1. Understanding Operating Systems Fifth Edition
Chapter 8 File Management

2. The File Manager
File Manager is also called the File management system and is the software responsible for:
Creating, deleting, modifying, controlling access to files
Provides support for libraries of programs to online users, for spooling operation, and for interactive computing
Understanding Operating Systems, Fifth Edition

3. Responsibilities of the File Manager
Four tasks
Keep track of where each file is stored
Implement a policy that will:
Determine where and how files are stored
Efficiently use available storage space
Provide efficient file access
Allocate each file when a user has been cleared for access to it, then record its use
File deallocation
File returned to storage
Communicate file availability to others waiting for it
Understanding Operating Systems, Fifth Edition

4. Responsibilities of the File Manager (continued)
File Manager’s Policy determines:
File storage location
System and user access files via device-independent commands
Who will have access to which file. This depends on two factors:
Flexibility of access to information (Factor 1)
Shared files
Providing distributed access
Allowing users to browse public directories
Understanding Operating Systems, Fifth Edition

5. Responsibilities of the File Manager (continued)
Subsequent protection (Factor 2)
Protect files against system malfunctions
Security checks
Account numbers, passwords, lockwords
File allocation
Activate secondary storage device, load file into memory, update records
File deallocation
Update file tables, rewrite file (if revised), notify waiting processes of file availability
Understanding Operating Systems, Fifth Edition

6. Definitions Field Record File
Group of related bytes (e.g. last_name, first_name, age) that can be identified by the user with a name, type, and size.
Record
Group of related fields
File
Group of related records that contains information to be used by specific application programs to generate reports. This type of file contains data and is sometimes called a flat file because it has no connections to other files and has no dimensionality.
Understanding Operating Systems, Fifth Edition

7. Definitions (continued)
Databases
Groups of related files
Interconnected at various levels
Give users flexibility of access to stored data
Program files
Contain instructions
Data files
Contain data
Directories
Are special files with listings of filenames and their attributes
Understanding Operating Systems, Fifth Edition

8. Definitions (continued)
Understanding Operating Systems, Fifth Edition

9. Interacting with the File Manager
User interacts with the File Manager with commands that are either:
Embedded in program
OPEN, CLOSE, READ, WRITE, MODIFY
Submitted interactively
CREATE, DELETE, RENAME, COPY
Device independent
Physical location knowledge not needed
Cylinder, surface, sector
Device medium knowledge not needed
Tape, magnetic disk, optical disc, flash storage
Network knowledge not needed
Understanding Operating Systems, Fifth Edition

10. Interacting with the File Manager (continued)
Understanding Operating Systems, Fifth Edition

11. Interacting with the File Manager (continued)
Each of the logical commands (READ, WRITE etc.) can be broken into lower-level signals.
Example: READ
Move read/write heads to record cylinder
Wait for rotational delay (sector containing record passes under read/write head)
Activate appropriate read/write head and read record
Transfer record to main memory
Send flag indicating free device for another request
File Manager does all of this and performs error checking and correction
No need for error-checking code in programs
Understanding Operating Systems, Fifth Edition

12. Typical Volume Configuration
Secondary storage unit can be removable (such as CDs, DVDs etc) or non-removable (such as hard disks). Each storage unit is considered a volume.
A multifile volume contains many files
Multivolume files
These are extremely large files spread across several volumes
Each volume is given a name
This is managed by the File manager and it writes this descriptive information in an easily accessible place
Innermost part of CD, beginning of tape, first sector of outermost track
Understanding Operating Systems, Fifth Edition

13. Typical Volume Configuration (continued)
Understanding Operating Systems, Fifth Edition

14. Typical Volume Configuration (continued)
Master file directory (MFD)
Is stored immediately after volume descriptor
MFD lists:
Names and characteristics of every file in volume
File names (program files, data files, system files)
Any subdirectories
If supported by file manager
The remainder of the volume
Is used for file storage
Understanding Operating Systems, Fifth Edition

15. Typical Volume Configuration (continued)
Single directory per volume
Supported by early operating systems
Disadvantages
Long search time for individual file
Directory space filled before disk storage space filled
Users cannot create subdirectories
Users cannot safeguard their files
Each program needs unique name
Even those serving many users
Understanding Operating Systems, Fifth Edition

16. About Subdirectories Newer File Managers Subdirectory
Create MFD for each volume
Contains file and subdirectory entries
Improvement over single directory scheme
Problems remain: users unable to logically group files by creating their own subdirectories.
Subdirectory
Is created when a user opens an account in the computer system
Is treated as a file
Though flagged in MFD as subdirectory
Has unique properties since its records are filenames pointing to files
Understanding Operating Systems, Fifth Edition

17. About Subdirectories (continued)
File managers today
Users create own subdirectories (folders)
Related files grouped together
Implemented as upside-down tree
Efficient system searching of individual directories
May require several directories to reach file
Understanding Operating Systems, Fifth Edition

18. About Subdirectories (continued)
Understanding Operating Systems, Fifth Edition

19. About Subdirectories (continued)
File descriptor
Filename: ASCII code
File type: organization and usage
System dependent
File size: for convenience
File location
First physical block identification
Date and time of creation
Owner
Protection information: access restrictions
Record size: fixed size, maximum size
Understanding Operating Systems, Fifth Edition

20. File-Naming Conventions
Filename components
Relative filename and extension
Complete filename (absolute filename)
Includes all path information
Relative filename
Name without path information
Appears in directory listings, folders
Provides filename differentiation within directory
Varies in length
One to many characters
Operating system specific
Understanding Operating Systems, Fifth Edition

21. File-Naming Conventions (continued)
Extensions
Appended to relative filename
Two to three characters
Separated by period
Identifies file type or contents
Example
BASIA_TUNE.MPG
Unknown extension
Requires user intervention
Understanding Operating Systems, Fifth Edition

22. File-Naming Conventions (continued)
Understanding Operating Systems, Fifth Edition

23. File-Naming Conventions (continued)
Operating system specifics
Windows
Drive label and directory name, relative name, and extension
Network with Open VMS Alpha
Node, volume or storage device, directory, subdirectory, relative name and extension, file version number
UNIX/Linux
Forward slash (root), first subdirectory, sub-subdirectory, file’s relative name
Understanding Operating Systems, Fifth Edition

24. File Organization Arrangement of records within files
All files composed of records
Modify command
Request to access record within a file
Understanding Operating Systems, Fifth Edition

25. Record Format Fixed-length records Variable-length records
Easiest to access directly
The record size is critical
Ideal for data files
Variable-length records
Difficult to access directly
No empty storage space and no character truncation
File descriptor stores record format
Used with files accessed sequentially
Text files, program files
Used with files using index to access records
Understanding Operating Systems, Fifth Edition

26. Record Format (continued)
Understanding Operating Systems, Fifth Edition

27. UNIX File Management (from Chapter 13)
Three file types
Directories
Ordinary files
Special files
Each enjoys certain privileges
Are files that maintain the hierarchical structure of file system
Users allowed to read information in directory files
Only system allowed directory file modification
Understanding Operating Systems, Fifth Edition 27

28. UNIX File Management (continued)
Ordinary files
Are those in which users store information
Their protection is based on user requests and are related to the read, write, execute, delete functions performed on a file
Special files
Device drivers providing I/O hardware interface
Appear as entries in directories
Part of file system (most in /dev directory)
The name of the special file indicates type of device association (e.g. /dev/hd, /dev/lp)
Understanding Operating Systems, Fifth Edition 28

29. UNIX File Management (continued)
Files stored as sequences of bytes
No structure imposed
Text files
Character strings
Lines delimited by line feed or new line character
Binary files
Sequences of binary digits (bits)
Grouped into words as they appear in memory during program execution
Structure of files
Controlled by programs using them and not by UNIX
Understanding Operating Systems, Fifth Edition 29

30. UNIX File Management (continued)
Organizes disk into blocks of 512 bytes each
Divides disk into four basic regions
First region (address 0): reserved for booting
Second region: contains disk size and other regions’ boundaries
Third region includes: list of file definitions called the “i-list”, which is a list of file descriptors (called “i-nodes”), one for each file. The position of an i-node is called an “i-number” and this uniquely identifies a file.
Remaining region: free blocks available for file storage
Files stored in contiguous empty blocks
Simple allocation and no need to compact
Understanding Operating Systems, Fifth Edition 30

31. UNIX File Management (continued)
“i-node”
Each entry in i-list called an “i-node” (or inode)
Contains 13 disk addresses
Contains specific file information
Owner’s identification
Protection bits, physical address, file size
Time of creation, last use, and last update
Number of links
File type
Directory, ordinary file, or special file
Understanding Operating Systems, Fifth Edition 31

32. UNIX File Management (continued)

33. UNIX File Naming Conventions
Case-sensitive filenames
255 character length
While UNIX does not impose any file naming conventions, system programs such as compilers expect files to have suffixes (“extensions”).
- E.g. prog1.c or prog1.java
Supports hierarchical tree file structure
Root directory identified by slash (/)
The names of other directories are preceded by the slash (/), which is used as a delimiter
E.g. /programs/pay/checks
where checks is the actual file (see figure 13.8)
The first slash indicates that this is an absolute path name that starts at the root directory. A relative path name would be pay/checks.
Understanding Operating Systems, Fifth Edition 33

34. UNIX File Naming Conventions (continued)
Understanding Operating Systems, Fifth Edition 34

35. UNIX File Naming Conventions (continued)
Path name rules
If a path name starts with a slash, the path starts at the root directory
Path name can be either:
One name or a list of names separated by slashes
Last name on a list is the name of the file requested
Using two periods (..) in path name
Moves you upward in hierarchy (closer to root). This is the only way to go up the hierarchy
All other path names go down tree
Spaces not allowed within path names
Understanding Operating Systems, Fifth Edition 35

36. UNIX Directory Listings
Table 13.2 shows a “long listing” of files in a directory with eight pieces of information for each file
First column
Shows file type and access privileges for each file
First character: nature of file or directory (‘d’ indicates a directory and ‘-’ indicates an ordinary file)
Next three characters: show the access privileges granted to the file owner (‘r’ indicates read, ‘w’ indicates write, and ‘x’ indicates execute)
Next three characters: show the access privileges granted to other members of the user’s group (e.g. students in a class could be a group)
Last three characters: show the access privileges granted to users at large (system-wide)
Understanding Operating Systems, Fifth Edition 36

37. UNIX Directory Listings (continued)
Second column
Indicates number of links (number of aliases)
Referring to same physical file
Aliases (links)
Important UNIX feature: support file sharing
Several users work together on same project
Shared files appear in different directories belonging to different users
Filename: may be different from directory to directory
Eventually number will indicate when file no longer needed: can be deleted
Understanding Operating Systems, Fifth Edition 37

38. UNIX Directory Listings (continued)
Understanding Operating Systems, Fifth Edition 38

39. UNIX Data Structures File descriptors divided into parts
Hierarchical directories
Contain filename and i-number
Pointer to another location: i-node
i-node
Contains rest of information
i-nodes stored in reserved part of device
Where directory resides
i-node has 13 pointers (0–12)
Understanding Operating Systems, Fifth Edition 39

40. UNX Data Structures (continued)
Understanding Operating Systems, Fifth Edition 40

41. UNIX Data Structures (continued)
When file created
i-node allocated to it
Directory entry with filename and i-node number created
Understanding Operating Systems, Fifth Edition 41

42. UNIX Data Structures (continued)
When file linked
Directory entry created with new name
Original i-node number and link-count field in the i-node incremented by one
When shared file deleted
Link-count field in i-node decremented by one
When count reaches zero
Directory entry erased
Deallocate all disk blocks, along with its i-node entry in the disk i-list
Understanding Operating Systems, Fifth Edition 42