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207 lines
9.2 KiB
XML
207 lines
9.2 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
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<refentry id="sd-id128"
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xmlns:xi="http://www.w3.org/2001/XInclude">
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<refentryinfo>
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<title>sd-id128</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>sd-id128</refentrytitle>
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<manvolnum>3</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>sd-id128</refname>
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<refname>SD_ID128_ALLF</refname>
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<refname>SD_ID128_CONST_STR</refname>
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<refname>SD_ID128_FORMAT_STR</refname>
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<refname>SD_ID128_FORMAT_VAL</refname>
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<refname>SD_ID128_MAKE</refname>
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<refname>SD_ID128_MAKE_STR</refname>
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<refname>SD_ID128_MAKE_UUID_STR</refname>
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<refname>SD_ID128_NULL</refname>
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<refname>SD_ID128_UUID_FORMAT_STR</refname>
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<refname>sd_id128_equal</refname>
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<refname>sd_id128_in_set</refname>
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<refname>sd_id128_in_set_sentinel</refname>
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<refname>sd_id128_in_setv</refname>
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<refname>sd_id128_is_allf</refname>
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<refname>sd_id128_is_null</refname>
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<refname>sd_id128_t</refname>
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<refpurpose>APIs for processing 128-bit IDs</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<funcsynopsis>
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<funcsynopsisinfo>#include <systemd/sd-id128.h></funcsynopsisinfo>
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</funcsynopsis>
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<cmdsynopsis>
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<command>pkg-config --cflags --libs libsystemd</command>
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</cmdsynopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para><filename>sd-id128.h</filename> provides APIs to process and generate 128-bit ID values. The
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128-bit ID values processed and generated by these APIs are a generalization of OSF UUIDs as defined by
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<ulink url="https://tools.ietf.org/html/rfc4122">RFC 4122</ulink> but use a simpler string format. These
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functions impose no structure on the used IDs, much unlike OSF UUIDs or Microsoft GUIDs, but are mostly
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compatible with those types of IDs.
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</para>
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<para>See
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<citerefentry><refentrytitle>sd_id128_to_string</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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and
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<citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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for more information about the implemented functions.</para>
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<para>A 128-bit ID is implemented as the following
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union type:</para>
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<programlisting>typedef union sd_id128 {
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uint8_t bytes[16];
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uint64_t qwords[2];
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} sd_id128_t;</programlisting>
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<para>This union type allows accessing the 128-bit ID as 16
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separate bytes or two 64-bit words. It is generally safer to
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access the ID components by their 8-bit array to avoid endianness
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issues. This union is intended to be passed call-by-value (as
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opposed to call-by-reference) and may be directly manipulated by
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clients.</para>
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<para>A couple of macros are defined to denote and decode 128-bit
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IDs:</para>
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<para><function>SD_ID128_MAKE()</function> may be used to denote a
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constant 128-bit ID in source code. A commonly used idiom is to
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assign a name to a 128-bit ID using this macro:</para>
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<programlisting>#define SD_MESSAGE_COREDUMP SD_ID128_MAKE(fc,2e,22,bc,6e,e6,47,b6,b9,07,29,ab,34,a2,50,b1)</programlisting>
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<para><constant>SD_ID128_NULL</constant> may be used to refer to the 128-bit ID consisting of only
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<constant>NUL</constant> bytes.</para>
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<para><function>SD_ID128_MAKE_STR()</function> is similar to <function>SD_ID128_MAKE()</function>, but creates a
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<type>const char*</type> expression that can be conveniently used in message formats and such:</para>
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<programlisting>#include <stdio.h>
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#define SD_MESSAGE_COREDUMP_STR SD_ID128_MAKE_STR(fc,2e,22,bc,6e,e6,47,b6,b9,07,29,ab,34,a2,50,b1)
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int main(int argc, char **argv) {
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puts("Match for coredumps: MESSAGE_ID=" SD_MESSAGE_COREDUMP_STR);
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}</programlisting>
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<para><function>SD_ID128_CONST_STR()</function> may be used to
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convert constant 128-bit IDs into constant strings for output. The
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following example code will output the string
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"fc2e22bc6ee647b6b90729ab34a250b1":</para>
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<programlisting>int main(int argc, char *argv[]) {
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puts("Match for coredumps: %s", SD_ID128_CONST_STR(SD_MESSAGE_COREDUMP));
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}</programlisting>
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<para><constant>SD_ID128_FORMAT_STR</constant> and <function>SD_ID128_FORMAT_VAL()</function> may
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be used to format a 128-bit ID in a
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<citerefentry project='man-pages'><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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format string, as shown in the following example:</para>
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<programlisting>int main(int argc, char *argv[]) {
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sd_id128_t id;
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id = SD_ID128_MAKE(ee,89,be,71,bd,6e,43,d6,91,e6,c5,5d,eb,03,02,07);
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printf("The ID encoded in this C file is " SD_ID128_FORMAT_STR ".\n", SD_ID128_FORMAT_VAL(id));
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return 0;
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}</programlisting>
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<para><constant>SD_ID128_UUID_FORMAT_STR</constant> and <function>SD_ID128_MAKE_UUID_STR()</function>
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are similar to
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<constant>SD_ID128_FORMAT_STR</constant> and <function>SD_ID128_MAKE_STR()</function>,
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but include separating hyphens to conform to the
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"<ulink url="https://en.wikipedia.org/wiki/Universally_unique_identifier#Format">canonical representation</ulink>".
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They format the string based on <ulink
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url="https://tools.ietf.org/html/rfc4122">RFC4122</ulink> Variant 1 rules, i.e. converting from Big
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Endian byte order. This matches behaviour of most other Linux userspace infrastructure. It's probably
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best to avoid UUIDs of other variants, in order to avoid unnecessary ambiguities. All 128-bit IDs
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generated by the sd-id128 APIs strictly conform to Variant 1 Version 4 UUIDs, as per RFC 4122.</para>
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<para>Use <function>sd_id128_equal()</function> to compare two 128-bit IDs:</para>
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<programlisting>int main(int argc, char *argv[]) {
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sd_id128_t a, b, c;
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a = SD_ID128_MAKE(ee,89,be,71,bd,6e,43,d6,91,e6,c5,5d,eb,03,02,07);
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b = SD_ID128_MAKE(f2,28,88,9c,5f,09,44,15,9d,d7,04,77,58,cb,e7,3e);
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c = a;
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assert(sd_id128_equal(a, c));
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assert(!sd_id128_equal(a, b));
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return 0;
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}</programlisting>
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<para>Use <function>sd_id128_is_null()</function> to check if an 128-bit ID consists of only
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<constant>NUL</constant> bytes:</para>
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<programlisting>assert(sd_id128_is_null(SD_ID128_NULL));</programlisting>
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<para>Similarly, use <function>sd_id128_is_allf()</function> to check if an 128-bit ID consists of only
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<constant>0xFF</constant> bytes (all bits on):</para>
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<programlisting>assert(sd_id128_is_allf(SD_ID128_ALLF));</programlisting>
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<para>For convenience, <function>sd_id128_in_set()</function> takes a list of IDs and
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returns true if any are equal to the first argument:</para>
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<programlisting>int main(int argc, char *argv[]) {
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sd_id12_t a = SD_ID128_MAKE(ee,89,be,71,bd,6e,43,d6,91,e6,c5,5d,eb,03,02,07);
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assert(sd_id128_in_set(a, a));
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assert(sd_id128_in_set(a, a, a));
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assert(!sd_id128_in_set(a));
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assert(!sd_id128_in_set(a,
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SD_ID128_MAKE(f2,28,88,9c,5f,09,44,15,9d,d7,04,77,58,cb,e7,3e)
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SD_ID128_MAKE(2f,88,28,5f,9c,44,09,9d,d7,15,77,04,bc,85,7e,e3)
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SD_ID128_ALLF));
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return 0;
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}
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</programlisting>
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<para><function>sd_id128_in_set()</function> is defined as a macro over
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<function>sd_id128_in_set_sentinel()</function>, adding the <constant>SD_ID128_NULL</constant>
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sentinel. Since <function>sd_id128_in_set_sentinel()</function> uses <constant>SD_ID128_NULL</constant>
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as the sentinel, <constant>SD_ID128_NULL</constant> cannot be otherwise placed in the argument list.
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</para>
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<para><function>sd_id128_in_setv()</function> is similar to
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<function>sd_id128_in_set_sentinel()</function>, but takes a <structname>struct varargs</structname>
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argument.</para>
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<para>Note that new, randomized IDs may be generated with
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<citerefentry><refentrytitle>systemd-id128</refentrytitle><manvolnum>1</manvolnum></citerefentry>'s
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<command>new</command> command.</para>
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</refsect1>
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<xi:include href="libsystemd-pkgconfig.xml" />
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<refsect1>
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<title>See Also</title>
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<para>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_id128_to_string</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_id128_randomize</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_id128_get_machine</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry project='man-pages'><refentrytitle>printf</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>journalctl</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
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<citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>machine-id</refentrytitle><manvolnum>5</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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