systemd/man/sd_journal_get_data.xml
Lennart Poettering 3c1668da62 journal: add ability to list values a specified field can take in all entries of the journal
The new 'unique' API allows listing all unique field values that a field
specified by a field name can take in all entries of the journal. This
allows answering queries such as "What units logged to the journal?",
"What hosts have logged into the journal?", "Which boot IDs have logged
into the journal?".

Ultimately this allows implementation of tools similar to lastlog based
on journal data.

Note that listing these field values will not work for journal files
created with older journald, as the field values are not indexed in
older files.
2012-10-18 03:35:18 +02:00

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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">
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<refentry id="sd_journal_get_data">
<refentryinfo>
<title>sd_journal_get_data</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_get_data</refentrytitle>
<manvolnum>3</manvolnum>
</refmeta>
<refnamediv>
<refname>sd_journal_get_data</refname>
<refname>sd_journal_enumerate_data</refname>
<refname>sd_journal_restart_data</refname>
<refname>SD_JOURNAL_FOREACH_DATA</refname>
<refpurpose>Read data fields from the current journal entry</refpurpose>
</refnamediv>
<refsynopsisdiv>
<funcsynopsis>
<funcsynopsisinfo>#include &lt;systemd/sd-journal.h&gt;</funcsynopsisinfo>
<funcprototype>
<funcdef>int <function>sd_journal_get_data</function></funcdef>
<paramdef>sd_journal* <parameter>j</parameter></paramdef>
<paramdef>const char* <parameter>field</parameter></paramdef>
<paramdef>const void** <parameter>data</parameter></paramdef>
<paramdef>size_t* <parameter>length</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>int <function>sd_journal_enumerate_data</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_data</function></funcdef>
<paramdef>sd_journal* <parameter>j</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef><function>SD_JOURNAL_FOREACH_DATA</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_get_data()</function> gets
the data object associated with a specific field from
the current journal entry. It takes four arguments:
the journal context object, a string with the field
name to request, plus a pair of pointers to
pointer/size variables where the data object and its
size shall be stored in. The field name should be an
entry field name. Well-known field names are listed in
<citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>. The
returned data is in a read-only memory map and is only
valid until the next invocation of
<function>sd_journal_get_data()</function> or
<function>sd_journal_enumerate_data()</function>, or
the read pointer is altered. Note that the data
returned will be prefixed with the field name and
'='.</para>
<para><function>sd_journal_enumerate_data()</function>
may be used to iterate through all fields of the
current entry. On each invocation the data for the
next field is returned. The order of these fields is
not defined. The data returned is in the same format
as with <function>sd_journal_get_data()</function> and
also follows the same life-time semantics.</para>
<para><function>sd_journal_restart_data()</function>
resets the data enumeration index to the beginning of
the entry. The next invocation of
<function>sd_journal_enumerate_data()</function> will return the first
field of the entry again.</para>
<para>Note that the
<function>SD_JOURNAL_FOREACH_DATA()</function> macro
may be used as a handy wrapper around
<function>sd_journal_restart_data()</function> and
<function>sd_journal_enumerate_data()</function>.</para>
<para>Note that these functions will not work before
<citerefentry><refentrytitle>sd_journal_next</refentrytitle><manvolnum>3</manvolnum></citerefentry>
(or related call) has been called at least
once, in order to position the read pointer at a valid entry.</para>
</refsect1>
<refsect1>
<title>Return Value</title>
<para><function>sd_journal_get_data()</function>
returns 0 on success or a negative errno-style error
code. If the current entry does not include the
specified field -ENOENT is returned. If
<citerefentry><refentrytitle>sd_journal_next</refentrytitle><manvolnum>3</manvolnum></citerefentry>
has not been called at least once -EADDRNOTAVAIL is
returned. <function>sd_journal_enumerate_data()</function>
returns a positive integer if the next field has been
read, 0 when no more fields are known, or a negative
errno-style error
code. <function>sd_journal_restart_data()</function>
returns nothing.</para>
</refsect1>
<refsect1>
<title>Notes</title>
<para>The <function>sd_journal_get_data()</function>,
<function>sd_journal_enumerate_data()</function> and
<function>sd_journal_restart_data()</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>See
<citerefentry><refentrytitle>sd_journal_next</refentrytitle><manvolnum>3</manvolnum></citerefentry>
for a complete example how to use
<function>sd_journal_get_data()</function>.</para>
<para>Use the
<function>SD_JOURNAL_FOREACH_DATA</function> macro to
iterate through all fields of the current journal
entry:</para>
<programlisting>...
int print_fields(sd_journal *j) {
const void *data;
size_t l;
SD_JOURNAL_FOREACH_DATA(j, data, length)
printf("%.*s\n", (int) length, data);
}
...</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_next</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_journal_get_realtime_usec</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_journal_query_unique</refentrytitle><manvolnum>3</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>