1. Recap
In the last class, you have learnt:
• How directories are deleted in a tree structure
• Advantages and disadvantages of tree structure
• Acyclic graph Directory Structure
• Its features
• Implementing shared directories and files
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2. • HOME PREVIOUS TOPIC NEXT
• PREVIOUS QUESTION PAPERS FOR
OS
• CPP TUTORIALS
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3. Objectives
On completion of this period, you would be able to
• Understand issues regarding deletion of files in an
acyclic-graph structure
• List advantages and disadvantages of acyclic-graph
structure
• Understand the structure of General Graph
Directories and problems
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4. Issues In Acyclic Graph Directories
• A file may have multiple path names
• Consequently distinct file names may refer to the same
file
• When traversing the entire file system we may come
across shared structures more than once
Ex: when we copy all files to backup storage same
file may get copied more than once
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5. Issues In Acyclic Graph Directories
• Another issue is deletion of shared files
• When can the space allocated to a shared file
be de allocated and reused ?
• Two approaches may be used
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6. Approach 1
• Remove the file whenever anyone deletes the
shared file
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7. Problems with Approach 1
• Problem is dangling pointers which point to the now-
nonexistent file
• Also, If the file pointers contain actual disk
addresses
• If the space is subsequently reused for other files,
• These dangling pointers may point into the middle of
other files
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8. How to handle problems with Approach 1?
• Easy to handle if sharing is implemented by
symbolic links
• If anyone deletes a shared file delete only the link
• Deletion of a link does not affect the original file
• If file entry itself is deleted, then space is de
allocated, leaving the links dangling
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9. How to handle problems with Approach 1?
• We can search for dangling links and remove them
• The search can be expensive unless a list of the
associated links is kept with each file
• Alternatively, we can leave the links until an attempt
is made to use them
• At that time, we can determine the file name given
by the symbolic link does not exist and can fail to
resolve the link name
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10. How to handle problems with Approach 1?
In Unix
• Symbolic links are left when file is deleted
• It is up to the user to realize that the original file
is gone or has been replaced with a new file
having the same name
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11. Approach 2
• Preserve the file until all references to it are
deleted
• To implement this approach, we need a
mechanism to determine that the last reference
is deleted
• A list of all references to a file can be kept
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12. Approach 2
• When a link or a directory entry is established, a
new entry is added to the file-reference list
• When a link or directory entry is deleted, we
remove its entry on the list
• The file is deleted when its file-reference list is
empty
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13. Problems with Approach 2
• Variable and potentially large size of the file-
reference list
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14. How to handle problems with
Approach 2?
• We need not keep the entire list
• Keep only a count of number of references
• A new link or directory entry increments the
reference count
• Deleting a link or entry decrements the count
• When count is zero, the file is physically deleted
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15. Summary
In this class, you have learnt:
• Various issues regarding acyclic graph directories
like deletion
• Different ways to handle deletion
• Advantages and disadvantages of acyclic graph
directories
• General graph directories and their disadvantages
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16. Frequently Asked Questions
1. Explain how deletion is handled in acyclic
graph directories
2. List the advantages and disadvantages of
acyclic graph directories
3. Explain general graph directories
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17. 1. Pick the correct statement about acyclic graph
directories
a) Does not allow shared files
b) Easy to handle deletion
c) Allows shared files
d) None of the above
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18. 2. An acyclic graph can contain
a) No cycles
b) Only one cycle
c) Two cycles
d) Any number of cycles
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19. 3. In an acyclic graph directory, a shared file can
have
a) Multiple relative path names
b) Multiple absolute path names
c) Both of the above
d) None of the above
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20. 4. A dangling pointer points to
a) a valid address
b) an invalid address
c) Sometimes a valid address and sometimes an
invalid address
d) None of the above
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21. State true or false
A link is a pointer to a file or subdirectory
True
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22. State true or false
It is very easy to perform deletion in acyclic graph
directories
False
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23. State true or false
Dangling pointers are desirable
False
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24. State true or false
A zero count in a file-reference list indicates that the file
is physically removed from the system
True
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25. State true or false
A general graph directory does not allow cycles
False
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26. State true or false
In a general graph directory certain operations may lead
to infinite looping
True
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27. Other subject materials
• Web designing
• Micro processors
• C++ tutorials
• java
home
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