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111 lines
5.2 KiB
XML
111 lines
5.2 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
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<refentry id="sd_journal_get_seqnum"
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xmlns:xi="http://www.w3.org/2001/XInclude">
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<refentryinfo>
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<title>sd_journal_get_seqnum</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>sd_journal_get_seqnum</refentrytitle>
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<manvolnum>3</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>sd_journal_get_seqnum</refname>
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<refpurpose>Read sequence number from the current journal entry</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<funcsynopsis>
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<funcsynopsisinfo>#include <systemd/sd-journal.h></funcsynopsisinfo>
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<funcprototype>
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<funcdef>int <function>sd_journal_get_seqnum</function></funcdef>
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<paramdef>sd_journal *<parameter>j</parameter></paramdef>
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<paramdef>uint64_t *<parameter>ret_seqnum</parameter></paramdef>
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<paramdef>sd_id128_t *<parameter>ret_seqnum_id</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para><function>sd_journal_get_seqnum()</function> returns the sequence number of the current journal
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entry. It takes three arguments: the journal context object, a pointer to a 64-bit unsigned integer to
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store the sequence number in, and a buffer to return the 128-bit sequence number ID in.</para>
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<para>When writing journal entries to disk each <command>systemd-journald</command> instance will number
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them sequentially, starting from 1 for the first entry written after subsystem initialization. Each such
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series of sequence numbers is associated with a 128-bit sequence number ID which is initialized randomly,
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once at <command>systemd-journal</command> initialization. Thus, while multiple instances of
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<command>systemd-journald</command> will assign the same sequence numbers to their written journal
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entries, they will have a distinct sequence number IDs. The sequence number is assigned at the moment of
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writing the entry to disk. If log entries are rewritten (for example because the volatile logs from
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<filename>/run/log/</filename> are flushed to <filename>/var/log/</filename> via
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<filename>systemd-journald-flush.service</filename>) they will get new sequence numbers assigned.</para>
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<para>Sequence numbers may be used to order entries (entries associated with the same sequence number ID
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and lower sequence numbers should be ordered chronologically before those with higher sequence numbers),
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and to detect lost entries. Note that journal service instances typically write to multiple journal files
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in parallel (for example because <varname>SplitMode=</varname> is used), in which case each journal file
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will only contain a subset of the sequence numbers. To recover the full stream of journal entries the
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files must be combined ("interleaved"), a process that primarily relies on the sequence numbers. When
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journal files are rotated (due to size or time limits), the series of sequence numbers is continued in
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the replacement files. All journal files generated from the same journal instance will carry the same
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sequence number ID.</para>
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<para>As the sequence numbers are assigned at the moment of writing the journal entries to disk they do
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not exist if storage is disabled via <varname>SplitMode=</varname>.</para>
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<para>The <varname>ret_seqnum</varname> and <varname>ret_seqnum_id</varname> parameters may be specified
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as <constant>NULL</constant> in which case the relevant data is not returned (but the call will otherwise
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succeed).</para>
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<para>Note that these functions will not work before
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<citerefentry><refentrytitle>sd_journal_next</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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(or related call) has been called at least
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once, in order to position the read pointer at a valid entry.</para>
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</refsect1>
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<refsect1>
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<title>Return Value</title>
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<para><function>sd_journal_get_seqnum()</function> returns 0 on success or a negative errno-style error
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code..</para>
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</refsect1>
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<refsect1>
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<title>Notes</title>
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<xi:include href="threads-aware.xml" xpointer="strict"/>
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<xi:include href="libsystemd-pkgconfig.xml" xpointer="pkgconfig-text"/>
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</refsect1>
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<refsect1>
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<title>History</title>
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<para><function>sd_journal_get_seqnum()</function> was added in version 254.</para>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<para>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_journal_open</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_journal_next</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_journal_get_data</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_journal_get_monotonic_usec</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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