1. Lecture 3 & 4: FILE SYSTEM

2. INTERNAL REPRESENTATION OF FILES
Every file on a UNIX system has a unique inode (contains information necessary for a process to access a file)
Inodes exist in a static form on disk and the kernel reads them into an in-core inode to manipulate them

3. Inodes
Consist of:
File owner identifier (individual owner & group owner)
File type (regular, directory, character or block special, or FIFO – pipes)
File access permissions (owner, group, other: read, write, execute)
File access time
Number of links to the file
Table of contents for the disk addresses of data in a file
File size

4. Sample Disk Inode Owner mjb Group os Type regular file Perms rwxr-xr-x
Accessed Oct :45 P.M.
Modified Oct :30 A.M.
Inode Oct :30 P.M.
Size 6030 bytes
Disk addresses

5. Algorithm for allocation of in-core inodes
algorithm iget
input: file system inode number
output: locked inode {
while (not done)
if (inode in inode cache)
if (inode locked)
sleep (event inode becomes unlocked);
continue; /*loop back to while */ }
/*special processing for mount points */
if (inode on inode free list)
remove from free list;
increment inode reference count;
return (inode);
/*inode not in inode cache */
if (no inodes on free list)
return (error);
remove new inode from free list;
reset inode number and fiel system;
remove inode from old hash queue, place on new one;
read inode from disk (algorithm bread);
initialise inode (e.g. reference count to 1);

6. Structure of a regular file
File A
File C
File B
……..
Block address 40 50 60 70
……..
File A
Free
File C
File B
…….. 40 50 60 70 81
Block address
Allocation of Contigiguous Files and Fregmentation of Free Space

7. Direct and Indirect Blocks in Inode
Single direct
Double direct
Triple direct

8. Directories Give the file system its hierarchical structure
Important role in conversion of a file name to an inode number
Initial access: open, chdir, link system calls
Algorithm namei uses intermediate indoes as it parses a path name

9. Algorithm for conversion of path name to an inode
algorithm namei /* convert path name to inode */
input: path name
output: locked inode {
if (path name starts from root)
working inode = root inode (algorithm iget);
else
working inode = current directory inode (algorithm iget);
while (there is more path name)
read next path name component from input;
verify that working inode is of directory, access permissions OK;
if (working inode is of root and component is “..”)
continue; /*loop back to while */
read directory (working inode) by repeated use of algorithms
bmap, bread and brelse;
if (component matches an entry in directory (working inode))
get inode number for matched component;
release working inode (algorithm iput);
working inode = inode of matched component (algorithm iget); }
else /*component not in directory */
return (no inode);
return (working node);

10. Super block Consists of: The size of the file system
The number of free blocks in the file system
A list of free blocks available on the file system
The index of the next free block in the free block list
The size of the inode list
The number of free inodes in the file system
A list of free inodes in the file system
The index of the next free inode in the free inode list
Lock fields for the free block and free inode lists
A flag indicating that the super block has been modified

11. Algorithm for assigning new inodes
algorithm ialloc /* allocate inode */
input: file system
output: locked inode
{ while (not done)
{ if (super blocked locked)
{ sleep (event super block becomes free);
continue; /*loop back to while */ }
if (inode list in super block is empty)
{ lock super block;
get remembered inode for free inode search;
search disk for free inodes until super block full, or no more
free inodes (algorithms bread and brelse);
unlock super block;
wake up (event super block becomes free);
if (no free inodes found on disk)
return (no inode);
set remembered inode for next free inode search;
/*there are inodes in super block inode list */
get inode number form super block inode list;
get inode (algorithm iget);
if (inode not free after all)
{ write inode to disk;
release inode (algorithm iput);
continue;
/*inode is free */
initialise inode;
write inode to disk;
decrement file system free inode count;
return (inode);}

12. Assigning Free Inode from Middle of List
Free inodes
Super Block Free Inode List 83 48
empty 18 19 20
index
array 1
array 2

13. Assigning Free Inode – Super Block List Empty
Super Block Free Inode List
470
empty
array 1
index
Remembered inode
Super Block Free Inode List
array 2
535
free inodes
476
475
471 48 49 50
index

14. The File System
The best way to learn about files is to play with them. Creating a small file:
$ ed a
Now is the time
For all good people .
w junk 36 q
$ ls –l junk
-rw-r--r– 1 you 36 Sept 27 06:11 junk $
$ cat junk

15. Basics
Cat shows what the file looks like. The command od (octal dump) prints a visible representation of all the bytes of a file. Try:
$ od –c junk
-c means “interpret bytes as characters”. Turning on the –b option will show the bytes as octal (base 8) numbers as well.
$ od -cb junk
Try something different: type some characters and then a ctr-d, rather than a return. This will immediately send the characters you have typed to the program that is reading from the terminal. (Ctr-d logs you out, as the shell sees no more input). What happens when you type ctl-d to ed?

16. What is in a file? The file command makes an educated guess:
$ file /bin /bin/ed /usr/src/cmd/ed.c /usr/man/man1/ed.1
/bin: directory
/bin/ed: pure executable
/usr/src/cmd/ed.c: c program text
/usr/man/man1/ed.1 roff, nroff, or eqn input text
-c means “interpret bytes as characters”. Turning on the –b option will show the bytes as octal (base 8) numbers as well.
$ sort /bin/ed
Directories and filenames:
$ ls
$ pwd
$ mkdir recipes
$ file *
$ ls recipes

17. Permissions
Every file has a set of permissions associated with it, which determine who can do what with the file.
The file /etc/passwd is the password file; it contains all the login information about each user. You can discover you uid and groupid, as does the system, by looking up your name in /etc/passwd
$ grep you /etc/passwd
you:gkmbCTrJ04COM:604:1:Y.O.A.People:/usr/you:
The –l option of ls command prints the permissions information, among other things:
$ ls –l /etc/passwd
-rw-r- - r root Aug :40 /etc/passwd
The chmod command changes permissions on files:
$ chmod permissions filenames….
$ chmod +x command
$ chmod –w command

18. Inodes
A file has several components: a name, contents and administrative information such as permissions and modification times. The administrative information is stored in the inode along with essential system data such as how long it is, where on the disc the contents of the file are stored and so on
There are three times in the inode: the time that the contents of the file were last modified (written); the time that the file was last used (read or executed); and the time that the inode itself was last changed, for example to set the permissions

19. Interesting directories/
Root of the file system
/bin
Essential programs in executable form (“binaries”)
/dev
Device files
/etc
System miscellany
/etc/motd
Login message of the day
/etc/passwd
Password file
/lib
Essenhtial libraries
/tmp
Temporary files; cleaned when system is restarted
/unix
Executable form of the operating system
/usr
User file system
/usr/adm
System administration: accounting info
/usr/bin
User binaries
/usr/dict
Dictionary and support for spell()
/usr/games
Game programs
/usr/include
Header files for C programs

20. Interesting directories
/usr/include/sys
System header files for C programs
/usr/lib
Libraries for C, etc
/usr/man
On-line manual
/usr/man/man1
Manual pages for section 1 of manual
/usr/mdec
Hardware diagnostics, bootstrap programs
/usr/news
Community service messages
/usr/pub
Public oddments
/usr/src
Source code for utilities and libraries
/usr/src/cmd
Source for commands
/usr/src/lib
Source code for subroutine libraries
/usr/spool
Working directories for communication programs
/usr/spool/lpd
Line printer temporary directory
/usr/spool/mail
Mail in-boxes
/usr/spool/uucp
Working directory for the uucp programs
/usr/sys
Source for the operating system kernel
/usr/tmp
Arternate temporary directory
/usr/you
Your login directory
/usr/you/bin
Your personal programs