Here is the code of a quine, a program that outputs its own source code:

quine.c:

#define B(x)void main(){puts("#define B(x)"x"\n#define X(s)S(s)\n#define S(s)#s\nB(X(B(x)))");}
#define X(s)S(s)
#define S(s)#s
B(X(B(x)))

How it works

Disclaimer: C processor macros are a pain to explain, I am not going into details.

Macro stringization

#define X(s) S(s)
#define S(s) #s

X is a preprocessor macro that takes an argument, expands the macros in it and transform the result to a string literal, thanks to the "stringize" operator (#). We need two two macro function because one alone cannot expand macro in its argument.

Example Supposing var is a defined by this line : #define var 42. S(var) → #var → "var": S(var) is replaced by the string literal "var". X(var) → S(42) → #42 → "42": X(var) is replaced by the expansion of var, also as a string literal.

Building the main function

#define B(x) void main(){puts("#define B(x) "x"\n#define X(s)S(s)\n#define S(s)#s\nB(X(B(x)))");}
B(X(B(x)))

The body of the main function is defined by B. The main function just calls puts to print its code, passed as a string literal. The tricky part is the x argument of B. We pass to B its own definition.

This is achieved by the line B(X(B(x))).

In the inner B call is passed the value x as argument. The x definition got replaced by the x passed as argument. B(x) is replaced by the exact definition of B:

B(x) → void main(){puts("#define B(x) "x"\n#define X(s)S(s)\n#define S(s)#s\nB(X(B(x)))");}

This gets transformed in a string literal by the X macro:

X(B(x)) → "void main(){puts(\"#define B(x) \"x\"\\n#define X(s)S(s)\\n#define S(s)#s\\nB(X(B(x)))\");}"

We pass this string literal to B again to give the program's main function:

B(X(B(x))) → void main(){puts("#define B(x) ""void main(){puts(\"#define B(x) \"x\"\\n#define X(s)S(s)\\n#define S(s)#s\\nB(X(B(x)))\");}""\n#define X(s)S(s)\n#define S(s)#s\nB(X(B(x)))");}

Remember that two adjacent string literals are concatenated in C. "abc" "def", or "abc""def" ends up being the same as "abcdef". So ignore the double double-quotes.

So now we have our main function, and the complete code passed to puts.

Links

This program is pretty simple. Peoples have done stuff a lot crazier, like a spiraling quine in perl, or a quine relay of 50 languages (hard to see 50 languages in that code, don't you think?).