A program that reproduces itself

[Robert Rotstein]

Following is a C program, taken from
http://en.wikipedia.org/wiki/Quine#Sample_quine_in_C,
which has the curious property that, when executed, it produces its own
source code as output.

#include <stdio.h>
char x[]="#include <stdio.h>%cch ar x[]=%c%s%c;%cint main()
{printf(x,10,34 ,x,34,10,10);re turn 0;}%c";
int main() {printf(x,10,34 ,x,34,10,10);re turn 0;}

But I can't figure it out! Specifically, I don't know the significance of
the hard-coded integers, nor how the % formatting characters work in this
instance. Can someone explain just how this works?

[Daniel Vallstrom]

"Robert Rotstein" <rr*******@veri zon.net> wrote in message news:<YUecd.153 3$B34.281@trndn y02>...

Following is a C program, taken from
http://en.wikipedia.org/wiki/Quine#Sample_quine_in_C,
which has the curious property that, when executed, it produces its own
source code as output.

#include <stdio.h>
char x[]="#include <stdio.h>%cch ar x[]=%c%s%c;%cint main()
{printf(x,10,34 ,x,34,10,10);re turn 0;}%c";
int main() {printf(x,10,34 ,x,34,10,10);re turn 0;}

But I can't figure it out! Specifically, I don't know the significance of
the hard-coded integers, nor how the % formatting characters work in this
instance. Can someone explain just how this works?

As others have said, they are hardcoded ASCII values for '\n' (10) and
'"' (34). As a result the wiki-program is not guaranteed to work because
the standard allows for different encodings. Below is a program that is
guaranteed to work. (Can it be improved (while keeping the lines short)?)

$ gcc -std=c99 -Wall -pedantic -O fixpoint.c
$ ./a.out
#include<stdio. h>
char*i="\\#incl ude<stdio.h>",n ='\n',q='"',*p=
"%s%cchar*i=%c% c%s%c,n='%cn',q ='%c',*p=%c%c%s %c,*m=%c%c%s%c% c;%s%c",*m=
"int main(){return!p rintf(p,i+1,n,q ,*i,i,q,*i,q,n, q,p,q,n,q,m,q,n ,m,n);}"
;int main(){return!p rintf(p,i+1,n,q ,*i,i,q,*i,q,n, q,p,q,n,q,m,q,n ,m,n);}
$ ./a.out | diff - fixpoint.c
$
Daniel Vallstrom