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<?xml version='1.0'?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!--
This file is part of systemd.
Copyright 2012 Lennart Poettering
systemd is free software; you can redistribute it and/or modify it
under the terms of the GNU Lesser General Public License as published by
the Free Software Foundation; either version 2.1 of the License, or
(at your option) any later version.
systemd is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public License
along with systemd; If not, see <http://www.gnu.org/licenses/>.
-->
<refentry id="sd_journal_next">
<refentryinfo>
<title>sd_journal_next</title>
<productname>systemd</productname>
<authorgroup>
<author>
<contrib>Developer</contrib>
<firstname>Lennart</firstname>
<surname>Poettering</surname>
<email>lennart@poettering.net</email>
</author>
</authorgroup>
</refentryinfo>
<refmeta>
<refentrytitle>sd_journal_next</refentrytitle>
<manvolnum>3</manvolnum>
</refmeta>
<refnamediv>
<refname>sd_journal_next</refname>
<refname>sd_journal_previous</refname>
<refname>sd_journal_next_skip</refname>
<refname>sd_journal_previous_skip</refname>
<refname>SD_JOURNAL_FOREACH</refname>
<refname>SD_JOURNAL_FOREACH_BACKWARDS</refname>
<refpurpose>Advance or set back the read pointer in the journal</refpurpose>
</refnamediv>
<refsynopsisdiv>
<funcsynopsis>
<funcsynopsisinfo>#include <systemd/sd-journal.h></funcsynopsisinfo>
<funcprototype>
<funcdef>int <function>sd_journal_next</function></funcdef>
<paramdef>sd_journal *<parameter>j</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>int <function>sd_journal_previous</function></funcdef>
<paramdef>sd_journal *<parameter>j</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>int <function>sd_journal_next_skip</function></funcdef>
<paramdef>sd_journal *<parameter>j</parameter></paramdef>
<paramdef>uint64_t <parameter>skip</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef>int <function>sd_journal_previous_skip</function></funcdef>
<paramdef>sd_journal *<parameter>j</parameter></paramdef>
<paramdef>uint64_t <parameter>skip</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef><function>SD_JOURNAL_FOREACH</function></funcdef>
<paramdef>sd_journal *<parameter>j</parameter></paramdef>
</funcprototype>
<funcprototype>
<funcdef><function>SD_JOURNAL_FOREACH_BACKWARDS</function></funcdef>
<paramdef>sd_journal *<parameter>j</parameter></paramdef>
</funcprototype>
</funcsynopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para><function>sd_journal_next()</function> advances the read
pointer into the journal by one entry. The only argument taken is
a journal context object as allocated via
<citerefentry><refentrytitle>sd_journal_open</refentrytitle><manvolnum>3</manvolnum></citerefentry>.
After successful invocation the entry may be read with functions
such as
<citerefentry><refentrytitle>sd_journal_get_data</refentrytitle><manvolnum>3</manvolnum></citerefentry>.</para>
<para>Similarly, <function>sd_journal_previous()</function> sets
the read pointer back one entry.</para>
<para><function>sd_journal_next_skip()</function> and
<function>sd_journal_previous_skip()</function> advance/set back
the read pointer by multiple entries at once, as specified in the
<varname>skip</varname> parameter.</para>
<para>The journal is strictly ordered by reception time, and hence
advancing to the next entry guarantees that the entry then
pointing to is later in time than then previous one, or has the
same timestamp.</para>
<para>Note that
<citerefentry><refentrytitle>sd_journal_get_data</refentrytitle><manvolnum>3</manvolnum></citerefentry>
and related calls will fail unless
<function>sd_journal_next()</function> has been invoked at least
once in order to position the read pointer on a journal
entry.</para>
<para>Note that the <function>SD_JOURNAL_FOREACH()</function>
macro may be used as a wrapper around
<citerefentry><refentrytitle>sd_journal_seek_head</refentrytitle><manvolnum>3</manvolnum></citerefentry>
and <function>sd_journal_next()</function> in order to make
iterating through the journal easier. See below for an example.
Similarly, <function>SD_JOURNAL_FOREACH_BACKWARDS()</function> may
be used for iterating the journal in reverse order.</para>
</refsect1>
<refsect1>
<title>Return Value</title>
<para>The four calls return the number of entries advanced/set
back on success or a negative errno-style error code. When the end
or beginning of the journal is reached, a number smaller than
requested is returned. More specifically, if
<function>sd_journal_next()</function> or
<function>sd_journal_previous()</function> reach the end/beginning
of the journal they will return 0, instead of 1 when they are
successful. This should be considered an EOF marker.</para>
</refsect1>
<refsect1>
<title>Notes</title>
<para>The <function>sd_journal_next()</function>,
<function>sd_journal_previous()</function>,
<function>sd_journal_next_skip()</function> and
<function>sd_journal_previous_skip()</function> interfaces are
available as a shared library, which can be compiled and linked to
with the
<constant>libsystemd</constant> <citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
file.</para>
</refsect1>
<refsect1>
<title>Examples</title>
<para>Iterating through the journal:</para>
<programlisting>#include <stdio.h>
#include <string.h>
#include <systemd/sd-journal.h>
int main(int argc, char *argv[]) {
int r;
sd_journal *j;
r = sd_journal_open(&j, SD_JOURNAL_LOCAL_ONLY);
if (r < 0) {
fprintf(stderr, "Failed to open journal: %s\n", strerror(-r));
return 1;
}
SD_JOURNAL_FOREACH(j) {
const char *d;
size_t l;
r = sd_journal_get_data(j, "MESSAGE", (const void **)&d, &l);
if (r < 0) {
fprintf(stderr, "Failed to read message field: %s\n", strerror(-r));
continue;
}
printf("%.*s\n", (int) l, d);
}
sd_journal_close(j);
return 0;
}</programlisting>
</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_get_data</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_journal_get_realtime_usec</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
<citerefentry><refentrytitle>sd_journal_get_cursor</refentrytitle><manvolnum>3</manvolnum></citerefentry>
</para>
</refsect1>
</refentry>
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