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Also, document this functionality more prominently, including with a reference from sd_event_exit(). This is mostly to make things complete, as previously we supported NULL callbacks only in _add_time() and _add_signal(). However, I think this makes snese for IO event sources too (think: when some fd such as a pipe end sees SIGHUP or so, exit), as well as defer or post event sources (i.e. exit once we got nothing else to do). This also adds support for inotify event sources, simply to complete things (I can't see the immediate use, but maybe someone else comes up with it). The only event source type that doesn't allow callback=NULL now are exit callbacks, but for them they make little sense, as the event loop is exiting then anyway.
339 lines
18 KiB
XML
339 lines
18 KiB
XML
<?xml version='1.0'?>
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1+ -->
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<refentry id="sd_event_add_child" xmlns:xi="http://www.w3.org/2001/XInclude">
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<refentryinfo>
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<title>sd_event_add_child</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>sd_event_add_child</refentrytitle>
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<manvolnum>3</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>sd_event_add_child</refname>
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<refname>sd_event_add_child_pidfd</refname>
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<refname>sd_event_source_get_child_pid</refname>
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<refname>sd_event_source_get_child_pidfd</refname>
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<refname>sd_event_source_get_child_pidfd_own</refname>
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<refname>sd_event_source_set_child_pidfd_own</refname>
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<refname>sd_event_source_get_child_process_own</refname>
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<refname>sd_event_source_set_child_process_own</refname>
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<refname>sd_event_source_send_child_signal</refname>
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<refname>sd_event_child_handler_t</refname>
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<refpurpose>Add a child process state change event source to an event loop</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<funcsynopsis>
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<funcsynopsisinfo>#include <systemd/sd-event.h></funcsynopsisinfo>
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<funcsynopsisinfo><token>typedef</token> struct sd_event_source sd_event_source;</funcsynopsisinfo>
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<funcprototype>
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<funcdef>typedef int (*<function>sd_event_child_handler_t</function>)</funcdef>
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<paramdef>sd_event_source *<parameter>s</parameter></paramdef>
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<paramdef>const siginfo_t *<parameter>si</parameter></paramdef>
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<paramdef>void *<parameter>userdata</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_event_add_child</function></funcdef>
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<paramdef>sd_event *<parameter>event</parameter></paramdef>
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<paramdef>sd_event_source **<parameter>source</parameter></paramdef>
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<paramdef>pid_t <parameter>pid</parameter></paramdef>
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<paramdef>int <parameter>options</parameter></paramdef>
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<paramdef>sd_event_child_handler_t <parameter>handler</parameter></paramdef>
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<paramdef>void *<parameter>userdata</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_event_add_child_pidfd</function></funcdef>
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<paramdef>sd_event *<parameter>event</parameter></paramdef>
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<paramdef>sd_event_source **<parameter>source</parameter></paramdef>
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<paramdef>int <parameter>pidfd</parameter></paramdef>
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<paramdef>int <parameter>options</parameter></paramdef>
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<paramdef>sd_event_child_handler_t <parameter>handler</parameter></paramdef>
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<paramdef>void *<parameter>userdata</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_event_source_get_child_pid</function></funcdef>
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<paramdef>sd_event_source *<parameter>source</parameter></paramdef>
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<paramdef>pid_t *<parameter>pid</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_event_source_get_child_pidfd</function></funcdef>
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<paramdef>sd_event_source *<parameter>source</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_event_source_get_child_pidfd_own</function></funcdef>
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<paramdef>sd_event_source *<parameter>source</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_event_source_set_child_pidfd_own</function></funcdef>
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<paramdef>sd_event_source *<parameter>source</parameter></paramdef>
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<paramdef>int <parameter>own</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_event_source_get_child_process_own</function></funcdef>
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<paramdef>sd_event_source *<parameter>source</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_event_source_set_child_process_own</function></funcdef>
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<paramdef>sd_event_source *<parameter>source</parameter></paramdef>
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<paramdef>int <parameter>own</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_event_source_send_child_signal</function></funcdef>
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<paramdef>sd_event_source *<parameter>source</parameter></paramdef>
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<paramdef>int <parameter>sig</parameter></paramdef>
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<paramdef>const siginfo_t *<parameter>info</parameter></paramdef>
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<paramdef>unsigned <parameter>flags</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para><function>sd_event_add_child()</function> adds a new child process state change event source to an
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event loop. The event loop object is specified in the <parameter>event</parameter> parameter, the event
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source object is returned in the <parameter>source</parameter> parameter. The <parameter>pid</parameter>
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parameter specifies the PID of the process to watch, which must be a direct child process of the invoking
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process. The <parameter>handler</parameter> must reference a function to call when the process changes
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state. The handler function will be passed the <parameter>userdata</parameter> pointer, which may be
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chosen freely by the caller. The handler also receives a pointer to a <structname>siginfo_t</structname>
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structure containing information about the child process event. The <parameter>options</parameter>
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parameter determines which state changes will be watched for. It must contain an OR-ed mask of
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<constant>WEXITED</constant> (watch for the child process terminating), <constant>WSTOPPED</constant>
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(watch for the child process being stopped by a signal), and <constant>WCONTINUED</constant> (watch for
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the child process being resumed by a signal). See <citerefentry
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project='man-pages'><refentrytitle>waitid</refentrytitle><manvolnum>2</manvolnum></citerefentry> for
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further information.</para>
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<para>Only a single handler may be installed for a specific
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child process. The handler is enabled for a single event
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(<constant>SD_EVENT_ONESHOT</constant>), but this may be changed
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with
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<citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
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If the handler function returns a negative error code, it will be
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disabled after the invocation, even if the
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<constant>SD_EVENT_ON</constant> mode was requested before.
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</para>
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<para>To destroy an event source object use
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<citerefentry><refentrytitle>sd_event_source_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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but note that the event source is only removed from the event loop
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when all references to the event source are dropped. To make sure
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an event source does not fire anymore, even when there's still a
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reference to it kept, consider setting the event source to
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<constant>SD_EVENT_OFF</constant> with
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<citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
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<para>The <constant>SIGCHLD</constant> signal must be blocked in all threads before this function is
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called (using <citerefentry
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project='man-pages'><refentrytitle>sigprocmask</refentrytitle><manvolnum>2</manvolnum></citerefentry> or
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<citerefentry
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project='man-pages'><refentrytitle>pthread_sigmask</refentrytitle><manvolnum>3</manvolnum></citerefentry>).</para>
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<para>If the second parameter of <function>sd_event_add_child()</function> is passed as
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<constant>NULL</constant> no reference to the event source object is returned. In this case the event
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source is considered "floating", and will be destroyed implicitly when the event loop itself is
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destroyed.</para>
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<para>Note that the <parameter>handler</parameter> function is
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invoked at a time where the child process is not reaped yet (and
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thus still is exposed as a zombie process by the kernel). However,
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the child will be reaped automatically after the function
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returns. Child processes for which no child process state change
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event sources are installed will not be reaped by the event loop
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implementation.</para>
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<para>If the <parameter>handler</parameter> parameter to <function>sd_event_add_child()</function> is
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<constant>NULL</constant>, and the event source fires, this will be considered a request to exit the
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event loop. In this case, the <parameter>userdata</parameter> parameter, cast to an integer, is passed as
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the exit code parameter to
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<citerefentry><refentrytitle>sd_event_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
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<para>If both a child process state change event source and a
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<constant>SIGCHLD</constant> signal event source is installed in
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the same event loop, the configured event source priorities decide
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which event source is dispatched first. If the signal handler is
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processed first, it should leave the child processes for which
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child process state change event sources are installed unreaped.</para>
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<para><function>sd_event_add_child_pidfd()</function> is similar to
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<function>sd_event_add_child()</function> but takes a file descriptor referencing the process ("pidfd")
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instead of the numeric PID. A suitable file descriptor may be acquired via <citerefentry
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project='man-pages'><refentrytitle>pidfd_open</refentrytitle><manvolnum>2</manvolnum></citerefentry> and
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related calls. The passed file descriptor is not closed when the event source is freed again, unless
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<function>sd_event_source_set_child_pidfd_own()</function> is used to turn this behaviour on. Note that
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regardless which of <function>sd_event_add_child()</function> and
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<function>sd_event_add_child_pidfd()</function> is used for allocating an event source, the watched
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process has to be a direct child process of the invoking process. Also in both cases
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<constant>SIGCHLD</constant> has to be blocked in the invoking process.</para>
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<para><function>sd_event_source_get_child_pid()</function>
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retrieves the configured PID of a child process state change event
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source created previously with
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<function>sd_event_add_child()</function>. It takes the event
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source object as the <parameter>source</parameter> parameter and a
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pointer to a <type>pid_t</type> variable to return the process ID
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in.
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</para>
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<para><function>sd_event_source_get_child_pidfd()</function> retrieves the file descriptor referencing
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the watched process ("pidfd") if this functionality is available. On kernels that support the concept the
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event loop will make use of pidfds to watch child processes, regardless if the individual event sources
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are allocated via <function>sd_event_add_child()</function> or
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<function>sd_event_add_child_pidfd()</function>. If the latter call was used to allocate the event
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source, this function returns the file descriptor used for allocation. On kernels that do not support the
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pidfd concept this function will fail with <constant>EOPNOTSUPP</constant>. This call takes the event
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source object as the <parameter>source</parameter> parameter and returns the numeric file descriptor.
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</para>
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<para><function>sd_event_source_get_child_pidfd_own()</function> may be used to query whether the pidfd
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the event source encapsulates shall be closed when the event source is freed. This function returns zero
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if the pidfd shall be left open, and positive if it shall be closed automatically. By default this
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setting defaults to on if the event source was allocated via <function>sd_event_add_child()</function>
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and off if it was allocated via <function>sd_event_add_child_pidfd()</function>. The
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<function>sd_event_source_set_child_pidfd_own()</function> function may be used to change the setting and
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takes a boolean parameter with the new setting.</para>
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<para><function>sd_event_source_get_child_process_own()</function> may be used to query whether the
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process the event source watches shall be killed (with <constant>SIGKILL</constant>) and reaped when the
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event source is freed. This function returns zero if the process shell be left running, and positive if
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it shall be killed and reaped automatically. By default this setting defaults to off. The
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<function>sd_event_source_set_child_process_own()</function> function may be used to change the setting
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and takes a boolean parameter with the new setting. Note that currently if the calling process is
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terminated abnormally the watched process might survive even thought the event source ceases to
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exist. This behaviour might change eventually.</para>
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<para><function>sd_event_source_send_child_signal()</function> may be used to send a UNIX signal to the
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watched process. If the pidfd concept is supported in the kernel, this is implemented via <citerefentry
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project='man-pages'><refentrytitle>pidfd_send_signal</refentrytitle><manvolnum>2</manvolnum></citerefentry>
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and otherwise via <citerefentry
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project='man-pages'><refentrytitle>rt_sigqueueinfo</refentrytitle><manvolnum>2</manvolnum></citerefentry>
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(or via <citerefentry
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project='man-pages'><refentrytitle>kill</refentrytitle><manvolnum>2</manvolnum></citerefentry> in case
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<parameter>info</parameter> is <constant>NULL</constant>). The specified parameters match those of these
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underlying system calls, except that the <parameter>info</parameter> is never modified (and is thus
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declared constant). Like for the underlying system calls, the <parameter>flags</parameter> parameter
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currently must be zero.</para>
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</refsect1>
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<refsect1>
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<title>Return Value</title>
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<para>On success, these functions return 0 or a positive
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integer. On failure, they return a negative errno-style error
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code.</para>
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<refsect2>
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<title>Errors</title>
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<para>Returned errors may indicate the following problems:</para>
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<variablelist>
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<varlistentry>
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<term><constant>-ENOMEM</constant></term>
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<listitem><para>Not enough memory to allocate an object.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>-EINVAL</constant></term>
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<listitem><para>An invalid argument has been passed. This includes
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specifying an empty mask in <parameter>options</parameter> or a mask
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which contains values different than a combination of
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<constant>WEXITED</constant>, <constant>WSTOPPED</constant>, and
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<constant>WCONTINUED</constant>.
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</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>-EBUSY</constant></term>
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<listitem><para>A handler is already installed for this child process, or
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<constant>SIGCHLD</constant> is not blocked.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>-ESTALE</constant></term>
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<listitem><para>The event loop is already terminated.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>-ECHILD</constant></term>
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<listitem><para>The event loop has been created in a different process.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>-EDOM</constant></term>
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<listitem><para>The passed event source is not a child process event source.</para></listitem>
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</varlistentry>
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<varlistentry>
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<term><constant>-EOPNOTSUPP</constant></term>
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<listitem><para>A pidfd was requested but the kernel does not support this concept.</para></listitem>
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</varlistentry>
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</variablelist>
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</refsect2>
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</refsect1>
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<xi:include href="libsystemd-pkgconfig.xml" />
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<refsect1>
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<title>See Also</title>
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<para>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_now</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_add_io</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_add_signal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_add_inotify</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_add_defer</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_source_set_priority</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_source_set_userdata</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_source_set_description</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_event_source_set_floating</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry project='man-pages'><refentrytitle>waitid</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
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<citerefentry project='man-pages'><refentrytitle>sigprocmask</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
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<citerefentry project='man-pages'><refentrytitle>pthread_sigmask</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry project='man-pages'><refentrytitle>pidfd_open</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
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<citerefentry project='man-pages'><refentrytitle>pidfd_send_signal</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
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<citerefentry project='man-pages'><refentrytitle>rt_sigqueueinfo</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
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<citerefentry project='man-pages'><refentrytitle>kill</refentrytitle><manvolnum>2</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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