Adding a Tap interface to a protocol allows it to do some useful things. In particular you can produce protocol statistics from the tap interface.

A tap is basically a way of allowing other items to see what’s happening as a protocol is dissected. A tap is registered with the main program, and then called on each dissection. Some arbitrary protocol specific data is provided with the routine that can be used.

To create a tap, you first need to register a tap. A tap is registered with an integer handle, and registered with the routine register_tap(). This takes a string name with which to find it again.

Initialising a tap. 

#include <epan/packet.h>
#include <epan/tap.h>

static int foo_tap = -1;

struct FooTap {
    gint packet_type;
    gint priority;
       ...
};

void proto_register_foo(void)
{
       ...
    foo_tap = register_tap("foo");

Whilst you can program a tap without protocol specific data, it is generally not very useful. Therefore it’s a good idea to declare a structure that can be passed through the tap. This needs to be a static structure as it will be used after the dissection routine has returned. It’s generally best to pick out some generic parts of the protocol you are dissecting into the tap data. A packet type, a priority or a status code maybe. The structure really needs to be included in a header file so that it can be included by other components that want to listen in to the tap.

Once you have these defined, it’s simply a case of populating the protocol specific structure and then calling tap_queue_packet, probably as the last part of the dissector.

Calling a protocol tap. 

static int
dissect_foo(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void *data _U_)
{
       ...
    fooinfo = wmem_alloc(wmem_packet_scope(), sizeof(struct FooTap));
    fooinfo->packet_type = tvb_get_guint8(tvb, 0);
    fooinfo->priority = tvb_get_ntohs(tvb, 8);
       ...
    tap_queue_packet(foo_tap, pinfo, fooinfo);

    return tvb_captured_length(tvb);
}

This now enables those interested parties to listen in on the details of this protocol conversation.