The Linux Programming Interface

Directories and Links 343

Figure 18-2: Representation of hard and symbolic links

Since a symbolic link refers to a filename, rather than an i-node number, it can be
used to link to a file in a different file system. Symbolic links also do not suffer the
other limitation of hard links: we can create symbolic links to directories. Tools
such as find and tar can tell the difference between hard and symbolic links, and
either don’t follow symbolic links by default, or avoid getting trapped in circular
references created using symbolic links.
It is possible to chain symbolic links (e.g., a is a symbolic link to b, which is a
symbolic link to c). When a symbolic link is specified in various file-related system
calls, the kernel dereferences the series of links to arrive at the final file.
SUSv3 requires that an implementation allow at least _POSIX_SYMLOOP_MAX deref-
erences of each symbolic link component of a pathname. The specified value for
_POSIX_SYMLOOP_MAX is 8. However, before kernel 2.6.18, Linux imposed a limit of 5
dereferences when following a chain of symbolic links. Starting with kernel 2.6.18,
Linux implements the SUSv3-specified minimum of 8 dereferences. Linux also
imposes a total of 40 dereferences for an entire pathname. These limits are
required to prevent extremely long symbolic link chains, as well as symbolic link
loops, from causing stack overflows in the kernel code that resolves symbolic links.

i-node

number

61

...

this 61

/home/erena directory

“/home/erena/

this”

data for soft link

/home/kiran/other

I-node table File data

... ...

that 61

... ...

regular file

file data

UID=kiran GID=users

type=symlink

perm=rwxrwxrwx

...
Data block pointers

link count = 1 size=16

UID=erena GID=users

type=file

perm=rw-r--r--

...
Data block pointers

link count = 2 size=518

/home/allyn directory

309

...

... ...

... ...

other 309

... ...

/home/kiran directory

Two hard

links to the

same file

A symbolic

link