man/sd_bus_default.xml

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   8   Copyright © 2014 Zbigniew Jędrzejewski-Szmek
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  10
  11 <refentry id="sd_bus_default" xmlns:xi="http://www.w3.org/2001/XInclude">
  12
  13   <refentryinfo>
  14     <title>sd_bus_default</title>
  15     <productname>systemd</productname>
  16   </refentryinfo>
  17
  18   <refmeta>
  19     <refentrytitle>sd_bus_default</refentrytitle>
  20     <manvolnum>3</manvolnum>
  21   </refmeta>
  22
  23   <refnamediv>
  24     <refname>sd_bus_default</refname>
  25     <refname>sd_bus_default_user</refname>
  26     <refname>sd_bus_default_system</refname>
  27
  28     <refname>sd_bus_open</refname>
  29     <refname>sd_bus_open_user</refname>
  30     <refname>sd_bus_open_system</refname>
  31     <refname>sd_bus_open_system_remote</refname>
  32     <refname>sd_bus_open_system_machine</refname>
  33
  34     <refpurpose>Acquire a connection to a system or user bus</refpurpose>
  35   </refnamediv>
  36
  37   <refsynopsisdiv>
  38     <funcsynopsis>
  39       <funcsynopsisinfo>#include &lt;systemd/sd-bus.h&gt;</funcsynopsisinfo>
  40
  41       <funcprototype>
  42         <funcdef>int <function>sd_bus_default</function></funcdef>
  43         <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
  44       </funcprototype>
  45
  46       <funcprototype>
  47         <funcdef>int <function>sd_bus_default_user</function></funcdef>
  48         <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
  49       </funcprototype>
  50
  51       <funcprototype>
  52         <funcdef>int <function>sd_bus_default_system</function></funcdef>
  53         <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
  54       </funcprototype>
  55
  56       <funcprototype>
  57         <funcdef>int <function>sd_bus_open</function></funcdef>
  58         <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
  59       </funcprototype>
  60
  61       <funcprototype>
  62         <funcdef>int <function>sd_bus_open_user</function></funcdef>
  63         <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
  64       </funcprototype>
  65
  66       <funcprototype>
  67         <funcdef>int <function>sd_bus_open_system</function></funcdef>
  68         <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
  69       </funcprototype>
  70
  71       <funcprototype>
  72         <funcdef>int <function>sd_bus_open_system_remote</function></funcdef>
  73         <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
  74         <paramdef>const char *<parameter>host</parameter></paramdef>
  75       </funcprototype>
  76
  77       <funcprototype>
  78         <funcdef>int <function>sd_bus_open_system_machine</function></funcdef>
  79         <paramdef>sd_bus **<parameter>bus</parameter></paramdef>
  80         <paramdef>const char *<parameter>machine</parameter></paramdef>
  81       </funcprototype>
  82
  83     </funcsynopsis>
  84   </refsynopsisdiv>
  85
  86   <refsect1>
  87     <title>Description</title>
  88
  89     <para><function>sd_bus_default()</function> acquires a bus
  90     connection object to the user bus when invoked in user context, or
  91     to the system bus otherwise. The connection object is associated
  92     with the calling thread. Each time the function is invoked from
  93     the same thread, the same object is returned, but its reference
  94     count is increased by one, as long as at least one reference is
  95     kept. When the last reference to the connection is dropped (using
  96     the
  97     <citerefentry><refentrytitle>sd_bus_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>
  98     call), the connection is terminated. Note that the connection is
  99     not automatically terminated when the associated thread ends. It
 100     is important to drop the last reference to the bus connection
 101     explicitly before the thread ends, as otherwise, the connection will
 102     leak. Also, queued but unread or unwritten messages keep the
 103     bus referenced, see below.</para>
 104
 105     <para><function>sd_bus_default_user()</function> returns a user
 106     bus connection object associated with the calling thread.
 107     <function>sd_bus_default_system()</function> is similar, but
 108     connects to the system bus. Note that
 109     <function>sd_bus_default()</function> is identical to these two
 110     calls, depending on the execution context.</para>
 111
 112     <para><function>sd_bus_open()</function> creates a new,
 113     independent bus connection to the user bus when invoked in user
 114     context, or the system bus
 115     otherwise. <function>sd_bus_open_user()</function> is similar, but
 116     connects only to the user bus.
 117     <function>sd_bus_open_system()</function> does the same, but
 118     connects to the system bus. In contrast to
 119     <function>sd_bus_default()</function>,
 120     <function>sd_bus_default_user()</function>, and
 121     <function>sd_bus_default_system()</function>, these calls return
 122     new, independent connection objects that are not associated with
 123     the invoking thread and are not shared between multiple
 124     invocations. It is recommended to share connections per thread to
 125     efficiently make use the available resources. Thus, it is
 126     recommended to use <function>sd_bus_default()</function>,
 127     <function>sd_bus_default_user()</function> and
 128     <function>sd_bus_default_system()</function> to connect to the
 129     user or system buses.</para>
 130
 131     <para>If the <varname>$DBUS_SESSION_BUS_ADDRESS</varname> environment
 132     variable is set
 133     (cf. <citerefentry project='man-pages'><refentrytitle>environ</refentrytitle><manvolnum>7</manvolnum></citerefentry>),
 134     it will be used as the address of the user bus. This variable can
 135     contain multiple addresses separated by <literal>;</literal>. If
 136     this variable is not set, a suitable default for the default user
 137     D-Bus instance will be used.</para>
 138
 139     <para>If the <varname>$DBUS_SYSTEM_BUS_ADDRESS</varname>
 140     environment variable is set, it will be used as the address of the
 141     system bus. This variable uses the same syntax as
 142     <varname>$DBUS_SESSION_BUS_ADDRESS</varname>. If this variable is
 143     not set, a suitable default for the default system D-Bus instance
 144     will be used.</para>
 145
 146     <para><function>sd_bus_open_system_remote()</function> connects to the system bus on
 147     the specified host using
 148     <citerefentry project='die-net'><refentrytitle>ssh</refentrytitle><manvolnum>1</manvolnum></citerefentry>.
 149     <parameter>host</parameter> consists of an optional user name followed by the
 150     <literal>@</literal> symbol, and the hostname, optionally followed by a
 151     <literal>:</literal> and a machine name. If the machine name is given, a connection
 152     is created to the system bus in the specified container on the remote machine, and
 153     otherwise a connection to the system bus on the specified host is created.</para>
 154
 155     <para>Note that entering a container is a privileged operation, and will likely only
 156     work for the root user on the remote machine.</para>
 157
 158     <para><function>sd_bus_open_system_machine()</function> connects
 159     to the system bus in the specified <parameter>machine</parameter>,
 160     where <parameter>machine</parameter> is the name of a local
 161     container.  See
 162     <citerefentry><refentrytitle>machinectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 163     for more information about the "machine" concept. Note that
 164     connections into local containers are only available to privileged
 165     processes at this time.</para>
 166
 167     <para>These calls allocate a bus connection object and initiate
 168     the connection to a well-known bus of some form. An alternative to
 169     using these high-level calls is to create an unconnected bus
 170     object with
 171     <citerefentry><refentrytitle>sd_bus_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 172     and to connect it with
 173     <citerefentry><refentrytitle>sd_bus_start</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
 174     </para>
 175
 176   </refsect1>
 177
 178   <refsect1>
 179     <title>Reference ownership</title>
 180     <para>The functions <function>sd_bus_open()</function>,
 181     <function>sd_bus_open_user()</function>,
 182     <function>sd_bus_open_system()</function>,
 183     <function>sd_bus_open_system_remote()</function>, and
 184     <function>sd_bus_open_system_machine()</function> return a new
 185     connection object and the caller owns the sole reference. When not
 186     needed anymore, this reference should be destroyed with
 187     <citerefentry><refentrytitle>sd_bus_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
 188     </para>
 189
 190     <para>The functions <function>sd_bus_default()</function>,
 191     <function>sd_bus_default_user()</function> and
 192     <function>sd_bus_default_system()</function> do not necessarily
 193     create a new object, but increase the connection reference of an
 194     existing connection object by one. Use
 195     <citerefentry><refentrytitle>sd_bus_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 196     to drop the reference.</para>
 197
 198     <para>Queued but unwritten/unread messages keep a reference to their bus connection object. For this reason, even
 199     if an application dropped all references to a bus connection, it might not get destroyed right away. Until all
 200     incoming queued messages are read, and until all outgoing unwritten messages are written, the bus object will stay
 201     alive. <function>sd_bus_flush()</function> may be used to write all outgoing queued messages so they drop their
 202     references. To flush the unread incoming messages, use <function>sd_bus_close()</function>, which will also close
 203     the bus connection. When using the default bus logic, it is a good idea to first invoke
 204     <function>sd_bus_flush()</function> followed by <function>sd_bus_close()</function> when a thread or process
 205     terminates, and thus its bus connection object should be freed.</para>
 206
 207     <para>Normally, slot objects (as created by
 208     <citerefentry><refentrytitle>sd_bus_add_match</refentrytitle><manvolnum>3</manvolnum></citerefentry> and similar
 209     calls) keep a reference to their bus connection object, too. Thus, as long as a bus slot object remains referenced
 210     its bus object will remain allocated too. Optionally, bus slot objects may be placed in "floating" mode. When in
 211     floating mode the life cycle of the bus slot object is bound to the bus object, i.e. when the bus object is freed
 212     the bus slot object is automatically unreferenced too. The floating state of a slot object may be controlled
 213     explicitly with
 214     <citerefentry><refentrytitle>sd_bus_slot_set_floating</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 215     though usually floating bus slot objects are created by passing <constant>NULL</constant> as the
 216     <parameter>slot</parameter> parameter of <function>sd_bus_add_match()</function> and related calls, thus indicating
 217     that the caller is not directly interested in referencing and managing the bus slot object.</para>
 218
 219     <para>The life cycle of the default bus connection should be the
 220     responsibility of the code that creates/owns the thread the
 221     default bus connection object is associated with. Library code
 222     should neither call <function>sd_bus_flush()</function> nor
 223     <function>sd_bus_close()</function> on default bus objects unless
 224     it does so in its own private, self-allocated thread. Library code
 225     should not use the default bus object in other threads unless it
 226     is clear that the program using it will life cycle the bus
 227     connection object and flush and close it before exiting from the
 228     thread. In libraries where it is not clear that the calling
 229     program will life cycle the bus connection object, it is hence
 230     recommended to use <function>sd_bus_open_system()</function>
 231     instead of <function>sd_bus_default_system()</function> and
 232     related calls.</para>
 233   </refsect1>
 234
 235   <refsect1>
 236     <title>Return Value</title>
 237
 238     <para>On success, these calls return 0 or a positive
 239     integer. On failure, these calls return a negative
 240     errno-style error code.</para>
 241   </refsect1>
 242
 243   <refsect1>
 244     <title>Errors</title>
 245
 246     <para>Returned errors may indicate the following problems:</para>
 247
 248     <variablelist>
 249
 250       <varlistentry>
 251         <term><constant>-EINVAL</constant></term>
 252
 253         <listitem><para>The specified parameters are invalid.</para></listitem>
 254       </varlistentry>
 255
 256       <varlistentry>
 257         <term><constant>-ENOMEM</constant></term>
 258
 259         <listitem><para>Memory allocation failed.</para></listitem>
 260       </varlistentry>
 261
 262       <varlistentry>
 263         <term><constant>-ESOCKTNOSUPPORT</constant></term>
 264
 265         <listitem><para>The protocol version required to connect to the selected bus is not supported.</para></listitem>
 266       </varlistentry>
 267     </variablelist>
 268
 269     <para>In addition, any further connection-related errors may be
 270     by returned. See <citerefentry><refentrytitle>sd_bus_send</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
 271   </refsect1>
 272
 273   <xi:include href="libsystemd-pkgconfig.xml" />
 274
 275   <refsect1>
 276     <title>See Also</title>
 277
 278     <para>
 279       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 280       <citerefentry><refentrytitle>sd-bus</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 281       <citerefentry><refentrytitle>sd_bus_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 282       <citerefentry><refentrytitle>sd_bus_ref</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 283       <citerefentry><refentrytitle>sd_bus_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 284       <citerefentry project='die-net'><refentrytitle>ssh</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 285       <citerefentry><refentrytitle>systemd-machined.service</refentrytitle><manvolnum>8</manvolnum></citerefentry>,
 286       <citerefentry><refentrytitle>machinectl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 287     </para>
 288   </refsect1>
 289
 290 </refentry>