Udev
Encyclopedia
udev is the device manager for the Linux kernel
. Primarily, it manages device nodes in /dev
. It is the successor of devfs and hotplug, which means that it handles the /dev directory and all user space
actions when adding/removing devices, including firmware
load.
The Linux kernel version 2.6.13 introduced or updated a new version of the uevent interface. A system using a new version of udev will not boot with kernels older than 2.6.13 unless udev is disabled and a traditional /dev directory is used for device access.
systems, where the device nodes in the /dev directory have been a static set of files, the Linux udev device manager dynamically provides only the nodes for the devices actually present on a system. Although devfs used to provide similar functionality, advocates of udev cited a number of reasons for preferring its implementation over devfs:
on a Linux system and listens (via netlink
socket) to uevents the kernel sends out if a new device is initialized or a device is removed from the system.
The system provides a set of rules that match against exported values of the event and properties of the discovered device.
A matching rule will possibly name and create a device node and run configured programs to set-up and configure the device.
udev rules can match on properties like the kernel subsystem, the kernel device name, the physical location of the device, or properties like the device's serial number. Rules can also request information from external programs to name a device or specify a custom name that will always be the same, regardless of the order devices are discovered by the system.
In the past a common way to use udev on Linux systems was to let it send events through a socket to HAL
, which would perform further device-specific actions. For example, HAL would notify other software running on the system that the new hardware had arrived by issuing a broadcast message on the D-Bus
IPC
system to all interested processes
. In this way, desktops such as GNOME
or KDE
could open a file browser to newly attached USB flash drive
s and SD cards
.
By the middle of 2011 HAL had been deprecated by most Linux distributions as well as by the KDE, GNOME and XFCE desktop environments, among others. The functionality previously embodied in HAL has been integrated into udev itself, or moved to separate software such as udisks and upower.
The system gets calls from the kernel via netlink socket. Earlier versions used hotplug, adding a link to themselves in /etc/hotplug.d/default with this purpose.
and Kay Sievers, with much help from Dan Stekloff, among others.
Linux kernel
The Linux kernel is an operating system kernel used by the Linux family of Unix-like operating systems. It is one of the most prominent examples of free and open source software....
. Primarily, it manages device nodes in /dev
Filesystem Hierarchy Standard
The Filesystem Hierarchy Standard defines the main directories and their contents in Linux operating systems. For the most part, it is a formalization and extension of the traditional BSD filesystem hierarchy....
. It is the successor of devfs and hotplug, which means that it handles the /dev directory and all user space
User space
A conventional computer operating system usually segregates virtual memory into kernel space and user space. Kernel space is strictly reserved for running the kernel, kernel extensions, and most device drivers...
actions when adding/removing devices, including firmware
Firmware
In electronic systems and computing, firmware is a term often used to denote the fixed, usually rather small, programs and/or data structures that internally control various electronic devices...
load.
History
udev was new in Linux 2.5.The Linux kernel version 2.6.13 introduced or updated a new version of the uevent interface. A system using a new version of udev will not boot with kernels older than 2.6.13 unless udev is disabled and a traditional /dev directory is used for device access.
Overview
Unlike traditional UnixUnix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
systems, where the device nodes in the /dev directory have been a static set of files, the Linux udev device manager dynamically provides only the nodes for the devices actually present on a system. Although devfs used to provide similar functionality, advocates of udev cited a number of reasons for preferring its implementation over devfs:
- udev supports persistent device naming, which does not depend on, for example, the order in which the devices are plugged into the system. The default udev setup provides persistent names for storage devices. Any hard disk is recognized by its unique filesystem id, the name of the disk and the physical location on the hardware it is connected to.
- udev executes entirely in user spaceUser spaceA conventional computer operating system usually segregates virtual memory into kernel space and user space. Kernel space is strictly reserved for running the kernel, kernel extensions, and most device drivers...
, as opposed to devfs' kernel space. One consequence is that udev moved the naming policy out of the kernel and can run arbitrary programs to compose a name for the device from the device's properties, before the node is created.
Operation
udev is a generic kernel device manager. It runs as a daemonDaemon (computer software)
In Unix and other multitasking computer operating systems, a daemon is a computer program that runs as a background process, rather than being under the direct control of an interactive user...
on a Linux system and listens (via netlink
Netlink
Netlink is a socket-like mechanism for IPC between the kernel and user space processes, as well as between user space processes alone or a mixture of multiple user space and kernel space processes...
socket) to uevents the kernel sends out if a new device is initialized or a device is removed from the system.
The system provides a set of rules that match against exported values of the event and properties of the discovered device.
A matching rule will possibly name and create a device node and run configured programs to set-up and configure the device.
udev rules can match on properties like the kernel subsystem, the kernel device name, the physical location of the device, or properties like the device's serial number. Rules can also request information from external programs to name a device or specify a custom name that will always be the same, regardless of the order devices are discovered by the system.
In the past a common way to use udev on Linux systems was to let it send events through a socket to HAL
HAL (software)
HAL was a software project providing a hardware abstraction layer for Unix-like computer systems.HAL is now deprecated on GNU/Linux systems, with functionality being merged into udev as of 2008–2010...
, which would perform further device-specific actions. For example, HAL would notify other software running on the system that the new hardware had arrived by issuing a broadcast message on the D-Bus
D-Bus
In computing, D-Bus is a simple inter-process communication open-source system for software applications to communicate with one another. Heavily influenced by KDE2–3's DCOP system, D-Bus has replaced DCOP in the KDE 4 release. An implementation of D-Bus supports most POSIX operating...
IPC
Inter-process communication
In computing, Inter-process communication is a set of methods for the exchange of data among multiple threads in one or more processes. Processes may be running on one or more computers connected by a network. IPC methods are divided into methods for message passing, synchronization, shared...
system to all interested processes
Process (computing)
In computing, a process is an instance of a computer program that is being executed. It contains the program code and its current activity. Depending on the operating system , a process may be made up of multiple threads of execution that execute instructions concurrently.A computer program is a...
. In this way, desktops such as GNOME
GNOME
GNOME is a desktop environment and graphical user interface that runs on top of a computer operating system. It is composed entirely of free and open source software...
or KDE
KDE
KDE is an international free software community producing an integrated set of cross-platform applications designed to run on Linux, FreeBSD, Microsoft Windows, Solaris and Mac OS X systems...
could open a file browser to newly attached USB flash drive
USB flash drive
A flash drive is a data storage device that consists of flash memory with an integrated Universal Serial Bus interface. flash drives are typically removable and rewritable, and physically much smaller than a floppy disk. Most weigh less than 30 g...
s and SD cards
Secure Digital
Secure Digital is a non-volatile memory card format developed by the SD Card Association for use in portable devices. The SD technology is used by more than 400 brands across dozens of product categories and more than 8,000 models, and is considered the de-facto industry standard.Secure Digital...
.
By the middle of 2011 HAL had been deprecated by most Linux distributions as well as by the KDE, GNOME and XFCE desktop environments, among others. The functionality previously embodied in HAL has been integrated into udev itself, or moved to separate software such as udisks and upower.
Architecture
The system is divided into three parts:- The library libudev, that allows access to device information.
- The daemon udevd, in user space, that manages the virtual /dev.
- The administrative command udevadm for diagnostics.
The system gets calls from the kernel via netlink socket. Earlier versions used hotplug, adding a link to themselves in /etc/hotplug.d/default with this purpose.
Authors
udev was developed by Greg Kroah-HartmanGreg Kroah-Hartman
Greg Kroah-Hartman is a Linux kernel developer. He is the current Linux kernel maintainer for the -stable branch with Chris Wright, the staging subsystem, USB, driver core, debugfs, kref, kobject, and the sysfs kernel subsystems, Userspace I/O and TTY layer. He is also the maintainer of the...
and Kay Sievers, with much help from Dan Stekloff, among others.