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/4.5/docbookx.dtd">
   4 <!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
   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     <refname>sd_bus_message_peek_type</refname>
  23
  24     <refpurpose>Read a sequence of values from a message</refpurpose>
  25   </refnamediv>
  26
  27   <refsynopsisdiv>
  28     <funcsynopsis>
  29       <funcsynopsisinfo>#include &lt;systemd/sd-bus.h&gt;</funcsynopsisinfo>
  30
  31       <funcprototype>
  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>
  36       </funcprototype>
  37
  38       <funcprototype>
  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>
  43       </funcprototype>
  44
  45       <funcprototype>
  46         <funcdef>int <function>sd_bus_message_peek_type</function></funcdef>
  47         <paramdef>sd_bus_message *<parameter>m</parameter></paramdef>
  48         <paramdef>char *<parameter>type</parameter></paramdef>
  49         <paramdef>const char **<parameter>contents</parameter></paramdef>
  50       </funcprototype>
  51     </funcsynopsis>
  52   </refsynopsisdiv>
  53
  54   <refsect1>
  55     <title>Description</title>
  56
  57     <para><function>sd_bus_message_read()</function> reads a sequence of fields from the D-Bus message object
  58     <parameter>m</parameter> and advances the read position in the message. The type string
  59     <parameter>types</parameter> describes the types of items expected in the message and the field arguments
  60     that follow. The type string may be <constant>NULL</constant> or empty, in which case nothing is read.
  61     </para>
  62
  63     <para>The type string is composed of the elements described in
  64     <citerefentry><refentrytitle>sd_bus_message_append</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
  65     i.e. basic and container types. It must contain zero or more single "complete types". The type string is
  66     <constant>NUL</constant>-terminated.</para>
  67
  68     <para>For each type specified in the type string, one or more arguments need to be specified after the
  69     <parameter>types</parameter> parameter, in the same order. The arguments must be pointers to appropriate
  70     types (a pointer to <type>int8_t</type> for a <literal>y</literal> in the type string, a pointer to
  71     <type>int32_t</type> for an <literal>i</literal>, a pointer to <type>const char*</type> for an
  72     <literal>s</literal>, ...)  which are set based on the values in the message. As an exception, in case of
  73     array and variant types, the first argument is an "input" argument that further specifies how the message
  74     should be read. See the table below for a complete list of allowed arguments and their types. Note that,
  75     if the basic type is a pointer (e.g., <type>const char *</type> in the case of a string), the argument is
  76     a pointer to a pointer, and also the pointer value that is written is only borrowed and the contents must
  77     be copied if they are to be used after the end of the message's lifetime. If the type is
  78     <literal>h</literal> (UNIX file descriptor), the descriptor is not duplicated by this call and the
  79     returned descriptor remains in possession of the message object, and needs to be duplicated by the caller
  80     in order to keep an open reference to it after the message object is freed.</para>
  81
  82     <para>Each argument may also be <constant>NULL</constant>, in which case the value is read and ignored.
  83     </para>
  84
  85     <table>
  86       <title>Item type specifiers</title>
  87
  88       <tgroup cols='5'>
  89         <colspec colname='specifier' />
  90         <colspec colname='constant' />
  91         <colspec colname='description' />
  92         <colspec colname='type1' />
  93         <colspec colname='type2' />
  94         <thead>
  95           <row>
  96             <entry>Specifier</entry>
  97             <entry>Constant</entry>
  98             <entry>Description</entry>
  99             <entry>Type of the first argument</entry>
 100             <entry>Types of the subsequent arguments, if any</entry>
 101           </row>
 102         </thead>
 103
 104         <tbody>
 105           <xi:include href="sd_bus_message_read_basic.xml" xpointer="xpointer(//table[@id='format-specifiers']//tbody/*)" />
 106
 107           <row>
 108             <entry><literal>a</literal></entry>
 109             <entry><constant>SD_BUS_TYPE_ARRAY</constant></entry>
 110             <entry>array</entry>
 111             <entry><type>int</type>, which specifies the expected length <parameter>n</parameter> of the array</entry>
 112             <entry><parameter>n</parameter> sets of arguments appropriate for the array element type</entry>
 113           </row>
 114
 115           <row>
 116             <entry><literal>v</literal></entry>
 117             <entry><constant>SD_BUS_TYPE_VARIANT</constant></entry>
 118             <entry>variant</entry>
 119             <entry>signature string</entry>
 120             <entry>arguments appropriate for the types specified by the signature</entry>
 121           </row>
 122
 123           <row>
 124             <entry><literal>(</literal></entry>
 125             <entry><constant>SD_BUS_TYPE_STRUCT_BEGIN</constant></entry>
 126             <entry>array start</entry>
 127             <entry morerows="1" namest="type1" nameend="type2">arguments appropriate for the structure elements</entry>
 128           </row>
 129           <row>
 130             <entry><literal>)</literal></entry>
 131             <entry><constant>SD_BUS_TYPE_STRUCT_END</constant></entry>
 132             <entry>array end</entry>
 133           </row>
 134
 135           <row>
 136             <entry><literal>{</literal></entry>
 137             <entry><constant>SD_BUS_TYPE_DICT_ENTRY_BEGIN</constant></entry>
 138             <entry>dictionary entry start</entry>
 139             <entry morerows="1">arguments appropriate for the first type in the pair</entry>
 140             <entry morerows="1">arguments appropriate for the second type in the pair</entry>
 141             </row>
 142           <row>
 143             <entry><literal>}</literal></entry>
 144             <entry><constant>SD_BUS_TYPE_DICT_ENTRY_END</constant></entry>
 145             <entry>dictionary entry end</entry>
 146           </row>
 147        </tbody>
 148       </tgroup>
 149     </table>
 150
 151     <para>If objects of the specified types are not present at the current position in the message, an error
 152     is returned.</para>
 153
 154     <para>The <function>sd_bus_message_readv()</function> is equivalent to the
 155     <function>sd_bus_message_read()</function>, except that it is called with a <literal>va_list</literal>
 156     instead of a variable number of arguments. This function does not call the <function>va_end()</function>
 157     macro. Because it invokes the <function>va_arg()</function> macro, the value of <parameter>ap</parameter>
 158     is undefined after the call.</para>
 159
 160     <para><function>sd_bus_message_peek_type()</function> determines the type of the next element in
 161     <parameter>m</parameter> to be read by <function>sd_bus_message_read()</function> or similar functions.
 162     On success, the type is stored in <parameter>type</parameter>, if it is not <constant>NULL</constant>.
 163     If the type is a container type, the type of its elements is stored in <parameter>contents</parameter>,
 164     if it is not <constant>NULL</constant>. If this function successfully determines the type of the next
 165     element in <parameter>m</parameter>, it returns a positive integer. If there are no more elements to be
 166     read, it returns zero.</para>
 167   </refsect1>
 168
 169   <refsect1>
 170     <title>Return Value</title>
 171
 172     <para>On success, these functions return a non-negative integer. On failure, they return a negative
 173     errno-style error code.</para>
 174
 175     <refsect2 id='errors'>
 176       <title>Errors</title>
 177
 178       <para>Returned errors may indicate the following problems:</para>
 179
 180       <variablelist>
 181         <xi:include href="sd_bus_message_read_basic.xml" xpointer="errors-einval"/>
 182         <xi:include href="sd_bus_message_read_basic.xml" xpointer="errors-enxio"/>
 183         <xi:include href="sd_bus_message_read_basic.xml" xpointer="errors-ebadmsg"/>
 184         <varlistentry>
 185           <term><constant>-EBUSY</constant></term>
 186
 187           <listitem><para>When reading from a container, this error will be returned if unread elements
 188           are left in the container.</para>
 189
 190           <xi:include href="version-info.xml" xpointer="v251"/></listitem>
 191         </varlistentry>
 192       </variablelist>
 193     </refsect2>
 194   </refsect1>
 195
 196   <xi:include href="libsystemd-pkgconfig.xml" />
 197
 198   <refsect1>
 199     <title>Examples</title>
 200
 201     <para>Read a single basic type (a 64-bit integer):
 202     </para>
 203
 204     <programlisting>sd_bus_message *m;
 205 int64_t x;
 206
 207 sd_bus_message_read(m, "x", &amp;x);</programlisting>
 208
 209     <para>Read a boolean value:</para>
 210
 211     <programlisting>sd_bus_message *m;
 212 int x; /* Do not use C99 'bool' type here, it's typically smaller
 213           in memory and would cause memory corruption */
 214
 215 sd_bus_message_read(m, "b", &amp;x);</programlisting>
 216
 217     <para>Read all types of integers:</para>
 218
 219     <programlisting>uint8_t y;
 220 int16_t n;
 221 uint16_t q;
 222 int32_t i;
 223 uint32_t u;
 224 int32_t x;
 225 uint32_t t;
 226 double d;
 227
 228 sd_bus_message_read(m, "ynqiuxtd", &amp;y, &amp;n, &amp;q, &amp;i, &amp;u, &amp;x, &amp;t, &amp;d);</programlisting>
 229
 230      <para>Read a structure composed of a string and a D-Bus path:</para>
 231
 232      <programlisting>const char *s, *p;
 233
 234 sd_bus_message_read(m, "(so)", &amp;s, &amp;p);
 235 </programlisting>
 236
 237      <para>Read a variant, with the real type "gt" (signature, unsigned integer):
 238      </para>
 239
 240      <programlisting>const char *s;
 241 uint64_t *v;
 242
 243 sd_bus_message_read(m, "v", "gt", &amp;s, &amp;v);</programlisting>
 244
 245      <para>Read a dictionary containing three pairs of type {integer=>string}:
 246      </para>
 247
 248      <programlisting>int i, j, k;
 249 const char *s, *t, *u;
 250
 251 sd_bus_message_read(m, "a{is}", 3, &amp;i, &amp;s, &amp;j, &amp;t, &amp;k, &amp;u);
 252      </programlisting>
 253
 254      <para>Read a single file descriptor, and duplicate it in order to keep it open after the message is
 255      freed.</para>
 256
 257      <programlisting>sd_bus_message *m;
 258 int fd, fd_copy;
 259
 260 sd_bus_message_read(m, "h", &amp;fd);
 261 fd_copy = fcntl(fd, FD_DUPFD_CLOEXEC, 3);</programlisting>
 262   </refsect1>
 263
 264   <refsect1>
 265     <title>History</title>
 266     <para><function>sd_bus_message_read()</function> and
 267     <function>sd_bus_message_readv()</function> were added in version 240.</para>
 268     <para><function>sd_bus_message_peek_type()</function> was added in version 246.</para>
 269   </refsect1>
 270
 271   <refsect1>
 272     <title>See Also</title>
 273
 274     <para><simplelist type="inline">
 275       <member><citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry></member>
 276       <member><citerefentry><refentrytitle>sd-bus</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 277       <member><citerefentry><refentrytitle>sd_bus_message_read_basic</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 278       <member><citerefentry><refentrytitle>sd_bus_message_skip</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 279       <member><citerefentry><refentrytitle>sd_bus_message_append</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 280       <member><citerefentry><refentrytitle>sd_bus_message_enter_container</refentrytitle><manvolnum>3</manvolnum></citerefentry></member>
 281     </simplelist></para>
 282   </refsect1>
 283
 284 </refentry>