man/sd_event_add_child.xml

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   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>options</parameter> parameter determines which state changes will be watched for.
 118     It must contain an OR-ed mask of <constant>WEXITED</constant> (watch for the child process terminating),
 119     <constant>WSTOPPED</constant> (watch for the child process being stopped by a signal), and
 120     <constant>WCONTINUED</constant> (watch for the child process being resumed by a signal). See
 121     <citerefentry project='man-pages'><refentrytitle>waitid</refentrytitle><manvolnum>2</manvolnum></citerefentry>
 122     for further information.</para>
 123
 124     <para>The <parameter>handler</parameter> must be a function to call when the process changes state or
 125     <constant>NULL</constant>. The handler function will be passed the <parameter>userdata</parameter>
 126     pointer, which may be chosen freely by the caller. The handler also receives a pointer to a
 127     <structname>siginfo_t</structname> structure containing information about the child process event. The
 128     handler may return negative to signal an error (see below), other return values are ignored. If
 129     <parameter>handler</parameter> is <constant>NULL</constant>, a default handler that calls
 130     <citerefentry><refentrytitle>sd_event_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry> will be
 131     used.</para>
 132
 133     <para>Only a single handler may be installed for a specific child process. The handler is enabled for a
 134     single event (<constant>SD_EVENT_ONESHOT</constant>), but this may be changed with
 135     <citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
 136     If the handler function returns a negative error code, it will either be disabled after the invocation,
 137     even if the <constant>SD_EVENT_ON</constant> mode was requested before, or it will cause the loop to
 138     terminate, see
 139     <citerefentry><refentrytitle>sd_event_source_set_exit_on_failure</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
 140     </para>
 141
 142     <para>To destroy an event source object use
 143     <citerefentry><refentrytitle>sd_event_source_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 144     but note that the event source is only removed from the event loop
 145     when all references to the event source are dropped. To make sure
 146     an event source does not fire anymore, even when there's still a
 147     reference to it kept, consider setting the event source to
 148     <constant>SD_EVENT_OFF</constant> with
 149     <citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
 150
 151     <para>The <constant>SIGCHLD</constant> signal must be blocked in all threads before this function is
 152     called (using <citerefentry
 153     project='man-pages'><refentrytitle>sigprocmask</refentrytitle><manvolnum>2</manvolnum></citerefentry> or
 154     <citerefentry
 155     project='man-pages'><refentrytitle>pthread_sigmask</refentrytitle><manvolnum>3</manvolnum></citerefentry>).</para>
 156
 157     <para>If the second parameter of <function>sd_event_add_child()</function> is passed as
 158     <constant>NULL</constant> no reference to the event source object is returned. In this case the event
 159     source is considered "floating", and will be destroyed implicitly when the event loop itself is
 160     destroyed.</para>
 161
 162     <para>Note that the <parameter>handler</parameter> function is
 163     invoked at a time where the child process is not reaped yet (and
 164     thus still is exposed as a zombie process by the kernel). However,
 165     the child will be reaped automatically after the function
 166     returns. Child processes for which no child process state change
 167     event sources are installed will not be reaped by the event loop
 168     implementation.</para>
 169
 170     <para>If the <parameter>handler</parameter> parameter to <function>sd_event_add_child()</function> is
 171     <constant>NULL</constant>, and the event source fires, this will be considered a request to exit the
 172     event loop. In this case, the <parameter>userdata</parameter> parameter, cast to an integer, is passed as
 173     the exit code parameter to
 174     <citerefentry><refentrytitle>sd_event_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
 175
 176     <para>If both a child process state change event source and a
 177     <constant>SIGCHLD</constant> signal event source is installed in
 178     the same event loop, the configured event source priorities decide
 179     which event source is dispatched first. If the signal handler is
 180     processed first, it should leave the child processes for which
 181     child process state change event sources are installed unreaped.</para>
 182
 183     <para><function>sd_event_add_child_pidfd()</function> is similar to
 184     <function>sd_event_add_child()</function> but takes a file descriptor referencing the process ("pidfd")
 185     instead of the numeric PID. A suitable file descriptor may be acquired via <citerefentry
 186     project='man-pages'><refentrytitle>pidfd_open</refentrytitle><manvolnum>2</manvolnum></citerefentry> and
 187     related calls. The passed file descriptor is not closed when the event source is freed again, unless
 188     <function>sd_event_source_set_child_pidfd_own()</function> is used to turn this behaviour on. Note that
 189     regardless which of <function>sd_event_add_child()</function> and
 190     <function>sd_event_add_child_pidfd()</function> is used for allocating an event source, the watched
 191     process has to be a direct child process of the invoking process. Also in both cases
 192     <constant>SIGCHLD</constant> has to be blocked in the invoking process.</para>
 193
 194     <para><function>sd_event_source_get_child_pid()</function>
 195     retrieves the configured PID of a child process state change event
 196     source created previously with
 197     <function>sd_event_add_child()</function>. It takes the event
 198     source object as the <parameter>source</parameter> parameter and a
 199     pointer to a <type>pid_t</type> variable to return the process ID
 200     in.
 201     </para>
 202
 203     <para><function>sd_event_source_get_child_pidfd()</function> retrieves the file descriptor referencing
 204     the watched process ("pidfd") if this functionality is available. On kernels that support the concept the
 205     event loop will make use of pidfds to watch child processes, regardless if the individual event sources
 206     are allocated via <function>sd_event_add_child()</function> or
 207     <function>sd_event_add_child_pidfd()</function>. If the latter call was used to allocate the event
 208     source, this function returns the file descriptor used for allocation. On kernels that do not support the
 209     pidfd concept this function will fail with <constant>EOPNOTSUPP</constant>. This call takes the event
 210     source object as the <parameter>source</parameter> parameter and returns the numeric file descriptor.
 211     </para>
 212
 213     <para><function>sd_event_source_get_child_pidfd_own()</function> may be used to query whether the pidfd
 214     the event source encapsulates shall be closed when the event source is freed. This function returns zero
 215     if the pidfd shall be left open, and positive if it shall be closed automatically. By default this
 216     setting defaults to on if the event source was allocated via <function>sd_event_add_child()</function>
 217     and off if it was allocated via <function>sd_event_add_child_pidfd()</function>. The
 218     <function>sd_event_source_set_child_pidfd_own()</function> function may be used to change the setting and
 219     takes a boolean parameter with the new setting.</para>
 220
 221     <para><function>sd_event_source_get_child_process_own()</function> may be used to query whether the
 222     process the event source watches shall be killed (with <constant>SIGKILL</constant>) and reaped when the
 223     event source is freed. This function returns zero if the process shell be left running, and positive if
 224     it shall be killed and reaped automatically. By default this setting defaults to off. The
 225     <function>sd_event_source_set_child_process_own()</function> function may be used to change the setting
 226     and takes a boolean parameter with the new setting. Note that currently if the calling process is
 227     terminated abnormally the watched process might survive even thought the event source ceases to
 228     exist. This behaviour might change eventually.</para>
 229
 230     <para><function>sd_event_source_send_child_signal()</function> may be used to send a UNIX signal to the
 231     watched process. If the pidfd concept is supported in the kernel, this is implemented via <citerefentry
 232     project='man-pages'><refentrytitle>pidfd_send_signal</refentrytitle><manvolnum>2</manvolnum></citerefentry>
 233     and otherwise via <citerefentry
 234     project='man-pages'><refentrytitle>rt_sigqueueinfo</refentrytitle><manvolnum>2</manvolnum></citerefentry>
 235     (or via <citerefentry
 236     project='man-pages'><refentrytitle>kill</refentrytitle><manvolnum>2</manvolnum></citerefentry> in case
 237     <parameter>info</parameter> is <constant>NULL</constant>). The specified parameters match those of these
 238     underlying system calls, except that the <parameter>info</parameter> is never modified (and is thus
 239     declared constant). Like for the underlying system calls, the <parameter>flags</parameter> parameter
 240     currently must be zero.</para>
 241   </refsect1>
 242
 243   <refsect1>
 244     <title>Return Value</title>
 245
 246     <para>On success, these functions return 0 or a positive
 247     integer. On failure, they return a negative errno-style error
 248     code.</para>
 249
 250     <refsect2>
 251       <title>Errors</title>
 252
 253       <para>Returned errors may indicate the following problems:</para>
 254
 255       <variablelist>
 256         <varlistentry>
 257           <term><constant>-ENOMEM</constant></term>
 258
 259           <listitem><para>Not enough memory to allocate an object.</para></listitem>
 260         </varlistentry>
 261
 262         <varlistentry>
 263           <term><constant>-EINVAL</constant></term>
 264
 265           <listitem><para>An invalid argument has been passed. This includes
 266           specifying an empty mask in <parameter>options</parameter> or a mask
 267           which contains values different than a combination of
 268           <constant>WEXITED</constant>, <constant>WSTOPPED</constant>, and
 269           <constant>WCONTINUED</constant>.
 270           </para></listitem>
 271
 272         </varlistentry>
 273
 274         <varlistentry>
 275           <term><constant>-EBUSY</constant></term>
 276
 277           <listitem><para>A handler is already installed for this child process, or
 278           <constant>SIGCHLD</constant> is not blocked.</para></listitem>
 279
 280         </varlistentry>
 281
 282         <varlistentry>
 283           <term><constant>-ESTALE</constant></term>
 284
 285           <listitem><para>The event loop is already terminated.</para></listitem>
 286
 287         </varlistentry>
 288
 289         <varlistentry>
 290           <term><constant>-ECHILD</constant></term>
 291
 292           <listitem><para>The event loop has been created in a different process, library or module instance.</para></listitem>
 293
 294         </varlistentry>
 295
 296         <varlistentry>
 297           <term><constant>-EDOM</constant></term>
 298
 299           <listitem><para>The passed event source is not a child process event source.</para></listitem>
 300         </varlistentry>
 301
 302         <varlistentry>
 303           <term><constant>-EOPNOTSUPP</constant></term>
 304
 305           <listitem><para>A pidfd was requested but the kernel does not support this concept.</para>
 306
 307           <xi:include href="version-info.xml" xpointer="v245"/></listitem>
 308         </varlistentry>
 309
 310       </variablelist>
 311     </refsect2>
 312   </refsect1>
 313
 314   <xi:include href="libsystemd-pkgconfig.xml" />
 315
 316   <refsect1>
 317     <title>Example</title>
 318
 319     <example>
 320       <title>Exit loop when the child terminates</title>
 321
 322       <programlisting><xi:include href="event-quick-child.c" parse="text" /></programlisting>
 323     </example>
 324   </refsect1>
 325
 326   <refsect1>
 327     <title>History</title>
 328     <para><function>sd_event_add_child()</function>,
 329     <function>sd_event_child_handler_t()</function>, and
 330     <function>sd_event_source_get_child_pid()</function> were added in version 217.</para>
 331     <para><function>sd_event_add_child_pidfd()</function>,
 332     <function>sd_event_source_get_child_pidfd()</function>,
 333     <function>sd_event_source_get_child_pidfd_own()</function>,
 334     <function>sd_event_source_set_child_pidfd_own()</function>,
 335     <function>sd_event_source_get_child_process_own()</function>,
 336     <function>sd_event_source_set_child_process_own()</function>, and
 337     <function>sd_event_source_send_child_signal()</function> were added in version 245.</para>
 338   </refsect1>
 339
 340   <refsect1>
 341     <title>See Also</title>
 342
 343     <para><simplelist type="inline">
 344       <member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
 345       <member><citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 346       <member><citerefentry><refentrytitle>sd_event_new</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 347       <member><citerefentry><refentrytitle>sd_event_now</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 348       <member><citerefentry><refentrytitle>sd_event_add_io</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 349       <member><citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 350       <member><citerefentry><refentrytitle>sd_event_add_signal</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 351       <member><citerefentry><refentrytitle>sd_event_add_inotify</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 352       <member><citerefentry><refentrytitle>sd_event_add_defer</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 353       <member><citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 354       <member><citerefentry><refentrytitle>sd_event_source_set_priority</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 355       <member><citerefentry><refentrytitle>sd_event_source_set_userdata</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 356       <member><citerefentry><refentrytitle>sd_event_source_set_description</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 357       <member><citerefentry><refentrytitle>sd_event_source_set_floating</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 358       <member><citerefentry project='man-pages'><refentrytitle>waitid</refentrytitle><manvolnum>2</manvolnum></citerefentry></member>
 359       <member><citerefentry project='man-pages'><refentrytitle>sigprocmask</refentrytitle><manvolnum>2</manvolnum></citerefentry></member>
 360       <member><citerefentry project='man-pages'><refentrytitle>pthread_sigmask</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 361       <member><citerefentry project='man-pages'><refentrytitle>pidfd_open</refentrytitle><manvolnum>2</manvolnum></citerefentry></member>
 362       <member><citerefentry project='man-pages'><refentrytitle>pidfd_send_signal</refentrytitle><manvolnum>2</manvolnum></citerefentry></member>
 363       <member><citerefentry project='man-pages'><refentrytitle>rt_sigqueueinfo</refentrytitle><manvolnum>2</manvolnum></citerefentry></member>
 364       <member><citerefentry project='man-pages'><refentrytitle>kill</refentrytitle><manvolnum>2</manvolnum></citerefentry></member>
 365     </simplelist></para>
 366   </refsect1>
 367
 368 </refentry>