HAL (software)

<h2 id="rationale">Rationale</h2>
Traditionally, the <a href="/facts/Operating_system/8XTuvMh2">operating system</a> <a href="/facts/Kernel_(operating_system)/uEj4U3Rz">kernel</a> was responsible for providing an abstract interface to the hardware the system ran on. Applications used the <a href="/facts/System_call/NW44AFJr">system call</a> interface, or performed file <a href="/facts/Input/output/WQTE8Jas">I/O</a> on <a href="/facts/Device_node/t8ISnXft">device nodes</a> 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 <a href="/facts/Peripheral/3lEBPE14">peripheral</a> types now common on both server and desktop computers. Most modern <a href="/facts/Bus_(computing)/R45TKwS7">buses</a> have also become <a href="/facts/Hot_swapping/Jn56Jw3h">hotplug</a>-capable and can have non-trivial <a href="/facts/Network_topology/rBYE0kke">topologies</a>. 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.<a class="footnote-ref" id="fnref:3" href="#fn:3">3</a>
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 <a href="/facts/Wireless_network/yr4SWTdl">wireless networks</a> only if they are logged into the video console).<a class="footnote-ref" id="fnref:4" href="#fn:4">4</a> Application authors resort to using <a href="/facts/Setuid/F2TnVj73">setuid</a> binaries or run service <a href="/facts/Daemon_(computing)/bzuEVGGX">daemons</a> to provide their own access control and privilege separation, potentially introducing security holes each time.

<h2 id="design">Design</h2>
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 <a href="/facts/D-Bus/gpzM6fjI">D-Bus</a> <a href="/facts/Inter-process_communication/cIQYyT6y">IPC</a> mechanism, which abstracts the hardware behind an <a href="/facts/Object-based_language/ACALqUev">object-based</a> <a href="/facts/Remote_procedure_call/uMMb9CQw">RPC</a> 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 <a href="/facts/Disk_partitioning/wrBk5RRS">disk partitions</a> 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 <a href="/facts/Digital_camera/LSGTserm">digital camera</a> being plugged in, an <a href="/facts/Optical_disc/vjVDB1GH">optical disc</a> spinning up or a laptop computer closing its lid.<a class="footnote-ref" id="fnref:5" href="#fn:5">5</a><a class="footnote-ref" id="fnref:6" href="#fn:6">6</a>

<h2 id="implementations-and-obsolescence">Implementations and obsolescence</h2>
On Linux, HAL uses <a href="/facts/Sysfs/YHLFf08v">/sys</a> (a <a href="/facts/Virtual_file_system/lRI31T4T">virtual file system</a> for <a href="/facts/Linux/9FvCKSTq">Linux</a> systems) to discover hardware and listen for kernel <a href="/facts/Hot_swapping/Jn56Jw3h">hotplug</a> events. Some Linux distributions also provide a <a href="/facts/Udev/89smRMUF">udev</a> rule to allow the udev daemon to notify HAL whenever new device nodes appear.

<h3>Deprecated</h3>
As of 2011<a href="https://en.wikipedia.org/w/index.php?title=HAL_(software)&action=edit">[update]</a>, Linux distributions such as <a href="/facts/Ubuntu_(operating_system)/0WDd7hkb">Ubuntu</a>,<a class="footnote-ref" id="fnref:7" href="#fn:7">7</a> <a href="/facts/Debian/cBd93qfJ">Debian</a>,<a class="footnote-ref" id="fnref:8" href="#fn:8">8</a> and <a href="/facts/Fedora_Linux/LJ2ocge5">Fedora</a> and on FreeBSD,<a class="footnote-ref" id="fnref:9" href="#fn:9">9</a> and projects such as KDE,<a class="footnote-ref" id="fnref:10" href="#fn:10">10</a> GNOME and <a href="/facts/X.Org_Server/7AaPmYtV">X.org</a> are in the process of <a href="/facts/Deprecation/1aQL7rnP">deprecating</a> HAL as it has "become a large monolithic unmaintainable mess".<a class="footnote-ref" id="fnref:11" href="#fn:11">11</a> 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.<a class="footnote-ref" id="fnref:12" href="#fn:12">12</a> In Linux, it is in the process of being merged into <a href="/facts/Udev/89smRMUF">udev</a> (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 <a href="/facts/DeviceKit/9H7juCrt">DeviceKit</a> 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.

<h2 id="see-also">See also</h2>
<ul><li><a href="/facts/Devfsd/6AngyreK">devfsd</a></li>
<li><a href="/facts/Eudev/zls5HoVh">eudev</a></li>
<li><a href="/facts/Udev/89smRMUF">udev</a></li>
<li><a href="/facts/UPower/BTHcrpVn">UPower</a></li></ul>
<ul>
<li>Free and open-source software portal</li></ul>

<h2 id="external-links">External links</h2>
<ul><li><a href="https://freedesktop.org/wiki/Software/hal/">HAL - Hardware Abstraction Layer</a></li></ul>

<h2 id="references">References</h2>

<ol>
<li id="fn:1">Pennington, Havoc (2003-07-10), Making Hardware Just Work <a href="http://ometer.com/hardware.html" target="_blank">http://ometer.com/hardware.html</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></li>
<li id="fn: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 <a href="https://cgit.freedesktop.org/hal/tree/COPYING" target="_blank">https://cgit.freedesktop.org/hal/tree/COPYING</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></li>
<li id="fn:3">Pennington, Havoc (2003-07-10), Making Hardware Just Work <a href="http://ometer.com/hardware.html" target="_blank">http://ometer.com/hardware.html</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></li>
<li id="fn:4">Pennington, Havoc (2003-07-10), Making Hardware Just Work <a href="http://ometer.com/hardware.html" target="_blank">http://ometer.com/hardware.html</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></li>
<li id="fn:5">Zeuthen, David (2009-11-01), HAL 0.5.14 Specification, freedesktop.org, retrieved 2017-01-15 <a href="https://people.freedesktop.org/~dkukawka/hal-spec-git/git-head/hal-spec.html" target="_blank">https://people.freedesktop.org/~dkukawka/hal-spec-git/git-head/hal-spec.html</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></li>
<li id="fn:6">"hal: doc/spec". 2010-03-16. Retrieved 2017-01-15. <a href="https://cgit.freedesktop.org/hal/tree/doc/spec" target="_blank">https://cgit.freedesktop.org/hal/tree/doc/spec</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></li>
<li id="fn:7">Halsectomy, ubuntu.com, 2013-05-10, retrieved 2017-01-15 <a href="https://wiki.ubuntu.com/Halsectomy" target="_blank">https://wiki.ubuntu.com/Halsectomy</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></li>
<li id="fn:8">"HALRemoval". debian.org. 2011-06-28. Retrieved 2017-01-15. <a href="https://wiki.debian.org/HALRemoval" target="_blank">https://wiki.debian.org/HALRemoval</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></li>
<li id="fn:9">"Features/HalRemoval". Fedora Project. 2012-01-09. Retrieved 2017-01-15. <a href="https://fedoraproject.org/wiki/Features/HalRemoval" target="_blank">https://fedoraproject.org/wiki/Features/HalRemoval</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></li>
<li id="fn:10">"Revision 1206281: Features in KDE Base". KDE. 2010-12-19. Retrieved 2017-01-15. <a href="https://commit-digest.kde.org/issues/2010-12-19/moreinfo/1206281/" target="_blank">https://commit-digest.kde.org/issues/2010-12-19/moreinfo/1206281/</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></li>
<li id="fn:11">Halsectomy, ubuntu.com, 2013-05-10, retrieved 2017-01-15 <a href="https://wiki.ubuntu.com/Halsectomy" target="_blank">https://wiki.ubuntu.com/Halsectomy</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></li>
<li id="fn:12">"TechnicalOverview: HAL removal". ubuntu.com. 2010-05-17. Retrieved 2017-01-15. <a href="https://wiki.ubuntu.com/LucidLynx/TechnicalOverview#HAL_removal" target="_blank">https://wiki.ubuntu.com/LucidLynx/TechnicalOverview#HAL_removal</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></li>
</ol>

HAL (software) open-in-new

HAL (software)