May 02, 2009

Working with binary data in Ruby

Recently I found my self having to work with binary data in Ruby, but have found myself confused and frustrated by a few things. Luckily this during recent experiment I discovers a few tricks that really help me.

View binary data in hex

When working in the Visual Studio debugger I prefer to view memory in hex, so I found it awkward to compare with the decimal escapes that Ruby uses.

But String#unpack has the H format. Since the documentation described it as “extract hex nibbles from each character” I didn’t expect it to do what I wanted, but it does. For example:

>> "\322\204\371\225Q".unpack("H*")
=> ["d284f99551"]

Parsing data into structs

While you can do this with String#unpack and Array#pack, that frankly blows. Luckily I came across Dion Mendel’s awesome binutils gem.

Where I was able to take this C struct:

struct ServerData
{
    WORD	 StructSize;     
    DWORD    ErrorCode;
    char	 ErrorMsg[100];  
    char	 ServerIPAddr[16];
    WORD	 ServerPort;     
    BYTE	 Reserved[100];  
    digest	 Signature;      
};

And rewrite it in ruby:

class ServerData < BinData::Record
  uint16le :struct_size, :value => lambda { num_bytes }
  uint32le :error_code
  string :error_msg, :length => 100
  string :server_ip_addr, :length => 16
  uint16le :server_port
  string :reserved, :length => 100
  string :signature, :length => 16
end

The only hard part in the process was figuring out what datatypes to use for the windows types. Here are a few I had to use:

As I was working my way thought the string I found it helpful to test the parsing one element at a time and make sure that is working. I would comment out all the elements but the first and try

class LittleExample < BinData::Record
  uint16le :word_var
end

>> a = LittleExample.new
>> a.read( "\370\000\364\001\000\000")
=> {"word_var"=>248}

One slick feature of BinData::Record is the ability to set default values. You can see in the ServerData example above I need to report the size of the struct, and since there are not dynamic length elements I can simply call the num_bytes method on the record:

class LengthData < BinData::Record
 uint16le :struct_size, :value => lambda { num_bytes }
end

>> len = LengthData.new
=> {"struct_size"=>2}
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