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333 lines
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333 lines
14 KiB
XML
<?xml version='1.0'?>
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!--
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SPDX-License-Identifier: LGPL-2.1+
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-->
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<refentry id="sd_event_wait" xmlns:xi="http://www.w3.org/2001/XInclude">
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<refentryinfo>
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<title>sd_event_wait</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>sd_event_wait</refentrytitle>
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<manvolnum>3</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>sd_event_wait</refname>
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<refname>sd_event_prepare</refname>
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<refname>sd_event_dispatch</refname>
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<refname>sd_event_get_state</refname>
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<refname>sd_event_get_iteration</refname>
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<refname>SD_EVENT_INITIAL</refname>
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<refname>SD_EVENT_PREPARING</refname>
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<refname>SD_EVENT_ARMED</refname>
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<refname>SD_EVENT_PENDING</refname>
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<refname>SD_EVENT_RUNNING</refname>
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<refname>SD_EVENT_EXITING</refname>
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<refname>SD_EVENT_FINISHED</refname>
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<refpurpose>Low-level event loop operations</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<funcsynopsis>
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<funcsynopsisinfo>#include <systemd/sd-event.h></funcsynopsisinfo>
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<funcsynopsisinfo><token>enum</token> {
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<constant>SD_EVENT_INITIAL</constant>,
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<constant>SD_EVENT_PREPARING</constant>,
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<constant>SD_EVENT_ARMED</constant>,
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<constant>SD_EVENT_PENDING</constant>,
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<constant>SD_EVENT_RUNNING</constant>,
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<constant>SD_EVENT_EXITING</constant>,
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<constant>SD_EVENT_FINISHED</constant>,
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};</funcsynopsisinfo>
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<funcprototype>
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<funcdef>int <function>sd_event_prepare</function></funcdef>
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<paramdef>sd_event *<parameter>event</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_event_wait</function></funcdef>
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<paramdef>sd_event *<parameter>event</parameter></paramdef>
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<paramdef>uint64_t <parameter>usec</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_event_dispatch</function></funcdef>
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<paramdef>sd_event *<parameter>event</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_event_get_state</function></funcdef>
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<paramdef>sd_event *<parameter>event</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_event_get_iteration</function></funcdef>
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<paramdef>sd_event *<parameter>event</parameter></paramdef>
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<paramdef>uint64_t *<parameter>ret</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para>The low-level <function>sd_event_prepare()</function>,
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<function>sd_event_wait()</function> and
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<function>sd_event_dispatch()</function> functions may be used to
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execute specific phases of an event loop. See
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<citerefentry><refentrytitle>sd_event_run</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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and
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<citerefentry><refentrytitle>sd_event_loop</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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for higher-level functions that execute individual but complete
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iterations of an event loop or run it continuously.</para>
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<para><function>sd_event_prepare()</function> checks for pending
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events and arms necessary timers. If any events are ready to be
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processed ("pending"), it returns a positive, non-zero value, and the caller
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should process these events with
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<function>sd_event_dispatch()</function>.</para>
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<para><function>sd_event_dispatch()</function> dispatches the
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highest priority event source that has a pending event. On
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success, <function>sd_event_dispatch()</function> returns either
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zero, which indicates that no further event sources may be
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dispatched and exiting of the event loop was requested via
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<citerefentry><refentrytitle>sd_event_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry>;
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or a positive non-zero value, which means that an event source was
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dispatched and the loop returned to its initial state, and the
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caller should initiate the next event loop iteration by invoking
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<function>sd_event_prepare()</function> again.</para>
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<para>In case <function>sd_event_prepare()</function> returned
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zero, <function>sd_event_wait()</function> should be called to
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wait for further events or a timeout. If any events are ready to
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be processed, it returns a positive, non-zero value, and the
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events should be dispatched with
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<function>sd_event_dispatch()</function>. Otherwise, the event
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loop returned to its initial state and the next event loop
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iteration should be initiated by invoking
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<function>sd_event_prepare()</function> again.</para>
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<para><function>sd_event_get_state()</function> may be used to
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determine the state the event loop is currently in. It returns one
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of the states described below.</para>
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<para><function>sd_event_get_iteration()</function> may be used to determine the current iteration of the event
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loop. It returns an unsigned 64bit integer containing a counter that increases monotonically with each iteration of
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the event loop, starting with 0. The counter is increased at the time of the
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<function>sd_event_prepare()</function> invocation.</para>
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<para>All five functions take, as the first argument, the event loop object <parameter>event</parameter> that has
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been created with <function>sd_event_new()</function>. The timeout for <function>sd_event_wait()</function> is
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specified in <parameter>usec</parameter> in microseconds. <constant>(uint64_t) -1</constant> may be used to
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specify an infinite timeout.</para>
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</refsect1>
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<refsect1>
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<title>State Machine</title>
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<para>The event loop knows the following states, that may be
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queried with <function>sd_event_get_state()</function>.</para>
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<variablelist>
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<varlistentry>
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<term><constant>SD_EVENT_INITIAL</constant></term>
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<listitem><para>The initial state the event loop is in,
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before each event loop iteration. Use
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<function>sd_event_prepare()</function> to transition the
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event loop into the <constant>SD_EVENT_ARMED</constant> or
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<constant>SD_EVENT_PENDING</constant> states.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>SD_EVENT_PREPARING</constant></term>
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<listitem><para>An event source is currently being prepared,
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i.e. the preparation handler is currently being executed, as
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set with
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<citerefentry><refentrytitle>sd_event_source_set_prepare</refentrytitle><manvolnum>3</manvolnum></citerefentry>. This
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state is only seen in the event source preparation handler
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that is invoked from the
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<function>sd_event_prepare()</function> call and is
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immediately followed by <constant>SD_EVENT_ARMED</constant> or
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<constant>SD_EVENT_PENDING</constant>.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>SD_EVENT_ARMED</constant></term>
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<listitem><para><function>sd_event_prepare()</function> has
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been called and no event sources were ready to be
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dispatched. Use <function>sd_event_wait()</function> to wait
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for new events, and transition into
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<constant>SD_EVENT_PENDING</constant> or back into
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<constant>SD_EVENT_INITIAL</constant>.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>SD_EVENT_PENDING</constant></term>
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<listitem><para><function>sd_event_prepare()</function> or
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<function>sd_event_wait()</function> have been called and
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there were event sources with events pending. Use
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<function>sd_event_dispatch()</function> to dispatch the
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highest priority event source and transition back to
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<constant>SD_EVENT_INITIAL</constant>, or
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<constant>SD_EVENT_FINISHED</constant>.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>SD_EVENT_RUNNING</constant></term>
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<listitem><para>A regular event source is currently being
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dispatched. This state is only seen in the event source
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handler that is invoked from the
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<function>sd_event_dispatch()</function> call, and is
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immediately followed by <constant>SD_EVENT_INITIAL</constant>
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or <constant>SD_EVENT_FINISHED</constant> as soon the event
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source handler returns. Note that during dispatching of exit
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event sources the <constant>SD_EVENT_EXITING</constant> state
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is seen instead.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>SD_EVENT_EXITING</constant></term>
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<listitem><para>Similar to
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<constant>SD_EVENT_RUNNING</constant> but is the state in
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effect while dispatching exit event sources. It is followed by
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<constant>SD_EVENT_INITIAL</constant> or
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<constant>SD_EVENT_FINISHED</constant> as soon as the event
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handler returns.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>SD_EVENT_FINISHED</constant></term>
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<listitem><para>The event loop has exited. All exit event
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sources have run. If the event loop is in this state it serves
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no purpose anymore, and should be freed.</para></listitem>
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</varlistentry>
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</variablelist>
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<para>A simplified flow chart of the states and the calls to
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transition between them is shown below. Note that
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<constant>SD_EVENT_PREPARING</constant>,
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<constant>SD_EVENT_RUNNING</constant> and
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<constant>SD_EVENT_EXITING</constant> are not shown here.</para>
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<programlisting>
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INITIAL -<---<---<---<---<---<---<---<---<---<---<---<---\
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| ^
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v ret == 0 |
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sd_event_prepare() >--->--->--->--->- ARMED |
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| | ^
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| ret > 0 | |
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| | |
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v v ret == 0 |
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PENDING <---<---<---<---<---< sd_event_wait() >--->--->--+
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| ret > 0 ^
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v |
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sd_event_dispatch() >--->--->--->--->--->--->--->--->--->--->/
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| ret > 0
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| ret == 0
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v
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FINISHED
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</programlisting>
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</refsect1>
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<refsect1>
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<title>Return Value</title>
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<para>On success, these functions return 0 or a positive integer.
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On failure, they return a negative errno-style error code. In case
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of <function>sd_event_prepare()</function> and
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<function>sd_event_wait()</function>, a positive, non-zero return
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code indicates that events are ready to be processed and zero
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indicates that no events are ready. In case of
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<function>sd_event_dispatch()</function>, a positive, non-zero
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return code indicates that the event loop returned to its initial
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state and zero indicates the event loop has
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exited. <function>sd_event_get_state()</function> returns a
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positive or zero state on success.</para>
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</refsect1>
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<refsect1>
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<title>Errors</title>
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<para>Returned errors may indicate the following problems:</para>
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<variablelist>
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<varlistentry>
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<term><constant>-EINVAL</constant></term>
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<listitem><para>The <parameter>event</parameter> parameter is
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invalid or <constant>NULL</constant>.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>-EBUSY</constant></term>
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<listitem><para>The event loop object is not in the right
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state.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>-ESTALE</constant></term>
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<listitem><para>The event loop is already terminated.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>-ECHILD</constant></term>
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<listitem><para>The event loop has been created in a different process.</para></listitem>
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</varlistentry>
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</variablelist>
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<para>Other errors are possible, too.</para>
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</refsect1>
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<xi:include href="libsystemd-pkgconfig.xml" />
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<refsect1>
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<title>See Also</title>
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<para>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_add_io</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_add_signal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_add_child</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_add_inotify</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_add_defer</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_run</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_get_fd</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_source_set_prepare</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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