C++ int undefined behaviour when stored as RTC_NOINIT_ATTR (ESP32)

Question

I'm programming an ESP32 using VSCode. I have the following simple script:

#include <Arduino.h>

RTC_DATA_ATTR int counter1 = 0;
RTC_NOINIT_ATTR int counter2 = 0;

void setup() {
Serial.begin(115200);
Serial.printf("RTC programme running, counter1 = %d; counter2 = %d\n",counter1,counter2);
delay(3000);
counter1++;
counter2++;
esp_restart();
}

void loop() {
  // nothing needed here
}

I'd expect the output to be:

RTC programme running, counter1 = 0; counter2 = 0
RTC programme running, counter1 = 0; counter2 = 1
RTC programme running, counter1 = 0; counter2 = 2
...

But instead I'm getting:

RTC programme running, counter1 = 0; counter2 = 109811943
RTC programme running, counter1 = 0; counter2 = 109811944
RTC programme running, counter1 = 0; counter2 = 109811945
...

(where the value of counter2 is a random value). I've tried various combinations of int , uint32_t etc. but still get the random value. It's caused by the RTC_NOINIT_ATTR definition but it's what I need for the eventual application. Anything I can be doing differently?

Answer 1

The RTC memory of the ESP32 is retained over software reset and deep sleep.

The RTC_DATA_ATTR and RTC_NOINIT_ATTR macros have linker directives to move the variables to addresses mapped into the RTC memory.

Variable with RTC_NOINIT_ATTR is not initialized at program start to not erase the value stored in the RTC memory. (RTC_DATA_ATTR variables value is available only in deep sleep stubs, which are small functions running in RTC memory right after wakeup before the normal program starts.)

To initialize the RTC_NOINIT_ATTR variable only on power-up, you can check the reset reason in setup() and initialize the variable only on some reset reasons .

Answer 2

I added this at the start of the setup() function:

  esp_reset_reason_t reason = esp_reset_reason();
  if ((reason != ESP_RST_DEEPSLEEP) && (reason != ESP_RST_SW))
  {
    counter2 = 0;
  }

It now works more or less as it should...