NAME

       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

SYNOPSIS

       #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);

DESCRIPTION

       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.

RETURN VALUE

       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.

   Errors
       Returned errors may indicate the following problems:

       -EINVAL
           m or contents are NULL or type is invalid.

       -EPERM
           The message m is already sealed.

       -ESTALE
           The message m is in an invalid state.

       -ENOMEM
           Memory allocation failed.

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

NOTES

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

EXAMPLES

       Example 1. Append an array of strings to a message

           /* SPDX-License-Identifier: CC0-1.0 */

           #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: CC0-1.0 */

           #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);
           }

SEE ALSO

       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]

NOTES

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