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()
