systemd/man/sd-login.xml

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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->

<refentry id="sd-login" conditional='HAVE_PAM'
  xmlns:xi="http://www.w3.org/2001/XInclude">

  <refentryinfo>
    <title>sd-login</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>sd-login</refentrytitle>
    <manvolnum>3</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>sd-login</refname>
    <refpurpose>APIs for
    tracking logins</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <funcsynopsis>
      <funcsynopsisinfo>#include &lt;systemd/sd-login.h&gt;</funcsynopsisinfo>
    </funcsynopsis>

    <cmdsynopsis>
      <command>pkg-config --cflags --libs libsystemd</command>
    </cmdsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para><filename>sd-login.h</filename> provides APIs to introspect
    and monitor seat, login session and user status information on the
    local system. </para>

    <para>Note that these APIs only allow purely passive access and
    monitoring of seats, sessions and users. To actively make changes
    to the seat configuration, terminate login sessions, or switch
    session on a seat you need to utilize the D-Bus API of
    systemd-logind, instead.</para>

    <para>These functions synchronously access data in
    <filename>/proc/</filename>, <filename>/sys/fs/cgroup/</filename>
    and <filename>/run/</filename>. All of these are virtual file
    systems, hence the runtime cost of the accesses is relatively
    cheap.</para>

    <para>It is possible (and often a very good choice) to mix calls
    to the synchronous interface of <filename>sd-login.h</filename>
    with the asynchronous D-Bus interface of systemd-logind. However,
    if this is done you need to think a bit about possible races since
    the stream of events from D-Bus and from
    <filename>sd-login.h</filename> interfaces such as the login
    monitor are asynchronous and not ordered against each
    other.</para>

    <para>If the functions return string arrays, these are generally
    <constant>NULL</constant> terminated and need to be freed by the
    caller with the libc
    <citerefentry project='man-pages'><refentrytitle>free</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    call after use, including the strings referenced therein.
    Similarly, individual strings returned need to be freed, as
    well.</para>

    <para>As a special exception, instead of an empty string array
    <constant>NULL</constant> may be returned, which should be treated
    equivalent to an empty string array.</para>

    <para>See
    <citerefentry><refentrytitle>sd_pid_get_session</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
    <citerefentry><refentrytitle>sd_uid_get_state</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
    <citerefentry><refentrytitle>sd_session_is_active</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
    <citerefentry><refentrytitle>sd_seat_get_active</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
    <citerefentry><refentrytitle>sd_get_seats</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
    <citerefentry><refentrytitle>sd_login_monitor_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    for more information about the functions
    implemented.</para>
  </refsect1>

  <refsect1>
    <title>Definition of Terms</title>

    <variablelist>
      <varlistentry>
        <term>seat</term>

        <listitem><para>A seat consists of all hardware devices assigned to a specific
        workplace. It consists of at least one graphics device, and usually also includes
        keyboard, mouse. It can also include video cameras, sound cards and more. Seats
        are identified by seat names, which are strings (&lt;= 255 characters), that start
        with the four characters <literal>seat</literal> followed by at least one
        character from the range [a-zA-Z0-9], <literal>_</literal> and
        <literal>-</literal>. They are suitable for use as file names. Seat names may or
        may not be stable and may be reused if a seat becomes available again.
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term>session</term>

        <listitem><para>A session is defined by the time a user is logged in until they
        log out. A session is bound to one or no seats (the latter for 'virtual' ssh
        logins). Multiple sessions can be attached to the same seat, but only one of them
        can be active, the others are in the background. A session is identified by a
        short string.</para>

        <para>
        <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>
        ensures that audit sessions are identical to systemd sessions, and uses the audit
        session ID as session ID in systemd (if auditing is enabled). In general the
        session identifier is a short string consisting only of [a-zA-Z0-9],
        <literal>_</literal> and <literal>-</literal>, suitable for use as a file name.
        Session IDs are unique on the local machine and are
        never reused as long as the machine is online. A user (the way we know it on UNIX)
        corresponds to the person using a computer. A single user can have multiple
        sessions open at the same time. A user is identified by a numeric user id (UID) or
        a user name (a string). A multi-session system allows multiple user sessions on
        the same seat at the same time. A multi-seat system allows multiple independent
        seats that can be individually and simultaneously used by different users.</para>
        </listitem>
      </varlistentry>
    </variablelist>

    <para>All hardware devices that are eligible to being assigned to a seat, are assigned
    to one. A device can be assigned to only one seat at a time. If a device is not
    assigned to any particular other seat it is implicitly assigned to the special default
    seat called <literal>seat0</literal>.</para>

    <para>Note that hardware like printers, hard disks or network cards is generally not
    assigned to a specific seat. They are available to all seats equally. (Well, with one
    exception: USB sticks can be assigned to a seat.)</para>

    <para><literal>seat0</literal> always exists.</para>
  </refsect1>

  <refsect1>
    <title>udev Rules</title>

    <para>Assignment of hardware devices to seats is managed inside the udev database, via
    settings on the devices:</para>

    <variablelist class='udev-directives'>
      <varlistentry>
        <term>Tag <literal>seat</literal></term>

        <listitem><para>When set, a device is eligible to be assigned to a seat. This tag
        is set for graphics devices, mice, keyboards, video cards, sound cards and
        more. Note that some devices like sound cards consist of multiple subdevices
        (i.e. a PCM for input and another one for output). This tag will be set only for
        the originating device, not for the individual subdevices. A UI for configuring
        assignment of devices to seats should enumerate and subscribe to all devices with
        this tag set and show them in the UI. Note that USB hubs can be assigned to a seat
        as well, in which case all (current and future) devices plugged into it will also
        be assigned to the same seat (unless they are explicitly assigned to another
        seat).
        </para></listitem>
      </varlistentry>

      <varlistentry>
        <term>Tag <literal>master-of-seat</literal></term>

        <listitem><para>When set, this device is enough for a seat to be considered
        existent. This tag is usually set for the framebuffer device of graphics cards. A
        seat hence consists of an arbitrary number of devices marked with the
        <literal>seat</literal> tag, but (at least) one of these devices needs to be
        tagged with <literal>master-of-seat</literal> before the seat is actually
        considered to be around.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term>Property <varname>ID_SEAT</varname></term>

        <listitem><para>This property specifies the name of the seat a specific device is
        assigned to. If not set the device is assigned to <literal>seat0</literal>. Also,
        to speed up enumeration of hardware belonging to a specific seat, the seat is also
        set as tag on the device. I.e. if the property
        <varname>ID_SEAT=seat-waldo</varname> is set for a device, the tag
        <literal>seat-waldo</literal> will be set as well. Note that if a device is
        assigned to <literal>seat0</literal>, it will usually not carry such a tag and you
        need to enumerate all devices and check the <varname>ID_SEAT</varname> property
        manually. Again, if a device is assigned to seat0 this is visible on the device in
        two ways: with a property <varname>ID_SEAT=seat0</varname> and with no property
        <varname>ID_SEAT</varname> set for it at all.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term>Property <varname>ID_AUTOSEAT</varname></term>

        <listitem><para>When set to <literal>1</literal>, this device automatically
        generates a new and independent seat, which is named after the path of the
        device. This is set for specialized USB hubs like the Pluggable devices, which when
        plugged in should create a hotplug seat without further configuration.</para>
        </listitem>
      </varlistentry>

      <varlistentry>
        <term>Property <varname>ID_FOR_SEAT</varname></term>

        <listitem><para>When creating additional (manual) seats starting from a graphics
        device this is a good choice to name the seat after. It is created from the path
        of the device. This is useful in UIs for configuring seats: as soon as you create
        a new seat from a graphics device, read this property and prefix it with
        <literal>seat-</literal> and use it as name for the seat.</para></listitem>
      </varlistentry>
    </variablelist>

    <para>A seat exists only and exclusively because a properly tagged device with the
    right <varname>ID_SEAT</varname> property exists. Besides udev rules there is no
    persistent data about seats stored on disk.</para>

    <para>Note that
    <citerefentry><refentrytitle>systemd-logind</refentrytitle><manvolnum>8</manvolnum></citerefentry>
    manages ACLs on a number of device classes, to allow user code to access the device
    nodes attached to a seat as long as the user has an active session on it. This is
    mostly transparent to applications. As mentioned above, for certain user software it
    might be a good idea to watch whether they can access device nodes instead of thinking
    about seats.</para>
  </refsect1>

  <xi:include href="libsystemd-pkgconfig.xml" />

  <refsect1>
    <title>See Also</title>
    <para>
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_pid_get_session</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_uid_get_state</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_session_is_active</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_seat_get_active</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_get_seats</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_login_monitor_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
    </para>

    <para>
      <ulink url="https://www.freedesktop.org/wiki/Software/systemd/multiseat">Multi-Seat on Linux</ulink>
      may also be of historical interest.
    </para>
  </refsect1>
</refentry>