o(1) based double linked list implementation

double_list.c

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

struct list {
    void *data;
    struct list *prev, *next;
};

static struct list *head, *last;

void list_init()
{
    head = last = NULL;
}

int list_add(void *data)
{
    struct list *elem = calloc(1, sizeof(struct list));
    if (!elem) {
        return-1;
    }
    elem->data = data;

    if (!head) {
        head = elem;
        last = elem;
    } else {
        last->next = elem;
        elem->prev = last;
        last = elem;
    }

    return 0;
}

int list_for_each_front(void (*callback)(void *data))
{
    struct list *elem;

    if (!callback)
        return -1;

    for (elem = head; elem; elem = elem->next) {
        callback(elem->data);
    }

    return 0;
}

int list_for_each_back(void (*callback)(void *data))
{
    struct list *elem;

    if (!callback)
        return -1;

    for (elem = last; elem != NULL; elem = elem->prev) {
        callback(elem->data);
    }

    return 0;
}

void list_delete(void *data)
{
    struct list *elem = head;

    if (head->data == data) {
        if (last == head) {
            last = NULL;
        }
        head = head->next;
        head->prev = NULL;
        free(elem);
    } else {
        for (elem = head->next; elem; elem = elem->next) {
            if (elem->data == data) {
                if (elem == last) {
                    last = elem->prev;
                    elem->prev->next = NULL;
                } else {
                    elem->prev->next = elem->next;
                    elem->next->prev = elem->prev;
                }
                free(elem);
                break;
            }
        }
    }
}

void print(void *data)
{
    printf("%d ", *(int *)(data));
}

int main()
{
    int elems[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
    int i;

    for (i = 0; i < 10; i ++) {
        list_add(&elems[i]);
    }

    list_for_each_front(print);
    printf("\n");
    list_for_each_back(print);
    printf("\n");

    list_delete(&elems[0]);
    list_for_each_front(print);
    printf("\n");
    list_for_each_back(print);
    printf("\n");

    list_delete(&elems[4]);
    list_for_each_front(print);
    printf("\n");
    list_for_each_back(print);
    printf("\n");

    list_delete(&elems[9]);
    list_for_each_front(print);
    printf("\n");
    list_for_each_back(print);
    printf("\n");
    return 0;
}