FAT File System — In-Memory Simulator

A from-scratch C++ implementation of a FAT (File Allocation Table) file system running entirely in shared memory. The project simulates a 64 MB virtual disk with 1 KB blocks and exposes a Unix-like shell (foosh) for navigating directories, copying files between the host and virtual disk, and inspecting metadata — all without touching real disk I/O.

Built as part of an Operating Systems course at IIT Kharagpur. See Problem Statement.pdf for the original specification.

Architecture

The system is split into three layers:

• foosh.cpp — Interactive shell (REPL) supporting cd, ls, mkdir, cp, prn
• diskutils.hpp / diskutils.cpp — Core FAT, bitmap, and directory logic, path resolution, three-way file copy (host→VD, VD→host, VD→VD)
• diskmanager.cpp — Creates and destroys the 64 MB SysV shared memory segment

Virtual disk layout (65,536 blocks × 1 KB):

Region	Blocks	Purpose
Superblock	0	Total blocks, free count, root dir pointer
Bitmap	1–8	One bit per block (free/allocated)
FAT	9–264	Linked-list chain pointers (4 bytes/block)
Data	265+	Root directory starts at block 265

Each directory entry is a packed 32-byte metadata struct: 1-byte type flag, 23-byte name, 4-byte size, 4-byte first-block pointer.

Building & Running

Prerequisites: Linux with SysV shared memory support, g++ with C++17.

make                       # builds diskmanager and foosh

./diskmanager -create      # allocate & format the 64 MB virtual disk in SHM
./foosh                    # launch the shell

# inside foosh:
md docs                   # create a directory
cp `README.md docs        # copy host file → virtual disk (backtick = host path)
ls docs                   # list directory contents
prn docs/README.md        # print file contents
cp docs/README.md `out.md # copy virtual disk → host
exit

./diskmanager -remove      # free the shared memory segment

Shell Commands

Command	Description
cd <path>	Change directory (supports ., .., absolute & relative paths)
md <path>	Create a new directory
ls [path]	Detailed listing (type, name, size, first block)
dir	Names-only listing of current directory
cp <src> <dst>	Copy file — prefix with ` for host paths
prn <file>	Print file contents to stdout
exit	Quit the shell

Key Concepts

• FAT data structure — block chaining via a file allocation table, the same scheme behind FAT12/16/32
• Bitmap block allocation — free-block tracking with a bit vector and randomized allocation
• SysV shared memory IPC — persistent in-memory disk image across process invocations using shmget/shmat/shmdt
• Path resolution — recursive traversal of absolute/relative paths through chained directory blocks
• Packed structs & raw memory — direct memcpy serialization with #pragma pack, no abstraction layers

License

MIT