Ruby - read bytes from a file, convert to integer

Question

Hey,

I'm trying to read unsigned integers from a file (stored as consecutive byte) and convert them to Integers. I've tried this:

file = File.new(filename,"r")
num = file.read(2).unpack("S") #read an unsigned short
puts num #value  will be less than expected

What am I doing wrong here?

Cheers,

Pete

Answer 1

What format are the numbers stored in the file? Is it in hex? Your code looks correct to me.

Answer 2

When dealing with binary data you need to be sure you're opening the file in binary mode if you're on Windows. This goes for both reading and writing.

open(filename, "rb") do |file|
  num = file.read(2).unpack("S")
  puts num
end

There may also be issues with "endian" encoding depending on the source platform. For instance, PowerPC-based machines, which include old Mac systems, IBM Power servers, PS3 clusters, or Sun Sparc servers.

Can you post an example of how it's "less"? Usually there's an obvious pattern to the data.

For example, if you want 0x1234 but you get 0x3412 it's an endian problem.

Answer 3

You're not reading enough bytes. As you say in the comment to tadman's answer, you get '202' instead of '3405691582'

Notice that the first 2 bytes of 0xCAFEBABE is 0xCA = 202

If you really want all 8 bytes in a single number, then you need to read more than the unsigned short

try

num = file.read(8).unpack("L_")

The underscore is assuming that the native long is going to be 8 bytes, which definitely is not guaranteed.

Answer 4

There are a couple of libraries that help with parsing binary data in Ruby, by letting you declare the data format in a simple high-level declarative DSL and then figure out all the packing, unpacking, bit-twiddling, shifting and endian-conversions by themselves.

I have never used one of these, but here's two examples. (There are more, but I don't know them):

• BitStruct
• BinData

Answer 5

Ok, I got it to work:

num = file.read(8).unpack("N")

Thanks for all of your help.