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     intro — introduction to devices and device drivers


     This section contains information related to devices, device drivers and miscellaneous

   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
     mechanism whereby the physical memory can be accessed using file access semantics.

     The device abstraction generally provides a common set of system calls, 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 a keyboard device is not likely to be useful.

     Aspects of the device abstraction have changed significantly in FreeBSD over the past two
     decades.  The section Historical Notes describes some of the more important differences.

   Accessing Devices
     Most of the devices in FreeBSD are accessed through device nodes, sometimes also called
     special files.  They are located within instances of the devfs(5) filesystem, which is
     conventionally mounted on the directory /dev in the file system hierarchy (see also

     The devfs(5) filesystem creates or removes device nodes automatically according to the
     physical hardware recognized as present at any given time.  For pseudo-devices, device nodes
     may be created and removed dynamically as required, depending on the nature of the device.

     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.  But
     since device nodes are not stored persistently between reboots, those file permissions are
     set at boot time from rules specified in devfs.conf(5), or dynamically according to rules
     defined in devfs.rules(5) or set using the devfs(8) command.  In the latter case, different
     rules may be used to make different sets of devices visible within different instances of
     the devfs(5) filesystem, which may be used, for example, to prevent jailed subsystems from
     accessing unsafe devices.  Manual changes to device node permissions may still be made, but
     will not persist.

   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 portions.  See also the
     files /usr/src/sys/conf/NOTES and /usr/src/sys/${ARCH}/conf/NOTES.

     Drivers need not be statically compiled into the kernel; they may also be loaded as modules,
     in which case any device nodes they provide will appear only after the module is loaded (and
     has attached to suitable hardware, if applicable).

   Historical Notes
     Prior to FreeBSD 6.0, device nodes could be created in the traditional way as persistent
     entries in the file system.  While such entries can still be created, they no longer
     function to access devices.

     Prior to FreeBSD 5.0, devices for disk and tape drives existed in two variants, known as
     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”.  Raw devices were traditionally named with a prefix
     of “r”, for example /dev/rda0 would denote the raw version of the disk whose buffered device
     was /dev/da0.  This is no longer the case; all disk devices are now “raw” in the traditional
     sense, even though they are not given “r” prefixes, and “buffered” devices no longer exist
     at all.

     Buffered devices were accessed through a buffer cache maintained by the operating system;
     historically this was the system's primary disk cache, but in FreeBSD this was rendered
     obsolete by the introduction of unified virtual memory management.  Buffered devices could
     be read or written at any byte position, with the buffer mechanism handling the reading and
     writing of disk blocks.  In contrast, raw disk devices can be read or written only at
     positions and lengths that are multiples of the underlying device block size, and write(2)
     calls are synchronous, not returning to the caller until the data has been handed off to the


     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 rewritten by Andrew Gierth from an earlier version written by Jörg
     Wunsch with initial input by David E. O'Brien.