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<!-- doc/src/sgml/uuid-ossp.sgml -->
<sect1 id="uuid-ossp" xreflabel="uuid-ossp">
<title>uuid-ossp</title>
<indexterm zone="uuid-ossp">
<primary>uuid-ossp</primary>
</indexterm>
<para>
The <filename>uuid-ossp</> module provides functions to generate universally
unique identifiers (UUIDs) using one of several standard algorithms. There
are also functions to produce certain special UUID constants.
</para>
<sect2>
<title><literal>uuid-ossp</literal> Functions</title>
<para>
<xref linkend="uuid-ossp-functions"> shows the functions available to
generate UUIDs.
The relevant standards ITU-T Rec. X.667, ISO/IEC 9834-8:2005, and RFC
4122 specify four algorithms for generating UUIDs, identified by the
version numbers 1, 3, 4, and 5. (There is no version 2 algorithm.)
Each of these algorithms could be suitable for a different set of
applications.
</para>
<table id="uuid-ossp-functions">
<title>Functions for UUID Generation</title>
<tgroup cols="2">
<thead>
<row>
<entry>Function</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry><function>uuid_generate_v1()</function><indexterm><primary>uuid_generate_v1</primary></indexterm></entry>
<entry>
<para>
This function generates a version 1 UUID. This involves the MAC
address of the computer and a time stamp. Note that UUIDs of this
kind reveal the identity of the computer that created the identifier
and the time at which it did so, which might make it unsuitable for
certain security-sensitive applications.
</para>
</entry>
</row>
<row>
<entry><function>uuid_generate_v1mc()</function><indexterm><primary>uuid_generate_v1mc</primary></indexterm></entry>
<entry>
<para>
This function generates a version 1 UUID but uses a random multicast
MAC address instead of the real MAC address of the computer.
</para>
</entry>
</row>
<row>
<entry><function>uuid_generate_v3(namespace uuid, name text)</function><indexterm><primary>uuid_generate_v3</primary></indexterm></entry>
<entry>
<para>
This function generates a version 3 UUID in the given namespace using
the specified input name. The namespace should be one of the special
constants produced by the <function>uuid_ns_*()</> functions shown
in <xref linkend="uuid-ossp-constants">. (It could be any UUID in theory.) The name is an identifier
in the selected namespace.
</para>
<para>
For example:
<programlisting>
SELECT uuid_generate_v3(uuid_ns_url(), 'http://www.postgresql.org');
</programlisting>
The name parameter will be MD5-hashed, so the cleartext cannot be
derived from the generated UUID.
The generation of UUIDs by this method has no random or
environment-dependent element and is therefore reproducible.
</para>
</entry>
</row>
<row>
<entry><literal>uuid_generate_v4()</literal></entry>
<entry>
<para>
This function generates a version 4 UUID, which is derived entirely
from random numbers.
</para>
</entry>
</row>
<row>
<entry><literal>uuid_generate_v5(namespace uuid, name text)</literal></entry>
<entry>
<para>
This function generates a version 5 UUID, which works like a version 3
UUID except that SHA-1 is used as a hashing method. Version 5 should
be preferred over version 3 because SHA-1 is thought to be more secure
than MD5.
</para>
</entry>
</row>
</tbody>
</tgroup>
</table>
<table id="uuid-ossp-constants">
<title>Functions Returning UUID Constants</title>
<tgroup cols="2">
<tbody>
<row>
<entry><literal>uuid_nil()</literal></entry>
<entry>
<para>
A <quote>nil</> UUID constant, which does not occur as a real UUID.
</para>
</entry>
</row>
<row>
<entry><literal>uuid_ns_dns()</literal></entry>
<entry>
<para>
Constant designating the DNS namespace for UUIDs.
</para>
</entry>
</row>
<row>
<entry><literal>uuid_ns_url()</literal></entry>
<entry>
<para>
Constant designating the URL namespace for UUIDs.
</para>
</entry>
</row>
<row>
<entry><literal>uuid_ns_oid()</literal></entry>
<entry>
<para>
Constant designating the ISO object identifier (OID) namespace for
UUIDs. (This pertains to ASN.1 OIDs, which are unrelated to the OIDs
used in <productname>PostgreSQL</>.)
</para>
</entry>
</row>
<row>
<entry><literal>uuid_ns_x500()</literal></entry>
<entry>
<para>
Constant designating the X.500 distinguished name (DN) namespace for
UUIDs.
</para>
</entry>
</row>
</tbody>
</tgroup>
</table>
</sect2>
<sect2>
<title>Building <filename>uuid-ossp</></title>
<para>
Historically this module depended on the OSSP UUID library, which accounts
for the module's name. While the OSSP UUID library can still be found
at <ulink url="http://www.ossp.org/pkg/lib/uuid/"></ulink>, it is not well
maintained, and is becoming increasingly difficult to port to newer
platforms. <filename>uuid-ossp</> can now be built without the OSSP
library on some platforms. On FreeBSD, NetBSD, and some other BSD-derived
platforms, suitable UUID creation functions are included in the
core <filename>libc</> library. On Linux, Mac OS X, and some other
platforms, suitable functions are provided in the <filename>libuuid</>
library, which originally came from the <literal>e2fsprogs</> project
(though on modern Linux it is considered part
of <literal>util-linux-ng</>). When invoking <filename>configure</>,
specify <option>--with-uuid=bsd</option> to use the BSD functions,
or <option>--with-uuid=e2fs</option> to
use <literal>e2fsprogs</>' <filename>libuuid</>, or
<option>--with-uuid=ossp</option> to use the OSSP UUID library.
More than one of these libraries might be available on a particular
machine, so <filename>configure</> does not automatically choose one.
</para>
<note>
<para>
If you only need randomly-generated (version 4) UUIDs,
consider using the <function>gen_random_uuid()</> function
from the <xref linkend="pgcrypto"> module instead.
</para>
</note>
</sect2>
<sect2>
<title>Author</title>
<para>
Peter Eisentraut <email>[email protected]</email>
</para>
</sect2>
</sect1>
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