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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 <systemd/sd-id128.h></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 128bit 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>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>
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