File Management In An Operating System

File Management In An Operating System




In this article I take a fleeting look at how the operating system(OS) deals with files.

Definition

Its a collection of data that typically is stored on a secondary storage device such as a hard disk or floppy diskette.

The operations performed on them

An OS must provide a number of operations associated with files so that users can safely store and retrieve data.

Typical operations are

Open

Close

Create

Copy

Rename

List

In addition, operations on single data elements within a file are supported by

Read

Write

Seek

File Control Blocks

File control blocks (FCB), sometimes referred to as file descriptors, are data structures that keep up information about a file. When an OS needs to access a file, it creates an associated file control block to manage the file.

The structure of the file control block differs between operating systems, but most file control blocks include the following parts

Name

Location on secondary storage

Length

Date and time or creation or last access

Naming

Each OS uses a specific convention or practice for naming them.

MS-DOS Uses eight character names, a dot, then a three-character extension that denotes the kind of file. Filenames are not case-sensitive.

UNIX Filenames can be up to 254 characters long and are case-sensitive.

Windows Filenames can be up to 255 characters long and are not case-sensitive.

Types

Types refer to classifying the content of the file, such as a program, text, executable program or data.

In Windows operating systems, the kind is derived from the filename extension. Typical types and their extensions are

Extension

kind

.bas

basic source program

.c

c source program

.dll

system library

.doc

information document

.exe

executable program

.txt

text

Windows associates applications (programs) with specific types. For example, the default application that opens to course of action a kind.txt is the Notepad editor.

How an operating system keep track of files

The hard disk is comprised of a large number of sequentially numbered sectors. As files are produced, free sectors are allocated to keep up the its contents and marked as allocated.

To keep track of the sectors and whether they are allocated or free, and to which file they belong, the OS maintains a number of tables.

Root file system

When the OS is first installed, it creates a root file system on the disk that specifies how many sectors are obtainable and how they will be allocated.

The root file system is a table of entries like a directory. In general, this is a fixed size, and once complete, no more entries can be additional.Each entry can be either a file or another directory table.

The Root file system entry

This is highly operating system specific, but an entry might look like,

Name

Beginning cluster number

Length in bytes

kind

Creation date and last alternation right

permissions (an access control list)




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