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

char* insert(char *src, char *s, char *dst)
{
   char *t = strdup(src + strlen(s));
   strcpy(src, dst);
   strcat(src, t);
   free(t);
   return src;
}

int main(void)
{
   char s[BUFSIZ] = "__Hello world";
   char *s1 = "Hi";
   char *s2 = "Hello";
   char *ptr = strchr(s, 'H');

   printf("%s\n", insert(ptr, s2, s1));
   printf("%s\n", insert(ptr, s1, s2));
   printf("%s\n", insert(ptr, s2, s2));
   printf("%s\n", s);

   return 0;
}