systemd/man/sd_journal_get_data.xml

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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>
                <refname>sd_journal_set_data_threshold</refname>
                <refname>sd_journal_get_data_threshold</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>

                        <funcprototype>
                                <funcdef>int <function>sd_journal_set_data_threshold</function></funcdef>
                                <paramdef>sd_journal* <parameter>j</parameter></paramdef>
                                <paramdef>size_t <parameter>sz</parameter></paramdef>
                        </funcprototype>

                        <funcprototype>
                                <funcdef>int <function>sd_journal_get_data_threshold</function></funcdef>
                                <paramdef>sd_journal* <parameter>j</parameter></paramdef>
                                <paramdef>size_t* <parameter>sz</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
                '='. Also note that by default data fields larger than
                64K might get truncated to 64K. This threshold may be
                changed and turned off with
                <function>sd_journal_set_data_threshold()</function> (see
                below).</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>

                <para><function>sd_journal_set_data_threshold()</function>
                may be used to change the data field size threshold
                for data returned by
                <function>sd_journal_get_data()</function>,
                <function>sd_journal_enumerate_data()</function> and
                <function>sd_journal_enumerate_unique()</function>. This
                threshold is a hint only: it indicates that the client
                program is interested only in the initial parts of the
                data fields, up to the threshold in size -- but the
                library might still return larger data objects. That
                means applications should not rely exclusively on this
                setting to limit the size of the data fields returned,
                but need to apply a explicit size limit on the
                returned data as well. This threshold defaults to 64K
                by default. To retrieve the complete data fields this
                threshold should be turned off by setting it to 0, so
                that the library always returns the complete data
                objects. It is recommended to set this threshold as
                low as possible since this relieves the library from
                having to decompress large compressed data objects in
                full.</para>

                <para><function>sd_journal_get_data_threshold()</function>
                returns the currently configured data field size
                threshold.</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. <function>sd_journal_set_data_threshold()</function>
                and <function>sd_journal_get_threshold()</function>
                return 0 on success or a negative errno-style error
                code.</para>
        </refsect1>

        <refsect1>
                <title>Notes</title>

                <para>The <function>sd_journal_get_data()</function>,
                <function>sd_journal_enumerate_data()</function>,
                <function>sd_journal_restart_data()</function>,
                <function>sd_journal_set_data_threshold()</function>
                and
                <function>sd_journal_get_data_threshold()</function>
                interfaces are available as a shared library, which can
                be compiled and linked to with the
                <constant>libsystemd-journal</constant> <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>