1. Operating Systems (CS 340 D)
Princess Nora University
Faculty of Computer & Information Systems
Computer science Department
Operating Systems (CS 340 D)

2. File System Interface

3. Chapter 11: File System Interface
File Concept
Access Methods
Directory Structure
File Sharing
Protection

4. OBJECTIVES:
To explain the function of file systems
To describe the interfaces to file systems
To discuss file-system design tradeoffs, including access methods, file sharing, file locking, and directory structures
To explore file-system protection

5. Background

6. a collection of files, each storing related data.
The file system consists of two distinct parts:
a collection of files, each storing related data.
a directory structure, which organizes and provides information about all the files in the system.

7. File Concept

8. File Concept
A file is a named collection of related information that is recorded on secondary storage.
Commonly, files represent :
programs (both source and object forms) and data.
Data files may be numeric, alphabetic, alphanumeric, or binary.
Files may be free form, such as text files, or may be formatted rigidly.
The information in a file is defined by its creator.

9. File Concept (cont.)
Many different types of information may be stored in a file
source programs, object programs, executable programs, numeric data, text, payroll records, graphic images, sound recordings, …..
A file has a certain defined structure, which depends on its type.
e.g: A text file is a sequence of characters organized into lines
A source file is a sequence of subroutines and functions, each of which is further organized as declarations followed by executable statements.
An object file is a sequence of bytes organized into blocks understandable by the system’s linker.
An executable file is a series of code sections that the
loader can bring into memory and execute.

10. File Attributes Name – only information kept in human-readable form
Identifier – unique tag (number) identifies file within file system
Type – needed for systems that support different types
Location – pointer to file location on device
Size – current file size
Protection – controls who can do reading, writing, executing
Time, date, and user identification – data for protection, security, and usage monitoring
Information about files are kept in the directory structure, which is maintained on the disk
The size of the directory itself may be megabytes.

11. File Operations File is an abstract data type Basic File Operations:
Creating a file.
Writing a file.
Reading a file.
Repositioning within a file.
Deleting a file.
Truncating a file.

12. File Operations (Cont.)
Create
Write
Two steps are necessary to create a file.
space in the file system must be found for the file… how?
an entry for the new file must be made in the directory.
To write a file, we make a system call specifying both the name of the file and the information to be written to the file.
Given the name of the file, the system searches the directory to find the file’s location.
The system must keep a write pointer to the location in the file where the next write is to take place. The write pointer must be updated whenever a write occurs.

13. File Operations (cont.)
Read
Delete
To read from a file, we use a system call that
specifies file name and where (in memory) the next block of the file should be put.
the directory is searched for the associated entry,
the system keeps a read pointer to the location in the file where the next read is to take place.
Once the read has taken place, the read pointer is updated
To delete a file
Provide name and search the directory for it
Having found it, we release all file space, so that it can be reused by other files, and erase the directory entry

14. File Operations (cont.)
Truncating a file
Repositioning within a file.
The user may want to erase the contents of a file but
keep its attributes. Rather than forcing the user to delete the file and then recreate it,
this function allows all attributes to remain unchanged—except for file length—but lets the file be reset to length zero
The directory is searched for the appropriate entry,
The current-file-position pointer is repositioned to a given value.
Repositioning within a file need not involve any actual I/O.
This file operation is also known as a file seek.

15. File Types
A common technique for implementing file types is to include the type as part of the file name.
The name is split into two parts—a name and an extension, usually separated by a period character . E.G.: resume.doc, Server.java .
The system uses the extension to indicate the type of the file and the type of operations that can be done on that file.
Only a file with a .com, .exe, or .bat extension can be executed,
Application programs also use extensions to indicate file types in which they are interested.
For example, the Microsoft Word expects its files to end with a (.docx) extension

16. File Types – Name, Extension

17. File Structure
File types also can be used to indicate the internal structure of the file.
Who decides the internal file structure??
Operating system or the application
APPROCH(1) : the operating system support multiple file structures
Disadvantages :
The resulting size of the operating system is huge.
If the operating system defines five different file structures, it needs to contain the code to support these file structures.
When new applications require information structured in ways not supported by the operating system, severe problems may result.
Two approaches

18. File Structure (Cont.)
APPROCH(2) : the operating system support minimum file structures
This approach has been adopted in UNIX, MS-DOS, and others.
UNIX considers each file to be a sequence of 8-bit bytes; no interpretation of these bits is made by the operating system.
This scheme provides maximum flexibility but little support.
Each application program must include its own code to interpret an input file as to the appropriate structure.
However, all operating systems must support at least one structure—that of an executable file—so that the system is able to load and run programs.

19. File Access Methods

20. Access Methods 1. Sequential Access 2. Direct Access
(or relative access)
Information in the file is processed in order, one record after the other.
This mode of access is by far the most common
read next, write next
reset
A file is made up of fixed length of logical records that allow programs to read and write records rapidly in no particular order.
read n, write n , position to n
read next, write next ,
n = relative block number

21. Simulation of Sequential Access on a Direct-access File

22. Directory and Disk Structure

23. Directory Structure
A storage device can be used in its entirety for a file system.
It can also be subdivided into partitions (each partition can hold a file system)
Partitioning is useful for limiting the sizes of individual file systems,
Partitions are also known as slices or (in the IBM world) minidisks.
Any entity containing a file system is generally known as a volume.

24. Directory Structure (cont..)
Each volume can be thought of as a virtual disk.
Each volume that contains a file system must also contain information about the files in the system. This is the device directory (simply as that directory)
records information—such as name, location, size, and type—for all files on that volume. file-system organization.

25. Directory Structure (cont..)
A collection of nodes containing information about all files
Directory
Files
F 1
F 2
F 3
F 4
F n
Both the directory structure and the files reside on disk
Backups of these two structures are kept on tapes

26. Organize the Directory (Logically) to Obtain
Schemes for defining the logical structure of a directory:
Single-Level Directory
Two-Level Directory

27. Single-Level Directory
Basic Idea: A single directory for all files and users.
Pros. (++): The simplest structure

28. Single-Level Directory (Cont.)
Cons. (--):
Naming problem: Since all files are in the same directory, they must have unique names .so it is difficult for user to find unique names for all files…why??
number of files is big ,specially when the system has more than one user.
Some OS’s have restriction on files’ names (e.g.: The MS-DOS operating system allows only 11-character file names; UNIX, in contrast, allows 255 characters. )
Grouping problem : user can’t organize his files into sub folders
Efficiency : OS must search the whole directory for one file

29. Two-Level Directory
Basic Idea: create Separate directory for each user
Each user has his own user file directory (UFD). The UFD lists only the files of a single user.
When a user job starts or a user logs in, the system’s master file directory (MFD) is searched. The MFD is indexed by user name or account number, and each entry points to the UFD for that user
When a user refers to a particular file, only his own UFD is searched.
Two-level directory can be thought of as a tree of height 2.

30. Two-Level Directory (Cont.)
Pros. (++):
Can have the same file name for different user………….(naming facility)
Searching is more efficient than single-level scheme. To create or delete a file for a user, the OS searches only that user’s UFD…(efficiency)
Every file in the system has a unique path name. (a user name and a file name defines a path in the tree from the root (the MFD) to a leaf (the specified file))
Cons. (--):
No grouping capability
Single user must create different names for all his files

31. Thank you The End