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intro - introduction to devices and device drivers
This section contains information related to devices, device drivers and
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.
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).
close(2), ioctl(2), mmap(2), open(2), read(2), select(2), socket(2),
write(2), devfs(5), hier(7), config(8)
This manual page first appeared in FreeBSD 2.1.
This man page has been written by Jörg Wunsch with initial input by David