converting userdata to number

length_octect=tvb_get_guint8(tvb,2)&0x3FF;(last ten bits offset value)

ti = proto_tree_add_text(data_tree, tvb, offset, length_octect, "data");

I used the above function in c for the offset length.How to get the length_octect in lua script

I tried something like this:

local length_octect=(buffer((offset,2) or 0x3FF):unit()

data=subtree:add(F.data,buffer(offset,length_octect))

but it is giving error as: Lua Error: C:\Program Files\Wireshark1\myproto.lua:104: No such 'unit' function/method/field for object type 'TvbRange'