offset &= chanb->buf_size - 1;
sbidx = offset >> chanb->subbuf_size_order;
id = shmp_index(handle, bufb->buf_wsb, sbidx)->id;
offset &= chanb->buf_size - 1;
sbidx = offset >> chanb->subbuf_size_order;
id = shmp_index(handle, bufb->buf_wsb, sbidx)->id;