/kwi:n/ n. [from the name of the logician Willard van Orman Quine, via Douglas Hofstadter] A program that generates a copy of its own source text as its complete output. Devising the shortest possible quine in some given programming language is a common hackish amusement. (We ignore some variants of BASIC in which a program consisting of a single empty string literal reproduces itself trivially.) Here is one classic quine:
((lambda (x) (list x (list (quote quote) x))) (quote (lambda (x) (list x (list (quote quote) x)))))
This one works in LISP or Scheme. It's relatively easy to write quines in other languages such as Postscript which readily handle programs as data; much harder (and thus more challenging!) in languages like C which do not. Here is a classic C quine for ASCII machines:
char*f="char*f=%c%s%c;main() printf(f,34,f,34,10);%c"; main()printf(f,34,f,34,10);
For excruciatingly exact quinishness, remove the interior line breaks. Here is another elegant quine in ANSI C:
#define q(k)main()return!puts(#k"\nq("#k")");* q(#define q(k)main()return!puts(#k"\nq("#k")");) *
Some infamous Obfuscated C Contest
entries have been quines that reproduced in exotic ways. There is an amusing Quine Home Page (http://www.nyx.org/~gthompso/quine.htm).