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systemd/man/sd_journal_query_unique.xml
Lennart Poettering 93b73b064c journal: by default do not decompress dat objects larger than 64K 
This introduces a new data threshold setting for sd_journal objects
which controls the maximum size of objects to decompress. This is
relieves the library from having to decompress full data objects even
if a client program is only interested in the initial part of them.

This speeds up "systemd-coredumpctl" drastically when invoked without
parameters.
2012-11-21 00:28:00 +01:00

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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">
<!--
This file is part of systemd.
Copyright 2012 Lennart Poettering
systemd is free software; you can redistribute it and/or modify it
under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 2.1 of the License, or
(at your option) any later version.
systemd is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public License
along with systemd; If not, see <http://www.gnu.org/licenses/>.
-->
<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 shared library, which can
be compiled and linked to with the
<literal>libsystemd-journal</literal>
<citerefentry><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>
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