How to know client (user) request cancel

Hi, I my module I will send the request challenge to get input from user via client. If user cancel the request, How to know it. I try to check it via *request->packet->code* DEBUG("<----------------Request code = %d--------------------->", request->packet->code); But it returning 1 in both user press ok or cancel. is in particular attribute tells the user cancel the request? thanks, Shivaprasad