man/sd_bus_add_object.xml

   1 <?xml version='1.0'?> <!--*-nxml-*-->
   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_add_object"
   7           xmlns:xi="http://www.w3.org/2001/XInclude">
   8
   9   <refentryinfo>
  10     <title>sd_bus_add_object</title>
  11     <productname>systemd</productname>
  12   </refentryinfo>
  13
  14   <refmeta>
  15     <refentrytitle>sd_bus_add_object</refentrytitle>
  16     <manvolnum>3</manvolnum>
  17   </refmeta>
  18
  19   <refnamediv>
  20     <refname>sd_bus_add_object</refname>
  21     <refname>sd_bus_add_fallback</refname>
  22     <refname>sd_bus_add_object_vtable</refname>
  23     <refname>sd_bus_add_fallback_vtable</refname>
  24     <refname>SD_BUS_VTABLE_START</refname>
  25     <refname>SD_BUS_VTABLE_END</refname>
  26     <refname>SD_BUS_METHOD_WITH_NAMES_OFFSET</refname>
  27     <refname>SD_BUS_METHOD_WITH_NAMES</refname>
  28     <refname>SD_BUS_METHOD_WITH_OFFSET</refname>
  29     <refname>SD_BUS_METHOD</refname>
  30     <refname>SD_BUS_SIGNAL_WITH_NAMES</refname>
  31     <refname>SD_BUS_SIGNAL</refname>
  32     <refname>SD_BUS_WRITABLE_PROPERTY</refname>
  33     <refname>SD_BUS_PROPERTY</refname>
  34     <refname>SD_BUS_PARAM</refname>
  35
  36     <refpurpose>Declare properties and methods for a D-Bus path</refpurpose>
  37   </refnamediv>
  38
  39   <refsynopsisdiv>
  40     <funcsynopsis>
  41       <funcsynopsisinfo>#include &lt;systemd/sd-bus-vtable.h&gt;</funcsynopsisinfo>
  42
  43       <funcprototype>
  44         <funcdef>typedef int (*<function>sd_bus_message_handler_t</function>)</funcdef>
  45         <paramdef>sd_bus_message *<parameter>m</parameter></paramdef>
  46         <paramdef>void *<parameter>userdata</parameter></paramdef>
  47         <paramdef>sd_bus_error *<parameter>ret_error</parameter></paramdef>
  48       </funcprototype>
  49
  50       <funcprototype>
  51         <funcdef>typedef int (*<function>sd_bus_property_get_t</function>)</funcdef>
  52         <paramdef>sd_bus *<parameter>bus</parameter></paramdef>
  53         <paramdef>const char *<parameter>path</parameter></paramdef>
  54         <paramdef>const char *<parameter>interface</parameter></paramdef>
  55         <paramdef>const char *<parameter>property</parameter></paramdef>
  56         <paramdef>sd_bus_message *<parameter>reply</parameter></paramdef>
  57         <paramdef>void *<parameter>userdata</parameter></paramdef>
  58         <paramdef>sd_bus_error *<parameter>ret_error</parameter></paramdef>
  59       </funcprototype>
  60
  61       <funcprototype>
  62         <funcdef>typedef int (*<function>sd_bus_property_set_t</function>)</funcdef>
  63         <paramdef>sd_bus *<parameter>bus</parameter></paramdef>
  64         <paramdef>const char *<parameter>path</parameter></paramdef>
  65         <paramdef>const char *<parameter>interface</parameter></paramdef>
  66         <paramdef>const char *<parameter>property</parameter></paramdef>
  67         <paramdef>sd_bus_message *<parameter>value</parameter></paramdef>
  68         <paramdef>void *<parameter>userdata</parameter></paramdef>
  69         <paramdef>sd_bus_error *<parameter>ret_error</parameter></paramdef>
  70       </funcprototype>
  71
  72       <funcprototype>
  73         <funcdef>typedef int (*<function>sd_bus_object_find_t</function>)</funcdef>
  74         <paramdef>const char *<parameter>path</parameter></paramdef>
  75         <paramdef>const char *<parameter>interface</parameter></paramdef>
  76         <paramdef>void *<parameter>userdata</parameter></paramdef>
  77         <paramdef>void **<parameter>ret_found</parameter></paramdef>
  78         <paramdef>sd_bus_error *<parameter>ret_error</parameter></paramdef>
  79       </funcprototype>
  80
  81       <funcprototype>
  82         <funcdef>int <function>sd_bus_add_object</function></funcdef>
  83         <paramdef>sd_bus *<parameter>bus</parameter></paramdef>
  84         <paramdef>sd_bus_slot **<parameter>slot</parameter></paramdef>
  85         <paramdef>const char *<parameter>path</parameter></paramdef>
  86         <paramdef>sd_bus_message_handler_t <parameter>callback</parameter></paramdef>
  87         <paramdef>void *<parameter>userdata</parameter></paramdef>
  88       </funcprototype>
  89
  90       <funcprototype>
  91         <funcdef>int <function>sd_bus_add_fallback</function></funcdef>
  92         <paramdef>sd_bus *<parameter>bus</parameter></paramdef>
  93         <paramdef>sd_bus_slot **<parameter>slot</parameter></paramdef>
  94         <paramdef>const char *<parameter>path</parameter></paramdef>
  95         <paramdef>sd_bus_message_handler_t <parameter>callback</parameter></paramdef>
  96         <paramdef>void *<parameter>userdata</parameter></paramdef>
  97       </funcprototype>
  98
  99       <funcprototype>
 100         <funcdef>int <function>sd_bus_add_object_vtable</function></funcdef>
 101         <paramdef>sd_bus *<parameter>bus</parameter></paramdef>
 102         <paramdef>sd_bus_slot **<parameter>slot</parameter></paramdef>
 103         <paramdef>const char *<parameter>path</parameter></paramdef>
 104         <paramdef>const char *<parameter>interface</parameter></paramdef>
 105         <paramdef>const sd_bus_vtable *<parameter>vtable</parameter></paramdef>
 106         <paramdef>void *<parameter>userdata</parameter></paramdef>
 107       </funcprototype>
 108
 109       <funcprototype>
 110         <funcdef>int <function>sd_bus_add_fallback_vtable</function></funcdef>
 111         <paramdef>sd_bus *<parameter>bus</parameter></paramdef>
 112         <paramdef>sd_bus_slot **<parameter>slot</parameter></paramdef>
 113         <paramdef>const char *<parameter>prefix</parameter></paramdef>
 114         <paramdef>const char *<parameter>interface</parameter></paramdef>
 115         <paramdef>const sd_bus_vtable *<parameter>vtable</parameter></paramdef>
 116         <paramdef>sd_bus_object_find_t <parameter>find</parameter></paramdef>
 117         <paramdef>void *<parameter>userdata</parameter></paramdef>
 118       </funcprototype>
 119
 120       <para>
 121         <constant>SD_BUS_VTABLE_START(<replaceable>flags</replaceable>)</constant>
 122       </para>
 123       <para>
 124         <constant>SD_BUS_VTABLE_END</constant>
 125       </para>
 126       <para>
 127         <constant>SD_BUS_METHOD_WITH_NAMES_OFFSET(
 128         <replaceable>member</replaceable>,
 129         <replaceable>signature</replaceable>,
 130         <replaceable>in_names</replaceable>,
 131         <replaceable>result</replaceable>,
 132         <replaceable>out_names</replaceable>,
 133         <replaceable>handler</replaceable>,
 134         <replaceable>offset</replaceable>,
 135         <replaceable>flags</replaceable>)
 136         </constant>
 137       </para>
 138       <para>
 139         <constant>SD_BUS_METHOD_WITH_NAMES(
 140         <replaceable>member</replaceable>,
 141         <replaceable>signature</replaceable>,
 142         <replaceable>in_names</replaceable>,
 143         <replaceable>result</replaceable>,
 144         <replaceable>out_names</replaceable>,
 145         <replaceable>handler</replaceable>,
 146         <replaceable>flags</replaceable>)
 147         </constant>
 148       </para>
 149       <para>
 150         <constant>SD_BUS_METHOD_WITH_OFFSET(
 151         <replaceable>member</replaceable>,
 152         <replaceable>signature</replaceable>,
 153         <replaceable>result</replaceable>,
 154         <replaceable>handler</replaceable>,
 155         <replaceable>offset</replaceable>,
 156         <replaceable>flags</replaceable>)
 157         </constant>
 158       </para>
 159       <para>
 160         <constant>SD_BUS_METHOD(
 161         <replaceable>member</replaceable>,
 162         <replaceable>signature</replaceable>,
 163         <replaceable>result</replaceable>,
 164         <replaceable>handler</replaceable>,
 165         <replaceable>flags</replaceable>)
 166         </constant>
 167       </para>
 168       <para>
 169         <constant>SD_BUS_SIGNAL_WITH_NAMES(
 170         <replaceable>member</replaceable>,
 171         <replaceable>signature</replaceable>,
 172         <replaceable>names</replaceable>,
 173         <replaceable>flags</replaceable>)
 174         </constant>
 175       </para>
 176       <para>
 177         <constant>SD_BUS_SIGNAL(
 178         <replaceable>member</replaceable>,
 179         <replaceable>signature</replaceable>,
 180         <replaceable>flags</replaceable>)
 181         </constant>
 182       </para>
 183       <para>
 184         <constant>SD_BUS_WRITABLE_PROPERTY(
 185         <replaceable>member</replaceable>,
 186         <replaceable>signature</replaceable>,
 187         <replaceable>get</replaceable>,
 188         <replaceable>set</replaceable>,
 189         <replaceable>offset</replaceable>,
 190         <replaceable>flags</replaceable>)
 191         </constant>
 192       </para>
 193       <para>
 194         <constant>SD_BUS_PROPERTY(
 195         <replaceable>member</replaceable>,
 196         <replaceable>signature</replaceable>,
 197         <replaceable>get</replaceable>,
 198         <replaceable>offset</replaceable>,
 199         <replaceable>flags</replaceable>)
 200         </constant>
 201       </para>
 202       <para>
 203         <constant>SD_BUS_PARAM(<replaceable>name</replaceable>)</constant>
 204       </para>
 205     </funcsynopsis>
 206   </refsynopsisdiv>
 207
 208   <refsect1>
 209     <title>Description</title>
 210
 211     <para><function>sd_bus_add_object_vtable()</function> is used to declare attributes for the
 212     object path <parameter>path</parameter> connected to the bus connection
 213     <parameter>bus</parameter> under the interface <parameter>interface</parameter>. The table
 214     <parameter>vtable</parameter> may contain property declarations using
 215     <constant>SD_BUS_PROPERTY()</constant> or <constant>SD_BUS_WRITABLE_PROPERTY()</constant>,
 216     method declarations using <constant>SD_BUS_METHOD()</constant>,
 217     <constant>SD_BUS_METHOD_WITH_NAMES()</constant>,
 218     <constant>SD_BUS_METHOD_WITH_OFFSET()</constant>, or
 219     <constant>SD_BUS_METHOD_WITH_NAMES_OFFSET()</constant>, and signal declarations using
 220     <constant>SD_BUS_SIGNAL_WITH_NAMES()</constant> or <constant>SD_BUS_SIGNAL()</constant>, see
 221     below. The <replaceable>userdata</replaceable> parameter contains a pointer that will be passed
 222     to various callback functions. It may be specified as <constant>NULL</constant> if no value is
 223     necessary. An interface can have any number of vtables attached to it.</para>
 224
 225     <para><function>sd_bus_add_fallback_vtable()</function> is similar to
 226     <function>sd_bus_add_object_vtable()</function>, but is used to register "fallback" attributes.
 227     When looking for an attribute declaration, bus object paths registered with
 228     <function>sd_bus_add_object_vtable()</function> are checked first. If no match is found, the
 229     fallback vtables are checked for each prefix of the bus object path, i.e. with the last
 230     slash-separated components successively removed. This allows the vtable to be used for an
 231     arbitrary number of dynamically created objects.</para>
 232
 233     <para>Parameter <replaceable>find</replaceable> is a function which is used to locate the target
 234     object based on the bus object path <replaceable>path</replaceable>. It must return
 235     <constant>1</constant> and set the <parameter>ret_found</parameter> output parameter if the
 236     object is found, return <constant>0</constant> if the object was not found, and return a
 237     negative errno-style error code or initialize the error structure
 238     <replaceable>ret_error</replaceable> on error. The pointer passed in
 239     <parameter>ret_found</parameter> will be used as the <parameter>userdata</parameter> parameter
 240     for the callback functions (offset by the <parameter>offset</parameter> offsets as specified in
 241     the vtable entries).</para>
 242
 243     <para><function>sd_bus_add_object()</function> attaches a callback directly to the object path
 244     <parameter>path</parameter>. An object path can have any number of callbacks attached to it.
 245     Each callback is prepended to the list of callbacks which are always called in order.
 246     <function>sd_bus_add_fallback()</function> is similar to
 247     <function>sd_bus_add_object()</function> but applies to fallback paths instead.</para>
 248
 249     <para>When a request is received, any associated callbacks are called sequentially until a
 250     callback returns a non-zero integer. Return zero from a callback to give other callbacks the
 251     chance to process the request. Callbacks are called in the following order: first, callbacks
 252     attached directly to the request object path are called, followed by any D-Bus method callbacks
 253     attached to the request object path, interface and member. Finally, the property callbacks
 254     attached to the request object path, interface and member are called. If the final callback
 255     returns zero, an error reply is sent back to the caller indicating no matching object for the
 256     request was found. Note that you can return a positive integer from a callback without
 257     immediately sending a reply. This informs sd-bus this callback will take responsibility for
 258     replying to the request without forcing the callback to produce a reply immediately. This allows
 259     a callback to perform any number of asynchronous operations required to construct a reply. Note
 260     that if producing a reply takes too long, the method call will time out at the caller.</para>
 261
 262     <para>If a callback was invoked to handle a request that expects a reply and the callback
 263     returns a negative value, the value is interpreted as a negative errno-style error code and sent
 264     back to the caller as a D-Bus error as if
 265     <citerefentry><refentrytitle>sd_bus_reply_method_errno</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 266     was called. Additionally, all callbacks take a <structname>sd_bus_error</structname> output
 267     parameter that can be used to provide more detailed error information. If
 268     <parameter>ret_error</parameter> is set when the callback finishes, the corresponding D-Bus
 269     error is sent back to the caller as if
 270     <citerefentry><refentrytitle>sd_bus_reply_method_error</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 271     was called. Any error stored in <parameter>ret_error</parameter> takes priority over any
 272     negative values returned by the same callback when determining which error to send back to
 273     the caller. Use
 274     <citerefentry><refentrytitle>sd_bus_error_set</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 275     or one of its variants to set <parameter>ret_error</parameter> and return a negative integer
 276     from a callback with a single function call. To send an error reply after a callback has already
 277     finished, use
 278     <citerefentry><refentrytitle>sd_bus_reply_method_errno</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 279     or one of its variants.</para>
 280
 281     <para>For all functions, a match slot is created internally. If the output parameter
 282     <replaceable>slot</replaceable> is <constant>NULL</constant>, a "floating" slot object is
 283     created, see
 284     <citerefentry><refentrytitle>sd_bus_slot_set_floating</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
 285     Otherwise, a pointer to the slot object is returned. In that case, the reference to the slot
 286     object should be dropped when the vtable is not needed anymore, see
 287     <citerefentry><refentrytitle>sd_bus_slot_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
 288     </para>
 289
 290     <refsect2>
 291       <title>The <structname>sd_bus_vtable</structname> array</title>
 292
 293       <para>The array consists of the structures of type <structname>sd_bus_vtable</structname>, but it
 294       should never be filled in manually, but through one of the following macros:</para>
 295
 296       <variablelist>
 297         <varlistentry>
 298           <term><constant>SD_BUS_VTABLE_START()</constant></term>
 299           <term><constant>SD_BUS_VTABLE_END</constant></term>
 300
 301           <listitem><para>Those must always be the first and last element.</para></listitem>
 302         </varlistentry>
 303
 304         <varlistentry>
 305           <term><constant>SD_BUS_METHOD_WITH_NAMES_OFFSET()</constant></term>
 306           <term><constant>SD_BUS_METHOD_WITH_NAMES()</constant></term>
 307           <term><constant>SD_BUS_METHOD_WITH_OFFSET()</constant></term>
 308           <term><constant>SD_BUS_METHOD()</constant></term>
 309
 310           <listitem><para>Declare a D-Bus method with the name <replaceable>member</replaceable>,
 311           parameter signature <replaceable>signature</replaceable>, result signature
 312           <replaceable>result</replaceable>. Parameters <replaceable>in_names</replaceable> and
 313           <replaceable>out_names</replaceable> specify the argument names of the input and output
 314           arguments in the function signature. The handler function
 315           <replaceable>handler</replaceable> must be of type
 316           <function>sd_bus_message_handler_t</function>. It will be called to handle the incoming
 317           messages that call this method. It receives a pointer that is the
 318           <replaceable>userdata</replaceable> parameter passed to the registration function offset
 319           by <replaceable>offset</replaceable> bytes. This may be used to pass pointers to different
 320           fields in the same data structure to different methods in the same vtable. To send a reply
 321           from <parameter>handler</parameter>, call
 322           <citerefentry><refentrytitle>sd_bus_reply_method_return</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 323           with the message the callback was invoked with. <replaceable>in_names</replaceable> and
 324           <replaceable>out_names</replaceable> should be created using the
 325           <constant>SD_BUS_PARAM()</constant> macro, see below. Parameter
 326           <replaceable>flags</replaceable> is a combination of flags, see below.</para>
 327
 328           <para><constant>SD_BUS_METHOD_WITH_NAMES()</constant>,
 329           <constant>SD_BUS_METHOD_WITH_OFFSET()</constant>, and <constant>SD_BUS_METHOD()</constant>
 330           are variants which specify zero offset (<replaceable>userdata</replaceable> parameter is
 331           passed with no change), leave the names unset (i.e. no parameter names), or both.</para>
 332           </listitem>
 333         </varlistentry>
 334
 335         <varlistentry>
 336           <term><constant>SD_BUS_SIGNAL_WITH_NAMES()</constant></term>
 337           <term><constant>SD_BUS_SIGNAL()</constant></term>
 338
 339           <listitem><para>Declare a D-Bus signal with the name <replaceable>member</replaceable>,
 340           parameter signature <replaceable>signature</replaceable>, and argument names
 341           <replaceable>names</replaceable>. <replaceable>names</replaceable> should be
 342           created using the <constant>SD_BUS_PARAM()</constant> macro, see below.
 343           Parameter <replaceable>flags</replaceable> is a combination of flags, see below.
 344           </para>
 345
 346           <para>Equivalent to <constant>SD_BUS_SIGNAL_WITH_NAMES()</constant> with the
 347           <replaceable>names</replaceable> parameter unset (i.e. no parameter names).</para>
 348           </listitem>
 349         </varlistentry>
 350
 351         <varlistentry>
 352           <term><constant>SD_BUS_WRITABLE_PROPERTY()</constant></term>
 353           <term><constant>SD_BUS_PROPERTY()</constant></term>
 354
 355           <listitem><para>Declare a D-Bus property with the name <replaceable>member</replaceable>
 356           and value signature <replaceable>signature</replaceable>. Parameters
 357           <replaceable>get</replaceable> and <replaceable>set</replaceable> are the getter and
 358           setter methods. They are called with a pointer that is the
 359           <replaceable>userdata</replaceable> parameter passed to the registration function offset
 360           by <replaceable>offset</replaceable> bytes. This may be used pass pointers to different
 361           fields in the same data structure to different setters and getters in the same vtable.
 362           Parameter <replaceable>flags</replaceable> is a combination of flags, see below.</para>
 363
 364           <para>The setter and getter methods may be omitted (specified as
 365           <constant>NULL</constant>), if the property is one of the basic types or
 366           <literal>as</literal> in case of read-only properties. In those cases, the
 367           <replaceable>userdata</replaceable> and <replaceable>offset</replaceable> parameters must
 368           together point to a valid variable of the corresponding type. A default setter and getter
 369           will be provided, which simply copy the argument between this variable and the message.
 370           </para>
 371
 372           <para><constant>SD_BUS_PROPERTY()</constant> is used to define a read-only property.
 373           </para></listitem>
 374         </varlistentry>
 375
 376         <varlistentry>
 377           <term><constant>SD_BUS_PARAM()</constant></term>
 378           <listitem><para>Parameter names should be wrapped in this macro, see the example below.
 379           </para></listitem>
 380         </varlistentry>
 381       </variablelist>
 382     </refsect2>
 383
 384     <refsect2>
 385       <title>Flags</title>
 386
 387       <para>The <replaceable>flags</replaceable> parameter is used to specify a combination of
 388       <ulink url="https://dbus.freedesktop.org/doc/dbus-specification.html#introspection-format">D-Bus annotations</ulink>.
 389       </para>
 390
 391       <variablelist>
 392         <varlistentry>
 393           <term><constant>SD_BUS_VTABLE_DEPRECATED</constant></term>
 394
 395           <listitem><para>Mark this vtable entry as deprecated using the
 396           <constant>org.freedesktop.DBus.Deprecated</constant> annotation in introspection data. If
 397           specified for <constant>SD_BUS_VTABLE_START()</constant>, the annotation is applied to the
 398           enclosing interface.</para></listitem>
 399         </varlistentry>
 400
 401         <varlistentry>
 402           <term><constant>SD_BUS_VTABLE_HIDDEN</constant></term>
 403
 404           <listitem><para>Make this vtable entry hidden. It will not be shown in introspection data.
 405           If specified for <constant>SD_BUS_VTABLE_START()</constant>, all entries in the array are
 406           hidden.</para></listitem>
 407         </varlistentry>
 408
 409         <varlistentry>
 410           <term><constant>SD_BUS_VTABLE_UNPRIVILEGED</constant></term>
 411
 412           <listitem><para>Mark this vtable entry as unprivileged. If not specified, the
 413           <constant>org.freedesktop.systemd1.Privileged</constant> annotation with value
 414           <literal>true</literal> will be shown in introspection data.</para></listitem>
 415         </varlistentry>
 416
 417         <varlistentry>
 418           <term><constant>SD_BUS_VTABLE_METHOD_NO_REPLY</constant></term>
 419
 420           <listitem><para>Mark his vtable entry as a method that will not return a reply using the
 421           <constant>org.freedesktop.DBus.Method.NoReply</constant> annotation in introspection data.
 422           </para></listitem>
 423         </varlistentry>
 424
 425         <varlistentry>
 426           <term><constant>SD_BUS_VTABLE_PROPERTY_CONST</constant></term>
 427           <term><constant>SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE</constant></term>
 428           <term><constant>SD_BUS_VTABLE_PROPERTY_EMITS_INVALIDATION</constant></term>
 429
 430           <listitem><para>Those three flags correspond to different values of the
 431           <constant>org.freedesktop.DBus.Property.EmitsChangedSignal</constant> annotation, which
 432           specifies whether the
 433           <constant>org.freedesktop.DBus.Properties.PropertiesChanged</constant> signal is emitted
 434           whenever the property changes. <constant>SD_BUS_VTABLE_PROPERTY_CONST</constant>
 435           corresponds to <constant>const</constant> and means that the property never changes during
 436           the lifetime of the object it belongs to, so no signal needs to be emitted.
 437           <constant>SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE</constant> corresponds to
 438           <constant>true</constant> and means that the signal is emitted.
 439           <constant>SD_BUS_VTABLE_PROPERTY_EMITS_INVALIDATION</constant> corresponds to
 440           <constant>invalidates</constant> and means that the signal is emitted, but the value is
 441           not included in the signal.</para></listitem>
 442         </varlistentry>
 443
 444         <varlistentry>
 445           <term><constant>SD_BUS_VTABLE_PROPERTY_EXPLICIT</constant></term>
 446
 447           <listitem><para>Mark this vtable property entry as requiring explicit request to for the
 448           value to be shown (generally because the value is large or slow to calculate). This entry
 449           cannot be combined with <constant>SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE</constant>, and will
 450           not be shown in property listings by default (e.g. <command>busctl introspect</command>).
 451           This corresponds to the <constant>org.freedesktop.systemd1.Explicit</constant> annotation
 452           in introspection data.</para></listitem>
 453         </varlistentry>
 454       </variablelist>
 455     </refsect2>
 456   </refsect1>
 457
 458   <refsect1>
 459     <title>Examples</title>
 460
 461     <example>
 462       <title>Create a simple listener on the bus</title>
 463
 464       <programlisting><xi:include href="vtable-example.c" parse="text" /></programlisting>
 465
 466       <para>This creates a simple client on the bus (the user bus, when run as normal user). We may
 467       use the D-Bus <constant>org.freedesktop.DBus.Introspectable.Introspect</constant> call to
 468       acquire the XML description of the interface:</para>
 469
 470       <programlisting><xi:include href="vtable-example.xml" parse="text" /></programlisting>
 471     </example>
 472   </refsect1>
 473
 474   <refsect1>
 475     <title>Return Value</title>
 476
 477     <para>On success, <function>sd_bus_add_object_vtable</function> and
 478     <function>sd_bus_add_fallback_vtable</function> calls return 0 or a positive integer. On
 479     failure, they return a negative errno-style error code.</para>
 480
 481     <refsect2>
 482       <title>Errors</title>
 483
 484       <para>Returned errors may indicate the following problems:</para>
 485
 486       <variablelist>
 487         <varlistentry>
 488           <term><constant>-EINVAL</constant></term>
 489
 490           <listitem><para>One of the required parameters is <constant>NULL</constant> or invalid. A
 491           reserved D-Bus interface was passed as the <replaceable>interface</replaceable> parameter.
 492           </para></listitem>
 493         </varlistentry>
 494
 495         <varlistentry>
 496           <term><constant>-ENOPKG</constant></term>
 497
 498           <listitem><para>The bus cannot be resolved.</para></listitem>
 499         </varlistentry>
 500
 501         <varlistentry>
 502           <term><constant>-ECHILD</constant></term>
 503
 504           <listitem><para>The bus was created in a different process.</para></listitem>
 505         </varlistentry>
 506
 507         <varlistentry>
 508           <term><constant>-ENOMEM</constant></term>
 509
 510           <listitem><para>Memory allocation failed.</para></listitem>
 511         </varlistentry>
 512
 513         <varlistentry>
 514           <term><constant>-EPROTOTYPE</constant></term>
 515
 516           <listitem><para><function>sd_bus_add_object_vtable</function> and
 517           <function>sd_bus_add_fallback_vtable</function> have been both called for the same bus
 518           object path, which is not allowed.</para></listitem>
 519         </varlistentry>
 520
 521         <varlistentry>
 522           <term><constant>-EEXIST</constant></term>
 523
 524           <listitem><para>This vtable has already been registered for this
 525           <replaceable>interface</replaceable> and <replaceable>path</replaceable>.
 526           </para></listitem>
 527         </varlistentry>
 528       </variablelist>
 529     </refsect2>
 530   </refsect1>
 531
 532   <xi:include href="libsystemd-pkgconfig.xml" />
 533
 534   <refsect1>
 535     <title>See Also</title>
 536
 537     <para>
 538       <citerefentry><refentrytitle>sd-bus</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 539       <citerefentry><refentrytitle>busctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 540     </para>
 541   </refsect1>
 542 </refentry>