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 1.2 2003/06/17 04:36:59 dillon Exp $ 28.\" 29.Dd January 20, 1996 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. There are also so-called 42.Em pseudo-devices 43where a device driver emulates the behaviour of a device in software 44without any particular underlying hardware. A typical example for 45the latter class is 46.Pa /dev/mem , 47a loophole where the physical memory can be accessed using the regular 48file access semantics. 49.Pp 50The device abstraction generally provides a common set of system calls 51layered on top of them, which are dispatched to the corresponding 52device driver by the upper layers of the kernel. The set of system 53calls available for devices is chosen from 54.Xr open 2 , 55.Xr close 2 , 56.Xr read 2 , 57.Xr write 2 , 58.Xr ioctl 2 , 59.Xr select 2 , 60and 61.Xr mmap 2 . 62Not all drivers implement all system calls, for example, calling 63.Xr mmap 2 64on terminal devices is likely to be not useful at all. 65.Ss Accessing Devices 66Most of the devices in a unix-like operating system are accessed 67through so-called 68.Em device nodes , 69sometimes also called 70.Em special files . 71They are usually located under the directory 72.Pa /dev 73in the file system hierarchy 74(see also 75.Xr hier 7 ) . 76.Pp 77Each device node must be created statically and 78independently of the existence of the associated device driver, 79usually by running 80.Xr MAKEDEV 8 . 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. In the first case, any attempt to reference the device 87through the device node will result in an error, returned by the upper 88layers of the kernel, usually 89.Er ENXIO . 90In the second case, the device node needs to be created before the 91driver and its device will be usable. 92.Pp 93Some devices come in two flavors: 94.Em block 95and 96.Em character 97devices, or to use better terms, buffered and unbuffered 98(raw) 99devices. The traditional names are reflected by the letters 100.Ql b 101and 102.Ql c 103as the file type identification in the output of 104.Ql ls -l . 105Buffered devices are being accessed through the buffer cache of the 106operating system, and they are solely intended to layer a file system 107on top of them. They are normally implemented for disks and disk-like 108devices only and, for historical reasons, for tape devices. 109.Pp 110Raw devices are available for all drivers, including those that also 111implement a buffered device. For the latter group of devices, the 112differentiation is conventionally done by prepending the letter 113.Ql r 114to the path name of the device node, for example 115.Pa /dev/rda0 116denotes the raw device for the first SCSI disk, while 117.Pa /dev/da0 118is the corresponding device node for the buffered device. 119.Pp 120Unbuffered devices should be used for all actions that are not related 121to file system operations, even if the device in question is a disk 122device. This includes making backups of entire disk partitions, or 123to 124.Em raw 125floppy disks 126(i.e. those used like tapes). 127.Pp 128Access restrictions to device nodes are usually subject to the regular 129file permissions of the device node entry, instead of being enforced 130directly by the drivers in the kernel. 131.Ss Drivers without device nodes 132Drivers for network devices do not use device nodes in order to be 133accessed. Their selection is based on other decisions inside the 134kernel, and instead of calling 135.Xr open 2 , 136use of a network device is generally introduced by using the system 137call 138.Xr socket 2 . 139.Ss Configuring a driver into the kernel 140For each kernel, there is a configuration file that is used as a base 141to select the facilities and drivers for that kernel, and to tune 142several options. See 143.Xr config 8 144for a detailed description of the files involved. The individual 145manual pages in this section provide a sample line for the 146configuration file in their synopsis portion. See also the sample 147config file 148.Pa /sys/i386/conf/LINT 149(for the 150.Em i386 151architecture). 152.Sh SEE ALSO 153.Xr close 2 , 154.Xr ioctl 2 , 155.Xr mmap 2 , 156.Xr open 2 , 157.Xr read 2 , 158.Xr select 2 , 159.Xr socket 2 , 160.Xr write 2 , 161.Xr hier 7 , 162.Xr config 8 , 163.Xr MAKEDEV 8 164.Sh AUTHORS 165.An -nosplit 166This man page has been written by 167.An J\(:org Wunsch 168with initial input by 169.An David E. O'Brien . 170.Sh HISTORY 171.Nm Intro 172appeared in 173.Fx 2.1 . 174