struct __freelist {
   size_t size;
   struct __freelist *next;
};

extern uint16_t *__brkval;
extern struct __freelist *__flp;
extern uint16_t __heap_start;
extern uint16_t __data_start;
extern uint16_t __bss_start;
extern uint16_t __bss_end;

void show_allocator() {
  if(__flp == 0) {
    Serial.println("There is no freelist.");
  } else {
    Serial.print("The freelist starts at 0x");
    Serial.println((uint16_t)__flp, HEX);
  }

  if(__brkval == 0) {
    Serial.println("No memory has been allocated yet");
    Serial.print("The heap starts at 0x");
    Serial.println((uint16_t)&__heap_start,HEX);
  } else {
    Serial.print("The current end of heap is at 0x");
    Serial.println((uint16_t)__brkval, HEX);
  }
  Serial.println();
}

void setup() {
  Serial.begin(9600);
  
  Serial.print(".data section starts at 0x");
  Serial.println((uint16_t)&__data_start, HEX);
  Serial.print(".bss section starts at 0x");
  Serial.println((uint16_t)&__bss_start, HEX);
  Serial.print("Together, they take up ");
  Serial.print((uint16_t)&__bss_end - (uint16_t)&__data_start);
  Serial.println(" bytes of memory\n");

  show_allocator();
  Serial.println("allocate 4 bytes");
  uint32_t* ptr = malloc(4);
  Serial.print("allocated block address = 0x");
  Serial.println((uint16_t)ptr, HEX);
  show_allocator();

  Serial.println("allocate 8 bytes");
  uint16_t* ptr2 = malloc(8);
  Serial.print("allocated block address = 0x");
  Serial.println((uint16_t)ptr2, HEX);
  show_allocator();

  Serial.println("free the first block (4 bytes)");
  free(ptr);
  show_allocator();

  Serial.println("free the second block (8 bytes)");
  free(ptr2);
  show_allocator();
}

void loop() {
  // put your main code here, to run repeatedly:

}