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252 lines
12 KiB
XML
252 lines
12 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!--
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SPDX-License-Identifier: LGPL-2.1+
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-->
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<refentry id="sd-login" conditional='HAVE_PAM'
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xmlns:xi="http://www.w3.org/2001/XInclude">
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<refentryinfo>
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<title>sd-login</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>sd-login</refentrytitle>
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<manvolnum>3</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>sd-login</refname>
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<refpurpose>APIs for
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tracking logins</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<funcsynopsis>
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<funcsynopsisinfo>#include <systemd/sd-login.h></funcsynopsisinfo>
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</funcsynopsis>
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<cmdsynopsis>
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<command>pkg-config --cflags --libs libsystemd</command>
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</cmdsynopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para><filename>sd-login.h</filename> provides APIs to introspect
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and monitor seat, login session and user status information on the
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local system. </para>
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<para>Note that these APIs only allow purely passive access and
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monitoring of seats, sessions and users. To actively make changes
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to the seat configuration, terminate login sessions, or switch
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session on a seat you need to utilize the D-Bus API of
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systemd-logind, instead.</para>
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<para>These functions synchronously access data in
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<filename>/proc</filename>, <filename>/sys/fs/cgroup</filename>
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and <filename>/run</filename>. All of these are virtual file
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systems, hence the runtime cost of the accesses is relatively
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cheap.</para>
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<para>It is possible (and often a very good choice) to mix calls
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to the synchronous interface of <filename>sd-login.h</filename>
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with the asynchronous D-Bus interface of systemd-logind. However,
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if this is done you need to think a bit about possible races since
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the stream of events from D-Bus and from
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<filename>sd-login.h</filename> interfaces such as the login
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monitor are asynchronous and not ordered against each
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other.</para>
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<para>If the functions return string arrays, these are generally
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<constant>NULL</constant> terminated and need to be freed by the
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caller with the libc
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<citerefentry project='man-pages'><refentrytitle>free</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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call after use, including the strings referenced therein.
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Similarly, individual strings returned need to be freed, as
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well.</para>
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<para>As a special exception, instead of an empty string array
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<constant>NULL</constant> may be returned, which should be treated
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equivalent to an empty string array.</para>
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<para>See
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<citerefentry><refentrytitle>sd_pid_get_session</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_uid_get_state</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_session_is_active</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_seat_get_active</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_get_seats</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_login_monitor_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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for more information about the functions
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implemented.</para>
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</refsect1>
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<refsect1>
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<title>Definition of Terms</title>
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<variablelist>
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<varlistentry>
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<term>seat</term>
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<listitem><para>A seat consists of all hardware devices assigned to a specific
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workplace. It consists of at least one graphics device, and usually also includes
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keyboard, mouse. It can also include video cameras, sound cards and more. Seats
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are identified by seat names, which are strings (<= 255 characters), that start
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with the four characters <literal>seat</literal> followed by at least one
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character from the range [a-zA-Z0-9], <literal>_</literal> and
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<literal>-</literal>. They are suitable for use as file names. Seat names may or
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may not be stable and may be reused if a seat becomes available again.
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</para></listitem>
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</varlistentry>
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<varlistentry>
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<term>session</term>
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<listitem><para>A session is defined by the time a user is logged in until they
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log out. A session is bound to one or no seats (the latter for 'virtual' ssh
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logins). Multiple sessions can be attached to the same seat, but only one of them
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can be active, the others are in the background. A session is identified by a
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short string.</para>
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<para>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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ensures that audit sessions are identical to systemd sessions, and uses the audit
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session ID as session ID in systemd (if auditing is enabled). In general the
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session identifier is a short string consisting only of [a-zA-Z0-9],
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<literal>_</literal> and <literal>-</literal>, suitable for use as a file name.
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Session IDs are unique on the local machine and are
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never reused as long as the machine is online. A user (the way we know it on UNIX)
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corresponds to the person using a computer. A single user can have multiple
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sessions open at the same time. A user is identified by a numeric user id (UID) or
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a user name (a string). A multi-session system allows multiple user sessions on
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the same seat at the same time. A multi-seat system allows multiple independent
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seats that can be individually and simultaneously used by different users.</para>
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</listitem>
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</varlistentry>
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</variablelist>
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<para>All hardware devices that are eligible to being assigned to a seat, are assigned
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to one. A device can be assigned to only one seat at a time. If a device is not
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assigned to any particular other seat it is implicitly assigned to the special default
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seat called <literal>seat0</literal>.</para>
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<para>Note that hardware like printers, hard disks or network cards is generally not
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assigned to a specific seat. They are available to all seats equally. (Well, with one
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exception: USB sticks can be assigned to a seat.)</para>
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<para><literal>seat0</literal> always exists.</para>
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</refsect1>
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<refsect1>
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<title>udev Rules</title>
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<para>Assignment of hardware devices to seats is managed inside the udev database, via
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settings on the devices:</para>
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<variablelist class='udev-directives'>
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<varlistentry>
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<term>Tag <literal>seat</literal></term>
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<listitem><para>When set, a device is eligible to be assigned to a seat. This tag
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is set for graphics devices, mice, keyboards, video cards, sound cards and
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more. Note that some devices like sound cards consist of multiple subdevices
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(i.e. a PCM for input and another one for output). This tag will be set only for
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the originating device, not for the individual subdevices. A UI for configuring
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assignment of devices to seats should enumerate and subscribe to all devices with
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this tag set and show them in the UI. Note that USB hubs can be assigned to a seat
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as well, in which case all (current and future) devices plugged into it will also
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be assigned to the same seat (unless they are explicitly assigned to another
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seat).
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</para></listitem>
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</varlistentry>
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<varlistentry>
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<term>Tag <literal>master-of-seat</literal></term>
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<listitem><para>When set, this device is enough for a seat to be considered
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existent. This tag is usually set for the framebuffer device of graphics cards. A
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seat hence consists of an arbitrary number of devices marked with the
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<literal>seat</literal> tag, but (at least) one of these devices needs to be
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tagged with <literal>master-of-seat</literal> before the seat is actually
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considered to be around.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term>Property <varname>ID_SEAT</varname></term>
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<listitem><para>This property specifies the name of the seat a specific device is
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assigned to. If not set the device is assigned to <literal>seat0</literal>. Also,
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to speed up enumeration of hardware belonging to a specific seat, the seat is also
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set as tag on the device. I.e. if the property
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<varname>ID_SEAT=seat-waldo</varname> is set for a device, the tag
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<literal>seat-waldo</literal> will be set as well. Note that if a device is
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assigned to <literal>seat0</literal>, it will usually not carry such a tag and you
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need to enumerate all devices and check the <varname>ID_SEAT</varname> property
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manually. Again, if a device is assigned to seat0 this is visible on the device in
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two ways: with a property <varname>ID_SEAT=seat0</varname> and with no property
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<varname>ID_SEAT</varname> set for it at all.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term>Property <varname>ID_AUTOSEAT</varname></term>
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<listitem><para>When set to <literal>1</literal>, this device automatically
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generates a new and independent seat, which is named after the path of the
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device. This is set for specialized USB hubs like the Plugable devices, which when
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plugged in should create a hotplug seat without further configuration.</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>Property <varname>ID_FOR_SEAT</varname></term>
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<listitem><para>When creating additional (manual) seats starting from a graphics
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device this is a good choice to name the seat after. It is created from the path
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of the device. This is useful in UIs for configuring seats: as soon as you create
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a new seat from a graphics device, read this property and prefix it with
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<literal>seat-</literal> and use it as name for the seat.</para></listitem>
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</varlistentry>
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</variablelist>
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<para>A seat exists only and exclusively because a properly tagged device with the
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right <varname>ID_SEAT</varname> property exists. Besides udev rules there is no
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persistent data about seats stored on disk.</para>
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<para>Note that
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<citerefentry><refentrytitle>systemd-logind</refentrytitle><manvolnum>8</manvolnum></citerefentry>
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manages ACLs on a number of device classes, to allow user code to access the device
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nodes attached to a seat as long as the user has an active session on it. This is
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mostly transparent to applications. As mentioned above, for certain user software it
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might be a good idea to watch whether they can access device nodes instead of thinking
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about seats.</para>
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</refsect1>
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<xi:include href="libsystemd-pkgconfig.xml" />
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<refsect1>
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<title>See Also</title>
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<para>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_pid_get_session</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_uid_get_state</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_session_is_active</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_seat_get_active</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_get_seats</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_login_monitor_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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</para>
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<para>
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<ulink url="https://www.freedesktop.org/wiki/Software/systemd/multiseat">Multi-Seat on Linux</ulink>
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for an introduction to multi-seat support on Linux and the background for this set of APIs.
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</para>
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</refsect1>
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</refentry>
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