The upstream code does not make any sense to me:
$ cat ImfDeepScanLineOutputFile.cpp
[...]
Int64 packedSampleCountSize = *(Int64 *) (&data[4]); // ???
Int64 packedDataSize = *(Int64 *) (&data[12]);
Int64 unpackedDataSize = *(Int64 *) (&data[20]);
const char * sampleCountTable = (char*)&data[0]+28;
[...]