man/sd_bus_default.xml

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