Creating pointer to a typedef stack - c

Question

I am not able to initialize all three pointers to struct S, and I don't know why.
I am using a fixed-length array as stack to store values.
The header file is created this way to hide information (struct S), and should be kept as generic as possible.
main.c
// main.c
#include <stdio.h>
#include "stack_exercise4.h"
int main(void) {
Stack *stack_1, *stack_2, *stack_3;
int a, b;
make_empty(stack_1);
make_empty(stack_2);
make_empty(stack_3);
return 0;
}
Problem is, after Stack *stack_1, *stack_2, *stack_3, only stack_2 has a valid address for Struct stack. stack_1 and stack_3 have some strange looking addresses, and I can't assign any values to stack_1->top, nor stack_3->top. What is the problem?
header file
// stack_exercise4.h
#ifndef STACK_EXERCISE4_H
#define STACK_EXERCISE4_H
#include <stdbool.h> /* C99 only */
typedef struct S Stack; /* incomplete type to hide the content
of S. */
void make_empty(Stack *s);
bool is_empty(const Stack *s);
bool is_full(const Stack *s);
void push(Stack *s, int i);
int pop(Stack *s);
#endif
stack source file
// stack_exercise4a.c
#include "stack_exercise4.h"
#include <stdio.h>
#define MAX_STACK_SIZE (10)
struct S {
int top;
int contents[MAX_STACK_SIZE];
};
void make_empty(Stack *s) {
s->top = 0;
}
bool is_empty(const Stack *s) {
return (s->top <= 0);
}
bool is_full(const Stack *s) {
return (s->top >= MAX_STACK_SIZE - 1);
}
void push(Stack *s, int i) {
if (!is_full(s)){
(s->contents)[s->top++] = i;
} else {
printf("Failed to push, Stack is full.\n");
}
}
int pop(Stack *s) {
return (s->contents)[s->top--];
}

Answer 1

The stack pointers must point on memory spaces before being dereferenced in make_empty(). Something like this could be the starting point: make_empty() allocates the memory space.
void make_empty(Stack **s) {
(*s) = (struct S *)malloc(sizeof(struct S));
(*s)->top = 0;
}
And so the initialization of the pointers would be:
make_empty(&stack_1);
make_empty(&stack_2);
make_empty(&stack_3);

Answer 2

Declare stack_X on stack instead.
#include <stdio.h>
#include "stack_exercise4.h"
int main(void) {
Stack stack_1 = {0}, stack_2 = {0}, stack_3 = {0};
int a, b;
make_empty(&stack_1);
make_empty(&stack_2);
make_empty(&stack_3);
return 0;
}
Otherwise, I't would need to have constructor/destructor for your Stack data structure e.g new_stack(Stack *ptr) del_stack(Stack *ptr). For beginner, I would recommend to use stack instead of heap (stay away from malloc).