1. Lecture 44 Syed Mansoor Sarwar
Operating Systems
Lecture 44
Syed Mansoor Sarwar

2. © Copyright Virtual University of Pakistan
Agenda for Today
Review of the previous lecture
File Sharing
File Protection
In-Memory Data Structures
Space Allocation Techniques
Contiguous, Linked, Index
UNIX Allocation
File Allocation Table (FAT)
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Review of Lecture 43
Directory Structures
Links in UNIX/Linux
File System Mounting
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File Sharing
Sharing of files on multi-user systems is desirable.
Sharing may be done through
Duplicating files
Common login for members of a team
Setting appropriate access permissions
Common groups for members of a team
Links
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Protection
File owner/creator should be able to control
What can be done
By whom
Types of access
Read
Write
Execute
Append
Delete
List
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UNIX Protection
Mode of access: read, write, and execute (r, w, x)
Three classes of users: owner, group, and others
r w x
a) Owner access: 7  b) Group access: 6  1 1 0
c) Public access: 1  0 0 1
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UNIX Protection
Ask system administrator to create a group and add some users to the group
For a particular file or subdirectory, set appropriate access permissions for proper protection
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Default Permissions
777 for executable files and directories
666 for text files
Can be changed with the umask command
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UNIX Protection
chmod game1
owner
group
others
Read Write Execute
Read Write
Execute
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UNIX Protection
chmod projectAthena
owner
group
others
Read Write Search
Read Search
Read Search
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UNIX Protection
Sample commands
chmod 700 ~
chmod 744 ~/file
chmod 755 ~/directory
ls –l ~
ls –ld ~
ls –l prog1.c
ls –ld ~/courses
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UNIX Protection
The umask Command
Sets default permissions on newly created files and directories as (default permissions – mask value)
umask
umask 022
touch temp1
ls –l temp1
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File Control Block
UNIX inode
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14. In-Memory Data Structures
The following upper-level data structures needed for file system support.
Mapping directory entry for the file to the file control block at the time of opening a file.
Mapping used while reading or writing a file.
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15. In-Memory Data Structures
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16. Per Process File Descriptor Table
File Descriptor to File Contents
Per Process File Descriptor Table
File Table
Inode Table
File Descriptor 1 2 3 4
OPEN_MAX — 1
File’s contents … … … … …
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17. Space Allocation Methods
Contiguous allocation
Linked allocation
Indexed allocation
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18. Contiguous Allocation
Each file occupies a set of contiguous blocks on the disk
Best-fit, first-fit, or worst-fit algorithm
Directory entry contains starting block number and file size (in blocks)
Good sequential and random access
External fragmentation
User needs to declare file size
Expensive file growth
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19. Contiguous Allocation
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Linked Allocation
Each file is a linked list of disk blocks: blocks may be scattered anywhere on the disk.
Simple – need only starting address
No wastage of space
No random access
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Linked Allocation
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Index Allocation
Brings all pointers together into the index block
Logical view
Index
table
Data
blocks
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Index Allocation
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Recap of Lecture
File Sharing
File Protection
In-Memory Data Structures
Space Allocation Techniques
Contiguous, Linked, Index
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Operating Systems
Lecture 44
Syed Mansoor Sarwar
19 November 2018
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