Lua: writing hexadecimal values as a binary file

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

Michal Kottman · Accepted Answer · 2012-02-04 10:37:59Z

32

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!

answered Feb 4, 2012 at 10:37

Michal Kottman

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3 Comments

Delusional Logic Over a year ago

This solution seems to add the hex value 0c0D (or 13) to some, seemingly random places. It was so close to what i needed.

Michal Kottman Over a year ago

Well, are you writing this on Windows? If yes, then the problem is in the line breaks. While on Unix/Linux/Mac OS X the newline is represented by single '\n' = 0x0A, on Windows when you type a new line, it embeds two bytes - 0x0D, 0x0A (13 10, CR+LF). This may be why you are seeing 0x0D at random places. Try to see if it is near 0x0A - if this is the case, use conversion tools to convert to Unix style (single '\n').

Delusional Logic Over a year ago

All i needed to do was open the file in Binary mode ("wb") instead of just writing mode ("w"). That fixed the problem.

John Zwinck · Accepted Answer · 2012-02-04 01:20:13Z

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.

answered Feb 4, 2012 at 1:20

John Zwinck

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1 Comment

Zerobinary99 Over a year ago

No, I don't want to convert the hexadecimal data, but I want to write a hexadecimal string as a hexadecimal value instead of a string. Let me enhance my above question.

jpjacobs · Accepted Answer · 2013-02-10 22:06:55Z

2

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.

edited Feb 10, 2013 at 22:06

answered Feb 4, 2012 at 8:56

jpjacobs

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5 Comments

Zerobinary99 Over a year ago

It doesn't work for hex-values that are stored as strings. Which format does "str" need to be for this to work properly?

jpjacobs Over a year ago

Well, yeah, you're right, I made a mistake in the code. Fixed it.

greatwolf Over a year ago

I think you mean str:gmatch'%x%x' since %X is the negation of what you're looking for.

jpjacobs Over a year ago

Absolutely right. If you have enough reputation, you can directly edit posts to fix such blunders ;). I'll correct it this time, thanks for reporting.

socketpair Over a year ago

for small numbers it will omit leading zeroes

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