DEV 4
NAME
dev - device files in /dev
DESCRIPTION
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 3 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 3 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 3, 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 , and
null 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.
The
null device is a data sink. It happily swallows any bytes written to it, and
returns nothing on a read.
"Floppy disk (major 2)"
The
fd0 , fd0p0 , fd0p1 , fd0p2 , and
fd0p3 block devices are the first floppy disk and the four partitions that may
exist on a that floppy disk. Likewise are
fd1 and
fd1p[0-3] 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
controller (4). "Controller 0 (major 3)"
The first hard disk on controller 0 can be accessed by block device
c0d0 . 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,
c0d0p0 , c0d0p1 , c0d0p2 , and
c0d0p3 . Each of these devices accesses a range of sectors on the
c0d0 device. It is customary to give each operating system on a disk a primary
partition. So the Windows C: "drive" can be on
c0d0p0 , and MINIX 3 can be on
c0d0p1 . MINIX 3 wants to have several partitions on its own, so
c0d0p1 can be further subdivided into the subpartitions
c0d0p1s0 , c0d0p1s1 , c0d0p1s2 , and
c0d0p1s3 . /dev contains devices for the first and second hard disk
( c0d0 and
c0d1 ), their primary partitions
( c0d[01]p[0-3] ) and subpartitions thereof
( c0d[01]p[0-3]s[0-3] ). More detail can be found in
controller (4). "Terminals (minor 4)"
The TTY driver manages the system console device, aptly named
console , the serial lines,
tty00 and
tty01 , and the pseudo ttys.
Through the console 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.
To allow remote login the devices with minor numbers of 128 or higher 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 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.
The
TCP/
IP task is not a kernel task, but a server like PM and VFS. It sits
between VFS and the drivers that manage the ethernet boards. Together
they implement the
TCP/
IP protocol.
"Controller 1 (major 8)"
Like controller 0 (major 3), but managing a second controller with devices
/dev/c1* . "Controller 2 (major 10)"
Like controller 0.
"Controller 3 (major 12)"
Like controller 0.
"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.
"uds (major 18)"
The
uds device is used to implement unix domain sockets.
FILES
10
/dev/* All MINIX 3 devices
"SEE ALSO"
read (2), write (2), lseek (2), ioctl (2), console (4), fd (4), controller (4), uds (4), tty (4), MAKEDEV (8). DIAGNOSTICS
There are five prominent errors that processes accessing device files may
provoke:
"ENODEV - No such device" 5
There is no driver managing the device class this device belongs to, typically
because it is not loaded.
"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 limits.
"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.
NOTES
Some devices are not present by default. The
MAKEDEV script knows how to make them.
The names of
MS-DOS/
Windows devices probably map to MINIX 3 devices as follows:
A: fd0
B: fd1
C: c0d0p0 (usually the first partition)
D: c0d1p0, c0d2p0 (if it's another disk)
D: c0d0p1s0 (if it's an extended partition)
D: c0d1, c0d2, c0d3 (if it's a CD-ROM)
CON console
COM1 tty00 (UNIX counts from 0)
LPT1 lp
AUTHOR
Kees J. Bot (kjb@cs.vu.nl)