Arduino number seq parsing

#include <Arduino.h>
#include "esp_system.h"

void reset_arr(int * const buf, int count)
{
  for(int i = 0; i < count; i++)
  {
    buf[i] = -1;
  }
}

void print_arr(const char * name, const char * text, const int * buf, size_t buf_len)
{
  Serial.printf("%s\n", name);
  Serial.printf("    LEN:  %d", buf_len);
  Serial.printf("    TEXT: \"%s\"\n", text);
  for (int i = 0; i < buf_len; i++)
  {
    Serial.printf("    ITEM[%d]: %d\n", i, buf[i]);
  }
}

void setup() {
  Serial.begin(9600);
  const String str_val = "2021,5,9,15,10,0";
  int int_vals[8];
  const int int_vals_count = sizeof(int_vals) / sizeof(int_vals[0]);
  reset_arr(
      (int *)int_vals,
      int_vals_count);
  print_arr(
    "Before parsing",
    str_val.c_str(),
    (int *)int_vals,
    int_vals_count);

  //variant1
  reset_arr(
    (int *)int_vals,
    int_vals_count);
  int parsed_elems=sscanf(str_val.c_str(), "%d,%d,%d,%d,%d,%d",
    &int_vals[0], &int_vals[1], &int_vals[2],
    &int_vals[3], &int_vals[4], &int_vals[5]);
  print_arr(
    "Parsed variant 1 (sscanf)",
    str_val.c_str(),
    (int *)int_vals,
    parsed_elems);

  //variant2
  reset_arr(
    (int *)int_vals,
    int_vals_count);

  int i_v2 = 0;
  const char * const str_p_v2 = str_val.c_str();
  const char * cur_p_v2 = str_p_v2;
  do {
    int_vals[i_v2] = atoi(cur_p_v2);
    cur_p_v2 = strstr(cur_p_v2, ",");
    while(cur_p_v2 != NULL && cur_p_v2[0] == ',') {
      cur_p_v2 += sizeof(char);
    }
    i_v2++;
  } while (cur_p_v2 != NULL);
  int parsed_elems_v2 = i_v2;
  print_arr(
    "Parsed variant 2 (strstr+atoi)",
    str_p_v2,
    (int *)int_vals,
    parsed_elems_v2);


  //variant3
  reset_arr(
    (int *)int_vals,
    int_vals_count);

  int i_v3 = 0;
  const char * const str_p_v3 = str_val.c_str();
  char * cur_p_v3;
  char * rest_p_v3 = NULL;

  for (cur_p_v3 = strtok_r((char *)str_p_v3, ",", &rest_p_v3);
    cur_p_v3 != NULL;
    cur_p_v3 = strtok_r(NULL, ",", &rest_p_v3)) {
      int_vals[i_v3++] = atoi(cur_p_v3);
  }

  int parsed_elems_v3 = i_v3;
  print_arr(
    "Parsed variant 3 (strtok_r+atoi)",
    str_p_v3,
    (int *)int_vals,
    parsed_elems_v3);
}

void loop() {

}