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Network file systems generally do not offer inotify() that would work across the network. We hence cannot rely on inotify() exclusiely in those case. Provide an API to determine these cases, and suggest doing manual regular rechecks. Note that this is not complete yet, as we need to rescan journal dirs on network file systems explicitly to find new/removed files
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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