man: document sd_journal_seek_head()

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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_seek_head">
+
+        <refentryinfo>
+                <title>sd_journal_seek_head</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_seek_head</refentrytitle>
+                <manvolnum>3</manvolnum>
+        </refmeta>
+
+        <refnamediv>
+                <refname>sd_journal_seek_head</refname>
+                <refname>sd_journal_seek_tail</refname>
+                <refname>sd_journal_seek_monotonic_usec</refname>
+                <refname>sd_journal_seek_realtime_usec</refname>
+                <refname>sd_journal_seek_cursor</refname>
+                <refpurpose>Seek to a position in the
+                journal</refpurpose>
+        </refnamediv>
+
+        <refsynopsisdiv>
+                <funcsynopsis>
+                        <funcsynopsisinfo>#include &lt;systemd/sd-journal.h&gt;</funcsynopsisinfo>
+
+                        <funcprototype>
+                                <funcdef>int <function>sd_journal_seek_head</function></funcdef>
+                                <paramdef>sd_journal* <parameter>j</parameter></paramdef>
+                        </funcprototype>
+
+                        <funcprototype>
+                                <funcdef>int <function>sd_journal_seek_tail</function></funcdef>
+                                <paramdef>sd_journal* <parameter>j</parameter></paramdef>
+                        </funcprototype>
+
+                        <funcprototype>
+                                <funcdef>int <function>sd_journal_seek_monotonic_usec</function></funcdef>
+                                <paramdef>sd_journal* <parameter>j</parameter></paramdef>
+                                <paramdef>sd_id128_t <parameter>boot_id</parameter></paramdef>
+                                <paramdef>uint64_t <parameter>usec</parameter></paramdef>
+                        </funcprototype>
+
+                        <funcprototype>
+                                <funcdef>int <function>sd_journal_seek_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_seek_cursor</function></funcdef>
+                                <paramdef>sd_journal* <parameter>j</parameter></paramdef>
+                                <paramdef>const char * <parameter>cursor</parameter></paramdef>
+                        </funcprototype>
+                </funcsynopsis>
+        </refsynopsisdiv>
+
+        <refsect1>
+                <title>Description</title>
+
+                <para><function>sd_journal_seek_head()</function>
+                seeks to the beginning of the journal, i.e. the oldest
+                avilable entry.</para>
+
+                <para>Similar,
+                <function>sd_journal_seek_tail()</function> may be
+                used to seek to the end of the journal, i.e. the most
+                recent available entry.</para>
+
+                <para><function>sd_journal_seek_monotonic_usec()</function>
+                seeks to the entry with the specified monotonic
+                timestamp, i.e. CLOCK_MONOOTONIC. Since monotonic time
+                restarts on every reboot a boot ID needs to be
+                specified as well.</para>
+
+                <para><function>sd_journal_seek_realtime_usec()</function>
+                seeks to the entry with the specified realtime
+                (wallclock) timestamp, i.e. CLOCK_REALTIME. Note that
+                the realtime clock is not necessary monotonic. If a
+                realtime timestamp is ambiguous it is not defined
+                which position is seeked to.</para>
+
+                <para><function>sd_journal_seek_cursor()</function>
+                seeks to the entry located at the specified cursor
+                string. For details on cursors see
+                <citerefentry><refentrytitle>sd_journal_get_cursor</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
+
+                <para>Note that these calls do not actually make any
+                entry the new current entry, this needs to be done in
+                a separate step with a subsequent
+                <citerefentry><refentrytitle>sd_journal_next</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+                invocation (or a similar call). Only then entry data
+                may be retrieved via
+                <citerefentry><refentrytitle>sd_journal_get_data</refentrytitle><manvolnum>3</manvolnum></citerefentry>. If
+                no entry exists that matches exactly the specified
+                seek address the next closest is seeked to. If
+                <citerefentry><refentrytitle>sd_journal_next</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+                is used the closest following entry will be seeked to,
+                if
+                <citerefentry><refentrytitle>sd_journal_previous</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+                is used the closest preceeding entry is seeked
+                to.</para>
+        </refsect1>
+
+        <refsect1>
+                <title>Return Value</title>
+
+                <para>The functions return 0 on success or a negative
+                errno-style error code.</para>
+        </refsect1>
+
+        <refsect1>
+                <title>Notes</title>
+
+                <para>The <function>sd_journal_seek_head()</function>,
+                <function>sd_journal_seek_tail()</function>,
+                <function>sd_journal_seek_monotonic_usec()</function>,
+                <function>sd_journal_seek_realtime_usec()</function>,
+                and <function>sd_journal_seek_cursor()</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_journal_get_cursor</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+                        <citerefentry><refentrytitle>sd_journal_get_realtime_usec</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+                </para>
+        </refsect1>
+
+</refentry>