static uint64_t get_cluster_offset(BlockDriverState *bs,
                                   uint64_t offset, int *num)
{
    BDRVQcowState *s = bs->opaque;
    int l1_index, l2_index;
    uint64_t l2_offset, *l2_table, cluster_offset;
    int l1_bits, c;
    int index_in_cluster, nb_available, nb_needed, nb_clusters;
    index_in_cluster = (offset >> 9) & (s->cluster_sectors - 1);
    nb_needed = *num + index_in_cluster;
    l1_bits = s->l2_bits + s->cluster_bits;
    /* compute how many bytes there are between the offset and
     * the end of the l1 entry
     */
    nb_available = (1 << l1_bits) - (offset & ((1 << l1_bits) - 1));
    /* compute the number of available sectors */
    nb_available = (nb_available >> 9) + index_in_cluster;
    cluster_offset = 0;
    /* seek the the l2 offset in the l1 table */
    l1_index = offset >> l1_bits;
    if (l1_index >= s->l1_size)
        goto out;
    l2_offset = s->l1_table[l1_index];
    /* seek the l2 table of the given l2 offset */
    if (!l2_offset)
        goto out;
    /* load the l2 table in memory */
    l2_offset &= ~QCOW_OFLAG_COPIED;
    l2_table = l2_load(bs, l2_offset);
    if (l2_table == NULL)
        return 0;
    /* find the cluster offset for the given disk offset */
    l2_index = (offset >> s->cluster_bits) & (s->l2_size - 1);
    cluster_offset = be64_to_cpu(l2_table[l2_index]);
    nb_clusters = size_to_clusters(s, nb_needed << 9);
    if (!cluster_offset) {
        /* how many empty clusters ? */
        c = count_contiguous_free_clusters(nb_clusters, &l2_table[l2_index]);
    } else {
        /* how many allocated clusters ? */
        c = count_contiguous_clusters(nb_clusters, s->cluster_size,
                &l2_table[l2_index], 0, QCOW_OFLAG_COPIED);
   nb_available = (c * s->cluster_sectors);
out:
    if (nb_available > nb_needed)
        nb_available = nb_needed;
    *num = nb_available - index_in_cluster;
    return cluster_offset & ~QCOW_OFLAG_COPIED;