<?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-or-later --> <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 <systemd/sd-bus.h></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>