Kaitai Struct

What is Kaitai Struct?

Kaitai Struct is a declarative language used for describing 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 programming 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 the 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 the 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 files usually have a .gif extension and use
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

See JavaScript notes in the documentation for a more complete quick start guide.

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

In Lua

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

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 a 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
• Lua
• Python
• Ruby

Downloading and installing

The easiest way to check out the whole Kaitai Struct project is to
download the 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_lua_runtime — for Lua
  • 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 in 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