Kaitai Struct

What is Kaitai Struct?

Kaitai Struct is a declarative language used for describe various binary data structures, laid out in files or in memory: i.e. binary file formats, network stream packet formats, etc.

The main idea is that a particular format is described in Kaitai Struct language only once and then can be compiled with a ksc into source files in one of the supported programm languages. These modules will include a generated code for a parser that can read described data structure from a file / stream and give access to it in a nice, easy-to-comprehend API.

What it's used for?

Have you ever found yourself writing repetitive, error-prone and hard-to-debug code that reads binary data structures from file / network stream and somehow represents them in memory for easier access?

Kaitai Struct tries to make this job easier — you only have to describe binary format once and then everybody can use it from their programming languages — cross-language, cross-platform.

Kaitai Struct includes a growing collection of format descriptions, available in formats submodule repository.

Can you give me a quick example?

Sure. Consider this simple .ksy format description file that describes header of a GIF file (a popular web image format):

meta:
  id: gif
  file-extension: gif
  endian: le
seq:
  - id: header
    type: header
  - id: logical_screen
    type: logical_screen
types:
  header:
    seq:
      - id: magic
        contents: 'GIF'
      - id: version
        size: 3
  logical_screen:
    seq:
      - id: image_width
        type: u2
      - id: image_height
        type: u2
      - id: flags
        type: u1
      - id: bg_color_index
        type: u1
      - id: pixel_aspect_ratio
        type: u1

It declares that GIF file usually has .gif extension and uses little-endian integer encoding. The file itself starts with two blocks: first comes header and then comes logical_screen:

• "Header" consists of "magic" string of 3 bytes ("GIF") that identifies that it's a GIF file starting and then there are 3 more bytes that identify format version (87a or 89a).
• "Logical screen descriptor" is a block of integers:
  • image_width and image_height are 2-byte unsigned ints
  • flags, bg_color_index and pixel_aspect_ratio take 1-byte unsigned int each

This .ksy file can be compiled it into Gif.cs / Gif.java / Gif.js / gif.py / gif.rb and then instantly one can load .gif file and access, for example, it's width and height.

In C#

Gif g = Gif.FromFile("path/to/some.gif");
Console.WriteLine("width = " + g.LogicalScreen.ImageWidth);
Console.WriteLine("height = " + g.LogicalScreen.ImageHeight);

In Java

Gif g = Gif.fromFile("path/to/some.gif");
System.out.println("width = " + g.logicalScreen().imageWidth());
System.out.println("height = " + g.logicalScreen().imageHeight());

In JavaScript

var g = new Gif(someArrayBuffer);
console.log("width = " + g.logicalScreen().imageWidth());
console.log("height = " + g.logicalScreen().imageHeight());

In Python

g = Gif.from_file("path/to/some.gif")
print "width = %d" % (g.logical_screen.image_width)
print "height = %d" % (g.logical_screen.image_height)

In Ruby

g = Gif.from_file("path/to/some.gif")
puts "width = #{g.logical_screen.image_width}"
puts "height = #{g.logical_screen.image_height}"

Of course, this example shows only very limited subset of what Kaitai Struct can do. Please refer to the tutorials and documentation for more insights.

Supported languages

Official Kaitai Struct compiler now supports compiling .ksy into source modules for the following languages:

• C#
• Java
• JavaScript
• Python
• Ruby

Downloading and installing

The easiest way to check out whole Kaitai Struct project is to download main project repository that already imports all other parts as sub-modules. Use:

git clone --recursive https://github.com/kaitai-io/kaitai_struct.git

Note the --recursive option.

Alternatively, one can check out individual sub-projects that consitute Kaitai Struct suite. They are:

• kaitai_struct_compiler — compiler that translates .ksy into a parser source code written in a target programming language
• kaitai_struct_tests — tests & specs to ensure that compiler work as planned
• Runtime libraries
  • kaitai_struct_cpp_stl_runtime — for C++/STL
  • kaitai_struct_csharp_runtime — for C#
  • kaitai_struct_java_runtime — for Java
  • kaitai_struct_javascript_runtime — for JavaScript
  • kaitai_struct_python_runtime — for Python
  • kaitai_struct_ruby_runtime — for Ruby
  • kaitai_struct_swift_runtime — for Swift
• kaitai_struct_formats — library of widely used formats and binary structures described as .ksy files

Using KS in your project

Typically, using formats described in KS in your project, involves the following steps:

• Describe the format — i.e. create a .ksy file
• Compile .ksy file into target language source file and include that file into your project
• Add KS runtime library for your particular language into your project (don't worry, it's small and it's there mostly to ensure readability of generated code)
• Use generated class(es) to parse your binary file / stream and access its components

Check out the tutorial and documentation for more information.

Licensing

• Compiler — GPLv3+
• Runtime libraries — MIT or Apache v2 (=> you can include generated code even into proprietary applications) — see individual libraries for details