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<refentry id="sd_event_wait">

<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>

<refpurpose>Run parts of the libsystemd event loop</refpurpose>

</refnamediv>

<refsynopsisdiv>

<funcsynopsis>

<funcsynopsisinfo>#include &lt;systemd/sd-event.h&gt;</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>timeout</parameter></paramdef>

</funcprototype>

<funcprototype>

<funcdef>int <function>sd_event_dispatch</function></funcdef>

<paramdef>sd_event *<parameter>event</parameter></paramdef>

</funcprototype>

</funcsynopsis>

</refsynopsisdiv>

<refsect1>

<title>Description</title>

<para>Functions described here form parts of an event loop.</para>

<para><function>sd_event_prepare</function> checks for pending

events and arms necessary timers. If any events are ready to be

processed, it returns a positive value, and the events should be

processed with <function>sd_event_dispatch</function>.

<function>sd_event_dispatch</function> runs a handler for one of

the events from the sources with the highest priority. On success,

<function>sd_event_dispatch</function> returns either 0, which

means that the loop is finished, or a positive value, which means

that the loop is again in the initial state and

<function>sd_event_prepare</function> should be called again.

</para>

<para>In case <function>sd_event_prepare</function> returned 0,

<function>sd_event_wait</function> should be called to wait for

events or a timeout. If any events are ready to be processed, it

returns a positive value, and the events should be processed with

<function>sd_event_dispatch</function>. Otherwise, the loop is

back in the initial state and <function>sd_event_prepare</function>

should be called again.</para>

<programlisting>

┌──────────┐

│ initial ├──←←←←←←←←←←←←←←←←←←←─┐

└───┬──────┘ ↑

│ ↑

sd_event_prepare ┌─────────┐ ↑

├ 0 →→→→→→→──┤ armed │ ↑

1 └───┬─────┘ ↑

↓ │ ↑

↓ sd_event_wait ↑

├───←←←←←←←─── 1 ┴─ 0 →→→→→→→─┘

┌───┴──────┐ ↑

│ pending │ ↑

└───┬──────┘ ↑

│ ↑

sd_event_dispatch ↑

↓ ↑

├ 1 ──────────→→→→→→→─────────┘

0

↓

┌───┴──────┐

│ finished │

└──────────┘

</programlisting>

<para>All three functions take, as the first argument, the event

loop object <parameter>event</parameter> that is created with

<function>sd_event_new</function>. The timeout for

<function>sd_event_wait</function> is specified with

<parameter>timeout</parameter> in milliseconds.

<constant>(uint64_t) -1</constant> may be used to specify an

infinite timeout.</para>

</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 value means that

events are ready to be processed and 0 means that no events are

ready. In case of <function>sd_event_dispatch</function>, a

positive value means that the loop is again in the initial state

and 0 means the loop is finished. For any of these functions, a

negative return value means the loop must be aborted.</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

<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>

<refsect1>

<title>Notes</title>

<para>Functions described here are available

as a shared library, which can be compiled and linked to with the

<constant>libsystemd</constant> <citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>

file.</para>

</refsect1>

<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_run</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>.

</para>

</refsect1>

</refentry>