Introduce the peek function

step3-peek.c

/* int_stack.h */

// ...

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

// Other function declarations...
int peek_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 (stack->top) {
        printf("`%d` was popped from the stack\n", pop_stack(stack));
    }
    free(stack); stack = NULL;
    return 0;
}

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

// Other definitions

int peek_stack(IntStack* stack)
{
    return stack->top->value;
}