Files and Directories in OS

1. Why Do We Need Them?

So far, the OS provides:

• Process → abstraction of CPU

• Address space → abstraction of memory

👉 Now we add:

• Persistent storage abstraction

💡 Unlike memory:

• Memory → temporary (lost on power off)

• Disk → permanent storage

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2. File: Basic Abstraction

🔷 What is a File?

A file is:

👉 A linear array of bytes that can be read or written

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🔷 Key Characteristics

• Stores data persistently on disk

• OS does not care about file content type

  • Could be text, image, video, etc.

• OS only ensures:
  👉 Data written = Data read later

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🔷 Inode (Low-Level Name)

• Each file has a unique identifier:
  👉 inode number

💡 Important:

• Users see names like file.txt

• OS internally uses inode numbers

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3. Directory: Naming Mechanism

🔷 What is a Directory?

A directory is:

👉 A special file that stores mappings

(user-readable name → inode number)

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🔷 Example

("foo", 10)

• foo → filename (user sees)

• 10 → inode (OS uses)

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🔷 Directory Structure

• Directories can contain:

  • Files

  • Other directories

👉 Forms a hierarchical tree

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🌳 Directory Tree Example

/
 ├── foo
 │    └── bar.txt
 └── bar
      └── foo
           └── bar.txt

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🔷 Path Names

Absolute Path

• Starts from root /

/foo/bar.txt

Key Point

• Same filename can exist in different directories

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4. Key Insight

👉 File system provides:

• Naming

• Organization

• Access mechanism

💡 Everything in UNIX is treated like a file:

• Devices

• Pipes

• Processes

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5. File System Interface (OS View)

The OS provides system calls to:

• Create files

• Read/write files

• Delete files

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6. Creating Files in OS

🔷 System Call: open()

File creation is done using:

int fd = open("foo", O_CREAT | O_WRONLY | O_TRUNC);

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🔷 Meaning of Flags

Flag	Meaning
O_CREAT	Create file if it doesn’t exist
O_WRONLY	Open for writing only
O_TRUNC	Clear existing content

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🔷 What Happens Internally?

When open() is called:

1. OS checks if file exists

2. If not → creates new file

3. Allocates:

   • inode

   • directory entry

4. Returns a file descriptor

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7. File Descriptor (Important Concept)

🔷 What is it?

A file descriptor (fd) is:

👉 An integer used by the process to access the file

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🔷 Key Features

• Unique per process

• Acts like a handle or pointer

• Used in:

  • read()

  • write()

  • close()

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🔷 Conceptual View

Process → File Descriptor → File Object → Disk Data

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8. Old Method: creat()

int fd = creat("foo");

👉 Equivalent to:

open("foo", O_CREAT | O_WRONLY | O_TRUNC)

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9. Important OS Perspective Points

     OS Responsibilities

• Maintain mapping:

  name → inode → disk blocks
  

• Ensure:

  • Persistence

  • Reliability

  • Correct data retrieval

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     OS Does NOT:

• Interpret file content

• Enforce file type (.txt, .jpg, etc.)

10. Key Takeaways

• File = linear sequence of bytes

• Directory = mapping of name → inode

• Inode = internal file identifier

• File descriptor = handle to access file

• open() is used to create files

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Final Insight

👉 File system provides abstraction + naming + persistence, making storage:

• Easy to use

• Organized

• Reliable