This is an implementation of a file system which I designed for my operating system classes. It combines idea of File Allocation Table and UNIX inodes.

Entire disk is divided into small sectors of predefined size (configured by FS_SECTOR_SIZE macro, 128 in current implementation).

The first sector is called bootstrap sector and contains all informations about file system neccessary to open it, including:

• Number of sectors
• Root node
• Index of first allocation table sector and number of sectors containing allocation table
• Index of sector with first cluster and number of clusters

Bootstrap sector is followed by allocation table which consist of few sectors depending on the file system size. Each table entry contains 4-byte information holding state of cluster:

• 0x00000000 - empty cluster
• 0xFFFFFFFE - end of file
• 0xFFFFFFFF - invalid cluster
• 0xFFFFFF00 - empty cluster holding nodes
• 0xFFFFFF__ - cluster holding nodes, __ indicates how many node structures are in use
• 0xFFFFFF08 - full cluster holding nodes
• other value - indicates index of next cluster containing continuation of data stored in this cluster

The rest of sectors in the file system are called clusters and they are used to hold file contents, directory structures or nodes.

Node is 16 byte structure holding information about file or directory stored in the file system:

• uint8_t flags - node flags, especially FS_NODE_FLAGS_IN_USE which indicates if entry is in use
• uint8_t type - type of node, either FS_NODE_TYPE_FILE or FS_NODE_TYPE_DIR
• uint16_t links_count - count of hard links to this node
• uint32_t size - size in bytes of file, if node is directory then total size of occupied clusters
• uint32_t cluster_index - first cluster containing data of file or directory
• uint32_t modification_time - last modification time, UNIX timestamp

Each node is indentified by its node number. Node number is 32 bit unsigned integer which contains exact location of node. Upper 24 bits are cluster index and lower 8 bits are index of structure within cluster.

Directory is simple list containing names paired with node indexes. Each entry occupy 32 bytes. First 28 bytes are reserved for name, terminated with null-terminator character. Last 4 bytes are occupied by node number. Each cluster can hold only 4 entries, thus next entires are stored in diffrent clusters and allocation table is used to indicate next part.

Node cluster is a cluster which can hold 8 node structures. When new node is requested, file system searches for existing node cluster with free entry. If not found, new cluster will be allocated for nodes and marked as node cluster.

Core file system logic is implemented in fs.c and fs.h files. main.c contains command line interface for manipulating file system and provides following commands:

• cp source destination - Copies file from source to destination.
• mv source destination - Moves file from soruce to destination.
• mkdir path - Creates directory. Allows nested directories.
• touch path - Creates empty file.
• ln file_path link_name - Creates hard link of link_name to file_path.
• rm path - Removes file or directory recursively.
• import real_source destination - Imports external file into file system.
• export source real_destination - Exports file from file system.
• edit file - Enters edit mode for specified file.
• cat file - Prints content of specified file
• ls [path] [-ds] - Lists specified directory. If path not specified then current directory is used. Flag -d - show detailed information (node index, links count, modification time). Flag -s - show size of the files and directories.
• cd dir - Change current directory.
• pwd - Prints path to current directory.
• exp file bytes - Expands file by specified amount of bytes
• trunc file bytes - Truncates file by specified amount of bytes
• fsinfo - Displays info about file system
• exit - Closes file system and exists application.
• help - Displays help

Use makefile

Create new file system: ./fs file_name size_in_bytes
Open existing file system: ./fs file_name

File system can be also used on real devices. In order to perform that, pass device path instead of file name and run fs with root privileges, example:
sudo ./fs /dev/sdb1 16384

This implementation is not well tested and usage for any real application is discouraged. Project has been made only for educational purposes.