Singly Linked List in C Programming

/* create_sll.c -- program creates a singly linked list */
#include <stdio.h>
#include <stdlib.h>
 
#define TRAVERSE 1
#define INSERT 2
#define QUIT   3
 
/* structure declaration */
typedef struct  NODE {
                      struct NODE *link;
                      int value;
        } Node;
 
void insert_sll(Node **, const int);
void traverse_sll(Node **);
 
int main(void)
{
    Node *root = 0;         /* root is 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 TRAVERSE, 2 FOR INSERT and 3 for QUIT : ");
    while (1) {
        while (scanf("%d", &op) == 1 &&
              (op == INSERT || op == QUIT || op == TRAVERSE)) {
 
            if (op == TRAVERSE) {
                traverse_sll(p2r);
            }
            else if (op == INSERT) {
                printf("User, enter an integer value: ");
                scanf("%d", &value);
                insert_sll(p2r, value);
            }
            else if (op == QUIT) {
                printf("\nThank You!\n");
                return 0;
            }
 
            printf("\nWant to do some more operations,\nenter 1 "
                   "for TRAVERSE, 2 for INSERT and 3 for QUIT : ");
        }
            printf("\n\aEntered is a WRONG choice,\nenter 1 "
                   "for TRAVERSE, 2 for INSERT, 3 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 newnde */
    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;
}
 
void traverse_sll(Node **linkp)
{
    Node *current = *linkp;
 
    /* traverse the list */
    if (current == NULL) {
        printf("\nList is EMPTY.\n");
    }
    else {
        printf("\nList contains values : ");
        while (current != NULL) {
            printf("%d ", current->value);
            current = current->link;
       }
        printf("\n");
    }
}