Kaitai Struct

A new way to develop parsers for binary structures.

Declarative: describe the very structure of the data, not how you read or write it

Language-neutral: write once, use in all supported languages:

  • C++/STL
  • C#
  • Go
  • Java
  • JavaScript
  • Lua
  • Nim
  • Perl
  • PHP
  • Python
  • Ruby

entry-level support

Packed with tools and samples: includes a compiler, an IDE, a visualizer and massive library of popular formats

Free & open source: feel free to use, modify and join the project

Reading and writing binary formats is hard, especially if it’s an interchange format that should work across a multitude of platforms and languages.

Have you ever found yourself writing repetitive, error-prone and hard-to-debug code that reads binary data structures from files or network streams 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.

What is Kaitai Struct?

Kaitai Struct is a declarative language used to 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 (.ksy file) and then can be compiled with ksc into source files in one of the supported programming languages. These modules will include a generated code for a parser that can read the described data structure from a file or stream and give access to it in a nice, easy-to-comprehend API.

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
  • Use a visualizer to debug the format and ensure that it parses data properly
  • Compile the .ksy file into a target language source file and include that file into your project
  • Add the 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 the generated class(es) to parse your binary file or stream and access its components

Check out the documentation for more information.

  id: tcp_segment
  endian: be
  - id: src_port
    type: u2
  - id: dst_port
    type: u2
  - id: seq_num
    type: u4
  - id: ack_num
    type: u4

public class TcpSegment extends KaitaiStruct {
    // ...
    private void _read() throws IOException {
        this.srcPort = _io.readU2be();
        this.dstPort = _io.readU2be();
        this.seqNum = _io.readU4be();
        this.ackNum = _io.readU4be();
    // ...

Quick start

Consider this simple .ksy format description file that describes the header of a GIF image file:

  id: gif
  file-extension: gif
  endian: le
  - id: header
    type: header
  - id: logical_screen
    type: logical_screen
      - id: magic
        contents: 'GIF'
      - id: version
        size: 3
      - 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 a GIF file usually has a .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 a “magic” string of 3 bytes (“GIF”) that identifies that it’s a GIF file starting and then there are 3 more bytes that identify the 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 ints each

This .ksy file can be compiled into gif.cpp / Gif.cs / gif.go / Gif.java / Gif.js / gif.lua / gif.nim / Gif.pm / Gif.php / gif.py / gif.rb and then one can instantly load a .gif file and access, for example, its width and height.

std::ifstream ifs("path/to/some.gif", std::ifstream::binary);
kaitai::kstream ks(&ifs);
gif_t g = gif_t(&ks);

std::cout << "width = " << g.logical_screen()->image_width() << std::endl;
std::cout << "height = " << g.logical_screen()->image_height() << std::endl;
Gif g = Gif.FromFile("path/to/some.gif");

Console.WriteLine("width = " + g.LogicalScreen.ImageWidth);
Console.WriteLine("height = " + g.LogicalScreen.ImageHeight);
file, err := os.Open("path/to/some.gif")
g := NewGif()
err = g.Read(kaitai.NewStream(file), nil, g)

fmt.Printf("width = %d\n", g.LogicalScreen.ImageWidth)
fmt.Printf("height = %d\n", g.LogicalScreen.ImageHeight)
Gif g = Gif.fromFile("path/to/some.gif");

System.out.println("width = " + g.logicalScreen().imageWidth());
System.out.println("height = " + g.logicalScreen().imageHeight());
var g = new Gif(new KaitaiStream(someArrayBuffer));

console.log("width = " + g.logicalScreen.imageWidth);
console.log("height = " + g.logicalScreen.imageHeight);
local g = Gif:from_file("path/to/some.gif")

print("width = " .. g.logical_screen.image_width)
print("height = " .. g.logical_screen.image_height)
let g = Gif.fromFile("path/to/some.gif")

echo "width = " & $g.logicalScreen.imageWidth
echo "height = " & $g.logicalScreen.imageHeight
my $g = Gif->from_file("path/to/some.gif");

print("width = ", $g->logical_screen()->image_width(), "\n");
print("height = ", $g->logical_screen()->image_height(), "\n");
$g = Gif::fromFile("path/to/some.gif");

print("width = " . $g->logicalScreen()->imageWidth() . "\n");
print("height = " . $g->logicalScreen()->imageHeight() . "\n");
g = Gif.from_file("path/to/some.gif")

print("width = %d" % (g.logical_screen.image_width))
print("height = %d" % (g.logical_screen.image_height))
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 documentation for more insights.

Downloading and installing

2021-05-02 — Temporarily using GitHub Releases for compiler distributions

As of , JFrog Bintray (distribution service where we hosted the compiler artifacts for years) has been sunset, so we moved all stable compiler versions to GitHub Releases in the kaitai_struct_compiler repository. The installation commands below have been updated accordingly. Development (unstable) builds that were hosted on Bintray (Linux .deb and Universal .zip) are not available right now (until we set up the new distribution system).

We're currently setting up an alternative repository to replace Bintray, which will be available on a custom domain packages.kaitai.io to be future-proof, so stay tuned!

The stable kaitai-struct-compiler versions are currently uploaded to https://github.com/kaitai-io/kaitai_struct_compiler/releases (see box above). Just download the .deb package and install it:

curl -LO https://github.com/kaitai-io/kaitai_struct_compiler/releases/download/0.10/kaitai-struct-compiler_0.10_all.deb
sudo apt-get install ./kaitai-struct-compiler_0.10_all.deb


  • .deb-based Linux distribution (Debian, Ubuntu, etc)

There is a Homebrew formula that you can use to install kaitai-struct-compiler:

brew install kaitai-struct-compiler


Windows versions are available as an MSI format installer. If you want a portable version that requires no installation, download our universal .zip build instead.

Download — stable v0.10, 7.7 MiB

Download — latest development (unstable) build


"Universal" builds are downloadable as a .zip file that includes all the required .jar files bundled and launcher scripts for Linux / macOS / Windows systems. No installation required, one can just unpack and run it.

Download — stable v0.10, 7.4 MiB

Downloadlatest development (unstable) build — currently not available (see box above)


If you prefer to build your tools from source, or just want to see how KS works, the easiest way to check out the whole project is to download the main (umbrella) repository that already includes all other parts as submodules. Use:

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

If you already cloned the project and forgot --recurse-submodules, run

git submodule update --init --recursive

Alternatively, one can check out individual sub-projects that consitute the Kaitai Struct suite. See the GitHub project page for details.



Kaitai Struct is free and open-source software, licensed under the following terms:

work in progress