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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.
276 lines
11 KiB
XML
276 lines
11 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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<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
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<refentry id="sd_bus_message_read"
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xmlns:xi="http://www.w3.org/2001/XInclude">
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<refentryinfo>
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<title>sd_bus_message_read</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</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</refname>
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<refname>sd_bus_message_readv</refname>
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<refname>sd_bus_message_peek_type</refname>
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<refpurpose>Read a sequence of values 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</function></funcdef>
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<paramdef>sd_bus_message *<parameter>m</parameter></paramdef>
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<paramdef>const char *<parameter>types</parameter></paramdef>
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<paramdef>...</paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_bus_message_readv</function></funcdef>
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<paramdef>sd_bus_message *<parameter>m</parameter></paramdef>
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<paramdef>const char *<parameter>types</parameter></paramdef>
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<paramdef>va_list <parameter>ap</parameter></paramdef>
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</funcprototype>
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<funcprototype>
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<funcdef>int <function>sd_bus_message_peek_type</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>const char **<parameter>contents</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_bus_message_read()</function> reads a sequence of fields from the D-Bus message object
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<parameter>m</parameter> and advances the read position in the message. The type string
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<parameter>types</parameter> describes the types of items expected in the message and the field arguments
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that follow. The type string may be <constant>NULL</constant> or empty, in which case nothing is read.
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</para>
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<para>The type string is composed of the elements described in
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<citerefentry><refentrytitle>sd_bus_message_append</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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i.e. basic and container types. It must contain zero or more single "complete types". The type string is
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<constant>NUL</constant>-terminated.</para>
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<para>For each type specified in the type string, one or more arguments need to be specified after the
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<parameter>types</parameter> parameter, in the same order. The arguments must be pointers to appropriate
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types (a pointer to <type>int8_t</type> for a <literal>y</literal> in the type string, a pointer to
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<type>int32_t</type> for an <literal>i</literal>, a pointer to <type>const char*</type> for an
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<literal>s</literal>, ...) which are set based on the values in the message. As an exception, in case of
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array and variant types, the first argument is an "input" argument that further specifies how the message
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should be read. See the table below for a complete list of allowed arguments and their types. Note that,
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if the basic type is a pointer (e.g., <type>const char *</type> in the case of a string), the argument is
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a pointer to a pointer, and also the pointer value that is written 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. If the type is
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<literal>h</literal> (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 caller
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in order to keep an open reference to it after the message object is freed.</para>
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<para>Each argument may also be <constant>NULL</constant>, in which case the value is read and ignored.
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</para>
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<table>
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<title>Item type specifiers</title>
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<tgroup cols='5'>
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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='type1' />
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<colspec colname='type2' />
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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>Type of the first argument</entry>
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<entry>Types of the subsequent arguments, if any</entry>
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</row>
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</thead>
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<tbody>
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<xi:include href="sd_bus_message_read_basic.xml" xpointer="xpointer(//table[@id='format-specifiers']//tbody/*)" />
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<row>
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<entry><literal>a</literal></entry>
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<entry><constant>SD_BUS_TYPE_ARRAY</constant></entry>
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<entry>array</entry>
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<entry><type>int</type>, which specifies the expected length <parameter>n</parameter> of the array</entry>
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<entry><parameter>n</parameter> sets of arguments appropriate for the array element type</entry>
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</row>
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<row>
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<entry><literal>v</literal></entry>
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<entry><constant>SD_BUS_TYPE_VARIANT</constant></entry>
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<entry>variant</entry>
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<entry>signature string</entry>
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<entry>arguments appropriate for the types specified by the signature</entry>
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</row>
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<row>
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<entry><literal>(</literal></entry>
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<entry><constant>SD_BUS_TYPE_STRUCT_BEGIN</constant></entry>
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<entry>array start</entry>
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<entry morerows="1" namest="type1" nameend="type2">arguments appropriate for the structure elements</entry>
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</row>
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<row>
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<entry><literal>)</literal></entry>
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<entry><constant>SD_BUS_TYPE_STRUCT_END</constant></entry>
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<entry>array end</entry>
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</row>
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<row>
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<entry><literal>{</literal></entry>
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<entry><constant>SD_BUS_TYPE_DICT_ENTRY_BEGIN</constant></entry>
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<entry>dictionary entry start</entry>
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<entry morerows="1">arguments appropriate for the first type in the pair</entry>
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<entry morerows="1">arguments appropriate for the second type in the pair</entry>
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</row>
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<row>
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<entry><literal>}</literal></entry>
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<entry><constant>SD_BUS_TYPE_DICT_ENTRY_END</constant></entry>
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<entry>dictionary entry end</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>If objects of the specified types are not present at the current position in the message, an error
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is returned.</para>
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<para>The <function>sd_bus_message_readv()</function> is equivalent to the
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<function>sd_bus_message_read()</function>, except that it is called with a <literal>va_list</literal>
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instead of a variable number of arguments. This function does not call the <function>va_end()</function>
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macro. Because it invokes the <function>va_arg()</function> macro, the value of <parameter>ap</parameter>
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is undefined after the call.</para>
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<para><function>sd_bus_message_peek_type()</function> determines the type of the next element in
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<parameter>m</parameter> to be read by <function>sd_bus_message_read()</function> or similar functions.
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On success, the type is stored in <parameter>type</parameter>, if it is not <constant>NULL</constant>.
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If the type is a container type, the type of its elements is stored in <parameter>contents</parameter>,
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if it is not <constant>NULL</constant>. If this function successfully determines the type of the next
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element in <parameter>m</parameter>, it returns a positive integer. If there are no more elements to be
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read, it returns zero.</para>
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</refsect1>
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<refsect1>
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<title>Return Value</title>
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<para>On success, these functions return a non-negative integer. On failure, they return a negative
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errno-style error code.</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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<xi:include href="sd_bus_message_read_basic.xml" xpointer="errors-einval"/>
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<xi:include href="sd_bus_message_read_basic.xml" xpointer="errors-enxio"/>
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<xi:include href="sd_bus_message_read_basic.xml" xpointer="errors-ebadmsg"/>
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<varlistentry>
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<term><constant>-EBUSY</constant></term>
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<listitem><para>When reading from a container, this error will be returned if unread elements
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are left in the container.</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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<xi:include href="libsystemd-pkgconfig.xml" />
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<refsect1>
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<title>Examples</title>
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<para>Read a single basic type (a 64-bit integer):
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</para>
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<programlisting>sd_bus_message *m;
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int64_t x;
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sd_bus_message_read(m, "x", &x);</programlisting>
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<para>Read a boolean value:</para>
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<programlisting>sd_bus_message *m;
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int x; /* Do not use C99 'bool' type here, it's typically smaller
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in memory and would cause memory corruption */
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sd_bus_message_read(m, "b", &x);</programlisting>
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<para>Read all types of integers:</para>
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<programlisting>uint8_t y;
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int16_t n;
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uint16_t q;
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int32_t i;
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uint32_t u;
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int32_t x;
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uint32_t t;
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double d;
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sd_bus_message_read(m, "ynqiuxtd", &y, &n, &q, &i, &u, &x, &t, &d);</programlisting>
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<para>Read a structure composed of a string and a D-Bus path:</para>
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<programlisting>const char *s, *p;
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sd_bus_message_read(m, "(so)", &s, &p);
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</programlisting>
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<para>Read a variant, with the real type "gt" (signature, unsigned integer):
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</para>
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<programlisting>const char *s;
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uint64_t *v;
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sd_bus_message_read(m, "v", "gt", &s, &v);</programlisting>
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<para>Read a dictionary containing three pairs of type {integer=>string}:
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</para>
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<programlisting>int i, j, k;
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const char *s, *t, *u;
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sd_bus_message_read(m, "a{is}", 3, &i, &s, &j, &t, &k, &u);
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</programlisting>
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<para>Read a single file descriptor, and duplicate it in order to keep it open after the message is
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freed.</para>
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<programlisting>sd_bus_message *m;
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int fd, fd_copy;
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sd_bus_message_read(m, "h", &fd);
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fd_copy = fcntl(fd, FD_DUPFD_CLOEXEC, 3);</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-bus</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_bus_message_read_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_append</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
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<citerefentry><refentrytitle>sd_bus_message_enter_container</refentrytitle><manvolnum>3</manvolnum></citerefentry>
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</para>
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</refsect1>
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</refentry>
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