man/sd_journal_get_data.xml - main - niceSOFT/systemd

man/sd_journal_get_data.xml
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noxiouz
sd-journal: make sd_journal_get_data() output params optional
08 апр 2026, 17:33
08 апр 2026, 17:33329233d
<?xml version='1.0'?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
  "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->

<refentry id="sd_journal_get_data" xmlns:xi="http://www.w3.org/2001/XInclude">

  <refentryinfo>
    <title>sd_journal_get_data</title>
    <productname>systemd</productname>
  </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_enumerate_available_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>int <function>sd_journal_enumerate_available_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 optional pointers to pointer/size variables where the data object and
    its size shall be stored in. Either pointer may be <constant>NULL</constant>, in which case the corresponding
    value is not returned (this is supported since version 261). 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>,
    but any field can be specified. 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>,
    <function>sd_journal_enumerate_data()</function>,
    <function>sd_journal_enumerate_available_data()</function>, or when the read pointer is altered. Note
    that the data returned will be prefixed with the field name and <literal>=</literal>. 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_enumerate_available_data()</function> is similar to
    <function>sd_journal_enumerate_data()</function>, but silently skips any fields which may be valid, but
    are too large or not supported by current implementation.</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_available_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. <function>sd_journal_enumerate_data()</function> and
    <function>sd_journal_enumerate_available_data()</function> return a positive integer if the next field
    has been read, 0 when no more fields remain, or a negative errno-style error code.
    <function>sd_journal_restart_data()</function> does not return anything.
    <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>

    <refsect2>
      <title>Errors</title>

      <para>Returned errors may indicate the following problems:</para>

      <variablelist>
        <varlistentry id='EINVAL'>
          <term><constant>-EINVAL</constant></term>

          <listitem><para>One of the required parameters is <constant>NULL</constant> or invalid.
          For <function>sd_journal_get_data()</function>, only the journal context object and field name
          are required non-<constant>NULL</constant>. For <function>sd_journal_enumerate_data()</function>
          and <function>sd_journal_enumerate_available_data()</function>,
          <parameter>ret_data</parameter> and <parameter>ret_size</parameter> are required as well.
          </para>

          <xi:include href="version-info.xml" xpointer="v246"/></listitem>
        </varlistentry>

        <varlistentry id='ECHILD'>
          <term><constant>-ECHILD</constant></term>

          <listitem><para>The journal object was created in a different process, library or module instance.</para>

          <xi:include href="version-info.xml" xpointer="v246"/></listitem>
        </varlistentry>

        <varlistentry id='EADDRNOTAVAIL'>
          <term><constant>-EADDRNOTAVAIL</constant></term>

          <listitem><para>The read pointer is not positioned at a valid entry;
          <citerefentry><refentrytitle>sd_journal_next</refentrytitle><manvolnum>3</manvolnum></citerefentry>
          or a related call has not been called at least once.</para>

          <xi:include href="version-info.xml" xpointer="v246"/></listitem>
        </varlistentry>

        <varlistentry id='ENOENT'>
          <term><constant>-ENOENT</constant></term>

          <listitem><para>The current entry does not include the specified field.</para>

          <xi:include href="version-info.xml" xpointer="v246"/>
          </listitem>
        </varlistentry>

        <varlistentry id='ENOMEM'>
          <term><constant>-ENOMEM</constant></term>

          <listitem><para>Memory allocation failed.</para>

          <xi:include href="version-info.xml" xpointer="v246"/></listitem>
        </varlistentry>

        <varlistentry id='ENOBUFS'>
          <term><constant>-ENOBUFS</constant></term>

          <listitem><para>A compressed entry is too large.</para>

          <xi:include href="version-info.xml" xpointer="v246"/></listitem>
        </varlistentry>

        <varlistentry id='E2BIG'>
          <term><constant>-E2BIG</constant></term>

          <listitem><para>The data field is too large for this computer architecture (e.g. above 4 GB on a
          32-bit architecture).</para>

          <xi:include href="version-info.xml" xpointer="v246"/></listitem>
        </varlistentry>

        <varlistentry id='EPROTONOSUPPORT'>
          <term><constant>-EPROTONOSUPPORT</constant></term>

          <listitem><para>The journal is compressed with an unsupported method or the journal uses an
          unsupported feature.</para>

          <xi:include href="version-info.xml" xpointer="v246"/></listitem>
        </varlistentry>

        <varlistentry id='EBADMSG'>
          <term><constant>-EBADMSG</constant></term>

          <listitem><para>The journal is corrupted (possibly just the entry being iterated over).
          </para>

          <xi:include href="version-info.xml" xpointer="v246"/></listitem>
        </varlistentry>

        <varlistentry id='EIO'>
          <term><constant>-EIO</constant></term>

          <listitem><para>An I/O error was reported by the kernel.</para>

          <xi:include href="version-info.xml" xpointer="v246"/></listitem>
        </varlistentry>
      </variablelist>
    </refsect2>
  </refsect1>

  <refsect1>
    <title>Notes</title>

    <xi:include href="threads-aware.xml" xpointer="strict"/>

    <xi:include href="libsystemd-pkgconfig.xml" xpointer="pkgconfig-text"/>
  </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>History</title>
    <para><function>sd_journal_get_data()</function>,
    <function>sd_journal_enumerate_data()</function>,
    <function>sd_journal_restart_data()</function>, and
    <function>SD_JOURNAL_FOREACH_DATA()</function> were added in version 187.</para>
    <para><function>sd_journal_set_data_threshold()</function> and
    <function>sd_journal_get_data_threshold()</function> were added in version 196.</para>
    <para><function>sd_journal_enumerate_available_data()</function> was added in version 246.</para>
  </refsect1>

  <refsect1>
    <title>See Also</title>

    <para><simplelist type="inline">
      <member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>sd_journal_open</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>sd_journal_next</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>sd_journal_get_realtime_usec</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
      <member><citerefentry><refentrytitle>sd_journal_query_unique</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
    </simplelist></para>
  </refsect1>

</refentry>