man: document size param of sd_journal_add_match

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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>.
    Parameter <parameter>data</parameter> must be of the form
    <literal><replaceable>FIELD</replaceable>=<replaceable>value</replaceable></literal>,
    where the <replaceable>FIELD</replaceable> 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 <replaceable>value</replaceable> part may be anything, including binary. Parameter
    <parameter>size</parameter> specifies the number of bytes in <parameter>data</parameter>
    (i.e. the length of <replaceable>FIELD</replaceable>, plus one, plus the length of
    <replaceable>value</replaceable>). Parameter <parameter>size</parameter> may also be
    specified as <constant>0</constant>, in which case <parameter>data</parameter>
    must be a <constant>NUL</constant>-terminated string, and the bytes before the terminating
    zero are used as the match.</para>

    <para>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>