man/sd_event_add_io.xml

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   4 <!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
   5
   6 <refentry id="sd_event_add_io" xmlns:xi="http://www.w3.org/2001/XInclude">
   7
   8   <refentryinfo>
   9     <title>sd_event_add_io</title>
  10     <productname>systemd</productname>
  11   </refentryinfo>
  12
  13   <refmeta>
  14     <refentrytitle>sd_event_add_io</refentrytitle>
  15     <manvolnum>3</manvolnum>
  16   </refmeta>
  17
  18   <refnamediv>
  19     <refname>sd_event_add_io</refname>
  20     <refname>sd_event_source_get_io_events</refname>
  21     <refname>sd_event_source_set_io_events</refname>
  22     <refname>sd_event_source_get_io_revents</refname>
  23     <refname>sd_event_source_get_io_fd</refname>
  24     <refname>sd_event_source_set_io_fd</refname>
  25     <refname>sd_event_source_get_io_fd_own</refname>
  26     <refname>sd_event_source_set_io_fd_own</refname>
  27     <refname>sd_event_source</refname>
  28     <refname>sd_event_io_handler_t</refname>
  29
  30     <refpurpose>Add an I/O 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_io_handler_t</function>)</funcdef>
  41         <paramdef>sd_event_source *<parameter>s</parameter></paramdef>
  42         <paramdef>int <parameter>fd</parameter></paramdef>
  43         <paramdef>uint32_t <parameter>revents</parameter></paramdef>
  44         <paramdef>void *<parameter>userdata</parameter></paramdef>
  45       </funcprototype>
  46
  47       <funcprototype>
  48         <funcdef>int <function>sd_event_add_io</function></funcdef>
  49         <paramdef>sd_event *<parameter>event</parameter></paramdef>
  50         <paramdef>sd_event_source **<parameter>source</parameter></paramdef>
  51         <paramdef>int <parameter>fd</parameter></paramdef>
  52         <paramdef>uint32_t <parameter>events</parameter></paramdef>
  53         <paramdef>sd_event_io_handler_t <parameter>handler</parameter></paramdef>
  54         <paramdef>void *<parameter>userdata</parameter></paramdef>
  55       </funcprototype>
  56
  57       <funcprototype>
  58         <funcdef>int <function>sd_event_source_get_io_events</function></funcdef>
  59         <paramdef>sd_event_source *<parameter>source</parameter></paramdef>
  60         <paramdef>uint32_t *<parameter>events</parameter></paramdef>
  61       </funcprototype>
  62
  63       <funcprototype>
  64         <funcdef>int <function>sd_event_source_set_io_events</function></funcdef>
  65         <paramdef>sd_event_source *<parameter>source</parameter></paramdef>
  66         <paramdef>uint32_t <parameter>events</parameter></paramdef>
  67       </funcprototype>
  68
  69       <funcprototype>
  70         <funcdef>int <function>sd_event_source_get_io_revents</function></funcdef>
  71         <paramdef>sd_event_source *<parameter>source</parameter></paramdef>
  72         <paramdef>uint32_t *<parameter>revents</parameter></paramdef>
  73       </funcprototype>
  74
  75       <funcprototype>
  76         <funcdef>int <function>sd_event_source_get_io_fd</function></funcdef>
  77         <paramdef>sd_event_source *<parameter>source</parameter></paramdef>
  78       </funcprototype>
  79
  80       <funcprototype>
  81         <funcdef>int <function>sd_event_source_set_io_fd</function></funcdef>
  82         <paramdef>sd_event_source *<parameter>source</parameter></paramdef>
  83         <paramdef>int <parameter>fd</parameter></paramdef>
  84       </funcprototype>
  85
  86       <funcprototype>
  87         <funcdef>int <function>sd_event_source_get_io_fd_own</function></funcdef>
  88         <paramdef>sd_event_source *<parameter>source</parameter></paramdef>
  89       </funcprototype>
  90
  91       <funcprototype>
  92         <funcdef>int <function>sd_event_source_set_io_fd_own</function></funcdef>
  93         <paramdef>sd_event_source *<parameter>source</parameter></paramdef>
  94         <paramdef>int <parameter>b</parameter></paramdef>
  95       </funcprototype>
  96
  97     </funcsynopsis>
  98   </refsynopsisdiv>
  99
 100   <refsect1>
 101     <title>Description</title>
 102
 103     <para><function>sd_event_add_io()</function> adds a new I/O event source to an event loop. The event loop
 104     object is specified in the <parameter>event</parameter> parameter, the event source object is returned in
 105     the <parameter>source</parameter> parameter. The <parameter>fd</parameter> parameter takes the UNIX file
 106     descriptor to watch, which may refer to a socket, a FIFO, a message queue, a serial connection, a
 107     character device, or any other file descriptor compatible with Linux <citerefentry
 108     project='man-pages'><refentrytitle>epoll</refentrytitle><manvolnum>7</manvolnum></citerefentry>. The
 109     <parameter>events</parameter> parameter takes a bit mask of events to watch for, a combination of the
 110     following event flags: <constant>EPOLLIN</constant>, <constant>EPOLLOUT</constant>,
 111     <constant>EPOLLRDHUP</constant>, <constant>EPOLLPRI</constant>, and <constant>EPOLLET</constant>, see
 112     <citerefentry
 113     project='man-pages'><refentrytitle>epoll_ctl</refentrytitle><manvolnum>2</manvolnum></citerefentry> for
 114     details. Note that not all file descriptors are compatible with epoll, for example regular file or
 115     directories are not. If this function is called with a file descriptor that does not support epoll,
 116     <constant>-EPERM</constant> is returned (also see below). In most cases such file descriptors may be
 117     treated as always-readable or always-writable, so that IO event watching is unnecessary.</para>
 118
 119     <para>The <parameter>handler</parameter> is a function to call when the event source is triggered or
 120     <constant>NULL</constant>. The <parameter>userdata</parameter> pointer will be passed to the handler
 121     function, and may be chosen freely by the caller. The handler will also be passed the file descriptor the
 122     event was seen on, as well as the actual event flags. It's generally a subset of the events watched,
 123     however may additionally include <constant>EPOLLERR</constant> and <constant>EPOLLHUP</constant>. The
 124     handler may return negative to signal an error (see below), other return values are ignored. If
 125     <parameter>handler</parameter> is <constant>NULL</constant>, a default handler that calls
 126     <citerefentry><refentrytitle>sd_event_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry> will be
 127     used.</para>
 128
 129     <para>By default, an event source will stay enabled continuously (<constant>SD_EVENT_ON</constant>), but
 130     this may be changed with
 131     <citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
 132     If the handler function returns a negative error code, it will either be disabled after the invocation,
 133     even if the <constant>SD_EVENT_ON</constant> mode was requested before, or it will cause the loop to
 134     terminate, see
 135     <citerefentry><refentrytitle>sd_event_source_set_exit_on_failure</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
 136     Note that an event source set to <constant>SD_EVENT_ON</constant> will fire continuously unless data is
 137     read from or written to the file descriptor to reset the mask of events seen.</para>
 138
 139     <para>Setting the I/O event mask to watch for to 0 does not mean
 140     that the event source won't be triggered anymore, as
 141     <constant>EPOLLHUP</constant> and <constant>EPOLLERR</constant>
 142     may be triggered even with a zero event mask. To temporarily
 143     disable an I/O event source use
 144     <citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 145     with <constant>SD_EVENT_OFF</constant> instead.</para>
 146
 147     <para>To destroy an event source object use
 148     <citerefentry><refentrytitle>sd_event_source_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 149     but note that the event source is only removed from the event loop
 150     when all references to the event source are dropped. To make sure
 151     an event source does not fire anymore, even if it is still referenced,
 152     disable the event source using
 153     <citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 154     with <constant>SD_EVENT_OFF</constant>.</para>
 155
 156     <para>If the second parameter of
 157     <function>sd_event_add_io()</function> is
 158     <constant>NULL</constant> no reference to the event source object
 159     is returned. In this case the event source is considered
 160     "floating", and will be destroyed implicitly when the event loop
 161     itself is destroyed.</para>
 162
 163     <para>If the <parameter>handler</parameter> to <function>sd_event_add_io()</function> is
 164     <constant>NULL</constant>, and the event source fires, this will be considered a request to exit the
 165     event loop. In this case, the <parameter>userdata</parameter> parameter, cast to an integer, is passed as
 166     the exit code parameter to
 167     <citerefentry><refentrytitle>sd_event_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
 168
 169     <para>Note that this call does not take possession of the file descriptor passed in, ownership (and thus
 170     the duty to close it when it is no longer needed) remains with the caller. However, with the
 171     <function>sd_event_source_set_io_fd_own()</function> call (see below) the event source may optionally
 172     take ownership of the file descriptor after the event source has been created. In that case the file
 173     descriptor is closed automatically as soon as the event source is released.</para>
 174
 175     <para>It is recommended to use
 176     <function>sd_event_add_io()</function> only in conjunction with
 177     file descriptors that have <constant>O_NONBLOCK</constant> set, to
 178     ensure that all I/O operations from invoked handlers are properly
 179     asynchronous and non-blocking. Using file descriptors without
 180     <constant>O_NONBLOCK</constant> might result in unexpected
 181     starvation of other event sources. See
 182     <citerefentry project='man-pages'><refentrytitle>fcntl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
 183     for details on enabling <constant>O_NONBLOCK</constant> mode.</para>
 184
 185     <para><function>sd_event_source_get_io_events()</function> retrieves
 186     the configured mask of watched I/O events of an event source created
 187     previously with <function>sd_event_add_io()</function>. It takes
 188     the event source object and a pointer to a variable to store the
 189     mask in.</para>
 190
 191     <para><function>sd_event_source_set_io_events()</function>
 192     configures the mask of watched I/O events of an event source created
 193     previously with <function>sd_event_add_io()</function>. It takes the
 194     event source object and the new event mask.</para>
 195
 196     <para><function>sd_event_source_get_io_revents()</function>
 197     retrieves the I/O event mask of currently seen but undispatched
 198     events from an event source created previously with
 199     <function>sd_event_add_io()</function>. It takes the event source
 200     object and a pointer to a variable to store the event mask
 201     in. When called from a handler function on the handler's event
 202     source object this will return the same mask as passed to the
 203     handler's <parameter>revents</parameter> parameter. This call is
 204     primarily useful to check for undispatched events of an event
 205     source from the handler of an unrelated (possibly higher priority)
 206     event source. Note the relation between
 207     <function>sd_event_source_get_pending()</function> and
 208     <function>sd_event_source_get_io_revents()</function>: both
 209     functions will report non-zero results when there's an event
 210     pending for the event source, but the former applies to all event
 211     source types, the latter only to I/O event sources.</para>
 212
 213     <para><function>sd_event_source_get_io_fd()</function> retrieves
 214     the UNIX file descriptor of an event source created previously
 215     with <function>sd_event_add_io()</function>. It takes the event
 216     source object and returns the non-negative file descriptor
 217     or a negative error number on error (see below).</para>
 218
 219     <para><function>sd_event_source_set_io_fd()</function> changes the UNIX file descriptor of an I/O event
 220     source created previously with <function>sd_event_add_io()</function>. It takes the event source object
 221     and the new file descriptor. If the event source takes the ownership of the previous file descriptor,
 222     that is, <function>sd_event_source_set_io_fd_own()</function> was called for the event source with a
 223     non-zero value, then the previous file descriptor will be closed and the event source will also take the
 224     ownership of the new file descriptor on success.</para>
 225
 226     <para><function>sd_event_source_set_io_fd_own()</function> controls whether the file descriptor of the
 227     event source shall be closed automatically when the event source is freed (or when the file descriptor
 228     assigned to the event source is replaced by <function>sd_event_source_set_io_fd()</function>), i.e.
 229     whether it shall be considered 'owned' by the event source object. By default it is not closed
 230     automatically, and the application has to do this on its own. The <parameter>b</parameter> parameter is a
 231     boolean parameter: if zero, the file descriptor is not closed automatically when the event source is
 232     freed, otherwise it is closed.</para>
 233
 234     <para><function>sd_event_source_get_io_fd_own()</function> may be used to query the current setting of the file
 235     descriptor ownership boolean flag as set with <function>sd_event_source_set_io_fd_own()</function>. It returns
 236     positive if the file descriptor is closed automatically when the event source is destroyed, zero if not, and
 237     negative on error.</para>
 238   </refsect1>
 239
 240   <refsect1>
 241     <title>Return Value</title>
 242
 243     <para>On success, these functions return 0 or a positive
 244     integer. On failure, they return a negative errno-style error
 245     code.</para>
 246
 247     <refsect2>
 248       <title>Errors</title>
 249
 250       <para>Returned values may indicate the following problems:</para>
 251
 252       <variablelist>
 253         <varlistentry>
 254           <term><constant>-ENOMEM</constant></term>
 255
 256           <listitem><para>Not enough memory to allocate an object.</para></listitem>
 257         </varlistentry>
 258
 259         <varlistentry>
 260           <term><constant>-EINVAL</constant></term>
 261
 262           <listitem><para>An invalid argument has been passed.</para></listitem>
 263
 264         </varlistentry>
 265
 266         <varlistentry>
 267           <term><constant>-ESTALE</constant></term>
 268
 269           <listitem><para>The event loop is already terminated.</para></listitem>
 270
 271         </varlistentry>
 272
 273         <varlistentry>
 274           <term><constant>-ECHILD</constant></term>
 275
 276           <listitem><para>The event loop has been created in a different process, library or module instance.</para></listitem>
 277         </varlistentry>
 278
 279         <varlistentry>
 280           <term><constant>-EDOM</constant></term>
 281
 282           <listitem><para>The passed event source is not an I/O event source.</para></listitem>
 283         </varlistentry>
 284
 285         <varlistentry>
 286           <term><constant>-EPERM</constant></term>
 287
 288           <listitem><para>The passed file descriptor does not support the <citerefentry
 289           project='man-pages'><refentrytitle>epoll</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 290           API, for example because it is a regular file or directory. See <citerefentry
 291           project='man-pages'><refentrytitle>epoll_ctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
 292           for details.</para>
 293
 294           <xi:include href="version-info.xml" xpointer="v255"/></listitem>
 295         </varlistentry>
 296       </variablelist>
 297     </refsect2>
 298   </refsect1>
 299
 300   <xi:include href="libsystemd-pkgconfig.xml" />
 301
 302   <refsect1>
 303     <title>History</title>
 304     <para><function>sd_event_io_handler_t()</function>,
 305     <function>sd_event_add_io()</function>,
 306     <function>sd_event_source_get_io_events()</function>,
 307     <function>sd_event_source_set_io_events()</function>,
 308     <function>sd_event_source_get_io_revents()</function>,
 309     <function>sd_event_source_get_io_fd()</function>, and
 310     <function>sd_event_source_set_io_fd()</function> were added in version 229.</para>
 311     <para><function>sd_event_source_get_io_fd_own()</function> and
 312     <function>sd_event_source_set_io_fd_own()</function> were added in version 239.</para>
 313   </refsect1>
 314
 315   <refsect1>
 316     <title>See Also</title>
 317
 318     <para><simplelist type="inline">
 319       <member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
 320       <member><citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 321       <member><citerefentry><refentrytitle>sd_event_new</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 322       <member><citerefentry><refentrytitle>sd_event_now</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 323       <member><citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 324       <member><citerefentry><refentrytitle>sd_event_add_signal</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 325       <member><citerefentry><refentrytitle>sd_event_add_child</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 326       <member><citerefentry><refentrytitle>sd_event_add_inotify</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 327       <member><citerefentry><refentrytitle>sd_event_add_defer</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 328       <member><citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 329       <member><citerefentry><refentrytitle>sd_event_source_set_priority</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 330       <member><citerefentry><refentrytitle>sd_event_source_set_userdata</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 331       <member><citerefentry><refentrytitle>sd_event_source_set_description</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 332       <member><citerefentry><refentrytitle>sd_event_source_get_pending</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 333       <member><citerefentry><refentrytitle>sd_event_source_set_floating</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 334       <member><citerefentry project='man-pages'><refentrytitle>epoll_ctl</refentrytitle><manvolnum>2</manvolnum></citerefentry></member>
 335       <member><citerefentry project='man-pages'><refentrytitle>epoll</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
 336     </simplelist></para>
 337   </refsect1>
 338
 339 </refentry>