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.\" $DragonFly: src/share/man/man4/intro.4,v 1.4 2006/10/24 17:09:45 swildner Exp $ 28.\" 29.Dd August 6, 2009 30.Dt INTRO 4 31.Os 32.Sh NAME 33.Nm intro 34.Nd introduction to devices and device drivers 35.Sh DESCRIPTION 36This section contains information related to devices, device drivers 37and miscellaneous hardware. 38.Ss The device abstraction 39Device is a term used mostly for hardware-related stuff that belongs 40to the system, like disks, printers, or a graphics display with its 41keyboard. 42There are also so-called 43.Em pseudo-devices 44where a device driver emulates the behaviour of a device in software 45without any particular underlying hardware. 46A typical example for 47the latter class is 48.Pa /dev/mem , 49a loophole where the physical memory can be accessed using the regular 50file access semantics. 51.Pp 52The device abstraction generally provides a common set of system calls 53layered on top of them, which are dispatched to the corresponding 54device driver by the upper layers of the kernel. 55The set of system 56calls available for devices is chosen from 57.Xr open 2 , 58.Xr close 2 , 59.Xr read 2 , 60.Xr write 2 , 61.Xr ioctl 2 , 62.Xr select 2 , 63and 64.Xr mmap 2 . 65Not all drivers implement all system calls, for example, calling 66.Xr mmap 2 67on terminal devices is likely to be not useful at all. 68.Ss Accessing Devices 69Most of the devices in a unix-like operating system are accessed 70through so-called 71.Em device nodes , 72sometimes also called 73.Em special files . 74They are usually located under the directory 75.Pa /dev 76in the file system hierarchy 77(see also 78.Xr hier 7 ) . 79.Pp 80Some devices come in two flavors: 81.Em block 82and 83.Em character 84devices, or to use better terms, buffered and unbuffered 85(raw) 86devices. 87The traditional names are reflected by the letters 88.Ql b 89and 90.Ql c 91as the file type identification in the output of 92.Ql ls -l . 93Buffered devices are being accessed through the buffer cache of the 94operating system, and they are solely intended to layer a file system 95on top of them. 96They are normally implemented for disks and disk-like 97devices only and, for historical reasons, for tape devices. 98.Pp 99Raw devices are available for all drivers, including those that also 100implement a buffered device. 101For the latter group of devices, the 102differentiation is conventionally done by prepending the letter 103.Ql r 104to the path name of the device node, for example 105.Pa /dev/rda0 106denotes the raw device for the first SCSI disk, while 107.Pa /dev/da0 108is the corresponding device node for the buffered device. 109.Pp 110Unbuffered devices should be used for all actions that are not related 111to file system operations, even if the device in question is a disk 112device. 113This includes making backups of entire disk partitions, or 114to 115.Em raw 116floppy disks 117(i.e. those used like tapes). 118.Pp 119Access restrictions to device nodes are usually subject to the regular 120file permissions of the device node entry, instead of being enforced 121directly by the drivers in the kernel. 122.Ss Drivers without device nodes 123Drivers for network devices do not use device nodes in order to be 124accessed. 125Their selection is based on other decisions inside the 126kernel, and instead of calling 127.Xr open 2 , 128use of a network device is generally introduced by using the system 129call 130.Xr socket 2 . 131.Ss Configuring a driver into the kernel 132For each kernel, there is a configuration file that is used as a base 133to select the facilities and drivers for that kernel, and to tune 134several options. 135See 136.Xr config 8 137for a detailed description of the files involved. 138The individual manual pages in this section provide a sample line for the 139configuration file in their synopsis portion. See also the sample 140config file 141.Pa /sys/config/LINT 142(for the 143.Em i386 144architecture). 145.Sh SEE ALSO 146.Xr close 2 , 147.Xr ioctl 2 , 148.Xr mmap 2 , 149.Xr open 2 , 150.Xr read 2 , 151.Xr select 2 , 152.Xr socket 2 , 153.Xr write 2 , 154.Xr hier 7 , 155.Xr config 8 156.Sh HISTORY 157The 158.Nm 159manual page first appeared in 160.Fx 2.1 . 161.Sh AUTHORS 162.An -nosplit 163This man page was written by 164.An J\(:org Wunsch 165with initial input by 166.An David E. O'Brien . 167