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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->

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

  <refentryinfo>
    <title>sd_journal_get_seqnum</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>sd_journal_get_seqnum</refentrytitle>
    <manvolnum>3</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>sd_journal_get_seqnum</refname>
    <refpurpose>Read sequence number 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_seqnum</function></funcdef>
        <paramdef>sd_journal *<parameter>j</parameter></paramdef>
        <paramdef>uint64_t *<parameter>ret_seqnum</parameter></paramdef>
        <paramdef>sd_id128_t *<parameter>ret_seqnum_id</parameter></paramdef>
      </funcprototype>
    </funcsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para><function>sd_journal_get_seqnum()</function> returns the sequence number of the current journal
    entry. It takes three arguments: the journal context object, a pointer to a 64-bit unsigned integer to
    store the sequence number in, and a buffer to return the 128-bit sequence number ID in.</para>

    <para>When writing journal entries to disk each <command>systemd-journald</command> instance will number
    them sequentially, starting from 1 for the first entry written after subsystem initialization. Each such
    series of sequence numbers is associated with a 128-bit sequence number ID which is initialized randomly,
    once at <command>systemd-journal</command> initialization. Thus, while multiple instances of
    <command>systemd-journald</command> will assign the same sequence numbers to their written journal
    entries, they will have a distinct sequence number IDs. The sequence number is assigned at the moment of
    writing the entry to disk. If log entries are rewritten (for example because the volatile logs from
    <filename>/run/log/</filename> are flushed to <filename>/var/log/</filename> via
    <filename>systemd-journald-flush.service</filename>) they will get new sequence numbers assigned.</para>

    <para>Sequence numbers may be used to order entries (entries associated with the same sequence number ID
    and lower sequence numbers should be ordered chronologically before those with higher sequence numbers),
    and to detect lost entries. Note that journal service instances typically write to multiple journal files
    in parallel (for example because <varname>SplitMode=</varname> is used), in which case each journal file
    will only contain a subset of the sequence numbers. To recover the full stream of journal entries the
    files must be combined ("interleaved"), a process that primarily relies on the sequence numbers. When
    journal files are rotated (due to size or time limits), the series of sequence numbers is continued in
    the replacement files. All journal files generated from the same journal instance will carry the same
    sequence number ID.</para>

    <para>As the sequence numbers are assigned at the moment of writing the journal entries to disk they do
    not exist if storage is disabled via <varname>SplitMode=</varname>.</para>

    <para>The <varname>ret_seqnum</varname> and <varname>ret_seqnum_id</varname> parameters may be specified
    as <constant>NULL</constant> in which case the relevant data is not returned (but the call will otherwise
    succeed).</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_seqnum()</function> returns 0 on success or a negative errno-style error
    code..</para>
  </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>See Also</title>

    <para>
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</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_data</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_journal_get_monotonic_usec</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>
