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91a03172 | 1 | <?xml version='1.0'?> <!--*-nxml-*--> |
3a54a157 | 2 | <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" |
12b42c76 | 3 | "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd"> |
0307f791 | 4 | <!-- SPDX-License-Identifier: LGPL-2.1+ --> |
91a03172 | 5 | |
929f5263 | 6 | <refentry id="sd_journal_get_fd" xmlns:xi="http://www.w3.org/2001/XInclude"> |
91a03172 | 7 | |
798d3a52 ZJS |
8 | <refentryinfo> |
9 | <title>sd_journal_get_fd</title> | |
10 | <productname>systemd</productname> | |
798d3a52 ZJS |
11 | </refentryinfo> |
12 | ||
13 | <refmeta> | |
14 | <refentrytitle>sd_journal_get_fd</refentrytitle> | |
15 | <manvolnum>3</manvolnum> | |
16 | </refmeta> | |
17 | ||
18 | <refnamediv> | |
19 | <refname>sd_journal_get_fd</refname> | |
20 | <refname>sd_journal_get_events</refname> | |
21 | <refname>sd_journal_get_timeout</refname> | |
22 | <refname>sd_journal_process</refname> | |
23 | <refname>sd_journal_wait</refname> | |
24 | <refname>sd_journal_reliable_fd</refname> | |
25 | <refname>SD_JOURNAL_NOP</refname> | |
26 | <refname>SD_JOURNAL_APPEND</refname> | |
27 | <refname>SD_JOURNAL_INVALIDATE</refname> | |
28 | <refpurpose>Journal change notification | |
29 | interface</refpurpose> | |
30 | </refnamediv> | |
31 | ||
32 | <refsynopsisdiv> | |
33 | <funcsynopsis> | |
34 | <funcsynopsisinfo>#include <systemd/sd-journal.h></funcsynopsisinfo> | |
35 | ||
36 | <funcprototype> | |
37 | <funcdef>int <function>sd_journal_get_fd</function></funcdef> | |
38 | <paramdef>sd_journal *<parameter>j</parameter></paramdef> | |
39 | </funcprototype> | |
40 | ||
41 | <funcprototype> | |
42 | <funcdef>int <function>sd_journal_get_events</function></funcdef> | |
43 | <paramdef>sd_journal *<parameter>j</parameter></paramdef> | |
44 | </funcprototype> | |
45 | ||
46 | <funcprototype> | |
47 | <funcdef>int <function>sd_journal_get_timeout</function></funcdef> | |
48 | <paramdef>sd_journal *<parameter>j</parameter></paramdef> | |
49 | <paramdef>uint64_t *<parameter>timeout_usec</parameter></paramdef> | |
50 | </funcprototype> | |
51 | ||
52 | <funcprototype> | |
53 | <funcdef>int <function>sd_journal_process</function></funcdef> | |
54 | <paramdef>sd_journal *<parameter>j</parameter></paramdef> | |
55 | </funcprototype> | |
56 | ||
57 | <funcprototype> | |
58 | <funcdef>int <function>sd_journal_wait</function></funcdef> | |
59 | <paramdef>sd_journal *<parameter>j</parameter></paramdef> | |
60 | <paramdef>uint64_t <parameter>timeout_usec</parameter></paramdef> | |
61 | </funcprototype> | |
62 | ||
63 | <funcprototype> | |
64 | <funcdef>int <function>sd_journal_reliable_fd</function></funcdef> | |
65 | <paramdef>sd_journal *<parameter>j</parameter></paramdef> | |
66 | </funcprototype> | |
67 | ||
68 | </funcsynopsis> | |
69 | </refsynopsisdiv> | |
70 | ||
71 | <refsect1> | |
72 | <title>Description</title> | |
73 | ||
74 | <para><function>sd_journal_get_fd()</function> returns a file | |
75 | descriptor that may be asynchronously polled in an external event | |
76 | loop and is signaled as soon as the journal changes, because new | |
77 | entries or files were added, rotation took place, or files have | |
78 | been deleted, and similar. The file descriptor is suitable for | |
79 | usage in | |
80 | <citerefentry><refentrytitle>poll</refentrytitle><manvolnum>2</manvolnum></citerefentry>. | |
81 | Use <function>sd_journal_get_events()</function> for an events | |
82 | mask to watch for. The call takes one argument: the journal | |
83 | context object. Note that not all file systems are capable of | |
84 | generating the necessary events for wakeups from this file | |
85 | descriptor for changes to be noticed immediately. In particular | |
86 | network files systems do not generate suitable file change events | |
87 | in all cases. Cases like this can be detected with | |
88 | <function>sd_journal_reliable_fd()</function>, below. | |
89 | <function>sd_journal_get_timeout()</function> will ensure in these | |
90 | cases that wake-ups happen frequently enough for changes to be | |
91 | noticed, although with a certain latency.</para> | |
92 | ||
93 | <para><function>sd_journal_get_events()</function> will return the | |
94 | <function>poll()</function> mask to wait for. This function will | |
95 | return a combination of <constant>POLLIN</constant> and | |
96 | <constant>POLLOUT</constant> and similar to fill into the | |
97 | <literal>.events</literal> field of <varname>struct | |
98 | pollfd</varname>.</para> | |
99 | ||
100 | <para><function>sd_journal_get_timeout()</function> will return a | |
101 | timeout value for usage in <function>poll()</function>. This | |
102 | returns a value in microseconds since the epoch of | |
103 | <constant>CLOCK_MONOTONIC</constant> for timing out | |
104 | <function>poll()</function> in <varname>timeout_usec</varname>. | |
105 | See | |
106 | <citerefentry><refentrytitle>clock_gettime</refentrytitle><manvolnum>2</manvolnum></citerefentry> | |
107 | for details about <constant>CLOCK_MONOTONIC</constant>. If there | |
108 | is no timeout to wait for, this will fill in <constant>(uint64_t) | |
109 | -1</constant> instead. Note that <function>poll()</function> takes | |
110 | a relative timeout in milliseconds rather than an absolute timeout | |
111 | in microseconds. To convert the absolute 'us' timeout into | |
112 | relative 'ms', use code like the following:</para> | |
113 | ||
114 | <programlisting>uint64_t t; | |
39c155ea LP |
115 | int msec; |
116 | sd_journal_get_timeout(m, &t); | |
117 | if (t == (uint64_t) -1) | |
798d3a52 | 118 | msec = -1; |
39c155ea | 119 | else { |
798d3a52 ZJS |
120 | struct timespec ts; |
121 | uint64_t n; | |
8feabc46 | 122 | clock_gettime(CLOCK_MONOTONIC, &ts); |
798d3a52 ZJS |
123 | n = (uint64_t) ts.tv_sec * 1000000 + ts.tv_nsec / 1000; |
124 | msec = t > n ? (int) ((t - n + 999) / 1000) : 0; | |
39c155ea LP |
125 | }</programlisting> |
126 | ||
798d3a52 ZJS |
127 | <para>The code above does not do any error checking for brevity's |
128 | sake. The calculated <varname>msec</varname> integer can be passed | |
129 | directly as <function>poll()</function>'s timeout | |
130 | parameter.</para> | |
131 | ||
132 | <para>After each <function>poll()</function> wake-up | |
133 | <function>sd_journal_process()</function> needs to be called to | |
134 | process events. This call will also indicate what kind of change | |
135 | has been detected (see below; note that spurious wake-ups are | |
136 | possible).</para> | |
137 | ||
138 | <para>A synchronous alternative for using | |
139 | <function>sd_journal_get_fd()</function>, | |
140 | <function>sd_journal_get_events()</function>, | |
141 | <function>sd_journal_get_timeout()</function> and | |
142 | <function>sd_journal_process()</function> is | |
143 | <function>sd_journal_wait()</function>. It will synchronously wait | |
144 | until the journal gets changed. The maximum time this call sleeps | |
145 | may be controlled with the <parameter>timeout_usec</parameter> | |
146 | parameter. Pass <constant>(uint64_t) -1</constant> to wait | |
147 | indefinitely. Internally this call simply combines | |
148 | <function>sd_journal_get_fd()</function>, | |
149 | <function>sd_journal_get_events()</function>, | |
150 | <function>sd_journal_get_timeout()</function>, | |
151 | <function>poll()</function> and | |
152 | <function>sd_journal_process()</function> into one.</para> | |
153 | ||
154 | <para><function>sd_journal_reliable_fd()</function> may be used to | |
155 | check whether the wakeup events from the file descriptor returned | |
156 | by <function>sd_journal_get_fd()</function> are known to be | |
157 | immediately triggered. On certain file systems where file change | |
158 | events from the OS are not available (such as NFS) changes need to | |
159 | be polled for repeatedly, and hence are detected only with a | |
160 | certain latency. This call will return a positive value if the | |
161 | journal changes are detected immediately and zero when they need | |
162 | to be polled for and hence might be noticed only with a certain | |
7ca41557 | 163 | latency. Note that there is usually no need to invoke this function |
798d3a52 ZJS |
164 | directly as <function>sd_journal_get_timeout()</function> on these |
165 | file systems will ask for timeouts explicitly anyway.</para> | |
166 | </refsect1> | |
167 | ||
168 | <refsect1> | |
169 | <title>Return Value</title> | |
170 | ||
171 | <para><function>sd_journal_get_fd()</function> returns a valid | |
172 | file descriptor on success or a negative errno-style error | |
173 | code.</para> | |
174 | ||
175 | <para><function>sd_journal_get_events()</function> returns a | |
176 | combination of <constant>POLLIN</constant>, | |
177 | <constant>POLLOUT</constant> and suchlike on success or a negative | |
178 | errno-style error code.</para> | |
179 | ||
180 | <para><function>sd_journal_reliable_fd()</function> returns a | |
181 | positive integer if the file descriptor returned by | |
182 | <function>sd_journal_get_fd()</function> will generate wake-ups | |
183 | immediately for all journal changes. Returns 0 if there might be a | |
184 | latency involved.</para> | |
185 | ||
aafcd252 LP |
186 | <para><function>sd_journal_process()</function> and <function>sd_journal_wait()</function> return a negative |
187 | errno-style error code, or one of <constant>SD_JOURNAL_NOP</constant>, <constant>SD_JOURNAL_APPEND</constant> or | |
188 | <constant>SD_JOURNAL_INVALIDATE</constant> on success:</para> | |
189 | ||
190 | <itemizedlist> | |
191 | <listitem><para>If <constant>SD_JOURNAL_NOP</constant> is returned, the journal did not change since the last | |
192 | invocation.</para></listitem> | |
193 | ||
194 | <listitem><para>If <constant>SD_JOURNAL_APPEND</constant> is returned, new entries have been appended to the end | |
195 | of the journal. In this case it is sufficient to simply continue reading at the previous end location of the | |
196 | journal, to read the newly added entries.</para></listitem> | |
197 | ||
198 | <listitem><para>If <constant>SD_JOURNAL_INVALIDATE</constant>, journal files were added to or removed from the | |
1b2ad5d9 | 199 | set of journal files watched (e.g. due to rotation or vacuuming), and thus entries might have appeared or |
aafcd252 LP |
200 | disappeared at arbitrary places in the log stream, possibly before or after the previous end of the log |
201 | stream. If <constant>SD_JOURNAL_INVALIDATE</constant> is returned, live-view UIs that want to reflect on screen | |
202 | the precise state of the log data on disk should probably refresh their entire display (relative to the cursor of | |
1b2ad5d9 | 203 | the log entry on the top of the screen). Programs only interested in a strictly sequential stream of log data may |
aafcd252 LP |
204 | treat <constant>SD_JOURNAL_INVALIDATE</constant> the same way as <constant>SD_JOURNAL_APPEND</constant>, thus |
205 | ignoring any changes to the log view earlier than the old end of the log stream.</para></listitem> | |
206 | </itemizedlist> | |
798d3a52 ZJS |
207 | </refsect1> |
208 | ||
89f552c0 ZJS |
209 | <refsect1> |
210 | <title>Signal safety</title> | |
211 | ||
2695b872 LP |
212 | <para>In general, <function>sd_journal_get_fd()</function>, <function>sd_journal_get_events()</function>, and |
213 | <function>sd_journal_get_timeout()</function> are <emphasis>not</emphasis> "async signal safe" in the meaning of | |
214 | <citerefentry | |
215 | project='man-pages'><refentrytitle>signal-safety</refentrytitle><manvolnum>7</manvolnum></citerefentry>. | |
216 | Nevertheless, only the first call to any of those three functions performs unsafe operations, so subsequent calls | |
217 | <emphasis>are</emphasis> safe.</para> | |
89f552c0 ZJS |
218 | |
219 | <para><function>sd_journal_process()</function> and <function>sd_journal_wait()</function> are not | |
220 | safe. <function>sd_journal_reliable_fd()</function> is safe.</para> | |
221 | </refsect1> | |
222 | ||
798d3a52 ZJS |
223 | <refsect1> |
224 | <title>Notes</title> | |
225 | ||
64a7ef8b LP |
226 | <xi:include href="threads-aware.xml" xpointer="strict"/> |
227 | ||
228 | <xi:include href="libsystemd-pkgconfig.xml" xpointer="pkgconfig-text"/> | |
798d3a52 ZJS |
229 | </refsect1> |
230 | ||
231 | <refsect1> | |
232 | <title>Examples</title> | |
233 | ||
234 | <para>Iterating through the journal, in a live view tracking all | |
235 | changes:</para> | |
236 | ||
929f5263 | 237 | <programlisting><xi:include href="journal-iterate-wait.c" parse="text" /></programlisting> |
85210bff | 238 | |
798d3a52 ZJS |
239 | <para>Waiting with <function>poll()</function> (this |
240 | example lacks all error checking for the sake of | |
241 | simplicity):</para> | |
85210bff | 242 | |
929f5263 | 243 | <programlisting><xi:include href="journal-iterate-poll.c" parse="text" /></programlisting> |
798d3a52 ZJS |
244 | </refsect1> |
245 | ||
246 | <refsect1> | |
247 | <title>See Also</title> | |
248 | ||
249 | <para> | |
250 | <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>, | |
251 | <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
252 | <citerefentry><refentrytitle>sd_journal_open</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
253 | <citerefentry><refentrytitle>sd_journal_next</refentrytitle><manvolnum>3</manvolnum></citerefentry>, | |
254 | <citerefentry><refentrytitle>poll</refentrytitle><manvolnum>2</manvolnum></citerefentry>, | |
255 | <citerefentry><refentrytitle>clock_gettime</refentrytitle><manvolnum>2</manvolnum></citerefentry> | |
256 | </para> | |
257 | </refsect1> | |
91a03172 LP |
258 | |
259 | </refentry> |