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514094f9 | 1 | <?xml version='1.0'?> |
42f1ab50 | 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 | <!-- | |
572eb058 | 6 | SPDX-License-Identifier: LGPL-2.1+ |
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7 | --> |
8 | ||
dc83f27a | 9 | <refentry id="sd_event_wait" xmlns:xi="http://www.w3.org/2001/XInclude"> |
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10 | |
11 | <refentryinfo> | |
12 | <title>sd_event_wait</title> | |
13 | <productname>systemd</productname> | |
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14 | </refentryinfo> |
15 | ||
16 | <refmeta> | |
17 | <refentrytitle>sd_event_wait</refentrytitle> | |
18 | <manvolnum>3</manvolnum> | |
19 | </refmeta> | |
20 | ||
21 | <refnamediv> | |
22 | <refname>sd_event_wait</refname> | |
23 | <refname>sd_event_prepare</refname> | |
24 | <refname>sd_event_dispatch</refname> | |
dc83f27a | 25 | <refname>sd_event_get_state</refname> |
60a3b1e1 | 26 | <refname>sd_event_get_iteration</refname> |
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27 | <refname>SD_EVENT_INITIAL</refname> |
28 | <refname>SD_EVENT_PREPARING</refname> | |
29 | <refname>SD_EVENT_ARMED</refname> | |
30 | <refname>SD_EVENT_PENDING</refname> | |
31 | <refname>SD_EVENT_RUNNING</refname> | |
32 | <refname>SD_EVENT_EXITING</refname> | |
33 | <refname>SD_EVENT_FINISHED</refname> | |
34 | ||
35 | <refpurpose>Low-level event loop operations</refpurpose> | |
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36 | </refnamediv> |
37 | ||
38 | <refsynopsisdiv> | |
39 | <funcsynopsis> | |
40 | <funcsynopsisinfo>#include <systemd/sd-event.h></funcsynopsisinfo> | |
41 | ||
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42 | <funcsynopsisinfo><token>enum</token> { |
43 | <constant>SD_EVENT_INITIAL</constant>, | |
44 | <constant>SD_EVENT_PREPARING</constant>, | |
45 | <constant>SD_EVENT_ARMED</constant>, | |
46 | <constant>SD_EVENT_PENDING</constant>, | |
47 | <constant>SD_EVENT_RUNNING</constant>, | |
48 | <constant>SD_EVENT_EXITING</constant>, | |
49 | <constant>SD_EVENT_FINISHED</constant>, | |
50 | };</funcsynopsisinfo> | |
51 | ||
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52 | <funcprototype> |
53 | <funcdef>int <function>sd_event_prepare</function></funcdef> | |
54 | <paramdef>sd_event *<parameter>event</parameter></paramdef> | |
55 | </funcprototype> | |
56 | ||
57 | <funcprototype> | |
58 | <funcdef>int <function>sd_event_wait</function></funcdef> | |
59 | <paramdef>sd_event *<parameter>event</parameter></paramdef> | |
dc83f27a | 60 | <paramdef>uint64_t <parameter>usec</parameter></paramdef> |
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61 | </funcprototype> |
62 | ||
63 | <funcprototype> | |
64 | <funcdef>int <function>sd_event_dispatch</function></funcdef> | |
65 | <paramdef>sd_event *<parameter>event</parameter></paramdef> | |
66 | </funcprototype> | |
67 | ||
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68 | <funcprototype> |
69 | <funcdef>int <function>sd_event_get_state</function></funcdef> | |
70 | <paramdef>sd_event *<parameter>event</parameter></paramdef> | |
71 | </funcprototype> | |
72 | ||
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73 | <funcprototype> |
74 | <funcdef>int <function>sd_event_get_iteration</function></funcdef> | |
75 | <paramdef>sd_event *<parameter>event</parameter></paramdef> | |
76 | <paramdef>uint64_t *<parameter>ret</parameter></paramdef> | |
77 | </funcprototype> | |
78 | ||
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79 | </funcsynopsis> |
80 | </refsynopsisdiv> | |
81 | ||
82 | <refsect1> | |
83 | <title>Description</title> | |
84 | ||
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85 | <para>The low-level <function>sd_event_prepare()</function>, |
86 | <function>sd_event_wait()</function> and | |
87 | <function>sd_event_dispatch()</function> functions may be used to | |
88 | execute specific phases of an event loop. See | |
89 | <citerefentry><refentrytitle>sd_event_run</refentrytitle><manvolnum>3</manvolnum></citerefentry> | |
90 | and | |
91 | <citerefentry><refentrytitle>sd_event_loop</refentrytitle><manvolnum>3</manvolnum></citerefentry> | |
92 | for higher-level functions that execute individual but complete | |
7f3fdb7f | 93 | iterations of an event loop or run it continuously.</para> |
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94 | |
95 | <para><function>sd_event_prepare()</function> checks for pending | |
42f1ab50 | 96 | events and arms necessary timers. If any events are ready to be |
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97 | processed ("pending"), it returns a positive, non-zero value, and the caller |
98 | should process these events with | |
99 | <function>sd_event_dispatch()</function>.</para> | |
100 | ||
101 | <para><function>sd_event_dispatch()</function> dispatches the | |
102 | highest priority event source that has a pending event. On | |
103 | success, <function>sd_event_dispatch()</function> returns either | |
104 | zero, which indicates that no further event sources may be | |
105 | dispatched and exiting of the event loop was requested via | |
106 | <citerefentry><refentrytitle>sd_event_exit</refentrytitle><manvolnum>3</manvolnum></citerefentry>; | |
107 | or a positive non-zero value, which means that an event source was | |
108 | dispatched and the loop returned to its initial state, and the | |
109 | caller should initiate the next event loop iteration by invoking | |
110 | <function>sd_event_prepare()</function> again.</para> | |
111 | ||
112 | <para>In case <function>sd_event_prepare()</function> returned | |
113 | zero, <function>sd_event_wait()</function> should be called to | |
114 | wait for further events or a timeout. If any events are ready to | |
115 | be processed, it returns a positive, non-zero value, and the | |
116 | events should be dispatched with | |
117 | <function>sd_event_dispatch()</function>. Otherwise, the event | |
118 | loop returned to its initial state and the next event loop | |
119 | iteration should be initiated by invoking | |
120 | <function>sd_event_prepare()</function> again.</para> | |
121 | ||
122 | <para><function>sd_event_get_state()</function> may be used to | |
123 | determine the state the event loop is currently in. It returns one | |
124 | of the states described below.</para> | |
125 | ||
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126 | <para><function>sd_event_get_iteration()</function> may be used to determine the current iteration of the event |
127 | loop. It returns an unsigned 64bit integer containing a counter that increases monotonically with each iteration of | |
128 | the event loop, starting with 0. The counter is increased at the time of the | |
129 | <function>sd_event_prepare()</function> invocation.</para> | |
130 | ||
131 | <para>All five functions take, as the first argument, the event loop object <parameter>event</parameter> that has | |
132 | been created with <function>sd_event_new()</function>. The timeout for <function>sd_event_wait()</function> is | |
133 | specified in <parameter>usec</parameter> in microseconds. <constant>(uint64_t) -1</constant> may be used to | |
134 | specify an infinite timeout.</para> | |
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135 | </refsect1> |
136 | ||
137 | <refsect1> | |
138 | <title>State Machine</title> | |
139 | ||
140 | <para>The event loop knows the following states, that may be | |
141 | queried with <function>sd_event_get_state()</function>.</para> | |
42f1ab50 | 142 | |
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143 | <variablelist> |
144 | <varlistentry> | |
145 | <term><constant>SD_EVENT_INITIAL</constant></term> | |
146 | ||
147 | <listitem><para>The initial state the event loop is in, | |
148 | before each event loop iteration. Use | |
149 | <function>sd_event_prepare()</function> to transition the | |
150 | event loop into the <constant>SD_EVENT_ARMED</constant> or | |
151 | <constant>SD_EVENT_PENDING</constant> states.</para></listitem> | |
152 | </varlistentry> | |
153 | ||
154 | <varlistentry> | |
155 | <term><constant>SD_EVENT_PREPARING</constant></term> | |
156 | ||
157 | <listitem><para>An event source is currently being prepared, | |
7f3fdb7f | 158 | i.e. the preparation handler is currently being executed, as |
dc83f27a | 159 | set with |
227bcd91 | 160 | <citerefentry><refentrytitle>sd_event_source_set_prepare</refentrytitle><manvolnum>3</manvolnum></citerefentry>. This |
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161 | state is only seen in the event source preparation handler |
162 | that is invoked from the | |
163 | <function>sd_event_prepare()</function> call and is | |
164 | immediately followed by <constant>SD_EVENT_ARMED</constant> or | |
165 | <constant>SD_EVENT_PENDING</constant>.</para></listitem> | |
166 | </varlistentry> | |
167 | ||
168 | <varlistentry> | |
169 | <term><constant>SD_EVENT_ARMED</constant></term> | |
170 | ||
171 | <listitem><para><function>sd_event_prepare()</function> has | |
172 | been called and no event sources were ready to be | |
173 | dispatched. Use <function>sd_event_wait()</function> to wait | |
174 | for new events, and transition into | |
175 | <constant>SD_EVENT_PENDING</constant> or back into | |
176 | <constant>SD_EVENT_INITIAL</constant>.</para></listitem> | |
177 | </varlistentry> | |
178 | ||
179 | <varlistentry> | |
180 | <term><constant>SD_EVENT_PENDING</constant></term> | |
181 | ||
182 | <listitem><para><function>sd_event_prepare()</function> or | |
183 | <function>sd_event_wait()</function> have been called and | |
184 | there were event sources with events pending. Use | |
185 | <function>sd_event_dispatch()</function> to dispatch the | |
186 | highest priority event source and transition back to | |
187 | <constant>SD_EVENT_INITIAL</constant>, or | |
188 | <constant>SD_EVENT_FINISHED</constant>.</para></listitem> | |
189 | </varlistentry> | |
190 | ||
191 | <varlistentry> | |
192 | <term><constant>SD_EVENT_RUNNING</constant></term> | |
193 | ||
194 | <listitem><para>A regular event source is currently being | |
195 | dispatched. This state is only seen in the event source | |
196 | handler that is invoked from the | |
197 | <function>sd_event_dispatch()</function> call, and is | |
198 | immediately followed by <constant>SD_EVENT_INITIAL</constant> | |
199 | or <constant>SD_EVENT_FINISHED</constant> as soon the event | |
200 | source handler returns. Note that during dispatching of exit | |
201 | event sources the <constant>SD_EVENT_EXITING</constant> state | |
202 | is seen instead.</para></listitem> | |
203 | </varlistentry> | |
204 | ||
205 | <varlistentry> | |
206 | <term><constant>SD_EVENT_EXITING</constant></term> | |
207 | ||
208 | <listitem><para>Similar to | |
209 | <constant>SD_EVENT_RUNNING</constant> but is the state in | |
210 | effect while dispatching exit event sources. It is followed by | |
211 | <constant>SD_EVENT_INITIAL</constant> or | |
212 | <constant>SD_EVENT_FINISHED</constant> as soon as the event | |
213 | handler returns.</para></listitem> | |
214 | </varlistentry> | |
215 | ||
216 | <varlistentry> | |
217 | <term><constant>SD_EVENT_FINISHED</constant></term> | |
218 | ||
219 | <listitem><para>The event loop has exited. All exit event | |
220 | sources have run. If the event loop is in this state it serves | |
221 | no purpose anymore, and should be freed.</para></listitem> | |
222 | </varlistentry> | |
223 | ||
224 | </variablelist> | |
225 | ||
226 | <para>A simplified flow chart of the states and the calls to | |
227 | transition between them is shown below. Note that | |
228 | <constant>SD_EVENT_PREPARING</constant>, | |
229 | <constant>SD_EVENT_RUNNING</constant> and | |
230 | <constant>SD_EVENT_EXITING</constant> are not shown here.</para> | |
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231 | |
232 | <programlisting> | |
dc83f27a LP |
233 | INITIAL -<---<---<---<---<---<---<---<---<---<---<---<---\ |
234 | | | | |
235 | | ^ | |
236 | | | | |
237 | v ret == 0 | | |
238 | sd_event_prepare() >--->--->--->--->- ARMED | | |
239 | | | ^ | |
240 | | ret > 0 | | | |
241 | | | | | |
242 | v v ret == 0 | | |
243 | PENDING <---<---<---<---<---< sd_event_wait() >--->--->--+ | |
244 | | ret > 0 ^ | |
245 | | | | |
246 | | | | |
247 | v | | |
248 | sd_event_dispatch() >--->--->--->--->--->--->--->--->--->--->/ | |
249 | | ret > 0 | |
250 | | ret == 0 | |
251 | | | |
252 | v | |
253 | FINISHED | |
42f1ab50 | 254 | </programlisting> |
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255 | </refsect1> |
256 | ||
257 | <refsect1> | |
258 | <title>Return Value</title> | |
259 | ||
260 | <para>On success, these functions return 0 or a positive integer. | |
261 | On failure, they return a negative errno-style error code. In case | |
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262 | of <function>sd_event_prepare()</function> and |
263 | <function>sd_event_wait()</function>, a positive, non-zero return | |
264 | code indicates that events are ready to be processed and zero | |
265 | indicates that no events are ready. In case of | |
266 | <function>sd_event_dispatch()</function>, a positive, non-zero | |
267 | return code indicates that the event loop returned to its initial | |
268 | state and zero indicates the event loop has | |
269 | exited. <function>sd_event_get_state()</function> returns a | |
270 | positive or zero state on success.</para> | |
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271 | </refsect1> |
272 | ||
273 | <refsect1> | |
274 | <title>Errors</title> | |
275 | ||
276 | <para>Returned errors may indicate the following problems:</para> | |
277 | ||
278 | <variablelist> | |
279 | <varlistentry> | |
280 | <term><constant>-EINVAL</constant></term> | |
281 | ||
a8eaaee7 | 282 | <listitem><para>The <parameter>event</parameter> parameter is |
dc83f27a | 283 | invalid or <constant>NULL</constant>.</para></listitem> |
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284 | </varlistentry> |
285 | ||
286 | <varlistentry> | |
287 | <term><constant>-EBUSY</constant></term> | |
288 | ||
289 | <listitem><para>The event loop object is not in the right | |
290 | state.</para></listitem> | |
291 | </varlistentry> | |
292 | ||
293 | <varlistentry> | |
294 | <term><constant>-ESTALE</constant></term> | |
295 | ||
296 | <listitem><para>The event loop is already terminated.</para></listitem> | |
297 | ||
298 | </varlistentry> | |
299 | ||
300 | <varlistentry> | |
301 | <term><constant>-ECHILD</constant></term> | |
302 | ||
303 | <listitem><para>The event loop has been created in a different process.</para></listitem> | |
304 | ||
305 | </varlistentry> | |
306 | ||
307 | </variablelist> | |
308 | ||
b938cb90 | 309 | <para>Other errors are possible, too.</para> |
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310 | </refsect1> |
311 | ||
dc83f27a | 312 | <xi:include href="libsystemd-pkgconfig.xml" /> |
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313 | |
314 | <refsect1> | |
315 | <title>See Also</title> | |
316 | ||
317 | <para> | |
318 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, | |
319 | <citerefentry><refentrytitle>sd_event_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
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320 | <citerefentry><refentrytitle>sd_event_add_io</refentrytitle><manvolnum>3</manvolnum></citerefentry>, |
321 | <citerefentry><refentrytitle>sd_event_add_time</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
322 | <citerefentry><refentrytitle>sd_event_add_signal</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
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323 | <citerefentry><refentrytitle>sd_event_add_child</refentrytitle><manvolnum>3</manvolnum></citerefentry>, |
324 | <citerefentry><refentrytitle>sd_event_add_inotify</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
42f1ab50 | 325 | <citerefentry><refentrytitle>sd_event_add_defer</refentrytitle><manvolnum>3</manvolnum></citerefentry>, |
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326 | <citerefentry><refentrytitle>sd_event_run</refentrytitle><manvolnum>3</manvolnum></citerefentry>, |
327 | <citerefentry><refentrytitle>sd_event_get_fd</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
328 | <citerefentry><refentrytitle>sd_event_source_set_prepare</refentrytitle><manvolnum>3</manvolnum></citerefentry> | |
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329 | </para> |
330 | </refsect1> | |
331 | ||
332 | </refentry> |