C语言更新双向链表中的数据

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

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

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

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

   printf("/n[head] <=>");
   //start from the beginning
   while(ptr != NULL) {        
      printf(" %d <=>",ptr->data);
      ptr = ptr->next;
   }

   printf(" [last]/n");
}

//Create Linked List
void insert(int data) {
   // Allocate memory for new node;
   struct node *link = (struct node*) malloc(sizeof(struct node));

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

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

   current = head;

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

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

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

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

   current = head;
   while(current!=NULL) {
      pos++;

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

      if(current->next != NULL)
         current = current->next;
      else
         break;
   }
   printf("%d does not exist in the list/n", old);
}

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

   printList();
   update_data(56, 99);   
   printList();

   return 0;
}