Inode on UNIX based systems is a data structure used
to represent a file system object. The inode contains the information required
by the process for accessing the files. Inodes exist in a static form on disk.
Whenever any modification is required the kernel reads disk inode into an
in-core inode.
The disk inode contains the following fields:
·
File owner identifier: Ownership is
divided between an individual and a group owner.
·
File type: Files may be of various types
as mentioned below:
o
Regular File
o
Directory
o
Character Device
o
Block Device
o
FIFO (named pipe)
o
Symbolic Link
o
Socket
·
File access permissions: The system
protects files according to three classes namely the owner, the group owner
& other users. Each class access rights (Read, write & Execute) can be
set individually.
·
File access times: This field specifies
the time the file was last modified, accessed and when the inode was last
modified.
·
Links: Number of hard links.
·
Table of contents for the disk addresses
of data in a file. Users treat the data in a file as a logical stream of bytes
but the kernel saves the data in discontiguous disk blocks. The inode
identifies the disk blocks that contain the file’s data.
·
File size
The in-core copy of the inode contains the following
fields in addition to the fields of the disk inode:
·
Status: The status of the in-core inode,
indicating whether:
- the inode is locked
- a process is waiting for the inode
- the file is a mount point
- the in-core representation of the inode differs from the disk copy as a result of a change to the data in the inode
- the in-core representation of the file differs from the disk copy as a result of a change to the file data
·
The logical device number of the file
system that contains the file.
·
The inode number. Inodes are stored in a
linear array, the kernel identifies the number of a disk inode by its position
in the array. The disk inode does not need this field.
·
Pointers to other in-core inodes.
·
A reference count indicating the number
of instances of the file that are active.
The VFS inode structure contains lot more fields.
Most interesting structures are inode_operations (i_op) & file_operations
(i_fop).
How
to get inode number for a file under Linux?
We can get the inode numbers using either of the
following commands:
·
ls -i
·
stat
Does
Windows have an inode number for uniquely identifying the file?
Windows has a structure named as
_FILE_INTERNAL_INFORMATION which contains the 8-byte file reference number for
the file. This number is assigned by the file system and is
file-system-specific. File reference numbers, also called file IDs, are
guaranteed to be unique only within a static file system.
