code: protected long checkRegions(long offset, int length) throws IOException {
          if (offset + length > filledUpTo)
              throw new IOException("Can't access memory outside the file size (" + filledUpTo
                      + " bytes). You've requested portion " + offset + "-" + (offset + length) + " bytes. File: "
                      + osFile.getAbsolutePath() + "\n" + "actual Filesize: " + size + "\n" + "actual filledUpTo: "
                      + filledUpTo);
          return offset;
      }
docs: checks if the accessed region is accessible
  
  @param offset
  the byte-offset where to start
  @param length
  the length of the region to access
  @return the offset, if the region is accessible
  @throws IOException