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_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 wakeup events from the file descriptor returned
    by <function>sd_journal_get_fd()</function> are known to be
    immediately 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
    certain latency. This call will return a positive value if the
    journal changes are detected immediately and zero when they need
    to be polled for and hence might be noticed only with a certain
    latency. Note that there is usually no need to invoke this function
    directly as <function>sd_journal_get_timeout()</function> on these
    file systems will ask for timeouts explicitly anyway.</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 one of
    <constant>SD_JOURNAL_NOP</constant>,
    <constant>SD_JOURNAL_APPEND</constant> or
    <constant>SD_JOURNAL_INVALIDATE</constant> on success or a
    negative errno-style error code. If
    <constant>SD_JOURNAL_NOP</constant> is returned, the journal did
    not change since the last invocation. If
    <constant>SD_JOURNAL_APPEND</constant> is returned, new entries
    have been appended to the end of the journal. If
    <constant>SD_JOURNAL_INVALIDATE</constant>, 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 <constant>SD_JOURNAL_APPEND</constant>, it is
    sufficient to simply continue reading at the previous end of the
    journal.</para>
  </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>

    <para>The <function>sd_journal_get_fd()</function>,
    <function>sd_journal_get_events()</function>,
    <function>sd_journal_reliable_fd()</function>,
    <function>sd_journal_process()</function> and
    <function>sd_journal_wait()</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, in a live view tracking all
    changes:</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;
  }
  for (;;)  {
    const void *d;
    size_t l;
    r = sd_journal_next(j);
    if (r &lt; 0) {
      fprintf(stderr, "Failed to iterate to next entry: %s\n", strerror(-r));
      break;
    }
    if (r == 0) {
      /* Reached the end, let's wait for changes, and try again */
      r = sd_journal_wait(j, (uint64_t) -1);
      if (r &lt; 0) {
        fprintf(stderr, "Failed to wait for changes: %s\n", strerror(-r));
        break;
      }
      continue;
    }
    r = sd_journal_get_data(j, "MESSAGE", &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, (const char*) d);
  }
  sd_journal_close(j);
  return 0;
}</programlisting>

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

    <programlisting>#include &lt;poll.h&gt;
#include &lt;systemd/sd-journal.h&gt;

int wait_for_changes(sd_journal *j) {
  struct pollfd pollfd;
  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;
  }

  pollfd.fd = sd_journal_get_fd(j);
  pollfd.events = sd_journal_get_events(j);
  poll(&amp;pollfd, 1, msec);
  return sd_journal_process(j);
}</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>