6f6c328eca52553e8c921565950fdddadc799bf7Christian Maeder<?xml version='1.0'?> <!--*-nxml-*-->

6f6c328eca52553e8c921565950fdddadc799bf7Christian Maeder<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"

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

<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><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>