A Quine in C

Originally published on: Tue, 28 Apr 2009 00:11:12 +0000

A quine is a program that produces its own source as its output.

I'd never written one before, so on Pi Day of this year ( 3/14 ) I gave it a go. Here's the result of my effort. You'll need to scroll waaaay over to the right ...

#include <stdio.h>
#define E(x) putchar(x)
char *s="#include <stdio.h>@10#define E(x) putchar(x)@10char *s=@34~main() {char *p=s;for(;*s;s++){if(*s==64){E((10*(s[1]-48))+(s[2]-48));s+=2;}else if(*s==126){while(*p){E(*p++);} E(34); E(59);E(10);}else{E(*s);}}}";
main() {char *p=s;for(;*s;s++){if(*s==64){E((10*(s[1]-48))+(s[2]-48));s+=2;}else if(*s==126){while(*p){E(*p++);} E(34); E(59);E(10);}else{E(*s);}}}

The issue that I found myself dealing with was the string metacharacters the slash and the double-quote. If I used any of the metacharacters in the source string, I would need to find a way to regenerate the character in its escaped form as well as the raw form.

My solution was to loop through the string s and use my own metacharacter @ to indicate that the next two digits will represent the ASCII character that is to be written. In checking for the metacharacter, I use the ASCII value of @ (64), so if you're trying this out on a machine that uses EBCDIC or something other than ASCII, the code won't work.

The appearance of @10 in the string will cause the program to output a character with an ASCII value of ten ( a newline. ) The appearance of @34 in the string will yield a double-quotation mark. @59 will yield a semicolon.

The next metacharacter my routine uses is the tilde ~ character to denote the end of the prologue of the output. The quine is really three sections wide.

The prologue code ( #include's and such and the beginning of the source string definition... )
The string containing the source code
The body of the program that produces the output.

I abbreviated putchar() as E() with a macro so that the source wouldn't be any lengthier than it needed to be.

To generate the quine, the code first outputs the prologue, then a double-quote, then the complete source string, then another double-quote, then a semicolon, then a preserved section of the source-string that appears after the tilde ( the main() function. )

Unless otherwise noted, all code and text entries are Copyright Š2009 by James K. Lawless

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