SimFS

SimFS — A simple user-space file system simulator.

A minimal Unix-like file system implementation that runs entirely in user space, featuring inode-based storage, directory hierarchy, and file operations.

Features

File System Structure

• Inode-based architecture with direct, indirect, and double-indirect block pointers
• Hierarchical directory structure with . and .. entries
• Bitmap-based allocation for inodes and data blocks
• Superblock containing file system metadata
• Configurable block size (default 4096 bytes)

File Operations

• File I/O: Create, open, read, write, seek, and close files
• Directory operations: Create (mkdir), remove (rmdir), and change (cd) directories
• File management: Create and remove files
• File descriptor management: Track open files with lsof
• Concurrency control: Prevents conflicting file access

Access Modes

• Read-only (r)
• Write-only (w)
• Read-write (rw)
• Append mode (a)

File System Limits

• Maximum file size: ~4GB (through double-indirect blocks)
• Maximum open files: 32 concurrent file descriptors
• Maximum filename length: 28 characters

Building

Run make to build the mkfs and shell utilities:

make

To clean build artifacts:

make clean

Usage

Creating a File System

Create the default file system image simfs.img:

./mkfs

Or create a file system image with a custom name:

./mkfs custom.img

Running the Shell

Start the interactive shell on the default image:

./shell

Or specify a custom image file:

./shell custom.img

Architecture

Disk Layout

+----------------+
| Boot Block (0) |
+----------------+
| Superblock (1) |
+----------------+
| Inode Bitmap   |
+----------------+
| Block Bitmap   |
+----------------+
| Inode Table    |
+----------------+
| Data Blocks    |
+----------------+

Inode Structure

Each inode contains:

• File metadata (mode, size, timestamps)
• 12 direct block pointers
• 1 indirect block pointer
• 1 double-indirect block pointer
• Link count and ownership information

Directory Entry Format

Each directory entry is 32 bytes:

• Inode number (4 bytes)
• Filename (28 bytes, null-terminated)