systemd-stable/man/sd_bus_message_read.xml

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<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
  "http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<!-- SPDX-License-Identifier: LGPL-2.1+ -->

<refentry id="sd_bus_message_read"
          xmlns:xi="http://www.w3.org/2001/XInclude">

  <refentryinfo>
    <title>sd_bus_message_read</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>sd_bus_message_read</refentrytitle>
    <manvolnum>3</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>sd_bus_message_read</refname>
    <refname>sd_bus_message_readv</refname>

    <refpurpose>Read a sequence of values from a message</refpurpose>
  </refnamediv>

  <refsynopsisdiv>
    <funcsynopsis>
      <funcsynopsisinfo>#include &lt;systemd/sd-bus.h&gt;</funcsynopsisinfo>

      <funcprototype>
        <funcdef>int <function>sd_bus_message_read</function></funcdef>
        <paramdef>sd_bus_message *<parameter>m</parameter></paramdef>
        <paramdef>const char *<parameter>types</parameter></paramdef>
        <paramdef>...</paramdef>
      </funcprototype>

      <funcprototype>
        <funcdef>int <function>sd_bus_message_readv</function></funcdef>
        <paramdef>sd_bus_message *<parameter>m</parameter></paramdef>
        <paramdef>const char *<parameter>types</parameter></paramdef>
        <paramdef>va_list <parameter>ap</parameter></paramdef>
      </funcprototype>
    </funcsynopsis>
  </refsynopsisdiv>

  <refsect1>
    <title>Description</title>

    <para><function>sd_bus_message_read()</function> reads a sequence of fields from
    the D-Bus message object <parameter>m</parameter> and advances the read position
    in the message. The type string <parameter>types</parameter> describes the types
    of items expected in the message and the field arguments that follow. The type
    string may be <constant>NULL</constant> or empty, in which case nothing is
    read.</para>

    <para>The type string is composed of the elements described in
    <citerefentry><refentrytitle>sd_bus_message_append</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
    i.e. basic and container types. It must contain zero or more single "complete
    types". The type string is <constant>NUL</constant>-terminated.</para>

    <para>For each type specified in the type string, one or more arguments need to be specified
    after the <parameter>types</parameter> parameter, in the same order. The arguments must be
    pointers to appropriate types (a pointer to <type>int8_t</type> for a <literal>y</literal> in
    the type string, a pointer to <type>int32_t</type> for an <literal>i</literal>, a pointer to
    <type>const char*</type> for an <literal>s</literal>, ...)  which are set based on the values in
    the message. As an exception, in case of array and variant types, the first argument is an
    "input" argument that further specifies how the message should be read. See the table below for
    a complete list of allowed arguments and their types. Note that, if the basic type is a pointer
    (e.g., <type>const char *</type> in the case of a string), the argument is a pointer to a
    pointer, and also the pointer value that is written is only borrowed and the contents must be
    copied if they are to be used after the end of the messages lifetime.</para>

    <para>Each argument may also be <constant>NULL</constant>, in which case the value is read and
    ignored.</para>

    <table>
      <title>Item type specifiers</title>

      <tgroup cols='5'>
        <colspec colname='specifier' />
        <colspec colname='constant' />
        <colspec colname='description' />
        <colspec colname='type1' />
        <colspec colname='type2' />
        <thead>
          <row>
            <entry>Specifier</entry>
            <entry>Constant</entry>
            <entry>Description</entry>
            <entry>Type of the first argument</entry>
            <entry>Types of the subsequent arguments, if any</entry>
          </row>
        </thead>

        <tbody>
          <xi:include href="sd_bus_message_read_basic.xml" xpointer="xpointer(//table[@id='format-specifiers']//tbody/*)" />

          <row>
            <entry><literal>a</literal></entry>
            <entry><constant>SD_BUS_TYPE_ARRAY</constant></entry>
            <entry>array</entry>
            <entry><type>int</type>, which specifies the expected length <parameter>n</parameter> of the array</entry>
            <entry><parameter>n</parameter> sets of arguments appropriate for the array element type</entry>
          </row>

          <row>
            <entry><literal>v</literal></entry>
            <entry><constant>SD_BUS_TYPE_VARIANT</constant></entry>
            <entry>variant</entry>
            <entry>signature string</entry>
            <entry>arguments appropriate for the types specified by the signature</entry>
          </row>

          <row>
            <entry><literal>(</literal></entry>
            <entry><constant>SD_BUS_TYPE_STRUCT_BEGIN</constant></entry>
            <entry>array start</entry>
            <entry morerows="1" namest="type1" nameend="type2">arguments appropriate for the structure elements</entry>
          </row>
          <row>
            <entry><literal>)</literal></entry>
            <entry><constant>SD_BUS_TYPE_STRUCT_END</constant></entry>
            <entry>array end</entry>
          </row>

          <row>
            <entry><literal>{</literal></entry>
            <entry><constant>SD_BUS_TYPE_DICT_ENTRY_BEGIN</constant></entry>
            <entry>dictionary entry start</entry>
            <entry morerows="1">arguments appropriate for the first type in the pair</entry>
            <entry morerows="1">arguments appropriate for the second type in the pair</entry>
            </row>
          <row>
            <entry><literal>}</literal></entry>
            <entry><constant>SD_BUS_TYPE_DICT_ENTRY_END</constant></entry>
            <entry>dictionary entry end</entry>
          </row>
       </tbody>
      </tgroup>
    </table>

    <para>If objects of the specified types are not present at the current position
    in the message, an error is returned.
    </para>

    <para>The <function>sd_bus_message_readv()</function> is equivalent to the
    <function>sd_bus_message_read()</function>, except that it is called with a
    <literal>va_list</literal> instead of a variable number of arguments. This
    function does not call the <function>va_end()</function> macro. Because it
    invokes the <function>va_arg()</function> macro, the value of
    <parameter>ap</parameter> is undefined after the call.</para>
  </refsect1>

  <refsect1>
    <title>Return Value</title>

    <para>On success, <function>sd_bus_message_read()</function> and
    <function>sd_bus_message_readv()</function> return 0 or a positive integer. On failure, they return a
    negative errno-style error code.</para>

    <xi:include href="sd_bus_message_read_basic.xml" xpointer="errors" />
  </refsect1>

  <xi:include href="libsystemd-pkgconfig.xml" />

  <refsect1>
    <title>Examples</title>

    <para>Read a single basic type (a 64-bit integer):
    </para>

    <programlisting>sd_bus_message *m;
int64_t x;

sd_bus_message_read(m, "x", &amp;x);</programlisting>

    <para>Read a boolean value:</para>

    <programlisting>sd_bus_message *m;
int x; /* Do not use C99 'bool' type here, it's typically smaller
          in memory and would cause memory corruption */

sd_bus_message_read(m, "b", &amp;x);</programlisting>

    <para>Read all types of integers:</para>

    <programlisting>uint8_t y;
int16_t n;
uint16_t q;
int32_t i;
uint32_t u;
int32_t x;
uint32_t t;
double d;

sd_bus_message_read(m, "ynqiuxtd", &amp;y, &amp;n, &amp;q, &amp;i, &amp;u, &amp;x, &amp;t, &amp;d);</programlisting>

     <para>Read a structure composed of a string and a D-Bus path:</para>

     <programlisting>const char *s, *p;

sd_bus_message_read(m, "(so)", &amp;s, &amp;p);
</programlisting>

     <para>Read a variant, with the real type "gt" (signature, unsigned integer):
     </para>

     <programlisting>const char *s;
uint64_t *v;

sd_bus_message_read(m, "v", "gt", &amp;s, &amp;v);</programlisting>

     <para>Read a dictionary containing three pairs of type {integer=>string}:
     </para>

     <programlisting>int i, j, k;
const char *s, *t, *u;

sd_bus_message_read(m, "a{is}", 3, &amp;i, &amp;s, &amp;j, &amp;t, &amp;k, &amp;u);
     </programlisting>
  </refsect1>

  <refsect1>
    <title>See Also</title>

    <para>
      <citerefentry><refentrytitle>systemd</refentrytitle><manvolnum>1</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd-bus</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_bus_message_read_basic</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_bus_message_skip</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_bus_message_append</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>