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   <section id="pseudo-file-system">
 <h1>Pseudo File System<a class="headerlink" href="#pseudo-file-system" title="Permalink to this heading"></a></h1>
 <div class="admonition warning">
 <p class="admonition-title">Warning</p>
 <p>Migrated from:
 <a class="reference external" href="https://cwiki.apache.org/confluence/display/NUTTX/Pseudo+File+System">https://cwiki.apache.org/confluence/display/NUTTX/Pseudo+File+System</a></p>
 </div>
 <section id="overview">
 <h2>Overview<a class="headerlink" href="#overview" title="Permalink to this heading"></a></h2>
 <section id="pseudo-root-file-system">
 <h3>Pseudo Root File System<a class="headerlink" href="#pseudo-root-file-system" title="Permalink to this heading"></a></h3>
 <p>NuttX includes an optional, scalable file system. As a minimum, this may
 be a simple in-memory, pseudo file system. This is an in-memory file
 system because it does not require any storage medium or block driver
 support. Rather, file system contents are generated on-the-fly as
 referenced via standard file system operations (open, close, read,
 write, etc.). In this sense, the file system is a pseudo file system
 (in the same sense that the Linux <code class="docutils literal notranslate"><span class="pre">/proc</span></code> file system is also referred
 to as a pseudo file system).</p>
 <p>Any user supplied data or logic can be accessed via the pseudo-file
 system. Built in support is provided for character, block, and MTD
 (Memory Technology Device) drivers in the <code class="docutils literal notranslate"><span class="pre">/dev</span></code> pseudo file system
 directory.</p>
 </section>
 <section id="special-files">
 <h3>Special Files<a class="headerlink" href="#special-files" title="Permalink to this heading"></a></h3>
 <p>NuttX does not support special files in the way that, say,
 Linux does. In fact, it is more correct to say that NuttX
 file systems do not support special files at all.</p>
 <p>NuttX does, however, support Linux-like special <cite>device node</cite>,
 character driver, and block driver files (as well as NuttX-specific
 mountpoint, named semaphore, message queue, and shared memory
 special files). However, these are not special files in sense
 that the term special files is used in a POSIX environment: In
 NuttX these special files may <cite>only</cite> be created in the root
 pseudo-file system. For the case of device nodes, see <a class="reference external" href="https://cwiki.apache.org/confluence/display/NUTTX/Device+Nodes">Device
 Nodes</a>
 for further information.</p>
 <p>In NuttX, the underlying principle is that all <cite>named resources</cite>
 appear as special files in the root pseudo-file system and are
 managed by the VFS.</p>
 </section>
 <section id="mounted-volumes">
 <h3>Mounted Volumes<a class="headerlink" href="#mounted-volumes" title="Permalink to this heading"></a></h3>
 <p>The simple in-memory file system can be extended by mounting block
 devices that provide access to true file systems backed up via
 some mass storage device. NuttX supports the standard mount()
 command that allows a block driver to be bound to a mount point
 within the pseudo file system and to a file system. At present,
 NuttX supports the standard VFAT and ROMFS file systems, a
 special, wear-levelling NuttX FLASH File System (NXFFS), as well
 as a Network File System client (NFS version 3, UDP).</p>
 </section>
 <section id="comparison-to-linux">
 <h3>Comparison to Linux<a class="headerlink" href="#comparison-to-linux" title="Permalink to this heading"></a></h3>
 <p>From a programming perspective, the NuttX file system appears
 very similar to a Linux file system. However, there is a
 fundamental difference: The NuttX root file system is a pseudo
 file system and true file systems may be mounted in the pseudo
 file system. In the typical Linux installation by comparison,
 the Linux root file system is a true file system and pseudo
 file systems may be mounted in the true, root file system.
 The approach selected by NuttX is intended to support greater
 scalability from the very tiny platform to the moderate platform.</p>
 </section>
 </section>
 <section id="faq">
 <h2>FAQ<a class="headerlink" href="#faq" title="Permalink to this heading"></a></h2>
 <p><strong>Question</strong>: I’m wondering why I can’t create a directory. If
 I try to create a dir.</p>
 <div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>mkdir<span class="w"> </span>/mnt
 </pre></div>
 </div>
 <p>I get this,</p>
 <div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>nsh:<span class="w"> </span>mkdir:<span class="w"> </span>mkdir<span class="w"> </span>failed:<span class="w"> </span><span class="m">2</span>
 </pre></div>
 </div>
 <p>although if I do this it creates both directories, mnt and sda</p>
 <div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>mount<span class="w"> </span>-t<span class="w"> </span>vfat<span class="w"> </span>/dev/mmcsd0<span class="w"> </span>/mnt/sda
 </pre></div>
 </div>
 <p><strong>Answer</strong>: This is because the top level directories are part of a
 <cite>pseudo-filesystem</cite> – like the Linux <code class="docutils literal notranslate"><span class="pre">proc/</span></code> or <code class="docutils literal notranslate"><span class="pre">sys/</span></code> file systems.
 But the NuttX pseudo-file system begins at the top level <code class="docutils literal notranslate"><span class="pre">/</span></code>.</p>
 <p>What that really means is that you do must have
 <code class="docutils literal notranslate"><span class="pre">CONFIG_DISABLE_PSEUDOFS_OPERATIONS</span></code> selected. Because you
 can normally create directories in the pseudo-filesystem
 with not problem:</p>
 <div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>NuttShell<span class="w"> </span><span class="o">(</span>NSH<span class="o">)</span><span class="w"> </span>NuttX-9.0.0
 nsh&gt;<span class="w"> </span>mkdir<span class="w"> </span>/mnt
 nsh&gt;<span class="w"> </span>ls
 /:
 dev/
 etc/
 mnt/
 proc/
 tmp/
 nsh&gt;<span class="w"> </span>ls<span class="w"> </span>mnt
 /mnt:
 nsh&gt;
 </pre></div>
 </div>
 <p>But lets assume that you do have operations on the pseudo-file
 system disabled. Why doesn’t it work? There is no <cite>real</cite> media
 there so you cannot create a file there or create any directories
 there. The <code class="docutils literal notranslate"><span class="pre">mount</span></code> command is special, it knows how to create mount
 points in the pseudo-file system.</p>
 <p>The pseudo-file system is just a tree structure in RAM.
 It serves two purposes: (1) you don’t have to have a real
 file system to use NuttX.
 It comes up out-of-the-box with usable (but limited)
 pseudo-file system. That allows a little more civilized
 programming environment on even very resource limited MCUs.
 And (2) this pseudo-file system is a place where all special
 NuttX files are retained: Character drivers, block drivers,
 and mount points.</p>
 <p>The NuttX top-level pseudo-filesystem creates the <cite>illusion</cite> of
 directories and provides a consistent, seamless semantic for
 interacting with mounted file systems. If there is a file
 called <code class="docutils literal notranslate"><span class="pre">hello.txt</span></code> in your volume mounted at <code class="docutils literal notranslate"><span class="pre">/mnt/sda</span></code>, then:</p>
 <p><code class="docutils literal notranslate"><span class="pre">/mnt</span></code> - is a <cite>node</cite> in the pseudo-filesystem that does
 nothing but contain the name mnt and provide links
 to things <cite>under</cite> <code class="docutils literal notranslate"><span class="pre">mnt</span></code>.</p>
 <p><code class="docutils literal notranslate"><span class="pre">/mnt/sda</span></code> - This refers to a node that contains the name
 sda that can be found <cite>under</cite> the node with the name mnt.
 This node is a special <cite>mountpoint node</cite> in the pseudo-filesystem.
 It contains the methods needed to interact will real file system.
 Everything <cite>below</cite> <code class="docutils literal notranslate"><span class="pre">/mnt/sda</span></code> is in the physical media.</p>
 <p><code class="docutils literal notranslate"><span class="pre">/mnt/sda/hello.txt</span></code> - This, then refers to the file
 <code class="docutils literal notranslate"><span class="pre">hello.txt</span></code> at the relative path <code class="docutils literal notranslate"><span class="pre">hello.txt</span></code> on the mounted media.
 The transition from the pseudo-filesystem to the
 real media is seamless.</p>
 <p>This is a little different from Linux: Linux always
 has to boot up with a <cite>real</cite> file system – even if it
 is only a initrd RAM disk.
 In Linux, these special files (links, drivers, pipes,
 etc.) reside on real media and can reside in any
 Linux-compatible filesystem.</p>
 <p>Normal <code class="docutils literal notranslate"><span class="pre">mkdir</span></code> can only work if there is a <cite>real</cite> filesystem
 at the location. There are no real directories in the
 pseudo-filesystem. The pseudo-filesystem does support
 <cite>nodes</cite> that look like directories and have some of the
 properties of directories (like the node <code class="docutils literal notranslate"><span class="pre">/mnt</span></code> mentioned
 above). But this is really an illusion.</p>
 <p>If <code class="docutils literal notranslate"><span class="pre">CONFIG_DISABLE_PSEUDOFS_OPERATIONS</span></code> is not enabled,
 then NuttX adds the capability to create new, empty <cite>nodes</cite>
 in the pseudo-filesystem using <code class="docutils literal notranslate"><span class="pre">mkdir</span></code>, completing the illusion.</p>
 <p>[On the other hand, all directories are really an
 <cite>illusion</cite> in a way and I suppose that in that sense
 these nodes the pseudo-filesystem are just as <cite>real</cite>
 as any other directory.]</p>
 <p>After you mount the SD card at <code class="docutils literal notranslate"><span class="pre">/mnt/sda</span></code>, then you can do:</p>
 <div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>mkdir<span class="w"> </span>/mnt/sda/newdir
 </pre></div>
 </div>
 <p>That should work fine and should create a directory at
 the relative path <code class="docutils literal notranslate"><span class="pre">newdir</span></code> in the mounted volume.</p>
 <p>There are a few other special NSH commands like mount that
 can change the pseudo-filesystem. Like <code class="docutils literal notranslate"><span class="pre">losetup</span></code>, <code class="docutils literal notranslate"><span class="pre">mkfifo</span></code>,
 <code class="docutils literal notranslate"><span class="pre">mkrd</span></code>, <code class="docutils literal notranslate"><span class="pre">umount</span></code>, etc.
 In fact, these commands <cite>only</cite> work in the pseudo-filesystem.
 Try them in <code class="docutils literal notranslate"><span class="pre">/mnt/sda</span></code>… they won’t work.</p>
 <p>But none of the <cite>normal</cite> commands that modify files or directories
 will work in the pseudo-filesystem: <code class="docutils literal notranslate"><span class="pre">mkdir</span></code>, <code class="docutils literal notranslate"><span class="pre">mv</span></code>, <code class="docutils literal notranslate"><span class="pre">rm</span></code>, <code class="docutils literal notranslate"><span class="pre">rmdir</span></code>.
 These all require real media. They will not work in the
 pseudo-filesystem, but will work in <code class="docutils literal notranslate"><span class="pre">/mnt/sda</span></code>.</p>
 <p>And trying to pipe to something in the pseudo-filesystem
 will also fail. You cannot do this, for example:</p>
 <div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>NuttShell<span class="w"> </span><span class="o">(</span>NSH<span class="o">)</span><span class="w"> </span>NuttX-6.20
 nsh&gt;<span class="w"> </span>cat<span class="w"> </span><span class="s2">&quot;Hello, World!&quot;</span><span class="w"> </span>&gt;/hello.text
 nsh:<span class="w"> </span>cat:<span class="w"> </span>open<span class="w"> </span>failed:<span class="w"> </span><span class="m">22</span>
 nsh&gt;
 </pre></div>
 </div>
 <p>See also NxFileSystem in
 <a class="reference external" href="https://cwiki.apache.org/confluence/display/NUTTX/Porting+Guide">Porting Guide</a></p>
 </section>
 </section>


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