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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 | ||
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8 | This file is part of systemd. |
9 | ||
10 | Copyright 2015 Zbigniew Jędrzejewski-Szmek | |
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11 | --> |
12 | ||
dc83f27a | 13 | <refentry id="sd_event_wait" xmlns:xi="http://www.w3.org/2001/XInclude"> |
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14 | |
15 | <refentryinfo> | |
16 | <title>sd_event_wait</title> | |
17 | <productname>systemd</productname> | |
18 | ||
19 | <authorgroup> | |
20 | <author> | |
21 | <contrib>Developer</contrib> | |
22 | <firstname>Tom</firstname> | |
23 | <surname>Gundersen</surname> | |
24 | <email>teg@jklm.no</email> | |
25 | </author> | |
26 | </authorgroup> | |
27 | </refentryinfo> | |
28 | ||
29 | <refmeta> | |
30 | <refentrytitle>sd_event_wait</refentrytitle> | |
31 | <manvolnum>3</manvolnum> | |
32 | </refmeta> | |
33 | ||
34 | <refnamediv> | |
35 | <refname>sd_event_wait</refname> | |
36 | <refname>sd_event_prepare</refname> | |
37 | <refname>sd_event_dispatch</refname> | |
dc83f27a | 38 | <refname>sd_event_get_state</refname> |
60a3b1e1 | 39 | <refname>sd_event_get_iteration</refname> |
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40 | <refname>SD_EVENT_INITIAL</refname> |
41 | <refname>SD_EVENT_PREPARING</refname> | |
42 | <refname>SD_EVENT_ARMED</refname> | |
43 | <refname>SD_EVENT_PENDING</refname> | |
44 | <refname>SD_EVENT_RUNNING</refname> | |
45 | <refname>SD_EVENT_EXITING</refname> | |
46 | <refname>SD_EVENT_FINISHED</refname> | |
47 | ||
48 | <refpurpose>Low-level event loop operations</refpurpose> | |
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49 | </refnamediv> |
50 | ||
51 | <refsynopsisdiv> | |
52 | <funcsynopsis> | |
53 | <funcsynopsisinfo>#include <systemd/sd-event.h></funcsynopsisinfo> | |
54 | ||
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55 | <funcsynopsisinfo><token>enum</token> { |
56 | <constant>SD_EVENT_INITIAL</constant>, | |
57 | <constant>SD_EVENT_PREPARING</constant>, | |
58 | <constant>SD_EVENT_ARMED</constant>, | |
59 | <constant>SD_EVENT_PENDING</constant>, | |
60 | <constant>SD_EVENT_RUNNING</constant>, | |
61 | <constant>SD_EVENT_EXITING</constant>, | |
62 | <constant>SD_EVENT_FINISHED</constant>, | |
63 | };</funcsynopsisinfo> | |
64 | ||
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65 | <funcprototype> |
66 | <funcdef>int <function>sd_event_prepare</function></funcdef> | |
67 | <paramdef>sd_event *<parameter>event</parameter></paramdef> | |
68 | </funcprototype> | |
69 | ||
70 | <funcprototype> | |
71 | <funcdef>int <function>sd_event_wait</function></funcdef> | |
72 | <paramdef>sd_event *<parameter>event</parameter></paramdef> | |
dc83f27a | 73 | <paramdef>uint64_t <parameter>usec</parameter></paramdef> |
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74 | </funcprototype> |
75 | ||
76 | <funcprototype> | |
77 | <funcdef>int <function>sd_event_dispatch</function></funcdef> | |
78 | <paramdef>sd_event *<parameter>event</parameter></paramdef> | |
79 | </funcprototype> | |
80 | ||
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81 | <funcprototype> |
82 | <funcdef>int <function>sd_event_get_state</function></funcdef> | |
83 | <paramdef>sd_event *<parameter>event</parameter></paramdef> | |
84 | </funcprototype> | |
85 | ||
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86 | <funcprototype> |
87 | <funcdef>int <function>sd_event_get_iteration</function></funcdef> | |
88 | <paramdef>sd_event *<parameter>event</parameter></paramdef> | |
89 | <paramdef>uint64_t *<parameter>ret</parameter></paramdef> | |
90 | </funcprototype> | |
91 | ||
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92 | </funcsynopsis> |
93 | </refsynopsisdiv> | |
94 | ||
95 | <refsect1> | |
96 | <title>Description</title> | |
97 | ||
dc83f27a LP |
98 | <para>The low-level <function>sd_event_prepare()</function>, |
99 | <function>sd_event_wait()</function> and | |
100 | <function>sd_event_dispatch()</function> functions may be used to | |
101 | execute specific phases of an event loop. See | |
102 | <citerefentry><refentrytitle>sd_event_run</refentrytitle><manvolnum>3</manvolnum></citerefentry> | |
103 | and | |
104 | <citerefentry><refentrytitle>sd_event_loop</refentrytitle><manvolnum>3</manvolnum></citerefentry> | |
105 | for higher-level functions that execute individual but complete | |
7f3fdb7f | 106 | iterations of an event loop or run it continuously.</para> |
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107 | |
108 | <para><function>sd_event_prepare()</function> checks for pending | |
42f1ab50 | 109 | events and arms necessary timers. If any events are ready to be |
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110 | processed ("pending"), it returns a positive, non-zero value, and the caller |
111 | should process these events with | |
112 | <function>sd_event_dispatch()</function>.</para> | |
113 | ||
114 | <para><function>sd_event_dispatch()</function> dispatches the | |
115 | highest priority event source that has a pending event. On | |
116 | success, <function>sd_event_dispatch()</function> returns either | |
117 | zero, which indicates that no further event sources may be | |
118 | dispatched and exiting of the event loop was requested via | |
119 | <citerefentry><refentrytitle>sd_event_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry>; | |
120 | or a positive non-zero value, which means that an event source was | |
121 | dispatched and the loop returned to its initial state, and the | |
122 | caller should initiate the next event loop iteration by invoking | |
123 | <function>sd_event_prepare()</function> again.</para> | |
124 | ||
125 | <para>In case <function>sd_event_prepare()</function> returned | |
126 | zero, <function>sd_event_wait()</function> should be called to | |
127 | wait for further events or a timeout. If any events are ready to | |
128 | be processed, it returns a positive, non-zero value, and the | |
129 | events should be dispatched with | |
130 | <function>sd_event_dispatch()</function>. Otherwise, the event | |
131 | loop returned to its initial state and the next event loop | |
132 | iteration should be initiated by invoking | |
133 | <function>sd_event_prepare()</function> again.</para> | |
134 | ||
135 | <para><function>sd_event_get_state()</function> may be used to | |
136 | determine the state the event loop is currently in. It returns one | |
137 | of the states described below.</para> | |
138 | ||
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139 | <para><function>sd_event_get_iteration()</function> may be used to determine the current iteration of the event |
140 | loop. It returns an unsigned 64bit integer containing a counter that increases monotonically with each iteration of | |
141 | the event loop, starting with 0. The counter is increased at the time of the | |
142 | <function>sd_event_prepare()</function> invocation.</para> | |
143 | ||
144 | <para>All five functions take, as the first argument, the event loop object <parameter>event</parameter> that has | |
145 | been created with <function>sd_event_new()</function>. The timeout for <function>sd_event_wait()</function> is | |
146 | specified in <parameter>usec</parameter> in microseconds. <constant>(uint64_t) -1</constant> may be used to | |
147 | specify an infinite timeout.</para> | |
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148 | </refsect1> |
149 | ||
150 | <refsect1> | |
151 | <title>State Machine</title> | |
152 | ||
153 | <para>The event loop knows the following states, that may be | |
154 | queried with <function>sd_event_get_state()</function>.</para> | |
42f1ab50 | 155 | |
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156 | <variablelist> |
157 | <varlistentry> | |
158 | <term><constant>SD_EVENT_INITIAL</constant></term> | |
159 | ||
160 | <listitem><para>The initial state the event loop is in, | |
161 | before each event loop iteration. Use | |
162 | <function>sd_event_prepare()</function> to transition the | |
163 | event loop into the <constant>SD_EVENT_ARMED</constant> or | |
164 | <constant>SD_EVENT_PENDING</constant> states.</para></listitem> | |
165 | </varlistentry> | |
166 | ||
167 | <varlistentry> | |
168 | <term><constant>SD_EVENT_PREPARING</constant></term> | |
169 | ||
170 | <listitem><para>An event source is currently being prepared, | |
7f3fdb7f | 171 | i.e. the preparation handler is currently being executed, as |
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172 | set with |
173 | <citerefentry><refentrytitle>sd_event_set_prepare</refentrytitle><manvolnum>3</manvolnum></citerefentry>. This | |
174 | state is only seen in the event source preparation handler | |
175 | that is invoked from the | |
176 | <function>sd_event_prepare()</function> call and is | |
177 | immediately followed by <constant>SD_EVENT_ARMED</constant> or | |
178 | <constant>SD_EVENT_PENDING</constant>.</para></listitem> | |
179 | </varlistentry> | |
180 | ||
181 | <varlistentry> | |
182 | <term><constant>SD_EVENT_ARMED</constant></term> | |
183 | ||
184 | <listitem><para><function>sd_event_prepare()</function> has | |
185 | been called and no event sources were ready to be | |
186 | dispatched. Use <function>sd_event_wait()</function> to wait | |
187 | for new events, and transition into | |
188 | <constant>SD_EVENT_PENDING</constant> or back into | |
189 | <constant>SD_EVENT_INITIAL</constant>.</para></listitem> | |
190 | </varlistentry> | |
191 | ||
192 | <varlistentry> | |
193 | <term><constant>SD_EVENT_PENDING</constant></term> | |
194 | ||
195 | <listitem><para><function>sd_event_prepare()</function> or | |
196 | <function>sd_event_wait()</function> have been called and | |
197 | there were event sources with events pending. Use | |
198 | <function>sd_event_dispatch()</function> to dispatch the | |
199 | highest priority event source and transition back to | |
200 | <constant>SD_EVENT_INITIAL</constant>, or | |
201 | <constant>SD_EVENT_FINISHED</constant>.</para></listitem> | |
202 | </varlistentry> | |
203 | ||
204 | <varlistentry> | |
205 | <term><constant>SD_EVENT_RUNNING</constant></term> | |
206 | ||
207 | <listitem><para>A regular event source is currently being | |
208 | dispatched. This state is only seen in the event source | |
209 | handler that is invoked from the | |
210 | <function>sd_event_dispatch()</function> call, and is | |
211 | immediately followed by <constant>SD_EVENT_INITIAL</constant> | |
212 | or <constant>SD_EVENT_FINISHED</constant> as soon the event | |
213 | source handler returns. Note that during dispatching of exit | |
214 | event sources the <constant>SD_EVENT_EXITING</constant> state | |
215 | is seen instead.</para></listitem> | |
216 | </varlistentry> | |
217 | ||
218 | <varlistentry> | |
219 | <term><constant>SD_EVENT_EXITING</constant></term> | |
220 | ||
221 | <listitem><para>Similar to | |
222 | <constant>SD_EVENT_RUNNING</constant> but is the state in | |
223 | effect while dispatching exit event sources. It is followed by | |
224 | <constant>SD_EVENT_INITIAL</constant> or | |
225 | <constant>SD_EVENT_FINISHED</constant> as soon as the event | |
226 | handler returns.</para></listitem> | |
227 | </varlistentry> | |
228 | ||
229 | <varlistentry> | |
230 | <term><constant>SD_EVENT_FINISHED</constant></term> | |
231 | ||
232 | <listitem><para>The event loop has exited. All exit event | |
233 | sources have run. If the event loop is in this state it serves | |
234 | no purpose anymore, and should be freed.</para></listitem> | |
235 | </varlistentry> | |
236 | ||
237 | </variablelist> | |
238 | ||
239 | <para>A simplified flow chart of the states and the calls to | |
240 | transition between them is shown below. Note that | |
241 | <constant>SD_EVENT_PREPARING</constant>, | |
242 | <constant>SD_EVENT_RUNNING</constant> and | |
243 | <constant>SD_EVENT_EXITING</constant> are not shown here.</para> | |
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244 | |
245 | <programlisting> | |
dc83f27a LP |
246 | INITIAL -<---<---<---<---<---<---<---<---<---<---<---<---\ |
247 | | | | |
248 | | ^ | |
249 | | | | |
250 | v ret == 0 | | |
251 | sd_event_prepare() >--->--->--->--->- ARMED | | |
252 | | | ^ | |
253 | | ret > 0 | | | |
254 | | | | | |
255 | v v ret == 0 | | |
256 | PENDING <---<---<---<---<---< sd_event_wait() >--->--->--+ | |
257 | | ret > 0 ^ | |
258 | | | | |
259 | | | | |
260 | v | | |
261 | sd_event_dispatch() >--->--->--->--->--->--->--->--->--->--->/ | |
262 | | ret > 0 | |
263 | | ret == 0 | |
264 | | | |
265 | v | |
266 | FINISHED | |
42f1ab50 | 267 | </programlisting> |
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268 | </refsect1> |
269 | ||
270 | <refsect1> | |
271 | <title>Return Value</title> | |
272 | ||
273 | <para>On success, these functions return 0 or a positive integer. | |
274 | On failure, they return a negative errno-style error code. In case | |
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275 | of <function>sd_event_prepare()</function> and |
276 | <function>sd_event_wait()</function>, a positive, non-zero return | |
277 | code indicates that events are ready to be processed and zero | |
278 | indicates that no events are ready. In case of | |
279 | <function>sd_event_dispatch()</function>, a positive, non-zero | |
280 | return code indicates that the event loop returned to its initial | |
281 | state and zero indicates the event loop has | |
282 | exited. <function>sd_event_get_state()</function> returns a | |
283 | positive or zero state on success.</para> | |
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284 | </refsect1> |
285 | ||
286 | <refsect1> | |
287 | <title>Errors</title> | |
288 | ||
289 | <para>Returned errors may indicate the following problems:</para> | |
290 | ||
291 | <variablelist> | |
292 | <varlistentry> | |
293 | <term><constant>-EINVAL</constant></term> | |
294 | ||
a8eaaee7 | 295 | <listitem><para>The <parameter>event</parameter> parameter is |
dc83f27a | 296 | invalid or <constant>NULL</constant>.</para></listitem> |
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297 | </varlistentry> |
298 | ||
299 | <varlistentry> | |
300 | <term><constant>-EBUSY</constant></term> | |
301 | ||
302 | <listitem><para>The event loop object is not in the right | |
303 | state.</para></listitem> | |
304 | </varlistentry> | |
305 | ||
306 | <varlistentry> | |
307 | <term><constant>-ESTALE</constant></term> | |
308 | ||
309 | <listitem><para>The event loop is already terminated.</para></listitem> | |
310 | ||
311 | </varlistentry> | |
312 | ||
313 | <varlistentry> | |
314 | <term><constant>-ECHILD</constant></term> | |
315 | ||
316 | <listitem><para>The event loop has been created in a different process.</para></listitem> | |
317 | ||
318 | </varlistentry> | |
319 | ||
320 | </variablelist> | |
321 | ||
b938cb90 | 322 | <para>Other errors are possible, too.</para> |
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323 | </refsect1> |
324 | ||
dc83f27a | 325 | <xi:include href="libsystemd-pkgconfig.xml" /> |
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326 | |
327 | <refsect1> | |
328 | <title>See Also</title> | |
329 | ||
330 | <para> | |
331 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, | |
332 | <citerefentry><refentrytitle>sd_event_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
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333 | <citerefentry><refentrytitle>sd_event_add_io</refentrytitle><manvolnum>3</manvolnum></citerefentry>, |
334 | <citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
335 | <citerefentry><refentrytitle>sd_event_add_signal</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
336 | <citerefentry><refentrytitle>sd_event_add_defer</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
337 | <citerefentry><refentrytitle>sd_event_add_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
dc83f27a LP |
338 | <citerefentry><refentrytitle>sd_event_add_post</refentrytitle><manvolnum>3</manvolnum></citerefentry>, |
339 | <citerefentry><refentrytitle>sd_event_run</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
340 | <citerefentry><refentrytitle>sd_event_get_fd</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
341 | <citerefentry><refentrytitle>sd_event_source_set_prepare</refentrytitle><manvolnum>3</manvolnum></citerefentry> | |
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342 | </para> |
343 | </refsect1> | |
344 | ||
345 | </refentry> |