Acyclic-graph directories use a graph structure without cycles to allow shared files and subdirectories that can be accessed by multiple users, which is useful for collaboration. They implement shared resources by either creating link entries that reference the shared file, ignoring the links to preserve the acyclic structure, or by duplicating file information in all sharing directories and ensuring consistency when modified.

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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