INTRO(4)                  BSD 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 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

     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).

     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
     E. O'Brien.

BSD                             January 20, 1996                             BSD