1.\" $NetBSD: intro.4,v 1.3 2017/12/20 09:58:36 wiz Exp $ 2.\" 3.\" Copyright (c) 1996 David E. O'Brien, Joerg Wunsch 4.\" 5.\" All rights reserved. 6.\" 7.\" Redistribution and use in source and binary forms, with or without 8.\" modification, are permitted provided that the following conditions 9.\" are met: 10.\" 1. Redistributions of source code must retain the above copyright 11.\" notice, this list of conditions and the following disclaimer. 12.\" 2. 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27.\" $FreeBSD: head/share/man/man4/intro.4 131530 2004-07-03 18:29:24Z ru $ 28.\" 29.Dd December 18, 2017 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 70.Ux Ns 71-like operating system are accessed 72through so-called 73.Em device nodes , 74sometimes also called 75.Em special files . 76They are usually located under the directory 77.Pa /dev 78in the file system hierarchy 79(see also 80.Xr hier 7 ) . 81.Pp 82Note that this could lead to an inconsistent state, where either there 83are device nodes that do not have a configured driver associated with 84them, or there may be drivers that have successfully probed for their 85devices, but cannot be accessed since the corresponding device node is 86still missing. 87In the first case, any attempt to reference the device 88through the device node will result in an error, returned by the upper 89layers of the kernel, usually 90.Er ENXIO . 91In the second case, the device node needs to be created before the 92driver and its device will be usable. 93.Pp 94Some devices come in two flavors: 95.Em block 96and 97.Em character 98devices, or to use better terms, buffered and unbuffered 99(raw) 100devices. 101The traditional names are reflected by the letters 102.Ql b 103and 104.Ql c 105as the file type identification in the output of 106.Ql ls -l . 107Buffered devices are being accessed through the buffer cache of the 108operating system, and they are solely intended to layer a file system 109on top of them. 110They are normally implemented for disks and disk-like 111devices only and, for historical reasons, for tape devices. 112.Pp 113Raw devices are available for all drivers, including those that also 114implement a buffered device. 115For the latter group of devices, the 116differentiation is conventionally done by prepending the letter 117.Ql r 118to the path name of the device node, for example 119.Pa /dev/rsd0[cd] 120denotes the raw device for the first SCSI disk, while 121.Pa /dev/sd0[cd] 122is the corresponding device node for the buffered device. 123.Pp 124Unbuffered devices should be used for all actions that are not related 125to file system operations, even if the device in question is a disk 126device. 127This includes making backups of entire disk partitions, or 128to 129.Em raw 130floppy disks 131(i.e., those used like tapes). 132.Pp 133Access restrictions to device nodes are usually subject to the regular 134file permissions of the device node entry, instead of being enforced 135directly by the drivers in the kernel. 136.Ss Drivers without device nodes 137Drivers for network devices do not use device nodes in order to be 138accessed. 139Their selection is based on other decisions inside the 140kernel, and instead of calling 141.Xr open 2 , 142use of a network device is generally introduced by using the system 143call 144.Xr socket 2 . 145.Ss Configuring a driver into the kernel 146For each kernel, there is a configuration file that is used as a base 147to select the facilities and drivers for that kernel, and to tune 148several options. 149See 150.Xr config 1 151for a detailed description of the files involved. 152The individual manual pages in this section provide a sample line for the 153configuration file in their synopsis portion. 154See also the sample config file 155.Pa /usr/src/sys/arch/i386/conf/GENERIC 156(for the 157.Em i386 158architecture). 159.Sh SEE ALSO 160.Xr config 1 , 161.Xr close 2 , 162.Xr ioctl 2 , 163.Xr mmap 2 , 164.Xr open 2 , 165.Xr read 2 , 166.Xr select 2 , 167.Xr socket 2 , 168.Xr write 2 , 169.\" .Xr devfs 5 , 170.Xr hier 7 171.Sh HISTORY 172This manual page first appeared in 173.Fx 2.1 174and 175.Nx 9 . 176.Sh AUTHORS 177.An -nosplit 178This man page has been written by 179.An J\(:org Wunsch 180with initial input by 181.An David E. O'Brien . 182