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NAME | SYNOPSIS | DESCRIPTION | TYPES STRING GRAMMAR | EXAMPLES | RETURN VALUE | NOTES | SEE ALSO | NOTES | COLOPHON |
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SD_BUS_MESSAGE_APPEND(3) sd_bus_message_append SD_BUS_MESSAGE_APPEND(3)
sd_bus_message_append, sd_bus_message_appendv - Attach fields to a
D-Bus message based on a type string
#include <systemd/sd-bus.h>
int sd_bus_message_append(sd_bus_message *m, const char *types,
...);
int sd_bus_message_appendv(sd_bus_message *m, const char *types,
va_list ap);
The sd_bus_message_append() function appends a sequence of fields
to the D-Bus message object m. The type string types 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.
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
NUL-terminated.
In case of a basic type, one argument of the corresponding type is
expected.
A structure is denoted by a sequence of complete types between "("
and ")". 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.
A variant is denoted by "v". Corresponding arguments must begin
with a type string denoting a complete type, and following that,
arguments corresponding to the specified type.
An array is denoted by "a" 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.
A dictionary is an array of dictionary entries, denoted by "a"
followed by a pair of complete types between "{" and "}". 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.
sd_bus_message_appendv() is equivalent to sd_bus_message_append(),
except that it is called with a "va_list" instead of a variable
number of arguments. This function does not call the va_end()
macro. Because it invokes the va_arg() macro, the value of ap is
undefined after the call.
For further details on the D-Bus type system, please consult the
D-Bus Specification[1].
Table 1. Item type specifiers
┌───────────┬──────────────────────────────┬────────────────┬──────────────┬─────────────┐
│ Specifier │ Constant │ Description │ Size │ Expected C │
│ │ │ │ │ Type │
├───────────┼──────────────────────────────┼────────────────┼──────────────┼─────────────┤
│ "y" │ SD_BUS_TYPE_BYTE │ unsigned │ 1 byte │ uint8_t │
│ │ │ integer │ │ │
├───────────┼──────────────────────────────┼────────────────┼──────────────┼─────────────┤
│ "b" │ SD_BUS_TYPE_BOOLEAN │ boolean │ 4 bytes │ int │
├───────────┼──────────────────────────────┼────────────────┼──────────────┼─────────────┤
│ "n" │ SD_BUS_TYPE_INT16 │ signed │ 2 bytes │ int16_t │
│ │ │ integer │ │ │
├───────────┼──────────────────────────────┼────────────────┼──────────────┼─────────────┤
│ "q" │ SD_BUS_TYPE_UINT16 │ unsigned │ 2 bytes │ uint16_t │
│ │ │ integer │ │ │
├───────────┼──────────────────────────────┼────────────────┼──────────────┼─────────────┤
│ "i" │ SD_BUS_TYPE_INT32 │ signed │ 4 bytes │ int32_t │
│ │ │ integer │ │ │
├───────────┼──────────────────────────────┼────────────────┼──────────────┼─────────────┤
│ "u" │ SD_BUS_TYPE_UINT32 │ unsigned │ 4 bytes │ uint32_t │
│ │ │ integer │ │ │
├───────────┼──────────────────────────────┼────────────────┼──────────────┼─────────────┤
│ "x" │ SD_BUS_TYPE_INT64 │ signed │ 8 bytes │ int64_t │
│ │ │ integer │ │ │
├───────────┼──────────────────────────────┼────────────────┼──────────────┼─────────────┤
│ "t" │ SD_BUS_TYPE_UINT64 │ unsigned │ 8 bytes │ uint64_t │
│ │ │ integer │ │ │
├───────────┼──────────────────────────────┼────────────────┼──────────────┼─────────────┤
│ "d" │ SD_BUS_TYPE_DOUBLE │ floating-point │ 8 bytes │ double │
├───────────┼──────────────────────────────┼────────────────┼──────────────┼─────────────┤
│ "s" │ SD_BUS_TYPE_STRING │ Unicode string │ variable │ char[] │
├───────────┼──────────────────────────────┼────────────────┼──────────────┼─────────────┤
│ "o" │ SD_BUS_TYPE_OBJECT_PATH │ object path │ variable │ char[] │
├───────────┼──────────────────────────────┼────────────────┼──────────────┼─────────────┤
│ "g" │ SD_BUS_TYPE_SIGNATURE │ signature │ variable │ char[] │
├───────────┼──────────────────────────────┼────────────────┼──────────────┼─────────────┤
│ "h" │ SD_BUS_TYPE_UNIX_FD │ UNIX file │ 4 bytes │ int │
│ │ │ descriptor │ │ │
├───────────┼──────────────────────────────┼────────────────┼──────────────┼─────────────┤
│ "a" │ SD_BUS_TYPE_ARRAY │ array │ determined │ int, │
│ │ │ │ by array │ followed by │
│ │ │ │ type and │ array │
│ │ │ │ size │ contents │
├───────────┼──────────────────────────────┼────────────────┼──────────────┼─────────────┤
│ "v" │ SD_BUS_TYPE_VARIANT │ variant │ determined │ signature │
│ │ │ │ by the type │ string, │
│ │ │ │ argument │ followed by │
│ │ │ │ │ variant │
│ │ │ │ │ contents │
├───────────┼──────────────────────────────┼────────────────┼──────────────┼─────────────┤
│ "(" │ SD_BUS_TYPE_STRUCT_BEGIN │ array start │ determined │ structure │
├───────────┼──────────────────────────────┼────────────────┤ by the │ contents │
│ ")" │ SD_BUS_TYPE_STRUCT_END │ array end │ nested types │ │
├───────────┼──────────────────────────────┼────────────────┼──────────────┼─────────────┤
│ "{" │ SD_BUS_TYPE_DICT_ENTRY_BEGIN │ dictionary │ │ │
│ │ │ entry start │ determined │ dictionary │
├───────────┼──────────────────────────────┼────────────────┤ by the │ contents │
│ "}" │ SD_BUS_TYPE_DICT_ENTRY_END │ dictionary │ nested types │ │
│ │ │ entry end │ │ │
└───────────┴──────────────────────────────┴────────────────┴──────────────┴─────────────┘
For types "s" and "g" (unicode string or signature), the pointer
may be NULL, which is equivalent to an empty string. For "h" (UNIX
file descriptor), the descriptor is duplicated by this call and
the passed descriptor stays in possession of the caller. See
sd_bus_message_append_basic(3) for the precise interpretation of
those and other types.
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 "}"
Append a single basic type (the string "a string"):
sd_bus_message *m;
...
sd_bus_message_append(m, "s", "a string");
Append all types of integers:
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);
Append a structure composed of a string and a D-Bus path:
sd_bus_message_append(m, "(so)", "a string", "/a/path");
Append an array of UNIX file descriptors:
sd_bus_message_append(m, "ah", 3, STDIN_FILENO, STDOUT_FILENO, STDERR_FILENO);
Append a variant, with the real type "g" (signature), and value
"sdbusisgood":
sd_bus_message_append(m, "v", "g", "sdbusisgood");
Append a dictionary containing the mapping {1=>"a", 2=>"b",
3=>""}:
sd_bus_message_append(m, "a{is}", 3, 1, "a", 2, "b", 3, NULL);
On success, these functions return a non-negative integer. On
failure, they return a negative errno-style error code.
Errors
Returned errors may indicate the following problems:
-EINVAL
Specified parameter is invalid.
-EPERM
Message has been sealed.
-ESTALE
Message is in invalid state.
-ENXIO
Message cannot be appended to.
-ENOMEM
Memory allocation failed.
Functions described here are available as a shared library, which
can be compiled against and linked to with the
libsystemd pkg-config(1) file.
The code described here uses getenv(3), which is declared to be
not multi-thread-safe. This means that the code calling the
functions described here must not call setenv(3) from a parallel
thread. It is recommended to only do calls to setenv() from an
early phase of the program when no other threads have been
started.
systemd(1), sd-bus(3), sd_bus_message_append_basic(3),
sd_bus_message_append_array(3), sd_bus_message_open_container(3)
1. D-Bus Specification
https://dbus.freedesktop.org/doc/dbus-specification.html#type-system
This page is part of the systemd (systemd system and service
manager) project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have a
bug report for this manual page, see
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
This page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2025-08-11. (At that
time, the date of the most recent commit that was found in the
repository was 2025-08-11.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
[email protected]
systemd 258~rc2 SD_BUS_MESSAGE_APPEND(3)
Pages that refer to this page: sd-bus(3), sd_bus_call(3), sd_bus_call_method(3), sd_bus_emit_signal(3), sd_bus_message_append_array(3), sd_bus_message_append_basic(3), sd_bus_message_append_strv(3), sd_bus_message_copy(3), sd_bus_message_open_container(3), sd_bus_message_read(3), sd_bus_message_verify_type(3), sd_bus_reply_method_return(3), sd_bus_set_property(3), systemd.directives(7), systemd.index(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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