1. Operating Systems Chapter 6: File Management
Instr: Yusuf Altunel
IKU Department of Computer Engineering

2. Content Introduction Files Directories File System Implementation
File Naming
File Types
File Operations
Directories
File System Implementation
Implementing Files
Implementing Directories

3. Introduction Long Term Information Storage It should be possible
To store a very large amount of information
The information must survive
Even after the termination of the process
Multiple processes must be able
to access the information concurrently

4. FILES Persistent storage of information Not affected by Can be located
the creation
and termination of processes
Can be located
on disks
or any other external media
Processes can
read and write data
Managed by the Operating System

5. File System Part of the operating system File system responsibilities:
to manage the files:
File system responsibilities:
Structuring the files
File Naming
File Access
File Usage
File Protection
File Implementation

6. Naming the Files Process gives a new name to the file it creates
Using the name
The same file can be accessed
by other processes even after the termination
using the name
File naming vary from one system to another
All systems allow
strings of 1 to 8 letters
Digits and special characters are also permitted
Many file systems support
names as long as 255 characters
Some systems (like UNIX)
distinguish between upper and lower case characters
While some others (like DOS and WINDOWs) does not

7. Naming the Files File Name Extensions
Many operating system support two-part file names
Separated by a period
In DOS the extension can be 3 characters
While UNIX allows longer extensions
More than one extension is possible

8. Some File Extensions Extension Meaning file.bak Backup file file.c
C source program
file.gif
Graphical Interchange Format image
file.html
World Wide Web HyperText Markup Language document
file.iso
ISO image of a CD-ROM (for burning to CD)
file.jpg
Still picture encoded with the JPEG standard
file.mp3
Music encoded in MPEG layer 3 audio format
file.mpg
Movie encoded with the MPEG standard
file.o
Object file (compiler output, not yet linked)
file.pdf
Portable Document Format file
file.ps
PostScript file
file.tex
Input for the TEX formatting program
file.txt
General text file
file.zip
Compressed archive

9. File Structure Byte Sequence File consist of
...
Byte Sequence
File consist of
unstructured sequences of bytes
OS does not know (or care)
what is in the file
Any meaning must be
imposed by user-level programs

10. File Structure Record Sequence File is structured Each record
as a sequence of fixed-length records
Each record
has its own internal structure
Each read operation
returns one record at a time
Record 1
Record 2
Record 3
...

11. File Structure Tree File consists of a tree of records
The length of the records
is not necessarily the same
Each record contain a key
Rapid searching is allowed
The OS decides
where to store new records
Ant
Fox
Pig
Cat
Cow
Dog
Goat
Lion
Owl
Pony
Rat
Worm
Hen
Ibis
Lamb

12. File Types Regular Files Directories Character Special Files
Contain user information
Data content would be any of
ASCII characters
Binary data (binary or hexadecimal)
Directories
Special System files
to keep the file system structures
Character Special Files
Related with I/O devices
Used to model serial I/O devices
Such as terminals, printers, networks
Block Special Files
Used to model disks

13. (a) An executable file (b) An archive
Examples of UNIX Files
(a) An executable file (b) An archive

14. File Access Sequential access Random access read all bytes/records
starting from the beginning
cannot jump around,
could rewind or back up
convenient for magnetic tapes
Random access
bytes/records read in any order
essential for data base systems
no limitataion on read types
move file marker (seek), then read …
read and then move file marker …

15. File Attributes OS may assign additional information to files
File craeation date and time
Last modification time
The size of the file
...
See the table
For a list of such attributes
Notice, not all OSs use them

16. File Attributes Attribute Meaning Protection
Who can access the file and in what way
Password
Password needed to access the file
Creator
ID of the person who created the file
Owner
Current owner
Read-only flag
0 for read/write; 1 for read only
Hidden flag
0 for normal; 1 for do not display in listings
System flag
0 for normal files; 1 for system files
Archive flag
0 for has been backed up; 1 for needs back up
ASCII/Binary flag
0 for ASCII file; 1 for for binary
Random access flag
0 for sequential access only; 1 for random access
Lock flags
0 for unlocked; nonzero for locked
Record length
Number of bytes in a record
Key position
Offset of the key within each record
Key length
Number of bytes in the key field
Creation time
Date and time the file was created
Time of last access
Date and time the file was last accessed
Time of last change
Date and time the file has last changed
Current size
Number of bytes in the file
Maximum Size
Number of bytes the file may grow to

17. File Operations Create Delete Open Close Read Write
Create a new file with no data
Delete
Delete the file when it is no longer needed
Open
fetch the attributes and disk addresses
before starting to use the file
Close
Free table space
when the file no longer used
Read
Data is transferred from the disk into the buffer
Write
Data is written to the file

18. File Operations Append Seek Get Attributes Set Attributes Rename
To write to the end of the file
increases the file size
Seek
To reposition the file pointer
to a specific place in the file
Get Attributes
To get the attributes of the file
Set Attributes
To change the attributes of the file
Rename
Change the name of the file

19. Directories Directory organizations: Single level: Two-level:
Only one directory is allowed
All files stored in this directory
Two-level:
A root and a second level directories To distinguish the user files
from other files
Hierarchical Directory Systems
A tree structure of directories
Helps the users
to organize their work

20. Directory Structures A B C A B C D E F G H Directory Examples
Root
directory
Root
directory
Root
directory A B C A B C D E F G H
Single level
directory system
Two- level
directory system
Hierarchical
directory system
Directory Examples

21. cp /usr/ast/mailbox /usr/ast/mailbox.bak
A UNIX Directory Tree
cp /usr/ast/mailbox /usr/ast/mailbox.bak

22. Directory Operations Create Delete Opendir Closedir
Create a new directory
Delete
Delete the directory.
Some systems requires
directories be empty before deleting
Opendir
To prepare for directory listing
Closedir
Free table space after read

23. Directory Operations Read Rename Link Unlink
To list the directory content
Rename
To change the directory name
Link
To appear a file
in more than one directory
Increases the counter in file’s i-node
Unlink
A file link to the directory is removed
If the count i-node was 1
file is removed from the file system

24. File System Implementation
File systems
are stored on disks
Disks can be divided
into one or more partitions
Independent file systems on each partition
MBR (Master Boot Record)
the first sector of the disk
Sector 0
the 512-byte boot sector
Partition Table
Keeps start-end address of each partition
One of the partitions is marked as active
Every partition starts with a boot block
Even if it contain no bootable OS

25. Content of File Partitions
Boot Block
The first block that BIOS will
read and execute
Used to load the OS located in that partition
Superblock
Comes after the boot block
Contains the key parameters about the file system
Magic number to identify the file system type
The number of blocks in the file system
Other administrative information
Loaded to memory
when the computer is booted

26. Content of File Partitions
Free Blocks
Comes after the superblock
Is generally a bitmap or a list of pointers
To identify the free blocks in the partition
i-Nodes
An array used to keep file blocks
Tells everything about the file
the address of disk blocks belonging the file
Other information for the file
Each file in the system has one i-node
Root Directory
Contains the top of the file system tree
Other directories and files

27. File System Layout
A file system layout

28. File Implementation Contiguous Allocation Linked List Allocation
i-nodes

29. Contiguous Allocation
keep files as contiguous disk blocks
On a disk with 1KB blocks
50KB file would allocate
50 consecutive blocks
Advantages
Simple to implement
The read performance is perfect
Drawbacks
Disk becomes fragmented
Needs to be compacted
The file’s final size must be known
Otherwise the file must be moved
File A
(4 blocks)
File B
(3 blocks)
File C
(6 blocks)
File E
File D
(5 blocks)
File F
File A
(4 blocks)
3 free
blocks
File C
(6 blocks)
6 free
File D
(5 blocks)
File F
After B and E are removed

30. Linked List Allocation
Keep files in linked lists of disk blocks
The first word of each block
is used as a pointer to the next block
The rest is data
No disk space is lost
as a fragmentation
It is sufficient for a directory entry of the file
To keep only the first block address
The rest can be found
after reading the blocks
Random access is too slow!

31. Linked List Allocation
Storing a file as a linked list of disk blocks

32. Linked List Allocation
FAT (File Allocation Table)
Linked list allocation can be modified
using a Table in memory
Pointer words of each block
are stored in a table in memory
Such a table is called FAT
The entire table must be in memory
it is hard to keep FAT in memory
if disc is large

33. FAT examples: Files A and B
FAT in Memory
Pyhsical Block
Pointer
File Name 1 2 10 3 11 4 7
File A 5 6
File B 8 9 12 14
NULL 13
FAT examples: Files A and B

34. i-Nodes The abbreviation of index-node To keep track of
which blocks are belonging to which file
Contains a list of
file attributes
Disk addresses of file blocks
If the file grows beyond the limit
some entries are reserved
for blocks of addresses

35. Example: i-Node On the Disk In Memory An i-node example
File Attributes
Address of file’s block 0
Address of file’s block 1
Address of file’s block 2
Address of file’s block 3
Address of file’s block 4
Address of file’s block 5
Address of file’s block 6
Address of block of pointers
In Memory
Address of file’s block 7
Address of file’s block 8
Address of file’s block 9
Address of file’s block 10
Address of file’s block 11
Address of file’s block 12
Address of file’s block 13
On the Disk
An i-node example

36. Implementing Directories
Directories used to keep file attributes
A simple design
A directory consist of
A list-of fixed size entries one per file
Containing a fixed-length file name
A structure of the attributes
Disk addresses to tell
where the disk blocks are
Systems use I-Node
File attributes are stored in the i-node
Directory entry for each file is shorter
A file name
An i-node number

37. Directory Implementation
(a) A simple directory
fixed size entries
disk addresses and attributes in directory entry
(b) Directory entry just refers to an i-node

38. Handling Long File Names
File 1 entry length
File 1 attributes
File 1 Name
File 2 entry length
File 2 attributes
File 2 Name
File 3 entry length
File 3 attributes
File 3 Name
The simplest approach:
Reserve a fixed amount of space
Typically 255 characters
Simple
but wastes too much space
Directory entries are free in size
Each directory entry contains
a fixed portion
holding the length of the entry
A fixed length header holds
The file attributes
The actual name of the file
Can be of any length

39. Long File Names
(a) In-line
(b) In a heap

40. End of Chapter 6
File Management