File Concept

Silberschatz, Galvin and Gagne 200211.1Operating System Concepts

File Concept

Contiguous logical address space Smallest user allocation Non-volatile Types:

Data numeric character binary

Program Source Script Binary


File Types – Name, Extension


File Structure

Levels of structuring None - sequence of words, bytes Simple record structure

Lines Fixed length Variable length

Complex structures Formatted document Relocatable load file

Can simulate last two with first method by inserting appropriate control characters.

Need at least executable binary and data Type indicated by

“type” extension Resource fork (Mac) Magic number (UNIX)

More complexity requires more OS support


File Attributes

Name – only information kept in human-readable form. Type – needed for systems that support different types. Location – pointer to file location on device. Size – current file size. Protection – controls who can do reading, writing,

executing. Time, date – creation, access, modification. User identification – data for protection, security, and

usage monitoring.

Information about files are kept in the directory structure, which is maintained on the disk.


Directory Structure

A collection of nodes containing information about all files.

F 1 F 2F 3

F 4

F n

Directory

Files

Both the directory structure and the files reside on disk.


File Operations

Create Allocate space Create directory entry

Open Copy the directory structure on disk to memory.

Read, Write Identified file Specified position Specified data Update directory

Reposition within file – file seek Close

Copy the directory structure in memory to disk. Delete

Release space Delete directory entry

Rename, Truncate, Append, Copy (can be formed from others)


Access Methods

Sequential Accessread nextwrite next reset (rewind)no read after last write

Direct Accessread nwrite nposition to nread nextwrite next n = relative block number (start, current, end)


Directory Operations

Search for a file By name/pattern By other attributes

Create a file Add a new entry

Delete a file Delete entry

List a directory Rename a file

Potentially moves file in directory structure Traverse the file system

E.g., for backups


Organize the Directory (Logically) to Obtain

Efficiency – locating a file quickly

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

Naming – convenient to users. Two users can have same name for different files. The same file can have several different names


Tree-Structured Directories


Tree-Structured Directories

Efficient searching

Grouping capability

Absolute and relative pathnames Relative to current directory Absolute to

Root Current (which absolute to root)

No sharing yet


Acyclic-Graph Directories


Acyclic-Graph Directories

Have shared subdirectories and files Allows multiple names for one file or directory Allows sharing of files and directories

Hard links Multiple directory nodes refer to the same file No precedence Problems of directory entry consistency - UNIX solves with inodes Delete file (only) when there are no links

Alternatively, have back pointers and delete all directory entries and file when one directory entry is deleted

Beware cyclic directory links (not permitted in UNIX now)

Symbolic (soft) links Gives name of another directory entry (could also be soft) Deletion just deletes the link Have to deal with hanging links when hard link is deleted Cyclic links are possible


General Graph Directory

How do we guarantee no cycles? Allow only links to file not subdirectories. Garbage collection. Every time a new link is added use a cycle detection

algorithm to determine whether it is OK.


Protection

File owner/creator should be able to control: what can be done by whom

Modes access: Files: read, write, execute, delete Directories: read, write, make current


Access Lists and Groups Access lists provide fine grained control (Windows NT)

Very large Requires variable size directory entries

Classes of usersRWX

a) owner access 7 1 1 1RWX

b) group access 6 1 1 0RWX

c) public access 1 0 0 1

Password protection Files (too many) Subdirectories (ala TOPS-20) Partition (ala VM/CMS) All user files

File Concept

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