How does this self outputting C program works?

Question

here is the smallest code that output itself. But can't grasp how this works. can somebody explain?

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

@PavelŠimerda If it ever were, it's no longer standards-compliant due to `main`'s prototype. — Medinoc, Apr 02 '14 at 09:13
Did you try reading http://en.wikipedia.org/wiki/Quine_(computing) ? — Ankur, Apr 02 '14 at 09:14
This also depends on the order `printf` arguments are evaluated, which is not standardized. — urzeit, Apr 02 '14 at 09:30

score 1 · Accepted Answer · answered Apr 02 '14 at 09:16

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I bet it won't work on a 64-bit platform unless its model is ILP64 (64-bit ints), because it relies on int being big enough to contain a char*.

It declares a variable a that contains a copy of the code minus the string itself, and uses printf()'s formatting codes to output both the code and the string. Do you need more details?

answered Apr 02 '14 at 09:16

Medinoc

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+1, but it works for most PC, `int` is big enough "in practice", for fun. – Aean Apr 02 '14 at 09:24
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It's a (dangerous) shortcut for declaring the `a` local variable: Here it's declared as a parameter of main, with its type implicitly `int`. Declaring it as a true `char*` inside the function body would have been the right thing to do, but would have made the quine longer. – Medinoc Apr 02 '14 at 15:05

score 1 · Answer 2 · answered Apr 02 '14 at 09:29

These codes called as quine codes.The computer languages supports this features till a fixed point. As per wikipedia

a fixed point (sometimes shortened to fixpoint, also known as an invariant point) of a function is an element of the function's domain that is mapped to itself by the function

means means f(f(...f(c)...)) = fn(c) = c where c is some constant for example

 f(x) = x^2 - 3 x + 4,
then 2 is a fixed point of f, because f(2) = 2

Lee Duhem · Answer 3 · 2014-04-02T09:37:15.580

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

can be rewritten to

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

These two versions are not equivalent, but you could use the second version to understand what is going on in the first version.

edited Apr 02 '14 at 09:37

answered Apr 02 '14 at 09:26

Lee Duhem

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How does this self outputting C program works?

3 Answers3