man/sd_bus_default.xml

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   6   SPDX-License-Identifier: LGPL-2.1+
   7
   8   This file is part of systemd.
   9
  10   Copyright 2014 Zbigniew Jędrzejewski-Szmek
  11 -->
  12
  13 <refentry id="sd_bus_default">
  14
  15   <refentryinfo>
  16     <title>sd_bus_default</title>
  17     <productname>systemd</productname>
  18
  19     <authorgroup>
  20       <author>
  21         <contrib>A monkey with a typewriter</contrib>
  22         <firstname>Zbigniew</firstname>
  23         <surname>Jędrzejewski-Szmek</surname>
  24         <email>zbyszek@in.waw.pl</email>
  25       </author>
  26     </authorgroup>
  27   </refentryinfo>
  28
  29   <refmeta>
  30     <refentrytitle>sd_bus_default</refentrytitle>
  31     <manvolnum>3</manvolnum>
  32   </refmeta>
  33
  34   <refnamediv>
  35     <refname>sd_bus_default</refname>
  36     <refname>sd_bus_default_user</refname>
  37     <refname>sd_bus_default_system</refname>
  38
  39     <refname>sd_bus_open</refname>
  40     <refname>sd_bus_open_user</refname>
  41     <refname>sd_bus_open_system</refname>
  42     <refname>sd_bus_open_system_remote</refname>
  43     <refname>sd_bus_open_system_machine</refname>
  44
  45     <refpurpose>Acquire a connection to a system or user bus</refpurpose>
  46   </refnamediv>
  47
  48   <refsynopsisdiv>
  49     <funcsynopsis>
  50       <funcsynopsisinfo>#include &lt;systemd/sd-bus.h&gt;</funcsynopsisinfo>
  51
  52       <funcprototype>
  53         <funcdef>int <function>sd_bus_default</function></funcdef>
  54         <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
  55       </funcprototype>
  56
  57       <funcprototype>
  58         <funcdef>int <function>sd_bus_default_user</function></funcdef>
  59         <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
  60       </funcprototype>
  61
  62       <funcprototype>
  63         <funcdef>int <function>sd_bus_default_system</function></funcdef>
  64         <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
  65       </funcprototype>
  66
  67       <funcprototype>
  68         <funcdef>int <function>sd_bus_open</function></funcdef>
  69         <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
  70       </funcprototype>
  71
  72       <funcprototype>
  73         <funcdef>int <function>sd_bus_open_user</function></funcdef>
  74         <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
  75       </funcprototype>
  76
  77       <funcprototype>
  78         <funcdef>int <function>sd_bus_open_system</function></funcdef>
  79         <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
  80       </funcprototype>
  81
  82       <funcprototype>
  83         <funcdef>int <function>sd_bus_open_system_remote</function></funcdef>
  84         <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
  85         <paramdef>const char *<parameter>host</parameter></paramdef>
  86       </funcprototype>
  87
  88       <funcprototype>
  89         <funcdef>int <function>sd_bus_open_system_machine</function></funcdef>
  90         <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
  91         <paramdef>const char *<parameter>machine</parameter></paramdef>
  92       </funcprototype>
  93
  94     </funcsynopsis>
  95   </refsynopsisdiv>
  96
  97   <refsect1>
  98     <title>Description</title>
  99
 100     <para><function>sd_bus_default()</function> acquires a bus
 101     connection object to the user bus when invoked in user context, or
 102     to the system bus otherwise. The connection object is associated
 103     with the calling thread. Each time the function is invoked from
 104     the same thread, the same object is returned, but its reference
 105     count is increased by one, as long as at least one reference is
 106     kept. When the last reference to the connection is dropped (using
 107     the
 108     <citerefentry><refentrytitle>sd_bus_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 109     call), the connection is terminated. Note that the connection is
 110     not automatically terminated when the associated thread ends. It
 111     is important to drop the last reference to the bus connection
 112     explicitly before the thread ends, as otherwise, the connection will
 113     leak. Also, queued but unread or unwritten messages keep the
 114     bus referenced, see below.</para>
 115
 116     <para><function>sd_bus_default_user()</function> returns a user
 117     bus connection object associated with the calling thread.
 118     <function>sd_bus_default_system()</function> is similar, but
 119     connects to the system bus. Note that
 120     <function>sd_bus_default()</function> is identical to these two
 121     calls, depending on the execution context.</para>
 122
 123     <para><function>sd_bus_open()</function> creates a new,
 124     independent bus connection to the user bus when invoked in user
 125     context, or the system bus
 126     otherwise. <function>sd_bus_open_user()</function> is similar, but
 127     connects only to the user bus.
 128     <function>sd_bus_open_system()</function> does the same, but
 129     connects to the system bus. In contrast to
 130     <function>sd_bus_default()</function>,
 131     <function>sd_bus_default_user()</function>, and
 132     <function>sd_bus_default_system()</function>, these calls return
 133     new, independent connection objects that are not associated with
 134     the invoking thread and are not shared between multiple
 135     invocations. It is recommended to share connections per thread to
 136     efficiently make use the available resources. Thus, it is
 137     recommended to use <function>sd_bus_default()</function>,
 138     <function>sd_bus_default_user()</function> and
 139     <function>sd_bus_default_system()</function> to connect to the
 140     user or system buses.</para>
 141
 142     <para>If the <varname>$DBUS_SESSION_BUS_ADDRESS</varname> environment
 143     variable is set
 144     (cf. <citerefentry project='man-pages'><refentrytitle>environ</refentrytitle><manvolnum>7</manvolnum></citerefentry>),
 145     it will be used as the address of the user bus. This variable can
 146     contain multiple addresses separated by <literal>;</literal>. If
 147     this variable is not set, a suitable default for the default user
 148     D-Bus instance will be used.</para>
 149
 150     <para>If the <varname>$DBUS_SYSTEM_BUS_ADDRESS</varname>
 151     environment variable is set, it will be used as the address of the
 152     system bus. This variable uses the same syntax as
 153     <varname>$DBUS_SESSION_BUS_ADDRESS</varname>. If this variable is
 154     not set, a suitable default for the default system D-Bus instance
 155     will be used.</para>
 156
 157     <para><function>sd_bus_open_system_remote()</function> connects to the system bus on
 158     the specified host using
 159     <citerefentry project='die-net'><refentrytitle>ssh</refentrytitle><manvolnum>1</manvolnum></citerefentry>.
 160     <parameter>host</parameter> consists of an optional user name followed by the
 161     <literal>@</literal> symbol, and the hostname, optionally followed by a
 162     <literal>:</literal> and a machine name. If the machine name is given, a connection
 163     is created to the system bus in the specified container on the remote machine, and
 164     otherwise a connection to the system bus on the specified host is created.</para>
 165
 166     <para>Note that entering a container is a privileged operation, and will likely only
 167     work for the root user on the remote machine.</para>
 168
 169     <para><function>sd_bus_open_system_machine()</function> connects
 170     to the system bus in the specified <parameter>machine</parameter>,
 171     where <parameter>machine</parameter> is the name of a local
 172     container.  See
 173     <citerefentry><refentrytitle>machinectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 174     for more information about the "machine" concept. Note that
 175     connections into local containers are only available to privileged
 176     processes at this time.</para>
 177
 178     <para>These calls allocate a bus connection object and initiate
 179     the connection to a well-known bus of some form. An alternative to
 180     using these high-level calls is to create an unconnected bus
 181     object with
 182     <citerefentry><refentrytitle>sd_bus_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 183     and to connect it with
 184     <citerefentry><refentrytitle>sd_bus_start</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
 185     </para>
 186
 187   </refsect1>
 188
 189   <refsect1>
 190     <title>Reference ownership</title>
 191     <para>The functions <function>sd_bus_open()</function>,
 192     <function>sd_bus_open_user()</function>,
 193     <function>sd_bus_open_system()</function>,
 194     <function>sd_bus_open_system_remote()</function>, and
 195     <function>sd_bus_open_system_machine()</function> return a new
 196     connection object and the caller owns the sole reference. When not
 197     needed anymore, this reference should be destroyed with
 198     <citerefentry><refentrytitle>sd_bus_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
 199     </para>
 200
 201     <para>The functions <function>sd_bus_default()</function>,
 202     <function>sd_bus_default_user()</function> and
 203     <function>sd_bus_default_system()</function> do not necessarily
 204     create a new object, but increase the connection reference of an
 205     existing connection object by one. Use
 206     <citerefentry><refentrytitle>sd_bus_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 207     to drop the reference.</para>
 208
 209     <para>Queued but unwritten/unread messages keep a reference to their bus connection object. For this reason, even
 210     if an application dropped all references to a bus connection, it might not get destroyed right away. Until all
 211     incoming queued messages are read, and until all outgoing unwritten messages are written, the bus object will stay
 212     alive. <function>sd_bus_flush()</function> may be used to write all outgoing queued messages so they drop their
 213     references. To flush the unread incoming messages, use <function>sd_bus_close()</function>, which will also close
 214     the bus connection. When using the default bus logic, it is a good idea to first invoke
 215     <function>sd_bus_flush()</function> followed by <function>sd_bus_close()</function> when a thread or process
 216     terminates, and thus its bus connection object should be freed.</para>
 217
 218     <para>Normally, slot objects (as created by
 219     <citerefentry><refentrytitle>sd_bus_add_match</refentrytitle><manvolnum>3</manvolnum></citerefentry> and similar
 220     calls) keep a reference to their bus connection object, too. Thus, as long as a bus slot object remains referenced
 221     its bus object will remain allocated too. Optionally, bus slot objects may be placed in "floating" mode. When in
 222     floating mode the life cycle of the bus slot object is bound to the bus object, i.e. when the bus object is freed
 223     the bus slot object is automatically unreferenced too. The floating state of a slot object may be controlled
 224     explicitly with
 225     <citerefentry><refentrytitle>sd_bus_slot_set_floating</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 226     though usually floating bus slot objects are created by passing <constant>NULL</constant> as the
 227     <parameter>slot</parameter> parameter of <function>sd_bus_add_match()</function> and related calls, thus indicating
 228     that the caller is not directly interested in referencing and managing the bus slot object.</para>
 229
 230     <para>The life cycle of the default bus connection should be the
 231     responsibility of the code that creates/owns the thread the
 232     default bus connection object is associated with. Library code
 233     should neither call <function>sd_bus_flush()</function> nor
 234     <function>sd_bus_close()</function> on default bus objects unless
 235     it does so in its own private, self-allocated thread. Library code
 236     should not use the default bus object in other threads unless it
 237     is clear that the program using it will life cycle the bus
 238     connection object and flush and close it before exiting from the
 239     thread. In libraries where it is not clear that the calling
 240     program will life cycle the bus connection object, it is hence
 241     recommended to use <function>sd_bus_open_system()</function>
 242     instead of <function>sd_bus_default_system()</function> and
 243     related calls.</para>
 244   </refsect1>
 245
 246   <refsect1>
 247     <title>Return Value</title>
 248
 249     <para>On success, these calls return 0 or a positive
 250     integer. On failure, these calls return a negative
 251     errno-style error code.</para>
 252   </refsect1>
 253
 254   <refsect1>
 255     <title>Errors</title>
 256
 257     <para>Returned errors may indicate the following problems:</para>
 258
 259     <variablelist>
 260
 261       <varlistentry>
 262         <term><constant>-EINVAL</constant></term>
 263
 264         <listitem><para>The specified parameters are invalid.</para></listitem>
 265       </varlistentry>
 266
 267       <varlistentry>
 268         <term><constant>-ENOMEM</constant></term>
 269
 270         <listitem><para>Memory allocation failed.</para></listitem>
 271       </varlistentry>
 272
 273       <varlistentry>
 274         <term><constant>-ESOCKTNOSUPPORT</constant></term>
 275
 276         <listitem><para>The protocol version required to connect to the selected bus is not supported.</para></listitem>
 277       </varlistentry>
 278     </variablelist>
 279
 280     <para>In addition, any further connection-related errors may be
 281     by returned. See <citerefentry><refentrytitle>sd_bus_send</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
 282   </refsect1>
 283
 284   <refsect1>
 285     <title>Notes</title>
 286
 287     <para><function>sd_bus_open_user()</function> and the other
 288     functions described here are available as a shared library, which
 289     can be compiled and linked to with the
 290     <constant>libsystemd</constant> <citerefentry
 291     project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 292     file.</para>
 293   </refsect1>
 294
 295   <refsect1>
 296     <title>See Also</title>
 297
 298     <para>
 299       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 300       <citerefentry><refentrytitle>sd-bus</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 301       <citerefentry><refentrytitle>sd_bus_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 302       <citerefentry><refentrytitle>sd_bus_ref</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 303       <citerefentry><refentrytitle>sd_bus_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 304       <citerefentry project='die-net'><refentrytitle>ssh</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 305       <citerefentry><refentrytitle>systemd-machined.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 306       <citerefentry><refentrytitle>machinectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 307     </para>
 308   </refsect1>
 309
 310 </refentry>