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93b73b064c
This introduces a new data threshold setting for sd_journal objects which controls the maximum size of objects to decompress. This is relieves the library from having to decompress full data objects even if a client program is only interested in the initial part of them. This speeds up "systemd-coredumpctl" drastically when invoked without parameters.
252 lines
13 KiB
XML
252 lines
13 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_data">
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<refentryinfo>
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<title>sd_journal_get_data</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_data</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_data</refname>
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<refname>sd_journal_enumerate_data</refname>
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<refname>sd_journal_restart_data</refname>
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<refname>SD_JOURNAL_FOREACH_DATA</refname>
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<refname>sd_journal_set_data_threshold</refname>
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<refname>sd_journal_get_data_threshold</refname>
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<refpurpose>Read data fields from the current journal entry</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_data</function></funcdef>
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<paramdef>sd_journal* <parameter>j</parameter></paramdef>
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<paramdef>const char* <parameter>field</parameter></paramdef>
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<paramdef>const void** <parameter>data</parameter></paramdef>
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<paramdef>size_t* <parameter>length</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_journal_enumerate_data</function></funcdef>
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<paramdef>sd_journal* <parameter>j</parameter></paramdef>
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<paramdef>const void** <parameter>data</parameter></paramdef>
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<paramdef>size_t* <parameter>length</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>void <function>sd_journal_restart_data</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><function>SD_JOURNAL_FOREACH_DATA</function></funcdef>
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<paramdef>sd_journal* <parameter>j</parameter></paramdef>
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<paramdef>const void* <parameter>data</parameter></paramdef>
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<paramdef>size_t <parameter>length</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_journal_set_data_threshold</function></funcdef>
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<paramdef>sd_journal* <parameter>j</parameter></paramdef>
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<paramdef>size_t <parameter>sz</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_journal_get_data_threshold</function></funcdef>
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<paramdef>sd_journal* <parameter>j</parameter></paramdef>
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<paramdef>size_t* <parameter>sz</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_data()</function> gets
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the data object associated with a specific field from
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the current journal entry. It takes four arguments:
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the journal context object, a string with the field
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name to request, plus a pair of pointers to
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pointer/size variables where the data object and its
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size shall be stored in. The field name should be an
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entry field name. Well-known field names are listed in
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<citerefentry><refentrytitle>systemd.journal-fields</refentrytitle><manvolnum>7</manvolnum></citerefentry>. The
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returned data is in a read-only memory map and is only
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valid until the next invocation of
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<function>sd_journal_get_data()</function> or
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<function>sd_journal_enumerate_data()</function>, or
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the read pointer is altered. Note that the data
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returned will be prefixed with the field name and
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'='. Also note that by default data fields larger than
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64K might get truncated to 64K. This threshold may be
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changed and turned off with
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<function>sd_journal_set_data_threshold()</function> (see
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below).</para>
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<para><function>sd_journal_enumerate_data()</function>
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may be used to iterate through all fields of the
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current entry. On each invocation the data for the
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next field is returned. The order of these fields is
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not defined. The data returned is in the same format
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as with <function>sd_journal_get_data()</function> and
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also follows the same life-time semantics.</para>
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<para><function>sd_journal_restart_data()</function>
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resets the data enumeration index to the beginning of
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the entry. The next invocation of
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<function>sd_journal_enumerate_data()</function> will return the first
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field of the entry again.</para>
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<para>Note that the
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<function>SD_JOURNAL_FOREACH_DATA()</function> macro
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may be used as a handy wrapper around
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<function>sd_journal_restart_data()</function> and
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<function>sd_journal_enumerate_data()</function>.</para>
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<para>Note that these functions will not work before
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<citerefentry><refentrytitle>sd_journal_next</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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(or related call) has been called at least
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once, in order to position the read pointer at a valid entry.</para>
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<para><function>sd_journal_set_data_threshold()</function>
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may be used to change the data field size threshold
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for data returned by
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<function>sd_journal_get_data()</function>,
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<function>sd_journal_enumerate_data()</function> and
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<function>sd_journal_enumerate_unique()</function>. This
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threshold is a hint only: it indicates that the client
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program is interested only in the initial parts of the
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data fields, up to the threshold in size -- but the
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library might still return larger data objects. That
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means applications should not rely exclusively on this
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setting to limit the size of the data fields returned,
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but need to apply a explicit size limit on the
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returned data as well. This threshold defaults to 64K
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by default. To retrieve the complete data fields this
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threshold should be turned off by setting it to 0, so
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that the library always returns the complete data
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objects. It is recommended to set this threshold as
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low as possible since this relieves the library from
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having to decompress large compressed data objects in
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full.</para>
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<para><function>sd_journal_get_data_threshold()</function>
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returns the currently configured data field size
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threshold.</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_data()</function>
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returns 0 on success or a negative errno-style error
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code. If the current entry does not include the
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specified field -ENOENT is returned. If
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<citerefentry><refentrytitle>sd_journal_next</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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has not been called at least once -EADDRNOTAVAIL is
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returned. <function>sd_journal_enumerate_data()</function>
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returns a positive integer if the next field has been
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read, 0 when no more fields are known, or a negative
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errno-style error
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code. <function>sd_journal_restart_data()</function>
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returns
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nothing. <function>sd_journal_set_data_threshold()</function>
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and <function>sd_journal_get_threshold()</function>
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return 0 on success or a negative errno-style error
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code.</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_data()</function>,
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<function>sd_journal_enumerate_data()</function>,
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<function>sd_journal_restart_data()</function>,
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<function>sd_journal_set_data_threshold()</function>
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and
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<function>sd_journal_get_data_threshold()</function>
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interfaces are available as shared library, which can
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be compiled and 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>See
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<citerefentry><refentrytitle>sd_journal_next</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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for a complete example how to use
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<function>sd_journal_get_data()</function>.</para>
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<para>Use the
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<function>SD_JOURNAL_FOREACH_DATA</function> macro to
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iterate through all fields of the current journal
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entry:</para>
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<programlisting>...
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int print_fields(sd_journal *j) {
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const void *data;
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size_t l;
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SD_JOURNAL_FOREACH_DATA(j, data, length)
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printf("%.*s\n", (int) length, data);
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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>systemd.journal-fields</refentrytitle><manvolnum>7</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>sd_journal_get_realtime_usec</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_journal_query_unique</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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