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Chapter 11: File-System Interface. File Concept Access Methods Directory Structure File System Mounting File Sharing Protection. File Concept. Contiguous logical address space Types: Data numeric character binary Program. File Structure. None - sequence of words, bytes

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Chapter 11 file system interface
Chapter 11: File-System Interface

  • File Concept

  • Access Methods

  • Directory Structure

  • File System Mounting

  • File Sharing

  • Protection

Operating System Concepts

File concept
File Concept

  • Contiguous logical address space

  • Types:

    • Data

      • numeric

      • character

      • binary

    • Program

Operating System Concepts

File structure
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

Operating System Concepts

File attributes
File Attributes

  • Name – only information kept in human-readable form.

  • 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.

Operating System Concepts

File types name extension
File Types – Name, Extension

Operating System Concepts

Access methods
Access Methods

  • Sequential Access

    read next

    write next


    no read after last write


  • Direct Access

    read n

    write n

    position to n

    read next

    write next

    rewrite n

    n = relative block number

Operating System Concepts

Sequential access file
Sequential-access File

Operating System Concepts

Example of index and relative files
Example of Index and Relative Files

Operating System Concepts

Directory structure
Directory Structure

  • Files in disks are organized in the following ways:

    • Disks are split into partitions (minidisks, volumes).

    • A device directory or volume table of contents records files information in each partition.

Operating System Concepts

Directory structure1
Directory Structure

  • A collection of nodes containing information about all 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.

Operating System Concepts

A typical file system organization
A Typical File-system Organization

Operating System Concepts

Information in a device directory
Information in a Device Directory

  • Name

  • Type

  • Address

  • Current length

  • Maximum length

  • Date last accessed (for archival)

  • Date last updated (for dump)

  • Owner ID (who pays)

  • Protection information (discuss later)

Operating System Concepts

Operations performed on directory
Operations Performed on Directory

  • Search for a file

  • Create a file: New files need to be added to the directory

  • Delete a file

  • List a directory

  • Rename a file: Renaming a file may also allow its position within the directory structure to be changed.

  • Traverse the file system

Operating System Concepts

Organize the directory logically to obtain
Organize the Directory (Logically) to Obtain

  • Efficiency – locating a file quickly.

  • Naming – convenient to users.

    • Two users can have same name for different files.

    • The same file can have several different names.

  • Grouping – logical grouping of files by properties, (e.g., all Java programs, all games, …)

Operating System Concepts

Single level directory
Single-Level Directory

  • A single directory for all users.

Naming problem

Grouping problem

Operating System Concepts

Two level directory
Two-Level Directory

  • Separate directory for each user (user file directory – UFD).

  • System has the master file directory (MFD).

  • Path name – A user and file name define it.

  • Can have the same file name for different user

  • The sequence of directories searched when a file is named is called the search path.

  • Efficient searching

  • No grouping capability

Operating System Concepts

Tree structured directories
Tree-Structured Directories

  • Properties of tree-structured directories:

    • A tree is the most common directory structure.

    • The tree has a root directory.

    • Every file has a unique path name.

    • A directory (or subdirectory) contains a set of files or subdirectories.

  • The current directory should contain most of the files that are of current interest to the user.

  • Efficient searching

  • Grouping Capability

  • Current directory (working directory)

    • cd /spell/mail/prog

    • type list

Operating System Concepts

Tree structured directories1
Tree-Structured Directories

Operating System Concepts

Tree structured directories cont
Tree-Structured Directories (Cont.)

  • Absolute or relative path name

    • An absolute path name begins at the root.

    • A relative path name defines a path from the current directory.

    • Some systems also allow users to define their own search paths.

  • Creating a new file is done in current directory.

  • Delete a file rm <file-name>

  • Creating a new subdirectory is done in current directory.

    mkdir <dir-name>

    Example: if in current directory /mail

    mkdir count







Deleting “mail”  deleting the entire subtree rooted by “mail”.

Operating System Concepts

Acyclic graph directories
Acyclic-Graph Directories

  • A tree structure prohibits the sharing of files or directories.

  • An acyclic graph (a generalization of the tree-structured) allows directories to have shared subdirectories and files.

Operating System Concepts

Acyclic graph directories cont
Acyclic-Graph Directories (Cont.)

  • Two different names (aliasing) point to the same file.

  • If dict deletes count  dangling pointer for spell/count.


    • Symbolic links

    • The file is deleted until all references to it are deleted.

    • Entry-hold-count solution: hard links in UNIX

Operating System Concepts

General graph directory
General Graph Directory

Operating System Concepts

General graph directory cont
General Graph Directory (Cont.)

  • If cycles are allowed to exist in the directory, repeated search could happen and the reference count may be nonzero (Figure 11.10).

  • How do we guarantee no cycles?

    • Allow only links to file not subdirectories.

    • Garbage collection.

    • Every time a new link is added use a cycle detectionalgorithm to determine whether it is OK.

Operating System Concepts

File system mounting
File System Mounting

  • A file can be opened. A file system must be mounted before it can be accessed.

  • Mount procedure:

    • Mount device to the location at which to attach the file system (mount point).

    • Verify the device contains a valid file system.

    • Note that a file system is mounted at the specified mount point.

  • A unmounted file system (I.e. Fig. 11-11(b)) is mounted at a mount point.

  • The Macintosh OS automatically mounts the floppy disk. The Microsoft Windows mount devices at boot time. In UNIX, the mount commands are explicit.

Operating System Concepts

A existing b unmounted partition
(a) Existing. (b) Unmounted Partition

Operating System Concepts

Mount point
Mount Point

Operating System Concepts

File sharing
File Sharing

  • Sharing of files on multi-user systems is desirable.

  • Most systems implement owner attributes by managing a list of user names and associated user identifiers (UIDs) and group identifiers (GIDs).

  • 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 file-sharing method.

Operating System Concepts

Remote file system
Remote File System

  • Networking allows the sharing of resources:

    • User can manually transfer files. ftp with anonymous access.

    • Distributed file system (DFS)

    • World Wide Web

  • To ease the management of client-server services, distributed information system, or distributed naming services can be used.

  • Domain name system (DNS) provides host-name-to-network-address translations for the entire Internet.

  • Sun Microsystems use yellow page (network information service (NIS)).

  • The industry is moving toward lightweight directory-access protocol (LDAP) as a secure, distributed naming mechanism.

Operating System Concepts

File sharing1
File Sharing

  • Redundant arrays of inexpensive disks (RAID) can prevent the loss of a disk from resulting in the loss of data.

  • Consistency semantics are directly related to the process synchronization algorithms.

  • The UNIX file system uses the following semantics:

    • Writes to an open file by a user are visible immediately to other users.

    • A file has a single image interleaving all accesses.

  • The session semantics enforces no constraints on scheduling accesses.

  • An immutable shared file is a file declared as shared, it cannot be modified.

Operating System Concepts


  • When information is kept in a computer system, we want to keep it safe from physical damage (reliability) and improper access (protection).

  • File owner/creator should be able to control:

    • what can be done

    • by whom

  • Types of access

    • Read

    • Write

    • Execute

    • Append

    • Delete

    • List

  • The most general access control is done by an access-control list (ACL), which specifies the user name and the types of access allowed for each users.

Operating System Concepts

Access lists and groups
Access Lists and Groups

  • Mode of access: read, write, execute

  • Three classes of users


    a) owner access 7  1 1 1 RWX

    b) group access 6  1 1 0


    c) public access 1  0 0 1

  • Ask manager to create a group (unique name), say G, and add some users to the group.

  • For a particular file (say game) or subdirectory, define an appropriate access.







Attach a group to a file chgrp G game

Operating System Concepts

Other protection approaches unix
Other Protection Approaches / UNIX

  • Associate a password with each file.

  • In UNIX, directory protection is handled similarly to file protection.

Operating System Concepts