#include<stdio.h>
#include<string.h>
#include<stdlib.h>

char* double_string(char* original){
	size_t len = strlen(original);
	char *new_string = malloc(len * 2 + 1);
	char *a = new_string;
	char *b = original;
	while(*a++ = *b++);

	a = new_string + len;
	b = original;
	while(*a++ = *b++);

	return new_string;
}


int main(){
	char *long_string = double_string("fox");
	puts(long_string);
	free(long_string);
	return 0;
}