Merge pull request #15253 from DaanDeMeyer/object-vtable-error-docs

sd-bus: Add error handling info to sd_bus_add_object_vtable docs

3 files changed, 171 insertions, 97 deletions

diff --git a/man/rules/meson.build b/man/rules/meson.build
index ee8c236744..5620ba1e5b 100644
--- a/man/rules/meson.build
+++ b/man/rules/meson.build
@@ -121,7 +121,7 @@ manpages = [
    'sd_bus_match_signal',
    'sd_bus_match_signal_async'],
   ''],
- ['sd_bus_add_object_vtable',
+ ['sd_bus_add_object',
   '3',
   ['SD_BUS_METHOD',
    'SD_BUS_METHOD_WITH_NAMES',
@@ -134,7 +134,9 @@ manpages = [
    'SD_BUS_VTABLE_END',
    'SD_BUS_VTABLE_START',
    'SD_BUS_WRITABLE_PROPERTY',
-   'sd_bus_add_fallback_vtable'],
+   'sd_bus_add_fallback',
+   'sd_bus_add_fallback_vtable',
+   'sd_bus_add_object_vtable'],
   ''],
  ['sd_bus_attach_event', '3', ['sd_bus_detach_event', 'sd_bus_get_event'], ''],
  ['sd_bus_call', '3', ['sd_bus_call_async'], ''],
diff --git a/man/sd-bus.xml b/man/sd-bus.xml
index c649bc46fe..a5f493b2ce 100644
--- a/man/sd-bus.xml
+++ b/man/sd-bus.xml
@@ -41,7 +41,10 @@
 
     <para>See
     <literallayout><citerefentry><refentrytitle>sd_bus_add_match</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+<citerefentry><refentrytitle>sd_bus_add_object</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 <citerefentry><refentrytitle>sd_bus_add_object_vtable</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+<citerefentry><refentrytitle>sd_bus_add_fallback</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+<citerefentry><refentrytitle>sd_bus_add_fallback_vtable</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 <citerefentry><refentrytitle>sd_bus_attach_event</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 <citerefentry><refentrytitle>sd_bus_call</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 <citerefentry><refentrytitle>sd_bus_call_async</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
diff --git a/man/sd_bus_add_object_vtable.xml b/man/sd_bus_add_object.xml
index 9d7e30a504..2abe2342f0 100644
--- a/man/sd_bus_add_object_vtable.xml
+++ b/man/sd_bus_add_object.xml
@@ -3,20 +3,22 @@
   "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
 <!-- SPDX-License-Identifier: LGPL-2.1+ -->
 
-<refentry id="sd_bus_add_object_vtable"
+<refentry id="sd_bus_add_object"
           xmlns:xi="http://www.w3.org/2001/XInclude">
 
   <refentryinfo>
-    <title>sd_bus_add_object_vtable</title>
+    <title>sd_bus_add_object</title>
     <productname>systemd</productname>
   </refentryinfo>
 
   <refmeta>
-    <refentrytitle>sd_bus_add_object_vtable</refentrytitle>
+    <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>
@@ -77,6 +79,24 @@
       </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>
@@ -188,40 +208,82 @@
   <refsect1>
     <title>Description</title>
 
-    <para><function>sd_bus_add_object_vtable()</function> is used to declare attributes for the path 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>,
+    <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.</para>
+    <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>For both 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
+    <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
+    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>
 
@@ -245,23 +307,28 @@
           <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. <replaceable>in_names</replaceable> and <replaceable>out_names</replaceable> should be
-          created using the <constant>SD_BUS_PARAM()</constant> macro, see below. Parameter
+          <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>
+          <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>
 
@@ -285,29 +352,31 @@
           <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 has 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 valid variable of the corresponding type. A default setter and
-          getters will be provided, which simply copy the argument between this variable and the message.
+          <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>
+          <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>
+          <listitem><para>Parameter names should be wrapped in this macro, see the example below.
+          </para></listitem>
         </varlistentry>
       </variablelist>
     </refsect2>
@@ -324,7 +393,7 @@
           <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
+          <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>
@@ -332,10 +401,9 @@
         <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>
+          <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>
@@ -343,8 +411,7 @@
 
           <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>
+          <literal>true</literal> will be shown in introspection data.</para></listitem>
         </varlistentry>
 
         <varlistentry>
@@ -361,27 +428,28 @@
           <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>
+          <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>
+          <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>
@@ -395,9 +463,9 @@
 
       <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>
+      <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>
@@ -407,8 +475,8 @@
     <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>
+    <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>
@@ -419,8 +487,9 @@
         <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>
+          <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>
@@ -445,8 +514,8 @@
           <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>
+          <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>