NAME¶
intro —
introduction to devices and
device drivers
DESCRIPTION¶
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 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
/dev/mem, 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
open(2),
close(2),
read(2),
write(2),
ioctl(2),
select(2), and
mmap(2). Not all drivers implement all system calls, for
example, calling
mmap(2) on terminal devices is likely to be
not useful at all.
Accessing Devices¶
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 in the file system hierarchy (see also
hier(7)).
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
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:
block and
character devices, or to use better terms, buffered and
unbuffered (raw) devices. The traditional names are reflected by the letters
‘
b’ and
‘
c’ as the file type identification in the
output of ‘
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 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
‘
r’ to the path name of the device node,
for example
/dev/rda0 denotes the raw device for the first
SCSI disk, while
/dev/da0 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
raw
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 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
open(2), use of a network device is generally
introduced by using the system call
socket(2).
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
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
/sys/i386/conf/LINT (for the
i386
architecture).
SEE ALSO¶
close(2),
ioctl(2),
mmap(2),
open(2),
read(2),
select(2),
socket(2),
write(2),
devfs(5),
hier(7),
config(8)
HISTORY¶
This manual page first appeared in
FreeBSD 2.1.
AUTHORS¶
This man page has been written by
Jörg Wunsch with
initial input by
David E. O'Brien.
