Adding upstream version 241.upstream/241upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 251 insertions, 0 deletions

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+<?xml version='1.0'?> <!--*-nxml-*-->
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
+  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
+
+<!--
+  SPDX-License-Identifier: LGPL-2.1+
+-->
+
+<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 Plugable 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>
+      for an introduction to multi-seat support on Linux and the background for this set of APIs.
+    </para>
+  </refsect1>
+
+</refentry>