replace str dynamic

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

int getCnt(const char *const s, const char *const ss) {
  int cnt = 0;
  char *p = strstr(s, ss);
  const int lenSS = strlen(ss);
  while (p) {
    cnt++;
    p = strstr(p + lenSS, ss);
  }
  return cnt;
}

char *string_replace_dynamic(const char *where, const char *const what, const char *const replace) {
  char *res = NULL;
  const int cnt = getCnt(where, what);
  if (cnt == 0) {
    return NULL;
  }
  const int lenWhat = strlen(what), lenRep = strlen(replace), whereLen = strlen(where);
  res = malloc(whereLen + cnt * (lenRep - lenWhat) + 1);
  int index = 0;
  while (*where) {
    if (strstr(where, what) == where) {
      strcpy(&res[index], replace);
      index += lenRep;
      where += lenWhat;
    } else {
      res[index] = *where;
      index++;
      where++;
    }
  }
  res[index] = 0;
  return res;
}//*/

int main(void) {
  char s1[] = "Acesta este un string si un string este terminat cu 0x00";
  char s2[] = "string";
  char s3[] = "sir";
  char *p = string_replace_dynamic(s1, s2, s3);
  if (p != NULL) {
    puts(p);
  }
  free(p);
  return 0;
}