Lua: writing hexadecimal values as a binary file - lua

Question

I have several hex-values that I try to write to a file. It seems that Lua doesn't support that out of the box, since they are all treated as strings instead of values. I figured I would have to break up a longer hex-value, for example AABBCC into AA, BB, CC and use string.char() on all of their decimal values consecutively to get the job done.
Is there a built in function that allows me to write such values directly without converting them first? I used escape characters such as "0xAA" and "\xAA", but those didn't work out.
Edit: Let me give you an example. I'm looking at a test file in a hex editor:
00000000 00 00 00 00 00 00 ......
And I want to write to it in the following fashion with the string "AABBCC":
00000000 AA BB CC 00 00 00 ......
What I get though with the escape characters is:
00000000 41 41 42 42 43 43 AABBCC

Answer 1

I use the following functions to convert between a hex string and a "raw binary":
function string.fromhex(str)
return (str:gsub('..', function (cc)
return string.char(tonumber(cc, 16))
end))
end
function string.tohex(str)
return (str:gsub('.', function (c)
return string.format('%02X', string.byte(c))
end))
end
They can be used as follows:
("Hello world!"):tohex() --> 48656C6C6F20776F726C6421
("48656C6C6F20776F726C6421"):fromhex() --> Hello world!

Answer 2

So you have a string like this:
value = 'AABBCC'
And you want to print it (or turn it into a string) like this?
'101010101011101111001100'
How about this?
function hex2bin(str)
local map = {
['0'] = '0000'
['1'] = '0001'
['2'] = '0010'
-- etc. up to 'F'
}
return str:gsub('[0-9A-F]', map)
end
Note that it leaves untouched any characters which could not be interpreted as hex.

Answer 3

There is so such function because it's that easy to write one.
function writeHex(str,fh)
for byte in str:gmatch'%x%x' do
fh:write(string.char(tonumber(byte,16)))
end
end
This just plainly writes the values to the file pointed to by the fh filehandle.