262 lines
7.8 KiB
Groff
262 lines
7.8 KiB
Groff
.TH DEV 4
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.SH NAME
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dev \- device files in /dev
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.SH DESCRIPTION
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Device files are the eyes and ears of the system. Through the device files
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one has access to the disks, terminals and other parts of the machine.
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Single bytes or disk blocks may be transferred to or from a device with
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ordinary
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.BR read (2)
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or
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.BR write (2)
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calls, byte positions set with
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.BR lseek (2),
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or more complicated control functions performed with
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.BR ioctl(2).
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.PP
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Device files as found in
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.B /dev
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have several attributes that must be considered. Here are two examples as
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.B "ls \-l"
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shows them:
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.PP
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.RS
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.nf
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.if t .ft C
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brw-rw-rw- 1 root operator 2, 1 Jun 10 1995 fd1
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crw--w---- 1 kjb tty 4, 0 May 11 09:41 console
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.if t .ft P
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.fi
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.RE
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.PP
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Most attributes are the same as for a regular file and have the same
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function. The file type and the major and minor device numbers are special
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to devices.
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.PP
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Character devices are marked with a
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.B c
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as a file type letter. Any I/O on a character device is sent down to the
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device driver without any interpretation. This means that a process doing
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the I/O must know the characteristics of the device and deal with them
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appropriately.
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.PP
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Block devices provoke the file system server into buffering the data on
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those devices. Data read or written by processes is passed through the file
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system block cache. Unaligned bytes read or written are extracted or
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reassembled by the file server from or to whole blocks in the cache. The
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file server transfers data to or from the device driver as blocks to
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positions at block size boundaries. These blocks are Minix blocks of 1024
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bytes, disk devices usually have a 512 byte block size. Only block devices
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can be mounted as part of the file system tree if they contain a Minix file
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system.
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.PP
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The major device number (2 for
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.B fd1
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and 4 for
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.BR console )
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are used by FS to find the device driver that manages a device. The minor
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device number (1 for
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.B fd1
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and 0 for
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.BR console )
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is passed to the driver to select a device among a number of related devices
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that are all managed by that driver. The device drivers are usually kernel
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tasks under Minix, small processes that are contained within the address
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space of the kernel. The following tasks and associated devices exist:
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.SS "Memory (major 1)"
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The
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.BR ram ,
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.BR mem ,
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.BR kmem ,
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and
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.BR null
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devices are managed by the memory task.
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The
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.B ram
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device is a block device for a chunk of memory that is the RAM disk. Any
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byte read from or written to the
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.B ram
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device is copied from or to that memory chunk.
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The
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.B mem
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device is a character device for the entire address space of the system, but
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.B kmem
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only for the kernel data area. These two devices allow programs like
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.BR ps (1)
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to hunt around the system looking for interesting bits.
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The
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.B null
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device is a data sink. It happily swallows any bytes written to it, and
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returns nothing on a read.
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.SS "Floppy disk (major 2)"
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The
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.BR fd0 ,
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.BR fd0p0 ,
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.BR fd0p1 ,
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.BR fd0p2 ,
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and
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.BR fd0p3
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block devices are the first floppy disk and the four partitions that may
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exist on a that floppy disk. Likewise are
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.BR fd1
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and
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.BR fd1p[0\-3]
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the device and partitions for the second floppy disk. The floppy disk
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devices are described in detail in
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.BR fd (4).
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Partitioning in general is explained in
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.BR controller (4).
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.SS "Controller 0 (major 3)"
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The first hard disk on controller 0 can be accessed by block device
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.BR c0d0 .
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This device addresses the entire hard disk from the first to the last
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sector. A hard disk is normally partitioned in up to four primary
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partitions,
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.BR c0d0p0 ,
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.BR c0d0p1 ,
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.BR c0d0p2 ,
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and
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.BR c0d0p3 .
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Each of these devices accesses a range of sectors on the
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.B c0d0
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device. It is customary to give each operating system on a disk a primary
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partition. So the Windows C: "drive" can be on
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.BR c0d0p0 ,
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and Minix can be on
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.BR c0d0p1 .
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Minix wants to have several partitions on its own, so
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.B c0d0p1
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can be further subdivided into the subpartitions
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.BR c0d0p1s0 ,
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.BR c0d0p1s1 ,
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.BR c0d0p1s2 ,
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and
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.BR c0d0p1s3 .
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.B /dev
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contains devices for the first and second hard disk
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.RB ( c0d0
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and
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.BR c0d1 ),
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their primary partitions
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.RB ( c0d[01]p[0\-3] )
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and subpartitions thereof
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.RB ( c0d[01]p[0\-3]s[0\-3] ).
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More detail can be found in
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.BR controller (4).
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.SS "Terminals (minor 4)"
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The TTY driver manages the system console device, aptly named
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.BR console ,
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the serial lines,
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.BR tty00
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and
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.BR tty01 ,
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and the pseudo ttys.
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Through the console device one can display characters on a screen attached
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to a monochrome, Hercules, color, or VGA adapter. The
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.BR ttyc1 ,
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.BR ttyc2 ,
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etc. devices are the so-called "virtual consoles" that share the one
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console display. One can select which virtual console is to be visible on
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the screen and take input from the keyboard.
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To allow remote login the devices with minor numbers of 128 or higher offer
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virtual terminals. These pseudo ttys come in tty, pty pairs that form a
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pipe between processes running under the tty, and a controlling process
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attached to the pty side.
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See also
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.BR console (4),
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and
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.BR tty (4).
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.SS "Anonymous TTY (major 5)"
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This is just one device named
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.BR tty
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that is a synonym for the controlling tty of a process. This device is not
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managed by any device driver, but is handled by FS itself. A process can
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get access to the terminal it is running under by using
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.BR /dev/tty .
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.SS "Line printer (major 6)"
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The
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.B lp
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device sends any bytes written to it to the printer.
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.SS "TCP/IP (major 7)"
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The TCP/IP task is not a kernel task, but a server like MM and FS. It sits
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between FS and the DP8390 task that manages the ethernet boards. Together
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they implement the TCP/IP protocol. See also
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.BR ip (4).
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.SS "Controller 1 (major 8)"
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Like controller 0 (major 3), but managing a second controller with devices
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.BR /dev/c1* .
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.SS "Controller 2 (major 10)"
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Like controller 0.
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.SS "Controller 3 (major 12)"
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Like controller 0.
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.SS "Audio (major 13)"
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The
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.B audio
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device can be used to produce or record air vibrations using a Soundblaster
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16 type audio card. See
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.BR audio (4).
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.SS "Mixer (major 14)"
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The
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.B mixer
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device is used to control the audio driver.
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.SH FILES
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.TP 10
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.B /dev/*
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All Minix devices
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.SH "SEE ALSO"
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.BR read (2),
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.BR write (2),
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.BR lseek (2),
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.BR ioctl (2),
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.BR console (4),
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.BR fd (4),
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.BR controller (4),
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.BR ip (4),
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.BR tty (4),
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.BR MAKEDEV (8).
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.SH DIAGNOSTICS
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There are five prominent errors that processes accessing device files may
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provoke:
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.IP "ENODEV \- No such device" 5
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There is no driver managing the device class this device belongs to. Either
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the driver is configured out, or it is not loaded (inet).
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.IP "ENXIO \- No such device or address"
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This device is not available. Either the driver does not support it at all,
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or the hardware isn't available, i.e. accessing the second disk on a system
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with only one disk.
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.IP "EACCES \- Permission denied"
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This error may cause a lot of head scratching if
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.B ls \-l
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shows a device file to be writable. The media you are trying to access is
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simply physically write protected!
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.IP "EINVAL \- Invalid argument"
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Devices may not like reads or writes that are not block multiples, or very
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big transfers, etc. The device manual page should list the limits.
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.IP "EIO \- I/O error"
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This may be a real I/O error, i.e. a read or write on the device failing due
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to a media error. But it may also be the result of an operation that a
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device can't do, or an empty tape drive, etc.
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.SH NOTES
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Some devices are not present by default. The
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.BR MAKEDEV
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script knows how to make them.
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.SS "MS-DOS/Windows equivalents"
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The names of MS-DOS/Windows devices probably map to Minix devices as follows:
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.PP
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.RS
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.nf
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.ta +\w'COM1mmm'u +\w'c0d1, c0d2, c0d3mmm'u
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A: fd0
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B: fd1
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C: c0d0p0 (usually the first partition)
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D: c0d1p0, c0d2p0 (if it's another disk)
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D: c0d0p1s0 (if it's an extended partition)
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D: c0d1, c0d2, c0d3 (if it's a CD-ROM)
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CON console
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COM1 tty00 (UNIX counts from 0)
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LPT1 lp
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.fi
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.RE
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.SH AUTHOR
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Kees J. Bot (kjb@cs.vu.nl)
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