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 an 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</constant> <citerefentry project='die-net'><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 length;
  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>