Converting string to bytes and writing to file in Lua

Question

I'm trying to convert and write string data into the file as bytes.

I have already tried something to, but instead of seeing 00 inside hexdump, im seeing 0x30 inside file which is hexadecimal value for character 0.

Here is what I wrote:

local data = "000000010000000100000004000000080000000100000000"
for i=1,#data,2 do
  file:write(tonumber(data:sub(i,i+1)))
end
io.close(file)

When I do hexdump of the file I'm getting this:

0000000 30 30 30 31 30 30 30 31 30 30 30 34 30 30 30 38  
0000010 30 30 30 31 30 30 30 30  
0000018

Expected is:

0000000 00 00 00 01 00 00 00 01 00 00 00 04 00 00 00 08  
0000010 00 00 00 01 00 00 00 00  
0000018

Use file:close() for consistency.

lhf
– lhf

2019-08-27 14:41:13 +00:00
Commented Aug 27, 2019 at 14:41 — lhf
– lhf, Commented Aug 27, 2019 at 14:41

Aki · Accepted Answer · 2019-08-27 13:31:28Z

3

You want to use string.char in one way:

local data = "000000010000000100000004000000080000000100000000"
for i=1,#data,2 do
  file:write(string.char(tonumber(data:sub(i,i+1), 16)))
end
io.close(file)

or another:

local data = string.char(0,0,0,1,0,0,0,1,0,0,0,4,0,0,0,8,0,0,0,1,0,0,0,0)
file:write(data)
io.close(file)

Note that strings in Lua may contain any bytes you want including null bytes. See Values and Types.

answered Aug 27, 2019 at 13:31

Aki

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Comments

lhf · Accepted Answer · 2019-08-27 13:25:46Z

1

Hint: Use string.char to convert numbers to bytes:

file:write(string.char(tonumber(data:sub(i,i+1))))

If the strings contains hexadecimal, use tonumber(...,16).

answered Aug 27, 2019 at 13:25

lhf

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