code: public long getHardLinkId(String src) throws IOException {
      byte[][] components = INode.getPathComponents(src);
      readLock();
      try {
        INodeFile node = this.getFileINode(components);
        if ((!exists(node)) || (!(node instanceof INodeHardLinkFile))) {
          throw new IOException(src + " is not a valid hardlink file");
        }
        return ((INodeHardLinkFile) node).getHardLinkID();
      } finally {
        readUnlock();
      }
    }
docs: Retrieves a the hardlink id for a given file.
  
  @param src
  the file to lookup
  @return the hardlink id
  @throws IOException
  if the specified file is not a valid hardlink file