FILES

PREPEARED BY- BIBHUDENDRA SARANGI

What are files?

^𝗈 In general, a file is a sequence of bits, bytes, lines or

records whose meaning is defined by the files

creator and user.

The file can be indexed, structured, etc. The file is an entry in a directory.

The file may have attributes (name, creator, date, type, permissions).

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File structure

^𝗈 File structure is a structure, which is according to a

required format that operating system can

understand.

^𝗈 A file has a certain defined structure according to its type.

^𝗈 A text file is a sequence of characters organized into lines.

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source file is a sequence of procedures and

functions.

^𝗈 An object file is a sequence of bytes organized into blocks that are understandable by the machine.

File attributes

^𝗈 Name – only information kept in human-readable form

^𝗈 Identifier – unique tag (number) identifies file within file system

^𝗈 Type – needed for systems that support different types

^𝗈 Location – pointer to file location on device

^𝗈 Size – current file size

^𝗈 Protection – controls who can do reading, writing, executing

^𝗈 Time, date, and user identification – data for protection, security, and usage monitoring

^𝗈 Information about files are kept in the directory structure, which is maintained on the disk

File operations

^𝗈 Create

^𝗈 Write

^𝗈 Read

^𝗈 Reposition within file

^𝗈 Delete

^𝗈 Truncate

File types

Functions of file system

^𝗈 File system in OS provide the way to create and access the files.

^𝗈 How the memory is allocated to files and how the addresses are saved regarding to each file.

File access methods

• There are several ways to access files
• Sequential access
• Direct/Random access
• Indexed sequential access

^𝗈 File access refers to the manner in which the records of a file may be accessed.

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Sequential access

^𝗈 A sequential access is that in which the records are accessed in some sequence i.e the information in the file is processed in order, one record after the other.

^𝗈 This access method is the most common one.

^𝗈 Example: Compilers usually access files in this fashion.

^𝗈 Retrieving from sequential file:

• To access records, it is necessary to read the file from beginning to examine each record in sequence until the desired record is located.

^𝗈 Appending a sequential file:

The append operation refers to adding more records to an existing file.

The following operations are done for the purpose of appending a file.

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• Open the file.

Read the file till EOF is encountered. Read the data to be added.

Write the data on the file. Close the file.

Direct / random access

^𝗈 Random access file organization provides, accessing the records directly.

^𝗈 Each record has its own address on the file with by the help of which it can be directly accessed for reading or writing.

^𝗈 The records need not be in any sequence within the file and they need not be in adjacent locations on the storage medium.

^𝗈 Method useful for disks.

^𝗈 There are no restrictions on which blocks are read/written in any order.

^𝗈 User now says "read n" rather than "read next".

Indexed sequential access

^𝗈 This approach combines the advantages of both sequential and direct file access.

^𝗈 An index is created for each file which contains pointers to various blocks.

^𝗈 Index is searched sequentially and its pointer is used to access the file directly.

Example to understand Indexed Sequential Access

(Prime Area)

File allocation methods

^𝗈 Files are allocated disk spaces by operating system.

^𝗈 Allocation methods are for

• Effective disk space utilization
• Allow fast file access

^𝗈 Operating systems deploy following three main

ways to allocate disk space to files.

• Contiguous Allocation
• Linked Allocation
• Indexed Allocation

Contiguous allocation

^𝗈 Each file occupies a set of contiguous blocks on the disk.

^𝗈 Simple – only starting location (block ) and length (number of blocks) are required.

^𝗈 Random access.

^𝗈 Can be used for both sequential and direct files.

Example:

^𝗈 If file is n block long and starts at location b, then, the blocks occupied are b, b+1, b+2…..b+n-1 .

^𝗈 The directory entry for file has

• Name
• Starting address

• Length

problems

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Finding space for a new file or a resized file.

^𝗈 Determining size requirements.

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^𝗈 External fragmentation of the hard disk.

Linked allocation

^𝗈 Each file is a linked list of disk blocks, scattered anywhere on the disk.

^𝗈 Directory contains link / pointer to first block of a file.

^𝗈 There's no external fragmentation since each request is for one block.

^𝗈 Method can only be effectively used for sequential files.

^𝗈 This method is inefficient for direct files.

EXAMPLE

Indexed allocation

^𝗈 Provides solutions to problems of contiguous and linked allocation.

^𝗈 A index block is created having all pointers to files.

^𝗈 Each file has its own index block which stores the addresses of disk space occupied by the file.

^𝗈 Directory contains the addresses of index blocks of files.

example

^𝗈 Method suffers from wasted space since, for small files, most of the index block is wasted

^𝗈 If the index block is too small, we can:

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Link several together Use a multilevel index Combined scheme

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protection

^𝗈 We want to keep our information safe from physical damage(reliability) and improper access(protection).

^𝗈 We can protect our information by controlled access.

^𝗈 Types of access

• Read
• Write
• Execute
• Delete etc.

Access control

^𝗈 This is the most common approach.

^𝗈 ALCs(access control lists) are maintained.

^𝗈 The main problem of ALCs is their length.

^𝗈 The technique has two undesirable consequences:

• Construction of list is tedious.
• Size of the directory once defined cannot be changed

^𝗈 This problem can be resolved by using condense access list.

^𝗈 Three classifications of users is defined:

• OWNER: the user who creates the file.
• GROUP: a set of user who are sharing the file and need similar access.
• UNIVERSE: all other users in the system.

^𝗈 Eg. UNIX system defines three fields i.e. rwx

• r- controls read access
• w- controls write access
• x- controls execution

^𝗈 DIFFICULTY: assigning the precedence when permission and ACLs conflict.

Other methods of protection

^𝗈 PROTECTION THROUGH PASSWORD:

• To associate password with each file.
• Use a separate password for each file. But it would be a tedious job for the user to remember all the passwords.
• Solution to this is to choose same password but if the password is once known to someone else than all the data will be revealed.
• Hence, the security in this method is on all or nothing basis.

Thank you