man/sd_event_add_io.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_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>Note that this call does not take possession of the file descriptor passed in, ownership (and thus
 165     the duty to close it when it is no longer needed) remains with the caller. However, with the
 166     <function>sd_event_source_set_io_fd_own()</function> call (see below) the event source may optionally
 167     take ownership of the file descriptor after the event source has been created. In that case the file
 168     descriptor is closed automatically as soon as the event source is released.</para>
 169
 170     <para>It is recommended to use
 171     <function>sd_event_add_io()</function> only in conjunction with
 172     file descriptors that have <constant>O_NONBLOCK</constant> set, to
 173     ensure that all I/O operations from invoked handlers are properly
 174     asynchronous and non-blocking. Using file descriptors without
 175     <constant>O_NONBLOCK</constant> might result in unexpected
 176     starvation of other event sources. See
 177     <citerefentry project='man-pages'><refentrytitle>fcntl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
 178     for details on enabling <constant>O_NONBLOCK</constant> mode.</para>
 179
 180     <para><function>sd_event_source_get_io_events()</function> retrieves
 181     the configured mask of watched I/O events of an event source created
 182     previously with <function>sd_event_add_io()</function>. It takes
 183     the event source object and a pointer to a variable to store the
 184     mask in.</para>
 185
 186     <para><function>sd_event_source_set_io_events()</function>
 187     configures the mask of watched I/O events of an event source created
 188     previously with <function>sd_event_add_io()</function>. It takes the
 189     event source object and the new event mask.</para>
 190
 191     <para><function>sd_event_source_get_io_revents()</function>
 192     retrieves the I/O event mask of currently seen but undispatched
 193     events from an event source created previously with
 194     <function>sd_event_add_io()</function>. It takes the event source
 195     object and a pointer to a variable to store the event mask
 196     in. When called from a handler function on the handler's event
 197     source object this will return the same mask as passed to the
 198     handler's <parameter>revents</parameter> parameter. This call is
 199     primarily useful to check for undispatched events of an event
 200     source from the handler of an unrelated (possibly higher priority)
 201     event source. Note the relation between
 202     <function>sd_event_source_get_pending()</function> and
 203     <function>sd_event_source_get_io_revents()</function>: both
 204     functions will report non-zero results when there's an event
 205     pending for the event source, but the former applies to all event
 206     source types, the latter only to I/O event sources.</para>
 207
 208     <para><function>sd_event_source_get_io_fd()</function> retrieves
 209     the UNIX file descriptor of an event source created previously
 210     with <function>sd_event_add_io()</function>. It takes the event
 211     source object and returns the non-negative file descriptor
 212     or a negative error number on error (see below).</para>
 213
 214     <para><function>sd_event_source_set_io_fd()</function>
 215     changes the UNIX file descriptor of an I/O event source created
 216     previously with <function>sd_event_add_io()</function>. It takes
 217     the event source object and the new file descriptor.</para>
 218
 219     <para><function>sd_event_source_set_io_fd_own()</function> controls whether the file descriptor of the event source
 220     shall be closed automatically when the event source is freed, i.e. whether it shall be considered 'owned' by the
 221     event source object. By default it is not closed automatically, and the application has to do this on its own. The
 222     <parameter>b</parameter> parameter is a boolean parameter: if zero, the file descriptor is not closed automatically
 223     when the event source is freed, otherwise it is closed.</para>
 224
 225     <para><function>sd_event_source_get_io_fd_own()</function> may be used to query the current setting of the file
 226     descriptor ownership boolean flag as set with <function>sd_event_source_set_io_fd_own()</function>. It returns
 227     positive if the file descriptor is closed automatically when the event source is destroyed, zero if not, and
 228     negative on error.</para>
 229   </refsect1>
 230
 231   <refsect1>
 232     <title>Return Value</title>
 233
 234     <para>On success, these functions return 0 or a positive
 235     integer. On failure, they return a negative errno-style error
 236     code.</para>
 237
 238     <refsect2>
 239       <title>Errors</title>
 240
 241       <para>Returned values may indicate the following problems:</para>
 242
 243       <variablelist>
 244         <varlistentry>
 245           <term><constant>-ENOMEM</constant></term>
 246
 247           <listitem><para>Not enough memory to allocate an object.</para></listitem>
 248         </varlistentry>
 249
 250         <varlistentry>
 251           <term><constant>-EINVAL</constant></term>
 252
 253           <listitem><para>An invalid argument has been passed.</para></listitem>
 254
 255         </varlistentry>
 256
 257         <varlistentry>
 258           <term><constant>-ESTALE</constant></term>
 259
 260           <listitem><para>The event loop is already terminated.</para></listitem>
 261
 262         </varlistentry>
 263
 264         <varlistentry>
 265           <term><constant>-ECHILD</constant></term>
 266
 267           <listitem><para>The event loop has been created in a different process.</para></listitem>
 268         </varlistentry>
 269
 270         <varlistentry>
 271           <term><constant>-EDOM</constant></term>
 272
 273           <listitem><para>The passed event source is not an I/O event source.</para></listitem>
 274         </varlistentry>
 275       </variablelist>
 276     </refsect2>
 277   </refsect1>
 278
 279   <xi:include href="libsystemd-pkgconfig.xml" />
 280
 281   <refsect1>
 282     <title>See Also</title>
 283
 284     <para>
 285       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 286       <citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 287       <citerefentry><refentrytitle>sd_event_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 288       <citerefentry><refentrytitle>sd_event_now</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 289       <citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 290       <citerefentry><refentrytitle>sd_event_add_signal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 291       <citerefentry><refentrytitle>sd_event_add_child</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 292       <citerefentry><refentrytitle>sd_event_add_inotify</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 293       <citerefentry><refentrytitle>sd_event_add_defer</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 294       <citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 295       <citerefentry><refentrytitle>sd_event_source_set_priority</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 296       <citerefentry><refentrytitle>sd_event_source_set_userdata</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 297       <citerefentry><refentrytitle>sd_event_source_set_description</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 298       <citerefentry><refentrytitle>sd_event_source_get_pending</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 299       <citerefentry><refentrytitle>sd_event_source_set_floating</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 300       <citerefentry project='man-pages'><refentrytitle>epoll_ctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
 301       <citerefentry project='man-pages'><refentrytitle>epoll</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 302     </para>
 303   </refsect1>
 304
 305 </refentry>