Quine

A quine is a program that prints out it's own listing. In other words, if it's a C program, you can do something like this:

$ gcc quine.c
$ ./a.out > quine2.c
$ gcc quine2.cc
$ ./a.out > quine3.c
$ diff quine3.c quine.c
$

Writing a quine is not as trivial as it sounds.  Here is the quine I wrote in C which takes advantage of the format codes in the printf statement to insert a partial listing of the program into itself:

#include <stdio.h> int main() {char listing[200]="#include <stdio.h> %cint main() {char listing[200]=%c%s%c; char delim=10; char delim2=34; printf(listing, delim, delim2, listing, delim2);}"; char delim=10; char delim2=34; printf(listing, delim, delim2, listing, delim2);}