick the first IPv4 address */
	for (ifa = ifap; ifa != NULL; ifa = ifa->ifa_next) {
		if (strcmp(ifa->ifa_name, ifname) == 0 &&
				ifa->ifa_addr->sa_family == AF_INET)
			break;
	}
	if (ifa == NULL) {
		debug(LOG_ERR, "%s: no IPv4 address assigned");
		goto out;
	}
	str = safe_strdup(inet_ntoa(
				((struct sockaddr_in *)ifa->ifa_addr)->sin_addr));
out:
	freeifaddrs(ifap);
	return str;
#else
	return safe_strdup("0.0.0.0");
#endif
}