Implementation of a XOR-Doubly linked list

xorlist.c

/*
 * Basic XORLIST-Implementation.
 * (c) 2019 Lennart Espe. All rights reserved.
 */
#include <stdlib.h>
#include <stdio.h>

struct item {
    uintptr_t both;
    int value;
};

struct list {
    struct item *head, *end;
    size_t size;
};

struct list *list_new() {
    return calloc(1, sizeof(struct list));
}

void list_add(struct list *list, int value) {
    struct item *item = malloc(sizeof(struct item));
    item->value = value;
    item->both = (uintptr_t) list->end;

    if (!list->head) list->head = item;
    if (list->end) list->end->both ^= (uintptr_t) item;
    list->end = item;
    list->size++;
};

int list_get(struct list *list, size_t index) {
    uintptr_t active = (uintptr_t) list->head,
              prev = (uintptr_t) NULL,
              tmp = (uintptr_t) NULL;
    for (size_t i = 0; i < index; i++) {
        tmp = active;
        active = ((struct item *) active)->both ^ prev;
        prev = tmp;
    }
    return ((struct item *) active)->value;
}

int main() {
    struct list *list = list_new();
    list_add(list, 12);
    list_add(list, 15);
    list_add(list, 30);

    for (size_t i = 0; i < list->size; i++) {
        printf("%d\n", list_get(list, i));
    }
}