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