man/sd_journal_next.xml

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<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>