systemd/man/sd_journal_get_fd.xml

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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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<refentry id="sd_journal_get_fd" xmlns:xi="http://www.w3.org/2001/XInclude">

  <refentryinfo>
    <title>sd_journal_get_fd</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>sd_journal_get_fd</refentrytitle>
    <manvolnum>3</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>sd_journal_get_fd</refname>
    <refname>sd_journal_get_events</refname>
    <refname>sd_journal_get_timeout</refname>
    <refname>sd_journal_process</refname>
    <refname>sd_journal_wait</refname>
    <refname>sd_journal_reliable_fd</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_get_events</function></funcdef>
        <paramdef>sd_journal *<parameter>j</parameter></paramdef>
      </funcprototype>

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

      <funcprototype>
        <funcdef>int <function>sd_journal_reliable_fd</function></funcdef>
        <paramdef>sd_journal *<parameter>j</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 signaled as soon as the journal changes, because new
    entries or files were added, rotation took place, or files have
    been deleted, and similar. The file descriptor is suitable for
    usage in
    <citerefentry><refentrytitle>poll</refentrytitle><manvolnum>2</manvolnum></citerefentry>.
    Use <function>sd_journal_get_events()</function> for an events
    mask to watch for. The call takes one argument: the journal
    context object. Note that not all file systems are capable of
    generating the necessary events for wakeups from this file
    descriptor for changes to be noticed immediately. In particular
    network files systems do not generate suitable file change events
    in all cases. Cases like this can be detected with
    <function>sd_journal_reliable_fd()</function>, below.
    <function>sd_journal_get_timeout()</function> will ensure in these
    cases that wake-ups happen frequently enough for changes to be
    noticed, although with a certain latency.</para>

    <para><function>sd_journal_get_events()</function> will return the
    <function>poll()</function> mask to wait for. This function will
    return a combination of <constant>POLLIN</constant> and
    <constant>POLLOUT</constant> and similar to fill into the
    <literal>.events</literal> field of <varname>struct
    pollfd</varname>.</para>

    <para><function>sd_journal_get_timeout()</function> will return a
    timeout value for usage in <function>poll()</function>. This
    returns a value in microseconds since the epoch of
    <constant>CLOCK_MONOTONIC</constant> for timing out
    <function>poll()</function> in <varname>timeout_usec</varname>.
    See
    <citerefentry><refentrytitle>clock_gettime</refentrytitle><manvolnum>2</manvolnum></citerefentry>
    for details about <constant>CLOCK_MONOTONIC</constant>. If there
    is no timeout to wait for, this will fill in <constant>(uint64_t)
    -1</constant> instead. Note that <function>poll()</function> takes
    a relative timeout in milliseconds rather than an absolute timeout
    in microseconds. To convert the absolute 'us' timeout into
    relative 'ms', use code like the following:</para>

    <programlisting>uint64_t t;
int msec;
sd_journal_get_timeout(m, &amp;t);
if (t == (uint64_t) -1)
  msec = -1;
else {
  struct timespec ts;
  uint64_t n;
  clock_gettime(CLOCK_MONOTONIC, &amp;ts);
  n = (uint64_t) ts.tv_sec * 1000000 + ts.tv_nsec / 1000;
  msec = t > n ? (int) ((t - n + 999) / 1000) : 0;
}</programlisting>

    <para>The code above does not do any error checking for brevity's
    sake. The calculated <varname>msec</varname> integer can be passed
    directly as <function>poll()</function>'s timeout
    parameter.</para>

    <para>After each <function>poll()</function> wake-up
    <function>sd_journal_process()</function> needs to be called to
    process events. 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>,
    <function>sd_journal_get_events()</function>,
    <function>sd_journal_get_timeout()</function> and
    <function>sd_journal_process()</function> is
    <function>sd_journal_wait()</function>. It will synchronously wait
    until the journal gets changed. The maximum time this call sleeps
    may be controlled with the <parameter>timeout_usec</parameter>
    parameter. Pass <constant>(uint64_t) -1</constant> to wait
    indefinitely. Internally this call simply combines
    <function>sd_journal_get_fd()</function>,
    <function>sd_journal_get_events()</function>,
    <function>sd_journal_get_timeout()</function>,
    <function>poll()</function> and
    <function>sd_journal_process()</function> into one.</para>

    <para><function>sd_journal_reliable_fd()</function> may be used to check whether the wake-up events from
    the file descriptor returned by <function>sd_journal_get_fd()</function> are known to be quickly
    triggered. On certain file systems where file change events from the OS are not available (such as NFS)
    changes need to be polled for repeatedly, and hence are detected only with a considerable latency. This
    call will return a positive value if the journal changes are detected quickly and zero when they need to
    be polled for. Note that there is usually no need to invoke this function directly as
    <function>sd_journal_get_timeout()</function> will request appropriate timeouts anyway.</para>

    <para>Note that all of the above change notification interfaces do not report changes
    instantly. Latencies are introduced for multiple reasons: as mentioned certain storage backends require
    time-based polling, in other cases wake-ups are optimized by coalescing events, and the OS introduces
    additional IO/CPU scheduling latencies.</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_get_events()</function> returns a
    combination of <constant>POLLIN</constant>,
    <constant>POLLOUT</constant> and suchlike on success or a negative
    errno-style error code.</para>

    <para><function>sd_journal_reliable_fd()</function> returns a
    positive integer if the file descriptor returned by
    <function>sd_journal_get_fd()</function> will generate wake-ups
    immediately for all journal changes. Returns 0 if there might be a
    latency involved.</para>

    <para><function>sd_journal_process()</function> and <function>sd_journal_wait()</function> return a negative
    errno-style error code, or one of <constant>SD_JOURNAL_NOP</constant>, <constant>SD_JOURNAL_APPEND</constant> or
    <constant>SD_JOURNAL_INVALIDATE</constant> on success:</para>

    <itemizedlist>
      <listitem><para>If <constant>SD_JOURNAL_NOP</constant> is returned, the journal did not change since the last
      invocation.</para></listitem>

      <listitem><para>If <constant>SD_JOURNAL_APPEND</constant> is returned, new entries have been appended to the end
      of the journal. In this case it is sufficient to simply continue reading at the previous end location of the
      journal, to read the newly added entries.</para></listitem>

      <listitem><para>If <constant>SD_JOURNAL_INVALIDATE</constant>, journal files were added to or removed from the
      set of journal files watched (e.g. due to rotation or vacuuming), and thus entries might have appeared or
      disappeared at arbitrary places in the log stream, possibly before or after the previous end of the log
      stream. If <constant>SD_JOURNAL_INVALIDATE</constant> is returned, live-view UIs that want to reflect on screen
      the precise state of the log data on disk should probably refresh their entire display (relative to the cursor of
      the log entry on the top of the screen). Programs only interested in a strictly sequential stream of log data may
      treat <constant>SD_JOURNAL_INVALIDATE</constant> the same way as <constant>SD_JOURNAL_APPEND</constant>, thus
      ignoring any changes to the log view earlier than the old end of the log stream.</para></listitem>
    </itemizedlist>
  </refsect1>

  <refsect1>
    <title>Signal safety</title>

    <para>In general, <function>sd_journal_get_fd()</function>, <function>sd_journal_get_events()</function>, and
    <function>sd_journal_get_timeout()</function> are <emphasis>not</emphasis> "async signal safe" in the meaning of
    <citerefentry
    project='man-pages'><refentrytitle>signal-safety</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
    Nevertheless, only the first call to any of those three functions performs unsafe operations, so subsequent calls
    <emphasis>are</emphasis> safe.</para>

    <para><function>sd_journal_process()</function> and <function>sd_journal_wait()</function> are not
    safe. <function>sd_journal_reliable_fd()</function> is safe.</para>
  </refsect1>

  <refsect1>
    <title>Notes</title>

    <xi:include href="threads-aware.xml" xpointer="strict"/>

    <xi:include href="libsystemd-pkgconfig.xml" xpointer="pkgconfig-text"/>
  </refsect1>

  <refsect1>
    <title>Examples</title>

    <para>Iterating through the journal, in a live view tracking all
    changes:</para>

    <programlisting><xi:include href="journal-iterate-wait.c" parse="text" /></programlisting>

    <para>Waiting with <function>poll()</function> (this
    example lacks all error checking for the sake of
    simplicity):</para>

    <programlisting><xi:include href="journal-iterate-poll.c" parse="text" /></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_next</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>poll</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>clock_gettime</refentrytitle><manvolnum>2</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>