HOW TO link_unlink_symlink

READ Chapter 11.8.6 - 11.8.9
			
0. POSTED sample solutions samples/directory:

   =========== Project Level-1 ==========
   LAB6       : mount_root; ls, cd, pwd
   mkdir_creat: mkdir, creat
   rmdir      : rmdir
   link_unlink: link, unlink, symlink, readlink
   ======================================


1. link oldFileName newFileName (link pathname parameter)

   creates a file newFileName which has the SAME inode (number) as that of
   oldFileName.


   Example: link     /a/b/c                      /x/y/z ==>
                     /a/b/   data block          /x/y    data block
                 ------------------------        -------------------------
                .. .|ino rlen nlen c|...        ....|ino rlen nlen z| ....
                ------|-----------------        ------|------------------
                      |                               |
                    INODE <----------------------------

                i_links_count = 1  <== INCrement i_links_count to 2



(1). get the INODE of /a/b/c into memory: mip->minode[ ]
                                               INODE of /a/b/c
                                               dev,ino
                                               .......
    mip = path2inode(pathname);
			    
(2). check /a/b/c is NOT a DIRectory (link to DIR is NOT allowed).

(3). break up paramter into parent (/x/y), child (z)
     check /x/y  exists and is a DIR but 'z' does not yet exist in /x/y/

(4). Add an entry [ino rec_len name_len z] to the data block of /x/y/
     This creates /x/y/z, which has the SAME ino as that of /a/b/c

(NOTE: both /a/b/c and /x/y/z must be on the SAME device; 
       link can not be across different devices).

(5). increment the i_links_count of INODE by 1

(6). write INODE back to disk      

===========================================================================
2.                     HOW TO unlink

     unlink pathname

(1). get pathname's INODE into memory

(2). verify it's a FILE (REG or LNK), can not be a DIR; 

(3). decrement INODE's i_links_count by 1;

(4). if i_links_count == 0 ==> rm pathname by

        deallocate its data blocks by:

     Write a truncate(INODE) function, which deallocates ALL the data blocks
     of INODE. This is similar to printing the data blocks of INODE.

        deallocate its INODE;
     
(5). Remove childName = basename(pathname) from the parent directory by

        rm_child(parentInodePtr, childName)

     which is the SAME as that in rmdir or unlink file operations.

3. ======================== HOW TO symlink ================================
   symlink oldNAME  newNAME    e.g. symlink /a/b/c /x/y/z

   ASSUME: oldNAME has <= 60 chars, inlcuding the ending NULL byte.

(1). verify oldNAME exists (either a DIR or a REG file)
(2). creat a FILE /x/y/z
(3). change /x/y/z's type to LNK (0120000)=(b1010.....)=0xA...
(4). write the string oldNAME into the i_block[ ], which has room for 60 chars.

     set /x/y/z file size = number of chars in oldName

(5). write the INODE of /x/y/z back to disk.
  

4. readlink pathname: return the contents of a symLink file

(1). get INODE of pathname into a minode[ ].
(2). check INODE is a symbolic Link file.
(3). return its string contents in INODE.i_block[ ].