File concept and access method

file concept

1. File concept & access method R.Pavithra MSc-IT Nadar saraswathi college of arts and science, Theni.

2. CONTENTS File Concept Access Methods

3. 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 structure. To explore file-system protection

4. File Concept Uniform logical view of information storage (no matter the medium) OS abstracts from physical properties into a logical storage unit, the file Files mapped onto physical devices, usually nonvolatile File is a collection of related information Smallest allotment of nameable storage Contiguous logical address space Types: Data numeric character binary Program May be free form or rigidly formed ( structured )

5. File Structure 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 Who decides: Operating system Program / programmer

6. File Attributes Name – only information kept in human-readable form Identifier – unique tag (number) identifies file within file system 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, and user identification – data for protection, security, and usage monitoring Information about files are kept in the directory structure, which is maintained on the disk Typically file’s name and identifier Identifier locates other file attributes Attributes may be > 1KB Directory structures may be > 1MB

7. File Operations File is an abstract data type Operations include the following (and usually more) Create – find space, add entry to directory Write – write data at current file position pointer location and update pointer Read – read file contents at pointer location, update pointer Reposition within file (seek) – change pointer location Delete – free space and remove entry from directory Truncate – delete data starting at pointer

8. Open Files Open( F i ) – allow process to access a file Returns a file handle for system call reference to the file Search the directory structure on disk for entry F i , and move the content or cache some of entry to memory Close(file handle) – end processes’ access to the file Move the content of entry F i in memory to directory structure on disk

9. Open File Data Structures Usually a global table containing process-independent open file information Size Access dates Disk location of the file: cache of data access information File-open count: counter of number of times a file is open To allow removal of data from open-file table when last processes closes it Per-process open file table contains pertinent info, plus pointer to entry in global open file table Current file position pointer: pointer to next read/write location Access rights: per-process access mode information read, write, append

10. Open File Locking Provided by some operating systems and file systems Mediates access to a file shared exclusive Mandatory or advisory: Mandatory – access is denied depending on locks held and requested Advisory – processes can find status of locks and decide what to do

11. File Types Most operating systems recognize file types Filename extension I.e. resume.doc, server.java, readerthread.c Most support them Automatically open a type of file via a specific application (.doc) Only execute files of a given extension (.exe, .com) Run files of a given type via a scripting language (.bat) Can get more advanced If source code modified since executable compiled, if attempt made to execute, recompile and then execute (TOPS-20) Mac OS encodes creating program’s name in file attributes Double clicking on file passes the file name to appropriate application Unix has magic number stored in file at first byte indicating file type

12. Access Methods Sequential Access – tape model of a file read next write next reset no read after last write ( rewrite) Direct Access – random access, relative access read n write n position to n read next write next rewrite n n = relative block number Can accommodate structured data in file by mapping record number to block number Oses usually support both kinds, sometimes require access method declaration during create()

13. Sequential-access File

14. Simulation of Sequential Access on Direct-access File

15. Example of Index and Relative Files

16. T H A N K I N G Y O U