Linked List in C with Examples

typedef struct NODE {
                     /* link is a ptr to type struct NODE */
                     struct NODE *link; 
                     int data;
    } Node;     /* Node new type for above structure pattern */

/* create_sll.c -- program creates a singly linked list */
#include <stdio.h>
#include <stdlib.h>
 
/* symbolic constants declared */
#define INSERT 1
#define QUIT   2
 
/* structure declaration */
typedef struct  NODE {
                      struct NODE *link;
                      int value;
        } Node;
 
void insert_sll(Node **, const int);
 
int main(void)
{
    Node *root = 0;         /* root is set to NULL */
    Node **p2r = &root;     /* pointer-to-root */
    int value;
    int op;
 
    puts("\n**Let's create a Singly Linked List**\n");
    printf("User, enter 1 for INSERT and 2 for QUIT : ");
    while (1) {
        while (scanf("%d", &op) == 1 && (op == INSERT || op == QUIT )) {
 
            if (op == INSERT) {
                printf("User, enter an integer value: ");
                scanf("%d", &value);
                insert_sll(p2r, value);
            }
            else if (op == QUIT) {
                printf("Thank You!\n");
                return 0;
            }
 
            printf("\nWant to insert more integer values,\nenter 1 "
                   "for INSERT, else 2 for QUIT : ");
        }
            puts("Entered is a WRONG choice, enter 1 "
                 "for INSERT, 2 for QUIT");
    }
}
 
void insert_sll(Node **linkp, const int value)
{
    Node *current = 0;
    Node *newnode = 0;
 
    /* Let's create an ordered singly linked list */
    /* firstly, ensure if value isn't already in the list */
    /* if value is already in the list, we won't add duplicate */
    while ((current = *linkp) != NULL && current->value < value)
        linkp = &current->link;
 
    /* if value is already in the list */
    if (current != NULL && current->value == value) {
        printf("\n\aValue %d is already in the list.\n", value);
        return ;
    }
 
    /* value isn't already in the list */
    /* Let's allocate memory to newnode */
    newnode = (Node *)malloc(sizeof(Node));
 
    /* If memory allocated to newnode successfully */
    if (newnode == NULL) {
        printf("Not sufficient Memory!\n");
      exit(EXIT_FAILURE);
    }
    /* write in value in value field of newnode */
    newnode->value = value;
 
    /* insert newnode in the list */
    /* between current and previous */
    newnode->link = current;
    *linkp = newnode;
}