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>It is recommended to use
 165     <function>sd_event_add_io()</function> only in conjunction with
 166     file descriptors that have <constant>O_NONBLOCK</constant> set, to
 167     ensure that all I/O operations from invoked handlers are properly
 168     asynchronous and non-blocking. Using file descriptors without
 169     <constant>O_NONBLOCK</constant> might result in unexpected
 170     starvation of other event sources. See
 171     <citerefentry project='man-pages'><refentrytitle>fcntl</refentrytitle><manvolnum>2</manvolnum></citerefentry>
 172     for details on enabling <constant>O_NONBLOCK</constant> mode.</para>
 173
 174     <para><function>sd_event_source_get_io_events()</function> retrieves
 175     the configured mask of watched I/O events of an event source created
 176     previously with <function>sd_event_add_io()</function>. It takes
 177     the event source object and a pointer to a variable to store the
 178     mask in.</para>
 179
 180     <para><function>sd_event_source_set_io_events()</function>
 181     configures the mask of watched I/O events of an event source created
 182     previously with <function>sd_event_add_io()</function>. It takes the
 183     event source object and the new event mask.</para>
 184
 185     <para><function>sd_event_source_get_io_revents()</function>
 186     retrieves the I/O event mask of currently seen but undispatched
 187     events from an event source created previously with
 188     <function>sd_event_add_io()</function>. It takes the event source
 189     object and a pointer to a variable to store the event mask
 190     in. When called from a handler function on the handler's event
 191     source object this will return the same mask as passed to the
 192     handler's <parameter>revents</parameter> parameter. This call is
 193     primarily useful to check for undispatched events of an event
 194     source from the handler of an unrelated (possibly higher priority)
 195     event source. Note the relation between
 196     <function>sd_event_source_get_pending()</function> and
 197     <function>sd_event_source_get_io_revents()</function>: both
 198     functions will report non-zero results when there's an event
 199     pending for the event source, but the former applies to all event
 200     source types, the latter only to I/O event sources.</para>
 201
 202     <para><function>sd_event_source_get_io_fd()</function> retrieves
 203     the UNIX file descriptor of an event source created previously
 204     with <function>sd_event_add_io()</function>. It takes the event
 205     source object and returns the non-negative file descriptor
 206     or a negative error number on error (see below).</para>
 207
 208     <para><function>sd_event_source_set_io_fd()</function>
 209     changes the UNIX file descriptor of an I/O event source created
 210     previously with <function>sd_event_add_io()</function>. It takes
 211     the event source object and the new file descriptor.</para>
 212
 213     <para><function>sd_event_source_set_io_fd_own()</function> controls whether the file descriptor of the event source
 214     shall be closed automatically when the event source is freed, i.e. whether it shall be considered 'owned' by the
 215     event source object. By default it is not closed automatically, and the application has to do this on its own. The
 216     <parameter>b</parameter> parameter is a boolean parameter: if zero, the file descriptor is not closed automatically
 217     when the event source is freed, otherwise it is closed.</para>
 218
 219     <para><function>sd_event_source_get_io_fd_own()</function> may be used to query the current setting of the file
 220     descriptor ownership boolean flag as set with <function>sd_event_source_set_io_fd_own()</function>. It returns
 221     positive if the file descriptor is closed automatically when the event source is destroyed, zero if not, and
 222     negative on error.</para>
 223   </refsect1>
 224
 225   <refsect1>
 226     <title>Return Value</title>
 227
 228     <para>On success, these functions return 0 or a positive
 229     integer. On failure, they return a negative errno-style error
 230     code.</para>
 231
 232     <refsect2>
 233       <title>Errors</title>
 234
 235       <para>Returned values may indicate the following problems:</para>
 236
 237       <variablelist>
 238         <varlistentry>
 239           <term><constant>-ENOMEM</constant></term>
 240
 241           <listitem><para>Not enough memory to allocate an object.</para></listitem>
 242         </varlistentry>
 243
 244         <varlistentry>
 245           <term><constant>-EINVAL</constant></term>
 246
 247           <listitem><para>An invalid argument has been passed.</para></listitem>
 248
 249         </varlistentry>
 250
 251         <varlistentry>
 252           <term><constant>-ESTALE</constant></term>
 253
 254           <listitem><para>The event loop is already terminated.</para></listitem>
 255
 256         </varlistentry>
 257
 258         <varlistentry>
 259           <term><constant>-ECHILD</constant></term>
 260
 261           <listitem><para>The event loop has been created in a different process.</para></listitem>
 262         </varlistentry>
 263
 264         <varlistentry>
 265           <term><constant>-EDOM</constant></term>
 266
 267           <listitem><para>The passed event source is not an I/O event source.</para></listitem>
 268         </varlistentry>
 269       </variablelist>
 270     </refsect2>
 271   </refsect1>
 272
 273   <xi:include href="libsystemd-pkgconfig.xml" />
 274
 275   <refsect1>
 276     <title>See Also</title>
 277
 278     <para>
 279       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 280       <citerefentry><refentrytitle>sd-event</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 281       <citerefentry><refentrytitle>sd_event_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 282       <citerefentry><refentrytitle>sd_event_now</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 283       <citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 284       <citerefentry><refentrytitle>sd_event_add_signal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 285       <citerefentry><refentrytitle>sd_event_add_child</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 286       <citerefentry><refentrytitle>sd_event_add_inotify</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 287       <citerefentry><refentrytitle>sd_event_add_defer</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 288       <citerefentry><refentrytitle>sd_event_source_set_enabled</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 289       <citerefentry><refentrytitle>sd_event_source_set_priority</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 290       <citerefentry><refentrytitle>sd_event_source_set_userdata</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 291       <citerefentry><refentrytitle>sd_event_source_set_description</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 292       <citerefentry><refentrytitle>sd_event_source_get_pending</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 293       <citerefentry project='man-pages'><refentrytitle>epoll_ctl</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
 294       <citerefentry project='man-pages'><refentrytitle>epoll</refentrytitle><manvolnum>7</manvolnum></citerefentry>
 295     </para>
 296   </refsect1>
 297
 298 </refentry>