File%20Systems

1
File Systems

• Chapter 6

6.1 Files 6.2 Directories 6.3 File system
implementation 6.4 Example file systems
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Long-term Information Storage

• Must store large amounts of data (virtual address
  space may not be big enough)
• Information stored must survive the termina-tion
  of the process using it, or if the computer
  crashes
• Multiple processes must be able to access the
  information concurrently
• Files are managed by the OS. Structure,
  management, naming, accessing, etc, are part of
  the file system.

3
File Naming

• Characters, digits, special characters
• Lower case and upper case letters
• File extensions (e.g. prog.c, prog.c.Z)
• Unix extensions mean nothing, however certain
  programs may require specific extensions
• Windows aware of extensions assigns meaning.
  Programs own extensions.

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File Naming

• Typical file extensions.

5
File Structure

• Three kinds of files
• byte sequence no meaning for OS, meaning at
  user level
• record sequence fixed length, structure (e.g.
  card images)
• tree each has a key field, tree is sorted on
  that key

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File Types

• Regular files the ones that contain user
  information
• Directories system files to maintain systems
  structure
• Unix Character and Block special files
• Regular files
• ASCII (plain text)
• binary (internal structure)
• Every OS must at least recognize its own
  executable file

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File Types

• (a) An executable file (b) An archive

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File Access

• Sequential access
• read all bytes/records from the beginning
• cannot jump around, could rewind or back up
• convenient when medium was magnetic tape
• Random access
• bytes/records read in any order
• essential for data base systems
• read can be done in 2 ways
• give the position in the file and initiate a read
• move file marker (seek) to set current position
  and then read

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File Attributes

• Possible file attributes

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File Operations

1. Create
2. Delete
3. Open
4. Close
5. Read
6. Write

1. Append
2. Seek
3. Get attributes
4. Set Attributes
5. Rename

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An Example Program Using File System Calls (1/2)
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An Example Program Using File System Calls (2/2)
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Memory-Mapped Files

• Motivation access files in a uniform way

• Segmented process before mapping files into its
  address space
• Process after mapping
• existing file abc into one segment
• creating new segment for xyz
• Problems the usual ones

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DirectoriesSingle-Level Directory Systems

• A single level directory system
• contains 4 files
• owned by 3 different people, A, B, and C
• Potential problem same filenames

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Two-level Directory Systems

• Each user gets a private directory letters
  indicate owners of the directories and files
• When a user tries to open a file, system knows
  which user is
• Users may want to access files in other users
  directories

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Hierarchical Directory Systems

• A hierarchical directory system

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Path Names

• If file system is a directory tree gt naming
  scheme?
• Two methods to specify file names
• absolute paths path from root to file (e.g.
  /usr/ast/mailbox)
• relative paths used in conjunction with the
  working or current directory. User (or process)
  specifies a directory as current and all paths
  (not beginning with the separator) are taken
  relative to that (e.g. ast/mailbox, if current
  /usr/)
• Each process has its own working directory
• Library procedures rarely change the working
  directory
• . (dot) and .. (dotdot) directories current and
  parent directories

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Path Names

• A UNIX directory tree

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Directory Operations

• Readdir
• Rename
• Link
• Unlink

1. Create
2. Delete
3. Opendir
4. Closedir