Directories and Links

Optional readings for this topic from Operating Systems: Principles and Practice: Sections 13.1-13.2.

Naming: how do users refer to their files? How does OS find file, given name?

First step: inode has to be stored on disk, so it will persist across system reboots.

Early UNIX versions: all inodes stored in a fixed-size array on disk.

  • Originally: entire inode array was at the outer edge of the disk. Result: long seeks between inodes and file data. First improvement: place inode array mid-way across disk. Second improvement: many small inode arrays spread across disk, so inodes can be near to file data.

Space for inodes is fixed when the disk is initialized, and can't be changed.

UNIX/Linux terms:

  • Index of inode in the inode array: i-number. Internally the OS uses the i-number as an identifier for the file.

When a file is open, its inode is kept in main memory. When the file is closed, the inode is written back to disk.

File naming: users want to use text names to refer to files. Special disk structures called directories are used to map names to i-numbers.

Early approaches to directory management:

  • A single directory for the entire disk:
    • Many early personal computers worked this way.
  • A single directory for each user (e.g. TOPS-10):
    • Avoids problems between users, but still makes it hard to organize information.

Modern systems support hierarchical directory structures. UNIX/Linux approach:

  • Directories are stored on disk just like regular files (i.e. inode with 14 pointers, etc.) except inode has special "type" bit set to indicate that it's a directory.
  • A simple approach: each directory contains <name, i-number> pairs in no particular order.
  • The file referred to by the i-number may be another directory. Hence, get hierarchical tree structure. Names have slashes separating the levels of the tree.
  • There is one special directory, called the root. This directory has no name; it has i-number 2 (i-numbers 0 and 1 have other special purposes).
  • Only the operating system can read and write directories.

Working directories

Cumbersome to specify the full path name for all files.

Have OS remember the i-number for one distinguished directory per process, called the working directory.

If a file name doesn't start with "/" then it is looked up starting in the working directory.

Names starting with "/" are looked up starting in the root directory.


UNIX hard links:

  • It is possible for more than one directory entry to refer to a single file.
  • UNIX uses reference counts in inodes to keep track of hard links referring to a file.
  • Files are deleted when the last directory entry goes away.
  • Must prevent circularities (don't allow links to directories).

Symbolic links:

  • A file whose contents are another file name.
  • Stored on disk like regular files, but with special flag set in inode.
  • If a symbolic link is encountered during file lookup, prepend link value to remaining path, continue lookup.