1.\" 2.\" Copyright (c) 1996 David E. O'Brien, Joerg Wunsch 3.\" 4.\" All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 15.\" THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``AS IS'' AND ANY EXPRESS OR 16.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES 17.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. 18.\" IN NO EVENT SHALL THE DEVELOPERS BE LIABLE FOR ANY DIRECT, INDIRECT, 19.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT 20.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 21.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 22.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 23.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF 24.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 25.\" 26.\" $FreeBSD: src/share/man/man4/intro.4,v 1.13.2.6 2002/01/09 15:36:51 ru Exp $ 27.\" 28.Dd January 20, 1996 29.Dt INTRO 4 30.Os 31.Sh NAME 32.Nm intro 33.Nd introduction to devices and device drivers 34.Sh DESCRIPTION 35This section contains information related to devices, device drivers 36and miscellaneous hardware. 37.Ss The device abstraction 38Device is a term used mostly for hardware-related stuff that belongs 39to the system, like disks, printers, or a graphics display with its 40keyboard. There are also so-called 41.Em pseudo-devices 42where a device driver emulates the behaviour of a device in software 43without any particular underlying hardware. A typical example for 44the latter class is 45.Pa /dev/mem , 46a loophole where the physical memory can be accessed using the regular 47file access semantics. 48.Pp 49The device abstraction generally provides a common set of system calls 50layered on top of them, which are dispatched to the corresponding 51device driver by the upper layers of the kernel. The set of system 52calls available for devices is chosen from 53.Xr open 2 , 54.Xr close 2 , 55.Xr read 2 , 56.Xr write 2 , 57.Xr ioctl 2 , 58.Xr select 2 , 59and 60.Xr mmap 2 . 61Not all drivers implement all system calls, for example, calling 62.Xr mmap 2 63on terminal devices is likely to be not useful at all. 64.Ss Accessing Devices 65Most of the devices in a unix-like operating system are accessed 66through so-called 67.Em device nodes , 68sometimes also called 69.Em special files . 70They are usually located under the directory 71.Pa /dev 72in the file system hierarchy 73(see also 74.Xr hier 7 ) . 75.Pp 76Each device node must be created statically and 77independently of the existence of the associated device driver, 78usually by running 79.Xr MAKEDEV 8 . 80.Pp 81Note that this could lead to an inconsistent state, where either there 82are device nodes that do not have a configured driver associated with 83them, or there may be drivers that have successfully probed for their 84devices, but cannot be accessed since the corresponding device node is 85still missing. In the first case, any attempt to reference the device 86through the device node will result in an error, returned by the upper 87layers of the kernel, usually 88.Er ENXIO . 89In the second case, the device node needs to be created before the 90driver and its device will be usable. 91.Pp 92Some devices come in two flavors: 93.Em block 94and 95.Em character 96devices, or to use better terms, buffered and unbuffered 97(raw) 98devices. The traditional names are reflected by the letters 99.Ql b 100and 101.Ql c 102as the file type identification in the output of 103.Ql ls -l . 104Buffered devices are being accessed through the buffer cache of the 105operating system, and they are solely intended to layer a file system 106on top of them. They are normally implemented for disks and disk-like 107devices only and, for historical reasons, for tape devices. 108.Pp 109Raw devices are available for all drivers, including those that also 110implement a buffered device. For the latter group of devices, the 111differentiation is conventionally done by prepending the letter 112.Ql r 113to the path name of the device node, for example 114.Pa /dev/rda0 115denotes the raw device for the first SCSI disk, while 116.Pa /dev/da0 117is the corresponding device node for the buffered device. 118.Pp 119Unbuffered devices should be used for all actions that are not related 120to file system operations, even if the device in question is a disk 121device. This includes making backups of entire disk partitions, or 122to 123.Em raw 124floppy disks 125(i.e. those used like tapes). 126.Pp 127Access restrictions to device nodes are usually subject to the regular 128file permissions of the device node entry, instead of being enforced 129directly by the drivers in the kernel. 130.Ss Drivers without device nodes 131Drivers for network devices do not use device nodes in order to be 132accessed. Their selection is based on other decisions inside the 133kernel, and instead of calling 134.Xr open 2 , 135use of a network device is generally introduced by using the system 136call 137.Xr socket 2 . 138.Ss Configuring a driver into the kernel 139For each kernel, there is a configuration file that is used as a base 140to select the facilities and drivers for that kernel, and to tune 141several options. See 142.Xr config 8 143for a detailed description of the files involved. The individual 144manual pages in this section provide a sample line for the 145configuration file in their synopsis portion. See also the sample 146config file 147.Pa /sys/i386/conf/LINT 148(for the 149.Em i386 150architecture). 151.Sh SEE ALSO 152.Xr close 2 , 153.Xr ioctl 2 , 154.Xr mmap 2 , 155.Xr open 2 , 156.Xr read 2 , 157.Xr select 2 , 158.Xr socket 2 , 159.Xr write 2 , 160.Xr hier 7 , 161.Xr config 8 , 162.Xr MAKEDEV 8 163.Sh AUTHORS 164.An -nosplit 165This man page has been written by 166.An J\(:org Wunsch 167with initial input by 168.An David E. O'Brien . 169.Sh HISTORY 170.Nm Intro 171appeared in 172.Fx 2.1 . 173