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<h3 class="section">14.4 Hard Links</h3>

<p><a name="index-hard-link-1456"></a><a name="index-link_002c-hard-1457"></a><a name="index-multiple-names-for-one-file-1458"></a><a name="index-file-names_002c-multiple-1459"></a>
In POSIX systems, one file can have many names at the same time.  All of
the names are equally real, and no one of them is preferred to the
others.

   <p>To add a name to a file, use the <code>link</code> function.  (The new name is
also called a <dfn>hard link</dfn> to the file.)  Creating a new link to a
file does not copy the contents of the file; it simply makes a new name
by which the file can be known, in addition to the file's existing name
or names.

   <p>One file can have names in several directories, so the organization
of the file system is not a strict hierarchy or tree.

   <p>In most implementations, it is not possible to have hard links to the
same file in multiple file systems.  <code>link</code> reports an error if you
try to make a hard link to the file from another file system when this
cannot be done.

   <p>The prototype for the <code>link</code> function is declared in the header
file <samp><span class="file">unistd.h</span></samp>. 
<a name="index-unistd_002eh-1460"></a>
<!-- unistd.h -->
<!-- POSIX.1 -->

<div class="defun">
&mdash; Function: int <b>link</b> (<var>const char *oldname, const char *newname</var>)<var><a name="index-link-1461"></a></var><br>
<blockquote><p>The <code>link</code> function makes a new link to the existing file named by
<var>oldname</var>, under the new name <var>newname</var>.

        <p>This function returns a value of <code>0</code> if it is successful and
<code>-1</code> on failure.  In addition to the usual file name errors
(see <a href="File-Name-Errors.html#File-Name-Errors">File Name Errors</a>) for both <var>oldname</var> and <var>newname</var>, the
following <code>errno</code> error conditions are defined for this function:

          <dl>
<dt><code>EACCES</code><dd>You are not allowed to write to the directory in which the new link is
to be written.

          <br><dt><code>EEXIST</code><dd>There is already a file named <var>newname</var>.  If you want to replace
this link with a new link, you must remove the old link explicitly first.

          <br><dt><code>EMLINK</code><dd>There are already too many links to the file named by <var>oldname</var>. 
(The maximum number of links to a file is <code>LINK_MAX</code><!-- /@w -->; see
<a href="Limits-for-Files.html#Limits-for-Files">Limits for Files</a>.)

          <br><dt><code>ENOENT</code><dd>The file named by <var>oldname</var> doesn't exist.  You can't make a link to
a file that doesn't exist.

          <br><dt><code>ENOSPC</code><dd>The directory or file system that would contain the new link is full
and cannot be extended.

          <br><dt><code>EPERM</code><dd>In the GNU system and some others, you cannot make links to directories. 
Many systems allow only privileged users to do so.  This error
is used to report the problem.

          <br><dt><code>EROFS</code><dd>The directory containing the new link can't be modified because it's on
a read-only file system.

          <br><dt><code>EXDEV</code><dd>The directory specified in <var>newname</var> is on a different file system
than the existing file.

          <br><dt><code>EIO</code><dd>A hardware error occurred while trying to read or write the to filesystem. 
</dl>
        </p></blockquote></div>

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