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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>