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<?xml version='1.0'?>
<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
<!-- SPDX-License-Identifier: LGPL-2.1-or-later -->
<refentry id="sd_bus_message_append"
<refpurpose>Attach fields to a D-Bus message based on a type string</refpurpose>
<funcsynopsisinfo>#include &lt;systemd/sd-bus.h&gt;</funcsynopsisinfo>
<funcdef>int sd_bus_message_append</funcdef>
<paramdef>sd_bus_message *<parameter>m</parameter></paramdef>
<paramdef>const char *<parameter>types</parameter></paramdef>
<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>
<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
<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
<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><function>sd_bus_message_appendv()</function> is equivalent to
<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="">D-Bus Specification</ulink>.
<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)" />
<xi:include href="sd_bus_message_append_basic.xml" xpointer="xpointer(//table[@id='format-specifiers']//tbody/*)" />
<entry>determined by array type and size</entry>
<entry>int, followed by array contents</entry>
<entry>determined by the type argument</entry>
<entry>signature string, followed by variant contents</entry>
<entry>array start</entry>
<entry morerows="1">determined by the nested types</entry>
<entry morerows="1">structure contents</entry>
<entry>array end</entry>
<entry>dictionary entry start</entry>
<entry morerows="1">determined by the nested types</entry>
<entry morerows="1">dictionary contents</entry>
<entry>dictionary entry end</entry>
<para>For types <literal>s</literal> and <literal>g</literal> (unicode string or signature), the pointer
may be <constant>NULL</constant>, which is equivalent to an empty string. For <literal>h</literal> (UNIX
file descriptor), the descriptor is duplicated by this call and the passed descriptor stays in possession
of the caller. See
for the precise interpretation of those and other types.</para>
<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 "}"
<para>Append a single basic type (the string <literal>a string</literal>):
<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");
<para>Append an array of UNIX file descriptors:</para>
<programlisting>sd_bus_message_append(m, "ah", 3, STDIN_FILENO, STDOUT_FILENO, STDERR_FILENO);
<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=>""}:
<programlisting>sd_bus_message_append(m, "a{is}", 3, 1, "a", 2, "b", 3, NULL);
<title>Return Value</title>
<para>On success, these functions return a non-negative integer. On failure, they return a
negative errno-style error code.</para>
<xi:include href="sd_bus_message_append_basic.xml" xpointer="errors" />
<xi:include href="libsystemd-pkgconfig.xml" />
<title>See Also</title>