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<?xml version='1.0'?>
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
<refentry id="sd_event_wait" xmlns:xi="http://www.w3.org/2001/XInclude">
<refentryinfo>
<title>sd_event_wait</title>
<productname>systemd</productname>
</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 64-bit 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>
<xi:include href="version-info.xml" xpointer="v229"/></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_source_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>
<xi:include href="version-info.xml" xpointer="v229"/></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>
<xi:include href="version-info.xml" xpointer="v229"/></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>
<xi:include href="version-info.xml" xpointer="v229"/></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>
<xi:include href="version-info.xml" xpointer="v229"/></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>
<xi:include href="version-info.xml" xpointer="v229"/></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>
<xi:include href="version-info.xml" xpointer="v229"/></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>
<refsect2>
<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, library or module instance.</para></listitem>
</varlistentry>
</variablelist>
<para>Other errors are possible, too.</para>
</refsect2>
</refsect1>
<xi:include href="libsystemd-pkgconfig.xml" />
<refsect1>
<title>History</title>
<para><function>sd_event_prepare()</function>,
<function>sd_event_wait()</function>,
<function>sd_event_dispatch()</function>, and
<function>sd_event_get_state()</function> were added in version 221.</para>
<para><function>sd_event_get_iteration()</function> was added in version 231.</para>
</refsect1>
<refsect1>
<title>See Also</title>
<para><simplelist type="inline">
<member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd_event_new</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd_event_add_io</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd_event_add_signal</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd_event_add_child</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd_event_add_inotify</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd_event_add_defer</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd_event_run</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd_event_get_fd</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
<member><citerefentry><refentrytitle>sd_event_source_set_prepare</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
</simplelist></para>
</refsect1>
</refentry>
|
