ITFN 2601 Introduction to Operating Systems Lecture 22 Files & Directories

Agenda Files Naming Structure Attributes Operations Directories Structure Operations

File Naming File names are used to reflect the content The name is useful to the user The extension(s) are knowledge for the OS DOS supported characters (a-z, _, -) in the filename 3 characters as an extension Most modern systems allow arbitrarily long filenames and extensions

File Structure (Byte) Byte Ordered File is a sequence of bytes Binary values System doesn’t care FAT32, NTFS, ext2fs

File Structure (Records) Files are kept as a set of records Each record contains data Records are typed Data in a record is expected to be the proper type

File Structure (Keyed) Files stored as a tree of records Each record has some “key” value Tree is restructured whenever it becomes unbalanced

File Attributes (Type) Files have a type ASCII Binary Either Executable or construct file Can be dictated by the file extension.exe is an executable windows file Can be appointed in the file All JAVA programs start with CAFÉ OS

File Attributes (Access) Files can be accessed in multiple methods Usually based on storage Sequential Access Reel-to-Reel storage media Movement is linear Random Access Platter-based media Movement is multi-dimensional

File Attributes (General) Informational Who owns the file Who can read the file System/Hidden flag Creation Time Modification Time Size Etc

File Operations Create Create a 0-size file Announces the file is coming Reserves the filename Delete Destroys the file Removes all OS information about the file

File Operations (2) Open Open the file for use Locks the file (if the open is for a write) Close Close the file after use Releases any locks held on the file

File Operations (3) Read Reads the next chunk from the file Starts from the file-cursor Write Writes the designated chunk to the file Starts at the file-cursor

File Operations (4) Append Special write that ALWAYS goes to the end of the file Seek Move the file-cursor to the location specified Rename Changes the files name Sometimes a copy-delete

File Operations (5) GetAttr Get Attributes Returns the list of file-attributes for the file Can request specific attributes SetAttr Set Attributes Sets the file-attributes Given an attribute-list to use

Directory Structure (Single Level) One directory listing All files at one level Two users can’t have the same filename Very restrictive

Directory Structure (Two-Level) Main listing shows user-directories Each user gets their own file-name-space Same filename can be used by every user! System files How does everyone get access? Copy to every user directory?

Hierarchical Directory Structures

Directory Structure (Hierarchical) Main directory lists other directories Uses paths to reach other directories for system or shared files At some level are “user” directories C:\documents and settings\jjones (Win 2k) /home/jjones (unix) User directories can have their own directory listings, too!

Directory Paths Each file is in some directory Path tells the OS how to find the file Absolute Path C:\windows\command.exe Relative Path..\windows\command.exe

Directory Operations Create Generates an empty directory Has two entries. – Current directory.. – Directory one level up Delete Removes the directory listing Generally only works on empty directories

Directory Operations (2) Opendir Opens the directory for reading by a program Necessary to retrieve a directory listing Closedir Closes the directory after a read

Directory Operations (3) Readdir Reads the next filename from the directory listing Starts at the directory-cursor

Directory Operations (4) Rename Rename the directory Link Create a special file that mimics the linked file Saves disk-space by not needing a true copy