Program which source code is exactly the same as its output [duplicate]

Question

The more I try to understand this perplexed enigma the more I want to give up.

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

How is happening for this one-line source code to generate exactly the same output when the program executes and is there any common notion for this kind of programs?

Answer 1

This is called a Quine.

So let's see what main() does:

printf(s,34,s,34);

34 is the ASCII code for the character " (double quote), so this is the same as:

printf(s, '"', s, '"');

The first argument to printf(3) is the format string. The string passed is:

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

So, printf(3) will output exactly that, but note the %c, %s and %c format specifiers, which instruct printf(3) to print a character, followed by a string, followed by another character in that place, which are respectively the 2nd, 3rd and 4th arguments.

The characters, as we saw, are both ", and the string is s again (the same string). So the program output is:

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

Where X is the string s in the program. So we get this as output:

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

Which, interestingly, is the program source itself.

Answer 2

take that string from first parameter of printf:

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

and do substitution where

%c = 34 = '"' //(same for both %c)
%s = 'char *s = %c%s%c; main(){printf(s,34,s,34);}'

printf will do substitution only once (not recursive), so result is:

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

Answer 3

To understand the code, first simplify and reformat:

char *s = "some format string";
main() {
    printf(s,34,s,34);
}

so it uses s as a formatting string to print three entities: 34, the string s itself and 34. In this case, the important part of the formatting string s is:

char *s = "... %c%s%c ..."

which means the two 34s become double-quotes (") and the formatting-string s is just printed as a normal string. Now you should see the the rest of the formatting-string s is just a copy of the whole program.