input
mem
misc
net
pci_bus
tty
Supported sysfs devices:
pci0000:00
platform
system
You can see from this listing the variety of system information available from sysfs. Many utilities use this information to determine the characteristics of system devices or to enforce system policies, such as power management and hot-plug capability.
9.9. Other File Systems
Numerous file systems are supported under Linux. Space does not permit coverage of all of them. However, you should be aware of some other important file systems frequently found in embedded systems.
The ramfs file system is best considered from the context of the Linux source code module that implements it. Listing 9-19 reproduces the first several lines of that file.
Listing 9-19. Linux ramfs Source Module Comments
/*
* Resizable simple ram filesystem for Linux.
*
* Copyright (C) 2000 Linus Torvalds.
* 2000 Transmeta Corp.
*
* Usage limits added by David Gibson, Linuxcare Australia.
* This file is released under the GPL.
*/
/*
* NOTE! This filesystem is probably most useful
* not as a real filesystem, but as an example of
* how virtual filesystems can be written.
*
* It doesn't get much simpler than this. Consider
* that this file implements the full semantics of
* a POSIX-compliant read-write filesystem.
This module was written primarily as an example of how virtual file systems can be written. One of the primary differences between this file system and the ramdisk facility found in modern Linux kernels is its capability to shrink and grow according to its use. A ramdisk does not have this property. This source module is compact and well written. It is presented here for its educational value. You are encouraged to study this good example.
The tmpfs file system is similar to and related to rams. Like ramfs, everything in tmpfs is stored in kernel virtual memory, and the contents of tmpfs are lost on power-down or reboot. The tmpfs file system is useful for fast temporary storage of files. I use tmpfs mounted on /tmp in a midi/audio application to speed up the creation and deletion of temporary objects required by the audio subsystem. This is also a great way to keep your /tmp directory cleanits contents are lost on every reboot. Mounting tmpfs is similar to any other virtual file system:
# mount -t tmpfs /tmpfs /tmp
As with other virtual file systems such as /proc, the first /tmpfs parameter in the previous mount command is a 'no-op'that is, it could be the word none and still function. However, it is a good reminder that you are mounting a virtual file system called tmpfs.
9.10. Building a Simple File System
It is straightforward to build a simple file system image. Here we demonstrate the use of the Linux kernel's loopback device. The loopback device enables the use of a regular file as a block device. In short, we build a file system image in a regular file and use the Linux loopback device to mount that file in the same way any other block device is mounted.
To build a simple root file system, start with a fixed-sized file containing all zeros:
# dd if=/dev/zero of=./my-new-fs-image bs=1k count=512
This command creates a file of 512KB containing nothing but zeros. We fill the file with zeros to aid in compression later and to have a consistent data pattern for uninitialized data blocks within the file system. Use caution with the dd command. Executing dd with no boundary (count=) or with an improper boundary can fill up your hard drive and possibly crash your system. dd is a powerful tool; use it with the respect it deserves. Simple typos in commands such as dd, executed as root, have destroyed countless file systems.
When we have the new image file, we actually format the file to contain the data structures defined by a given file system. In this example, we build an ext2 file system. Listing 9-20 details the procedure.
Listing 9-20. Creating an ext2 File System Image
# /sbin/mke2fs ./my-new-fs-image
mke2fs 1.35 (28-Feb-2004)
./my-new-fs-image is not a block special device.
Proceed anyway? (y,n) y
Filesystem label=
OS type: Linux
Block size=1024 (log=0)
Fragment size=1024 (log=0)
64 inodes, 512 blocks
25 blocks (4.88%) reserved for the super user
First data block=1
1 block group
8192 blocks per group, 8192 fragments per group
64 inodes per group
Writing inode tables: done
Writing superblocks and filesystem accounting information: done
This filesystem will be automatically checked every 24 mounts or
180 days, whichever comes first. Use tune2fs -c or -i to override.
#
As with dd, the mke2fs command can destroy your system, so use it with care. In this example, we asked
