Operating Systems - File Abstraction - File Operation | CMSC 412, Study notes of Operating Systems

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Reading Chapter 11

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File Abstraction

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What is a file?–

A named collection of information stored on secondary storage

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Properties of a file–

non-volatile

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can read, read, or update it

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has meta-data to describe attributes of the file

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File Attributes–

name: a way to describe the file

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type: some information about what is stored in the file

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location: how to find the file on disk

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size: number of bytes

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protection: access control

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may be different for read, write, execute, append, etc.

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time: access, modification, creation

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version: how many times has the file changed

File Operations (cont)

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write– indicate what file to write (either name of handle)– provide data to write– specify where to write the data within the file

• generally this is implicit (file pointer)• could be explicit (direct access)

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read– indicate what file to read (either name of handle)– provide place to put information read– indicate how much to read– specify where to write the data within the file

• generally this is implicit (file pointer)• could be explicit (direct access)

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fsync (synchronize disk version with in-core version)– ensure any previous writes to the file are stored on disk

File Operations (cont)

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seek– move the implicit file pointer to a new offset in the file

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delete– remove named file

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truncate– remove the data in the file from the current position to end

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close– unlock the file (if open locked it)– update meta data about time– free system resources (file descriptors, buffers)

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read meta data– get file size, time, owner, etc.

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update meta data– change file size, time owner, etc.

Tree Directories

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create a tree of files

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each directory can contain files or directory entries

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each process has a current directory– can name files relative to that directory– can change directories as needed

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hollings

user

user

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gdb

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system

users

mail

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Acylic Graph Directories

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Permit users to share subdirectories

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hollings

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gdb

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system

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Does the OS know what is stored in a file? z^

needs to know about some types of files–

directories

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executables

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should other file types be visible to the OS?–

Example: word processing file vs. spreadsheet

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Advantages:

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OS knows what application to run

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Automatic make (tops-20)

• if source changed, re-compile before running

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Problems:

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to add new type, need to extend OS

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OS vs. application features are blurred

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what if a file is several types

• consider a compressed postscript file

Example of File Types

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Macintosh– has a file type that is part of file meta-data– also has an application associated with each file type

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Windows 95/NT– has a file type in the extension of the file name– has a table (per user) to map extensions to applications

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Unix– can use last part of filename like an extension– applications can decide what (if anything) to do with it