Provided by: libsystemd-dev_256.5-2ubuntu3.1_amd64 
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 integer │ 1 byte │ uint8_t │
├──────────┼──────────────────────────────┼──────────────────┼──────────────────┼──────────────────┤
│"b" │ SD_BUS_TYPE_BOOLEAN │ boolean │ 4 bytes │ int │
├──────────┼──────────────────────────────┼──────────────────┼──────────────────┼──────────────────┤
│"n" │ SD_BUS_TYPE_INT16 │ signed integer │ 2 bytes │ int16_t │
├──────────┼──────────────────────────────┼──────────────────┼──────────────────┼──────────────────┤
│"q" │ SD_BUS_TYPE_UINT16 │ unsigned integer │ 2 bytes │ uint16_t │
├──────────┼──────────────────────────────┼──────────────────┼──────────────────┼──────────────────┤
│"i" │ SD_BUS_TYPE_INT32 │ signed integer │ 4 bytes │ int32_t │
├──────────┼──────────────────────────────┼──────────────────┼──────────────────┼──────────────────┤
│"u" │ SD_BUS_TYPE_UINT32 │ unsigned integer │ 4 bytes │ uint32_t │
├──────────┼──────────────────────────────┼──────────────────┼──────────────────┼──────────────────┤
│"x" │ SD_BUS_TYPE_INT64 │ signed integer │ 8 bytes │ int64_t │
├──────────┼──────────────────────────────┼──────────────────┼──────────────────┼──────────────────┤
│"t" │ SD_BUS_TYPE_UINT64 │ unsigned integer │ 8 bytes │ uint64_t │
├──────────┼──────────────────────────────┼──────────────────┼──────────────────┼──────────────────┤
│"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 by │ int, followed by │
│ │ │ │ array type and │ array contents │
│ │ │ │ size │ │
├──────────┼──────────────────────────────┼──────────────────┼──────────────────┼──────────────────┤
│"v" │ SD_BUS_TYPE_VARIANT │ variant │ determined by │ signature │
│ │ │ │ the type │ string, followed │
│ │ │ │ argument │ by variant │
│ │ │ │ │ contents │
├──────────┼──────────────────────────────┼──────────────────┼──────────────────┼──────────────────┤
│"(" │ SD_BUS_TYPE_STRUCT_BEGIN │ array start │ determined by │ structure │
├──────────┼──────────────────────────────┼──────────────────┤ the nested types │ contents │
│")" │ SD_BUS_TYPE_STRUCT_END │ array end │ │ │
├──────────┼──────────────────────────────┼──────────────────┼──────────────────┼──────────────────┤
│"{" │ SD_BUS_TYPE_DICT_ENTRY_BEGIN │ dictionary entry │ │ │
│ │ │ start │ determined by │ dictionary │
├──────────┼──────────────────────────────┼──────────────────┤ the nested types │ contents │
│"}" │ SD_BUS_TYPE_DICT_ENTRY_END │ dictionary 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