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2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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6 SPDX-License-Identifier: LGPL-2.1+
9 <refentry id="sd_bus_default" xmlns:xi="http://www.w3.org/2001/XInclude">
12 <title>sd_bus_default</title>
13 <productname>systemd</productname>
17 <refentrytitle>sd_bus_default</refentrytitle>
18 <manvolnum>3</manvolnum>
22 <refname>sd_bus_default</refname>
23 <refname>sd_bus_default_user</refname>
24 <refname>sd_bus_default_system</refname>
26 <refname>sd_bus_open</refname>
27 <refname>sd_bus_open_user</refname>
28 <refname>sd_bus_open_system</refname>
29 <refname>sd_bus_open_system_remote</refname>
30 <refname>sd_bus_open_system_machine</refname>
32 <refpurpose>Acquire a connection to a system or user bus</refpurpose>
37 <funcsynopsisinfo>#include <systemd/sd-bus.h></funcsynopsisinfo>
40 <funcdef>int <function>sd_bus_default</function></funcdef>
41 <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
45 <funcdef>int <function>sd_bus_default_user</function></funcdef>
46 <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
50 <funcdef>int <function>sd_bus_default_system</function></funcdef>
51 <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
55 <funcdef>int <function>sd_bus_open</function></funcdef>
56 <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
60 <funcdef>int <function>sd_bus_open_user</function></funcdef>
61 <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
65 <funcdef>int <function>sd_bus_open_system</function></funcdef>
66 <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
70 <funcdef>int <function>sd_bus_open_system_remote</function></funcdef>
71 <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
72 <paramdef>const char *<parameter>host</parameter></paramdef>
76 <funcdef>int <function>sd_bus_open_system_machine</function></funcdef>
77 <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
78 <paramdef>const char *<parameter>machine</parameter></paramdef>
85 <title>Description</title>
87 <para><function>sd_bus_default()</function> acquires a bus
88 connection object to the user bus when invoked in user context, or
89 to the system bus otherwise. The connection object is associated
90 with the calling thread. Each time the function is invoked from
91 the same thread, the same object is returned, but its reference
92 count is increased by one, as long as at least one reference is
93 kept. When the last reference to the connection is dropped (using
95 <citerefentry><refentrytitle>sd_bus_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>
96 call), the connection is terminated. Note that the connection is
97 not automatically terminated when the associated thread ends. It
98 is important to drop the last reference to the bus connection
99 explicitly before the thread ends, as otherwise, the connection will
100 leak. Also, queued but unread or unwritten messages keep the
101 bus referenced, see below.</para>
103 <para><function>sd_bus_default_user()</function> returns a user
104 bus connection object associated with the calling thread.
105 <function>sd_bus_default_system()</function> is similar, but
106 connects to the system bus. Note that
107 <function>sd_bus_default()</function> is identical to these two
108 calls, depending on the execution context.</para>
110 <para><function>sd_bus_open()</function> creates a new,
111 independent bus connection to the user bus when invoked in user
112 context, or the system bus
113 otherwise. <function>sd_bus_open_user()</function> is similar, but
114 connects only to the user bus.
115 <function>sd_bus_open_system()</function> does the same, but
116 connects to the system bus. In contrast to
117 <function>sd_bus_default()</function>,
118 <function>sd_bus_default_user()</function>, and
119 <function>sd_bus_default_system()</function>, these calls return
120 new, independent connection objects that are not associated with
121 the invoking thread and are not shared between multiple
122 invocations. It is recommended to share connections per thread to
123 efficiently make use the available resources. Thus, it is
124 recommended to use <function>sd_bus_default()</function>,
125 <function>sd_bus_default_user()</function> and
126 <function>sd_bus_default_system()</function> to connect to the
127 user or system buses.</para>
129 <para>If the <varname>$DBUS_SESSION_BUS_ADDRESS</varname> environment
131 (cf. <citerefentry project='man-pages'><refentrytitle>environ</refentrytitle><manvolnum>7</manvolnum></citerefentry>),
132 it will be used as the address of the user bus. This variable can
133 contain multiple addresses separated by <literal>;</literal>. If
134 this variable is not set, a suitable default for the default user
135 D-Bus instance will be used.</para>
137 <para>If the <varname>$DBUS_SYSTEM_BUS_ADDRESS</varname>
138 environment variable is set, it will be used as the address of the
139 system bus. This variable uses the same syntax as
140 <varname>$DBUS_SESSION_BUS_ADDRESS</varname>. If this variable is
141 not set, a suitable default for the default system D-Bus instance
144 <para><function>sd_bus_open_system_remote()</function> connects to the system bus on
145 the specified host using
146 <citerefentry project='die-net'><refentrytitle>ssh</refentrytitle><manvolnum>1</manvolnum></citerefentry>.
147 <parameter>host</parameter> consists of an optional user name followed by the
148 <literal>@</literal> symbol, and the hostname, optionally followed by a
149 <literal>:</literal> and a machine name. If the machine name is given, a connection
150 is created to the system bus in the specified container on the remote machine, and
151 otherwise a connection to the system bus on the specified host is created.</para>
153 <para>Note that entering a container is a privileged operation, and will likely only
154 work for the root user on the remote machine.</para>
156 <para><function>sd_bus_open_system_machine()</function> connects
157 to the system bus in the specified <parameter>machine</parameter>,
158 where <parameter>machine</parameter> is the name of a local
160 <citerefentry><refentrytitle>machinectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
161 for more information about the "machine" concept. Note that
162 connections into local containers are only available to privileged
163 processes at this time.</para>
165 <para>These calls allocate a bus connection object and initiate
166 the connection to a well-known bus of some form. An alternative to
167 using these high-level calls is to create an unconnected bus
169 <citerefentry><refentrytitle>sd_bus_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>
170 and to connect it with
171 <citerefentry><refentrytitle>sd_bus_start</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
177 <title>Reference ownership</title>
178 <para>The functions <function>sd_bus_open()</function>,
179 <function>sd_bus_open_user()</function>,
180 <function>sd_bus_open_system()</function>,
181 <function>sd_bus_open_system_remote()</function>, and
182 <function>sd_bus_open_system_machine()</function> return a new
183 connection object and the caller owns the sole reference. When not
184 needed anymore, this reference should be destroyed with
185 <citerefentry><refentrytitle>sd_bus_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
188 <para>The functions <function>sd_bus_default()</function>,
189 <function>sd_bus_default_user()</function> and
190 <function>sd_bus_default_system()</function> do not necessarily
191 create a new object, but increase the connection reference of an
192 existing connection object by one. Use
193 <citerefentry><refentrytitle>sd_bus_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>
194 to drop the reference.</para>
196 <para>Queued but unwritten/unread messages keep a reference to their bus connection object. For this reason, even
197 if an application dropped all references to a bus connection, it might not get destroyed right away. Until all
198 incoming queued messages are read, and until all outgoing unwritten messages are written, the bus object will stay
199 alive. <function>sd_bus_flush()</function> may be used to write all outgoing queued messages so they drop their
200 references. To flush the unread incoming messages, use <function>sd_bus_close()</function>, which will also close
201 the bus connection. When using the default bus logic, it is a good idea to first invoke
202 <function>sd_bus_flush()</function> followed by <function>sd_bus_close()</function> when a thread or process
203 terminates, and thus its bus connection object should be freed.</para>
205 <para>Normally, slot objects (as created by
206 <citerefentry><refentrytitle>sd_bus_add_match</refentrytitle><manvolnum>3</manvolnum></citerefentry> and similar
207 calls) keep a reference to their bus connection object, too. Thus, as long as a bus slot object remains referenced
208 its bus object will remain allocated too. Optionally, bus slot objects may be placed in "floating" mode. When in
209 floating mode the life cycle of the bus slot object is bound to the bus object, i.e. when the bus object is freed
210 the bus slot object is automatically unreferenced too. The floating state of a slot object may be controlled
212 <citerefentry><refentrytitle>sd_bus_slot_set_floating</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
213 though usually floating bus slot objects are created by passing <constant>NULL</constant> as the
214 <parameter>slot</parameter> parameter of <function>sd_bus_add_match()</function> and related calls, thus indicating
215 that the caller is not directly interested in referencing and managing the bus slot object.</para>
217 <para>The life cycle of the default bus connection should be the
218 responsibility of the code that creates/owns the thread the
219 default bus connection object is associated with. Library code
220 should neither call <function>sd_bus_flush()</function> nor
221 <function>sd_bus_close()</function> on default bus objects unless
222 it does so in its own private, self-allocated thread. Library code
223 should not use the default bus object in other threads unless it
224 is clear that the program using it will life cycle the bus
225 connection object and flush and close it before exiting from the
226 thread. In libraries where it is not clear that the calling
227 program will life cycle the bus connection object, it is hence
228 recommended to use <function>sd_bus_open_system()</function>
229 instead of <function>sd_bus_default_system()</function> and
230 related calls.</para>
234 <title>Return Value</title>
236 <para>On success, these calls return 0 or a positive
237 integer. On failure, these calls return a negative
238 errno-style error code.</para>
242 <title>Errors</title>
244 <para>Returned errors may indicate the following problems:</para>
249 <term><constant>-EINVAL</constant></term>
251 <listitem><para>The specified parameters are invalid.</para></listitem>
255 <term><constant>-ENOMEM</constant></term>
257 <listitem><para>Memory allocation failed.</para></listitem>
261 <term><constant>-ESOCKTNOSUPPORT</constant></term>
263 <listitem><para>The protocol version required to connect to the selected bus is not supported.</para></listitem>
267 <para>In addition, any further connection-related errors may be
268 by returned. See <citerefentry><refentrytitle>sd_bus_send</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
271 <xi:include href="libelogind-pkgconfig.xml" />
274 <title>See Also</title>
277 <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
278 <citerefentry><refentrytitle>sd-bus</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
279 <citerefentry><refentrytitle>sd_bus_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
280 <citerefentry><refentrytitle>sd_bus_ref</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
281 <citerefentry><refentrytitle>sd_bus_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
282 <citerefentry project='die-net'><refentrytitle>ssh</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
283 <citerefentry><refentrytitle>systemd-machined.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
284 <citerefentry><refentrytitle>machinectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>