<?xml version='1.0'?> <!--*- Mode: nxml; nxml-child-indent: 2; indent-tabs-mode: nil -*--> <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN" "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> <!-- This file is part of systemd. Copyright 2015 Zbigniew Jędrzejewski-Szmek systemd is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2.1 of the License, or (at your option) any later version. systemd is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details. You should have received a copy of the GNU Lesser General Public License along with systemd; If not, see <http://www.gnu.org/licenses/>. --> <refentry id="sd_event_wait" xmlns:xi="http://www.w3.org/2001/XInclude"> <refentryinfo> <title>sd_event_wait</title> <productname>systemd</productname> <authorgroup> <author> <contrib>Developer</contrib> <firstname>Tom</firstname> <surname>Gundersen</surname> <email>teg@jklm.no</email> </author> </authorgroup> </refentryinfo> <refmeta> <refentrytitle>sd_event_wait</refentrytitle> <manvolnum>3</manvolnum> </refmeta> <refnamediv> <refname>sd_event_wait</refname> <refname>sd_event_prepare</refname> <refname>sd_event_dispatch</refname> <refname>sd_event_get_state</refname> <refname>sd_event_get_iteration</refname> <refname>SD_EVENT_INITIAL</refname> <refname>SD_EVENT_PREPARING</refname> <refname>SD_EVENT_ARMED</refname> <refname>SD_EVENT_PENDING</refname> <refname>SD_EVENT_RUNNING</refname> <refname>SD_EVENT_EXITING</refname> <refname>SD_EVENT_FINISHED</refname> <refpurpose>Low-level event loop operations</refpurpose> </refnamediv> <refsynopsisdiv> <funcsynopsis> <funcsynopsisinfo>#include <systemd/sd-event.h></funcsynopsisinfo> <funcsynopsisinfo><token>enum</token> { <constant>SD_EVENT_INITIAL</constant>, <constant>SD_EVENT_PREPARING</constant>, <constant>SD_EVENT_ARMED</constant>, <constant>SD_EVENT_PENDING</constant>, <constant>SD_EVENT_RUNNING</constant>, <constant>SD_EVENT_EXITING</constant>, <constant>SD_EVENT_FINISHED</constant>, };</funcsynopsisinfo> <funcprototype> <funcdef>int <function>sd_event_prepare</function></funcdef> <paramdef>sd_event *<parameter>event</parameter></paramdef> </funcprototype> <funcprototype> <funcdef>int <function>sd_event_wait</function></funcdef> <paramdef>sd_event *<parameter>event</parameter></paramdef> <paramdef>uint64_t <parameter>usec</parameter></paramdef> </funcprototype> <funcprototype> <funcdef>int <function>sd_event_dispatch</function></funcdef> <paramdef>sd_event *<parameter>event</parameter></paramdef> </funcprototype> <funcprototype> <funcdef>int <function>sd_event_get_state</function></funcdef> <paramdef>sd_event *<parameter>event</parameter></paramdef> </funcprototype> <funcprototype> <funcdef>int <function>sd_event_get_iteration</function></funcdef> <paramdef>sd_event *<parameter>event</parameter></paramdef> <paramdef>uint64_t *<parameter>ret</parameter></paramdef> </funcprototype> </funcsynopsis> </refsynopsisdiv> <refsect1> <title>Description</title> <para>The low-level <function>sd_event_prepare()</function>, <function>sd_event_wait()</function> and <function>sd_event_dispatch()</function> functions may be used to execute specific phases of an event loop. See <citerefentry><refentrytitle>sd_event_run</refentrytitle><manvolnum>3</manvolnum></citerefentry> and <citerefentry><refentrytitle>sd_event_loop</refentrytitle><manvolnum>3</manvolnum></citerefentry> for higher-level functions that execute individual but complete iterations of an event loop or run it continuously.</para> <para><function>sd_event_prepare()</function> checks for pending events and arms necessary timers. If any events are ready to be processed ("pending"), it returns a positive, non-zero value, and the caller should process these events with <function>sd_event_dispatch()</function>.</para> <para><function>sd_event_dispatch()</function> dispatches the highest priority event source that has a pending event. On success, <function>sd_event_dispatch()</function> returns either zero, which indicates that no further event sources may be dispatched and exiting of the event loop was requested via <citerefentry><refentrytitle>sd_event_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry>; or a positive non-zero value, which means that an event source was dispatched and the loop returned to its initial state, and the caller should initiate the next event loop iteration by invoking <function>sd_event_prepare()</function> again.</para> <para>In case <function>sd_event_prepare()</function> returned zero, <function>sd_event_wait()</function> should be called to wait for further events or a timeout. If any events are ready to be processed, it returns a positive, non-zero value, and the events should be dispatched with <function>sd_event_dispatch()</function>. Otherwise, the event loop returned to its initial state and the next event loop iteration should be initiated by invoking <function>sd_event_prepare()</function> again.</para> <para><function>sd_event_get_state()</function> may be used to determine the state the event loop is currently in. It returns one of the states described below.</para> <para><function>sd_event_get_iteration()</function> may be used to determine the current iteration of the event loop. It returns an unsigned 64bit integer containing a counter that increases monotonically with each iteration of the event loop, starting with 0. The counter is increased at the time of the <function>sd_event_prepare()</function> invocation.</para> <para>All five functions take, as the first argument, the event loop object <parameter>event</parameter> that has been created with <function>sd_event_new()</function>. The timeout for <function>sd_event_wait()</function> is specified in <parameter>usec</parameter> in microseconds. <constant>(uint64_t) -1</constant> may be used to specify an infinite timeout.</para> </refsect1> <refsect1> <title>State Machine</title> <para>The event loop knows the following states, that may be queried with <function>sd_event_get_state()</function>.</para> <variablelist> <varlistentry> <term><constant>SD_EVENT_INITIAL</constant></term> <listitem><para>The initial state the event loop is in, before each event loop iteration. Use <function>sd_event_prepare()</function> to transition the event loop into the <constant>SD_EVENT_ARMED</constant> or <constant>SD_EVENT_PENDING</constant> states.</para></listitem> </varlistentry> <varlistentry> <term><constant>SD_EVENT_PREPARING</constant></term> <listitem><para>An event source is currently being prepared, i.e. the preparation handler is currently being executed, as set with <citerefentry><refentrytitle>sd_event_set_prepare</refentrytitle><manvolnum>3</manvolnum></citerefentry>. This state is only seen in the event source preparation handler that is invoked from the <function>sd_event_prepare()</function> call and is immediately followed by <constant>SD_EVENT_ARMED</constant> or <constant>SD_EVENT_PENDING</constant>.</para></listitem> </varlistentry> <varlistentry> <term><constant>SD_EVENT_ARMED</constant></term> <listitem><para><function>sd_event_prepare()</function> has been called and no event sources were ready to be dispatched. Use <function>sd_event_wait()</function> to wait for new events, and transition into <constant>SD_EVENT_PENDING</constant> or back into <constant>SD_EVENT_INITIAL</constant>.</para></listitem> </varlistentry> <varlistentry> <term><constant>SD_EVENT_PENDING</constant></term> <listitem><para><function>sd_event_prepare()</function> or <function>sd_event_wait()</function> have been called and there were event sources with events pending. Use <function>sd_event_dispatch()</function> to dispatch the highest priority event source and transition back to <constant>SD_EVENT_INITIAL</constant>, or <constant>SD_EVENT_FINISHED</constant>.</para></listitem> </varlistentry> <varlistentry> <term><constant>SD_EVENT_RUNNING</constant></term> <listitem><para>A regular event source is currently being dispatched. This state is only seen in the event source handler that is invoked from the <function>sd_event_dispatch()</function> call, and is immediately followed by <constant>SD_EVENT_INITIAL</constant> or <constant>SD_EVENT_FINISHED</constant> as soon the event source handler returns. Note that during dispatching of exit event sources the <constant>SD_EVENT_EXITING</constant> state is seen instead.</para></listitem> </varlistentry> <varlistentry> <term><constant>SD_EVENT_EXITING</constant></term> <listitem><para>Similar to <constant>SD_EVENT_RUNNING</constant> but is the state in effect while dispatching exit event sources. It is followed by <constant>SD_EVENT_INITIAL</constant> or <constant>SD_EVENT_FINISHED</constant> as soon as the event handler returns.</para></listitem> </varlistentry> <varlistentry> <term><constant>SD_EVENT_FINISHED</constant></term> <listitem><para>The event loop has exited. All exit event sources have run. If the event loop is in this state it serves no purpose anymore, and should be freed.</para></listitem> </varlistentry> </variablelist> <para>A simplified flow chart of the states and the calls to transition between them is shown below. Note that <constant>SD_EVENT_PREPARING</constant>, <constant>SD_EVENT_RUNNING</constant> and <constant>SD_EVENT_EXITING</constant> are not shown here.</para> <programlisting> INITIAL -<---<---<---<---<---<---<---<---<---<---<---<---\ | | | ^ | | v ret == 0 | sd_event_prepare() >--->--->--->--->- ARMED | | | ^ | ret > 0 | | | | | v v ret == 0 | PENDING <---<---<---<---<---< sd_event_wait() >--->--->--+ | ret > 0 ^ | | | | v | sd_event_dispatch() >--->--->--->--->--->--->--->--->--->--->/ | ret > 0 | ret == 0 | v FINISHED </programlisting> </refsect1> <refsect1> <title>Return Value</title> <para>On success, these functions return 0 or a positive integer. On failure, they return a negative errno-style error code. In case of <function>sd_event_prepare()</function> and <function>sd_event_wait()</function>, a positive, non-zero return code indicates that events are ready to be processed and zero indicates that no events are ready. In case of <function>sd_event_dispatch()</function>, a positive, non-zero return code indicates that the event loop returned to its initial state and zero indicates the event loop has exited. <function>sd_event_get_state()</function> returns a positive or zero state on success.</para> </refsect1> <refsect1> <title>Errors</title> <para>Returned errors may indicate the following problems:</para> <variablelist> <varlistentry> <term><constant>-EINVAL</constant></term> <listitem><para>The <parameter>event</parameter> parameter is invalid or <constant>NULL</constant>.</para></listitem> </varlistentry> <varlistentry> <term><constant>-EBUSY</constant></term> <listitem><para>The event loop object is not in the right state.</para></listitem> </varlistentry> <varlistentry> <term><constant>-ESTALE</constant></term> <listitem><para>The event loop is already terminated.</para></listitem> </varlistentry> <varlistentry> <term><constant>-ECHILD</constant></term> <listitem><para>The event loop has been created in a different process.</para></listitem> </varlistentry> </variablelist> <para>Other errors are possible, too.</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_event_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>, <citerefentry><refentrytitle>sd_event_add_io</refentrytitle><manvolnum>3</manvolnum></citerefentry>, <citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry>, <citerefentry><refentrytitle>sd_event_add_signal</refentrytitle><manvolnum>3</manvolnum></citerefentry>, <citerefentry><refentrytitle>sd_event_add_defer</refentrytitle><manvolnum>3</manvolnum></citerefentry>, <citerefentry><refentrytitle>sd_event_add_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry>, <citerefentry><refentrytitle>sd_event_add_post</refentrytitle><manvolnum>3</manvolnum></citerefentry>, <citerefentry><refentrytitle>sd_event_run</refentrytitle><manvolnum>3</manvolnum></citerefentry>, <citerefentry><refentrytitle>sd_event_get_fd</refentrytitle><manvolnum>3</manvolnum></citerefentry>, <citerefentry><refentrytitle>sd_event_source_set_prepare</refentrytitle><manvolnum>3</manvolnum></citerefentry> </para> </refsect1> </refentry>