Systems Programming File Systems Project

A (very) simple file system implemented in native c.

Requirements

File Allocation Table
Read, Write, Modify Operations
Write to disk
All files are pre-declared 'poems' (defined in poetry_map.cpp)
'Map' function to show disk usage.

Design

The project is written primarily in c, except for functions to make the instructor-provided c++ available in c. The third-party library argtable3 is used to create the CLI. A wrapping function is used to create the file handle that the disk.c functions use. Finally, each command is carried out by its own function, which calls disk.c functions for the heavy lifting.

Argparse allows a program to compare its arguments against templates for each of its various functions. The program uses Argparse to compare the input to each command template. By counting the number of errors produced from each parse, the correct command is chosen (0 errors). If no command runs, then all errors are explained to the user.

The project uses Make, g++, and gcc for compilation. It's meant to compile in a unix environment. Both the c and c++ source files are compiled respectively then linked. This relies on the extern modifiers on the linking functions. See both poetry_map.cpp and main.c for these functions. The final output is a single executable that relies on the standard libm.so library only.

The program maintains a relatively small memory footprint through its operations. The largest memory consumption is during the poetry map initialization. This is done by using native c file operations wherever possible. Instead of storing the entire disk in memory, the program uses lseek(), read(), and write() to transfer minimal amount of data at a time. In general, only a block (4096 bytes) of the disk is held at time. The FAT and directory entries read and write small in smaller increments. This is most visible in the disk.c functions; especially in append_poem() and first_avail_block(). No profiling has been done on this, and the program could likely be optimized to reduce file operations without increase the memory use by much.

Usage

Build

Build with make (no arguments). The build requires g++, gcc, make, and libm.so.

make

Also, clean with make clean.

Run

First, run ./phile format to create and setup the disk file.
Then, ./phile map can be used at any time to see the usage of the disk. The map displays an array of characters with the following key:

F - FAT table
D - Directory
* - Occupied block
_ - Free block

./phile write <filename> <poemname> will create a file on the disk with the contents of the given poem. Filenames can be any valid c-string with a max length of 26 characters. The available poem names are: (Poem names are case-sensitive, and strictly matched.)

"Jaberwocky"
"TheLama"
"Eldorado"
"SongOfTheOpenRoad"

./phile read <filename> will print the contents of the file to stdout.
./phile append <filename> <poemname> will append the contents of the given poem to an existing file. It will also create a new file if the file does not exist. Available poems are as above.

Source Files

main.c: CLI parsing, disk-file handling, and command functions.
disk.c: Functions for reading and modifying the disk
argtable3.c: Third-party tool for CLI parsing
poetry_map.cpp: Mapping of poem names to content. Includes linking functions.