> From: Wireshark-dev [mailto:[email protected]] On Behalf Of
> David Aldrich
> Sent: Thursday, September 6, 2018 7:01 AM
> To: [email protected]
> Subject: Re: [Wireshark-dev] Lua dissector: How to set sub-field bit widths
> using preferences?
>
> Hi Chris
>
> Thanks very much for your reply and suggested code. I have a couple of
> questions:
You're welcome.
> 1) I need to declare RuPortId_F somehow. Previously I had:
>
> RuPortId_F = ProtoField.uint16("my_protocol.RuPortId", "RU Port ID",
> base.HEX, NULL, 0x000F)
>
> but now the masking is done in your function. So how should I declare it?
It hardly matters because the mask specified here isn't being used anymore. I
would probably just use:
RuPortId_F = ProtoField.uint16("my_protocol.RuPortId", "RU Port ID", base.HEX)
By the way, you should use nil instead of NULL in Lua.
From https://www.lua.org/manual/5.2/manual.html:
There are eight basic types in Lua: nil, boolean, number, string, function,
userdata, thread, and table. Nil is the type of the value nil, whose main
property is to be different from any other value; it usually represents the
absence of a useful value.
> 2) What does this line do?
>
> table.insert(t, (bit.band(bit.rshift(val, i), 1) == 1 and '1') or '0')
This shifts val right i number of bits, where i is a value between 16 and 1,
and then tests that bit to see if it's set or not. If it is set, it inserts a
'1' into the table t; otherwise it inserts a '0' into the table t.
> 3) Your function makes things much tidier but, as I think Jeff points out,
> the preference still gets read in the header code. This means that if the
> user changes the value of the preference, your function does not reflect the
> change until Wireshark is restarted (and the dissector reloaded). Do you
> agree? Is there any way around this?
No. You can dynamically change the preference and it will take immediate
affect without needing to restart Wireshark.
- Chris
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