How are file sharing and protection implemented?
To execute file sharing and protection the system have to maintain more file and directory attributes than are needed on a single user system. The most generally used concept is file or directory, owner or and group. The owner is the user who are able to change attributes and who can grant access. The owner as well has the most control over the file.
The group attribute defines a subset of users who are able to share access to the file. The group IDs of a file (or directory) and owner are stored with the other file attributes. When a user desires an operation on a file then the user ID can be compared with the owner attribute to determine if the requesting user is the owner of the file. Similarly, the group IDs is able to be compared. The result point to which permissions are applicable. The system after that applies those permissions to the requested operation and allows or denies it.