Linked lists - Insert at

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

typedef struct _item {
  int value;
  struct _item* next;
} Item;

void print_list(Item* head);

// Aggiunge alla fine
Item* append(Item* head, int value);

// Aggiunge all'inizio
Item* prepend(Item* head, int value);

// Aggiunge all'indirizzo
Item* insert_at(Item* head, int value, size_t index);

// Adds element without ruining sorting property
// Only works for previusly sorted arrays
// Example:
// -> 15
// 1 9 10 (15) 29 32
Item* insert_sorted(Item* head, int value);

int main() {
  // ...
  // Insert solution here
  // ...

  Item* list = NULL;
  list = insert_at(list, 1, 0);
  print_list(list);

  list = insert_at(list, 2, 1);
  print_list(list);

  list = insert_at(list, 3, 0);
  print_list(list);

  list = insert_at(list, 4, 1);
  print_list(list);

  list = insert_at(list, 5, 1);
  print_list(list);

  list = insert_at(list, 6, 1);
  print_list(list);

  return 0;
}

// Aggiunge alla fine
Item* append(Item* head, int value) { return insert_at(head, value, INT_MAX); }

// Aggiunge all'inizio
Item* prepend(Item* head, int value) { return insert_at(head, value, 0); }

// Aggiunge all'indirizzo
Item* insert_at(Item* head, int value, size_t index) {
  // Se head è null, quindi se non è definita una lista
  if (head == NULL) {
    Item* t_head = (Item*)malloc(sizeof(Item));
    t_head->value = value;
    t_head->next = NULL;

    // Returning new head to be replaced
    return t_head;
  }

  if (index == 0) {
    Item* t_head = (Item*)malloc(sizeof(Item));
    t_head->value = value;
    t_head->next = head;

    // Returning new head to be replaced
    return t_head;
  }

  size_t position = 0;
  Item* _iterator = head;

  // Finds the position for the item to be inserted
  while (_iterator->next != NULL && ++position < index) {
    _iterator = _iterator->next;
  }

  // _iterator -> next == NULL or position == index
  if (_iterator->next == NULL) {
    _iterator->next = (Item*)malloc(sizeof(Item));
    _iterator->next->value = value;
    _iterator->next->next = NULL;

    return head;
  }

  // _iterator -> next != NULL
  Item* old_next = _iterator->next;
  _iterator->next = (Item*)malloc(sizeof(Item));
  _iterator->next->value = value;
  _iterator->next->next = old_next;

  return head;
}

void print_list(Item* head) {
  for (Item* _iterator = head; _iterator != NULL; _iterator = _iterator->next) {
    printf("%d -> ", _iterator->value);
  }

  printf("NULL\n");
}