Navigating File Systems

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Navigating File Systems

• CS 241 Discussion Section
• Week 8
• 3/26/07 3/30/07
• Stephen Kloder

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Outline

• UNIX file systems
• inodes
• directories
• UNIX File Operations
• Directory
• File Status
• Links

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UNIX File Systems

• inode 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

• inode (continued)
• Storing Large Files

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Directories are files too!

• Directories, like files, have inodes with
  attributes and pointers to disk blocks
• Each directory contains the name and i-node for
  each file in the directory.

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Directory functions

• 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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Directory reading functions

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

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Example 1 ds8-p1.c

• Use opendir and readdir to print all the
  filenames in the current directory
• include ltdirent.hgt
• DIR dir
• struct dirent entry
• dir opendir(.)
• while(entry readdir(dir))
• printf(s\n,entry-gtd_name)
• closedir(dir)
• Remember to include error checking!!

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Whats in a directory entry?

• struct dirent
• Member Fields
• char d_name
• Null-terminated file name
• ino_t d_fileno
• inode number
• unsigned char d_namlen
• Length of file name
• unsigned char d_type
• Type of file
• DT_REG, DT_DIR, DT_FIFO, DT_SOCK, DT_CHR, DT_BLK,
  DT_UNKNOWN

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Example 2 ds8-p2.c

• Modify ds8-p1.c to use the member fields of
  struct dirent to display the inode for each file,
  as well as whether the file is a directory or a
  regular file.
• if (entry.d_type DT_DIR)
• // File is a directory

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More Directory Functions

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

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Warning! Warning!

• opendir and readdir are NOT thread-safe.
• DO NOT open two directories at the same time!

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How to recursively traverse a directory tree

• Open the directory (opendir)
• Read each entry (readdir)
• If the file is a directory (d_type), store it
  (e.g. in an array of strings).
• Close the directory (closedir)
• Traverse each saved subdirectory, EXCEPT '.' and
  '..'

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File information stat

• Use the stat functions to view the files inodes
  attributes.
• include ltsys/stat.hgt
• include ltsys/types.hgt
• include ltunistd.hgt
• For a file
• int stat(const char restrict path, struct stat
  restrict buf)
• For a link
• int lstat(const char restrict path, struct stat
  restrict buf)
• For a file descriptor
• int fstat(int fildes, struct stat buf)

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Example 3 ds8-p3.c

• Modify ds8-p2.c to also give file information
  about each file.
• How large is each file?
• Which files are world-readable?
• Which files have been modified in the last 24
  hours?
• Hint man 2 stat

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Useful fields and macros in struct stat

• stat.st_size
• File size
• stat.st_mode
• File type
• User permissions
• Etc.
• stat.st_mtime
• Time of last modification
• S_ISDIR(stat.st_mode)
• Is this a directory?

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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
• To create a new link
• int link(const char oldpath, const char
  newpath)
• Same as ln
• To removes an entry from the directory
• int unlink(const char path)
• Same as rm
• Returns 0 if successful, -1 with errno set if
  unsuccessful

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Hard Link 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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Hard Link Example (contd)

• Q What happens if /dirA/name1 is deleted and
  recreated?

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Hard Link Example (contd)

• A /dirA/name1 and /dirB/name2 are now two
  distinct files.

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Symbolic Link Function

• include ltunistd.hgt
• To create a symbolic link
• int symlink(const char oldpath,
• const char newpath)
• Same function as command ln s
• Returns 0 if successful, -1 with errno set if
  unsuccessful

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Soft Link 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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Soft Link Example (contd)

• Q What happens if /dirA/name1 to is deleted and
  recreated?

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Soft Link Example (contd)

• A /dirA/name1 has a different inode, but
  /dir/name2 still links to it.

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Link number

• The link number (the st_nlink field in stat)
  tells how many directory entries link to this
  inode. The link number is
• Set to 1 when a file is created
• Incremented when link is called
• Decremented when unlink is called
• The link number appears in the second column of
  the output of ls l. Try it!
• The link number only counts hard links, not soft
  links.

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Exercise

• What happens in the following figure if
• rm /dirA/name1
• is executed?

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Summary

• UNIX File Systems
• i-nodes
• directories
• File operations
• Directory
• opendir, readdir, closedir
• File Status
• stat, fstat, lstat
• Links
• link, symlink, unlink