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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">
<!--
SPDX-License-Identifier: LGPL-2.1+
-->
<refentry id="sd_journal_next" xmlns:xi="http://www.w3.org/2001/XInclude">
<refentryinfo>
<title>sd_journal_next</title>
<productname>systemd</productname>
</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>All functions listed here are thread-agnostic and only a single thread may operate
on a given <structname>sd_journal</structname> object.</para>
<xi:include href="libsystemd-pkgconfig.xml" xpointer="pkgconfig-text"/>
</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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