introduction to devices and device drivers
This section contains information related to devices, device drivers and
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 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
, a loophole where the physical
memory can be accessed using the regular file access semantics.
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
Not all drivers implement all system calls, for example, calling
on terminal devices is likely to be not useful at all.
Most of the devices in a UNIX
-like operating system are
accessed through so-called device nodes
sometimes also called special files
. They are
usually located under the directory /dev
the file system hierarchy (see also
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
. In the second case, the device node
needs to be created before the driver and its device will be usable.
Some devices come in two flavors: block
devices, or to use better terms,
buffered and unbuffered (raw) devices. The traditional names are reflected by
the letters ‘
’ as the file type identification in
the output of ‘
’. 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 and, for historical reasons,
for tape devices.
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 letter
’ to the path name of the device node,
for example /dev/rda0
denotes the raw
device for the first SCSI disk, while
is the corresponding device node
for the buffered device.
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
floppy disks (i.e., those used like tapes).
Access restrictions to device nodes are usually subject to the regular file
permissions of the device node entry, instead of being enforced directly by
the drivers in the kernel.
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
use of a network device is generally introduced by using the system call
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
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
This manual page first appeared in FreeBSD 2.1
This man page has been written by Jörg
with initial input by David E.