Directories and Links

• This is chapter 18, you should read it.
• A directory is just a different type of file.
  • It contains a mapping between file names and inode numbers.
• But you can't use standard i/o on directories.
• Hard links/directories and inodes
  • Opening a file creates a file, and an association between the allocated inode and the file name.
  • making a hard link (ln oldfile newfile) will create an entry in a directory and increase the link count.
  • removing a file will decrease the link count
    • If there are multiple links, this is simply removing an entry in a directory.
  • When the link count goes to 0, the associated inode is freed.
• Do we have any questions about hard/soft links?
• The table on 345 summarizes how different system calls deal with symbolic links.
• System calls with hard links.
  • Making a hard link

    •        #include <unistd.h>
      
             int link(const char *oldpath, const char *newpath);
      	

    • Creates a hard link between two files.
    • Fails if newpath exists.
    • Behaves different in linux with symbolic links.
  • Removing a hard link, and perhaps more

    •        #include <unistd.h>
      
             int unlink(const char *pathname);
          

    • Removes files or symbolic links.
    • If this was the last reference to an inode, the inode is freed
    • But only after the last file descriptor referencing it is deleted.
  • Moving a hard link

    •        #include <stdio.h>
      
             int rename(const char *oldpath, const char *newpath);
          

    • newpath will be replaced usually
      • Directories can only be renamed to non-existing files.
      • Link counts are not changed.
• Symbolic Links.

  •        #include 
           int symlink(const char *target, const char *linkpath);
    	

  • Allow links across file systems. I always forget this.
  • Can point to nothing.
  • Permissions are irrelevant.
  • If the target exists, it will not be overwritten.

  •        #include 
           ssize_t readlink(const char *pathname, char *buf, size_t bufsiz);
    	

  • Returns the contents of the symbolic link file.
  • Will truncate the contents if necessary