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177 lines
7.6 KiB
XML
177 lines
7.6 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/4.5/docbookx.dtd">
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<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
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<refentry id="sd_journal_next" xmlns:xi="http://www.w3.org/2001/XInclude">
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<refentryinfo>
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<title>sd_journal_next</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_next</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_next</refname>
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<refname>sd_journal_previous</refname>
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<refname>sd_journal_step_one</refname>
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<refname>sd_journal_next_skip</refname>
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<refname>sd_journal_previous_skip</refname>
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<refname>SD_JOURNAL_FOREACH</refname>
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<refname>SD_JOURNAL_FOREACH_BACKWARDS</refname>
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<refpurpose>Advance or set back the read pointer in the journal</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_next</function></funcdef>
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<paramdef>sd_journal *<parameter>j</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_journal_previous</function></funcdef>
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<paramdef>sd_journal *<parameter>j</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_journal_step_one</function></funcdef>
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<paramdef>sd_journal *<parameter>j</parameter></paramdef>
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<paramdef>int <parameter>advanced</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_journal_next_skip</function></funcdef>
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<paramdef>sd_journal *<parameter>j</parameter></paramdef>
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<paramdef>uint64_t <parameter>skip</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_journal_previous_skip</function></funcdef>
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<paramdef>sd_journal *<parameter>j</parameter></paramdef>
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<paramdef>uint64_t <parameter>skip</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef><function>SD_JOURNAL_FOREACH</function></funcdef>
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<paramdef>sd_journal *<parameter>j</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef><function>SD_JOURNAL_FOREACH_BACKWARDS</function></funcdef>
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<paramdef>sd_journal *<parameter>j</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_next()</function> advances the read
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pointer into the journal by one entry. The only argument taken is
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a journal context object as allocated via
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<citerefentry><refentrytitle>sd_journal_open</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
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After successful invocation the entry may be read with functions
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such as
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<citerefentry><refentrytitle>sd_journal_get_data</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
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<para>Similarly, <function>sd_journal_previous()</function> sets
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the read pointer back one entry.</para>
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<para><function>sd_journal_step_one()</function> also moves the read pointer. If the current location
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is the head of the journal, e.g. when this is called following
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<function>sd_journal_seek_head()</function>, then this is equivalent to
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<function>sd_journal_next()</function>, and the argument <varname>advanced</varname> will be ignored.
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Similarly, if the current location is the tail of the journal, e.g. when this is called following
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<function>sd_journal_seek_tail()</function>, then this is equivalent to
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<function>sd_journal_previous()</function>, and <varname>advanced</varname> will be ignored. Otherwise,
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this is equivalent to <function>sd_journal_next()</function> when <varname>advanced</varname> is
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non-zero, and <function>sd_journal_previous()</function> when <varname>advanced</varname> is zero.</para>
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<para><function>sd_journal_next_skip()</function> and
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<function>sd_journal_previous_skip()</function> advance/set back the read pointer by multiple
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entries at once, as specified in the <varname>skip</varname> parameter. The <varname>skip</varname>
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parameter must be less than or equal to 2147483647 (2³¹-1).</para>
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<para>The journal is strictly ordered by reception time, and hence
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advancing to the next entry guarantees that the entry then
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pointing to is later in time than then previous one, or has the
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same timestamp.</para>
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<para>Note that
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<citerefentry><refentrytitle>sd_journal_get_data</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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and related calls will fail unless
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<function>sd_journal_next()</function> has been invoked at least
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once in order to position the read pointer on a journal
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entry.</para>
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<para>Note that the <function>SD_JOURNAL_FOREACH()</function>
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macro may be used as a wrapper around
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<citerefentry><refentrytitle>sd_journal_seek_head</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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and <function>sd_journal_next()</function> in order to make
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iterating through the journal easier. See below for an example.
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Similarly, <function>SD_JOURNAL_FOREACH_BACKWARDS()</function> may
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be used for iterating the journal in reverse order.</para>
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</refsect1>
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<refsect1>
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<title>Return Value</title>
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<para>The four calls return the number of entries advanced/set
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back on success or a negative errno-style error code. When the end
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or beginning of the journal is reached, a number smaller than
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requested is returned. More specifically, if
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<function>sd_journal_next()</function> or
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<function>sd_journal_previous()</function> reach the end/beginning
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of the journal they will return 0, instead of 1 when they are
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successful. This should be considered an EOF marker.</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>Examples</title>
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<para>Iterating through the journal:</para>
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<programlisting><xi:include href="journal-iterate-foreach.c" parse="text" /></programlisting>
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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_next()</function>,
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<function>sd_journal_previous()</function>,
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<function>sd_journal_next_skip()</function>,
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<function>sd_journal_previous_skip()</function>,
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<function>SD_JOURNAL_FOREACH()</function>, and
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<function>SD_JOURNAL_FOREACH_BACKWARDS()</function> were added in version 187.</para>
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<para><function>sd_journal_step_one()</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><simplelist type="inline">
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<member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>sd_journal_open</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>sd_journal_get_data</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>sd_journal_get_realtime_usec</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
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<member><citerefentry><refentrytitle>sd_journal_get_cursor</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
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</simplelist></para>
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</refsect1>
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</refentry>
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