man/sd_bus_default.xml

   1 <?xml version='1.0'?>
   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
   3   "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
   4 <!-- SPDX-License-Identifier: LGPL-2.1+ -->
   5
   6 <refentry id="sd_bus_default" xmlns:xi="http://www.w3.org/2001/XInclude">
   7
   8   <refentryinfo>
   9     <title>sd_bus_default</title>
  10     <productname>systemd</productname>
  11   </refentryinfo>
  12
  13   <refmeta>
  14     <refentrytitle>sd_bus_default</refentrytitle>
  15     <manvolnum>3</manvolnum>
  16   </refmeta>
  17
  18   <refnamediv>
  19     <refname>sd_bus_default</refname>
  20     <refname>sd_bus_default_user</refname>
  21     <refname>sd_bus_default_system</refname>
  22
  23     <refname>sd_bus_open</refname>
  24     <refname>sd_bus_open_with_description</refname>
  25     <refname>sd_bus_open_user</refname>
  26     <refname>sd_bus_open_user_with_description</refname>
  27     <refname>sd_bus_open_system</refname>
  28     <refname>sd_bus_open_system_with_description</refname>
  29     <refname>sd_bus_open_system_remote</refname>
  30     <refname>sd_bus_open_system_machine</refname>
  31
  32     <refpurpose>Acquire a connection to a system or user bus</refpurpose>
  33   </refnamediv>
  34
  35   <refsynopsisdiv>
  36     <funcsynopsis>
  37       <funcsynopsisinfo>#include &lt;systemd/sd-bus.h&gt;</funcsynopsisinfo>
  38
  39       <funcprototype>
  40         <funcdef>int <function>sd_bus_default</function></funcdef>
  41         <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
  42       </funcprototype>
  43
  44       <funcprototype>
  45         <funcdef>int <function>sd_bus_default_user</function></funcdef>
  46         <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
  47       </funcprototype>
  48
  49       <funcprototype>
  50         <funcdef>int <function>sd_bus_default_system</function></funcdef>
  51         <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
  52       </funcprototype>
  53
  54       <funcprototype>
  55         <funcdef>int <function>sd_bus_open</function></funcdef>
  56         <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
  57       </funcprototype>
  58
  59       <funcprototype>
  60         <funcdef>int <function>sd_bus_open_with_description</function></funcdef>
  61         <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
  62         <paramdef>const char *<parameter>description</parameter></paramdef>
  63       </funcprototype>
  64
  65       <funcprototype>
  66         <funcdef>int <function>sd_bus_open_user</function></funcdef>
  67         <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
  68       </funcprototype>
  69
  70       <funcprototype>
  71         <funcdef>int <function>sd_bus_open_user_with_description</function></funcdef>
  72         <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
  73         <paramdef>const char *<parameter>description</parameter></paramdef>
  74       </funcprototype>
  75
  76       <funcprototype>
  77         <funcdef>int <function>sd_bus_open_system</function></funcdef>
  78         <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
  79       </funcprototype>
  80
  81       <funcprototype>
  82         <funcdef>int <function>sd_bus_open_system_with_description</function></funcdef>
  83         <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
  84         <paramdef>const char *<parameter>description</parameter></paramdef>
  85       </funcprototype>
  86
  87       <funcprototype>
  88         <funcdef>int <function>sd_bus_open_system_remote</function></funcdef>
  89         <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
  90         <paramdef>const char *<parameter>host</parameter></paramdef>
  91       </funcprototype>
  92
  93       <funcprototype>
  94         <funcdef>int <function>sd_bus_open_system_machine</function></funcdef>
  95         <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
  96         <paramdef>const char *<parameter>machine</parameter></paramdef>
  97       </funcprototype>
  98
  99     </funcsynopsis>
 100   </refsynopsisdiv>
 101
 102   <refsect1>
 103     <title>Description</title>
 104
 105     <para><function>sd_bus_default()</function> acquires a bus
 106     connection object to the user bus when invoked in user context, or
 107     to the system bus otherwise. The connection object is associated
 108     with the calling thread. Each time the function is invoked from
 109     the same thread, the same object is returned, but its reference
 110     count is increased by one, as long as at least one reference is
 111     kept. When the last reference to the connection is dropped (using
 112     the
 113     <citerefentry><refentrytitle>sd_bus_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 114     call), the connection is terminated. Note that the connection is
 115     not automatically terminated when the associated thread ends. It
 116     is important to drop the last reference to the bus connection
 117     explicitly before the thread ends, as otherwise, the connection will
 118     leak. Also, queued but unread or unwritten messages keep the
 119     bus referenced, see below.</para>
 120
 121     <para><function>sd_bus_default_user()</function> returns a user
 122     bus connection object associated with the calling thread.
 123     <function>sd_bus_default_system()</function> is similar, but
 124     connects to the system bus. Note that
 125     <function>sd_bus_default()</function> is identical to these two
 126     calls, depending on the execution context.</para>
 127
 128     <para><function>sd_bus_open()</function> creates a new,
 129     independent bus connection to the user bus when invoked in user
 130     context, or the system bus
 131     otherwise. <function>sd_bus_open_user()</function> is similar, but
 132     connects only to the user bus.
 133     <function>sd_bus_open_system()</function> does the same, but
 134     connects to the system bus. In contrast to
 135     <function>sd_bus_default()</function>,
 136     <function>sd_bus_default_user()</function>, and
 137     <function>sd_bus_default_system()</function>, these calls return
 138     new, independent connection objects that are not associated with
 139     the invoking thread and are not shared between multiple
 140     invocations. It is recommended to share connections per thread to
 141     efficiently make use the available resources. Thus, it is
 142     recommended to use <function>sd_bus_default()</function>,
 143     <function>sd_bus_default_user()</function> and
 144     <function>sd_bus_default_system()</function> to connect to the
 145     user or system buses.</para>
 146
 147     <para><function>sd_bus_open_with_description()</function>,
 148     <function>sd_bus_open_user_with_description()</function>, and
 149     <function>sd_bus_open_system_with_description()</function> are similar to
 150     <function>sd_bus_open()</function>, <function>sd_bus_open_user()</function>, and
 151     <function>sd_bus_open_system()</function>, but allow a description string to be set, see
 152     <citerefentry><refentrytitle>sd_bus_set_description</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
 153     <parameter>description</parameter> may be <constant>NULL</constant>, in which case this function
 154     is equivalent to <function>sd_bus_open()</function>. This description string is used in log
 155     messages about the bus object, and including a "name" for the bus makes them easier to
 156     understand. Some messages are emitted during bus initialization, hence using this function is
 157     prefereable to setting the description later with
 158     <function>sd_bus_open_with_description()</function>. The argument is copied internally and will
 159     not be referenced after the function returns.</para>
 160
 161     <para>If the <varname>$DBUS_SESSION_BUS_ADDRESS</varname> environment
 162     variable is set
 163     (cf. <citerefentry project='man-pages'><refentrytitle>environ</refentrytitle><manvolnum>7</manvolnum></citerefentry>),
 164     it will be used as the address of the user bus. This variable can
 165     contain multiple addresses separated by <literal>;</literal>. If
 166     this variable is not set, a suitable default for the default user
 167     D-Bus instance will be used.</para>
 168
 169     <para>If the <varname>$DBUS_SYSTEM_BUS_ADDRESS</varname>
 170     environment variable is set, it will be used as the address of the
 171     system bus. This variable uses the same syntax as
 172     <varname>$DBUS_SESSION_BUS_ADDRESS</varname>. If this variable is
 173     not set, a suitable default for the default system D-Bus instance
 174     will be used.</para>
 175
 176     <para><function>sd_bus_open_system_remote()</function> connects to the system bus on
 177     the specified host using
 178     <citerefentry project='die-net'><refentrytitle>ssh</refentrytitle><manvolnum>1</manvolnum></citerefentry>.
 179     <parameter>host</parameter> consists of an optional user name followed by the
 180     <literal>@</literal> symbol, and the hostname, optionally followed by a
 181     <literal>:</literal> and a port, optionally followed by a
 182     <literal>/</literal> and a machine name. If the machine name is given, a connection
 183     is created to the system bus in the specified container on the remote machine, and
 184     otherwise a connection to the system bus on the specified host is created.</para>
 185
 186     <para>Note that entering a container is a privileged operation, and will likely only
 187     work for the root user on the remote machine.</para>
 188
 189     <para><function>sd_bus_open_system_machine()</function> connects
 190     to the system bus in the specified <parameter>machine</parameter>,
 191     where <parameter>machine</parameter> is the name of a local
 192     container.  See
 193     <citerefentry><refentrytitle>machinectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 194     for more information about the "machine" concept. Note that
 195     connections into local containers are only available to privileged
 196     processes at this time.</para>
 197
 198     <para>These calls allocate a bus connection object and initiate
 199     the connection to a well-known bus of some form. An alternative to
 200     using these high-level calls is to create an unconnected bus
 201     object with
 202     <citerefentry><refentrytitle>sd_bus_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 203     and to connect it with
 204     <citerefentry><refentrytitle>sd_bus_start</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
 205     </para>
 206
 207   </refsect1>
 208
 209   <refsect1>
 210     <title>Reference ownership</title>
 211     <para>The functions <function>sd_bus_open()</function>,
 212     <function>sd_bus_open_user()</function>,
 213     <function>sd_bus_open_system()</function>,
 214     <function>sd_bus_open_system_remote()</function>, and
 215     <function>sd_bus_open_system_machine()</function> return a new
 216     connection object and the caller owns the sole reference. When not
 217     needed anymore, this reference should be destroyed with
 218     <citerefentry><refentrytitle>sd_bus_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
 219     </para>
 220
 221     <para>The functions <function>sd_bus_default()</function>,
 222     <function>sd_bus_default_user()</function> and
 223     <function>sd_bus_default_system()</function> do not necessarily
 224     create a new object, but increase the connection reference of an
 225     existing connection object by one. Use
 226     <citerefentry><refentrytitle>sd_bus_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 227     to drop the reference.</para>
 228
 229     <para>Queued but unwritten/unread messages keep a reference to their bus connection object. For this reason, even
 230     if an application dropped all references to a bus connection, it might not get destroyed right away. Until all
 231     incoming queued messages are read, and until all outgoing unwritten messages are written, the bus object will stay
 232     alive. <function>sd_bus_flush()</function> may be used to write all outgoing queued messages so they drop their
 233     references. To flush the unread incoming messages, use <function>sd_bus_close()</function>, which will also close
 234     the bus connection. When using the default bus logic, it is a good idea to first invoke
 235     <function>sd_bus_flush()</function> followed by <function>sd_bus_close()</function> when a thread or process
 236     terminates, and thus its bus connection object should be freed.</para>
 237
 238     <para>Normally, slot objects (as created by
 239     <citerefentry><refentrytitle>sd_bus_add_match</refentrytitle><manvolnum>3</manvolnum></citerefentry> and similar
 240     calls) keep a reference to their bus connection object, too. Thus, as long as a bus slot object remains referenced
 241     its bus object will remain allocated too. Optionally, bus slot objects may be placed in "floating" mode. When in
 242     floating mode the life cycle of the bus slot object is bound to the bus object, i.e. when the bus object is freed
 243     the bus slot object is automatically unreferenced too. The floating state of a slot object may be controlled
 244     explicitly with
 245     <citerefentry><refentrytitle>sd_bus_slot_set_floating</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 246     though usually floating bus slot objects are created by passing <constant>NULL</constant> as the
 247     <parameter>slot</parameter> parameter of <function>sd_bus_add_match()</function> and related calls, thus indicating
 248     that the caller is not directly interested in referencing and managing the bus slot object.</para>
 249
 250     <para>The life cycle of the default bus connection should be the
 251     responsibility of the code that creates/owns the thread the
 252     default bus connection object is associated with. Library code
 253     should neither call <function>sd_bus_flush()</function> nor
 254     <function>sd_bus_close()</function> on default bus objects unless
 255     it does so in its own private, self-allocated thread. Library code
 256     should not use the default bus object in other threads unless it
 257     is clear that the program using it will life cycle the bus
 258     connection object and flush and close it before exiting from the
 259     thread. In libraries where it is not clear that the calling
 260     program will life cycle the bus connection object, it is hence
 261     recommended to use <function>sd_bus_open_system()</function>
 262     instead of <function>sd_bus_default_system()</function> and
 263     related calls.</para>
 264   </refsect1>
 265
 266   <refsect1>
 267     <title>Return Value</title>
 268
 269     <para>On success, these calls return 0 or a positive
 270     integer. On failure, these calls return a negative
 271     errno-style error code.</para>
 272
 273     <refsect2>
 274       <title>Errors</title>
 275
 276       <para>Returned errors may indicate the following problems:</para>
 277
 278       <variablelist>
 279
 280         <varlistentry>
 281           <term><constant>-EINVAL</constant></term>
 282
 283           <listitem><para>The specified parameters are invalid.</para></listitem>
 284         </varlistentry>
 285
 286         <varlistentry>
 287           <term><constant>-ENOMEM</constant></term>
 288
 289           <listitem><para>Memory allocation failed.</para></listitem>
 290         </varlistentry>
 291
 292         <varlistentry>
 293           <term><constant>-ESOCKTNOSUPPORT</constant></term>
 294
 295           <listitem><para>The protocol version required to connect to the selected bus is not
 296           supported.</para></listitem>
 297         </varlistentry>
 298       </variablelist>
 299
 300       <para>In addition, any further connection-related errors may be by returned. See
 301       <citerefentry><refentrytitle>sd_bus_send</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
 302     </refsect2>
 303   </refsect1>
 304
 305   <xi:include href="libsystemd-pkgconfig.xml" />
 306
 307   <refsect1>
 308     <title>See Also</title>
 309
 310     <para>
 311       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 312       <citerefentry><refentrytitle>sd-bus</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 313       <citerefentry><refentrytitle>sd_bus_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 314       <citerefentry><refentrytitle>sd_bus_ref</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 315       <citerefentry><refentrytitle>sd_bus_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 316       <citerefentry project='die-net'><refentrytitle>ssh</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 317       <citerefentry><refentrytitle>systemd-machined.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 318       <citerefentry><refentrytitle>machinectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 319     </para>
 320   </refsect1>
 321
 322 </refentry>