Stack using pointer

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

#define MAX 50
int size;

// Defining the stack structure
struct stack {
    int arr[MAX];
    int top;
};

// Initializing the stack(i.e., top=-1)
void init_stk(struct stack *st)
{
    st->top = -1;
}

// Entering the elements into stack
void push(struct stack *st, int num)
{
    if (st->top == size - 1)
    {
        printf("\nStack overflow(i.e., stack full).");
        return;
    }
    st->top++;
    st->arr[st->top] = num;
}

//Deleting an element from the stack.
int pop(struct stack *st)
{
    int num;
    if (st->top == -1)
    {
        printf("\nStack underflow(i.e., stack empty).");
        return NULL;
    }
    num = st->arr[st->top];
    st->top--;
    return num;
}

void display(struct stack *st)
{
    int i;
    for (i = st->top; i >= 0; i--)
        printf("\n%d", st->arr[i]);
}

int main()
{
    int element, opt, val;
    struct stack ptr;
    init_stk(&ptr);
    printf("\nEnter Stack Size :");
    scanf("%d", &size);

    printf("\n\nSTACK PRIMITIVE OPERATIONS");
    printf("\n1.PUSH");
    printf("\n2.POP");
    printf("\n3.DISPLAY");
    printf("\n4.QUIT");
    printf("\n");

    while (1)
    {
        printf("\nEnter your option : ");
        scanf("%d", &opt);
        switch (opt)
        {
            case 1:
                printf("\nEnter the element into stack:");
                scanf("%d", &val);
                push(&ptr, val);
                break;
            case 2:
                element = pop(&ptr);
                printf("\nThe element popped from stack is : %d", element);
                break;
            case 3:
                printf("\nThe current stack elements are:");
                display(&ptr);
                break;
            case 4:
                exit(0);
            default:
                printf("\nEnter correct option!Try again.");
        }
    }
    return 0;
}

Output : 

Enter Stack Size :3


STACK PRIMITIVE OPERATIONS
1.PUSH
2.POP
3.DISPLAY
4.QUIT

Enter your option : 1

Enter the element into stack:10

Enter your option : 1

Enter the element into stack:30

Enter your option : 1

Enter the element into stack:20

Enter your option : 1

Enter the element into stack:12

Stack overflow(i.e., stack full).
Enter your option : 2

The element popped from stack is : 20
Enter your option : 3

The current stack elements are:
30
10
Enter your option : 4