In multiuser systems, effective file sharing involves addressing access rights and managing simultaneous access.
Access Rights:
The file system must provide flexible mechanisms for file sharing by defining various access rights for users or groups. These rights control how files can be accessed and modified. Here’s a summary of typical access rights:
1. None: The user cannot access or even see the file, as the file does not appear in the user's directory listings. This ensures total privacy.
2. Knowledge: The user can see that the file exists and who owns it but cannot access its contents. They can request additional access from the file owner.
3. Execution: The user can execute a program but cannot copy it. This is often used for proprietary software to allow running without duplication.
4. Reading: The user can view and copy the file’s contents. Some systems differentiate between just viewing and copying, where viewing displays content without permitting duplication.
5.Appending: The user can add data to the file, typically only at the end. This is useful for log files or accumulating data from multiple sources, without modifying existing data.
6. Updating: The user can modify, delete, or add data to the file. This includes creating, rewriting, and removing parts of the file. Some systems provide varying levels of update rights.
7. Changing Protection: The user can alter access rights for others. Typically, only the file owner can change these rights, though they may delegate this ability. Owners can specify which rights are modifiable.
8. Deletion: The user can remove the file from the file system entirely.
These rights are hierarchical. For instance, having updating rights implies having all rights below it, such as knowledge, execution, reading, and appending.
Files are owned by the creator, who holds all access rights and can grant permissions to others. Access can be set for:
- Specific Users: Individual users identified by their user ID.
- User Groups: Groups of users, with system mechanisms tracking group membership.
- All Users: Public files accessible to anyone on the system.
Simultaneous Access
When multiple users have rights to append or update a file, the system must manage access to prevent conflicts and maintain integrity. This involves:
- File Locking: A simple approach where a user locks the entire file during updates to prevent others from accessing it simultaneously.
- Record Locking: A more refined method where individual records within the file are locked, allowing multiple users to access different parts of the file concurrently.
These methods address the readers/writers problem, focusing on mutual exclusion and deadlock prevention to ensure smooth and consistent file access.
