This was my submission to the 20th [International Obfuscated C Code Contest] IOCCC; this repository is the one I was doing my actual work in, so there it contains a certain amount of information about how things were put together, in the revision history and in various notes and precursors to the final program, which were not part of the contest submission itself. If you didn't want spoilers for such things, it's probably already too late.
The final program in final.c; the accompanying Makefile may be of
use but is trivial. The remarks.md file explains what the program
does and how to build and use it; it also explains how it does that
thing, and describes the ways in which that functioning is obfuscated,
but these sections were written vaguely enough that they were not
expected to be helpful in understanding the program. There are a few
other files of interest; in particular, prototype.c grew into the
final functionality, and then a few layers of cosmetic obfuscation /
decoration were applied to obtain the submitted version.
It lost.
These programs is released into the public domain, as required by the IOCCC submission rules. Specifically, I release it under CC0, although that license and the beginning of this paragraph are equivalent up to the noninvolvement of lawyers.
Of course, as a losing entry, its existence will be steadfastly denied by the IOCCC organizers — also as specified by the contest rules — so I suppose I could theoretically turn around and claim copyright interest on it and they couldn't stop me. But if I did that, then I'd effectively also be denying that it was an IOCCC entry, which kind of defeats the point.
So, public domain it is. Indeed, if anyone finds a way to make use of this, I might actually prefer not to be publicly associated with the result in any way.
The section of the remarks which documents the few known nasal demons is incomplete; there is one more known environmental dependency which I forgot about until several hours after the contest deadline. However, it is satisfied by essentially every known C environment, and I'm told that it may also be guaranteed by POSIX (as an extension of ANSI C proper).
I should probably write up an actual explanation of what's going on here (i.e., a “spoiler” in the Nethack sense). It's not quite as complicated as it seems.