man/sd_bus_message_read.xml

   1 <?xml version='1.0'?>
   2 <!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
   3 "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
   4
   5 <!--
   6   SPDX-License-Identifier: LGPL-2.1+
   7 -->
   8
   9 <refentry id="sd_bus_message_read"
  10           xmlns:xi="http://www.w3.org/2001/XInclude">
  11
  12   <refentryinfo>
  13     <title>sd_bus_message_read</title>
  14     <productname>systemd</productname>
  15   </refentryinfo>
  16
  17   <refmeta>
  18     <refentrytitle>sd_bus_message_read</refentrytitle>
  19     <manvolnum>3</manvolnum>
  20   </refmeta>
  21
  22   <refnamediv>
  23     <refname>sd_bus_message_read</refname>
  24     <refname>sd_bus_message_readv</refname>
  25
  26     <refpurpose>Read a sequence of values from a message</refpurpose>
  27   </refnamediv>
  28
  29   <refsynopsisdiv>
  30     <funcsynopsis>
  31       <funcsynopsisinfo>#include &lt;systemd/sd-bus.h&gt;</funcsynopsisinfo>
  32
  33       <funcprototype>
  34         <funcdef>int <function>sd_bus_message_read</function></funcdef>
  35         <paramdef>sd_bus_message *<parameter>m</parameter></paramdef>
  36         <paramdef>char char *<parameter>types</parameter></paramdef>
  37         <paramdef>...</paramdef>
  38       </funcprototype>
  39
  40       <funcprototype>
  41         <funcdef>int <function>sd_bus_message_readv</function></funcdef>
  42         <paramdef>sd_bus_message *<parameter>m</parameter></paramdef>
  43         <paramdef>char char *<parameter>types</parameter></paramdef>
  44         <paramdef>va_list <parameter>ap</parameter></paramdef>
  45       </funcprototype>
  46     </funcsynopsis>
  47   </refsynopsisdiv>
  48
  49   <refsect1>
  50     <title>Description</title>
  51
  52     <para><function>sd_bus_message_read()</function> reads a sequence of fields from
  53     the D-Bus message object <parameter>m</parameter> and advances the read position
  54     in the message. The type string <parameter>types</parameter> describes the types
  55     of items expected in the message and the field arguments that follow. The type
  56     string may be <constant>NULL</constant> or empty, in which case nothing is
  57     read.</para>
  58
  59     <para>The type string is composed of the elements described in
  60     <citerefentry><refentrytitle>sd_bus_message_append</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
  61     i.e. basic and container types. It must contain zero or more single "complete
  62     types". The type string is <constant>NUL</constant>-terminated.</para>
  63
  64     <para>For each type specified in the type string, one or more arguments need to be specified
  65     after the <parameter>types</parameter> parameter, in the same order. The arguments must be
  66     pointers to appropriate types (a pointer to <code>int8_t</code> for a <literal>y</literal> in
  67     the type string, a pointer to <code>int32_t</code> for an <literal>i</literal>, a pointer to
  68     <code>const char*</code> for an <literal>s</literal>, ...)  which are set based on the values in
  69     the message. As an exception, in case or array and variant types, the first argument is an
  70     "input" argument that further specifies how the message should be read. See the table below for
  71     a complete list of allowed arguments and their types. Note that, if the basic type is a pointer
  72     (e.g., <code>const char *</code> in the case of a string), the argument is a pointer to a
  73     pointer, and also the pointer value that is written is only borrowed and the contents must be
  74     copied if they are to be used after the end of the messages lifetime.</para>
  75
  76     <para>Each argument may also be <constant>NULL</constant>, in which case the value is read and
  77     ignored.</para>
  78
  79     <table>
  80       <title>Item type specifiers</title>
  81
  82       <tgroup cols='5'>
  83         <colspec colname='specifier' />
  84         <colspec colname='constant' />
  85         <colspec colname='description' />
  86         <colspec colname='type1' />
  87         <colspec colname='type2' />
  88         <thead>
  89           <row>
  90             <entry>Specifier</entry>
  91             <entry>Constant</entry>
  92             <entry>Description</entry>
  93             <entry>Type of the first argument</entry>
  94             <entry>Types of the subsequent arguments, if any</entry>
  95           </row>
  96         </thead>
  97
  98         <tbody>
  99           <xi:include href="sd_bus_message_read_basic.xml" xpointer="xpointer(//table[@id='format-specifiers']//tbody/*)" />
 100
 101           <row>
 102             <entry><literal>a</literal></entry>
 103             <entry><constant>SD_BUS_TYPE_ARRAY</constant></entry>
 104             <entry>array</entry>
 105             <entry>int, which specifies the expected length <parameter>n</parameter> of the array</entry>
 106             <entry><parameter>n</parameter> sets of arguments appropriate for the array element type</entry>
 107           </row>
 108
 109           <row>
 110             <entry><literal>v</literal></entry>
 111             <entry><constant>SD_BUS_TYPE_VARIANT</constant></entry>
 112             <entry>variant</entry>
 113             <entry>signature string</entry>
 114             <entry>arguments appropriate for the types specified by the signature</entry>
 115           </row>
 116
 117           <row>
 118             <entry><literal>(</literal></entry>
 119             <entry><constant>SD_BUS_TYPE_STRUCT_BEGIN</constant></entry>
 120             <entry>array start</entry>
 121             <entry morerows="1" namest="type1" nameend="type2">arguments appropriate for the structure elements</entry>
 122           </row>
 123           <row>
 124             <entry><literal>)</literal></entry>
 125             <entry><constant>SD_BUS_TYPE_STRUCT_END</constant></entry>
 126             <entry>array end</entry>
 127           </row>
 128
 129           <row>
 130             <entry><literal>{</literal></entry>
 131             <entry><constant>SD_BUS_TYPE_DICT_ENTRY_BEGIN</constant></entry>
 132             <entry>dictionary entry start</entry>
 133             <entry morerows="1">arguments appropriate for the first type in the pair</entry>
 134             <entry morerows="1">arguments appropriate for the second type in the pair</entry>
 135             </row>
 136           <row>
 137             <entry><literal>}</literal></entry>
 138             <entry><constant>SD_BUS_TYPE_DICT_ENTRY_END</constant></entry>
 139             <entry>dictionary entry end</entry>
 140           </row>
 141        </tbody>
 142       </tgroup>
 143     </table>
 144
 145     <para>If objects of the specified types are not present at the current position
 146     in the message, an error is returned.
 147     </para>
 148
 149     <para>The <function>sd_bus_message_readv()</function> is equivalent to the
 150     <function>sd_bus_message_read()</function>, except that it is called with a
 151     <literal>va_list</literal> instead of a variable number of arguments. This
 152     function does not call the <function>va_end()</function> macro. Because it
 153     invokes the <function>va_arg()</function> macro, the value of
 154     <parameter>ap</parameter> is undefined after the call.</para>
 155   </refsect1>
 156
 157   <refsect1>
 158     <title>Return Value</title>
 159
 160     <para>
 161       On success, <function>sd_bus_message_read()</function> and
 162       <function>sd_bus_message_readv()</function> return 0 or a positive integer. On
 163       failure, they return a negative errno-style error code.
 164     </para>
 165   </refsect1>
 166
 167   <xi:include href="sd_bus_message_read_basic.xml" xpointer="errors" />
 168
 169   <xi:include href="libsystemd-pkgconfig.xml" />
 170
 171   <refsect1>
 172     <title>Examples</title>
 173
 174     <para>Read a single basic type (a 64-bit integer):
 175     </para>
 176
 177     <programlisting>sd_bus_message *m;
 178 int64_t x;
 179
 180 sd_bus_message_read(m, "x", &amp;x);</programlisting>
 181
 182     <para>Read all types of integers:</para>
 183
 184     <programlisting>uint8_t y;
 185 int16_t n;
 186 uint16_t q;
 187 int32_t i;
 188 uint32_t u;
 189 int32_t x;
 190 uint32_t t;
 191 double d;
 192
 193 sd_bus_message_read(m, "ynqiuxtd", &amp;y, &amp;n, &amp;q, &amp;i, &amp;u, &amp;x, &amp;t, &amp;d);</programlisting>
 194
 195      <para>Read a structure composed of a string and a D-Bus path:</para>
 196
 197      <programlisting>const char *s, *p;
 198
 199 sd_bus_message_read(m, "(so)", &amp;s, &amp;p);
 200 </programlisting>
 201
 202      <para>Read a variant, with the real type "gt" (signature, unsigned integer):
 203      </para>
 204
 205      <programlisting>const char *s;
 206 uint64_t *v;
 207
 208 sd_bus_message_read(m, "v", "gt", &amp;s, &amp;v);</programlisting>
 209
 210      <para>Read a dictionary containing three pairs of type {integer=>string}:
 211      </para>
 212
 213      <programlisting>int i, j, k;
 214 const char *s, *t, *u;
 215
 216 sd_bus_message_read(m, "a{is}", 3, &amp;i, &amp;s, &amp;j, &amp;t, &amp;k, &amp;u);
 217      </programlisting>
 218   </refsect1>
 219
 220   <refsect1>
 221     <title>See Also</title>
 222
 223     <para>
 224       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 225       <citerefentry><refentrytitle>sd-bus</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 226       <citerefentry><refentrytitle>sd_bus_message_read_basic</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 227       <citerefentry><refentrytitle>sd_bus_message_append</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 228     </para>
 229   </refsect1>
 230
 231 </refentry>