Values vanishing after leaving function

Question

I'm writing a function to insert into a tree and the data can be seen (printed) after insertion within the function but when trying to see it in main nothing happens.

node *tree = NULL;
insert(tree, 4321);
printf("outer: %d\n", tree->data);

void insert(node* tree, int data) {
  if (tree == NULL) {
    tree = new_node(data);
    printf("inner: %d\n", tree->data);
  }
}

This gives:

inner: 4321

It should be:

inner: 4321
outer: 4321

Answer 1

The problem is, tree is passed as an argument to the function, and by-status which is a pass-by-value in C. Any change made to tree itself inside the function will not be reflected back to the caller.

You need to pass a pointer to tree, if you want to modify tree from the called function.

Something like

void insert(node** tree, int data) {
  if (*tree == NULL) {
    *tree = new_node(data);
    printf("inner: %d\n", (*tree)->data);
  }
}

and call it like

insert(&tree, 4321);