Learning outcomes
- Explain what files are, list common file attributes and operations.
Unit 5 · I/O, Disk Scheduling & File Systems
Lecture 6: File concept, attributes & operations
Lecture Notes
Main content
File System
A file system is the part of the operating system that provides a convenient and organized way to store, retrieve, manage, protect, and share data. It hides the complexity of physical storage devices and presents data to the user and programs in the form of files and directories.
File Concept
Access Methods
Directories
File Sharing
Implementation
Protection
Security
1. File Concept
1.1 Meaning of a file
A file is a named collection of related information stored on secondary storage. The file system allows users and programs to treat stored data as organized units rather than as raw disk blocks.
A file may contain text, numbers, executable code, images, audio, video, or structured records. In practice, a file is one of the most basic abstractions provided by the operating system.
Simple definition: A file is a logical storage unit used to store related data under one name.
1.2 File attributes
A file has not only contents but also metadata, that is, information about the file itself.
| Attribute | Meaning |
|---|---|
| Name | Human-readable file identifier |
| Type | Text, binary, executable, directory, etc. |
| Location | Pointers to data blocks on storage |
| Size | Current file length |
| Owner / Group | Who owns the file |
| Protection | Read, write, execute permissions / ACLs |
| Timestamps | Create, access, and modification dates |
The text’s file-control block example explicitly includes items such as file permissions, file dates, owner/group/ACL, file size, and block pointers. :contentReference[oaicite:2]{index=2}
1.3 Basic file operations
A file system typically supports operations such as:
- Create a file
- Open a file
- Read from a file
- Write to a file
- Reposition within a file (seek)
- Close a file
- Delete a file
- Truncate a file
The OS text notes that repositioning is also known as file seek, and that many systems maintain an open-file table so repeated directory searching is avoided once a file is opened. :contentReference[oaicite:3]{index=3}
Figure 1: File operations typically go through the logical file system and open-file table before reaching disk blocks.
2. File Organization and Access Mechanisms
2.1 File organization
File organization refers to how the contents of a file are logically arranged. Two common ways to view file organization are:
- Byte stream / character stream – file is treated as a sequence of bytes
- Record-based organization – file consists of records, either fixed-length or variable-length
Modern systems such as UNIX usually treat files as byte streams, leaving interpretation to applications.
2.2 Access methods
The access method defines how data is read from or written to a file.
| Access Method | Meaning | Typical Use |
|---|---|---|
| Sequential Access | Records or bytes are processed in order, one after another. | Text files, logs, media streams |
| Direct / Random Access | Any block or record can be accessed directly without reading all previous data. | Databases, indexed files |
| Indexed Access | An index is used to locate records quickly. | Large structured datasets |
Sequential Access Example
A text editor reading a text file line by line uses sequential access.
Direct Access Example
A database jumping directly to record number 5000 uses direct access.
2.3 Repositioning / Seek
In random access, the current file-position pointer may be moved to a new location. This is called seek or repositioning. The OS text notes that seek often does not require actual I/O, since it may simply change a pointer. :contentReference[oaicite:4]{index=4}
3. File Directories
3.1 Meaning of a directory
A directory is a special structure that stores information about files. It provides a way to organize files and locate them by name.
Some operating systems treat a directory as a special kind of file, while others treat it as a separate structure. The OS text explicitly notes that UNIX treats a directory like a file with a special type field, while some systems like Windows treat directories separately. :contentReference[oaicite:5]{index=5}
3.2 Main directory operations
- Create directory
- Delete directory
- Search for a file
- List directory contents
- Rename a file or directory
- Traverse the directory tree
3.3 Directory structures
| Directory Structure | Idea | Strength | Weakness |
|---|---|---|---|
| Single-Level Directory | All files are stored in one directory. | Very simple | Name conflicts, poor organization |
| Two-Level Directory | Each user has a separate directory. | Reduces name conflicts | Limited sharing |
| Tree-Structured Directory | Directories can contain subdirectories. | Good organization and hierarchy | More complex traversal |
| Acyclic Graph Directory | Shared files / subdirectories allowed but cycles avoided. | Supports sharing | Needs reference management |
Figure 2: A simple tree-structured directory organization.
4. File Sharing
4.1 Meaning
File sharing means that more than one user or process may access the same file. This is important in multiuser systems, networked systems, and collaborative environments.
4.2 Types of sharing
- Local sharing – multiple processes on the same machine access a file
- Remote sharing – files are accessed across a network from a server
The text discusses remote file systems in the client–server model, where a server exports files and a client accesses them over the network. :contentReference[oaicite:6]{index=6}
4.3 Sharing issues
| Issue | Meaning |
|---|---|
| Concurrency | Two users may try to update the same file simultaneously. |
| Consistency | All users should see valid and predictable file contents. |
| Locking | Used to control simultaneous access. |
| Authentication | Remote sharing must ensure that only authorized users or clients get access. |
Example
In a shared office document, if one user is editing while another is reading, the system must manage permissions, consistency, and possibly file locking.
5. File-System Implementation Issues
5.1 Main implementation idea
Internally, the file system must map logical files to physical disk blocks. It must also keep metadata such as file size, block addresses, ownership, permissions, and open-file information.
5.2 File Control Block (FCB) / inode idea
Each file is represented internally by a control structure, often called a File Control Block (FCB) or inode in UNIX-like systems. The FCB stores the metadata needed to manage the file.
5.3 Open-file tables
The text explains that once a file is opened, its information is copied into a system-wide open-file table, and a per-process table entry points to that information. This avoids repeated searching of the directory. :contentReference[oaicite:7]{index=7}
| Structure | Purpose |
|---|---|
| System-wide open-file table | Stores FCB-related information and tracks how many processes have the file open. |
| Per-process open-file table | Stores process-specific information such as current file pointer and access mode. |
5.4 File allocation methods
File allocation defines how file blocks are placed on disk.
| Allocation Method | Main Idea | Strength | Weakness |
|---|---|---|---|
| Contiguous Allocation | File occupies consecutive disk blocks. | Fast sequential and direct access | External fragmentation, hard file growth |
| Linked Allocation | Each block points to the next block. | No external fragmentation | Poor direct access, pointer overhead |
| Indexed Allocation | Index block stores pointers to all file blocks. | Supports direct access well | Index overhead |
5.5 Free-space management
The file system must track which disk blocks are free and which are allocated. Common techniques include:
- bit maps / bit vectors,
- linked free lists,
- grouping,
- counting.
5.6 VFS idea
In modern systems, a Virtual File System (VFS) layer provides a common interface for different file systems. The text explains that VFS can call the appropriate function for a file object without needing to know whether it is a local file, directory file, or remote file. :contentReference[oaicite:8]{index=8}
Main implementation issue: The file system must efficiently map names and metadata to physical storage blocks while supporting fast access, sharing, and protection.
6. File-System Protection and Security
6.1 Need for protection
A multiuser operating system must ensure that one user cannot access, modify, or delete another user’s files without permission. Therefore, the file system must provide protection and security.
6.2 Protection mechanisms
| Mechanism | Meaning |
|---|---|
| Permissions | Read, write, execute rights for owner, group, others |
| Access Control List (ACL) | More detailed list of users / groups and allowed operations |
| Password / Authentication | Ensures only authorized users log in |
| File locking | Controls simultaneous access to shared files |
| Backup and recovery | Helps restore data after failure or attack |
6.3 Security concerns
- Unauthorized access
- Accidental deletion or corruption
- Malware or ransomware attacks
- Spoofing in remote file systems
- Improper sharing permissions
The text notes that remote file-sharing security involves authentication problems, including client identification and possible spoofing, and that stronger security may require encrypted keys and secure authentication methods. :contentReference[oaicite:9]{index=9}
Important: Protection controls who may access a file. Security protects the file system against unauthorized or malicious actions.
7. Quick Comparison Snapshot
| Topic | Main idea | What to remember |
|---|---|---|
| File concept | Named logical unit of related data | Files have contents and metadata |
| Access methods | Ways to read/write file data | Sequential, direct, indexed |
| Directories | Structures used to organize and locate files | Single-level, two-level, tree, graph |
| File sharing | Multiple users/processes access the same file | Needs locking, consistency, protection |
| Implementation | Maps logical files to physical storage | FCB, open-file table, allocation methods, free-space management |
| Protection & security | Restrict access and defend files | Permissions, ACLs, authentication, backup |
8. One-Page Summary
A file is a named logical collection of related information stored on secondary storage. File attributes include name, size, owner, permissions, timestamps, and block pointers.
Access methods define how file data is processed. Common methods are sequential access, direct access, and indexed access.
Directories organize files and support file naming, searching, listing, and hierarchical storage. Tree-structured directories are widely used because they provide logical organization.
File sharing allows multiple users or processes to access the same file, but it introduces issues such as consistency, locking, and protection.
File-system implementation uses structures such as file-control blocks, open-file tables, allocation methods, and free-space management to map logical files to physical disk blocks.
Protection and security ensure that files are accessed only by authorized users and are protected against accidental damage and malicious attacks.
9. Likely University Questions
- What is a file? Explain file attributes and basic file operations.
- Differentiate between sequential, direct, and indexed access methods.
- Explain directory structures with suitable examples.
- What is file sharing? What problems arise in shared file access?
- Explain file allocation methods: contiguous, linked, and indexed.
- What is the role of the open-file table in file-system implementation?
- What is free-space management? Explain common methods.
- Explain file-system protection and security mechanisms.