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a1a03fa54b
* sd-bus/man: document EBUSY error in bus_message_read The EBUSY error can be returned from sd_bus_exit_container(), and, if that happens, it will be propogated upwards towards bus_message_read. In terms of documentation, this means that bus_message_read's man page can't just include the error text for sd_bus_message_read_basic, as reading basic types exclusively doesn't have the potential for this error. sd_bus_message_read_basic's error documentation isn't incorrect when applied to sd_bus_message_read, it's just incomplete. While EBUSY is documented in sd_bus_message_open_container.xml, it's explanation is unique to the sd_bus_message_exit_container function and makes for poor documentation of the general read API.
238 lines
8.5 KiB
XML
238 lines
8.5 KiB
XML
<?xml version='1.0'?>
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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
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"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
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<!--
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SPDX-License-Identifier: LGPL-2.1-or-later
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Copyright © 2016 Julian Orth
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-->
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<refentry id="sd_bus_message_read_basic">
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<refentryinfo>
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<title>sd_bus_message_read_basic</title>
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<productname>systemd</productname>
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</refentryinfo>
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<refmeta>
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<refentrytitle>sd_bus_message_read_basic</refentrytitle>
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<manvolnum>3</manvolnum>
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</refmeta>
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<refnamediv>
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<refname>sd_bus_message_read_basic</refname>
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<refpurpose>Read a basic type from a message</refpurpose>
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</refnamediv>
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<refsynopsisdiv>
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<funcsynopsis>
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<funcsynopsisinfo>#include <systemd/sd-bus.h></funcsynopsisinfo>
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<funcprototype>
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<funcdef>int <function>sd_bus_message_read_basic</function></funcdef>
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<paramdef>sd_bus_message *<parameter>m</parameter></paramdef>
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<paramdef>char <parameter>type</parameter></paramdef>
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<paramdef>void *<parameter>p</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>
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<function>sd_bus_message_read_basic()</function> reads a basic type from a
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message and advances the read position in the message. The set of basic
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types and their ascii codes passed in <parameter>type</parameter> are
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described in the <ulink
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url="https://dbus.freedesktop.org/doc/dbus-specification.html">D-Bus
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Specification</ulink>.
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</para>
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<para>
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If <parameter>p</parameter> is not <constant>NULL</constant>, it should contain a pointer to an
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appropriate object. For example, if <parameter>type</parameter> is <constant>'y'</constant>, the object
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passed in <parameter>p</parameter> should have type <type>uint8_t *</type>. If
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<parameter>type</parameter> is <constant>'s'</constant>, the object passed in <parameter>p</parameter>
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should have type <type>const char **</type>. Note that, if the basic type is a pointer (e.g.,
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<type>const char *</type> in the case of a string), the pointer is only borrowed and the contents must
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be copied if they are to be used after the end of the message's lifetime. Similarly, during the
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lifetime of such a pointer, the message must not be modified. If <parameter>type</parameter> is
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<constant>'h'</constant> (UNIX file descriptor), the descriptor is not duplicated by this call and the
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returned descriptor remains in possession of the message object, and needs to be duplicated by the
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caller in order to keep an open reference to it after the message object is freed (for example by
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calling <literal>fcntl(fd, FD_DUPFD_CLOEXEC, 3)</literal>). See the table below for a complete list of
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allowed types.
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</para>
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<table id='format-specifiers'>
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<title>Item type specifiers</title>
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<tgroup cols='4'>
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<colspec colname='specifier' />
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<colspec colname='constant' />
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<colspec colname='description' />
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<colspec colname='ctype' />
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<thead>
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<row>
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<entry>Specifier</entry>
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<entry>Constant</entry>
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<entry>Description</entry>
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<entry>Expected C Type</entry>
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</row>
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</thead>
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<tbody>
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<row>
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<entry><literal>y</literal></entry>
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<entry><constant>SD_BUS_TYPE_BYTE</constant></entry>
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<entry>8bit unsigned integer</entry>
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<entry><type>uint8_t *</type></entry>
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</row>
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<row>
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<entry><literal>b</literal></entry>
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<entry><constant>SD_BUS_TYPE_BOOLEAN</constant></entry>
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<entry>boolean</entry>
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<entry><type>int *</type> (NB: not <type>bool *</type>)</entry>
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</row>
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<row>
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<entry><literal>n</literal></entry>
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<entry><constant>SD_BUS_TYPE_INT16</constant></entry>
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<entry>16bit signed integer</entry>
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<entry><type>int16_t *</type></entry>
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</row>
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<row>
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<entry><literal>q</literal></entry>
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<entry><constant>SD_BUS_TYPE_UINT16</constant></entry>
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<entry>16bit unsigned integer</entry>
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<entry><type>uint16_t *</type></entry>
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</row>
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<row>
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<entry><literal>i</literal></entry>
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<entry><constant>SD_BUS_TYPE_INT32</constant></entry>
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<entry>32bit signed integer</entry>
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<entry><type>int32_t *</type></entry>
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</row>
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<row>
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<entry><literal>u</literal></entry>
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<entry><constant>SD_BUS_TYPE_UINT32</constant></entry>
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<entry>32bit unsigned integer</entry>
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<entry><type>uint32_t *</type></entry>
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</row>
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<row>
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<entry><literal>x</literal></entry>
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<entry><constant>SD_BUS_TYPE_INT64</constant></entry>
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<entry>64bit signed integer</entry>
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<entry><type>int64_t *</type></entry>
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</row>
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<row>
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<entry><literal>t</literal></entry>
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<entry><constant>SD_BUS_TYPE_UINT64</constant></entry>
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<entry>64bit unsigned integer</entry>
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<entry><type>uint64_t *</type></entry>
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</row>
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<row>
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<entry><literal>d</literal></entry>
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<entry><constant>SD_BUS_TYPE_DOUBLE</constant></entry>
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<entry>IEEE 754 double precision floating-point</entry>
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<entry><type>double *</type></entry>
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</row>
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<row>
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<entry><literal>s</literal></entry>
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<entry><constant>SD_BUS_TYPE_STRING</constant></entry>
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<entry>UTF-8 string</entry>
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<entry><type>const char **</type></entry>
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</row>
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<row>
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<entry><literal>o</literal></entry>
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<entry><constant>SD_BUS_TYPE_OBJECT_PATH</constant></entry>
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<entry>D-Bus object path string</entry>
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<entry><type>const char **</type></entry>
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</row>
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<row>
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<entry><literal>g</literal></entry>
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<entry><constant>SD_BUS_TYPE_SIGNATURE</constant></entry>
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<entry>D-Bus signature string</entry>
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<entry><type>const char **</type></entry>
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</row>
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<row>
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<entry><literal>h</literal></entry>
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<entry><constant>SD_BUS_TYPE_UNIX_FD</constant></entry>
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<entry>UNIX file descriptor</entry>
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<entry><type>int *</type></entry>
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</row>
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</tbody>
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</tgroup>
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</table>
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<para>
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If there is no object of the specified type at the current position in the
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message, an error is returned.
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</para>
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</refsect1>
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<refsect1>
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<title>Return Value</title>
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<para>
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On success, <function>sd_bus_message_read_basic()</function> returns a positive integer.
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If the end of the currently opened array has been reached, it returns 0.
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On failure, it returns a negative errno-style error code.
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</para>
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<refsect2 id='errors'>
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<title>Errors</title>
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<para>Returned errors may indicate the following problems:</para>
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<variablelist>
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<varlistentry id="errors-einval">
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<term><constant>-EINVAL</constant></term>
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<listitem><para>Specified type string is invalid or the message parameter is
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<constant>NULL</constant>.</para></listitem>
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</varlistentry>
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<varlistentry id="errors-enxio">
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<term><constant>-ENXIO</constant></term>
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<listitem><para>The message does not contain the specified type at current position.
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</para></listitem>
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</varlistentry>
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<varlistentry id="errors-ebadmsg">
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<term><constant>-EBADMSG</constant></term>
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<listitem><para>The message cannot be parsed.</para></listitem>
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</varlistentry>
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</variablelist>
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</refsect2>
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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-bus</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_bus_message_append_basic</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_bus_message_skip</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_bus_message_read</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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