code: private long readSubheaderSignature(Long subheaderPointerOffset) throws IOException {
          int intOrLongLength = sasFileProperties.isU64() ? BYTES_IN_LONG : BYTES_IN_INT;
          Long[] subheaderOffsetMass = {subheaderPointerOffset};
          Integer[] subheaderLengthMass = {intOrLongLength};
          List<byte[]> subheaderSignatureMass = getBytesFromFile(subheaderOffsetMass,
                  subheaderLengthMass);
          return bytesToLong(subheaderSignatureMass.get(0));
      }
docs: The function to read a subheader signature at the offset known from its ({@link SubheaderPointer}).
  
  @param subheaderPointerOffset the offset at which the subheader is located.
  @return - the subheader signature to search for in the {@link SasFileParser#SUBHEADER_SIGNATURE_TO_INDEX}
  mapping later.
  @throws IOException if reading from the {@link SasFileParser#sasFileStream} stream is impossible.