<?xml version='1.0'?>
<!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_slot_set_userdata" xmlns:xi="http://www.w3.org/2001/XInclude">

  <refentryinfo>
    <title>sd_bus_slot_set_userdata</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>sd_bus_slot_set_userdata</refentrytitle>
    <manvolnum>3</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>sd_bus_slot_set_userdata</refname>
    <refname>sd_bus_slot_get_userdata</refname>

    <refpurpose>Set and query the value in the "userdata" field</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <funcsynopsis>
      <funcsynopsisinfo>#include &lt;systemd/sd-bus.h&gt;</funcsynopsisinfo>

      <funcprototype>
        <funcdef>void* <function>sd_bus_slot_set_userdata</function></funcdef>
        <paramdef>sd_bus_slot* <parameter>slot</parameter></paramdef>
        <paramdef>void* <parameter>userdata</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>void* <function>sd_bus_slot_get_userdata</function></funcdef>
        <paramdef>sd_bus_slot* <parameter>slot</parameter></paramdef>
      </funcprototype>

    </funcsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para>The userdata pointer allows data to be passed between the point where a callback is
    registered, for example when a filter is added using
    <citerefentry><refentrytitle>sd_bus_add_filter</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    or an asynchronous function call is made using
    <citerefentry><refentrytitle>sd_bus_call_async</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
    and the point where the callback is called, without having any global state. The pointer has
    type <type>void*</type> and is not used by the sd-bus functions in any way, except to pass to
    the callback function.</para>

    <para>Usually, the userdata field is set when the slot object is initially
    registered. <function>sd_bus_slot_set_userdata()</function> may be used to change it later for
    the bus slot object <parameter>slot</parameter>. Previous value of the field is returned. The
    argument and returned value may be <constant>NULL</constant>. It will be passed as the
    <parameter>userdata</parameter> argument to the callback function attached to the slot.</para>

    <para><function>sd_bus_slot_set_userdata()</function> gets the value of the userdata field in
    the bus slot object <parameter>slot</parameter>.</para>
  </refsect1>

  <refsect1>
    <title>Return Value</title>

    <para>On success, these functions return the value of the userdata field before the function
    call. If the <parameter>slot</parameter> object is <constant>NULL</constant>,
    <constant>NULL</constant> will be returned to signify an error, but this is not distinguishable
    from the userdata field value being <constant>NULL</constant>.</para>
  </refsect1>

  <xi:include href="libsystemd-pkgconfig.xml" />

  <refsect1>
    <title>See Also</title>

    <para>
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd-bus</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_bus_slot_set_destroy_callback</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_bus_add_match</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_bus_slot_get_current_userdata</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>
