Page 237 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes This design could be used, for example, on a multi-user computer or on a simple network of
personal computers that shared a common file server over a local area network. Implicit in this
design is that when a user tries to open a file, the operating system knows which user it is in
order to know which directory to search. As a consequence, some kind of login procedure is
needed, in which the user specifies a login name or identification, something not required with
a single-level directory system. When this system is implemented in its most basic form, users
can only access files in their own directories.

6.12.2 Hierarchical Directory Systems
The two-level hierarchy eliminates file name conflicts between users. But another problem is that
users with many files may want to group them in smaller subgroups, for instance a professor
might want to separate handouts for a class from drafts of chapters of a new textbook. What
is needed is a general hierarchy (i.e., a tree of directories). With this approach, each user can
have as many directories as are needed so that files can be grouped together in natural ways.
This approach is shown in Fig. 6.14 (c). Here, the directories A, B, and C contained in the root
directory each belong to a different user, two of whom have created subdirectories for projects
they are working on.

The ability to create an arbitrary number of subdirectories provides a powerful structuring tool
for users to organize their work. For this reason nearly all modern PC and server file systems
are organized this way.
However, as we have pointed out before, history often repeats itself with new technologies.
Digital cameras have to record their images somewhere, usually on a flash memory card. The
very first digital cameras had a single directory and named the files DSC0001.JPG, DSC0002.
JPG, etc. However, it did not take very long for camera manufacturers to build file systems with
multiple directories, as in Fig. 6.14 b). What difference does it make that none of the camera
owners understand how to use multiple directories, and probably could not conceive of any use
for this feature even if they did understand it? It is only (embedded) software, after all, and thus
costs the camera manufacturer next to nothing to provide. Can digital cameras with full-blown
hierarchical file systems, multiple login names, and 255-character file names be far behind?
6.12.3 Path Names
When the file system is organized as a directory tree, some way is needed for specifying file
names. Two different methods are commonly used. In the first method, each file is given an
absolute path name consisting of the path from the root directory to the file. As an example,
the path /usr/ast/mailbox means that the root directory contains a subdirectory usr/, which in
turn contains a subdirectory ast/, which contains the file mailbox. Absolute path names always
start at the root directory and are unique. In UNIX the components of the path are separated
by /. In Windows the separator is \ . Thus the same path name would be written as follows
in these two systems:
Windows \usr\ast\mailbox
UNIX /usr/ast/mailbox
No matter which character is used, if the first character of the path name is the separator, then
the path is absolute.
The other kind of name is the relative path name. This is used in conjunction with the concept of
the working directory (also called the current directory). A user can designate one directory as
the current working directory, in which case all path names not beginning at the root directory
are taken relative to the working directory. For example, if the current working directory is /
usr/ast, then the file whose absolute path is /usr/ast/mailbox can be referenced simply as
mailbox. In other words, the UNIX command cp /usr/ast/mailbox /usr/ast/mailbox.bak nand

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