sd-bus: Add note about sd_bus_reply_method_return to SD_BUS_METHOD docs

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+<?xml version='1.0'?> <!--*-nxml-*-->
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
+  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
+<!-- SPDX-License-Identifier: LGPL-2.1+ -->
+
+<refentry id="sd_bus_add_object"
+          xmlns:xi="http://www.w3.org/2001/XInclude">
+
+  <refentryinfo>
+    <title>sd_bus_add_object</title>
+    <productname>systemd</productname>
+  </refentryinfo>
+
+  <refmeta>
+    <refentrytitle>sd_bus_add_object</refentrytitle>
+    <manvolnum>3</manvolnum>
+  </refmeta>
+
+  <refnamediv>
+    <refname>sd_bus_add_object</refname>
+    <refname>sd_bus_add_fallback</refname>
+    <refname>sd_bus_add_object_vtable</refname>
+    <refname>sd_bus_add_fallback_vtable</refname>
+    <refname>SD_BUS_VTABLE_START</refname>
+    <refname>SD_BUS_VTABLE_END</refname>
+    <refname>SD_BUS_METHOD_WITH_NAMES_OFFSET</refname>
+    <refname>SD_BUS_METHOD_WITH_NAMES</refname>
+    <refname>SD_BUS_METHOD_WITH_OFFSET</refname>
+    <refname>SD_BUS_METHOD</refname>
+    <refname>SD_BUS_SIGNAL_WITH_NAMES</refname>
+    <refname>SD_BUS_SIGNAL</refname>
+    <refname>SD_BUS_WRITABLE_PROPERTY</refname>
+    <refname>SD_BUS_PROPERTY</refname>
+    <refname>SD_BUS_PARAM</refname>
+
+    <refpurpose>Declare properties and methods for a D-Bus path</refpurpose>
+  </refnamediv>
+
+  <refsynopsisdiv>
+    <funcsynopsis>
+      <funcsynopsisinfo>#include &lt;systemd/sd-bus-vtable.h&gt;</funcsynopsisinfo>
+
+      <funcprototype>
+        <funcdef>typedef int (*<function>sd_bus_message_handler_t</function>)</funcdef>
+        <paramdef>sd_bus_message *<parameter>m</parameter></paramdef>
+        <paramdef>void *<parameter>userdata</parameter></paramdef>
+        <paramdef>sd_bus_error *<parameter>ret_error</parameter></paramdef>
+      </funcprototype>
+
+      <funcprototype>
+        <funcdef>typedef int (*<function>sd_bus_property_get_t</function>)</funcdef>
+        <paramdef>sd_bus *<parameter>bus</parameter></paramdef>
+        <paramdef>const char *<parameter>path</parameter></paramdef>
+        <paramdef>const char *<parameter>interface</parameter></paramdef>
+        <paramdef>const char *<parameter>property</parameter></paramdef>
+        <paramdef>sd_bus_message *<parameter>reply</parameter></paramdef>
+        <paramdef>void *<parameter>userdata</parameter></paramdef>
+        <paramdef>sd_bus_error *<parameter>ret_error</parameter></paramdef>
+      </funcprototype>
+
+      <funcprototype>
+        <funcdef>typedef int (*<function>sd_bus_property_set_t</function>)</funcdef>
+        <paramdef>sd_bus *<parameter>bus</parameter></paramdef>
+        <paramdef>const char *<parameter>path</parameter></paramdef>
+        <paramdef>const char *<parameter>interface</parameter></paramdef>
+        <paramdef>const char *<parameter>property</parameter></paramdef>
+        <paramdef>sd_bus_message *<parameter>value</parameter></paramdef>
+        <paramdef>void *<parameter>userdata</parameter></paramdef>
+        <paramdef>sd_bus_error *<parameter>ret_error</parameter></paramdef>
+      </funcprototype>
+
+      <funcprototype>
+        <funcdef>typedef int (*<function>sd_bus_object_find_t</function>)</funcdef>
+        <paramdef>const char *<parameter>path</parameter></paramdef>
+        <paramdef>const char *<parameter>interface</parameter></paramdef>
+        <paramdef>void *<parameter>userdata</parameter></paramdef>
+        <paramdef>void **<parameter>ret_found</parameter></paramdef>
+        <paramdef>sd_bus_error *<parameter>ret_error</parameter></paramdef>
+      </funcprototype>
+
+      <funcprototype>
+        <funcdef>int <function>sd_bus_add_object</function></funcdef>
+        <paramdef>sd_bus *<parameter>bus</parameter></paramdef>
+        <paramdef>sd_bus_slot **<parameter>slot</parameter></paramdef>
+        <paramdef>const char *<parameter>path</parameter></paramdef>
+        <paramdef>sd_bus_message_handler_t <parameter>callback</parameter></paramdef>
+        <paramdef>void *<parameter>userdata</parameter></paramdef>
+      </funcprototype>
+
+      <funcprototype>
+        <funcdef>int <function>sd_bus_add_fallback</function></funcdef>
+        <paramdef>sd_bus *<parameter>bus</parameter></paramdef>
+        <paramdef>sd_bus_slot **<parameter>slot</parameter></paramdef>
+        <paramdef>const char *<parameter>path</parameter></paramdef>
+        <paramdef>sd_bus_message_handler_t <parameter>callback</parameter></paramdef>
+        <paramdef>void *<parameter>userdata</parameter></paramdef>
+      </funcprototype>
+
+      <funcprototype>
+        <funcdef>int <function>sd_bus_add_object_vtable</function></funcdef>
+        <paramdef>sd_bus *<parameter>bus</parameter></paramdef>
+        <paramdef>sd_bus_slot **<parameter>slot</parameter></paramdef>
+        <paramdef>const char *<parameter>path</parameter></paramdef>
+        <paramdef>const char *<parameter>interface</parameter></paramdef>
+        <paramdef>const sd_bus_vtable *<parameter>vtable</parameter></paramdef>
+        <paramdef>void *<parameter>userdata</parameter></paramdef>
+      </funcprototype>
+
+      <funcprototype>
+        <funcdef>int <function>sd_bus_add_fallback_vtable</function></funcdef>
+        <paramdef>sd_bus *<parameter>bus</parameter></paramdef>
+        <paramdef>sd_bus_slot **<parameter>slot</parameter></paramdef>
+        <paramdef>const char *<parameter>prefix</parameter></paramdef>
+        <paramdef>const char *<parameter>interface</parameter></paramdef>
+        <paramdef>const sd_bus_vtable *<parameter>vtable</parameter></paramdef>
+        <paramdef>sd_bus_object_find_t <parameter>find</parameter></paramdef>
+        <paramdef>void *<parameter>userdata</parameter></paramdef>
+      </funcprototype>
+
+      <para>
+        <constant>SD_BUS_VTABLE_START(<replaceable>flags</replaceable>)</constant>
+      </para>
+      <para>
+        <constant>SD_BUS_VTABLE_END</constant>
+      </para>
+      <para>
+        <constant>SD_BUS_METHOD_WITH_NAMES_OFFSET(
+        <replaceable>member</replaceable>,
+        <replaceable>signature</replaceable>,
+        <replaceable>in_names</replaceable>,
+        <replaceable>result</replaceable>,
+        <replaceable>out_names</replaceable>,
+        <replaceable>handler</replaceable>,
+        <replaceable>offset</replaceable>,
+        <replaceable>flags</replaceable>)
+        </constant>
+      </para>
+      <para>
+        <constant>SD_BUS_METHOD_WITH_NAMES(
+        <replaceable>member</replaceable>,
+        <replaceable>signature</replaceable>,
+        <replaceable>in_names</replaceable>,
+        <replaceable>result</replaceable>,
+        <replaceable>out_names</replaceable>,
+        <replaceable>handler</replaceable>,
+        <replaceable>flags</replaceable>)
+        </constant>
+      </para>
+      <para>
+        <constant>SD_BUS_METHOD_WITH_OFFSET(
+        <replaceable>member</replaceable>,
+        <replaceable>signature</replaceable>,
+        <replaceable>result</replaceable>,
+        <replaceable>handler</replaceable>,
+        <replaceable>offset</replaceable>,
+        <replaceable>flags</replaceable>)
+        </constant>
+      </para>
+      <para>
+        <constant>SD_BUS_METHOD(
+        <replaceable>member</replaceable>,
+        <replaceable>signature</replaceable>,
+        <replaceable>result</replaceable>,
+        <replaceable>handler</replaceable>,
+        <replaceable>flags</replaceable>)
+        </constant>
+      </para>
+      <para>
+        <constant>SD_BUS_SIGNAL_WITH_NAMES(
+        <replaceable>member</replaceable>,
+        <replaceable>signature</replaceable>,
+        <replaceable>names</replaceable>,
+        <replaceable>flags</replaceable>)
+        </constant>
+      </para>
+      <para>
+        <constant>SD_BUS_SIGNAL(
+        <replaceable>member</replaceable>,
+        <replaceable>signature</replaceable>,
+        <replaceable>flags</replaceable>)
+        </constant>
+      </para>
+      <para>
+        <constant>SD_BUS_WRITABLE_PROPERTY(
+        <replaceable>member</replaceable>,
+        <replaceable>signature</replaceable>,
+        <replaceable>get</replaceable>,
+        <replaceable>set</replaceable>,
+        <replaceable>offset</replaceable>,
+        <replaceable>flags</replaceable>)
+        </constant>
+      </para>
+      <para>
+        <constant>SD_BUS_PROPERTY(
+        <replaceable>member</replaceable>,
+        <replaceable>signature</replaceable>,
+        <replaceable>get</replaceable>,
+        <replaceable>offset</replaceable>,
+        <replaceable>flags</replaceable>)
+        </constant>
+      </para>
+      <para>
+        <constant>SD_BUS_PARAM(<replaceable>name</replaceable>)</constant>
+      </para>
+    </funcsynopsis>
+  </refsynopsisdiv>
+
+  <refsect1>
+    <title>Description</title>
+
+    <para><function>sd_bus_add_object_vtable()</function> is used to declare attributes for the
+    object path <parameter>path</parameter> connected to the bus connection
+    <parameter>bus</parameter> under the interface <parameter>interface</parameter>. The table
+    <parameter>vtable</parameter> may contain property declarations using
+    <constant>SD_BUS_PROPERTY()</constant> or <constant>SD_BUS_WRITABLE_PROPERTY()</constant>,
+    method declarations using <constant>SD_BUS_METHOD()</constant>,
+    <constant>SD_BUS_METHOD_WITH_NAMES()</constant>,
+    <constant>SD_BUS_METHOD_WITH_OFFSET()</constant>, or
+    <constant>SD_BUS_METHOD_WITH_NAMES_OFFSET()</constant>, and signal declarations using
+    <constant>SD_BUS_SIGNAL_WITH_NAMES()</constant> or <constant>SD_BUS_SIGNAL()</constant>, see
+    below. The <replaceable>userdata</replaceable> parameter contains a pointer that will be passed
+    to various callback functions. It may be specified as <constant>NULL</constant> if no value is
+    necessary. An interface can have any number of vtables attached to it.</para>
+
+    <para><function>sd_bus_add_fallback_vtable()</function> is similar to
+    <function>sd_bus_add_object_vtable()</function>, but is used to register "fallback" attributes.
+    When looking for an attribute declaration, bus object paths registered with
+    <function>sd_bus_add_object_vtable()</function> are checked first. If no match is found, the
+    fallback vtables are checked for each prefix of the bus object path, i.e. with the last
+    slash-separated components successively removed. This allows the vtable to be used for an
+    arbitrary number of dynamically created objects.</para>
+
+    <para>Parameter <replaceable>find</replaceable> is a function which is used to locate the target
+    object based on the bus object path <replaceable>path</replaceable>. It must return
+    <constant>1</constant> and set the <parameter>ret_found</parameter> output parameter if the
+    object is found, return <constant>0</constant> if the object was not found, and return a
+    negative errno-style error code or initialize the error structure
+    <replaceable>ret_error</replaceable> on error. The pointer passed in
+    <parameter>ret_found</parameter> will be used as the <parameter>userdata</parameter> parameter
+    for the callback functions (offset by the <parameter>offset</parameter> offsets as specified in
+    the vtable entries).</para>
+
+    <para><function>sd_bus_add_object()</function> attaches a callback directly to the object path
+    <parameter>path</parameter>. An object path can have any number of callbacks attached to it.
+    Each callback is prepended to the list of callbacks which are always called in order.
+    <function>sd_bus_add_fallback()</function> is similar to
+    <function>sd_bus_add_object()</function> but applies to fallback paths instead.</para>
+
+    <para>When a request is received, any associated callbacks are called sequentially until a
+    callback returns a non-zero integer. Return zero from a callback to give other callbacks the
+    chance to process the request. Callbacks are called in the following order: first, callbacks
+    attached directly to the request object path are called, followed by any D-Bus method callbacks
+    attached to the request object path, interface and member. Finally, the property callbacks
+    attached to the request object path, interface and member are called. If the final callback
+    returns zero, an error reply is sent back to the caller indicating no matching object for the
+    request was found. Note that you can return a positive integer from a callback without
+    immediately sending a reply. This informs sd-bus this callback will take responsibility for
+    replying to the request without forcing the callback to produce a reply immediately. This allows
+    a callback to perform any number of asynchronous operations required to construct a reply. Note
+    that if producing a reply takes too long, the method call will time out at the caller.</para>
+
+    <para>If a callback was invoked to handle a request that expects a reply and the callback
+    returns a negative value, the value is interpreted as a negative errno-style error code and sent
+    back to the caller as a D-Bus error as if
+    <citerefentry><refentrytitle>sd_bus_reply_method_errno</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+    was called. Additionally, all callbacks take a <structname>sd_bus_error</structname> output
+    parameter that can be used to provide more detailed error information. If
+    <parameter>ret_error</parameter> is set when the callback finishes, the corresponding D-Bus
+    error is sent back to the caller as if
+    <citerefentry><refentrytitle>sd_bus_reply_method_error</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+    was called. Any error stored in <parameter>ret_error</parameter> takes priority over any
+    negative values returned by the same callback when determining which error to send back to
+    the caller. Use
+    <citerefentry><refentrytitle>sd_bus_error_set</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+    or one of its variants to set <parameter>ret_error</parameter> and return a negative integer
+    from a callback with a single function call. To send an error reply after a callback has already
+    finished, use
+    <citerefentry><refentrytitle>sd_bus_reply_method_errno</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+    or one of its variants.</para>
+
+    <para>For all functions, a match slot is created internally. If the output parameter
+    <replaceable>slot</replaceable> is <constant>NULL</constant>, a "floating" slot object is
+    created, see
+    <citerefentry><refentrytitle>sd_bus_slot_set_floating</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
+    Otherwise, a pointer to the slot object is returned. In that case, the reference to the slot
+    object should be dropped when the vtable is not needed anymore, see
+    <citerefentry><refentrytitle>sd_bus_slot_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
+    </para>
+
+    <refsect2>
+      <title>The <structname>sd_bus_vtable</structname> array</title>
+
+      <para>The array consists of the structures of type <structname>sd_bus_vtable</structname>, but it
+      should never be filled in manually, but through one of the following macros:</para>
+
+      <variablelist>
+        <varlistentry>
+          <term><constant>SD_BUS_VTABLE_START()</constant></term>
+          <term><constant>SD_BUS_VTABLE_END</constant></term>
+
+          <listitem><para>Those must always be the first and last element.</para></listitem>
+        </varlistentry>
+
+        <varlistentry>
+          <term><constant>SD_BUS_METHOD_WITH_NAMES_OFFSET()</constant></term>
+          <term><constant>SD_BUS_METHOD_WITH_NAMES()</constant></term>
+          <term><constant>SD_BUS_METHOD_WITH_OFFSET()</constant></term>
+          <term><constant>SD_BUS_METHOD()</constant></term>
+
+          <listitem><para>Declare a D-Bus method with the name <replaceable>member</replaceable>,
+          parameter signature <replaceable>signature</replaceable>, result signature
+          <replaceable>result</replaceable>. Parameters <replaceable>in_names</replaceable> and
+          <replaceable>out_names</replaceable> specify the argument names of the input and output
+          arguments in the function signature. The handler function
+          <replaceable>handler</replaceable> must be of type
+          <function>sd_bus_message_handler_t</function>. It will be called to handle the incoming
+          messages that call this method. It receives a pointer that is the
+          <replaceable>userdata</replaceable> parameter passed to the registration function offset
+          by <replaceable>offset</replaceable> bytes. This may be used to pass pointers to different
+          fields in the same data structure to different methods in the same vtable. To send a reply
+          from <parameter>handler</parameter>, call
+          <citerefentry><refentrytitle>sd_bus_reply_method_return</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+          with the message the callback was invoked with. <replaceable>in_names</replaceable> and
+          <replaceable>out_names</replaceable> should be created using the
+          <constant>SD_BUS_PARAM()</constant> macro, see below. Parameter
+          <replaceable>flags</replaceable> is a combination of flags, see below.</para>
+
+          <para><constant>SD_BUS_METHOD_WITH_NAMES()</constant>,
+          <constant>SD_BUS_METHOD_WITH_OFFSET()</constant>, and <constant>SD_BUS_METHOD()</constant>
+          are variants which specify zero offset (<replaceable>userdata</replaceable> parameter is
+          passed with no change), leave the names unset (i.e. no parameter names), or both.</para>
+          </listitem>
+        </varlistentry>
+
+        <varlistentry>
+          <term><constant>SD_BUS_SIGNAL_WITH_NAMES()</constant></term>
+          <term><constant>SD_BUS_SIGNAL()</constant></term>
+
+          <listitem><para>Declare a D-Bus signal with the name <replaceable>member</replaceable>,
+          parameter signature <replaceable>signature</replaceable>, and argument names
+          <replaceable>names</replaceable>. <replaceable>names</replaceable> should be
+          created using the <constant>SD_BUS_PARAM()</constant> macro, see below.
+          Parameter <replaceable>flags</replaceable> is a combination of flags, see below.
+          </para>
+
+          <para>Equivalent to <constant>SD_BUS_SIGNAL_WITH_NAMES()</constant> with the
+          <replaceable>names</replaceable> parameter unset (i.e. no parameter names).</para>
+          </listitem>
+        </varlistentry>
+
+        <varlistentry>
+          <term><constant>SD_BUS_WRITABLE_PROPERTY()</constant></term>
+          <term><constant>SD_BUS_PROPERTY()</constant></term>
+
+          <listitem><para>Declare a D-Bus property with the name <replaceable>member</replaceable>
+          and value signature <replaceable>signature</replaceable>. Parameters
+          <replaceable>get</replaceable> and <replaceable>set</replaceable> are the getter and
+          setter methods. They are called with a pointer that is the
+          <replaceable>userdata</replaceable> parameter passed to the registration function offset
+          by <replaceable>offset</replaceable> bytes. This may be used pass pointers to different
+          fields in the same data structure to different setters and getters in the same vtable.
+          Parameter <replaceable>flags</replaceable> is a combination of flags, see below.</para>
+
+          <para>The setter and getter methods may be omitted (specified as
+          <constant>NULL</constant>), if the property is one of the basic types or
+          <literal>as</literal> in case of read-only properties. In those cases, the
+          <replaceable>userdata</replaceable> and <replaceable>offset</replaceable> parameters must
+          together point to a valid variable of the corresponding type. A default setter and getter
+          will be provided, which simply copy the argument between this variable and the message.
+          </para>
+
+          <para><constant>SD_BUS_PROPERTY()</constant> is used to define a read-only property.
+          </para></listitem>
+        </varlistentry>
+
+        <varlistentry>
+          <term><constant>SD_BUS_PARAM()</constant></term>
+          <listitem><para>Parameter names should be wrapped in this macro, see the example below.
+          </para></listitem>
+        </varlistentry>
+      </variablelist>
+    </refsect2>
+
+    <refsect2>
+      <title>Flags</title>
+
+      <para>The <replaceable>flags</replaceable> parameter is used to specify a combination of
+      <ulink url="https://dbus.freedesktop.org/doc/dbus-specification.html#introspection-format">D-Bus annotations</ulink>.
+      </para>
+
+      <variablelist>
+        <varlistentry>
+          <term><constant>SD_BUS_VTABLE_DEPRECATED</constant></term>
+
+          <listitem><para>Mark this vtable entry as deprecated using the
+          <constant>org.freedesktop.DBus.Deprecated</constant> annotation in introspection data. If
+          specified for <constant>SD_BUS_VTABLE_START()</constant>, the annotation is applied to the
+          enclosing interface.</para></listitem>
+        </varlistentry>
+
+        <varlistentry>
+          <term><constant>SD_BUS_VTABLE_HIDDEN</constant></term>
+
+          <listitem><para>Make this vtable entry hidden. It will not be shown in introspection data.
+          If specified for <constant>SD_BUS_VTABLE_START()</constant>, all entries in the array are
+          hidden.</para></listitem>
+        </varlistentry>
+
+        <varlistentry>
+          <term><constant>SD_BUS_VTABLE_UNPRIVILEGED</constant></term>
+
+          <listitem><para>Mark this vtable entry as unprivileged. If not specified, the
+          <constant>org.freedesktop.systemd1.Privileged</constant> annotation with value
+          <literal>true</literal> will be shown in introspection data.</para></listitem>
+        </varlistentry>
+
+        <varlistentry>
+          <term><constant>SD_BUS_VTABLE_METHOD_NO_REPLY</constant></term>
+
+          <listitem><para>Mark his vtable entry as a method that will not return a reply using the
+          <constant>org.freedesktop.DBus.Method.NoReply</constant> annotation in introspection data.
+          </para></listitem>
+        </varlistentry>
+
+        <varlistentry>
+          <term><constant>SD_BUS_VTABLE_PROPERTY_CONST</constant></term>
+          <term><constant>SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE</constant></term>
+          <term><constant>SD_BUS_VTABLE_PROPERTY_EMITS_INVALIDATION</constant></term>
+
+          <listitem><para>Those three flags correspond to different values of the
+          <constant>org.freedesktop.DBus.Property.EmitsChangedSignal</constant> annotation, which
+          specifies whether the
+          <constant>org.freedesktop.DBus.Properties.PropertiesChanged</constant> signal is emitted
+          whenever the property changes. <constant>SD_BUS_VTABLE_PROPERTY_CONST</constant>
+          corresponds to <constant>const</constant> and means that the property never changes during
+          the lifetime of the object it belongs to, so no signal needs to be emitted.
+          <constant>SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE</constant> corresponds to
+          <constant>true</constant> and means that the signal is emitted.
+          <constant>SD_BUS_VTABLE_PROPERTY_EMITS_INVALIDATION</constant> corresponds to
+          <constant>invalidates</constant> and means that the signal is emitted, but the value is
+          not included in the signal.</para></listitem>
+        </varlistentry>
+
+        <varlistentry>
+          <term><constant>SD_BUS_VTABLE_PROPERTY_EXPLICIT</constant></term>
+
+          <listitem><para>Mark this vtable property entry as requiring explicit request to for the
+          value to be shown (generally because the value is large or slow to calculate). This entry
+          cannot be combined with <constant>SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE</constant>, and will
+          not be shown in property listings by default (e.g. <command>busctl introspect</command>).
+          This corresponds to the <constant>org.freedesktop.systemd1.Explicit</constant> annotation
+          in introspection data.</para></listitem>
+        </varlistentry>
+      </variablelist>
+    </refsect2>
+  </refsect1>
+
+  <refsect1>
+    <title>Examples</title>
+
+    <example>
+      <title>Create a simple listener on the bus</title>
+
+      <programlisting><xi:include href="vtable-example.c" parse="text" /></programlisting>
+
+      <para>This creates a simple client on the bus (the user bus, when run as normal user). We may
+      use the D-Bus <constant>org.freedesktop.DBus.Introspectable.Introspect</constant> call to
+      acquire the XML description of the interface:</para>
+
+      <programlisting><xi:include href="vtable-example.xml" parse="text" /></programlisting>
+    </example>
+  </refsect1>
+
+  <refsect1>
+    <title>Return Value</title>
+
+    <para>On success, <function>sd_bus_add_object_vtable</function> and
+    <function>sd_bus_add_fallback_vtable</function> calls return 0 or a positive integer. On
+    failure, they return a negative errno-style error code.</para>
+
+    <refsect2>
+      <title>Errors</title>
+
+      <para>Returned errors may indicate the following problems:</para>
+
+      <variablelist>
+        <varlistentry>
+          <term><constant>-EINVAL</constant></term>
+
+          <listitem><para>One of the required parameters is <constant>NULL</constant> or invalid. A
+          reserved D-Bus interface was passed as the <replaceable>interface</replaceable> parameter.
+          </para></listitem>
+        </varlistentry>
+
+        <varlistentry>
+          <term><constant>-ENOPKG</constant></term>
+
+          <listitem><para>The bus cannot be resolved.</para></listitem>
+        </varlistentry>
+
+        <varlistentry>
+          <term><constant>-ECHILD</constant></term>
+
+          <listitem><para>The bus was created in a different process.</para></listitem>
+        </varlistentry>
+
+        <varlistentry>
+          <term><constant>-ENOMEM</constant></term>
+
+          <listitem><para>Memory allocation failed.</para></listitem>
+        </varlistentry>
+
+        <varlistentry>
+          <term><constant>-EPROTOTYPE</constant></term>
+
+          <listitem><para><function>sd_bus_add_object_vtable</function> and
+          <function>sd_bus_add_fallback_vtable</function> have been both called for the same bus
+          object path, which is not allowed.</para></listitem>
+        </varlistentry>
+
+        <varlistentry>
+          <term><constant>-EEXIST</constant></term>
+
+          <listitem><para>This vtable has already been registered for this
+          <replaceable>interface</replaceable> and <replaceable>path</replaceable>.
+          </para></listitem>
+        </varlistentry>
+      </variablelist>
+    </refsect2>
+  </refsect1>
+
+  <xi:include href="libsystemd-pkgconfig.xml" />
+
+  <refsect1>
+    <title>See Also</title>
+
+    <para>
+      <citerefentry><refentrytitle>sd-bus</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+      <citerefentry><refentrytitle>busctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+    </para>
+  </refsect1>
+</refentry>