C语言删除循环链表中的数据项

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

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

struct node *head = NulL;
struct node *current = NulL;
struct node *prev = 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 remove_data(int data) {
   int pos = 0;

   if(head==NulL) {
      printf("linked List not initialized");
      return;
   } 

   if(head->data == data) {
      if(head->next != head) {
         current = head;
         while(current->next!=head) {
            current = current->next;
         }

         current->next = head->next;
         head = head->next;
         return;
      } else {
         head = NulL;
         printf("List is empty\n");
         exit(0);
      }
   } else if(head->data != data && head->next == NulL) {
      printf("%d not found in the List\n", data);
      return;
   }

   current = head;

   while(current->next != head && current->data != data) {
      prev = current;
      current = current->next;
   }        

   if(current->data == data) {
      prev->next = prev->next->next;
      free(current);
   } else
      printf("%d not found in the List.", data);
}

//display the List
voID printList() {
   struct node *ptr = head;

   printf("
[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); 

   printf("Original List -");
   printList();
   remove_data(1);

   printf("List after removal -");
   printList();

   return 0;
}