summaryrefslogtreecommitdiff
path: root/man/sd_event_wait.xml
blob: f2aea00e985c0dc05cf358445111918f0d922c85 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
<?xml version='1.0'?> <!--*- Mode: nxml; nxml-child-indent: 2; indent-tabs-mode: nil -*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

<!--
  This file is part of systemd.

  Copyright 2015 Zbigniew Jędrzejewski-Szmek

  systemd is free software; you can redistribute it and/or modify it
  under the terms of the GNU Lesser General Public License as published by
  the Free Software Foundation; either version 2.1 of the License, or
  (at your option) any later version.

  systemd is distributed in the hope that it will be useful, but
  WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  Lesser General Public License for more details.

  You should have received a copy of the GNU Lesser General Public License
  along with systemd; If not, see <http://www.gnu.org/licenses/>.
-->

<refentry id="sd_event_wait" xmlns:xi="http://www.w3.org/2001/XInclude">

  <refentryinfo>
    <title>sd_event_wait</title>
    <productname>systemd</productname>

    <authorgroup>
      <author>
        <contrib>Developer</contrib>
        <firstname>Tom</firstname>
        <surname>Gundersen</surname>
        <email>teg@jklm.no</email>
      </author>
    </authorgroup>
  </refentryinfo>

  <refmeta>
    <refentrytitle>sd_event_wait</refentrytitle>
    <manvolnum>3</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>sd_event_wait</refname>
    <refname>sd_event_prepare</refname>
    <refname>sd_event_dispatch</refname>
    <refname>sd_event_get_state</refname>
    <refname>SD_EVENT_INITIAL</refname>
    <refname>SD_EVENT_PREPARING</refname>
    <refname>SD_EVENT_ARMED</refname>
    <refname>SD_EVENT_PENDING</refname>
    <refname>SD_EVENT_RUNNING</refname>
    <refname>SD_EVENT_EXITING</refname>
    <refname>SD_EVENT_FINISHED</refname>

    <refpurpose>Low-level event loop operations</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <funcsynopsis>
      <funcsynopsisinfo>#include &lt;systemd/sd-event.h&gt;</funcsynopsisinfo>

      <funcsynopsisinfo><token>enum</token> {
        <constant>SD_EVENT_INITIAL</constant>,
        <constant>SD_EVENT_PREPARING</constant>,
        <constant>SD_EVENT_ARMED</constant>,
        <constant>SD_EVENT_PENDING</constant>,
        <constant>SD_EVENT_RUNNING</constant>,
        <constant>SD_EVENT_EXITING</constant>,
        <constant>SD_EVENT_FINISHED</constant>,
};</funcsynopsisinfo>

      <funcprototype>
        <funcdef>int <function>sd_event_prepare</function></funcdef>
        <paramdef>sd_event *<parameter>event</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_event_wait</function></funcdef>
        <paramdef>sd_event *<parameter>event</parameter></paramdef>
        <paramdef>uint64_t <parameter>usec</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_event_dispatch</function></funcdef>
        <paramdef>sd_event *<parameter>event</parameter></paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_event_get_state</function></funcdef>
        <paramdef>sd_event *<parameter>event</parameter></paramdef>
      </funcprototype>

    </funcsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para>The low-level <function>sd_event_prepare()</function>,
    <function>sd_event_wait()</function> and
    <function>sd_event_dispatch()</function> functions may be used to
    execute specific phases of an event loop. See
    <citerefentry><refentrytitle>sd_event_run</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    and
    <citerefentry><refentrytitle>sd_event_loop</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    for higher-level functions that execute individual but complete
    iterations of an event loop or run it continuously.</para>

    <para><function>sd_event_prepare()</function> checks for pending
    events and arms necessary timers. If any events are ready to be
    processed ("pending"), it returns a positive, non-zero value, and the caller
    should process these events with
    <function>sd_event_dispatch()</function>.</para>

    <para><function>sd_event_dispatch()</function> dispatches the
    highest priority event source that has a pending event. On
    success, <function>sd_event_dispatch()</function> returns either
    zero, which indicates that no further event sources may be
    dispatched and exiting of the event loop was requested via
    <citerefentry><refentrytitle>sd_event_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry>;
    or a positive non-zero value, which means that an event source was
    dispatched and the loop returned to its initial state, and the
    caller should initiate the next event loop iteration by invoking
    <function>sd_event_prepare()</function> again.</para>

    <para>In case <function>sd_event_prepare()</function> returned
    zero, <function>sd_event_wait()</function> should be called to
    wait for further events or a timeout. If any events are ready to
    be processed, it returns a positive, non-zero value, and the
    events should be dispatched with
    <function>sd_event_dispatch()</function>. Otherwise, the event
    loop returned to its initial state and the next event loop
    iteration should be initiated by invoking
    <function>sd_event_prepare()</function> again.</para>

    <para><function>sd_event_get_state()</function> may be used to
    determine the state the event loop is currently in. It returns one
    of the states described below.</para>

    <para>All four functions take, as the first argument, the event
    loop object <parameter>event</parameter> that has been created
    with <function>sd_event_new()</function>. The timeout for
    <function>sd_event_wait()</function> is specified in
    <parameter>usec</parameter> in milliseconds.  <constant>(uint64_t)
    -1</constant> may be used to specify an infinite timeout.</para>
</refsect1>

  <refsect1>
    <title>State Machine</title>

    <para>The event loop knows the following states, that may be
    queried with <function>sd_event_get_state()</function>.</para>

    <variablelist>
      <varlistentry>
        <term><constant>SD_EVENT_INITIAL</constant></term>

        <listitem><para>The initial state the event loop is in,
        before each event loop iteration. Use
        <function>sd_event_prepare()</function> to transition the
        event loop into the <constant>SD_EVENT_ARMED</constant> or
        <constant>SD_EVENT_PENDING</constant> states.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>SD_EVENT_PREPARING</constant></term>

        <listitem><para>An event source is currently being prepared,
        i.e. the preparation handler is currently being executed, as
        set with
        <citerefentry><refentrytitle>sd_event_set_prepare</refentrytitle><manvolnum>3</manvolnum></citerefentry>. This
        state is only seen in the event source preparation handler
        that is invoked from the
        <function>sd_event_prepare()</function> call and is
        immediately followed by <constant>SD_EVENT_ARMED</constant> or
        <constant>SD_EVENT_PENDING</constant>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>SD_EVENT_ARMED</constant></term>

        <listitem><para><function>sd_event_prepare()</function> has
        been called and no event sources were ready to be
        dispatched. Use <function>sd_event_wait()</function> to wait
        for new events, and transition into
        <constant>SD_EVENT_PENDING</constant> or back into
        <constant>SD_EVENT_INITIAL</constant>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>SD_EVENT_PENDING</constant></term>

        <listitem><para><function>sd_event_prepare()</function> or
        <function>sd_event_wait()</function> have been called and
        there were event sources with events pending. Use
        <function>sd_event_dispatch()</function> to dispatch the
        highest priority event source and transition back to
        <constant>SD_EVENT_INITIAL</constant>, or
        <constant>SD_EVENT_FINISHED</constant>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>SD_EVENT_RUNNING</constant></term>

        <listitem><para>A regular event source is currently being
        dispatched. This state is only seen in the event source
        handler that is invoked from the
        <function>sd_event_dispatch()</function> call, and is
        immediately followed by <constant>SD_EVENT_INITIAL</constant>
        or <constant>SD_EVENT_FINISHED</constant> as soon the event
        source handler returns. Note that during dispatching of exit
        event sources the <constant>SD_EVENT_EXITING</constant> state
        is seen instead.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>SD_EVENT_EXITING</constant></term>

        <listitem><para>Similar to
        <constant>SD_EVENT_RUNNING</constant> but is the state in
        effect while dispatching exit event sources. It is followed by
        <constant>SD_EVENT_INITIAL</constant> or
        <constant>SD_EVENT_FINISHED</constant> as soon as the event
        handler returns.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>SD_EVENT_FINISHED</constant></term>

        <listitem><para>The event loop has exited. All exit event
        sources have run. If the event loop is in this state it serves
        no purpose anymore, and should be freed.</para></listitem>
      </varlistentry>

    </variablelist>

    <para>A simplified flow chart of the states and the calls to
    transition between them is shown below. Note that
    <constant>SD_EVENT_PREPARING</constant>,
    <constant>SD_EVENT_RUNNING</constant> and
    <constant>SD_EVENT_EXITING</constant> are not shown here.</para>

    <programlisting>
          INITIAL -&lt;---&lt;---&lt;---&lt;---&lt;---&lt;---&lt;---&lt;---&lt;---&lt;---&lt;---&lt;---\
             |                                                     |
             |                                                     ^
             |                                                     |
             v                 ret == 0                            |
      sd_event_prepare() &gt;---&gt;---&gt;---&gt;---&gt;- ARMED                  |
             |                                |                    ^
             | ret > 0                        |                    |
             |                                |                    |
             v                                v          ret == 0  |
          PENDING &lt;---&lt;---&lt;---&lt;---&lt;---&lt; sd_event_wait() &gt;---&gt;---&gt;--+
             |           ret > 0                                   ^
             |                                                     |
             |                                                     |
             v                                                     |
      sd_event_dispatch() &gt;---&gt;---&gt;---&gt;---&gt;---&gt;---&gt;---&gt;---&gt;---&gt;---&gt;/
             |                             ret > 0
             | ret == 0
             |
             v
          FINISHED
    </programlisting>
  </refsect1>

  <refsect1>
    <title>Return Value</title>

    <para>On success, these functions return 0 or a positive integer.
    On failure, they return a negative errno-style error code. In case
    of <function>sd_event_prepare()</function> and
    <function>sd_event_wait()</function>, a positive, non-zero return
    code indicates that events are ready to be processed and zero
    indicates that no events are ready. In case of
    <function>sd_event_dispatch()</function>, a positive, non-zero
    return code indicates that the event loop returned to its initial
    state and zero indicates the event loop has
    exited. <function>sd_event_get_state()</function> returns a
    positive or zero state on success.</para>
  </refsect1>

  <refsect1>
    <title>Errors</title>

    <para>Returned errors may indicate the following problems:</para>

    <variablelist>
      <varlistentry>
        <term><constant>-EINVAL</constant></term>

        <listitem><para>The <parameter>event</parameter> parameter is
        invalid or <constant>NULL</constant>.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>-EBUSY</constant></term>

        <listitem><para>The event loop object is not in the right
        state.</para></listitem>
      </varlistentry>

      <varlistentry>
        <term><constant>-ESTALE</constant></term>

        <listitem><para>The event loop is already terminated.</para></listitem>

      </varlistentry>

      <varlistentry>
        <term><constant>-ECHILD</constant></term>

        <listitem><para>The event loop has been created in a different process.</para></listitem>

      </varlistentry>

    </variablelist>

    <para>Other errors are possible, too.</para>
  </refsect1>

  <xi:include href="libsystemd-pkgconfig.xml" />

  <refsect1>
    <title>See Also</title>

    <para>
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_event_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_event_add_io</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_event_add_signal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_event_add_defer</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_event_add_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_event_add_post</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_event_run</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_event_get_fd</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_event_source_set_prepare</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>