What are Udev, HAL, Dbus and Netlink ?

Hot-plugging (which is the word used to describe the process of inserting devices into a running system) is achieved in a Linux distribution by a combination of three components: Udev, HAL, and Dbus.

Udev is a userspace daemon, that supplies a dynamic device directory containing only the nodes for devices which are connected to the system. It creates or removes the device node files in the /dev directory as they are plugged in or taken out. Dbus is like a system bus which is used for inter-process communication. The HAL gets information from the Udev service, when a device is attached to the system and it creates an XML representation of that device. It then notifies the corresponding desktop application like Nautilus through the Dbus and Nautilus will open the mounted device files.

Dbus is an IPC mechanism, which allows applications to register for system device events.

Udev is the device manager for the Linux 2.6 kernel that creates/removes device nodes in the /dev directory dynamically. It runs in userspace and the user can change device names using Udev rules.

Udev depends on the sysfs file system which was introduced in the 2.5 kernel. It is sysfs which makes devices visible in user space. When a device is added or removed, kernel events are produced which will notify Udev in userspace. Udev directly listens to Netlink socket to know about device state change events (kernel uevents).


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