SELF PRINTERS IN C

The definition of self printers is straightforward --- they are simply programs
that print themselves or print their source codes, that is.  You may have seen
these programs before.  Self-printers sparked  my interest when I was in high-
school.  My friend asked me if could make a PASCAL program that could print itself.
I stood up to his challenge and four hours later, I came up with something like:

begin write('begin write(''begin write(''begin write(''...

...and a conclusion: it's a waste of time.  Days later I tried to do the same
trick in C and viola! I came up with the program below after a few minutes.

listing 1. self01.c
char*s="char*s=%s;main(){printf(s,s);}{";main(){printf(s,s);}

What a program like this really needs is something that does double work, a literal
that can be made to print itself twice.  In this case, the variable s which is a
character string.  Using printf(), it is possible to do this because of the unique
way that this function formats its output.  For example, given the following code
fragment:

char *d = "x%s";
printf(d, d);

you should be able to produce the output:

xx%s

clearly demonstrating that d does the double work of specifying the print format
and printing itself.  So given that, the program in listing 1 should be able to print
itself.  Or so I thought.  So what's wrong with it?  As can be seen in a sample run of
the program:

output of self01.c
char*s=char*s=%s;main(){printf(s,s);}{;main(){printf(s,s);}

it is clear that it's missing the double qoutes that delimit the literal value of
char*s.  In the next program, I fixed the bug and came up with a correct output.  It
should work on systems that maps the numeric value 34 to the character \" (ASCII).

listing 2. self02.c
char*s="char*s=%c%s%c;int q=34;main(){printf(s,q,s,q);}";
int q=34;main(){printf(s,q,s,q);}

I won't bother to show the output of this and the other programs below, they're self-
printers all.

listing 3. self03.c
char*s="char*s=%c%s%c;%cint q=34,n=10;main(){printf(s,q,s,q,n);}";
int q=34,n=10;main(){printf(s,q,s,q,n);}

listing 4. self04.c
char *s1="char *s1=%c%s%c;%cchar *s2=%c%s%c;%cchar *s3=%c%s%c;%c";
char *s2="int q=34,n=10;%cmain()%c{%c  printf(s1,q,s1,q,n,q,s2,q,n,q,s3,q,n);";
char *s3="%c  printf(s2,n,n,n);%c  printf(s3,n,n,n);%c}";
int q=34,n=10;
main()
{
  printf(s1,q,s1,q,n,q,s2,q,n,q,s3,q,n);
  printf(s2,n,n,n);
  printf(s3,n,n,n);
}


The program in listing 2 is the shortest I could ever code.  If you have something
shorter, please tell me about it.

- Mark J. P. Documento
javelinexxx@yahoo.com

Any loss of sanity effected by the perusal of the above article will not be a responsibility of the author.
God knows he has enough of this problem himself.

The following response came from mail to the article above which I published at the Planet Source Code.

Mark,

Since you have an interest in self-producing 'C' code, you 
might be interested in http://www.ioccc.org/.  It's the 
home page for the International Obfuscated C Code Contest. 
It's a contest to see who can produce the ugliest, most 
confusing piece of functional c code.  

They used to run a shortest self-producing C code contest 
until someone won by submitting a blank cpp file.  Because 
of an old Unix compiler glitch you could compile a blank 
file, and it would produce an executable that ran and did 
nothing.  And since you can direct input from a program to 
a file in the Unix world.  You could direct the results of 
this program to a file, and the result would be an empty 
file.  So this kind of end that contest.  

And the point of this long e-mail is...... They have an 
archive of all the code from past winners, and you will 
find some nice examples of self-replicating code.
Along with a bunch of interesting/ugly c and cpp code.

Sorry, I didn't intend this e-mail to be this long.

Ryan


Ryan Smulan <smulanry@uregina.ca>

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