From 38183534a3a0c2027ad2ea04e63169292790c1eb Mon Sep 17 00:00:00 2001
From: Zhiyuan Shao <zyshao@hust.edu.cn>
Date: Mon, 8 May 2023 15:41:02 +0800
Subject: [PATCH] init commit of lab4_3

---
 Makefile             |   2 +-
 kernel/hostfs.c      |  13 ++++
 kernel/hostfs.h      |   2 +
 kernel/proc_file.c   |  14 +++++
 kernel/proc_file.h   |   3 +
 kernel/rfs.c         | 141 +++++++++++++++++++++++++++++++++++++++++++
 kernel/rfs.h         |   2 +
 kernel/syscall.c     |  22 +++++++
 kernel/syscall.h     |   3 +
 kernel/vfs.c         |  99 ++++++++++++++++++++++++++++++
 kernel/vfs.h         |   8 +++
 user/app_directory.c |  60 ------------------
 user/app_hardlink.c  |  94 +++++++++++++++++++++++++++++
 user/user_lib.c      |  14 +++++
 user/user_lib.h      |   5 ++
 15 files changed, 421 insertions(+), 61 deletions(-)
 delete mode 100644 user/app_directory.c
 create mode 100644 user/app_hardlink.c

diff --git a/Makefile b/Makefile
index 11faa01..980e035 100644
--- a/Makefile
+++ b/Makefile
@@ -68,7 +68,7 @@ USER_CPPS 		:= user/*.c
 USER_CPPS  		:= $(wildcard $(USER_CPPS))
 USER_OBJS  		:= $(addprefix $(OBJ_DIR)/, $(patsubst %.c,%.o,$(USER_CPPS)))
 
-USER_TARGET 	:= $(OBJ_DIR)/app_directory
+USER_TARGET 	:= $(OBJ_DIR)/app_hardlink
 #------------------------targets------------------------
 $(OBJ_DIR):
 	@-mkdir -p $(OBJ_DIR)	
diff --git a/kernel/hostfs.c b/kernel/hostfs.c
index 62fc763..03348d8 100644
--- a/kernel/hostfs.c
+++ b/kernel/hostfs.c
@@ -24,6 +24,8 @@ const struct vinode_ops hostfs_i_ops = {
     .viop_write_back_vinode = hostfs_write_back_vinode,
 
     // not implemented
+    .viop_link = hostfs_link,
+    .viop_unlink = hostfs_unlink,
     .viop_readdir = hostfs_readdir,
     .viop_mkdir = hostfs_mkdir,
 };
@@ -233,6 +235,17 @@ int hostfs_lseek(struct vinode *f_inode, ssize_t new_offset, int whence,
   return -1;
 }
 
+int hostfs_link(struct vinode *parent, struct dentry *sub_dentry,
+                struct vinode *link_node) {
+  panic("hostfs_link not implemented!\n");
+  return -1;
+}
+
+int hostfs_unlink(struct vinode *parent, struct dentry *sub_dentry, struct vinode *unlink_node) {
+  panic("hostfs_unlink not implemented!\n");
+  return -1;
+}
+
 int hostfs_readdir(struct vinode *dir_vinode, struct dir *dir, int *offset) {
   panic("hostfs_readdir not implemented!\n");
   return -1;
diff --git a/kernel/hostfs.h b/kernel/hostfs.h
index c788463..9909bd2 100644
--- a/kernel/hostfs.h
+++ b/kernel/hostfs.h
@@ -28,6 +28,8 @@ struct vinode *hostfs_lookup(struct vinode *parent, struct dentry *sub_dentry);
 struct vinode *hostfs_create(struct vinode *parent, struct dentry *sub_dentry);
 int hostfs_lseek(struct vinode *f_inode, ssize_t new_offset, int whence,
                   int *offset);
+int hostfs_link(struct vinode *parent, struct dentry *sub_dentry, struct vinode *link_node);
+int hostfs_unlink(struct vinode *parent, struct dentry *sub_dentry, struct vinode *unlink_node);
 int hostfs_hook_open(struct vinode *f_inode, struct dentry *f_dentry);
 int hostfs_hook_close(struct vinode *f_inode, struct dentry *dentry);
 int hostfs_readdir(struct vinode *dir_vinode, struct dir *dir, int *offset);
diff --git a/kernel/proc_file.c b/kernel/proc_file.c
index 7575e9e..3d99cbb 100644
--- a/kernel/proc_file.c
+++ b/kernel/proc_file.c
@@ -207,3 +207,17 @@ int do_closedir(int fd) {
   struct file *pfile = get_opened_file(fd);
   return vfs_closedir(pfile);
 }
+
+//
+// create hard link to a file
+//
+int do_link(char *oldpath, char *newpath) {
+  return vfs_link(oldpath, newpath);
+}
+
+//
+// remove a hard link to a file
+//
+int do_unlink(char *path) {
+  return vfs_unlink(path);
+}
diff --git a/kernel/proc_file.h b/kernel/proc_file.h
index e233d0c..2cfb051 100644
--- a/kernel/proc_file.h
+++ b/kernel/proc_file.h
@@ -21,6 +21,9 @@ int do_readdir(int fd, struct dir *dir);
 int do_mkdir(char *pathname);
 int do_closedir(int fd);
 
+int do_link(char *oldpath, char *newpath);
+int do_unlink(char *path);
+
 void fs_init(void);
 
 // data structure that manages all openned files in a PCB
diff --git a/kernel/rfs.c b/kernel/rfs.c
index b1f63db..e32cefb 100644
--- a/kernel/rfs.c
+++ b/kernel/rfs.c
@@ -25,6 +25,8 @@ const struct vinode_ops rfs_i_ops = {
     .viop_create = rfs_create,
     .viop_lseek = rfs_lseek,
     .viop_disk_stat = rfs_disk_stat,
+    .viop_link = rfs_link,
+    .viop_unlink = rfs_unlink,
     .viop_lookup = rfs_lookup,
 
     .viop_readdir = rfs_readdir,
@@ -571,6 +573,145 @@ int rfs_disk_stat(struct vinode *vinode, struct istat *istat) {
   return 0;
 }
 
+//
+// create a hard link under a direntry "parent" for an existing file of "link_node"
+//
+int rfs_link(struct vinode *parent, struct dentry *sub_dentry, struct vinode *link_node) {
+  // TODO (lab4_3): we now need to establish a hard link to an existing file whose vfs
+  // inode is "link_node". To do that, we need first to know the name of the new (link)
+  // file, and then, we need to increase the link count of the existing file. Lastly, 
+  // we need to make the changes persistent to disk. To know the name of the new (link)
+  // file, you need to stuty the structure of dentry, that contains the name member;
+  // To incease the link count of the existing file, you need to study the structure of
+  // vfs inode, since it contains the inode information of the existing file.
+  //
+  // hint: to accomplish this experiment, you need to:
+  // 1) increase the link count of the file to be hard-linked;
+  // 2) append the new (link) file as a dentry to its parent directory; you can use 
+  //    rfs_add_direntry here.
+  // 3) persistent the changes to disk. you can use rfs_write_back_vinode here.
+  //
+  panic("You need to implement the code for creating a hard link in lab4_3.\n" );
+}
+
+//
+// remove a hard link with "name" under a direntry "parent"
+//
+int rfs_unlink(struct vinode *parent, struct dentry *sub_dentry, struct vinode *unlink_vinode) {
+  struct rfs_device *rdev = rfs_device_list[parent->sb->s_dev->dev_id];
+
+  // ** find the direntry in the directory file
+  int total_direntrys = parent->size / sizeof(struct rfs_direntry);
+  int one_block_direntrys = RFS_BLKSIZE / sizeof(struct rfs_direntry);
+
+  struct rfs_direntry *p_direntry = NULL;
+  int delete_index;
+  for (delete_index = 0; delete_index < total_direntrys; ++delete_index) {
+    // read in the disk block at boundary
+    if (delete_index % one_block_direntrys == 0) {
+      rfs_r1block(rdev, parent->addrs[delete_index / one_block_direntrys]);
+      p_direntry = (struct rfs_direntry *)rdev->iobuffer;
+    }
+    if (strcmp(p_direntry->name, sub_dentry->name) == 0) {  // found
+      break;
+    }
+    ++p_direntry;
+  }
+
+  int inum = p_direntry->inum;
+
+  if (delete_index == total_direntrys) {
+    sprint("unlink: file %s not found.\n", sub_dentry->name);
+    return -1;
+  }
+
+  // ** read the disk inode of the file to be unlinked
+  struct rfs_dinode *unlink_dinode = rfs_read_dinode(rdev, inum);
+
+  // if this assertion fails, it indicates that the previous modification to nlinks
+  // was not written back to disk, which is not allowed
+  assert(unlink_vinode->nlinks == unlink_dinode->nlinks);
+
+  // ** decrease vinode nlinks by 1
+  unlink_vinode->nlinks--;
+
+  // ** update disk inode nlinks
+  unlink_dinode->nlinks = unlink_vinode->nlinks;
+
+  // ** if nlinks == 0, free the disk inode and disk blocks
+  if (unlink_dinode->nlinks == 0) {
+    // free disk blocks
+    for (int i = 0; i < unlink_dinode->blocks; ++i) {
+      rfs_free_block(parent->sb, unlink_dinode->addrs[i]);
+    }
+    // free disk inode
+    unlink_dinode->type = R_FREE;
+  }
+  // ** write the disk inode back to disk
+  rfs_write_dinode(rdev, unlink_dinode, inum);
+  free_page(unlink_dinode);
+
+  // ** remove the direntry from the directory
+
+  // handle the first block
+  int delete_block_index = delete_index / one_block_direntrys;
+  rfs_r1block(rdev, parent->addrs[delete_block_index]);
+
+  int offset = delete_index % one_block_direntrys;
+  memmove(rdev->iobuffer + offset * sizeof(struct rfs_direntry),
+          rdev->iobuffer + (offset + 1) * sizeof(struct rfs_direntry),
+          (one_block_direntrys - offset - 1) * sizeof(struct rfs_direntry));
+
+  struct rfs_direntry *previous_block = (struct rfs_direntry *)alloc_page();
+  memcpy(previous_block, rdev->iobuffer, RFS_BLKSIZE);
+
+  for (int i = delete_block_index + 1; i < parent->blocks; i++) {
+    rfs_r1block(rdev, parent->addrs[i]);
+    struct rfs_direntry *this_block = (struct rfs_direntry *)alloc_page();
+    memcpy(this_block, rdev->iobuffer, RFS_BLKSIZE);
+
+    // copy the first direntry of this block to the last direntry 
+    // of previous block
+    memcpy(previous_block + one_block_direntrys - 1, rdev->iobuffer,
+           sizeof(struct rfs_direntry));
+
+    // move the direntry in this block forward by one
+    memmove(this_block, this_block + 1,
+            (one_block_direntrys - 1) * sizeof(struct rfs_direntry));
+
+    // write the previous block back to disk
+    memcpy(rdev->iobuffer, previous_block, RFS_BLKSIZE);
+    rfs_w1block(rdev, parent->addrs[i - 1]);
+
+    // update previous block
+    free_page(previous_block);
+    previous_block = this_block;
+  }
+
+  // write the last block back to disk
+  memcpy(rdev->iobuffer, previous_block, RFS_BLKSIZE);
+  rfs_w1block(rdev, parent->addrs[parent->blocks - 1]);
+  free_page(previous_block);
+
+  // if the last block is empty, free it
+  total_direntrys--;
+  if (total_direntrys % one_block_direntrys == 0 && parent->blocks > 1) {
+    rfs_free_block(parent->sb, parent->addrs[parent->blocks - 1]);
+    parent->blocks--;
+  }
+
+  // ** update the directory file's size
+  parent->size -= sizeof(struct rfs_direntry);
+
+  // ** write the directory file's inode back to disk
+  if (rfs_write_back_vinode(parent) != 0) {
+    sprint("rfs_unlink: rfs_write_back_vinode failed");
+    return -1;
+  }
+
+  return 0;
+}
+
 //
 // when a directory is opened, the contents of the directory file are read
 // into the memory for directory read operations
diff --git a/kernel/rfs.h b/kernel/rfs.h
index 5e8971b..923b20c 100644
--- a/kernel/rfs.h
+++ b/kernel/rfs.h
@@ -79,6 +79,8 @@ struct vinode *rfs_lookup(struct vinode *parent, struct dentry *sub_dentry);
 struct vinode *rfs_create(struct vinode *parent, struct dentry *sub_dentry);
 int rfs_lseek(struct vinode *f_inode, ssize_t new_offset, int whence, int *offset);
 int rfs_disk_stat(struct vinode *vinode, struct istat *istat);
+int rfs_link(struct vinode *parent, struct dentry *sub_dentry, struct vinode *link_node);
+int rfs_unlink(struct vinode *parent, struct dentry *sub_dentry, struct vinode *unlink_vinode);
 
 int rfs_hook_opendir(struct vinode *dir_vinode, struct dentry *dentry);
 int rfs_hook_closedir(struct vinode *dir_vinode, struct dentry *dentry);
diff --git a/kernel/syscall.c b/kernel/syscall.c
index 0efcade..4ef7a33 100644
--- a/kernel/syscall.c
+++ b/kernel/syscall.c
@@ -196,6 +196,23 @@ ssize_t sys_user_closedir(int fd){
   return do_closedir(fd);
 }
 
+//
+// lib call to link
+//
+ssize_t sys_user_link(char * vfn1, char * vfn2){
+  char * pfn1 = (char*)user_va_to_pa((pagetable_t)(current->pagetable), (void*)vfn1);
+  char * pfn2 = (char*)user_va_to_pa((pagetable_t)(current->pagetable), (void*)vfn2);
+  return do_link(pfn1, pfn2);
+}
+
+//
+// lib call to unlink
+//
+ssize_t sys_user_unlink(char * vfn){
+  char * pfn = (char*)user_va_to_pa((pagetable_t)(current->pagetable), (void*)vfn);
+  return do_unlink(pfn);
+}
+
 //
 // [a0]: the syscall number; [a1] ... [a7]: arguments to the syscalls.
 // returns the code of success, (e.g., 0 means success, fail for otherwise)
@@ -239,6 +256,11 @@ long do_syscall(long a0, long a1, long a2, long a3, long a4, long a5, long a6, l
       return sys_user_mkdir((char *)a1);
     case SYS_user_closedir:
       return sys_user_closedir(a1);
+    // added @lab4_3
+    case SYS_user_link:
+      return sys_user_link((char *)a1, (char *)a2);
+    case SYS_user_unlink:
+      return sys_user_unlink((char *)a1);
     default:
       panic("Unknown syscall %ld \n", a0);
   }
diff --git a/kernel/syscall.h b/kernel/syscall.h
index 886f58e..0486c81 100644
--- a/kernel/syscall.h
+++ b/kernel/syscall.h
@@ -27,6 +27,9 @@
 #define SYS_user_readdir  (SYS_user_base + 25)
 #define SYS_user_mkdir    (SYS_user_base + 26)
 #define SYS_user_closedir (SYS_user_base + 27)
+// added @lab4_3
+#define SYS_user_link   (SYS_user_base + 28)
+#define SYS_user_unlink (SYS_user_base + 29)
 
 long do_syscall(long a0, long a1, long a2, long a3, long a4, long a5, long a6, long a7);
 
diff --git a/kernel/vfs.c b/kernel/vfs.c
index 2088fb0..c2d1c9d 100644
--- a/kernel/vfs.c
+++ b/kernel/vfs.c
@@ -255,6 +255,105 @@ int vfs_disk_stat(struct file *file, struct istat *istat) {
   return viop_disk_stat(file->f_dentry->dentry_inode, istat);
 }
 
+//
+// make hard link to the file specified by "oldpath" with the name "newpath"
+// return: -1 on failure, 0 on success.
+//
+int vfs_link(const char *oldpath, const char *newpath) {
+  struct dentry *parent = vfs_root_dentry;
+  char miss_name[MAX_PATH_LEN];
+
+  // lookup oldpath
+  struct dentry *old_file_dentry =
+      lookup_final_dentry(oldpath, &parent, miss_name);
+  if (!old_file_dentry) {
+    sprint("vfs_link: cannot find the file!\n");
+    return -1;
+  }
+
+  if (old_file_dentry->dentry_inode->type != FILE_I) {
+    sprint("vfs_link: cannot link a directory!\n");
+    return -1;
+  }
+
+  parent = vfs_root_dentry;
+  // lookup the newpath
+  // note that parent is changed to be the last directory entry to be accessed
+  struct dentry *new_file_dentry =
+      lookup_final_dentry(newpath, &parent, miss_name);
+  if (new_file_dentry) {
+    sprint("vfs_link: the new file already exists!\n");
+    return -1;
+  }
+
+  char basename[MAX_PATH_LEN];
+  get_base_name(newpath, basename);
+  if (strcmp(miss_name, basename) != 0) {
+    sprint("vfs_link: cannot create file in a non-exist directory!\n");
+    return -1;
+  }
+
+  // do the real hard-link
+  new_file_dentry = alloc_vfs_dentry(basename, old_file_dentry->dentry_inode, parent);
+  int err =
+      viop_link(parent->dentry_inode, new_file_dentry, old_file_dentry->dentry_inode);
+  if (err) return -1;
+
+  // make a new dentry for the new link
+  hash_put_dentry(new_file_dentry);
+
+  return 0;
+}
+
+//
+// unlink (delete) a file specified by "path".
+// return: -1 on failure, 0 on success.
+//
+int vfs_unlink(const char *path) {
+  struct dentry *parent = vfs_root_dentry;
+  char miss_name[MAX_PATH_LEN];
+
+  // lookup the file, find its parent direntry
+  struct dentry *file_dentry = lookup_final_dentry(path, &parent, miss_name);
+  if (!file_dentry) {
+    sprint("vfs_unlink: cannot find the file!\n");
+    return -1;
+  }
+
+  if (file_dentry->dentry_inode->type != FILE_I) {
+    sprint("vfs_unlink: cannot unlink a directory!\n");
+    return -1;
+  }
+
+  if (file_dentry->d_ref > 0) {
+    sprint("vfs_unlink: the file is still opened!\n");
+    return -1;
+  }
+
+  // do the real unlink
+  struct vinode *unlinked_vinode = file_dentry->dentry_inode;
+  int err = viop_unlink(parent->dentry_inode, file_dentry, unlinked_vinode);
+  if (err) return -1;
+
+  // remove the dentry from the hash table
+  hash_erase_dentry(file_dentry);
+  free_vfs_dentry(file_dentry);
+  unlinked_vinode->ref--; 
+
+  // if this inode has been removed from disk
+  if (unlinked_vinode->nlinks == 0) {
+    // no one will remember a dead inode
+    assert(unlinked_vinode->ref == 0);
+
+    // we don't write back the inode, because it has disappeared from the disk
+    hash_erase_vinode(unlinked_vinode);
+    free_page(unlinked_vinode);  // free the vinode
+  }
+  
+
+  return 0;
+}
+
 //
 // close a file at vfs layer.
 //
diff --git a/kernel/vfs.h b/kernel/vfs.h
index 6f030d5..12f4d28 100644
--- a/kernel/vfs.h
+++ b/kernel/vfs.h
@@ -28,6 +28,8 @@ ssize_t vfs_write(struct file *file, const char *buf, size_t count);
 ssize_t vfs_lseek(struct file *file, ssize_t offset, int whence);
 int vfs_stat(struct file *file, struct istat *istat);
 int vfs_disk_stat(struct file *file, struct istat *istat);
+int vfs_link(const char *oldpath, const char *newpath);
+int vfs_unlink(const char *path);
 int vfs_close(struct file *file);
 
 // directory interfaces
@@ -140,6 +142,10 @@ struct vinode_ops {
   struct vinode *(*viop_create)(struct vinode *parent, struct dentry *sub_dentry);
   int (*viop_lseek)(struct vinode *node, ssize_t new_off, int whence, int *off);
   int (*viop_disk_stat)(struct vinode *node, struct istat *istat);
+  int (*viop_link)(struct vinode *parent, struct dentry *sub_dentry,
+                   struct vinode *link_node);
+  int (*viop_unlink)(struct vinode *parent, struct dentry *sub_dentry,
+                     struct vinode *unlink_node);
   struct vinode *(*viop_lookup)(struct vinode *parent,
                                 struct dentry *sub_dentry);
 
@@ -171,6 +177,8 @@ struct vinode_ops {
 #define viop_create(node, name)                (node->i_ops->viop_create(node, name))
 #define viop_lseek(node, new_off, whence, off) (node->i_ops->viop_lseek(node, new_off, whence, off))
 #define viop_disk_stat(node, istat)            (node->i_ops->viop_disk_stat(node, istat))
+#define viop_link(node, name, link_node)       (node->i_ops->viop_link(node, name, link_node))
+#define viop_unlink(node, name, unlink_node)   (node->i_ops->viop_unlink(node, name, unlink_node))
 #define viop_lookup(parent, sub_dentry)        (parent->i_ops->viop_lookup(parent, sub_dentry))
 #define viop_readdir(dir_vinode, dir, offset)  (dir_vinode->i_ops->viop_readdir(dir_vinode, dir, offset))
 #define viop_mkdir(dir, sub_dentry)            (dir->i_ops->viop_mkdir(dir, sub_dentry))
diff --git a/user/app_directory.c b/user/app_directory.c
deleted file mode 100644
index 8636756..0000000
--- a/user/app_directory.c
+++ /dev/null
@@ -1,60 +0,0 @@
-#include "user_lib.h"
-#include "util/string.h"
-#include "util/types.h"
-
-void ls(char *path) {
-  int dir_fd = opendir_u(path);
-  printu("------------------------------\n");
-  printu("ls \"%s\":\n", path);
-  printu("[name]               [inode_num]\n");
-  struct dir dir;
-  int width = 20;
-  while(readdir_u(dir_fd, &dir) == 0) {
-    // we do not have %ms :(
-    char name[width + 1];
-    memset(name, ' ', width + 1);
-    name[width] = '\0';
-    if (strlen(dir.name) < width) {
-      strcpy(name, dir.name);
-      name[strlen(dir.name)] = ' ';
-      printu("%s %d\n", name, dir.inum);
-    }
-    else
-      printu("%s %d\n", dir.name, dir.inum);
-  }
-  printu("------------------------------\n");
-  closedir_u(dir_fd);
-}
-
-int main(int argc, char *argv[]) {
-  char str[] = "hello world";
-  int fd;
-  
-  printu("\n======== Test 1: open and read dir ========\n");
-
-  ls("/RAMDISK0");
-  
-  printu("\n======== Test 2: make dir ========\n");
-
-  mkdir_u("/RAMDISK0/sub_dir");
-  printu("make: /RAMDISK0/sub_dir\n");
-
-  ls("/RAMDISK0");
-
-  // try to write a file in the new dir
-  printu("write: /RAMDISK0/sub_dir/ramfile\n");
-
-  fd = open("/RAMDISK0/sub_dir/ramfile", O_RDWR | O_CREAT);
-  printu("file descriptor fd: %d\n", fd);
-
-  write_u(fd, str, strlen(str));
-  printu("write content: \n%s\n", str);
-
-  close(fd);
-
-  ls("/RAMDISK0/sub_dir");
-
-  printu("\nAll tests passed!\n\n");
-  exit(0);
-  return 0;
-}
diff --git a/user/app_hardlink.c b/user/app_hardlink.c
new file mode 100644
index 0000000..7e209e5
--- /dev/null
+++ b/user/app_hardlink.c
@@ -0,0 +1,94 @@
+#include "user_lib.h"
+#include "util/string.h"
+#include "util/types.h"
+
+void ls(char *path) {
+  int dir_fd = opendir_u(path);
+  printu("------------------------------\n");
+  printu("ls \"%s\":\n", path);
+  printu("[name]               [inode_num]\n");
+  struct dir dir;
+  int width = 20;
+  while(readdir_u(dir_fd, &dir) == 0) {
+    // we do not have %ms :(
+    char name[width + 1];
+    memset(name, ' ', width + 1);
+    name[width] = '\0';
+    if (strlen(dir.name) < width) {
+      strcpy(name, dir.name);
+      name[strlen(dir.name)] = ' ';
+      printu("%s %d\n", name, dir.inum);
+    }
+    else
+      printu("%s %d\n", dir.name, dir.inum);
+  }
+  printu("------------------------------\n");
+  closedir_u(dir_fd);
+}
+
+int main(int argc, char *argv[]) {
+  int MAXBUF = 512;
+  char str[] = "hello world";
+  char buf[MAXBUF];
+  int fd1, fd2;
+  
+  printu("\n======== establish the file ========\n");
+
+  fd1 = open("/RAMDISK0/ramfile", O_RDWR | O_CREAT);
+  printu("create file: /RAMDISK0/ramfile\n");
+  close(fd1);
+
+  printu("\n======== Test 1: hard link ========\n");
+
+  link_u("/RAMDISK0/ramfile", "/RAMDISK0/ramfile2");
+  printu("create hard link: /RAMDISK0/ramfile2 -> /RAMDISK0/ramfile\n");
+
+  fd1 = open("/RAMDISK0/ramfile", O_RDWR);
+  fd2 = open("/RAMDISK0/ramfile2", O_RDWR);
+  
+  printu("file descriptor fd1 (ramfile): %d\n", fd1);
+  printu("file descriptor fd2 (ramfile2): %d\n", fd2);
+
+  // check the number of hard links to ramfile on disk
+  struct istat st;
+  disk_stat_u(fd1, &st);
+  printu("ramfile hard links: %d\n", st.st_nlinks);
+  if (st.st_nlinks != 2) {
+    printu("ERROR: the number of hard links to ramfile should be 2, but it is %d\n",
+             st.st_nlinks);
+    exit(-1);
+  }
+  
+  write_u(fd1, str, strlen(str));
+  printu("/RAMDISK0/ramfile write content: \n%s\n", str);
+
+  read_u(fd2, buf, MAXBUF);
+  printu("/RAMDISK0/ramfile2 read content: \n%s\n", buf);
+
+  close(fd1);
+  close(fd2);
+
+  printu("\n======== Test 2: unlink ========\n");
+
+  ls("/RAMDISK0");
+
+  unlink_u("/RAMDISK0/ramfile");
+  printu("unlink: /RAMDISK0/ramfile\n");
+
+  ls("/RAMDISK0");
+
+  // check the number of hard links to ramfile2 on disk
+  fd2 = open("/RAMDISK0/ramfile2", O_RDWR);
+  disk_stat_u(fd2, &st);
+  printu("ramfile2 hard links: %d\n", st.st_nlinks);
+  if (st.st_nlinks != 1) {
+    printu("ERROR: the number of hard links to ramfile should be 1, but it is %d\n",
+             st.st_nlinks);
+    exit(-1);
+  }
+  close(fd2);
+
+  printu("\nAll tests passed!\n\n");
+  exit(0);
+  return 0;
+}
diff --git a/user/user_lib.c b/user/user_lib.c
index ca9ba1b..1c4f6d1 100644
--- a/user/user_lib.c
+++ b/user/user_lib.c
@@ -147,6 +147,20 @@ int closedir_u(int fd) {
   return do_user_call(SYS_user_closedir, fd, 0, 0, 0, 0, 0, 0);
 } 
 
+//
+// lib call to link
+//
+int link_u(const char *fn1, const char *fn2){
+  return do_user_call(SYS_user_link, (uint64)fn1, (uint64)fn2, 0, 0, 0, 0, 0);
+}
+
+//
+// lib call to unlink
+//
+int unlink_u(const char *fn){
+  return do_user_call(SYS_user_unlink, (uint64)fn, 0, 0, 0, 0, 0, 0);
+}
+
 //
 // lib call to close
 //
diff --git a/user/user_lib.h b/user/user_lib.h
index cee321f..95a686d 100644
--- a/user/user_lib.h
+++ b/user/user_lib.h
@@ -29,4 +29,9 @@ int readdir_u(int fd, struct dir *dir);
 int mkdir_u(const char *pathname);
 int closedir_u(int fd);
 
+// added @ lab4_3
+int link_u(const char *fn1, const char *fn2);
+int unlink_u(const char *fn);
+
+
 #endif