2 <!DOCTYPE refentry PUBLIC
"-//OASIS//DTD DocBook XML V4.5//EN"
3 "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
4 <!-- SPDX-License-Identifier: LGPL-2.1+ -->
6 <refentry id=
"sd_bus_message_read"
7 xmlns:
xi=
"http://www.w3.org/2001/XInclude">
10 <title>sd_bus_message_read
</title>
11 <productname>systemd
</productname>
15 <refentrytitle>sd_bus_message_read
</refentrytitle>
16 <manvolnum>3</manvolnum>
20 <refname>sd_bus_message_read
</refname>
21 <refname>sd_bus_message_readv
</refname>
22 <refname>sd_bus_message_peek_type
</refname>
24 <refpurpose>Read a sequence of values from a message
</refpurpose>
29 <funcsynopsisinfo>#include
<systemd/sd-bus.h
></funcsynopsisinfo>
32 <funcdef>int
<function>sd_bus_message_read
</function></funcdef>
33 <paramdef>sd_bus_message *
<parameter>m
</parameter></paramdef>
34 <paramdef>const char *
<parameter>types
</parameter></paramdef>
35 <paramdef>...
</paramdef>
39 <funcdef>int
<function>sd_bus_message_readv
</function></funcdef>
40 <paramdef>sd_bus_message *
<parameter>m
</parameter></paramdef>
41 <paramdef>const char *
<parameter>types
</parameter></paramdef>
42 <paramdef>va_list
<parameter>ap
</parameter></paramdef>
46 <funcdef>int
<function>sd_bus_message_peek_type
</function></funcdef>
47 <paramdef>char *
<parameter>type
</parameter></paramdef>
48 <paramdef>const char **
<parameter>contents
</parameter></paramdef>
54 <title>Description
</title>
56 <para><function>sd_bus_message_read()
</function> reads a sequence of fields from the D-Bus message object
57 <parameter>m
</parameter> and advances the read position in the message. The type string
58 <parameter>types
</parameter> describes the types of items expected in the message and the field arguments
59 that follow. The type string may be
<constant>NULL
</constant> or empty, in which case nothing is read.
62 <para>The type string is composed of the elements described in
63 <citerefentry><refentrytitle>sd_bus_message_append
</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
64 i.e. basic and container types. It must contain zero or more single
"complete types". The type string is
65 <constant>NUL
</constant>-terminated.
</para>
67 <para>For each type specified in the type string, one or more arguments need to be specified after the
68 <parameter>types
</parameter> parameter, in the same order. The arguments must be pointers to appropriate
69 types (a pointer to
<type>int8_t
</type> for a
<literal>y
</literal> in the type string, a pointer to
70 <type>int32_t
</type> for an
<literal>i
</literal>, a pointer to
<type>const char*
</type> for an
71 <literal>s
</literal>, ...) which are set based on the values in the message. As an exception, in case of
72 array and variant types, the first argument is an
"input" argument that further specifies how the message
73 should be read. See the table below for a complete list of allowed arguments and their types. Note that,
74 if the basic type is a pointer (e.g.,
<type>const char *
</type> in the case of a string), the argument is
75 a pointer to a pointer, and also the pointer value that is written is only borrowed and the contents must
76 be copied if they are to be used after the end of the messages lifetime.
</para>
78 <para>Each argument may also be
<constant>NULL
</constant>, in which case the value is read and ignored.
82 <title>Item type specifiers
</title>
85 <colspec colname='specifier'
/>
86 <colspec colname='constant'
/>
87 <colspec colname='description'
/>
88 <colspec colname='type1'
/>
89 <colspec colname='type2'
/>
92 <entry>Specifier
</entry>
93 <entry>Constant
</entry>
94 <entry>Description
</entry>
95 <entry>Type of the first argument
</entry>
96 <entry>Types of the subsequent arguments, if any
</entry>
101 <xi:include href=
"sd_bus_message_read_basic.xml" xpointer=
"xpointer(//table[@id='format-specifiers']//tbody/*)" />
104 <entry><literal>a
</literal></entry>
105 <entry><constant>SD_BUS_TYPE_ARRAY
</constant></entry>
107 <entry><type>int
</type>, which specifies the expected length
<parameter>n
</parameter> of the array
</entry>
108 <entry><parameter>n
</parameter> sets of arguments appropriate for the array element type
</entry>
112 <entry><literal>v
</literal></entry>
113 <entry><constant>SD_BUS_TYPE_VARIANT
</constant></entry>
114 <entry>variant
</entry>
115 <entry>signature string
</entry>
116 <entry>arguments appropriate for the types specified by the signature
</entry>
120 <entry><literal>(
</literal></entry>
121 <entry><constant>SD_BUS_TYPE_STRUCT_BEGIN
</constant></entry>
122 <entry>array start
</entry>
123 <entry morerows=
"1" namest=
"type1" nameend=
"type2">arguments appropriate for the structure elements
</entry>
126 <entry><literal>)
</literal></entry>
127 <entry><constant>SD_BUS_TYPE_STRUCT_END
</constant></entry>
128 <entry>array end
</entry>
132 <entry><literal>{
</literal></entry>
133 <entry><constant>SD_BUS_TYPE_DICT_ENTRY_BEGIN
</constant></entry>
134 <entry>dictionary entry start
</entry>
135 <entry morerows=
"1">arguments appropriate for the first type in the pair
</entry>
136 <entry morerows=
"1">arguments appropriate for the second type in the pair
</entry>
139 <entry><literal>}
</literal></entry>
140 <entry><constant>SD_BUS_TYPE_DICT_ENTRY_END
</constant></entry>
141 <entry>dictionary entry end
</entry>
147 <para>If objects of the specified types are not present at the current position in the message, an error
150 <para>The
<function>sd_bus_message_readv()
</function> is equivalent to the
151 <function>sd_bus_message_read()
</function>, except that it is called with a
<literal>va_list
</literal>
152 instead of a variable number of arguments. This function does not call the
<function>va_end()
</function>
153 macro. Because it invokes the
<function>va_arg()
</function> macro, the value of
<parameter>ap
</parameter>
154 is undefined after the call.
</para>
156 <para><function>sd_bus_message_peek_type()
</function> determines the type of the next element in
157 <parameter>m
</parameter> to be read by
<function>sd_bus_message_read()
</function> or similar functions.
158 On success, the type is stored in
<parameter>type
</parameter>, if it is not
<constant>NULL
</constant>.
159 If the type is a container type, the type of its elements is stored in
<parameter>contents
</parameter>,
160 if it is not
<constant>NULL
</constant>. If this function successfully determines the type of the next
161 element in
<parameter>m
</parameter>, it returns a positive integer. If there are no more elements to be
162 read, it returns zero.
</para>
166 <title>Return Value
</title>
168 <para>On success, these functions return a non-negative integer. On failure, they return a negative
169 errno-style error code.
</para>
171 <xi:include href=
"sd_bus_message_read_basic.xml" xpointer=
"errors" />
174 <xi:include href=
"libsystemd-pkgconfig.xml" />
177 <title>Examples
</title>
179 <para>Read a single basic type (a
64-bit integer):
182 <programlisting>sd_bus_message *m;
185 sd_bus_message_read(m,
"x",
&x);
</programlisting>
187 <para>Read a boolean value:
</para>
189 <programlisting>sd_bus_message *m;
190 int x; /* Do not use C99 'bool' type here, it's typically smaller
191 in memory and would cause memory corruption */
193 sd_bus_message_read(m,
"b",
&x);
</programlisting>
195 <para>Read all types of integers:
</para>
197 <programlisting>uint8_t y;
206 sd_bus_message_read(m,
"ynqiuxtd",
&y,
&n,
&q,
&i,
&u,
&x,
&t,
&d);
</programlisting>
208 <para>Read a structure composed of a string and a D-Bus path:
</para>
210 <programlisting>const char *s, *p;
212 sd_bus_message_read(m,
"(so)",
&s,
&p);
215 <para>Read a variant, with the real type
"gt" (signature, unsigned integer):
218 <programlisting>const char *s;
221 sd_bus_message_read(m,
"v",
"gt",
&s,
&v);
</programlisting>
223 <para>Read a dictionary containing three pairs of type {integer=
>string}:
226 <programlisting>int i, j, k;
227 const char *s, *t, *u;
229 sd_bus_message_read(m,
"a{is}",
3,
&i,
&s,
&j,
&t,
&k,
&u);
234 <title>See Also
</title>
237 <citerefentry><refentrytitle>systemd
</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
238 <citerefentry><refentrytitle>sd-bus
</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
239 <citerefentry><refentrytitle>sd_bus_message_read_basic
</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
240 <citerefentry><refentrytitle>sd_bus_message_skip
</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
241 <citerefentry><refentrytitle>sd_bus_message_append
</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
242 <citerefentry><refentrytitle>sd_bus_message_enter_container
</refentrytitle><manvolnum>3</manvolnum></citerefentry>