Arduino MKR WiFi 1010 memory management

Question

according to the official documentation of the Arduino, the SRAM memory (which is allocated for the local variables) should be automatically released after calling any function. For example, I have a function memory_allocate_test2, which declares a large char array and this function is called in the loop(). I expect, that every time, after the function memory_allocate_test2 is executed, the SRAM memory that I allocated for the local variables within that function should be freed or released. However, it is not working like that. When I call that function in a loop() the microcontroller hangs and stops working because it runs out of the available SRAM memory. Let me provide an example code:

void setup() {
  //Initialize serial and wait for port to open:
  Serial.begin(9600);
  
  while (!Serial) {
    ; // wait for serial port to connect. Needed for native USB port only - development mode. It has to be removed when releasing to live!!!
  }
}

void loop() {
  delay(2000);
  memory_allocate_test2();
}

void memory_allocate_test2() {
  Serial.println("memory_allocate_test2 STARTING ");
  display_freeram();
  Serial.println("------------------------------------------------------------");
  Serial.println("Allocating the memory for the MKR: ");
  char* memory_usage_test = new char[2000];
  Serial.println("Allocated memory!");
  display_freeram();
  Serial.println("------------------------------------------------------------");
}

extern "C" char* sbrk(int incr);

void display_freeram(){
  Serial.print(F("- SRAM left: "));
  Serial.println(freeRam());
}

int freeRam() {
  char top;
  return &top - reinterpret_cast<char*>(sbrk(0));
}

why it doesn't work as it states in the documentation? what I'm doing wrong? why the SRAM memory is not released each time after the memory_allocate_test2 function execution?

https://www.seeedstudio.com/blog/2021/04/26/managing-arduino-memory-flash-sram-eeprom/

when I execute that program, here is what happens:

memory_allocate_test2 STARTING

• SRAM left: 7163

---

Allocating the memory for the MKR: Allocated memory!

• SRAM left: 5155

---

memory_allocate_test2 STARTING

• SRAM left: 5155

---

Allocating the memory for the MKR: Allocated memory!

• SRAM left: 3147

---

memory_allocate_test2 STARTING

• SRAM left: 3147

---

Allocating the memory for the MKR: Allocated memory!

• SRAM left: 1139

---

memory_allocate_test2 STARTING

• SRAM left: 1139

---

Allocating the memory for the MKR: Allocated memory!

• SRAM left: -869

---

memory_allocate_test2 STARTING

• SRAM left: -869

---

Allocating the memory for the MKR:

at that point the device just hangs and stops working. That's not what I expect!

since the functionality differs for each board and microcontroller, here are my specs: the board name is "Arduino® MKR WiFi 1010", the microcontroller name is "SAMD21 Cortex®-M0+ 32bit low power ARM MCU"

Answer 1

You are mixing two different ways of memory allocation.

The automatic memory allocation and release you cite works with defined local variables. These variables are allocated on the stack. At the end of their scope, their memory is released. To mimic your example:

void memory_allocate_test3() {
  Serial.println("memory_allocate_test3 STARTING ");
  display_freeram();
  Serial.println("------------------------------------------------------------");
  Serial.println("Allocating the memory for the MKR: ");
  char memory_usage_test[2000] = "Some content...";
  Serial.println("Allocated memory!");
  display_freeram();
  Serial.println("------------------------------------------------------------");
}

But what you use is heap-allocated memory because of the new operator. This kind of allocation is under complete responsibility of the programmer, this is you. Since you only allocate and never release, each loop reduces the remaining RAM. Use delete (actually delete[]) to release:

void memory_allocate_test2() {
  Serial.println("memory_allocate_test2 STARTING ");
  display_freeram();
  Serial.println("------------------------------------------------------------");
  Serial.println("Allocating the memory for the MKR: ");
  char* memory_usage_test = new char[2000];
  Serial.println("Allocated memory!");
  display_freeram();
  Serial.println("------------------------------------------------------------");
  delete[] memory_usage_test;
}

---

Beware: There are different allocation methods, malloc/calloc/alloca/free and new/delete. The former method uses functions of the C standard library, the latter method uses keywords of C++.

The pointers returned by the different methods are not compatible. You cannot release memory by free that you allocated by new. The same holds true for delete/malloc.

However, you can use both methods in the same program.

Please learn about memory allocation before attempting to use it. C and C++ are error-prone to the unsuspecting user.

As a side note, C and C++ are different languages, even though you can build a mixed program.