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5ca99dfabd
The thing with blocking SIGCHLD is rather annoying. I think we could/should make this automatic.
353 lines
19 KiB
XML
353 lines
19 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-or-later -->
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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>options</parameter> parameter determines which state changes will be watched for.
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It must contain an OR-ed mask of <constant>WEXITED</constant> (watch for the child process terminating),
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<constant>WSTOPPED</constant> (watch for the child process being stopped by a signal), and
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<constant>WCONTINUED</constant> (watch for the child process being resumed by a signal). See
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<citerefentry project='man-pages'><refentrytitle>waitid</refentrytitle><manvolnum>2</manvolnum></citerefentry>
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for further information.</para>
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<para>The <parameter>handler</parameter> must be a function to call when the process changes state or
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<constant>NULL</constant>. The handler function will be passed the <parameter>userdata</parameter>
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pointer, which may be chosen freely by the caller. The handler also receives a pointer to a
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<structname>siginfo_t</structname> structure containing information about the child process event. The
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handler may return negative to signal an error (see below), other return values are ignored. If
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<parameter>handler</parameter> is <constant>NULL</constant>, a default handler that calls
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<citerefentry><refentrytitle>sd_event_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry> will be
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used.</para>
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<para>Only a single handler may be installed for a specific child process. The handler is enabled for a
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single event (<constant>SD_EVENT_ONESHOT</constant>), but this may be changed 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 either be disabled after the invocation,
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even if the <constant>SD_EVENT_ON</constant> mode was requested before, or it will cause the loop to
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terminate, see
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<citerefentry><refentrytitle>sd_event_source_set_exit_on_failure</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
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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>Example</title>
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<example>
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<title>Exit loop when the child terminates</title>
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<programlisting><xi:include href="event-quick-child.c" parse="text" /></programlisting>
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</example>
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</refsect1>
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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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