summaryrefslogtreecommitdiff
path: root/man/sd_journal_get_fd.xml
blob: 16aeea2dc777801cd10b4d228df3c357431ac488 (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
347
348
349
<?xml version='1.0'?> <!--*-nxml-*-->
<!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 2012 Lennart Poettering

  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_journal_get_fd">

        <refentryinfo>
                <title>sd_journal_get_fd</title>
                <productname>systemd</productname>

                <authorgroup>
                        <author>
                                <contrib>Developer</contrib>
                                <firstname>Lennart</firstname>
                                <surname>Poettering</surname>
                                <email>lennart@poettering.net</email>
                        </author>
                </authorgroup>
        </refentryinfo>

        <refmeta>
                <refentrytitle>sd_journal_get_fd</refentrytitle>
                <manvolnum>3</manvolnum>
        </refmeta>

        <refnamediv>
                <refname>sd_journal_get_fd</refname>
                <refname>sd_journal_get_events</refname>
                <refname>sd_journal_get_timeout</refname>
                <refname>sd_journal_process</refname>
                <refname>sd_journal_wait</refname>
                <refname>sd_journal_reliable_fd</refname>
                <refname>SD_JOURNAL_NOP</refname>
                <refname>SD_JOURNAL_APPEND</refname>
                <refname>SD_JOURNAL_INVALIDATE</refname>
                <refpurpose>Journal change notification
                interface</refpurpose>
        </refnamediv>

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

                        <funcprototype>
                                <funcdef>int <function>sd_journal_get_fd</function></funcdef>
                                <paramdef>sd_journal* <parameter>j</parameter></paramdef>
                        </funcprototype>

                        <funcprototype>
                                <funcdef>int <function>sd_journal_get_events</function></funcdef>
                                <paramdef>sd_journal* <parameter>j</parameter></paramdef>
                        </funcprototype>

                        <funcprototype>
                                <funcdef>int <function>sd_journal_get_timeout</function></funcdef>
                                <paramdef>sd_journal* <parameter>j</parameter></paramdef>
                                <paramdef>uint64_t* <parameter>timeout_usec</parameter></paramdef>
                        </funcprototype>

                        <funcprototype>
                                <funcdef>int <function>sd_journal_process</function></funcdef>
                                <paramdef>sd_journal* <parameter>j</parameter></paramdef>
                        </funcprototype>

                        <funcprototype>
                                <funcdef>int <function>sd_journal_wait</function></funcdef>
                                <paramdef>sd_journal* <parameter>j</parameter></paramdef>
                                <paramdef>uint64_t <parameter>timeout_usec</parameter></paramdef>
                        </funcprototype>

                        <funcprototype>
                                <funcdef>int <function>sd_journal_reliable_fd</function></funcdef>
                                <paramdef>sd_journal* <parameter>j</parameter></paramdef>
                        </funcprototype>

                </funcsynopsis>
        </refsynopsisdiv>

        <refsect1>
                <title>Description</title>

                <para><function>sd_journal_get_fd()</function> returns
                a file descriptor that may be asynchronously polled in
                an external event loop and is signaled as soon as the
                journal changes, because new entries or files were
                added, rotation took place, or files have been
                deleted, and similar. The file descriptor is suitable
                for usage in
                <citerefentry><refentrytitle>poll</refentrytitle><manvolnum>2</manvolnum></citerefentry>. Use
                <function>sd_journal_get_events()</function> for an
                events mask to watch for. The call takes one argument:
                the journal context object. Note that not all file
                systems are capable of generating the necessary events
                for wakeups from this file descriptor for changes to
                be noticed immediately. In particular network files
                systems do not generate suitable file change events in
                all cases. Cases like this can be detected with
                <function>sd_journal_reliable_fd()</function>,
                below. <function>sd_journal_get_timeout()</function>
                will ensure in these cases that wake-ups happen
                frequently enough for changes to be noticed, although
                with a certain latency.</para>

                <para><function>sd_journal_get_events()</function>
                will return the <function>poll()</function> mask to
                wait for. This function will return a combination of
                <constant>POLLIN</constant> and
                <constant>POLLOUT</constant> and similar to fill into
                the <literal>.events</literal> field of
                <varname>struct pollfd</varname>.</para>

                <para><function>sd_journal_get_timeout()</function>
                will return a timeout value for usage in
                <function>poll()</function>. This returns a value in
                microseconds since the epoch of
                <constant>CLOCK_MONOTONIC</constant> for timing out
                <function>poll()</function> in
                <varname>timeout_usec</varname>. See
                <citerefentry><refentrytitle>clock_gettime</refentrytitle><manvolnum>2</manvolnum></citerefentry>
                for details about
                <constant>CLOCK_MONOTONIC</constant>. If there is no
                timeout to wait for, this will fill in
                <constant>(uint64_t) -1</constant> instead. Note that
                <function>poll()</function> takes a relative timeout
                in milliseconds rather than an absolute timeout in
                microseconds. To convert the absolute 'us' timeout
                into relative 'ms', use code like the
                following:</para>

                <programlisting>uint64_t t;
int msec;
sd_journal_get_timeout(m, &amp;t);
if (t == (uint64_t) -1)
        msec = -1;
else {
        struct timespec ts;
        uint64_t n;
        clock_getttime(CLOCK_MONOTONIC, &amp;ts);
        n = (uint64_t) ts.tv_sec * 1000000 + ts.tv_nsec / 1000;
        msec = t > n ? (int) ((t - n + 999) / 1000) : 0;
}</programlisting>

                <para>The code above does not do any error checking
                for brevity's sake. The calculated <varname>msec</varname>
                integer can be passed directly as
                <function>poll()</function>'s timeout
                parameter.</para>

                <para>After each <function>poll()</function> wake-up
                <function>sd_journal_process()</function> needs to be
                called to process events. This call will also indicate
                what kind of change has been detected (see below; note
                that spurious wake-ups are possible).</para>

                <para>A synchronous alternative for using
                <function>sd_journal_get_fd()</function>,
                <function>sd_journal_get_events()</function>,
                <function>sd_journal_get_timeout()</function> and
                <function>sd_journal_process()</function> is
                <function>sd_journal_wait()</function>. It will
                synchronously wait until the journal gets changed. The
                maximum time this call sleeps may be controlled with
                the <parameter>timeout_usec</parameter>
                parameter. Pass <constant>(uint64_t) -1</constant> to
                wait indefinitely. Internally this call simply
                combines <function>sd_journal_get_fd()</function>,
                <function>sd_journal_get_events()</function>,
                <function>sd_journal_get_timeout()</function>,
                <function>poll()</function> and
                <function>sd_journal_process()</function> into
                one.</para>

                <para><function>sd_journal_reliable_fd()</function>
                may be used to check whether the wakeup events from
                the file descriptor returned by
                <function>sd_journal_get_fd()</function> are known to
                be immediately triggered. On certain file systems
                where file change events from the OS are not available
                (such as NFS) changes need to be polled for
                repeatedly, and hence are detected only with a certain
                latency. This call will return a positive value if the
                journal changes are detected immediately and zero when
                they need to be polled for and hence might be noticed
                only with a certain latency. Note that there's usually
                no need to invoke this function directly as
                <function>sd_journal_get_timeout()</function> on these
                file systems will ask for timeouts explicitly
                anyway.</para>
        </refsect1>

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

                <para><function>sd_journal_get_fd()</function> returns
                a valid file descriptor on success or a negative
                errno-style error code.</para>

                <para><function>sd_journal_get_events()</function>
                returns a combination of <constant>POLLIN</constant>,
                <constant>POLLOUT</constant> and suchlike on success or
                a negative errno-style error code.</para>

                <para><function>sd_journal_reliable_fd()</function>
                returns a positive integer if the file descriptor
                returned by <function>sd_journal_get_fd()</function>
                will generate wake-ups immediately for all journal
                changes. Returns 0 if there might be a latency
                involved.</para>

                <para><function>sd_journal_process()</function> and
                <function>sd_journal_wait()</function> return one of
                <constant>SD_JOURNAL_NOP</constant>,
                <constant>SD_JOURNAL_APPEND</constant> or
                <constant>SD_JOURNAL_INVALIDATE</constant> on success or
                a negative errno-style error code. If
                <constant>SD_JOURNAL_NOP</constant> is returned, the
                journal did not change since the last invocation. If
                <constant>SD_JOURNAL_APPEND</constant> is returned, new
                entries have been appended to the end of the
                journal. If <constant>SD_JOURNAL_INVALIDATE</constant>,
                journal files were added or removed (possibly due to
                rotation). In the latter event, live-view UIs should
                probably refresh their entire display, while in the
                case of <constant>SD_JOURNAL_APPEND</constant>, it is
                sufficient to simply continue reading at the previous
                end of the journal.</para>
        </refsect1>

        <refsect1>
                <title>Notes</title>

                <para>The <function>sd_journal_get_fd()</function>,
                <function>sd_journal_get_events()</function>,
                <function>sd_journal_reliable_fd()</function>,
                <function>sd_journal_process()</function> and
                <function>sd_journal_wait()</function> interfaces are
                available as a shared library, which can be compiled and
                linked to with the
                <constant>libsystemd</constant> <citerefentry><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
                file.</para>
        </refsect1>

        <refsect1>
                <title>Examples</title>

                <para>Iterating through the journal, in a live view tracking all changes:</para>

                <programlisting>#include &lt;stdio.h&gt;
#include &lt;string.h&gt;
#include &lt;systemd/sd-journal.h&gt;

int main(int argc, char *argv[]) {
        int r;
        sd_journal *j;
        r = sd_journal_open(&amp;j, SD_JOURNAL_LOCAL_ONLY);
        if (r &lt; 0) {
                fprintf(stderr, "Failed to open journal: %s\n", strerror(-r));
                return 1;
        }
        for (;;)  {
                const void *d;
                size_t l;
                r = sd_journal_next(j);
                if (r &lt; 0) {
                        fprintf(stderr, "Failed to iterate to next entry: %s\n", strerror(-r));
                        break;
                }
                if (r == 0) {
                        /* Reached the end, let's wait for changes, and try again */
                        r = sd_journal_wait(j, (uint64_t) -1);
                        if (r &lt; 0) {
                                fprintf(stderr, "Failed to wait for changes: %s\n", strerror(-r));
                                break;
                        }
                        continue;
                }
                r = sd_journal_get_data(j, "MESSAGE", &amp;d, &amp;l);
                if (r &lt; 0) {
                        fprintf(stderr, "Failed to read message field: %s\n", strerror(-r));
                        continue;
                }
                printf("%.*s\n", (int) l, (const char*) d);
        }
        sd_journal_close(j);
        return 0;
}</programlisting>

                <para>Waiting with <function>poll()</function> (this
                example lacks all error checking for the sake of
                simplicity):</para>

                <programlisting>#include &lt;sys/poll.h&gt;
#include &lt;systemd/sd-journal.h&gt;

int wait_for_changes(sd_journal *j) {
        struct pollfd pollfd;
        int msec;

        sd_journal_get_timeout(m, &amp;t);
        if (t == (uint64_t) -1)
                msec = -1;
        else {
                struct timespec ts;
                uint64_t n;
                clock_getttime(CLOCK_MONOTONIC, &amp;ts);
                n = (uint64_t) ts.tv_sec * 1000000 + ts.tv_nsec / 1000;
                msec = t > n ? (int) ((t - n + 999) / 1000) : 0;
        }

        pollfd.fd = sd_journal_get_fd(j);
        pollfd.events = sd_journal_get_events(j);
        poll(&amp;pollfd, 1, msec);
        return sd_journal_process(j);
}</programlisting>
        </refsect1>

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

                <para>
                        <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>sd_journal_open</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>sd_journal_next</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>poll</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>clock_gettime</refentrytitle><manvolnum>2</manvolnum></citerefentry>
                </para>
        </refsect1>

</refentry>