C language concatenate strings without library functions

/*
Example: concatenate two strings without standard library in C language
*/

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

void main(void)
{
  char str1[25] = "hello ";
  char str2[25] = "world";
  int i=0,j=0;

  while(str1[i]!='\0')
    i++;
  while(str2[j]!='\0')
  {
    str1[i]=str2[j];
    j++;
    i++;
  }
  str1[i]='\0';
  printf("\nconcatenated string: %s",str1);
}