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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 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 as the first argument take the event
    loop object <parameter>event</parameter> that is created with 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 those 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>Parameter <parameter>event</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>
