根据传入的接口名获取相应的mac地址

//传入的参数是接口名，如eth0，返回值是mac地址
char * get_iface_mac(const char *ifname)
{
#if defined(__linux__)
	int r, s;
	struct ifreq ifr;
	char *hwaddr, mac[13];

	strcpy(ifr.ifr_name, ifname);

	s = socket(PF_INET, SOCK_DGRAM, 0);
	if (-1 == s) {
		debug(LOG_ERR, "get_iface_mac socket: %s", strerror(errno));
		return NULL;
	}

	r = ioctl(s, SIOCGIFHWADDR, &ifr);
	if (r == -1) {
		debug(LOG_ERR, "get_iface_mac ioctl(SIOCGIFHWADDR): %s", strerror(errno));
		close(s);
		return NULL;
	}

	hwaddr = ifr.ifr_hwaddr.sa_data;
	close(s);
	snprintf(mac, sizeof(mac), "%02X%02X%02X%02X%02X%02X", 
			hwaddr[0] & 0xFF,
			hwaddr[1] & 0xFF,
			hwaddr[2] & 0xFF,
			hwaddr[3] & 0xFF,
			hwaddr[4] & 0xFF,
			hwaddr[5] & 0xFF
		);

	return safe_strdup(mac);
#elif defined(__NetBSD__)
	struct ifaddrs *ifa, *ifap;
	const char *hwaddr;
	char mac[13], *str = NULL;
	struct sockaddr_dl *sdl;

	if (getifaddrs(&ifap) == -1) {
		debug(LOG_ERR, "getifaddrs(): %s", strerror(errno));
		return NULL;
	}
	for (ifa = ifap; ifa != NULL; ifa = ifa->ifa_next) {
		if (strcmp(ifa->ifa_name, ifname) == 0 &&
				ifa->ifa_addr->sa_family == AF_LINK)
			break;
	}
	if (ifa == NULL) {
		debug(LOG_ERR, "%s: no link-layer address assigned");
		goto out;
	}
	sdl = (struct sockaddr_dl *)ifa->ifa_addr;
	hwaddr = LLADDR(sdl);
	snprintf(mac, sizeof(mac), "%02X%02X%02X%02X%02X%02X",
			hwaddr[0] & 0xFF, hwaddr[1] & 0xFF,
			hwaddr[2] & 0xFF, hwaddr[3] & 0xFF,
			hwaddr[4] & 0xFF, hwaddr[5] & 0xFF);

	str = safe_strdup(mac);
out:
	freeifaddrs(ifap);
	return str;
#else
	return NULL;
#endif
}