随机获取UDID

(NSString *)uuidString
{
CFUUIDRef uuid_ref = CFUUIDCreate(NULL);
CFStringRef uuid_string_ref= CFUUIDCreateString(NULL, uuid_ref);
NSString *uuid = [NSString stringWithString:(__bridge NSString *)uuid_string_ref];
CFRelease(uuid_ref);
CFRelease(uuid_string_ref);
return [uuid lowercaseString];
}