поведение strtoul между C и Arduino

У меня есть код C, который компилирует OK на моем ноутбуке

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

unsigned int crc32(const void *m, size_t len) {
  const unsigned char *message = m;
  size_t i;
  int j;
  unsigned int byte, crc, mask;

  i = 0;
  crc = 0xFFFFFFFF;
  while (i < len) {
    byte = message[i];
    crc = crc ^ byte;
    for (j = 7; j >= 0; j--) {
      mask = -(crc & 1);
      crc = (crc >> 1) ^ (0xEDB88320 & mask);
    }
    i = i + 1;
  }
  return ~crc;
}

int main() {
  char str[] =
      "620004";
  size_t len = strlen(str);
  unsigned int crc = crc32(str, len);

  size_t len2 = (len + 1) / 2;
  unsigned char arr2[len2];
  for (size_t i = 0; i < len; i += 2) {
    arr2[i / 2] = strtoul((char[3]) {str[i], str[i + 1], '\0'}, 0, 16);
  }
  crc = crc32(arr2, len2);
  printf("CRC: 0x%X\n", crc);

  return 0;
}

И я пытаюсь портировать его на Arduino

#include <CRC32.h>

void setup() {
  Serial.begin(115200);
  uint8_t byteBuffer[] = "620004";
  size_t numBytes = sizeof(byteBuffer) - 1;
  size_t len2 = (numBytes + 1) / 2;
  unsigned char arr2[len2];
  CRC32 crc;
  for (size_t i = 0; i < numBytes; i += 2) {
    arr2[i / 2] = strtoul((char[3]) {byteBuffer[i], byteBuffer[i + 1], '\0'}, 0, 16);
  }
  for (size_t i = 0; i < numBytes; i++){
    crc.update(byteBuffer[i]);
  }

  uint32_t checksum = crc.finalize();
  Serial.print("CRC: ");
  Serial

Но я получаю

Arduino: 1.8.9 (Linux), Board: "Arduino/Genuino Uno"

/home/nico/Arduino/crc32/crc32.ino: In function 'void setup()':
/home/nico/Arduino/crc32/crc32.ino:11:50: warning: narrowing conversion of 'byteBuffer[i]' from 'uint8_t {aka unsigned char}' to 'char' inside { } [-Wnarrowing]
     arr2[i / 2] = strtoul((char[3]) {byteBuffer[i], byteBuffer[i + 1], '\0'}, 0, 16);
                                                  ^
/home/nico/Arduino/crc32/crc32.ino:11:69: warning: narrowing conversion of 'byteBuffer[(i + 1u)]' from 'uint8_t {aka unsigned char}' to 'char' inside { } [-Wnarrowing]
     arr2[i / 2] = strtoul((char[3]) {byteBuffer[i], byteBuffer[i + 1], '\0'}, 0, 16);
                                                                     ^
crc32:11:84: error: taking address of temporary array
     arr2[i / 2] = strtoul((char[3]) {byteBuffer[i], byteBuffer[i + 1], '\0'}, 0, 16);
                                                                                    ^
exit status 1
taking address of temporary array

This report would have more information with
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И я не совсем понимаю, в чем проблема с функцией strtoul и ее другим поведением на Arduino