Dereferencing a string pointer from a FreeRTOS queue

Question

I'm trying to use the queue API that FreeRTOS provides to read a string data from an Interrupt Service Routine (ISR) on an ESP32 device.

As strings are quite large data, I actually send the address of the string using a pointer. This seems to work as I can read the correct pointer adress (see example below).

However, I'm stuck to understand why I'm unable to get the string value when dereferencing this pointer. I'm not used to deal with pointers but reading various examples on the web, I don't see what I'm doing wrong.

QueueHandle_t qGsmEventData;

void IRAM_ATTR ISR_GSM_RI(){
  BaseType_t xHigherPriorityTaskWoken = pdFALSE;
  String sGsmEventData = "String sent from ISR";
  String * pGsmEventData = &sGsmEventData;

  Serial.print("Pointer address added to queue: ");
  Serial.println((unsigned int)pGsmEventData);
  Serial.print("String length: ");
  Serial.println(sGsmEventData.length());

  xQueueSendToBackFromISR(qGsmEventData, &pGsmEventData, &xHigherPriorityTaskWoken);
}

void setup() {
  [...]
  
  qGsmEventData = xQueueCreate(10, sizeof(String *));
  attachInterrupt(digitalPinToInterrupt(GSM_INT_PIN), ISR_GSM_RI, RISING);
}

void loop() {
  String *pGsmEventData;
  xQueueReceive(qGsmEventData, &(pGsmEventData), portMAX_DELAY);

  Serial.print("Pointer address get from queue: ");
  Serial.println((unsigned int)pGsmEventData);

  String sGsmEventData = *pGsmEventData;
  Serial.print("String length: ");
  Serial.println(sGsmEventData.length());

  delay(500);
}

and i'm getting on the serial console output :

Pointer address added to queue: 1073475016
String length: 20
Pointer address get from queue: 1073475016
String length: 0

So the queue seems to work as I get the correct pointer address but why I get a 0 length string when dereferencing this pointer ? How to get the string value ?

score 1 · Accepted Answer · answered Jul 09 '22 at 13:00

Basically the pointers outlive the objects they point to.

Let's break the ISR down as an example:


void IRAM_ATTR ISR_GSM_RI(){
  BaseType_t xHigherPriorityTaskWoken = pdFALSE;
  String sGsmEventData = "String sent from ISR";
  //string is created on the stack
  String * pGsmEventData = &sGsmEventData;
  //its address is taken
  Serial.print("Pointer address added to queue: ");
  Serial.println((unsigned int)pGsmEventData);
  Serial.print("String length: ");
  Serial.println(sGsmEventData.length());
  //String object is valid thus so are the logs

  xQueueSendToBackFromISR(qGsmEventData, &pGsmEventData, &xHigherPriorityTaskWoken);
  //its address is sent
}  //string's destructor is called, it no longer exists
   //pointer points to now empty place in memory

After reception, you're dereferencing the no longer valid pointer, so anything can happen really, the behaviour is undefined. And this is generally true for objects of any type, save string literals.

Thanks for your explanation. It is very clear now. A fix is then to declare sGsmEventData outside the ISR and it should be fine as I use the Queue and won't access this value directly outside the ISR. — Loïc G., Jul 09 '22 at 13:06
@LoïcG. a possible one, yes. Question is if in such case there's still a need to pass the pointer. — alagner, Jul 09 '22 at 13:09
Yes because the device has not enough RAM to store a such string Queue — Loïc G., Jul 09 '22 at 13:17

Dereferencing a string pointer from a FreeRTOS queue

1 Answers1