C language: Releasing memory of pointers to struct

Question

Say I have declared a pointer to a struct and assign it with malloc() using this definition

typedef struct node {
    int info;
    struct node *next;
} NODE         


        

Answer 1

When you do

node1 = malloc(sizeof(NODE));

you have something like

+-------+      +-----------------------------+
| node1 | ---> | memory for a NODE structure |
+-------+      +-----------------------------+

After the assignment node2 = node1 you have instead this:

+-------+
| node1 | -\
+-------+   \    +-----------------------------+
             >-> | memory for a NODE structure |
+-------+   /    +-----------------------------+
| node2 | -/
+-------+

In other words you have two pointers pointing to the same memory.

Attempting to call free using either of the two pointer variable will invalidate both pointers.

Answer 2

You should pass the address returned by malloc() and family to free() in-order to free the allocated memory.

In your case you are just assigning the returned address to some other pointer and using that in free which is fine.

You shouldn't do

node2 = node1;
node2 = node2 +1;

free(node2);

So you can use one of them in your case to free the memory

free(node1) and free(node2) are same in your case

Answer 3

node1 is a pointer, meaning the value of it is the virtual memory address of the allocated struct.

Assigning node2 = node1 just copies that address.

As a result free(node1) and free(node2) are equivalent.

Answer 4

You release neither node1 nor node2. You release the memory they point to.

With that in mind, it should become clear why you should only call free once