C语言在循环链表更新项目

#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;

}

void update_data(int old, int new) {
   int pos = 0;

   if(head==NULL) {
      printf(Linked List not initialized);
      return;
   } 

   current = head;

   while(current->next!=head) {
      if(current->data == old) {
         current->data = new;
         printf(\n%d found at position %d, replaced with %d\n, old, pos, new);
         return;
      }
      current = current->next;
      pos++;
   }

   if(current->data == old) {
      current->data = new;
      printf(\n%d found at position %d, replaced with %d\n, old, pos, new);
      return;
   }      

   printf(%d does not exist in the list\n, old);
}

//display the list
void printList() {
   struct node *ptr = head;

   printf(\n[head] =>);

   //start from the beginning
   while(ptr->next != head) {        
      printf( %d =>,ptr->data);
      ptr = ptr->next;
   }

   printf( %d =>,ptr->data);
   printf( [head]\n);
}

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

   printList();
   update_data(56,11);
   printList();

   return 0;
}