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+
8 This file is part of systemd.
10 Copyright 2012 Lennart Poettering
13 <refentry id=
"sd_journal_get_fd" xmlns:
xi=
"http://www.w3.org/2001/XInclude">
16 <title>sd_journal_get_fd
</title>
17 <productname>systemd
</productname>
21 <contrib>Developer
</contrib>
22 <firstname>Lennart
</firstname>
23 <surname>Poettering
</surname>
24 <email>lennart@poettering.net
</email>
30 <refentrytitle>sd_journal_get_fd
</refentrytitle>
31 <manvolnum>3</manvolnum>
35 <refname>sd_journal_get_fd
</refname>
36 <refname>sd_journal_get_events
</refname>
37 <refname>sd_journal_get_timeout
</refname>
38 <refname>sd_journal_process
</refname>
39 <refname>sd_journal_wait
</refname>
40 <refname>sd_journal_reliable_fd
</refname>
41 <refname>SD_JOURNAL_NOP
</refname>
42 <refname>SD_JOURNAL_APPEND
</refname>
43 <refname>SD_JOURNAL_INVALIDATE
</refname>
44 <refpurpose>Journal change notification
45 interface
</refpurpose>
50 <funcsynopsisinfo>#include
<systemd/sd-journal.h
></funcsynopsisinfo>
53 <funcdef>int
<function>sd_journal_get_fd
</function></funcdef>
54 <paramdef>sd_journal *
<parameter>j
</parameter></paramdef>
58 <funcdef>int
<function>sd_journal_get_events
</function></funcdef>
59 <paramdef>sd_journal *
<parameter>j
</parameter></paramdef>
63 <funcdef>int
<function>sd_journal_get_timeout
</function></funcdef>
64 <paramdef>sd_journal *
<parameter>j
</parameter></paramdef>
65 <paramdef>uint64_t *
<parameter>timeout_usec
</parameter></paramdef>
69 <funcdef>int
<function>sd_journal_process
</function></funcdef>
70 <paramdef>sd_journal *
<parameter>j
</parameter></paramdef>
74 <funcdef>int
<function>sd_journal_wait
</function></funcdef>
75 <paramdef>sd_journal *
<parameter>j
</parameter></paramdef>
76 <paramdef>uint64_t
<parameter>timeout_usec
</parameter></paramdef>
80 <funcdef>int
<function>sd_journal_reliable_fd
</function></funcdef>
81 <paramdef>sd_journal *
<parameter>j
</parameter></paramdef>
88 <title>Description
</title>
90 <para><function>sd_journal_get_fd()
</function> returns a file
91 descriptor that may be asynchronously polled in an external event
92 loop and is signaled as soon as the journal changes, because new
93 entries or files were added, rotation took place, or files have
94 been deleted, and similar. The file descriptor is suitable for
96 <citerefentry><refentrytitle>poll
</refentrytitle><manvolnum>2</manvolnum></citerefentry>.
97 Use
<function>sd_journal_get_events()
</function> for an events
98 mask to watch for. The call takes one argument: the journal
99 context object. Note that not all file systems are capable of
100 generating the necessary events for wakeups from this file
101 descriptor for changes to be noticed immediately. In particular
102 network files systems do not generate suitable file change events
103 in all cases. Cases like this can be detected with
104 <function>sd_journal_reliable_fd()
</function>, below.
105 <function>sd_journal_get_timeout()
</function> will ensure in these
106 cases that wake-ups happen frequently enough for changes to be
107 noticed, although with a certain latency.
</para>
109 <para><function>sd_journal_get_events()
</function> will return the
110 <function>poll()
</function> mask to wait for. This function will
111 return a combination of
<constant>POLLIN
</constant> and
112 <constant>POLLOUT
</constant> and similar to fill into the
113 <literal>.events
</literal> field of
<varname>struct
114 pollfd
</varname>.
</para>
116 <para><function>sd_journal_get_timeout()
</function> will return a
117 timeout value for usage in
<function>poll()
</function>. This
118 returns a value in microseconds since the epoch of
119 <constant>CLOCK_MONOTONIC
</constant> for timing out
120 <function>poll()
</function> in
<varname>timeout_usec
</varname>.
122 <citerefentry><refentrytitle>clock_gettime
</refentrytitle><manvolnum>2</manvolnum></citerefentry>
123 for details about
<constant>CLOCK_MONOTONIC
</constant>. If there
124 is no timeout to wait for, this will fill in
<constant>(uint64_t)
125 -
1</constant> instead. Note that
<function>poll()
</function> takes
126 a relative timeout in milliseconds rather than an absolute timeout
127 in microseconds. To convert the absolute 'us' timeout into
128 relative 'ms', use code like the following:
</para>
130 <programlisting>uint64_t t;
132 sd_journal_get_timeout(m,
&t);
133 if (t == (uint64_t) -
1)
138 clock_gettime(CLOCK_MONOTONIC,
&ts);
139 n = (uint64_t) ts.tv_sec *
1000000 + ts.tv_nsec /
1000;
140 msec = t
> n ? (int) ((t - n +
999) /
1000) :
0;
143 <para>The code above does not do any error checking for brevity's
144 sake. The calculated
<varname>msec
</varname> integer can be passed
145 directly as
<function>poll()
</function>'s timeout
148 <para>After each
<function>poll()
</function> wake-up
149 <function>sd_journal_process()
</function> needs to be called to
150 process events. This call will also indicate what kind of change
151 has been detected (see below; note that spurious wake-ups are
154 <para>A synchronous alternative for using
155 <function>sd_journal_get_fd()
</function>,
156 <function>sd_journal_get_events()
</function>,
157 <function>sd_journal_get_timeout()
</function> and
158 <function>sd_journal_process()
</function> is
159 <function>sd_journal_wait()
</function>. It will synchronously wait
160 until the journal gets changed. The maximum time this call sleeps
161 may be controlled with the
<parameter>timeout_usec
</parameter>
162 parameter. Pass
<constant>(uint64_t) -
1</constant> to wait
163 indefinitely. Internally this call simply combines
164 <function>sd_journal_get_fd()
</function>,
165 <function>sd_journal_get_events()
</function>,
166 <function>sd_journal_get_timeout()
</function>,
167 <function>poll()
</function> and
168 <function>sd_journal_process()
</function> into one.
</para>
170 <para><function>sd_journal_reliable_fd()
</function> may be used to
171 check whether the wakeup events from the file descriptor returned
172 by
<function>sd_journal_get_fd()
</function> are known to be
173 immediately triggered. On certain file systems where file change
174 events from the OS are not available (such as NFS) changes need to
175 be polled for repeatedly, and hence are detected only with a
176 certain latency. This call will return a positive value if the
177 journal changes are detected immediately and zero when they need
178 to be polled for and hence might be noticed only with a certain
179 latency. Note that there is usually no need to invoke this function
180 directly as
<function>sd_journal_get_timeout()
</function> on these
181 file systems will ask for timeouts explicitly anyway.
</para>
185 <title>Return Value
</title>
187 <para><function>sd_journal_get_fd()
</function> returns a valid
188 file descriptor on success or a negative errno-style error
191 <para><function>sd_journal_get_events()
</function> returns a
192 combination of
<constant>POLLIN
</constant>,
193 <constant>POLLOUT
</constant> and suchlike on success or a negative
194 errno-style error code.
</para>
196 <para><function>sd_journal_reliable_fd()
</function> returns a
197 positive integer if the file descriptor returned by
198 <function>sd_journal_get_fd()
</function> will generate wake-ups
199 immediately for all journal changes. Returns
0 if there might be a
200 latency involved.
</para>
202 <para><function>sd_journal_process()
</function> and
<function>sd_journal_wait()
</function> return a negative
203 errno-style error code, or one of
<constant>SD_JOURNAL_NOP
</constant>,
<constant>SD_JOURNAL_APPEND
</constant> or
204 <constant>SD_JOURNAL_INVALIDATE
</constant> on success:
</para>
207 <listitem><para>If
<constant>SD_JOURNAL_NOP
</constant> is returned, the journal did not change since the last
208 invocation.
</para></listitem>
210 <listitem><para>If
<constant>SD_JOURNAL_APPEND
</constant> is returned, new entries have been appended to the end
211 of the journal. In this case it is sufficient to simply continue reading at the previous end location of the
212 journal, to read the newly added entries.
</para></listitem>
214 <listitem><para>If
<constant>SD_JOURNAL_INVALIDATE
</constant>, journal files were added to or removed from the
215 set of of journal files watched (e.g. due to rotation or vacuuming), and thus entries might have appeared or
216 disappeared at arbitrary places in the log stream, possibly before or after the previous end of the log
217 stream. If
<constant>SD_JOURNAL_INVALIDATE
</constant> is returned, live-view UIs that want to reflect on screen
218 the precise state of the log data on disk should probably refresh their entire display (relative to the cursor of
219 the log entry on the top of the screen). Programs only interested in a strictly sequencial stream of log data may
220 treat
<constant>SD_JOURNAL_INVALIDATE
</constant> the same way as
<constant>SD_JOURNAL_APPEND
</constant>, thus
221 ignoring any changes to the log view earlier than the old end of the log stream.
</para></listitem>
226 <title>Signal safety
</title>
228 <para>In general,
<function>sd_journal_get_fd()
</function>,
<function>sd_journal_get_events()
</function>, and
229 <function>sd_journal_get_timeout()
</function> are
<emphasis>not
</emphasis> "async signal safe" in the meaning of
231 project='man-pages'
><refentrytitle>signal-safety
</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
232 Nevertheless, only the first call to any of those three functions performs unsafe operations, so subsequent calls
233 <emphasis>are
</emphasis> safe.
</para>
235 <para><function>sd_journal_process()
</function> and
<function>sd_journal_wait()
</function> are not
236 safe.
<function>sd_journal_reliable_fd()
</function> is safe.
</para>
242 <para>The
<function>sd_journal_get_fd()
</function>,
243 <function>sd_journal_get_events()
</function>,
244 <function>sd_journal_reliable_fd()
</function>,
245 <function>sd_journal_process()
</function> and
246 <function>sd_journal_wait()
</function> interfaces are available as
247 a shared library, which can be compiled and linked to with the
248 <constant>libsystemd
</constant> <citerefentry project='die-net'
><refentrytitle>pkg-config
</refentrytitle><manvolnum>1</manvolnum></citerefentry>
253 <title>Examples
</title>
255 <para>Iterating through the journal, in a live view tracking all
258 <programlisting><xi:include href=
"journal-iterate-wait.c" parse=
"text" /></programlisting>
260 <para>Waiting with
<function>poll()
</function> (this
261 example lacks all error checking for the sake of
264 <programlisting><xi:include href=
"journal-iterate-poll.c" parse=
"text" /></programlisting>
268 <title>See Also
</title>
271 <citerefentry><refentrytitle>systemd
</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
272 <citerefentry><refentrytitle>sd-journal
</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
273 <citerefentry><refentrytitle>sd_journal_open
</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
274 <citerefentry><refentrytitle>sd_journal_next
</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
275 <citerefentry><refentrytitle>poll
</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
276 <citerefentry><refentrytitle>clock_gettime
</refentrytitle><manvolnum>2</manvolnum></citerefentry>