HAL (software)

Last updated
HAL
Stable release
0.5.14 / November 30, 2009;13 years ago (2009-11-30)
Operating system Linux, FreeBSD, NetBSD, OpenSolaris, Solaris
Platform UNIX
Type System software
License GNU General Public License and Academic Free License
Website freedesktop.org/wiki/Software/hal/

HAL (Hardware Abstraction Layer or rather Hardware Annotation Library) is a software subsystem for UNIX-like operating systems providing hardware abstraction.

Contents

HAL is now deprecated on most Linux distributions and on FreeBSD. Functionality is being merged into udev on Linux as of 2008–2010 and devd on FreeBSD.[ citation needed ] Previously, HAL was built on top of udev.[ citation needed ]

Some other operating systems which don't have an alternative like udev or devd still use HAL.

The purpose of the hardware abstraction layer was to allow desktop applications to discover and use the hardware of the host system through a simple, portable and abstract API, regardless of the type of the underlying hardware. [1]

HAL for Linux OS was originally envisioned by Havoc Pennington. It became a freedesktop.org project, and was a key part of the software stack of the GNOME and KDE desktop environments. It is free software, dual-licensed under both the GNU General Public License and the Academic Free License. [2]

HAL is unrelated to the concept of Windows NT kernel HALs, which handle some platform-specific core functionality within the kernel, such as interrupt routing.

Rationale

Traditionally, the operating system kernel was responsible for providing an abstract interface to the hardware the system ran on. Applications used the system call interface, or performed file I/O on device nodes in order to communicate with hardware through these abstractions. This sufficed for the simple hardware of early desktop computing.

Computer hardware, however, has increased in complexity and the abstractions provided by Unix kernels have not kept pace with the proliferating number of device and peripheral types now common on both server and desktop computers. Most modern buses have also become hotplug-capable and can have non-trivial topologies. As a result, devices are discovered or change state in ways which can be difficult to track through the system call interface or Unix IPC. The complexity of doing so forces application authors to re-implement hardware support logic. [1]

Some devices also require privileged helper programs to prepare them for use. These must often be invoked in ways that can be awkward to express with the Unix permissions model (for example, allowing users to join wireless networks only if they are logged into the video console). [1] Application authors resort to using setuid binaries or run service daemons to provide their own access control and privilege separation, potentially introducing security holes each time.

Design

HAL is a single daemon responsible for discovering, enumerating and mediating access to most of the hardware on the host computer. Applications communicate with HAL through the D-Bus IPC mechanism, which abstracts the hardware behind an object-based RPC mechanism.

Each logical hardware device is represented as a D-Bus object, and its bus address is used as a unique identifier. Devices include abstractions like disk partitions and visible wireless networks. The device's functionality is exposed through D-Bus interfaces, and its state accessed through properties, a set of key-value pairs.

HAL broadcasts hardware events as signals on these objects; applications can listen for signals and react to the hardware events that they signify—events such as a digital camera being plugged in, an optical disc spinning up or a laptop computer closing its lid. [3] [4]

Implementations and obsolescence

On Linux, HAL uses /sys (a virtual file system for Linux systems) to discover hardware and listen for kernel hotplug events. Some Linux distributions also provide a udev rule to allow the udev daemon to notify HAL whenever new device nodes appear.

Deprecated

As of 2011, Linux distributions such as Ubuntu, [5] Debian, [6] and Fedora and on FreeBSD, [7] and projects such as KDE, [8] GNOME and X.org are in the process of deprecating HAL as it has "become a large monolithic unmaintainable mess". [5] The process is largely complete, but some use of HAL remains – Debian squeeze (Feb 2011) and Ubuntu version 10.04 remove HAL from the basic system and boot process. [9] In Linux, it is in the process of being merged into udev (main udev, libudev, and udev-extras) and existing udev and kernel functionality. The replacement for non-Linux systems such as FreeBSD is devd.

Initially a new daemon DeviceKit was planned to replace certain aspects of HAL, but in March 2009, DeviceKit was deprecated in favor of adding the same code to udev as package udev-extras, and some functions have now moved to udev proper.

See also

Related Research Articles

<span class="mw-page-title-main">Common Desktop Environment</span> Desktop environment for Unix, Unix-like, and OpenVMS operating systems

The Common Desktop Environment (CDE) is a desktop environment for Unix and OpenVMS, based on the Motif widget toolkit. It was part of the UNIX 98 Workstation Product Standard, and was for a long time the Unix desktop associated with commercial Unix workstations. It helped to influence early implementations of successor projects such as KDE and GNOME desktop environment, which largely replaced CDE following the turn of the century.

<span class="mw-page-title-main">Linux distribution</span> Operating system based on the Linux kernel

A Linux distribution is an operating system made from a software collection that includes the Linux kernel, and often a package management system. Linux users usually obtain their operating system by downloading one of the Linux distributions, which are available for a wide variety of systems ranging from embedded devices and personal computers to powerful supercomputers.

<span class="mw-page-title-main">Windowing system</span> Software that manages separately different parts of display screens

In computing, a windowing system is software that manages separately different parts of display screens. It is a type of graphical user interface (GUI) which implements the WIMP paradigm for a user interface.

The Open Sound System (OSS) is an interface for making and capturing sound in Unix and Unix-like operating systems. It is based on standard Unix devices system calls. The term also sometimes refers to the software in a Unix kernel that provides the OSS interface; it can be thought of as a device driver for sound controller hardware. The goal of OSS is to allow the writing of sound-based applications that are agnostic of the underlying sound hardware.

glibc Standard C Library of the GNU Project

The GNU C Library, commonly known as glibc, is the GNU Project's implementation of the C standard library. Despite its name, it now also directly supports C++. It was started in the 1980s by the Free Software Foundation (FSF) for the GNU operating system.

freedesktop.org (fd.o) is a project to work on interoperability and shared base technology for free-software desktop environments for the X Window System (X11) and Wayland on Linux and other Unix-like operating systems. It was founded by Havoc Pennington, a GNOME developer working for Red Hat in March 2000. Some of the project's servers are hosted by Portland State University, sponsored by Hewlett-Packard, Intel, and Google.

udev is a device manager for the Linux kernel. As the successor of devfsd and hotplug, udev primarily manages device nodes in the /dev directory. At the same time, udev also handles all user space events raised when hardware devices are added into the system or removed from it, including firmware loading as required by certain devices.

In computing, D-Bus is a message-oriented middleware mechanism that allows communication between multiple processes running concurrently on the same machine. D-Bus was developed as part of the freedesktop.org project, initiated by Havoc Pennington from Red Hat to standardize services provided by Linux desktop environments such as GNOME and KDE.

sysfs is a pseudo file system provided by the Linux kernel that exports information about various kernel subsystems, hardware devices, and associated device drivers from the kernel's device model to user space through virtual files. In addition to providing information about various devices and kernel subsystems, exported virtual files are also used for their configuration.

<span class="mw-page-title-main">NetworkManager</span> Software

NetworkManager is a daemon that sits on top of libudev and other Linux kernel interfaces and provides a high-level interface for the configuration of the network interfaces.

<span class="mw-page-title-main">GNOME Keyring</span>

GNOME Keyring is a software application designed to store security credentials such as usernames, passwords, and keys, together with a small amount of relevant metadata. The sensitive data is encrypted and stored in a keyring file in the user's home directory. The default keyring uses the login password for encryption, so users don't need to remember another password.

<span class="mw-page-title-main">PulseAudio</span> Sound server for Unix-like operating systems

PulseAudio is a network-capable sound server program distributed via the freedesktop.org project. It runs mainly on Linux, various BSD distributions such as FreeBSD and OpenBSD, macOS, as well as Illumos distributions and the Solaris operating system.

<span class="mw-page-title-main">Linux</span> Family of Unix-like operating systems

Linux is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, which includes the kernel and supporting system software and libraries, many of which are provided by the GNU Project. Many Linux distributions use the word "Linux" in their name, but the Free Software Foundation uses the name "GNU/Linux" to emphasize the importance of GNU software, causing some controversy.

libusb

libusb is a library that provides applications with access for controlling data transfer to and from USB devices on Unix and non-Unix systems, without the need for kernel-mode drivers.

GVfs is GNOME's userspace virtual filesystem designed to work with the I/O abstraction of GIO, a library available in GLib since version 2.15.1. It installs several modules that are automatically used by applications using the APIs of libgio. There is also FUSE support that allows applications not using GIO to access the GVfs filesystems.

DeviceKit is a modular hardware abstraction layer designed for use in Linux systems that is designed to simplify device management and replace the current monolithic Linux HAL. DeviceKit includes the ability to enumerate system devices and send notifications when hardware is added or removed from the computer system.

<span class="mw-page-title-main">Wayland (protocol)</span> Display system intended to replace X11

Wayland is a communication protocol that specifies the communication between a display server and its clients, as well as a C library implementation of that protocol. A display server using the Wayland protocol is called a Wayland compositor, because it additionally performs the task of a compositing window manager.

systemd Suite of system components for Linux

systemd is a software suite that provides an array of system components for Linux operating systems. The main aim is to unify service configuration and behavior across Linux distributions. Its primary component is a "system and service manager" – an init system used to bootstrap user space and manage user processes. It also provides replacements for various daemons and utilities, including device management, login management, network connection management, and event logging. The name systemd adheres to the Unix convention of naming daemons by appending the letter d. It also plays on the term "System D", which refers to a person's ability to adapt quickly and improvise to solve problems.

<span class="mw-page-title-main">UPower</span>

UPower is a piece of middleware for power management on Linux systems. It enumerates power sources, maintains statistics and history data on them and notifies about status changes. It consists of a daemon (upowerd), an application programming interface and a set of command line tools. The daemon provides its functionality to applications over the system bus. PolicyKit restricts access to the UPower functionality for initiating hibernate mode or shutting down the operating system (freedesktop.upower.policy). The command-line client program upower can be used to query and monitor information about the power supply devices in the system. Graphical user interfaces to the functionality of UPower include the GNOME Power Manager and the Xfce Power Manager.

References

  1. 1 2 3 Pennington, Havoc (2003-07-10), Making Hardware Just Work
  2. ↑ HAL source code license text, HAL is licensed to you under your choice of the Academic Free License version 2.1, or the GNU General Public License version 2
  3. ↑ Zeuthen, David (2009-11-01), HAL 0.5.14 Specification, freedesktop.org , retrieved 2017-01-15
  4. ↑ "hal: doc/spec". 2010-03-16. Retrieved 2017-01-15.
  5. 1 2 Halsectomy, ubuntu.com, 2013-05-10, retrieved 2017-01-15
  6. ↑ "HALRemoval". debian.org. 2011-06-28. Retrieved 2017-01-15.
  7. ↑ "Features/HalRemoval". Fedora Project. 2012-01-09. Retrieved 2017-01-15.
  8. ↑ "Revision 1206281: Features in KDE Base". KDE. 2010-12-19. Retrieved 2017-01-15.
  9. ↑ "TechnicalOverview: HAL removal". ubuntu.com. 2010-05-17. Retrieved 2017-01-15.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.