How are file sharing and protection implemented?
To implement file protection and sharing, the system ought 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 user) and group. The owner is the user who is 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 is able to share access to the file. The group IDs and owner of a file (or directory) are stored with the other file attributes. When a user (owner) requests an operation on a file the user ID can be evaluate 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.