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INTRO(4)	       FreeBSD Kernel Interfaces Manual		      INTRO(4)

     intro -- 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 key-
     board.  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	avail-
     able 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 identifi-
     cation 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	imple-
     mented 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	dif-
     ferentiation 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 Jorg Wunsch with	initial	input by David
     E.	O'Brien.

FreeBSD	8.4-stable	       January 20, 1996		    FreeBSD 8.4-stable


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