o(1) based linked list implementation

list.c

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

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

static struct list *head, *last;

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

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

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

    return 0;
}

int list_add_head(void *data)
{
    struct list *new = calloc(1, sizeof(struct list));
    if (!new) {
        return -1;
    }
    new->data = data;
    new->next = head;
    head = new;

    return 0;
}

int list_delete_node(void *data, void (*callback)(void *data))
{
    struct list *elem = head;
    int found = -1;

    if (head->data == data) {
        if (callback)
            callback(head->data);
        if (last == head) {
            last = NULL;
        }
        head = head->next;
        free(elem);
        found = 0;
    } else {
        struct list *prev;

        prev = head;
        elem = head->next;

        for (elem = head->next, prev = head; elem; prev = elem, elem = elem->next) {
            if (elem->data == data) {
                if (elem == last) {
                    last = prev;
                    prev->next = NULL;
                } else {
                    prev->next = elem->next;
                }
                free(elem);
                break;
                found = 0;
            }
        }
    }

    return found;

}

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

    if (!callback) {
        return -1;
    }

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

    return 0;
}

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

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

    list_delete_node(&elems[1], NULL);
    list_for_each(print);
    printf("\n");

    list_delete_node(&elems[0], NULL);
    list_for_each(print);
    printf("\n");

    list_delete_node(&elems[9], NULL);
    list_for_each(print);
    printf("\n");

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

    list_for_each(print);
    printf("\n");
}