FILE MANAGEMENT

ALL ABOUT FILE MANAGEMENT
In this section we take a brief look at how the operating system deals with files.

What is a file?
A file of a collection of data that normally is stored on a secondary storage device such as a hard disk or floppy diskette.

What are the typical operations performed on files?
An operating system 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

What are File Control Blocks?
File control blocks (FCB), sometimes referred to as file descriptors, are data structures that hold information about a file. When an operating system 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

Filename
Location of file on secondary storage
Length of file
Date and time or creation or last access

What about how we name files?
Each operating system uses a specific convention or practice for naming files.
MS-DOS Uses eight character file names, a dot, then a three-character extension that denotes the type 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.
How does an operating system keep track of files?
The hard disk is comprised of a large number of sequentially numbered sectors. As files are created, free sectors are allocated to hold the file 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 operating system maintains a number of tables.

What is a root file system?
When the operating system is first installed, it creates a root file system on the disk that specifies how many sectors are available 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 full, no more entries can be added.
Each entry can be either a file or another directory table. The following table depicts this structure.

What does a root file system entry look like?
This is highly operating system specific, but an entry might look like,

Name of file
Beginning cluster number
Length of file in bytes
Type of file
Creation date and last modified right
File permissions (an access control list)