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   to you under the Apache License, Version 2.0 (the
   "License"); you may not use this file except in compliance
   with the License.  You may obtain a copy of the License at

   http://www.apache.org/licenses/LICENSE-2.0

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   "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
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.. include:: ../../../../common.defs

.. _developer-plugins-blacklist-access-process-txn:

Accessing the Transaction Being Processed
*****************************************

A continuation's handler function is of type ``TSEventFunc``; the
prototype is as follows:

``static int function_name (TSCont contp, TSEvent event, void *edata)``

In general, the return value of the handler function is not used. The
continuation argument is the continuation being called back, the event
is the event being sent to the continuation, and the data pointed to by
``void *edata`` depends on the type of event. The data types for each
event type are listed in :doc:`Writing Handler
Functions <../../continuations/writing-handler-functions.en>`

The key here is that if the event is an HTTP transaction event, then the
data passed to the continuation's handler is of type ``TSHttpTxn`` (a
data type that represents HTTP transactions). Your plugin can then do
things with the transaction. Here's how it looks in the code for the
Blacklist plugin's handler:

.. code-block:: c

   static int
   blacklist_plugin (TSCont contp, TSEvent event, void *edata)
   {
      TSHttpTxn txnp = (TSHttpTxn) edata;
      switch (event) {
         case TS_EVENT_HTTP_OS_DNS:
            handle_dns (txnp, contp);
            return 0;
         case TS_EVENT_HTTP_SEND_RESPONSE_HDR:
            handle_response (txnp);
            return 0;
         default:
            break;
      }
      return 0;
   }

For example: when the origin server DNS lookup event is sent,
``blacklist_plugin`` can call ``handle_dns``\ and pass ``txnp`` as an
argument.
