00ef9f43b3
- Remove issues that no longer apply thanks to devfs - Add language pointing out devfs's role and referencing its config - Add a "historical notes" section and move discussion of block vs character devs to it, including pointing out the removal of block devs - Modernize some examples MFC after: 1 week PR: 236970 Submitted by: andrew@tao173.riddles.org.uk Reviewed by: 0mp Differential Revision: https://reviews.freebsd.org/D19799
219 lines
6.9 KiB
Groff
219 lines
6.9 KiB
Groff
.\"
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.\" Copyright (c) 1996 David E. O'Brien, Joerg Wunsch
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.\" Copyright (c) 2019 Andrew Gierth
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.\"
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE DEVELOPERS ``AS IS'' AND ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
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.\" IN NO EVENT SHALL THE DEVELOPERS BE LIABLE FOR ANY DIRECT, INDIRECT,
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd April 3, 2019
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.Dt INTRO 4
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.Os
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.Sh NAME
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.Nm intro
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.Nd introduction to devices and device drivers
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.Sh DESCRIPTION
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This section contains information related to devices, device drivers
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and miscellaneous hardware.
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.Ss The device abstraction
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Device is a term used mostly for hardware-related stuff that belongs
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to the system, like disks, printers, or a graphics display with its
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keyboard.
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There are also so-called
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.Em pseudo-devices
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where a device driver emulates the behaviour of a device in software
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without any particular underlying hardware.
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A typical example for
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the latter class is
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.Pa /dev/mem ,
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a mechanism whereby the physical memory can be accessed using file
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access semantics.
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.Pp
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The device abstraction generally provides a common set of system
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calls, which are dispatched to the corresponding device driver by the
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upper layers of the kernel.
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The set of system calls available for devices is chosen from
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.Xr open 2 ,
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.Xr close 2 ,
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.Xr read 2 ,
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.Xr write 2 ,
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.Xr ioctl 2 ,
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.Xr select 2 ,
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and
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.Xr mmap 2 .
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Not all drivers implement all system calls; for example, calling
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.Xr mmap 2
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on a keyboard device is not likely to be useful.
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.Pp
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Aspects of the device abstraction have changed significantly in
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.Fx
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over the past two decades.
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The section
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.Sx Historical Notes
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describes some of the more important differences.
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.Ss Accessing Devices
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Most of the devices in
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.Fx
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are accessed through
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.Em device nodes ,
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sometimes also called
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.Em special files .
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They are located within instances of the
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.Xr devfs 5
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filesystem, which is conventionally mounted on the directory
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.Pa /dev
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in the file system hierarchy
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(see also
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.Xr hier 7 ) .
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.Pp
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The
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.Xr devfs 5
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filesystem creates or removes device nodes automatically according to
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the physical hardware recognized as present at any given time.
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For pseudo-devices, device nodes may be created and removed dynamically
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as required, depending on the nature of the device.
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.Pp
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Access restrictions to device nodes are usually subject to the regular
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file permissions of the device node entry, instead of being enforced
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directly by the drivers in the kernel.
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But since device nodes are not stored persistently between reboots,
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those file permissions are set at boot time from rules specified in
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.Xr devfs.conf 5 ,
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or dynamically according to rules defined in
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.Xr devfs.rules 5
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or set using the
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.Xr devfs 8
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command.
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In the latter case, different rules may be used to make different sets
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of devices visible within different instances of the
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.Xr devfs 5
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filesystem, which may be used, for example, to prevent jailed
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subsystems from accessing unsafe devices.
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Manual changes to device
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node permissions may still be made, but will not persist.
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.Ss Drivers without device nodes
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Drivers for network devices do not use device nodes in order to be
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accessed.
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Their selection is based on other decisions inside the
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kernel, and instead of calling
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.Xr open 2 ,
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use of a network device is generally introduced by using the system
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call
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.Xr socket 2 .
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.Ss Configuring a driver into the kernel
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For each kernel, there is a configuration file that is used as a base
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to select the facilities and drivers for that kernel, and to tune
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several options.
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See
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.Xr config 8
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for a detailed description of the files involved.
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The individual manual pages in this section provide a sample line for the
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configuration file in their synopsis portions.
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See also the files
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.Pa /usr/src/sys/conf/NOTES
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and
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.Pa /usr/src/sys/${ARCH}/conf/NOTES .
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.Pp
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Drivers need not be statically compiled into the kernel; they may also be
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loaded as modules, in which case any device nodes they provide will appear
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only after the module is loaded (and has attached to suitable hardware,
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if applicable).
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.Ss Historical Notes
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Prior to
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.Fx 6.0 ,
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device nodes could be created in the traditional way as persistent
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entries in the file system.
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While such entries can still be created, they no longer function to
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access devices.
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.Pp
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Prior to
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.Fx 5.0 ,
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devices for disk and tape drives existed in two variants, known as
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.Em block
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and
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.Em character
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devices, or to use better terms, buffered and unbuffered
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(raw)
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devices.
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The traditional names are reflected by the letters
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.Dq Li b
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and
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.Dq Li c
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as the file type identification in the output of
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.Dq Li ls -l .
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Raw devices were traditionally named with a prefix of
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.Dq Li r ,
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for example
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.Pa /dev/rda0
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would denote the raw version of the disk whose buffered device was
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.Pa /dev/da0 .
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.Em This is no longer the case ;
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all disk devices are now
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.Dq raw
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in the traditional sense, even though they are not given
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.Dq Li r
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prefixes, and
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.Dq buffered
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devices no longer exist at all.
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.Pp
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Buffered devices were accessed through a buffer cache maintained by
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the operating system; historically this was the system's primary disk
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cache, but in
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.Fx
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this was rendered obsolete by the introduction of unified virtual
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memory management.
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Buffered devices could be read or written at any
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byte position, with the buffer mechanism handling the reading and
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writing of disk blocks.
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In contrast, raw disk devices can be read or
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written only at positions and lengths that are multiples of the
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underlying device block size, and
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.Xr write 2
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calls are
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.Em synchronous ,
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not returning to the caller until the data has been handed off to the
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device.
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.Sh SEE ALSO
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.Xr close 2 ,
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.Xr ioctl 2 ,
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.Xr mmap 2 ,
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.Xr open 2 ,
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.Xr read 2 ,
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.Xr select 2 ,
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.Xr socket 2 ,
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.Xr write 2 ,
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.Xr devfs 5 ,
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.Xr hier 7 ,
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.Xr config 8
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.Sh HISTORY
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This manual page first appeared in
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.Fx 2.1 .
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.Sh AUTHORS
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.An -nosplit
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This man page has been rewritten by
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.An Andrew Gierth
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from an earlier version written by
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.An J\(:org Wunsch
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with initial input by
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.An David E. O'Brien .
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