C语言查找循环链表大小

#include <stdio.h>
#include <stdlib.h>

struct node {
   int data;
   struct node *next;
};

struct node *head = NULL;
struct node *current = NULL;

//insert link at the first location
void insert(int data) {
   // Allocate memory for new node;
   struct node *link = (struct node*) malloc(sizeof(struct node));

   link->data = data;
   link->next = NULL;

   // If head is empty, create new list
   if(head==NULL) {
      head = link;
      head->next = link;
      return;
   }

   current = head;

   // move to the end of the list
   while(current->next != head)
      current = current->next;

   // Insert link at the end of the list
   current->next = link;

   // Link the last node back to head
   link->next = head;

}

//display the list
void list_size(struct node *list) {
   int size = 0;

   if(head==NULL) {
      printf(List size : %d , size);
      return;
   } 

   current = head;
   size = 1;

   while(current->next != head) {
      current = current->next;
      size++;
   }

   printf(List size : %d , size);   
}

int main() {
   insert(10);
   insert(20);
   insert(30);
   insert(1);
   insert(40);
   insert(56); 

   list_size(head);

   return 0;
}