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11a1589223
Files which are installed as-is (any .service and other unit files, .conf files, .policy files, etc), are left as is. My assumption is that SPDX identifiers are not yet that well known, so it's better to retain the extended header to avoid any doubt. I also kept any copyright lines. We can probably remove them, but it'd nice to obtain explicit acks from all involved authors before doing that.
276 lines
11 KiB
XML
276 lines
11 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
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<function>sd_journal_wait()</function> return one of
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<constant>SD_JOURNAL_NOP</constant>,
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<constant>SD_JOURNAL_APPEND</constant> or
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<constant>SD_JOURNAL_INVALIDATE</constant> on success or a
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negative errno-style error code. If
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<constant>SD_JOURNAL_NOP</constant> is returned, the journal did
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not change since the last invocation. If
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<constant>SD_JOURNAL_APPEND</constant> is returned, new entries
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have been appended to the end of the journal. If
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<constant>SD_JOURNAL_INVALIDATE</constant>, journal files were
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added or removed (possibly due to rotation). In the latter event,
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live-view UIs should probably refresh their entire display, while
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in the case of <constant>SD_JOURNAL_APPEND</constant>, it is
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sufficient to simply continue reading at the previous end of the
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journal.</para>
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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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