man/sd_bus_message_read.xml

   1 <?xml version='1.0'?>
   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+ -->
   5
   6 <refentry id="sd_bus_message_read"
   7           xmlns:xi="http://www.w3.org/2001/XInclude">
   8
   9   <refentryinfo>
  10     <title>sd_bus_message_read</title>
  11     <productname>systemd</productname>
  12   </refentryinfo>
  13
  14   <refmeta>
  15     <refentrytitle>sd_bus_message_read</refentrytitle>
  16     <manvolnum>3</manvolnum>
  17   </refmeta>
  18
  19   <refnamediv>
  20     <refname>sd_bus_message_read</refname>
  21     <refname>sd_bus_message_readv</refname>
  22
  23     <refpurpose>Read a sequence of values from a message</refpurpose>
  24   </refnamediv>
  25
  26   <refsynopsisdiv>
  27     <funcsynopsis>
  28       <funcsynopsisinfo>#include &lt;systemd/sd-bus.h&gt;</funcsynopsisinfo>
  29
  30       <funcprototype>
  31         <funcdef>int <function>sd_bus_message_read</function></funcdef>
  32         <paramdef>sd_bus_message *<parameter>m</parameter></paramdef>
  33         <paramdef>const char *<parameter>types</parameter></paramdef>
  34         <paramdef>...</paramdef>
  35       </funcprototype>
  36
  37       <funcprototype>
  38         <funcdef>int <function>sd_bus_message_readv</function></funcdef>
  39         <paramdef>sd_bus_message *<parameter>m</parameter></paramdef>
  40         <paramdef>const char *<parameter>types</parameter></paramdef>
  41         <paramdef>va_list <parameter>ap</parameter></paramdef>
  42       </funcprototype>
  43     </funcsynopsis>
  44   </refsynopsisdiv>
  45
  46   <refsect1>
  47     <title>Description</title>
  48
  49     <para><function>sd_bus_message_read()</function> reads a sequence of fields from
  50     the D-Bus message object <parameter>m</parameter> and advances the read position
  51     in the message. The type string <parameter>types</parameter> describes the types
  52     of items expected in the message and the field arguments that follow. The type
  53     string may be <constant>NULL</constant> or empty, in which case nothing is
  54     read.</para>
  55
  56     <para>The type string is composed of the elements described in
  57     <citerefentry><refentrytitle>sd_bus_message_append</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
  58     i.e. basic and container types. It must contain zero or more single "complete
  59     types". The type string is <constant>NUL</constant>-terminated.</para>
  60
  61     <para>For each type specified in the type string, one or more arguments need to be specified
  62     after the <parameter>types</parameter> parameter, in the same order. The arguments must be
  63     pointers to appropriate types (a pointer to <type>int8_t</type> for a <literal>y</literal> in
  64     the type string, a pointer to <type>int32_t</type> for an <literal>i</literal>, a pointer to
  65     <type>const char*</type> for an <literal>s</literal>, ...)  which are set based on the values in
  66     the message. As an exception, in case of array and variant types, the first argument is an
  67     "input" argument that further specifies how the message should be read. See the table below for
  68     a complete list of allowed arguments and their types. Note that, if the basic type is a pointer
  69     (e.g., <type>const char *</type> in the case of a string), the argument is a pointer to a
  70     pointer, and also the pointer value that is written is only borrowed and the contents must be
  71     copied if they are to be used after the end of the messages lifetime.</para>
  72
  73     <para>Each argument may also be <constant>NULL</constant>, in which case the value is read and
  74     ignored.</para>
  75
  76     <table>
  77       <title>Item type specifiers</title>
  78
  79       <tgroup cols='5'>
  80         <colspec colname='specifier' />
  81         <colspec colname='constant' />
  82         <colspec colname='description' />
  83         <colspec colname='type1' />
  84         <colspec colname='type2' />
  85         <thead>
  86           <row>
  87             <entry>Specifier</entry>
  88             <entry>Constant</entry>
  89             <entry>Description</entry>
  90             <entry>Type of the first argument</entry>
  91             <entry>Types of the subsequent arguments, if any</entry>
  92           </row>
  93         </thead>
  94
  95         <tbody>
  96           <xi:include href="sd_bus_message_read_basic.xml" xpointer="xpointer(//table[@id='format-specifiers']//tbody/*)" />
  97
  98           <row>
  99             <entry><literal>a</literal></entry>
 100             <entry><constant>SD_BUS_TYPE_ARRAY</constant></entry>
 101             <entry>array</entry>
 102             <entry><type>int</type>, which specifies the expected length <parameter>n</parameter> of the array</entry>
 103             <entry><parameter>n</parameter> sets of arguments appropriate for the array element type</entry>
 104           </row>
 105
 106           <row>
 107             <entry><literal>v</literal></entry>
 108             <entry><constant>SD_BUS_TYPE_VARIANT</constant></entry>
 109             <entry>variant</entry>
 110             <entry>signature string</entry>
 111             <entry>arguments appropriate for the types specified by the signature</entry>
 112           </row>
 113
 114           <row>
 115             <entry><literal>(</literal></entry>
 116             <entry><constant>SD_BUS_TYPE_STRUCT_BEGIN</constant></entry>
 117             <entry>array start</entry>
 118             <entry morerows="1" namest="type1" nameend="type2">arguments appropriate for the structure elements</entry>
 119           </row>
 120           <row>
 121             <entry><literal>)</literal></entry>
 122             <entry><constant>SD_BUS_TYPE_STRUCT_END</constant></entry>
 123             <entry>array end</entry>
 124           </row>
 125
 126           <row>
 127             <entry><literal>{</literal></entry>
 128             <entry><constant>SD_BUS_TYPE_DICT_ENTRY_BEGIN</constant></entry>
 129             <entry>dictionary entry start</entry>
 130             <entry morerows="1">arguments appropriate for the first type in the pair</entry>
 131             <entry morerows="1">arguments appropriate for the second type in the pair</entry>
 132             </row>
 133           <row>
 134             <entry><literal>}</literal></entry>
 135             <entry><constant>SD_BUS_TYPE_DICT_ENTRY_END</constant></entry>
 136             <entry>dictionary entry end</entry>
 137           </row>
 138        </tbody>
 139       </tgroup>
 140     </table>
 141
 142     <para>If objects of the specified types are not present at the current position
 143     in the message, an error is returned.
 144     </para>
 145
 146     <para>The <function>sd_bus_message_readv()</function> is equivalent to the
 147     <function>sd_bus_message_read()</function>, except that it is called with a
 148     <literal>va_list</literal> instead of a variable number of arguments. This
 149     function does not call the <function>va_end()</function> macro. Because it
 150     invokes the <function>va_arg()</function> macro, the value of
 151     <parameter>ap</parameter> is undefined after the call.</para>
 152   </refsect1>
 153
 154   <refsect1>
 155     <title>Return Value</title>
 156
 157     <para>On success, <function>sd_bus_message_read()</function> and
 158     <function>sd_bus_message_readv()</function> return 0 or a positive integer. On failure, they return a
 159     negative errno-style error code.</para>
 160
 161     <xi:include href="sd_bus_message_read_basic.xml" xpointer="errors" />
 162   </refsect1>
 163
 164   <xi:include href="libsystemd-pkgconfig.xml" />
 165
 166   <refsect1>
 167     <title>Examples</title>
 168
 169     <para>Read a single basic type (a 64-bit integer):
 170     </para>
 171
 172     <programlisting>sd_bus_message *m;
 173 int64_t x;
 174
 175 sd_bus_message_read(m, "x", &amp;x);</programlisting>
 176
 177     <para>Read a boolean value:</para>
 178
 179     <programlisting>sd_bus_message *m;
 180 int x; /* Do not use C99 'bool' type here, it's typically smaller
 181           in memory and would cause memory corruption */
 182
 183 sd_bus_message_read(m, "b", &amp;x);</programlisting>
 184
 185     <para>Read all types of integers:</para>
 186
 187     <programlisting>uint8_t y;
 188 int16_t n;
 189 uint16_t q;
 190 int32_t i;
 191 uint32_t u;
 192 int32_t x;
 193 uint32_t t;
 194 double d;
 195
 196 sd_bus_message_read(m, "ynqiuxtd", &amp;y, &amp;n, &amp;q, &amp;i, &amp;u, &amp;x, &amp;t, &amp;d);</programlisting>
 197
 198      <para>Read a structure composed of a string and a D-Bus path:</para>
 199
 200      <programlisting>const char *s, *p;
 201
 202 sd_bus_message_read(m, "(so)", &amp;s, &amp;p);
 203 </programlisting>
 204
 205      <para>Read a variant, with the real type "gt" (signature, unsigned integer):
 206      </para>
 207
 208      <programlisting>const char *s;
 209 uint64_t *v;
 210
 211 sd_bus_message_read(m, "v", "gt", &amp;s, &amp;v);</programlisting>
 212
 213      <para>Read a dictionary containing three pairs of type {integer=>string}:
 214      </para>
 215
 216      <programlisting>int i, j, k;
 217 const char *s, *t, *u;
 218
 219 sd_bus_message_read(m, "a{is}", 3, &amp;i, &amp;s, &amp;j, &amp;t, &amp;k, &amp;u);
 220      </programlisting>
 221   </refsect1>
 222
 223   <refsect1>
 224     <title>See Also</title>
 225
 226     <para>
 227       <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
 228       <citerefentry><refentrytitle>sd-bus</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 229       <citerefentry><refentrytitle>sd_bus_message_read_basic</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 230       <citerefentry><refentrytitle>sd_bus_message_skip</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
 231       <citerefentry><refentrytitle>sd_bus_message_append</refentrytitle><manvolnum>3</manvolnum></citerefentry>
 232     </para>
 233   </refsect1>
 234
 235 </refentry>