Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition, Chapter 10: File-System Interface

10.2 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Chapter 10: File-System Interface File Concept Access Methods Directory Structure File-System Mounting File Sharing Protection

10.3 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Objectives To explain the function of file systems To describe the interfaces to file systems To discuss file-system design tradeoffs, including access methods, file sharing, file locking, and directory structures To explore file-system protection

10.4 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition File Concept Contiguous logical address space Types: Data  numeric  character  binary Program

10.5 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition File Structure Possible Structures None -- sequence of words, bytes Simple record structure  Lines  Fixed length  Variable length Complex structures  Formatted document  Relocatable load file Who decides: Operating system Program

10.6 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition File Attributes Attributes of a file Name – only information kept in human-readable form Identifier – unique tag (number) identifies file within file system  e.g. i-node number Type – needed for systems that support different types Location – pointer to file location on device  Block number Size – current file size Protection – controls who can do reading, writing, executing Time, date, and user identification – data for protection, security, and usage monitoring Initial information about files are kept in the directory structure, which is maintained on the disk

10.7 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition File Operations File is an abstract data type Create Write Write at current file-position pointer (cp), update cp Read Read at cfp, update cp Reposition within file Reposition the cp to a given value Delete Truncate – reset file length to 0, keep all other file attributes Open(F) – search the directory structure on disk for entry F, and move the content of entry to memory Close (F) – move the content of entry F in memory to directory structure on disk

10.8 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Open Files Several pieces of data are needed to manage open files: File pointer: pointer to last read/write location, per process that has the file open File-open count: counter of number of times a file is open – to allow removal of data from open-file table when last process closes it Disk location of the file: cache of data access information Access rights: per-process access mode information

10.9 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition File Types – Name, Extension If an operating system recognizes the type of a file, it can then operate on the file in reasonable ways. For example, only a file with a.com,.exe, or.bat extension can be executed Application programs also use extensions to indicate file types in which they are interested Microsoft Word word processor expects its files to end with a.doc extension The UNIX system uses a crude magic number stored at the beginning of some files to indicate roughly the type of the file -- executable program, batch file (or shell script), PostScript file, and so on. Not all files have magic numbers

10.10 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition File Types – Name, Extension

10.11 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Access Methods Sequential access read next write next reset Direct access read n write n n = relative block number

10.12 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Sequential-access File

10.13 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Simulation of Sequential Access on Direct-access File

10.14 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Directory Similar to telephone directory (also called a telephone book) A collection of nodes containing information about all files Both the directory structure and the files reside on disk F 1 F 2 F 3 F 4 F n Directory Files

10.15 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Disk Structure Disk can be subdivided into partitions Disk or partition can be used raw – without a file system, or formatted with a file system Entity containing file system known as a volume Each volume containing file system also tracks that file system’s info in device directory or volume table of contents As well as general-purpose file systems there are many special-purpose file systems, frequently all within the same operating system or computer

10.16 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition A Typical File-System Organization

10.17 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Operations Performed on Directory Search for a file Create a file Delete a file List a directory Rename a file Traverse the file system

10.18 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Organize the Directory (Logically) to Obtain Efficiency – locating a file quickly Naming – convenient to users Two users can use same name for different files The same file can have several different names Grouping – logical grouping of files by properties, (e.g., all Java programs, all games,…)

10.19 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Single-Level Directory A single directory for all users Solve the naming problem? Solve the grouping problem?

10.20 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Two-Level Directory Separate directory for each user Path name  Example: /userb/test Can have the same file name for different users  Files ‘test’, ‘data’… No grouping capability

10.21 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Tree-Structured Directories

10.22 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Tree-Structured Directories (Cont) Users can create their own subdirectories Grouping capability  E.g., separate directories for files associated with different topics Path name Two types of path name: absolute or relative path name Each file or directory has a unique absolute path name

10.23 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Tree-Structured Directories (Cont) Current directory (working directory) – the directory that is of current interest to the process cd /spell/mail/prog type list Delete a file ‘in’ the current directory rm Solve the naming problem? ‘The same file can have several different names’

10.24 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Acyclic-Graph Directories A directory or file can be shared: it can have names that appear in different directories – can have distinct absolute path names

10.25 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Acyclic-Graph Directories Solve the naming problem?

10.26 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition General Graph Directory

10.27 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition File System Mounting A file system must be mounted before it can be accessed To mount, the OS  asks the device driver to read the device directory  verifies that the directory has the expected format  notes in its directory structure that a file system is mounted at the specified mount point An unmounted file system is mounted at a mount point

10.28 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Existing partition Unmounted Partition

10.29 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Mount Point

10.30 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition File Sharing Sharing of files on multi-user systems is desirable Sharing may be done through a protection scheme On distributed systems, files may be shared across a network Network File System (NFS) is a common distributed file-sharing method

10.31 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition File Sharing – Multiple Users Notions User IDs identify users, allowing permissions and protections to be per- user Group IDs allow users to be in groups, permitting group access rights

10.32 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Protection File owner/creator should be able to control: what can be done (types of access) by whom Types of access Read Write Execute Append Delete List (for directory)

10.33 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Access Lists and Groups Mode of access: read, write, execute Three classes of users: owner (who created the file), group (work group), and public (all other users) RWX a) owner access1 1 1  7 b) group access1 1 0  6 c) public access0 0 1  1 Example. Ask manager to create a group (unique name), say G, and add some users to the group For a particular file (say game) or subdirectory, define an appropriate access. ownergrouppublic chmod761game

10.34 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Windows XP Access-Control List Management

10.35 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition A Sample UNIX Directory Listing

Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition, End of Chapter 10