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.