man/sd_event_add_io.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_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
 104     source to an event loop. The event loop object is specified in the
 105     <parameter>event</parameter> parameter, the event source object is
 106     returned in the <parameter>source</parameter> parameter. The
 107     <parameter>fd</parameter> parameter takes the UNIX file descriptor
 108     to watch, which may refer to a socket, a FIFO, a message queue, a
 109     serial connection, a character device, or any other file descriptor
 110     compatible with Linux
 111     <citerefentry project='man-pages'><refentrytitle>epoll</refentrytitle><manvolnum>7</manvolnum></citerefentry>. The
 112     <parameter>events</parameter> parameter takes a bit mask of events
 113     to watch for, a combination of the following event flags:
 114     <constant>EPOLLIN</constant>, <constant>EPOLLOUT</constant>,
 115     <constant>EPOLLRDHUP</constant>, <constant>EPOLLPRI</constant>,
 116     and <constant>EPOLLET</constant>, see
 117     <citerefentry project='man-pages'><refentrytitle>epoll_ctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
 118     for details. The <parameter>handler</parameter> shall reference a
 119     function to call when the event source is triggered. The
 120     <parameter>userdata</parameter> pointer will be passed to the
 121     handler function, and may be chosen freely by the caller. The
 122     handler will also be passed the file descriptor the event was seen
 123     on, as well as the actual event flags. It's generally a subset of
 124     the events watched, however may additionally include
 125     <constant>EPOLLERR</constant> and <constant>EPOLLHUP</constant>.
 126     </para>
 127
 128     <para>By default, an event source will stay enabled
 129     continuously (<constant>SD_EVENT_ON</constant>), but this may be
 130     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 be
 133     disabled after the invocation, even if the
 134     <constant>SD_EVENT_ON</constant> mode was requested before. Note
 135     that an event source set to <constant>SD_EVENT_ON</constant> will
 136     fire continuously unless data is read from or written to the file
 137     descriptor to reset the mask of events seen.
 138     </para>
 139
 140     <para>Setting the I/O event mask to watch for to 0 does not mean
 141     that the event source won't be triggered anymore, as
 142     <constant>EPOLLHUP</constant> and <constant>EPOLLERR</constant>
 143     may be triggered even with a zero event mask. To temporarily
 144     disable an I/O event source use
 145     <citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 146     with <constant>SD_EVENT_OFF</constant> instead.</para>
 147
 148     <para>To destroy an event source object use
 149     <citerefentry><refentrytitle>sd_event_source_unref</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 150     but note that the event source is only removed from the event loop
 151     when all references to the event source are dropped. To make sure
 152     an event source does not fire anymore, even if it is still referenced,
 153     disable the event source using
 154     <citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 155     with <constant>SD_EVENT_OFF</constant>.</para>
 156
 157     <para>If the second parameter of
 158     <function>sd_event_add_io()</function> is
 159     <constant>NULL</constant> no reference to the event source object
 160     is returned. In this case the event source is considered
 161     "floating", and will be destroyed implicitly when the event loop
 162     itself is destroyed.</para>
 163
 164     <para>If the <parameter>handler</parameter> to <function>sd_event_add_io()</function> is
 165     <constant>NULL</constant>, and the event source fires, this will be considered a request to exit the
 166     event loop. In this case, the <parameter>userdata</parameter> parameter, cast to an integer, is passed as
 167     the exit code parameter to
 168     <citerefentry><refentrytitle>sd_event_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
 169
 170     <para>Note that this call does not take possession of the file descriptor passed in, ownership (and thus
 171     the duty to close it when it is no longer needed) remains with the caller. However, with the
 172     <function>sd_event_source_set_io_fd_own()</function> call (see below) the event source may optionally
 173     take ownership of the file descriptor after the event source has been created. In that case the file
 174     descriptor is closed automatically as soon as the event source is released.</para>
 175
 176     <para>It is recommended to use
 177     <function>sd_event_add_io()</function> only in conjunction with
 178     file descriptors that have <constant>O_NONBLOCK</constant> set, to
 179     ensure that all I/O operations from invoked handlers are properly
 180     asynchronous and non-blocking. Using file descriptors without
 181     <constant>O_NONBLOCK</constant> might result in unexpected
 182     starvation of other event sources. See
 183     <citerefentry project='man-pages'><refentrytitle>fcntl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
 184     for details on enabling <constant>O_NONBLOCK</constant> mode.</para>
 185
 186     <para><function>sd_event_source_get_io_events()</function> retrieves
 187     the configured mask of watched I/O events of an event source created
 188     previously with <function>sd_event_add_io()</function>. It takes
 189     the event source object and a pointer to a variable to store the
 190     mask in.</para>
 191
 192     <para><function>sd_event_source_set_io_events()</function>
 193     configures the mask of watched I/O events of an event source created
 194     previously with <function>sd_event_add_io()</function>. It takes the
 195     event source object and the new event mask.</para>
 196
 197     <para><function>sd_event_source_get_io_revents()</function>
 198     retrieves the I/O event mask of currently seen but undispatched
 199     events from an event source created previously with
 200     <function>sd_event_add_io()</function>. It takes the event source
 201     object and a pointer to a variable to store the event mask
 202     in. When called from a handler function on the handler's event
 203     source object this will return the same mask as passed to the
 204     handler's <parameter>revents</parameter> parameter. This call is
 205     primarily useful to check for undispatched events of an event
 206     source from the handler of an unrelated (possibly higher priority)
 207     event source. Note the relation between
 208     <function>sd_event_source_get_pending()</function> and
 209     <function>sd_event_source_get_io_revents()</function>: both
 210     functions will report non-zero results when there's an event
 211     pending for the event source, but the former applies to all event
 212     source types, the latter only to I/O event sources.</para>
 213
 214     <para><function>sd_event_source_get_io_fd()</function> retrieves
 215     the UNIX file descriptor of an event source created previously
 216     with <function>sd_event_add_io()</function>. It takes the event
 217     source object and returns the non-negative file descriptor
 218     or a negative error number on error (see below).</para>
 219
 220     <para><function>sd_event_source_set_io_fd()</function>
 221     changes the UNIX file descriptor of an I/O event source created
 222     previously with <function>sd_event_add_io()</function>. It takes
 223     the event source object and the new file descriptor.</para>
 224
 225     <para><function>sd_event_source_set_io_fd_own()</function> controls whether the file descriptor of the event source
 226     shall be closed automatically when the event source is freed, i.e. whether it shall be considered 'owned' by the
 227     event source object. By default it is not closed automatically, and the application has to do this on its own. The
 228     <parameter>b</parameter> parameter is a boolean parameter: if zero, the file descriptor is not closed automatically
 229     when the event source is freed, otherwise it is closed.</para>
 230
 231     <para><function>sd_event_source_get_io_fd_own()</function> may be used to query the current setting of the file
 232     descriptor ownership boolean flag as set with <function>sd_event_source_set_io_fd_own()</function>. It returns
 233     positive if the file descriptor is closed automatically when the event source is destroyed, zero if not, and
 234     negative on error.</para>
 235   </refsect1>
 236
 237   <refsect1>
 238     <title>Return Value</title>
 239
 240     <para>On success, these functions return 0 or a positive
 241     integer. On failure, they return a negative errno-style error
 242     code.</para>
 243
 244     <refsect2>
 245       <title>Errors</title>
 246
 247       <para>Returned values may indicate the following problems:</para>
 248
 249       <variablelist>
 250         <varlistentry>
 251           <term><constant>-ENOMEM</constant></term>
 252
 253           <listitem><para>Not enough memory to allocate an object.</para></listitem>
 254         </varlistentry>
 255
 256         <varlistentry>
 257           <term><constant>-EINVAL</constant></term>
 258
 259           <listitem><para>An invalid argument has been passed.</para></listitem>
 260
 261         </varlistentry>
 262
 263         <varlistentry>
 264           <term><constant>-ESTALE</constant></term>
 265
 266           <listitem><para>The event loop is already terminated.</para></listitem>
 267
 268         </varlistentry>
 269
 270         <varlistentry>
 271           <term><constant>-ECHILD</constant></term>
 272
 273           <listitem><para>The event loop has been created in a different process.</para></listitem>
 274         </varlistentry>
 275
 276         <varlistentry>
 277           <term><constant>-EDOM</constant></term>
 278
 279           <listitem><para>The passed event source is not an I/O event source.</para></listitem>
 280         </varlistentry>
 281       </variablelist>
 282     </refsect2>
 283   </refsect1>
 284
 285   <xi:include href="libsystemd-pkgconfig.xml" />
 286
 287   <refsect1>
 288     <title>See Also</title>
 289
 290     <para>
 291       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 292       <citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 293       <citerefentry><refentrytitle>sd_event_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 294       <citerefentry><refentrytitle>sd_event_now</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 295       <citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 296       <citerefentry><refentrytitle>sd_event_add_signal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 297       <citerefentry><refentrytitle>sd_event_add_child</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 298       <citerefentry><refentrytitle>sd_event_add_inotify</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 299       <citerefentry><refentrytitle>sd_event_add_defer</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 300       <citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 301       <citerefentry><refentrytitle>sd_event_source_set_priority</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 302       <citerefentry><refentrytitle>sd_event_source_set_userdata</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 303       <citerefentry><refentrytitle>sd_event_source_set_description</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 304       <citerefentry><refentrytitle>sd_event_source_get_pending</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 305       <citerefentry><refentrytitle>sd_event_source_set_floating</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 306       <citerefentry project='man-pages'><refentrytitle>epoll_ctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
 307       <citerefentry project='man-pages'><refentrytitle>epoll</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 308     </para>
 309   </refsect1>
 310
 311 </refentry>