1typedef struct node {
 2  struct node *next;
 3  void *data;
 4} Node;
 5
 6typedef struct {
 7  Node *head;
 8} Stack;
 9
10Stack *stack_init();
11void *stack_peek(Stack *self);
12int stack_size(Stack *self);
13void stack_push(Stack *self, void *data);
14void *stack_pop(Stack *self);