The annual International Obfuscated C Code Contest is open again, and you can submit your weird program coded in C, that only you understand, all the way to January 12th, 2012 – the contest closes at 12:12, of course.
Some cool winners from the previous years include 2006/hamre, which shows a nice source file layout. Also of note is the very funny abuse of the contest rules in 1994, which produces the world’s smallest self-reproducing program: The source is empty, and when the compiled program runs, it prints out nothing… exactly matching the original source code. The contest rules have since 1994 included a blurb on not trying to beat the record for the world’s smallest self-reproducing program.
The most beautiful program I have seen on that site so far, must be 2005/anon. Take a look at the code. Note that constructs such as do, if, switch and case have all been redefined to some weird series of parentheses. Reading the explanation of the code tells you, that almost no normal C-constructs have been used at all. The only variables are argc and argv. The code calls main() recursively a few times, in order to get more variables on the stack. Then it abuses the allocated stack as a variable space. Logic is implemented only by short-circuiting boolean expressions, and almost reads like a functional programming language.
On the subject of abusing C code, don’t forget the magnificent Duff’s Device. Amazing abuse of the C standard by an assembler programmer- but hey, it works!
I should probably also mention the Underhanded C Contest, where contestant compete to create normal-looking programs, which actually have backdoors hidden in them. The very best example of this is this story, where a Trojan horse is inserted into the C compiler itself, which is then removed from the source. Every time this “patched” C compiler is compiling the “login” program on Unix, it will insert a backdoor into the compiled program. But the kicker is, that if the C compiler compiles itself, it will insert code into the compiled program, so that the new C compiler will still insert code into itself and the “login” program – without having these instructions in the source. I urge you to read that story to understand more about security and Trojan horses.
Ever wondered which pen to choose? Wonder no longer, 
