sd_bus_message_append - Man Page

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);

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 descriptor	4 bytes	int
"a"	SD_BUS_TYPE_ARRAY	array	determined by array type and size	int, followed by array contents
"v"	SD_BUS_TYPE_VARIANT	variant	determined by the type argument	signature string, followed by variant contents
"("	SD_BUS_TYPE_STRUCT_BEGIN	array start	determined by the nested types	structure contents
")"	SD_BUS_TYPE_STRUCT_END	array end
"{"	SD_BUS_TYPE_DICT_ENTRY_BEGIN	dictionary entry start	determined by the nested types	dictionary 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.