How to checkout a Bit in a Byte?

Question

Hi, just a short simple question:

How can i checkout a specific Bit in a Byte? I tryed something like this:

for i = Value_Start, Value_Stop do
  local c = buffer(i,1):uint()  <--- is this the right valuetype???
if c and 2^1        == 1 then …..
if c.1              == 1 then …..
if c(1)             == 1 then …..
if c[1]             == 1 then …..
if c:1              == 1 then …..
if c and 0x00000001 == 1 then …..
if c and 128        == 1 then …..
end

can anybody give me a short hint, please

Greets from Hamburg Pfanne

Answer 1

TvbRange supports a bitfield function that extracts the specified number of bits from an offset.

Also, Wireshark Lua natively supports Lua BitOp (without downloading any external libraries). I recommend using the native support instead of your own Lua bitops functions.

The documentation from those links should be clear enough.

Answer 2

Lua doesn't support bit operations by default. You may need to download a library like the one from

http://luaforge.net/projects/bit/

Number of bitwise operations are supported

Also I'm not sure how to add a tag but this question should have been tagged with "lua" as well.

Answer 3

Hi izopizo,

thanks for your hint. I have solved my problem like this.

-- Convert Byte to BitArray ####################################################
local function to_bits(n)
 --check_int(n)
 --if(n < 0) then
  -- negative
  --return to_bits(bit.bnot(math.abs(n)) + 1)
 --end
– to bits table
local tbl = {}
local cnt = 1
while (n > 0) do
local last = math.mod(n,2)
if(last == 1) then
tbl[cnt] = 1
else
tbl[cnt] = 0
end
n = (n-last)/2
cnt = cnt + 1
end
return tbl
end
for i = Value_Start, Value_Stop do
local c = buffer(i,1):uint()
local BitTable = to_bits(c)  

if BitTable[8] == 1 then Bit8 = Bit8 .. " 1  " else Bit8 = Bit8 .. " .  " end
if BitTable[7] == 1 then Bit7 = Bit7 .. " 1  " else Bit7 = Bit7 .. " .  " end
if BitTable[6] == 1 then Bit6 = Bit6 .. " 1  " else Bit6 = Bit6 .. " .  " end
if BitTable[5] == 1 then Bit5 = Bit5 .. " 1  " else Bit5 = Bit5 .. " .  " end
if BitTable[4] == 1 then Bit4 = Bit4 .. " 1  " else Bit4 = Bit4 .. " .  " end
if BitTable[3] == 1 then Bit3 = Bit3 .. " 1  " else Bit3 = Bit3 .. " .  " end
if BitTable[2] == 1 then Bit2 = Bit2 .. " 1  " else Bit2 = Bit2 .. " .  " end
if BitTable[1] == 1 then Bit1 = Bit1 .. " 1  " else Bit1 = Bit1 .. " .  " end
end

I´am a little bid amezed, becouse Lua dosen´t support this comparatively simple operation.

Greets From Hamburg Pfanne