215 lines
9.5 KiB
XML
215 lines
9.5 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!--
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This file is part of systemd.
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Copyright 2012 Lennart Poettering
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systemd is free software; you can redistribute it and/or modify it
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under the terms of the GNU Lesser General Public License as published by
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the Free Software Foundation; either version 2.1 of the License, or
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(at your option) any later version.
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systemd is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with systemd; If not, see <http://www.gnu.org/licenses/>.
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-->
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<refentry id="sd_journal_next">
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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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<authorgroup>
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<author>
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<contrib>Developer</contrib>
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<firstname>Lennart</firstname>
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<surname>Poettering</surname>
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<email>lennart@poettering.net</email>
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</author>
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</authorgroup>
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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_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_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
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the read pointer into the journal by one entry. The
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only argument taken is a journal context object as
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allocated via
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<citerefentry><refentrytitle>sd_journal_open</refentrytitle><manvolnum>3</manvolnum></citerefentry>. After
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successful invocation the entry may be read with
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functions such as
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<citerefentry><refentrytitle>sd_journal_get_data</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
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<para>Similar, <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_next_skip()</function> and
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<function>sd_journal_previous_skip()</function>
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advance/set back the read pointer by multiple entries
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at once, as specified in the <varname>skip</varname>
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parameter.</para>
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<para>The journal is strictly ordered by reception
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time, and hence advancing to the next entry guarantees
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that the entry then pointing to is later in time than
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then previous one, or has the 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
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invoked at least once in order to position the read
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pointer on a journal entry.</para>
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<para>Note that the
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<function>SD_JOURNAL_FOREACH()</function> macro may be used
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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
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make iterating through the journal easier. See below
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for an example. Similar,
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<function>SD_JOURNAL_FOREACH_BACKWARDS()</function>
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may be used for iterating the journal in reverse
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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
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advanced/set back on success or a negative errno-style
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error code. When the end or beginning of the journal
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is reached, a number smaller than requested is
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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
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end/beginning of the journal they will return 0,
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instead of 1 when they are successful. This should be
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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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<para>The <function>sd_journal_next()</function>, <function>sd_journal_previous()</function>,
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<function>sd_journal_next_skip()</function> and
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<function>sd_journal_previous_skip()</function> interfaces are
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available as shared library, which can be compiled and
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linked to with the
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<literal>libsystemd-journal</literal>
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<citerefentry><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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file.</para>
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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>#include <stdio.h>
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#include <string.h>
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#include <systemd/sd-journal.h>
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int main(int argc, char *argv[]) {
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int r;
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sd_journal *j;
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r = sd_journal_open(&j, SD_JOURNAL_LOCAL_ONLY);
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if (r < 0) {
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fprintf(stderr, "Failed to open journal: %s\n", strerror(-r));
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return 1;
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}
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SD_JOURNAL_FOREACH(j) {
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const char *d;
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size_t l;
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r = sd_journal_get_data(j, "MESSAGE", &d, &l);
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if (r < 0) {
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fprintf(stderr, "Failed to read message field: %s\n", strerror(-r));
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continue;
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}
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printf("%.*s\n", (int) l, d);
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}
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sd_journal_close(j);
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return 0;
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}</programlisting>
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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_get_data</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_journal_get_realtime_usec</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_journal_get_cursor</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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