Comparing 2 UInt64s in Lua for Null Value

edit

From reading the guide I believe the code is correct. This branch should be taken:

  if value == 18446744073709551615 then
    return "Cross Id: No Value ("..value..")"

And No Value should be printed. However, Wireshark dissection prints 18446744073709551615. I have tried a few iterations of comparing UInt64s but none of my guesses worked. Any ideas on exactly what changes need to be made to make Wireshark recognize 64bit null values? Note this code work fine for 32 bit integers.

I still don't understand what you mean by a null value. null implies there is no value at all, but that's not the case. Here you're comparing against a specific value, except that you're not performing the comparison correctly. You need to compare a UInt64 against another UInt64, but you're not doing that. You need something like this:

-- Display: Cross Id
display.cross_id = function(value)
    local maxvalue = UInt64(0xffffffff, 0xffffffff)

    -- Check if field has value
    if value == maxvalue then
        return "Cross Id: No Value ("..value..")"
    end

    return "Cross Id: "..value
end

But for some reason you're still displaying value even if it is "null", so maybe you just want this?

return "Cross Id: No Value"

This works:

-- Display: Cross Id
display.cross_id = function(value)
   -- Check if field has value
   if value == UInt64(0xFFFFFFFF, 0xFFFFFFFF) then
    return "Cross Id: No Value ("..value..")"
  end

  return "Cross Id: "..value
end

Thanks.

For these binary fields that are always on the wire, CME uses a sentinel value to mark null/optional fields. I have generally included the value in parenthesis in the dissectors, however I am changing my mind as

No Value (18446744073709551615)

is not exactly the most elegant display.