273 lines
12 KiB
XML
273 lines
12 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_reliable_fd</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_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_reliable_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_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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</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 readable as
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soon as the journal changes, because new entries or
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files were added, rotation took place, or files have
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been deleted, and similar. The file descriptor is
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suitable for usage in
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<citerefentry><refentrytitle>poll</refentrytitle><manvolnum>2</manvolnum></citerefentry>
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where it will yield POLLIN on changes. The call takes
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one argument: the journal context object. Note that
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not all file systems are capable of generating the
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necessary events for wakeups from this file descriptor
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to be enirely reliable. In particular network files
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systems do not generate suitable file change events in
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all cases. In such a case an application should not
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rely alone on wake-ups from this file descriptor but
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wake up and recheck the journal in regular time
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intervals, for example every 2s. To detect
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cases where this is necessary, use
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<function>sd_journal_reliable_fd()</function>,
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below.</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 sufficient
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to track changes to the journal. If this call returns
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0, it is necessary to regularly recheck for journal
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changes (suggestion: every 2s). If this call returns a
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positive integer this is not necessary, and wakeups
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from the file descriptor returned by
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<function>sd_journal_get_fd()</function> are
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sufficient as only source for wake-ups.</para>
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<para>After each POLLIN wake-up
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<function>sd_journal_process()</function> needs to be
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called to process events and reset the readable state
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of the file descriptor. 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_reliable_fd()</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,
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possibly using a 2s time-out if this is necessary (see
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above). In either way the maximum time this call
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sleeps may be controlled with the
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<parameter>timeout_usec</parameter> parameter. Pass
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<literal>(uint64_t) -1</literal> to wait
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indefinitely. Internally this call simply combines
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<function>sd_journal_get_fd()</function>,
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<function>sd_journal_reliable_fd()</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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</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 a valid file descriptor on success or a negative errno-style error
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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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is sufficient as sole wake-up source for journal
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change events. Returns 0 if it is not sufficient and
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the journal needs to be checked manually in regular
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time intervals for changes. Returns a negative
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errno-style error code on failure.</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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<literal>SD_JOURNAL_NOP</literal>,
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<literal>SD_JOURNAL_APPEND</literal> or
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<literal>SD_JOURNAL_INVALIDATE</literal> on success or
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a negative errno-style error code. If
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<literal>SD_JOURNAL_NOP</literal> is returned the
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journal didn't change since the last invocation. If
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<literal>SD_JOURNAL_APPEND</literal> is returned new
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entries have been appended to the end of the
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journal. If <literal>SD_JOURNAL_INVALIDATE</literal>
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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 <literal>SD_JOURNAL_APPEND</literal> 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_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 shared library, which can be compiled and
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linked to with the
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<literal>libsystemd-journal</literal>
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<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 char *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, 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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pollfd.fd = sd_journal_get_fd();
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pollfd.events = POLLIN;
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poll(&pollfd, 1, sd_journal_reliable_fd() > 0 ? -1 : 2000);
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return sd_journal_process(j);
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}
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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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</para>
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</refsect1>
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</refentry>
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