mp4ff_set_sample_position(c->mp4ff, c->track, chunk_num);
offset = c->fpos;
ss = mp4ff_read_sample_getsize(c->mp4ff, c->track, chunk_num);
mp4ff_set_sample_position(c->mp4ff, c->track, chunk_num);
offset = c->fpos;
ss = mp4ff_read_sample_getsize(c->mp4ff, c->track, chunk_num);