It is actually quite difficult to write a self-reproducing program that is truly portable, due particularly to quoting and character set difficulties.

Here is a classic example (which ought to be presented on one line, although it will fix itself the first time it's run):

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

main(){printf(s,34,s,34);}

(This program has a few deficiencies, among other things neglecting to #include <stdio.h>, and assuming that the double-quote character " has the value 34, as it does in ASCII.)

#define q(k)main(){return!puts(#k"nq("#k")");}

q(#define q(k)main(){return!puts(#k"nq("#k")");})