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<section id="linux-directory-notification">
<h1>Linux Directory Notification<a class="headerlink" href="#linux-directory-notification" title="Link to this heading">¶</a></h1>
<blockquote>
<div><p>Stephen Rothwell <<a class="reference external" href="mailto:sfr%40canb.auug.org.au">sfr<span>@</span>canb<span>.</span>auug<span>.</span>org<span>.</span>au</a>></p>
</div></blockquote>
<p>The intention of directory notification is to allow user applications
to be notified when a directory, or any of the files in it, are changed.
The basic mechanism involves the application registering for notification
on a directory using a fcntl(2) call and the notifications themselves
being delivered using signals.</p>
<p>The application decides which “events” it wants to be notified about.
The currently defined events are:</p>
<blockquote>
<div><table class="docutils align-default">
<tbody>
<tr class="row-odd"><td><p>DN_ACCESS</p></td>
<td><p>A file in the directory was accessed (read)</p></td>
</tr>
<tr class="row-even"><td><p>DN_MODIFY</p></td>
<td><p>A file in the directory was modified (write,truncate)</p></td>
</tr>
<tr class="row-odd"><td><p>DN_CREATE</p></td>
<td><p>A file was created in the directory</p></td>
</tr>
<tr class="row-even"><td><p>DN_DELETE</p></td>
<td><p>A file was unlinked from directory</p></td>
</tr>
<tr class="row-odd"><td><p>DN_RENAME</p></td>
<td><p>A file in the directory was renamed</p></td>
</tr>
<tr class="row-even"><td><p>DN_ATTRIB</p></td>
<td><p>A file in the directory had its attributes
changed (chmod,chown)</p></td>
</tr>
</tbody>
</table>
</div></blockquote>
<p>Usually, the application must reregister after each notification, but
if DN_MULTISHOT is or’ed with the event mask, then the registration will
remain until explicitly removed (by registering for no events).</p>
<p>By default, SIGIO will be delivered to the process and no other useful
information. However, if the F_SETSIG fcntl(2) call is used to let the
kernel know which signal to deliver, a siginfo structure will be passed to
the signal handler and the si_fd member of that structure will contain the
file descriptor associated with the directory in which the event occurred.</p>
<p>Preferably the application will choose one of the real time signals
(SIGRTMIN + <n>) so that the notifications may be queued. This is
especially important if DN_MULTISHOT is specified. Note that SIGRTMIN
is often blocked, so it is better to use (at least) SIGRTMIN + 1.</p>
<section id="implementation-expectations-features-and-bugs">
<h2>Implementation expectations (features and bugs :-))<a class="headerlink" href="#implementation-expectations-features-and-bugs" title="Link to this heading">¶</a></h2>
<p>The notification should work for any local access to files even if the
actual file system is on a remote server. This implies that remote
access to files served by local user mode servers should be notified.
Also, remote accesses to files served by a local kernel NFS server should
be notified.</p>
<p>In order to make the impact on the file system code as small as possible,
the problem of hard links to files has been ignored. So if a file (x)
exists in two directories (a and b) then a change to the file using the
name “a/x” should be notified to a program expecting notifications on
directory “a”, but will not be notified to one expecting notifications on
directory “b”.</p>
<p>Also, files that are unlinked, will still cause notifications in the
last directory that they were linked to.</p>
</section>
<section id="configuration">
<h2>Configuration<a class="headerlink" href="#configuration" title="Link to this heading">¶</a></h2>
<p>Dnotify is controlled via the CONFIG_DNOTIFY configuration option. When
disabled, fcntl(fd, F_NOTIFY, ...) will return -EINVAL.</p>
</section>
<section id="example">
<h2>Example<a class="headerlink" href="#example" title="Link to this heading">¶</a></h2>
<p>See tools/testing/selftests/filesystems/dnotify_test.c for an example.</p>
</section>
<section id="note">
<h2>NOTE<a class="headerlink" href="#note" title="Link to this heading">¶</a></h2>
<p>Beginning with Linux 2.6.13, dnotify has been replaced by inotify.
See <a class="reference internal" href="inotify.html"><span class="doc">Inotify - A Powerful yet Simple File Change Notification System</span></a> for more information on it.</p>
</section>
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