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+<?xml version='1.0'?> <!--*-nxml-*-->
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
+ "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
+<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
+
+<refentry id="sd_id128_to_string" xmlns:xi="http://www.w3.org/2001/XInclude">
+
+ <refentryinfo>
+ <title>sd_id128_to_string</title>
+ <productname>systemd</productname>
+ </refentryinfo>
+
+ <refmeta>
+ <refentrytitle>sd_id128_to_string</refentrytitle>
+ <manvolnum>3</manvolnum>
+ </refmeta>
+
+ <refnamediv>
+ <refname>sd_id128_to_string</refname>
+ <refname>SD_ID128_TO_STRING</refname>
+ <refname>SD_ID128_STRING_MAX</refname>
+ <refname>sd_id128_to_uuid_string</refname>
+ <refname>SD_ID128_TO_UUID_STRING</refname>
+ <refname>SD_ID128_UUID_STRING_MAX</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>
+
+ <funcsynopsisinfo>#define SD_ID128_STRING_MAX 33U</funcsynopsisinfo>
+
+ <funcsynopsisinfo>#define SD_ID128_UUID_STRING_MAX 37U</funcsynopsisinfo>
+
+ <funcsynopsisinfo>#define SD_ID128_TO_STRING(id) …</funcsynopsisinfo>
+
+ <funcsynopsisinfo>#define SD_ID128_TO_UUID_STRING(id) …</funcsynopsisinfo>
+
+ <funcprototype>
+ <funcdef>char *<function>sd_id128_to_string</function></funcdef>
+ <paramdef>sd_id128_t <parameter>id</parameter>, char <parameter>s</parameter>[static SD_ID128_STRING_MAX]</paramdef>
+ </funcprototype>
+
+ <funcprototype>
+ <funcdef>char *<function>sd_id128_uuid_string</function></funcdef>
+ <paramdef>sd_id128_t <parameter>id</parameter>, char <parameter>s</parameter>[static SD_ID128_UUID_STRING_MAX]</paramdef>
+ </funcprototype>
+
+ <funcprototype>
+ <funcdef>int <function>sd_id128_from_string</function></funcdef>
+ <paramdef>const char *<parameter>s</parameter>, 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 a character string. It expects
+ the ID and a string array capable of storing 33 characters
+ (<constant>SD_ID128_STRING_MAX</constant>). The ID will be formatted as 32 lowercase hexadecimal digits
+ and be terminated by a <constant>NUL</constant> byte.</para>
+
+ <para><function>SD_ID128_TO_STRING()</function> is a macro that wraps
+ <function>sd_id128_to_string()</function> and passes an appropriately sized buffer as second argument,
+ allocated as C99 compound literal. Each use will thus implicitly acquire a suitable buffer on the stack
+ which remains valid until the end of the current code block. This is usually the simplest way to acquire
+ a string representation of a 128-bit ID in a buffer that is valid in the current code block.</para>
+
+ <para><function>sd_id128_to_uuid_string()</function> and <function>SD_ID128_TO_UUID_STRING()</function>
+ are similar to these two functions/macros, but format the 128-bit values as RFC4122 UUIDs, i.e. a series
+ of 36 lowercase hexadeciaml digits and dashes, terminated by a <constant>NUL</constant> byte.</para>
+
+ <para><function>sd_id128_from_string()</function> implements the reverse operation: it takes a 33
+ character string with 32 hexadecimal digits (either lowercase or uppercase, terminated by
+ <constant>NUL</constant>) and parses them back into a 128-bit ID returned in
+ <parameter>ret</parameter>. Alternatively, this call can also parse a 37-character string with a 128-bit
+ ID formatted as RFC UUID. If <parameter>ret</parameter> is passed as <constant>NULL</constant> the
+ function will validate the passed ID string, but not actually return it in parsed form.</para>
+
+ <para>Note that when formatting and parsing 36 character UUIDs this is done strictly in Big Endian byte order,
+ i.e. according to <ulink url="https://tools.ietf.org/html/rfc4122">RFC4122</ulink> Variant 1 rules, even
+ if the UUID encodes a different variant. This matches behaviour in various other Linux userspace
+ tools. It's probably wise to avoid UUIDs of other variant types.</para>
+
+ <para>For more information about the <literal>sd_id128_t</literal> type see
+ <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>. Note that
+ these calls operate the same way on all architectures, i.e. the results do not depend on
+ endianness.</para>
+
+ <para>When formatting a 128-bit ID into a string, it is often easier to use a format string for
+ <citerefentry
+ project='man-pages'><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry>. This
+ is easily done using the <constant>SD_ID128_FORMAT_STR</constant> and
+ <function>SD_ID128_FORMAT_VAL()</function> macros. For more information see
+ <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</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>
+
+ <xi:include href="libsystemd-pkgconfig.xml" />
+
+ <refsect1>
+ <title>History</title>
+ <para><function>sd_id128_to_string()</function> and
+ <function>sd_id128_from_string()</function> were added in version 187.</para>
+ <para><function>sd_id128_uuid_string()</function> was added in version 251.</para>
+ </refsect1>
+
+ <refsect1>
+ <title>See Also</title>
+
+ <para>
+ <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>sd-id128</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+ <citerefentry project='man-pages'><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+ </para>
+ </refsect1>
+
+</refentry>