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350 lines
16 KiB
XML
350 lines
16 KiB
XML
<?xml version='1.0'?> <!--*-nxml-*-->
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!--
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This file is part of systemd.
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Copyright 2012 Lennart Poettering
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systemd is free software; you can redistribute it and/or modify it
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under the terms of the GNU Lesser General Public License as published by
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the Free Software Foundation; either version 2.1 of the License, or
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(at your option) any later version.
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systemd is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser General Public License
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along with systemd; If not, see <http://www.gnu.org/licenses/>.
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-->
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<refentry id="sd_journal_get_fd">
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<refentryinfo>
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<title>sd_journal_get_fd</title>
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<productname>systemd</productname>
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<authorgroup>
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<author>
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<contrib>Developer</contrib>
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<firstname>Lennart</firstname>
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<surname>Poettering</surname>
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<email>lennart@poettering.net</email>
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</author>
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</authorgroup>
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</refentryinfo>
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<refmeta>
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<refentrytitle>sd_journal_get_fd</refentrytitle>
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<manvolnum>3</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>sd_journal_get_fd</refname>
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<refname>sd_journal_get_events</refname>
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<refname>sd_journal_get_timeout</refname>
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<refname>sd_journal_process</refname>
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<refname>sd_journal_wait</refname>
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<refname>sd_journal_reliable_fd</refname>
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<refname>SD_JOURNAL_NOP</refname>
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<refname>SD_JOURNAL_APPEND</refname>
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<refname>SD_JOURNAL_INVALIDATE</refname>
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<refpurpose>Journal change notification
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interface</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<funcsynopsis>
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<funcsynopsisinfo>#include <systemd/sd-journal.h></funcsynopsisinfo>
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<funcprototype>
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<funcdef>int <function>sd_journal_get_fd</function></funcdef>
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<paramdef>sd_journal* <parameter>j</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_journal_get_events</function></funcdef>
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<paramdef>sd_journal* <parameter>j</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_journal_get_timeout</function></funcdef>
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<paramdef>sd_journal* <parameter>j</parameter></paramdef>
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<paramdef>uint64_t* <parameter>timeout_usec</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_journal_process</function></funcdef>
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<paramdef>sd_journal* <parameter>j</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_journal_wait</function></funcdef>
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<paramdef>sd_journal* <parameter>j</parameter></paramdef>
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<paramdef>uint64_t <parameter>timeout_usec</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_journal_reliable_fd</function></funcdef>
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<paramdef>sd_journal* <parameter>j</parameter></paramdef>
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</funcprototype>
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</funcsynopsis>
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</refsynopsisdiv>
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<refsect1>
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<title>Description</title>
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<para><function>sd_journal_get_fd()</function> returns
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a file descriptor that may be asynchronously polled in
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an external event loop and is signaled as soon as the
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journal changes, because new entries or files were
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added, rotation took place, or files have been
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deleted, and similar. The file descriptor is suitable
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for usage in
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<citerefentry><refentrytitle>poll</refentrytitle><manvolnum>2</manvolnum></citerefentry>. Use
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<function>sd_journal_get_events()</function> for an
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events mask to watch for. The call takes one argument:
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the journal context object. Note that not all file
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systems are capable of generating the necessary events
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for wakeups from this file descriptor for changes to
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be noticed immediately. In particular network files
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systems do not generate suitable file change events in
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all cases. Cases like this can be detected with
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<function>sd_journal_reliable_fd()</function>,
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below. <function>sd_journal_get_timeout()</function>
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will ensure in these cases that wake-ups happen
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frequently enough for changes to be noticed, although
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with a certain latency.</para>
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<para><function>sd_journal_get_events()</function>
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will return the <function>poll()</function> mask to
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wait for. This function will return a combination of
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<constant>POLLIN</constant> and
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<constant>POLLOUT</constant> and similar to fill into
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the <literal>.events</literal> field of
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<varname>struct pollfd</varname>.</para>
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<para><function>sd_journal_get_timeout()</function>
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will return a timeout value for usage in
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<function>poll()</function>. This returns a value in
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microseconds since the epoch of
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<constant>CLOCK_MONOTONIC</constant> for timing out
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<function>poll()</function> in
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<varname>timeout_usec</varname>. See
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<citerefentry><refentrytitle>clock_gettime</refentrytitle><manvolnum>2</manvolnum></citerefentry>
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for details about
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<constant>CLOCK_MONOTONIC</constant>. If there is no
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timeout to wait for, this will fill in
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<constant>(uint64_t) -1</constant> instead. Note that
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<function>poll()</function> takes a relative timeout
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in milliseconds rather than an absolute timeout in
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microseconds. To convert the absolute 'us' timeout
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into relative 'ms', use code like the
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following:</para>
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<programlisting>uint64_t t;
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int msec;
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sd_journal_get_timeout(m, &t);
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if (t == (uint64_t) -1)
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msec = -1;
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else {
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struct timespec ts;
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uint64_t n;
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clock_getttime(CLOCK_MONOTONIC, &ts);
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n = (uint64_t) ts.tv_sec * 1000000 + ts.tv_nsec / 1000;
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msec = t > n ? (int) ((t - n + 999) / 1000) : 0;
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}</programlisting>
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<para>The code above does not do any error checking
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for brevity's sake. The calculated <varname>msec</varname>
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integer can be passed directly as
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<function>poll()</function>'s timeout
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parameter.</para>
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<para>After each <function>poll()</function> wake-up
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<function>sd_journal_process()</function> needs to be
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called to process events. This call will also indicate
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what kind of change has been detected (see below; note
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that spurious wake-ups are possible).</para>
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<para>A synchronous alternative for using
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<function>sd_journal_get_fd()</function>,
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<function>sd_journal_get_events()</function>,
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<function>sd_journal_get_timeout()</function> and
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<function>sd_journal_process()</function> is
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<function>sd_journal_wait()</function>. It will
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synchronously wait until the journal gets changed. The
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maximum time this call sleeps may be controlled with
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the <parameter>timeout_usec</parameter>
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parameter. Pass <constant>(uint64_t) -1</constant> to
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wait indefinitely. Internally this call simply
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combines <function>sd_journal_get_fd()</function>,
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<function>sd_journal_get_events()</function>,
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<function>sd_journal_get_timeout()</function>,
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<function>poll()</function> and
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<function>sd_journal_process()</function> into
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one.</para>
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<para><function>sd_journal_reliable_fd()</function>
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may be used to check whether the wakeup events from
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the file descriptor returned by
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<function>sd_journal_get_fd()</function> are known to
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be immediately triggered. On certain file systems
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where file change events from the OS are not available
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(such as NFS) changes need to be polled for
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repeatedly, and hence are detected only with a certain
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latency. This call will return a positive value if the
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journal changes are detected immediately and zero when
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they need to be polled for and hence might be noticed
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only with a certain latency. Note that there's usually
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no need to invoke this function directly as
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<function>sd_journal_get_timeout()</function> on these
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file systems will ask for timeouts explicitly
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anyway.</para>
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</refsect1>
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<refsect1>
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<title>Return Value</title>
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<para><function>sd_journal_get_fd()</function> returns
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a valid file descriptor on success or a negative
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errno-style error code.</para>
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<para><function>sd_journal_get_events()</function>
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returns a combination of <constant>POLLIN</constant>,
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<constant>POLLOUT</constant> and suchlike on success or
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a negative errno-style error code.</para>
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<para><function>sd_journal_reliable_fd()</function>
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returns a positive integer if the file descriptor
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returned by <function>sd_journal_get_fd()</function>
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will generate wake-ups immediately for all journal
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changes. Returns 0 if there might be a latency
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involved.</para>
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<para><function>sd_journal_process()</function> and
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<function>sd_journal_wait()</function> return one of
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<constant>SD_JOURNAL_NOP</constant>,
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<constant>SD_JOURNAL_APPEND</constant> or
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<constant>SD_JOURNAL_INVALIDATE</constant> on success or
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a negative errno-style error code. If
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<constant>SD_JOURNAL_NOP</constant> is returned, the
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journal did not change since the last invocation. If
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<constant>SD_JOURNAL_APPEND</constant> is returned, new
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entries have been appended to the end of the
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journal. If <constant>SD_JOURNAL_INVALIDATE</constant>,
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journal files were added or removed (possibly due to
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rotation). In the latter event, live-view UIs should
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probably refresh their entire display, while in the
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case of <constant>SD_JOURNAL_APPEND</constant>, it is
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sufficient to simply continue reading at the previous
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end of the journal.</para>
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</refsect1>
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<refsect1>
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<title>Notes</title>
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<para>The <function>sd_journal_get_fd()</function>,
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<function>sd_journal_get_events()</function>,
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<function>sd_journal_reliable_fd()</function>,
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<function>sd_journal_process()</function> and
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<function>sd_journal_wait()</function> interfaces are
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available as a shared library, which can be compiled and
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linked to with the
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<constant>libsystemd</constant> <citerefentry><refentrytitle>pkg-config</refentrytitle><manvolnum>1</manvolnum></citerefentry>
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file.</para>
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</refsect1>
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<refsect1>
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<title>Examples</title>
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<para>Iterating through the journal, in a live view tracking all changes:</para>
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<programlisting>#include <stdio.h>
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#include <string.h>
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#include <systemd/sd-journal.h>
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int main(int argc, char *argv[]) {
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int r;
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sd_journal *j;
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r = sd_journal_open(&j, SD_JOURNAL_LOCAL_ONLY);
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if (r < 0) {
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fprintf(stderr, "Failed to open journal: %s\n", strerror(-r));
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return 1;
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}
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for (;;) {
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const void *d;
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size_t l;
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r = sd_journal_next(j);
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if (r < 0) {
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fprintf(stderr, "Failed to iterate to next entry: %s\n", strerror(-r));
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break;
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}
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if (r == 0) {
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/* Reached the end, let's wait for changes, and try again */
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r = sd_journal_wait(j, (uint64_t) -1);
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if (r < 0) {
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fprintf(stderr, "Failed to wait for changes: %s\n", strerror(-r));
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break;
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}
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continue;
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}
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r = sd_journal_get_data(j, "MESSAGE", &d, &l);
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if (r < 0) {
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fprintf(stderr, "Failed to read message field: %s\n", strerror(-r));
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continue;
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}
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printf("%.*s\n", (int) l, (const char*) d);
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}
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sd_journal_close(j);
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return 0;
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}</programlisting>
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<para>Waiting with <function>poll()</function> (this
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example lacks all error checking for the sake of
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simplicity):</para>
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<programlisting>#include <sys/poll.h>
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#include <systemd/sd-journal.h>
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int wait_for_changes(sd_journal *j) {
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struct pollfd pollfd;
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int msec;
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sd_journal_get_timeout(m, &t);
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if (t == (uint64_t) -1)
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msec = -1;
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else {
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struct timespec ts;
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uint64_t n;
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clock_getttime(CLOCK_MONOTONIC, &ts);
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n = (uint64_t) ts.tv_sec * 1000000 + ts.tv_nsec / 1000;
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msec = t > n ? (int) ((t - n + 999) / 1000) : 0;
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}
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pollfd.fd = sd_journal_get_fd(j);
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pollfd.events = sd_journal_get_events(j);
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poll(&pollfd, 1, msec);
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return sd_journal_process(j);
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}</programlisting>
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</refsect1>
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<refsect1>
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<title>See Also</title>
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<para>
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<citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd-journal</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_journal_open</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_journal_next</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>poll</refentrytitle><manvolnum>2</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>clock_gettime</refentrytitle><manvolnum>2</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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