Opaque pointer valgrind

Question

I'm currently writing a binary tree structure with Opaque Pointer. However, I have Invalid writes with Valgrind.

Tree.c :

struct node{
    int key;
    struct node *left, *right, *parent;
};

NODE nodeInit(void)
{
    NODE tree = (NODE) malloc(sizeof(NODE));
    tree->key = -1;
    //Error with the 3 lines below
    tree->right = NULL;
    tree->left = NULL;
    tree->parent = NULL;
    return tree;
}

Tree.h :

typedef struct node* NODE;

Note : I mustn't change the header file.

Answer 1

You're allocating memory for just the pointer.

typedef struct node* NODE means that from now on NODE is an alias for "a pointer to struct node". Thus malloc(sizeof(NODE)) allocates sizeof struct node * bytes of memory - enough memory to hold a pointer to your structure, not enough to memory to contain your structure.

Use either:

NODE tree = malloc(sizeof *tree);

or

NODE tree = malloc(sizeof (struct node));

The former is perhaps better, because it implicitly means "allocate enough memory to contain that which is pointed to by the pointer tree".

---

P.S., do not cast the result of malloc.

Answer 2

NODE tree = (NODE) malloc(sizeof(NODE)); this line is incorrect. 

It should be

NODE tree = (NODE) malloc(sizeof(struct node));