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.

Debian                                          January 20, 1996                                        INTRO(4)