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<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 '='. Note
    that this call is subject to the data field size threshold as
    controlled by
    <function>sd_journal_set_data_threshold()</function>.</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>

    <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> 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>All functions listed here are thread-agnostic and only a single thread may operate
    on a given <structname>sd_journal</structname> object.</para>

    <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 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>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 void *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, (const char*) 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_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>