_STACK *sk_dup(_STACK *sk)
{
    _STACK *ret;
    char **s;

    if ((ret = sk_new(sk->comp)) == NULL)
        goto err;
    s = OPENSSL_realloc((char *)ret->data,
                        (unsigned int)sizeof(char *) * sk->num_alloc);
    if (s == NULL)
        goto err;
    ret->data = s;

    ret->num = sk->num;
    memcpy(ret->data, sk->data, sizeof(char *) * sk->num);
    ret->sorted = sk->sorted;
    ret->num_alloc = sk->num_alloc;
    ret->comp = sk->comp;
    return (ret);
 err:
    sk_free(ret);
    return (NULL);
}