systemd/man/sd_event_add_child.xml
Zbigniew Jędrzejewski-Szmek 5ca99dfabd man: add example of sd_event_add_child()
The thing with blocking SIGCHLD is rather annoying. I think we could/should
make this automatic.
2022-01-05 15:19:13 +01:00

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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>