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">
6 SPDX-License-Identifier: LGPL-2.1+
9 <refentry id=
"sd_journal_get_fd" xmlns:
xi=
"http://www.w3.org/2001/XInclude">
12 <title>sd_journal_get_fd
</title>
13 <productname>systemd
</productname>
17 <contrib>Developer
</contrib>
18 <firstname>Lennart
</firstname>
19 <surname>Poettering
</surname>
20 <email>lennart@poettering.net
</email>
26 <refentrytitle>sd_journal_get_fd
</refentrytitle>
27 <manvolnum>3</manvolnum>
31 <refname>sd_journal_get_fd
</refname>
32 <refname>sd_journal_get_events
</refname>
33 <refname>sd_journal_get_timeout
</refname>
34 <refname>sd_journal_process
</refname>
35 <refname>sd_journal_wait
</refname>
36 <refname>sd_journal_reliable_fd
</refname>
37 <refname>SD_JOURNAL_NOP
</refname>
38 <refname>SD_JOURNAL_APPEND
</refname>
39 <refname>SD_JOURNAL_INVALIDATE
</refname>
40 <refpurpose>Journal change notification
41 interface
</refpurpose>
46 <funcsynopsisinfo>#include
<systemd/sd-journal.h
></funcsynopsisinfo>
49 <funcdef>int
<function>sd_journal_get_fd
</function></funcdef>
50 <paramdef>sd_journal *
<parameter>j
</parameter></paramdef>
54 <funcdef>int
<function>sd_journal_get_events
</function></funcdef>
55 <paramdef>sd_journal *
<parameter>j
</parameter></paramdef>
59 <funcdef>int
<function>sd_journal_get_timeout
</function></funcdef>
60 <paramdef>sd_journal *
<parameter>j
</parameter></paramdef>
61 <paramdef>uint64_t *
<parameter>timeout_usec
</parameter></paramdef>
65 <funcdef>int
<function>sd_journal_process
</function></funcdef>
66 <paramdef>sd_journal *
<parameter>j
</parameter></paramdef>
70 <funcdef>int
<function>sd_journal_wait
</function></funcdef>
71 <paramdef>sd_journal *
<parameter>j
</parameter></paramdef>
72 <paramdef>uint64_t
<parameter>timeout_usec
</parameter></paramdef>
76 <funcdef>int
<function>sd_journal_reliable_fd
</function></funcdef>
77 <paramdef>sd_journal *
<parameter>j
</parameter></paramdef>
84 <title>Description
</title>
86 <para><function>sd_journal_get_fd()
</function> returns a file
87 descriptor that may be asynchronously polled in an external event
88 loop and is signaled as soon as the journal changes, because new
89 entries or files were added, rotation took place, or files have
90 been deleted, and similar. The file descriptor is suitable for
92 <citerefentry><refentrytitle>poll
</refentrytitle><manvolnum>2</manvolnum></citerefentry>.
93 Use
<function>sd_journal_get_events()
</function> for an events
94 mask to watch for. The call takes one argument: the journal
95 context object. Note that not all file systems are capable of
96 generating the necessary events for wakeups from this file
97 descriptor for changes to be noticed immediately. In particular
98 network files systems do not generate suitable file change events
99 in all cases. Cases like this can be detected with
100 <function>sd_journal_reliable_fd()
</function>, below.
101 <function>sd_journal_get_timeout()
</function> will ensure in these
102 cases that wake-ups happen frequently enough for changes to be
103 noticed, although with a certain latency.
</para>
105 <para><function>sd_journal_get_events()
</function> will return the
106 <function>poll()
</function> mask to wait for. This function will
107 return a combination of
<constant>POLLIN
</constant> and
108 <constant>POLLOUT
</constant> and similar to fill into the
109 <literal>.events
</literal> field of
<varname>struct
110 pollfd
</varname>.
</para>
112 <para><function>sd_journal_get_timeout()
</function> will return a
113 timeout value for usage in
<function>poll()
</function>. This
114 returns a value in microseconds since the epoch of
115 <constant>CLOCK_MONOTONIC
</constant> for timing out
116 <function>poll()
</function> in
<varname>timeout_usec
</varname>.
118 <citerefentry><refentrytitle>clock_gettime
</refentrytitle><manvolnum>2</manvolnum></citerefentry>
119 for details about
<constant>CLOCK_MONOTONIC
</constant>. If there
120 is no timeout to wait for, this will fill in
<constant>(uint64_t)
121 -
1</constant> instead. Note that
<function>poll()
</function> takes
122 a relative timeout in milliseconds rather than an absolute timeout
123 in microseconds. To convert the absolute 'us' timeout into
124 relative 'ms', use code like the following:
</para>
126 <programlisting>uint64_t t;
128 sd_journal_get_timeout(m,
&t);
129 if (t == (uint64_t) -
1)
134 clock_gettime(CLOCK_MONOTONIC,
&ts);
135 n = (uint64_t) ts.tv_sec *
1000000 + ts.tv_nsec /
1000;
136 msec = t
> n ? (int) ((t - n +
999) /
1000) :
0;
139 <para>The code above does not do any error checking for brevity's
140 sake. The calculated
<varname>msec
</varname> integer can be passed
141 directly as
<function>poll()
</function>'s timeout
144 <para>After each
<function>poll()
</function> wake-up
145 <function>sd_journal_process()
</function> needs to be called to
146 process events. This call will also indicate what kind of change
147 has been detected (see below; note that spurious wake-ups are
150 <para>A synchronous alternative for using
151 <function>sd_journal_get_fd()
</function>,
152 <function>sd_journal_get_events()
</function>,
153 <function>sd_journal_get_timeout()
</function> and
154 <function>sd_journal_process()
</function> is
155 <function>sd_journal_wait()
</function>. It will synchronously wait
156 until the journal gets changed. The maximum time this call sleeps
157 may be controlled with the
<parameter>timeout_usec
</parameter>
158 parameter. Pass
<constant>(uint64_t) -
1</constant> to wait
159 indefinitely. Internally this call simply combines
160 <function>sd_journal_get_fd()
</function>,
161 <function>sd_journal_get_events()
</function>,
162 <function>sd_journal_get_timeout()
</function>,
163 <function>poll()
</function> and
164 <function>sd_journal_process()
</function> into one.
</para>
166 <para><function>sd_journal_reliable_fd()
</function> may be used to
167 check whether the wakeup events from the file descriptor returned
168 by
<function>sd_journal_get_fd()
</function> are known to be
169 immediately triggered. On certain file systems where file change
170 events from the OS are not available (such as NFS) changes need to
171 be polled for repeatedly, and hence are detected only with a
172 certain latency. This call will return a positive value if the
173 journal changes are detected immediately and zero when they need
174 to be polled for and hence might be noticed only with a certain
175 latency. Note that there is usually no need to invoke this function
176 directly as
<function>sd_journal_get_timeout()
</function> on these
177 file systems will ask for timeouts explicitly anyway.
</para>
181 <title>Return Value
</title>
183 <para><function>sd_journal_get_fd()
</function> returns a valid
184 file descriptor on success or a negative errno-style error
187 <para><function>sd_journal_get_events()
</function> returns a
188 combination of
<constant>POLLIN
</constant>,
189 <constant>POLLOUT
</constant> and suchlike on success or a negative
190 errno-style error code.
</para>
192 <para><function>sd_journal_reliable_fd()
</function> returns a
193 positive integer if the file descriptor returned by
194 <function>sd_journal_get_fd()
</function> will generate wake-ups
195 immediately for all journal changes. Returns
0 if there might be a
196 latency involved.
</para>
198 <para><function>sd_journal_process()
</function> and
<function>sd_journal_wait()
</function> return a negative
199 errno-style error code, or one of
<constant>SD_JOURNAL_NOP
</constant>,
<constant>SD_JOURNAL_APPEND
</constant> or
200 <constant>SD_JOURNAL_INVALIDATE
</constant> on success:
</para>
203 <listitem><para>If
<constant>SD_JOURNAL_NOP
</constant> is returned, the journal did not change since the last
204 invocation.
</para></listitem>
206 <listitem><para>If
<constant>SD_JOURNAL_APPEND
</constant> is returned, new entries have been appended to the end
207 of the journal. In this case it is sufficient to simply continue reading at the previous end location of the
208 journal, to read the newly added entries.
</para></listitem>
210 <listitem><para>If
<constant>SD_JOURNAL_INVALIDATE
</constant>, journal files were added to or removed from the
211 set of journal files watched (e.g. due to rotation or vacuuming), and thus entries might have appeared or
212 disappeared at arbitrary places in the log stream, possibly before or after the previous end of the log
213 stream. If
<constant>SD_JOURNAL_INVALIDATE
</constant> is returned, live-view UIs that want to reflect on screen
214 the precise state of the log data on disk should probably refresh their entire display (relative to the cursor of
215 the log entry on the top of the screen). Programs only interested in a strictly sequential stream of log data may
216 treat
<constant>SD_JOURNAL_INVALIDATE
</constant> the same way as
<constant>SD_JOURNAL_APPEND
</constant>, thus
217 ignoring any changes to the log view earlier than the old end of the log stream.
</para></listitem>
222 <title>Signal safety
</title>
224 <para>In general,
<function>sd_journal_get_fd()
</function>,
<function>sd_journal_get_events()
</function>, and
225 <function>sd_journal_get_timeout()
</function> are
<emphasis>not
</emphasis> "async signal safe" in the meaning of
227 project='man-pages'
><refentrytitle>signal-safety
</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
228 Nevertheless, only the first call to any of those three functions performs unsafe operations, so subsequent calls
229 <emphasis>are
</emphasis> safe.
</para>
231 <para><function>sd_journal_process()
</function> and
<function>sd_journal_wait()
</function> are not
232 safe.
<function>sd_journal_reliable_fd()
</function> is safe.
</para>
238 <para>The
<function>sd_journal_get_fd()
</function>,
239 <function>sd_journal_get_events()
</function>,
240 <function>sd_journal_reliable_fd()
</function>,
241 <function>sd_journal_process()
</function> and
242 <function>sd_journal_wait()
</function> interfaces are available as
243 a shared library, which can be compiled and linked to with the
244 <constant>libsystemd
</constant> <citerefentry project='die-net'
><refentrytitle>pkg-config
</refentrytitle><manvolnum>1</manvolnum></citerefentry>
249 <title>Examples
</title>
251 <para>Iterating through the journal, in a live view tracking all
254 <programlisting><xi:include href=
"journal-iterate-wait.c" parse=
"text" /></programlisting>
256 <para>Waiting with
<function>poll()
</function> (this
257 example lacks all error checking for the sake of
260 <programlisting><xi:include href=
"journal-iterate-poll.c" parse=
"text" /></programlisting>
264 <title>See Also
</title>
267 <citerefentry><refentrytitle>systemd
</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
268 <citerefentry><refentrytitle>sd-journal
</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
269 <citerefentry><refentrytitle>sd_journal_open
</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
270 <citerefentry><refentrytitle>sd_journal_next
</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
271 <citerefentry><refentrytitle>poll
</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
272 <citerefentry><refentrytitle>clock_gettime
</refentrytitle><manvolnum>2</manvolnum></citerefentry>