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Operating Systems Lecture No.
Operating Systems
Lecture No. 42
Reading Material
Chapter 11 of the textbook Lecture 42 on Virtual TV
Summary
File Concept File Types File Operations Access Methods Directories Directory Operations Directory Structure
The File Concept
Computers can store information on several different storage media, such as magnetic disks, magnetic tapes and optical disks. The operating system abstracts from the physical properties of its storage devices to define a logical storage unit (the file). Files are mapped by the OS onto physical devices. These storage devices are usually non-volatile, so the contents are persistent through power failures, etc. A file is a named collection of related information that is recorded on secondary storage. Data cannot be written to secondary storage unless they are within a file. Commonly, files represent programs (source and object forms) and data. Data files may be numeric, alphabetic, alphanumeric or binary. In essence it is a contiguous logical address space.
File Structure
A file has certain defined structure characteristics according to its type. A few common types of file structures are: NNoonnee – file is a sequence of words, bytes SSiimmppllee rreeccoorrdd ssttrruuccttuurree Lines Fixed length Variable length CCoommpplleexx SSttrruuccttuurreess Formatted document Relocatable load file
UNIX considers each file to be a sequence of bytes; no interpretation of these bytes is made by the OS. This scheme provides maximum flexibility but little support. Each application program must include its own code to interpret an input file into the
appropriate structure. However all operating systems must support at least one structure- that of an executable file-so that the system is able to load and run programs.
File Attributes
Every file has certain attributes, which vary from one OS to another, but typically consist of these: Name: The symbolic file name is the only information kept in human-readable form Type: This information is needed for those systems that support different types. Location: This location is a pointer to a device and to the location of the file on that device. Size: The current size of the file (in bytes, words or blocks) and possibly the maximum allowed size are included in this attribute. Protection: Access control information determines who can do reading , writing, etc. Owner Time and date created: useful for security, protection and usage monitoring. Time and date last updated: useful for security, protection and usage monitoring. Read/write pointer value
Where are Attributes Stored? File attributes are stored in the directory structure, as part of the ddiirreeccttoorryy eennttrryy for a file, e.g., in DOS, Windows, or in a separate data structure; in UNIX/Linux this structure is known as the ininooddee for the file.
Directory Entry
A file is represented in a directory by its directory entry. Contents of a directory entry vary from system to system. For example, in DOS/Windows a directory entry consists of file name and its attributes. In UNIX/Linux, a directory entry consists of file name and inode number. Name can be up to 255 characters in BSD UNIX compliant systems. Inode number is used to access file’s inode. The following diagrams show directory entries for DOS/Windows and UNIX/Linux systems.
DO DOSS//WWiinnddoowwss
UUNNIIXX//LLiinnuuxx
File Operations
Various operations can be performed on files. Here are some of the commonly supported operations. In parentheses are written UNIX/Linux system calls for the corresponding operations.
FAT
Poi
File Name Date …
Inode Number
File Name
The following tables shows some of the commonly supported file extensions on different operating systems.
Common file types
File Types in UNIX
UNIX does not support supports seven types of file: Ordinary file : used to store data on secondary storage device, e.g., a source program(in C), an executable program. Every file is a sequence of bytes. Directory : contains the names of other files and/or directories. Block-special file : correspond to block oriented devices such as a disk. They are used to access such hardware devices. Character-special file : correspond to character oriented devices, such as keyboard Link file (created with the ln –s command): is created by the system when a symbolic link is created to an existing file, allowing you to rename the existing file and share it without duplicating its contents without FIFO (created with the mkfifo or mknod commands or system calls): enable processes to communicate with each other. A FIFO(name pipe) is an area in the kernel that allows two processes to communicate with each other provided they are running on the same system , but the processes do not have to be related to each other. Socket (in BSD-compliant systems—socket): can be used by the process on the same computer or on different computers to communicate with each other.
File Access
FFiilleess ssttoorree iinnffoorrmmaattiioonn tthhaatt ccaann bbee aacccceesssseedd iinn sseevveerraall wwaayyss::
SSeeqquueennttiiaall AAcccceessss Information in the file is processed in order, one record after the other. A read operation reads the next potion of the file and automatically advances a file pointer which tracks the I/O location. Similarly, a write operation appends to the end of the file and advances to the end of the newly written material. Such a file can be rest to the beginning and on some systems; a program may be able to skip forward or backward, n records.
Sequential Access File
DDiirreecctt AAcccceessss A file is made up of fixed length logical record that allow program to read and write records in no particular order. For the direct-access method, the file operations must be modified to include the block number as a parameter (read n (n = relative block number), write n for instance). An alternate approach is to retain read next and write next and to add an operation, position file to n , where n is the block number. The block number provided by the user to the OS is normally a relative block number , an index relative to the beginning of the file.
Sequential Access on a Direct Access File
Directory Structure
It is a collection of directory entries. To manage all the data, first disks are split into one or more partitions. Each partition contains information about files within it. This information is kept within device directory or volume table of contents.
F 1 F 2 F 3 F 4 F n
Directory
Files and Directories
When a user refers to a particular file, only his own user file directory (UFD) is searched. Thus different users can have the same file name as long as the file names within each UFD are unique. This directory structure allows efficient searching. However, this structure effectively isolates one user from another, hence provides no grouping capability.
Tree Directory Here is the tree directory structure. Each user has his/her own directory (known as user’s home directory) under which he/she can create a complete directory tree of his/her own.
Tree directory structure
The tree has a root directory. Every file in the system has a unique pathname. A path name is the path from the root, through al the subdirectories to a specified file. A directory/subdirectory contains a set of files or subdirectories. In normal use, each user has a current directory. The current directory should contain most of the files that are of current interest to the user. When a reference to a file is made, the current directory is searched. If a file is needed that is not in the current directory, then the user must either specify a path name or change the directory to the directory holding the file( using the cd system call).This structure hence supports efficient searching. Allowing the user to define his own subdirectories permits him to impose a structure on his files.A lso users can access files of other users.
UNIX / Linux Notations and Concepts Root directory (/) Home directory o ~, $HOME, $home o cd ~ o cd Current/working directory ( .. ) o pwd Parent of Current Directory ( .... ) Absolute Pathname o Starts with the root directory o For example, /etc, /bin, /usr/bin, /etc/passwd, /home/students/ibraheem Relative Pathname
o Starts with the current directory or a user’s home directory o For example, ~/courses/cs604, ./a.out
