User-level File System

The goal of this exercise is to design and implement an User-level File System similar to file systems discussed in class (e.g. FAT and UNIX), but relying on a virtual disk instead of a real I/O subsystem.

Virtual Disk

The virtual disk upon which the user-level file system will be implemented is a standard file on the hosting operating system. The file will be 100 MB in size and will be accessed in chunks (i.e. logical blocks) of 1 KB through the following interface:

• int read(int n, void * buf)
  Reads the block of order n (first block = 0) from the virtual disk into the buffer pointed by buf. The function returns "0" on success and "-1" on error.
• int write(int n, void * buf)
  Writes the data pointed by buf into the n-th block (first block = 0) of the virtual disk. The function returns "0" on success and "-1" on error.

FS API

The User-level File System must implement, at least, the following interface:

• int open(const char * pathname, int flags)
  Opens the file designated by pathname or create it case it does not exist. Files can be open in tree modes according to flags:
  1. O_RDONLY: read-only;
  2. O_WRONLY: write-only;
  3. O_RDWR: read/write.
  The function returns a file descriptor on success or "-1" on failure.
• int close(int fd)
  Closes the file designated by fd. In case of success, this function returns "0", otherwise, "-1".
• int read(int fd, void * buf, int count)
  Attempts to read up to count bytes from the file designated by fd into the buffer starting at buf. On success, the number of bytes read is returned (zero indicates end of file), and the file position is advanced by this number. On error, "-1" is returned.
• int write(int fd, const void * buf, int count)
  Writes up to count bytes to the file referenced by the file descriptor fd from the buffer starting at buf. On success, the number of bytes written are returned (zero indicates nothing was written). On error, "-1" is returned.
• int stat(const char * pathname, struct stat * buf)
  Gets information about the file designated by pathname and returns it as a stat structure in buf. In case of success, this function returns "0", otherwise, "-1".
• int mkdir(const char * pathname)
  Attempts to create a directory named pathname. In case of success, this function returns "0", otherwise, "-1".
• int rmdir(const char * pathname)
  Deletes an empty directory designated by pathname. In case of success, this function returns "0", otherwise, "-1".