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1 | <?xml version='1.0'?> <!--*-nxml-*--> | |
2 | <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN" | |
3 | "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> | |
4 | ||
5 | <!-- | |
6 | SPDX-License-Identifier: LGPL-2.1+ | |
7 | ||
8 | Copyright 2010 Lennart Poettering | |
9 | --> | |
10 | ||
11 | <refentry id="sd-login" conditional='HAVE_PAM' | |
12 | xmlns:xi="http://www.w3.org/2001/XInclude"> | |
13 | ||
14 | <refentryinfo> | |
15 | <title>sd-login</title> | |
16 | <productname>systemd</productname> | |
17 | ||
18 | <authorgroup> | |
19 | <author> | |
20 | <contrib>Developer</contrib> | |
21 | <firstname>Lennart</firstname> | |
22 | <surname>Poettering</surname> | |
23 | <email>lennart@poettering.net</email> | |
24 | </author> | |
25 | </authorgroup> | |
26 | </refentryinfo> | |
27 | ||
28 | <refmeta> | |
29 | <refentrytitle>sd-login</refentrytitle> | |
30 | <manvolnum>3</manvolnum> | |
31 | </refmeta> | |
32 | ||
33 | <refnamediv> | |
34 | <refname>sd-login</refname> | |
35 | <refpurpose>APIs for | |
36 | tracking logins</refpurpose> | |
37 | </refnamediv> | |
38 | ||
39 | <refsynopsisdiv> | |
40 | <funcsynopsis> | |
41 | <funcsynopsisinfo>#include <systemd/sd-login.h></funcsynopsisinfo> | |
42 | </funcsynopsis> | |
43 | ||
44 | <cmdsynopsis> | |
45 | <command>pkg-config --cflags --libs libsystemd</command> | |
46 | </cmdsynopsis> | |
47 | </refsynopsisdiv> | |
48 | ||
49 | <refsect1> | |
50 | <title>Description</title> | |
51 | ||
52 | <para><filename>sd-login.h</filename> provides APIs to introspect | |
53 | and monitor seat, login session and user status information on the | |
54 | local system. </para> | |
55 | ||
56 | <para>Note that these APIs only allow purely passive access and | |
57 | monitoring of seats, sessions and users. To actively make changes | |
58 | to the seat configuration, terminate login sessions, or switch | |
59 | session on a seat you need to utilize the D-Bus API of | |
60 | systemd-logind, instead.</para> | |
61 | ||
62 | <para>These functions synchronously access data in | |
63 | <filename>/proc</filename>, <filename>/sys/fs/cgroup</filename> | |
64 | and <filename>/run</filename>. All of these are virtual file | |
65 | systems, hence the runtime cost of the accesses is relatively | |
66 | cheap.</para> | |
67 | ||
68 | <para>It is possible (and often a very good choice) to mix calls | |
69 | to the synchronous interface of <filename>sd-login.h</filename> | |
70 | with the asynchronous D-Bus interface of systemd-logind. However, | |
71 | if this is done you need to think a bit about possible races since | |
72 | the stream of events from D-Bus and from | |
73 | <filename>sd-login.h</filename> interfaces such as the login | |
74 | monitor are asynchronous and not ordered against each | |
75 | other.</para> | |
76 | ||
77 | <para>If the functions return string arrays, these are generally | |
78 | <constant>NULL</constant> terminated and need to be freed by the | |
79 | caller with the libc | |
80 | <citerefentry project='man-pages'><refentrytitle>free</refentrytitle><manvolnum>3</manvolnum></citerefentry> | |
81 | call after use, including the strings referenced therein. | |
82 | Similarly, individual strings returned need to be freed, as | |
83 | well.</para> | |
84 | ||
85 | <para>As a special exception, instead of an empty string array | |
86 | <constant>NULL</constant> may be returned, which should be treated | |
87 | equivalent to an empty string array.</para> | |
88 | ||
89 | <para>See | |
90 | <citerefentry><refentrytitle>sd_pid_get_session</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
91 | <citerefentry><refentrytitle>sd_uid_get_state</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
92 | <citerefentry><refentrytitle>sd_session_is_active</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
93 | <citerefentry><refentrytitle>sd_seat_get_active</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
94 | <citerefentry><refentrytitle>sd_get_seats</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
95 | <citerefentry><refentrytitle>sd_login_monitor_new</refentrytitle><manvolnum>3</manvolnum></citerefentry> | |
96 | for more information about the functions | |
97 | implemented.</para> | |
98 | </refsect1> | |
99 | ||
100 | <refsect1> | |
101 | <title>Definition of Terms</title> | |
102 | ||
103 | <variablelist> | |
104 | <varlistentry> | |
105 | <term>seat</term> | |
106 | ||
107 | <listitem><para>A seat consists of all hardware devices assigned to a specific | |
108 | workplace. It consists of at least one graphics device, and usually also includes | |
109 | keyboard, mouse. It can also include video cameras, sound cards and more. Seats | |
110 | are identified by seat names, which are strings (<= 255 characters), that start | |
111 | with the four characters <literal>seat</literal> followed by at least one | |
112 | character from the range [a-zA-Z0-9], <literal>_</literal> and | |
113 | <literal>-</literal>. They are suitable for use as file names. Seat names may or | |
114 | may not be stable and may be reused if a seat becomes available again. | |
115 | </para></listitem> | |
116 | </varlistentry> | |
117 | ||
118 | <varlistentry> | |
119 | <term>session</term> | |
120 | ||
121 | <listitem><para>A session is defined by the time a user is logged in until they | |
122 | log out. A session is bound to one or no seats (the latter for 'virtual' ssh | |
123 | logins). Multiple sessions can be attached to the same seat, but only one of them | |
124 | can be active, the others are in the background. A session is identified by a | |
125 | short string.</para> | |
126 | ||
127 | <para> | |
128 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry> | |
129 | ensures that audit sessions are identical to systemd sessions, and uses the audit | |
130 | session ID as session ID in systemd (if auditing is enabled). In general the | |
131 | session identifier is a short string consisting only of [a-zA-Z0-9], | |
132 | <literal>_</literal> and <literal>-</literal>, suitable for use as a file name. | |
133 | Session IDs are unique on the local machine and are | |
134 | never reused as long as the machine is online. A user (the way we know it on UNIX) | |
135 | corresponds to the person using a computer. A single user can have multiple | |
136 | sessions open at the same time. A user is identified by a numeric user id (UID) or | |
137 | a user name (a string). A multi-session system allows multiple user sessions on | |
138 | the same seat at the same time. A multi-seat system allows multiple independent | |
139 | seats that can be individually and simultaneously used by different users.</para> | |
140 | </listitem> | |
141 | </varlistentry> | |
142 | </variablelist> | |
143 | ||
144 | <para>All hardware devices that are eligible to being assigned to a seat, are assigned | |
145 | to one. A device can be assigned to only one seat at a time. If a device is not | |
146 | assigned to any particular other seat it is implicitly assigned to the special default | |
147 | seat called <literal>seat0</literal>.</para> | |
148 | ||
149 | <para>Note that hardware like printers, hard disks or network cards is generally not | |
150 | assigned to a specific seat. They are available to all seats equally. (Well, with one | |
151 | exception: USB sticks can be assigned to a seat.)</para> | |
152 | ||
153 | <para><literal>seat0</literal> always exists.</para> | |
154 | </refsect1> | |
155 | ||
156 | <refsect1> | |
157 | <title>udev Rules</title> | |
158 | ||
159 | <para>Assignment of hardware devices to seats is managed inside the udev database, via | |
160 | settings on the devices:</para> | |
161 | ||
162 | <variablelist> | |
163 | <varlistentry> | |
164 | <term>Tag <literal>seat</literal></term> | |
165 | ||
166 | <listitem><para>When set, a device is eligible to be assigned to a seat. This tag | |
167 | is set for graphics devices, mice, keyboards, video cards, sound cards and | |
168 | more. Note that some devices like sound cards consist of multiple subdevices | |
169 | (i.e. a PCM for input and another one for output). This tag will be set only for | |
170 | the originating device, not for the individual subdevices. A UI for configuring | |
171 | assignment of devices to seats should enumerate and subscribe to all devices with | |
172 | this tag set and show them in the UI. Note that USB hubs can be assigned to a seat | |
173 | as well, in which case all (current and future) devices plugged into it will also | |
174 | be assigned to the same seat (unless they are explicitly assigned to another | |
175 | seat). | |
176 | </para></listitem> | |
177 | </varlistentry> | |
178 | ||
179 | <varlistentry> | |
180 | <term>Tag <literal>master-of-seat</literal></term> | |
181 | ||
182 | <listitem><para>When set, this device is enough for a seat to be considered | |
183 | existent. This tag is usually set for the framebuffer device of graphics cards. A | |
184 | seat hence consists of an arbitrary number of devices marked with the | |
185 | <literal>seat</literal> tag, but (at least) one of these devices needs to be | |
186 | tagged with <literal>master-of-seat</literal> before the seat is actually | |
187 | considered to be around.</para></listitem> | |
188 | </varlistentry> | |
189 | ||
190 | <varlistentry> | |
191 | <term>Property <varname>ID_SEAT</varname></term> | |
192 | ||
193 | <listitem><para>This property specifies the name of the seat a specific device is | |
194 | assigned to. If not set the device is assigned to <literal>seat0</literal>. Also, | |
195 | to speed up enumeration of hardware belonging to a specific seat, the seat is also | |
196 | set as tag on the device. I.e. if the property | |
197 | <varname>ID_SEAT=seat-waldo</varname> is set for a device, the tag | |
198 | <literal>seat-waldo</literal> will be set as well. Note that if a device is | |
199 | assigned to <literal>seat0</literal>, it will usually not carry such a tag and you | |
200 | need to enumerate all devices and check the <varname>ID_SEAT</varname> property | |
201 | manually. Again, if a device is assigned to seat0 this is visible on the device in | |
202 | two ways: with a property <varname>ID_SEAT=seat0</varname> and with no property | |
203 | <varname>ID_SEAT</varname> set for it at all.</para></listitem> | |
204 | </varlistentry> | |
205 | ||
206 | <varlistentry> | |
207 | <term>Property <varname>ID_AUTOSEAT</varname></term> | |
208 | ||
209 | <listitem><para>When set to <literal>1</literal>, this device automatically | |
210 | generates a new and independent seat, which is named after the path of the | |
211 | device. This is set for specialized USB hubs like the Plugable devices, which when | |
212 | plugged in should create a hotplug seat without further configuration.</para> | |
213 | </listitem> | |
214 | </varlistentry> | |
215 | ||
216 | <varlistentry> | |
217 | <term>Property <varname>ID_FOR_SEAT</varname></term> | |
218 | ||
219 | <listitem><para>When creating additional (manual) seats starting from a graphics | |
220 | device this is a good choice to name the seat after. It is created from the path | |
221 | of the device. This is useful in UIs for configuring seats: as soon as you create | |
222 | a new seat from a graphics device, read this property and prefix it with | |
223 | <literal>seat-</literal> and use it as name for the seat.</para></listitem> | |
224 | </varlistentry> | |
225 | </variablelist> | |
226 | ||
227 | <para>A seat exists only and exclusively because a properly tagged device with the | |
228 | right <varname>ID_SEAT</varname> property exists. Besides udev rules there is no | |
229 | persistent data about seats stored on disk.</para> | |
230 | ||
231 | <para>Note that | |
232 | <citerefentry><refentrytitle>systemd-logind</refentrytitle><manvolnum>8</manvolnum></citerefentry> | |
233 | manages ACLs on a number of device classes, to allow user code to access the device | |
234 | nodes attached to a seat as long as the user has an active session on it. This is | |
235 | mostly transparent to applications. As mentioned above, for certain user software it | |
236 | might be a good idea to watch whether they can access device nodes instead of thinking | |
237 | about seats.</para> | |
238 | </refsect1> | |
239 | ||
240 | <xi:include href="libsystemd-pkgconfig.xml" /> | |
241 | ||
242 | <refsect1> | |
243 | <title>See Also</title> | |
244 | <para> | |
245 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, | |
246 | <citerefentry><refentrytitle>sd_pid_get_session</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
247 | <citerefentry><refentrytitle>sd_uid_get_state</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
248 | <citerefentry><refentrytitle>sd_session_is_active</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
249 | <citerefentry><refentrytitle>sd_seat_get_active</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
250 | <citerefentry><refentrytitle>sd_get_seats</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
251 | <citerefentry><refentrytitle>sd_login_monitor_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
252 | <citerefentry><refentrytitle>sd-daemon</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
253 | <citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry> | |
254 | </para> | |
255 | ||
256 | <para> | |
257 | <ulink url="https://www.freedesktop.org/wiki/Software/systemd/multiseat">Multi-Seat on Linux</ulink> | |
258 | for an introduction to multi-seat support on Linux and the background for this set of APIs. | |
259 | </para> | |
260 | </refsect1> | |
261 | ||
262 | </refentry> |