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5aded36978
It is annoying when we have dead links on fd.o. Add project='man-pages|die-net|archlinux' to <citerefentry>-ies. In generated html, add external links to http://man7.org/linux/man-pages/man, http://linux.die.net/man/, https://www.archlinux.org/. By default, pages in sections 2 and 4 go to man7, since Michael Kerrisk is the autorative source on kernel related stuff. The rest of links goes to linux.die.net, because they have the manpages. Except for the pacman stuff, since it seems to be only available from archlinux.org. Poor gummiboot gets no link, because gummitboot(8) ain't to be found on the net. According to common wisdom, that would mean that it does not exist. But I have seen Kay using it, so I know it does, and deserves to be found. Can somebody be nice and put it up somewhere?
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 project='die-net'><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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