sd_bus_message_open_container - Man Page

#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​.

Functions described here are available as a shared library, which can be compiled against and linked to with the libsystemd ​pkg-config(1) file​.

All functions listed here are thread-agnostic and only a single thread may operate on a given object at any given time​. Different threads may access the same object at different times​. Multiple independent objects may be used from different threads in parallel​.

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) {
  const char *s;
  int r;

  r = sd_bus_message_open_container(m, 'a', "s");
  if (r < 0)
    return r;

  for (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)
      break;

    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]

1.

The D-Bus specification
https://dbus.freedesktop.org/doc/dbus-specification.html