When the door turns weird, scanf

#include <stdio.h>
#include <stdlib.h>
#defineMAX_STACK_SIZE 100 // Declare Global Variables
char stack[MAX_STACK_SIZE];
int top = -1;        
void stackFull() {
    fprintf(stderr, "Stack is full, cannot add element");
    exit(1);
}
void stackEmpty() {
    fprintf(stderr, "Stack is Empty");
    exit(1);
}
void push(char a)
{
    if (top >= MAX_STACK_SIZE - 1)
        stackFull();
        stack[++top] = a;  
}
char pop(char a)
{
    if (top == -1)
        stackEmpty();
    return stack[top--];
}
void showStack() // output values in the stack
{
    int i;
    if (top == -1) printf ("No value stored in the stack").\n");
    else {
        printf ("List of values stored in the stack\n");
        for (i = 0; i < top+1; i++) {
            printf("%03d : %c\n", i + 1, stack[i]);
        }
        printf("%d values are stored.\n",top);
    }
}
void main()
{
    int select=0;
    char a;
    while (select != 4) {
        printf("\n1. Put value in stack\n");
        printf("2. Pull value from stack\n");
        printf("3. Check the value stored in the stack\n");
        printf("4. Exit\n\n");
        printf ("Enter : ");
        scanf_s("%d", &select); 
        switch (select) {
        Case 1: // Save the entered value to the stack.
            printf("Enter a value to save: ");"
            a = getchar();
            push(a);
            break;
        Case 2: // Gets the integer value stored in the stack.
            printf("Import value : %c", pop(a));
            break;
        Case 3: // shows the values stored in the stack.
            showStack();
            break;
        }
    }
}