File Attributes

• Inodes (14.4)
  • On a disk, each file has a unique record called an i-node
  • This contains information about the file
    • type
    • owner id,
    • group id
    • permissions
    • times last: accessed, modified, status change
    • number of hard links to the file
    • size in bytes
    • Blocks allocated to the file
    • Pointer to the data blocks of the file.
  • Look at the picture on page 258.
• Hard Links.
  • This is really not a unique file type.
  • Multiple file names can point to the same file.
  • Because, as we will find out, a directory associates a sting with a i-node number
  • Think of a directory on the wall in a building.

  • 	     Adelgren   	152
    	     Baker		159
    	     Bennett		158
    
    	

  • Ann Quinn is a exception.

  • 	     Chairperson	107
    	     Quinn		107
    	

  • This is the idea of a hard link.
  • A hard link associates a file name with a i-node number
  • ln oldfile newfile
    • The files must be on the same file system.
    • ls -li
    • do this.
• Symbolic links
  • Hard links are limited to file systems.
  • To overcome this, symbolic links are provided.
  • This can cross file system boundaries.
  • This is a new file type.
  • ln -s oldfile newfile
    • newfile, the link will hold the path to the old file.
    • If oldfile goes away, the link will still exist, but will be an error
  • There is a limit to the number of symbolic links in a chain (surprisingly not one).
  • These are used all over the place.
  • By the way, some system calls will behave differently for symbolic links than for regular files.
• Character and Block Special files.
  • These are the interfaces to the devices on the system.
  • Character files are for character style i/o
    • Think mouse, keyboard, terminal
  • Block files are for disks and block writes.
  • Each has a major number, which is the routine to call when accessing the device.
  • Each has a minor number, which is the parameter to pass to the routine when accessing the device.
  • That is mostly what we will do with these.
  • mknod is used to create them.
  • File systems are associated with block/special devices via the mount command.
• Sockets
  • These are for unix-domain sockets.
  • A communications mechanism.
  • There is no direct way to create them at the command line
  • A call to bind with the proper arguments will create them.
  • probably one in /tmp/.X11-unix