tail_add.c

int list_add_tail(void *data)
{
    struct linked_list *t;

    t = malloc(sizeof(struct linked_list));
    if (!t) {
        return -1;
    }

    t->data = data;

    if (!head) { // no head element yet..
        head = t;
        tail = t;
    } else { // add it at the tail..
        tail->next = t;
        tail = t;
    }

    return 0;
}