.\" .\" Copyright (c) 1996 David E. O'Brien, Joerg Wunsch .\" .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" .\" THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``AS IS'' AND ANY EXPRESS OR .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. .\" IN NO EVENT SHALL THE DEVELOPERS BE LIABLE FOR ANY DIRECT, INDIRECT, .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. .\" .\" $FreeBSD$ .\" .Dd January 20, 1996 .Dt INTRO 4 .Os FreeBSD 2.1 .Sh NAME .Nm intro .Nd introduction to devices and device drivers .Sh DESCRIPTION This section contains information related to devices, device driver and miscellaneous hardware. .Ss The device abstraction Device is a term used mostly for hardware-related stuff that belongs to the system, like disks, printers, or a graphics display with its keyboard. There are also so-called .Em pseudo-devices where a device driver emulates the behaviour of a device in software without any particular underlying hardware. A typical example for the latter class is .Pa /dev/mem , a loophole where the physical memory can be accessed using the regular file access semantics. .Pp The device abstraction generally provides a common set of system calls layered on top of them, which are dispatched to the corresponding device driver by the upper layers of the kernel. The set of system calls available for devices is chosen from .Xr open 2 , .Xr close 2 , .Xr read 2 , .Xr write 2 , .Xr ioctl 2 , .Xr select 2 , and .Xr mmap 2 . Not all drivers implement all system calls, for example, calling .Xr mmap 2 on a terminal devices is likely to be not useful at all. .Ss Accessing Devices Most of the devices in a unix-like operating system are accessed through so-called .Em device nodes , sometimes also called .Em special files . They are usually located under the directory .Pa /dev in the file system hierarchy .Pq see also Xr hier 7 . .Pp Until .Xr devfs 5 is fully functional, each device node must be created statically and independently of the existence of the associated device driver, usually by running .Xr MAKEDEV 8 . .Pp Note that this could lead to an inconsistent state, where either there are device nodes that do not have a configured driver associated with them, or there may be drivers that have successfully probed for their devices, but cannot be accessed since the corresponding device node is still missing. In the first case, any attempt to reference the device through the device node will result in an error, returned by the upper layers of the kernel, usually .Ql ENXIO . In the second case, the device node needs to be created before the driver and its device will be usable. .Pp Some devices come in two flavors: .Em block and .Em character devices, or by a better name, buffered and unbuffered .Pq raw devices. The traditional names are reflected by the letters .Ql b and .Ql c as the file type identification in the output of .Ql ls -l . Buffered devices are being accessed through the buffer cache of the operating system, and they are solely intended to layer a file system on top of them. They are normally implemented for disks and disk-like devices only, for historical reasons also for tape devices. .Pp Raw devices are available for all drivers, including those that also implement a buffered device. For the latter group of devices, the differentiation is conventionally done by prepending the latter .Ql r to the path name of the device node, for example .Pa /dev/rsd0 denotes the raw device for the first SCSI disk, while .Pa /dev/sd0 is the corresponding device node for the buffered device. .Pp Unbuffered devices should be used for all actions that are not related to file system operations, even if the device in question is a disk device. This includes making backups of entire disk partitions, or to .Em raw floppy disks .Pq i.e. those used like tapes . .Pp Access restrictions to device nodes are usually subject of the regular file permissions of the device node entry, instead of being implied directly by the drivers in the kernel. .Ss Drivers without device nodes Drivers for network devices do not use device nodes in order to be accessed. Their selection is based on other decisions inside the kernel, and instead of calling .Xr open 2 , use of a network device is generally introduced by using the system call .Xr socket 2 . .Ss Configuring a driver into the kernel For each kernel, there is a configuration file that is used as a base to select the facilities and drivers for that kernel, and to tune several options. See .Xr config 8 for a detailed description of the files involved. The individual manual pages in this section provide a sample line for the configuration file in their synopsis portion. See also the sample config file .Pa /sys/i386/conf/LINT .Po for the .Em i386 architecture .Pc . .Sh SEE ALSO .Xr close 2 , .Xr ioctl 2 , .Xr mmap 2 , .Xr open 2 , .Xr read 2 , .Xr select 2 , .Xr socket 2 , .Xr write 2 , .Xr devfs 5 , .Xr hier 7 , .Xr MAKEDEV 8 , .Xr config 8 . .Sh AUTHORS This man page has been written by .if t J\(:org Wunsch .if n Joerg Wunsch with initial input by David E. O'Brien. .Sh HISTORY .Nm intro appeared in .Fx 2.1 .