1. Topics
1-Record blocking
2-File Management of
Secondary Memory
3-Reallocation and
Dynamic allocations
By: Noor Mustafa Soomro

2. 1 Record Blocking

3. Record:
• It is the collection of related data values and items. It describes
the entities and attributes.
• e.g: Employee records, Files
• , Folders etc.
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Block:
• A block sometimes called a physical record.
• It is the sequence of bytes or bits , usually containing whole numbers of
records, having max length, a block size.
• Data thus structures are said to be blocked.
• The Process of putting data into blocks is called blocking.
• While deblocking is the process of extracting data from blocks.
• Blocked Data is normally stored in a data buffer and read written a whole
at a time.

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Three methods of blocking:
•Fixed blocking
•Variable-length spanned blocking
•Variable length un-spanned blocking

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Fixed Blocking
• Fixed length records are used and an integral number of records are
stored in a block
• There may be unused space at the end of each block (internal
fragmentation).

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Variable Length Spanned Blocking
• Variable length records are used and are packed into blocks with no
unused space
• Two records may span across two blocks with the continuation
indicated by a pointer to the successor block
• Wastes space only at the end of the file

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Variable Length Un-spanned Blocking
• Variable length records are used, but spanning is not employed.
• There is wasted space inmost blocks because of the inability to use the
remainder of a block if the next record is larger than the remaining
unused space

11. 2 File Management of
Secondary Memory

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A file issequenceof logical records i.e. a sequence of bits and
bytes.
What is File?
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FileAttributes:
A file hasvariousattributes like nam e, type, location, size,
protection, time and data of creation, user informationetc.

13. File naming=File name + File
extension
Example : Student.doc
In this example the Student is the
file name and doc is thefile
extension
File Naming
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Some Common File extensions
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File Attributes
 A file has a nameand data.
Otherthan this, it contains information likedate
and time of file creation, file’s current size etc.
witha file . These are knownas attributes of afile.

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Operations that can be performed on
files are:
1. CREATE: A blank file iscreated.
2. DELETE: The purposeof this system call is todelete this file
3. OPEN: Opena fileeither forreading orwriting.
4. CLOSE: When a file is no longeraccessed.
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 READ: When a file isonly to beread.
 Write: Towrite some dataon file.
 Append: Toadd somedata totheend of the file.
 Rename: Torename thefile.
• Note: A file can be made read-only, hidden, system-file, and so-on by setting its read onlyflag to 1, its
hidden flag to 1, itssystem flag to 1 and so on.

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File Access Methods
 Sequential Access: in this access method, data records are
retrieved in the same order in which they have been stored on the
disk. E.g data stored on magneticdisk.
 Random Access: In case of random access the record is searched
from thedisk based on its direct address information. The
technique used is Hashing. In hashing every record is associated
with a key number to preprocess the address calculation. Hash
function is used to obtain absolute address of a particularrecord.

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SequentialAccess
RandomAccess

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Index Access method:
•Indexed file approach is helpful with multiple
attribute fields like indatabase files. In these files,
every field is associatedwith an index key. While
querying data the index key is kept in the memory
and related records are fetched from the disk.

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Index Access Method

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Directory Structure
 A directory contains informationabout files.
 Adirectory is used asa means togroup the files owned by auser.
Hierarchical Directory Systems

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Access paths
• Two possible methods for access path are:
• It is a listing of the directories and files from the root directory to the intended
file. For example, the path‘c:/windows/programs/spss.exe’ means that the
root directory contains a subdirectory ‘windows’, which furthercontains a
subdirectory
• ‘programs’, that contains an executable“spss.exe”.
1. Absolute path name:

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2. Relative path name:
• This uses theconcept of current directory (also known asworking directory).
A user can specify a particular directory as his current working directory
and all the path names instead of being specified from the root directory are
specified relative to theworking directory. For example, if the current
working directory is ‘usercurr’, then the file whose absolute path is
‘usercurrstudent’ can be referred simply as‘student’.

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Directory Operations
 CREATE: A directory iscreated.
 DELETE: A directory isdeleted.
 OPENDIR: Directories can be opened forreading it.
 CLOSEDIR: When a directory has been read, it should beclosed to free up
internal table space in main memory.
 RENAME: Directories can be renamed justlike files.

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File Protection
File systems often contain information that is highly
valuable to theirusers.
Protecting this information againstunauthorized
usage is therefore, a major concern of all the file
systems.

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1. File protection through Access Control
 Following are the few file operations thatcan be controlled:
 Read
 Write
 Append
 Delete
 List
 Rename
 Edit
 Copy
- Read a file
- Write thefile
- Append afile
- Delete afile
- List the nameand attributesof a file
- Rename afile
- Changing the contents of afile
- Make a copy of afile.

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Access control list
 This list may contain the user name and the types of access allowed foreach
user. The operating system checks this access control list (associated with a file)
whenevera userrequestsan access toa particular file.
• Butone limitationof Accesscontrol list is their length, sowe
creategroups.

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• Classify the usersof a file into three types:
1. Owner - The userwho created the file.
2.Group - A setof userswhoaresharing the fileand need similaraccess.
3. Universe - All remaining users in thesystem constitute universe

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2. File protection through password
•Files can be protected bya password . The
owner of a file can control its access by
assigning a password. Thus, only those users
who know the password, can access a particular
file.

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File System Implementation
• There are 3 ways of File system implementation. They are thefollowing:
1. Contiguousallocation
2. Linked listallocation
3. Linked list allocation using anindex

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1. Contiguous allocation
• Thesimplestallocation
scheme is tostoreeach file
as a contiguous block of
data onthedisk. Thus,
on a disk having blocks
size 1k, a 25k file would be
allocated 25 consecutive
blocks.

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2. Linked list allocation
• Thesecond
method forstoring
files is to keepeach
oneas a linked list
of disk blocks, as
shown in Figure
below. The first
word of each block
is used as a pointer
to the next one .
The restof the
block is used for
storing data.

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In this technique instead
of having a pointer, an
index is maintained.
3. Linked list allocation using an index

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Free Space Management
• Freespace management is used to reuse thedisk space created after deleting the
files.
• Wehave 4 techniques for Free space management, Which are the
following:
1. Bit map
2. Linked list
3. Grouping
4. Counting

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1. Bit map
 The free space list is implemented as a bit map. Every
bit represents a block on thedisk. The bit fora block is
1 if it is freeand it is 0 if the block is allocated.

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2. Linked list
 Thisapproach maintainsa linked listof all the free
disk blocks. The first free block in the list can be
pointed out by a head pointer, which is kept in a
special location on thedisk.

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3. Grouping
 Another approach is to
store the addresses of n
free blocks in the first
free block. The last
block contains the
addressesof other n
free blocksand soon.
4. Counting
 In this technique, instead of keeping a listof addresses of n
free blocks, it is more convenient to keep the address of the
first free blocks and the number n of freecontiguous
blocks that follow the first block.

38. 3 Static allocation and
Dynamic allocations

39. Memory allocation
• The placement of blocks of information in a memory
system is called memory allocation.
• To allocate memory it is necessary to keep in information
of available memory in the system
• If sufficient memory is not available, swapping of blocks is
done.
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40. Types of memory allocation
•There are two (2) types of memory allocation
•Static memory allocation
•Dynamic memory allocation
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41. Static memory allocation
• In static memory allocation, size of the memory is required
for the calculation that must be required for the calculation
that must be define before loading and executing the
program
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42. Example:
int[] arr =new int[6];

43. Dynamic Memory Allocation
• In static memory allocation, size of the memory is required
for the calculation that must be required for the calculation
that must be define before loading and executing the
program
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44. Dynamic Memory Allocation
• Dynamic Memory Management/Allocation Dynamic Memory
Allocation is sometimes called as Manual Memory Management
• In DMA the memory is allocated at run time.
• It is allocated whenever program, application, data, variable
demands with required amount of bytes.
• We are going present, manual memory allocation
using ‘C ’ programming language because it will very easy
to show allocation and de -allocation.
The programmer has direct access to memory at run time to
control over memory using DMA

45. Dynamic Memory Allocations Functions
• It is mostly explicit call to heap management functions.
• ‘C ’ programming language has supports malloc(), calloc() and
realloc() functions to allocate memory for our program or
applications. These functions are called dynamic memory allocation
functions.
• The two key dynamic memory functions are malloc() and free().
• It returns a pointer to the allocated memory.
• If the allocation fails, it returns NULL.
• The prototype for the standard library function is like this:
void *malloc(size_t size);

46. Dynamic Memory Allocations Functions
• The free() function takes the pointer returned by malloc() and de -
allocates the memory.
• No indication of success or failure is returned.
• The function prototype is like this: void free(void *pointer);
• There are two other variants of the malloc() function: calloc() and
realloc().
• The allocated memory is also initialized to zeros.
• Here is the prototype:
void *calloc(size_t nelements, size_t elementSize);

47. Dynamic Memory Allocations Functions
• The realloc() function resizes a memory allocation previously made
by malloc().
• It takes as parameters a pointer to the memory area and the new size
that is required.
• If the size is reduced, data may be lost.
• If the size is increased and the function is unable to extend the
existing allocation, it will automatically allocate a new memory area
and copy data across.
• In any case, it returns a pointer to the allocated memory.
• Here is the prototype:
• void *realloc(void *pointer, size_t size);

48. Thank You.