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+<?xml version='1.0'?> <!--*-nxml-*-->
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
+        "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
+
+<!--
+  This file is part of systemd.
+
+  Copyright 2012 Lennart Poettering
+
+  systemd is free software; you can redistribute it and/or modify it
+  under the terms of the GNU Lesser General Public License as published by
+  the Free Software Foundation; either version 2.1 of the License, or
+  (at your option) any later version.
+
+  systemd is distributed in the hope that it will be useful, but
+  WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+  Lesser General Public License for more details.
+
+  You should have received a copy of the GNU Lesser General Public License
+  along with systemd; If not, see <http://www.gnu.org/licenses/>.
+-->
+
+<refentry id="sd_journal_query_unique">
+
+        <refentryinfo>
+                <title>sd_journal_query_unique</title>
+                <productname>systemd</productname>
+
+                <authorgroup>
+                        <author>
+                                <contrib>Developer</contrib>
+                                <firstname>Lennart</firstname>
+                                <surname>Poettering</surname>
+                                <email>lennart@poettering.net</email>
+                        </author>
+                </authorgroup>
+        </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_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_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>. Field
+                names must be specified without a trailing '='. After
+                this function has been executed successfully the field
+                values may be queried using
+                <function>sd_journal_enumerate_unique()</function>. Invoking
+                this call a second time 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 context object, plus a pair of pointers to
+                pointer/size variables where the data object and its
+                size shall be stored in. 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 '='.</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_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>
+        </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>
+                returns a positive integer if the next field data has
+                been read, 0 when no more fields are known, or a
+                negative errno-style error
+                code. <function>sd_journal_restart_unique()</function>
+                returns nothing.</para>
+        </refsect1>
+
+        <refsect1>
+                <title>Notes</title>
+
+                <para>The <function>sd_journal_query_unique()</function>,
+                <function>sd_journal_enumerate_unique()</function> and
+                <function>sd_journal_restart_unique()</function>
+                interfaces are available as shared library, which can
+                be compiled and linked to with the
+                <literal>libsystemd-journal</literal>
+                <citerefentry><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
+                file.</para>
+        </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. The following example lists all unit names
+                referenced in the journal:</para>
+
+                <programlisting>#include &lt;stdio.h&gt;
+#include &lt;string.h&gt;
+#include &lt;systemd/sd-journal.h&gt;
+
+int main(int argc, char *argv[]) {
+        sd_journal *j;
+        const char *d;
+        size_t l;
+        int r;
+
+        r = sd_journal_open(&amp;j, SD_JOURNAL_LOCAL_ONLY);
+        if (r &lt; 0) {
+                fprintf(stderr, "Failed to open journal: %s\n", strerror(-r));
+                return 1;
+        }
+        r = sd_journal_query_unique(j, "_SYSTEMD_UNIT");
+        if (r &lt; 0) {
+                fprintf(stderr, "Failed to query journal: %s\n", strerror(-r));
+                return 1;
+        }
+        SD_JOURNAL_FOREACH_UNIQUE(j, d, l)
+                printf("%.*s\n", (int) l, d);
+        sd_journal_close(j);
+        return 0;
+}</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_get_data</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+                        <citerefentry><refentrytitle>sd_journal_add_match</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+                </para>
+        </refsect1>
+
+</refentry>