The document discusses file concepts and access methods. It defines what a file is, including its logical view, mapping to physical storage, and common types. File attributes like name, size, and protection are also described. The functions of file systems are explained, including creating, writing, reading, and deleting files. Open file data structures are outlined, as are sequential and direct access methods. File locking, types, and the simulation of sequential access on direct-access files are also summarized.

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(Fi) – 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
Fi, 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 Fi 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
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