Files

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).

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.

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

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

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.

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

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.

 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.

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.

 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".

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.

Example to understand Indexed
Sequential Access
(Prime Area)

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

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.

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

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

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.

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.

 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

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.

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

 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.

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.

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