FILE CONCEPTIt provides the mechanism for online storage of and access to both data and programs of OS and all the users of the computer system.

The file system consists of two distinct parts:

collection of files each storing related data

directory structure which organizes and provides information about all the files in the system.

FILE CONCEPTA file is a named collection of related information that is recorded on secondary storage. A file is a sequence of bits, bytes, lines or records. Files represent programs and data.Data files may be numeric, alphabetic, alphanumeric or binary.Files may be free form such as text files or may be formatted rigidly.

Many different types of information may be stored in a file source programs, object programs, executable programs, numeric data, text etc. A file has a certain defined structure which depends on its type. Text file sequence of characters organized into lines Source file sequence of sub routines and functions each of which is further organized as declarations followed by executable statements.

Object file sequence of bytes organized into blocks understandable by the systems linker.

Executable file series of code sections that the loader can bring into memory and execute.

File Attributes A file is referred to by its name. A name is usually a string of characters. A files attributes vary from one OS to another but consist of these Name: symbolic file name is the only information kept in human readable form. Identifier: number which identifies the file within the file system; it is the non human readable name for the file. Type: information is needed for systems that support different types of files.

File Attributes Location: this information is a pointer to a device and to the location of the file on that device. Size: the current size of the file Protection: Access control information determines who can do reading, writing, executing etc. Time, date and user identification: This information may be kept for creation, last modification and last use.

File Attributes The information about all files is kept in the directory structure which resides on secondary storage. A directory entry consists of the files name and its unique identifier. The identifier in turn locates the other file attributes.

File Operations A file is an abstract data type. OS can provide system calls to create, write, read, reposition, delete and truncate files. Creating a file First space in the file system must be found for the file. Second, an entry for the new file must be made in the directory.

File Operations Writing a file To write a file, specify both the name of the file and the information to be written to the file. The system must keep a write pointer to the location in the file where the next write is to take place. Reading a file To read from a file, directory is searched for the associated entry and the system needs to keep a read pointer to the location in the file where the next read is to take place.

File Operations Because a process is either reading from or writing to a file, the current operation location can be kept as a per process current file position pointer.Repositioning within a file Directory is searched for the appropriate entry and the current file position pointer is repositioned to a given value. This operation is also known as file seek. Deleting a file To delete a file, search the directory for the named file. When found, release all file space and erase the directory entry.

File Operations Truncating a file User may want to erase the contents of a file but keep its attributes. This function allows all attributes to remain unchanged except for file length.

OS keeps a small table called the open file table containing information about all open files. The open () call can also accept access mode information create, read only, read write, append only, etc. This mode is checked against files permissions

File Operations

OS uses two levels of internal tables a per process table and a system wide table. The per process table tracks all files that a process has open. Stored in this table is information regarding the use of the file by the process.

File Operations

Each entry in the per process table points to a system wide open file table.

The system wide table contains process independent information.

Once a file has been opened by one process, the system wide table includes an entry for the file.

The open file table also has an open count associated with each file to indicate how many processes have the file open.

File Operations File pointer System must keep track of the last read write location as a current file position pointer.

File open count As files are closed, OS must reuse its open file entries or it could run out of space in the table. File open counter tracks the number of opens and closes and reaches zero on the last close.

File Operations

Disk location of the file The information needed to locate the file on disk is kept in memory so that the system does not have to read it from disk for each operation.

Access rights Each process opens a file a file in an access mode. This information is stored on the per process table so the OS can allow or deny subsequent I/O requests.

FILE OPERATIONSFile locks allow one process to lock a file and prevent other processes from gaining access to it. File locks are useful for files that are shared by several processes. A shared lock is where several processes can acquire the lock concurrently. An exclusive lock is where only one process at a time can acquire such a lock.

File types

A common technique for implementing file types is to include the type as part of the file name.The name is split into two parts a name and an extension separated by a period character.The system uses the extension to indicatethe type of the file andtype of operations performed on that file.

File StructureFile types also can be used to indicate the internal structure of the file.Hence some OSs impose a minimal number of file structures.MAC OS also supports a minimal number of file structures. It expects files to contain two parts a resource fork and a data fork. The resource fork contains information of interest to the user. The data fork contains program code or data traditional file contents.

Internal file structureFiles store information. When it is used, this information must be accessed and read into computer memory.

Acyclic graph directories

A tree structure prohibits the sharing of files and directories. An acyclic graph i.e. a graph with no cycles allows directories to share subdirectories and files. The same file or subdirectory may be in two different directories.

Acyclic graph directoriesWith a shared file, only one actual file exists, so anychanges made by one person are immediately visible to the other.Sharing is particularly important for subdirectories; a new file created by one person will automatically appear in all the shared subdirectories.

A common way, is to create a new directory entry called a link. A link is effectively a pointer to another file or subdirectory. For example, a link may be implemented as an absolute or a relative path name. When a reference to a file is made, we search the directory. If the directory entry is marked as a link, then the name of the real file is included in the link information. We resolve the link by using that path name to locate the real file.

Another common approach for implementing shared files is simply to duplicate all information about them in both sharing directories. Thus, both entries are identical and equal. The link is clearly different from the original directory entry; thus, the two are not equal. A major problem with duplicate directory entries is maintaining consistency when a file is modified.

An acyclic-graph directory structure is more flexible than is a simple tree structure, but it is also more complex.A file may now have multiple absolute path names. Consequently, distinct file names may refer to the same file. If we are trying to traverse the entire file system-to find a file to copy all files to backup storage-this problem becomes significant, since we do not want to traverse shared structures more than once.

Another problem involves deletion. When can the space allocated to a shared file be deallocated and reused?One possibility is to remove the file whenever anyone deletes it, but this action may leave dangling pointers to the now-nonexistent file. Worse, if the remaining file pointers contain actual diskaddresses, and the space is subsequently reused for other files, these dangling pointers may point into the middle of other files.

File ConceptContiguous logical address space

Types: DatanumericcharacterbinaryProgram

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File StructureNone - sequence of words, bytesSimple record structureLines Fixed lengthVariable lengthComplex StructuresFormatted documentRelocatable load fileCan simulate last two with first method by inserting appropriate control charactersWho decides:Operating systemProgram

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File AttributesName only information kept in human-readable formIdentifier unique tag (number) identifies file within file systemType needed for systems that support different typesLocation pointer to file location on deviceSize current file sizeProtection controls who can do reading, writing, executingTime, date, and user identification data for protection, security, and usage monitoringInformation about files are kept in the directory structure, which is maintained on the disk

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File OperationsFile is an abstract data typeCreateWriteReadReposition within fileDeleteTruncateOpen(Fi) search the directory structure on disk for entry Fi, and move the content of entry to memoryClose (Fi) move the content of entry Fi in memory to directory structure on disk

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Open FilesSeveral pieces of data are needed to manage open files:File pointer: pointer to last read/write location, per process that has the file openFile-open count: counter of number of times a file is open to allow removal of data from open-file table when last processes closes itDisk location of the file: cache of data access informationAccess rights: per-process access mode information

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Open File LockingProvided by some operating systems and file systems

Mediates access to a file

Mandatory or advisory:Mandatory access is denied depending on locks held and requestedAdvisory processes can find status of locks and decide what to do

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File Types Name, Extension

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Access MethodsSequential Access

read nextwrite next resetno read after last write(rewrite)Direct Access

read nwrite nposition to nread nextwrite next rewrite nn = relative block number

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Sequential-access File

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Simulation of Sequential Access on Direct-access File

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Example of Index and Relative Files

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Directory StructureA collection of nodes containing information about all files

F 1F 2F 3F 4F nDirectoryFilesBoth the directory structure and the files reside on diskBackups of these two structures are kept on tapes

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Disk StructureDisk can be subdivided into partitionsDisks or partitions can be RAID protected against failureDisk or partition can be used raw without a file system, or formatted with a file systemPartitions also known as minidisks, slicesEntity containing file system known as a volumeEach volume containing file system also tracks that file systems info in device directory or volume table of contentsAs well as general-purpose file systems there are many special-purpose file systems, frequently all within the same operating system or computer

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A Typical File-system Organization

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Operations Performed on DirectorySearch for a file

Create a file

Delete a file

List a directory

Rename a file

Traverse the file system

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Organize the Directory (Logically) to ObtainEfficiency locating a file quickly

Naming convenient to usersTwo users can have same name for different filesThe same file can have several different names

Grouping logical grouping of files by properties, (e.g., all Java programs, all games, )

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Single-Level DirectoryA single directory for all users

Naming problem

Grouping problem

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Two-Level DirectorySeparate directory for each user

Path nameCan have the same file name for different userEfficient searchingNo grouping capability

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Tree-Structured Directories

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Tree-Structured Directories (Cont.)Efficient searching

Grouping Capability

Current directory (working directory)cd /spell/mail/progtype list

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Tree-Structured Directories (Cont)Absolute or relative path nameCreating a new file is done in current directoryDelete a file

rm Creating a new subdirectory is done in current directory

mkdir Example: if in current directory /mailmkdir countmailprogcopyprtexpcountDeleting mail deleting the entire subtree rooted by mail

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Acyclic-Graph DirectoriesHave shared subdirectories and files

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Acyclic-Graph Directories (Cont.)Two different names (aliasing)

If dict deletes list dangling pointer

Solutions:Backpointers, so we can delete all pointersVariable size records a problemBackpointers using a daisy chain organizationEntry-hold-count solution

New directory entry typeLink another name (pointer) to an existing fileResolve the link follow pointer to locate the file

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General Graph Directory

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General Graph Directory (Cont.)How do we guarantee no cycles?Allow only links to file not subdirectoriesGarbage collectionEvery time a new link is added use a cycle detection algorithm to determine whether it is OK

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File System MountingA file system must be mounted before it can be accessed

A unmounted file system (i.e., Fig. 11-11(b)) is mounted at a mount point

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(a) Existing (b) Unmounted Partition

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Mount Point

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File Sharing File sharing is desirable for users who want to collaborate and to reduce the effort required to achieve a computing goal.

Multiple users: When an OS accommodates multiple users, the issues of file sharing, file naming and file protection become preeminent.

System mediates file sharing.

The system can either allow a user to access the files of other users by default or require that a user specifically grant access to the files.

File SharingSharing 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

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File Sharing Multiple UsersUser IDs identify users, allowing permissions and protections to be per-user

Group IDs allow users to be in groups, permitting group access rights

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File Sharing Remote File SystemsUses networking to allow file system access between systemsManually via programs like FTPAutomatically, seamlessly using distributed file systemsSemi automatically via the world wide webClient-server model allows clients to mount remote file systems from serversServer can serve multiple clientsClient and user-on-client identification is insecure or complicatedNFS is standard UNIX client-server file sharing protocolCIFS is standard Windows protocolStandard operating system file calls are translated into remote callsDistributed Information Systems (distributed naming services) such as LDAP, DNS, NIS, Active Directory implement unified access to information needed for remote computing

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File Sharing Failure ModesRemote file systems add new failure modes, due to network failure, server failure

Recovery from failure can involve state information about status of each remote request

Stateless protocols such as NFS include all information in each request, allowing easy recovery but less security

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Protection

When information is stored in a computer system, it should be kept safe from physical damage (reliability) and improper access (protection). Reliability is provided by duplicate copies of files. Protection can be provided in many ways such as physically removing the floppy disks and locking them up.

Types of Access

Complete protection to files can be provided by prohibiting access. Systems that do not permit access to the files of other users do not need protection.Both these approaches are extreme. Hence controlled access is required.

Protection mechanisms provide controlled access by limiting the types of file access that can be made.

Several different types of operations may be controlled i. Read ii. Write iii. Execute iv. Append v. Delete vi. List

Other operations such as renaming, copying etc may also be controlled.

Access Control

The most common approach to the protection problem is to make access dependent on the identity of the user. The most general scheme to implement identity- dependent access is to associate with each file and directory an access- control list (ACL) specifying user names and the types of access allowed for each user.

Access Control

The main problem with access lists is their length.

Three classifications of users in connection with each file: a) Owner user who created the file b) Group set of users who are sharing the file and need similar access c) Universe all other users in the system

Access Lists and GroupsMode of access: read, write, executeThree classes of users

RWXa) owner access 71 1 1RWXb) group access 6 1 1 0RWXc) public access1 0 0 1Ask 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.

ownergrouppublicchmod761gameAttach a group to a file chgrp G game

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Access Control

With the more limited protection classification, only three fields are needed to define protection. Each field is a collection of bits and each bit either allows or prevents the access associated with it. A separate field is kept for the file owner for the files group and for all the other users.

Other Protection Approaches Another approach to protection problem is to associate a password with each file. If the passwords are chosen randomly and changed often, this scheme may be effective in limiting access to a file. Use of passwords has certain disadvantages The number of passwords that a user needs to remember may become large making the scheme impractical. If only one password is used for all the files, then once it is discovered, all files are accessible.

Silberschatz, Galvin and Gagne 2009Operating System Concepts 8th Edition

End of Chapter 10

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