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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>
        </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 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_get_data</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
                        <citerefentry><refentrytitle>sd_journal_add_match</refentrytitle><manvolnum>3</manvolnum></citerefentry>
                </para>
        </refsect1>

</refentry>
