systemd/man/sd_event_add_defer.xml

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<refentry id="sd_event_add_defer" xmlns:xi="http://www.w3.org/2001/XInclude">

  <refentryinfo>
    <title>sd_event_add_defer</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>sd_event_add_defer</refentrytitle>
    <manvolnum>3</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>sd_event_add_defer</refname>
    <refname>sd_event_add_post</refname>
    <refname>sd_event_add_exit</refname>
    <refname>sd_event_handler_t</refname>

    <refpurpose>Add static event sources to an event loop</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <funcsynopsis>
      <funcsynopsisinfo>#include &lt;systemd/sd-event.h&gt;</funcsynopsisinfo>

      <funcsynopsisinfo><token>typedef</token> struct sd_event_source sd_event_source;</funcsynopsisinfo>

      <funcprototype>
        <funcdef>typedef int (*<function>sd_event_handler_t</function>)</funcdef>
        <paramdef>sd_event_source *<parameter>s</parameter></paramdef>
        <paramdef>void *<parameter>userdata</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_event_add_defer</function></funcdef>
        <paramdef>sd_event *<parameter>event</parameter></paramdef>
        <paramdef>sd_event_source **<parameter>source</parameter></paramdef>
        <paramdef>sd_event_handler_t <parameter>handler</parameter></paramdef>
        <paramdef>void *<parameter>userdata</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_event_add_post</function></funcdef>
        <paramdef>sd_event *<parameter>event</parameter></paramdef>
        <paramdef>sd_event_source **<parameter>source</parameter></paramdef>
        <paramdef>sd_event_handler_t <parameter>handler</parameter></paramdef>
        <paramdef>void *<parameter>userdata</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_event_add_exit</function></funcdef>
        <paramdef>sd_event *<parameter>event</parameter></paramdef>
        <paramdef>sd_event_source **<parameter>source</parameter></paramdef>
        <paramdef>sd_event_handler_t <parameter>handler</parameter></paramdef>
        <paramdef>void *<parameter>userdata</parameter></paramdef>
      </funcprototype>

    </funcsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para>These three functions add new static event sources to an event loop. The event loop object is
    specified in the <parameter>event</parameter> parameter, the event source object is returned in the
    <parameter>source</parameter> parameter. The event sources are enabled statically and will "fire" when
    the event loop is run and the conditions described below are met.</para>

    <para>The <parameter>handler</parameter> is a function to call or <constant>NULL</constant>. The handler
    function will be passed the <parameter>userdata</parameter> pointer, which may be chosen freely by the
    caller. The handler may return negative to signal an error (see below), other return values are
    ignored. If <parameter>handler</parameter> is <constant>NULL</constant>, a default handler that calls
    <citerefentry><refentrytitle>sd_event_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry> will be
    used.</para>

    <para><function>sd_event_add_defer()</function> adds a new event
    source that will be dispatched instantly, before the event loop
    goes to sleep again and waits for new events. By default, the
    handler will be called once
    (<constant>SD_EVENT_ONESHOT</constant>). Note that if the event
    source is set to <constant>SD_EVENT_ON</constant> the event loop
    will never go to sleep again, but continuously call the handler,
    possibly interleaved with other event sources.</para>

    <para><function>sd_event_add_post()</function> adds a new event
    source that is run before the event loop will sleep and wait
    for new events, but only after at least one other non-post event
    source was dispatched. By default, the source is enabled
    permanently (<constant>SD_EVENT_ON</constant>). Note that this
    event source type will still allow the event loop to go to sleep
    again, even if set to <constant>SD_EVENT_ON</constant>, as long as
    no other event source is ever triggered.</para>

    <para><function>sd_event_add_exit()</function> adds a new event
    source that will be dispatched when the event loop is terminated
    with <citerefentry><refentrytitle>sd_event_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>

    <para>The
    <citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    function may be used to enable the event source permanently
    (<constant>SD_EVENT_ON</constant>) or to make it fire just once
    (<constant>SD_EVENT_ONESHOT</constant>).</para>

    <para>If the handler function returns a negative error code, it will either be disabled after the
    invocation, even if the <constant>SD_EVENT_ON</constant> mode was requested before, or it will cause the
    loop to terminate, see
    <citerefentry><refentrytitle>sd_event_source_set_exit_on_failure</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
    </para>

    <para>To destroy an event source object use
    <citerefentry><refentrytitle>sd_event_source_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
    but note that the event source is only removed from the event loop
    when all references to the event source are dropped. To make sure
    an event source does not fire anymore, even when there's still a
    reference to it kept, consider setting the event source to
    <constant>SD_EVENT_OFF</constant> with
    <citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>

    <para>If the second parameter of these functions is passed as <constant>NULL</constant> no reference to
    the event source object is returned. In this case the event source is considered "floating", and will be
    destroyed implicitly when the event loop itself is destroyed.</para>

    <para>If the <parameter>handler</parameter> parameter to <function>sd_event_add_defer()</function> or
    <function>sd_event_add_post()</function> is <constant>NULL</constant>, and the event source fires, this
    will be considered a request to exit the event loop. In this case, the <parameter>userdata</parameter>
    parameter, cast to an integer, is passed as the exit code parameter to
    <citerefentry><refentrytitle>sd_event_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry>. Similar
    functionality is not available for <function>sd_event_add_exit()</function>, as these types of event
    sources are only dispatched when exiting anyway.</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.</para>

    <refsect2>
      <title>Errors</title>

      <para>Returned errors may indicate the following problems:</para>

      <variablelist>
        <varlistentry>
          <term><constant>-ENOMEM</constant></term>

          <listitem><para>Not enough memory to allocate an object.</para></listitem>
        </varlistentry>

        <varlistentry>
          <term><constant>-EINVAL</constant></term>

          <listitem><para>An invalid argument has been passed.</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>
    </refsect2>
  </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</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_event_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_event_now</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_child</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_event_add_inotify</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_event_source_set_priority</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_event_source_set_userdata</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_event_source_set_description</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_event_source_set_floating</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_event_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>