Files are sequences of data organized in a structure defined by their creator. They have attributes like a name, size, and permissions. There are different file structures - text files are sequences of characters organized into lines, while object files contain machine-readable blocks. Files are stored in a directory and accessed using methods like sequential, direct, or indexed sequential access. The operating system allocates disk space to files using allocation methods like contiguous, linked, or indexed allocation and provides protection through access controls and permissions.

1. files

2. What are files?
 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).

3. File structure
 File structure is a structure, which is according to a
required format that operating system can
understand.
 A file has a certain defined structure according to its
type.
 A text file is a sequence of characters organized into
lines.
 A source file is a sequence of procedures and
functions.
 An object file is a sequence of bytes organized into
blocks that are understandable by the machine.

4. File attributes
 Name – only information kept in human-readable form
 Identifier – unique tag (number) identifies file within file system
 Type – needed for systems that support different types
 Location – pointer to file location on device
 Size – current file size
 Protection – controls who can do reading, writing, executing
 Time, date, and user identification – data for protection,
security, and usage monitoring
 Information about files are kept in the directory structure, which is
maintained on the disk

5. File operations
 Create
Write
 Read
 Reposition within file
 Delete
 Truncate

6. File types

7. Functions of file system
 File system in OS provide the way to create and
access the files.
 How the memory is allocated to files and how the
addresses are saved regarding to each file.

8. File access methods
 File access refers to the manner in which the
records of a file may be accessed.
 There are several ways to access files
 Sequential access
 Direct/Random access
 Indexed sequential access

9. Sequential access
 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.

10.  This access method is the most common one.
 Example: Compilers usually access files in this fashion.
 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.
 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.
 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.

11. Direct / random access
 Random access file organization provides, accessing the records
directly.
 Each record has its own address on the file with by the help of
which it can be directly accessed for reading or writing.
 The records need not be in any sequence within the file and they
need not be in adjacent locations on the storage medium.

12.  Method useful for disks.
 There are no restrictions on which blocks are read/written
in any order.
 User now says "read n" rather than "read next".

14. Indexed sequential access
 This approach combines the advantages of both
sequential and direct file access.
 An index is created for each file which contains
pointers to various blocks.
 Index is searched sequentially and its pointer is
used to access the file directly.

15. Example to understand Indexed
Sequential Access
(Prime Area)

16. File allocation methods
 Files are allocated disk spaces by operating system.
 Allocation methods are for
 Effective disk space utilization
 Allow fast file access
 Operating systems deploy following three main
ways to allocate disk space to files.
 Contiguous Allocation
 Linked Allocation
 Indexed Allocation

17. Contiguous allocation
 Each file occupies a set of contiguous blocks
on the disk.
Simple – only starting location (block ) and
length (number of blocks) are required.
Random access.
Can be used for both sequential and direct
files.

18. Example:
 If file is n block long and starts at location b, then, the blocks
occupied are b, b+1, b+2…..b+n-1 .
 The directory entry for file has
 Name
 Starting address
 Length

19. problems
 Finding space for a new file or a resized file.
 Determining size requirements.
 External fragmentation of the hard disk.

20. Linked allocation
 Each file is a linked list of disk blocks, scattered
anywhere on the disk.
 Directory contains link / pointer to first block of a
file.
 There's no external fragmentation since each
request is for one block.
 Method can only be effectively used for sequential
files.
 This method is inefficient for direct files.

21. EXAMPLE

22. Indexed allocation
 Provides solutions to problems of contiguous and
linked allocation.
 A index block is created having all pointers to files.
 Each file has its own index block which stores the
addresses of disk space occupied by the file.
 Directory contains the addresses of index blocks of
files.

23. example

24.  Method suffers from wasted space since, for small files, most of the index block is
wasted
 If the index block is too small, we can:
 Link several together
 Use a multilevel index
 Combined scheme

25. protection
 We want to keep our information safe from
physical damage(reliability) and improper
access(protection).
 We can protect our information by controlled
access.
 Types of access
 Read
 Write
 Execute
 Delete etc.

26. Access control
 This is the most common approach.
 ALCs(access control lists) are maintained.
 The main problem of ALCs is their length.
 The technique has two undesirable consequences:
 Construction of list is tedious.
 Size of the directory once defined cannot be
changed

27.  This problem can be resolved by using condense
access list.
 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.
 Eg. UNIX system defines three fields i.e. rwx
 r- controls read access
 w- controls write access
 x- controls execution
 DIFFICULTY: assigning the precedence when
permission and ACLs conflict.

28. Other methods of protection
 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.

29. Thank you