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aafcd2523b
This rewords the section, explicitly distuingishing the cases of clients that only want a continious log stream (which can simply treat SD_JOURNAL_INVALIDATE the same way as SD_JOURNAL_APPEND) and those which want to represent on screen the full state of the log data on disk. This is an alternative to a part of PR #9060, but keeps an explanation of the destinction of handling depending on the type of client. Fixes: #8963
281 lines
12 KiB
XML
281 lines
12 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!--
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SPDX-License-Identifier: LGPL-2.1+
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This file is part of systemd.
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Copyright 2012 Lennart Poettering
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-->
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<refentry id="sd_journal_get_fd" xmlns:xi="http://www.w3.org/2001/XInclude">
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<refentryinfo>
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<title>sd_journal_get_fd</title>
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<productname>systemd</productname>
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<authorgroup>
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<author>
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<contrib>Developer</contrib>
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<firstname>Lennart</firstname>
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<surname>Poettering</surname>
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<email>lennart@poettering.net</email>
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</author>
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</authorgroup>
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</refentryinfo>
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<refmeta>
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<refentrytitle>sd_journal_get_fd</refentrytitle>
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<manvolnum>3</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>sd_journal_get_fd</refname>
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<refname>sd_journal_get_events</refname>
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<refname>sd_journal_get_timeout</refname>
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<refname>sd_journal_process</refname>
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<refname>sd_journal_wait</refname>
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<refname>sd_journal_reliable_fd</refname>
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<refname>SD_JOURNAL_NOP</refname>
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<refname>SD_JOURNAL_APPEND</refname>
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<refname>SD_JOURNAL_INVALIDATE</refname>
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<refpurpose>Journal change notification
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interface</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<funcsynopsis>
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<funcsynopsisinfo>#include <systemd/sd-journal.h></funcsynopsisinfo>
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<funcprototype>
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<funcdef>int <function>sd_journal_get_fd</function></funcdef>
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<paramdef>sd_journal *<parameter>j</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_journal_get_events</function></funcdef>
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<paramdef>sd_journal *<parameter>j</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_journal_get_timeout</function></funcdef>
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<paramdef>sd_journal *<parameter>j</parameter></paramdef>
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<paramdef>uint64_t *<parameter>timeout_usec</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_journal_process</function></funcdef>
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<paramdef>sd_journal *<parameter>j</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_journal_wait</function></funcdef>
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<paramdef>sd_journal *<parameter>j</parameter></paramdef>
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<paramdef>uint64_t <parameter>timeout_usec</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_journal_reliable_fd</function></funcdef>
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<paramdef>sd_journal *<parameter>j</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para><function>sd_journal_get_fd()</function> returns a file
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descriptor that may be asynchronously polled in an external event
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loop and is signaled as soon as the journal changes, because new
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entries or files were added, rotation took place, or files have
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been deleted, and similar. The file descriptor is suitable for
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usage in
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<citerefentry><refentrytitle>poll</refentrytitle><manvolnum>2</manvolnum></citerefentry>.
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Use <function>sd_journal_get_events()</function> for an events
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mask to watch for. The call takes one argument: the journal
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context object. Note that not all file systems are capable of
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generating the necessary events for wakeups from this file
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descriptor for changes to be noticed immediately. In particular
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network files systems do not generate suitable file change events
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in all cases. Cases like this can be detected with
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<function>sd_journal_reliable_fd()</function>, below.
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<function>sd_journal_get_timeout()</function> will ensure in these
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cases that wake-ups happen frequently enough for changes to be
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noticed, although with a certain latency.</para>
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<para><function>sd_journal_get_events()</function> will return the
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<function>poll()</function> mask to wait for. This function will
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return a combination of <constant>POLLIN</constant> and
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<constant>POLLOUT</constant> and similar to fill into the
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<literal>.events</literal> field of <varname>struct
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pollfd</varname>.</para>
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<para><function>sd_journal_get_timeout()</function> will return a
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timeout value for usage in <function>poll()</function>. This
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returns a value in microseconds since the epoch of
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<constant>CLOCK_MONOTONIC</constant> for timing out
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<function>poll()</function> in <varname>timeout_usec</varname>.
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See
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<citerefentry><refentrytitle>clock_gettime</refentrytitle><manvolnum>2</manvolnum></citerefentry>
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for details about <constant>CLOCK_MONOTONIC</constant>. If there
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is no timeout to wait for, this will fill in <constant>(uint64_t)
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-1</constant> instead. Note that <function>poll()</function> takes
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a relative timeout in milliseconds rather than an absolute timeout
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in microseconds. To convert the absolute 'us' timeout into
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relative 'ms', use code like the following:</para>
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<programlisting>uint64_t t;
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int msec;
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sd_journal_get_timeout(m, &t);
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if (t == (uint64_t) -1)
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msec = -1;
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else {
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struct timespec ts;
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uint64_t n;
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clock_gettime(CLOCK_MONOTONIC, &ts);
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n = (uint64_t) ts.tv_sec * 1000000 + ts.tv_nsec / 1000;
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msec = t > n ? (int) ((t - n + 999) / 1000) : 0;
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}</programlisting>
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<para>The code above does not do any error checking for brevity's
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sake. The calculated <varname>msec</varname> integer can be passed
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directly as <function>poll()</function>'s timeout
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parameter.</para>
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<para>After each <function>poll()</function> wake-up
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<function>sd_journal_process()</function> needs to be called to
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process events. This call will also indicate what kind of change
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has been detected (see below; note that spurious wake-ups are
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possible).</para>
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<para>A synchronous alternative for using
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<function>sd_journal_get_fd()</function>,
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<function>sd_journal_get_events()</function>,
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<function>sd_journal_get_timeout()</function> and
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<function>sd_journal_process()</function> is
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<function>sd_journal_wait()</function>. It will synchronously wait
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until the journal gets changed. The maximum time this call sleeps
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may be controlled with the <parameter>timeout_usec</parameter>
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parameter. Pass <constant>(uint64_t) -1</constant> to wait
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indefinitely. Internally this call simply combines
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<function>sd_journal_get_fd()</function>,
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<function>sd_journal_get_events()</function>,
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<function>sd_journal_get_timeout()</function>,
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<function>poll()</function> and
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<function>sd_journal_process()</function> into one.</para>
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<para><function>sd_journal_reliable_fd()</function> may be used to
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check whether the wakeup events from the file descriptor returned
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by <function>sd_journal_get_fd()</function> are known to be
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immediately triggered. On certain file systems where file change
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events from the OS are not available (such as NFS) changes need to
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be polled for repeatedly, and hence are detected only with a
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certain latency. This call will return a positive value if the
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journal changes are detected immediately and zero when they need
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to be polled for and hence might be noticed only with a certain
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latency. Note that there is usually no need to invoke this function
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directly as <function>sd_journal_get_timeout()</function> on these
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file systems will ask for timeouts explicitly anyway.</para>
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</refsect1>
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<refsect1>
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<title>Return Value</title>
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<para><function>sd_journal_get_fd()</function> returns a valid
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file descriptor on success or a negative errno-style error
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code.</para>
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<para><function>sd_journal_get_events()</function> returns a
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combination of <constant>POLLIN</constant>,
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<constant>POLLOUT</constant> and suchlike on success or a negative
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errno-style error code.</para>
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<para><function>sd_journal_reliable_fd()</function> returns a
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positive integer if the file descriptor returned by
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<function>sd_journal_get_fd()</function> will generate wake-ups
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immediately for all journal changes. Returns 0 if there might be a
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latency involved.</para>
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<para><function>sd_journal_process()</function> and <function>sd_journal_wait()</function> return a negative
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errno-style error code, or one of <constant>SD_JOURNAL_NOP</constant>, <constant>SD_JOURNAL_APPEND</constant> or
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<constant>SD_JOURNAL_INVALIDATE</constant> on success:</para>
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<itemizedlist>
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<listitem><para>If <constant>SD_JOURNAL_NOP</constant> is returned, the journal did not change since the last
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invocation.</para></listitem>
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<listitem><para>If <constant>SD_JOURNAL_APPEND</constant> is returned, new entries have been appended to the end
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of the journal. In this case it is sufficient to simply continue reading at the previous end location of the
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journal, to read the newly added entries.</para></listitem>
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<listitem><para>If <constant>SD_JOURNAL_INVALIDATE</constant>, journal files were added to or removed from the
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set of of journal files watched (e.g. due to rotation or vacuuming), and thus entries might have appeared or
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disappeared at arbitrary places in the log stream, possibly before or after the previous end of the log
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stream. If <constant>SD_JOURNAL_INVALIDATE</constant> is returned, live-view UIs that want to reflect on screen
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the precise state of the log data on disk should probably refresh their entire display (relative to the cursor of
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the log entry on the top of the screen). Programs only interested in a strictly sequencial stream of log data may
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treat <constant>SD_JOURNAL_INVALIDATE</constant> the same way as <constant>SD_JOURNAL_APPEND</constant>, thus
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ignoring any changes to the log view earlier than the old end of the log stream.</para></listitem>
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</itemizedlist>
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</refsect1>
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<refsect1>
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<title>Signal safety</title>
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<para>In general, <function>sd_journal_get_fd()</function>, <function>sd_journal_get_events()</function>, and
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<function>sd_journal_get_timeout()</function> are <emphasis>not</emphasis> "async signal safe" in the meaning of
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<citerefentry
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project='man-pages'><refentrytitle>signal-safety</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
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Nevertheless, only the first call to any of those three functions performs unsafe operations, so subsequent calls
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<emphasis>are</emphasis> safe.</para>
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<para><function>sd_journal_process()</function> and <function>sd_journal_wait()</function> are not
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safe. <function>sd_journal_reliable_fd()</function> is safe.</para>
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</refsect1>
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<refsect1>
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<title>Notes</title>
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<para>The <function>sd_journal_get_fd()</function>,
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<function>sd_journal_get_events()</function>,
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<function>sd_journal_reliable_fd()</function>,
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<function>sd_journal_process()</function> and
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<function>sd_journal_wait()</function> interfaces are available as
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a shared library, which can be compiled and linked to with the
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<constant>libsystemd</constant> <citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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file.</para>
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</refsect1>
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<refsect1>
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<title>Examples</title>
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<para>Iterating through the journal, in a live view tracking all
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changes:</para>
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<programlisting><xi:include href="journal-iterate-wait.c" parse="text" /></programlisting>
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<para>Waiting with <function>poll()</function> (this
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example lacks all error checking for the sake of
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simplicity):</para>
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<programlisting><xi:include href="journal-iterate-poll.c" parse="text" /></programlisting>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<para>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_journal_open</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_journal_next</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>poll</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>clock_gettime</refentrytitle><manvolnum>2</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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