sd_bus_message_open_container(3) — Linux manual page



NAME         top

       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         top

       #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         top

       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,
       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

       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_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.

RETURN VALUE         top

       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.

NOTES         top

       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

EXAMPLES         top

       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);

SEE ALSO         top

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

NOTES         top

        1. The D-Bus specification

COLOPHON         top

       This page is part of the systemd (systemd system and service
       manager) project.  Information about the project can be found at
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       This page was obtained from the project's upstream Git repository
       ⟨⟩ on 2023-06-23.  (At that
       time, the date of the most recent commit that was found in the
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       problems in this HTML version of the page, or you believe there
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       (which is not part of the original manual page), send a mail to

systemd 253                             SD_BUS_MESSAGE_OPEN_CONTAINER(3)

Pages that refer to this page: sd-bus(3)sd_bus_message_append(3)sd_bus_message_read(3)systemd.directives(7)systemd.index(7)