When using ESP32 with the Arduino IDE, it does not support the native watchdog timer. The following gist does not use the official watchdog timer but employs a normal timer to reset the CPU if wdt_reset() is not called in time.

esp32_wdt.cpp

#include "esp32_wdt.h"
#include <esp32-hal-timer.h>

static hw_timer_t *timer = NULL;

/**
 * iInterrupt service routine called when the timer expires.
 */
void IRAM_ATTR resetModule() {
  ets_printf("watchdog reboot\n");
  esp_restart_noos();
}

void wdt_enable(const unsigned long durationMs) {
  //timer 0, div 80
  timer = timerBegin(0, 80, true);
  timerAttachInterrupt(timer, &resetModule, true);
  //set time in us
  timerAlarmWrite(timer, durationMs * 1000, false);
  //enable interrupt
  timerAlarmEnable(timer);
}

void wdt_disable() {
  if (timer != NULL) {
    //disable interrupt
    timerDetachInterrupt(timer);
    timerEnd(timer);
    timer = NULL;
  }
}

void wdt_reset() {
  //reset timer (feed watchdog)
  if (timer != NULL) {
    timerWrite(timer, 0);
  }
}

esp32_wdt.h

/**
   When using ESP32 with the Arduino IDE, it does not support the native watchdog timer.
   The following watchdog implementation does not use the official watchdog timer
   but employs a normal timer to reset the CPU if wdt_reset() is not called in time.

   Inspired by the following discussion:
   https://github.com/espressif/arduino-esp32/issues/841

   Credit goes to @me-no-dev.
*/
#ifndef WDT_H
#define WDT_H

/**
   Enable the watchdog timer, configuring it for expiry
   after durationMs milliseconds.
*/
void wdt_enable(const unsigned long durationMs);

/**
   Disable the watchdog timer.
*/
void wdt_disable();

/**
   Reset the watchdog timer. When the watchdog timer is enabled,
   a call to this method is required before the timer expires,
   otherwise a watchdog-initiated device reset will occur.
*/
void wdt_reset();

#endif