Page 223 - DCAP103_Principle of operating system

Principles of Operating Systems

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6.7 File Sharing

In the previous sections, we explored the motivation for file sharing and some of the difficulties
involved in allowing users to share files. Such file sharing is very desirable for users who want
to collaborate and to reduce the effort required to achieve a computing goal. Therefore, user
oriented operating systems must accommodate the need to share files in spite of the inherent
difficulties.

In this section, we examine more aspects of file sharing. First is the topic of multiple users and
the sharing methods possible. Once multiple users are allowed to share files, the challenge is
to extend sharing to multiple file systems, including remote file systems. Finally, there can be
several interpretations of conflicting actions occurring on shared files. For instance, if multiple
users are writing to the file, should all the writes be allowed to occur, or should the operating
system protect the user actions from each other.
6.7.1 Multiple Users

When an operating system accommodates multiple users, the issues of file sharing, file naming,
and file protection become preeminent. Given a directory structure that allows files to be shared
by users, the system must mediate the file sharing. The system either can allow a user to access
the files of other users by default, or it may require that a user specifically grant access to the files.
These are the issues of access control and protection, which are covered below. To implement
sharing and protection, the system must maintain more file and directory attributes than on a
single-user system. Although there have been many approaches to this topic historically, most
systems have evolved to the concepts of file/directory owner (or user) and group. The owner
is the user who may change attributes, grant access, and has the most control over the file or
directory. The group attribute of a file is used to define a subset of users who may share access
to the file. For example, the owner of a file on a UNIX system may issue all operations on a file,
while members of the file’s group may execute one subset of those operations, and all other users
may execute another subset of operations. Exactly which operations can be executed by group
members and other users is definable by the file’s owner. More details on permission attributes
are included in the next section. Most systems implement owner attributes by managing a list
of user names and associated user identifiers (user IDS). In Windows NT parlance, this is a
Security ID (SID). These numerical IDS are unique, one per user. When a user logs in to the
system, the authentication stage determines the appropriate user ID for the user. That user ID

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