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<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_add_conjunction</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_add_conjunction</function></funcdef>
                                <paramdef>sd_journal *<parameter>j</parameter></paramdef>
                        </funcprototype>

                        <funcprototype>
                                <funcdef>void <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 since the last invocation of
                <function>sd_journal_add_disjunction()</function> or
                <function>sd_journal_add_conjunction()</function> are
                combined in an OR with all matches added afterwards,
                until
                <function>sd_journal_add_disjunction()</function> or
                <function>sd_journal_add_conjunction()</function> is
                invoked again to begin the next OR or AND
                term. </para>

                <para><function>sd_journal_add_conjunction()</function>
                may be used to insert a conjunction (i.e. logical AND)
                in the match list. If this call is invoked, all
                previously added matches since the last invocation of
                <function>sd_journal_add_conjunction()</function> are
                combined in an AND with all matches added afterwards,
                until
                <function>sd_journal_add_conjunction()</function> is
                invoked again to begin the next AND term. The
                combination of
                <function>sd_journal_add_match()</function>,
                <function>sd_journal_add_disjunction()</function> and
                <function>sd_journal_add_conjunction()</function> may
                be used to build complex search terms, even though
                full logical expressions are not available. Note that
                <function>sd_journal_add_conjunction()</function>
                operates one level 'higher' than
                <function>sd_journal_add_disjunction()</function>. It
                is hence possible to build an expression of AND terms,
                consisting of OR terms, consisting of AND terms,
                consisting of OR terms of matches (the latter OR
                expression is implicitly created for matches with the
                same field name, see above).</para>

                <para><function>sd_journal_flush_matches()</function>
                may be used to flush all matches, disjunction and
                conjunction 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
                the 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>,
                <function>sd_journal_add_disjunction()</function> and
                <function>sd_journal_add_conjunction()</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>,
                <function>sd_journal_add_conjunction()</function> and
                <function>sd_journal_flush_matches()</function>
                interfaces are available as a shared library, which can
                be compiled and linked to with the
                <constant>libsystemd</constant> <citerefentry project='die-net'><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", 0);
        sd_journal_add_match(j, "PRIORITY=0", 0);
        sd_journal_add_match(j, "PRIORITY=1", 0);
        sd_journal_add_match(j, "PRIORITY=2", 0);
        sd_journal_add_match(j, "PRIORITY=3", 0);
        sd_journal_add_disjunction(j);
        sd_journal_add_match(j, "MESSAGE_ID=03bb1dab98ab4ecfbf6fff2738bdd964", 0);
}</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>
