second stack inside reverse function with pointers warning - c

Question

I have the following stack implementation using doubly linked list, and I want to use a second stack inside the reverse function but I get errors. how to do that, let's say I want to return s2.
#include <stdio.h>
#include <stdlib.h>
typedef struct Node {
int data;
struct Node* prev;
struct Node* next;
} Node;
//---------------------Stack---------------------
typedef struct Stack {
int size;
Node* head;
Node* tail;
int top;
} Stack;
const Stack stack_init = { .size = 0, .head = NULL, .tail = NULL, .top = -1 };
Node* create_node(int elm) {
Node* node = malloc(sizeof * node);
if (!node) return node;
node->data = elm;
node->prev = NULL;
node->next = NULL;
return node;
}
int is_empty_s(Stack *s) {
return s->tail == NULL;
}
void push(Stack *s, int elm) {
Node* updated_head = create_node(elm);
if (!s->head) {
s->head = updated_head;
s->tail = s->head;
} else {
updated_head->next = s->head;
s->head->prev = updated_head;
s->head = updated_head;
}
s->size++;
s->top = s->head->data;
}
int pop(Stack *s) {
if (!is_empty_s(s)) {
Node* node = s->head;
int elm = node->data;
s->head = s->head->next;
if (s->head) {
s->head->prev = NULL;
s->top = s->head->data;
}
else {
s->tail = NULL;
s->top = -1;
}
s->size--;
free(node);
return elm;
}
}
Stack* reverse_s(Stack *s) { // iterative: using another stack, queue
Stack *s2 = stack_init;
while (s->tail) {
push(s2, pop(s));
}
return s2;
}
int main() {
Stack s1 = stack_init;
// Queue queue1 = queue_init; { .size = 0, .head = NULL, .tail = NULL, .front = -1 };
push(&s1, 5);
push(&s1, 4);
return 0;
}
As you can see the reverse function is not yet completed, I am new to C and this syntax I don't know how to handle it.

Answer 1

In reverse_s, s2 is of type pointer-to-Stack. stack_init has the structure type Stack. The assignment of a Stack value to a Stack * variable is incompatible.
A few options for reverse_s:
Return a Stack structure.
Stack reverse_s(Stack *s) {
Stack s2 = stack_init;
while (s->tail)
push(&s2, pop(s));
return s2;
}
Return a pointer-to-Stack, dynamically allocating memory for the structure.
Stack *reverse_s(Stack *s) {
Stack *s2 = malloc(sizeof *s2);
*s2 = stack_init;
while (s->tail)
push(s2, pop(s));
return s2;
}
Modify the original structure.
void reverse_s(Stack *s) {
Stack s2 = stack_init;
while (s->tail)
push(&s2, pop(s));
memcpy(s, &s2, sizeof *s);
}