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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_journal_query_unique" xmlns:xi="http://www.w3.org/2001/XInclude">
+
+ <refentryinfo>
+ <title>sd_journal_query_unique</title>
+ <productname>systemd</productname>
+ </refentryinfo>
+
+ <refmeta>
+ <refentrytitle>sd_journal_query_unique</refentrytitle>
+ <manvolnum>3</manvolnum>
+ </refmeta>
+
+ <refnamediv>
+ <refname>sd_journal_query_unique</refname>
+ <refname>sd_journal_enumerate_unique</refname>
+ <refname>sd_journal_enumerate_available_unique</refname>
+ <refname>sd_journal_restart_unique</refname>
+ <refname>SD_JOURNAL_FOREACH_UNIQUE</refname>
+ <refpurpose>Read unique data fields from the journal</refpurpose>
+ </refnamediv>
+
+ <refsynopsisdiv>
+ <funcsynopsis>
+ <funcsynopsisinfo>#include &lt;systemd/sd-journal.h&gt;</funcsynopsisinfo>
+
+ <funcprototype>
+ <funcdef>int <function>sd_journal_query_unique</function></funcdef>
+ <paramdef>sd_journal *<parameter>j</parameter></paramdef>
+ <paramdef>const char *<parameter>field</parameter></paramdef>
+ </funcprototype>
+
+ <funcprototype>
+ <funcdef>int <function>sd_journal_enumerate_available_unique</function></funcdef>
+ <paramdef>sd_journal *<parameter>j</parameter></paramdef>
+ <paramdef>const void **<parameter>data</parameter></paramdef>
+ <paramdef>size_t *<parameter>length</parameter></paramdef>
+ </funcprototype>
+
+ <funcprototype>
+ <funcdef>int <function>sd_journal_enumerate_unique</function></funcdef>
+ <paramdef>sd_journal *<parameter>j</parameter></paramdef>
+ <paramdef>const void **<parameter>data</parameter></paramdef>
+ <paramdef>size_t *<parameter>length</parameter></paramdef>
+ </funcprototype>
+
+ <funcprototype>
+ <funcdef>void <function>sd_journal_restart_unique</function></funcdef>
+ <paramdef>sd_journal *<parameter>j</parameter></paramdef>
+ </funcprototype>
+
+ <funcprototype>
+ <funcdef><function>SD_JOURNAL_FOREACH_UNIQUE</function></funcdef>
+ <paramdef>sd_journal *<parameter>j</parameter></paramdef>
+ <paramdef>const void *<parameter>data</parameter></paramdef>
+ <paramdef>size_t <parameter>length</parameter></paramdef>
+ </funcprototype>
+
+ </funcsynopsis>
+ </refsynopsisdiv>
+
+ <refsect1>
+ <title>Description</title>
+
+ <para><function>sd_journal_query_unique()</function> queries the journal for all unique values the
+ specified field can take. It takes two arguments: the journal to query and the field name to look
+ for. Well-known field names are listed on
+ <citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
+ but any field can be specified. Field names must be specified without a trailing
+ <literal>=</literal>. After this function has been executed successfully the field values may be queried
+ using <function>sd_journal_enumerate_unique()</function> and
+ <function>sd_journal_enumerate_available_unique()</function>. Invoking one of those calls will change the
+ field name being queried and reset the enumeration index to the first field value that matches.</para>
+
+ <para><function>sd_journal_enumerate_unique()</function> may be used to iterate through all data fields
+ which match the previously selected field name as set with
+ <function>sd_journal_query_unique()</function>. On each invocation the next field data matching the field
+ name is returned. The order of the returned data fields is not defined. It takes three arguments: the
+ journal object, plus a pair of pointers to pointer/size variables where the data object and its size
+ shall be stored. The returned data is in a read-only memory map and is only valid until the next
+ invocation of <function>sd_journal_enumerate_unique()</function>. Note that the data returned will be
+ prefixed with the field name and <literal>=</literal>. Note that this call is subject to the data field
+ size threshold as controlled by <function>sd_journal_set_data_threshold()</function> and only the initial
+ part of the field up to the threshold is returned. An error is returned for fields which cannot be
+ retrieved. See the error list below for details.</para>
+
+ <para><function>sd_journal_enumerate_available_unique()</function> is similar to
+ <function>sd_journal_enumerate_unique()</function>, but silently skips any fields which may be valid, but
+ are too large or not supported by current implementation.</para>
+
+ <para><function>sd_journal_restart_unique()</function> resets the
+ data enumeration index to the beginning of the list. The next
+ invocation of <function>sd_journal_enumerate_unique()</function>
+ will return the first field data matching the field name
+ again.</para>
+
+ <para>Note that the <function>SD_JOURNAL_FOREACH_UNIQUE()</function> macro may be used as a handy wrapper
+ around <function>sd_journal_restart_unique()</function> and
+ <function>sd_journal_enumerate_available_unique()</function>.</para>
+
+ <para>Note that these functions currently are not influenced by
+ matches set with <function>sd_journal_add_match()</function> but
+ this might change in a later version of this software.</para>
+
+ <para>To enumerate all field names currently in use (and thus all suitable field parameters for
+ <function>sd_journal_query_unique()</function>), use the
+ <citerefentry><refentrytitle>sd_journal_enumerate_fields</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+ call.</para>
+ </refsect1>
+
+ <refsect1>
+ <title>Return Value</title>
+
+ <para><function>sd_journal_query_unique()</function> returns 0 on success or a negative errno-style error
+ code. <function>sd_journal_enumerate_unique()</function> and
+ <function>sd_journal_query_available_unique()</function> return a positive integer if the next field data
+ has been read, 0 when no more fields remain, or a negative errno-style error code.
+ <function>sd_journal_restart_unique()</function> doesn't return anything.</para>
+
+ <refsect2>
+ <title>Errors</title>
+
+ <para>Returned errors may indicate the following problems:</para>
+
+ <variablelist>
+ <xi:include href="sd_journal_get_data.xml" xpointer="EINVAL"/>
+ <xi:include href="sd_journal_get_data.xml" xpointer="ECHILD"/>
+ <xi:include href="sd_journal_get_data.xml" xpointer="EADDRNOTAVAIL"/>
+ <xi:include href="sd_journal_get_data.xml" xpointer="ENOENT"/>
+ <xi:include href="sd_journal_get_data.xml" xpointer="ENOBUFS"/>
+ <xi:include href="sd_journal_get_data.xml" xpointer="E2BIG"/>
+ <xi:include href="sd_journal_get_data.xml" xpointer="EPROTONOSUPPORT"/>
+ <xi:include href="sd_journal_get_data.xml" xpointer="EBADMSG"/>
+ <xi:include href="sd_journal_get_data.xml" xpointer="EIO"/>
+ </variablelist>
+ </refsect2>
+ </refsect1>
+
+ <refsect1>
+ <title>Notes</title>
+
+ <xi:include href="threads-aware.xml" xpointer="strict"/>
+
+ <xi:include href="libsystemd-pkgconfig.xml" xpointer="pkgconfig-text"/>
+ </refsect1>
+
+ <refsect1>
+ <title>Examples</title>
+
+ <para>Use the <function>SD_JOURNAL_FOREACH_UNIQUE()</function> macro to iterate through all values a field
+ of the journal can take (and which can be accessed on the given architecture and are not compressed with
+ an unsupported mechanism). The following example lists all unit names referenced in the journal:</para>
+
+ <programlisting><xi:include href="journal-iterate-unique.c" parse="text" /></programlisting>
+ </refsect1>
+
+ <refsect1>
+ <title>See Also</title>
+
+ <para>
+ <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>sd_journal_open</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>sd_journal_enumerate_fields</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>sd_journal_get_data</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+ <citerefentry><refentrytitle>sd_journal_add_match</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+ </para>
+ </refsect1>
+
+</refentry>