- introduction to devices and device drivers
This section contains information related to devices, device drivers
and miscellaneous hardware.
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
There are also so-called
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 chosen from
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
-like operating system are accessed
sometimes also called
They are usually located under the directory
in the file system hierarchy
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
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
Er ENXIO .
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:
devices, or to use better terms, buffered and unbuffered
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
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
This includes making backups of entire disk partitions, or
(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 without device nodes
Drivers for network devices do not use device nodes in order to be
Their selection is based on other decisions inside the
kernel, and instead of calling
use of a network device is generally introduced by using the system
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
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