Question

Seems like I still didn’t get the pointers in C right.

I want the length of the global array (pointer) j being dynamic.

I have this (Arduino) code

unsigned int* j;

void setup() {

  Serial.begin(9600);

  initj();

  Serial.println(j[0]); //111 -> right
  Serial.println(j[0]); //768 -> wrong!
  Serial.println(j[1]); //32771 -> wrong!
}

void initj() {
  unsigned int i[2];
  i[0] = 111;
  i[1] = 222;

  j = i;

  Serial.println(j[0]); // 111 -> right
  Serial.println(j[1]); // 222 -> right
}

void loop() {
}

How can I do this right?

Thank you in advance!

Answer 1

Your initj() function sets j to point at a local array. The lifetime of this array is limited to the function call itself, as soon as the function returns, the pointer is no longer valid. Attempting to dereference j is therefore undefined behaviour.

I don’t know exactly what you want to do, but three possibilities are:

1. Declare i at global scope instead.
2. Declare i as static.
3. Dynamically allocate the array using malloc (maybe unsuitable for an embedded platform).