1<?xml version='1.0'?>
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-or-later -->
5
6<refentry id="sd_bus_slot_set_userdata" xmlns:xi="http://www.w3.org/2001/XInclude">
7
8  <refentryinfo>
9    <title>sd_bus_slot_set_userdata</title>
10    <productname>systemd</productname>
11  </refentryinfo>
12
13  <refmeta>
14    <refentrytitle>sd_bus_slot_set_userdata</refentrytitle>
15    <manvolnum>3</manvolnum>
16  </refmeta>
17
18  <refnamediv>
19    <refname>sd_bus_slot_set_userdata</refname>
20    <refname>sd_bus_slot_get_userdata</refname>
21
22    <refpurpose>Set and query the value in the "userdata" field</refpurpose>
23  </refnamediv>
24
25  <refsynopsisdiv>
26    <funcsynopsis>
27      <funcsynopsisinfo>#include &lt;systemd/sd-bus.h&gt;</funcsynopsisinfo>
28
29      <funcprototype>
30        <funcdef>void* <function>sd_bus_slot_set_userdata</function></funcdef>
31        <paramdef>sd_bus_slot* <parameter>slot</parameter></paramdef>
32        <paramdef>void* <parameter>userdata</parameter></paramdef>
33      </funcprototype>
34
35      <funcprototype>
36        <funcdef>void* <function>sd_bus_slot_get_userdata</function></funcdef>
37        <paramdef>sd_bus_slot* <parameter>slot</parameter></paramdef>
38      </funcprototype>
39
40    </funcsynopsis>
41  </refsynopsisdiv>
42
43  <refsect1>
44    <title>Description</title>
45
46    <para>The userdata pointer allows data to be passed between the point where a callback is
47    registered, for example when a filter is added using
48    <citerefentry><refentrytitle>sd_bus_add_filter</refentrytitle><manvolnum>3</manvolnum></citerefentry>
49    or an asynchronous function call is made using
50    <citerefentry><refentrytitle>sd_bus_call_async</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
51    and the point where the callback is called, without having any global state. The pointer has
52    type <type>void*</type> and is not used by the sd-bus functions in any way, except to pass to
53    the callback function.</para>
54
55    <para>Usually, the userdata field is set when the slot object is initially
56    registered. <function>sd_bus_slot_set_userdata()</function> may be used to change it later for
57    the bus slot object <parameter>slot</parameter>. Previous value of the field is returned. The
58    argument and returned value may be <constant>NULL</constant>. It will be passed as the
59    <parameter>userdata</parameter> argument to the callback function attached to the slot.</para>
60
61    <para><function>sd_bus_slot_set_userdata()</function> gets the value of the userdata field in
62    the bus slot object <parameter>slot</parameter>.</para>
63  </refsect1>
64
65  <refsect1>
66    <title>Return Value</title>
67
68    <para>On success, these functions return the value of the userdata field before the function
69    call. If the <parameter>slot</parameter> object is <constant>NULL</constant>,
70    <constant>NULL</constant> will be returned to signify an error, but this is not distinguishable
71    from the userdata field value being <constant>NULL</constant>.</para>
72  </refsect1>
73
74  <xi:include href="libsystemd-pkgconfig.xml" />
75
76  <refsect1>
77    <title>See Also</title>
78
79    <para>
80      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
81      <citerefentry><refentrytitle>sd-bus</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
82      <citerefentry><refentrytitle>sd_bus_slot_set_destroy_callback</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
83      <citerefentry><refentrytitle>sd_bus_add_match</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
84      <citerefentry><refentrytitle>sd_bus_slot_get_current_userdata</refentrytitle><manvolnum>3</manvolnum></citerefentry>
85    </para>
86  </refsect1>
87
88</refentry>
89