File Directory

operating-systems file-management

Definition

File Directory

A directory (or folder) is a special system file that contains mapping information to translate user-defined file names into references to the physical data.

Structure

• Flat List: A single list of entries for the entire partition. Found in very early or simple operating systems.
• Hierarchical Tree: A nested structure where directories can contain files and other sub-directories. This is the standard for modern operating systems.

Pathnames

• Absolute Pathname: Specifies the location of a file starting from the root directory (e.g., /home/user/notes.txt in Unix or C:\Users\User\notes.txt in Windows).
• Relative Pathname: Locates a file relative to the Current Working Directory (CWD).
  • . (Dot): Refers to the current directory.
  • .. (Dot-dot): Refers to the parent directory.

Implementation Variations

Operating systems differ in how they store file attributes relative to the directory:

• Attributes in Directory: The directory entry contains the file name, all its attributes, and the physical address (e.g., in early Windows/MS-DOS systems).
  • Long Filenames: In Windows 95/98, long filenames are stored using “extension” directory entries marked with a special attribute byte (0x0F).
• Attributes in i-nodes: The directory entry is kept minimal, containing only the file name and an i-node number. The attributes and block pointers are stored in the referenced i-node (standard in Unix/Linux).

Operations

Standard directory operations provided by the OS include:

• create / delete: Creating or removing a directory.
• opendir / closedir: Opening or closing a directory stream.
• readdir: Reading the next entry in a directory.
• rename: Changing the name of a directory.
• link / unlink: Managing hard links to files.