C语言更新单链表中的数据项

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

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

struct node *head = NulL;
struct node *current = NulL;

//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->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; 
}

voID display() {
   struct node *ptr = head;

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

   printf(" [null]\n");
}

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

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

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

      current = current->next;
      pos++;
   }

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

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

   display();
   update_data(40, 44);
   display();

   return 0;
}