dev - device files in /dev
Device files are the eyes and ears of the system. Through the device
files one has access to the disks, terminals and other parts of the
machine. Single bytes or disk blocks may be transferred to or from a
device with ordinary read(2) or write(2) calls, byte positions set with
lseek(2), or more complicated control functions performed with ioctl(2).
Device files as found in /dev have several attributes that must be
considered. Here are two examples as ls -l shows them:
brw-rw-rw- 1 root operator 2, 1 Jun 10 1995 fd1
crw--w---- 1 kjb tty 4, 0 May 11 09:41 console
Most attributes are the same as for a regular file and have the same
function. The file type and the major and minor device numbers are
special to devices.
Character devices are marked with a c as a file type letter. Any I/O on
a character device is sent down to the device driver without any
interpretation. This means that a process doing the I/O must know the
characteristics of the device and deal with them appropriately.
Block devices provoke the file system server into buffering the data on
those devices. Data read or written by processes is passed through the
file system block cache. Unaligned bytes read or written are extracted
or reassembled by the file server from or to whole blocks in the cache.
The file server transfers data to or from the device driver as blocks to
positions at block size boundaries. These blocks are Minix blocks of
1024 bytes, disk devices usually have a 512 byte block size. Only block
devices can be mounted as part of the file system tree if they contain a
Minix file system.
The major device number (2 for fd1 and 4 for console) are used by FS to
find the device driver that manages a device. The minor device number (1
for fd1 and 0 for console) is passed to the driver to select a device
among a number of related devices that are all managed by that driver.
The device drivers are usually kernel tasks under Minix, small processes
that are contained within the address space of the kernel. The following
tasks and associated devices exist:
Memory (major 1)
The ram, mem, kmem, null, and zero devices are managed by the memory
task. The ram device is a block device for a chunk of memory that is the
RAM disk. Any byte read from or written to the ram device is copied from
or to that memory chunk. The mem device is a character device for the
entire address space of the system, but kmem only for the kernel data
area. These two devices allow programs like ps(1) to hunt around the
system looking for interesting bits. Note that mem accesses virtual
memory as the kernel sees it, i.e. the first so-many megabytes are mapped
1 to 1 to physical memory, everything above is virtual and may read as
zeros if there is no real memory behind the virtual pages. The null and
zero devices are a data sink and a zero byte source respectively. Both
devices happily swallow any bytes written to them. Nothing is returned
on a read from null, and as many zeros as requested are returned by zero.
Floppy disk (major 2)
The fd0, fd0a, fd0b, fd0c, and fd0d block devices are the first floppy
disk and the four partitions that may exist on a that floppy disk.
Likewise are fd1 and fd1[a-d] the device and partitions for the second
floppy disk. The floppy disk devices are described in detail in fd(4).
Partitioning in general is explained in hd(4).
Hard disk (major 3)
The first hard disk can be accessed by block device hd0. This device
addresses the entire hard disk from the first to the last sector. A hard
disk is normally partitioned in up to four primary partitions, hd1, hd2,
hd3, and hd4. Each of these devices accesses a range of sectors on the
hd0 device. It is customary to give each operating system on a disk a
primary partition. So the MS-DOS C: "drive" can be on hd1, and Minix can
be on hd2. Minix wants to have several partitions on its own, so hd2 can
be further subdivided into the subpartitions hd2a, hd2b, hd2c, and hd2d.
/dev contains devices for the first and second hard disk (hd0 and hd5)
their primary partitions (hd[1-46-9]) and subpartitions thereof
(hd[1-46-9][a-d]). More detail can be found in hd(4).
Console (minor 4)
The console driver manages the system console device, aptly named
console. Through this device one can display characters on a screen
attached to a monochrome, Hercules, color, or VGA adapter. The ttyc1,
ttyc2, etc. devices are the so-called "virtual consoles" that share the
one console display. One can select which virtual console is to be
visible on the screen and take input from the keyboard. Other devices
managed by the console driver are profile, a device used to profile
Minix, log, used by daemon processes to write log messages to the
console, vga and kbd, used by an X11 server to get raw access to the VGA
screen and keyboard, and lastly kbdaux, the auxiliary keyboard device,
usually a PS/2 type mouse. See also console(4), and tty(4).
Anonymous TTY (major 5)
This is just one device named tty that is a synonym for the controlling
tty of a process. This device is not managed by any device driver, but
is handled by FS itself. A process can get access to the terminal it is
running under by using /dev/tty.
Line printer (major 6)
The lp device sends any bytes written to it to the printer. See lp(4).
TCP/IP (major 7)
The TCP/IP task is not a kernel task, but a separate server. It starts
its life as a normal process named inet, executes a few system calls to
become a server like MM and FS, and then signs up with FS to manage the
TCP/IP devices that happen to be assigned major 7. There is a kernel
task involved in this scenario though, the ethernet boards are managed by
the kernel task DP8390. (So the TCP/IP task is not completely
freestanding.) See also ip(4).
CD-ROM (major 8)
This is the CD-ROM driver for the Mitsumi proprietary CD-ROM interface.
The cd0 device addresses the whole CD, with extra cd[1-4] and
cd[1-4][a-d] devices for if the CD also contains partitions with Minix
Pseudo ttys (major 9)
To allow remote login the devices under major 9 offer virtual terminals.
These pseudo ttys come in tty, pty pairs that form a pipe between
processes running under the tty, and a controlling process attached to
the pty side. See tty(4).
SCSI disks and tapes (major 10)
The sd* devices are disks in the same way as the hd* devices. Except
that these disks are SCSI disks attached to an Adaptec 1540 controller or
compatible. The driver also manages the rst* and nrst* tape devices
(rewinding or non-rewinding). See sd(4).
Serial lines (major 11)
The serial lines can be accessed through tty00, tty01, tty02, and tty03.
A serial mouse is often attached to tty00 with a modem on tty01, unless
you put them somewhere else, but then you probably know where. See
DOS virtual disk (major 12)
If one creates a really big file on an MS-DOS file system then this
driver can use it as a disk. This DOS virtual disk can be accessed
through dosd0, with partitions dosd1, dosd2, dosd3, and dosd4. There are
no subpartitions offered by this driver. One can use a another DOS file
through devices dosd[5-9]. See dosd(4).
Audio (major 13)
The audio device can be used to produce or record air vibrations using a
Soundblaster 16 type audio card. See audio(4).
Mixer (major 14)
The mixer device is used to control the audio driver.
Virtual disks (major 15)
The virtual disk driver can be used to concatenate any two disk devices
to form one big disk. These disks are named vd0, vd1, etc. There are no
partitions as it makes no sense to first concatenate disks and then to
partition the result. See vd(4).
/dev/* All Minix devices
read(2), write(2), lseek(2), ioctl(2), console(4), dosd(4), fd(4), hd(4),
ip(4), lp(4), sd(4), tty(4), vd(4), MAKEDEV(8).
There are five prominent errors that processes accessing device files may
ENODEV - No such device
There is no driver managing the device class this device belongs to.
Either the driver is configured out, or it is not loaded (inet).
ENXIO - No such device or address
This device is not available. Either the driver does not support it
at all, or the hardware isn't available, i.e. accessing the second
disk on a system with only one disk.
EACCES - Permission denied
This error may cause a lot of head scratching if ls -l shows a
device file to be writable. The media you are trying to access is
simply physically write protected!
EINVAL - Invalid argument
Devices may not like reads or writes that are not block multiples,
or very big transfers, etc. The device manual page should list the
EIO - I/O error
This may be a real I/O error, i.e. a read or write on the device
failing due to a media error. But it may also be the result of an
operation that a device can't do, or an empty tape drive, etc.
Some devices are not present by default. The MAKEDEV script knows how to
The names of MS-DOS devices map to Minix devices as follows:
C: hd1, sd1 (usually the first partition)
D: hd6, sd1, sd6 (or an extended partition)
COM1 tty00 (UNIX counts from 0)
Rumours claiming that the zero device upsets the balance of zeros and
ones in the universe are unfounded and are only vile attempts to
discredit the industry.
Kees J. Bot (firstname.lastname@example.org)