The author originally wrote a paper provin that printf is Turing Complete [0].

Here is program itself and an actual, excellent write up [1].

@dang, you might consider switching the link to the GitHub. I don't think the Hackaday article actually adds anything new.

[0]: https://www.usenix.org/system/files/conference/usenixsecurit...

[1]: https://github.com/carlini/printf-tac-toe

There’s a scanf in there...

Note that the #define statements spell "noughts and crosses" when read vertically.

It's the little touches.