Separate declarations and definitions with forward declarations

step1.c

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

typedef struct int_node IntNode; // Forward declaration for struct int_node

typedef struct int_stack {
    IntNode* top;
} IntStack;

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

static void push_to_stack(IntStack* stack, int value); // Forward declaration
static int pop_stack(IntStack* stack); // Forward declaration
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", stack->top->value);
    while (stack->top) {
        printf("`%d` was popped from the stack\n", pop_stack(stack));
    }
    free(stack); stack = NULL;
    return 0;
}

void push_to_stack(IntStack* stack, int value)
{
    // Implementation...
}

int pop_stack(IntStack* stack)
{
    // Implementation...
}