man/sd_event_add_child.xml

   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+ -->
   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_source_get_child_pid</refname>
  21     <refname>sd_event_child_handler_t</refname>
  22
  23     <refpurpose>Add a child process state change event source to an event loop</refpurpose>
  24   </refnamediv>
  25
  26   <refsynopsisdiv>
  27     <funcsynopsis>
  28       <funcsynopsisinfo>#include &lt;systemd/sd-event.h&gt;</funcsynopsisinfo>
  29
  30       <funcsynopsisinfo><token>typedef</token> struct sd_event_source sd_event_source;</funcsynopsisinfo>
  31
  32       <funcprototype>
  33         <funcdef>typedef int (*<function>sd_event_child_handler_t</function>)</funcdef>
  34         <paramdef>sd_event_source *<parameter>s</parameter></paramdef>
  35         <paramdef>const siginfo_t *<parameter>si</parameter></paramdef>
  36         <paramdef>void *<parameter>userdata</parameter></paramdef>
  37       </funcprototype>
  38
  39       <funcprototype>
  40         <funcdef>int <function>sd_event_add_child</function></funcdef>
  41         <paramdef>sd_event *<parameter>event</parameter></paramdef>
  42         <paramdef>sd_event_source **<parameter>source</parameter></paramdef>
  43         <paramdef>pid_t <parameter>pid</parameter></paramdef>
  44         <paramdef>int <parameter>options</parameter></paramdef>
  45         <paramdef>sd_event_child_handler_t <parameter>handler</parameter></paramdef>
  46         <paramdef>void *<parameter>userdata</parameter></paramdef>
  47       </funcprototype>
  48
  49       <funcprototype>
  50         <funcdef>int <function>sd_event_source_get_child_pid</function></funcdef>
  51         <paramdef>sd_event_source *<parameter>source</parameter></paramdef>
  52         <paramdef>pid_t *<parameter>pid</parameter></paramdef>
  53       </funcprototype>
  54
  55     </funcsynopsis>
  56   </refsynopsisdiv>
  57
  58   <refsect1>
  59     <title>Description</title>
  60
  61     <para><function>sd_event_add_child()</function> adds a new child
  62     process state change event source to an event loop. The event loop
  63     object is specified in the <parameter>event</parameter> parameter,
  64     the event source object is returned in the
  65     <parameter>source</parameter> parameter. The
  66     <parameter>pid</parameter> parameter specifies the PID of the
  67     process to watch. The <parameter>handler</parameter> must
  68     reference a function to call when the process changes state. The
  69     handler function will be passed the
  70     <parameter>userdata</parameter> pointer, which may be chosen
  71     freely by the caller. The handler also receives a pointer to a
  72     <structname>siginfo_t</structname> structure containing
  73     information about the child process event. The
  74     <parameter>options</parameter> parameter determines which state
  75     changes will be watched for. It must contain an OR-ed mask of
  76     <constant>WEXITED</constant> (watch for the child process
  77     terminating), <constant>WSTOPPED</constant> (watch for the child
  78     process being stopped by a signal), and
  79     <constant>WCONTINUED</constant> (watch for the child process being
  80     resumed by a signal). See <citerefentry
  81     project='man-pages'><refentrytitle>waitid</refentrytitle><manvolnum>2</manvolnum></citerefentry>
  82     for further information.</para>
  83
  84     <para>Only a single handler may be installed for a specific
  85     child process. The handler is enabled for a single event
  86     (<constant>SD_EVENT_ONESHOT</constant>), but this may be changed
  87     with
  88     <citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
  89     If the handler function returns a negative error code, it will be
  90     disabled after the invocation, even if the
  91     <constant>SD_EVENT_ON</constant> mode was requested before.
  92     </para>
  93
  94     <para>To destroy an event source object use
  95     <citerefentry><refentrytitle>sd_event_source_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
  96     but note that the event source is only removed from the event loop
  97     when all references to the event source are dropped. To make sure
  98     an event source does not fire anymore, even when there's still a
  99     reference to it kept, consider setting the event source to
 100     <constant>SD_EVENT_OFF</constant> with
 101     <citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
 102
 103     <para>If the second parameter of
 104     <function>sd_event_add_child()</function> is passed as NULL no
 105     reference to the event source object is returned. In this case the
 106     event source is considered "floating", and will be destroyed
 107     implicitly when the event loop itself is destroyed.</para>
 108
 109     <para>Note that the <parameter>handler</parameter> function is
 110     invoked at a time where the child process is not reaped yet (and
 111     thus still is exposed as a zombie process by the kernel). However,
 112     the child will be reaped automatically after the function
 113     returns. Child processes for which no child process state change
 114     event sources are installed will not be reaped by the event loop
 115     implementation.</para>
 116
 117     <para>If both a child process state change event source and a
 118     <constant>SIGCHLD</constant> signal event source is installed in
 119     the same event loop, the configured event source priorities decide
 120     which event source is dispatched first. If the signal handler is
 121     processed first, it should leave the child processes for which
 122     child process state change event sources are installed unreaped.</para>
 123
 124     <para><function>sd_event_source_get_child_pid()</function>
 125     retrieves the configured PID of a child process state change event
 126     source created previously with
 127     <function>sd_event_add_child()</function>. It takes the event
 128     source object as the <parameter>source</parameter> parameter and a
 129     pointer to a <type>pid_t</type> variable to return the process ID
 130     in.
 131     </para>
 132   </refsect1>
 133
 134   <refsect1>
 135     <title>Return Value</title>
 136
 137     <para>On success, these functions return 0 or a positive
 138     integer. On failure, they return a negative errno-style error
 139     code.</para>
 140
 141     <refsect2>
 142       <title>Errors</title>
 143
 144       <para>Returned errors may indicate the following problems:</para>
 145
 146       <variablelist>
 147         <varlistentry>
 148           <term><constant>-ENOMEM</constant></term>
 149
 150           <listitem><para>Not enough memory to allocate an object.</para></listitem>
 151         </varlistentry>
 152
 153         <varlistentry>
 154           <term><constant>-EINVAL</constant></term>
 155
 156           <listitem><para>An invalid argument has been passed. This includes
 157           specifying an empty mask in <parameter>options</parameter> or a mask
 158           which contains values different than a combination of
 159           <constant>WEXITED</constant>, <constant>WSTOPPED</constant>, and
 160           <constant>WCONTINUED</constant>.
 161           </para></listitem>
 162
 163         </varlistentry>
 164
 165         <varlistentry>
 166           <term><constant>-EBUSY</constant></term>
 167
 168           <listitem><para>A handler is already installed for this
 169           child process.</para></listitem>
 170
 171         </varlistentry>
 172
 173         <varlistentry>
 174           <term><constant>-ESTALE</constant></term>
 175
 176           <listitem><para>The event loop is already terminated.</para></listitem>
 177
 178         </varlistentry>
 179
 180         <varlistentry>
 181           <term><constant>-ECHILD</constant></term>
 182
 183           <listitem><para>The event loop has been created in a different process.</para></listitem>
 184
 185         </varlistentry>
 186
 187         <varlistentry>
 188           <term><constant>-EDOM</constant></term>
 189
 190           <listitem><para>The passed event source is not a child process event source.</para></listitem>
 191         </varlistentry>
 192
 193       </variablelist>
 194     </refsect2>
 195   </refsect1>
 196
 197   <xi:include href="libsystemd-pkgconfig.xml" />
 198
 199   <refsect1>
 200     <title>See Also</title>
 201
 202     <para>
 203       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 204       <citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 205       <citerefentry><refentrytitle>sd_event_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 206       <citerefentry><refentrytitle>sd_event_now</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 207       <citerefentry><refentrytitle>sd_event_add_io</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 208       <citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 209       <citerefentry><refentrytitle>sd_event_add_signal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 210       <citerefentry><refentrytitle>sd_event_add_inotify</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 211       <citerefentry><refentrytitle>sd_event_add_defer</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 212       <citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 213       <citerefentry><refentrytitle>sd_event_source_set_priority</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 214       <citerefentry><refentrytitle>sd_event_source_set_userdata</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 215       <citerefentry><refentrytitle>sd_event_source_set_description</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 216       <citerefentry project='man-pages'><refentrytitle>waitid</refentrytitle><manvolnum>2</manvolnum></citerefentry>
 217     </para>
 218   </refsect1>
 219
 220 </refentry>