1 <?xml version='1.0'?> <!--*- Mode: nxml; nxml-child-indent: 2; indent-tabs-mode: nil -*-->
2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
3 "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
6 SPDX-License-Identifier: LGPL-2.1+
9 <refentry id="sd_event_add_child" xmlns:xi="http://www.w3.org/2001/XInclude">
12 <title>sd_event_add_child</title>
13 <productname>systemd</productname>
17 <contrib>More text</contrib>
18 <firstname>Zbigniew</firstname>
19 <surname>Jędrzejewski-Szmek</surname>
20 <email>zbyszek@in.waw.pl</email>
26 <refentrytitle>sd_event_add_child</refentrytitle>
27 <manvolnum>3</manvolnum>
31 <refname>sd_event_add_child</refname>
32 <refname>sd_event_source_get_child_pid</refname>
33 <refname>sd_event_child_handler_t</refname>
35 <refpurpose>Add a child process state change event source to an event loop</refpurpose>
40 <funcsynopsisinfo>#include <systemd/sd-event.h></funcsynopsisinfo>
42 <funcsynopsisinfo><token>typedef</token> struct sd_event_source sd_event_source;</funcsynopsisinfo>
45 <funcdef>typedef int (*<function>sd_event_child_handler_t</function>)</funcdef>
46 <paramdef>sd_event_source *<parameter>s</parameter></paramdef>
47 <paramdef>const siginfo_t *<parameter>si</parameter></paramdef>
48 <paramdef>void *<parameter>userdata</parameter></paramdef>
52 <funcdef>int <function>sd_event_add_child</function></funcdef>
53 <paramdef>sd_event *<parameter>event</parameter></paramdef>
54 <paramdef>sd_event_source **<parameter>source</parameter></paramdef>
55 <paramdef>pid_t <parameter>pid</parameter></paramdef>
56 <paramdef>int <parameter>options</parameter></paramdef>
57 <paramdef>sd_event_child_handler_t <parameter>handler</parameter></paramdef>
58 <paramdef>void *<parameter>userdata</parameter></paramdef>
62 <funcdef>int <function>sd_event_source_get_child_pid</function></funcdef>
63 <paramdef>sd_event_source *<parameter>source</parameter></paramdef>
64 <paramdef>pid_t *<parameter>pid</parameter></paramdef>
71 <title>Description</title>
73 <para><function>sd_event_add_child()</function> adds a new child
74 process state change event source to an event loop. The event loop
75 object is specified in the <parameter>event</parameter> parameter,
76 the event source object is returned in the
77 <parameter>source</parameter> parameter. The
78 <parameter>pid</parameter> parameter specifies the PID of the
79 process to watch. The <parameter>handler</parameter> must
80 reference a function to call when the process changes state. The
81 handler function will be passed the
82 <parameter>userdata</parameter> pointer, which may be chosen
83 freely by the caller. The handler also receives a pointer to a
84 <structname>siginfo_t</structname> structure containing
85 information about the child process event. The
86 <parameter>options</parameter> parameter determines which state
87 changes will be watched for. It must contain an OR-ed mask of
88 <constant>WEXITED</constant> (watch for the child process
89 terminating), <constant>WSTOPPED</constant> (watch for the child
90 process being stopped by a signal), and
91 <constant>WCONTINUED</constant> (watch for the child process being
92 resumed by a signal). See <citerefentry
93 project='man-pages'><refentrytitle>waitid</refentrytitle><manvolnum>2</manvolnum></citerefentry>
94 for further information.</para>
96 <para>Only a single handler may be installed for a specific
97 child process. The handler is enabled for a single event
98 (<constant>SD_EVENT_ONESHOT</constant>), but this may be changed
100 <citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
101 If the handler function returns a negative error code, it will be
102 disabled after the invocation, even if the
103 <constant>SD_EVENT_ON</constant> mode was requested before.
106 <para>To destroy an event source object use
107 <citerefentry><refentrytitle>sd_event_source_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
108 but note that the event source is only removed from the event loop
109 when all references to the event source are dropped. To make sure
110 an event source does not fire anymore, even when there's still a
111 reference to it kept, consider setting the event source to
112 <constant>SD_EVENT_OFF</constant> with
113 <citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
115 <para>If the second parameter of
116 <function>sd_event_add_child()</function> is passed as NULL no
117 reference to the event source object is returned. In this case the
118 event source is considered "floating", and will be destroyed
119 implicitly when the event loop itself is destroyed.</para>
121 <para>Note that the <parameter>handler</parameter> function is
122 invoked at a time where the child process is not reaped yet (and
123 thus still is exposed as a zombie process by the kernel). However,
124 the child will be reaped automatically after the function
125 returns. Child processes for which no child process state change
126 event sources are installed will not be reaped by the event loop
127 implementation.</para>
129 <para>If both a child process state change event source and a
130 <constant>SIGCHLD</constant> signal event source is installed in
131 the same event loop, the configured event source priorities decide
132 which event source is dispatched first. If the signal handler is
133 processed first, it should leave the child processes for which
134 child process state change event sources are installed unreaped.</para>
136 <para><function>sd_event_source_get_child_pid()</function>
137 retrieves the configured PID of a child process state change event
138 source created previously with
139 <function>sd_event_add_child()</function>. It takes the event
140 source object as the <parameter>source</parameter> parameter and a
141 pointer to a <type>pid_t</type> variable to return the process ID
147 <title>Return Value</title>
149 <para>On success, these functions return 0 or a positive
150 integer. On failure, they return a negative errno-style error
155 <title>Errors</title>
157 <para>Returned errors may indicate the following problems:</para>
161 <term><constant>-ENOMEM</constant></term>
163 <listitem><para>Not enough memory to allocate an object.</para></listitem>
167 <term><constant>-EINVAL</constant></term>
169 <listitem><para>An invalid argument has been passed. This includes
170 specifying an empty mask in <parameter>options</parameter> or a mask
171 which contains values different than a combination of
172 <constant>WEXITED</constant>, <constant>WSTOPPED</constant>, and
173 <constant>WCONTINUED</constant>.
179 <term><constant>-EBUSY</constant></term>
181 <listitem><para>A handler is already installed for this
182 child process.</para></listitem>
187 <term><constant>-ESTALE</constant></term>
189 <listitem><para>The event loop is already terminated.</para></listitem>
194 <term><constant>-ECHILD</constant></term>
196 <listitem><para>The event loop has been created in a different process.</para></listitem>
201 <term><constant>-EDOM</constant></term>
203 <listitem><para>The passed event source is not a child process event source.</para></listitem>
209 <xi:include href="libelogind-pkgconfig.xml" />
212 <title>See Also</title>
215 <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
216 <citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
217 <citerefentry><refentrytitle>sd_event_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
218 <citerefentry><refentrytitle>sd_event_now</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
219 <citerefentry><refentrytitle>sd_event_add_io</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
220 <citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
221 <citerefentry><refentrytitle>sd_event_add_signal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
222 <citerefentry><refentrytitle>sd_event_add_inotify</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
223 <citerefentry><refentrytitle>sd_event_add_defer</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
224 <citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
225 <citerefentry><refentrytitle>sd_event_source_set_priority</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
226 <citerefentry><refentrytitle>sd_event_source_set_userdata</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
227 <citerefentry><refentrytitle>sd_event_source_set_description</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
228 <citerefentry project='man-pages'><refentrytitle>waitid</refentrytitle><manvolnum>2</manvolnum></citerefentry>