UNIX Filesystems: Evolution, Design, and Implementation

Symbolic Links and Hard Links

Symbolic links and hard links can be created using the ln command, which in turn maps onto the link() and symlink() system calls. Both prototypes are shown below:

#include <unistd.h>

int link(const char *existing, const char *new);
int symlink(const char *name1, const char *name2);

The section Truncating and Removing Files earlier in this chapter describes hard links and showed the effects that link() and unlink() have on the underlying file. Symbolic links are managed in a very different manner by the filesystem as the following example shows:

$ echo “Hello world” > myfile
$ ls -l myfile
-rw-r--r-      1 spate    fcf                         12 Mar 15 12:17 myfile
$ cat myfile
Hello world
$ strace ln -s myfile mysymlink 2>&1 | grep link
execve(“/bin/ln”, [“ln”, “-s”, “myfile”,
“mysymlink”],  [/* 39 vars */]) = 0
lstat(“mysymlink”, 0xbffff660) = -1 ENOENT (No such file/directory)
symlink(“myfile”, “mysymlink”) = 0
$ ls -l my*
-rw-r--r-     1 spate     fcf    12 Mar 15 12:17 myfile
lrwxrwxrwx    1 spate     fcf     6 Mar 15 12:18 mysymlink -> myfile
$ cat mysymlink
Hello world
$ rm myfile
$ cat mysymlink
cat: mysymlink: No such file or directory

The ln command checks to see if a file called mysymlink already exists and then calls symlink() to create the symbolic link. There are two things to notice here. First of all, after the symbolic link is created, the link count of myfile does not change. Secondly, the size of mysymlink is 6 bytes, which is the length of the string myfile.

Because creating a symbolic ...