Add the given digit to a number stored in a linked list

// C Program 
// Add the given digit to a number stored in a linked list
#include <stdio.h>
#include <stdlib.h> //for malloc function

// Linked List LinkNode
struct LinkNode
{
	int data;
	struct LinkNode *next;
};
// Singly linked list 
struct SingleLL
{
	struct LinkNode *head;
	struct LinkNode *tail;
};
// Returns the new linked list
struct SingleLL *newLinkedList()
{
	// Create memory of head and tail Nodes
	struct SingleLL *sll = (struct SingleLL *) malloc(sizeof(struct SingleLL));
	if (sll == NULL)
	{
		printf("Memory overflow\n");
	}
	else
	{
		sll->head = NULL;
	}
	return sll;
}
// Returns a new Node of linked list
struct LinkNode *createLinkNode(int data)
{
	// Create dynamic node
	struct LinkNode *node = (struct LinkNode *) malloc(sizeof(struct LinkNode));
	if (node == NULL)
	{
		printf("Memory overflow\n");
	}
	else
	{
		// Set initial node value
		node->data = data;
		node->next = NULL;
	}
	return node;
}
// Add new Node at end of linked list 
void addNode(struct SingleLL *sll, int data)
{
	if (data < 0)
	{
		printf("\n Number is negative : %d \n", data);
		return;
	}
	else if (data > 9)
	{
		printf("\n Two digit number is not allowed : %d \n", data);
		return;
	}
	struct LinkNode *node = createLinkNode(data);
	if (sll->head == NULL)
	{
		sll->head = node;
	}
	else
	{
		struct LinkNode *temp = sll->head;
		// Find last node
		while (temp->next != NULL)
		{
			temp = temp->next;
		}
		// Append the node at last position
		temp->next = node;
	}
}
// Display linked list element
void display(struct SingleLL *sll)
{
	if (sll->head == NULL)
	{
		printf("\n Empty linked list\n");
		return;
	}
	struct LinkNode *temp = sll->head;
	// iterating linked list elements
	while (temp != NULL)
	{
		if (temp != sll->head)
		{
			printf("→");
		}
		printf(" %d ", temp->data);
		// Visit to next node
		temp = temp->next;
	}
	printf("→ NULL\n");
}
// Add a single digit, in number represent of linked list
void addDigit(struct SingleLL *sll, int digit)
{
	if (digit < 0 || digit > 9)
	{
		printf("\n Invalid digit : %d\n", digit);
		return;
	}
	else if (sll->head == NULL)
	{
		printf("\n Empty linked list\n");
		return;
	}
	struct LinkNode *temp = sll->head;
	struct LinkNode *auxiliary = NULL;
	// Find last node and 
	// Detect last node which is contain value of less than 9, 
	// If it is exist
	while (temp->next != NULL)
	{
		if (temp->data < 9)
		{
			auxiliary = temp;
		}
		// Visit to next node
		temp = temp->next;
	}
	temp->data = temp->data + digit;
	if (temp->data > 9)
	{
		// When need to update linked list
		if (auxiliary == NULL)
		{
			// When auxiliary is NULL then assign first node to this
			auxiliary = sll->head;
			// Create a new node
			struct LinkNode *node = createLinkNode(1);
			// Add node at begining
			node->next = sll->head;
			// Make new head
			sll->head = node;
		}
		// Manage last node
		temp->data = temp->data % 10;
		// Update the node value from auxiliary to second last node
		while (auxiliary->next != NULL)
		{
			auxiliary->data = (auxiliary->data + 1) % 10;
			// Visit to next node
			auxiliary = auxiliary->next;
		}
	}
}
int main()
{
	// Create a empty linked list
	struct SingleLL *sll = newLinkedList();
	//  Constructed linked list
	//  9 → 9 → 9 → 8 → 9 → NULL
	addNode(sll, 9);
	addNode(sll, 9);
	addNode(sll, 9);
	addNode(sll, 8);
	addNode(sll, 9);
	display(sll);
	int digit = 9;
	printf(" Add digit : %d \n", digit);
	addDigit(sll, digit);
	display(sll);
	digit = 6;
	printf(" Add digit : %d \n", digit);
	addDigit(sll, digit);
	display(sll);
	digit = 5;
	printf(" Add digit : %d \n", digit);
	addDigit(sll, digit);
	display(sll);
	return 0;
}

Output

 9 → 9 → 9 → 8 → 9 → NULL
 Add digit : 9
 9 → 9 → 9 → 9 → 8 → NULL
 Add digit : 6
 1 → 0 → 0 → 0 → 0 → 4 → NULL
 Add digit : 5
 1 → 0 → 0 → 0 → 0 → 9 → NULL