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_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
 154     <function>sd_event_add_child()</function> is passed as NULL no
 155     reference to the event source object is returned. In this case the
 156     event source is considered "floating", and will be destroyed
 157     implicitly when the event loop itself is destroyed.</para>
 158
 159     <para>Note that the <parameter>handler</parameter> function is
 160     invoked at a time where the child process is not reaped yet (and
 161     thus still is exposed as a zombie process by the kernel). However,
 162     the child will be reaped automatically after the function
 163     returns. Child processes for which no child process state change
 164     event sources are installed will not be reaped by the event loop
 165     implementation.</para>
 166
 167     <para>If both a child process state change event source and a
 168     <constant>SIGCHLD</constant> signal event source is installed in
 169     the same event loop, the configured event source priorities decide
 170     which event source is dispatched first. If the signal handler is
 171     processed first, it should leave the child processes for which
 172     child process state change event sources are installed unreaped.</para>
 173
 174     <para><function>sd_event_add_child_pidfd()</function> is similar to
 175     <function>sd_event_add_child()</function> but takes a file descriptor referencing the process ("pidfd")
 176     instead of the numeric PID. A suitable file descriptor may be acquired via <citerefentry
 177     project='man-pages'><refentrytitle>pidfd_open</refentrytitle><manvolnum>2</manvolnum></citerefentry> and
 178     related calls. The passed file descriptor is not closed when the event source is freed again, unless
 179     <function>sd_event_source_set_child_pidfd_own()</function> is used to turn this behaviour on. Note that
 180     regardless which of <function>sd_event_add_child()</function> and
 181     <function>sd_event_add_child_pidfd()</function> is used for allocating an event source, the watched
 182     process has to be a direct child process of the invoking process. Also in both cases
 183     <constant>SIGCHLD</constant> has to be blocked in the invoking process.</para>
 184
 185     <para><function>sd_event_source_get_child_pid()</function>
 186     retrieves the configured PID of a child process state change event
 187     source created previously with
 188     <function>sd_event_add_child()</function>. It takes the event
 189     source object as the <parameter>source</parameter> parameter and a
 190     pointer to a <type>pid_t</type> variable to return the process ID
 191     in.
 192     </para>
 193
 194     <para><function>sd_event_source_get_child_pidfd()</function> retrieves the file descriptor referencing
 195     the watched process ("pidfd") if this functionality is available. On kernels that support the concept the
 196     event loop will make use of pidfds to watch child processes, regardless if the individual event sources
 197     are allocated via <function>sd_event_add_child()</function> or
 198     <function>sd_event_add_child_pidfd()</function>. If the latter call was used to allocate the event
 199     source, this function returns the file descriptor used for allocation. On kernels that do not support the
 200     pidfd concept this function will fail with <constant>EOPNOTSUPP</constant>. This call takes the event
 201     source object as the <parameter>source</parameter> parameter and returns the numeric file descriptor.
 202     </para>
 203
 204     <para><function>sd_event_source_get_child_pidfd_own()</function> may be used to query whether the pidfd
 205     the event source encapsulates shall be closed when the event source is freed. This function returns zero
 206     if the pidfd shall be left open, and positive if it shall be closed automatically. By default this
 207     setting defaults to on if the event source was allocated via <function>sd_event_add_child()</function>
 208     and off if it was allocated via <function>sd_event_add_child_pidfd()</function>. The
 209     <function>sd_event_source_set_child_pidfd_own()</function> function may be used to change the setting and
 210     takes a boolean parameter with the new setting.</para>
 211
 212     <para><function>sd_event_source_get_child_process_own()</function> may be used to query whether the
 213     process the event source watches shall be killed (with <constant>SIGKILL</constant>) and reaped when the
 214     event source is freed. This function returns zero if the process shell be left running, and positive if
 215     it shall be killed and reaped automatically. By default this setting defaults to off. The
 216     <function>sd_event_source_set_child_process_own()</function> function may be used to change the setting
 217     and takes a boolean parameter with the new setting. Note that currently if the calling process is
 218     terminated abnormally the watched process might survive even thought the event source ceases to
 219     exist. This behaviour might change eventually.</para>
 220
 221     <para><function>sd_event_source_send_child_signal()</function> may be used to send a UNIX signal to the
 222     watched process. If the pidfd concept is supported in the kernel, this is implemented via <citerefentry
 223     project='man-pages'><refentrytitle>pidfd_send_signal</refentrytitle><manvolnum>2</manvolnum></citerefentry>
 224     and otherwise via <citerefentry
 225     project='man-pages'><refentrytitle>rt_sigqueueinfo</refentrytitle><manvolnum>2</manvolnum></citerefentry>
 226     (or via <citerefentry
 227     project='man-pages'><refentrytitle>kill</refentrytitle><manvolnum>2</manvolnum></citerefentry> in case
 228     <parameter>info</parameter> is <constant>NULL</constant>). The specified parameters match those of these
 229     underlying system calls, except that the <parameter>info</parameter> is never modified (and is thus
 230     declared constant). Like for the underlying system calls, the <parameter>flags</parameter> parameter
 231     currently must be zero.</para>
 232   </refsect1>
 233
 234   <refsect1>
 235     <title>Return Value</title>
 236
 237     <para>On success, these functions return 0 or a positive
 238     integer. On failure, they return a negative errno-style error
 239     code.</para>
 240
 241     <refsect2>
 242       <title>Errors</title>
 243
 244       <para>Returned errors may indicate the following problems:</para>
 245
 246       <variablelist>
 247         <varlistentry>
 248           <term><constant>-ENOMEM</constant></term>
 249
 250           <listitem><para>Not enough memory to allocate an object.</para></listitem>
 251         </varlistentry>
 252
 253         <varlistentry>
 254           <term><constant>-EINVAL</constant></term>
 255
 256           <listitem><para>An invalid argument has been passed. This includes
 257           specifying an empty mask in <parameter>options</parameter> or a mask
 258           which contains values different than a combination of
 259           <constant>WEXITED</constant>, <constant>WSTOPPED</constant>, and
 260           <constant>WCONTINUED</constant>.
 261           </para></listitem>
 262
 263         </varlistentry>
 264
 265         <varlistentry>
 266           <term><constant>-EBUSY</constant></term>
 267
 268           <listitem><para>A handler is already installed for this child process, or
 269           <constant>SIGCHLD</constant> is not blocked.</para></listitem>
 270
 271         </varlistentry>
 272
 273         <varlistentry>
 274           <term><constant>-ESTALE</constant></term>
 275
 276           <listitem><para>The event loop is already terminated.</para></listitem>
 277
 278         </varlistentry>
 279
 280         <varlistentry>
 281           <term><constant>-ECHILD</constant></term>
 282
 283           <listitem><para>The event loop has been created in a different process.</para></listitem>
 284
 285         </varlistentry>
 286
 287         <varlistentry>
 288           <term><constant>-EDOM</constant></term>
 289
 290           <listitem><para>The passed event source is not a child process event source.</para></listitem>
 291         </varlistentry>
 292
 293         <varlistentry>
 294           <term><constant>-EOPNOTSUPP</constant></term>
 295
 296           <listitem><para>A pidfd was requested but the kernel does not support this concept.</para></listitem>
 297         </varlistentry>
 298
 299       </variablelist>
 300     </refsect2>
 301   </refsect1>
 302
 303   <xi:include href="libsystemd-pkgconfig.xml" />
 304
 305   <refsect1>
 306     <title>See Also</title>
 307
 308     <para>
 309       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 310       <citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 311       <citerefentry><refentrytitle>sd_event_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 312       <citerefentry><refentrytitle>sd_event_now</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 313       <citerefentry><refentrytitle>sd_event_add_io</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 314       <citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 315       <citerefentry><refentrytitle>sd_event_add_signal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 316       <citerefentry><refentrytitle>sd_event_add_inotify</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 317       <citerefentry><refentrytitle>sd_event_add_defer</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 318       <citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 319       <citerefentry><refentrytitle>sd_event_source_set_priority</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 320       <citerefentry><refentrytitle>sd_event_source_set_userdata</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 321       <citerefentry><refentrytitle>sd_event_source_set_description</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 322       <citerefentry><refentrytitle>sd_event_source_set_floating</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 323       <citerefentry project='man-pages'><refentrytitle>waitid</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
 324       <citerefentry project='man-pages'><refentrytitle>sigprocmask</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
 325       <citerefentry project='man-pages'><refentrytitle>pthread_sigmask</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 326       <citerefentry project='man-pages'><refentrytitle>pidfd_open</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
 327       <citerefentry project='man-pages'><refentrytitle>pidfd_send_signal</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
 328       <citerefentry project='man-pages'><refentrytitle>rt_sigqueueinfo</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
 329       <citerefentry project='man-pages'><refentrytitle>kill</refentrytitle><manvolnum>2</manvolnum></citerefentry>
 330     </para>
 331   </refsect1>
 332
 333 </refentry>