1 <?xml version='1.0'?> <!--*- Mode: nxml; nxml-child-indent: 2; indent-tabs-mode: nil -*-->
2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
3 "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
6 SPDX-License-Identifier: LGPL-2.1+
8 Copyright © 2014 Zbigniew Jędrzejewski-Szmek
11 <refentry id="sd_bus_default" xmlns:xi="http://www.w3.org/2001/XInclude">
14 <title>sd_bus_default</title>
15 <productname>systemd</productname>
19 <contrib>A monkey with a typewriter</contrib>
20 <firstname>Zbigniew</firstname>
21 <surname>Jędrzejewski-Szmek</surname>
22 <email>zbyszek@in.waw.pl</email>
28 <refentrytitle>sd_bus_default</refentrytitle>
29 <manvolnum>3</manvolnum>
33 <refname>sd_bus_default</refname>
34 <refname>sd_bus_default_user</refname>
35 <refname>sd_bus_default_system</refname>
37 <refname>sd_bus_open</refname>
38 <refname>sd_bus_open_user</refname>
39 <refname>sd_bus_open_system</refname>
40 <refname>sd_bus_open_system_remote</refname>
41 <refname>sd_bus_open_system_machine</refname>
43 <refpurpose>Acquire a connection to a system or user bus</refpurpose>
48 <funcsynopsisinfo>#include <systemd/sd-bus.h></funcsynopsisinfo>
51 <funcdef>int <function>sd_bus_default</function></funcdef>
52 <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
56 <funcdef>int <function>sd_bus_default_user</function></funcdef>
57 <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
61 <funcdef>int <function>sd_bus_default_system</function></funcdef>
62 <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
66 <funcdef>int <function>sd_bus_open</function></funcdef>
67 <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
71 <funcdef>int <function>sd_bus_open_user</function></funcdef>
72 <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
76 <funcdef>int <function>sd_bus_open_system</function></funcdef>
77 <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
81 <funcdef>int <function>sd_bus_open_system_remote</function></funcdef>
82 <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
83 <paramdef>const char *<parameter>host</parameter></paramdef>
87 <funcdef>int <function>sd_bus_open_system_machine</function></funcdef>
88 <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
89 <paramdef>const char *<parameter>machine</parameter></paramdef>
96 <title>Description</title>
98 <para><function>sd_bus_default()</function> acquires a bus
99 connection object to the user bus when invoked in user context, or
100 to the system bus otherwise. The connection object is associated
101 with the calling thread. Each time the function is invoked from
102 the same thread, the same object is returned, but its reference
103 count is increased by one, as long as at least one reference is
104 kept. When the last reference to the connection is dropped (using
106 <citerefentry><refentrytitle>sd_bus_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>
107 call), the connection is terminated. Note that the connection is
108 not automatically terminated when the associated thread ends. It
109 is important to drop the last reference to the bus connection
110 explicitly before the thread ends, as otherwise, the connection will
111 leak. Also, queued but unread or unwritten messages keep the
112 bus referenced, see below.</para>
114 <para><function>sd_bus_default_user()</function> returns a user
115 bus connection object associated with the calling thread.
116 <function>sd_bus_default_system()</function> is similar, but
117 connects to the system bus. Note that
118 <function>sd_bus_default()</function> is identical to these two
119 calls, depending on the execution context.</para>
121 <para><function>sd_bus_open()</function> creates a new,
122 independent bus connection to the user bus when invoked in user
123 context, or the system bus
124 otherwise. <function>sd_bus_open_user()</function> is similar, but
125 connects only to the user bus.
126 <function>sd_bus_open_system()</function> does the same, but
127 connects to the system bus. In contrast to
128 <function>sd_bus_default()</function>,
129 <function>sd_bus_default_user()</function>, and
130 <function>sd_bus_default_system()</function>, these calls return
131 new, independent connection objects that are not associated with
132 the invoking thread and are not shared between multiple
133 invocations. It is recommended to share connections per thread to
134 efficiently make use the available resources. Thus, it is
135 recommended to use <function>sd_bus_default()</function>,
136 <function>sd_bus_default_user()</function> and
137 <function>sd_bus_default_system()</function> to connect to the
138 user or system buses.</para>
140 <para>If the <varname>$DBUS_SESSION_BUS_ADDRESS</varname> environment
142 (cf. <citerefentry project='man-pages'><refentrytitle>environ</refentrytitle><manvolnum>7</manvolnum></citerefentry>),
143 it will be used as the address of the user bus. This variable can
144 contain multiple addresses separated by <literal>;</literal>. If
145 this variable is not set, a suitable default for the default user
146 D-Bus instance will be used.</para>
148 <para>If the <varname>$DBUS_SYSTEM_BUS_ADDRESS</varname>
149 environment variable is set, it will be used as the address of the
150 system bus. This variable uses the same syntax as
151 <varname>$DBUS_SESSION_BUS_ADDRESS</varname>. If this variable is
152 not set, a suitable default for the default system D-Bus instance
155 <para><function>sd_bus_open_system_remote()</function> connects to the system bus on
156 the specified host using
157 <citerefentry project='die-net'><refentrytitle>ssh</refentrytitle><manvolnum>1</manvolnum></citerefentry>.
158 <parameter>host</parameter> consists of an optional user name followed by the
159 <literal>@</literal> symbol, and the hostname, optionally followed by a
160 <literal>:</literal> and a machine name. If the machine name is given, a connection
161 is created to the system bus in the specified container on the remote machine, and
162 otherwise a connection to the system bus on the specified host is created.</para>
164 <para>Note that entering a container is a privileged operation, and will likely only
165 work for the root user on the remote machine.</para>
167 <para><function>sd_bus_open_system_machine()</function> connects
168 to the system bus in the specified <parameter>machine</parameter>,
169 where <parameter>machine</parameter> is the name of a local
171 <citerefentry><refentrytitle>machinectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
172 for more information about the "machine" concept. Note that
173 connections into local containers are only available to privileged
174 processes at this time.</para>
176 <para>These calls allocate a bus connection object and initiate
177 the connection to a well-known bus of some form. An alternative to
178 using these high-level calls is to create an unconnected bus
180 <citerefentry><refentrytitle>sd_bus_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>
181 and to connect it with
182 <citerefentry><refentrytitle>sd_bus_start</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
188 <title>Reference ownership</title>
189 <para>The functions <function>sd_bus_open()</function>,
190 <function>sd_bus_open_user()</function>,
191 <function>sd_bus_open_system()</function>,
192 <function>sd_bus_open_system_remote()</function>, and
193 <function>sd_bus_open_system_machine()</function> return a new
194 connection object and the caller owns the sole reference. When not
195 needed anymore, this reference should be destroyed with
196 <citerefentry><refentrytitle>sd_bus_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
199 <para>The functions <function>sd_bus_default()</function>,
200 <function>sd_bus_default_user()</function> and
201 <function>sd_bus_default_system()</function> do not necessarily
202 create a new object, but increase the connection reference of an
203 existing connection object by one. Use
204 <citerefentry><refentrytitle>sd_bus_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>
205 to drop the reference.</para>
207 <para>Queued but unwritten/unread messages keep a reference to their bus connection object. For this reason, even
208 if an application dropped all references to a bus connection, it might not get destroyed right away. Until all
209 incoming queued messages are read, and until all outgoing unwritten messages are written, the bus object will stay
210 alive. <function>sd_bus_flush()</function> may be used to write all outgoing queued messages so they drop their
211 references. To flush the unread incoming messages, use <function>sd_bus_close()</function>, which will also close
212 the bus connection. When using the default bus logic, it is a good idea to first invoke
213 <function>sd_bus_flush()</function> followed by <function>sd_bus_close()</function> when a thread or process
214 terminates, and thus its bus connection object should be freed.</para>
216 <para>Normally, slot objects (as created by
217 <citerefentry><refentrytitle>sd_bus_add_match</refentrytitle><manvolnum>3</manvolnum></citerefentry> and similar
218 calls) keep a reference to their bus connection object, too. Thus, as long as a bus slot object remains referenced
219 its bus object will remain allocated too. Optionally, bus slot objects may be placed in "floating" mode. When in
220 floating mode the life cycle of the bus slot object is bound to the bus object, i.e. when the bus object is freed
221 the bus slot object is automatically unreferenced too. The floating state of a slot object may be controlled
223 <citerefentry><refentrytitle>sd_bus_slot_set_floating</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
224 though usually floating bus slot objects are created by passing <constant>NULL</constant> as the
225 <parameter>slot</parameter> parameter of <function>sd_bus_add_match()</function> and related calls, thus indicating
226 that the caller is not directly interested in referencing and managing the bus slot object.</para>
228 <para>The life cycle of the default bus connection should be the
229 responsibility of the code that creates/owns the thread the
230 default bus connection object is associated with. Library code
231 should neither call <function>sd_bus_flush()</function> nor
232 <function>sd_bus_close()</function> on default bus objects unless
233 it does so in its own private, self-allocated thread. Library code
234 should not use the default bus object in other threads unless it
235 is clear that the program using it will life cycle the bus
236 connection object and flush and close it before exiting from the
237 thread. In libraries where it is not clear that the calling
238 program will life cycle the bus connection object, it is hence
239 recommended to use <function>sd_bus_open_system()</function>
240 instead of <function>sd_bus_default_system()</function> and
241 related calls.</para>
245 <title>Return Value</title>
247 <para>On success, these calls return 0 or a positive
248 integer. On failure, these calls return a negative
249 errno-style error code.</para>
253 <title>Errors</title>
255 <para>Returned errors may indicate the following problems:</para>
260 <term><constant>-EINVAL</constant></term>
262 <listitem><para>The specified parameters are invalid.</para></listitem>
266 <term><constant>-ENOMEM</constant></term>
268 <listitem><para>Memory allocation failed.</para></listitem>
272 <term><constant>-ESOCKTNOSUPPORT</constant></term>
274 <listitem><para>The protocol version required to connect to the selected bus is not supported.</para></listitem>
278 <para>In addition, any further connection-related errors may be
279 by returned. See <citerefentry><refentrytitle>sd_bus_send</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
282 <xi:include href="libelogind-pkgconfig.xml" />
285 <title>See Also</title>
288 <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
289 <citerefentry><refentrytitle>sd-bus</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
290 <citerefentry><refentrytitle>sd_bus_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
291 <citerefentry><refentrytitle>sd_bus_ref</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
292 <citerefentry><refentrytitle>sd_bus_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
293 <citerefentry project='die-net'><refentrytitle>ssh</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
294 <citerefentry><refentrytitle>systemd-machined.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
295 <citerefentry><refentrytitle>machinectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>