Wireshark-dev: Re: [Wireshark-dev] How to only output certain selected segment of a packet?
From: Stephen Fisher <[email protected]>
Date: Wed, 13 Feb 2008 19:44:35 -0700
On Thu, Feb 14, 2008 at 12:09:08AM +0000, yenan wrote:

> I am trying to output a paclet with a index field whihc is 20-bits 
> long , If I use "guint32" to define the variable for "INDEX" field, 
> and have the index value as "12345678", how can I chop the "678" off, 
> and display only "12345" to the screen?

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It sounds like you need to use a bitmask in the field registration (from 

If the field is a bitfield, then the bitmask is the mask which will
leave only the bits needed to make the field when ANDed with a value.
The proto_tree routines will calculate 'bitshift' automatically
from 'bitmask', by finding the rightmost set bit in the bitmask.
If the field is not a bitfield, then bitmask should be set to 0.

This bitmask (in hex) would go where the 0x0 usually is in a field 
registration.  See epan/dissectors/packet-vnc.c for a number of examples 
(such as in &hf_vnc_zrle_rle).