Re: Passing further information between dissections

[Alan Bowman <alan.michael.bowman () gmail com>]

Perfect, that's exactly what I was after.  Thanks very much.

Alan

On 26 July 2010 09:07, Tarjei Knapstad <tarjei.knapstad () gmail com> wrote:
> On 23 July 2010 13:49, Alan Bowman <alan.michael.bowman () gmail com> wrote:
> > I have written two dissectors for some protocols I am using, one of
> > which is wrapped inside the other.  Protocol B is used if a particular
> > byte (A6, below) in protocol A is a particular value.  I have
> > registered A using register_dissector_table().  I can create a subset
> > tvb and use dissector_try_port() to get protocol B dissector to be
> > called and to have the data (B0..BN) passed to it.  However, B's
> > dissection also depends on knowing whether A was sent as a broadcast
> > packet or as a direct response to a request, which is stored in A's
> > header (A2).  (Not my design!).  Can anyone suggest how I should pass
> > this information to B?  Should I tack it onto the start of the tvb as
> > a composite buffer, or is there some way of attaching metadata to it?
> > Is there a similar protocol I should look at?
> >
> > A0 A1 A2 A3 A4 A5 A6 B0 B1 B2 ... BN
> >
> > Thanks in advance
>
> Alan,
>
> I think this is usually achieved by passing the data along in the
> packet_info->private_data field. In your case I guess it would suffice
> to just pass along a boolean. Hopefully someone will correct me if I'm
> wrong here, but I think the standard procedure is along these lines:
>
>  void* saved_private_data = pinfo->private_data;
>  gboolean isBroadcastPacket = check_if_broadcast_packet(/*...*/);
>  pinfo->private_data = &isBroadcastPacket;
>  dissector_try_port(/*...*/);
>  pinfo->private_data = saved_private_data;
>
> Regards,
> Tarjei
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