Lua dissector: How to set sub-field bit widths using preferences?

[David Aldrich <david.aldrich.ntml () gmail com>]

Our protocol includes a 16-bit field which is sub-divided into 4
sub-fields.  The width of those sub-fields is variable so I want to specify
the widths using Wireshark preferences.  I understand how to create and
read  Wireshark preferences but I am unsure of how to apply them in this
circumstance.

My code structure looks like this:

my_protocol = Proto("...", "...")

-- Create a preference
my_protocol.prefs.ru_port_id_width = Pref.uint( "RU_Port_ID width ", 4 )

-- Read the preference
ru_port_id_width = my_protocol.prefs.ru_port_id_width

-- Specify a field using the preference
RuPortId_F = ProtoField.uint16("...", "...", base.HEX, NULL, ((2^
ru_port_id_width)-1))

-- Create a fields table and add the field to it
xran_protocol.fields = {RuPortId_F}

-- Then specify the dissector function
function my_protocol.dissector(buffer, pinfo,tree)

-- Show the field in a subtree
local RtcidPcid_range = buffer(4,2) -- the 16-bit field
local ecpriRtcidPcid = ecpriRtcidPcid_range:uint()
subtree:add(RuPortId_F, RtcidPcid_range, RtcidPcid) -- the variable width
sub-field
The trouble with this is that the preference is only read at startup. A new
value can't be specified by the user without restarting Wireshark to apply
it.

Is there a better sequence of code that I code use to overcome this
limitation?

Best regards

David

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