Virtualization

• Time-sharing: Each user has the impression of working on a dedicated computer.
• Virtual memory: Each process has the impression of a dedicated address space.

Files*

• An array of bytes that persist.
• Can be found via a path.
  • Paths can be used to look up files…
  • …but files and paths change independently
    • file contents can change w/o changing path
    • path can change w/o changing file
• No user-meaningful names.

Directories

• Set of entries $(n \to i)$
  • $n$: user-meaningful name
  • $i$: file’s inode
  • e.g.: foo.c$\to 925551$
• Basically just contains a list (entry name, inode number) of pairs.
• Maps names to inodes.
• For each file or directory in a given directory, there is a string and a number in the data blocks(s) of the directory.

Example:

Directory dir (inode #5) has three files:
- foo (inode #12)
- bar (inode #13)
- foobar (inode #24)

The on-disk data for dir might look like this:

inum	reclen	strlen	name
5	12	2	.
2	12	3	..
12	12	4	foo
13	12	4	bar
24	36	28	foobar

Each directory has two extra entries:

• “dot” .: current directory (dir in this case).
• “dot-dot” ..: parent directory (root in this case).

inode

• index node: Integer index that names a location on a disk. The true and realest name a file has.
• Data structure that describes the structure that holds the metadata for a given file or directory (length, permissions, location).
• Used to identify files and directories.

Process File Abstractions

Can use syscalls to open, close, read, write, etc.

/* Open or close a file: */
int      open(const char *, int, ...);
int      close(int);

/* Sequential reading and writing: */
ssize_t  read(int, void *, size_t);
ssize_t  write(int, const void *, size_t);

/* Random reading and writing: */
off_t    lseek(int, off_t, int);
ssize_t  pread(int, void *, size_t, off_t);
ssize_t  pwrite(int, const void *, size_t, off_t);

/* Directories and metadata: */
int      mkdir(const char *, mode_t);
int      rename(const char *, const char *);
int      stat(const char *, struct stat *);
int      unlink(const char *);

File I/O SysCalls

From a process’ perspective

• syscalls are C functions.
• files are named by file descriptors (e.g., FD 3)

File Descriptors

• An integer that acts as a reference to open a file within a process.
• Indices into a kernel array.
• Each process has its own file descriptor array
  • i.e., my FD 4 != your FD 4