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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 <systemd/sd-journal.h></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>