Stack and Queue

#include <stdio.h>
#include <stdlib.h>
typedef struct list_t list_t;
struct list_t {
  int data;
  list_t *link;
};
list_t *new_list(int data) {
  list_t *new = (list_t *) malloc(sizeof(list_t));
  new->data = data;
  new->link = NULL;
  return new;
}
list_t *top = NULL;
void push(int data) {
  list_t *new = new_list(data);
  if (!top) {
    top = new;
    return;
  }
  new->link = top;
  top = new;
}
int pop(void) {
  int data;
  if (!top) {
    fprintf(stderr, "can't pop\n");
    return -1;
  }
  data = top->data;
  top = top->link;
  return data;
}
list_t *head = NULL, *tail = NULL;
void enqueue(int data) {
  list_t *new = new_list(data);
  if (!head) head = new;
  if (!tail) tail = new;
  head->link = new;
  head = new;
}
int dequeue(void) {
  int data;
  if (!tail) {
    fprintf(stderr, "can't dequeue\n");
    return -1;
  }
  data = tail->data;
  tail = tail->link;
  return data;
}
int main(void) {
  int i;
  for (i = 0; i < 5; i++) push(i);
  for (i = 0; i < 6; i++) printf("%d, ", pop());
  puts("");
  for (i = 0; i < 5; i++) enqueue(i);
  for (i = 0; i < 6; i++) printf("%d, ", dequeue());
  puts("");
  return 0;
}