sd_bus_message_append

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## Name

sd_bus_message_append — Attach parts of message based on a format string

## Synopsis

    #include <systemd/sd-bus.h>

`int sd_bus_message_append(` | sd_bus_message *m,
| const char *types,
| ...`)`;

## Description

The `sd_bus_message_append` function appends a sequence of items to message `m`. The format string `types` describes the types of arguments that follow.

The format 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, a variant type code, an array with its element type, or a dictionary with its entry type. The format 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`". Correspoding arguments must include a format 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 include the size of the array, and then repeated this number of times, arguments corresponding to the nested type.

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 include the size of the dictionary, and then repeated this number of times, arguments corresponding to each of the two nested types.

Table 1. Item format specifiers

Specifier | Constant | Description | Size
--- | --- | --- | ---
"`y`" | `SD_BUS_TYPE_BYTE` | unsigned interger | 1 byte
"`b`" | `SD_BUS_TYPE_BOOLEAN` | boolean | 4 bytes
"`n`" | `SD_BUS_TYPE_INT16` | signed integer | 2 bytes
"`q`" | `SD_BUS_TYPE_UINT16` | unsigned integer | 2 bytes
"`i`" | `SD_BUS_TYPE_INT32` | signed integer | 4 bytes
"`u`" | `SD_BUS_TYPE_UINT32` | unsigned integer | 4 bytes
"`x`" | `SD_BUS_TYPE_INT64` | signed integer | 8 bytes
"`t`" | `SD_BUS_TYPE_UINT64` | unsigned integer | 8 bytes
"`d`" | `SD_BUS_TYPE_DOUBLE` | floating-point | 8 bytes
"`s`" | `SD_BUS_TYPE_STRING` | Unicode string | variable
"`o`" | `SD_BUS_TYPE_OBJECT_PATH` | object path | variable
"`g`" | `SD_BUS_TYPE_SIGNATURE` | signature | variable
"`h`" | `SD_BUS_TYPE_UNIX_FD` | UNIX file descriptor | 4 bytes
"`a`" | `SD_BUS_TYPE_ARRAY` | array | determined by array type and size
"`v`" | `SD_BUS_TYPE_VARIANT` | variant | determined by the type argument
"`(`" | `SD_BUS_TYPE_STRUCT_BEGIN` | array start | determined by the nested types
"`)`" | `SD_BUS_TYPE_STRUCT_END` | array end
"`{`" | `SD_BUS_TYPE_DICT_ENTRY_BEGIN` | dictionary entry start | determined by the nested types
"`}`" | `SD_BUS_TYPE_DICT_ENTRY_END` | dictionary entry end

## 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 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, this call returns 0 or a positive integer. On failure, this call returns 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

`sd_bus_open_user()` and other functions described here are available as a shared library, which can be compiled and linked to with the `libsystemd-bus` pkg-config(1) file.

## See Also

systemd(1), sd-bus(3), sd_bus_new(3), sd_bus_ref(3), sd_bus_unref(3), ssh(1), systemd-machined.service(8), machinectl(1)