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sd_bus_message_open_container, sd_bus_message_close_container, sd_bus_message_enter_container, sd_bus_message_exit_container - Create and move between containers in D-Bus messages


#include <systemd/sd-bus.h>

int sd_bus_message_open_container(sd_bus_message *m, char type, const char *contents);

int sd_bus_message_close_container(sd_bus_message *m);

int sd_bus_message_enter_container(sd_bus_message *m, char type, const char *contents);

int sd_bus_message_exit_container(sd_bus_message *m);


sd_bus_message_open_container() appends a new container to the message m. After opening a new container, it can be filled with content using sd_bus_message_append(3) and similar functions. Containers behave like a stack. To nest containers inside each other, call sd_bus_message_open_container() multiple times without calling sd_bus_message_close_container() in between. Each container will be nested inside the previous container. type represents the container type and should be one of "r", "a", "v" or "e" as described in sd_bus_message_append(3). Instead of literals, the corresponding constants SD_BUS_TYPE_STRUCT, SD_BUS_TYPE_ARRAY, SD_BUS_TYPE_VARIANT or SD_BUS_TYPE_DICT_ENTRY can also be used. contents describes the type of the container's elements and should be a D-Bus type string following the rules described in sd_bus_message_append(3).

sd_bus_message_close_container() closes the last container opened with sd_bus_message_open_container(). On success, the write pointer of the message m is positioned after the closed container in its parent container or in m itself if there is no parent container.

sd_bus_message_enter_container() enters the next container of the message m for reading. It behaves mostly the same as sd_bus_message_open_container(). Entering a container allows reading its contents with sd_bus_message_read(3) and similar functions. type and contents are the same as in sd_bus_message_open_container().

sd_bus_message_exit_container() exits the scope of the last container entered with sd_bus_message_enter_container(). It behaves mostly the same as sd_bus_message_close_container(). Note that sd_bus_message_exit_container() may only be called after iterating through all members of the container, i.e. reading or skipping over them. Use sd_bus_message_skip(3) to skip over fields of a container in order to be able to exit the container with sd_bus_message_exit_container() without reading all members.


On success, these functions return a non-negative integer. sd_bus_message_open_container() and sd_bus_message_close_container() return 0. sd_bus_message_enter_container() returns 1 if it successfully opened a new container, and 0 if that was not possible because the end of the currently open container or message was reached. sd_bus_message_exit_container() returns 1 on success. On failure, all of these functions return a negative errno-style error code.


Returned errors may indicate the following problems:


m or contents are NULL or type is invalid.


Message m has invalid structure.


Message m does not have a container of type type at the current position, or the contents do not match contents.


The message m is already sealed.


The message m is in an invalid state.


Memory allocation failed.


sd_bus_message_exit_container() was called but there are unread members left in the container.


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.


Example 1. Append an array of strings to a message

/* SPDX-License-Identifier: MIT-0 */
#include <systemd/sd-bus.h>
int append_strings_to_message(sd_bus_message *m, const char *const *arr) {

int r;
r = sd_bus_message_open_container(m, 'a', "s");
if (r < 0)
return r;
for (const char *s = *arr; *s; s++) {
r = sd_bus_message_append(m, "s", s);
if (r < 0)
return r;
return sd_bus_message_close_container(m); }

Example 2. Read an array of strings from a message

/* SPDX-License-Identifier: MIT-0 */
#include <stdio.h>
#include <systemd/sd-bus.h>
int read_strings_from_message(sd_bus_message *m) {

int r;
r = sd_bus_message_enter_container(m, 'a', "s");
if (r < 0)
return r;
for (;;) {
const char *s;
r = sd_bus_message_read(m, "s", &s);
if (r < 0)
return r;
if (r == 0)
printf("%s\n", s);
return sd_bus_message_exit_container(m); }


systemd(1), sd-bus(3), sd_bus_message_append(3), sd_bus_message_read(3), sd_bus_message_skip(3), The D-Bus specification[1]


The D-Bus specification
systemd 254