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

  <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>
    <refname>sd_event_child_handler_t</refname>

    <refpurpose>Add a child process state change event source 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_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_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>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
    process state change event source 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
    <parameter>pid</parameter> parameter specifies the PID of 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>siginfo_t</structname> structure containing
    information about the child process 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 process
    terminating), <constant>WSTOPPED</constant> (watch for the child
    process being stopped by a signal), and
    <constant>WCONTINUED</constant> (watch for the child process being
    resumed by a signal). See <citerefentry
    project='man-pages'><refentrytitle>waitid</refentrytitle><manvolnum>2</manvolnum></citerefentry>
    for further information.</para>

    <para>Only a single handler may be installed for a specific
    child process. 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 the
    <constant>SD_EVENT_ON</constant> mode was requested before.
    </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
    <function>sd_event_add_child()</function> is passed as NULL 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>Note that the <parameter>handler</parameter> function is
    invoked at a time where the child process is not reaped yet (and
    thus still is exposed as a zombie process by the kernel). However,
    the child will be reaped automatically after the function
    returns. Child processes for which no child process state change
    event sources are installed will not be reaped by the event loop
    implementation.</para>

    <para>If both a child process state change event source and a
    <constant>SIGCHLD</constant> signal event source is installed in
    the same event loop, the configured event source priorities decide
    which event source is dispatched first. If the signal handler is
    processed first, it should leave the child processes for which
    child process state change event sources are installed unreaped.</para>

    <para><function>sd_event_source_get_child_pid()</function>
    retrieves the configured PID of a child process 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 a <type>pid_t</type> variable to return the process ID
    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>A handler is already installed for this
        child process.</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>

      <varlistentry>
        <term><constant>-EDOM</constant></term>

        <listitem><para>The passed event source is not a child process event source.</para></listitem>
      </varlistentry>

    </variablelist>
  </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_defer</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 project='man-pages'><refentrytitle>waitid</refentrytitle><manvolnum>2</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>
