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| <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" |
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| <!-- 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 <systemd/sd-login.h></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 (<= 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> |