sd_bus_message_append(3) — Linux manual page
SD_BUS_MESSAGE_APPEND(3) sd_bus_message_append SD_BUS_MESSAGE_APPEND(3)
NAME
sd_bus_message_append, sd_bus_message_appendv - Attach fields to
a D-Bus message based on a type string
SYNOPSIS
#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);
DESCRIPTION
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 STRING GRAMMAR
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 "}"
EXAMPLES
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);
RETURN VALUE
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.
NOTES
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.
SEE ALSO
systemd(1), sd-bus(3), sd_bus_message_append_basic(3),
sd_bus_message_append_array(3), sd_bus_message_open_container(3)
NOTES
1. D-Bus Specification
https://dbus.freedesktop.org/doc/dbus-specification.html#type-system
COLOPHON
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This page was obtained from the project's upstream Git repository
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systemd 257~devel 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)