struct list {
  list* prev;
  list* next;
}

struct element {
  int value;
  struct list* link;
}

void list_insert(list* list, list* element) {
    element->prev = list;
    element->next = list->next;
    list->next = element;
    element->next->prev = element;
}

// Initialize empty list
struct list mylist;
mylist.next = &mylist;
mylist.prev = &mylist;

// Initialize element
struct element e;
e.value = 42;

// Add element to list
list_insert(&mylist, &e);

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

// element within list
typedef struct element {
    int value;                          // data value
    struct element *next;               // pointer to next element in list
    struct element *prev;               // pointer to previous element in list
} element_t;

// list of elements
typedef struct list {
    element_t *head;                    // pointer to first element in list
    element_t *tail;                    // pointer to last element in list
} list_t;

// new_element -- get new element
element_t *
new_element(int value)
{
    element_t *newptr;

    newptr = calloc(1,sizeof(element_t));

    if (newptr == NULL) {
        printf("new_element: malloc failuren");
        exit(1);
    }

    newptr->value = value;

    return newptr;
}

// list_insert -- insert before list head
void
list_insert(list_t *list,element_t *newptr)
{
    element_t *head;

    head = list->head;

    newptr->prev = NULL;
    newptr->next = head;

    // insert at head of empty list
    if (head == NULL)
        list->tail = newptr;

    // insert at head of non-empty list
    else {
        newptr->next = head;
        head->prev = newptr;
    }

    // make new element the head of the list
    list->head = newptr;
}

// list_insert_unique -- insert before list head
void
list_insert_unique(list_t *list,element_t *newptr)
{
    element_t *curptr;

    // search for pre-existing element with the same value
    for (curptr = list->head;  curptr != NULL;  curptr = curptr->next) {
        if (curptr->value == newptr->value)
            break;
    }

    // only insert if we did _not_ find one
    if (curptr == NULL)
        list_insert(list,newptr);
    else
        free(newptr);
}

// list_print -- print the list
void
list_print(list_t *list)
{
    element_t *curptr;

    printf("list_print:");

    // search for pre-existing element with the same value
    for (curptr = list->head;  curptr != NULL;  curptr = curptr->next)
        printf(" %d",curptr->value);

    printf("n");
}

// list_print -- print the list in reverse order
void
list_rprint(list_t *list)
{
    element_t *curptr;

    printf("list_rprint:");

    // search for pre-existing element with the same value
    for (curptr = list->tail;  curptr != NULL;  curptr = curptr->prev)
        printf(" %d",curptr->value);

    printf("n");
}

// main -- main program
int
main(void)
{
    list_t mylist;
    element_t *e;

    // initialize list
    mylist.head = NULL;
    mylist.tail = NULL;

    // Add unique element to list
    e = new_element(42);
    list_insert_unique(&mylist,e);

    e = new_element(23);
    list_insert_unique(&mylist,e);

    e = new_element(17);
    list_insert_unique(&mylist,e);

    // try to insert a duplicate
    e = new_element(17);
    list_insert_unique(&mylist,e);

    // try to insert a duplicate
    e = new_element(42);
    list_insert_unique(&mylist,e);

    // print list in order [checks the head/next pointers]
    list_print(&mylist);

    // print list in reverse order [checks the tail/prev pointers]
    list_rprint(&mylist);

    return 0;
}