Accessing struct from double pointer

Question

I'm currently trying to implement a doubly linked list in C, and am not understanding how to access the struct from a double pointer.

Here is my simple struct:

typedef struct node {
  int val;
  struct node * next;
  struct node * prev;
} node;

Here is a simple method where I try to push a value to the front of the list:

void push_front(node ** head, int newVal) 
{
  node * newNode = malloc(sizeof(node));

  newNode->val = newVal;
  newNode->next = head;

  *head->prev = newNode;

  *head = newNode;
}

However, the line *head->prev = newNode gives me an error, saying that the left of ->prev must point to a struct/union. I'm just learning C so maybe I'm overlooking something really easy, but isn't head the pointer to the pointer of my head node? And *head is the pointer to my head node. Which I would assume means *head->prev should work?

Greg Hewgill · Accepted Answer · 2015-06-05 03:47:06Z

6

Yes, head is a pointer to the pointer of your head node. So you can access ->prev by doing:

(*head)->prev = newNode;

Without the parentheses, the operator precedence rules of C parse your statement as

*(head->prev) = newNode;

which is not what you want.

answered Jun 5, 2015 at 3:47

Greg Hewgill

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Gopi · Accepted Answer · 2015-06-05 03:59:24Z

0

node ** head

Here head is a pointer to a pointer and (*head) is a pointer. So you need to access pointer like

(*head)->prev = newnode;

edited Jun 5, 2015 at 3:59

answered Jun 5, 2015 at 3:47

Gopi

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