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 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         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.

           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

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

EXAMPLES         top

       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);

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
       ⟨⟩.  If you have
       a bug report for this manual page, see
       This page was obtained from the project's upstream Git repository
       ⟨⟩ on 2021-04-01.  (At that
       time, the date of the most recent commit that was found in the
       repository was 2021-04-01.)  If you discover any rendering
       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 248                             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)