simfs - Simulated File System

A simulated file system implemented in C for CSC 4420 (Computer Operating Systems) at Wayne State University.

Overview

simfs is a single-directory file system stored inside one ordinary Unix file. It supports basic file operations with limits on how many files (MAXFILES) and how many blocks (MAXBLOCKS) the image can hold. Metadata and data-block layout are defined in starter_code/simfstypes.h.

Building

cd starter_code
make

The default Makefile required by the assignmnet enables AddressSanitizer (-fsanitize=address). On some macOS + Apple Clang setups, the sanitizer runtime can abort immediately with a check in sanitizer_malloc_mac.inc (before the program runs). That is a toolchain/runtime issue that can be worked around.

If that happens, build without AddressSanitizer:

make clean
make FLAGS='-Wall -Werror -g'

Usage

./simfs -f <filesystem-image> <command> [args...]

initfs creates (or overwrites) the image file. Run it once per new disk image.

Successful commands such as initfs and createfile produce no stdout; errors go to stderr and exit non-zero.

Simulated filenames are limited to 11 characters (see fentry.name in simfstypes.h).

writefile reads exactly length bytes from standard input. The byte count you pass must match what you pipe in (for example, echo -n omits a trailing newline, so lengths differ from plain echo).

Commands

Command	Arguments	Description
initfs	none	Initialize a new file system (overwrites the image if it exists)
printfs	none	Print file system metadata
createfile	filename	Create an empty file
deletefile	filename	Delete a file and free its blocks
readfile	filename start length	Read bytes from a file to stdout
writefile	filename start length	Read length bytes from stdin and write at start

Examples

cd starter_code

# New image (creates or clobbers myfs)
./simfs -f myfs initfs

./simfs -f myfs createfile hello

# Two bytes at offset 0; "hi" from echo -n is exactly 2 bytes
echo -n "hi" | ./simfs -f myfs writefile hello 0 2
./simfs -f myfs readfile hello 0 2

# "Hello World\n" is 12 bytes with default echo
echo "Hello World" | ./simfs -f myfs writefile hello 0 12
./simfs -f myfs readfile hello 0 12

./simfs -f myfs deletefile hello

./simfs -f myfs printfs

From the repository root, you can run the bundled test script after building starter_code/simfs:

cd starter_code
../test_simfs.sh

File system structure (brief)

• fentry: Directory entry — name (max 11 characters), logical size, index of first fnode in the chain (-1 if empty).
• fnode: Describes one block of file data — disk block index and index of the next fnode (-1 at end of chain). Free fnodes use a negative blockindex whose magnitude is the fnode index.
• Metadata is written at the beginning of the image; file data lives in fixed-size blocks indexed by block number.

Source layout

• starter_code/simfs.c — CLI and command dispatch
• starter_code/simfs_ops.c — createfile, deletefile, readfile, writefile
• starter_code/initfs.c — initialize a new image
• starter_code/printfs.c — print metadata
• starter_code/simfstypes.h — constants and structs (do not change per assignment)
• starter_code/simfs.h — shared prototypes