man/sd_bus_message_append.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_append"
          xmlns:xi="http://www.w3.org/2001/XInclude">

  <refentryinfo>
    <title>sd_bus_message_append</title>
    <productname>systemd</productname>
  </refentryinfo>

  <refmeta>
    <refentrytitle>sd_bus_message_append</refentrytitle>
    <manvolnum>3</manvolnum>
  </refmeta>

  <refnamediv>
    <refname>sd_bus_message_append</refname>
    <refname>sd_bus_message_appendv</refname>

    <refpurpose>Attach fields to a D-Bus message based on a type
    string</refpurpose>
  </refnamediv>

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

      <funcprototype>
        <funcdef>int sd_bus_message_append</funcdef>
        <paramdef>sd_bus_message *<parameter>m</parameter></paramdef>
        <paramdef>const char *<parameter>types</parameter></paramdef>
        <paramdef>…</paramdef>
      </funcprototype>

      <funcprototype>
          <funcdef>int sd_bus_message_appendv</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>The <function>sd_bus_message_append()</function> function
    appends a sequence of fields to the D-Bus message object
    <parameter>m</parameter>. The type string
    <parameter>types</parameter> describes the types of the field
    arguments that follow. For each type specified in the type string,
    one or more arguments need to be specified, in the same order as
    declared in the type string.</para>

    <para>The type string is composed of the elements shown in the
    table below. It contains zero or more single "complete types".
    Each complete type may be one of the basic types or a fully
    described container type. A container type may be a structure with
    the contained types, a variant, an array with its element type, or
    a dictionary entry with the contained types. The type string is
    <constant>NUL</constant>-terminated.</para>

    <para>In case of a basic type, one argument of the corresponding
    type is expected.</para>

    <para>A structure is denoted by a sequence of complete types
    between <literal>(</literal> and <literal>)</literal>. This
    sequence cannot be empty — it must contain at least one type.
    Arguments corresponding to this nested sequence follow the same
    rules as if they were not nested.</para>

    <para>A variant is denoted by <literal>v</literal>. Corresponding
    arguments must begin with a type string denoting a complete type,
    and following that, arguments corresponding to the specified type.
    </para>

    <para>An array is denoted by <literal>a</literal> followed by a
    complete type. Corresponding arguments must begin with the number of
    entries in the array, followed by the entries themselves,
    matching the element type of the array.</para>

    <para>A dictionary is an array of dictionary entries, denoted by
    <literal>a</literal> followed by a pair of complete types between
    <literal>{</literal> and <literal>}</literal>. The first of those
    types must be a basic type. Corresponding arguments must begin
    with the number of dictionary entries, followed by a pair of
    values for each entry matching the element type of
    the dictionary entries.</para>

    <para>The <function>sd_bus_message_appendv()</function> is equivalent to the
    <function>sd_bus_message_append()</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>

    <para>For further details on the D-Bus type system, please consult
    the <ulink
    url="http://dbus.freedesktop.org/doc/dbus-specification.html#type-system">D-Bus
    Specification</ulink>.</para>

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

      <tgroup cols='5'>
        <xi:include href="sd_bus_message_append_basic.xml" xpointer="xpointer(//table[@id='format-specifiers'])//colspec" />
        <xi:include href="sd_bus_message_append_basic.xml" xpointer="xpointer(//table[@id='format-specifiers']//thead)" />

        <tbody>
          <xi:include href="sd_bus_message_append_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>determined by array type and size</entry>
            <entry>int, followed by array contents</entry>
          </row>

          <row>
            <entry><literal>v</literal></entry>
            <entry><constant>SD_BUS_TYPE_VARIANT</constant></entry>
            <entry>variant</entry>
            <entry>determined by the type argument</entry>
            <entry>signature string, followed by variant contents</entry>
          </row>

          <row>
            <entry><literal>(</literal></entry>
            <entry><constant>SD_BUS_TYPE_STRUCT_BEGIN</constant></entry>
            <entry>array start</entry>
            <entry morerows="1">determined by the nested types</entry>
            <entry morerows="1">structure contents</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">determined by the nested types</entry>
            <entry morerows="1">dictionary contents</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>For types "s" and "g" (unicode string or signature), the pointer may be
    <constant>NULL</constant>, which is equivalent to an empty string. See
    <citerefentry><refentrytitle>sd_bus_message_append_basic</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    for the precise interpretation of those and other types.</para>

  </refsect1>

  <refsect1>
    <title>Types String Grammar</title>

    <programlisting>types ::= complete_type*
complete_type ::= basic_type | variant | structure | array | dictionary
basic_type ::= "y" | "n" | "q" | "u" | "i" | "x" | "t" | "d" |
               "b" | "h" |
               "s" | "o" | "g"
variant ::= "v"
structure ::= "(" complete_type+ ")"
array ::= "a" complete_type
dictionary ::= "a" "{" basic_type complete_type "}"
</programlisting>
  </refsect1>

  <refsect1>
    <title>Examples</title>

    <para>Append a single basic type (the string <literal>a string</literal>):
    </para>

    <programlisting>sd_bus_message *m;
…
sd_bus_message_append(m, "s", "a string");</programlisting>

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

    <programlisting>uint8_t y = 1;
int16_t n = 2;
uint16_t q = 3;
int32_t i = 4;
uint32_t u = 5;
int32_t x = 6;
uint32_t t = 7;
double d = 8.0;
sd_bus_message_append(m, "ynqiuxtd", y, n, q, i, u, x, t, d);</programlisting>

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

     <programlisting>sd_bus_message_append(m, "(so)", "a string", "/a/path");
</programlisting>

     <para>Append an array of UNIX file descriptors:</para>

     <programlisting>sd_bus_message_append(m, "ah", 3, STDIN_FILENO, STDOUT_FILENO, STDERR_FILENO);
</programlisting>

     <para>Append a variant, with the real type "g" (signature),
     and value "sdbusisgood":</para>

     <programlisting>sd_bus_message_append(m, "v", "g", "sdbusisgood");</programlisting>

     <para>Append a dictionary containing the mapping {1=>"a", 2=>"b", 3=>""}:
     </para>

     <programlisting>sd_bus_message_append(m, "a{is}", 3, 1, "a", 2, "b", 3, NULL);
     </programlisting>
  </refsect1>

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

    <para>On success, these functions return 0 or a positive integer. On failure, they return a negative
    errno-style error code.</para>

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

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

  <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_append_basic</refentrytitle><manvolnum>3</manvolnum></citerefentry>,
      <citerefentry><refentrytitle>sd_bus_message_append_array</refentrytitle><manvolnum>3</manvolnum></citerefentry>
    </para>
  </refsect1>

</refentry>