mirror of
https://github.com/systemd/systemd.git
synced 2024-11-01 17:51:22 +03:00
260 lines
12 KiB
XML
260 lines
12 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
|
|
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
|
|
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
|
|
<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
|
|
|
|
<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 <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_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, &t);
|
|
if (t == (uint64_t) -1)
|
|
msec = -1;
|
|
else {
|
|
struct timespec ts;
|
|
uint64_t n;
|
|
clock_gettime(CLOCK_MONOTONIC, &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>
|