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-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.</para>
119
120    <para>The <parameter>handler</parameter> is a function to call when the event source is triggered or
121    <constant>NULL</constant>. The <parameter>userdata</parameter> pointer will be passed to the handler
122    function, and may be chosen freely by the caller. The handler will also be passed the file descriptor the
123    event was seen on, as well as the actual event flags. It's generally a subset of the events watched,
124    however may additionally include <constant>EPOLLERR</constant> and <constant>EPOLLHUP</constant>. The
125    handler may return negative to signal an error (see below), other return values are ignored. If
126    <parameter>handler</parameter> is <constant>NULL</constant>, a default handler that calls
127    <citerefentry><refentrytitle>sd_event_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry> will be
128    used.</para>
129
130    <para>By default, an event source will stay enabled continuously (<constant>SD_EVENT_ON</constant>), but
131    this may be changed 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 either be disabled after the invocation,
134    even if the <constant>SD_EVENT_ON</constant> mode was requested before, or it will cause the loop to
135    terminate, see
136    <citerefentry><refentrytitle>sd_event_source_set_exit_on_failure</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
137    Note that an event source set to <constant>SD_EVENT_ON</constant> will fire continuously unless data is
138    read from or written to the file descriptor to reset the mask of events seen.</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>
312