An Operating Systems Vade Mecum

200 File Structures Chapter 6file, either in the directory entry for it or as part of the file itself. (We will return to thischoice soon.) When a subject opens a file, the file manager checks that the subject islisted in the access list for the file and has the necessary privileges.Once again, this alternative has awkward features. The set of all subjects is likelyto be quite large, and many subjects will have identical privileges over the object. It isconvenient to group subjects into classes. Each subject carries both a user identifier anda group identifier. All members of the same group can be given the same privileges forthe object. The few subjects that don’t fit neatly into a class can be dealt with separately.Many operating systems follow this approach. They partition subjects into four classes:Owner of the fileUsers in the same group as the ownerOther usersUtility programs that periodically back up disk storage onto tape. (We discuss filedumps later in this chapter.)For example, Mary might set the access list for slide1.text toselfgroupworldread, writereadreadWe assume that Professor Troy, Mary’s adviser, is a member of her group. The thesischapter, chapter1.text, would have the following access list:selfgroupworldread, writereadno accessFinally, her mail file would have the following access list:selfgroupworldread, writeappendappendMultics allows the access list to contain individual user names as well as larger groupings.If Mary belongs to the same group as all of Professor Troy’s students, she may notwant to grant Read privilege over chapter1.text to the entire group. Instead, shemight set the access list like this: