device file

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A device file is an interface for a device driver that appears in a filesystem as if it were an ordinary file. Such files allow software to interact with a device driver. They are found in the /dev directory.

The /dev directory

Most Linux users understand that /dev/sda1 is just a fast way of referring to the first partition on the first disk that the kernel found. That's pretty easy, right?

But consider hotpluggable devices like USB, IEEE1394, hot-swappable PCI, etc. What is the first device for each of these? And for how long? What will the other devices be named when the first one disappears? How will that affect ongoing transactions? Wouldn't it be fun if a printing job were suddenly moved from a high-end laser printer to an almost-dead matrix printer just because someone's mom decided to pull the plug on the laser printer (which just happened to be the first printer)?

Enter the device manager. A modern device manager (including udev and eudev) must:

  • Run in userspace;
  • Dynamically create and remove device files;
  • Provide consistent device naming;
  • Provide a userspace application program interface (API).

Every time a change happens within the device structure, the kernel emits a uevent which gets picked up by the device manager. The device manager then follows the rules declared in the /etc/udev/rules.d, /run/udev/rules.d and /lib/udev/rules.d directories. Based on the information contained within the uevent, it finds the rule or rules it needs to trigger and performs the required actions. These actions may involve the creation or deletion of device files, and may also trigger the loading of particular firmware files into kernel memory.

Installation

Device files in Gentoo are managed by eudev, which will take care of installation of any required files.

Alternatives to eudev include:

Usage

Creation

Each device (either internal or peripheral) has a corresponding device file. During kernel boot time the kernel detects devices and creates device files in the virtual devtmpfs filesystem. Then eudev takes over the device files and stores them in /dev. From this point on, eudev is in charge of creating new device files and deleting unavailable ones.

Information can be obtained by using udevadm:

Get device info using udevadm info followed by the device path.

user $udevadm info -p /devices/pci0000:00/0000:00:1d.7

Permissions

Like other files, access to device file is restricted by filesystem permissions. The permission to access a device file have to be granted first to a user:

  • Add the user to the group the device file belongs to.
  • Setup ConsoleKit to grant access to some device files (e.g. sound device files) to the user of the active, local session.

Symlinks

eudev creates for some device classes additional symlinks. The device file /dev/cdrom (first CD-ROM drive) and /dev/dvd (first DVD drive) are just symlinks to the device file /dev/sr0 (first optical drive). You can use the symlinks in programs and config files like every other device file. Other examples are /dev/input for input devices or /dev/disk for storage devices.

External resources