[Nicholas Carlini] programmed a C implementation of two-player Tic Tac Toe, and he did it in a single call to
printf(). The arguments for that single function call get mind-bendingly complex, so it may come as no surprise that it was written for The International Obfuscated C Code Contest (IOCCC).
Most of us are aware that
printf() is one of those functions that is considerably more complex under the hood, and capable of far more, than it may appear to be. But did you know that it is capable of Turing-complete computation?
[Nicholas] clearly steps through the theory, so give it a read. In short, a maze of arguments handles the logic of the game while an embedded
scanf() reads user input, and printing the game board is always preceded by an escape code to clear the screen.
[Nicholas] is certainly no stranger to in-depth understandings; we’ve seen his work before in demonstrating how to fool speech recognition with hidden commands, including a powerful example showing how two virtually identical-sounding audio files transcribe entirely differently.
George Mallory, a famous English mountaineer, once suggested that it was of no use to climb mountains. Instead, he posited, the only reason to climb a mountain is because it is there. Likewise, when you become an expert in nurse call systems like those found in hospitals, you may find that you do things with them that are of similar use. Making a Turing-complete nurse call system is something you do because you can.
[Erik] has been working on this particular call system, known as Netrix, and used Wireshark to sniff out all of its protocols. With this information he realized that it would be possible to use the system’s routing features to perform all of the tasks that any Turing complete system can do: conditional branching and memory access. He set up a virtual machine and set about implementing all of these tasks using the nurse call system’s features.
The setup for this project is impressive, and belies an extensive knowledge of this one proprietary system but also of computer science in general. It’s interesting to see how something can be formed into a working computer system from parts that otherwise might not be used that way. Even things that aren’t electronic can be used as Turing-complete computers.
Photo via Wikimedia Commons