Pseudo File System

Warning

Migrated from: https://cwiki.apache.org/confluence/display/NUTTX/Pseudo+File+System

Overview

Pseudo Root File System

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 /proc file system is also referred to as a pseudo file system).

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 /dev pseudo file system directory.

Special Files

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.

NuttX does, however, support Linux-like special device node, 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 only be created in the root pseudo-file system. For the case of device nodes, see Device Nodes for further information.

In NuttX, the underlying principle is that all named resources appear as special files in the root pseudo-file system and are managed by the VFS.

Mounted Volumes

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).

Comparison to Linux

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.

FAQ

Question: I’m wondering why I can’t create a directory. If I try to create a dir.

mkdir /mnt

I get this,

nsh: mkdir: mkdir failed: 2

although if I do this it creates both directories, mnt and sda

mount -t vfat /dev/mmcsd0 /mnt/sda

Answer: This is because the top level directories are part of a pseudo-filesystem – like the Linux proc/ or sys/ file systems. But the NuttX pseudo-file system begins at the top level /.

What that really means is that you do must have CONFIG_DISABLE_PSEUDOFS_OPERATIONS selected. Because you can normally create directories in the pseudo-filesystem with not problem:

NuttShell (NSH) NuttX-9.0.0
nsh> mkdir /mnt
nsh> ls
/:
dev/
etc/
mnt/
proc/
tmp/
nsh> ls mnt
/mnt:
nsh>

But lets assume that you do have operations on the pseudo-file system disabled. Why doesn’t it work? There is no real media there so you cannot create a file there or create any directories there. The mount command is special, it knows how to create mount points in the pseudo-file system.

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.

The NuttX top-level pseudo-filesystem creates the illusion of directories and provides a consistent, seamless semantic for interacting with mounted file systems. If there is a file called hello.txt in your volume mounted at /mnt/sda, then:

/mnt - is a node in the pseudo-filesystem that does nothing but contain the name mnt and provide links to things under mnt.

/mnt/sda - This refers to a node that contains the name sda that can be found under the node with the name mnt. This node is a special mountpoint node in the pseudo-filesystem. It contains the methods needed to interact will real file system. Everything below /mnt/sda is in the physical media.

/mnt/sda/hello.txt - This, then refers to the file hello.txt at the relative path hello.txt on the mounted media. The transition from the pseudo-filesystem to the real media is seamless.

This is a little different from Linux: Linux always has to boot up with a real 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.

Normal mkdir can only work if there is a real filesystem at the location. There are no real directories in the pseudo-filesystem. The pseudo-filesystem does support nodes that look like directories and have some of the properties of directories (like the node /mnt mentioned above). But this is really an illusion.

If CONFIG_DISABLE_PSEUDOFS_OPERATIONS is not enabled, then NuttX adds the capability to create new, empty nodes in the pseudo-filesystem using mkdir, completing the illusion.

[On the other hand, all directories are really an illusion in a way and I suppose that in that sense these nodes the pseudo-filesystem are just as real as any other directory.]

After you mount the SD card at /mnt/sda, then you can do:

mkdir /mnt/sda/newdir

That should work fine and should create a directory at the relative path newdir in the mounted volume.

There are a few other special NSH commands like mount that can change the pseudo-filesystem. Like losetup, mkfifo, mkrd, umount, etc. In fact, these commands only work in the pseudo-filesystem. Try them in /mnt/sda… they won’t work.

But none of the normal commands that modify files or directories will work in the pseudo-filesystem: mkdir, mv, rm, rmdir. These all require real media. They will not work in the pseudo-filesystem, but will work in /mnt/sda.

And trying to pipe to something in the pseudo-filesystem will also fail. You cannot do this, for example:

NuttShell (NSH) NuttX-6.20
nsh> cat "Hello, World!" >/hello.text
nsh: cat: open failed: 22
nsh>