man: document sd_journal_add_match()

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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_add_match">
+
+        <refentryinfo>
+                <title>sd_journal_add_match</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_add_match</refentrytitle>
+                <manvolnum>3</manvolnum>
+        </refmeta>
+
+        <refnamediv>
+                <refname>sd_journal_add_match</refname>
+                <refname>sd_journal_add_disjunction</refname>
+                <refname>sd_journal_flush_matches</refname>
+                <refpurpose>Add or remove entry matches</refpurpose>
+        </refnamediv>
+
+        <refsynopsisdiv>
+                <funcsynopsis>
+                        <funcsynopsisinfo>#include &lt;systemd/sd-journal.h&gt;</funcsynopsisinfo>
+
+                        <funcprototype>
+                                <funcdef>int <function>sd_journal_add_match</function></funcdef>
+                                <paramdef>sd_journal* <parameter>j</parameter></paramdef>
+                                <paramdef>const void* <parameter>data</parameter></paramdef>
+                                <paramdef>size_t <parameter>size</parameter></paramdef>
+                        </funcprototype>
+
+                        <funcprototype>
+                                <funcdef>int <function>sd_journal_add_disjunction</function></funcdef>
+                                <paramdef>sd_journal* <parameter>j</parameter></paramdef>
+                        </funcprototype>
+
+                        <funcprototype>
+                                <funcdef>int <function>sd_journal_flush_matches</function></funcdef>
+                                <paramdef>sd_journal* <parameter>j</parameter></paramdef>
+                        </funcprototype>
+                </funcsynopsis>
+        </refsynopsisdiv>
+
+        <refsect1>
+                <title>Description</title>
+
+                <para><function>sd_journal_add_match()</function> adds
+                a match by which to filter the entries of the journal
+                file. Matches applied with this call will filter what
+                can be iterated through and read from the journal file
+                via calls like
+                <citerefentry><refentrytitle>sd_journal_next</refentrytitle><manvolnum>3</manvolnum></citerefentry>
+                and
+                <citerefentry><refentrytitle>sd_journal_get_data</refentrytitle><manvolnum>3</manvolnum></citerefentry>. Matches
+                are of the form <literal>FIELD=value</literal>, where
+                the field part is a short uppercase string consisting
+                only of 0-9, A-Z and the underscore. It may not begin
+                with two underscores or be the empty string. The value
+                part may be any value, including binary. If a match is
+                applied only entries with this field set will be
+                iterated. Multiple matches may be active at the same
+                time: if they apply to different fields only entries
+                with both fields set like this will be iterated, if
+                they apply to the same fields only entries where the
+                field takes one of the specified values will be
+                iterated. Well known fields are documented in
+                <citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>. Whenever
+                a new match is added the current entry position is
+                reset, and
+                <citerefentry><refentrytitle>sd_journal_next</refentrytitle><manvolnum>3</manvolnum></citerefentry> (or a similar call)
+                needs to be called before entries can be read
+                again.</para>
+
+                <para><function>sd_journal_add_disjunction()</function>
+                may be used to insert a disjunction (i.e. logical OR)
+                in the match list. If this call is invoked all
+                previously added matches are combined in an OR with
+                all matches added afterwards, until
+                <function>sd_journal_add_disjunction()</function> is
+                invoked again to begin the next OR term. The
+                combination of
+                <function>sd_journal_add_match()</function> and
+                <function>sd_journal_add_disjunction()</function> may
+                be used to build complex search terms, even though
+                full logical expressions are not available.</para>
+
+                <para><function>sd_journal_flush_matches()</function>
+                may be used to flush all matches and disjunction terms
+                again. After this call all filtering is removed and
+                all entries in the journal will be iterated
+                again.</para>
+
+                <para>Note that filtering via matches only applies to
+                way the journal is read, it has no effect on storage
+                on disk.</para>
+        </refsect1>
+
+        <refsect1>
+                <title>Return Value</title>
+
+                <para><function>sd_journal_add_match()</function> and
+                <function>sd_journal_add_disjunction()</function>
+                return 0 on success or a negative errno-style error
+                code. <function>sd_journal_flush_matches()</function>
+                returns nothing.</para>
+        </refsect1>
+
+        <refsect1>
+                <title>Notes</title>
+
+                <para>The <function>sd_journal_add_match()</function>,
+                <function>sd_journal_add_disjunction()</function> and
+                <function>sd_journal_flush_matches()</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>The following example adds matches to a journal
+                context object to iterate only through messages
+                generated by the Avahi service at the four error log
+                levels, plus all messages of the message ID
+                03bb1dab98ab4ecfbf6fff2738bdd964 coming from any
+                service (this example lacks the necessary error
+                checking):</para>
+
+                <programlisting>...
+int add_matches(sd_journal *j) {
+        sd_journal_add_match(j, "_SYSTEMD_UNIT=avahi-daemon.service");
+        sd_journal_add_match(j, "PRIORITY=0");
+        sd_journal_add_match(j, "PRIORITY=1");
+        sd_journal_add_match(j, "PRIORITY=2");
+        sd_journal_add_match(j, "PRIORITY=3");
+        sd_journal_add_disjunction(j);
+        sd_journal_add_match(j, "MESSAGE_ID=03bb1dab98ab4ecfbf6fff2738bdd964");
+}</programlisting>
+
+        </refsect1>
+
+        <refsect1>
+                <title>See Also</title>
+
+                <para>
+                        <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</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_next</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+                        <citerefentry><refentrytitle>sd_journal_get_data</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
+                        <citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>
+                </para>
+        </refsect1>
+
+</refentry>