man/sd_event_add_child.xml

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