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Full Discussion: Hard link
Top Forums UNIX for Advanced & Expert Users Hard link Post 302168034 by Perderabo on Saturday 16th of February 2008 03:04:29 PM
Old 02-16-2008
A directory must have two special entries in it. A entry called . must exist and be a link to the directory itself. And an entry called .. must exist and be an link to the parent directory. The link system call does not perform these operations. In the old days, it took 3 link operations to create a directory. Now there is just a single mkdir system call to invoke. And root should be using mkdir, not link. Also by using silly combinations of link and unlink it is possible to break pieces of the file system off from the tree structure, create loops and even remove . or .. from a directory. I doubt that you could recover from
unlink /usr/bin
or something like that.
 

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LINK(2) 							System Calls Manual							   LINK(2)

NAME
link - make a hard link to a file SYNOPSIS
link(name1, name2) char *name1, *name2; DESCRIPTION
A hard link to name1 is created; the link has the name name2. Name1 must exist. With hard links, both name1 and name2 must be in the same file system. Unless the caller is the super-user, name1 must not be a directory. Both the old and the new link share equal access and rights to the underlying object. RETURN VALUE
Upon successful completion, a value of 0 is returned. Otherwise, a value of -1 is returned and errno is set to indicate the error. ERRORS
Link will fail and no link will be created if one or more of the following are true: [ENOTDIR] A component of either path prefix is not a directory. [EINVAL] Either pathname contains a character with the high-order bit set. [ENAMETOOLONG] A component of either pathname exceeded 255 characters, or entire length of either path name exceeded 1023 characters. [ENOENT] A component of either path prefix does not exist. [EACCES] A component of either path prefix denies search permission. [EACCES] The requested link requires writing in a directory with a mode that denies write permission. [ELOOP] Too many symbolic links were encountered in translating one of the pathnames. [ENOENT] The file named by name1 does not exist. [EEXIST] The link named by name2 does exist. [EPERM] The file named by name1 is a directory and the effective user ID is not super-user. [EXDEV] The link named by name2 and the file named by name1 are on different file systems. [ENOSPC] The directory in which the entry for the new link is being placed cannot be extended because there is no space left on the file system containing the directory. [EDQUOT] The directory in which the entry for the new link is being placed cannot be extended because the user's quota of disk blocks on the file system containing the directory has been exhausted. [EIO] An I/O error occurred while reading from or writing to the file system to make the directory entry. [EROFS] The requested link requires writing in a directory on a read-only file system. [EFAULT] One of the pathnames specified is outside the process's allocated address space. SEE ALSO
symlink(2), unlink(2) 4th Berkeley Distribution August 26, 1985 LINK(2)
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