1. ACYCLIC-GRAPH DIRECTORIES
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2. Acyclic-Graph Directories
Features
• Use a graph structure without any cycles
– A shared file or subdirectory can be created
– Which can be used by two or more users
• The above feature is useful where a group of persons
working for the same project need common
information
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3. Acyclic-Graph Directories
Example
Refer to fig. 1
The file circled with red mark is a shared file
It is contained in two subdirectories
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4. Fig. 1 Acyclic-Graph Directories
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5. Acyclic-Graph Directories
Important
• A shared file or subdirectory will exist in the file
system in two ( or more ) places at once
• A shared file (or subdirectory) is not the same as two
copies of the file
• Difference is explained in next slide
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6. Difference Between Two Copies of the File and
Shared File
• With two copies
– Each user views only the copy, not original
– If a user changes the file, the changes will not
appear in the other’s copy
• With a shared file
– There is only one file
– Any changes made by one user would be
automatically available to other users
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7. Two methods of Implementing Shared Files and
Subdirectories
1. Create a new directory entry called link
Used by many UNIX systems
2. Duplicate all information about shared files in both
sharing directories
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8. Method 1
• For a shared file the directory entry is marked as a
link
• A link may be implemented as an absolute path
name or relative path name ( a symbolic link )
• When a shared file is referenced , the link is resolved
• Links are identified by their special format or by their
having a special type
• Operating system ignores the links when traversing
the directory to preserve the acyclic structure
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9. Method 2
• Maintain duplicate entries about the shared file in
all the sharing directories
• Duplicate entries make the original and copy
indistinguishable
• Major problem is maintaining consistency if file
modified
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10. Summary
In this class, you have learnt
• Deletion of a directory from tree-structured
directories
• Advantages and limitations of tree-structured
directories
• Acyclic-graph directory and its features
• Implementation of shared files
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