b5ae094776014b09c5d8b9f3fc91cbe3041211d8Christian Maeder<?xml version='1.0'?> <!--*-nxml-*-->

b5ae094776014b09c5d8b9f3fc91cbe3041211d8Christian Maeder<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"

b5ae094776014b09c5d8b9f3fc91cbe3041211d8Christian Maeder "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">

b5ae094776014b09c5d8b9f3fc91cbe3041211d8Christian Maeder

<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 &lt;systemd/sd-journal.h&gt;</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 &lt;stdio.h&gt;

#include &lt;string.h&gt;

#include &lt;systemd/sd-journal.h&gt;

int main(int argc, char *argv[]) {

int r;

sd_journal *j;

r = sd_journal_open(&amp;j, SD_JOURNAL_LOCAL_ONLY);

if (r &lt; 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 **)&amp;d, &amp;l);

if (r &lt; 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>