Global array doesn't get assigned the space it would take up in memory

Question

I have this sketch:

const uint32_t len = 65536;
uint8_t arr[len];

void setup() {}

void loop() {}

The length of the array should fit into the dynamic memory. However, when I compile the sketch it says this in the output window:

Sketch uses 10528 bytes (0%) of program storage space. Maximum is 2031616 bytes.
Global variables use 13504 bytes (1%) of dynamic memory, leaving 1034224 bytes for local variables. Maximum is 1048576 bytes.

If I change len to 10, that doesn't change. The compiler is set to "Debug", so it should not get optimized away. This indicates, that the memory isn't actually being allocated. What seems to be the problem here?

I am compiling for the Teensy 4.0, but the output is the same in essence for the Arduino Uno and other boards (with lower memory sizes).

Answer 1

There is no problem here. Only the compiler is a bit smarter than you anticipated. It sees, that you are not using arr anywhere, so it just optimizes it out.

There are two ways to prevent that, if you really want to:

• I think you can use flags to tell the compiler, that this shouldn't be optimized out. I'm no expert in this, others might know more about this and add their own answer
• You can just start using it.