GF_Err gf_isom_modify_cts_offset(GF_ISOFile *the_file, u32 trackNumber, u32 sample_number, u32 offset)
{
	GF_TrackBox *trak = gf_isom_get_track_from_file(the_file, trackNumber);
	if (!trak) return GF_BAD_PARAM;
	if (!trak->Media->information->sampleTable->CompositionOffset) return GF_BAD_PARAM;
	if (!trak->Media->information->sampleTable->CompositionOffset->unpack_mode) return GF_BAD_PARAM;
	/*we're in unpack mode: one entry per sample*/
	trak->Media->information->sampleTable->CompositionOffset->entries[sample_number - 1].decodingOffset = offset;
	return GF_OK;
}