systemd-stable/README

udev - userspace device management

For more information see the files in the docs/ directory.

Important Note:
  Integrating udev in the system is a whole lot of work, has complex dependencies
  and differs a lot from distro to distro. All reasonable distros depend on udev
  these days and the system will not work without it.

  The upstream udev project does not support or recomend to replace a distro's udev
  installation with the upstream version. The installation of a unmodified upstream
  version may render your system unusable! There is no "default" setup or a set
  of "default" rules provided by the upstream udev version.

Requirements:
  - 2.6.x version of the Linux kernel. See the RELEASE-NOTES file in the
    udev tree and the Documentation/Changes in the kernel source tree for
    the actual dependency.

  - The kernel must have sysfs and unix domain socket enabled.
    (unix domain sockets (CONFIG_UNIX) as a loadable kernel module may work,
     but it is completely silly, don't complain if anything goes wrong.)

  - The proc filesystem must be mounted on /proc.

  - The sysfs filesystem must be mounted at /sys. No other location
    will be supported by udev.


Operation:
  Udev creates and removes device nodes in /dev, based on events the kernel
  sends out on device discovery or removal.

  - Early in the boot process, /dev should get a tmpfs filesystem
    mounted, which is populated from scratch by udev. Created nodes or
    changed permissions will not survive a reboot, which is intentional.

  - The content of /lib/udev/devices directory which contains the nodes,
    symlinks and directories, which are always expected to be in /dev, should
    be copied over to the tmpfs mounted /dev, to provide the required nodes
    to initialize udev and continue booting.

  - The udevd daemon must be started by an init script to receive netlink
    uevents from the kernel driver core.

  - From kernel version 2.6.15 on, the hotplug helper /sbin/hotplug should
    be disabled with an init script before actions like loading kernel
    modules are taken, which may cause a lot of events.

  - All kernel events are matched against a set of specified rules in
    /etc/udev/rules.d/ which make it possible to hook into the event
    processing to load required kernel modules and setup devices. For all
    devices the kernel requests a device node, udev will create one with
    the default name or the one specified by a matching udev rules.


Compile Options:
  DESTDIR
  	Prefix of install target, used for package building.
  USE_LOG
  	If set to 'true', udev is able to pass errors or debug information
  	to syslog. This is very useful to see what udev is doing or not doing.
  	It is enabled by default, don't expect any useful answer, if you
  	need to hunt a bug, but you can't enable syslog.
  DEBUG
  	If set to 'true', very verbose debugging messages will be compiled
  	into the udev binaries. The actual level of debugging is specified
  	in the udev config file.
  STRIPCMD
  	If udev is compiled for packaging an empty string can be passed
  	to disable the stripping of the binaries.
  USE_SELINUX
  	If set to 'true', udev will be built with SELinux support
  	enabled.  This is disabled by default.
  USE_KLIBC
  	If set to 'true', udev is built and linked against klibc.
  	Default value is 'false'. KLCC specifies the klibc compiler
  	wrapper, usually located at /usr/bin/klcc.
  EXTRAS
  	If set, will build the "extra" helper programs as specified
  	as listed (see below for an example).

If you want to build the udev helper programs:
  make EXTRAS="extras/cdrom_id extras/scsi_id extras/volume_id"


Installation:
  - The install target intalls the udev binaries in the default locations,
    All at boot time reqired binaries will be installed in /sbin.

  - The default location for scripts and binaries that are called from
    rules is /lib/udev. Other packages who install udev rules, should use
    that directory too.

  - It is recommended to use the /lib/udev/devices directory to place
    device nodes and symlinks in, which are copied to /dev at every boot.
    That way, nodes for broken subsystems or devices which can't be
    detected automatically by the kernel, will always be available.

  - Copies of the rules files for all major distros are in the etc/udev
    directory (you may look there how others distros are doing it).

  - The persistent disk links in /dev/disk are the de facto standard
    on Linux and should be installed with every default udev installation.
    The devfs naming scheme rules are not recommended and not supported.

Please direct any comment/question/concern to the linux-hotplug-devel mailing list at:
  linux-hotplug-devel@lists.sourceforge.net