advance_position
static mblk_t *advance_position(mblk_t *, off_t, uchar_t **);
mp1 = advance_position(mp, input->cd_offset, &start);
mp2 = advance_position(mp, input->cd_offset +
mp1 = advance_position(mp, input->cd_offset, &start);
mp2 = advance_position(mp, input->cd_offset + input->cd_length,
mp1 = advance_position(mp, out->cd_offset, &start);
mp2 = advance_position(mp, out->cd_offset +