My first job out of Uni was as a programmer for a small company which manufactured network switches. One of the other programmers there challenged me to write a program \u00a0which output itself without referring to the file containing the source code. The following piece of ugly C code is what I came up with.<\/p>\n
<\/p>\n
#include <stdio.h>\r\nint c=10;int q=34;int p=37;\r\nchar h[]=\"#include <stdio.h>%cint c=10;int q=34;int p=37;%cchar h[]=%c%cs%c;\";\r\nchar b[]=\"%s%cchar b[]=%c%cs%c;%cchar s[1024];char o[1024];%cint main(){sprintf(s,h,c,c,q,p,q);sprintf(o,b,s,c,q,p,q,c,c,c);printf(o,h,b);}%c\";\r\nchar s[1024];char o[1024];\r\nint main(){sprintf(s,h,c,c,q,p,q);sprintf(o,b,s,c,q,p,q,c,c,c);printf(o,h,b);}<\/pre>\nIs an explanation necessary… I guess so. All variable name are kept as short as possible and formatting for to make the code readable is all but eliminated.<\/p>\n
After the necessary includes, I define useful character codes. Carriage return, quote, percent.<\/p>\n
int c=10;int q=34;int p=37;<\/pre>\nThen I craft a format string that will allow the header part of the source code to be output.<\/p>\n
char h[]=\"#include %cint c=10;int q=34;int p=37;%cchar h[]=%c%cs%c;\";<\/pre>\nThen another format string which will be used to output the body of the source code.<\/p>\n
char b[]=\"%s%cchar b[]=%c%cs%c;%cchar s[1024];char o[1024];%cint main(){sprintf(s,h,c,c,q,p,q);sprintf(o,b,s,c,q,p,q,c,c,c);printf(o,h,b);}%c\";<\/pre>\nNow define a couple of working strings.<\/p>\n
char s[1024];char o[1024];<\/pre>\nAnd finally the main function which builds the output string.<\/p>\n
int main(){sprintf(s,h,c,c,q,p,q);sprintf(o,b,s,c,q,p,q,c,c,c);printf(o,h,b);}<\/pre>\nBuild the header string which will result in a copy of itself.<\/p>\n
sprintf(s,h,c,c,q,p,q);<\/pre>\nBuild the body string with %s place holders for head string and the body format string.<\/p>\n
sprintf(o,b,s,c,q,p,q,c,c,c);<\/pre>\nOutput the finished string for the code.<\/p>\n
printf(o,h,b);<\/pre>\nI don’t understand how it works either!<\/p>\n","protected":false},"excerpt":{"rendered":"
My first job out of Uni was as a programmer for a small company which manufactured network switches. One of the other programmers there challenged me to write a program \u00a0which output itself without referring to the file containing the … Continue reading