Introduce the is_empty function

step-3-is_empty.c

/* int_stack.h */

#include <stdbool.h>

typedef struct int_node IntNode;
typedef struct int_stack {
    IntNode* top;
} IntStack;

// Other function declarations...
bool is_empty_stack(IntStack* stack);

/* main.c */
#include "int_stack.h"

// ...

int main()
{
    IntStack* stack = malloc(sizeof (IntStack));
    stack->top = NULL;
    for (int i = 5; i > 0; i--) {
        push_to_stack(stack, i);
    }
    printf("At the top of the stack is `%d`\n\n", peek_stack(stack));
    while (!is_empty_stack(stack)) {
        printf("`%d` was popped from the stack\n", pop_stack(stack));
    }
    free(stack); stack = NULL;
    return 0;
}

/* int_stack.c */
#include "int_stack.h"

// ...

struct int_node {
    int  value;
    struct int_node* next;
};

// Other definitions

bool is_empty_stack(IntStack* stack)
{
    return !stack->top;
}