Kaitai Struct: declarative binary format parsing language
19 comments
·October 14, 2025theLiminator
Is the main difference from https://github.com/google/wuffs being that Kaitai is declarative?
nigeltao
See https://github.com/google/wuffs/blob/main/doc/related-work.m...
> Kaitai Struct is in a similar space, generating safe parsers for multiple target programming languages from one declarative specification. Again, Wuffs differs in that it is a complete (and performant) end to end implementation, not just for the structured parts of a file format. Repeating a point in the previous paragraph, the difficulty in decoding the GIF format isn't in the regularly-expressible part of the format, it's in the LZW compression. Kaitai's GIF parser returns the compressed LZW data as an opaque blob.
Taking PNG as an example, Kaitai will tell you the image's metadata (including width and height) and that the compressed pixels are in the such-and-such part of the file. But unlike Wuffs, Kaitai doesn't actually decode the compressed pixels.
---
Wuffs' generated C code also doesn't need any capabilities, including the ability to malloc or free. Its example/mzcat program (equivalent to /bin/bzcat or /bin/zcat, for decoding BZIP2 or GZIP) self-imposes a SECCOMP_MODE_STRICT sandbox, which is so restrictive (and secure!) that it prohibits any syscalls other than read, write, _exit and sigreturn.
(I am the Wuffs author.)
Sesse__
They overlap, but none does strictly more than the other.
Kaitai is for describing, encoding and decoding file formats. Wuffs is for decoding images (which includes decoding certain file formats). Kaitai is multi-language, Wuffs compiles to C only. If you wrote a parser for PNGs, your Kaitai implementation could tell you what the resolution was, where the palette information was (if any), what the comments look like and on what byte the compressed pixel chunk started. Your Wuffs implementation would give you back the decoded pixels (OK, and the resolution).
Think of Kaitai as an IDL generator for file formats, perhaps. It lets you parse the file into some sort of language-native struct (say, a series of nested objects) but doesn't try to process it beyond the parse.
dgan
Wow this is good. My only complaint is annoyingly verbose yaml. What if I would like to use Kaitai instead of protobuffs, my .proto file is already a thousand lines, splitting each od these lines into 3-4 yaml indented lines is hurting readability
zzlk
I wanted to use this a long time ago but the rust support wasn't there. I can see now that it's on the front page with apparently first class support so looks like I can give it a go again.
okanat
Even if you don't want to use it since it is not as efficient as a hand-written specialized parser, Kaitai Struct gives a perfect way of documenting file formats. I love the idea and every bit of the project!
jdp
I also like Protodata [1]. It's complementary as an exploration and transformation tool when working with binary data formats.
setheron
Great timing! I just published https://github.com/fzakaria/nix-nar-kaitai-spec and contributed kaitai C++ STL runtime to nixpkgs https://github.com/NixOS/nixpkgs/pull/454243
woodruffw
Kaitai Struct is really great. I've used it several times over the years to quickly pull in a parser that I'd otherwise have to hand-roll (and almost certainly get subtly wrong).
Their reference parsers for Mach-O and DER work quite nicely in abi3audit[1].
[1]: https://github.com/pypa/abi3audit/tree/main/abi3audit/_vendo...
layoric
I discovered this project recently and used it for Himawari Standard Data format and it made it so much easier. Definitely recommend using this if you need to create binary readers for uncommon formats.
imtringued
https://en.wikipedia.org/wiki/Data_Format_Description_Langua...
DFDL is heavily encroaching on Kaitai structs territory.
ginko
No pure C backend?
vendiddy
It's not C but we have sponsored a Zig target for Kaitai. If anyone reading this knows Zig well, please comment because would love to get a code review of the generated code!
dhsysusbsjsi
This would be great for most projects as Swift for example is abandoned & 6+ years since last commit.
Kaitai is absolutely one of my favorite projects. I use it for work (parsing scientific formats, prototyping and exploring those formats, etc) as well as for fun (reverse engineering games, formats for DOSbox core dumps, etc).
I gave a guest lecture in a friend's class last week where we used Kaitai to back out the file format used in "Where in Time is Carmen Sandiego" and it was a total blast. (For me. Not sure that the class agreed? Maybe.) The Web IDE made this super easy -- https://ide.kaitai.io/ .
(On my youtube page I've got recordings of streams where I work with Kaitai to do projects like these, but somehow I am not able to work up the courage to link them here.)