1<?xml version='1.0'?>
2<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
3  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
4<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
5
6<refentry id="sd_event_add_child" xmlns:xi="http://www.w3.org/2001/XInclude">
7
8  <refentryinfo>
9    <title>sd_event_add_child</title>
10    <productname>systemd</productname>
11  </refentryinfo>
12
13  <refmeta>
14    <refentrytitle>sd_event_add_child</refentrytitle>
15    <manvolnum>3</manvolnum>
16  </refmeta>
17
18  <refnamediv>
19    <refname>sd_event_add_child</refname>
20    <refname>sd_event_add_child_pidfd</refname>
21    <refname>sd_event_source_get_child_pid</refname>
22    <refname>sd_event_source_get_child_pidfd</refname>
23    <refname>sd_event_source_get_child_pidfd_own</refname>
24    <refname>sd_event_source_set_child_pidfd_own</refname>
25    <refname>sd_event_source_get_child_process_own</refname>
26    <refname>sd_event_source_set_child_process_own</refname>
27    <refname>sd_event_source_send_child_signal</refname>
28    <refname>sd_event_child_handler_t</refname>
29
30    <refpurpose>Add a child process state change event source to an event loop</refpurpose>
31  </refnamediv>
32
33  <refsynopsisdiv>
34    <funcsynopsis>
35      <funcsynopsisinfo>#include &lt;systemd/sd-event.h&gt;</funcsynopsisinfo>
36
37      <funcsynopsisinfo><token>typedef</token> struct sd_event_source sd_event_source;</funcsynopsisinfo>
38
39      <funcprototype>
40        <funcdef>typedef int (*<function>sd_event_child_handler_t</function>)</funcdef>
41        <paramdef>sd_event_source *<parameter>s</parameter></paramdef>
42        <paramdef>const siginfo_t *<parameter>si</parameter></paramdef>
43        <paramdef>void *<parameter>userdata</parameter></paramdef>
44      </funcprototype>
45
46      <funcprototype>
47        <funcdef>int <function>sd_event_add_child</function></funcdef>
48        <paramdef>sd_event *<parameter>event</parameter></paramdef>
49        <paramdef>sd_event_source **<parameter>source</parameter></paramdef>
50        <paramdef>pid_t <parameter>pid</parameter></paramdef>
51        <paramdef>int <parameter>options</parameter></paramdef>
52        <paramdef>sd_event_child_handler_t <parameter>handler</parameter></paramdef>
53        <paramdef>void *<parameter>userdata</parameter></paramdef>
54      </funcprototype>
55
56      <funcprototype>
57        <funcdef>int <function>sd_event_add_child_pidfd</function></funcdef>
58        <paramdef>sd_event *<parameter>event</parameter></paramdef>
59        <paramdef>sd_event_source **<parameter>source</parameter></paramdef>
60        <paramdef>int <parameter>pidfd</parameter></paramdef>
61        <paramdef>int <parameter>options</parameter></paramdef>
62        <paramdef>sd_event_child_handler_t <parameter>handler</parameter></paramdef>
63        <paramdef>void *<parameter>userdata</parameter></paramdef>
64      </funcprototype>
65
66      <funcprototype>
67        <funcdef>int <function>sd_event_source_get_child_pid</function></funcdef>
68        <paramdef>sd_event_source *<parameter>source</parameter></paramdef>
69        <paramdef>pid_t *<parameter>pid</parameter></paramdef>
70      </funcprototype>
71
72      <funcprototype>
73        <funcdef>int <function>sd_event_source_get_child_pidfd</function></funcdef>
74        <paramdef>sd_event_source *<parameter>source</parameter></paramdef>
75      </funcprototype>
76
77      <funcprototype>
78        <funcdef>int <function>sd_event_source_get_child_pidfd_own</function></funcdef>
79        <paramdef>sd_event_source *<parameter>source</parameter></paramdef>
80      </funcprototype>
81
82      <funcprototype>
83        <funcdef>int <function>sd_event_source_set_child_pidfd_own</function></funcdef>
84        <paramdef>sd_event_source *<parameter>source</parameter></paramdef>
85        <paramdef>int <parameter>own</parameter></paramdef>
86      </funcprototype>
87
88      <funcprototype>
89        <funcdef>int <function>sd_event_source_get_child_process_own</function></funcdef>
90        <paramdef>sd_event_source *<parameter>source</parameter></paramdef>
91      </funcprototype>
92
93      <funcprototype>
94        <funcdef>int <function>sd_event_source_set_child_process_own</function></funcdef>
95        <paramdef>sd_event_source *<parameter>source</parameter></paramdef>
96        <paramdef>int <parameter>own</parameter></paramdef>
97      </funcprototype>
98
99      <funcprototype>
100        <funcdef>int <function>sd_event_source_send_child_signal</function></funcdef>
101        <paramdef>sd_event_source *<parameter>source</parameter></paramdef>
102        <paramdef>int <parameter>sig</parameter></paramdef>
103        <paramdef>const siginfo_t *<parameter>info</parameter></paramdef>
104        <paramdef>unsigned <parameter>flags</parameter></paramdef>
105      </funcprototype>
106
107    </funcsynopsis>
108  </refsynopsisdiv>
109
110  <refsect1>
111    <title>Description</title>
112
113    <para><function>sd_event_add_child()</function> adds a new child process state change event source to an
114    event loop. The event loop object is specified in the <parameter>event</parameter> parameter, the event
115    source object is returned in the <parameter>source</parameter> parameter. The <parameter>pid</parameter>
116    parameter specifies the PID of the process to watch, which must be a direct child process of the invoking
117    process. The <parameter>options</parameter> parameter determines which state changes will be watched for.
118    It must contain an OR-ed mask of <constant>WEXITED</constant> (watch for the child process terminating),
119    <constant>WSTOPPED</constant> (watch for the child process being stopped by a signal), and
120    <constant>WCONTINUED</constant> (watch for the child process being resumed by a signal). See
121    <citerefentry project='man-pages'><refentrytitle>waitid</refentrytitle><manvolnum>2</manvolnum></citerefentry>
122    for further information.</para>
123
124    <para>The <parameter>handler</parameter> must be a function to call when the process changes state or
125    <constant>NULL</constant>. The handler function will be passed the <parameter>userdata</parameter>
126    pointer, which may be chosen freely by the caller. The handler also receives a pointer to a
127    <structname>siginfo_t</structname> structure containing information about the child process event. The
128    handler may return negative to signal an error (see below), other return values are ignored. If
129    <parameter>handler</parameter> is <constant>NULL</constant>, a default handler that calls
130    <citerefentry><refentrytitle>sd_event_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry> will be
131    used.</para>
132
133    <para>Only a single handler may be installed for a specific child process. The handler is enabled for a
134    single event (<constant>SD_EVENT_ONESHOT</constant>), but this may be changed with
135    <citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
136    If the handler function returns a negative error code, it will either be disabled after the invocation,
137    even if the <constant>SD_EVENT_ON</constant> mode was requested before, or it will cause the loop to
138    terminate, see
139    <citerefentry><refentrytitle>sd_event_source_set_exit_on_failure</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
140    </para>
141
142    <para>To destroy an event source object use
143    <citerefentry><refentrytitle>sd_event_source_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
144    but note that the event source is only removed from the event loop
145    when all references to the event source are dropped. To make sure
146    an event source does not fire anymore, even when there's still a
147    reference to it kept, consider setting the event source to
148    <constant>SD_EVENT_OFF</constant> with
149    <citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
150
151    <para>The <constant>SIGCHLD</constant> signal must be blocked in all threads before this function is
152    called (using <citerefentry
153    project='man-pages'><refentrytitle>sigprocmask</refentrytitle><manvolnum>2</manvolnum></citerefentry> or
154    <citerefentry
155    project='man-pages'><refentrytitle>pthread_sigmask</refentrytitle><manvolnum>3</manvolnum></citerefentry>).</para>
156
157    <para>If the second parameter of <function>sd_event_add_child()</function> is passed as
158    <constant>NULL</constant> no reference to the event source object is returned. In this case the event
159    source is considered "floating", and will be destroyed implicitly when the event loop itself is
160    destroyed.</para>
161
162    <para>Note that the <parameter>handler</parameter> function is
163    invoked at a time where the child process is not reaped yet (and
164    thus still is exposed as a zombie process by the kernel). However,
165    the child will be reaped automatically after the function
166    returns. Child processes for which no child process state change
167    event sources are installed will not be reaped by the event loop
168    implementation.</para>
169
170    <para>If the <parameter>handler</parameter> parameter to <function>sd_event_add_child()</function> is
171    <constant>NULL</constant>, and the event source fires, this will be considered a request to exit the
172    event loop. In this case, the <parameter>userdata</parameter> parameter, cast to an integer, is passed as
173    the exit code parameter to
174    <citerefentry><refentrytitle>sd_event_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
175
176    <para>If both a child process state change event source and a
177    <constant>SIGCHLD</constant> signal event source is installed in
178    the same event loop, the configured event source priorities decide
179    which event source is dispatched first. If the signal handler is
180    processed first, it should leave the child processes for which
181    child process state change event sources are installed unreaped.</para>
182
183    <para><function>sd_event_add_child_pidfd()</function> is similar to
184    <function>sd_event_add_child()</function> but takes a file descriptor referencing the process ("pidfd")
185    instead of the numeric PID. A suitable file descriptor may be acquired via <citerefentry
186    project='man-pages'><refentrytitle>pidfd_open</refentrytitle><manvolnum>2</manvolnum></citerefentry> and
187    related calls. The passed file descriptor is not closed when the event source is freed again, unless
188    <function>sd_event_source_set_child_pidfd_own()</function> is used to turn this behaviour on. Note that
189    regardless which of <function>sd_event_add_child()</function> and
190    <function>sd_event_add_child_pidfd()</function> is used for allocating an event source, the watched
191    process has to be a direct child process of the invoking process. Also in both cases
192    <constant>SIGCHLD</constant> has to be blocked in the invoking process.</para>
193
194    <para><function>sd_event_source_get_child_pid()</function>
195    retrieves the configured PID of a child process state change event
196    source created previously with
197    <function>sd_event_add_child()</function>. It takes the event
198    source object as the <parameter>source</parameter> parameter and a
199    pointer to a <type>pid_t</type> variable to return the process ID
200    in.
201    </para>
202
203    <para><function>sd_event_source_get_child_pidfd()</function> retrieves the file descriptor referencing
204    the watched process ("pidfd") if this functionality is available. On kernels that support the concept the
205    event loop will make use of pidfds to watch child processes, regardless if the individual event sources
206    are allocated via <function>sd_event_add_child()</function> or
207    <function>sd_event_add_child_pidfd()</function>. If the latter call was used to allocate the event
208    source, this function returns the file descriptor used for allocation. On kernels that do not support the
209    pidfd concept this function will fail with <constant>EOPNOTSUPP</constant>. This call takes the event
210    source object as the <parameter>source</parameter> parameter and returns the numeric file descriptor.
211    </para>
212
213    <para><function>sd_event_source_get_child_pidfd_own()</function> may be used to query whether the pidfd
214    the event source encapsulates shall be closed when the event source is freed. This function returns zero
215    if the pidfd shall be left open, and positive if it shall be closed automatically. By default this
216    setting defaults to on if the event source was allocated via <function>sd_event_add_child()</function>
217    and off if it was allocated via <function>sd_event_add_child_pidfd()</function>. The
218    <function>sd_event_source_set_child_pidfd_own()</function> function may be used to change the setting and
219    takes a boolean parameter with the new setting.</para>
220
221    <para><function>sd_event_source_get_child_process_own()</function> may be used to query whether the
222    process the event source watches shall be killed (with <constant>SIGKILL</constant>) and reaped when the
223    event source is freed. This function returns zero if the process shell be left running, and positive if
224    it shall be killed and reaped automatically. By default this setting defaults to off. The
225    <function>sd_event_source_set_child_process_own()</function> function may be used to change the setting
226    and takes a boolean parameter with the new setting. Note that currently if the calling process is
227    terminated abnormally the watched process might survive even thought the event source ceases to
228    exist. This behaviour might change eventually.</para>
229
230    <para><function>sd_event_source_send_child_signal()</function> may be used to send a UNIX signal to the
231    watched process. If the pidfd concept is supported in the kernel, this is implemented via <citerefentry
232    project='man-pages'><refentrytitle>pidfd_send_signal</refentrytitle><manvolnum>2</manvolnum></citerefentry>
233    and otherwise via <citerefentry
234    project='man-pages'><refentrytitle>rt_sigqueueinfo</refentrytitle><manvolnum>2</manvolnum></citerefentry>
235    (or via <citerefentry
236    project='man-pages'><refentrytitle>kill</refentrytitle><manvolnum>2</manvolnum></citerefentry> in case
237    <parameter>info</parameter> is <constant>NULL</constant>). The specified parameters match those of these
238    underlying system calls, except that the <parameter>info</parameter> is never modified (and is thus
239    declared constant). Like for the underlying system calls, the <parameter>flags</parameter> parameter
240    currently must be zero.</para>
241  </refsect1>
242
243  <refsect1>
244    <title>Return Value</title>
245
246    <para>On success, these functions return 0 or a positive
247    integer. On failure, they return a negative errno-style error
248    code.</para>
249
250    <refsect2>
251      <title>Errors</title>
252
253      <para>Returned errors may indicate the following problems:</para>
254
255      <variablelist>
256        <varlistentry>
257          <term><constant>-ENOMEM</constant></term>
258
259          <listitem><para>Not enough memory to allocate an object.</para></listitem>
260        </varlistentry>
261
262        <varlistentry>
263          <term><constant>-EINVAL</constant></term>
264
265          <listitem><para>An invalid argument has been passed. This includes
266          specifying an empty mask in <parameter>options</parameter> or a mask
267          which contains values different than a combination of
268          <constant>WEXITED</constant>, <constant>WSTOPPED</constant>, and
269          <constant>WCONTINUED</constant>.
270          </para></listitem>
271
272        </varlistentry>
273
274        <varlistentry>
275          <term><constant>-EBUSY</constant></term>
276
277          <listitem><para>A handler is already installed for this child process, or
278          <constant>SIGCHLD</constant> is not blocked.</para></listitem>
279
280        </varlistentry>
281
282        <varlistentry>
283          <term><constant>-ESTALE</constant></term>
284
285          <listitem><para>The event loop is already terminated.</para></listitem>
286
287        </varlistentry>
288
289        <varlistentry>
290          <term><constant>-ECHILD</constant></term>
291
292          <listitem><para>The event loop has been created in a different process.</para></listitem>
293
294        </varlistentry>
295
296        <varlistentry>
297          <term><constant>-EDOM</constant></term>
298
299          <listitem><para>The passed event source is not a child process event source.</para></listitem>
300        </varlistentry>
301
302        <varlistentry>
303          <term><constant>-EOPNOTSUPP</constant></term>
304
305          <listitem><para>A pidfd was requested but the kernel does not support this concept.</para></listitem>
306        </varlistentry>
307
308      </variablelist>
309    </refsect2>
310  </refsect1>
311
312  <xi:include href="libsystemd-pkgconfig.xml" />
313
314  <refsect1>
315    <title>Example</title>
316
317    <example>
318      <title>Exit loop when the child terminates</title>
319
320      <programlisting><xi:include href="event-quick-child.c" parse="text" /></programlisting>
321    </example>
322  </refsect1>
323
324  <refsect1>
325    <title>See Also</title>
326
327    <para>
328      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
329      <citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
330      <citerefentry><refentrytitle>sd_event_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
331      <citerefentry><refentrytitle>sd_event_now</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
332      <citerefentry><refentrytitle>sd_event_add_io</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
333      <citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
334      <citerefentry><refentrytitle>sd_event_add_signal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
335      <citerefentry><refentrytitle>sd_event_add_inotify</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
336      <citerefentry><refentrytitle>sd_event_add_defer</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
337      <citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
338      <citerefentry><refentrytitle>sd_event_source_set_priority</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
339      <citerefentry><refentrytitle>sd_event_source_set_userdata</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
340      <citerefentry><refentrytitle>sd_event_source_set_description</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
341      <citerefentry><refentrytitle>sd_event_source_set_floating</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
342      <citerefentry project='man-pages'><refentrytitle>waitid</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
343      <citerefentry project='man-pages'><refentrytitle>sigprocmask</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
344      <citerefentry project='man-pages'><refentrytitle>pthread_sigmask</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
345      <citerefentry project='man-pages'><refentrytitle>pidfd_open</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
346      <citerefentry project='man-pages'><refentrytitle>pidfd_send_signal</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
347      <citerefentry project='man-pages'><refentrytitle>rt_sigqueueinfo</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
348      <citerefentry project='man-pages'><refentrytitle>kill</refentrytitle><manvolnum>2</manvolnum></citerefentry>
349    </para>
350  </refsect1>
351
352</refentry>
353