man: document libsystemd-id128

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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_id128_to_string">
+
+        <refentryinfo>
+                <title>sd_id128_to_string</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_id128_to_string</refentrytitle>
+                <manvolnum>3</manvolnum>
+        </refmeta>
+
+        <refnamediv>
+                <refname>sd_id128_to_string</refname>
+                <refname>sd_id128_from_string</refname>
+                <refpurpose>Format or parse 128 bit IDs as strings</refpurpose>
+        </refnamediv>
+
+        <refsynopsisdiv>
+                <funcsynopsis>
+                        <funcsynopsisinfo>#include &lt;systemd/sd-id128.h&gt;</funcsynopsisinfo>
+
+                        <funcprototype>
+                                <funcdef>char* <function>sd_id128_to_string</function></funcdef>
+                                <paramdef>sd_id128_t <parameter>id</parameter>, char <parameter>s</parameter>[33]</paramdef>
+                        </funcprototype>
+
+                        <funcprototype>
+                                <funcdef>int <function>sd_id128_from_string</function></funcdef>
+                                <paramdef>const char <parameter>s</parameter>[33], sd_id128_t* <parameter>ret</parameter></paramdef>
+                        </funcprototype>
+
+                </funcsynopsis>
+        </refsynopsisdiv>
+
+        <refsect1>
+                <title>Description</title>
+
+                <para><function>sd_id128_to_string()</function>
+                formats a 128 bit ID as character string. It expects
+                the ID and a string array capable of storing 33
+                characters. The ID will be formatted as 32 lowercase
+                hexadecimal digits and be terminated by a NUL
+                byte.</para>
+
+                <para><function>sd_id128_from_string()</function>
+                implements the reverse operation: it takes a 33
+                character array with 32 hexadecimial digits
+                (terminated by NUL) and parses them back into an
+                128 bit ID returned in
+                <parameter>ret</parameter>.</para>
+
+                <para>For more information about the
+                <literal>sd_id128_t</literal> type see
+                <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
+
+                <para>When formatting a 128 bit ID into a string it is
+                often easier to use a format string for
+                <citerefentry><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry>. This
+                is easly done using the
+                <function>SD_ID128_FORMAT_STR</function> and
+                <function>SD_ID128_FORMAT_VAL()</function> macros. For
+                more information see
+                <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>7</manvolnum></citerefentry>.</para>
+        </refsect1>
+
+        <refsect1>
+                <title>Return Value</title>
+
+                <para><function>sd_id128_to_string()</function> always
+                succeeds and returns a pointer to the string array
+                passed in.  <function>sd_id128_from_string</function>
+                returns 0 on success (in which case
+                <parameter>ret</parameter> is filled in), or a negative
+                errno-style error code.</para>
+        </refsect1>
+
+        <refsect1>
+                <title>Notes</title>
+
+                <para>The <function>sd_id128_to_string()</function>
+                and <function>sd_id128_from_string()</function> interfaces are
+                available as shared library, which can be compiled and
+                linked to with the <literal>libsystemd-id128</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-id128</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
+                        <citerefentry><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+                </para>
+        </refsect1>
+
+</refentry>