A Left Node Right (LNR) linked list - Learning note

/*  
A Left Node Right (LNR) linked list.
*/
#include 
#include 

struct linked_list {
 double item;
 struct linked_list *left;
 struct linked_list *right;
};

void insert_left(struct linked_list **main_list, double item_to_insert)
{
 struct linked_list *new_item;
 new_item = malloc(sizeof(struct linked_list));
 if (new_item == NULL) {
  fprintf(stderr, "Out of memory: insert_left\n");
  exit(1);
 }
 new_item->item = item_to_insert;
 if (*main_list == NULL) {
  new_item->left = NULL;
  new_item->right = NULL;
  *main_list = new_item;
 } else {
  new_item->left = (*main_list)->left;
  new_item->right = NULL;
  (*main_list)->left = new_item;
 }
}

void insert_right(struct linked_list **main_list, double item_to_insert)
{
 struct linked_list *new_item;
 new_item = malloc(sizeof(struct linked_list));
 if (new_item == NULL) {
  fprintf(stderr, "Out of memory: insert_right\n");
  exit(2);
 }
 new_item->item = item_to_insert;
 if (*main_list == NULL) {
  new_item->left = NULL;
  new_item->right = NULL;
  *main_list = new_item;
 } else {
  new_item->left = NULL;
  new_item->right = (*main_list)->right;
  (*main_list)->right = new_item;
 }

}

void print_LNR(struct linked_list *main_list)
{
 if (main_list == NULL)
  return;
 print_LNR(main_list->left);
 printf("%f\n", main_list->item);
 print_LNR(main_list->right);

}

void print_NLR(struct linked_list *main_list)
{
 if (main_list == NULL)
  return;
 printf("%f\n", main_list->item);
 print_NLR(main_list->left);
 print_NLR(main_list->right);
}

void print_LRN(struct linked_list *main_list)
{
 if (main_list == NULL)
  return;
 print_LRN(main_list->left);
 print_LRN(main_list->right);
 printf("%f\n", main_list->item);
}

void free_LRN(struct linked_list *main_list)
{
 if (main_list == NULL)
  return;
 free_LRN(main_list->left);
 free_LRN(main_list->right);
 /*  printf("%f\n",main_list->item);   */
 free(main_list);
 main_list = NULL;

}

int main()
{
 struct linked_list *main_list = NULL;
 insert_right(&main_list, 0.0);
 insert_left(&main_list, -5);
 insert_right(&main_list, 5);

 print_LRN(main_list);
 free_LRN(main_list);

 return 0;
}