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<table class="head">
<tr>
<td class="head-ltitle">FDESCFS(4)</td>
<td class="head-vol">Device Drivers Manual</td>
<td class="head-rtitle">FDESCFS(4)</td>
</tr>
</table>
<div class="manual-text">
<section class="Sh">
<h1 class="Sh" id="NAME"><a class="permalink" href="#NAME">NAME</a></h1>
<p class="Pp"><code class="Nm">fdescfs</code> —
<span class="Nd">file-descriptor file system</span></p>
</section>
<section class="Sh">
<h1 class="Sh" id="SYNOPSIS"><a class="permalink" href="#SYNOPSIS">SYNOPSIS</a></h1>
<div class="Bd Li">
<pre>fdescfs /dev/fd fdescfs rw 0 0</pre>
</div>
</section>
<section class="Sh">
<h1 class="Sh" id="DESCRIPTION"><a class="permalink" href="#DESCRIPTION">DESCRIPTION</a></h1>
<p class="Pp">The file-descriptor file system, or
<code class="Nm">fdescfs</code>, provides access to the per-process file
descriptor namespace in the global file system namespace. The conventional
mount point is <span class="Pa">/dev/fd</span>.</p>
<p class="Pp">The file system's contents appear as a list of numbered files
which correspond to the open files of the process reading the directory. The
files <span class="Pa">/dev/fd/0</span> through
<span class="Pa">/dev/fd/#</span> refer to file descriptors which can be
accessed through the file system.</p>
<p class="Pp">The following mount options can be used when mounting
<code class="Nm">fdescfs</code> filesystem:</p>
<dl class="Bl-tag">
<dt id="nodup"><a class="permalink" href="#nodup"><code class="Cm">nodup</code></a></dt>
<dd>For file descriptors referencing vnodes, instead of the
<a class="Xr">dup(2)</a> semantic described above, implement re-opening of
the referenced vnode. See below for more details.</dd>
<dt id="linrdlnk"><a class="permalink" href="#linrdlnk"><code class="Cm">linrdlnk</code></a></dt>
<dd>Report the type of the <code class="Nm">fdescfs</code> vnode as
<code class="Dv">VLNK</code> instead of <span class="Ux">FreeBSD</span>
traditional <code class="Dv">VCHR</code>. For <a class="Xr">linux(4)</a>
ABI compatibility mount <code class="Nm">fdescfs</code> volume with the
<code class="Cm">linrdlnk</code> option.</dd>
<dt id="rdlnk"><a class="permalink" href="#rdlnk"><code class="Cm">rdlnk</code></a></dt>
<dd>Treat <code class="Nm">fdescfs</code> vnodes as symbolic links
consistently, in particular, follow the resolved name for the name
lookups. This option is strictly stronger than the
<code class="Cm">linrdlnk</code> option, it changes not only the type
returned by <a class="Xr">stat(2)</a>, but also causes the
<code class="Nm">fdescfs</code> files to behave as symlinks.</dd>
</dl>
<p class="Pp">For <code class="Nm">fdescfs</code> mounted without the
<code class="Cm">nodup</code> mount option, if the file descriptor is open
and the mode the file is being opened with is a subset of the mode of the
existing descriptor, the call:</p>
<div class="Bd Pp Bd-indent Li">
<pre>fd = open("/dev/fd/0", mode);</pre>
</div>
<p class="Pp">and the call:</p>
<div class="Bd Pp Bd-indent Li">
<pre>fd = fcntl(0, F_DUPFD, 0);</pre>
</div>
<p class="Pp">are equivalent. Flags to the <a class="Xr">open(2)</a> call other
than <code class="Dv">O_RDONLY</code>, <code class="Dv">O_WRONLY</code> and
<code class="Dv">O_RDWR</code> are ignored.</p>
<p class="Pp">For <code class="Nm">fdescfs</code> mounted with the
<code class="Cm">nodup</code> option, and file descriptor referencing a
vnode, the call:</p>
<div class="Bd Pp Bd-indent Li">
<pre>fd = open("/dev/fd/0", mode);</pre>
</div>
<p class="Pp" id="open">reopens the referenced vnode with the specified
<var class="Fa">mode</var>. In other words, the
<a class="permalink" href="#open"><code class="Fn">open</code></a>() call
above is equivalent to</p>
<div class="Bd Pp Bd-indent Li">
<pre>fd = openat(0, "", O_EMPTY_PATH, mode);</pre>
</div>
<p class="Pp" id="open~2">In particular, if the file descriptor was opened with
the <code class="Dv">O_PATH</code> flag, then either
<code class="Dv">O_EMPTY_PATH</code> or
<a class="permalink" href="#open~2"><code class="Fn">open</code></a>() over
<code class="Nm">fdescfs</code> mount with <code class="Cm">nodup</code>
option allows one to convert it to a regularly opened file, assuming that
the current permissions allow the requested <var class="Fa">mode</var>.</p>
<p class="Pp" id="Note:"><a class="permalink" href="#Note:"><i class="Em">Note:</i></a>
<span class="Pa">/dev/fd/0</span>, <span class="Pa">/dev/fd/1</span> and
<span class="Pa">/dev/fd/2</span> files are created by default when devfs
alone is mounted. <code class="Nm">fdescfs</code> creates entries for all
file descriptors opened by the process.</p>
</section>
<section class="Sh">
<h1 class="Sh" id="FILES"><a class="permalink" href="#FILES">FILES</a></h1>
<dl class="Bl-tag Bl-compact">
<dt><span class="Pa">/dev/fd/#</span></dt>
<dd style="width: auto;"> </dd>
</dl>
</section>
<section class="Sh">
<h1 class="Sh" id="EXAMPLES"><a class="permalink" href="#EXAMPLES">EXAMPLES</a></h1>
<p class="Pp">To mount a <code class="Nm">fdescfs</code> volume located on
<span class="Pa">/dev/fd</span>:</p>
<p class="Pp"></p>
<div class="Bd Bd-indent"><code class="Li">mount -t fdescfs none
/dev/fd</code></div>
<p class="Pp">For <a class="Xr">linux(4)</a> ABI compatibility:</p>
<p class="Pp"></p>
<div class="Bd Bd-indent"><code class="Li">mount -t fdescfs -o linrdlnk none
/compat/linux/dev/fd</code></div>
<p class="Pp">For substitute of <code class="Dv">O_EMPTY_PATH</code> flag
use:</p>
<p class="Pp"></p>
<div class="Bd Bd-indent"><code class="Li">mount -t fdescfs -o nodup none
/dev/fdpath</code></div>
</section>
<section class="Sh">
<h1 class="Sh" id="SEE_ALSO"><a class="permalink" href="#SEE_ALSO">SEE
ALSO</a></h1>
<p class="Pp"><a class="Xr">devfs(4)</a>, <a class="Xr">mount(8)</a></p>
</section>
<section class="Sh">
<h1 class="Sh" id="HISTORY"><a class="permalink" href="#HISTORY">HISTORY</a></h1>
<p class="Pp">The <code class="Nm">fdescfs</code> file system first appeared in
<span class="Ux">4.4BSD</span>. The <code class="Nm">fdescfs</code> manual
page first appeared in <span class="Ux">FreeBSD 2.2</span>.</p>
</section>
<section class="Sh">
<h1 class="Sh" id="AUTHORS"><a class="permalink" href="#AUTHORS">AUTHORS</a></h1>
<p class="Pp">The <code class="Nm">fdescfs</code> manual page was written by
<span class="An">Mike Pritchard</span>
<<a class="Mt" href="mailto:mpp@FreeBSD.org">mpp@FreeBSD.org</a>>, and
was based on the manual page written by <span class="An">Jan-Simon
Pendry</span>.</p>
</section>
</div>
<table class="foot">
<tr>
<td class="foot-date">July 11, 2023</td>
<td class="foot-os">FreeBSD 15.0</td>
</tr>
</table>
|
