Ethereal-users: RE: [Ethereal-users] Bit test in a Display Filter

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From: Wes <wes_r@xxxxxxxxx>
Date: Mon, 29 Mar 2004 07:07:19 -0800 (PST)
Thanks Olivier and thanks for implementing this. It is
very useful.

Wes
--- Biot Olivier <Olivier.Biot@xxxxxxxxxxx> wrote:
> |From: Martin Regner
> |
> |Wes wrote
> |> I noticed with release 0.10.3 Ethereal:
> |>
> |>  "Display filters now support the bitwise and (&)
> |> operator."
> |>
> |> This sounds like it might allow me to do the
> binary
> |> display filter I was looking for, but I haven't
> been
> |> able to find any description or example how to
> use it.
> |> I tried some variations like below, but it didn't
> seem
> |> to work.
> |>
> |> rtp.payload[4] and 2 == 0
> |>
> |> I am not a programmer and I'm probably missing
> |> something obvious.
> 
> The HTML help pages of the Win32 build of Ethereal
> 0.10.3 are broken. Hence
> you won't find much information in those.
> 
> |> Can someone provide any insight on what this
> feature
> |> is and how to use it?
> 
> |The "bitwise and" operator is "&" or "bitwise_and"
> |
> |However it seems not currently possible to do a
> filter like 
> |"rtp.payload[4] & 2 == 0".
> |
> |Maybe the following filter will work for what you
> want to do:
> |rtp.payload and !(rtp.payload[4] & 2).
> 
> This is correct. Today a display filter expression
> returns a boolean, so the
> example "rtp.payload[4] & 2 == 0" is interpreted as:
> 1. (rtp.payload[4] & 2) ==> { True, False }
> 2. { True, False } == 0 ==> Syntax Error (today at
> least)
> 
> Think of a display filter expression as a *boolean*
> expression. Every test
> you write WILL/MUST return True/False.
> 
> Regards,
> 
> Olivier
> 
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