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+<?xml version='1.0'?>
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
+ "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
+<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
+
+<refentry id="sd_event_add_child" xmlns:xi="http://www.w3.org/2001/XInclude">
+
+ <refentryinfo>
+ <title>sd_event_add_child</title>
+ <productname>systemd</productname>
+ </refentryinfo>
+
+ <refmeta>
+ <refentrytitle>sd_event_add_child</refentrytitle>
+ <manvolnum>3</manvolnum>
+ </refmeta>
+
+ <refnamediv>
+ <refname>sd_event_add_child</refname>
+ <refname>sd_event_add_child_pidfd</refname>
+ <refname>sd_event_source_get_child_pid</refname>
+ <refname>sd_event_source_get_child_pidfd</refname>
+ <refname>sd_event_source_get_child_pidfd_own</refname>
+ <refname>sd_event_source_set_child_pidfd_own</refname>
+ <refname>sd_event_source_get_child_process_own</refname>
+ <refname>sd_event_source_set_child_process_own</refname>
+ <refname>sd_event_source_send_child_signal</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_add_child_pidfd</function></funcdef>
+ <paramdef>sd_event *<parameter>event</parameter></paramdef>
+ <paramdef>sd_event_source **<parameter>source</parameter></paramdef>
+ <paramdef>int <parameter>pidfd</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>
+
+ <funcprototype>
+ <funcdef>int <function>sd_event_source_get_child_pidfd</function></funcdef>
+ <paramdef>sd_event_source *<parameter>source</parameter></paramdef>
+ </funcprototype>
+
+ <funcprototype>
+ <funcdef>int <function>sd_event_source_get_child_pidfd_own</function></funcdef>
+ <paramdef>sd_event_source *<parameter>source</parameter></paramdef>
+ </funcprototype>
+
+ <funcprototype>
+ <funcdef>int <function>sd_event_source_set_child_pidfd_own</function></funcdef>
+ <paramdef>sd_event_source *<parameter>source</parameter></paramdef>
+ <paramdef>int <parameter>own</parameter></paramdef>
+ </funcprototype>
+
+ <funcprototype>
+ <funcdef>int <function>sd_event_source_get_child_process_own</function></funcdef>
+ <paramdef>sd_event_source *<parameter>source</parameter></paramdef>
+ </funcprototype>
+
+ <funcprototype>
+ <funcdef>int <function>sd_event_source_set_child_process_own</function></funcdef>
+ <paramdef>sd_event_source *<parameter>source</parameter></paramdef>
+ <paramdef>int <parameter>own</parameter></paramdef>
+ </funcprototype>
+
+ <funcprototype>
+ <funcdef>int <function>sd_event_source_send_child_signal</function></funcdef>
+ <paramdef>sd_event_source *<parameter>source</parameter></paramdef>
+ <paramdef>int <parameter>sig</parameter></paramdef>
+ <paramdef>const siginfo_t *<parameter>info</parameter></paramdef>
+ <paramdef>unsigned <parameter>flags</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, which must be a direct child process of the invoking
+ process. 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>The <parameter>handler</parameter> must be a function to call when the process changes state 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 also receives a pointer to a
+ <structname>siginfo_t</structname> structure containing information about the child process event. 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>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 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>The <constant>SIGCHLD</constant> signal must be blocked in all threads before this function is
+ called (using <citerefentry
+ project='man-pages'><refentrytitle>sigprocmask</refentrytitle><manvolnum>2</manvolnum></citerefentry> or
+ <citerefentry
+ project='man-pages'><refentrytitle>pthread_sigmask</refentrytitle><manvolnum>3</manvolnum></citerefentry>).</para>
+
+ <para>If the second parameter of <function>sd_event_add_child()</function> 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>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 the <parameter>handler</parameter> parameter to <function>sd_event_add_child()</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>.</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_add_child_pidfd()</function> is similar to
+ <function>sd_event_add_child()</function> but takes a file descriptor referencing the process ("pidfd")
+ instead of the numeric PID. A suitable file descriptor may be acquired via <citerefentry
+ project='man-pages'><refentrytitle>pidfd_open</refentrytitle><manvolnum>2</manvolnum></citerefentry> and
+ related calls. The passed file descriptor is not closed when the event source is freed again, unless
+ <function>sd_event_source_set_child_pidfd_own()</function> is used to turn this behaviour on. Note that
+ regardless which of <function>sd_event_add_child()</function> and
+ <function>sd_event_add_child_pidfd()</function> is used for allocating an event source, the watched
+ process has to be a direct child process of the invoking process. Also in both cases
+ <constant>SIGCHLD</constant> has to be blocked in the invoking process.</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>
+
+ <para><function>sd_event_source_get_child_pidfd()</function> retrieves the file descriptor referencing
+ the watched process ("pidfd") if this functionality is available. On kernels that support the concept the
+ event loop will make use of pidfds to watch child processes, regardless if the individual event sources
+ are allocated via <function>sd_event_add_child()</function> or
+ <function>sd_event_add_child_pidfd()</function>. If the latter call was used to allocate the event
+ source, this function returns the file descriptor used for allocation. On kernels that do not support the
+ pidfd concept this function will fail with <constant>EOPNOTSUPP</constant>. This call takes the event
+ source object as the <parameter>source</parameter> parameter and returns the numeric file descriptor.
+ </para>
+
+ <para><function>sd_event_source_get_child_pidfd_own()</function> may be used to query whether the pidfd
+ the event source encapsulates shall be closed when the event source is freed. This function returns zero
+ if the pidfd shall be left open, and positive if it shall be closed automatically. By default this
+ setting defaults to on if the event source was allocated via <function>sd_event_add_child()</function>
+ and off if it was allocated via <function>sd_event_add_child_pidfd()</function>. The
+ <function>sd_event_source_set_child_pidfd_own()</function> function may be used to change the setting and
+ takes a boolean parameter with the new setting.</para>
+
+ <para><function>sd_event_source_get_child_process_own()</function> may be used to query whether the
+ process the event source watches shall be killed (with <constant>SIGKILL</constant>) and reaped when the
+ event source is freed. This function returns zero if the process shell be left running, and positive if
+ it shall be killed and reaped automatically. By default this setting defaults to off. The
+ <function>sd_event_source_set_child_process_own()</function> function may be used to change the setting
+ and takes a boolean parameter with the new setting. Note that currently if the calling process is
+ terminated abnormally the watched process might survive even thought the event source ceases to
+ exist. This behaviour might change eventually.</para>
+
+ <para><function>sd_event_source_send_child_signal()</function> may be used to send a UNIX signal to the
+ watched process. If the pidfd concept is supported in the kernel, this is implemented via <citerefentry
+ project='man-pages'><refentrytitle>pidfd_send_signal</refentrytitle><manvolnum>2</manvolnum></citerefentry>
+ and otherwise via <citerefentry
+ project='man-pages'><refentrytitle>rt_sigqueueinfo</refentrytitle><manvolnum>2</manvolnum></citerefentry>
+ (or via <citerefentry
+ project='man-pages'><refentrytitle>kill</refentrytitle><manvolnum>2</manvolnum></citerefentry> in case
+ <parameter>info</parameter> is <constant>NULL</constant>). The specified parameters match those of these
+ underlying system calls, except that the <parameter>info</parameter> is never modified (and is thus
+ declared constant). Like for the underlying system calls, the <parameter>flags</parameter> parameter
+ currently must be zero.</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. 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, or
+ <constant>SIGCHLD</constant> is not blocked.</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>
+
+ <varlistentry>
+ <term><constant>-EOPNOTSUPP</constant></term>
+
+ <listitem><para>A pidfd was requested but the kernel does not support this concept.</para></listitem>
+ </varlistentry>
+
+ </variablelist>
+ </refsect2>
+ </refsect1>
+
+ <xi:include href="libsystemd-pkgconfig.xml" />
+
+ <refsect1>
+ <title>Example</title>
+
+ <example>
+ <title>Exit loop when the child terminates</title>
+
+ <programlisting><xi:include href="event-quick-child.c" parse="text" /></programlisting>
+ </example>
+ </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_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_inotify</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><refentrytitle>sd_event_source_set_floating</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+ <citerefentry project='man-pages'><refentrytitle>waitid</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
+ <citerefentry project='man-pages'><refentrytitle>sigprocmask</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
+ <citerefentry project='man-pages'><refentrytitle>pthread_sigmask</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+ <citerefentry project='man-pages'><refentrytitle>pidfd_open</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
+ <citerefentry project='man-pages'><refentrytitle>pidfd_send_signal</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
+ <citerefentry project='man-pages'><refentrytitle>rt_sigqueueinfo</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
+ <citerefentry project='man-pages'><refentrytitle>kill</refentrytitle><manvolnum>2</manvolnum></citerefentry>
+ </para>
+ </refsect1>
+
+</refentry>