How to Use Typedef Structure Name with Pointer

Question

I am trying to implement a linked list using the given structure for a bigger project. The structure is defined below:

typedef struct node { unint32_t size; // = size of the node struct node * link; // = .next pointer } * ListNode;

I was able to implement a linked list using struct node *. But when I attempt to use ListNode like in the following program:

typedef struct node { unint32_t size; struct node * link; } * ListNode; void insert_node (ListNode * head, unint32_t size) { ListNode new_node = (ListNode) malloc (sizeof(ListNode)); new_node->size = size; new_node->link = NULL; if (head == NULL) { head = &new_node; } else { ListNode current = *head; while (current->link != NULL) { current = current->link; } current->link = new_node; } } int main (int argc, char const * argv[]) { ListNode head = NULL; insert_node (&head, 10); insert_node(&head, 20); ListNode ptr = head; while (ptr != NULL) { printf ("%d ", ptr->size); } printf ("\n"); return 0; }

I get a segmentation fault. Why is that? It even says that struct node * and ListNode are incompatible pointers/types. I thought they were the same struct just named differently.

Side note: In main, your print loop needs: ptr = ptr->next; at the bottom after the printf — Craig Estey
– Craig Estey, Commented Feb 19, 2022 at 0:50

pm100 · Accepted Answer · 2022-02-19 00:25:43Z

A little clarification

typedef struct node { unint32_t size; struct node * link; } *ListNode;

creates a type called ListNode. It is a pointer to a struct node. It is not a struct node

So when you do

sizeof(ListNode)

you get the size of a pointer, not the size of struct node

You needed to do

sizeof(struct node)

A very common thing to do is this

typedef struct node { uint32_ size; struct node* link; } *PListNode, ListNode;

this creates 2 types

PlistNode which is a pointer to a struct node
ListNode which is a struct node

the 'P' is a reminder that this is a pointer

so now you can do

PListNode pn = malloc(sizeof(ListNode));

Ted Lyngmo · Accepted Answer · 2022-02-19 00:49:13Z

Since you supply a struct node** (a ListNode*) to insert_node, you need to dereference it to assign to it.
You malloc the size of a struct node* (a ListNode) but you need to malloc the size of a struct node.
You also need to do ptr = ptr->link in the loop in main.

Example:

void insert_node(ListNode* head, uint32_t size) { // corrected malloc, you don't want the sizeof a pointer but the // size of a `node`: ListNode new_node = malloc(sizeof *new_node); new_node->size = size; new_node->link = NULL; if (*head == NULL) { // corrected check (dereference head) *head = new_node; // corrected assignment } else { ListNode current = *head; while (current->link != NULL) { current = current->link; } current->link = new_node; } } int main() { ListNode head = NULL; insert_node(&head, 10); insert_node(&head, 20); // the below loop had no exit condition before: for(ListNode ptr = head; ptr; ptr = ptr->link) { printf("%d ", ptr->size); } printf("\n"); }

Demo

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