Question

I read Ken Thompson’s classic paper Reflections on Trusting Trust in which he prompts users to write a Quine as an introduction to his argument (highly recommended read).

A quine is a computer program which takes no input and produces a copy of its own source code as its only output.

The naive approach is simply to want to say:

print "[insert this program's source here]"

But one quickly sees that this is impossible. I ended up writing one myself using Python but still have trouble explaining “the trick”. I’m looking for an excellent explanation of why Quines are possible.

Answer 1

The normal trick is to use printf such that the format-string represents the structure of the program, with a place-holder for the string itself to get the recursion you need:

The standard C example from http://www.nyx.net/~gthompso/quine.htm illustrates this quite well:

char*f="char*f=%c%s%c;main(){printf(f,34,f,34,10);}%c";main(){printf(f,34,f,34,10);}

edit: After writing this, I did a bit of searching: http://www.madore.org/~david/computers/quine.html gives a very good, more theoretical, description of what exactly quines are and why they work.