man: document sd_journal_get_realtime_usec()

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+<?xml version='1.0'?> <!--*-nxml-*-->
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
+        "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
+
+<!--
+  This file is part of systemd.
+
+  Copyright 2012 Lennart Poettering
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+
+  systemd is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+-->
+
+<refentry id="sd_journal_get_realtime_usec">
+
+        <refentryinfo>
+                <title>sd_journal_get_realtime_usec</title>
+                <productname>systemd</productname>
+
+                <authorgroup>
+                        <author>
+                                <contrib>Developer</contrib>
+                                <firstname>Lennart</firstname>
+                                <surname>Poettering</surname>
+                                <email>lennart@poettering.net</email>
+                        </author>
+                </authorgroup>
+        </refentryinfo>
+
+        <refmeta>
+                <refentrytitle>sd_journal_get_realtime_usec</refentrytitle>
+                <manvolnum>3</manvolnum>
+        </refmeta>
+
+        <refnamediv>
+                <refname>sd_journal_get_realtime_usec</refname>
+                <refname>sd_journal_get_monotonic_usec</refname>
+                <refpurpose>Read timestamps from the current journal entry</refpurpose>
+        </refnamediv>
+
+        <refsynopsisdiv>
+                <funcsynopsis>
+                        <funcsynopsisinfo>#include &lt;systemd/sd-journal.h&gt;</funcsynopsisinfo>
+
+                        <funcprototype>
+                                <funcdef>int <function>sd_journal_get_realtime_usec</function></funcdef>
+                                <paramdef>sd_journal* <parameter>j</parameter></paramdef>
+                                <paramdef>uint64_t* <parameter>usec</parameter></paramdef>
+                        </funcprototype>
+
+                        <funcprototype>
+                                <funcdef>int <function>sd_journal_get_monotonic_usec</function></funcdef>
+                                <paramdef>sd_journal* <parameter>j</parameter></paramdef>
+                                <paramdef>uint64_t* <parameter>usec</parameter></paramdef>
+                                <paramdef>sd_id128_t* <parameter>boot_id</parameter></paramdef>
+                        </funcprototype>
+
+                </funcsynopsis>
+        </refsynopsisdiv>
+
+        <refsect1>
+                <title>Description</title>
+
+                <para><function>sd_journal_get_realtime_usec()</function>
+                gets the realtime (wallclock) timestamp of the the
+                current journal entry.  It takes two arguments: the
+                journal context object and a pointer to a 64 Bit
+                unsigned integer to store the timestamp in. The
+                timestamp is in microseconds since the epoch,
+                i.e. CLOCK_REALTIME.</para>
+
+                <para><function>sd_journal_get_realtime_usec()</function>
+                gets the monotonic timestamp of the the current
+                journal entry.  It takes three arguments: the journal
+                context object, a pointer to a 64 Bit unsigned integer
+                to store the timestamp in as well as a 128 Bit ID
+                buffer to store the boot ID of the monotonic timestamp
+                in. The timestamp is in microseconds since boot-up of
+                the specific boot, i.e. CLOCK_MONOTONIC. Since the
+                monotonic clock begins new with every reboot it only
+                defines a well-defined point in time when used
+                together with a identifier identifying the boot, see
+                <citerefentry><refentrytitle>sd_id128_get_boot</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+                for more information. If the boot ID parameter is
+                passed NULL the function will fail if the monotonic
+                timestamp of the current entry is not of the current
+                system boot.</para>
+
+                <para>Note that these functions will not work before
+                <citerefentry><refentrytitle>sd_journal_next</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+                (or related call) has been called at least
+                once.</para>
+        </refsect1>
+
+        <refsect1>
+                <title>Return Value</title>
+
+                <para><function>sd_journal_get_realtime_usec()</function>
+                and
+                <function>sd_journal_get_monotonic_usec()</function>
+                returns 0 on success or a negative errno-style error
+                code. If the boot ID parameter was passed NULL and the
+                monotonic timestamp of the current journal entry is
+                not of the current system boot, -ESTALE is returned by <function>sd_journal_get_monotonic_usec()</function>.</para>
+        </refsect1>
+
+        <refsect1>
+                <title>Notes</title>
+
+                <para>The
+                <function>sd_journal_get_realtime_usec()</function>
+                and
+                <function>sd_journal_get_monotonic_usec()</function>
+                interfaces are available as shared library, which can
+                be compiled and linked to with the
+                <literal>libsystemd-journal</literal>
+                <citerefentry><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+                file.</para>
+        </refsect1>
+
+        <refsect1>
+                <title>See Also</title>
+
+                <para>
+                        <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+                        <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+                        <citerefentry><refentrytitle>sd_journal_open</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+                        <citerefentry><refentrytitle>sd_journal_next</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+                        <citerefentry><refentrytitle>sd_journal_get_data</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+                        <citerefentry><refentrytitle>sd_id128_get_boot</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+                        <citerefentry><refentrytitle>clock_gettime</refentrytitle><manvolnum>2</manvolnum></citerefentry>
+                </para>
+        </refsect1>
+
+</refentry>