systemd/man/sd_journal_get_fd.xml

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<refentry id="sd_journal_get_fd">

        <refentryinfo>
                <title>sd_journal_get_fd</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_get_fd</refentrytitle>
                <manvolnum>3</manvolnum>
        </refmeta>

        <refnamediv>
                <refname>sd_journal_get_fd</refname>
                <refname>sd_journal_process</refname>
                <refname>sd_journal_wait</refname>
                <refname>SD_JOURNAL_NOP</refname>
                <refname>SD_JOURNAL_APPEND</refname>
                <refname>SD_JOURNAL_INVALIDATE</refname>
                <refpurpose>Journal change notification
                interface</refpurpose>
        </refnamediv>

        <refsynopsisdiv>
                <funcsynopsis>
                        <funcsynopsisinfo>#include &lt;systemd/sd-journal.h&gt;</funcsynopsisinfo>

                        <funcprototype>
                                <funcdef>int <function>sd_journal_get_fd</function></funcdef>
                                <paramdef>sd_journal* <parameter>j</parameter></paramdef>
                        </funcprototype>

                        <funcprototype>
                                <funcdef>int <function>sd_journal_process</function></funcdef>
                                <paramdef>sd_journal* <parameter>j</parameter></paramdef>
                        </funcprototype>

                        <funcprototype>
                                <funcdef>int <function>sd_journal_wait</function></funcdef>
                                <paramdef>sd_journal* <parameter>j</parameter></paramdef>
                                <paramdef>uint64_t <parameter>timeout_usec</parameter></paramdef>
                        </funcprototype>

                </funcsynopsis>
        </refsynopsisdiv>

        <refsect1>
                <title>Description</title>

                <para><function>sd_journal_get_fd()</function> returns
                a file descriptor that may be asynchronously polled in
                an external event loop and is signalled readable as
                soon as the journal changes, for example because new
                entries were added. The file descriptor is suitable
                for usage in
                <citerefentry><refentrytitle>poll</refentrytitle><manvolnum>2</manvolnum></citerefentry>
                where it will yield POLLIN on all changes. The call
                takes one argument: the journal context object.</para>

                <para>After each POLLIN wake-up
                <function>sd_journal_process()</function> needs to be
                called to process events and reset the readable state
                of the file descriptor. This call will also indicate
                what kind of change has been detected (see below; note
                that spurious wake-ups are possible).</para>

                <para>A synchronous alternative for using
                <function>sd_journal_get_fd()</function> and
                <function>sd_journal_process()</function> is
                <function>sd_journal_wait()</function>. It will
                synchronously wait until the journal gets changed up
                to a certain time-out as specified in its second
                argument. Pass <literal>(uint64_t) -1</literal> to
                wait indefinitely. Internally this call simply
                combines <function>sd_journal_get_fd()</function>,
                <function>poll()</function> and
                <function>sd_journal_process()</function> into
                one.</para>

        </refsect1>

        <refsect1>
                <title>Return Value</title>

                <para><function>sd_journal_get_fd()</function> returns a valid file descriptor on success or a negative errno-style error
                code.</para>

                <para><function>sd_journal_process()</function> and
                <function>sd_journal_wait()</function> return one of
                <literal>SD_JOURNAL_NOP</literal>,
                <literal>SD_JOURNAL_APPEND</literal> or
                <literal>SD_JOURNAL_INVALIDATE</literal> on success or
                a negative errno-style error code. If
                <literal>SD_JOURNAL_NOP</literal> is returned the
                journal didn't change since the last invocation. If
                <literal>SD_JOURNAL_APPEND</literal> is returned new
                entries have been appended to the end of the
                journal. If <literal>SD_JOURNAL_INVALIDATE</literal>
                journal files were added or removed (possibly due to
                rotation). In the latter event live-view UIs should
                probably refresh their entire display while in the
                case of <literal>SD_JOURNAL_APPEND</literal> it is
                sufficient to simply continue reading at the previous
                end of the journal.</para>
        </refsect1>

        <refsect1>
                <title>Notes</title>

                <para>The <function>sd_journal_get_fd()</function>,
                <function>sd_journal_process()</function> and
                <function>sd_journal_wait()</function> interfaces are
                available as shared library, which can be compiled and
                linked to with the
                <literal>libsystemd-journal</literal>
                <citerefentry><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
                file.</para>
        </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_next</refentrytitle><manvolnum>3</manvolnum></citerefentry>
                </para>
        </refsect1>

</refentry>