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1 | <?xml version='1.0'?> <!--*-nxml-*--> |
2 | <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN" | |
3 | "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> | |
4 | ||
5 | <!-- | |
6 | This file is part of systemd. | |
7 | ||
8 | Copyright 2012 Lennart Poettering | |
9 | ||
10 | systemd is free software; you can redistribute it and/or modify it | |
11 | under the terms of the GNU Lesser General Public License as published by | |
12 | the Free Software Foundation; either version 2.1 of the License, or | |
13 | (at your option) any later version. | |
14 | ||
15 | systemd is distributed in the hope that it will be useful, but | |
16 | WITHOUT ANY WARRANTY; without even the implied warranty of | |
17 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU | |
18 | Lesser General Public License for more details. | |
19 | ||
20 | You should have received a copy of the GNU Lesser General Public License | |
21 | along with systemd; If not, see <http://www.gnu.org/licenses/>. | |
22 | --> | |
23 | ||
24 | <refentry id="sd_journal_get_fd"> | |
25 | ||
26 | <refentryinfo> | |
27 | <title>sd_journal_get_fd</title> | |
28 | <productname>systemd</productname> | |
29 | ||
30 | <authorgroup> | |
31 | <author> | |
32 | <contrib>Developer</contrib> | |
33 | <firstname>Lennart</firstname> | |
34 | <surname>Poettering</surname> | |
35 | <email>lennart@poettering.net</email> | |
36 | </author> | |
37 | </authorgroup> | |
38 | </refentryinfo> | |
39 | ||
40 | <refmeta> | |
41 | <refentrytitle>sd_journal_get_fd</refentrytitle> | |
42 | <manvolnum>3</manvolnum> | |
43 | </refmeta> | |
44 | ||
45 | <refnamediv> | |
46 | <refname>sd_journal_get_fd</refname> | |
ee531d94 | 47 | <refname>sd_journal_get_events</refname> |
39c155ea | 48 | <refname>sd_journal_get_timeout</refname> |
19125c20 LP |
49 | <refname>sd_journal_process</refname> |
50 | <refname>sd_journal_wait</refname> | |
39c155ea | 51 | <refname>sd_journal_reliable_fd</refname> |
95694951 LP |
52 | <refname>SD_JOURNAL_NOP</refname> |
53 | <refname>SD_JOURNAL_APPEND</refname> | |
54 | <refname>SD_JOURNAL_INVALIDATE</refname> | |
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55 | <refpurpose>Journal change notification |
56 | interface</refpurpose> | |
57 | </refnamediv> | |
58 | ||
59 | <refsynopsisdiv> | |
60 | <funcsynopsis> | |
61 | <funcsynopsisinfo>#include <systemd/sd-journal.h></funcsynopsisinfo> | |
62 | ||
63 | <funcprototype> | |
64 | <funcdef>int <function>sd_journal_get_fd</function></funcdef> | |
65 | <paramdef>sd_journal* <parameter>j</parameter></paramdef> | |
66 | </funcprototype> | |
67 | ||
ee531d94 LP |
68 | <funcprototype> |
69 | <funcdef>int <function>sd_journal_get_events</function></funcdef> | |
70 | <paramdef>sd_journal* <parameter>j</parameter></paramdef> | |
71 | </funcprototype> | |
72 | ||
85210bff | 73 | <funcprototype> |
39c155ea | 74 | <funcdef>int <function>sd_journal_get_timeout</function></funcdef> |
85210bff | 75 | <paramdef>sd_journal* <parameter>j</parameter></paramdef> |
39c155ea | 76 | <paramdef>uint64_t* <parameter>timeout_usec</parameter></paramdef> |
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77 | </funcprototype> |
78 | ||
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79 | <funcprototype> |
80 | <funcdef>int <function>sd_journal_process</function></funcdef> | |
81 | <paramdef>sd_journal* <parameter>j</parameter></paramdef> | |
82 | </funcprototype> | |
83 | ||
84 | <funcprototype> | |
85 | <funcdef>int <function>sd_journal_wait</function></funcdef> | |
86 | <paramdef>sd_journal* <parameter>j</parameter></paramdef> | |
87 | <paramdef>uint64_t <parameter>timeout_usec</parameter></paramdef> | |
88 | </funcprototype> | |
89 | ||
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90 | <funcprototype> |
91 | <funcdef>int <function>sd_journal_reliable_fd</function></funcdef> | |
92 | <paramdef>sd_journal* <parameter>j</parameter></paramdef> | |
93 | </funcprototype> | |
94 | ||
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95 | </funcsynopsis> |
96 | </refsynopsisdiv> | |
97 | ||
98 | <refsect1> | |
99 | <title>Description</title> | |
100 | ||
101 | <para><function>sd_journal_get_fd()</function> returns | |
102 | a file descriptor that may be asynchronously polled in | |
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103 | an external event loop and is signaled as soon as the |
104 | journal changes, because new entries or files were | |
105 | added, rotation took place, or files have been | |
106 | deleted, and similar. The file descriptor is suitable | |
107 | for usage in | |
108 | <citerefentry><refentrytitle>poll</refentrytitle><manvolnum>2</manvolnum></citerefentry>. Use | |
109 | <function>sd_journal_get_events()</function> for an | |
110 | events mask to watch for. The call takes one argument: | |
111 | the journal context object. Note that not all file | |
112 | systems are capable of generating the necessary events | |
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113 | for wakeups from this file descriptor for changes to |
114 | be noticed immediately. In particular network files | |
115 | systems do not generate suitable file change events in | |
116 | all cases. Cases like this can be detected with | |
85210bff | 117 | <function>sd_journal_reliable_fd()</function>, |
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118 | below. <function>sd_journal_get_timeout()</function> |
119 | will ensure in these cases that wake-ups happen | |
120 | frequently enough for changes to be noticed, although | |
121 | with a certain latency.</para> | |
85210bff | 122 | |
ee531d94 | 123 | <para><function>sd_journal_get_events()</function> |
dace83cb LP |
124 | will return the <function>poll()</function> mask to |
125 | wait for. This function will return a combination of | |
126 | <literal>POLLIN</literal> and | |
127 | <literal>POLLOUT</literal> and similar to fill into | |
128 | the <literal>.events</literal> field of | |
ee531d94 LP |
129 | <literal>struct pollfd</literal>.</para> |
130 | ||
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131 | <para><function>sd_journal_get_timeout()</function> |
132 | will return a timeout value for usage in <function>poll()</function>. This returns a value in microseconds since the epoch of CLOCK_MONOTONIC for timing out <function>poll()</function> in <literal>timeout_usec</literal>. See | |
133 | <citerefentry><refentrytitle>clock_gettime</refentrytitle><manvolnum>2</manvolnum></citerefentry> | |
134 | for details about | |
135 | <literal>CLOCK_MONOTONIC</literal>. If there's no | |
136 | timeout to wait for this will fill in | |
137 | <literal>(uint64_t) -1</literal> instead. Note that | |
138 | <function>poll()</function> takes a relative timeout | |
139 | in milliseconds rather than an absolute timeout in | |
140 | microseconds. To convert the absolute 'us' timeout into | |
141 | relative 'ms', use code like the following:</para> | |
142 | ||
143 | <programlisting>uint64_t t; | |
144 | int msec; | |
145 | sd_journal_get_timeout(m, &t); | |
146 | if (t == (uint64_t) -1) | |
147 | msec = -1; | |
148 | else { | |
149 | struct timespec ts; | |
150 | uint64_t n; | |
151 | clock_getttime(CLOCK_MONOTONIC, &ts); | |
152 | n = (uint64_t) ts.tv_sec * 1000000 + ts.tv_nsec / 1000; | |
153 | msec = t > n ? (int) ((t - n + 999) / 1000) : 0; | |
154 | }</programlisting> | |
155 | ||
156 | <para>The code above does not do any error checking | |
157 | for brevity's sake. The calculated <literal>msec</literal> | |
158 | integer can be passed directly as | |
159 | <function>poll()</function>'s timeout | |
160 | parameter.</para> | |
91a03172 | 161 | |
dace83cb | 162 | <para>After each <function>poll()</function> wake-up |
91a03172 | 163 | <function>sd_journal_process()</function> needs to be |
ee531d94 | 164 | called to process events. This call will also indicate |
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165 | what kind of change has been detected (see below; note |
166 | that spurious wake-ups are possible).</para> | |
167 | ||
168 | <para>A synchronous alternative for using | |
85210bff | 169 | <function>sd_journal_get_fd()</function>, |
ee531d94 | 170 | <function>sd_journal_get_events()</function>, |
39c155ea | 171 | <function>sd_journal_get_timeout()</function> and |
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172 | <function>sd_journal_process()</function> is |
173 | <function>sd_journal_wait()</function>. It will | |
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174 | synchronously wait until the journal gets changed. The |
175 | maximum time this call sleeps may be controlled with | |
176 | the <parameter>timeout_usec</parameter> | |
177 | parameter. Pass <literal>(uint64_t) -1</literal> to | |
178 | wait indefinitely. Internally this call simply | |
179 | combines <function>sd_journal_get_fd()</function>, | |
ee531d94 | 180 | <function>sd_journal_get_events()</function>, |
39c155ea | 181 | <function>sd_journal_get_timeout()</function>, |
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182 | <function>poll()</function> and |
183 | <function>sd_journal_process()</function> into | |
184 | one.</para> | |
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185 | |
186 | <para><function>sd_journal_reliable_fd()</function> | |
187 | may be used to check whether the wakeup events from | |
188 | the file descriptor returned by | |
189 | <function>sd_journal_get_fd()</function> are known to | |
190 | be immediately triggered. On certain file systems | |
191 | where file change events from the OS are not available | |
192 | (such as NFS) changes need to be polled for | |
193 | repeatedly, and hence are detected only with a certain | |
194 | latency. This call will return a positive value if the | |
195 | journal changes are detected immediately and zero when | |
196 | they need to be polled for and hence might be noticed | |
197 | only with a certain latency. Note that there's usually | |
198 | no need to invoke this function directly as | |
199 | <function>sd_journal_get_timeout()</function> on these | |
200 | file systems will ask for timeouts explicitly | |
201 | anyway.</para> | |
91a03172 LP |
202 | </refsect1> |
203 | ||
204 | <refsect1> | |
205 | <title>Return Value</title> | |
206 | ||
ee531d94 LP |
207 | <para><function>sd_journal_get_fd()</function> returns |
208 | a valid file descriptor on success or a negative | |
209 | errno-style error code.</para> | |
210 | ||
211 | <para><function>sd_journal_get_events()</function> | |
dace83cb LP |
212 | returns a combination of <literal>POLLIN</literal>, |
213 | <literal>POLLOUT</literal> and suchlike on success or | |
214 | a negative errno-style error code.</para> | |
91a03172 | 215 | |
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216 | <para><function>sd_journal_reliable_fd()</function> |
217 | returns a positive integer if the file descriptor | |
218 | returned by <function>sd_journal_get_fd()</function> | |
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219 | will generate wake-ups immediately for all journal |
220 | changes. Returns 0 if there might be a latency | |
221 | involved.</para> | |
85210bff | 222 | |
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223 | <para><function>sd_journal_process()</function> and |
224 | <function>sd_journal_wait()</function> return one of | |
225 | <literal>SD_JOURNAL_NOP</literal>, | |
226 | <literal>SD_JOURNAL_APPEND</literal> or | |
227 | <literal>SD_JOURNAL_INVALIDATE</literal> on success or | |
228 | a negative errno-style error code. If | |
229 | <literal>SD_JOURNAL_NOP</literal> is returned the | |
230 | journal didn't change since the last invocation. If | |
231 | <literal>SD_JOURNAL_APPEND</literal> is returned new | |
232 | entries have been appended to the end of the | |
233 | journal. If <literal>SD_JOURNAL_INVALIDATE</literal> | |
234 | journal files were added or removed (possibly due to | |
235 | rotation). In the latter event live-view UIs should | |
236 | probably refresh their entire display while in the | |
237 | case of <literal>SD_JOURNAL_APPEND</literal> it is | |
238 | sufficient to simply continue reading at the previous | |
239 | end of the journal.</para> | |
240 | </refsect1> | |
241 | ||
242 | <refsect1> | |
243 | <title>Notes</title> | |
244 | ||
245 | <para>The <function>sd_journal_get_fd()</function>, | |
ee531d94 | 246 | <function>sd_journal_get_events()</function>, |
85210bff | 247 | <function>sd_journal_reliable_fd()</function>, |
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248 | <function>sd_journal_process()</function> and |
249 | <function>sd_journal_wait()</function> interfaces are | |
250 | available as shared library, which can be compiled and | |
251 | linked to with the | |
252 | <literal>libsystemd-journal</literal> | |
253 | <citerefentry><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry> | |
254 | file.</para> | |
255 | </refsect1> | |
256 | ||
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257 | <refsect1> |
258 | <title>Examples</title> | |
259 | ||
260 | <para>Iterating through the journal, in a live view tracking all changes:</para> | |
261 | ||
262 | <programlisting>#include <stdio.h> | |
263 | #include <string.h> | |
264 | #include <systemd/sd-journal.h> | |
265 | ||
266 | int main(int argc, char *argv[]) { | |
267 | int r; | |
268 | sd_journal *j; | |
269 | r = sd_journal_open(&j, SD_JOURNAL_LOCAL_ONLY); | |
270 | if (r < 0) { | |
271 | fprintf(stderr, "Failed to open journal: %s\n", strerror(-r)); | |
272 | return 1; | |
273 | } | |
274 | for (;;) { | |
1d98d9a6 | 275 | const void *d; |
85210bff LP |
276 | size_t l; |
277 | r = sd_journal_next(j); | |
278 | if (r < 0) { | |
279 | fprintf(stderr, "Failed to iterate to next entry: %s\n", strerror(-r)); | |
280 | break; | |
281 | } | |
282 | if (r == 0) { | |
283 | /* Reached the end, let's wait for changes, and try again */ | |
284 | r = sd_journal_wait(j, (uint64_t) -1); | |
285 | if (r < 0) { | |
286 | fprintf(stderr, "Failed to wait for changes: %s\n", strerror(-r)); | |
287 | break; | |
288 | } | |
289 | continue; | |
290 | } | |
291 | r = sd_journal_get_data(j, "MESSAGE", &d, &l); | |
292 | if (r < 0) { | |
293 | fprintf(stderr, "Failed to read message field: %s\n", strerror(-r)); | |
294 | continue; | |
295 | } | |
1d98d9a6 | 296 | printf("%.*s\n", (int) l, (const char*) d); |
85210bff LP |
297 | } |
298 | sd_journal_close(j); | |
299 | return 0; | |
300 | }</programlisting> | |
301 | ||
302 | <para>Waiting with <function>poll()</function> (this | |
303 | example lacks all error checking for the sake of | |
304 | simplicity):</para> | |
305 | ||
306 | <programlisting>#include <sys/poll.h> | |
307 | #include <systemd/sd-journal.h> | |
308 | ||
309 | int wait_for_changes(sd_journal *j) { | |
310 | struct pollfd pollfd; | |
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311 | int msec; |
312 | ||
313 | sd_journal_get_timeout(m, &t); | |
314 | if (t == (uint64_t) -1) | |
315 | msec = -1; | |
316 | else { | |
317 | struct timespec ts; | |
318 | uint64_t n; | |
319 | clock_getttime(CLOCK_MONOTONIC, &ts); | |
320 | n = (uint64_t) ts.tv_sec * 1000000 + ts.tv_nsec / 1000; | |
321 | msec = t > n ? (int) ((t - n + 999) / 1000) : 0; | |
322 | } | |
323 | ||
1d98d9a6 | 324 | pollfd.fd = sd_journal_get_fd(j); |
ee531d94 | 325 | pollfd.events = sd_journal_get_events(j); |
39c155ea | 326 | poll(&pollfd, 1, msec); |
85210bff LP |
327 | return sd_journal_process(j); |
328 | } | |
329 | </programlisting> | |
330 | </refsect1> | |
331 | ||
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332 | <refsect1> |
333 | <title>See Also</title> | |
334 | ||
335 | <para> | |
336 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, | |
337 | <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
338 | <citerefentry><refentrytitle>sd_journal_open</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
85210bff | 339 | <citerefentry><refentrytitle>sd_journal_next</refentrytitle><manvolnum>3</manvolnum></citerefentry>, |
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340 | <citerefentry><refentrytitle>poll</refentrytitle><manvolnum>2</manvolnum></citerefentry>, |
341 | <citerefentry><refentrytitle>clock_gettime</refentrytitle><manvolnum>2</manvolnum></citerefentry> | |
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342 | </para> |
343 | </refsect1> | |
344 | ||
345 | </refentry> |