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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 them. Use sd_bus_message_skip(3) to skip over felds 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. On failure, they return a negative errno-style error code.

Returned errors may indicate the following problems:


m or contents are NULL or type is invalid.


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.

These APIs are implemented as a shared library, which can be compiled and linked to with the libsystemd pkg-config(1) file.

Example 1. Append an array of strings to a message
#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

#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 246