Re: Extracting field values in a C post-dissector

[Graham Bloice <graham.bloice () trihedral com>]

On 3 June 2016 at 08:47, Paul Offord <Paul.Offord () advance7 com> wrote:

> I want to rewrite a LUA post-dissector in C.  How can my code get
> dissected data values, e.g. tcp.len, into a C variable?  In LUA you define
> a field extractor.  Is there an equivalent in C?
>
>
>
> I’ve looked at the wiki and I’ve looked at the MATE code but I haven’t
> found how to do it.
>
>
>
> Thanks and regards…Paul

doc\README.dissector is your guide for C-based dissectors.

Are you aiming to get values from dissectors called before yours, e.g. your
protocol runs on tcp, so tcp dissection is done first, then the data from
the tcp segment is handed to your dissector?

If so, then the pinfo structure passed to your dissector has a lot of info
about previous dissection, see epan\packet_info.h for the structure members.

Is your reference to tcp.len just a general example or an actual value you
*think* you need?  Generally in tcp based dissectors, the length of any
individual tcp segment is immaterial, you just process the data handed to
your dissector in the *tvb.  This is especially the case when a protocol
PDU can either be spread over multiple tcp segments, or multiple PDU's in a
single segment.  This is where tcp reassembly/desegmentation is used.


-- 
Graham Bloice

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