Many operating systems, including Unix, are multiuser. As such, it is necessary to have mechanisms which protect a user's file from unwanted access. An integral part of an access control mechanism in a multiuser operating system is the concept of user ownership of a file and, very often, the concept of group ownership. However, many operating systems, such as those for personal computers, are single-user operating systems. In a single-user environment, there is no need for file access mechanisms which protect files from unwanted access by others because, conceptually, there is only one user. Nonetheless, some single-user operating systems permit a file to be marked as read-only to protect the file from inadvertent modification or removal.
Both single-user and multiuser operating systems have been used as servers providing transparent file access. Most multiuser operating systems are already well suited for use as a file server since access protection is already an integral part of its design. For a single-user operating system, it is necessary that some sort of access control mechanism be implemented when such an operating system is used to provide file service. This is usually accomplished by creating a separate partition for each user's files. Thus, user identification is not associated with an individual file but with a partition on the server's disk. File servers which have single-user operating systems may also have partitions that are accessible by anyone.
Network environments include both servers based on multiuser operating systems and servers based on single-user operating systems. Application programs on clients may have transparent access to servers which are based on single-user operating systems which may neither provide owner information nor group ownership information for a file, but are capable of imposing some level of file access control. That file access control may be no more than a read/write permission that applies to any user. A very large group of applications are able to function in such an environment.