Symbolic Links and Loops because of links(4.16)

A symbolic link is an indirect pointer to a file, unlike the hard links from the previous section, which pointed directly to the i-node of the file. Symbolic links were introduced to get around the limitations of hard links.

• Hard links normally require that the link and the file reside in the same file system

• Only the superuser can create a hard link to a directory

There are no file system limitations on a symbolic link and what it points to, and anyone can create a symbolic link to a directory. Symbolic links are typically used to move a file or an entire directory hierarchy to another location on a system.

When using functions that refer to a file by name, we always need to know whether the function follows a symbolic link. If the function follows a symbolic link, a pathname argument to the function refers to the file pointed to by the symbolic link. Otherwise, a pathname argument refers to the link itself, not the file pointed to by the link. Figure 4.17 summarizes whether the functions described in this chapter follow a symbolic link. The functions mkdir, mkfifo, mknod, and rmdir are not in this figure, as they return an error when the pathname is a symbolic link. Also, the functions that take a file descriptor argument, such as fstat and fchmod, are not listed, as the handling of a symbolic link is done by the function that returns the file descriptor (usually open). Whether or not chown follows a symbolic link depends on the implementation.

One exception to Figure 4.17 is when the open function is called with both O_CREAT and O_EXCL set. In this case, if the pathname refers to a symbolic link, open will fail with errno set to EEXIST. This behavior is intended to close a security hole so that privileged processes can't be fooled into writing to the wrong files.

　　　　　　　　　　　　　　　　　　　　　　　　Figure 4.17. Treatment of symbolic links by various functions

Function	Does not follow symbolic link	Follows symbolic link
access		•
chdir		•
chmod		•
chown	•	•
creat		•
exec		•
lchown	•
link		•
lstat	•
open		•
opendir		•
pathconf		•
readlink	•
remove	•
rename	•
stat		•
truncate		•
unlink	•

Example

It is possible to introduce loops into the file system by using symbolic links. Most functions that look up a pathname return an errno of ELOOP when this occurs. Consider the following commands:

     $ mkdir foo                   make a new directory
     $ touch foo/a                 create a 0-length file
     $ ln -s ../foo foo/testdir    create a symbolic link("../foo" is the content written into foo/testdir)
     $ ls -l foo
     total 0
     -rw-r----- 1 sar            0 Jan 22 00:16 a
     lrwxrwxrwx 1 sar            6 Jan 22 00:16 testdir -> ../foo

This creates a directory foo that contains the file a and a symbolic link that points to foo. If we write a simple program that uses the standard function ftw(3) on Solaris to descend through a file hierarchy, printing each pathname encountered, the output is

    foo
    foo/a
    foo/testdir
    foo/testdir/a
    foo/testdir/testdir
    foo/testdir/testdir/a
    foo/testdir/testdir/testdir
    foo/testdir/testdir/testdir/a

　　(many more lines until we encounter an ELOOP error)

In Section 4.21, we provide our own version of the ftw function that uses lstat instead of stat, to prevent it from following symbolic links.

Note that on Linux, the ftw function uses lstat, so it doesn't display this behavior.

A loop of this form is easy to remove. We are able to unlink the file foo/testdir, as unlink does not follow a symbolic link. But if we create a hard link that forms a loop of this type, its removal is much more difficult. This is why the link function will not form a hard link to a directory unless the process has superuser privileges(in Centos, even you have superuser privileges, hardlink to a directory is still not allowed).

Indeed, Rich Stevens did this on his own system as an experiment while writing the original version of this section. The file system got corrupted and the normal fsck(1) utility couldn't fix things. The deprecated tools clri(8) and dcheck(8) were needed to repair the file system.

The need for hard links to directories has long since passed. With symbolic links and the mkdir function, there is no longer any need for users to create hard links to directories.

When we open a file, if the pathname passed to open specifies a symbolic link, open follows the link to the specified file. If the file pointed to by the symbolic link doesn't exist, open returns an error saying that it can't open the file. This can confuse users who aren't familiar with symbolic links. For example,

     $ ln -s /no/such/file myfile            create a symbolic link
     $ ls myfile
     myfile                                  ls says it's there
     $ cat myfile                            so we try to look at it
     cat: myfile: No such file or directory
     $ ls -l myfile                          try -l option
     lrwxrwxrwx 1 sar        13 Jan 22 00:26 myfile -> /no/such/file

The file myfile does exist, yet cat says there is no such file, because myfile is a symbolic link and the file pointed to by the symbolic link doesn't exist. The -l option to ls gives us two hints: the first character is an l, which means a symbolic link, and the sequence -> also indicates a symbolic link. The ls command has another option (-F) that appends an at-sign(@) to filenames that are symbolic links, which can help spot symbolic links in a directory listing without the -l option.