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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">
<!--
SPDX-License-Identifier: LGPL-2.1+
-->
<refentry id="sd_journal_query_unique" xmlns:xi="http://www.w3.org/2001/XInclude">
<refentryinfo>
<title>sd_journal_query_unique</title>
<productname>systemd</productname>
</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 <systemd/sd-journal.h></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>
<xi:include href="threads-aware.xml" xpointer="strict"/>
<xi:include href="libsystemd-pkgconfig.xml" xpointer="pkgconfig-text"/>
</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><xi:include href="journal-iterate-unique.c" parse="text" /></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>
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