man: document message timestamping/sequence number API

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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 2013 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_bus_message_get_monotonic_usec">
+
+        <refentryinfo>
+                <title>sd_bus_message_get_monotonic_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_bus_message_get_monotonic_usec</refentrytitle>
+                <manvolnum>3</manvolnum>
+        </refmeta>
+
+        <refnamediv>
+                <refname>sd_bus_message_get_monotonic_usec</refname>
+                <refname>sd_bus_message_get_realtime_usec</refname>
+                <refname>sd_bus_message_get_seqnum</refname>
+                <refpurpose>Retrieve the sender timestamps and sequence number of a message</refpurpose>
+        </refnamediv>
+
+        <refsynopsisdiv>
+                <funcsynopsis>
+                        <funcsynopsisinfo>#include &lt;systemd/sd-bus.h&gt;</funcsynopsisinfo>
+
+                        <funcprototype>
+                                <funcdef>int <function>sd_bus_message_get_monotonic_usec</function></funcdef>
+                                <paramdef>sd_bus_message* <parameter>message</parameter></paramdef>
+                                <paramdef>uint64_t* <parameter>usec</parameter></paramdef>
+                        </funcprototype>
+
+                        <funcprototype>
+                                <funcdef>int <function>sd_bus_message_get_realtime_usec</function></funcdef>
+                                <paramdef>sd_bus_message* <parameter>message</parameter></paramdef>
+                                <paramdef>uint64_t* <parameter>usec</parameter></paramdef>
+                        </funcprototype>
+
+                        <funcprototype>
+                                <funcdef>int <function>sd_bus_message_get_seqnum</function></funcdef>
+                                <paramdef>sd_bus_message* <parameter>message</parameter></paramdef>
+                                <paramdef>uint64_t* <parameter>seqnum</parameter></paramdef>
+                        </funcprototype>
+                </funcsynopsis>
+        </refsynopsisdiv>
+
+        <refsect1>
+                <title>Description</title>
+
+                <para><function>sd_bus_message_get_monotonic_usec()</function>
+                returns the monotonic timestamp of the time the
+                message was sent. This value is in microseconds since
+                the <literal>CLOCK_MONOTONIC</literal> epoch, see
+                <citerefentry><refentrytitle>clock_gettime</refentrytitle><manvolnum>2</manvolnum></citerefentry>
+                for details.</para>
+
+                <para>Similar,
+                <function>sd_bus_message_get_realtime_usec()</function>
+                returns the realtime (wallclock) timestamp of the time
+                the message was sent. This value is in microseconds
+                since Jan 1st, 1970, i.e. in the
+                <literal>CLOCK_REALTIME</literal> clock.</para>
+
+                <para><function>sd_bus_message_get_seqnum()</function>
+                returns the kernel-assigned sequence number of the
+                message. The kernel assigns a global monotonically increasing
+                sequence number to all messages sent on the local
+                system. This sequence number is useful for determining
+                message send order, even across different busses of
+                the local system. The sequence number combined with
+                the boot ID of the system (as returned by
+                <citerefentry><refentrytitle>sd_id128_get_boot</refentrytitle><manvolnum>3</manvolnum></citerefentry>)
+                is a suitable globally unique identifier for bus
+                messages.</para>
+
+                <para>These timestamps and the sequence number are
+                attached to each message by the kernel and cannot be
+                manipulated by the sending userspace.</para>
+
+                <para>Note that these timestamps are only available on
+                some bus transports, and only after support for them
+                has been negotiated with the
+                <citerefentry><refentrytitle>sd_bus_negotiate_timestamp</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+                call.</para>
+        </refsect1>
+
+        <refsect1>
+                <title>Return Value</title>
+
+                <para>On success, these calls return 0 or a positive
+                integer. On failure, these calls return a negative
+                errno-style error code.</para>
+
+                <para>On success, the timestamp or sequence number is
+                returned in the specified 64-bit unsigned integer
+                variable.</para>
+        </refsect1>
+
+        <refsect1>
+                <title>Errors</title>
+
+                <para>Returned errors may indicate the following problems:</para>
+
+                <variablelist>
+                        <varlistentry>
+                                <term><varname>-EINVAL</varname></term>
+
+                                <listitem><para>A specified parameter
+                                is invalid.</para></listitem>
+                        </varlistentry>
+
+                        <varlistentry>
+                                <term><varname>-ENODATA</varname></term>
+
+                                <listitem><para>No timestamp or
+                                sequence number information is
+                                attached to the passed message. This
+                                error is returned if the underlying
+                                transport does not support
+                                timestamping or assigning of sequence
+                                numbers, or if this feature has not
+                                been negotiated with
+                                <citerefentry><refentrytitle>sd_bus_negotiate_timestamp</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para></listitem>
+                        </varlistentry>
+                </variablelist>
+        </refsect1>
+
+        <refsect1>
+                <title>Notes</title>
+
+                <para>The
+                <function>sd_bus_message_get_monotonic_usec()</function>,
+                <function>sd_bus_message_get_realtime_usec()</function>,
+                and <function>sd_bus_message_get_seqnum()</function>
+                interfaces are available as a shared library, which
+                can be compiled and linked to with the
+                <constant>libsystemd</constant> <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-bus</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+                        <citerefentry><refentrytitle>sd_bus_new</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+                        <citerefentry><refentrytitle>sd_bus_negotiate_timestamp</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+                        <citerefentry><refentrytitle>clock_gettime</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
+                        <citerefentry><refentrytitle>sd_id128_get_boot</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+                </para>
+        </refsect1>
+
+</refentry>