<?xml version='1.0'?> <!--*-nxml-*-->
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
- "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
+ "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!--
+ SPDX-License-Identifier: LGPL-2.1+
+
This file is part of systemd.
Copyright 2012 Lennart Poettering
+-->
- systemd is free software; you can redistribute it and/or modify it
- under the terms of the GNU Lesser General Public License as published by
- the Free Software Foundation; either version 2.1 of the License, or
- (at your option) any later version.
+<refentry id="sd_journal_get_fd" xmlns:xi="http://www.w3.org/2001/XInclude">
- systemd is distributed in the hope that it will be useful, but
- WITHOUT ANY WARRANTY; without even the implied warranty of
- MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
- Lesser General Public License for more details.
+ <refentryinfo>
+ <title>sd_journal_get_fd</title>
+ <productname>systemd</productname>
- You should have received a copy of the GNU Lesser General Public License
- along with systemd; If not, see <http://www.gnu.org/licenses/>.
--->
+ <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 <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 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><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>
-<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>
+ <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>
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