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Linux Filesystem Hierarchy:
Prev Chapter 1. Linux Filesystem Hierarchy Next

1.5. /dev

/dev is the location of special or device files. It is a very interesting directory that highlights one important aspect of the Linux filesystem - everything is a file or a directory. Look through this directory and you should hopefully see hda1, hda2 etc.... which represent the various partitions on the first master drive of the system. /dev/cdrom and /dev/fd0 represent your CD-ROM drive and your floppy drive. This may seem strange but it will make sense if you compare the characteristics of files to that of your hardware. Both can be read from and written to. Take /dev/dsp, for instance. This file represents your speaker device. Any data written to this file will be re-directed to your speaker. If you try 'cat /boot/vmlinuz > /dev/dsp' (on a properly configured system) you should hear some sound on the speaker. That's the sound of your kernel! A file sent to /dev/lp0 gets printed. Sending data to and reading from /dev/ttyS0 will allow you to communicate with a device attached there - for instance, your modem.

The majority of devices are either block or character devices; however other types of devices exist and can be created. In general, 'block devices' are devices that store or hold data, 'character devices' can be thought of as devices that transmit or transfer data. For example, diskette drives, hard drives and CD-ROM drives are all block devices while serial ports, mice and parallel printer ports are all character devices. There is a naming scheme of sorts but in the vast majority of cases these are completely illogical.


total 724
lrwxrwxrwx    1 root     root           13 Sep 28 18:06 MAKEDEV -> /sbin/MAKEDEV
crw-rw----    1 root     audio     14,  14 Oct  7 16:26 admmidi0
crw-rw----    1 root     audio     14,  30 Oct  7 16:26 admmidi1
lrwxrwxrwx    1 root     root           11 Oct  7 16:26 amidi -> /dev/amidi0
crw-rw----    1 root     audio     14,  13 Oct  7 16:26 amidi0
crw-rw----    1 root     audio     14,  29 Oct  7 16:26 amidi1
crw-rw----    1 root     audio     14,  11 Oct  7 16:26 amixer0
crw-rw----    1 root     audio     14,  27 Oct  7 16:26 amixer1
drwxr-xr-x    2 root     root         4096 Sep 28 18:05 ataraid
lrwxrwxrwx    1 root     root           11 Oct  7 16:26 audio -> /dev/audio0
crw-rw----    1 root     audio     14,   4 Oct  7 16:26 audio0
crw-rw----    1 root     audio     14,  20 Oct  7 16:26 audio1
crw-rw----    1 root     audio     14,   7 Mar 15  2002 audioctl
lrwxrwxrwx    1 root     root            9 Oct 14 22:51 cdrom -> /dev/scd1
lrwxrwxrwx    1 root     root            9 Oct 14 22:52 cdrom1 -> /dev/scd0
crw-------    1 root     tty        5,   1 Jan 19 20:47 console
lrwxrwxrwx    1 root     root           11 Sep 28 18:06 core -> /proc/kcore
crw-rw----    1 root     audio     14,  10 Oct  7 16:26 dmfm0
crw-rw----    1 root     audio     14,  26 Oct  7 16:26 dmfm1
crw-rw----    1 root     audio     14,   9 Oct  7 16:26 dmmidi0
crw-rw----    1 root     audio     14,  25 Oct  7 16:26 dmmidi1
lrwxrwxrwx    1 root     root            9 Oct  7 16:26 dsp -> /dev/dsp0
crw-rw----    1 root     audio     14,   3 Oct  7 16:26 dsp0
crw-rw----    1 root     audio     14,  19 Oct  7 16:26 dsp1
crw--w----    1 root     video     29,   0 Mar 15  2002 fb0
crw--w----    1 root     video     29,   1 Mar 15  2002 fb0autodetect
crw--w----    1 root     video     29,   0 Mar 15  2002 fb0current
crw--w----    1 root     video     29,  32 Mar 15  2002 fb1
crw--w----    1 root     video     29,  33 Mar 15  2002 fb1autodetect
crw--w----    1 root     video     29,  32 Mar 15  2002 fb1current
lrwxrwxrwx    1 root     root           13 Sep 28 18:05 fd -> /proc/self/fd
brw-rw----    1 root     floppy     2,   0 Mar 15  2002 fd0
brw-rw----    1 root     floppy     2,   1 Mar 15  2002 fd1
crw--w--w-    1 root     root       1,   7 Sep 28 18:06 full
brw-rw----    1 root     disk       3,   0 Mar 15  2002 hda
brw-rw----    1 root     disk       3,  64 Mar 15  2002 hdb
brw-rw----    1 root     disk      22,   0 Mar 15  2002 hdc
brw-rw----    1 root     disk      22,  64 Mar 15  2002 hdd
drwxr-xr-x    2 root     root        12288 Sep 28 18:05 ida
prw-------    1 root     root            0 Jan 19 20:46 initctl
brw-rw----    1 root     disk       1, 250 Mar 15  2002 initrd
drwxr-xr-x    2 root     root         4096 Sep 28 18:05 input
crw-rw----    1 root     dialout   45, 128 Mar 15  2002 ippp0
crw-rw----    1 root     dialout   45,   0 Mar 15  2002 isdn0
crw-rw----    1 root     dialout   45,  64 Mar 15  2002 isdnctrl0
crw-rw----    1 root     dialout   45, 255 Mar 15  2002 isdninfo
crw-------    1 root     root      10,   4 Mar 15  2002 jbm
crw-r-----    1 root     kmem       1,   2 Sep 28 18:06 kmem
brw-rw----    1 root     cdrom     24,   0 Mar 15  2002 lmscd
crw-------    1 root     root      10,   0 Mar 15  2002 logibm
brw-rw----    1 root     disk       7,   0 Sep 28 18:06 loop0
brw-rw----    1 root     disk       7,   1 Sep 28 18:06 loop1
crw-rw----    1 root     lp         6,   0 Mar 15  2002 lp0
crw-rw----    1 root     lp         6,   1 Mar 15  2002 lp1
crw-rw----    1 root     lp         6,   2 Mar 15  2002 lp2
crw-r-----    1 root     kmem       1,   1 Sep 28 18:06 mem
lrwxrwxrwx    1 root     root           10 Oct  7 16:26 midi -> /dev/midi0
crw-rw----    1 root     audio     14,   2 Oct  7 16:26 midi0
crw-rw----    1 root     audio     14,  18 Oct  7 16:26 midi1
lrwxrwxrwx    1 root     root           11 Oct  7 16:26 mixer -> /dev/mixer0
crw-rw-rw-    1 root     root      14,   0 Nov 11 16:22 mixer0
crw-rw----    1 root     audio     14,  16 Oct  7 16:26 mixer1
lrwxrwxrwx    1 root     root           11 Oct  7 06:50 modem -> /dev/ttyLT0
crw-rw----    1 root     audio     31,   0 Mar 15  2002 mpu401data
crw-rw----    1 root     audio     31,   1 Mar 15  2002 mpu401stat
crw-rw----    1 root     audio     14,   8 Oct  7 16:26 music
crw-rw-rw-    1 root     root       1,   3 Sep 28 18:06 null
crw-rw-rw-    1 root     root     195,   0 Jan  6 03:03 nvidia0
crw-rw-rw-    1 root     root     195,   1 Jan  6 03:03 nvidia1
crw-rw-rw-    1 root     root     195, 255 Jan  6 03:03 nvidiactl
crw-rw----    1 root     lp         6,   0 Mar 15  2002 par0
crw-rw----    1 root     lp         6,   1 Mar 15  2002 par1
crw-rw----    1 root     lp         6,   2 Mar 15  2002 par2
-rw-r--r--    1 root     root       665509 Oct  7 16:41 pcm
crw-r-----    1 root     kmem       1,   4 Sep 28 18:06 port
crw-rw----    1 root     dip      108,   0 Sep 28 18:07 ppp
crw-------    1 root     root      10,   1 Mar 15  2002 psaux
crw-rw-rw-    1 root     root       1,   8 Sep 28 18:06 random
crw-rw----    1 root     root      10, 135 Mar 15  2002 rtc
brw-rw----    1 root     cdrom     11,   0 Mar 15  2002 scd0
brw-rw----    1 root     cdrom     11,   1 Mar 15  2002 scd1
brw-rw----    1 root     disk       8,   0 Mar 15  2002 sda
brw-rw----    1 root     disk       8,   1 Mar 15  2002 sda1
brw-rw----    1 root     disk       8,   2 Mar 15  2002 sda2
brw-rw----    1 root     disk       8,   3 Mar 15  2002 sda3
brw-rw----    1 root     disk       8,   4 Mar 15  2002 sda4
brw-rw----    1 root     disk       8,  16 Mar 15  2002 sdb
brw-rw----    1 root     disk       8,  17 Mar 15  2002 sdb1
brw-rw----    1 root     disk       8,  18 Mar 15  2002 sdb2
brw-rw----    1 root     disk       8,  19 Mar 15  2002 sdb3
brw-rw----    1 root     disk       8,  20 Mar 15  2002 sdb4
crw-rw----    1 root     audio     14,   1 Oct  7 16:26 sequencer
lrwxrwxrwx    1 root     root           10 Oct  7 16:26 sequencer2 -> /dev/music
lrwxrwxrwx    1 root     root            4 Sep 28 18:05 stderr -> fd/2
lrwxrwxrwx    1 root     root            4 Sep 28 18:05 stdin -> fd/0
lrwxrwxrwx    1 root     root            4 Sep 28 18:05 stdout -> fd/1
crw-rw-rw-    1 root     tty        5,   0 Sep 28 18:06 tty
crw-------    1 root     root       4,   0 Sep 28 18:06 tty0
crw-------    1 root     root       4,   1 Jan 19 14:59 tty1
crw-rw----    1 root     dialout   62,  64 Oct  7 06:50 ttyLT0
crw-rw----    1 root     dialout    4,  64 Mar 15  2002 ttyS0
crw-rw----    1 root     dialout    4,  65 Mar 15  2002 ttyS1
crw-rw----    1 root     dialout    4,  66 Mar 15  2002 ttyS2
crw-rw----    1 root     dialout    4,  67 Mar 15  2002 ttyS3
crw-rw----    1 root     dialout  188,   0 Mar 15  2002 ttyUSB0
crw-rw----    1 root     dialout  188,   1 Mar 15  2002 ttyUSB1
cr--r--r--    1 root     root       1,   9 Jan 19 20:46 urandom
drwxr-xr-x    2 root     root         4096 Sep 28 18:05 usb
prw-r-----    1 root     adm             0 Jan 19 14:58 xconsole
crw-rw-rw-    1 root     root       1,   5 Sep 28 18:06 zero

Some common device files as well as their equivalent counterparts under Windows that you may wish to remember are:

/dev/ttyS0 (First communications port, COM1)

First serial port (mice, modems).

/dev/psaux (PS/2)

PS/2 mouse connection (mice, keyboards).

/dev/lp0 (First printer port, LPT1)

First parallel port (printers, scanners, etc).

/dev/dsp (First audio device)

The name DSP comes from the term digital signal processor, a specialized processor chip optimized for digital signal analysis. Sound cards may use a dedicated DSP chip, or may implement the functions with a number of discrete devices. Other terms that may be used for this device are digitized voice and PCM.

/dev/usb (USB Devices)

This subdirectory contains most of the USB device nodes. Device name allocations are fairly simplistic so no elaboration is be necessary.

/dev/sda (C:\, SCSI device)

First SCSI device (HDD, Memory Sticks, external mass storage devices such as CD-ROM drives on laptops, etc).

/dev/scd (D:\, SCSI CD-ROM device)

First SCSI CD-ROM device.

/dev/js0 (Standard gameport joystick)

First joystick device.

Devices are defined by type, such as 'block' or 'character', and 'major' and 'minor' number. The major number is used to categorize a device and the minor number is used to identify a specific device type. For example, all IDE device connected to the primary controller have a major number of 3. Master and slave devices, as well as individual partitions are further defined by the use of minor numbers. These are the two numbers precede the date in the following display:

# ls -l /dev/hd*


brw-rw----    1 root     disk       3,   0 Mar 15  2002 /dev/hda
brw-rw----    1 root     disk       3,   1 Mar 15  2002 /dev/hda1
brw-rw----    1 root     disk       3,  10 Mar 15  2002 /dev/hda10
brw-rw----    1 root     disk       3,  11 Mar 15  2002 /dev/hda11
brw-rw----    1 root     disk       3,  12 Mar 15  2002 /dev/hda12
brw-rw----    1 root     disk       3,  13 Mar 15  2002 /dev/hda13
brw-rw----    1 root     disk       3,  14 Mar 15  2002 /dev/hda14
brw-rw----    1 root     disk       3,  15 Mar 15  2002 /dev/hda15
brw-rw----    1 root     disk       3,  16 Mar 15  2002 /dev/hda16
brw-rw----    1 root     disk       3,  17 Mar 15  2002 /dev/hda17
brw-rw----    1 root     disk       3,  18 Mar 15  2002 /dev/hda18
brw-rw----    1 root     disk       3,  19 Mar 15  2002 /dev/hda19
brw-rw----    1 root     disk       3,   2 Mar 15  2002 /dev/hda2
brw-rw----    1 root     disk       3,  20 Mar 15  2002 /dev/hda20
brw-rw----    1 root     disk       3,   3 Mar 15  2002 /dev/hda3
brw-rw----    1 root     disk       3,   4 Mar 15  2002 /dev/hda4
brw-rw----    1 root     disk       3,   5 Mar 15  2002 /dev/hda5
brw-rw----    1 root     disk       3,   6 Mar 15  2002 /dev/hda6
brw-rw----    1 root     disk       3,   7 Mar 15  2002 /dev/hda7
brw-rw----    1 root     disk       3,   8 Mar 15  2002 /dev/hda8
brw-rw----    1 root     disk       3,   9 Mar 15  2002 /dev/hda9
brw-rw----    1 root     disk       3,  64 Mar 15  2002 /dev/hdb
brw-rw----    1 root     disk       3,  65 Mar 15  2002 /dev/hdb1
brw-rw----    1 root     disk       3,  74 Mar 15  2002 /dev/hdb10
brw-rw----    1 root     disk       3,  75 Mar 15  2002 /dev/hdb11
brw-rw----    1 root     disk       3,  76 Mar 15  2002 /dev/hdb12
brw-rw----    1 root     disk       3,  77 Mar 15  2002 /dev/hdb13
brw-rw----    1 root     disk       3,  78 Mar 15  2002 /dev/hdb14
brw-rw----    1 root     disk       3,  79 Mar 15  2002 /dev/hdb15
brw-rw----    1 root     disk       3,  80 Mar 15  2002 /dev/hdb16
brw-rw----    1 root     disk       3,  81 Mar 15  2002 /dev/hdb17
brw-rw----    1 root     disk       3,  82 Mar 15  2002 /dev/hdb18
brw-rw----    1 root     disk       3,  83 Mar 15  2002 /dev/hdb19
brw-rw----    1 root     disk       3,  66 Mar 15  2002 /dev/hdb2
brw-rw----    1 root     disk       3,  84 Mar 15  2002 /dev/hdb20
brw-rw----    1 root     disk       3,  67 Mar 15  2002 /dev/hdb3
brw-rw----    1 root     disk       3,  68 Mar 15  2002 /dev/hdb4
brw-rw----    1 root     disk       3,  69 Mar 15  2002 /dev/hdb5
brw-rw----    1 root     disk       3,  70 Mar 15  2002 /dev/hdb6
brw-rw----    1 root     disk       3,  71 Mar 15  2002 /dev/hdb7
brw-rw----    1 root     disk       3,  72 Mar 15  2002 /dev/hdb8
brw-rw----    1 root     disk       3,  73 Mar 15  2002 /dev/hdb9
brw-rw----    1 root     disk      22,   0 Mar 15  2002 /dev/hdc
brw-rw----    1 root     disk      22,  64 Mar 15  2002 /dev/hdd

The major number for both hda and hdb devices is 3. Of course, the minor number changes for each specific partition. The definition of each major number category can be examined by looking at the contents of the /usr/src/linux/include/linux/major.h file. The devices.txt also documents major and minor numbers. It is located in the /usr/src/linux/Documentation directory. This file defines the major numbers. Almost all files devices are created by default at the install time. However, you can always create a device using the mknod command or the MAKEDEV script which is located in the /dev directory itself. Devices can be created with this utility by supplying the device to be created, the device type (block or character) and the major and minor numbers. For example, let's say you have accidentally deleted /dev/ttyS0 (COM1 under Windows), it can be recreated using the following command

# mknod ttyS0 c 4 64

For those of us who are rather lazy you can simply run the MAKEDEV script as such

# MAKEDEV *

which will create all devices known.

If is possible that /dev may also contain a MAKEDEV.local for the creation of any local device files.

In general and as required by the FSSTND, MAKEDEV will have provisions for creating any device that may be found on the system, not just those that a particular implementation installs.

For those of you who are wondering why Linux is using such a primitive system to reference devices its because we haven't been able to devise a sufficiently sophisticated mechanism which provides enough advantages over the current system in order to achieve widespread adoption.

To date (as of kernel version 2.4), the best attempt has been made by Richard Gooch of the CSIRO. It's called devfsd and has been a part of the kernel for a number of years now. It has been sanctioned by the kernel developers and Linus himself and details of its implementation can be found at /usr/src/linux/Documentation/filesystems/devfs/README. Below is an excerpt from this document.

Devfs is an alternative to "real" character and block special devices on your root filesystem. Kernel device drivers can register devices by name rather than major and minor numbers. These devices will appear in devfs automatically, with whatever default ownership and protection the driver specified. A daemon (devfsd) can be used to override these defaults. Devfs has been in the kernel since 2.3.46.

NOTE that devfs is entirely optional. If you prefer the old disc-based device nodes, then simply leave CONFIG_DEVFS_FS=n (the default). In this case, nothing will change. ALSO NOTE that if you do enable devfs, the defaults are such that full compatibility is maintained with the old devices names.

There are two aspects to devfs: one is the underlying device namespace, which is a namespace just like any mounted filesystem. The other aspect is the filesystem code which provides a view of the device namespace. The reason I make a distinction is because devfs can be mounted many times, with each mount showing the same device namespace. Changes made are global to all mounted devfs filesystems. Also, because the devfs namespace exists without any devfs mounts, you can easily mount the root filesystem by referring to an entry in the devfs namespace.

The cost of devfs is a small increase in kernel code size and memory usage. About 7 pages of code (some of that in __init sections) and 72 bytes for each entry in the namespace. A modest system has only a couple of hundred device entries, so this costs a few more pages. Compare this with the suggestion to put /dev on a ramdisc.

On a typical machine, the cost is under 0.2 percent. On a modest system with 64 MBytes of RAM, the cost is under 0.1 percent. The accusations of "bloatware" levelled at devfs are not justified.

As of kernel version 2.6, devfs has been marked obsolete and has now been replaced by udev. A system very similar (at least from a the end user's point of view) to devfs but which works entirely in userspace. An overview of udev can be found at http://www.kroah.com/linux/talks/ols_2003_udev_paper/Reprint-Kroah-Hartman-OLS2003.pdf


 
 
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