man/sd_journal_get_fd.xml

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   8
   9 <refentry id="sd_journal_get_fd" xmlns:xi="http://www.w3.org/2001/XInclude">
  10
  11   <refentryinfo>
  12     <title>sd_journal_get_fd</title>
  13     <productname>systemd</productname>
  14   </refentryinfo>
  15
  16   <refmeta>
  17     <refentrytitle>sd_journal_get_fd</refentrytitle>
  18     <manvolnum>3</manvolnum>
  19   </refmeta>
  20
  21   <refnamediv>
  22     <refname>sd_journal_get_fd</refname>
  23     <refname>sd_journal_get_events</refname>
  24     <refname>sd_journal_get_timeout</refname>
  25     <refname>sd_journal_process</refname>
  26     <refname>sd_journal_wait</refname>
  27     <refname>sd_journal_reliable_fd</refname>
  28     <refname>SD_JOURNAL_NOP</refname>
  29     <refname>SD_JOURNAL_APPEND</refname>
  30     <refname>SD_JOURNAL_INVALIDATE</refname>
  31     <refpurpose>Journal change notification
  32     interface</refpurpose>
  33   </refnamediv>
  34
  35   <refsynopsisdiv>
  36     <funcsynopsis>
  37       <funcsynopsisinfo>#include &lt;systemd/sd-journal.h&gt;</funcsynopsisinfo>
  38
  39       <funcprototype>
  40         <funcdef>int <function>sd_journal_get_fd</function></funcdef>
  41         <paramdef>sd_journal *<parameter>j</parameter></paramdef>
  42       </funcprototype>
  43
  44       <funcprototype>
  45         <funcdef>int <function>sd_journal_get_events</function></funcdef>
  46         <paramdef>sd_journal *<parameter>j</parameter></paramdef>
  47       </funcprototype>
  48
  49       <funcprototype>
  50         <funcdef>int <function>sd_journal_get_timeout</function></funcdef>
  51         <paramdef>sd_journal *<parameter>j</parameter></paramdef>
  52         <paramdef>uint64_t *<parameter>timeout_usec</parameter></paramdef>
  53       </funcprototype>
  54
  55       <funcprototype>
  56         <funcdef>int <function>sd_journal_process</function></funcdef>
  57         <paramdef>sd_journal *<parameter>j</parameter></paramdef>
  58       </funcprototype>
  59
  60       <funcprototype>
  61         <funcdef>int <function>sd_journal_wait</function></funcdef>
  62         <paramdef>sd_journal *<parameter>j</parameter></paramdef>
  63         <paramdef>uint64_t <parameter>timeout_usec</parameter></paramdef>
  64       </funcprototype>
  65
  66       <funcprototype>
  67         <funcdef>int <function>sd_journal_reliable_fd</function></funcdef>
  68         <paramdef>sd_journal *<parameter>j</parameter></paramdef>
  69       </funcprototype>
  70
  71     </funcsynopsis>
  72   </refsynopsisdiv>
  73
  74   <refsect1>
  75     <title>Description</title>
  76
  77     <para><function>sd_journal_get_fd()</function> returns a file
  78     descriptor that may be asynchronously polled in an external event
  79     loop and is signaled as soon as the journal changes, because new
  80     entries or files were added, rotation took place, or files have
  81     been deleted, and similar. The file descriptor is suitable for
  82     usage in
  83     <citerefentry><refentrytitle>poll</refentrytitle><manvolnum>2</manvolnum></citerefentry>.
  84     Use <function>sd_journal_get_events()</function> for an events
  85     mask to watch for. The call takes one argument: the journal
  86     context object. Note that not all file systems are capable of
  87     generating the necessary events for wakeups from this file
  88     descriptor for changes to be noticed immediately. In particular
  89     network files systems do not generate suitable file change events
  90     in all cases. Cases like this can be detected with
  91     <function>sd_journal_reliable_fd()</function>, below.
  92     <function>sd_journal_get_timeout()</function> will ensure in these
  93     cases that wake-ups happen frequently enough for changes to be
  94     noticed, although with a certain latency.</para>
  95
  96     <para><function>sd_journal_get_events()</function> will return the
  97     <function>poll()</function> mask to wait for. This function will
  98     return a combination of <constant>POLLIN</constant> and
  99     <constant>POLLOUT</constant> and similar to fill into the
 100     <literal>.events</literal> field of <varname>struct
 101     pollfd</varname>.</para>
 102
 103     <para><function>sd_journal_get_timeout()</function> will return a
 104     timeout value for usage in <function>poll()</function>. This
 105     returns a value in microseconds since the epoch of
 106     <constant>CLOCK_MONOTONIC</constant> for timing out
 107     <function>poll()</function> in <varname>timeout_usec</varname>.
 108     See
 109     <citerefentry><refentrytitle>clock_gettime</refentrytitle><manvolnum>2</manvolnum></citerefentry>
 110     for details about <constant>CLOCK_MONOTONIC</constant>. If there
 111     is no timeout to wait for, this will fill in <constant>(uint64_t)
 112     -1</constant> instead. Note that <function>poll()</function> takes
 113     a relative timeout in milliseconds rather than an absolute timeout
 114     in microseconds. To convert the absolute 'us' timeout into
 115     relative 'ms', use code like the following:</para>
 116
 117     <programlisting>uint64_t t;
 118 int msec;
 119 sd_journal_get_timeout(m, &amp;t);
 120 if (t == (uint64_t) -1)
 121   msec = -1;
 122 else {
 123   struct timespec ts;
 124   uint64_t n;
 125   clock_gettime(CLOCK_MONOTONIC, &amp;ts);
 126   n = (uint64_t) ts.tv_sec * 1000000 + ts.tv_nsec / 1000;
 127   msec = t > n ? (int) ((t - n + 999) / 1000) : 0;
 128 }</programlisting>
 129
 130     <para>The code above does not do any error checking for brevity's
 131     sake. The calculated <varname>msec</varname> integer can be passed
 132     directly as <function>poll()</function>'s timeout
 133     parameter.</para>
 134
 135     <para>After each <function>poll()</function> wake-up
 136     <function>sd_journal_process()</function> needs to be called to
 137     process events. This call will also indicate what kind of change
 138     has been detected (see below; note that spurious wake-ups are
 139     possible).</para>
 140
 141     <para>A synchronous alternative for using
 142     <function>sd_journal_get_fd()</function>,
 143     <function>sd_journal_get_events()</function>,
 144     <function>sd_journal_get_timeout()</function> and
 145     <function>sd_journal_process()</function> is
 146     <function>sd_journal_wait()</function>. It will synchronously wait
 147     until the journal gets changed. The maximum time this call sleeps
 148     may be controlled with the <parameter>timeout_usec</parameter>
 149     parameter. Pass <constant>(uint64_t) -1</constant> to wait
 150     indefinitely. Internally this call simply combines
 151     <function>sd_journal_get_fd()</function>,
 152     <function>sd_journal_get_events()</function>,
 153     <function>sd_journal_get_timeout()</function>,
 154     <function>poll()</function> and
 155     <function>sd_journal_process()</function> into one.</para>
 156
 157     <para><function>sd_journal_reliable_fd()</function> may be used to
 158     check whether the wakeup events from the file descriptor returned
 159     by <function>sd_journal_get_fd()</function> are known to be
 160     immediately triggered. On certain file systems where file change
 161     events from the OS are not available (such as NFS) changes need to
 162     be polled for repeatedly, and hence are detected only with a
 163     certain latency. This call will return a positive value if the
 164     journal changes are detected immediately and zero when they need
 165     to be polled for and hence might be noticed only with a certain
 166     latency. Note that there is usually no need to invoke this function
 167     directly as <function>sd_journal_get_timeout()</function> on these
 168     file systems will ask for timeouts explicitly anyway.</para>
 169   </refsect1>
 170
 171   <refsect1>
 172     <title>Return Value</title>
 173
 174     <para><function>sd_journal_get_fd()</function> returns a valid
 175     file descriptor on success or a negative errno-style error
 176     code.</para>
 177
 178     <para><function>sd_journal_get_events()</function> returns a
 179     combination of <constant>POLLIN</constant>,
 180     <constant>POLLOUT</constant> and suchlike on success or a negative
 181     errno-style error code.</para>
 182
 183     <para><function>sd_journal_reliable_fd()</function> returns a
 184     positive integer if the file descriptor returned by
 185     <function>sd_journal_get_fd()</function> will generate wake-ups
 186     immediately for all journal changes. Returns 0 if there might be a
 187     latency involved.</para>
 188
 189     <para><function>sd_journal_process()</function> and <function>sd_journal_wait()</function> return a negative
 190     errno-style error code, or one of <constant>SD_JOURNAL_NOP</constant>, <constant>SD_JOURNAL_APPEND</constant> or
 191     <constant>SD_JOURNAL_INVALIDATE</constant> on success:</para>
 192
 193     <itemizedlist>
 194       <listitem><para>If <constant>SD_JOURNAL_NOP</constant> is returned, the journal did not change since the last
 195       invocation.</para></listitem>
 196
 197       <listitem><para>If <constant>SD_JOURNAL_APPEND</constant> is returned, new entries have been appended to the end
 198       of the journal. In this case it is sufficient to simply continue reading at the previous end location of the
 199       journal, to read the newly added entries.</para></listitem>
 200
 201       <listitem><para>If <constant>SD_JOURNAL_INVALIDATE</constant>, journal files were added to or removed from the
 202       set of journal files watched (e.g. due to rotation or vacuuming), and thus entries might have appeared or
 203       disappeared at arbitrary places in the log stream, possibly before or after the previous end of the log
 204       stream. If <constant>SD_JOURNAL_INVALIDATE</constant> is returned, live-view UIs that want to reflect on screen
 205       the precise state of the log data on disk should probably refresh their entire display (relative to the cursor of
 206       the log entry on the top of the screen). Programs only interested in a strictly sequential stream of log data may
 207       treat <constant>SD_JOURNAL_INVALIDATE</constant> the same way as <constant>SD_JOURNAL_APPEND</constant>, thus
 208       ignoring any changes to the log view earlier than the old end of the log stream.</para></listitem>
 209     </itemizedlist>
 210   </refsect1>
 211
 212   <refsect1>
 213     <title>Signal safety</title>
 214
 215     <para>In general, <function>sd_journal_get_fd()</function>, <function>sd_journal_get_events()</function>, and
 216     <function>sd_journal_get_timeout()</function> are <emphasis>not</emphasis> "async signal safe" in the meaning of
 217     <citerefentry
 218     project='man-pages'><refentrytitle>signal-safety</refentrytitle><manvolnum>7</manvolnum></citerefentry>.
 219     Nevertheless, only the first call to any of those three functions performs unsafe operations, so subsequent calls
 220     <emphasis>are</emphasis> safe.</para>
 221
 222     <para><function>sd_journal_process()</function> and <function>sd_journal_wait()</function> are not
 223     safe. <function>sd_journal_reliable_fd()</function> is safe.</para>
 224   </refsect1>
 225
 226   <refsect1>
 227     <title>Notes</title>
 228
 229     <para>The <function>sd_journal_get_fd()</function>,
 230     <function>sd_journal_get_events()</function>,
 231     <function>sd_journal_reliable_fd()</function>,
 232     <function>sd_journal_process()</function> and
 233     <function>sd_journal_wait()</function> interfaces are available as
 234     a shared library, which can be compiled and linked to with the
 235     <constant>libsystemd</constant> <citerefentry project='die-net'><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
 236     file.</para>
 237   </refsect1>
 238
 239   <refsect1>
 240     <title>Examples</title>
 241
 242     <para>Iterating through the journal, in a live view tracking all
 243     changes:</para>
 244
 245     <programlisting><xi:include href="journal-iterate-wait.c" parse="text" /></programlisting>
 246
 247     <para>Waiting with <function>poll()</function> (this
 248     example lacks all error checking for the sake of
 249     simplicity):</para>
 250
 251     <programlisting><xi:include href="journal-iterate-poll.c" parse="text" /></programlisting>
 252   </refsect1>
 253
 254   <refsect1>
 255     <title>See Also</title>
 256
 257     <para>
 258       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 259       <citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 260       <citerefentry><refentrytitle>sd_journal_open</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 261       <citerefentry><refentrytitle>sd_journal_next</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 262       <citerefentry><refentrytitle>poll</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
 263       <citerefentry><refentrytitle>clock_gettime</refentrytitle><manvolnum>2</manvolnum></citerefentry>
 264     </para>
 265   </refsect1>
 266
 267 </refentry>