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1 | <?xml version='1.0'?> <!--*- Mode: nxml; nxml-child-indent: 2; indent-tabs-mode: nil -*--> |
2 | <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN" | |
12b42c76 | 3 | "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> |
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4 | |
5 | <!-- | |
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6 | SPDX-License-Identifier: LGPL-2.1+ |
7 | ||
96b2fb93 | 8 | Copyright © 2015 Zbigniew Jędrzejewski-Szmek |
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9 | --> |
10 | ||
dc83f27a | 11 | <refentry id="sd_event_wait" xmlns:xi="http://www.w3.org/2001/XInclude"> |
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12 | |
13 | <refentryinfo> | |
14 | <title>sd_event_wait</title> | |
15 | <productname>systemd</productname> | |
16 | ||
17 | <authorgroup> | |
18 | <author> | |
19 | <contrib>Developer</contrib> | |
20 | <firstname>Tom</firstname> | |
21 | <surname>Gundersen</surname> | |
22 | <email>teg@jklm.no</email> | |
23 | </author> | |
24 | </authorgroup> | |
25 | </refentryinfo> | |
26 | ||
27 | <refmeta> | |
28 | <refentrytitle>sd_event_wait</refentrytitle> | |
29 | <manvolnum>3</manvolnum> | |
30 | </refmeta> | |
31 | ||
32 | <refnamediv> | |
33 | <refname>sd_event_wait</refname> | |
34 | <refname>sd_event_prepare</refname> | |
35 | <refname>sd_event_dispatch</refname> | |
dc83f27a | 36 | <refname>sd_event_get_state</refname> |
60a3b1e1 | 37 | <refname>sd_event_get_iteration</refname> |
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38 | <refname>SD_EVENT_INITIAL</refname> |
39 | <refname>SD_EVENT_PREPARING</refname> | |
40 | <refname>SD_EVENT_ARMED</refname> | |
41 | <refname>SD_EVENT_PENDING</refname> | |
42 | <refname>SD_EVENT_RUNNING</refname> | |
43 | <refname>SD_EVENT_EXITING</refname> | |
44 | <refname>SD_EVENT_FINISHED</refname> | |
45 | ||
46 | <refpurpose>Low-level event loop operations</refpurpose> | |
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47 | </refnamediv> |
48 | ||
49 | <refsynopsisdiv> | |
50 | <funcsynopsis> | |
51 | <funcsynopsisinfo>#include <systemd/sd-event.h></funcsynopsisinfo> | |
52 | ||
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53 | <funcsynopsisinfo><token>enum</token> { |
54 | <constant>SD_EVENT_INITIAL</constant>, | |
55 | <constant>SD_EVENT_PREPARING</constant>, | |
56 | <constant>SD_EVENT_ARMED</constant>, | |
57 | <constant>SD_EVENT_PENDING</constant>, | |
58 | <constant>SD_EVENT_RUNNING</constant>, | |
59 | <constant>SD_EVENT_EXITING</constant>, | |
60 | <constant>SD_EVENT_FINISHED</constant>, | |
61 | };</funcsynopsisinfo> | |
62 | ||
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63 | <funcprototype> |
64 | <funcdef>int <function>sd_event_prepare</function></funcdef> | |
65 | <paramdef>sd_event *<parameter>event</parameter></paramdef> | |
66 | </funcprototype> | |
67 | ||
68 | <funcprototype> | |
69 | <funcdef>int <function>sd_event_wait</function></funcdef> | |
70 | <paramdef>sd_event *<parameter>event</parameter></paramdef> | |
dc83f27a | 71 | <paramdef>uint64_t <parameter>usec</parameter></paramdef> |
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72 | </funcprototype> |
73 | ||
74 | <funcprototype> | |
75 | <funcdef>int <function>sd_event_dispatch</function></funcdef> | |
76 | <paramdef>sd_event *<parameter>event</parameter></paramdef> | |
77 | </funcprototype> | |
78 | ||
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79 | <funcprototype> |
80 | <funcdef>int <function>sd_event_get_state</function></funcdef> | |
81 | <paramdef>sd_event *<parameter>event</parameter></paramdef> | |
82 | </funcprototype> | |
83 | ||
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84 | <funcprototype> |
85 | <funcdef>int <function>sd_event_get_iteration</function></funcdef> | |
86 | <paramdef>sd_event *<parameter>event</parameter></paramdef> | |
87 | <paramdef>uint64_t *<parameter>ret</parameter></paramdef> | |
88 | </funcprototype> | |
89 | ||
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90 | </funcsynopsis> |
91 | </refsynopsisdiv> | |
92 | ||
93 | <refsect1> | |
94 | <title>Description</title> | |
95 | ||
dc83f27a LP |
96 | <para>The low-level <function>sd_event_prepare()</function>, |
97 | <function>sd_event_wait()</function> and | |
98 | <function>sd_event_dispatch()</function> functions may be used to | |
99 | execute specific phases of an event loop. See | |
100 | <citerefentry><refentrytitle>sd_event_run</refentrytitle><manvolnum>3</manvolnum></citerefentry> | |
101 | and | |
102 | <citerefentry><refentrytitle>sd_event_loop</refentrytitle><manvolnum>3</manvolnum></citerefentry> | |
103 | for higher-level functions that execute individual but complete | |
7f3fdb7f | 104 | iterations of an event loop or run it continuously.</para> |
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105 | |
106 | <para><function>sd_event_prepare()</function> checks for pending | |
42f1ab50 | 107 | events and arms necessary timers. If any events are ready to be |
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108 | processed ("pending"), it returns a positive, non-zero value, and the caller |
109 | should process these events with | |
110 | <function>sd_event_dispatch()</function>.</para> | |
111 | ||
112 | <para><function>sd_event_dispatch()</function> dispatches the | |
113 | highest priority event source that has a pending event. On | |
114 | success, <function>sd_event_dispatch()</function> returns either | |
115 | zero, which indicates that no further event sources may be | |
116 | dispatched and exiting of the event loop was requested via | |
117 | <citerefentry><refentrytitle>sd_event_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry>; | |
118 | or a positive non-zero value, which means that an event source was | |
119 | dispatched and the loop returned to its initial state, and the | |
120 | caller should initiate the next event loop iteration by invoking | |
121 | <function>sd_event_prepare()</function> again.</para> | |
122 | ||
123 | <para>In case <function>sd_event_prepare()</function> returned | |
124 | zero, <function>sd_event_wait()</function> should be called to | |
125 | wait for further events or a timeout. If any events are ready to | |
126 | be processed, it returns a positive, non-zero value, and the | |
127 | events should be dispatched with | |
128 | <function>sd_event_dispatch()</function>. Otherwise, the event | |
129 | loop returned to its initial state and the next event loop | |
130 | iteration should be initiated by invoking | |
131 | <function>sd_event_prepare()</function> again.</para> | |
132 | ||
133 | <para><function>sd_event_get_state()</function> may be used to | |
134 | determine the state the event loop is currently in. It returns one | |
135 | of the states described below.</para> | |
136 | ||
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137 | <para><function>sd_event_get_iteration()</function> may be used to determine the current iteration of the event |
138 | loop. It returns an unsigned 64bit integer containing a counter that increases monotonically with each iteration of | |
139 | the event loop, starting with 0. The counter is increased at the time of the | |
140 | <function>sd_event_prepare()</function> invocation.</para> | |
141 | ||
142 | <para>All five functions take, as the first argument, the event loop object <parameter>event</parameter> that has | |
143 | been created with <function>sd_event_new()</function>. The timeout for <function>sd_event_wait()</function> is | |
144 | specified in <parameter>usec</parameter> in microseconds. <constant>(uint64_t) -1</constant> may be used to | |
145 | specify an infinite timeout.</para> | |
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146 | </refsect1> |
147 | ||
148 | <refsect1> | |
149 | <title>State Machine</title> | |
150 | ||
151 | <para>The event loop knows the following states, that may be | |
152 | queried with <function>sd_event_get_state()</function>.</para> | |
42f1ab50 | 153 | |
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154 | <variablelist> |
155 | <varlistentry> | |
156 | <term><constant>SD_EVENT_INITIAL</constant></term> | |
157 | ||
158 | <listitem><para>The initial state the event loop is in, | |
159 | before each event loop iteration. Use | |
160 | <function>sd_event_prepare()</function> to transition the | |
161 | event loop into the <constant>SD_EVENT_ARMED</constant> or | |
162 | <constant>SD_EVENT_PENDING</constant> states.</para></listitem> | |
163 | </varlistentry> | |
164 | ||
165 | <varlistentry> | |
166 | <term><constant>SD_EVENT_PREPARING</constant></term> | |
167 | ||
168 | <listitem><para>An event source is currently being prepared, | |
7f3fdb7f | 169 | i.e. the preparation handler is currently being executed, as |
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170 | set with |
171 | <citerefentry><refentrytitle>sd_event_set_prepare</refentrytitle><manvolnum>3</manvolnum></citerefentry>. This | |
172 | state is only seen in the event source preparation handler | |
173 | that is invoked from the | |
174 | <function>sd_event_prepare()</function> call and is | |
175 | immediately followed by <constant>SD_EVENT_ARMED</constant> or | |
176 | <constant>SD_EVENT_PENDING</constant>.</para></listitem> | |
177 | </varlistentry> | |
178 | ||
179 | <varlistentry> | |
180 | <term><constant>SD_EVENT_ARMED</constant></term> | |
181 | ||
182 | <listitem><para><function>sd_event_prepare()</function> has | |
183 | been called and no event sources were ready to be | |
184 | dispatched. Use <function>sd_event_wait()</function> to wait | |
185 | for new events, and transition into | |
186 | <constant>SD_EVENT_PENDING</constant> or back into | |
187 | <constant>SD_EVENT_INITIAL</constant>.</para></listitem> | |
188 | </varlistentry> | |
189 | ||
190 | <varlistentry> | |
191 | <term><constant>SD_EVENT_PENDING</constant></term> | |
192 | ||
193 | <listitem><para><function>sd_event_prepare()</function> or | |
194 | <function>sd_event_wait()</function> have been called and | |
195 | there were event sources with events pending. Use | |
196 | <function>sd_event_dispatch()</function> to dispatch the | |
197 | highest priority event source and transition back to | |
198 | <constant>SD_EVENT_INITIAL</constant>, or | |
199 | <constant>SD_EVENT_FINISHED</constant>.</para></listitem> | |
200 | </varlistentry> | |
201 | ||
202 | <varlistentry> | |
203 | <term><constant>SD_EVENT_RUNNING</constant></term> | |
204 | ||
205 | <listitem><para>A regular event source is currently being | |
206 | dispatched. This state is only seen in the event source | |
207 | handler that is invoked from the | |
208 | <function>sd_event_dispatch()</function> call, and is | |
209 | immediately followed by <constant>SD_EVENT_INITIAL</constant> | |
210 | or <constant>SD_EVENT_FINISHED</constant> as soon the event | |
211 | source handler returns. Note that during dispatching of exit | |
212 | event sources the <constant>SD_EVENT_EXITING</constant> state | |
213 | is seen instead.</para></listitem> | |
214 | </varlistentry> | |
215 | ||
216 | <varlistentry> | |
217 | <term><constant>SD_EVENT_EXITING</constant></term> | |
218 | ||
219 | <listitem><para>Similar to | |
220 | <constant>SD_EVENT_RUNNING</constant> but is the state in | |
221 | effect while dispatching exit event sources. It is followed by | |
222 | <constant>SD_EVENT_INITIAL</constant> or | |
223 | <constant>SD_EVENT_FINISHED</constant> as soon as the event | |
224 | handler returns.</para></listitem> | |
225 | </varlistentry> | |
226 | ||
227 | <varlistentry> | |
228 | <term><constant>SD_EVENT_FINISHED</constant></term> | |
229 | ||
230 | <listitem><para>The event loop has exited. All exit event | |
231 | sources have run. If the event loop is in this state it serves | |
232 | no purpose anymore, and should be freed.</para></listitem> | |
233 | </varlistentry> | |
234 | ||
235 | </variablelist> | |
236 | ||
237 | <para>A simplified flow chart of the states and the calls to | |
238 | transition between them is shown below. Note that | |
239 | <constant>SD_EVENT_PREPARING</constant>, | |
240 | <constant>SD_EVENT_RUNNING</constant> and | |
241 | <constant>SD_EVENT_EXITING</constant> are not shown here.</para> | |
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242 | |
243 | <programlisting> | |
dc83f27a LP |
244 | INITIAL -<---<---<---<---<---<---<---<---<---<---<---<---\ |
245 | | | | |
246 | | ^ | |
247 | | | | |
248 | v ret == 0 | | |
249 | sd_event_prepare() >--->--->--->--->- ARMED | | |
250 | | | ^ | |
251 | | ret > 0 | | | |
252 | | | | | |
253 | v v ret == 0 | | |
254 | PENDING <---<---<---<---<---< sd_event_wait() >--->--->--+ | |
255 | | ret > 0 ^ | |
256 | | | | |
257 | | | | |
258 | v | | |
259 | sd_event_dispatch() >--->--->--->--->--->--->--->--->--->--->/ | |
260 | | ret > 0 | |
261 | | ret == 0 | |
262 | | | |
263 | v | |
264 | FINISHED | |
42f1ab50 | 265 | </programlisting> |
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266 | </refsect1> |
267 | ||
268 | <refsect1> | |
269 | <title>Return Value</title> | |
270 | ||
271 | <para>On success, these functions return 0 or a positive integer. | |
272 | On failure, they return a negative errno-style error code. In case | |
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273 | of <function>sd_event_prepare()</function> and |
274 | <function>sd_event_wait()</function>, a positive, non-zero return | |
275 | code indicates that events are ready to be processed and zero | |
276 | indicates that no events are ready. In case of | |
277 | <function>sd_event_dispatch()</function>, a positive, non-zero | |
278 | return code indicates that the event loop returned to its initial | |
279 | state and zero indicates the event loop has | |
280 | exited. <function>sd_event_get_state()</function> returns a | |
281 | positive or zero state on success.</para> | |
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282 | </refsect1> |
283 | ||
284 | <refsect1> | |
285 | <title>Errors</title> | |
286 | ||
287 | <para>Returned errors may indicate the following problems:</para> | |
288 | ||
289 | <variablelist> | |
290 | <varlistentry> | |
291 | <term><constant>-EINVAL</constant></term> | |
292 | ||
a8eaaee7 | 293 | <listitem><para>The <parameter>event</parameter> parameter is |
dc83f27a | 294 | invalid or <constant>NULL</constant>.</para></listitem> |
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295 | </varlistentry> |
296 | ||
297 | <varlistentry> | |
298 | <term><constant>-EBUSY</constant></term> | |
299 | ||
300 | <listitem><para>The event loop object is not in the right | |
301 | state.</para></listitem> | |
302 | </varlistentry> | |
303 | ||
304 | <varlistentry> | |
305 | <term><constant>-ESTALE</constant></term> | |
306 | ||
307 | <listitem><para>The event loop is already terminated.</para></listitem> | |
308 | ||
309 | </varlistentry> | |
310 | ||
311 | <varlistentry> | |
312 | <term><constant>-ECHILD</constant></term> | |
313 | ||
314 | <listitem><para>The event loop has been created in a different process.</para></listitem> | |
315 | ||
316 | </varlistentry> | |
317 | ||
318 | </variablelist> | |
319 | ||
b938cb90 | 320 | <para>Other errors are possible, too.</para> |
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321 | </refsect1> |
322 | ||
dc83f27a | 323 | <xi:include href="libsystemd-pkgconfig.xml" /> |
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324 | |
325 | <refsect1> | |
326 | <title>See Also</title> | |
327 | ||
328 | <para> | |
329 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, | |
330 | <citerefentry><refentrytitle>sd_event_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
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331 | <citerefentry><refentrytitle>sd_event_add_io</refentrytitle><manvolnum>3</manvolnum></citerefentry>, |
332 | <citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
333 | <citerefentry><refentrytitle>sd_event_add_signal</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
334 | <citerefentry><refentrytitle>sd_event_add_defer</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
335 | <citerefentry><refentrytitle>sd_event_add_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
dc83f27a LP |
336 | <citerefentry><refentrytitle>sd_event_add_post</refentrytitle><manvolnum>3</manvolnum></citerefentry>, |
337 | <citerefentry><refentrytitle>sd_event_run</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
338 | <citerefentry><refentrytitle>sd_event_get_fd</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
339 | <citerefentry><refentrytitle>sd_event_source_set_prepare</refentrytitle><manvolnum>3</manvolnum></citerefentry> | |
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340 | </para> |
341 | </refsect1> | |
342 | ||
343 | </refentry> |