CS241 System Programming

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CS241 System Programming

• File Systems
• 10/30/06-11/3/06

2
Quiz

• LMP1-(10/30/06)-QuizA password 1066

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• 1 Autograder    50 for total 50
• 4 Compass Quiz (4 of them for LPM1) 5 each for
  total 20
• 2 MpQuiz for Discussion Section (2 per week) 10
  each for total 20
• 1 TA Evaluation 10

4
Outline

• UNIX File Systems
• Directory
• File Status
• Links
• UNIX File Systems vs. Windows File Systems

5
Review

• include ltunistd.hgt
• Change the directory
• int chdir(const char path)
• Get the current working directory
• char getcwd(char buf, size_t size)
• Get the maximum path length
• long fpathconf(int fildes, int name)
• long pathconf(const char path, int name)
• long sysconf(int name)

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Review

• include ltdirent.hgt
• Open the directory
• DIR opendir(const char dirname)
• Close the directory
• int closedir(DIR dirp)
• void rewinddir(DIR dirp)
• Read the directory
• struct dirent readdir(DIR dirp)

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Directory Entry

• Struct dirent
• Member Fields
• char d_name
• Null-terminated File Name
• ino_t d_fileno
• File Serial Number
• unsigned char d_namlen
• Length of a File Name
• unsigned char d_type
• Type of the File
• DT_REG, DT_DIR, DT_FIFO, DT_SOCK, DT_CHR, DT_BLK,
  DT_UNKNOWN

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More Directory Functions

• include ltdirent.hgt
• Set the position of next readdir
• void seekdir(DIR dir, off_t offset)
• Get the current location of directory stream
• off_t telldir (DIR dir)

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Review

• include ltsys/stat.hgt
• Get the status of a file
• int lstat(const char restrict path, struct stat
  restrict buf)
• int stat(const char restrict path, struct stat
  restrict buf)
• int fstat(int fildes, struct stat buf)
• st_mode
• Access permission
• Type

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Example

• Write a function isdirectory that returns nonzero
  if path is a directory, 0 otherwise
• include ltstdio.hgt
• include lttime.hgt
• include ltsys/stat.hgt
• int isdirectory(char path)
• struct stat statbuf
• if (stat(path, statbuf) -1)
• return 0
• else
• return S_ISDIR(statbuf.st_mode)

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Links

• Hard Link
• Directory Entry
• e.g. all regular files
• Symbolic Link
• Also called a Soft Link
• A special file that serves as a reference to
  another file

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Link Functions

• include ltunistd.hgt
• Creates additional links
• int link(const char path1, const char path2)
• Removes the directory entry
• int unlink(const char path)
• Same function as commands ln and rm, respectively
• Returns 0 if successful, -1 with errno set if
  unsuccessful

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Example

• Command Line
• ln /dirA/name1 /dirB/name2
• C Code Segments
• if (link("/dirA/name1", "/dirB/name2") -1)
• perror("Failed to make a new link in /dirB")

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Exercise

• What happens to the previous figure after the
  following sequence of edit operations? (Exercise
  5.17)
• Open the file /dirA/name1.
• Read the entire file into memory.
• Close /dirA/name1.
• Modify the memory image of the file.
• Unlink /dirA/name1.
• Open the file /dirA/name1 (create and write
  flags).
• Write the contents of memory to the file.
• Close /dirA/name1.

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Exercise (Continued)

• What happens to the previous figure after the
  following sequence of edit operations? (Exercise
  5.17)

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Symbolic Link Function

• include ltunistd.hgt
• Creates a symbolic link
• int symlink(const char path1, const char
  path2)
• Same function as commands ln -s
• Returns 0 if successful, -1 with errno set if
  unsuccessful

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Example

• Command Line
• ln s /dirA/name1 /dirB/name2
• C Code Segments
• if (symlink("/dirA/name1", "/dirB/name2") -1)
• perror("Failed to create a symbolic link in
  /dirB")

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Exercise

• What happens to the previous figure after the
  same sequence of edit operations as Exercise
  5.17? (Exercise 5.21)
• Open the file /dirA/name1.
• Read the entire file into memory.
• Close /dirA/name1.
• Modify the memory image of the file.
• Unlink /dirA/name1.
• Open the file /dirA/name1 (create and write
  flags).
• Write the contents of memory to the file.
• Close /dirA/name1.

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Exercise (Continued)

• What happens to the previous figure after the
  same sequence of edit operations as Exercise
  5.17? (Exercise 5.21)

20
Exercise

• What happens if rm /dirA/name1 is executed to the
  following figure? (Exercise 5.26)

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UNIX File Systems

• I-node per-file data structure
• Advantage
• Efficient for small files
• Flexible if the size changes
• Disadvantage
• File must fit in a single disk partition

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UNIX File Systems

• I-node (continued)
• Storing Large Files

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Windows File Systems

• FAT File Allocation Table
• Advantage
• Random access is faster
• Disadvantage
• FAT should be in memory
• FAT16, FAT32
• Number of bits to identify blocks on a disk

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Windows File Systems

• NTFS
• 64-bit index
• MFT Master File Table
• MFT record example
• Run represents one or multiple consecutive
  blocks

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Windows File Systems

• NTFS (continued)
• Storing Large Files

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Summary

• UNIX File Systems
• Directory
• File Status
• Links
• File Systems
• UNIX File Systems
• Windows File Systems
• FAT, NTFS