How to measure free RAM on an Arduino Due in Runtime?

Question

I use this Library to measure my free RAM on a Mega in runtime. But this Library does not work on the Due.

• Mikael Patel: Try this library github.com/mpflaga/Arduino-MemoryFree

Answer 1

As mentioned by Mikael Patel in a comment, the library from Arduino-MemoryFree should do it. A minimal sketch that seems to work on the Due:

extern "C" char* sbrk(int incr);
int freeMemory() {
 char top;
 return &top - reinterpret_cast<char*>(sbrk(0));
}
void setup ()
{
 Serial.begin (115200);
 Serial.println ();
 Serial.print ("Free memory is: ");
 Serial.println (freeMemory ());
} // end of setup
void loop () { }

I'm not totally convinced that this gives the correct figure. The figure returned by the library above for my Uno was too high (2297 bytes).

• Edgar Bonet: It seems to work fine after a call to malloc(), which is needed to initialize __brkval. I would return &top - ( __brkval ? __brkval : &__heap_start); in order to have something that works also when not using malloc().
• Nick Gammon: I thought it might be something like that. After all, not everyone uses malloc.

Answer 2

Try to use this method:

int freeRAM() {
 extern int __heap_start, *__brkval;
 int v;
 return (int) &v - (__brkval == 0 ? (int) &__heap_start : (int) __brkval);
}

• Nick Gammon: The question was about a Due. When I tried your code on my Due I got -28 bytes of RAM, which is hard to believe.