diff --git a/src/sfs.rs b/src/sfs.rs
index efd0dad..4c4d5f2 100644
--- a/src/sfs.rs
+++ b/src/sfs.rs
@@ -4,6 +4,7 @@ use core::cell::{RefCell, RefMut};
 use core::mem::{uninitialized, size_of};
 use core::slice;
 use core::fmt::{Debug, Formatter, Error};
+use core::any::Any;
 use dirty::Dirty;
 use structs::*;
 use vfs::{self, Device};
@@ -354,6 +355,25 @@ impl vfs::INode for INode {
 
         Ok(())
     }
+    fn link(&mut self, name: &str, other:&mut vfs::INode) -> vfs::Result<()> {
+        let fs = self.fs.upgrade().unwrap();
+        let info = self.info().unwrap();
+        assert_eq!(info.type_, vfs::FileType::Dir);
+        assert!(info.nlinks>0);
+        assert!(self.get_file_inode_id(name).is_none(), "file name exist");
+        let child = other.downcast_mut::<INode>().unwrap();
+        assert!(Rc::ptr_eq(&fs,&child.fs.upgrade().unwrap()));
+        assert!(child.info().unwrap().type_!=vfs::FileType::Dir);
+        let entry = DiskEntry {
+            id: child.id as u32,
+            name: Str256::from(name),
+        };
+        let old_size=self._size();
+        self._resize(old_size + BLKSIZE).unwrap();
+        self._write_at(old_size, entry.as_buf()).unwrap();
+        child.nlinks_inc();
+        Ok(())
+    }
     fn lookup(&self, path: &str) -> vfs::Result<Ptr<vfs::INode>> {
         let fs = self.fs.upgrade().unwrap();
         let info = self.info().unwrap();
@@ -390,6 +410,12 @@ impl vfs::INode for INode {
     fn fs(&self) -> Weak<vfs::FileSystem> {
         self.fs.clone()
     }
+    fn as_any_ref(&self) -> &Any {
+        self
+    }
+    fn as_any_mut(&mut self) -> &mut Any {
+        self
+    }
 }
 
 impl Drop for INode {
diff --git a/src/tests.rs b/src/tests.rs
index c550f2d..f2ac9f6 100644
--- a/src/tests.rs
+++ b/src/tests.rs
@@ -181,6 +181,21 @@ fn kernel_image_file_unlink(){
     sfs.sync().unwrap();
 }
 
+
+#[test]
+fn hard_link(){
+    let sfs = _create_new_sfs();
+    let root = sfs.root_inode();
+    let file1 = root.borrow_mut().create("file1", FileType::File).unwrap();
+    use core::ops::DerefMut;
+    root.borrow_mut().link("file2",file1.borrow_mut().deref_mut()).unwrap();
+    let file2 = root.borrow_mut().lookup("file2").unwrap();
+    file1.borrow_mut().resize(100);
+    assert_eq!(file2.borrow().info().unwrap().size,100);
+
+    sfs.sync().unwrap();
+}
+
 #[test]
 fn nlinks(){
     let sfs = _create_new_sfs();
@@ -192,8 +207,17 @@ fn nlinks(){
     let dir1 = root.borrow_mut().create("dir1", FileType::Dir).unwrap();
     assert_eq!(dir1.borrow().info().unwrap().nlinks,2);
     assert_eq!(root.borrow().info().unwrap().nlinks,3);
+    use core::ops::DerefMut;
+    dir1.borrow_mut().link("file2",file1.borrow_mut().deref_mut()).unwrap();
+    assert_eq!(dir1.borrow().info().unwrap().nlinks,2);
+    assert_eq!(root.borrow().info().unwrap().nlinks,3);
+    assert_eq!(file1.borrow().info().unwrap().nlinks,2);
     root.borrow_mut().unlink("file1").unwrap();
+    assert_eq!(file1.borrow().info().unwrap().nlinks,1);
+    assert_eq!(root.borrow().info().unwrap().nlinks,3);
+    dir1.borrow_mut().unlink("file2").unwrap();
     assert_eq!(file1.borrow().info().unwrap().nlinks,0);
+    assert_eq!(dir1.borrow().info().unwrap().nlinks,2);
     assert_eq!(root.borrow().info().unwrap().nlinks,3);
     root.borrow_mut().unlink("dir1").unwrap();
     assert_eq!(dir1.borrow().info().unwrap().nlinks,0);
diff --git a/src/vfs.rs b/src/vfs.rs
index 1b5396c..84e7f9d 100644
--- a/src/vfs.rs
+++ b/src/vfs.rs
@@ -3,6 +3,7 @@ use core::cell::RefCell;
 use core::mem::size_of;
 use core;
 use core::fmt::Debug;
+use core::any::Any;
 
 /// Interface for FS to read & write
 ///     TODO: use std::io::{Read, Write}
@@ -12,7 +13,7 @@ pub trait Device {
 }
 
 /// Abstract operations on a inode.
-pub trait INode: Debug {
+pub trait INode: Debug + Any {
     fn open(&mut self, flags: u32) -> Result<()>;
     fn close(&mut self) -> Result<()>;
     fn read_at(&self, offset: usize, buf: &mut [u8]) -> Result<usize>;
@@ -23,12 +24,28 @@ pub trait INode: Debug {
 //    fn reclaim(&mut self) -> Result<()>;
     fn resize(&mut self, len: usize) -> Result<()>;
     fn create(&mut self, name: &str, type_: FileType) -> Result<INodePtr>;
-    // since we do not have link, unlink==remove
     fn unlink(&mut self, name: &str) -> Result<()>;
+    /// user of the vfs api should call borrow_mut by itself
+    fn link(&mut self, name: &str, other:&mut INode) -> Result<()>;
     fn lookup(&self, path: &str) -> Result<INodePtr>;
     fn list(&self) -> Result<Vec<String>>;
 //    fn io_ctrl(&mut self, op: u32, data: &[u8]) -> Result<()>;
     fn fs(&self) -> Weak<FileSystem>;
+    /// this is used to implement dynamics cast
+    /// simply return self in the implement of the function
+    fn as_any_ref(&self) -> &Any;
+    /// this is used to implement dynamics cast
+    /// simply return self in the implement of the function
+    fn as_any_mut(&mut self) -> &mut Any;
+}
+
+impl INode{
+    pub fn downcast_ref<T:INode>(&self) -> Option<&T> {
+        self.as_any_ref().downcast_ref::<T>()
+    }
+    pub fn downcast_mut<T:INode>(&mut self) -> Option<&mut T> {
+        self.as_any_mut().downcast_mut::<T>()
+    }
 }
 
 #[derive(Debug, Eq, PartialEq)]