Princess Nora University
Faculty of Computer & Information Systems
Computer science Department
Operating Systems
(CS 340 D)
(Chapter-11)
File System Interface
Chapter 11: File System Interface
1. File Concept
2. Access Methods
3. Directory Structure
4. File Sharing
5. Protection
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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
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Background
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The file system consists of two distinct parts:
1.
a collection of files, each storing related data, and
2.
a directory structure, which organizes and provides
information about all the files in the system.
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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.
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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.
o
o
o
o
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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.
File Structure
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File Structure





None - sequence of words, bytes
Simple record structure
 Lines
 Fixed length
 Variable length
Complex Structures
 Formatted document
 Relocatable load file
Can simulate last two with first method by inserting
appropriate control characters
Who decides:
 Operating system
 Program
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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
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File Operations
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File Operations

File is an abstract data type
Create
Two steps are necessary to
create a file.
Write
To write a file, we make a system
call specifying both the name of the
file and the information to be
 First, space in the file system must written to the file.
be found for the file how?
 Second, an entry for the new file
must be made in the directory.
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 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
mustbe updated whenever a write occurs.
File Operations (cont.)
Read
Delete
To read from a file, we use a system call To delete a file
that
Provide name and search the
1. specifies file name and where (in directory for it
memory) the next block of the file  Having found it, we release all file
should be put.
space, so that it can be reused by
2. the directory is searched for the other files, and erase the directory
associated entry,
entry
3. the system keeps a read pointer to
the location in the file where the next
read is to take place.
4. Once the read has taken place, the
read pointer is updated
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File Operations (cont.)
Truncating 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
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File Types – Name, Extension
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File Access Methods
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Access Methods
1. Sequential Access
2. Direct Access
(or relative access)
 Information in the file is  A file is made up of fixed length
processed in order, one
of logical records that allow
record after the other.
programs to read and write
records rapidly in no particular
 This mode of access is by
order.
far the most common
 read next, write next
 reset , no read after last write
 (rewrite)
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 read n, write n , position to n
 read next, write next , rewrite n
n = relative block number
Simulation of Sequential Access on a Directaccess File
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Directory and Disk
Structure
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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.
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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.
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Directory Structure (cont..)

A collection of nodes containing information about all files
Directory
Files
F1
F2
F3
F4
Fn
Both the directory structure and the files reside on disk
Backups of these two structures are kept on tapes
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A Typical File-system Organization
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Operations Performed on Directory






Search for a file
Create a file
Delete a file
List a directory
Rename a file
Traverse the file system
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Organize the Directory (Logically) to Obtain



Efficiency – locating a file quickly
Naming – convenient to users
most common schemes for defining the logical structure of a
directory.
 Single-Level Directory
 Two-Level Directory
 Tree-Structured Directories
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Single-Level Directory



A single directory for all users
The simplest structure
Limitation
 Naming problem: when the number of files increases or when
the system has more than one user. Since all files are in the same
directory, they must have unique names
e.g :The MS-DOS operating system allows only 11-character file names; UNIX, in contrast, allows 255
characters.
 Grouping problem : Keeping track of so many files is unaffordable
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Two-Level Directory

Separate directory for each user
 Path name
 Can have the same file name for different user
 Efficient searching
 No grouping capability
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Tree-Structured Directories
 Efficient searching
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 Grouping Capability
File Share and Protection
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File Sharing

Sharing of files on multi-user systems is desirable

Sharing may be done through a protection scheme

On distributed systems, files may be shared across a
network

Network File System (NFS) is a common distributed filesharing method
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File Sharing – Multiple Users

User IDs identify users, allowing permissions and
protections to be per-user

Group IDs allow users to be in groups, permitting group
access rights
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Protection

File owner/creator should be able to control:



what can be done
by whom
Types of access






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Read
Write
Execute
Append
Delete
List
Thank you
End of
Chapter 11
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Dr. Abeer Mahmoud