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<refentry id="sd_event_add_child">

  <refentryinfo>
    <title>sd_event_add_child</title>
    <productname>systemd</productname>

    <authorgroup>
      <author>
        <contrib>More text</contrib>
        <firstname>Zbigniew</firstname>
        <surname>Jędrzejewski-Szmek</surname>
        <email>zbyszek@in.waw.pl</email>
      </author>
    </authorgroup>
  </refentryinfo>

  <refmeta>
    <refentrytitle>sd_event_add_child</refentrytitle>
    <manvolnum>3</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>sd_event_add_child</refname>
    <refname>sd_event_source_get_child_pid</refname>

    <refpurpose>Add a child state change event source to an event loop</refpurpose>
  </refnamediv>

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

      <funcprototype>
        <funcdef>int <function>sd_event_add_child</function></funcdef>
        <paramdef>sd_event *<parameter>event</parameter></paramdef>
        <paramdef>sd_event_source **<parameter>source</parameter></paramdef>
        <paramdef>pid_t <parameter>pid</parameter></paramdef>
        <paramdef>int <parameter>options</parameter></paramdef>
        <paramdef>sd_event_child_handler_t <parameter>handler</parameter></paramdef>
        <paramdef>void *<parameter>userdata</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>typedef int (*<function>sd_event_child_handler_t</function>)</funcdef>
        <paramdef>sd_event_source *<parameter>s</parameter></paramdef>
        <paramdef>const siginfo_t *<parameter>si</parameter></paramdef>
        <paramdef>void *<parameter>userdata</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_event_source_get_child_pid</function></funcdef>
        <paramdef>sd_event_source *<parameter>source</parameter></paramdef>
        <paramdef>pid_t *<parameter>pid</parameter></paramdef>
      </funcprototype>

    </funcsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para><function>sd_event_add_child()</function> adds a new child
    state change event source to an event loop object. The event loop
    is specified in <parameter>event</parameter>, the event source is
    returned in the <parameter>source</parameter> parameter. The
    <parameter>pid</parameter> parameter specifies the process to
    watch. The <parameter>handler</parameter> must reference a
    function to call when the process changes state. The handler
    function will be passed the <parameter>userdata</parameter>
    pointer, which may be chosen freely by the caller. The handler
    also receives a pointer to a <structname>const
    siginfo_t</structname> structure containing the information about
    the event. The <parameter>options</parameter> parameter determines
    which state changes will be watched for. It must contain an OR-ed
    mask of <constant>WEXITED</constant> (watch for the child
    terminating), <constant>WSTOPPED</constant> (watch for the child
    being stopped by a signal), and <constant>WCONTINUED</constant>
    (watch for the child being resumed by a signal). See
    <citerefentry><refentrytitle>waitid</refentrytitle><manvolnum>2</manvolnum></citerefentry>
    for further information.</para>

    <para>Only a single handler may be installed for a specific
    child. The handler is enabled
    for a single event (<constant>SD_EVENT_ONESHOT</constant>),
    but this may be
    changed with
    <citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
    If the handler function returns a negative error code, it will be
    disabled after the invocation, even if
    <constant>SD_EVENT_ON</constant> mode is set.
    </para>

    <para><function>sd_event_source_get_child_pid()</function>
    retrieves the configured <parameter>pid</parameter> of a child
    state change event source created previously with
    <function>sd_event_add_child()</function>. It takes the event
    source object as the <parameter>source</parameter> parameter and a
    pointer to <type>pid_t</type> to return the result in.
    </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>
  </refsect1>

  <refsect1>
    <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. This includes
        specifying an empty mask in <parameter>options</parameter> or a mask
        which contains values different than a combination of
        <constant>WEXITED</constant>, <constant>WSTOPPED</constant>, and
        <constant>WCONTINUED</constant>.
        </para></listitem>

      </varlistentry>

      <varlistentry>
        <term><constant>-EBUSY</constant></term>

        <listitem><para>An handler is already installed for this
        child.</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>
  </refsect1>

  <refsect1>
    <title>Notes</title>

    <para><function>sd_event_add_child()</function> and the other 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</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_event_new</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_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>
