getdns_cancel_callback -- cancel an outstanding asyn getdns request

       To  cancel  an outstanding callback, use getdns_cancel_callback. This causes the API to call the callback
       with a callback_type of GETDNS_CALLBACK_CANCEL if the callback for this transaction_id  has  not  already
       been  called.  The  callback  code  for cancellation should clean up any memory related to the identified
       call, such as to deallocate the memory for the userarg. getdns_cancel_callback() may return  immediately,
       even before the callback finishes its work and returns.

       context see getdns_context (3)

       transaction_id   populated   by   the   API   and   used   to  identify  the  callback  (for  example  to
          getdns_cancel_callback), can be NULL, set to 0 if the function fails

       TBD

/etc/hosts/etc/resolv.conf

       DNS Resolver library (libgetdns, -lgetdns)

getdns_cancel_callback -- cancel an outstanding asyn getdns request

       Upon successful completion the functions return GETDNS_RETURN_GOOD , otherwise the following error values
       are returned:

       GETDNS_RETURN_INVALID_PARAMETER if context == NULL

       GETDNS_RETURN_UNKNOWN_TRANSACTION if the transaction_id that is unknown or belongs to a callback that has
       already been called

libgetdns(3), getdns_context(3), getdns_general(3), getdns_hostname(3), getdns_service(3),

getdns 1.6.0                                      December 2015                        getdns_cancel_callback(3)

       #include <getdns.h>

       getdns_return_t
       getdns_cancel_callback (getdns_context_t context,
          getdns_transaction_t *transaction_id)