As the title of the topic is not so clear, now here i can clear my problem...

I have a program source code, main.c . In which i m intending to do following thing with out changing the source code...
#include <stdio.h>
int main(int argc,char **argv)
char* backup;
char* arg = argv && argc > 0 ? argv[1] : NULL;


if (strcmp(arg, backup) != 0)
printf("Yeah! u success; as u have manipulated arg");
printf("You Failed!; no change in arg; arg and backup both are same");

void foo(char *arg)
arg="what can i do in it ??????? " ;
I am beginner in C, I want to do some thing in this funtion so i would be able to change the arg, without returning arg from this method, as i m not supposed to change in the main method, i can only do things in foo function.
waiting for ur suggestions..
thanks for your concentration.
I m using freebsd7.1 and compiler is gcc.